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PCM Light Weight Concrete Masonry Units for Use in Reactive Building Envelopes

Final Report for Phase 1

W. Mark McGinley Department of Civil Engineering University of Louisville Louisville, Kentucky

November 2013

ACKNOWLEDGEMENTS

The majority of this report has been taken from the Ph.D. Dissertation authored by Jeffery Kiesel in November 2013 at the University of Louisville.

Executive Summary PCM Light Weight Concrete Masonry Units for Use in Reactive Building Envelopes November 2013 . To address the increasing energy use in North America and projected decreases in energy supply, building codes and design guidelines across the various sectors are being changed to raise minimum energy performance standards. This increase in energy performance and the growth in sustainable design focus has made providing cost effective thermally efficient exterior wall systems a major design consideration. It has become difficult to meet the minimum envelop requirements with single wythe masonry wall systems, especially where most of the cores are grouted to provide structural support for the building. Although some increases in thermal resistance are possible through the use of core loose fill insulations, insulation inserts or expanding foam systems, these systems are of limited effectiveness due to thermal bridging. In many climates, a single wythe masonry wall will not meet the prescriptive code requirements for thermal resistance. The only solution in these cases is to apply a continuous layer of insulation over the CMU assembly.

This continuous insulation layer requires a covering and

significantly impacts the cost effectiveness of the wall system, its aesthetics, durability, and fire resistance. There is a need increase the thermal performance of a single wythe masonry wall systems in alternative ways. Thermal mass incorporated into the building envelope has the ability to attenuate peak interior diurnal temperature fluctuations and energy flow, provide interior thermal phase shifts, and absorb surplus energy from solar gains as well as internal gains created

iii

by occupants, lighting, appliances and electronics. Heating and cooling demands can be reduced and delayed by thermally massive building envelopes. Historically, thermal mass has been achieved using thick, dense building envelopes such as stone, adobe, and mass concrete. These massive walls absorb heat during the day keeping the interior cool, then release the stored heat during the night maintaining interior comfort. The thermal mass of a building envelope, in this case concrete, can be further increased with the incorporation of Phase Change Materials (PCMs). In the first phase of this research, Concrete Masonry Units (CMUs) were examined for improved thermal energy performance using the enhanced thermal properties provided by PCMs. Specifically this investigation evaluated whether PCMs could be incorporated into in concrete suitable for use to form CMUs with higher energy storage capability than standard CMUs. Once viable concrete mixes were formed and tested analytical studies evaluated how PCM/CMU’s might impact building energy use in a few building configurations. The results of this investigation indicate that Phase Change Materials can be successfully incorporated into concrete mixes appropriate for forming CMUs. Several viable PCM concrete mixes with compression strength equal or greater that the minimum 13.1 MPa (1900 psi) described in ASTM C 90 were developed. Thermal testing was performed on the PCMs as well as the PCM concrete to characterize the performance of these PCM concrete mixes. These characteristics were used to evaluate the performance of PCM concrete as a building envelope component using holistic energy analyses. The EnergyPlus holistic building analyses showed that

using PCM concrete with 10% to 15% PCM by weight provided equivalent energy performance to the ASHRAE 90.1 prescribed continuous interior insulation requirements in Climate Zones 2 through 4.

Single-wythe PCM concrete masonry walls with

polyurethane core insulation can be used as an acceptable alternative to interior insulated CMU walls. Though currently the cost of PCM makes it more these single wythe PCM/CMU’s more costly, with wide scale implementation, the cost of PCM concrete can be reduced to a price at which it is competitive with traditional masonry wall configurations. It is was estimated that if PCM price can halved to $2.20/kg ($1/lb), then PCM concrete masonry has the potential to compete with standard concrete masonry with interior insulation. Further, there are many potential benefits related to the additional effective thermal mass associated with PCM concrete masonry. These include buffering of interior temperatures associated with heat gains such as electronics, occupants, and solar gains as well as buffering of the interior air mass. Buffering of the heat gains from electronics, occupants, and solar gain can reduce summer overheating. Mass buffering can also alleviate heat losses due to drafts related to periodically opened fenestrations and help to regulate interior temperatures to improve HVAC performance and reduce cycling.

the HVAC system has fewer cycles with longer run times, performance efficiency can be improved. Although this investigation demonstrated performance enhancements with PCM concrete masonry walls, further research is need to realize all of the potential benefits of PCM Concrete Masonry Units. We recommend Phase 2 be conducted. This further experimental work is needed to quantify the behavior of PCM concrete masonry wall

systems. It is recommended that research be performed on PCM concrete masonry wall sections in the Dynamic Hot Box Apparatus to fully quantify the steady-state and transient multi-dimensional heat transfer properties of a PCM CMU wall section with mortar, grout, and cores. This would help to provide the heat storage and transfer characteristics for holistic building energy modeling. This is important in the further development of PCM concrete masonry modeling and will indicate whether simplified models are applicable or if data from the Dynamic Hot Box Apparatus is required for accurate modeling. It is also expected that examination of ventilation and passive solar strategies should provide significant performance improvements beyond that which was found in this research. Ground coupling and solar radiation can be used to capitalize on the effective thermal mass of PCM concrete masonry to store and release thermal energy. In the winter months this can be used to store thermal energy from daytime solar gains for release during the nights. In, addition, strategies can be developed to store solar thermal energy during summer days to reduce internal loads. Finally, the hollow cores of PCM concrete masonry can be coupled with earth tubes or similar structures so that cool air can be introduced to extract heat from the walls during the summer providing envelope cooling. During the winter, if the ground is warm enough, ground coupling with earth tubes or solar panels can introduce heat to the PCM masonry walls for radiant heating.

TABLE OF CONTENTS

ACKNOWLEDGEMENTS ................................................................................................ ii CHAPTER ONE: INTRODUCTION ................................................................................. 1 1.1

Overview .............................................................................................................. 1

1.2

Research Objectives and Scope of Research Work ............................................. 7

1.3

Background on Thermal Mass and Phase Change Materials (PCMs) ............... 10

1.5

PCM Characteristics and Types ......................................................................... 23

1.6

Encapsulation Methods ...................................................................................... 28

1.7

Transient Energy Storage Theory ...................................................................... 33

1.8

Mathematical Modeling of Latent Heat of PCMs .............................................. 35

CHAPTER TWO: PCM CONCRETE DEVELOPMENT AND TESTING.................... 41 2.1

PCM Concrete Development ............................................................................. 41

2.1.1

Aggregate Analysis ..................................................................................... 44

2.1.2

PCM Choice and Preparation...................................................................... 46

2.1.3

PCM Encapsulation .................................................................................... 50

2.1.4

Concrete Mix Design .................................................................................. 57

2.2.1

PCM Content .............................................................................................. 67

2.2.2

Concrete Density......................................................................................... 68

2.2.3

Compression Testing .................................................................................. 70

2.2.4

PCM Concrete Mix Optimization ............................................................... 72

CHAPTER THREE: THERMAL CHARACTERIZATION OF PCMS AND PCM CONCRETES ................................................................................................................... 76 3.1

Thermal Testing ................................................................................................. 76

3.2

Digital Scanning Calorimetry............................................................................. 78

3.2.1

DSC Methods .............................................................................................. 81

3.2.2

DSC Analysis .............................................................................................. 83

3.2.3

Enthalpy Measurement Results................................................................... 84 vii

3.3

Dynamic Hot Box Apparatus ........................................................................... 107

3.3.1 Development and Fabrication ......................................................................... 108 3.3.2

Sensor Calibration..................................................................................... 111

3.3.3

Thermal Conductivity ............................................................................... 120

3.3.4

Heat Capacity ............................................................................................ 122

3.3.4

Diurnal Cycling......................................................................................... 127

CHAPTER FOUR: PCM MODELING .......................................................................... 147 4.1

One-Dimensional Finite Difference Model...................................................... 147

4.2

Holistic Building Energy Model ...................................................................... 152

4.2.1

CODYBA and EnergyPlus ....................................................................... 155

4.2.2

CODYBA Analysis Results ...................................................................... 158

Figure 4.8: 7432 m2 (80K ft2) Louisville, KY (4A) ................................................ 159 Commercial Building Energy Use (kWh) ............................................................... 159 Figure 4.9: 7432 m2 (80K ft2) Louisville, KY (4A) ................................................ 160 Commercial Building Energy Savings ($)............................................................... 160 4.2.3 4.3

EnergyPlus Analysis Results .................................................................... 160

Economic Analysis ........................................................................................... 179

CHAPTER FIVE: SUMMARY, CONCLUSIONS AND RECOMMENDATIONS .... 190 5.1

Summary and Conclusions ............................................................................... 190

5.2

Recommendations ............................................................................................ 193

REFERENCES CITED................................................................................................... 196 APPENDIX A: SIEVE ANALYSIS RESULTS ............................................................ 203 APPENDIX B: HEAT FLUX SENSOR CALIBRATION ............................................ 210

viii

NOMENCLATURE

Cp E g H h k L m ρ t T v V x y z

= = = = = = = = = = = = = = = =

specific heat capacity (kJ kg-1 K-1) energy storage (kJ) liquid phase fraction enthalpy (kJ kg-1) heat transfer coefficient (radiation/convection) thermal conductivity (W m-1 K-1) latent heat of fusion (kJ kg-1) mass (kg) density (kg m-3) time (s) Temperature (°C or K) volume (m3) micro-voltage (µV) X spacial coordinate (m) Y spacial coordinate (m) Z spacial coordinate (m)

LIST OF TABLES Table 1.1 ............................................................................................................................25 Table 1.2 ............................................................................................................................26 Table 1.3 ............................................................................................................................27 Table 1.4 ............................................................................................................................28 Table 2.1 ............................................................................................................................45 Table 2.2 ............................................................................................................................45 Table 2.3 ............................................................................................................................46 Table 2.4 ............................................................................................................................56 Table 2.5 ............................................................................................................................59 Table 2.6 ............................................................................................................................68 Table 2.7 ............................................................................................................................69 Table 3.1 ............................................................................................................................85 Table 3.2 ............................................................................................................................88 Table 3.3 ..........................................................................................................................102 Table 3.4 ..........................................................................................................................102 Table 3.5 ..........................................................................................................................104 Table 3.6 ..........................................................................................................................119 Table 3.7 ..........................................................................................................................120 Table 3.8 ..........................................................................................................................134 Table 3.9 ..........................................................................................................................143 Table 4.1 ..........................................................................................................................164 Table 4.2 ..........................................................................................................................164 Table 4.3 ..........................................................................................................................165 Table 4.4 ..........................................................................................................................174 Table 4.5 ..........................................................................................................................182 Table 4.6 ..........................................................................................................................183 Table 4.7 ..........................................................................................................................183 Table 4.8 ..........................................................................................................................184 Table 4.9 ..........................................................................................................................184 Table 4.10 ........................................................................................................................185 x

Table 4.11 ........................................................................................................................185 Table 4.12 ........................................................................................................................186 Table 4.13 ........................................................................................................................186 Table 4.14 ........................................................................................................................187 Table 4.15 ........................................................................................................................187 Table 4.16 ........................................................................................................................189

LIST OF FIGURES Figure 1.1 ............................................................................................................................5 Figure 1.2 ............................................................................................................................6 Figure 1.3 ............................................................................................................................6 Figure 1.4 ..........................................................................................................................18 Figure 1.5 ..........................................................................................................................19 Figure 1.6 ..........................................................................................................................20 Figure 1.7 ..........................................................................................................................22 Figure 1.8 ..........................................................................................................................29 Figure 1.9 ..........................................................................................................................31 Figure 1.10 ........................................................................................................................31 Figure 1.11 ........................................................................................................................32 Figure 1.12 ........................................................................................................................32 Figure 1.13 ........................................................................................................................39 Figure 2.1 ..........................................................................................................................48 Figure 2.2 ..........................................................................................................................48 Figure 2.3 ..........................................................................................................................48 Figure 2.4 ..........................................................................................................................48 Figure 2.5 ..........................................................................................................................49 Figure 2.6 ..........................................................................................................................52 Figure 2.7 ..........................................................................................................................53 Figure 2.8 ..........................................................................................................................54 Figure 2.9 ..........................................................................................................................60 Figure 2.10 ........................................................................................................................60 Figure 2.11 ........................................................................................................................61 Figure 2.12 ........................................................................................................................61 Figure 2.13 ........................................................................................................................62 Figure 2.14 ........................................................................................................................63 Figure 2.15 ........................................................................................................................65 xii

Figure 2.16 ........................................................................................................................71 Figure 2.17 ........................................................................................................................71 Figure 2.18 ........................................................................................................................72 Figure 2.19 ........................................................................................................................73 Figure 3.1 ..........................................................................................................................78 Figure 3.2 ..........................................................................................................................80 Figure 3.3 ..........................................................................................................................82 Figure 3.4 ..........................................................................................................................82 Figure 3.5 ..........................................................................................................................83 Figure 3.6 ..........................................................................................................................86 Figure 3.7 ..........................................................................................................................90 Figure 3.8 ..........................................................................................................................90 Figure 3.9 ..........................................................................................................................91 Figure 3.10 ........................................................................................................................91 Figure 3.11 ........................................................................................................................92 Figure 3.12 ........................................................................................................................92 Figure 3.13 ........................................................................................................................93 Figure 3.14 ........................................................................................................................93 Figure 3.15 ........................................................................................................................94 Figure 3.16 ........................................................................................................................94 Figure 3.17 ........................................................................................................................95 Figure 3.18 ........................................................................................................................95 Figure 3.19 ........................................................................................................................96 Figure 3.20 ........................................................................................................................96 Figure 3.21 ........................................................................................................................97 Figure 3.22 ........................................................................................................................99 Figure 3.23 ........................................................................................................................99 xiii

Figure 3.24 ......................................................................................................................100 Figure 3.25 ......................................................................................................................100 Figure 3.26 ......................................................................................................................105 Figure 3.27 ......................................................................................................................105 Figure 3.28 ......................................................................................................................106 Figure 3.29 ......................................................................................................................106 Figure 3.30 ......................................................................................................................110 Figure 3.31 ......................................................................................................................110 Figure 3.32 ......................................................................................................................110 Figure 3.33 ......................................................................................................................111 Figure 3.34 ......................................................................................................................114 Figure 3.35 ......................................................................................................................115 Figure 3.36 ......................................................................................................................115 Figure 3.37 ......................................................................................................................117 Figure 3.38 ......................................................................................................................123 Figure 3.39 ......................................................................................................................123 Figure 3.40 ......................................................................................................................124 Figure 3.41 ......................................................................................................................124 Figure 3.42 ......................................................................................................................125 Figure 3.43 ......................................................................................................................125 Figure 3.44 ......................................................................................................................127 Figure 3.45 ......................................................................................................................129 Figure 3.46 ......................................................................................................................129 Figure 3.47 ......................................................................................................................130 Figure 3.48 ......................................................................................................................130 Figure 3.49 ......................................................................................................................131 Figure 3.50 ......................................................................................................................131 xiv

Figure 3.51 ......................................................................................................................132 Figure 3.52 ......................................................................................................................132 Figure 3.53 ......................................................................................................................133 Figure 3.54 ......................................................................................................................133 Figure 3.55 ......................................................................................................................134 Figure 3.56 ......................................................................................................................138 Figure 3.57 ......................................................................................................................138 Figure 3.58 ......................................................................................................................139 Figure 3.59 ......................................................................................................................139 Figure 3.60 ......................................................................................................................141 Figure 3.61 ......................................................................................................................141 Figure 3.62 ......................................................................................................................142 Figure 3.63 ......................................................................................................................142 Figure 3.64 ......................................................................................................................144 Figure 3.65 ......................................................................................................................145 Figure 3.66 ......................................................................................................................145 Figure 4.1 ........................................................................................................................148 Figure 4.2 ........................................................................................................................149 Figure 4.3 ........................................................................................................................151 Figure 4.4 ........................................................................................................................151 Figure 4.5 ........................................................................................................................154 Figure 4.6 ........................................................................................................................156 Figure 4.7 ........................................................................................................................159 Figure 4.8 ........................................................................................................................159 Figure 4.9 ........................................................................................................................160 Figure 4.10 ......................................................................................................................164 Figure 4.11 ......................................................................................................................165 xv

Figure 4.12 ......................................................................................................................166 Figure 4.13 ......................................................................................................................166 Figure 4.14 ......................................................................................................................166 Figure 4.15 ......................................................................................................................166 Figure 4.16 ......................................................................................................................167 Figure 4.17 ......................................................................................................................167 Figure 4.18 ......................................................................................................................168 Figure 4.19 ......................................................................................................................168 Figure 4.20 ......................................................................................................................169 Figure 4.21 ......................................................................................................................169 Figure 4.22 ......................................................................................................................170 Figure 4.23 ......................................................................................................................170 Figure 4.24 ......................................................................................................................174 Figure 4.25 ......................................................................................................................175 Figure 4.26 ......................................................................................................................175 Figure 4.27 ......................................................................................................................176 Figure 4.28 ......................................................................................................................176 Figure 4.29 ......................................................................................................................177 Figure 4.30 ......................................................................................................................180 Figure 4.31 ......................................................................................................................181

xvi

CHAPTER ONE: INTRODUCTION

1.1

Overview Governor Steve Beshear reported that Kentucky uses significant amounts of

energy, well above the national average in all segments of the State’s economy. Much of this high energy use is being attributed to State’s low energy costs resulting in there being little economic incentive to conserve energy (Beshear, 2008). Beshear goes on to report that Kentucky’s energy use is expected to rise more than 40 percent over current levels by the year 2025.

This significant increase in energy demand will likely not only

increase energy costs, but could potentially result in a large increase in green-house gas emission in the State, significantly impacting quality of life and the State’s economic health. To address this projected energy and environmental concern, the government of Kentucky has set a goal of reducing protected energy needs by 18 percent through increased energy efficiencies. This energy use growth and the need to conserve are not exclusive to the state of Kentucky and has become a priority throughout the nation. In fact, building codes and design guidelines across the various sectors are being changed to raise minimum energy performance standards (ASHRAE, 2010). One of these changes included an increase in prescriptive minimum envelope requirements in the model building codes that are becoming difficult to meet with single wythe masonry wall systems. In many climates, a single wythe masonry wall will not meet the prescriptive code requirements for thermal resistance. There is a need increase the thermal performance of a single wythe masonry wall systems in alternative ways.

Currently, there are two philosophies used for building envelope design. The first uses highly insulated lightweight construction with minimal air infiltration, the second method uses thermally massive construction with optimized window location and shading to allow solar gain in the winter months and mitigate solar gain in the summer. While highly insulated structures minimize heat penetration, they lack the ability of thermally massive materials to absorb surplus energy, dampen interior temperature fluctuations, and offset peak energy consumption. In addition, once thermal energy has penetrated a highly insulated structure with low thermal mass, it meets the same resistance when the temperature differential reverses. The continued increase in prescriptive minimum envelope requirements in the model building codes, the growth in sustainable design focus and new legislative mandates for energy efficiency has made providing cost effective thermally efficient exterior wall systems a major design consideration. It has become difficult to meet the minimum envelop requirements with single wythe masonry wall systems, especially where most of the cores are grouted to provide structural support for the building. Although some increases in thermal resistance are possible through the use of core loose fill insulations, insulation inserts or expanding foam systems, these systems are of limited effectiveness due to thermal bridging. In many climates, a single wythe masonry wall will not meet the prescriptive code requirements for thermal resistance. The only current solution in these cases is to apply a continuous layer of insulation over the CMU assembly. This continuous insulation layer requires a covering and significantly impacts the cost effectiveness of the wall system, its aesthetics, durability, and fire resistance.

There is a need to increase the thermal performance of a single wythe masonry wall systems in alternative ways. Materials with high volumetric heat capacities can store significant amounts of energy and can significantly reduce heating and cooling loads in conditioned environments

(Brown,

1990),(Marceau

et-

al,

2007),

(Khudhair

and

Farid,

2004),(Richardson and Woods, 2008). This reduction in energy use is in addition to a significant reduction in HVAC system size and costs due to peak load reduction. Thermal mass has a significant potential to cost effectively reduce energy use in buildings. Phase change materials (PCM) have the ability to significantly increase heat capacities of materials, to store energy, to control temperature of wall surfaces and to moderate the perceived temperatures in the interior environment (Khudhair and Farid, 2004),(Richardson and Woods, 2008), (Zlaba et al ,2003). These materials have been incorporated into a variety of thermal storage devices and systems, as well as into a variety of building materials (Khudhair and Farid, 2004), [Richardson and Woods, 20080, (Alawadhi, 2008), (Taygi and Buddhi, 2007).

Phase change materials can be

used in active systems where they are isolated and used to store thermal energy for later use in an active conditioning system (Taygi and Buddhi, 2007). Salt hydrates and a variety of organic phase solid-liquid change materials have been investigated for application in building systems (Khudhair and Farid, 2004). However, few of these systems are in wide spread use since most have high costs, are flammable, are corrosive, have deteriorating performance in storage efficiency, or have difficulty in controlling the transfer of heat energy in both cooling and heating mode.

The PCM performance in the built environment is also highly dependent on how these materials are incorporated in building systems and the environment to which they are exposed. Some experimental and analytical studies have shown that little overall energy has been saved where PCM systems are used in direct contact with an interior environment that is not allowed to cycle through significant temperature ranges, or used where direct solar gains cannot be used for later heating (Khudhair and Farid, 2004),(Stoville and Tomlinson, 1995). Others, have shown significant reductions in energy used for interior conditioning, where direct solar energy can be captured for later heating (Little, 2002), or if the PCM materials are used in contact or near where temperatures cycle significantly (Ismail and Castro, 1997), (Elewadhi, 2008),(Castalone, 2006). It is clear that further investigation into this behavior is needed to fully realize the potential of PCM materials as a mechanism for improved energy efficiency in the built environment. The use of PCM’s with masonry units has been investigated by others (Khudhair and Farid, 2004) and there is even a patented PCM/masonry unit that was developed in the US in the late 1990’s (Salyer, 1998). This masonry unit primarily uses composite PCM material cast into the CMU cores and inserts (see Figure 1.3). Since this unit is currently not commercially available, it is likely quite costly to make. In addition, the construction industry in the US, and the masonry industry in particular, is quite conservative. Since the inserts will likely make it harder for the mason to construct walls with these units, especially if they are to be reinforced and grouted, they are not likely looked on favorably by the industry. Thus, a unit that is more expensive and will require

a significant change in the manufacturing and in construction techniques is unlikely to get significant market penetration.

7 5 8 in.

1 1 2 in. 1 1 2 in. 1 1 2 in.

1 1 4 in. 1 1 4 in. 1 1 4 in.

15 5 8 in.

Figure 1.1 Typical 8 in. Hollow Concrete Masonry Unit (CMU) (NCMA, Tek Note 0201A, 2002)

The development of a hollow PCM/concrete masonry unit for use in loadbearing and non-loadbearing exterior wall systems appears to have potential as an energy efficient cost effective single wythe masonry wall, especially if low cost PCM materials can be incorporated into a common hollow concrete masonry units (CMU), like that shown in Figure 1.1. Since hollow CMU’s are typically applied in residential, industrial and institutional applications and their low cost, good thermal mass, durability, high fire resistance and the ability to be used for both building structure and envelope make them attractive for thermal enhancement. Figure 1.2 shows a typical exterior single wythe (layer) wall configuration used in a variety of applications.

Roof sheathing Exterior sheathing/ soffit vent

Bond beam/lintel Roof anchor

Window

Window framing Sill Solid backing unit

Window flashing with drip edge

Bed joint reinforcement, as required Vertical reinforcement, as required

Exposed concrete masonry wall system

Interior finish (varies) Isolation joint Concrete slab Vapor retarder, as required Granular base

Footing Perimeter insulation, as required

Figure 1.2 Typical Single Wythe Masonry Wall Application [NCMA Details 03A, 2009].

Figure 1.3 Patented PCM Block [Salyer, 1998].

1.2

Research Objectives and Scope of Research Work The objective of this investigation was to attempt to develop a PCM/CMU unit

that can be produced cost effectively in a typical block plant and, when used in an exterior wall, will produce comparable thermal performance to a conventional CMU unit with continuous insulation. The investigation was conducted using a two phase approach. The first phase of this investigation (the subject of this report) was to develop a PCM/concrete mixture that can be incorporated into a conventional CMU concrete mix and to evaluate how these mixtures affect the compressive strength and thermal characteristics of the units, and energy use performance of buildings with exterior single wythe walls constructed with PCM /CMU walls. The results of these tests were also used to determine the amount of PCM materials possible in the mix without adversely affecting the strength or handling characteristics of the concrete mix. This investigation used an integrated experimental and numerical approach. The experimental data was used to validate and improve a numerical model. The numerical analyses were used to explore the role of PCM’s on the energy performance of a typical 8 in. CMU unit and exterior wall assembly. Optimization methodologies were used to find the optimal PCM percentage that provides comparable performance to insulated wall systems at a minimum cost. In order to develop a viable PCM-CMU a series of steps must be completed. First, a viable concrete mix design must be developed. This mix must ASTM C90 Standard Specification for Loadbearing Concrete Masonry Units (compression strength initially) while also allowing significant volumes of PCM to be incorporated. This must 7

be done while also retaining the PCM while it is in its liquid state. Proper mix design and PCM containment are essential. In addition, proper PCM selection is critical for energy storage as well as minimal chemical reactivity with the cement matrix, reinforcement, and wall coatings. Analysis of the effects of enhanced thermal energy storage characteristics on the energy performance of structures constructed with these PCM CMUs were conducted. These analyses were performed to evaluate the relative energy efficiency performance of single-wythe PCM concrete masonry as compared to standard configurations comprising insulation and conventional concrete masonry. Characterization of the PCM thermal properties is necessary to quantify the thermal performance of the PCM concrete mix. The enthalpy and transition temperatures are required to understand the energy storage properties so that the mix can be optimized. Optimum transition temperature depends on the climate, interior gains, and HVAC set points since the PCM must cycle through its transition temperature regularly in order to be effective. After PCM selection and successful mix development, the composite material thermal properties must be quantified to evaluate thermal performance.

Thermal

conductivity must be determined in order to evaluate the steady state R-value. R-value is a steady-state value quantifying a material’s one-dimensional thermal resistance to heat flux for a temperature differential applied to the bounding surfaces. High R-value can be an attribute during prolonged periods of cold weather, when the PCM remains frozen. Also, high R-value helps to prevent thermal penetration during hot summer periods when the PCM may remain in a liquid state. On the other hand, during cold periods with solar

gain, conductivity must be high enough to allow solar thermal energy storage of the PCM during sunny days for release at night. In addition, in cooling climates with large diurnal temperature swings, higher conductivity can allow cooler nighttime temperatures to penetrate the envelope and thus precool the building envelope for daytime cooling. The transient thermal mass properties must also be evaluated to quantify the energy storage properties of the PCM CMUs since thermal mass is only effective when accompanied by diurnal fluctuations. To study the transient properties, a Dynamic Hot Box Apparatus was constructed. The device is capable of applying steady state as well as dynamic temperature profiles to a material test sample. The apparatus can perform steady state R-value testing as well as simulate diurnal climate cycling, in addition to performing ramp and step profiles. This allowed testing configurations that emulate both prolonged temperature extremes, temperature cycling, as well as other dynamic profiles needed for testing. A series of analytical models were developed and applied to evaluate the performance of PCM CMU walls.

A one-dimensional transient conduction finite

difference model was used to examine the response of PCMs in a layered building envelope. The dynamic hot box diurnal cycling results were used to validate the model beyond simplified theoretical analyses. Two holistic building analyses software programs were used to examine the potential energy savings provided by PCM in the building envelope. These were a modified version of CODYBA and EnergyPlus 8.0. The programs were used to simulate building envelopes using PCM CMU walls and compare these responses to conventional

wall configurations.

Whole building energy analyses were performed for wall

configurations in varying climates Finally, an economic analysis was performed. The initial additional upfront cost associated with inclusion of PCM was compared to that of standard configurations. From this comparison the required cost of PCM was determined for economic viability. If viability of the CMU/PCM mix is proven in Phase 1, at least three concrete mixes will be used to fabricate typical hollow concrete masonry units. During the second phase of the investigation, the concrete mix proportions will be evaluated for ease of manufacture. The mix proportions and materials may be adjusted to optimize fabrication and performance. Hot box testing and further analysis will allow characterization of the CMU/PCM Units in a typical wall configuration and further refine and optimize the unit performance. This report is presented in five chapters; Introduction and Background (Chapter 1), PCM Concrete Development and Testing (Chapter 2), Thermal Characterization of PCMs and PCM Concretes (Chapter 3), PCM Energy Modeling (Chapter 4), and finally Summary, Conclusions and Recommendations (Chapter 5).

1.3

Background on Thermal Mass and Phase Change Materials (PCMs) Historically thermal mass has been achieved with a thick, dense building

envelope. An example is adobe construction. The massive walls absorb penetrating heat during the day keeping the interior cool, then release the stored thermal energy during the cool nights and thus maintain interior comfort. Alternatively, significant thermal mass can be incorporated into lightweight construction though the use of PCMs. Lightweight 10

frame wall construction can incorporate PCMs as a thin layer, placed in pouches, packets or cylinders, or be suspended in insulation.

PCMs can also be incorporated into

thermally massive materials such as concrete to further increase the thermal mass, thus improving thermal performance. PCMs can be used to increase the effective thermal mass of a structure without significantly increasing the actual weight of the structure. PCMs store energy as they change phase from a solid to a liquid during a nearly isothermal process. This occurs when the material reaches the transition temperature of the PCM. Energy is stored until the latent heat storage potential is consumed. Once melting is completed the temperature of the material can once again rise. When the material cools and returns to the transition temperature, the PCM solidifies releasing the stored thermal energy, again in a nearly isothermal process. Once the latent heat is released, the material can begin to cool again. PCMs can be used to enhance building energy performance. This can be achieved by placing the PCM in the interior so that solar gain though windows is absorbed and stored. PCM can also be placed in the building envelope to absorb excess solar gains (Athientis et al. 1997, Drake 1987, Neeper 2000) as well as to slow thermal penetration (Richardson and Woods 2008). In addition, energy storage tanks utilizing PCM can be an effective way to store excess solar thermal energy for use at night or to store thermal energy produced in other ways until needed. PCM impregnated wallboard has seen the widest research and application since it is easy to apply as a retrofit and is non-structural. Other materials such as floor tile, concrete flooring, brick, and concrete walls have also been studied (Khudhair et al. 2004).

Flooring has been utilized for solar gain applications. Wallboard, brick, and

concrete walls have also been studied for solar gain applications, as well as for their building envelope performance improvement capabilities. Further, all of these PCM composites have the ability to buffer internal temperature fluctuations for improved occupant comfort. PCM composite flooring can be placed in an area of a structure with good fenestration exposure on the southern face. During the winter, the PCM flooring will absorb and store daytime solar gains. The stored thermal energy is released at night to maintain a regulated temperature until daybreak. During summer it can help attenuate peak interior temperatures and prevent overheating. A study was performed to examine the passive use of PCM floor tile in a south facing sunspace for mitigation of overheating and for energy savings. It was found that PCM floor tile in that application has the potential of at least 24% annual heating energy savings (Hittle 2002). PCM concrete flooring was also studied by Entrop et al. (2011). It was found that with southern solar exposure, PCM concrete flooring provides evening and nighttime heating utilizing the solar thermal energy stored during the day. PCM wallboard in contact with the interior environment has the ability to absorb interior thermal gains which can be solar, occupant, or from electronics and appliances like PCM floor tiles, wallboard can be thermally charged during the winter days in order to maintain nighttime comfort and can buffer interior temperatures in the warm months (Athientis et al. 1997, Drake 1987, Neeper 2000). There are two favored methods of incorporating PCM into the wallboard; mixing micro-encapsulated PCMs with the gypsum matrix during manufacturing and immersing finished wallboard in molten PCM.

Immersion is more economical while Micro-

encapsulation provides stability to the liquid state of the PCM. Both methods have their advantages and disadvantages. Immersion requires submerging concrete in liquid PCM in order to imbibe it within in concrete void space which may allow long-term stability problems. Micro-encapsulation involves polymer encapsulation of PCM spheres with the benefit of stability at added cost of the encapsulation process and materials. Masonry has seen extensive use dating back 1000’s of years with cut stone being used in manly early European, South and Central America structures, as well as in the Egyptian pyramids of Northern Africa, and finally the adobe brick constructions of southwestern North America as well as the Mesopotamian beehives in Asia to name a few. The thick thermally massive exterior of these structures provided human comfort prior to modern insulation. Remnants of these great masonry structures still stand to this day.

Due to the consistent and continued use of masonry, as well as its resilience and

thermal dampening, much research has focused upon it. Though masonry’s thermal mass is substantial, PCMs can improve it. Thus, researchers have investigated whether modern day masonry such as clay brick and concrete units will have improved thermal performance with the incorporation of PCMs. Brick incorporating PCM has been studied and has indicated a reduction of thermal penetration, dampening of indoor temperature fluctuations, and improved thermal comfort (Alawadhi 2008, Castell et al. 2010, and Zhang et al. 2011). Since clay brick must be kiln fired, PCM core inserts must be inserted, or pouches must be applied to the exterior or interior of envelope after firing. Inserts are an economically difficult solution due to the mechanization that inserts require and the feasibility of internal or external PCM containment.

Prior PCM research in concrete has involved incorporating PCM into mass concrete and concrete masonry units in several ways; micro-encapsulated PCM spheres in the concrete matrix, soaking of the cured porous concrete in liquid PCM, and placement of PCM modules within the cores of CMUs.

These methods all have

advantages and disadvantages. Micro-encapsulation is effective and has been favored due to its ease of incorporation. It is added as a fine aggregate to the materials during mixing. In addition, the capsules provide retention of the PCM in its liquid phase. However, since the PCM is placed within the cement matrix, significant volumes of PCM cannot be incorporated without displacing substantial quantities of the cement and matrix, thus adversely affecting the concrete compression strength. Finally, the dispersion and coating process that forms the microcapsules adds to costs. Soaking involves infusing the PCM into a porous concrete matrix (Hawes et al. 1991). However, surface tension and molecular bonds are all that prevents the PCM from seeping from the matrix. Long term stability of the composite is questionable. PCM modules placed in the cores of concrete masonry units have also been studied (Sayler and Kumar 1995). These modules effectively mitigate the seepage issues. However, a portion of the cores of masonry walls are needed for reinforcement in almost all structural applications. Thus, rendering the cores unusable has significant structural implications in masonry applications.

In addition, using the cores for grout and

reinforcement replaces a substantial portion of the PCM, lowering PCM quantities. Further, these large PCM cores can experience the difficulties of lack of nucleation sites and low thermal conductivity commonly associated with macro-encapsulation.

Some research has focused on vacuum impregnation of porous aggregates. The primary porous aggregates that have been examined are perlite and vermiculite due to their large volume of voids and interconnected pore structure. However, the low strength of perlite and vermiculite is deleterious to concrete in structural applications. Diatomaceous earth has also been examined as a supporting aggregate (Karaman et al. 2011). However, it has a similar negative effects as micro-encapsulated PCM in that it displaces cement and thus reduces concrete strength with increased PCM content. Expanded clay aggregates have been studied as a porous PCM media (Zhang et al. 2003). The expanded clay provides interconnected pore space for retention of the PCM. Like the PCM immersed concrete, the PCM is retained by surface tension. However, since the PCM aggregates are then used to make PCM concrete, the cement matrix encapsulates the aggregates thus preventing leakage of the PCM. The energy storage capacity of PCMs is substantial and thus its use has been extended to many uses beyond building applications. PCMs have been studied in fabrics, bridges, dams, space craft, as well as buildings for temperature regulation. Fabrics have been examined for temperature regulating clothing as well as tents that maintain occupant comfort. This research has largely been facilitated by the military in an effort to provide thermal comfort for soldiers. Ultimately this research has led to PCM clothing being commercialized with socks, shirts, and other clothing items for sale to the public.

Bridges have been studied to see if the incorporation of PCM in road decks can reduce freeze / thaw cycles (Stoll et al. 1996). The repetitive freezing and thawing of concrete bridge decks causes cracking over time. These cracks then allow water to penetrate.

With each additional freeze cycle, the water expands and thus enlarges

existing cracks as well as developing new cracks due to water in the pore space. This process increases maintenance costs as well as shortening the time before replacement is required. PCM concrete shows promise in minimizing bridge deck maintenance and maximizing life span due to its thermal stabilization properties. Concrete dams involve placement of mass concrete.

The heat of hydration

minimizes the volume of concrete that can be placed at a given time in order to allow for dissipation of the heat. Otherwise, large temperature differentials within the concrete mass would cause thermal stresses and thus cracking of a material that is to be impervious to large hydraulic pressures. PCMs may be able to absorb excess thermal energy from the exothermic reaction, providing a thermal buffer to allow for placement of larger volumes of mass concrete at a given time. This has the potential to speed construction and improve economics of mass concrete work (Hunger et al. 2009). The National Aeronautical Space Administration (NASA) performed a significant amount of research on PCMs in the 1960s to regulate the interior temperature of spacecraft (Bannister 1967). Being in space, a capsule is exposed to solar radiation when in direct sunlight. When shadowed by the earth or moon, the capsule radiates the thermal energy back out to space. PCMs have the ability to store the excess solar thermal energy and release it during periods of radiative cooling, thus maintaining occupant comfort.

The vast variety of uses for energy storage and temperature regulation for which PCMs have been studied, make it evident that they have significant potential for these applications.

While other uses have been studied, it seems that the most practical

application may be for the building environment. PCMs can be used either in energy storage tanks or in the building envelope itself, in both passive and active applications. The term thermal mass is used to signify the ability of a material to store large quantities of energy and delay thermal penetration through the building envelope. It is related to the heat capacity of the material which refers to the quantity of heat required to raise the temperature of a given mass by one degree (ASHRAE 1997), hence the term thermal mass. Thus, it is related to mass and specific heat of the building envelope material. Increasing mass and/or density increases the thermal mass of a structure. Thermally massive structures have the ability to attenuate and delay peak interior temperatures in relation to the exterior climatic driving temperatures. demonstrated in Figure 1.4.

This is

Thermal mass helps to maintain consistent interior

temperatures and reduces temperature differential across the building envelope thus reducing thermal transmittance. In addition, the peak amplitude phase shift has the potential to shift peak energy use to off-peak energy generation times. This shift can help to buffer peak energy generation by power plants, therefore mitigating the need for new peak related generation plants. In nonresidential buildings, thermal mass is often more effective in reducing cooling loads than heating loads. In some climates, thermally massive buildings can perform significantly better than low mass building regardless of their insulation levels (Wilcox et al. 1985).

Figure 1.4: Thermal Mass Delay and Attenuation (educate-sustainability.eu 2013) Kosny et al. (2009) have demonstrated that in comparison to materials with low thermal mass and high thermal resistance, comparably resistive materials with high thermal mass have the ability to improve the thermal performance of the building envelope through transient energy storage via specific heat, or in the case of PCMs, latent heat.

Thermal mass energy storage has the potential to significantly reduce heating,

ventilation, and air-conditioning (HVAC) energy use in many climates. This has led to the development of a Dynamic Benefit for Massive Systems (DBMS) factor that when multiplied by the steady-state R-value, provides the Dynamic R-value Equivalent (DRE) (Kosny et al. 2001) rating which equates to a dynamic equivalent of massive structures to the steady-state R-value of lightweight construction. As shown in Figure 1.5 this value

provides a measure of the required insulation that a low mass, frame construction structure would need to provide the same energy savings of a massive structure with equivalent steady-state insulation. It is a function of the material configuration and the climatic conditions (Kosny et al. 2009).

Figure 1.5: Relationship between Attic R-Value and Energy Consumption in Bakersfield, CA (Kosny et al. 2009) Efficiency of thermal mass depends on; building configuration, type of building structure, amount of thermal mass in the building, internal heat sources, climate, steadystate wall R-value, and wall materials configuration. Thermal mass in contact with the conditioned interior is most effective while thermal mass placed on the exterior is the least effective as shown in Figure 1.6 (Kosny et al. 2001).

Conc. Insulation Conc. Interior Mass Insulation Conc. Insulation Exterior Mass

Figure 1.6: Dynamic Benefit for Massive Systems (DBMS) for House in Washington D.C. (Kosny et al. 2001)

Incorporation of PCMs into the building envelope effectively increases the thermal mass of the structure. As with sensible energy storage, PCM latent heat storage has the benefit of reducing energy consumption, decreasing temperature fluctuations, as well as shifting peak energy use to off-peak hours. However, the PCM must go through the transition temperature and change phase in order to provide energy storage and release.

Theoretically, with a thick enough wall and/or enough PCM, energy from the exterior would never fully penetrate the building envelope during diurnal cycling. However, this would require an uneconomical envelope thickness and/or quantity of PCM. Instead, a more economical use of PCM is one that delays the penetration of heat

into the building envelope. This has the effect of lowering the peak interior and exterior wall temperature amplitudes. In a study by Castellón et al. (2007) in Lleida, Spain, 2.64 meter square concrete cubicles were constructed. One cubicle had walls of concrete while the other had walls of concrete incorporating the commercial product, BASF Micronal PCM at 5% mass fraction.

Air temperature, interior wall surface temperatures, and heat flux were

monitored in each cubicle. Under free floating interior temperatures, it was found that interior wall temperatures for the PCM cubicle were up to 4°C cooler than the concrete control cubicle during summer. In addition, a phase lag of approximately two hours was observed in the PCM cubicle. Further studies by Castellón et al. (2009) involved construction of additional cubicles constructed of brick and alveolar brick with and without insulation as well as with and without PCM.

All of the cubicles had sensors measuring interior air

temperature, interior wall temperatures, and interior wall heat flux along with an outside weather station and pyrometer. Free floating temperatures were tested for all cubicles; concrete, brick, alveolar brick, brick with insulation, along with all aforementioned material types with PCM. Heat pumps with metering of energy usage were installed for all but the concrete cubicles. Winter configurations included trombe walls while summer did not. For the winter trombe wall cases it was found that thermal energy could be stored at lower wall temperatures.

The significance of this is that a lower wall

temperature produces lower heat transfer losses with the exterior environment during cold weather, thus providing a more efficient system. Under controlled temperature settings in the summer, it was found that alveolar brick with PCM performed better than

an insulated brick cubicle. A 14.8% improvement in energy efficiency in contrast to insulated brick was found (Castellón et al. 2009). Reduced peak interior temperature and phase lag is shown in Figure 1.7.

Figure 1.7: Wall Temperature with and without PCM (Castellón et al. 2007)

Finally, thermal mass and thus PCMs have the ability to shift peak cooling loads. This has the potential to improve HVAC performance as well as shift peak energy usage to off-peak hours. Since peak cooling energy consumption can be shifted to nighttime, condensing unit performance can be improved due a larger temperature differential between the condensing unit and the ambient air temperature as compared to the warmer daytime differential. Further, shifting peak energy consumption to off-peak hours helps to buffer peak energy use and thus can mitigate the need for new power plants built for peak hour demand.

1.5

PCM Characteristics and Types Ideal PCMs store and release large quantities of thermal energy in a nearly

isothermal process in addition to their sensible heat. Significant thermal energy is stored and released at the transition temperature via latent heat. Latent heat has much larger energy storage compared to the typical sensible heat values associated with temperature change. A measure of latent heat as compared to the sensible heat of a PCM is the Stephan number, Ste. It provides a ratio of energy storage of sensible heat for a given temperature range divided by the latent heat. A lower Stephan Number indicates higher latent heat storage.

Ste =

C p ∆T

(1.1)

L Ste: Stephan Number

(unitless)

C p: Specific Heat

(kJ kg-1 K-1)

ΔT: Temperature Change

(°C or K )

L: Latent Heat

(kJ kg-1)

PCMs must provide certain characteristics for effective building envelope applications.

These characteristics include significant latent heat energy storage,

adequate thermal conductivity with respect to the confining medium, a transition temperature in the range of human comfort, transition range must be reasonably narrow, low reactivity, and the ability to nucleate (Baetens et al. 2010).

There are two main categories of PCMs. These are organic and inorganic PCMs. Organic PCMs include linear alkanes, carboxylic acids (fatty acids), polyethylene glycol (PEG) and other carbon based molecules that provide a change of phase between solid and liquid state (as is the case in typical building envelope applications). Inorganic PCMs typically include hydrated salts of various chemical compositions. Overall, inorganic PCMs are cheaper, have higher latent heat storage, are more conductive, and are not flammable. On the converse, they have a large volume change and are highly corrosive which causes them to be detrimental to building materials. In addition, they are prone to undercooling and phase separation which affects the long-term efficacy (Baetens et al. 2010). Although organic PCMs are more costly, flammable, and have poorer thermal performance than inorganic PCMs, their long-term stability and consistent thermal properties throughout the life cycle hold promise for their use in building materials (Baetens et al. 2010).

An added benefit of organic PCMs is that their transition

temperature can be tailored to the application. With adequate encapsulation or fire retardant treatments, flammability can be controlled. Thus organic PCMs have potential in the building envelope energy storage sector. The properties of a number of organic and inorganic PCMs are shown in Tables 1.1 and 1.2, respectively. The advantages and disadvantages of organic and inorganic PCMs are in Table 1.3 and 1.4, respectively.

Table 1.1: Organic Phase Change Materials (PCMs) Organic PCMs Transition Temp. (°C)

Latent Heat (J/gm)

Hexadecane

200

Octadecane

200

18 to 28

< 200

Capric Acid / Myristic Acid Eutectic

200

Capric Acid / Stearic Acid Eutectic

180

Butyl Stearate

140

PEG 600

130

PureTemp 23

200

PCM Compound

Hexadecane / Octadecane Blends

Table 1.2: Inorganic Phase Change Materials (PCMs) Adapted from (Cabeza and Heinz 2004) Inorganic PCMs PCM Compound

Transition Temp. (°C) 18.5

KF 4 H20

Latent Heat (J/gm) 231

Mn(No3)2 6 H20

25.8

126

CaCl2 6H20

29.7

192

LiNo3 3H20

296

51-55% Cu(NO3)3 6 H20 + 45-49% LiNO3 3H20

16.5

250

45-52% LiNO3 3H20 + 48-55% Zn(NO3)2 6H20

17.2

220

55-65% LiNO3 3H20 + 35-45% Ni(NO3)2 6H2O

24.2

230

66.6% CaCl2 6H2O + 35-45% Ni(NO3)2 6H20

127

45% Ca(NO3)2 6H2O + 33.3% MgCl2 6H2O

130

26.8

188

136

48% CaCl2 + 4.3% NaCl + 0.4% kCl + 47.3% H20 67% Ca(NO3)2 4H2O + 33% Mg(NO3)2 6H20

Table 1.3: Advantages and Disadvantages of Organic Phase Change Materials (PCMs). Adapted from (Kuznik et al. 2011) Organic PCMs Advantages

Disadvantages

•

availability in a large temperature range

•

freeze without significant super

amended with graphite) •

cooling •

ability to melt congruently

•

self-nucleating properties

•

compatibility with many conventional

no segregation

•

chemically stable

•

relatively high heat of fusion

•

safe and non-reactive

•

recyclable

lower volumetric latent heat storage capacity

•

flammable (depending on containment)

construction materials •

low thermal conductivity (can be

Table 1.4: Advantages and Disadvantages of Inorganic Phase Change Materials (PCMs). Adapted from (Kuznik et al. 2011) Inorganic PCMs Advantages

Disadvantages

•

high volumetric latent heat storage

•

large volume change

capacity

•

undercooling

•

low cost and easy availability

•

corrosive (especially to steel)

•

sharp phase change

•

poor thermal stability

•

high thermal conductivity

•

phase separation

•

non-flammable

1.6

Encapsulation Methods Due to the changing physical nature of PCMs encapsulation is necessary for

stabilization during the liquid state.

There are two predominant methods of

encapsulation; macro-encapsulation and Micro-encapsulation.

Macro-encapsulation

involves encasem*nt of PCM in metal or plastic containment typically a centimeter or larger in diameter. Pouches or canisters are used to prevent PCM seepage during the liquid phase. While this method is economical to produce, there are issues with the lack of nucleation sites to promote crystallization due to a lower surface area to volume ratio. In addition, the large volume has a tendency to inhibit a full phase change throughout the PCM due to differing thermal conductivities of the solid and liquid states.

Micro-encapsulation provides PCM capsules in the range of 20µm to 100µm in diameter. These microcapsules are capable of repeated cycling without degradation. As shown in Figure 1.8, the shell becomes smoother with increased cycling due to thermal expansion and constraction. In addition, the small size mitigates the nucleation and conductivity issues. However, typical micro-encapsulation techniques create a spherical polymer coating in a dispersion process which increases associated encapsulation costs. Further, the small size creates a significant volume of encapsulation product as compared to the PCM. Core to shell ratio tends to fall in the range of 80% PCM core to 20% shell. Thus a significant portion of the PCM is replaced by the shell. Finally, when mixed into a structural material such as concrete, micro-encapsulated PCMs displace the cement matrix thus, creating a reduction of strength with increased PCM content.

Figure 1.8: Micro-encapsulated PCM at Different Cycles under SEM (Khudhair et al. 2004) 29

Another process which has been the focus of this research is vacuum impregnation of the PCM within porous aggregate. This process is attractive because it encapsulates the PCM within the confinement of the aggregate pore space. When utilized in concrete the vacuum impregnated PCM aggregates can be used as lightweight concrete aggregate. IN this application the PCM does not displace the cement matrix. The cement matrix surrounds the PCM impregnated aggregates thereby encapsulating them so that retention is not left to surface tension within the pore space. Finally, the microscopic surface irregularities of the pore space provide nucleation sites for PCM crystallization. Zhang et al. (2004) studied vacuum impregnation of expanded clay and shale. They found that pore diameter and connectivity are essential for absorption of PCM. While expanded shale was capable of incorporating and retaining PCM, expanded clay held significantly larger quantities of PCM. They were able to achieve a significant mass fraction of PCM in the “superlight” expanded clay aggregate up to 68%, 15% for the “normal” expanded clay aggregate, and 6.7% for expanded shale. While the 68% mass fraction uptake is quite significant, the aggregate density is low at 0.76 gm/cm3 and porosity is 75.6%. Thus it is uncertain whether this aggregate can be used for structural concrete applications. Further, compression testing results were not provided. This is most likely due to the choice of butyl stearate as a PCM. Butyl stearate is a fatty acid and thus deleteriously reacts with cement thereby significantly reducing concrete strength. Figure 1.9 shows macro-encapsulated PCMs. encapsulated PCMs.

Figure 1.10 shows micro-

Figures 1.11 and 1.12 compares micro-encapsulated PCMs to

vacuum impregnated PCMs in a concrete matrix.

Figure 1.9: Macro-Encapsulated PCM

Figure 1.10: Micro-Encapsulated PCM

Micro-Encapsulated PCM in Cement

Figure 1.11: Representation of Micro-Encapsulated PCM in Cement Matrix (Not to Scale)

Vacuum Impregnated PCM in Pore Space

Figure 1.12: Representation of Vacuum Impregnated PCM in Aggregate Pores (Not to Scale)

1.7

Transient Energy Storage Theory The fundamental equation for energy storage in building materials is the transient

heat flow equation. The equation relates energy storage to the material conductivity and for the case of PCMs, latent heat storage, as a function of time and position. The rate of storage of energy of a material is equal to the sum of the differential of heat entering through the bounding surfaces with respect to position and the rate of energy generation as shown in the following equation (Ozisik 1992). For the rectangular coordinate system (x, y, z) the equation is described as;

Cpρ

∂T ∂  ∂T  ∂  ∂T  ∂  ∂T   + k = k  + k +E ∂t ∂x  ∂x  ∂y  ∂y  ∂z  ∂z 

where; Cp: Specific Heat Capacity (kJ kg-1 K-1) ρ: Density (kg m-3) E: Energy Storage (kJ) k: Thermal Conductivity (w m-1 °K-1 T: Temperature (°K) x: X Coordinate (m) y: Y Coordinate (m) z: Z Coordinate (m)

(1.1)

For the assumption of one-dimensional heat flow this equation simplifies to;

Cpρ

∂T ∂  ∂T  = k +E ∂t ∂x  ∂x 

(1.2)

For the case of PCM, energy storage, E, is equal to the product of the latent heat, L, the density, ρ, and the fraction, g, of the PCM in liquid state with respect to time;

E = − Lρ pcm

∂g l ∂t

(1.3)

Thus the transient heat equation becomes;

Cpρ

∂g ∂T ∂  ∂T  = k  − Lρ pcm ∂t ∂t ∂x  ∂x 

(1.4)

where; Cp: Specific Heat Capacity (kJ kg-1 K-1) ρ: Density (kg m-3) ρpcm: Density of PCM (kg m-3) g: Liquid Phase Fraction k: Thermal Conductivity (w m-1 °K-1 L: Latent Heat of Fusion (kJ) t: Time (s) T: Temperature (°K) x: X Coordinate (m) 34

The equation is non-hom*ogeneous. For a simple case an exact solution can be obtained through separation of variables. For complicated scenarios a numerical solution method is required. Typically a conduction finite difference approximation is used to solve the equation.

1.8

Mathematical Modeling of Latent Heat of PCMs As stated above, the exact solution to phase change problems is obtainable for

only a few idealized situations. These cases are mainly one-dimension infinite and semiinfinite regions with simple boundary conditions. Exact solutions are obtainable only if a similarity solution can be developed allowing the two independent variables x and t to merge into a single similarity variable x/t1/2 (Ozisik 1992). The geometric configuration and boundary conditions of building envelope analysis do not allow exact solutions. Thus numerical methods must be used to solve these phase change problems. The transient heat equation is discretized in order to obtain a solution. Strong form solutions allow determination of the phase change boundary. However, this process is quite complex. Weak form solutions avoid the explicit nature of the moving boundary. These weak solution methods are the apparent capacity method, the effective capacity method, the heat integration method, the source based method, and the enthalpy method to name a few (Hu and Argyropoulos 1996). The following are explanations of the above referenced solution methods presented by Hu and Argyropoulos (1996).

The Apparent Heat Capacity method accounts for phase change by varying the heat capacity of the material. The heat capacity is provided for three regions; the solid phase, the transition from solid to liquid, and the liquid phase as follows; Cs Cin Cl

Capp =

T < Ts Ts < T < Tl T > Tl

Solid Phase Solid/Liquid Phase Liquid Phase

(1.5)

where; Capp: Apparent Heat Capacity (kJ kg-1 K-1) Cs: Solid Heat Capacity (kJ kg-1 K-1) Cin: Solid / Liquid Transition Heat Capacity (kJ kg-1 K-1) Cl: Liquid Heat Capacity (kJ kg-1 K-1) T: Temperature (°K) Ts: Solid Phase Transition Temperature (°K) Tl: Liquid Phase Transition Temperature (°K)

then;

{∫ C(T )dT + L} = Tl

Cin

(1.6)

(Tl − Ts )

where; C: Specific Heat Capacity (kJ kg-1 K-1) L: Latent Heat (kJ kg-1)

thus;

Capp ρ

∂T ∂  ∂T  = k  ∂t ∂x  ∂x 

(1.7)

where; k: Thermal Conductivity (w m-1 °K-1) x: X Coordinate (m) t: Time (s)

The Effective Capacity method improves upon the apparent capacity method.

The

temperature profile is assumed between nodes. An effective capacity is calculated based integration through the control volume as show below. Ceff =

(∫ C

app

)

(1.8)

where;

Ceff : Effective Heat Capacity Capp : Apparent Heat Capacity V : Control Volume

The Heat Integration method is performed by monitoring the temperatures of the control volumes.

For the melting case, when the temperature of a control volume

exceeds the melting temperature, the material in that control volume is assumed to 37

undergo phase change. The temperature of that control volume is then reset to the melting temperature and the amount of heat due to resetting the temperature is added to an enthalpy account for that control volume. Once the enthalpy account equals the latent heat, the temperature is allowed to rise.

The Source Based method allows the latent heat to be modeled as an additional term representing a source or sink. This allows existing codes to be easily adapted to include latent heat. The heat equation with the source term is as follows;

Cpρ

∂T ∂  ∂T  = k +E ∂t ∂x  ∂x 

(1.9)

where; Cp: Specific Heat Capacity (kJ kg-1 K-1) ρ: Density (kg m-3) E: Energy Storage (kJ) k: Thermal Conductivity (w m-1 °K-1 T: Temperature (°K) t: Time (s) x: X Coordinate (m)

The Enthalpy method is based on the relationship between enthalpy and temperature. An enthalpy-temperature curve is created such as is shown below in Figures 1.13 (a) and (b).

Figure 1.13: Enthalpy as a function of temperature for (a) isothermal phase change; (b) Non-isothermal phase change (Hu and Argyropoulos 1996)

For isothermal phase change, Ts = Tl;

CsT ClT+L

T ≤ Ts T > Tl

Solid Phase Liquid Phase

T < Ts

Solid Phase

Ts < T < Tl

Solid/Liquid Phase

T > Tl

Liquid Phase

(1.10)

For non-isothermal, linear phase change; CsT

CinT +

H f (T − Ts )

(Tl − Ts ) ClT+L+Cin(Tl-Ts)

where; Cs: Solid Heat Capacity (kJ kg-1 K-1) Cin: Solid / Liquid Transition Heat Capacity (kJ kg-1 K-1) Cl: Liquid Heat Capacity (kJ kg-1 K-1) L: Latent Heat (kJ/kg) T: Temperature (°K) Ts: Solid Phase Transition Temperature (°K) Tl: Liquid Phase Transition Temperature (°K)

(1.11)

All of the above described methods allow the formulation of latent heat in the transient heat conduction equation. The the equatoion developed in the Apparent Heat Capacity method can be easily discretized and solved nemerically. The Effective Heat Capacity method was developed to improve upon he Apparent Heat Capacity method. However, it is computationally intensive. The Heat Integration method, also called thpost-iterative method is simple and computational efficient.

However, accuracy

depends on the time step. The Source Based method allows a heat source or sink to be added to the general form of the transient heat eqation as an additional term. Therefor, xisting numerical code can be easilyt addapted. In addition, it provides good accuracy and is computionally efficient. Finally, the Enthalpy method is accurate and the solution is independent of the time step. However, it si complex and computationally intensive (Hu and Argyropoulos 1996).

CHAPTER TWO: PCM CONCRETE DEVELOPMENT AND TESTING

2.1

PCM Concrete Development As the objective of this research was to develop a concrete mix that can be used to

create Concrete Masonry Units (CMUs), a commercial lightweight concrete masonry mix design was obtained from a local block manufacturer. The aggregate gradation, cement types, proportions and mix water were examined for this mix. Although mix design was based on this local manufacturer’s mix, other lightweight aggregates and proportions (following recommended ESCSI standards) were needed to enable PCM incorporation.

Sieve analyses of all of the aggregates that could be used in the PCM concrete mix were conducted. ASTM standard, round, 20 cm (8 inch) diameter sieves were used for all of the sieve analyses.

The standard sieve sizes for concrete masonry gradation

are 3/8 inch (9.50 mm), #4 (6.35 mm), #8 (3.18 mm), #16 (1.59 mm), #30 (0.85 mm), #50 0.51 mm), and #100 (0.25 mm).

The next step in mix development was the identification of potential PCMs for incorporation into concrete mixes. Initial PCM choices were determined according to their latent heat and transition temperature.

This entailed selecting PCMs with a

reasonably large latent heat value typically ranging from 150 KJ/Kg to 250 KJ/Kg, and a

transition temperature within ±5°C of room temperature (~20°C, 68° F). Though, several PCM

that

exceeded the ±5°C from room set point limits were also studied. Next, PCMs were examined for reactivity and thus salt hydrates were excluded. The prime focus of the PCM evaluation was therefore restricted to organic PCMs.

Once a variety of PCMs were selected, encapsulation methods were studied. Three encapsulation methods were studied including Micro-encapsulation, formstabilized diatomaceous earth, and form-stabilized vacuum impregnated porous expanded aggregates.

Micro-encapsulation and form-stabilized diatomaceous earth PCMs are

commercially available while vacuum impregnation of aggregates was done in the laboratory. Micro-encapsulation is the most common of the PCM encapsulation methods due to its ease of use, although it does add to cost. Diatomaceous earth form-stabilization is less common than Micro-encapsulation. However, the use of earthen materials is less expensive than other forms of micro-encapsulation. The downside of this is that the diatomaceous earth encapsulation has a large volume in comparison to other forms of PCM encapsulation. Vacuum impregnation reduces costs by also using earthen materials, and there is a significant volume of the composite incorporated into the aggregate for little or no additional volume in the mix. The vacuum impregnated aggregates can be incorporated as a structural constituent of the PCM concrete mix if the aggregates are significantly strong. Thus vacuum impregnation holds significant promise for PCM Concrete Masonry Unit development.

Having examined the aggregates, chosen the PCMs, and the encapsulation types, the next step was designing mixes that used all three forms of encapsulation and a variety

of PCMs. All mixes began with a standard block mix modified to incorporate microencapsulated PCMs, the diatomaceous earth form stabilized PCMs and finally, vacuum impregnated PCM aggregates.

Once a total of 24 PCM concrete mixes were developed and prepared, the physical and mechanical properties of the aggregates and concrete mixes were examined and density was measured for the cast concrete samples. Finally, compressive strength was determined for the cast PCM concrete samples.

2.1.1

Aggregate Analysis The initial step in the PCM concrete mix design was to determine gradation and

bulk density of all of the aggregates that could be used in masonry block mix. The local block manufacturer mix aggregates included crushed limestone, crushed expanded shale, and dredged river sand. A sieve analysis was performed for each of the aggregate types. The bulk density for each aggregate sieve fraction was determined for all of the aggregates. These are summarized in Tables 2.1 and 2.2.

In addition to the provided mix aggregates, other lightweight aggregates were obtained and analyzed.

These included several expanded clay blends of varying

gradations, several vermiculite blends, expanded perlite, and graphite. Sieve analysis and bulk density were determined for each aggregate type. These results are also shown in Tables 2.1 and 2.2.

Table 2.1: Aggregate Sieve Analysis Results Percent Retained Expanded Expanded Clay Expanded Clay Expanded Clay Sieve Limestone Shale Sand (Fines) (Intermediates) (10% Blend) 3/8 0.00% 0.00% 0.00% 0.00% 0.00% 0.04% #4 2.65% 1.88% 2.20% 0.11% 37.92% 7.69% #8 23.12% 25.22% 9.46% 13.87% 34.59% 12.03% #16 33.41% 26.55% 14.41% 32.77% 16.64% 30.21% #30 21.90% 17.37% 27.83% 21.78% 6.53% 19.73% #50 11.39% 11.28% 37.73% 15.96% 2.62% 17.98% #100 4.98% 6.86% 7.37% 10.94% 1.15% 7.34% Pan 2.55% 10.84% 0.99% 4.58% 0.54% 4.99%

Table 2.2: Additional Aggregate Sieve Analysis Results Percent Retained Ottawa Vermiculite Vermiculite Vermiculite Sieve Sand (Fine) (Medium) (Coarse) Perlite Zonolite 3/8 0.00% 0.00% 0.00% 3.11% 0.00% 0.00% #4 0.00% 0.00% 2.68% 59.33% 5.54% 0.00% #8 0.00% 3.55% 31.77% 26.89% 43.24% 0.00% #16 0.00% 42.45% 49.66% 7.55% 28.82% 0.67% #30 0.97% 41.33% 11.86% 1.55% 9.98% 54.53% #50 84.74% 9.33% 2.68% 0.45% 5.10% 34.21% #100 13.89% 2.00% 0.67% 0.45% 2.88% 7.28% Pan 0.40% 1.33% 0.67% 0.67% 4.44% 3.31%

Individual sieve results for each aggregate can be found in Appendix A. Aggregate density test results are shown in Table 2.3.

Table 2.3: Aggregate Density Results Sieve Size

Limestone

Expanded Shale

Expanded Clay

Sand

Vermiculite

Perlite

Graphite

gm/ml lb/ft gm/ml lb/ft gm/ml lb/ft3 gm/ml lb/ft gm/ml lb/ft gm/ml lb/ft gm/ml lb/ft #4 1.29 80.50 0.61 37.86 0.55 34.23 1.52 94.54 0.11 6.69 0.12 7.24 n.a. n.a. #8 1.39 87.03 0.64 39.78 0.58 36.25 1.53 95.47 0.15 9.32 0.15 9.20 n.a. n.a. #16 1.47 91.57 0.66 41.27 0.59 36.59 1.51 94.52 0.15 9.53 0.13 8.24 0.169 10.556 #30 1.48 92.57 0.69 42.98 0.60 37.44 1.54 95.81 0.14 8.68 0.11 6.86 0.165 10.282 #50 1.51 94.15 0.77 47.91 0.62 38.87 1.56 97.14 0.16 9.98 0.15 9.54 0.135 8.430 #100 1.43 89.14 0.81 50.60 0.63 39.43 1.55 96.56 0.25 15.39 0.19 12.11 0.183 11.402 Pan 1.11 69.26 0.95 59.39 0.64 40.01 1.64 102.34 0.38 23.71 0.17 10.31 0.247 15.392 3

From the density results, it can be seen that sand has the highest bulk density. This is logical since sand, being typically it formed from quartz. Limestone also has a higher density than expanded shale and expanded clay.

Expanded vermiculite and

expanded perlite are much lower in density than the other aggregates. Graphite has a slightly higher density than vermiculite in the #16 and # 30 sizes while lower for the # 50, #100 and pan sizes.

2.1.2

PCM Choice and Preparation A variety of PCMs were evaluated during the development of a viable PCM

concrete mix. These included linear alkanes, as well as fatty acids. Encapsulation systems included commercial Micro-encapsulation, commercially available PCM form stabilized in diatomaceous earth, and bulk PCM vacuum impregnated into porous aggregate in our lab.

Three micro-encapsulated PCMs were evaluated. The first PCM studied was micro-encapsulated BASF Micronal 5001 which is a proprietary organic PCM with a 46

transition temperature of 26°C (79°F). Next were the commercial micro-encapsulated PCM products, Microtek 18d and 28d. Microtek 18d is comprised of pure n-Hexadecane and has a transition temperature of approximately 18°C (64°F). Microtek 28d consists of n-Octadecane and has a transition temperature of approximately 28°C (82°F). Finally, two products from Entropy Solutions were examined. These were bulk PureTemp 23 (PT23) as well as PureTemp 23 form-stabilized in diatomaceous earth. Both of the PureTemp 23 PCMs have a transition temperature of approximately 23°C (73°F).

Several other bulk linear alkane PCMs were evaluated during the PCM concrete development. As shown in Figures 2.1 and 2.2 these were n-Hexadecane, n-Octadecane, and a proprietary linear alkane blend from Microtek Laboratory with a transition temperature of 24°C (75°F). These linear alkanes consist of a chain of saturated carbon atoms with the associated single bonded hydrogen atoms. In addition, bulk Capric acid (Decanoic acid) and Myristic acid (Tetradecanioic acid) were also examined (See Figures 2.3 and 2.4). These are both fatty acids (carboxylic acids) consisting of saturated carbon chains and a carboxyl group. All of the bulk PCMs were encapsulated using the vacuum aggregate impregnation method. Graphical representations of these PCMs are shown in Figures 2.1 through 2.4.

Figure 2.1: n-Hexadecane CH3 (CH2)14CH3 (3DChem 2013)

Figure 2.2 n-Octadecane CH3 (CH2)16 CH3 (3DChem 2013)

Figure 2.3: Capric Acid (n-Decanoic Acid) Figure 2.4 Myristic Acid CH3 (CH2)8 COOH (3DChem 2013) CH3 (CH2)12 COOH (3DChem 2013) The Capric acid (Decanoic Acid) and Myristic acid (Tetradecanioic Acid) PCMs were blended to a 74% Capric acid and 26% Myristic acid ratio by mass according to Figure 2.5. As shown in Figure 2.5, this is the eutectic point for the two PCMs with a melting point of 21°C (70°F) (Karaipekli and Sari 2008). Proportioned quantities of Capric acid and Myristic acid were weighed and combined. The solid PCM flakes were slowly heated and stirred until both materials had melted. The mixture was then stirred for approximately 10 minutes to ensure complete mixing.

CA: Capric Acid MA: Myristic Acid

Figure 2.5: Eutectic Proportioning of Capric acid and Myristic acid (Karaipekli and Sari, 2008)

Organic PCMs were used because inorganic PCMs are typically hydrated salts which are inclined to react strongly with metals. Since the goal of this research is to develop a viable PCM concrete mix that can be used in concrete masonry units (CMUs), salt related corrosion with metal is a significant concern since most masonry structures are reinforced with steel reinforcing bars. Furthermore, even if a salt hydrate PCM concrete that does not react with metals could be developed, the conservative construction industry would likely still not adopt it due to the bad past experience with corrosion of reinforcement.

2.1.3

PCM Encapsulation Several methods of encapsulation were used during the PCM concrete mix

development process.

These were Micro-encapsulation, diatomaceous earth form

stabilization, and vacuum impregnation of expanded porous aggregate form stabilization. Micro-encapsulated PCM was used in the initial trials due its commercial availability, and ease of use, with PCM contained within polymer spheres. Micro-encapsulation allowed the PCM to be incorporated as part of the mix aggregates.

Initially, viable micro-encapsulated PCM mixes were difficult to obtain. This was because the micro-encapsulated PCM spheres are very small, being less than 100µm in diameter. This created a large surface area with the micro-encapsulated PCM which in turn required significant quantities of mix water to obtain a workable mix. The resultant water to cement ratios were initially around 1.0, which is quite high. In addition, the mechanical agitation of the concrete mixer had a tendency to rupture the PCM capsules. Thus between excess mix water and PCM from ruptured capsules entering the cement matrix, initial mixes had poor strength.

With the addition of a water-reducing

superplasticizing agent and the delayed addition of the micro-encapsulated PCMs to the final stages of mixing, a viable mix was developed. This mix contained 9% microencapsulated PCM by mass.

It should be noted that 20% of the PCM is the

encapsulation, so only approximately 7% PCM was incorporated in the mix by mass fraction.

Because of the added cost of Micro-encapsulation and the relatively low PCM percentages possible with micro-encapsulated PCMs, alternative encapsulated methods were examined. One alternative was presented by Entropy Solutions. This was a fatty acid form stabilized PCM in diatomaceous earth with a mass fraction of approximately 40% PCM. Like micro-encapsulated PCM, the diatomaceous earth form stabilized PCM also displaced the cement in the concrete mix. This PCM composite was mixed within the concrete matrix in a similar fashion to the micro-encapsulated PCM. Surface tension maintains stability of the product prior to mixing. Due to the significant cement matrix displacement and introduction of unconfined PCMs, a viable mix was not obtained using this form of PCM.

Focus was then placed on vacuum impregnation of porous aggregates. This process has the benefit of placing the PCM within the aggregate pore space and not in the cement matrix. Cement is not displaced by the PCM, thus reducing the amount of cement needed to achieve a mix that has the potential to meet ASTM C90 strength requirements while also containing significant quantities of PCM.

Several expanded porous aggregates were examined for potential use in vacuum impregnation. These included expanded vermiculite, expanded perlite, expanded shale, and expanded clay. Due to their compressibility, expanded vermiculite and perlite did not provide the strength needed to create a structural mix. Expanded shale did not provide good PCM uptake, most likely due to the unconnected nature of its pore space. However, expanded clay was found to have considerable PCM retention capabilities.

Thus, expanded clay aggregates were used for the vacuum impregnation process in all subsequent mix designs. Expanded clay is manufactured by heating pellets of clay at high temperatures. As shown in Figure 2.6, this causes the moisture trapped in the pore space to expand creating a lightweight ceramic aggregate with a large volume of interconnected pore space. Interconnected pore space is important for the vacuum impregnation process since the PCM must be able to penetrate deeply within the aggregate and the aggregate must be able to retain a significant quantity of PCM. In addition, expanded clay is already used as an aggregate in many commercial lightweight concrete and CMU mixes. Figure 2.6 shows a typical expanded clay aggregate section.

Pore Space

Figure 2.6: Section of Porous Expanded Clay Aggregate

The process of vacuum impregnation involves drawing the air from the aggregate pore space and then replacing it with PCM. In order to do this a vacuum impregnation device was developed. As shown in Figures 2.7 and 2.8 the principle components of this device were a vacuum pump, a pressure vessel to hold the aggregate, a vacuum regulated funnel to hold the PCM under vacuum pressure prior to impregnation, and a water-filled vacuum trap to protect the vacuum pump from fumes. In addition, a hotplate kept the pressure vessel and the aggregate warm. A heat lamp kept the PCM molten in the funnel. Initially a 2000 ml flask was used as the pressure vessel during initial development. In order to increase production, a stainless steel pressure vessel was constructed to achieve a larger yield for PCM concrete production.

Heat Lamp Funnel Vacuum Trap

Vacuum Pump Flask w/ Aggregate

Figure 2.7: Initial Vacuum Impregnation Apparatus

Figure 2.8: View of Enlarged Vacuum Impregnation Apparatus

The vacuum impregnation procedure was as follows; (1) The PCM was heated to a liquid state. (2) The PCM was then introduced into the vacuum regulated funnel with the PCM temperature maintained via a heat lamp. (3) Dry aggregate was placed in the pressure vessel and warmed with the hotplate.

(4) A vacuum was applied to the sealed system for 10 minutes with the aggregate and PCM separate in order to remove the air from the aggregate pores. Heating of the aggregate and PCM combined during the air removal process would have blocked the pore openings at the surface and thus would have inhibited removal of air from the pore space. Ten minutes was deemed appropriate since the air bubbles filtering through the vacuum trap had substantially dissipated indicating that the air in aggregate pores had been removed. (5) The liquid PCM was introduced into the pressure vessel and mixed with the aggregate.

An additional 10 minutes of vacuum was

maintained to remove any air that may have been introduced with the PCM in solution. (6) The vacuum was released and the PCM was allowed to penetrate the pore space. Again, 10 minutes was allowed for impregnation into the pore space. (7) The pressure vessel was opened, the extra PCM was decanted, and the PCM impregnated aggregate was allowed to drain on a sieve.

After draining, the PCM aggregates were rinsed in a solution of 50% water and 50% ethanol to remove the PCM from the surface of the aggregate to facilitate bonding with cement during concrete mixing. This solution was considered strong enough to remove the PCM surface coating but not remove substantial PCM from the pores. The PCM aggregate was then placed on drying pans and allowed to dry under forced

convection. Excellent PCM retention was achieved. It is believed, however, that with additional study, this process could possibly be improved to increase PCM retention.

In order to determine the amount of PCM retained, the dry aggregate was weighed prior to vacuum impregnation.

The PCM aggregate was again weighed after

impregnation and drying. The difference in weight after impregnation divided by the preimpregnation weight provided the mass ratio of PCM retained in the aggregate. The impregnation process was performed on the aggregates retained on the #4 (6.35 mm), #8 (3.18 mm), #16 (1.59 mm), and #30 (0.85 mm) sieves. Multiple samples with the exception of PureTemp 23 were examined for impregnation efficiency. The average PCM impregnation results for each sieve size are listed in Table 2.4. Smaller sized aggregates were found to hold little PCM and were difficult to handle, especially when sieving and rinsing, and thus were not used. Further research may identify an economical way to impregnate all of the aggregates and thus improve total PCM in the mix.

Table 2.4: Average Vacuum Impregnation Results PCM Type Sieve

#4 (6.35mm) #8 (3.18 mm) #16 (1.59 mm) #30 (0.85 mm)

Capric/ Myristic Acid Blend 24%

nnHexadecane Octadecane

PureTemp 23

27%

Linear Alkane Blend 24%

23%

17%

25%

20%

30%

34%

32%

26%

17%

23%

36%

31%

37%

41%

Impregnation percentages were similar for all of the PCMs. The linear alkanes were consistent with the exception of the #30 (0.85 mm) sieve fraction. The values of impregnation for n-Octadecane, the Linear Alkane Blend, and PureTemp 23 ranged from 31% to 37% while the Capric/Myristic Acid Blend and n-Hexadecane had retentions of 17% and 23% respectively. The discrepancies for this sieve fraction are likely due to the small aggregate size creating difficulties in the decanting and rinsing process for that size. The small diameter #30 (0.85 mm) sieve fraction aggregates caused a situation where close aggregate proximity created small aggregate interstices and thus allowed surface tension to dominate. This created a situation where it was difficult fully rinse the aggregates.

Higher uptake percentages likely indicate excess surficial PCM on the

aggregate which was not removed during washing.

Beyond the #30 (0.85 mm) aggregates, the Capric acid / Myristic acid eutectic provided similar results to the linear alkanes. However, PureTemp 23 retention was quite high for the #4 (6.35 mm) and #30 (0.85 mm) sieve sizes. The PureTemp 23 was the last PCM evaluation during testing and only one vacuum impregnation was performed for this PCM. Additional, impregnations may provide an average that is closer to the other PCMs. It is likely that PCM viscosity plays a significant role in the retention values.

2.1.4

Concrete Mix Design The initial concrete mix design was based upon a mix developed by a local

concrete masonry unit manufacturer which is shown in Table 2.5.. The mix contained limestone, dredged river sand, and expanded shale aggregates along with Type III

Portland cement and Slag cement as well as a waterproofing admixture, all of which were constituents in the manufacturers mix. As shown in Figures 2.9 and 2.10, the mix was proportioned according to aggregate weights. By mass, the manufacturers mix did not remain in the bounds of the Expanded Shale, Clay, and Slate Institute (ESCSI) gradation limits as is common for masonry block mixes (Holm 1997). Due to the significant variations of density between different aggregates, the mix gradation was examined according to volumetric fractions rather than by weight.

Using typical aggregate

densities, the mix volume fractions did fall within the ESCSI gradation as shown in Figures 2.11 and 2.12.

For the micro-encapsulated PCM concrete, the commercial mix was altered by removing the fines (sieve size #50 (0.5 mm, 1/50th of an inch) and lower) in the limestone, expanded clay, and sand aggregates. These sieve fractions were replaced with the micro-encapsulated PCM capsules. Two micro-encapsulated PCM percentages were examined; approximately 9% and 17% by mass. These values include the mass of the PCM and the polymer capsules, and thus, provided lower actual percentages of PCM. Type III Portland cement proportions were varied in the mix to optimize strength. A viable mix with 9% micro-encapsulated PCM was obtained by increasing the Type III Portland cement weight by 150% of the original mix and by using a water-reducing superplasticizer. This mix provided approximately 7% PCM by mass after correction for the polymer capsule mass.

Figure 2.9 shows the ESCSI gradation limits. Figure 2.10 shows the commercial lightweight concrete masonry block mix gradation by mass. Figure 2.11 shows the commercial lightweight concrete masonry block mix gradation by volume.

Finally,

Figure 2.12 shows the proportion of the commercial lightweight block mix replaced by micro-encapsulated PCMs.

Table 2.5: Commercial Block Mix Proportions by Weight

Mix Component Type III Cement Grade 100 Slag Cement Expanded Shale Manufactured Limestone Concrete Sand M.P. Additive

Percent Weight (%)

Density (kg/m3)

Density (pcf)

9.89% 1.77% 31.80% 24.95% 31.58% 0.01%

1505.7 1505.7 789.7 1525.0 1499.3 1000.0

94 94 49.3 95.2 93.6 62.4

Figure 2.9: ESCSI Minimum and Maximum Gradation Limits

ESCSI Upper Limit

ESCSI Lower Limit

Figure 2.10: Commercial Block Mix by Mass 60

ESCSI Upper Limit

ESCSI Lower Limit

Figure 2.11: Commercial Block Mix by Volume

Fractions Replaced by Micro-encapsulated

Figure 2.12: Commercial Block Mix w/ Volume Replaced by Micro-encapsulated PCM

A similar concrete mix was utilized with the form stabilized PureTemp 23 PCM in diatomaceous earth. Instead of just the fine fractions the entire aggregate range was utilized. For this mix 20% form stabilized PCM composite by mass was added to the standard mix.

Initially the diatomaceous earth PCM clumped. This occurred because during transport it had exceeded 23°C (73°F) causing melting, and then cooled and solidified. Removal of the PCM from the container provided large chunks that could not be used in the mix. In order to remove the clumps, the PCM material was warmed and then mixed with the coarse aggregates. This mitigated the clumping. Finally, the remainder of the aggregates, cement and water were added to the mix.

These difficulties caused

abandonment of this PCM incorporation method.

In an effort to improve the PCM concrete mix economics, mixes incorporating vacuum impregnated aggregates were developed. These mixes used a variety of PCMs including n-Hexadecane, n-Octadecane, a proprietary linear alkane blend with a transition temperature of 24°C (73°F), a eutectic mixture of Capric acid and Myristic acid, and bulk liquid PureTemp 23.

Due to the significant differences in the aggregate densities, the mixes were developed using volume fractions rather than the traditional mass proportions. The bulk density values of the aggregates were used to convert the original mix fractions to

volumes. This conversion is necessary when mixing heavy aggregates, such as sand and limestone with lightweight aggregates such as expanded clay and expanded vermiculite.

The ESCSI gradation limits were used as bounds to create a mix that maximized PCM incorporation but maintained workability. PCM was incorporated in the expanded clay aggregate #4 (6.35 mm), #8 (3.18 mm), #16 (1.59 mm), and #30 (0.85 mm) sizes. A new mix was designed by sieve fractions incorporating expanded clay with and without PCM, crushed limestone, and sand as shown in Figure 2.13. This mix had significant PCM content and provided a workable mix for both the controls and PCM mixes.

Figure 2.13: Vacuum Impregnated Expanded Clay PCM Concrete Mix by Volume

A concrete mix was also designed using PCM impregnated expanded clay and expanded vermiculite aggregates. Figure 2.14 shows gradation for this mix.

Figure 2.14: Vacuum Impregnated Expanded Clay and Vermiculite PCM Concrete Mix by Mass Initially, there was considerable difficulty creating a vacuum impregnated PCM concrete mix that met the compressive strength requirements. At the time, a PCM blend of Capric acid and Myristic acid was being used. Surface cracking of the test cylinders at 2 to 3 weeks after casting suggested a possible cause for the low compressive strengths for the Capric acid and Myristic acid mixes.

At first, the cracking was thought to be caused by hydration related thermal stress. A thermocouple was placed in the center of a sample after casting to determine if this might be the cause. It was found that the interior temperature of the PCM concrete test cylinder raised only slightly during curing. This is probably due to energy storage capacity of the PCM. Further investigation found that there is a reaction between fatty acids and Portland cement. The fatty acid glycerides are broken down by hydrolysis in their constituent acid and alcohol components. This is mainly brought about by the action of aqueous solutions of mineral acids or hydroxides. In alkaline hydrolysis the 64

metal of the hydroxide forms a salt with the fatty acid. This is the process where solutions of sodium or potassium hydroxide act on fats and oils and is used to produce salts, or soaps.

This process is known as saponification (Lea and Desch 1956).

Saponification cracking is what can be seen in Figure 2.15.

Saponification Related Surface Cracking

Figure 2.15: Surface Cracking of Cylinders due to Saponification

Once it was determined that saponification was the cause of poor compressive strengths, alternative PCMs were examined. The linear alkanes such as n-Hexadecane, nOctadecane, and the Microtek 24°C blend were used.

Once these PCMs were

incorporated into the concrete mixes, strengths in excess of the ASTM C90 requirements were obtained. The PureTemp 23 PCM was also investigated. Although PureTemp 23 is vegetable derived and most likely a fatty acid, it had less of a reaction with the cement

matrix possibly due to modification of the carboxyl group, reducing reactivity. This PCM has the advantage of having a high heat of fusion and a relatively low cost.

2.2.1

PCM Content The PCM content was quantified for all of the PCM mixes. For the micro-

encapsulated mixes the PCM / capsule content was the mass of the PCM powder incorporated into the concrete mix divided by the PCM concrete mix mass. The microencapsulated PCMs were assumed to be 20% resin capsule and 80% PCM. Thus the PCM for micro-encapsulated mixes was 80% of the mass added to the mix. The 20% value was estimated based on thermal testing of the micro-encapsulated PCMs and their pure forms.

The form stabilized diatomaceous earth PCM content was determined similarly to the micro-encapsulated PCM. The PCM / diatomaceous earth content was calculated as the mass of the material incorporated into the concrete mix divided by the PCM concrete mix mass. The diatomaceous earth was approximately 60% of the mix. Thus, the PCM for the diatomaceous earth stabilized earth mixes was 40% of the mass added to the mix.

The PCM content of the vacuum impregnated expanded porous aggregate PCM concrete was more difficult to quantify. PCM was incorporated into the #4, #8, #16, and #30 aggregates.

Each aggregate retained different quantities of PCM.

In addition,

variations in PCM retention were observed for different vacuum impregnation batches of the same PCM. This was a result of slight variations in liquid PCM temperature and thus viscosity, PCM / aggregate mixing, and rinsing of the PCM aggregates. Therefore each mix had slightly different proportions of PCM for each aggregate size. Thus, PCM content was dependent on the individual aggregate impregnations for each of the sieve

sizes and the proportion of each aggregate incorporated into the PCM concrete mix. Table 2.6 summarizes the estimated PCM contents by mass for each concrete mix and PCM encapsulation method. The designations 1.0X, 1.5X, and 2.0X refer to the ratio of the amount of cement in the vacuum impregnated mix as compared to the initial mix obtained from the local block manufacturer.

Table 2.6: PCM Percentage of Concrete Mixes Mix

PCM Percent

Microtek Micro-encapsulated PCM

Hexadecane (1.5X Cement)

11%

24°C Alkane Blend (1.0X Cement)

14%

24°C Alkane Blend (1.5X Cement)

13%

24°C Alkane Blend (2.0X Cement)

10%

Octadecane (1.5X Cement)

14%

PureTemp 23 (1.5X)

12%

2.2.2

Concrete Density The concrete density was determined for the control and PCM concrete mixes.

This density is needed to accurately evaluate the transient thermal mass behavior of the material because both specific heat and latent heat are dependent on the material mass.

Density was determined during mixing. Each cylinder was weighed full, after consolidation. The weight divided by the cylinder volume provided the density. Initially the cylinders were weighed a second time after the 28 day curing period. However, very

little deviation was found so density measurements were only performed at the mixing stage. Density was dependent upon the mix compaction. Therefore, due to variability in compaction, density was not used in the determination of PCM incorporation. Rather, the PCM mix design values were adjusted for PCM content as a ratio of the projected yield to the actual volumetric yield. Thus, the volume of the entire batch produced was measured to adjust the PCM content. Table 2.7 summarizes the results of the measured densities for all of the concrete mixes.

Table 2.7: Concrete Densities

Mix Local Block Mix BASF 9% BASF 17% Microtek 18D 9% Microtek 18D 17% Microtek 28D 9% Microtek 28D 17% PT 23 Diatom. Earth Exp. Clay Vac. Imp. Control (1.5 Cement) Exp. Clay Vac. Imp. Control (2X Cement) Exp. Clay Vac. Imp. CA-MA (2X Cement) Exp. Clay Vac Imp. n-Hexadecane (1.5X Cement) Exp. Clay Vac. Imp. 24C Alkane (1.5X Cement) Exp. Clay Vac. Imp. n-Octadecane (1.5X Cement) Exp. Clay Vac. Imp. 24C Alkane (2X Cement) Exp. Clay Vac. Imp. PT 23 (1.5X Cement)

Density (kg/m3) 1791 1440 1302 1645 1398 1661 1175 1469 1497 1419 1446 1561 1537 1506 1658 1769

Density (pcf) 112 90 81 103 87 104 73 92 93 89 90 97 96 94 103 110

By examining the density data it can be seen that the local block mix had the highest density. This is a result of the large amounts of crushed limestone and sand, both

being high in density as was shown in Table 2.4. The mixes incorporating BASF and Microtek micro-encapsulated PCM decreased in density with increased PCM content. This is because the micro-encapsulated PCM is replacing a portion of the mix matrix.

The expanded clay controls for the vacuum impregnation mixes contained an anomaly. The mix with 1.5X cement had a higher density than the 2.0X cement control mix. This is likely due to variability in the compaction process during casting. While the vacuum impregnated mixes contained significant quantities of lightweight expanded clay aggregate, the PCM impregnated into them increased their density significantly and thus increased the mix density as well.

2.2.3

Compression Testing Compressive strength testing of all PCM concrete mixes was performed according

to the ASTM C39 Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens. As shown in Figure 2.16, 7.62 cm (3”) diameter by 15.24 cm (6”) height cylinders were tested in a Satec Systems, 270 KN (60 kip), universal testing machine after curing for 28 days in a moist room having a temperature of 20°C (68°F) a relative humidity of 98% ± 2%. A minimum of three samples were tested until failure with the resulting maximum loading recorded. The maximum load divided by the sample area was calculated to determine the compressive strength.

Failed specimens were

photographed and failure mechanisms were noted. Typical concrete sample specimens are shown in Figure 2.16. The typical crushing pattern of failure is shown in Figure 2.17. Figure 2.18 shows the average compression test results for each mix as well as the

minimum strength required by ASTM C 90. It can be seen that a number of the mixes meet the compressive strength requirements.

Figure 2.16: Concrete Test Cylinders

Figure 2.17: Failed Concrete Test Cylinder

Vacuum Impregnated Diatomaceous Earth

MicroEncapsulated

ASTM C90 13.1 MPa (1900 psi)

Figure 2.18: PCM Concrete Compression Testing Results

2.2.4

PCM Concrete Mix Optimization There is an optimal balance of cement and PCM for economics, maximum PCM

incorporation, and preserving structural compressive strength and thermal performance. While additional cement increases strength, it also displaces PCM carrying aggregates thus reducing the PCM fraction, which in turn reduces energy storage. In addition, other than PCM, cement is the most expensive component in a PCM concrete mix. Thus reducing cement improves economics in two ways. It is therefore imperative to balance PCM and cement through optimization. To facilitate this optimization, compressive strength and PCM percent were plotted in Figure 2.19 with respect to the percentage of Type III cement in the mixes. These cement proportions were 17%, 24%, and 31%

respectively by mass. The 17% percent value relates to the 1X designation (same amount of type III cement as commercial mix), 24% is 1.5X (50% more type III cement than the commercial mix, and 31% is 2X (twice the type III cement as the commercial mix).

Figure 2.19: Optimization of PCM and PCM Percentage as a Function of Type III Cement Proportion It was found that vacuum impregnated mixes had PCM percentages that ranged from approximately 10% to 14%. While the 10% PCM mix produced strengths that exceeded 34.5 MPa (5000 psi), the 14% PCM mix did not meet minimum compressive strength requirements.

A second order equation for compressive strength as a function of the Type III cement was developed from the data using a regression analysis.

C = −9.17.02 x 2 + 629.87 x − 71.995

(2.1)

In addition, a linear equation for PCM ratio with respect to Type III cement was developed from the data using a linear regression analysis.

P = −0.2587 x + 0.1827

(2.2)

Where;

C : Compressive Strength (MPa)

P : Ratio of PCM mass to PCM concrete mass

x : Ratio of Type III cement mass to PCM concrete mass

Since the minimum allowed compressive strength allowed by ASTM C90 is 13.1 MPa (1900 psi);

C = 13.1 MPa

Therefore;

P = 0.134 (13.4 % PCM) x = 0.135 (13.5% Type III Cement)

Using a minimum compressive strength of 13.1 MPa (1900 psi) and the solving the two equations simultaneously, a PCM percentage of 14.7% PCM can be provided by a mix containing 13.5% Type III cement. This value can most likely be increased if vacuum impregnation retention can be improved through further process enhancement. At this point, a PCM percentage range between 10% and 15% seems to be the upper working range for PCM mix combinations. 74

In summary, viable PCM concrete mixes were developed meeting the ASTM C90 structural compressive strength of 13.1 MPa (1900 psi). These mixes included microencapsulated PCM as well as vacuum impregnated PCM aggregates.

PCM mix

incorporation ranged from 7% PCM by mass for the micro-encapsulated PCM to almost 14% PCM by mass for the vacuum impregnated PCM. If the vacuum impregnation method can be improved, it may be possible to exceed 15% PCM by mass in the PCM concrete.

CHAPTER THREE: THERMAL CHARACTERIZATION OF PCMS AND PCM CONCRETES

3.1

Thermal Testing Thermal testing was performed in order to determine the thermal properties of the

PCMs, as well as the thermal properties of the PCM concrete. During this portion of the investigation Digital Scanning Calorimetry was performed on small PCM samples typically ranging in mass from 10 mg to 20 mg. Larger PCM concrete and control samples in excess of 150 gm were tested in the Dynamic Hot Box Apparatus.

Pure PCM, micro-encapsulated PCM, and vacuum impregnated PCM aggregates were tested in the DSC since the samples were of small enough size to fit in the 6 mm (1/4 inch) DSC pans. In addition, two PCM concrete mixes with cement paste, fine aggregates, and BASF micro-encapsulated PCM were studied. For small samples, the DSC allows accurate analysis of specific heat, latent heat and transition temperature. Two DSC methods were utilized in these tests. These methods were the dynamic method and the isothermal step method. The two methods different methods were used because DSC results are dependent on sample size and ramping rate. The dynamic method provides accurate values of latent while the isothermal step method provides improved accuracy for the transition temperatures and the phase fraction curve (Gunter et al. 2009).

Larger samples such as concrete specimens required the use of the Dynamic Hot Box Apparatus. This testing device is large enough to test concrete samples. Due to the composite nature of concrete having relatively large, varying aggregates sizes, types, and random material distribution, samples must be of substantial enough volume in order to provide a representative sample.

Thus, 7.62 cm (3 inch) diameter samples with a

thickness of 2.54 cm (1 inch) were the smallest concrete sample size deemed acceptable for thermal testing.

The Dynamic Hot Box Apparatus was used to test the control and composite PCM concrete thermal conductivity. Thermal conductivity is required for simulation models in order to account for steady-state and transient heat conduction.

Further

characterization performed using the Dynamic Hot Box apparatus involved the determination of heat capacity. By utilizing the isothermal step method, the specific heat, latent heat, and phase transition temperature were determined for PCM concrete samples. This analysis provided composite PCM concrete heat capacity values and thus when latent heat values were compared, validated the mass fraction PCM incorporation values determined during PCM concrete preparation.

In addition, the Dynamic Hot Box Apparatus was used to simulate diurnal cycling of the control and PCM concrete materials. This provided data that could be examined to quantify peak temperature and flux reductions as well as phase shifts provided by the incorporated PCMs. The diurnal cycling provides tangible data as well as the ability to visualize the PCM performance.

3.2

Digital Scanning Calorimetry Digital Scanning Calorimetry was performed on the PCM samples in order to

determine the latent heat, transition temperature, and specific heat of the materials. As shown in Figure 3.1, a TA Instruments Q20 Digital Scanning Calorimeter (DSC) with an RSC400 cooling apparatus was used to perform the measurements. This apparatus is a heat flux DSC. The TA Universal Analysis software provided with the Q20 DSC was used to determine the latent heat and transition temperatures of samples in the case of the dynamic method For the isothermal step method it was used to integrate the individual peaks in order to develop enthalpy curves quantifying the specific heat, latent heat, and transition temperature.

Cooling Device

DSC

Figure 3.1: Digital Scanning Calorimeter (DSC) with Cooling Device 78

A heat flux DSC determines the amount of heat absorbed by a sample as a function of temperature change. The DSC is calibrated using the enthalpy of a standard reference material prior to testing. In this case, sapphire was used as the reference material for calibration.

In a typical DSC scan, a test sample and a reference are symmetrically placed in the calibrated DSC furnace. A representation of a heat flux DSC is shown in Figure 3.2. The temperature of the sample is compared to the temperature of the reference which in this case was an empty pan. The temperature difference determines the heat flux between the sample and the furnace (Castellon et al 2008). The enthalpy is determined by the integration of the specific heat as a function of temperature according to the following equation; T

h(T ) = ∫ C p (T )dT

(3.1)

Where: h(T): Enthalpy as a Function of Temperature (kJ kg-1) Cp: Specific Heat Capacity (kJ kg-1 °K-1) T: Temperature (°K) To: Onset Temperature (°K)

Furnace reference

sample

Temperature Sensors

Figure 3.2: Heat Flux DSC Furnace (Castellon et al 2008)

Both the dynamic method and isothermal step method were used in DSC testing. The dynamic method was used to determine the latent heat. The isothermal step method was used to determine the enthalpy curve from which the PCM phase fraction and specific heat were obtained. This is because the dynamic method produces more accurate results for latent heat, while the step method produces a more accurate enthalpy curve profile for development of the phase fraction (Gunter et al. 2009). The latent heat and transition temperature results from the two methods were compared to examine the variation of the results

Initially DSC measurements were performed on micro-encapsulated PCMs. BASF and Microtek PCMs were tested for latent heat values. At the time of the first tests, the DSC was capable of heating only. Samples of powder form BASF Micronal 5001, Microtek 18d, Microtek 28d, as well as PCM / concrete composites of with BASF at 7% and 16% mass fraction were tested under heating only conditions. The samples were initially tested with the dynamic method at a rate of 10°C/minute. It was found that 80

this ramping rate is much larger than is experienced in a building envelope environment. Though the ramp rate was large, the total measured latent heat is valid (Gunter et al. 2009).

However, the transition range for fast ramp rates is wider than would be

experienced in an actual building application. Thus, lower ramp rate rates were used to provide tighter transition ranges indicative of building envelope applications. A ramping rate of 1°C/minute was used for all subsequent dynamic DSC testing.

3.2.1

DSC Methods The method of enthalpy measurement has an impact on the accuracy of the DSC

results. The dynamic method is the most commonly used method for determining the melting and solidification latent heats (Gunter et al. 2009). This is most likely due to its ease of use and simplicity due to the constant ramping rate and integration of a single peak when determining the latent heat. In addition, it provides results relatively quickly. However, the dynamic method suffers complications with increased sample size and faster ramping rates. Smaller sample sizes cause weak signals and thus inaccuracies, while larger sample sizes exhibit temperature differentials across the sample as shown in Figure 3.3. A sample experiencing heating will be warmer at the surface than the interior. A sample experiencing cooling will be cooler at the surface than the interior. This temperature differential creates a shift in measured phase transition temperature range; towards the warmer temperatures for heating, and towards cooler temperatures for cooling. This can be seen in Figure 3.3. The larger the sample, the larger the transition temperature range shift. In addition to sample size, faster ramping rates further the temperature shift (Castellon et al. 2008). For energy storage applications, both the latent

heat and sensible heat must be quantified. Thus sensitivity to small signals and low ramping rates must be addressed for the specific heat measurements. The dynamic method at 1°C/minute ramp rate for both heating and cooling is shown for PureTemp 23 in Figure 3.4. The phase transition shift is quite pronounced.

Figure 3.3: DSC Dynamic Method Sample Temperature Differential (Castellon et al. 2008)

Sample: Pure Temp 23 Size: 18.1000 mg

File: C:...\PT 23 Liquid-(Heating&Cooling).010 Operator: JDK Run Date: 14-Dec-2012 14:14 Instrument: DSC Q20 V24.4 Build 116

DSC

Comment: Eq. at 0 deg. Iso 10 min,Ramp 1 deg. C/min to 40C cool 2

Latent Heat of Crystallization

18.84°C

Heat Flow (W/g)

204.8J/g

21.63°C 199.8J/g

-1

Latent Heat of Fusion -2 Exo Up

24.26°C

Temperature (°C)

35 Universal V4.5A TA Instruments

Figure 3.4: Heating / Cooling Analysis with Dynamic Method 82

One way to address this effect related to sample size and forcing temperature profile is to use the isothermal step method. This method alleviates the issues with ramping rate and sample size related temperature differentials by using a step-wise temperature profile.

The temperature is increased and then held constant until

equilibrium is achieved for a series of steps. The enthalpy of each step is integrated to create discrete points in a plot of the melting enthalpy curve. The same is done under cooling conditions to create the crystallization enthalpy curve (Castellon et al, 2008). The isothermal step method in heating is shown for PureTemp 23 in Figure 3.5. This figure shows the individual peaks prior to their integration.

Sample: Puretemp 23 1C Step Cooling Size: 17.4000 mg Method: Step Method Heating 0 to 30C Comment: Step 0-30C 1C/Step

File: C:...\PT23-Step Heating.001 Operator: JDK Run Date: 03-Apr-2013 08:27 Instrument: DSC Q20 V24.4 Build 116

DSC

0.0

Heat Flow (W/g)

-0.1

-0.2

Enthalpy for each step is integrated

-0.3

-0.4

-0.5

Exo Up

Temperature (°C)

35 Universal V4.5A TA Instruments

Figure 3.5: Heating Analysis with Isothermal Step Method

3.2.2

DSC Analysis The TA Universal Analysis 2000 software was used for the DSC analyses of

latent heat and enthalpy curves. For the dynamic test method the software was used to 83

integrate the sample latent energy over the total melting / solidification range to determine the latent heat. For the step method the energy absorbed or released during each step is integrated to create points in the fusion and crystallization enthalpy curves.

3.2.3

Enthalpy Measurement Results As explained previously, enthalpy measurement results were from two different

DSC test procedures. The latent heat and transition temperatures were determined via the dynamic method. The relatively quick test procedure allowed multiple test samples to be tested in order to obtain an average latent heat value. The step method provided specific heat values and the latent heat phase fraction curve for energy modeling. The results of the tests on pure PCMs and a composite BASF PCM and fine aggregate concrete mix are shown in Table 3.1 which shows the transition temperatures of melting and solidification. These represent the temperatures where the melting and solidification phase transitions begin. The latent heat of melting and solidification characterize the energy storage capabilities for the PCM during the phase transition. The column showing the latent heat average is an average value of the melting and solidification latent heats.

Table: 3.1: PCM Latent Heat and Transition Temperature from Dynamic Method

PCM Sample

Transition Temp. of Melting (°C)

Latent Heat of Melting (KJ/Kg)

Transition Temp. of Solid. (°C)

Latent Heat of Solid. (KJ/Kg)

Latent Heat Average (KJ/Kg)

BASF Micronal 5001

22.3

99.0

24.6

100.0

99.5

Cement/9% BASF

23.4

9.1

N/A

9.1

Cement/17% BASF Microtek 18D

23.0

17.6

N/A

17.6

13.3

158.8

13.8

156.0

157.4

Microtek 28D

23.6

144.4

25.3

147.5

146.0

Capric / Myristic Acid n-Hexadecane

21.0

138.1

19.3

131.3

134.7

14.6

199.7

13.7

185.9

192.8

25% Octa / 75% Hexa 50% Octa / 50% Hexa 75% Octa / 25% Hexa n-Octadecane

13.2

114.6

14.6

125.1

119.9

14.3

123.0

17.5

120.5

121.8

18.3

101.4

20.8

106.9

104.2

23.1

198.9

23.9

203.0

200.9

17.1 150.6 20.89 140.9 145.8 Microtek Alkane Blend 21.6 202.2 ~20 (est.)* 202.2 202.2 PureTemp 23 Liquid 22.4 100.9 ~20 (est.)* 99.7 100.3 PureTemp 23 Diatom Matrix *The transition temperature of solidification for Pure Temp 23 was estimated due to supercooling of the sample which prevented its determination using the DSC software

The linear alkanes, n-Hexadecane and n-Octadecane were blended in varying proportions to examine the effects on latent heat and transition temperature.

The

variations in transition temperature and latent were measured using the dynamic method for blend ratios of 25%, 50%, and 75% n-Octadecane along with pure n-Hexadecane and n-Octadecane. As shown in Figure 3.6, the transition temperature varied somewhat proportionally with increased n-Octadecane while latent decreased for blended samples in comparison to pure samples.

Figure 3.6: n-Hexadecane and n-Octadecane Blend DSC Results

Examination of the blends of n-Hexadecane and n-Octadecane shows that blending of these linear alkanes provides a feasible method of tailoring transition temperatures. As shown in Figure 3.6 blending the two alkanes to provide a transition 86

temperature comes at the cost of reduced latent heat energy storage. From the results it appears that the Microtek linear alkane blend PCM with a stated 24°C (75°F) transition temperature (actually tested to be approximately 21°C (70°F)) is a 25% n-Hexadecane and 75% n-Octadecane alkane blend. The transition temperatures are extremely close. The latent heat was slightly lower for the in-house blend. This may be due to material and measurement variability.

Dynamic DSC testing was also performed on vacuum impregnated expanded clay aggregates. Aggregates of #4 (6.35 mm), #8 (3.18 mm), #16 (1.59 mm), and #30 (0.85 mm) sizes were tested with n-Hexadecane, n-Octadecane and the linear alkane blend. The DSC testing of the expanded clay aggregates provided performance data for the composite PCM and aggregate.

In addition, the PCM aggregate data provided

confirmation of the amount of PCM impregnation in each of the aggregate sizes. Comparing the PCM aggregate latent heat to the pure PCM latent heat provides data about the PCM impregnation ratios.

Wide phase transition ranges and incongruent phase transition during cooling of the pure linear alkanes also led to further study of these PCMs vacuum impregnated into the porous expanded clay aggregates to see if performance improvements could be obtained with vacuum impregnation. It was hoped that vacuum impregnation of the pure PCMs in the expanded clay aggregate pore space would provide nucleation sites in order to improve PCM performance. The pure PCMs tended to exhibit two peaks during cooling.

Table 3.2 summarizes the results of the tests of PCM / expanded clay composite samples.

Listed in the table are the average latent heat of melting, latent heat of

solidification, average of the latent heats of melting and solidification, and the ratio of the average latent heat of the PCM / expanded clay composite to that of the pure PCM materials. Table 3.2: Vacuum Impregnated Expanded Clay Aggregate DSC Results PCM Aggregate

Latent Heat of Latent Heat of Latent Heat Melting Solidification Average (KJ/Kg) (KJ/Kg) (KJ/Kg)

Latent Heat of Vacuum Impregnated PCM / Pure PCM (%)

n-Hexadecane #4

26.5

26.2

26.4

14%

n-Hexadecane #8

36.9

36.1

36.4

19%

n-Hexadecane #16

39.6

38.9

39.2

20%

n-Hexadecane #30

43.6

42.6

43.1

22%

Alkane Blend #4

22.4

22.5

15%

Alkane Blend #8

26.9

25.4

26.2

18%

Alkane Blend #16

31.1

30.5

30.8

21%

Alkane Blend #30

33.2

31.9

32.5

22%

n-Octadecane #4

35.1

35.8

35.4

18%

n-Octadecane #8

39.7

40.5

40.1

20%

n-Octadecane #16

30.8

31.2

31.0

15%

n-Octadecane #30

37.0

37.5

37.3

19%

Average

19%

Examination of Table 3.2 indicates that the average latent heat of the vacuum impregnated expanded clay aggregates range from 14% to 22% with an average value of 19%. The standard deviation is 3%. The values shown in Table 3.2 are for averages of 3 or more repeated test results. Higher latent heat percentages indicate higher impregnation efficiency of the aggregates. Considering that the PCM vacuum impregnation by mass fraction in Table 2.4 ranged from 23% to 36% with an average of 28% for the linear alkane impregnated PCM aggregates, it can be determined that the values of 14% to 22% as shown in Table 3.2 are reasonable and validate the mix values. Since the PCM aggregates were stored for several months before DSC testing there was sufficient time for the aggregates to repeatedly cycle through their transition temperatures prior to testing. Immediate incorporation of the PCM aggregates into PCM concrete would likely have mitigated the PCM loss and would therefore have provided higher latent heat for the individual aggregates. Thus, it can be determined that immediate incorporation of PCM aggregates into the concrete mix or cold storage of PCM aggregates is essential to longterm stability prior to concrete mixing.

Figures 3.7 through 3.21 show typical behavior observed for this range of samples during DSC testing.

The results of each PCM first show typical melting and

solidification curves for pure PCM and then curves for the #4 (6.35 mm) through the #30 (0.85 mm) vacuum impregnated aggregates are shown for each PCM. In addition, the latent heat comparisons shown in Table 3.2 are illustrated.

Sample: Hexadecane Size: 12.5000 mg

DSC

Comment: Equilbriate at 0 ramp 1 C/min to 40 return 1 C/min

File: 18C MT Hexadecane 3-(Heating&Cooling)... Operator: Joshua Wilcox Run Date: 11-Sep-2012 14:23 Instrument: DSC Q20 V24.4 Build 116

1.5 13.05°C

1.0

Heat Flow (W/g)

0.5 13.45°C 188.6J/g

0.0 14.24°C 194.8J/g

-0.5

-1.0 16.98°C

-1.5 -10

Exo Up

50 Universal V4.5A TA Instruments

Temperature (°C)

Figure 3.7: Typical Pure n-Hexadecane DSC Test Result (100% PCM)

Sample: MT 18 #4 23% Size: 14.3000 mg

DSC

Comment: Eq. at 0 C Ramp 1 deg per minute to 40 C and return

File: C:...\Aggregate\#4\18C MT-Agg-4-23.002 Operator: JDK Run Date: 03-Oct-2013 10:28 Instrument: DSC Q20 V24.4 Build 116

0.3

0.2

Heat Flow (W/g)

0.1

11.30°C 13.66°C 26.34J/g

4.56°C

0.0 18.35°C 14.23°C 26.44J/g

-0.1

16.91°C

-0.2

-0.3 -10 Exo Up

Temperature (°C)

50 Universal V4.5A TA Instruments

Figure 3.8: Composite #4 Aggregate / n-Hexadecane DSC Test Result (14% PCM) 90

Sample: MT 18 #8 25% Size: 24.4000 mg

DSC

Comment: Eq. at 0 C Ramp 1 deg per minute to 40 C and return

File: C:...\Aggregate\#8\18C MT-Agg-8-25.002 Operator: JDK Run Date: 02-Oct-2013 08:38 Instrument: DSC Q20 V24.4 Build 116

0.3

0.2

11.33°C

Heat Flow (W/g)

0.1 13.72°C 35.26J/g 5.32°C

0.0 19.68°C 14.37°C 36.33J/g

-0.1

18.00°C

-0.2

-0.3 -10

Exo Up

50 Universal V4.5A TA Instruments

Temperature (°C)

Figure 3.9: Composite #8 Aggregate / n-Hexadecane DSC Test Result (19% PCM)

Sample: MT 18 #16 30.5% Size: 21.7000 mg

DSC

Comment: Eq. at 0 C Ramp 1 deg per minute to 40 C and return

File: C:...\Aggregate\#16\18C MT-Agg-16-30.501 Operator: JDK Run Date: 01-Oct-2013 08:39 Instrument: DSC Q20 V24.4 Build 116

0.3

0.2 11.33°C

Heat Flow (W/g)

0.1 13.78°C 38.39J/g

7.36°C

0.0 19.19°C 14.55°C 39.10J/g

-0.1

-0.2 17.14°C

-0.3 -10 Exo Up

Temperature (°C)

50 Universal V4.5A TA Instruments

Figure 3.10: Composite #16 Aggregate / n-Hexadecane DSC Test Result (20% PCM) 91

Sample: MT 18 #30 27% Size: 21.5000 mg

DSC

Comment: Eq. at 0 C Ramp 1 deg per minute to 40 C and return

File: C:...\Aggregate\#30\18C MT-Agg-30-27.001 Operator: JDK Run Date: 26-Sep-2013 13:24 Instrument: DSC Q20 V24.4 Build 116

0.3

0.2

12.43°C

Heat Flow (W/g)

0.1 13.85°C 42.45J/g

7.51°C

0.0 18.77°C

14.61°C 43.58J/g

-0.1

-0.2

-0.3 -10

17.20°C

Exo Up

50 Universal V4.5A TA Instruments

Temperature (°C)

Figure 3.11: Composite #30 Aggregate / n-Hexadecane DSC Test Result (22% PCM)

Sample: 24C Alkane Blend Size: 10.2000 mg

DSC

Comment: Eq. at 0 deg. Iso 10 min,Ramp 1deg. C/min to 40C return.

File: 24C MT Alkane Blend-(Heating&Cooling)... Operator: JDK Run Date: 10-Jan-2013 13:19 Instrument: DSC Q20 V24.4 Build 116

1.5

1.0 20.17°C

Heat Flow (W/g)

0.5 20.82°C 152.3J/g

0.0

17.06°C 161.5J/g

-0.5

21.73°C

-1.0

-1.5 -10 Exo Up

Temperature (°C)

50 Universal V4.5A TA Instruments

Figure 3.12 Typical Pure Alkane Blend DSC Test Result (100% PCM) 92

Sample: MT 24 #4 24% Size: 12.1000 mg

File: C:...\Aggregate\#4\24C MT-Agg-4-24.010 Operator: JDK Run Date: 25-Sep-2013 12:47 Instrument: DSC Q20 V24.4 Build 116

DSC

Comment: Eq. at 0 C Ramp 1 deg per minute to 40 C and return 0.3

0.2

19.35°C

Heat Flow (W/g)

0.1

20.96°C 22.68J/g

0.0

17.87°C 21.51J/g

-0.1

21.67°C

-0.2

-0.3 -10

Exo Up

50 Universal V4.5A TA Instruments

Temperature (°C)

Figure 3.13: Composite #4 Aggregate / Alkane Blend DSC Test Result (15% PCM)

Sample: MT 24 #8 29.5% Size: 24.7000 mg

File: C:...\Aggregate\#8\24C MT-Agg-8-29.5%0 Operator: jk Run Date: 23-Sep-2013 08:45 Instrument: DSC Q20 V24.4 Build 116

DSC

Comment: Eq. at 0 C Ramp 1 C/min to 40 deg return 0.3

0.2

18.89°C

Heat Flow (W/g)

0.1

21.45°C 26.56J/g

0.0

18.18°C 28.42J/g

-0.1

23.18°C

-0.2

-0.3 -10 Exo Up

Temperature (°C)

50 Universal V4.5A TA Instruments

Figure 3.14: Composite #8 Aggregate / Alkane Blend DSC Test Result (18% PCM) 93

Sample: MT 24 #16 32% Size: 28.9000 mg

DSC

Comment: Eq. at 0 C Ramp 1 C/min to 40 deg return

File: C:...\#16\24C MT-Agg-16-32%.003 Operator: jk Run Date: 20-Sep-2013 08:36 Instrument: DSC Q20 V24.4 Build 116

0.3

0.2 18.63°C

Heat Flow (W/g)

0.1 20.84°C 31.81J/g

0.0

17.59°C 31.80J/g

-0.1

22.24°C

-0.2

-0.3 -10

Exo Up

50 Universal V4.5A TA Instruments

Temperature (°C)

Figure 3.15: Composite #16 Aggregate / Alkane Blend DSC Test Result (21% PCM)

Sample: MT 24 #30 34.5% Size: 17.3000 mg

DSC

Comment: Eq. at 0 C Ramp 1 C/min to 40 deg, iso 10 min. Ramp 1 C/min to 0

File: 24C MT-Agg-30-34.5%-(Heating&Cooling)... Operator: JDK Run Date: 16-Sep-2013 13:13 Instrument: DSC Q20 V24.4 Build 116

0.3

0.2 19.23°C

Heat Flow (W/g)

0.1 21.26°C 33.10J/g

0.0

-0.1

17.91°C 34.29J/g

22.11°C

-0.2

-0.3 -10 Exo Up

Temperature (°C)

50 Universal V4.5A TA Instruments

Figure 3.16: Composite #30 Aggregate / Alkane Blend DSC Test Result (22% PCM) 94

File: 28C MT Octadecane-(Heating&Cooling).006 Operator: Jeff Kiesel Run Date: 30-Nov-2012 10:36 Instrument: DSC Q20 V24.4 Build 116

Sample: Octadecane Size: 12.3000 mg DSC Method: Ramp 1C per Minute 0-40C-0 Comment: Eq. at 0 deg. Ramp 1 deg. C/min to 40 and return (cooling on) 1.5

1.0

23.18°C

Heat Flow (W/g)

0.5

23.98°C 195.9J/g

0.0

23.06°C 195.2J/g

-0.5

-1.0

-1.5 -10

26.61°C

Exo Up

50 Universal V4.5A TA Instruments

Temperature (°C)

Figure 3.17: Typical Pure n-Octadecane DSC Test Result (100% PCM)

Sample: Octadecane / EC Aggregate #4-27% Size: 17.1000 mg

File: 28C MT Octa-Agg4-27%-(Heating&Cooling... Operator: JDK Run Date: 04-Mar-2013 12:16 Instrument: DSC Q20 V24.4 Build 116

DSC

Comment: Eq. at 0 deg. Iso 10 min,Ramp 1deg. C/min to 40C return. 0.3

0.2 22.61°C

Heat Flow (W/g)

0.1 24.72°C 40.10J/g

0.0

-0.1

23.46°C 38.98J/g

-0.2 26.53°C

-0.3 -10 Exo Up

Temperature (°C)

50 Universal V4.5A TA Instruments

Figure 3.18: Composite #4 Aggregate / n-Octadecane DSC Test Result (18% PCM) 95

Sample: Octadecane / EC Aggregate #8-25% Size: 19.1000 mg

File: 28C MT Octa-Agg8-25%-(Heating&Cooling... Operator: JDK Run Date: 19-Feb-2013 13:35 Instrument: DSC Q20 V24.4 Build 116

DSC

Comment: Eq. at 0 deg. Iso 10 min,Ramp 1deg. C/min to 40C return. 0.3

0.2

22.64°C

Heat Flow (W/g)

0.1 24.92°C 37.39J/g

0.0

23.43°C 36.76J/g

-0.1

26.88°C

-0.2

-0.3 -10

Exo Up

50 Universal V4.5A TA Instruments

Temperature (°C)

Figure 3.19: Composite #8 Aggregate / n-Octadecane DSC Test Result (20% PCM)

Sample: Octadecane / EC Aggregate-33.5% Size: 20.1000 mg

File: 28C MT Octa-Agg-33.5%-(Heating&Coolin... Operator: JDK Run Date: 28-Jan-2013 12:06 Instrument: DSC Q20 V24.4 Build 116

DSC

Comment: Eq. at 0 deg. Iso 10 min,Ramp 1deg. C/min to 40C return. 0.3

0.2

21.36°C

Heat Flow (W/g)

0.1

24.27°C 27.44J/g

0.0

21.40°C 27.75J/g

-0.1

26.14°C

-0.2

-0.3 -10 Exo Up

Temperature (°C)

50 Universal V4.5A TA Instruments

Figure 3.20: Composite #16 Aggregate / n-Octadecane DSC Test Result (15% PCM) 96

Sample: Octadecane / EC Aggregate #30-36 Size: 17.2000 mg

File: 28C MT Octa-Agg-36%-(Heating&Cooling)... Operator: JDK Run Date: 08-Mar-2013 10:00 Instrument: DSC Q20 V24.4 Build 116

DSC

Comment: Eq. at 0 deg. Iso 10 min,Ramp 1deg. C/min to 40C return. 0.3

0.2 22.95°C

Heat Flow (W/g)

0.1 24.89°C 38.37J/g

0.0

-0.1

23.16°C 37.28J/g

-0.2 26.70°C

-0.3 -10

Exo Up

Temperature (°C)

50 Universal V4.5A TA Instruments

Figure 3.21: Composite #30 Aggregate / n-Octadecane DSC Test Result (19% PCM)

Reviewing the above DSC test results for the pure PCMs as well as the composite expanded clay / PCM aggregates, it can be seen that the composite latent heat proportions are consistent with the PCM impregnation mass ratios shown in Table 2.4. Figure 3.7 shows that for the pure n-Hexadecane a single melting and solidification curve without multiple peaks is exhibited.

This remains true for the n-Hexadecane / aggregate

composites as shown in Figures 3.8 through 3.11.

The pure alkane blend melting and solidification curves both exhibit two peaks as shown in Figure 3.12.

This is likely due to the mixture of alkanes with different

transition temperatures. Impregnating the alkane blend in the aggregate improves the

PCM performance by providing a single melting and solidification peak most likely due to increased nucleation sites as shown in the above Figures 3.13 through 3.16.

The pure n-Octadecane sample has a second peak during cooling as shown in Figure 3.17. This is likely due to super cooling. While the #4 (6.35 mm) aggregate / PCM composite exhibits a similar DSC curve as compared to the pure n-Octadecane, the smaller #8 (3.18 mm) through # 30 (0.85 mm) aggregates exhibit a smoothed cooling curve as compared with the pure PCM. This is likely due to increased nucleation sites found in the porous aggregate pore space. With smaller aggregate size it is evident that nucleation is improved. Overall, as shown in Figures 3.18 through 3.21, it appears that the performance of n-Octadecane is improved when incorporated into expanded clay aggregates and appears viable for use in PCM concrete production.

The step method data was used to develop the enthalpy curves for n-Hexadecane, n-Octadecane, the Microtek linear alkane blend, and PureTemp 23 PCMs.

The

temperature steps were typically 1°C. However 0.5°C steps were also used in the melting region when equilibrium was not reached with the 1°C steps.

These values were

averaged to create consistent 1°C steps for graphing. From the step method data, the phase fraction curves were determined. Figures 3.22 through 3.25 show typical test results from the step method.

The latent heat of fusion and crystallization were

determined by integrating the peaks.

Figure 3.22: Pure PureTemp 23 DSC Step Method Results

Figure 3.23: Pure n-Hexadecane DSC Step Method Results

Figure 3.24: Pure n-Octadecane DSC Step Method Results

Figure 3.25: Pure Microtek Alkane Blend DSC Step Method Results

100

From the step method it can be seen that the PureTemp 23 and n-Hexadecane samples exhibit good melting and solidification performance as shown in Figures 3.22 and 3.23. The phase change region occurs in a reasonably narrow range and the initial transition temperature for melting and solidification are relative close.

Further, in

comparison to the dynamic method, the PureTemp 23 sample shows cooling and heating transition temperatures that are considerably closer than that measured using the dynamic method.

As shown in Figure 3.24, the n-Octadecane appears to either exhibit

supercooling or performance issues due to impurities as can be seen by a solidification profile that exhibits two peaks. This may be due to nucleation issues. Though, as noted previously, incorporation into expanded clay aggregates allow the peaks to somewhat merge.

The linear alkane blend exhibits a smeared profile for melting and solidification as shown on Figure 3.25. The transition range for both melting and solidification occurs over a wide temperature range. In addition, the total latent heat is lower than the pure alkane materials. This is confirmed by the results of the blends of n-Hexadecane and nOctadecane in Figure 3.6. Table 3.3 summarizes the results from the step method tests. Table 3.4 compares the dynamic method to the step method test results.

101

Table 3.3: PCM Latent Heat and Transition Temperature from Step Method PCM Sample

Transition Temp. of Melting (°C)

Latent Heat of Melting (KJ/Kg)

Transition Temp. of Solid. (°C)

Latent Heat of Solid. (kJ/Kg)

Latent Heat Average (kJ/Kg)

n-Hexadecane

13.75

195

14.5

180

187.5

n-Octadecane

22.25

200

24.5

190

195

Microtek Alkane Blend

16.25

180

22.5

150

165

PureTemp 23 Liquid

20.5

220

20.5

220

Table 3.4: Comparison of Dynamic Method to Step Method PCM Sample

Transition Temp. of Transition Temp. of Latent Heat Average Melting (°C) Solid. (°C) (kJ/kg) Dynamic Method

Step Method

Dynamic Method

Step Method

Dynamic Method

Step Method

n-Hexadecane

14.53

13.75

13.68

14.5

190.1

187.5

n-Octadecane

22.41

22.25

23.77

24.5

183.6

195

Microtek Alkane Blend

18.62

16.25

21.63

22.5

124.1

165

PureTemp 23 Liquid

22.36

20.5

204.5

220

A comparison of the dynamic method values to the step method values indicates that there is fairly good agreement in transition temperatures. Variations in transition temperatures are most likely due to the step method not providing continuous data from 102

which an accurate tangent can be obtained from. Latent heat values are also fairly close for all but the Microtek Alkane Blend with a 24°C (75°F) transition temperature. However, this blended PCM smeared the latent heat across a large temperature range which creates difficulties in integrating the enthalpy to determine latent heat.

Specific heat from the step method provided data that is consistent with published results (Kuznik et al. 2011). Linear alkanes and fatty acids are documented as having a specific around 2 kJ/kg °C depending on solid or liquid state. All of the PCMs tested produced measurements around this value or slightly higher. However it was difficult to determine a solid phase specific heat for the Microtek alkane blend. This was because the latent heat of the blend continued well into the cooler temperatures of the test procedure. The lowest value measured during the step method test was 2.6 kJ/kg °C and thus, the specific heat lies at or below this value. Starting the DSC test at a lower temperature would likely have provided more accurate specific heat values. Specific heat values are shown in Table 3.5.

103

Table 3.5: Specific Heat from Step Method PCM Sample

Cp Solid (kJ/kg °C)

Cp Liquid (kJ/kg °C)

n-Hexadecane

2.5

2.1

n-Octadecane

2.5

2.3

Microtek Alkane Blend

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References