Encapsulation of phase change materials for heat storage

ENCAPSULATION OF PHASE CHANGE MATERIALS (PCMS) FOR HEAT STORAGE MYA MYA KHIN (B.E., Yangon Technological University) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING DEPARTMENT OF CHEMICAL & ENVIRONMENTAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2003 Acknowledgement ACKNOWLEDGEMENT I wish to express my thanks to the following institution and persons, without whose assistance and guidance this thesis would not have been possible. ƒ To the National University of Singapore, for the postgraduate research scholarship, without which I would not be able to continue my higher degree studies. ƒ To my supervisors, Associate Professor M.S Uddin and Associate Professor M.N.A Hawlader for their constant guidance, kindness, forgiveness, care, concern shown throughout the project and time taken to read the manuscript. ƒ To all the technical and clerical staff in the Chemical & Environmental Engineering Department for their patience and help. ƒ To Dr Zhu Haijun for giving me some informations and literature for this project. ƒ To all my colleagues from E4A-07-07 especially Mr. Peng Zanguo for their help on different occasions, discussion and for their encouragement during my tenure at NUS. ƒ To my parents and family members and my best friend Miss Thin Thin Aye for their continuous love and encouragement throughout the study. Last but not least, my thanks to all who have contributed in one-way or another to make this thesis possible. i TABLE OF CONTENTS ACKNOWLEDGEMENT i TABLE OF CONTENTS ii SUMMARY v LIST OF FIGURES viii LIST OF TABLES xi CHAPTER 1 INTRODUCTION 1 1.1 General background 1 1.2 Objective and scope of thesis 4 CHAPTER 2 LITERATURE REVIEW 6 2.1 Thermal energy storage 6 2.2 Thermal energy storage techniques 7 2.3 Candidate heat storage materials 15 2.4 Factors affecting the energy storage capacity of PCM 20 2.5 Encapsulation of phase change materials 20 2.6 Methods of microencapsulation 25 2.7 Thermal cycling test for encapsulated PCM 30 2.8 Heat transfer of PCMs 31 2.9 Scope of the present work 34 CHAPTER 3 MATERIALS AND EXPERIMENTAL METHODS 3.1 Materials 34 36 36 ii 3.2 Characteristics of core and coating materials 37 3.3 Experiments 40 3.3.1 Complex coacervation 40 3.3.2 Spray drying 42 3.4 Characteristics and performance of microcapsules 44 3.4.1 Energy storage and release capacities 44 3.4.2 Thermogravimetric analysis 45 3.4.3 Surface morphology and characterization of inner 46 structure 3.4.4 Microencapsulation efficiency 47 3.4.5 Estimation of core to coating ratio 47 3.4.6 Chemical structure stability evaluation 48 3.5 Accelerated test process 48 3.6 Fluidized bed heat exchanger for microencapsulated PCM 51 CHAPTER 4 RESULTS AND DISCUSSION 54 4.1 Encapsulation efficiency 54 4.2 Estimation of core to coating ratio 63 4.3 Thermal performance 64 4.4 Surface morphology and inner structure characterization 68 4.4.1 Surface morphology 68 4.4.2 Inner structure 69 4.5 Thermogravimetric analysis 70 4.6 Thermal cyclic test 75 iii 4.7 Structural stability 88 4.8 Thermal performance of microencapsulated PCM in 91 fluidized bed heat exchanger 4.8.1 Temperature profiles 91 4.8.2 Total heat storage and release 94 CHAPTER 5 CONCLUSIONS AND RECOMMENDATIONS 96 5.1 Conclusions 96 5.2 Recommendations 99 REFERENCES 100 APPENDICES 113 iv SUMMARY Microencapsulated PCMs are micron size phase change materials enclosed in a protective wrapping. The microcapsule prevents the leakage of the material during its phase change. It also provides larger heat transfer area per unit volume of heat storage vessel. It could be used in solar energy storage, waste heat utilization, and space heating and cooling. This study investigated the use of complex coacervation and spray drying methods for microencapsulating paraffin wax by polymeric materials (gelatin and acacia) in an aqueous system. Experiments on operational variables to select suitable conditions were carried out for complex coacervation. Encapsulation efficiency was found to be higher when the products had lower core to coating ratios. The optimum condition for various core to coating ratios was found to be 10 minutes homogenizing time and the amount of cross-linking agent 6~8 ml. Non-linear regression was used to correlate the encapsulation efficiency and the parameters studied. In spray-drying method, decrease in encapsulation efficiency with increase in the ratio of core to coating was observed. The optimum core to coating ratio was found to be 1:2. In the studies on thermal performance analysis by Differential Scanning Calorimetery (DSC), the effect of core to coating ratio on energy storage/release capacities was investigated. The higher paraffin wax content in the sample gave the higher energy storage/release capacities. The energy storage/release capacities of the coacervated microcapsules were higher than those of the spray-dried samples. Energy storage/release v capacities were found to be in the range of 91-239 J/g for microencapsulated PCMs prepared under different conditions. Further characterization for both coacervated and spray-dried samples focused on surface morphology and inner structure by using microtone and Scanning Electron Microscopy (SEM). SEM analysis showed that spray-dried samples were more regular and spherical in shape compared to coacervated samples. Both samples contained a few small globules. Size of coacervated particles ranged from 3.3 to10.5 µm. Spray-dried microcapsules had a diameter ranging from 1.3-10.1 µm. The inner structure characterization showed that both coacervated and spray-dried samples consisted of a polymeric matrix surrounding numerous globules. The thermal stability of both coacervated and spray-dried samples was estimated by using Thermogravimetry (TG) analysis. Thermal decomposition temperatures of core and coating materials were determined from TG output curves. The decomposition temperature of paraffin wax existed between 200 and 300°C, and the decomposition temperature of polymer network (gelatin and acacia) was observed between 300 and 400°C. The TG output curve for spray-dried samples had two peaks between 300 and 400°C. The second extra peak showed the decomposition temperature of unreacted coating materials. Thermal stability of microencapsulated PCMs was also checked through accelerated thermal (melt/freeze) cyclic tests. Both samples were subjected to thermal cycle tests up vi to 2000 cycles. DSC analysis was carried out to measure energy storage/release capacities, melting temperature and specific heat capacity after specific number of cycles. Both the coacervated and spray-dried samples showed good thermal stability throughout cycling process. Fourier Transform Infrared Spectophotometery (FTIR) analysis also confirmed distinct chemical stability of both samples throughout thermal cycling. Finally, the thermal performance of the PCM was carried out in as fluidized bed heat exchanger. Heat transfer between the spray-dried encapsulated PCM and air was studied during heating and cooling process. It was found that the time taken for charging and discharging the capsules was about 760 and 600 seconds, respectively. Total energy and release amount were found to be 2953 and 2431 J. Therefore, it was observed that it was efficient heat exchange system. vii LIST OF FIGURES Figure 2.1: Areas of research in thermal energy storage system 8 Figure 2.2: Thermal energy storage strategies 9 Figure 2.3: Various forms of capsules 21 Figure 3.1: Molecular structure of paraffinic hydrocarbons 37 Figure 3.2: Molecular structure for gelatin 38 Figure 3.3: Schematic representation of the coacervation process 41 Figure 3.4: Photograph of mini spray dryer used in microencapsulation of paraffin wax 43 Figure 3.5: Schematic diagram of thermal cyclic system 50 Figure 3.6: Actual thermal cyclic system in laboratory 51 Figure 3.7: Schematic diagram of experimenatal set up for fluidized bed 53 Figure 4.1: Effect of homogenizing time and the amount of cross-linking 58 agent on encapsulation efficiency at 2:1 core to coating ratio (HCHO) Figure 4.2: Effect of homogenizing time and the amount of cross-linking 58 agent on encapsulation efficiency at 1:1 core to coating ratio (HCHO) Figure 4.3: Effect of homogenizing time and the amount of cross-linking 59 agent on encapsulation efficiency at 1:2 core to coating ratio (HCHO) Figure 4.4: Effect of homogenizing time and the amount of cross-linking agent on encapsulation efficiency at 2:1 core to coating ratio (CH3CHO) 59 Figure 4.5: Effect of homogenizing time and the amount of cross-linking agent on encapsulation efficiency at 1:1 core to coating ratio (CH3CHO) 60 Figure 4.6: Effect of homogenizing time and the amount of cross-linking agent on encapsulation efficiency at 1:2 core to coating ratio (CH3CHO) 60 viii Figure 4.7: The output curve of DSC for spray-dried sample 2:1 67 Figure 4.8: The output curve of DSC for coacervated sample 2:1 67 Figure 4.9: SEM profile of the coacervated samples 68 Figure 4.10: SEM profile of the spray-dried samples 68 Figure 4.11: Inner structure of spray-dried samples (fresh) 71 Figure 4.12: Inner structure of spray-dried samples (cycled) 71 Figure 4.13: Inner structure of coacervated samples (fresh) 72 Figure 4.14: Inner structure of coacervated samples (cycled) 72 Figure 4.15: TG thermogram of coacervated microcapsules 74 Figure 4.16: TG thermogram of spray-dried microcapsules 74 Figure 4.17: DSC output curve for 1:1 coacervated sample after 500 cycles 79 Figure 4.18: DSC output curve for 1:1 coacervated sample after 2000 cycles 79 Figure 4.19: DSC output curve for 2:1 spray-dried sample at 0 cycle 80 Figure 4.20: DSC output curve for 2:1 spray-dried sample after 1500 cycles 80 Figure 4.21: DSC measurement of melting temperature of microencapsulated paraffin wax (spray-dried 1:1) after 500 cycles 84 Figure 4.22: DSC measurement of melting temperature of microencapsulated paraffin wax (spray-dried 1:1) after 2000 cycles 84 Figure 4.23: DSC measurement of melting temperature of microencapsulated paraffin wax (coacervated 2:1) at 0 cycle 85 Figure 4.24: DSC measurement of melting temperature of microencapsulated paraffin wax (coacervated 2:1) after 2000 cycles 85 Figure 4.25: DSC measurement of specific heat capacity of microencapsulated paraffin wax (spray-dried 1:2) at 0 cycle 86 Figure 4.26: DSC measurement of specific heat capacity of microencapsulated paraffin wax (spray-dried 1:2) after 1000 cycles 86 ix Figure 4.27: DSC measurement of specific heat capacity of microencapsulated paraffin wax (coacervated 1:2) after 500 cycles 87 Figure 4.28: DSC measurement of specific heat capacity of microencapsulated paraffin wax (coacervated 1:2) after 2000 cycles 87 Figure 4.29: FTIR output curve for spray-dried samples with 1:1 core to coating ratio 90 Figure 4.30: FTIR output curve for coacervated samples with 1:1 core to coating ratio 90 Figure 4.31: Temperature profiles during heat storage stage 93 Figure 4.32: temperature profiles during heat release stage 93 Figure 4.33: Heat storage with time during heat storage stage 95 Figure 4.34: Heat release with time during heat release stage 95 x LIST OF TABLES Table 2.1: Comparison of various heat storage media 13 Table 2.2: Physical properties of some PCMs 16 Table 2.3: Comparison of organic and inorganic materials for heat storage 19 Table 2.4: Important characteristics of energy storage materials 19 Table 2.5: List of published encapsulated PCM systems 24 Table 3.1: Materials used in the microencapsulating of paraffin wax 36 Table 3.2: Operating conditions for spray drying microencapsulation 44 Table 4.1: Encapsulation efficiency of coacervated capsules 55 Table 4.2: Encapsulation efficiency of coacervated capsules with 36% formaldehyde 55 Table 4.3: Encapsulation efficiency of coacervated capsules with 50% gluteraldehyde 56 Table 4.4: Non-linear regression analysis of encapsulation efficiency using formaldehyde 62 Table 4.5: Non-linear regression analysis of encapsulation efficiency using gluteraldehyde 62 Table 4.6: Encapsulation efficiency of spray-dried samples 63 Table 4.7: Comparison of experimentally measured core to coating ratio with designed values 64 Table 4.8: Energy storage and release capacities for coacervated and spray-dried 66 microencapsulated paraffin wax Table 4.9: Energy storage and release capacities for microencapsulated paraffin wax (2:1 coacervated sample) 77 Table 4.10: Energy storage and release capacities for microencapsulated paraffin wax (1:1 coacervated sample) 77 Table 4.11: Energy storage and release capacities for microencapsulated paraffin wax (1:2 coacervated sample) 77 xi Table 4.12: Energy storage and release capacities for microencapsulated paraffin wax (2:1 spray-dried sample) 78 Table 4.13: Energy storage and release capacities for microencapsulated paraffin wax (1:1 spray-dried sample) 78 Table 4.14: Energy storage and release capacities for microencapsulated paraffin wax (1:2 spray-dried sample) 78 Table 4.15: Thermophysical properties of microencapsulated paraffin wax (coacervated 2:1) with test cycles 82 Table 4.16: Thermophysical properties of microencapsulated paraffin wax (coacervated 1:1) with test cycles 82 Table 4.17: Thermophysical properties of microencapsulated paraffin wax (coacervated 1:2) with test cycles 82 Table 4.18: Thermophysical properties of microencapsulated paraffin wax (spray-dried 2:1) with test cycles 83 Table 4.19: Thermophysical properties of microencapsulated paraffin wax (spray-dried 1:1) with test cycles 83 Table 4.20: Thermophysical properties of microencapsulated paraffin wax (spray-dried 1:2) with test cycles 83 xii Chapter 1 Introduction CHAPTER 1 INTRODUCTION 1.1 General background Renewable energy has been used over the last two decades to save the costs and adverse environmental pollution effects of fossil fuel (Klass, 2003). Today, use of the renewable energy provides electricity and it has been used to improve solar water heating and space application of an advanced power system over the past few years (Fath, 1995). However, the main problems for renewable energy are: (1) Solar radiation is intermittent by its nature; its total available value is a factor of time, weather condition and latitude. (2) Energy sources and the demands, in general, do not match each other. Therefore, scientists investigated technically to solve these problems. Finally, they found that energy storage is one of possible solutions for energy conservation and leveling of energy demand patterns. Thermal energy storage (TES) is considered by many to be one of the energy storage technologies (Dincer and Dost, 1996). TES contains a thermal storage mass, and can store heat or cool. Basically, it can be classified as latent, sensible and thermo-chemical energy. Among these energy storage types, the most attractive form is latent heat storage in phase change material (PCM) because of the advantages of high storage capacity in a small volume and charging/discharging heat from the system at a nearly constant temperature (Abhat, 1983). 1 Chapter 1 Introduction In a latent heat energy storage system, one of the main elements is the PCM and its selection criteria. Most investigations were focused on salt hydrates, paraffin, nonparaffin organic acids, clathrates and eutectic organic and inorganic compounds (Lane, 1986). Among those materials, paraffin wax offers more desirable properties such as nonpoisonous, chemically stable, self-nucleating, negligible supercooling, low vapor pressure in the melt, no phase segregation and commercially available at reasonable cost (Abhat, 1978). Therefore, in this study, paraffin wax was emphasized. The application of conventional paraffin wax for heat storage has some limitations. They are as follows: (1) paraffin wax has low thermal conductivity approximately 0.18 W/m K (Abhat and Malatids, 1981) that leads to low heat transfer; (2) energy withdrawn from paraffin wax during cooling is limited by the fact that the storage medium begins to solidify on the surface of heat exchangers, the layer of solid material can act as insulating material; (3) large volume change during phase transition; (4) if heat transport medium is air, oxidation of paraffin wax produces complex compounds, aldehydes, ketones, etc. that can lead to toxic to our environment (Lane, 1986); (5) conventional particles of paraffin waxes are slightly sticky and can stick together to form large lumps, clogging occurs in a heat storage system, resulting in failure to circulate heat transport fluid through the system (Winsters, 1991). These limitations can result in decreasing in energy storage capacity. 2 Chapter 1 Introduction In order to overcome these problems, Patel (1968), Patenkar (1980), Fouda et al. (1984), Garg et al. (1985) and Yanadori et al. (1989) have identified heat transfer enhancement concepts such as the use of agitators, scrapers and slurries in heat exchangers. The disadvantage of their heat exchanger development is increasing the cost and complexity of thermal energy storage devices. In order to solve these problems, both material investigation and heat exchanger development should be performed. Therefore, the studies focused on both cases were investigated (Hawlader et al., 2000). They observed and reported that PCM should be bounded within a secondary supporting structure and the application of a packed/fluidized bed heat exchanger is a better way of heat transfer enhancement. Therefore, the progress in latent heat storage systems mainly depends on heat storage material investigations and on the development of heat exchangers that assure a high effective heat transfer rate to allow rapid charging and discharging. The required heat transfer surfaces should be large to maintain a low temperature gradient during these processes (Banaszek et al., 1999). Microencapsulation refers to a process where droplets of liquids, solids, or gases (core) are coated by thin films (coatings) that protect the core material (Sheu and Rosenburg, 1995). The National Cash Register for commercially applying in carbonless copy paper started encapsulation process at 1930 (Green and Schleicher, 1956). Recently, encapsulation processes have been developed in various fields such as pharmaceutical industry, food industry, biomedical field, coating of PCMs for better heat storage system and so on. The advantages of using microencapsulated paraffin wax in fluidized bed heat 3 Chapter 1 Introduction exchanger are that it provides large heat transfer area per unit volume and provides higher heat transfer rate due to low thermal resistance between the heat transfer fluid and the PCM and high convective heat transfer by heat transfer fluid in a fluidized bed (Hawlader et al., 2000). 1.2 Objective and scope of thesis For many applications, encapsulated PCMs were produced by researchers. Inaba et al., 1997 prepared encapsulated paraffin wax by using interfacial polymerization and integrated the samples with building materials to reduce overheating in summer and to take effect storage discharge by ventilation. Xiao et al., 2000 prepared matrix type microcapsules (paraffin wax) by using interfacial polymerization and used them as latent heat storage materials for thermal storage units. Hawlader et al., 2002 prepared the microcapsules by using complex coacervation method and studied thermal performance in packed bed heat exchanger. All preliminary studies showed that encapsulated paraffin wax was prepared by interfacial polymerization and complex coacervation methods. However, not much work was reported on the inner structure of the microcapsules, thermal cycles test on microcapsules and the thermal performance of the encapsulated PCM in fluidized bed heat exchanger. The overall objective is to synthesize microencapsulated paraffin wax and to evaluate thermal performance of microencapsulated paraffin wax in fluidized bed heat exchanger. The scope encompasses the following aspects of work: 4 Chapter 1 Introduction (1) preparation of microencapsulated paraffin wax (2) characteristic evaluation of encapsulated paraffin wax (3) to study the effects of thermal cycling on the thermal properties of microencapsulated paraffin wax (4) to study heat transfer behavior in fluidized bed exchanger. This thesis is presented by organizing five chapters including introduction, Chapter 1. In chapter 2, literature review on the renewable energy and its application, thermal energy storage techniques and materials are presented. Furthermore, literature review on encapsulation of PCMs, encapsulation techniques and heat transfer development for PCMs are also presented in this chapter. Chapter 3 lists the materials used in this experiment, the experimental detail procedures, the evaluation techniques and the equipments used for characterization of microencapsulated paraffin wax. Chapter 4 presents experimental results and discussion on characterization of microencapsulated paraffin wax, the effect of thermal cyclic test on thermal properties and heat transfer in fluidized bed heat exchanger. Chapter 5 summarizes the conclusions of the present work and recommendation for future work. 5 Chapter 2 Literature Review CHAPTER 2 LITERATURE REVIEW In this chapter, literature review on the most challenging techniques of thermal energy storage and the advantages and disadvantages of each storage techniques are presented. Moreover, the study of heat storage materials, encapsulation of PCMs and encapsulation techniques, laboratory test of freeze-melt behaviour of recent researches on heat transfer enhancement of PCMs has been included. 2.1 Thermal energy storage Renewable energy is an intermittent energy source. For example, intermittence of solar energy is caused by day-night cycles, seasons and weather conditions. Similar problems arise for waste heat recovery systems, where the waste heat availability and utilization periods are different. Therefore, thermal energy storage (TES) is an essential technique for thermal energy utilization to solve the intermittence problems and levelling energy supply and demand. A large volume of TES materials can store the entire daily and annual energy requirement. The optimum size is mainly dependent upon meteorological conditions, storage temperature, storage heat losses, economic viability of storage medium, collector area and efficiency. (Rosen, 1992). Irrespective of their sizes, all TES system must satisfy certain characteristics. The desired characteristics of TES are as follows: ƒ compact, large storage capacity per unit mass and volume; ƒ heat storage medium with suitable properties in the operating temperature range; 6 Chapter 2 ƒ Literature Review capability to charge and discharge with largest heat input/output rates but without large temperature gradients; ƒ able to undergo large number of charging/discharging cycles without loss in performance and storage capacity; ƒ small self-discharging rate i.e. negligible heat losses due to surroundings; ƒ long life; ƒ inexpensive. The research areas for TES systems are shown in the Figure 2.1. New TES concepts and improvements required in the performance of TES system, the design of compact TES systems, and the use of TES in practical energy applications are outlined. Research on TES has been broad based and productive, and directed towards the resolution of specific TES issues and new TES material. 2.2 Thermal energy storage techniques For thermal energy storage, there are two alternatives: ƒ sensible heat utilization; ƒ latent heat utilization. An overview of major TES techniques is presented in Figure 2.2. The storage techniques, materials and their advantages and disadvantages are described in the following section. 7 Chapter 2 Literature Review Thermal energy storage Materials research Heat exchanger development Selection of materials in appropriate range Selection of type of exchanger and parameters Thermal analysis Thermal storage material Thermophysical property data Melting-solidifying characteristics DSC (Differential Scanning Calorimetry) TA (Thermal Analysis) Construction of material Compatibility of materials Simulation Experimental research Laboratory models Prototypes Short term behavior Pilot Plants Long term behavior Thermal cycle Useful life Incorporation into heating/cooling systems Field tests Final cost analysis Commercial product Figure 2.1 Areas of research in thermal energy storage systems (Zalba et al., 2003) 8 Chapter 2 Literature Review Thermal energy storage Thermochemical Sensible heat Latent heat Gas-liquid Solid-gas Solid-liquid Solid-solid Organics Eutectics single temperature Inorganics Mixtures temperature interval Paraffins (alkanes mixtures) Commercial grade Eutectics single temperature Fatty acids Mixtures temperature interval Hydrated salts Analytical grade Figure 2.2 Thermal energy storage strategies (Zalba et al., 2003) 9 Chapter 2 Literature Review Sensible heat storage Sensible heat storage medium is carried out by adding energy to a material to increase its temperature without changing its phase. The amount of heat released or absorbed (Q), as the medium is cooled or heated between temperatures T1 and T2, can be mathematically illustrated by Equation 2.1. T2 Q = ∫ mcpdT (2.1) T1 Q = the amount of heat released or absorbed (J) m = the mass of heat storage/release material (g) cp = specific heat capacity of heat storage/release material (J kg-1°K-1) T1 and T2 = initial and final temperatures of heat storage medium (°C) Sensible heat storage media can be classified on the basis of storage media as (1) liquid storage media (water, oil-based fluids, molten salts, etc.) and (2) solid media storage (rocks, metals and others) (Duffy and Beckman, 1989). Water as storage material has the advantages of being inexpensive and readily available, of having excellent heat transfer characteristics. Hot water is required for washing, bathing, etc. and it is commonly employed in radiators for space heating. Water also can be used as storage and as a transport medium of energy in a solar energy system. Consequently, it is the most widely used storage medium today for solar based warm water and space heating applications. However, its major drawbacks include difficulties: (1) system corrosion and leakage, (2) due to its high vapor pressure, it requires costly 10 Chapter 2 Literature Review insulation and pressure withstanding containment for high temperature applications and (3) large size and large temperature swing during the addition and extraction of energy (Wyman et al., 1980). The most commonly proposed substitutes for water are petroleum based oils and molten salts. The heat capacities are 25-40% of that of water on a weight basis. However, these substitutes have lower vapor pressure than water and are capable of operating at high temperatures exceeding 300°C. However, it can be limited due to stability and safety reasons and high cost. In addition, it is highly corrosive, and there is a difficulty in containing it at high temperatures (Hasnain, 1998). For a low as well as high temperature thermal energy storage, solid materials such as rocks, metals, concrete, sand and bricks etc. can be used. In this case, the energy can be stored at low or high temperatures, since these materials will not freeze or boil. The difficulties of high vapor pressure of water and the limitations of other liquids can be avoided by storing thermal energy as sensible heat in solids. Moreover, solids do not leak from the container. The pebble bed or rock pile consists of a bed of loosely packed rock material through which the heat transport fluid can flow. The thermal energy is stored in the packed bed by forcing heated air into the bed and utilized again by recirculating ambient air into the heated bed. The energy stored in a packed bed storage system depends, apart from the thermophysical properties of the material, on several parameters, including rock size and shape, packing density, heat transfer fluid etc. 11 Chapter 2 Literature Review Probably more important than rock size is uniformity of size. If there is too much variation, the smaller stones will fill in the voids between the larger stones, thus increasing air blower power requirement. When those types of rock tend to scale and flake, the resulting dust will be picked up by the heat transfer air and either clogs the furnace filters and, if the furnace is by-passed, dust is blown directly into the heating area (Hasnain, 1998). Latent heat storage The term “latent heat storage” can be generally described as the storage of heat in the form of latent heat of fusion, vaporization and sublimation that can undergo phase separation at a desired temperature level. The heat storage process using such a phase-change medium can be represented mathematically, by the following Equation 2.2. Tm Q= ∫ T1 T2 mcpdT + m∆Hfusion + ∫ mcpdT (2.2) Tm Q = total amount of heat storage/release (J) m = mass of heat storage material (g) cp = specific heat capacity of heat storage material (J kg-1 °K-1) Tm = melting temperature of heat storage material (°C) T1 and T2 = initial and final temperatures of heat storage medium (°C) ∆H fusion = heat required to change from solid phase to liquid phase (J/g) 12 Chapter 2 Literature Review In a latent heat storage system, the sensible component of the heat storage is kept low. This enables the system to be operated at low temperature resulting in high efficiency of the solar energy collection system in renewable energy application. As shown in Table 2.1, latent heat storage media (PCMs) can store large quantity of heat in a smaller weight and volume of material in comparison with sensible heat storage media. Therefore, latent heat storage media offers the following advantages: (1) it provides high-energy storage capacity, (2) it can operate at narrow range of temperature and (3) heat store for phase transition is significantly greater than sensible heat. Table 2.1 Comparison of various heat storage media (stored energy = 106 KJ, ∆T = 15 °K) (Hasnain, 1998) Sensible heat storage media Latent heat storage media Property rock water organic PCM inorganic PCM Storage mass for storing 106KJ (kg) Storage volume for storing 106KJ (m3) 67, 000 16, 000 5, 300 4, 350 30 16 6.6 2.7 Solid-solid, liquid-gas, and solid-liquid transformations can be found in PCM. Heat can be stored as the heat of crystallization, as the substance is transformed from one solid phase to another solid phase in solid-solid PCM. Relatively few solid-solid PCMs have been identified that have heats of crystallization and transition temperatures suitable for thermal energy storage applications. Liquid-gas PCMs usually have high heats of transformations, however, due to the large volume change during transformation, they are not usually 13 Chapter 2 Literature Review considered for practical applications. Therefore, solid-liquid transformation is commonly utilized and the energy stored could be discharged at a constant crystallization temperature. Basic technology for latent heat storage system design should be considered. Any latent heat thermal energy storage system must possess at least the three following basic components: (1) a heat storage substance that undergoes a solid-to-liquid phase transition in the required operating temperature range and where the bulk of heat added is stored as the latent heat of fusion; (2) a container for holding the storage substance; (3) a heat exchanging surface for transferring heat from the heat source to the PCM and from the later to heat sink. The type of the heat-exchanging surface plays an important role in the design of the system, as it strongly influences the temperature gradients for charging and discharging of the storage. Therefore, the development of a latent heat thermal energy storage system involves two essentially diverse subjects: (1) screening of heat storage materials (PCMs); (2) heat exchangers for better heat transfer (Abhat, 1983). A large number of organic and inorganic substances are known to melt with a high heat of fusion in any required temperature range, e.g. 0-120°C. However, for their employment as heat storage materials in latent heat thermal energy storage systems, these phase change materials must exhibit certain desirable thermodynamic, kinetic and chemical properties. 14 Chapter 2 Literature Review Moreover, economic considerations of cost and large-scale availability of the materials must be considered. 2.3 Heat storage materials Within operating temperature range of 0-120°C, candidates PCMs are grouped into two subfamilies: organics and inorganics. Organic families include paraffin and non-paraffin organics. Paraffins are substances having a waxy consistency at room temperature. Paraffins contain in them one major component called alkanes, characterized by CnH2n+2; the n-alkane content in paraffin waxes usually exceeds 75% and may reach 100%. The melting point of alkanes increases with the increasing number of carbon atoms; alkanes containing 14-40 carbon atoms possess melting points between 6 and 80°C and are generally termed as paraffins. Commercial waxes, on the other hand, may have a range of about 8-15 carbon numbers. Table 2.2 lists thermophysical data for some technical grade paraffin wax materials, which are paraffin mixtures and are not completely refined of oil, some fatty acids and salt hydrates. Physical properties are also included in this table. Paraffins qualify as heat-of fusion storage materials due to their availability in a large temperature range and their reasonably high heat of fusion. Furthermore, they are known 15 Chapter 2 Literature Review Table 2.2 Physical properties of some PCMs (Abhat, 1983) Distribution of Catoms C16C28 C20 – C33 C22C45 Caprilic acid Palmitic acid CaCl2.6 H2O MgCl2.6 H2O Oil content (%) Freezing point range (°C) Heat of fusion Density at (KJ/kg) (KJ/m3) (kg/m3) 0.912 (20°C) 0.769 (70°C) 0.915 (20°C) 0.790 (70°C) 0.930 (20°C) 0.830 (70°C) 1.033 (10°C) 0.862 (80°C) 0.847 (80°C) Specific heat at 100°C (KJ/kg K) Thermal conductivity (solid phase) 2.1 (W/m K) 0.21 2.1 0.21 2.1 0.21 - 0.148 - 0.165 5 42– 44 240 146 [...]... total amount of heat storage/ release (J) m = mass of heat storage material (g) cp = specific heat capacity of heat storage material (J kg-1 °K-1) Tm = melting temperature of heat storage material (°C) T1 and T2 = initial and final temperatures of heat storage medium (°C) ∆H fusion = heat required to change from solid phase to liquid phase (J/g) 12 Chapter 2 Literature Review In a latent heat storage system,... subjects: (1) screening of heat storage materials (PCMs); (2) heat exchangers for better heat transfer (Abhat, 1983) A large number of organic and inorganic substances are known to melt with a high heat of fusion in any required temperature range, e.g 0-120°C However, for their employment as heat storage materials in latent heat thermal energy storage systems, these phase change materials must exhibit... Temperature profiles during heat storage stage 93 Figure 4.32: temperature profiles during heat release stage 93 Figure 4.33: Heat storage with time during heat storage stage 95 Figure 4.34: Heat release with time during heat release stage 95 x LIST OF TABLES Table 2.1: Comparison of various heat storage media 13 Table 2.2: Physical properties of some PCMs 16 Table 2.3: Comparison of organic and inorganic materials. .. furnace is by-passed, dust is blown directly into the heating area (Hasnain, 1998) Latent heat storage The term “latent heat storage can be generally described as the storage of heat in the form of latent heat of fusion, vaporization and sublimation that can undergo phase separation at a desired temperature level The heat storage process using such a phase- change medium can be represented mathematically,... supporting structure and the application of a packed/fluidized bed heat exchanger is a better way of heat transfer enhancement Therefore, the progress in latent heat storage systems mainly depends on heat storage material investigations and on the development of heat exchangers that assure a high effective heat transfer rate to allow rapid charging and discharging The required heat transfer surfaces should be... storage substance; (3) a heat exchanging surface for transferring heat from the heat source to the PCM and from the later to heat sink The type of the heat- exchanging surface plays an important role in the design of the system, as it strongly influences the temperature gradients for charging and discharging of the storage Therefore, the development of a latent heat thermal energy storage system involves... materials for heat storage 19 Table 2.4: Important characteristics of energy storage materials 19 Table 2.5: List of published encapsulated PCM systems 24 Table 3.1: Materials used in the microencapsulating of paraffin wax 36 Table 3.2: Operating conditions for spray drying microencapsulation 44 Table 4.1: Encapsulation efficiency of coacervated capsules 55 Table 4.2: Encapsulation efficiency of coacervated... component of the heat storage is kept low This enables the system to be operated at low temperature resulting in high efficiency of the solar energy collection system in renewable energy application As shown in Table 2.1, latent heat storage media (PCMs) can store large quantity of heat in a smaller weight and volume of material in comparison with sensible heat storage media Therefore, latent heat storage. .. latent heat storage media offers the following advantages: (1) it provides high-energy storage capacity, (2) it can operate at narrow range of temperature and (3) heat store for phase transition is significantly greater than sensible heat Table 2.1 Comparison of various heat storage media (stored energy = 106 KJ, ∆T = 15 °K) (Hasnain, 1998) Sensible heat storage media Latent heat storage media Property... technology for latent heat storage system design should be considered Any latent heat thermal energy storage system must possess at least the three following basic components: (1) a heat storage substance that undergoes a solid-to-liquid phase transition in the required operating temperature range and where the bulk of heat added is stored as the latent heat of fusion; (2) a container for holding the storage ... total amount of heat storage/ release (J) m = mass of heat storage material (g) cp = specific heat capacity of heat storage material (J kg-1 °K-1) Tm = melting temperature of heat storage material... directly into the heating area (Hasnain, 1998) Latent heat storage The term “latent heat storage can be generally described as the storage of heat in the form of latent heat of fusion, vaporization... energy storage 2.2 Thermal energy storage techniques 2.3 Candidate heat storage materials 15 2.4 Factors affecting the energy storage capacity of PCM 20 2.5 Encapsulation of phase change materials