Thermal Energy Storage using Latent Heat of Phase Change Materials

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1 Thermal Energy Storage using Latent Heat of Phase Change Materials BY MUKUND BHASKAR B.E., Anna University, INDIA, 2013 THESIS Submitted as partial fulfillment of the requirements for the degree of Master of Science in Mechanical Engineering in the Graduate College of the University of Illinois at Chicago, 2016 Chicago, Illinois Defense Committee: Yayue Pan, Chair and Advisor Houshang Darabi Said Al Hallaj, Chemical Engineering

2 ACKNOWLEDGEMENT I take this opportunity to thank my thesis advisor Dr. Yayue Pan for stretching her arm out and constantly motivating me towards overcoming challenges and Professor Dr. Said Al Hallaj for guiding me and giving me a wonderful team to work and learn with at AllCell Technologies, which has become my second home. I also would like to express my extreme gratitude to my supervisor, friend and mentor Mr. Siddique Khateeb at AllCell Technologies, for being patient and giving me the space and time to blossom and excel and for having faith in my work, for giving me a vision and being my role model in this journey of hard work and overcoming numerous challenges, also to my R&D team members Mr. Ben Schweitzer and Mr. Stephen Wilke for their constant support and backing during tough times. A special thanks to my friend and colleague Mr. Denard Harper, the Lead Manufacturing Engineer at AllCell Technologies who has helped me machine and build many parts, despite his busy schedule. Last but not the least a special and emotional mention to my family back home in India, my parents, grandparents, cousins and to my friends, families here in the US, without them I would have never been able to achieve my dream and thanks for their support, in always having my back. ii

3 Table of Contents List of abbreviations... viii Chapter 1 Introduction Energy Consumption and Expense Renewable Technologies Research Goal and Tasks... 5 Chapter 2 Literature Review Thermal Energy Storage Thermal Energy Storage Techniques Sensible Thermal Energy Storage Latent TES Heat from Chemical Reactions Phase Change Materials for Thermal Energy Storage Ice Storage-A Conventional Method of Cold Storage Other Phase Change Materials used for Thermal Energy Storage Units Thermal Enhancements Materials Chapter 3 Finite Element Analysis using COMSOL Proposed Phase Change Composite Conventional PCM Finite Element Analysis Model Equations for the Model Mesh Construction Input Parameters for Numerical Study Flow Velocity of Heat Transfer Fluid Specific Heat of the Material Thermal Conductivity of PCC/PCM Density of PCC/PCM Properties of Ethylene Glycol Temperature Range Simulation Assumptions Chapter 4 Prototype Setup Phase Change Materials Preparation Procurement Prototype Setup for the TES Units iii

4 4.3 Chiller Phase Change Materials Copper Serpentine Thermocouples Working of the System Pre Charge/Discharge Mode Charge/Discharge Mode Chapter 5 Results and Discussions Results from Finite Element Analysis Effect of the TES Units at Low Flow Rates Effect of TES Units at High Flow Rates Experimental Testing of the TES Units Results from a High Discharge Rate Results from a Medium Discharge Rate Results from a Low Flow Rate Validation of Experimental Results with FEA Model Low Flow Rates Validation for the TES Units Medium Flow Rates Validation for the TES Units Chapter 6 Prototype Design and Development Effect of Air Gap for the PCC TES Unit Low Flow Rates Medium Flow Rate Effect of Thermal Conductivity for PCC Design Optimization Effect of Copper Pitch Effect of Longer Slab Design on TES Performance Design Optimization Optimized Design for Ice TES Optimized Design for the PCC TES Prototype Cost Analysis of Optimum Designs of TES Units Chapter 7 Conclusion Chapter 8 Bibliography Chapter 9 Appendix Chapter 10 VITA iv

5 List of Figures Figure 1. Average energy consumption in Figure 2. Weighted load of energy consumption of a household... 1 Figure 3. Demand profile curve for a day... 2 Figure 4. Electricity pricing and period... 3 Figure 5. Installed capacity of solar energy from 2007 to Figure 6. Research motivation... 6 Figure 7. Methods of thermal energy storage... 8 Figure 8. Energy storage as a function of temperature... 9 Figure 9.Sensible TES methods... 9 Figure 10. Temperature addition and energy storage for Latent TES Figure 11. Types of PCM Figure 12. Sub-cooling phenomenon Figure 13. Ice TES unit schematic Figure 14. 3D Model of PCC Slabs with a two-inch long copper serpentine Figure 15. Two-inch long copper serpentine design Figure 16. FEA Meshing Figure 17. Cp as a function of temperature for PCC Figure 18. Cp as a function of temperature for ice Figure 19. Cp as a function of temperature for PCM Figure 20. TES experimental setup schematic Figure 21. Julabo chiller F25ME Figure 22. PCM/ICE TES prototype Figure 23. PCC TES prototype Figure 24. Copper serpentine Figure 25. Thermocouple map Figure 26. Representation of the pre charge/discharge mode Figure 27. Representation of the charge / discharge mode Figure 28. PCC TES at low flow rates simulated Figure 29. PCM TES at low flow rates simulated Figure 30.Ice TES at low flow rates simulated Figure 31. PCC TES at higher flow rates simulated Figure 32. PCM TES at higher flow rate simulated Figure 33. Ice TES unit at higher flow rate simulated Figure 34. Performance of the PCC TES unit at high flow rates Figure 35. Performance of ice TES at high flow rates Figure 36. Performance of PCM TES at high flow rate Figure 37. PCC TES at medium flow rate Figure 38. Ice TES at medium flow rate Figure 39. PCM TES at medium flow rate Figure 40. PCC TES at low flow rate Figure 41. Ice TES at low flow rate Figure 42. PCM TES at Low flow rate Figure 43. Experimental validation for ice TES at low flow rate Figure 44. Experimental validation for PCM TES at low flow rate Figure 45. Experimental validation for PCC TES at low flow rates v

6 Figure 46. Experimental validation for ice TES at medium flow rate Figure 47. Experimental validation for PCM TES at medium flow rate Figure 48. Experimental validation of PCC TES at medium flow rates Figure 49. Comparison of PCC TES with and without air gap at medium flow rate Figure 50. PCC slab orientation of current design Figure 51. Variation of PCC slab orientation to study effect of thermal conductivity Figure 52. PCC TES with variation in thermal conductivity at low flow rate Figure 53. Copper serpentine designs for one-inch pitch Figure 54. PCC TES for one-inch copper serpentine pitch at low flow rate Figure 55. Ice TES for a one-inch copper serpentine design at low flow rate Figure 56. Design of a TES unit for increased PCC/ice content Figure 57. PCC TES with increased thermal capacity at low flow rate Figure 58. Ice TES with increased thermal capacity at low flow rate Figure 59. Optimized ice TES design Figure 60. Discharge profile for optimized ice TES at low flow rate Figure 61 PCC slab's variation in thermal conductivity Figure 62 Discharge for PCC slab at 12.5 GPH Figure 63 Dynamic viscosity of ethylene glycol water mixture as a function of temperature Figure 64 Thermal conductivity of ethylene glycol water mixture as a function of temperature Figure 65 Specific Heat as a function of temperature for ethylene glycol water mixture Figure 66 Density as a function of temperature for ethylene glycol water mixture Figure 67 LVDT setup for measuring thermal expansion Figure 68 Temperature of evaporator coil as a function of dry bulb temperature of air vi

7 List of tables Table 1. Limitations of sensible TES units Table 2. Properties of commonly used PCM Table 3. Comparison of PCM TES Table 4. Comparison of commonly used heat spreaders Table 5. Comparison between PCM and PCC Properties Table 6. Flow rates for the experimental and simulation tests Table 7. Thermal conductivity of materials Table 8 Density of materials Table 9. Temperature range for experimental and simulation tests Table 10. Raw material for TES unit Table 11. Components of the TES unit Table 12. Quantity of PCM/PCC content for TES unit Table 13. Simulated PCC TES at low flow rates Table 14. Simulated PCM TES at Low flow rates Table 15. Simulated ice TES unit at low flow rates Table 16. Simulated PCC TES at higher flow rate Table 17. Simulated PCM TES at higher flow rate Table 18 Simulated ice TES unit at higher flow rates Table 19. Comparison of TES units at high flow rate Table 20. Comparison of TES units at medium flow rates Table 21. Comparison of TES units at low flow rates Table 22. Comparison of experiment and simulation for ice TES at low flow rate Table 23. Comparison of experiment and simulation for PCM TES at low flow rate Table 24. Comparison of experiment and simulation for PCC TES at low flow rate Table 25. Comparison of TES units at low flow rate from simulation Table 26. Comparison of experiment and simulation for ice TES unit at medium flow rate Table 27. Comparison of experimental and simulation validation Table 28. Comparison of experiment and simulation for PCC TES at medium flow rate Table 29. Comparison of TES units at medium flow rate Table 30. Comparison of PCC TES with and without air gap at low flow rate Table 31. Comparison of PCC TES with and without air gap at medium flow rates Table 32. Comparison of PCC TES units with varying thermal conductivities Table 33. Comparison of PCC TES with varying copper pitch Table 34. Comparison of ice TES with varying copper pitch Table 35. Test conditions for TES unit with increased thermal capacity Table 36. Comparison of PCC TES units on design for thermal capacity Table 37. Comparison of ice TES unit on design for thermal capacity Table 38. Comparison of ice TES design for optimization Table 39. Performance comparison for optimized ice TES with experimental design Table 40. Comparison of optimized ice and PCC TES units Table 41.Prototype cost analysis of ice and PCC TES units vii

8 List of abbreviations A Cp,pcm Cp,eg Eg k h L m PCM Q T TES w η λ α Area Specific heat at constant pressure for PCM specific heat at constant pressure for ethylene glycol Ethylene glycol Heat transfer coefficient of conduction Enthalpy length mass Phase Change Material Heat transfer rate Temperature Thermal energy storage Width Thermal energy utilization Latent energy Thermal diffusivity viii

9 SUMMARY The major energy consumption comes from domestic and tertiary buildings and there is about 40% consumption by just space conditioning units, like air conditioners (AC). In this study, the objective is to reduce the load from the AC units during peak periods, by using phase change materials based thermal energy storage (TES) units. In this project, a phase change composite (PCC) has been fabricated. Compared to conventional phase change materials, the proposed phase change composite is capable of storing cold thermal energy with a higher thermal conductivity. To verify and validate the developed phase change composite, the performance of thermal energy storage units based on the proposed phase change composite was measured and compared to existing thermal energy storage units using conventional phase change materials. Especially, experiments were performed to test the functioning of the TES units on varying rates of cold discharge. In addition, Finite Element Analysis (FEA) on the TES unit was carried out. Key factors that affect performance such as surface area, thermal conductivity and geometry of the TES, were identified through the numerical analysis and experimental tests. After that, a commercially successful ice TES prototype unit and a PCC TES prototype unit were developed. Then design optimization was carried out to compare the performance of the ice and the PCC TES units. Finally, with equal performance rates, a cost analysis was carried out to evaluate the two TES technologies. ix

10 % Energy Consumption Chapter 1 Introduction 1.1 Energy Consumption and Expense Nowadays, meeting energy demands is a struggle in society, due to the growing population, and limited resources. Figure 1 [1], shows the energy utilization by sectors for the year The domestic buildings account for 37% of the total energy being consumed, having been identified as the sector that has the highest energy consumption. Efforts have been made to identify the root cause of major energy consuming appliances and devices in buildings. For instance, Figure 2 shows the electrical load consumptions in a household [2] Industrial processes Industrial buildings Buildings (domestic & tertiary) Agriculture Transport Figure 1. Average energy consumption in % 2% 2% 4% 4% 2% 4% 21% Water Heater Air Conditioner 5% 18% 37% Refrigerator + Freezer Computer Lighting Figure 2. Weighted load of energy consumption of a household 1

11 Demand (kwh) As shown in Figure 2, the air conditioners account for about 37% of total energy demanded in a household, followed by water heaters and refrigerators. To create people's awareness of the energy consumption in a household and to encourage people to shift to renewable energy, utilities are usually charged by the demand in most places in North America. These demand charges can be divided into peak and off peak rates. The peak rates are priced anywhere from two to six times of the rates of off peak periods. 12 am 6 am Time of the day 6 pm 12 am Figure 3. Demand profile curve for a day Figure 3 [3] shows a typical energy demand profile in a day and figure 4 [4] shows the electricity pricing in Ontario, Canada. As shown in Figure 4, the typical peak period in summer weekdays is from Noon to 6 p.m., and the energy consumption is charged by a rate of 16 cents/kwh, twice of the rate for off peak period. 2

12 Figure 4. Electricity pricing and period 1.2 Renewable Technologies Renewable technologies have been used as an alternate energy source in households/ industries to meet the demand for energy globally. It has been applied increasingly in recent years. Figure 5 shows the installed capacity of solar devices from the year 2008 to 2013 [5]. 3

13 Installed Capacity (MW) Year Figure 5. Installed capacity of solar energy from 2007 to 2013 Though renewable energy has been applied to solve the energy demand in many applications, many disadvantages and challenges still exist, as listed below: (1) It is not a perennial source for energy. (2) The capital cost for the device and technology is high. (3) The pay back periods often vary from one to 3 years. (4) The harnessing of renewable energy is dependent on external factors. (5) Service and maintenance is not easy. (6) The generated energy is not standardized. Especially, due to the non-uniform energy generating characteristics, the renewable energy as a powering source may not be able to solve the energy crisis entirely. To address this problem, scientists and researchers suggest storing of energy obtained from renewable sources, thereby excess energy generated in a given day can help supplement a day when energy produced is insufficient. With such energy storage techniques, the overall system efficiency can be improved greatly [6]. 4

14 Among the many energy storage techniques, thermal energy storage technology has been implemented widely to reduce energy demanded by the heating and cooling household appliance, such as air conditioners, refrigerators, and heaters. As discussed in previous section, those appliances consume the majority of energy demanded by buildings. This technology harnesses thermal energy directly and stores it by various methodologies such as sensible thermal energy storage, latent thermal energy storage and thermal energy storage from chemical reactions. It directly provides the necessary heating or cooling load required by an application, unlike other energy storage mediums that have to convert energy to different forms with energy losses while functioning in an application. Hence, Thermal Energy Storage (TES) unit is particularly useful in applications of space cooling and heating. These units are almost similar to how battery systems work. The storage unit needs to be charged and once charged can be discharged. Thus, heat or cold energy from various processes can be stored during off-peak periods and released during peak periods. As discussed in previous section, appliances such as air conditioner, heater and refrigerator, consume a great amount of energy daily and the energy consumption is usually priced by demand. Thermal energy storage unit therefore has the potential to be an effective alternate renewable energy to solve this energy demand challenge [7,8]. 1.3 Research Goal and Tasks Against this background, the goal of this project is to reduce the energy consumption of air conditioners by implementing thermal energy storage technology. To achieve this goal, a novel TES unit design has been developed. Various design parameters were investigated, by using finite element analysis method. After that, a commercially successful ice TES prototype unit and a PCC TES prototype unit were developed. Then design optimization was carried out to compare the performance of the ice and the PCC TES units. Finally, with equal performance rates, an economic analysis was carried out to evaluate the effectiveness of the two TES technologies on reducing energy consumption. The flow chart below 5

15 shows the identified research problem, the defined research scope, and the planned research tasks to achieve the research goal. Background and Research Scope 40% energy demand from Space cooling which costs 2-6 times more than regular cost of energy. Thermal Energy Storage for cooling systems to offset peak electric demand Investigate TES unit design TES unit design optimization for cold storage and release Research Task Finite Element Analysis using COMSOL Experimental study Initial proof of concept of TES Design and optimization of TES unit Figure 6. Research motivation 6

16 Chapter 2 Literature Review 2.1 Thermal Energy Storage Thermal energy storage (TES) is a technique for storing heat or cold in a medium for later use. Demand for energy is effectively managed using TES. The TES overcomes the drawback of non-standardized energy production from renewable energy sources. TES system brings down the energy consumption through constant supply during the peak period thereby resulting in an efficient energy system. Some of the requirements of a good TES unit are as follows [9,10]: (1) High energy storage density. (2) Ability to release heat/cold immediately. (3) Ability to work well and compatible with other materials. (4) Eco friendly technology. (5) Low thermal losses. (6) Easy to store and release thermal energy. There are numerous ways of storing thermal energy by: (i) Physical processes; and (ii) chemical processes. Figure 7 below shows a classification of thermal energy storage methods. 7

17 METHODS FOR THERMAL ENERGY STORAGE PHYSICAL PROCESS CHEMICAL PROCESS SENSIBLE LATENT Figure 7. Methods of thermal energy storage 2.2 Thermal Energy Storage Techniques The thermal energy storage methods are going to be discussed in the following section. This discussion will identify the pros and cons with regard to every thermal energy storage unit Sensible Thermal Energy Storage The most common mode of thermal energy storage is provided by sensible heat addition into a medium, which is utilized at a later stage. The ratio of stored Heat Q to T (Temperature Rise) can be defined as the heat capacity (C) of the storage medium. [11] The heat transfer equation for the process is: (1) Figure 8 provides an illustration of sensible heat storage when temperature is added sensibly. 8

18 Temperature K Energy Stored kwh Figure 8. Energy storage as a function of temperature For this mode of thermal storage, the amount of energy stored is directly proportional to the heat capacity and amount of material available. The materials that can be used for sensible TES are provided from the research conducted by A.I.Fernandez et al. [12], M.E. Navarro et al. [13],.S.khare et al. [14],Doerte Laing et al [15]. Another common mode of sensible thermal storage for high temperature applications is in the form of molten salts and liquids for high temperature applications [16, 17]. Some common methods of energy utilization by this technique are by ground aquifers, boreholes etc. Figure 9 shows a flow chart of a few methods by which sensible thermal storage is achieved. Sensible Seasonal Thermal Energy storage Liquid form Solid form Hot water TES Acquifier TES Borehole TES Figure 9.Sensible TES methods 9

19 Figure 9 provides a list of common methods of storing sensible thermal energy. Though this technology is proven capable of reducing energy demand and has been widely commercialized, there are still some limitations of this technique such as need for geographical factors to be suitable, corrosion due to molten salts [18-21] etc. and can be explained in Table 1. S.no. Storage Concept Geological Requirements 1. Hot water 1.Existence of a stable underground storage space 2.Preferable no ground water 3. 5 to 15 m deep storage tank or reservoir 2. Aquifers 1.Existence of a natural storage space underground 2.A wall thickness of 20-50m 3.Absence of ground water flow inside the aquifer 3. Bore hole 1.Existence of ground space of depth m 2.Preferable location with availability of ground water Table 1. Limitations of sensible TES units The difficulties and cost in building and maintaining various sensible thermal energy storage units are listed in Table 1. To avoid these challenges, other types of thermal energy storage units, latent TES and TES from chemical reactions are considered Latent TES The concept of Latent TES is implemented by utilizing the latent energy available from phase change materials. The material undergoes phase transition from a solid to a liquid, by overcoming the latent heat of fusion. Generally, the amount of energy that can be stored in a material depends on its latent energy, which is the amount of energy needed or the enthalpy difference from a solid to a liquid phase. When 10

20 Temperature K a material is undergoing melting say from solid to liquid, it does so by absorbing the latent energy of fusion. Once it has absorbed the latent energy, by releasing the same it can go back to its previous state. The phase change phenomenon is a reversible process, which is a basic requirement of an energy storage medium. When comparing with sensible thermal energy storage from the previous section, the latent thermal energy storage is easier and straightforward to implement [22]. From thermodynamics, the process of phase change can be explained below. Sensible Storage Latent Storage T m Sensible Storage Energy Storage kwh Figure 10. Temperature addition and energy storage for Latent TES The relation between thermal energy added and energy storage for a phase change material is shown in Figure 10. Initially thermal energy is added to a cold (solid state- frozen) phase change material, where heat is added sensibly till the temperature rises to the melting point temperature Tm. Anymore thermal energy addition is observed to occur at the constant melting point temperature Tm, until the latent heat of fusion of the phase change material is overcome. This region of thermal heat addition at a constant temperature is the latent thermal energy storage region. The end of this phase is shown by a complete 11

21 melting of the material from a solid to a liquid. Thermal energy beyond the latent region is added in a sensible manner. [22] The characteristic equation for the latent energy heat addition can be given by: (2) Higher the latent heat for a material, higher is the thermal energy storage. Based on the melting point of the phase change material, a variety of applications [23-24] for heating and cooling is available. PHASE CHANGE MATERIALS Organic matierials Inorganic matierals Eutectics materials Paraffin Non-Paraffin Salt Hydrates Metallics Organic-Organic Inorganic-Inorganic Organic-Inorganic Figure 11. Types of PCM Figure 11 shows the type of phase change materials that are available. Primarily they can be divided into three categories based on the kind of materials they are derived from- organic, inorganic, and eutectic materials. The organic materials are further divided into paraffin and non-paraffin materials, the inorganic materials are further divided as salt hydrates and metallic, followed by eutectic materials that are a mixture of the above forms. Advantages of using phase change materials include: (1) Low cost for implementation of TES; (2) High energy density offered from latent energy storage; (3) Easy functionality of phase change materials as energy storage units; (4) Long life for energy storage with less maintenance and; (5) Wide range of applications based on melting temperature chosen. 12

22 Disadvantages of using phase change materials include: (1) Material has a low thermal conductivity; (2) Phase change from liquid to solid may result in super-cooling, which is explained in the following section. The material must overcome these drawbacks in order for improving the performance of phase change materials as latent thermal storage units Heat from Chemical Reactions The thermal energy storage from chemical reactions employs the enthalpy difference, between the initial and final states of a chemical reaction. The enthalpy difference is the basis for the thermal energy storage. The enthalpy difference is called the heat of the reaction. An endothermic reaction would tend to absorb heat as the reaction takes place, while an exothermic reaction would tend to release heat as the reaction takes place. In general, higher the heat of the reaction, higher is the thermal capacity and thermal energy storage obtained. The equation that characterizes the heat from the chemical reaction is: (3) The heat of a reaction is based on the enthalpy difference from the start to the end of a chemical reaction and when designing a thermal energy storage unit, a high value for heat of a reaction is preferred, so that higher is the energy storage available. For a chemical reaction to be considered for thermal energy storage, the following conditions have to be favorable [25,26]: (1) The reaction has to be in equilibrium. (2) The reaction must yield a lot of energy. (3) The reaction should not consume the reactants. 13

23 (4) The reaction time must be short and energy yield must be high. (5) The reaction should be economical. The major disadvantage of the thermal energy storage from chemical reactions lies in controlling the chemical reaction. Often times to predict the chemical reaction is not an easy task, and controlling the energy released or absorbed by the reaction can lead to failure in thermal energy storage. The various thermal energy storage techniques have been identified. From the three thermal storage methods discussed, it is clear that the latent heat of thermal energy storage using phase change materials is the best amongst the three techniques listed. The sensible thermal energy storage unit is extremely difficult and is not economical to build, while the thermal storage from the chemical reaction is hard to control and sustain. 2.3 Phase Change Materials for Thermal Energy Storage Phase change materials (PCM) are chemicals that undergo phase change, and this change in phase is utilized for storing heat. In nature, there is a wide variety of phase change materials. There are also phase changing materials with desired properties fabricated by man. Based on the phase change temperature, a variety of applications can be targeted. S.no. Material Melting Fusion Heat (kj/l) Temperature (oc) 1. Water Salt Hydrates Sugar Alcohols Nitrates

24 5. Hydroxides Chlorides Carbonates Fluorides Table 2. Properties of commonly used PCM Table 2 lists some phase change materials (PCM) with their melting temperature and enthalpy of fusion. In order to carry out cooling and refrigeration; phase change materials that melt below 12 o C are chosen, such as paraffin, fatty acids, polyethylene glycols, hydrated salts and ice/water [27-35]. A few points to be kept in mind to achieve cold storage implementing PCM are: (1) Melting temperature range from 5-12 o C at atmospheric pressure is required. (2) A high value for the latent heat of fusion corresponds to high thermal storage capacity. (3) Reproducible phase change phenomenon or cycling stability is required. (4) Ability of the phase change material to work with other components. (5) Stable chemical properties over long usage. (6) Minimal super cooling or sub-cooling. Figure 12 explains sub-cooling or super cooling process: 15

25 Temperature (K) Tm Sub-cooling Time (sec) Figure 12. Sub-cooling phenomenon Sub-cooling or super cooling is the phenomenon of rapid temperature drop as a material begins freezing, without undergoing proper freezing. This phenomenon leads to improper cold storage, as the latent thermal transition by the material is incomplete Ice Storage- A Conventional Method of Cold Storage Many active interests in utilizing ice as a thermal energy storage unit had resulted in thermal energy storage units, which are built by utilizing water as a phase change material. Many companies have developed thermal energy storage units from ice/water, which have attained commercial success. The TES units work with roof top air conditioning units to offset peak electric demand. A few commercially established companies are Ice Bear, Evapco, Trane, Calmac, etc. The two forms of ice storage are (1) Static ice build. (2) Dynamic ice builds. Static ice build is building ice on coils or in between plates that have water stored in them by gradual cooling. Dynamic ice building systems build ice with continuous removal and transportation. The principle behind ice storage technology is utilizing the electrical energy during off peak hours to store cold and utilizing the stored cold during peak hours, for meeting the 16

26 cooling load demanded by buildings [36-39]. An Ice thermal energy storage system has been shown in Figure 13. fan Heat exchanger coil Chiller Ice Thermal Energy Storage Unit Cool air Figure 13. Ice TES unit schematic Figure 13 shows the schematic of an ice thermal energy storage unit. This unit operates by utilizing ice. Ice is statically built around coils during nighttime when electricity rates are low priced, to store cold. During off peak rates, the chiller cools a refrigerant that is passed through a tank containing water with coils, and the water freezes to ice. The stored ice is utilized during peak hours, to meet the cooling loads of the chiller. The refrigerant relies on the stored cold, to meet the cooling loads of the chiller without depending on the electrical energy available for the chiller [40]. Advantages of ice storage are: (1) Large cooling capacity to cold storage volume is achieved. (2) Ability to retrofit the TES unit to existing roof top AC units. (3) Accurate design available, based on the thermal properties of ice. (4) Low material cost. 17

27 (5) Pollution free and zero ozone depletion. Disadvantages of ice storage: (1) Very low temperatures needed to charge the TES unit. (2) The process to charge / discharge is slow due to the poor thermal conductivity. (3) The phase change material undergoes super cooling Other Phase Change Materials used for Thermal Energy Storage Units Some other thermal energy storage units are also used but not as well- known as the ice storage units. These units are the eutectic salt cold storage units. Eutectic salts are a mixture of inorganic salts, water and nucleating agents, these nucleating and stabilizing agents prevent sub cooling from taking place. They have phase change temperatures from 4-6 o C. The coefficient of performance (COP) of this thermal energy storage unit is higher than that of the ice thermal energy storage unit. However, they have not reached commercial success as the ice TES unit [41]. A comparison between the two systems is shown in Table 3 S.no. Parameter Ice Storage Unit Eutectic Salt TES 1. Latent Heat (kj/kg) Charging Temperature ( o C) -3 to -6 4 to Heating capacity to tank volume (m 3 /kwh) Chiller Charging Efficiency (COP) Discharging Temperature ( o C) to to to to 3 9 to 10 18

28 6. Storage installed cost ($/kwh) 14 to to 43 Table 3. Comparison of PCM TES Table 3 compares the eutectic salt thermal energy storage unit to the ice thermal energy storage unit; it is observable that the cost to build and manufacture the ice thermal energy storage is half the cost of the eutectic salt thermal energy storage unit. The COP of the eutectic salt thermal energy storage unit is double of the COP of the ice thermal energy storage unit. The various reasons for a higher COP are: (1) the eutectic salt thermal energy storage units do not undergo super cooling; (2) the eutectic salt TES units do not require to be cooled to subzero conditions and; (3) eutectic thermal energy storage unit s properties can be varied significantly due to additives, targeted for particular cooling applications. A common disadvantage with conventional thermal energy storage unit is they have low thermal conductivity. Heat transfer optimization is often carried out at the expense of design innovations; that add cost to the unit. The low thermal conductivity also results in extended durations for charge and discharge. The inefficiency in the material must be countered by economical thermal enhancement techniques. 2.4 Thermal Enhancement Techniques Thermal enhancement is necessary for improving heat transfer. Various techniques have been implemented in the past such as addition of conductive fins, carbon Nano fiber insertions, and metal or graphite impregnation. Thermal enhancement solution must be cost effective. In the past, researchers and industries have been successful with metal insertion, such as aluminum and copper mesh or adding fins made from the metals to improve heat transfer. In recent times, graphite was found to be a cheap alternative to these techniques, and had been employed for thermal management of 19

29 electronics. Heat spreaders are made from graphite sheets. The reason why graphite exhibits a high level of thermal conductivity is that graphite has unique structural orientation. When graphite blocks are being compacted from expanded natural graphite fibers, they tend to have a bi directional thermal conductivity. They exhibit high thermal conductivity along perpendicular to the direction of compaction and they seem to have a lower value of thermal conductivity along the direction of compaction. Due to this anisotropic nature of thermal conductivity in graphite, they can behave as heat spreaders and insulators. Apart from graphite, some other materials could also act as heat spreaders, such as copper and aluminum. A review of the properties of commonly used heat spreaders is listed in Table 4 [42]. S.no Property Density (g/cm 3 ) Thermal conductivity (W/(mK)) Specific Heat (J/kgK) Aluminu Expanded Natural Copper m 1100 Graphite alloy alloy (In plane)* 3-10 (through plane)* ($/lb) Table 4. Comparison of commonly used heat spreaders * Refer to appendix 20

30 Table 4 shows a comparison of the common heat spreaders employed in industries. It is clear that the cheapest amongst the three heat spreaders is the expanded natural graphite. It will be interesting to investigate the behavior of a phase change material with and without improved thermal conductivity. Thus from literature review, it is clear that phase change materials have achieved significant success. However, utilization of phase change materials needs enhancement with regard to thermal properties. Thermal enhancement can be a significant step towards the future of thermal energy storage industry. The following sections identify a thermal improvement process for a phase change material. 21

31 Chapter 3 Finite Element Analysis using COMSOL From the previous chapter, it is clear that steps towards thermal enhancement must be carried out in order to improve the properties of phase change materials. A few techniques discussed involved addition of graphite to enhance the thermal conductivity. Once the enhanced thermally conducting phase change composite (PCC) is obtained, the next step is to conduct a finite element analysis (FEA) to provide a proof of concept for the improvement of performance by comparing with some conventional TES units. This chapter presents a finite element analysis (FEA) study, which is carried out using the commercial software, COMSOL. A working model of the TES unit was built and defined with initial and boundary conditions. After that, meshes were constructed for computation. 3.1 Proposed Phase Change Composite According to the literature review, it is clear that enhancement of thermal conductivity for PCM is in great need and need to be cost effective. There are numerous techniques available to enhance thermal conductivity, such as additions of metal fins, carbon Nano fibers, and graphite [43-48]. In this study, the use of the expanded natural graphite matrix is investigated, considering its many advantages over other heat spreaders. A method developed by Andrew et al. [49] on using high melting phase change materials with a graphite matrix to enhance thermal conductivity of the material is considered. However, the application is targeted at an entirely new field of energy storage. For the purpose of space cooling and from previous literature review on material selection, a commercially available 6 o C melting organic phase change material, called OM06P manufactured by Rgees LLC, North Carolina, is considered. The organic phase change material is impregnated to an expanded natural graphite block. The level of impregnation obtained in the graphite was 74%. The prepared composite has an increased thermal conductivity about times of its original value. Table 22

32 5 compares the properties of the phase change material, and the phase change composite derived from the phase change material. S.no. Parameter PCC Material PCM Material 1. Latent Heat (kj/kg) Thermal Conductivity (W/mK) Density (kg/m 3 ) Table 5. Comparison between PCM and PCC Properties Table 5 shows, a variation in thermal conductivity from W/mK for the PCC material. This variation can be attributed to the structural variation in the graphite matrix. 3.2 Conventional PCM The performance of the PCC TES unit was compared to conventional TES units, such as the ice storage unit and the 6 o C organic PCM (OM06P) storage unit. Ice TES with density of 1000kg/m 3, thermal conductivity ranging from 1.5 W/mK (below 5 o C) to 0.1 W/mK (above 5 o C) was employed, as it had received commercial success in the TES market. The direct variation in thermal conductivity is highlighted by comparing 6 o C PCM TES with PCC TES unit. The properties of the 6 o C organic PCM are listed in Table Finite Element Analysis Model The next step was to design a geometry constructed with, the PCM/ PCC material, to conduct an FEA study of the TES unit. The model for the simulation was created from computer-aided-design (CAD) software, as shown in the Figure 14 and

33 The TES unit design comprised of, pure PCM / PCC slabs with grooves, and a copper serpentine resting on the grooves. The slabs were completely charged (frozen), and the copper serpentine was used in transporting the heat transfer fluid to carry out the necessary charge/discharge for the slabs. Since the cold release/discharge of the TES unit was of primary interest for analysis, an assumption was made that the TES unit was always at 100% state of charge at the beginning of discharge. 4 All Dimensions are in cm Figure 14. 3D Model of PCC Slabs with a two-inch long copper serpentine All units are in cm Figure 15. Two-inch long copper serpentine design 24

34 3.4 Equations for the Model The fundamental equation of the cold storage for the PCC/PCM TES unit is: (kwh) (4) where (kg) is mass of the PCM or PCC material, (J/gK) is the PCM/PCC material s heat transfer coefficient given as a function of temperature, T (K) is the temperature difference between the initial state and final state of the TES unit and λ (J/kg) is the latent capacity of the TES unit. The above equation (4) represents the heat (Q) stored in the PCM or PCC material; where the temperature difference was identified as a major driving force for the reaction. The rate of heat propagation into the material is dependent on and λ. The unit for the above expression is kwh. The major mode of heat transfer through the PCM/PCC material is conduction, and it is expressed by: (W) (5) The above equation represents the rate of heat transfer (q) through the PCM/PCC slab, with T being the difference in temperature between the various points on the PCM/PCC slab, A being the cross sectional area of heat flow and x (m) being the distance between the source point and the destination. Thermal conductivity (W/mK) of the material can be found from the measurement of thermal diffusivity by conducting the laser flash experiment. A plane parallel sample is heated by an energy pulse on one side and temperature rise monitored on the opposite side of the material as a function of time to obtain the thermal diffusivity of the material. From the thermal diffusivity, the thermal conductivity of the material is obtained. From equation (5), it is clear that the rate of heat transferred through the material, is dependent on thermal conductivity of the material, the cross sectional area for heat transfer and the temperature variation between the source and the destination. The improved 25

35 thermal conductivity correlates to improved rate of heat transfer and improved performance of the TES unit. The model uses a constant temperature fluid, which is flowing into the TES unit to be discharged. The heat transfer fluid is an ethylene glycol-water mixture (50-50%). The properties of the heat transfer fluid are found in the appendix. The heat transferred by the fluid (Qeg), is shown by: (kwh) (6) where m eg is the mass flow rate of the refrigerant (kg/sec); C p eg is the specific heat of the refrigerant (J/gK); and T is the temperature difference between the inlet refrigerant temperature entering into the TES unit and the outlet refrigerant temperature (K) flowing out of the TES unit. The final unit is expressed in kwh. The system efficiency is calculated, as shown by equation (7). Efficiency (η) (7) The thermal storage capacity of the PCC/PCM is given by (4) and the heat released by the refrigerant can be calculated by equation (6). Based on these equations, the efficiency of the thermal energy storage unit is calculated. (P/ E) rate = (8) Heat release rate = (9) 26

36 Equation (8) is used to calculate, the average cooling power available from the TES unit, which is the average value obtained by finding the product of mass flow rate, specific heat capacity of the fluid and the temperature difference between the inlet and outlet refrigerant flowing through the TES unit, for the duration of discharge. Dividing the average cooling power to stored capacity, the (P/ E) rate is obtained. The equation (9) shows an expression for the heat release rate by a TES unit, which can be obtained by, dividing equation (6) by the duration for discharge. Based on the above equations, a system can be described. The system analysis will identify the best TES unit. 3.5 Mesh Construction With a 3D design of the TES unit, the geometry built in Solidworks is imported to COMSOL Multiphysics. The initial and final boundary conditions are defined in COMSOL. The input parameters required for the model are defined in the following section. Another important aspect for solving the problem is the mesh construction. A fine tetrahedral mesh was built, with a boundary layer developed for the fluid domain, as illustrated in Figure 16. The size of the mesh affects the accuracy of the solution. In general, a more fine mesh has a higher accuracy solution, than a coarse mesh. The drawback with a fine mesh is longer time for analysis. The total number of mesh elements for the analysis is 985,000 elements, as shown in Figure

37 Figure 16. FEA Meshing 3.6 Input Parameters for Numerical Study The various input parameters that affect performance, and essential for defining the problem are listed below Flow Velocity of Heat Transfer Fluid The flow velocity is an important design parameter. Based on the flow rate of the heat transfer fluid, the discharge rate can be faster or slower, resulting in varying temperature gradient between the inlet and outlet fluid in the TES unit. A range for flow velocity has been investigated and its effect on the flow rate was calibrated experimentally. Table 6 shows the values for flow velocity. Flow rate for Flow rate for Flow rate for S.no. Flow Description PCC TES PCM TES Ice TES (GPH) (GPH) (GPH) 1. Low Medium

38 Cp (J/gK) 3. High Table 6. Flow rates for the experimental and simulation tests The flow velocity range selected above can be divided into three ranges, as low, medium, and high values. The flow values for a particular range are observed to be different for each TES unit, due to varying thermal heat capacity values of the material. The ranges for the velocity were influenced by the manual flow meter used in the experimental set up. The flow meter has a limited range of flow detection, from 0.1 Lpm to 2.2 Lpm (1.5 GPH to 34 GPH). Due to the limitation of the flow meter, the flow ranges were restricted to the above values Specific Heat of the Material The specific heat of a material is defined as the amount of energy (J), needed for one gram of a substance to raise its temperature by 1 o C. The DSC (Differential Scanning Calorimetry) analysis of the material provides the Cp (J/g o C) as a function of temperature, the details of the experiment, and the instrument used are provided in the research conducted by Ben Schweitzer et al. [50] Temperature (K) Figure 17. Cp as a function of temperature for PCC 29

39 Cp (J/gC) Figure 17 shows the specific heat as a function of temperature for the PCC material. The curve has a peak value of Cp at temperature close to 279 K, and the value for the peak is 120 (J/g K). The Cp (J/g K) has been input as a function of temperature, in the FEA model Temperature (C) Figure 18. Cp as a function of temperature for ice Figure 18 shows the Cp curve as a function of temperature for the ice. The curve has a peak value of 170(J/gK) at K, which is close to the melting temperature of ice. 30

40 Cp (J/gC) Temperature (K) Figure 19. Cp as a function of temperature for PCM Figure 19 depicts the Cp (J/gK) for the 6 o C organic PCM as a function of temperature and the peak value for Cp is 104 (J/gK) at 279.5K Thermal Conductivity of PCC/PCM The thermal conductivity (k) of the material is the ability of a material to conduct heat and is defined by the Fourier s Law of heat transfer. The thermal conductivity (k) is determined from the laser flash experiment, where the thermal diffusivity, α (m 2 /s) is obtained, defined as the material s ability to conduct thermal energy, relative to its ability to store thermal energy. From thermal diffusivity, thermal conductivity is obtained by equation (10) as follows: (10) From the experiment, two values of thermal conductivity were obtained for the PCC material as shown in Table 7. 31

41 S.no. Thermal Conductivity (W/mK) Orientation Material In-plane PCC 2. 9 Through-plane PCC Isotropic PCM Isotropic Ice Table 7. Thermal conductivity of materials Density of PCC/PCM The density of a phase change material is temperature dependent. Generally phase change materials show a 10% variation in volume, when they change from one phase to another. However, for the simulation a constant value for density was assumed, for the material. Table 8 shows the density of the various materials. S.no. Density (Kg/m 3 ) Temperature ( o C) Material PCC PCC PCM PCM Ice Ice Table 8 Density of materials 32

42 3.6.5 Properties of Ethylene Glycol The properties of ethylene glycol are input, as a function of temperature; namely the dynamic viscosity (Pa/sec), density (kg/m3), specific heat (J/kgK), and thermal conductivity (W/mK). (Refer Appendix for property) Temperature Range The temperature for each process varies from one material to the other as shown in Table 9. The PCC TES unit requires a higher temperature for charging, when compared to the ice TES unit and the 6 o C PCM TES unit. The discharge temperature for all the TES units is 15 o C. (Refer Appendix). S.no. Process Temperature ( o C) 1. Charging for ice TES Charging for PCM TES Charging for PCC TES Discharging for TESs End of discharge when outlet fluid reached 13 Table 9. Temperature range for experimental and simulation tests The reason for the low temperatures to charge ice TES and PCM TES, were due to their ability to undergo super cooling. The PCC material does not show super cooling, hence a temperature of -2 o C was chosen. 33

43 3.7 Simulation Assumptions The key assumptions in the model were: (1) The TES unit is an adiabatic system. (2) The thermocouples are located as shown in figure 26 in the layer between the two slabs. (3) Flow is continuous and laminar from the start of the discharge. (4) There is no variation in density for the phase change materials. (5) The change in volume after phase change is not accounted. With these assumptions, the model is all ready to be solved. The following sections presents details of the experimental tests and simulation results. 34

44 Chapter 4 Prototype Setup This chapter presents the fabrication of the TES units. An experimental test setup was built with various components to measure the performance of the TES units, in order to identify the best TES unit. 4.1 Phase Change Materials The materials for preparing the PCM are listed in Table 10. S.no. Material Procurement 1. Water Water is available from household line o C PCM (OM06P), Manufactured by Rgees LLC, North Carolina o C PCC Table 10. Raw material for TES unit Graphite blocks provided by AllCell Technologies, 6 o PCM- Rgees LLC. Water required for the ice TES unit, is collected from household water lines. The 6 o C PCM, is commercially called OM06P, it is obtained from a PCM manufacturing company- Rgees LLC. The 6 o C PCC is an expanded natural graphite block containing the 6 o C PCM material. The graphite blocks obtained for the PCC fabrication, were from AllCell Technologies. The 6 o C PCC material is fabricated by soaking the graphite block, in a bath containing the 6 o C PCM. The soaking is carried out until the PCM content reaches 74% by weight, in the graphite block. The properties of the materials are listed in Tables 4, 5, 6, and 7. 35

45 4.2 Prototype Setup for the TES Units The testing methodology and the test setup for the TES units are both described. The result from testing of these different TES units gives an overview on factors, which influence performance, and identifies the best performing TES unit. The test setup for the TES units is described below. The test setup for the TES prototype comprises of various functional components integrated, a schematic of the test setup is provided for easy understanding of the different components, with their functionality. The schematic of the TES test setup is illustrated in Figure 20. The components are numbered to explain the modes of operation for the test set up. 3-Port PVC Ball Valve 3 Tubing PCC Slabs with Copper Serpentine Thermal Insulation 5 6 Panel-Mount Flowmeter with Flow-Control Chiller Figure 20. TES experimental setup schematic Table 11 lists the functionality of the various components listed in Figure

46 S.no. Component Functionality 1 Thermal energy storage unit 2 Chiller unit with an in-built Pump Thermal energy storage fabricated from PCM/PCC. To maintain constant flow and controlled temperature of the heat transfer fluid during charge/ discharge. 3 Flow meter To measure the flow velocity of the heat transfer fluid. 4 Two Way three position valve 5 Connecting tubes/tubing To direct and control the flow of the fluid in the experimental set up. To connect various components of the TES test setup. 6 T type thermocouples To measure temperature at various points of the PCM/PCC and the inlet and outlet fluid, through the TES unit. 7 Data logger To log the data from the thermocouples. 8 Ethylene Glycol water 50-50% mixture Table 11. Components of the TES unit Refrigerant fluid that causes charging/ discharging in the TES unit. A more detailed description of the components is provided in the sections that follow. 4.3 Chiller As listed in Table 11, the chiller unit is manufactured by JULABO, the F25 ME chiller unit; functions to provide a continuous temperature controlled heat transfer fluid to the TES unit; to carry out the necessary charge/discharge, to the TES unit. The temperatures for charge/discharge are listed in Table 9. The chiller is filled with glycol water (50-50% mix). The chiller unit contains an inbuilt pump, which can supply the fluid to the TES unit, at a flow rate from 11 to 16 liters per minute. The temperature range capable by the chiller is from -28 o C to 200 o C. The chiller can be programmed to run at a particular flow rate, with a varying time dependent temperature settings. The chiller unit is connected to the TES unit by means of rubber tubing of diameter 3/8. 37

47 The flow monitoring is essential for the TES unit. A digital flow meter, the Digiflow 6710 manufactured by Savant, was employed. It comprises of a paddle wheel that rotates to show fluid velocity and records the total volumetric flow occurring, with an accuracy of ± 10%. The flow of liters per minute is controlled by a manual flow meter mounted in the circuit, which has flow controlling capabilities, as shown in the schematic. The flow meter was manufactured by King instruments which has a flow controlling range of 0-30 gallons per hour. With the manual flow meter, the flow velocity in the circuit was controlled. The flow velocity reading was taken from the digital flow meter. Figure 21. Julabo chiller F25ME 4.4 Phase Change Materials From Chapter 3, the properties of the PCM materials employed for the TES unit have been mentioned. This section describes details on the TES unit fabrication, and methods employed for containing the liquid PCM in the TES unit. Table 12 provides details on the amount of material contributing towards the building of each TES unit. 38

48 S.no Material Amount of material (kg) 1. Water o C PCM o C PCC 2.42 Table 12. Quantity of PCM/PCC content for TES unit Figure 22. PCM/ICE TES prototype Figure 22 illustrates an apparatus made from a plastic box, which holds the copper serpentine in place, similar to the CAD design, for containing the pure PCM. The PCC TES unit as it is not in a liquid state, it does not require any apparatus to contain the material, as shown in Figure 23. The TES units require very good insulation, the insulation for the thermal storage units were provided by standard R 4.0, of thickness ½ and fiber wool material. The chiller was connected to the TES unit by rubber tubing of diameter 3/8. These lines were insulated by standard piping insulation. 39

49 Figure 23. PCC TES prototype 4.5 Copper Serpentine A copper serpentine was built, based on the CAD design with a pitch of 2 inches, internal and external diameter of inches, and inches. The total center tube length was 200cm. The weight of the copper serpentine was 0.38kg. The reason for choosing a two-inch long pitch for the copper serpentine is that the standard heat exchangers employ 1.5 to 2 inches. One of the factors influencing the performance of the TES unit is the pitch of the copper serpentine. Figure 24 illustrates the copper serpentine design. Figure 23 shows the copper serpentine used for the experiment. The influence of a lower pitch is considered in Chapter 6. 40

50 All units are in cm 32 Figure 24. Copper serpentine Inlet Outlet Figure 25. Thermocouple map 41

51 4.6 Thermocouples To monitor the temperature profiles during the discharge of the TES unit, thermocouples were employed. The thermocouple employed for the measurement of temperature was a T type thermocouple. A data logger manufactured by Yokogowa, GP 20 was used. The data logging interval was set at five seconds. Figure 25 shows the location of the thermocouples at different points on the PCC or PCM TES unit, with two thermocouples measuring the temperature of the inlet and outlet flowing heat transfer fluid. 4.7 Working of the System The TES unit test setup has two modes of operation. In these modes of operation, the heat transfer fluid circulates in a closed loop, either to the chiller or to the TES unit from the chiller. The modes of operation are described below. The modes of operation are implemented by the use of three way two position valves, which directs the flow to or away from the TES unit Pre Charge/Discharge Mode 6 inlet 4 6 outlet 2 1 Figure 26. Representation of the pre charge/discharge mode 42

52 The pre charge/discharge mode is the first process in the cycle. Figure 26, illustrates the path of the heat transfer fluid. The numbering in Figure 26 refers to the components, of the schematic test set up as shown in Figure 20. The current mode operates, to bring the heat transfer to a fixed temperature. The pre charge temperature is -2 o C, while the pre discharge temperature is 15 o C. This mode brings the heat transfer fluid in the chiller to a steady temperature and maintains the temperature. The chiller has an inbuilt condenser and a heating unit, to maintain the temperature of the heat transfer fluid. In this mode, the fluid is brought to a set point temperature, and until it reaches a set point temperature, it is made to circulate within a closed circuit only to the chiller Charge/Discharge Mode 6 inlet outlet Figure 27. Representation of the charge / discharge mode The charge/discharge mode is the second mode of operation. The objective of this mode is to carry out charge/discharge to the TES unit. The pre charged/discharged fluid flows from the chiller to the TES unit and carries out the necessary charge/discharge, the end of each process is marked by temperature readings of the thermocouples. The end of charge is marked by the TES unit reaching the end of charge temperatures, shown in Table 9 and the end of discharge is marked by the outlet fluid temperature 43

53 reaching 13 o C. Figure 27 shows the path taken by the heat transfer fluid. The numbering in Figure 27 represents the components from the TES schematic test setup, shown by Figure 20. Having outlined the TES test setup, results from the finite element analysis and the experimental testing are discussed and compared in Chapter 5. 44

54 Chapter 5 Results and Discussions In this Chapter, the TES unit s performance, both the experimental results and simulated results from FEA are presented. Important factors that affect performance are identified, thereby allowing design optimization of the TES units. 5.1 Results from Finite Element Analysis The FEA was carried out to observe the PCC TES unit s performance. The performance of the PCC TES unit was compared with the performance of the ice TES unit and 6 o C PCM TES unit. The factors that affect performance were identified. This aided to design an optimal TES unit Effect of the TES Units at Low Flow Rates The low flow rates provide the highest heat transfer for the TES units. Thus, easy comparison of the TES units is enabled. The observations at the low flow rates are shown in the sections that follow PCC TES unit The PCC TES unit s performance is shown in Figure 28. The low flow rate used for the simulation was 1.86 GPH. 45

55 Temperature (C) Inlet Outlet PCC1 PCC5 PCC3 PCC2 PCC Time (sec) Figure 28. PCC TES at low flow rates simulated Figure 28 identifies the performance of an adiabatic PCC TES unit. The discharge profile shows a temperature difference of less than 2 o C between the outlet fluid and the thermocouple located at PCC 4. Until about 1000 seconds, the melting profile or the discharge profile of the PCC TES unit was in the sensible region. Then from seconds, the PCC TES unit was in the latent region. From 5000 seconds onwards, it has completely melted and any more heat addition follows the sensible heat addition. The results for the PCC TES unit are summarized in Table

56 Parameter PCC TES Efficiency (%) 55 Heat release rate (kwh/min) (P/E) Rate (kw/kwh) 0.59 Duration of discharge (sec) 6600 Power density (kw/l) Energy density (kwh/l) 0.03 Flow rate (GPH) 1.86 Table 13. Simulated PCC TES at low flow rates As shown in Table 13, the duration for discharge is 6600 seconds, with an efficiency of 55%, a P/E rate of 0.59/hr. and a power density of kw/l. The outlet fluid flowing from the TES reaches 3 C during the discharge PCM TES unit The 6 o C PCM TES unit s discharge was carried out at a low flow rate of 2.24 GPH. The discharge profile for the PCM TES unit is illustrated in Figure 29. The performance of the PCM TES unit was compared to the performance of the PCC TES unit at low flow rate. 47

57 Temperature (C) Inlet Outlet PCM#1 PCM#5 PCM#3 PCM# Time (sec) Figure 29. PCM TES at low flow rates simulated The duration for the discharge is 16,800 sec; however, the initial sensible heat addition in the TES lasted for almost 6000 seconds, followed by latent heat addition until 19,800 seconds. The duration for discharge is 2.5 times longer; the cooling energy and the power density of the PCM TES unit are much lower when compared to the PCC TES unit. Table 14 provides the performance of the PCM TES unit. The PCM TES unit has 36% efficiency; a (P/ E) rate of 0.10/hr. and the cooling power density is 0.007kW/L. When comparing with PCC TES unit, the PCC TES had 61% higher cooling power density. Thus, graphite addition in the PCM has been justified. Table 14 summarizes the performance of the PCM TES unit. 48

58 Parameter PCM TES Efficiency (%) 36 Heat release rate (kwh/min) (P/E) Rate (kw/kwh) 0.10 Duration of discharge (sec) Power density (kw/l) Energy density (kwh/l) Flow rate (GPH) 2.24 Table 14. Simulated PCM TES at Low flow rates Ice TES unit The discharge profile for the ice TES unit is shown in Figure 30.The low flow rate chosen for the ice TES unit was 4 GPH. The performance of the ice TES unit was compared to the PCC and PCM TES units. 49

59 Temperature (C) Inlet Outlet Ice#1 Ice#5 Ice#3 Ice# Time (sec) Figure 30 Ice TES at low flow rates simulated From Figure 30, the performance of the ice TES unit is shown. The duration of discharge for the ice TES unit is almost seconds. The duration for discharge is 2.7 times longer, than that of the PCC TES unit. The initial sensible heat addition to the ice TES unit lasts for almost 6000 seconds, with latent heat addition until 16,000 seconds. The ice TES unit has an efficiency of 52%, a (P/ E) rate of 0.105/hr. and a power density of 0.011kW/L. The cooling power and the power density of the ice TES unit are much lower compared to the PCC TES unit. This can be owed to the higher thermal conductivity of the PCC TES unit. The PCC TES unit had 38% higher cooling density, compared to the ice TES unit. Ice TES unit has 36% higher cooling power density, compared to the PCM TES unit. Table 15 lists the performance of the ice TES unit. 50

60 Parameter Ice TES Efficiency (%) 52 Heat release rate (kwh/min) (P/E) Rate (kw/kwh) Duration of discharge (sec) Power density (kw/l) Energy density (kwh/l) Flow rate (GPH) 4 Table 15. Simulated ice TES unit at low flow rates Thus amongst the three TES units at low flow rates, the PCC TES unit has the best performance in comparison to the PCM TES and the ice TES units. The performance of the PCC TES unit indicated a significant improvement in the thermal conductivity Effect of TES Units at High Flow Rates After determining the performance of the PCC TES unit, at low flow rates, as the best performing TES unit, the current section evaluates PCC TES unit s performance at high flow rates, with a performance comparison of the ice and the PCM TES units at high flow rates PCC TES The flow rate chosen for the PCC TES unit was 4.86 GPH. Figure 31, shows the discharge profile for the TES unit. 51

61 Temperature (C) Inlet Outlet PCC#1 pcc#5 pcc#3 pcc#2 pccc# Time (sec) Figure 31. PCC TES at higher flow rates simulated The duration for discharge is reduced by half for the high flow rate, when compared to the duration of discharge at low flow rate, for the PCC TES unit. The initial sensible heat addition is for 350 seconds, followed by the latent heat addition for 2600 seconds. It is clear that the temperature difference between the outlet temperature of the heat transfer fluid from the TES unit, and the thermocouple located at PCC 5 is higher, compared to the TES unit s discharge profile at low flow rates. Table 16, shows the TES unit s performance. The power density was increased by almost two times due to the high flow rate, when comparing to the TES unit s performance at low flow rate. 52

62 Parameter PCC TES Efficiency (%) 56 Heat release rate (kwh/min) (P/E) Rate (kw/kwh) 0.71 Duration of discharge (sec) 3344 Power Density (kw/l) 0.03 Energy Density (kwh/l) Flow rate (GPH) 4.65 Table 16. Simulated PCC TES at higher flow rate The results from Table 16 shows, the TES unit has an efficiency of 56%, a (P/ E) rate of 0.71/hr. and a power density of 0.03kW/L. The performance with PCM and ice TES are shown in the section that follows PCM TES The PCM TES unit had a flow rate value of 5.61 GPH, based on the thermal capacity of the material. Figure 32 illustrates the discharge profile for the PCM TES unit. 53

63 Temperature (C) Inlet Outlet PCM1 PCM3 PCM Time (sec) Figure 32. PCM TES at higher flow rate simulated The temperature difference between the outlet fluid and the thermocouple located at PCM 5 is higher compared to the PCM TES unit at the low flow rate. The duration for the sensible heat addition to the PCM TES is for 1700 seconds, followed by the latent heat addition for 17,095 seconds. Table 17 summarizes the performance of the TES unit. 54

64 Parameter PCM TES Efficiency (%) 32 Heat release rate (kwh/min) (P/E) Rate (kw/kwh) 0.16 Duration of discharge (sec) 7200 Power Density (kw/l) Energy Density (kwh/l) Flow rate (GPH) 5.60 Table 17. Simulated PCM TES at higher flow rate The PCM TES unit has an efficiency of 32%, a (P/E) Rate of 0.16/hr. The PCC TES unit had a higher cooling power density when compared to the PCM TES unit by 64%. The duration of discharge for the PCM TES unit is two times that of the PCC TES unit. Thus, the effect of the improved thermal conductivity is significant by the performance of the PCC TES unit Ice TES The high flow rate value for the ice TES was 10 GPH, based on the thermal capacity for the material. Figure 33, illustrates the discharge profile for the ice TES unit. 55

65 Temperature (C) Inlet Outlet Water1 Water3 Water Time (sec) Figure 33 Ice TES unit at higher flow rate simulated The ice TES unit has a discharge profile as shown in the Figure 33. The duration of discharge is 8400 sec; with up to 2000 seconds of the sensible heat addition, followed by seconds of latent heat addition in the ice TES unit. The ice TES unit has an efficiency of 45%, a (P/E) Rate of 0.171/hr. and a cooling power density was kw/l. The duration for discharge is 3.1 times longer, compared to the PCC TES unit. The cooling power density for the PCC TES unit was 68% higher. Table 18 summarizes the performance of the ice TES unit: 56

66 Parameter Ice TES Efficiency (%) 45 Heat release rate (kwh/min) (P/E) Rate (kw/kwh) Duration of discharge (sec) 8400 Power Density (kw/l) Energy Density (kwh/l) Flow rate (GPH) 10 Table 18 Simulated ice TES unit at higher flow rates It is clear that the PCC TES unit was once again the best performing TES unit, when compared to the ice TES unit at high flow rates. The next section discusses the experimental results. 5.2 Experimental Testing of the TES Units The current section identifies the performance of the TES units experimentally, at different flow rates. The section also identified the true performance of the TES units, and helps in revising the FEA model to incorporate any factors that may affect performance, which were earlier neglected Results from a High Discharge Rate The section shows the discharge performance of the TES units at high flow rates of the heat transfer fluid. Refer Table 5 for the values of the flow rate being considered PCC TES The performance of the PCC TES unit at the highest flow rate was considered. The flow rate for the PCC TES unit considered is GPH. 57

67 Temperature (C) PCC#1 PCC#2 PCC#3 INLET PCC#5 OUTLET Time (sec) Figure 34. Performance of the PCC TES unit at high flow rates Figure 34 illustrates the experimental discharge profile, for a flow rate of 12.5 GPH, the thermocouples on the PCC TES unit are depicted as PCC#1- PCC#5, as shown in Figure 25. The cooling profile at the high flow rate is not significant, as the temperature gradient between the inlet and outlet is very small. The heat transfer fluid reaches a temperature of 13.5 o C, which marks the end of discharge. The duration of discharge is for 1.3 hrs. The melting profile for the PCC TES unit shows that the sensible heat addition occurs until 750seconds, followed by the latent heat addition until 5300 seconds. The duration for discharge at the highest flow rate is significantly longer than the simulated cases considered in section 5.1. Hence, the FEA model assumptions need to be reviewed, to validate the experimental performance of the TES unit. The PCC TES unit shows 54% efficiency, a (P/E) rate of 0.42/hr. The low precision flow meter along with the inconsistent flow results in an inaccuracy having 40% error in the 58

68 Temperature (C) calculated performance. A solution to this can be found by simulating the experimental performance of the TES unit, by revising the FEA model Ice TES The ice TES unit has a flow rate of 26.8 GPH, based on the thermal capacity of the TES unit. Ice#5 outlet inlet Ice#1 Ice# Time (sec) Figure 35. Performance of ice TES at high flow rates Figure 35 shows the discharge profile for the ice TES unit. The duration for discharge is very short, and the temperature gradient between the inlet and the outlet fluid suggests that there is no cooling obtained from the TES unit. The duration of the experiment was close to 12 mins. The reason for the poor cooling performance of the TES unit is due to the low thermal conductivity of ice and the high flow rate used for experimental analysis. The ice TES unit shows 2000 seconds of sensible heat addition followed by 11,000 seconds of latent heat addition. As there is no cooling obtained, the system performance cannot be identified. 59

69 Temperature (C) PCM TES The PCM TES unit has an equivalent flow rate of GPH. The performance of the TES is as shown below: 20 PCM 5 C Outlet C Inlet C PCM 1 C PCM 3 C Time (sec) Figure 36. PCM TES at high flow rate Figure 36 shows the discharge profile for the PCM TES unit. The duration of discharge is close to 11mins, and there was no cooling obtained from the TES unit. Table 19 shows the comparative performance of the TES units at high flow rates. The high flow rates selected for the experiment were not feasible for the study, as observed from the zero cooling in the PCM and the ice TES units. Thus, the high flow rate will be omitted from further analysis. The experiment at high flow rate suggests that the PCC TES unit has the best performance. However, further studies at low flow rates need to be carried out. 60

70 S.no. Property Ice TES PCM TES PCC TES 1. Flow rate (GPH) Stored Capacity (kj) Duration of useful cooling (sec) Efficiency (%) na% (cut off 13.5 o C) na% (cutoff 13.5 o C) 54% (cutoff T 13.5 o C) 5. Maximum del T between inlet and outlet 1 o C 1.0 o C 1.8 o C Table 19. Comparison of TES units at high flow rate Results from a Medium Discharge Rate This section identifies the performance of the PCC TES units, at a medium flow rate and compares with the conventional TES units, to identify the best TES unit. From the previous section, at high flow rates, the PCC TES unit had outperformed the ice and the PCM TES units; Table 5 lists the values of the flow rate PCC TES The medium flow rate of 4.8 GPH was considered for the PCC TES unit. The discharge profile is as shown in Figure

71 Temperature (C) Inlet Outlet pcc1 pcc3 pcc Time (sec) Figure 37. PCC TES at medium flow rate The duration for discharge was 1.86 hrs. The results revealed an efficiency of 71%, a (P/E) rate of 0.37/hr. The discharge profile suggested that the cooling power is lowered, with an increased duration for discharge. The calculated system performance cannot be taken as a true measure of performance due to the low precision flow meter; resulting in error (%) of ± 40% based on flow rate chosen. The duration of discharge is increased by two times compared to the FEA results at similar flow. Thus, FEA assumptions have to be reviewed to validate with the experimental performance Ice TES The ice TES unit for the medium flow has a flow velocity of 12.7 GPH. The performance of the TES unit is shown in Figure

72 Temperature (C) 20 Ice#5 Outlet Inlet Ice#1 Ice# Time (sec) Figure 38. Ice TES at medium flow rate The duration of discharge of the ice TES, is longer, and the cooling performance much lower in comparison to the PCC TES unit. The reason for this trend is attributed to the lower value of thermal conductivity of the ice TES unit. The duration of discharge lasted for 4 hr. The efficiency of the TES unit is 61% and the (P/ E) rate is 0.15 /hr. The calculated results, as mentioned before, have some inaccuracy in them, which is corrected by reviewing the assumptions of the FEA model and validating with the experimental results PCM TES The equivalent flow rate of 6.34 GPH, for the PCM TES unit is shown in the section. Based on the flow rate, the TES performance is as shown in Figure

73 Temperature C 20 PCM 5 C Outlet C Inlet C PCM 1 C PCM 3 C Time (s) Figure 39. PCM TES at medium flow rate Property Ice TES PCM TES PCC TES Flow rate (GPH) Stored Capacity (kj) Duration (sec) Effective Heat utilization Maximum del T between inlet and outlet 56% (cut off 13.5 C) (+/- 40%) 23% (cutoff 13.4C) (+/- 40%) 71% ( cutoff T 13.5C) (+/-40%) 2 C 1.5 C 3.5 C (P/E) Rate (kw/kwh) Heat release rate (kwh/min) Table 20. Comparison of TES units at medium flow rates 64

74 The duration of discharge is close to 1.5hrs. The efficiency of the system is 23% and the (P/ E) rate is 0.23/hr. The system performances calculated from the experiment are further validated by re-developing our FEA model, to match the experimental results. Table 20 however shows an estimate of the performance of the various TES units. Table 20 shows, the highest efficiency for the PCC TES unit, followed by ice and PCM TES units. The heat release rates of the PCM TES unit and the ice TES unit are higher than the PCC TES unit, due to the high temperature difference between, the TES unit s initial temperature at start of discharge, and the inlet fluid s temperature. The (P/ E) rate was highest for the PCC TES, followed by PCM TES and then the ice TES units, due to the improved thermal conductivity of the PCC material. The low precision flow meter results in a range of ± 40% error in performance numbers, based on the flow value assumed. Thus, a more accurate performance of the TES units must be obtained by validating the experimental with a revised FEA model Results from a Low Discharge Rate This section evaluates the performance of the TES units at low flow rates. Table 5 lists values used for flow of the heat transfer fluid PCC TES This section shows the performance for a low flow rate discharge for the PCC TES unit. The low flow rate for the PCC TES unit considered was 1.86 GPH. 65

75 Temperature (C) 20 Inlet Outlet PCC#1 PCC#3 PCC# Time (sec) Figure 40. PCC TES at low flow rate From Figure 40, the duration of discharge is 10,700 seconds. The performance of the TES unit shows that the efficiency is 75% and the (P/ E) rate is 0.3/hr. The performance comparison of the experiment TES unit with the simulation, shows the experiment s discharge duration is twice as long, and the (P/ E) rate is half the value of the simulation. Thus, further analysis to validate the experiment with the FEA results must be carried out Ice TES The ice TES unit at lower flow rate has a flow value of 3.38 GPH, based on the thermal capacity. The experimental performance of the TES unit is as shown in Figure

76 Temperature (C) Ice#5 C Outlet C Inlet C Ice#3 C Ice#4 C Time (mins) Figure 41. Ice TES at low flow rate Figure 41 shows, the performance of the ice TES unit. The TES unit has an efficiency of 44% and a (P/ E) rate of 0.14/hr. The duration for discharge is close to 16,200 sec. When comparing the performance with the PCC TES unit, the (P/ E) rate is halved for the ice TES unit. The simulation shows a good resemblance to the true behavior of the TES unit experimentally PCM TES The PCM TES unit has a low flow rate of 2.24 GPH. The discharge performance of the TES unit at the low flow rate is shown in Figure

77 Temperature (C) PCM#4 Outlet Inlet PCM# Time(sec) Figure 42. PCM TES at Low flow rate The duration for discharge is seconds. The efficiency of the TES unit is 42%, with a (P/ E) rate of 0.11 /hr. Table 21 summarizes the results from the experiment conducted at low flow rates. The best performance was observed from the PCC TES unit; the (P/ E) rate of the PCC TES unit is two times the value of the ice and PCM TES units. The PCC TES unit has 1.5 times quicker discharge duration compared to the ice TES unit. The low precision flow meter results in a ± 40% error range, in performance parameters. Thus, the true performance of the TES units must be obtained by validating the experimental with a revised FEA model. 68

78 Property Ice TES PCM TES PCC TES Flow rate (GPH) Stored Capacity (kj) Duration (sec) 16,200 12,060 10,666 Efficiency (%) Maximum del T between inlet and outlet ( o C) 44% (13.5 o C cut-off temperature) (+/- 40%) 42% (13.5 o C cutoff temperature) (+/- 40%) 75% (13.5 o C cutoff temperature) (+/-40%) 3.1 o C 2.3 o C 5.8 o C (P/E) Rate (kw/kwh) 0.14(+/-) 0.11(+/-) 0.3(+/-) Heat Release rate (kwh/min) Table 21. Comparison of TES units at low flow rates Thus from this section, the following can be concluded: 1. Improved thermal conductivity in a TES unit improves the performance and reduces the duration of the charge/discharge cycle. 2. Heat transfer was much effective at lower flow rates. 3. Amongst the three TES units, it was observed that the PCC TES unit is the best performing followed by the ice TES unit. 4. The average cooling power to stored capacity or the (P/ E) rate was consistently high for the PCC TES unit. 69

79 One of the major limitations of the experimental setup was the low precision flow meter. The resultant efficiency calculated had an error percentage of (+/-40%). Thus to identify true performance of the system, the FEA is validated with the experimental results. 5.3 Validation of Experimental Results with FEA model This section identifies a revised FEA model of the TES units. The main objective of this section is to develop a working FEA model, which validated the experimental results. The model aids in understanding the true performance of the TES units. Factors that affect performance can be identified; in order to implement design optimization Low Flow Rates Validation for the TES Units This section identifies a FEA model that validates the ice, PCM and the PCC TES units at low flow rates Ice TES unit This section shows a FEA model that validates the ice TES unit at low flow rates. Figure 43 shows the discharge profile. The experimental and simulation profiles of ice TES unit were overlaid, as shown in Figure 43. The discontinuous lines represent the experimental curve, and the smooth solid lines represent the simulation curve. Variations in performance were a result of inconsistent flow, disturbances in the early data collection. The average error (%) when comparing simulated results with the experimental results showed an 8% error. However as the simulation replicated the experimental curve, the results can be used to estimate the efficiency and the system performance. Table 22 summarizes the results of overlaying the simulation and the experimental curves. 70

80 Temperature (C) Outlet Inlet Ice#1 Ice#4 Inlet Outlet Ice#1 Ice# Time Time (mins.) (sec) Figure 43. Experimental validation for ice TES at low flow rate Parameter Experiment Simulation Error (%) Outlet Temperature ( o C) ICE Temperature ( o C) Duration of Phase Change (mins) Table 22. Comparison of experiment and simulation for ice TES at low flow rate Having validated the ice TES unit, the system performance is found. The results are tabulated at the end of the section. 71

81 Temperature C PCM TES This section validates the experimental results with a FEA model for the PCM TES unit. The experimental and simulation curves are overlaid one on top of the other, as shown in Figure 44. The solid lines in Figure 44 represent the simulated curves; the discontinuous lines represent the experimental curves. 20 PCM#4 Outlet Inlet PCM#3 Inlet Outlet PCM#1 PCM# Time (sec) Figure 44. Experimental validation for PCM TES at low flow rate Parameter Experiment Simulation Error (%) Outlet temperature ( o C) PCM Temperature ( o C) Duration of Phase change (sec) Table 23. Comparison of experiment and simulation for PCM TES at low flow rate 72

82 Temperature (C) Table 23 provides a comparison of the simulated and the experimental results, when the curves are overlaid one on top of the other. It shows that the simulation has an error of 7 %. The error can be attributed to the low-level precision of the flow meter. The model predicts the behavior of the phase change and the exit temperature of the glycol to a good extent. Based on the validated model, the efficiency and other system parameters were found. The system performance is tabulated at the end of the section, comparing the ice and the PCC TES units PCC TES The section validates the experimental results with a refined FEA model for the PCC TES unit. The simulated curve overlaid to validate the experimental curve can be shown below. The solid lines represent the simulated curve, while the discontinuous lines represent the experimental curve. 20 Inlet Outlet PCC#4 PCC#1 Inlet Simulated Outlet Simulated PCC1 Simulated PCC3 Simulated PCC5 simulated Time (sec) Figure 45. Experimental validation for PCC TES at low flow rates 73

83 Figure 45 shows the simulated curve overlaid on the experimental curve. Table 24 shows the comparison of overlaying the simulation curve on the experimental curve. Parameter Experiment Simulation Error (%) Outlet temperature ( o C) PCC Temperature ( o C) Duration of Phase change (sec) Table 24. Comparison of experiment and simulation for PCC TES at low flow rate The error (%) between the simulated and the experimental results are under 5%, this shows that the model has estimated the true performance of the TES unit to a good extent. The reasons for the error can be attributed to a varying air gap between the copper and the PCC material. Numerous values were chosen for the air gap adjustment, to validate the simulation and the experimental case. The most successful simulation was achieved with an air gap of 0.1mm Summary of Results Table 25 provides a summary on the performance of the TES units from the validated FEA model. The efficiency for the three TES units are shown, with the highest efficiency for the PCC TES unit, followed by the ice TES and the PCM TES units. Efficiency is directly proportional to the cooling performance; the highest cooling performance is from the PCC TES unit. Another measure, for performance is the (P/ E) rate. The highest cooling power is delivered by the PCC TES unit. The PCC TES showed a 22% increase in efficiency, compared to the PCM TES unit. The reason PCM TES unit has the lowest efficiency, can be attributed to the low thermal conductivity. The ice TES unit due to its low thermal conductivity performs only with an efficiency of 50%. 74

84 Parameter Ice TES PCM TES PCC TES Energy storage (kwh) Heat Release Rate (kwh/min) (P/E) Rate (kw/kwh) Efficiency (%) Power density (kw/l) Energy density (kwh/l) Table 25. Comparison of TES units at low flow rate from simulation Thus, the effect of improved thermal conductivity by graphite addition is justified based on the performance of the TES units. However, the PCC TES unit has losses present due to the layer of air between the serpentine and the PCC slab, which must also be accounted for improving system performance Medium Flow Rates Validation for the TES Units This section is to validate the experimental performance of the TES units, at the medium flow rates with a FEA model. The FEA model is reviewed, to predict the actual system performance Ice TES This section is to validate the ice TES unit s experimental performance at the medium flow rate. The experimental performance of the ice TES is compared with the results from a revised FEA model; the validated model will be used for further analysis. 75

85 Temperature (C) 20 Water#5 Outlet Inlet Water#1 Inlet Outlet Water1 Water Time (sec) Figure 46. Experimental validation for ice TES at medium flow rate Figure 46 shows the experimental curve overlaid on the simulation curve for the ice TES unit, at the medium flow rate. At higher flow rates, the accuracy on the reading of the digital flow meter is more dependable, due to the continuous and steady flow. The table 26 compares the experimental and simulated results. Parameter Experiment Simulation Error (%) Outlet temperature ( o C) PCM Temperature ( o C) Duration of Phase change (min) Table 26. Comparison of experiment and simulation for ice TES unit at medium flow rate 76

86 Temperature (C) Results from Table 26 show there is a good and close correlation between the experimental and the simulation results. The average error (%) was found to be as low as 3%. The system performance is tabulated, and compared with other TES units at the end of the section PCM TES This section provides details on the validation of the simulated PCM TES unit s performance, with the experimental result at the medium flow rate. The validation will result in finding the true performance of the TES unit, and further optimize the TES unit. 20 PCM 5 C Outlet C Inlet C PCM 3 C Inlet Outlet PCM1 PCM Time (sec) Figure 47. Experimental validation for PCM TES at medium flow rate Figure 47 shows the PCM TES unit s FEA results overlaid on the experimental results and Table 27 lists the comparison between experimental and simulation results. 77

87 Parameter Experiment Simulation Error (%) Outlet Temperature of glycol ( o C) PCC Temperature ( o C) Duration for phase change (sec) Table 27. Comparison of experimental and simulation validation Table 27 shows there is an average of 5% error, when comparing the experiment and the simulation results; the error can be attributed to the low precision flow meter. The system performance is tabulated at the end of the section, to compare with the ice and the PCC TES units PCC TES The section shows the refined FEA model, which validates the experimental performance of the PCC TES unit at high flow rates. At higher flow rates, the comparison between the experimental and the simulation show a reduced error percentage. This reduction in error percentage can be once again attributed to the steady flow. The overlaid curves of the experiment and the simulation, at higher flow rate are shown in Figure 48. It is observed that there is a good similarity between the curves. However, a major reason for the error is due to the assumption for the air layer between the PCC and the copper serpentine. The air gap used for the simulation was 0.1mm. Table 28 shows the comparison between the experiment and the simulation results. 78

88 Temperature (C) PCC#1 PCC#3 PCC#5 Outlet Inlet simulated Outlet Time (sec) Figure 48. Experimental validation of PCC TES at medium flow rates Parameter Experiment Simulation Error (%) Outlet Temperature of glycol ( o C) PCC Temperature ( o C) Duration for phase change (sec) Table 28. Comparison of experiment and simulation for PCC TES at medium flow rate From Table 28, the average error is 5%; and this model can be considered an estimate, for actual performance of the TES unit; and can be used for future analysis. The section that follows gives a summary of the performance of the TES units at medium flow rates. 79

89 Summary of Medium Flow Rates This section highlights the results from the simulation of the medium flow rates of the three TES units. The Table 29, lists the summary of the results from the simulation: Parameter Ice TES PCM TES PCC TES Energy storage (kwh) Heat Release Rate (kwh/min) (P/E) Rate (kw/kwh) Efficiency (%) Power density (kw/l) Energy density (kwh/l) Flow rate (GPH) Table 29. Comparison of TES units at medium flow rate As shown in Table 29, the highest efficiency is for the PCC TES unit; the (P/ E) rate was very comparable for the three TES units. As mentioned before, there are considerable losses for the PCC TES unit, due to the layer of air that is present between the copper and the PCC slab. Hence, TES unit without air gap is considered for further analysis. Since the results at the low flow rate gives the highest cooling power and the best efficiency, further analysis for optimal design is carried out at the low flow rates. 80

90 Chapter 6 Prototype Design and Development In this Chapter, optimal design parameters are identified for best performance. This chapter begins with a comparison of the performance of the PCC TES units, with and without air gaps, followed by a comparison of ice and PCC TES units, two TES units that are performing equally. For a fair comparison, the Ice TES unit was compared with the PCC TES unit without air gaps. After that, design optimization by FEA was performed. Lastly, the chapter concludes with a cost analysis of equally performing ice TES and PCC TES units. 6.1 Effect of Air Gap for the PCC TES unit From the previous experimental analysis, the PCC TES unit is found to be the best cold storage unit. However, the performance suggested that the TES unit had some losses at the system level. When fabricating the specimen, special care must be taken in manufacturing the slabs and assembling them in order to avoid the formation of the air layer. The air gap caused in the manufacturing and assembly process leads to losses in the heat transferred between the PCC and the copper serpentine, resulting in lower cooling and longer duration for discharge. The following sections show and compare the performance of the TES unit without air gap, to the fabricated PCC TES unit Low Flow Rates The initial analysis compared the PCC TES units with and without air gap at the low flow rate. Figure 28 illustrates the discharge profile of the TES unit without air gaps, while the Figure 45 illustrates the discharge profile of the TES unit with air gaps. The figures indicate that the discharge process is quicker and the performance of the cooling is superior for the PCC TES unit without air gaps. The comparisons between the TES units with and without air gap are compared in Table

91 Parameter PCC TES without air gap PCC TES with air gap Efficiency (%) Heat release rate (kwh/min) (P/E) Rate (kw/kwh) Duration of discharge (sec) Power density (kw/l) Energy density (kwh/l) Flow rate (GPH) Table 30. Comparison of PCC TES with and without air gap at low flow rate From Table 30, it is clear that the duration of discharge is reduced by half for the PCC TES unit without air gap when compared to the TES unit with air gap. The efficiency dropped from 63% to 55% due to an increase in heat release rate by two times for the PCC TES unit without air gap, when compared to the TES unit with air gap. The cooling power delivered is 72% more for PCC TES without air gap when compared to the PCC TES unit with air gap. Thus, air layer is an impediment to heat transfer. A comparison at medium flow rates is shown for the PCC TES unit Medium Flow Rate The comparison of performance for the PCC TES unit with and without air gap at the medium flow rate is considered. Figure 31 illustrates the discharge of the TES unit without air gap at the medium flow rate, while Figure 48 illustrates the discharge of the PCC TES unit with air gap, at the medium flow rate. Table 31 shows the effect of air gap by comparing the performances of the PCC TES unit. 82

92 Parameter PCC TES without air gap PCC TES with air gap Efficiency (%) Heat release rate (kwh/min) (P/E) Rate (kw/kwh) Duration of discharge (sec) Power Density (kw/l) Energy Density (kwh/l) Flow rate (GPH) Table 31. Comparison of PCC TES with and without air gap at medium flow rates From Table 31, the PCC TES unit without air gap shows improved performance when compared to the PCC TES unit with air gap. The efficiencies are quite similar; however, the (P/ E) rate for the PCC TES has increased by 59%. The duration of discharge is reduced by half for the PCC TES unit without air gap, in comparison to the unit with air gaps. Hence, the air gap is an impediment to heat transfer. Figure 49 shows the overlaid curves of the PCC TES unit with and without air gaps. The solid lines represent the TES unit with air gap, while the discontinuous lines represent the TES unit without air gaps. 83

93 Temperature (C) Inlet Simulated Outlet NAG pcc#5 NAG Outlet PCC Time (sec) Figure 49. Comparison of PCC TES with and without air gap at medium flow rate From Figure 49, it is clear that the PCC TES with no air gap provides a quicker and a higher cooling performance. 6.2 Effect of Thermal Conductivity for PCC The bi-directional thermal conductivity of the PCC slabs exists due to the bi-direction thermal conductivity of the graphite block that has undergone compaction from graphite fibers. This bidirectional thermal conductivity, for the PCC material, can result in performance variation based on the orientation of the slabs. The thermal conductivity variation is as shown in the image below. 84

94 Direction of compaction y In-plane (xy plane) x Figure 50. PCC slab orientation of current design From Figure 50, the direction of compaction is shown. The plane perpendicular to the direction of compaction is called the in plane. The in-plane region is the region of high thermal conductivity, while the plane parallel to the direction of compaction is the through plane, which has a lower thermal conductivity value. The thermal conductivity of the in plane is 22 W/mK, while the thermal conductivity of the through plane is 9W/mK. Based on the variation in thermal conductivity, there can be two orientations for the PCC slab. The current design has an orientation as shown in Figure 50. The new design is oriented as shown below in Figure 51: 85

95 Temperature(C) Through plane (xy plane) y x Figure 51. Variation of PCC slab orientation to study effect of thermal conductivity Figure 51 shows a new design obtained by orienting the through plane in the XY axis, and the in plane along the XZ axis. Based on the new orientation, the results for the TES unit s discharge is shown in Figure 52. Inlet Outlet PCC1 PCC5 PCC Time (sec) Figure 52. PCC TES with variation in thermal conductivity at low flow rate 86

96 The duration for discharge is seconds, the performance shows an efficiency of 81%, a heat release rate of kwh/min, and a (P/ E) rate of 0.30 /hr. Table 32 lists parameters to compare the two orientations of the PCC TES unit. Parameter Thermal conductivity (W/mK) PCC TES new orientation of slab K x =9 K y =9 K z =22 PCC TES original orientation of slab K x =22 K y =22 K z =9 Efficiency (%) Heat release rate (kwh/min) (P/E) Rate (kw/kwh) Duration of discharge (sec) Power density (kw/l) Energy density (kwh/l) Flow rate (GPH) Table 32. Comparison of PCC TES units with varying thermal conductivities From Table 32, it is clear that the efficiency is higher for the TES design with the new orientation as it has a slower discharge rate. The (P/ E) rate is twice for the original PCC TES unit when compared to the new oriented PCC TES unit. The original design has an improved and quicker discharge performance, compared to the new oriented design. 87

97 6.3 Design Optimization This section identifies an optimal design for optimal performance of the TES units. Once an optimized design is obtained, cost analysis for the TES units is carried out Effect of Copper Pitch The first design variation considers the effect of copper pitch on the performance of the TES units. The current design that is employed has a two-inch long copper serpentine pitch; the design variation considered has a one-inch long copper serpentine pitch design. This design variation will have more copper; and the serpentine will have twice the volume, compared to the two-inch long copper serpentine pitch design. This would mean there is a longer residence time of the fluid in the copper serpentine. The design for copper serpentine pitch is shown in Figure Figure 53. Copper serpentine designs for one-inch pitch 88

98 Temperature (C) Based on the design variation in copper pitch, a comparison can be made on the performance. The copper with a one-inch pitch showed a better performance; due to the higher surface area available for heat transfer. The results based on the one-inch copper pitch are as shown below PCC TES The PCC TES unit s performance was studied with a one-inch copper serpentine pitch design. Figure 54 shows the discharge profile for the TES unit with a one-inch copper serpentine pitch. Inlet PCC1 PCC2 PCC3 PCC4 PCC5 Outlet Time (sec) Figure 54. PCC TES for one-inch copper serpentine pitch at low flow rate From Figure 54, the duration of discharge for the PCC TES unit considered is much quicker than the two-inch copper serpentine pitch, design. A marginal temperature gradient between the thermocouple located at PCC5 and the outlet, implies an improved cooling performance. The reason for the improvement in performance is due to greater residence time of the fluid, inside the serpentine. 89

99 Parameter PCC with1 copper design PCC with 2 copper design Efficiency (%) Heat release rate (kwh/min) (P/E) Rate (kw/kwh) Duration for discharge (sec) Power density (kw/l) Energy density (kwh/l) Flow rate (GPH) Table 33. Comparison of PCC TES with varying copper pitch From Table 33, the comparison between the PCC one-inch copper serpentine pitch design and the twoinch copper serpentine pitch design is shown. The efficiency and the (P/E) rate have both increased by 11 and 16% respectively. The reason can be attributed to the increased surface area for heat transfer; the new design had twice the area compared to the two-inch copper serpentine pitch design. The duration for discharge had also reduced by 276 seconds. Thus, the one-inch copper serpentine improves the overall performance of the TES unit Ice TES The effect of copper serpentine pitch design to identify the performance of the ice TES unit is shown in this section. Figure 55 shows the discharge performance of the ice TES unit with the one-inch copper serpentine design. 90

100 Temperature (C) Inlet Ice#1 Ice#3 Ice#5 Outlet Time (sec) Figure 55. Ice TES for a one-inch copper serpentine design at low flow rate The performance illustrated an improved discharge profile when comparing with the two-inch copper serpentine pitch design. However, the duration is still long, in comparison with the PCC TES unit. Table 34 compares the performances of the ice TES unit, of one-inch copper serpentine pitch design, with an ice TES unit of two-inch copper serpentine pitch design. 91

101 Parameter Ice with 1 copper design Ice with 2 copper Design Efficiency (%) Duration for discharge (sec) Heat release rate (kwh/min) (P/E) Rate (kw/kwh) Power density (kw/l) Energy Density (kwh/l) Table 34. Comparison of ice TES with varying copper pitch The one inch copper serpentine design, shows an efficiency increase by 29% and the (P/ E) rate increased by a factor of two times, when compared to the two inch copper serpentine pitch design. The effect of the one-inch copper pitch on the ice TES unit is significant both in the duration of discharge, and the performance. It is clear, that increased surface area available for heat transfer, improves the performance of the TES unit Effect of Longer Slab Design on TES Performance This section identifies a new design, where the overall PCC/PCM content of the TES unit is increased. The new design is as shown below in Figure

102 4 27 All units are in cm Figure 56. Design of a TES unit for increased PCC/ice content Figure 56 illustrates the design for an increased PCC/PCM TES unit; the slabs are longer compared to the previous case, with increased thermal capacities. The flow rate has to be accounted for the increased thermal capacity. Table 35 summarizes the test condition for the TES units. Parameter PCC Long slab design Ice long slab design Thermal capacity (kj) Flow rate (GPH) Inlet Temperature ( o C) Table 35. Test conditions for TES unit with increased thermal capacity Based on Table 35 the performances of the PCC and ice TES units are shown. 93

103 Temperature (C) PCC TES The discharge profile for the increased PCC content with reduced copper in the TES is shown in figure 57. inlet Outlet PCC#1 PCC#2 PCC#3 PCC# Time (sec) Figure 57. PCC TES with increased thermal capacity at low flow rate The effect of long slab, on the PCC discharge performance is as shown in Figure 57. The discharge performance is longer, for the equivalent flow rate. This extended duration for discharge is due to the higher thermal capacity. The table 36 below compares the original design to the long slab design of the PCC TES unit. 94

104 Parameter PCC Long slab PCC 2 copper pitch Duration for discharge (sec) 10, Efficiency (%) Heat release rate (kwh/min) (P/E) Rate (kw/kwh) Power density (kw/l) Energy Density (kwh/l) Table 36. Comparison of PCC TES units on design for thermal capacity From Table 36, the TES unit with the long slab shows a 35% reduction in the (P/ E) rate, which can be attributed to the reduced surface area, in the long slab design. The long slab PCC TES unit has a 40% higher efficiency, with improved heat release rate by 70% when compared to the two-inch copper serpentine pitch design for the PCC TES unit Ice TES unit The increased ice content of the TES unit is considered in this section. The discharge profile of the TES unit is as shown in Figure

105 Temperature (C) Inlet Outlet Ice#5 Ice#1 Ice# Time (Sec) Figure 58. Ice TES with increased thermal capacity at low flow rate The effect of the long slab design, on performance of the ice TES unit is as shown in Figure 58. The duration for discharge is slow. The cooling performance is minimal, due to the extreme temperature gradient between the outlet and the thermocouple located at Ice#5. The reason for the poor performance, and the longer duration is attributed to lower thermal conductivity of the ice and the increased thermal capacity in the TES unit. Table 37 shows the performance of the Ice TES unit. 96

106 Parameter Long slab ice TES 2 copper ice TES Duration for discharge (sec) Efficiency (%) Heat release rate (kwh/min) (P/E) Ratio (kw/kwh) Power Density (kw/l) Energy Density (kwh/l) Table 37. Comparison of ice TES unit on design for thermal capacity Table 37, compares the performance of the ice TES unit with original and long slab design. The long slab ice TES unit shows a 30 % increase in efficiency, due to a slower discharge process, because of increased thermal capacity and the average cooling power delivered is reduced by 6%, when compared to the two-inch copper serpentine design. 6.4 Design Optimization The design optimization brings together system performance, with cost estimation analysis, based on design. Based on two equally performing TES units (ice TES unit and PCC TES unit) a technoeconomic report on the two systems had been created, to identify an optimum performance, and an economical TES unit. From the above trends, it is clear that the TES units have a better performance with increased copper content in the system; however, this comes with a drawback of increasing the overall system cost. Thus, the overall goal in design optimization is to design a unit, that is efficient and at the same time cost effective. 97

107 For the ice TES unit, the cooling performance is shown to increase by a one-inch copper serpentine pitch design, due to increased surface area available for heat transfer. Adding more coils and more copper in the TES unit increases the overall cost of the system; however as water is inexpensive, analysis must be done to see if the addition of extra copper to improve performance is justified. The PCC TES unit has good performance compared to ice TES unit, but the cost for the raw material for fabricating the PCC is more expensive than the cost of ice; it is clear that, analysis must be done to reduce system cost to a minimum Optimized Design for ice TES Unit For the ice TES unit, the two-inch copper serpentine design shows the following performance parameters: (1) The efficiency of the TES unit is 50%. (2) It has the lowest (P/ E) rate of 0.082/hr. The major reason for the poor performance can be attributed to the low thermal conductivity; the performance improvement is affected by a one- inch copper serpentine pitch design, when doing so a few highlights from the design modification were: (1) The average cooling power to stored capacity increased by a factor of two times. (2) The duration for discharge reduced by a factor of almost two. (3) The average cooling power is still four times lower compared to the PCC 2 copper serpentine. (4) The duration for discharge, for the ice TES is still 1.5 times longer than the PCC 2 copper serpentine. 98

108 It is clear that, in order for ice TES unit to reach a comparable performance as the PCC TES unit, an equal average cooling power to stored capacity or (P/ E) rate as the PCC TES unit, must be obtained. The ice TES unit must also be designed to discharge at a faster rate. To achieve these two criterions, the increased copper serpentine design along with a reduced thermal capacity for ice TES is considered. Based on this, a new design is considered as shown in Figure All dimensions are in cm Figure 59. Optimized ice TES design The thermal capacity is reduced by half and the slabs are now half their original thickness. The performance for an equivalent low flow rate for discharge is shown in Figure 60, and a summary of the test conditions is shown in Table

109 Temperature (C) Parameter Ice TES Optimized design Ice TES 2 copper pitch design Thermal capacity (kwh) Flow rate (GPH) Copper pitch 2 Thickness (cm) 2 4 Table 38. Comparison of ice TES design for optimization The discharge profile is shown in Figure 60, based on test conditions in Table Inlet Water1 Water2 Water3 Water5 Outlet Time (sec) Figure 60. Discharge profile for optimized ice TES at low flow rate 100

110 Figure 60 shows that the ice TES unit has a reduced duration for discharge in the range of 6000 sec sec, with an efficiency of 70-81%. The heat release rate increases due to a reduced duration for discharge. Table 39 compares the performance of the optimized design to the two-inch copper serpentine pitch design. Parameter Ice Optimized TES design Ice 2 copper pitch serpentine Efficiency (%) Heat release rate (kwh/min) Cooling power/ stored capacity (kw/kwh) Mass of copper (kg) Mass of ice(kg) Power density (kw/l) Energy density (kwh/l) Table 39. Performance comparison for optimized ice TES with experimental design From Table 39, a few highlights from the optimized design are, the average cooling power to the stored capacity shows an increase by a factor of 4.5 times to the two inch copper serpentine pitch design along with an increase in efficiency by 26%. As the exact nature of the extrapolated curve is unknown, a range of efficiency is predicted Optimized Design for the PCC TES The PCC TES unit had consistent good performance, compared to the ice TES unit. However, one thing to remember for the analysis of the PCC TES unit is to reduce the system cost to a minimum; this can be achieved by a reduced copper content in the system. This would mean a compromise in system 101

111 performance. The main reason to reduce the cost of the system is due to the high price of the PCC material. Reduced copper involves choosing the TES with increased PCC content. Table 36 compares the PCC long slab design to the PCC two-inch copper serpentine pitch design. The efficiency of the increased PCC content in the TES unit shows a 40% increase when compared to the TES two-inch copper serpentine design; the (P/ E) rate shows a reduction of 37% for the long slab design when compared to the two- inch copper serpentine design. The reduced (P/ E) rate is due to the low copper content and reduction in effective surface area available for heat transfer. The system cost for the TES unit with a long slab design was significantly lower when compared to the PCC two-inch copper serpentine design. The next step in the analysis is to pick a case of equal performance for both the PCC TES and the ice TES units, and compare the costs from the system level. 6.5 Prototype Cost Analysis of Optimized Designs of TES Units This section identifies the two TES units with equal (P/ E) rate and the comparison was made on their economics. The optimized design for the ice TES unit is compared to the long slab design of the PCC TES unit. The optimizations for the ice and the PCC TES unit were performed for different reasons; in the case of the ice TES, the optimization was to increase the cooling power to stored capacity; and the PCC TES unit s design optimization was to reduce the overall cost of the unit. Based on the two designs the comparison of performance is shown in Table

112 Parameter Optimized ice TES PCC long slab Efficiency (%) (P/E) Rate (kw/kwh) Heat release rate (kwh/min) Storage capacity (kwh) % of Copper by mass in TES unit Duration for discharge (sec) Power density (kw/l) Energy density (kwh/l) Table 40. Comparison of optimized ice and PCC TES units Table 40, shows there is a reduction of 36% in the duration of discharge for the ice TES unit when compared to the PCC TES unit. The ice TES has about 2.5 times more copper content by mass. Only 40% by mass of the ice TES unit is occupied for thermal storage, the remaining mass is contributed by copper. The efficiency for the discharge process is higher for PCC TES unit. Based on the results, cost estimation is done for the TES units, by considering the following factors: (1) Cost of the copper for the analysis is 9.937$/kg. (2) Cost of the ice/water is taken to be 0.0$/kg. (3) Cost of the PCC is taken to be 4 $/kg (cost at high volumes). Table 41 shows results from a detailed calculation and a summary on the economics of the system for a one Ton- hr. thermal storage capacity. 103

113 Ice TES PCC TES PCC TES. Thermal capacity kj Weight of raw material kg Amount of Cu kg Cost of raw material /kg $ cost of copper /kg $ total cost for two slabs $ cost/thermal capacity $/kwh 1ton Hr $ Volume of PCM/PCC L Volume of Cu L Total volume L % Cu 15% 3% 3% Total mass of unit kg Weight % of PCM 75% 85% % difference in cost 13% 32% % savings 16% 39% Table 41.Prototype cost analysis of ice and PCC TES units From Table 41, the PCC TES unit is found to be more economical to the ice TES unit. The ice TES unit has 5.4% higher cost per kwh than the PCC TES unit. The two TES units are of different sizes. The PCC TES unit is nearly three times larger than the ice TES unit; however, the performance is comparable with the ice TES unit with higher percentage of copper by mass at equal (P/ E) rates. Experimental analysis on soaking the graphite blocks, in the PCM bath revealed that, the highest weight fraction attained was around 83-84% of PCM, in the composite. Based on the analysis the thermal capacity of the unit would rise, thereby reducing the cost/kwh. A total savings of almost 40% is obtained by choosing the PCC TES unit over the ice TES unit. 104

114 Chapter 7 Conclusion A research on latent thermal energy storage has been carried out in this thesis. The main crux of the thesis involved developing a TES unit, to provide a solution to improving the performance of conventional TES units in a cost effective manner. Major findings in this thesis are as follows: 1) Effect of graphite addition for improving thermal conductivity of a phase change material is very significant. Addition of 25% graphite into the phase change material reduces the duration of discharge by half and increases the (P/E) rate by a factor of four times compared to conventional TES units. 2) Air layer of 0.1mm, if present in the TES unit can cause deterioration of performance by reducing the cooling obtained by up to 30-40% and increasing the duration of discharge by almost twice. 3) An optimized design for the ice TES unit to match the performance of a PCC TES unit must perform with a reduced thermal capacity and an increase of 35% copper content compared to the fabricated PCC TES unit. 4) The overall cost comparison of equally performing ice TES and PCC TES shows that the PCC TES unit is economical, and has 40% savings than the ice TES unit. 5) This research has revealed a new thermal energy storage material, a C melting PCM-graphite composite, which shows performance far superior to conventional TES units. To conclude, this study shows that the PCC TES unit fabricated by using the developed thermal energy storage material has the potential to offset peak electric demands in households in an effective and economical manner. 105

115 Chapter 8 Bibliography [1] "Average energy consumption in 2013," [Online]. Available: glassforeurope.com. [2] N. A.Arteconi, "state of the art of thermal storage for demand side management," applied energy, vol. 93, pp , [3] "energy demand vs time," [Online]. Available: [4] "ontario energy board," [Online]. [5] L. W. John Rogers, "Solar Power on the Rise," Union of concerned scientists, [6] D. B. R. L. S. anant shukla, "SOlar water heaters with phase change material thermal energy storage medium : A review," renewable and sustainable energy reviews, vol. 13, no. 8, pp , [7] V. V. t. C. R. C. D. b. atul sharma, "Review on thermal energy storage with phase change materials and applications," Renewable and Sustainable energy reviews, vol. 13, no. 2, pp , [8] J. B. G. C. J. D. Y. L. Huili Zhang, "Thermal energy storage: Recent developments and practical aspects," Progress in Energy and Combustion Science, vol. 53, pp. 1-40, [9] R. M. D. I, Thermal Energy Storage- Systems and Aplications, John Wiley & Sons, [10] J. T. D. Y. G. M. M. R. E. K. S. Sarada Kuravi, thermal energy storage technologies and systems for concentrating solar power plants, elsevier, [11] L. F. C. Harald Mehling, Heat and cold storage with PCM, BERLIN: SPRINGER, [12] M. M. M. S. I. M. L. C. A.I. Fernandez, "Selection of materials with potential in sensible thermal energy storage," Solar Energy Materials and Solar Cells, vol. 94, no. 10, pp , [13] M. M. A. G. A. F. L. C. R. O. X. P. M.E. Navarro, "Selection and characterization of recycled materials for sensible thermal energy storage," Solar Energy Materials and Solar Cells, vol. 107, pp , [14] M. D. C. K. S. M. S. Khare, "Selection of materials for high temperature sensible energy storage," Solar Energy Materials and Solar Cells, vol. 115, pp , [15] W.-D. S. R. T. C. R. Doerte Laing, "Solid media thermal storage for parabolic trough power plants," Solar Energy, vol. 80, no. 10, pp ,

116 [16] A. F. M. G. S. Ushak, "3 Using molten salts and other liquid sensible storage media in thermal energy storage (TES) systems," Advances in thermal energy storage systems, pp , [17] N. P.,. N. B.,. M. E. L. S. K. Thomas Bauer, "Material aspects of Solar Salt for sensible heat storage," Applied Energy, vol. 111, pp , [18] L. G. SOCACIU, "Seasonal Sensible Thermal Energy Storage Solutions," Journal of Practices and Technologies, no. 19, pp , [19] A. F. A. R. J. L. E. V. P. E. C. B.,. N. C. X. P. Stéphanie Guillot, "Corrosion effects between molten salts and thermal storage material for concentrated solar power plants," Applied Energy, vol. 94, pp , [20] R. I. Olivares, "The thermal stability of molten nitrite/nitrates salt for solar thermal energy storage in different atmospheres," Solar Energy, vol. 86, no. 9, pp , [21] Y. A. R. O. Xavier Py, "Concentrated solar power: Current technologies, major innovative issues and applicability to West African countries," Renewable and Sustainable Energy Reviews, vol. 18, pp , [22] l. F. c. harald mehling, heat and cold storage with PCM, berlin: springer, [23] h. A, "Sorption theory for thermal energy storage," Thermal energy storage for sustainable energy consumption fundamentals, case studies and, vol. 234, pp , [24] A. S. Ali Karaipekli, "Development and thermal performance of pumice/organic PCM/gypsum composite plasters for thermal energy storage in buildings," Solar Energy Materials and Solar Cells, vol. 149, pp , [25] L. P. O. A. P.A.J. Donkers, "Experimental studies for the cyclability of salt hydrates for thermochemical heat storage," Journal of Energy Storage, vol. 5, pp , [26] unesco-eolss, "storage of thermal energy," [Online]. Available: [27] Y. H. R. R. Gang Li, "review of cold storage materials for air conditioning applications," international journal of refrigeration, vol. 35, no. 8, pp , [28] I. D.,. M. G. Sayem Zafar, "Experimental testing and analysis of R134a clathrates based PCMs for cooling applications," International Jounral of Heat and Mass Transfer, vol. 91, pp , [29] J. J. C. B. R. Y. M. M. F. Reza Barzin, "Application of PCM energy storage in combination with night ventilation for space cooling," Applied Energy, vol. 158, pp , [30] L. G. F. C. I. F. M. R. A.H. Mosaffa, "Energy and exergy evaluation of a multiple-pcm thermal storage unit for free cooling applications," Renewable Energy, vol. 68, pp ,

117 [31] C. I. F. M. R. F. T. A.H. Mosaffa, "Thermal performance optimization of free cooling systems using enhanced latent heat thermal storage unit," Applied Thermal Energy, vol. 59, no. 1-2, pp , [32] M. M. F. Amar M. Khudhair, "A review on energy conservation in building applications with thermal storage by latent heat using phase change materials," Energy Conversion and Management, vol. 45, no. 2, pp , [33] S. S. J. S. A. Felix Regin, "Heat transfer characteristics of thermal energy storage system using PCM capsules: A review," Renewable and Sustainable Energy Reviews, vol. 12, no. 9, pp , [34] R. I. Olivares, "The thermal stability of molten nitrite/nitrates salt for solar thermal energy storage in different atmospheres," Solar Energy, vol. 86, no. 9, pp , [35] S. T. B. A. M. K. J. G. C. Brian D. Iverson, "Thermal and mechanical properties of nitrate thermal storage salts in the solid-phase," Solar Energy, vol. 86, no. 10, pp , [36] "REVIEW ON SUSTAINABLE THERMAL ENERGY STORAGE PART II :COLD STORAGE," ENERGY MANAGEMENT, pp , [37] "Performance assessment of some ice TES systems," International Jounral of Thermal Sciences, vol. 48, no. 12, pp , [38] M. K. M. J. B. Gregor P. Henze, "Guidelines for improved performance of ice storage systems," Energy and Buildings, vol. 35, no. 2, pp , [39] V. N. Michael J. Kazmierczak, "Heat transfer augmentation for external ice-on-tube TES systems using porous copper mesh to increase volumetric ice production," International Journal of Refrigeration, vol. 29, no. 6, pp , [40] N. N. Motoi Yamaha, "Thermal Energy Storage Tanks Using PCM in HVAC," Intech, [Online]. Available: [41] J. M. M. L. F. C. H. M. Belén Zalba, "Review on thermal energy storage with phase change: materials, heat transfer analysis and applications," Applied Thermal Engineering, vol. 23, no. 3, pp , [42] G. S. C. G. N. A. R. I. Martin Smalc, "THERMAL PERFORMANCE OF NATURAL GRAPHITE HEAT SPREADERS," in Interpack , San Francisco, [43] T. N. M. T. N. O. T. A. Teppei Oya, "Thermal conductivity enhancement of erythritol as PCM by using graphite and nickel particles," Applied Thermal Engineering, vol. 61, no. 2, pp ,

118 [44] H. K. S. M. P. K. Y. R.,. S.-J. P. Seul-Yi Lee, "Thermal characterization of erythritol/expanded graphite composites for high thermal storage capacity," Carbon, vol. 68, pp , [45] N. O. T. A. Takahiro Nomura, "Impregnation of porous material with phase change material for thermal energy storage," Materials Chemistry and Physics, vol. 115, no. 2-3, pp , [46] [47] [48] T. N. C. Z. N. S. N. O. T. A. Ryo Fukahori, "Thermal analysis of Al Si alloys as hightemperature phase-change material and their corrosion properties with ceramic materials," Applied Energy, vol. 163, pp. 1-8, K.L. Ahmed Elgafy, Effect of carbon nanofiber additives on thermal behavior, CARBON, vol. 43, pp ,2005. N.N.A.A.A.R.K.M.S.T.M.M.Sadegh Motahar, A novel phase change material containing mesoporous silica nanoparticles for thermal storage: A study on thermal conductivity and viscosity, International Communications in Heat and Mass Transfer,vol. 5,pp , [49] M. F. J. S. S. A.-H. Andrew Mills, "Thermal conductivity enhancement of phase change materials using a graphite matrix," Applied Thermal Engineering, vol. 26, no , pp , [50] S. W. S. K. S. A.-H. Ben Schweitzer, "Experimental validation of a 0-D numerical model for phase change thermal management systems in lithium-ion batteries," Journal of Power Sources, vol. 287, pp ,

119 Chapter 9 Appendix 1. In plane and through plane surfaces Direction of Compaction of the block Y (through plane) Direction of lower thermal conductivity (9W/m K) Direction of High Thermal Conductivity (22 W/m K) X (In plane) Figure 61 PCC slab's variation in thermal conductivity 2. Effect of pure PCM simulation, 12.5 Gph 110

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