Design of Heat Exchanger for Thermal Energy

The University of Sharjah College of Engineering Sustainable and Renewable Energy Engineering Program

Senior Design Project Report

Design of Heat Exchanger for Thermal Energy Storage Using High Temperature Phase Change Material ‫تصميم م بادل حراري باستخذام مواد متغيرة الحالت‬ ‫راث درجت حرارة عاليت لغرض تخزين الطاقت‬ ‫الحراريت‬

Project Group: Marwa Ibrahim Albanna Noorah Khaled Altamimi Reem Ahemed Abuhamra Project Examination Committee Supervisor Prof./Dr. Ahmed Amine Hachicha

Table of Contents Table of Contents ........................................................................................................................ i List of Figures ........................................................................................................................... iv List of Tables ............................................................................................................................ vi Symbols sheet ..........................................................................................................................vii Problem Statement .................................................................................................................... ix Abstract ...................................................................................................................................... x 1

Motivation ........................................................................................................................ 1

2

Introduction ...................................................................................................................... 2 2.1

Introduction to the topic .......................................................................................... 2

2.2

Literature Review .................................................................................................... 3 2.2.1 Storage systems ........................................................................................... 3 2.2.2 Phase change materials ................................................................................ 7

3

2.3

Design Criteria ........................................................................................................ 9

2.4

Our Design ............................................................................................................ 12

Alternative Solutions ...................................................................................................... 13 3.1

Introduction ........................................................................................................... 13

3.2

Thermal energy storage system............................................................................. 13

3.3

Phase change material ........................................................................................... 14 3.3.1 Type of containments: ............................................................................... 14 3.3.2 Selection: ................................................................................................... 15

4.3

Heat transfer fluid ................................................................................................. 19

3.5

Heat exchanger design .......................................................................................... 20 3.5.1 Type ........................................................................................................... 20 3.5.2 Material of components ............................................................................. 23

3.6 4

Heat transfer enhancement methods ..................................................................... 25

Design calculations ......................................................................................................... 29 4.1

Technical background ........................................................................................... 29

4.2

Prototype Calculations .......................................................................................... 30 4.2.1 Steady state analysis .................................................................................. 31 4.2.2 Transient analysis ...................................................................................... 34 4.2.3 PCM: ......................................................................................................... 34 i

4.3

Large-scale calculations ........................................................................................ 36 4.3.1 SHAMS 1 .................................................................................................. 36 4.3.2 Design calculations.................................................................................... 37

5

6

Proposed Design ............................................................................................................. 43 5.1

Prototype proposed design .................................................................................... 43

5.2

Large scale proposed design ................................................................................. 46

ANSYS Simulation ........................................................................................................ 48 6.1

Geometry:.............................................................................................................. 48

6.2

Mesh:..................................................................................................................... 48

6.3

Setup: .................................................................................................................... 49 6.3.1 General: ..................................................................................................... 50 6.3.2 Models: ...................................................................................................... 50 6.3.3 Materials: ................................................................................................... 50 6.3.4 Phases: ....................................................................................................... 51 6.3.5 Cell zone conditions: ................................................................................. 51 6.3.6 Boundary conditions: ................................................................................ 51 6.3.7 Solution method: ....................................................................................... 52 6.3.8 Solution Initialization: ............................................................................... 52 6.3.9 Calculation activity: .................................................................................. 52 6.3.10 Run calculation: ....................................................................................... 52

6.4

Results: .................................................................................................................. 53 6.4.1 Charging: ................................................................................................... 53 6.4.2 Discharging: .............................................................................................. 55

7

List of Constraints .......................................................................................................... 58 7.1

System constraints & limitations: ......................................................................... 58 7.1.1 Phase change material constraints ............................................................. 58 7.1.2 Heat transfer fluid constraints ................................................................... 63

7.2

Environmental Concerns ....................................................................................... 64

7.3

Ethical and Social Issues ....................................................................................... 64

7.4

Economic Issues .................................................................................................... 65

8

Cost and economic analysis............................................................................................ 66

9

List of Specifications ...................................................................................................... 69 9.1

Parts List and Estimated Budget ........................................................................... 69 ii

10

9.2

Conclusion ............................................................................................................ 70

9.3

Recommendations ................................................................................................. 70

References ...................................................................................................................... 71

Acknowledgments.................................................................................................................... 77 Appendices ............................................................................................................................... 78 Appendix A: SYLTHERM800 Specification sheet ..................................................... 78 Appendix B: MATLAB Algorithm ............................................................................. 80 Appendix C: PCMs specification ................................................................................ 84 Appendix D: Carbon steel pipes properties and maximum allowable pressure .......... 87 Appendix E: Therminol VP-1 specification sheet ....................................................... 88 Appendix F: Mineral wool thermal conductivity ........................................................ 88

iii

List of Figures Figure 1: temperature versus stored heat graph, illustrating the temperature of phase change where energy is stored as latent heat.......................................................................................... 2 Figure 2: a CSP plant with integrated thermal energy storage system charging and discharging processes................................................................................................................. 3 Figure 3: two-tank thermal energy storage system .................................................................... 4 Figure 4: single-tank thermal energy storage system ................................................................ 5 Figure 5: thermal stratification effect in a tank. Retrieved from Invalid source specified. ....... 5 Figure 6: PCM groups with their heat of fusion and melting temperature range [54] ............. 16 Figure 7: Classification of PCM with examples of materials used [54] .................................. 16 Figure 8: Heat capacity (a) and media cost (b) of high melting point PCMS [24] .................. 17 Figure 9: Screw heat exchanger sechematic [29] .................................................................... 20 Figure10: shell and tube heat exchanger .................................................................................. 22 Figure 11: module heat exchanger [64]. .................................................................................. 22 Figure 12: Heat transfer enhancement methods [22] ............................................................... 26 Figure 13: Heat pipes with different configuration [28] .......................................................... 27 Figure 14: Heat transfer in radial direction through the pipe (upper view) [65] ..................... 29 Figure 15: schematic diagram of the design, showing the change of temperature with the zaxis in the inlet, outlet and separation between the tanks ........................................................ 31 Figure 16: SHAMS 1 operation process [46] .......................................................................... 36 Figure 17: Storage system foundation ..................................................................................... 40 Figure 18: Thermal resistance representation in r-axis. ........................................................... 41 Figure 19: Top thermal resistance representation .................................................................... 42 Figure 20: 3D design of the heat exchanger ............................................................................ 43 Figure 21: CSP Plant with the thermal storage tank ................................................................ 44 Figure 22: 50 mm diameter x150mm length heater used to simulate the effect of HTF ......... 44 Figure 23: the experimental setup (Prototype consists of storage tank, temperature controller and an exhaust fan) .................................................................................................................. 45 Figure 24: (A) the insulation material (Ceramic Fiber roll), (B) the insulated tank ................ 45 Figure 25: Flow diagram of SHAMS1 with storage system .................................................... 47 Figure 26: Geometry of the storage tank on ANSYS Fluid Flow (Fluent)-Design Modeler .. 48

iv

Figure 27: the generated mesh (it shows a reasonable density all over the geometry) ............ 49 Figure 28: the named selection (input and output) .................................................................. 49 Figure 29: defining of the properties (piecewise-linear for temperature dependent properties) .................................................................................................................................................. 50 Figure 30: the primary and secondary phases in the fluent solver setup ................................. 51 Figure 31: the inlet boundary conditions setup ........................................................................ 51 Figure 32: the insulation boundary condition (thermal condition-convection) ....................... 52 Figure 33: the initial state temperature contour, where the initial temperature of the PCMs is 26 ⁰C ........................................................................................................................................ 53 Figure 34: temperature contour after 1000 s ............................................................................ 53 Figure 35: temperature contour after 7900s (2.2 hours) .......................................................... 54 Figure 36: temperature contour after 5000 s (1.4 hours) ......................................................... 54 Figure 37: the temperatue contour for the discharging process when it starts the simulation (initial state) ............................................................................................................................. 55 Figure 38: temperature contour at 1300s ................................................................................. 55 Figure 39: temperature contour at 7500s (2.1 hours) ............................................................... 56 Figure 40: temperature contour at 6200 s (1.7 hours ) ............................................................. 56 Figure 41: Calorimeter device components. retrieved from [6] .............................................. 58 Figure 42: XRD pattern of unpurified with and without cycling [40] ..................................... 59 Figure 43: XRD pattern of purified with and without cycling [40] ......................................... 59 Figure 44: XRD image of Sodium Nitrate ............................................................................... 60 Figure 45: XRD image of solar salt. ........................................................................................ 60 Figure 46: XRD image of Sodium nitrite. ............................................................................... 61 Figure 47: Scanning electron microscopy images of the samples before heating. .................. 61 Figure 48: Scanning electron microscopy images of the samples after heating. ..................... 61 Figure 49: SEM image of solar salt, NaNO3 and NaNO2 (from left to right). ........................ 62 Figure 50: Total cost of two tank storage system [54]............................................................. 66

v

List of Tables Table 1: Comparison between organic and inorganic PCMs properties [25] .......................... 15 Table 2: Chosen PCMs properties and cost ............................................................................. 17 Table 3: Comparison between different HTFs......................................................................... 20 Table 4: Comparasion between heat exchanger designs.......................................................... 23 Table 5: Comparsion between possible shell materials [39]. .................................................. 24 Table 6: Comparison between insulation materials [44] ......................................................... 25 Table 7: Two storage tank and proposed design dimensions and required mass .................... 67 Table 8: Cost comparison between two storage tank and proposed design ............................. 67 Table 9: Parts List and Estimated Budget ................................................................................ 69

vi

Symbols sheet Symbol

̇ ̇

Physical quantity Temperature Melting temperature Surface temperature Fluid temperature Density Volume Volumetric flow rate Mass flow rate Diameter height Kinematic viscosity Dynamic viscosity Mass Area Cross sectional area Surface area

Unit

Heat transfer convection coefficient. Velocity Specific heat W

Work

J

Specific enthalpy of evaporation Capital costs Operating and Maintenance costs Fuel cost

t

Energy input Average capacity factor Average plant efficiency Life of plant Pressure Stress Time

-

Losses due to fraction Gravitational acceleration vii

Thermal Diffusivity Critical radius Thermal conductivity

̇

Heat of fusion Length Molecular weight Rayleigh number Nusslet number Prandtl number Grashof number Volume change Thermal energy storage capacity Energy per time Thermal resistance

J

viii

Problem Statement Concentrated solar thermal plant (CSP) is one of the unique and most promising applications that can generate electricity with significant potential to meet part of future energy demand. One of most significant barriers to CSP technologies is the variability of the solar power source according to the latitude and season, which results in a mismatch between the energy supply and the electricity demand. By coupling the CSP plant with an efficient thermal energy storage design the mismatch problem can be avoided and the best use of the energy produced will be made. One of the promising forms to store thermal energy is latent heat; that can significantly enhance the performance of the CSP system, afford a smaller volume, lower cost, and more efficient energy storage system compared to the other forms. Latent energy storage technology uses phase change materials (PCM); substances of a high heat of fusion that can melt and solidify at certain temperatures, meanwhile storing or releasing large amounts of energy, to provide higher storage capacities and target-oriented discharging temperatures. In our project, a new prototype of a heat exchanger storage system using more than one PCM will be designed. The choice of the appropriate PCM and the relative proportions suitable for the temperature range will be selected. The heat transfer flow direction and the arrangement of the PCM inside the system will be studied and a preliminary proof of concept will be conducted. Our aim is to extract the best out of the existing storage technologies to come up with a new high-temperature phase change thermal storage system, that is cost-effective, efficient and affordable and finally, the ability of integrating the storage design with a system will be studied.

ix

Abstract A thermal energy storage system (TES) is one of the important facilities required in concentrated solar power plants (CSP) that can contribute in solving the energy mismatch issues between solar energy source availability and energy demand. In this report, a study to design a heat exchanger with high-temperature phase change materials (PCM) for thermal energy storage in CSP plants has been conducted. The alternatives of the storage system type, PCMs, heat exchanger design and materials used were viewed and discussed. Based on sets of criteria the components and material has been chosen in order to maximize heat transfer rate, extend storage duration and reduce the cost. Moreover, energy balance equations were applied to obtain a rough estimate of the design parameters. Besides, the design was up scaled to be integrated in SHAMS1 CSP plant considering its required electrical output. The constraints associated with the materials and the used components have been discussed and considered. Additionally, to prove that a heat exchanger for thermal energy storage system using PCM is more economically viable the reduction in cost of integrating it in SHAMS1 has been studied and compared to the current state of art storage system, which is two storage tank. An ANSYS software simulation was also conducted to study the heat transfer flow of the system, the temperature gradient in the heat transfer fluid and the phase change materials and charging and discharging duration.

‫ملخص المشروع‬ ‫ و ذلك‬،‫نظام تخزٌن الطاقة الحرارٌة ٌعد أحد من المرافق األساسٌة و المهمة فً أنظمة محطات الطاقة الشمسٌة المركزة‬ ً‫ ف‬،‫لفعالٌتها فً حل المشاكل المترتبة عن عدم التطابق بٌن توفر مصدر الطاقة الشمسٌة و احتٌاجات الشبكة الكهربائٌة‬ ‫ تمت دراسة تصمٌم مبادل حرارة ٌستخدم مواد متغٌرة الحالة ذات درجة حرارة عالٌة ألنظمة تخزٌن الطاقة‬، ‫هذا البحث‬ ‫ باإلضافة إلى دراسة البدائل المتعلقة بنوع نظام التخزٌن و أنواع المواد متغٌرة‬،‫الحرارٌة فً محطات الطاقة الشمسٌة‬ .‫الحالة و التصمٌم المقترح للمبادل الحراري و المواد المستخدمة فٌه‬ ‫بنا ًء على مجموعة من المعاٌٌر تم اختٌار التصمٌم األنسب و المواد ذات الكفاءة من أجل زٌادة معدل انتقال الحرارة و مدة‬ ‫ تمت دراسة التصمٌم المقترح و استنتاج تقدٌر تقرٌبً لمعاٌٌر‬،‫ عالو ًة على ذلك‬،‫التخزٌن باإلضافة إلى خفض التكلفة‬ ‫ كما تمت دراسة نسبة‬،”‫التصمٌم و درجات الحرارة المتعلقة بالنظام من خالل تطبٌق و حل“معادالت توازن الطاقة‬ ً ‫مقارنة بأنظمة التخزٌن الحالٌة‬ "1 ‫انخفاض الكلفة اإلجمالٌة لدمج نظام التخزٌن المقترح فً محطة الطاقة الشمسٌة "شمس‬ ً ‫ لمحاكاة و دراسة تدفق الحرارة و طرٌقة انتقالها بٌن‬SYANA ‫إضافة إلى استخدام برنامج‬ )‫(خزانان لتخزٌن الطاقة‬ .‫المائع الحراري و المواد متغٌرة الحالة و مدة شحن و تفرٌغ الطاقة المخزنة فً النظام‬

x

1 Motivation The implementation of renewable energy sources in electricity producing technologies is required nowadays due to the environmental issues associated with the usage of fossil fuels, natural gas and coal. Solar energy is one of the most available renewable energy sources, where variable techniques are used to extract it and generate electricity. However, the main issue linked with it is the mismatch between electricity demand and its availability that can be avoided when using concentrated solar power (CSP) technology, since it can be coupled with thermal energy storage system, unlike other solar energy technologies. CSP has great international attention because of its high capacity factor and the ability of producing dispatchable electricity that eases grid integration and economic competitiveness. Most of the thermal energy storage systems coupled with CSP plants stores heat in two forms: latent heat and sensible heat. Currently, most of the world CSP plants uses two sensible storage tank, one is for cold molten salts and the other for hot molten salts, which is costly due the large volume required for storage tanks. However, latent heat compared to sensible heat has high energy density, leading to lower cost of the storage media and volume required. PCMs are promising to be integrated into latent heat TES systems since a large amount of heat is transferred during their phase change. In addition to their ability to maintain at the melting temperature for a certain period of time, which increases the quality of the HTF provided when CSP plant is out of operation. Moreover, the variation of the temperature during charging and discharging cycles is low. On the other hand, PCMs has low thermal conductivity, which affects the heat transfer [1]. Also, insufficient thermal diffusivity may act as a reason for insufficiently produced power [2]. Therefore, designing a heat exchanger for TES system, the design must be optimized and able to overcome or at least reduce the issues associated when using PCMs. The aim of this project is to design efficient heat exchanger for TES using appropriate PCMs with most effective cost. In addition to decreasing the issues associated with the PCM by improving current designs and integrate heat transfer enhancement techniques within the system. Finally, is to be expandable and thus applicable in CSP plants.

1

2 Introduction 2.1 Introduction to the topic Thermal energy storage using phase change materials (PCMs) is a very effective way of storing energy. As shown in (Figure 1) PCMs are latent heat storage units, they store and release energy at a constant temperature as they change phase (e.g., solid liquid, liquid-solid, liquid-gas and solid-gas). They have high energy density as they store a large amount of energy in a small volume. Another advantage of using phase changing materials is that there are enormous numbers of them that change phase (melts, solidify or evaporates) at a wide range of temperatures, which makes them attractive in many applications. One of the applications they are used for in the energy storage in CSP plants is to solve the mismatch problem between energy supply and electricity generating in the case of cloudy weather or at night. In the CSP plant, the sunlight is concentrated in a large number of mirrors, which is converted into high-temperature heat in a fluid called the heat transfer fluid (HTF). After being heated, HTF transfers through well-insulated pipes to a heat exchanger (i.e., a boiler), where steam is produced, and HTF is cooled down and circulated back to the CSP. The steam drives a heat engine (usually a steam turbine), which in turn drives an electric generator to generate electricity. In between the solar field and the power block lays the storage system, where the HTF passes through the storage medium to store energy for later use. When using PCMs as the storage medium (i.e., solid-liquid), heat is transferred from HTF to the PCM raising its temperature to the phase change temperature (which is called melting point in the case of solid-liquid phase change). PCM keeps melting until it is transferred into a complete liquid indicating the end of the phase change process, hence the charging process as well. The opposite happens in the discharging process, at night or in cloudy weather, the concentrating mirrors are not capable of producing heat since no sunlight is available. The storage system takes place when the cooled HTF flows in the opposite direction into the storage tank. In this process, the PCM releases energy it has gained in the charging process to the heat transfer fluid which in turn heats up and travels out of the storage tank at a high temperature; to produce electricity (Figure 2).

Figure 1: temperature versus stored heat graph, illustrating the temperature of phase change where energy is stored as latent heat

2

Figure 2: a CSP plant with integrated thermal energy storage system charging and discharging processes.

2.2 Literature Review 2.2.1 Storage systems Solar energy is one of the most important and reliable energy sources in the world. It is harvested by different technologies such as solar collectors and photovoltaics. Concentrating solar power (CSP) is one of the solar thermal energy technologies; it converts sunlight to high-temperature heat by concentrating it in mirrors and lenses (from 25 to 3000 times of the sunlight intensity) [3], for large-scale power production. CSP technologies include: parabolic troughs, solar power towers, concentrating linear Fresnel reflectors and dish sterling. Although sunlight is a safe, clean and sustainable energy source, it has drawbacks as it is considered as a discontinuous source of energy; since the orbit, planet rotation and the weather conditions affect sunlight’s intensity. This instability leads to a mismatch between the energy demand (i.e., electricity) and the energy supply. By integrating a thermal energy storage system, the problem of mismatch will be avoided and full-load operation will be sustained [4]. An efficient thermal energy storage system plays an important role in keeping the stability of the power generation, thus improving the efficiency of the CSP plant. Thermal energy can be stored as sensible heat, latent heat and bond (thermochemical) energy. In sensible heat, energy is stored in or extracted from a substance without undergoing a phase change by increasing or decreasing its temperature, respectively. In addition to the temperature difference, the amount of energy stored by sensible heat depends on the specific heat of the substance. On the other hand, in latent heat, energy storage or extraction involve a phase change in the substance at constant temperature called the phase change temperature (i.e., melting point for solid-liquid phase change). The third type of storing thermal energy is the bond energy which is a mainly absorbing and releasing chemical energy by changing temperature and pressure to shift equilibrium [5]. Both sensible and latent heats are used as 3

methods of storing energy in CSP plants by different techniques. In current CSP plants, the techniques used to store energy are two-tanks and a single-tank. The two tank-storage technique uses two tanks; one at high temperature and the other is at low temperature. It can be classified into direct and indirect storage system. In the direct system, the heat transfer fluid flows from the low-temperature tank to the CSP. Once it’s heated up, it flows to the high-temperature tank for storage. Afterwards, HTF flows through a heat exchanger to generate steam, which drives a steam turbine to generate electricity. On the other hand, the indirect system uses two different fluids for storage and heat transfer. Indirect systems are used when the heat transfer fluid is very expensive or not suited to be stored. The storage fluid flows from the low-temperature tank to an additional heat exchanger to be heated by the high-temperature heat transfer fluid. Afterwards, the heated storage fluid flows to the high-temperature storage tanks, while the cooled HTF flows back to the CSP to be heated again. Then, electricity is generated by the high-temperature heat transfer fluid in the same manner of the direct storage system. However, using indirect storage system increases the cost; since it uses an additional heat exchanger. Figure 3 illustrates a two-tank storage system integrated in a CSP plant (i.e., parabolic trough).

Figure 3: two-tank thermal energy storage system

The single-tank thermal storage system, stores energy in a single tank and a solid medium as shown in (figure 4). In the tank, there is a temperature gradient such that the top of the medium is at high temperature while the bottom is at low temperature at any time. The temperature gradient which is called a “thermocline”, maintains the separation between the hot and cold parts. For storing energy, a high-temperature HTF flows from the top of the tank to the bottom where it exits at a low temperature. Energy is then transferred from HTF to the storage medium and the thermocline is moved downward. By reversing the process, HTF 4

flows from the bottom at low temperature and exists the tank through the top at high temperature, moving the thermocline upward and releasing the thermal energy stored before to generate steam for electricity generation. Moreover, the thermocline is maintained and stabilized by the thermal stratification (Figure 5), which is created within the tank by the buoyancy effect. It is clearly seen that the single-tank storage system has a lower cost than the two-tank system; since it uses one tank only, which reduces the cost [6].

Figure 4: single-tank thermal energy storage system

Figure 5: thermal stratification effect in a tank. Retrieved from Invalid source specified.

The dominant technology for thermal energy storage that is commercially available for current CSP plants is the two-tank molten salt storage tanks. One of the very first CSP plants with integrated thermal energy storage system is the solar electric generating 1 (SEGS 1) [7], 5

which was started in 1984 in California’s Mojave Desert. The plant combined with the other 8 plants in Mojave Desert produced more than 350 megawatts, while it has an integrated twotank direct storage system with a capacity of three hours. Another well-known CSP plant is Andasol-1 [8], which is the first CSP plant that uses parabolic troughs in Europe. Andasol-1 started operating in 2008 in southern Spain. Moreover, it produces over 50 megawatts and uses the two-tank indirect technology to store thermal energy in 28,500 tons of molten salt for 7.5 hours storage capacity. Andasol-1 was followed by Andasol-2 and Andasol-3 in 2009 and 2011, respectively, where the three plants use the two-tank indirect thermal energy storage of molten salts. Termosol-1 is another concentrating solar power plant [9] that is located in Spain and was started in 2013. It produces around 50 MW and has a two-tank indirect storage system with a capacity of up to 9 hours, which uses molten salt as the storage medium (i.e., Sodium and Potassium Nitrate). In 2015, NOOR 1 CSP plant was built in QuarzazateMorocco to produce energy of 160 MW. The plant has a storage capacity of 3 hours, which is by using molten salts in two-tank indirect storage system. It is clearly seen that molten salts in two-tank storage systems are almost the only commercial thermal storage system available in the market. Recently, the attention is shifted from two tanks of molten salts toward phase change materials (PCM) as a thermal energy storage medium. As PCMs stores energy in the form of latent heat, the storage materials are minimized which results in lower cost compared to that in sensible heat storage. Several low temperature PCMs have been studied for numerous applications [10]. On the other hand, high temperature phase change materials are still under developments and studies. PCMs with high melting temperatures are very beneficial, as they would increase the thermal capacitance of the system. Researches nowadays, are concentrating on the high melting temperature phase change materials for thermal energy storage in CSP plants. Verlaj et al. [11] conducted a study and proposed a technique for enhancing the phase change materials heat transfer. Esakkimuthu et al. [12] studied the storing and releasing of excess solar energy through an inorganic salt based phase change material. They developed a mathematical model to study the melting and solidification process using different methodologies [13]. Park and Chun [14] used the finite difference method for discretization by using a fixed grid approach. Later on, a fixed grid was developed by Voller [15], on the phase change process that is conduction controlled, by implementing an implicit technique. The results of his study showed that this technique is faster than the previous ones. Further study by Zhang and Khodadadi [16] on the effects of convection driven by buoyancy on the melting of phase change materials in spherical containers was done by the use of iterative finite-volume method. They used Darcy’s law to account for the convection by considering a porous media. The result of their study was that the melting process is accelerated by convection (i.e., buoyancy-driven) compared to the melting by diffusion. Moreover, the solidification of several phase change materials was studied by Moraes and Ismail [17] in cylindrical and spherical shells at a constant surface temperature. They concluded that the surface temperature, diameter and material affect the solidification process.

6

Using these studies and researches, multiple designs of thermal storage system using PCMs were proposed and investigated. Screw heat exchanger (SHE) is one of the proposed design [18]. The system consists of a drum and hollow shafts in parallel to each other. The heat transfer fluid passes through the hollow heating or cooling the PCM during charge or discharge, respectively, until the PCM reaches the phase change temperature and the phase change takes place. Moreover, the system is a two-tank storage system that uses PCM instead of the conventional molten salts. Another design of thermal storage using PCM, is the conventional shell and tube heat exchanger (STHE) [19]. It has a simple structure and it is the most considerable type of heat exchangers using PCM. STHE is said to be a very promising technique for latent heat storage by phase change materials. One more design using PCM’s latent heat is the heat exchanger with module (HEM) [20]. The design uses Macroencapsulated PCMs that are placed in cylindrical of spherical modules where the HTF flows in between. Although this design is not commercial for high melting temperatures PCMs yet, it is said to be able to solve the volume change and the material compatibility issues. 2.2.2 Phase change materials Phase change materials (PCM), are substances that store high amount of latent energy during melting and solidifying at their specified temperature called melting point. Within the past decade PCM has been effective in storing thermal energy for different cooling / heating applications, especially for concentrated solar power plants systems (CSP). The uses of PCM in storing the thermal energy of a CSP system increase its efficiency and performance by increasing the storage capacity and providing more isothermal behavior. The latent heat can be stored and released in two states, charging and discharging, respectively. During the charging process the PCM change its phase from solid to liquid storing the latent energy that has been absorbed from the heat transfer fluid received by the solar field. While in discharging process the opposite take place where the PCM solidifies releasing the latent energy to the heat transfer fluid. 

History review

The use of PCM in energy storage plants and buildings has been investigated long time ago. Earlier on 1940, Dr.Telkes introduced the application of PCM in energy storage technologies [21]. The integrating of PCM was mostly for heating or cooling buildings such as implementing it on the building walls where a passive solar system was one of the earliest applications. Later on, more researches were developed for investigating the use of PCM in thermal storage plants. Throughout 1980s, encapsulated paraffinic PCM had been produced. Studies show that taking the benefit of PCM’s storage capacity enhances the thermal performance of the system [20]. 

Classifications

Phase change materials can be classified into three different families depending on their properties of different temperature range. Including organic, inorganic, and eutectic. 7

-

Organic:

Organic materials melt and solidify without phase separation (segregation), they are noncorrosive, crystallize with little or no super-cooling, and melt with a congruent or a little change in volume. Organics can be categorized into two types: Paraffin’s and Non-paraffin’s. Paraffin is a compound of hydrocarbon molecules of general formula (CnH2n+2), and mostly straight chain n-alkanes (CH3–(CH2)–CH3), where CH3 chain release high amount of latent energy when it crystallize. Paraffins in general are available in a wide range of temperature, have high heat of fusion, non-reactive, chemically stable below 500 , and reliable. As amount of carbon atoms increases in the paraffin alkane, its melting point increases in which it become practical for CSP thermal energy storage. On the other hand, parrafins have low thermal conductivity, flammable, and incompatible with plastics. When choosing a paraffin material, technical grade materials are selected and that is due to the cost. Non-Paraffins are composed of carboxylic acid, and have a general formula of (CH3(CH2)2nCOOH ). These materials represent the widest sections of the phase change materials for energy storage, that each of them have its own properties when compared to paraffins, where most of them have similar characteristics. Non-paraffins can be further classified into Fatty acids and other non paraffins materials. However, the most suitable paraffin materials for thermal storage are fatty acids, glycols and alcohols. When compared to paraffins , fatty acids has much higher heat of fusion values , but a 2 – 2.5 times higher cost. The level of toxicity of fatty acids is mainly variable. In addition, they are unstable when exposed to high temperature and have a low thermal conductivity. -

Inorganic

Inorganic materials are classified into two categories: Salt hydrates of general formula (MnH2O), and metallic. Most of in-organics are unrecyclable, and have no large enough super-cooling. Salt hydrates are mainly two alloys of water and inorganic salt forming a crystalline solid that separate into anhydrous salt and water, or into a lower hydrate and water at the melting point. Most salt hydrates have inconsistent melting, because the water released is not capable enough to dissolve all solid particles in the solution due to the difference in density, where the anhydrous salt is being collected down in the container. Other problems with salt hydrates is their poor nucleating properties, in which it can be solved by adding an effective nucleating agents than can provide the initial nucleon to form a crystal. In addition, salt hydrates melt incongruently, and to overcome this problem, different methods can be applied including:  Adding mechanical stirring.  Encapsulating the PCM to reduce separation.  Adding of the thickening agents that prevent setting of the solid salts. 8

 Use of excess of water so that melted crystals do not produce supersaturated solution.  Modifying the chemical composition. On the other hand, salt hydrates are attractive materials due to their high thermal conductivities unlike the paraffin’s and fatty acids from organics; they are also compatible with plastics and non-corrosive. In addition, salt hydrates perform a slight change in volume when melting with a high heat of fusion. Metallic's have the highest thermal conductivity among all PCMs but, they are not being seriously used in PCM developments due to their large weight. Metallic's can also provide high heat of fusion, and low vapor pressure, but a low specific heat capacity. -

Eutectics

Eutectics are composed of two or more components: organic-organic, nonorganicnonorganic, and organic-nonorganic. Those compounds solidify and liquefy with no segregation, they also provide a slightly higher storage density volume than organics. The problem with eutectics is the limited data available about their properties due to the combined structure. Furthermore, eutectics represent the highest cost among the phase change materials [22].

2.3 Design Criteria Since our thermal energy storage system consists of multiple components, sets of design criteria for each component will be different. In general, we aim to maximize the heat transfer with the most effective cost using available and simple components. The established criteria for each component will meet these goals with different approaches. 

Overall thermal energy storage system criteria

As mentioned before, thermal energy storage system plays a key role in the stability of power generation and meeting the energy demand in CSP plants; however, there is only a few tested energy storage systems with high temperature around the world. Thermal energy storage system must meet the following criteria [23]:  High storage capacity or energy density.  Chemically and mechanically stable (the ability to store and release energy as heat without degradation for number of cycles.  Efficient heat transfer between the heat transfer fluid and the storage medium.  Compatibility (The heat transfer fluid, heat exchanger and storage medium must be compatible with each other).  Reversibility (the storage medium must extract the maximum amount of energy from the HTF during charging and release this energy during discharging then return to the initial state).  Minimum thermal losses.

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 Cost-effective (the benefit from the system most compensate for its cost and more, otherwise money will be spent uselessly). 

PCM selection criteria

The selection of a phase change material is mainly upon its melting point that depends on the operating temperature range of the HTF. However, other main desirable characteristics need to be in a phase change material for a thermal storage system to avoid heat transfer losses and enhance the effectiveness of the system. The required properties for the design criteria can be classified into thermodynamic, chemical, economic and nucleation properties. The most important ones of these includes [5]:  High latent heat of fusion: represent the need of high variation in the enthalpy produced when heat is applied to change the material phase at a constant pressure.  Chemically stable: represent of how stable is the material when exposed to different chemical variations in its surrounded environment.  Non-corrosive: this insures that the material will not damage or destroy other substances that are in contact with along high temperature and high-pressure cycles.  Non -toxic: represent the requirement of not causing harm on the environment society and organism.  High heat storage capacity: represents the ability of the material to store high amount of energy per its volume and mass.  Congruent melting: to ensure that the PCM will melt and solidify in equal and uniform compositions.  Non-flammable: to ensure that the material doesn’t burn or cause combustion.  Compatible: represents the ability of the material to exist with used design materials without any problems.  Low-priced.  High thermal conductivity: represent the material fast rate in conducting the heat to other substances.  Avoid sub-cooling: the criteria of avoiding liquefying below its boiling point.  Avoid super-cooling: the criteria of avoiding to solidify above the meting point.  No degradation: represents avoiding to be destroyed during the process of phase change. However, the most important criteria’s that needs to be highly considered and existed in the PCM material to ensure an appropriate design are: the high thermal conductivity, the Noncorrosiveness, and low cost. In addition, the melting point range and the availability of the PCM has to be investigated. 

Heat transfer fluid selection criteria

The selection of heat transfer fluid will be based on the following criteria: 10

 Availability: this criterion determines the commercial availability of HTF.  Stability: this criterion indicates the stability of the HTF at operating temperature over the lifetime.  Handling complexity: this criterion detects the handling of the HTF in the system and wither its possible or not.  Phase change temperatures & vapor pressure: this criterion indicates wither the HTF suites the system operating temperature and pressure. 

Heat exchanger design criteria

When choosing heat exchanger type design criteria are prioritized as following:  Availability: this criterion determines the commercial availability of components and material.  Compatibility: this criterion indicates if the component is compatible with other components in the system so that no corrosion or contamination will occur.  Implementation complexity: this criterion detects the implementation of the system complexities and handling issues.  Cost effectiveness: this criterion studies the worth of benefits in terms of money and investigates either it is desirable or not.  Performance efficiency: this criterion studies heat exchanger factors and parameters that affect performance efficiency. When choosing heat exchanger components material, sets of criterion are prioritized as following:  Availability: this criterion determines the commercial availability of components and material.  Compatibility & Stability: this criterion indicates if the component is compatible with other components in the system so that no corrosion, contamination or failure will occur.  Thermal conductivity: this criterion defines the ability of material to conduct heat.  Cost: this criterion represents the cost of material per unit of mass or volume.  Density: this criterion indicates the size and portability of the design. When choosing heat exchanger insulating material, sets of criterion are prioritized as following:  Availability: this criterion determines the commercial availability of components and material.  Thermal conductivity: this criterion defines the ability of material to conduct heat.  Maximum operating temperature: this criterion indicates the maximum temperature that the insulation material can handle.  Cost: this criterion represents the cost of material per unit area. 11



Heat transfer enhancement design criteria

When choosing heat exchanger and its components material, criteria are prioritized as following:  Applicability: this criterion indicates either the method is applicable or not.  Compatibility: this criterion indicates if the component is compatible with other components in the system so that no corrosion or contamination will occur.  Implementation complexity: this criterion detects the implementation of the system complexities.

2.4 Our Design In our design, a heat exchanger with PCM thermal energy storage system will be implemented to store heat in the form of latent energy for later use. In this report, alternatives of storage system, heat exchanger design, PCMs and the materials required to build heat exchanger will be compared. Then, the choice will be based on design criteria priority. Later, calculations will be conducted in order to obtain optimum design parameters. After that, the design will be up-scaled in order to be integrated in CSP plant. Moreover, ANSYS simulation will be conducted and the prototype will be built and tested.

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3 Alternative Solutions 3.1 Introduction The performance of thermal energy storage system depends on the design of heat exchanger, PCMs, and heat transfer fluid (HTF). There are many researches that compare different heat exchanger designs performance. In order to design an efficient thermal energy storage system, the alternatives of heat exchanger design, PCMs and HTFs will be compared based on the mentioned criteria above to come up with an optimum final design decision.

3.2 Thermal energy storage system As mentioned before, thermal energy storage can be by sensible heat, latent heat or thermochemical reaction. Sensible and latent heats can be implemented by several techniques such as two-tank and single-tank storage systems in CSP plants. Two-tank thermal energy storage system is divided into direct or indirect storage system. 

Thermal energy storage by sensible heat and latent heat:

The energy is stored in sensible heat form, by a temperature difference without undergoing a phase change. While the substance undergoes a phase change when the energy is stored as latent heat, which occurs at a constant temperature called the phase change temperature. The thermal losses are minimized in the latent heat storage, since there is no temperature difference and the energy is stored and released at almost a constant temperature. In addition, the phase change materials have high energy density, where they store large amount of energy in a small volume, which makes the latent heat energy storage very attractive. 

Two-tank direct storage system:

Two-tank direct storage system uses the same fluid as a heat transfer fluid and a storage fluid while they are circulated in a hot and a cold temperatures tank. Although it is a commercially available storage technique in current CSP plants, two-tank technique requires large volume and large amounts of materials, which make it costly. 

Two-tank indirect storage system:

Two-tank indirect storage system (i.e. hot and cold temperatures tanks) is similar to the direct system, but it uses two different fluids for the transfer and storage. This due to the heat transfer fluid being expensive. The system uses an extra heat exchanger to heat the storage fluid by the hot heat transfer fluid. The addition of the heat exchanger adds cost to the system. 

Single-tank storage system:

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In single-tank storage system the temperature is distributed such that the top of the tank is at high temperature, while the bottom is at low temperature. The temperature gradient, which is also called the thermocline, is maintained by the thermal stratification. Reducing the number of tanks, thus the materials used, reduces the cost considerably, which makes the single-tank storage system attractive from the cost perspective. Comparing the alternatives of the thermal storage system techniques, the latent heat seems to be better than sensible heat in terms of the minimized thermal losses and the small sized system with high-energy storage. However, comparing the three technologies, the single-tank storage system appears to be cost effective in terms of the reduced number of tanks and materials. From this perspective, we chose our system to be a single-tank that stores latent heat in phase change materials.

3.3 Phase change material 3.3.1 Type of containments: In the industrial market, phase change materials are being integrated within the design in two main ways: 

Encapsulation:

Encapsulation is one of the methods that help in preventing the leakage of the liquid PCM phase, controlling the volume change and reducing the interaction of the PCM within the environment. The encapsulation material used has to meet some main requirements that include: strength, corrosion resistance, and appropriate surface cover for heat transfer, flexibility and stability. However, encapsulating the PCM within the storage system is classified in three types including: Bulk storage encapsulation, microencapsulation, and macro-encapsulation. Bulk storage encapsulation can be applied for tank storage designs that consist of tank heat exchanger for which PCM spherical capsules are immersed inside it. This type of design is limited by the need of large heat transfer are between the PCM and the working fluid. This problem can be overcome by the addition of fins and fibers in the PCM side. The macro-encapsulation consists of a spherical or rod shaped thin polymeric film, where the PCM is being engulfed inside it. The diameter of microcapsules varies from 1 m to 1 mm. On the other hand, macroencapsulation is the most common method of encapsulating the PCM, because it's easy to be applied in manufacturing and it has more than 70% higher density. The PCM can be packed in the forms of tubes, spheres and panels where the diameter of microcapsules is larger than 1mm. [24] 

Bulk storage in tank:

In bulk tank storage, such as the shell and tube the PCM is applied by filling in the inner shell space around the tube. The heat exchange happens between the PCM and the HTF when the 14

fluid flows within the tube. Furthermore, Pipes or tubes materials should be highly conductive to enhance the rate of heat transfer. 

PCM Categories:

Relating to the design criteria mentioned earlier, a comparison analysis between the main selections categories was applied in the following table: Table 1: Comparison between organic and inorganic PCMs properties [25] Properties/PCM type

Heat of fusion Specific heat

Organic Increases with chain length and higher than salt hydrates

Corrosion

Higher specific heat capable of storing more sensible heat Not corrosive

Stability

Chemically stable and unreactive

Flammability

Thermal conductivity Toxicity

Super-cooling Cost and Availability

In-Organic Increases with degree of hydrates but higher hydrates have incongruent and semi congruent melting Lower specific heat than organic PCM Corrosive in the presence of H2O Less chemical stability, degrades by oxidation, by hydration, thermal decomposition Low flammability based on material

Flammable in the presence of O2 at elevated temperature due to law vapor pressure low fire hazard Low thermal conductivity Has higher thermal conductivity Innocuous irritants

neither

toxic

nor Less toxic. Irritation occurs and contact with skin should be avoided No or little super cooling occurs Degree of super cooling is greater Higher initial cost than inorganics Lower cost and easily available and less availability

Eutectic material properties depend on the type of its combination, it could take most of inorganic properties if it’s an inorganic eutectic or organic properties if it’s organic eutectic, it can also show both properties if it was a combination of both inorganic and organic. Subcooling and segregation in eutectic materials can hinder the heat transfer rate and decrease the latent heat capability along the thermal cycles due to the resulted irreversible process along the thermal cycles.

3.3.2 Selection:

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In the design, phase change materials are being selected from different categories. The melting temperature and the availability of the PCM is one of the main selection concerns. Figure 6 below shows a result of a research that has studied the melting temperature range for

Figure 6: PCM groups with their heat of fusion and melting temperature range [54] different PCM types:

As shown in the Figure 6, not all PCM’s are suitable for our thermal storage design where the temperature range needed falls from 320 . However, inorganics are most suitable for our storage design where their melting temperature range is above 150 . In addition, inorganic molten salts are the most available ones and include the most appropriate design criteria. Another study to classify the PCM types Figure 7: Classification of PCM with examples of materials used [54] including their melting temperature is shown in ( Figure 7). 16

To illustrate, the most common PCM alternatives according to the melting point and storage capacity is shown in the Figure 8 below:

Figure 8: Heat capacity (a) and media cost (b) of high melting point PCMS [24] Our choice of the phase change materials was based on the melting point, availability, and the optimum design criteria analysis for each PCM. As a result, the following three PCM’s where selected:   

Sodium Nitrate (NaNO3), inorganic. Sodium Nitrite (NaNO2), inorganic. Sodium Nitrate and Potassium Nitrate compound (40%KNO3-60%NaNO3), Eutectic.

Thermal and chemical properties data of each PCM selected is shown in Table 2, for further details refer to Appendix C. Table 2: Chosen PCMs properties and cost Name

Sodium Nitrate

Sodium Nitrite

Sodium Nitrate17

Material

Properties

Chemical NaNO3 formula Classification Inorganic Melting temperature 308 (°C) [26] Decomposition temperature 380 (°C) [26] Molecular weight (g/mol) 84.99 [26] Volume change 10.7 (∆V/Vs) [27] Heat of fusion (KJ/Kg) 200 [27] Specific heat Cp=1822.7 capacity For600= 4000) disp('Flow is turbulent') rel.rough=(0.05*10^-3)/dpipe disp('find friction factor from mody chart at these values')

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disp(Re) disp(rel.rough) ff=input('friction factor value:') if (pr >=0.5 && pr