1. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Chapter 4: Thermal Energy Storage and Transport (First Part) D. Yogi Goswami, Frank Kreith, Jan F. Kreider Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering

2. D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Outline  Introduction Introduction  Thermal Energy Storage Types Thermal Energy Storage Types  Sensible Heat Storage Sensible Heat Storage  Latent Heat Storage Latent Heat Storage  Thermochemical Storage Thermochemical Storage  Design of Storage System Design of Storage System  Selection of Storage Material Selection of Storage Material  Design of Containment Design of Containment  Heat Exchange Design Heat Exchange Design  Energy Transport Subsystems Energy Transport Subsystems  Piping Systems Piping Systems  Heat Exchangers Heat Exchangers  Thermochemical Storage Thermochemical Storage Thermal Energy Storage Outline

3. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Energy Needs Introduction  Storage is necessary whenever there is a mismatch between the energy available and the demand.  Storage is especially important in solar applications because of the seasonal, diurnal, and intermittent nature of solar energy.  Energy storage is accomplished by devices or physical media that store some form of energy to perform some useful operation at a later time  All forms of energy can be stored except electromagnetic radiation. Thermal Energy Storage Introduction  This chapter analyzes in detail the storage of thermal energy.

4. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering

5. D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Important Considerations for TES  For selecting a thermal energy storage system it is important to consider the design and operating conditions of the system.  Important design criteria are:  Duration of storage  Energy density (kJ/m 3 ) or specific energy (kJ/kg)  Charging and discharging or storage and recovery  Economics  The energy density is a critical factor for the size of a system.  The rate of charging and discharging depends on thermophysical properties such as thermal conductivity and design of the system.  The thermal energy storage system design must be compatible with the application, have low thermal losses and have high efficiency. Thermal Energy Storage Introduction

6. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Types of Thermal Energy Storage Thermal Energy may be stored as Sensible Heat, Latent Heat or as Heat of reaction (Thermochemical) For moderate temperature changes, such as in solar space and water heating systems, the density and specific heat may be considered constant.

7. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering

8. D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Sensible Heat Storage Thermal Energy Storage Types of Thermal Energy Storage  Water has the highest specific heat (4.19 kJ/kg.K) and is the most common medium for storing sensible heat for low and medium solar heating and cooling systems Advantages and Disadvantages of using water as storage medium

9. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Types of Storage Latent Heat Storage: Thermal energy may be stored as latent heat if a material undergoes phase transition at temperature that is useful for the application. If a material with a phase change temperature of T m is heated from T 1 to T 2 such that T 1 < T m < T 2, the thermal energy Q stored in a mass m is Where  heat of phase transition. Types of Phase transitions: Solid Liquid Solid Vapor Liquid Vapor Solid

10. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Latent Heat Storage Thermal Energy Storage Types of Thermal Energy Storage  Latent thermal energy is stored and retrieved at a fixed temperature known as transition temperature.  Most common phase change materials for solar energy storage undergo solid ↔ liquid transformation and their stored thermal energy is given by

11. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Latent Heat Storage Thermal Energy Storage Types of Thermal Energy Storage Physical properties of latent heat storage materials or PCMs

12. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Thermochemical Energy Storage Types of Thermal Energy Storage Introduction  A large amount of heat can be stored in a small quantity of material as chemical reaction is a highly energetic process.

13. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Thermochemical Energy Storage Thermal Energy Storage Types of Thermal Energy Storage  Some examples of reactions include decomposition of metal hydrides, oxides, peroxides, ammoniated salts, carbonates, sulfur trioxide etc. Properties of thermochemical storage media

14. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Selection of Storage Material Design of a storage system involves  selection of storage material,  design of containment,  design of heat exchangers for charging and discharging Selection of storage material is the most important part of the design and it depends on,  solar collection system,  Application, and  additional considerations Thermal Energy Storage Design of Storage System

15. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Selection of Storage Material  A. Solar collection system determines the temperature at which the storage material will be charged and the maximum rate of charge.  For liquid type flat plate and moderately concentrating solar systems, water and glycol-water mixtures are used as the most common storage materials.  For parabolic trough concentrators, high temperature oils or molten salts may be used.  For central receiver tower, molten salts or metals may be used.  Molten nitrate salt, (50 wt% NaNO 3 /50 wt% KNO 3 ) has been used in the first commercial demonstration of generating power with storage at Albuquereque, NM and also in Solar –Two project at Barstow, CA. Thermal Energy Storage Design of Storage System

16. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Selection of Storage Material  B. Application determines the temperature at which the storage will be charged and discharged and the maximum rate of discharge.  For hot water applications and moderate industrial process heat, water or PCM can be used  For heating and cooling of buildings, PCMs can be used. The containment of PCMs can become integral part of the building  For space-heating applications, PCMs (mostly salt hydrates) contained in tubes, rods, trays, panels, canisters and tiles have been studied in 1970s and 1980s.  Paraffin mixtures have been used for thermal storage in wall boards Thermal Energy Storage Design of Storage System

17. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Selection of Storage Material Thermal Energy Storage Design of Storage System C. Additional Considerations:  Space Requirements  Long term Cycling stability  Corrosivity  Complexity of Containment System  Complexity of Heat Exchanger Design for maximum charge and discharge rates

18. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Design of Containment  Microencapsulation involves very small particles of a PCM dispersed in a single phase matrix, usually used for high TES in buildings.  Some examples include PCMs encapsulates in concrete, floor tiles, and wallboard.  Wax pellets were used for development of a composite wall board in a research sponsored by USDOE.  Most successful developments have been made in macroencapsulation of PCMs. Thermal Energy Storage Design of Storage System

19. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering

20. D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Design of Containment Thermal Energy Storage Design of Storage System Examples of PCM containment

21. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Design of Containment Thermal Energy Storage Design of Storage System Thermal conductivity of potential containment materials

22. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Heat Exchanger Design Thermal Energy Storage Design of Storage System  For liquid storage media, heat exchangers of the shell and tube type Or submerged coil type are generally used  For solid storage media or macro-encapsulated PCMs, normally a packed bed type storage configuration is designed. In this case, the heat is transferred to or from a heat transfer fluid by flowing the heat transfer fluid through the voids in the bed

23. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Heat Exchanger Design Thermal Energy Storage Design of Storage System Pebble bed storage system (left) and TES unit with PCM encapsulated in tubes Average void fraction for packed beds

24. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Heat Exchanger Design Thermal Energy Storage Design of Storage System  For predicting the heat transfer coefficient in packed beds, the following correlation by Beasley and Clark can be used for Re of 10-10,000

25. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Heat Exchanger Design Thermal Energy Storage Design of Storage System Or the mass flow rate of air divided by the minimum free flow area.

26. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Heat Exchanger Design Thermal Energy Storage Design of Storage System To find the pressure drop in packed beds, the following equation developed by Ergun [10] may be used:

27. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Heat Exchanger Design Thermal Energy Storage Design of Storage System Schematic of packed bed storage Rocked bed performance map  Alternately, pressure drop can also be found from figures below  For flow across tube bank,

28. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Piping Systems Thermal Energy Storage Energy Transport Subsystems  A. Heat Exchangers: Heat exchangers are devices in which two fluid streams exchange thermal energy: one stream is heated while the other one is cooled.  To transport solar heat from a solar collector to storage, and then to an end use, an energy transport subsystem is used. It consists of pipes, pumps, expansion tanks, valves and heat exchangers.

29. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Piping Systems Thermal Energy Storage Energy Transport Subsystems Double Pipe Heat Exchanger  Heat transfer coefficient is based on outside area of the inner tube  Local rate of the heat transfer across the tube is

30. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Piping Systems Thermal Energy Storage Energy Transport Subsystems  The flow arrangement can be parallel flow or counter flow type. Temperature Distribution for Parallel Flow (left) & Counter Flow Heat Exchanger (right)  The rate of heat transfer for the method employing a mean temperature between the fluids is given by:  If a counter flow arrangement is made very long, it approaches the thermodynamically most efficient possible heat transfer condition.

31. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Piping Systems Thermal Energy Storage Energy Transport Subsystems The other method is called the Effectiveness – NTU method Effectiveness

32. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Piping Systems Thermal Energy Storage Energy Transport Subsystems  For hot fluid, if C h = C min  For cold fluid, if C c = C min  The fluid that undergoes the maximum temperature is the one that has minimum value of heat capacity  The maximum heat transfer rate is given by:

33. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering

34. D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering

35. D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Piping Systems Thermal Energy Storage Energy Transport Subsystems Use of Heat Exchanger to isolate a Collector The heat exchange factor is given by, - heat exchanger effectiveness U c - Collector loss coefficient - Mean collector operating temperature

36. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Piping Systems Thermal Energy Storage Energy Transport Subsystems  B. Pressure drop: Friction at pipe walls, bends, valves etc. results in flow resistance and pressure drop.  Pumps are used to overcome the pressure drop and the sizing of pumps is done by calculating two quantities, the friction factor, f, and pipe length, L.  Two parasitic losses occur in pipes – pressure drop and heat loss.  Darcy friction factor is defined as

37. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Piping Systems Thermal Energy Storage Energy Transport Subsystems  For engineering calculations, the following figure is used for estimation of friction factor Friction factor for pipe flow as a function of Re

38. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Piping Systems Thermal Energy Storage Energy Transport Subsystems  The viscosity values of some common heat transfer fluids used in high temperature solar systems range from 0.2 to 5.0 cp Roughness factor Properties of common heat transfer fluids

39. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Piping Systems Thermal Energy Storage Energy Transport Subsystems

40. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Piping Systems Thermal Energy Storage Energy Transport Subsystems Properties of pipe insulation for elevated temperatures