Principles of Solar Engineering
Principles of Solar Engineering
Chapter 4: Thermal Energy Storage and Transport
(First Part)
D. Yogi Goswami, Frank Kreith, Jan F. Kreider
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Thermal Energy Storage
Outline
Outline
 Introduction
 Thermal Energy Storage Types
 Sensible Heat Storage
 Latent Heat Storage
 Thermochemical Storage
 Design of Storage System
 Selection of Storage Material
 Design of Containment
 Heat Exchange Design
 Energy Transport Subsystems
 Piping Systems
 Heat Exchangers
 Thermochemical Storage
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Thermal Energy Storage
Introduction
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
electromagnetic radiation.
be
stored
except
 This chapter analyzes in detail the storage of thermal
energy.
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Energy Needs
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D. Y. Goswami, F. Kreith, J. F. Kreider
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Thermal Energy Storage
Introduction
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/m3) 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.
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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.
D. Y. Goswami, F. Kreith, J. F. Kreider
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D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
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Thermal Energy Storage
Types of Thermal Energy Storage
Sensible Heat 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
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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 Tm is heated from T1 to T2
such that T1 < Tm < T2, the thermal energy Q stored in a mass m is
Where l = heat of phase transition.
Types of Phase transitions:
Solid Liquid
Solid
Vapor
Liquid Vapor
Solid
Solid
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Thermal Energy Storage
Types of Thermal Energy Storage
Latent Heat 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
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Thermal Energy Storage
Types of Thermal Energy Storage
Latent Heat Storage
Physical properties of latent heat storage materials or PCMs
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Types of Thermal Energy Storage
Introduction
Thermochemical Energy Storage
 A large amount of heat can be stored in a small quantity of material as chemical
reaction is a highly energetic process.
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Thermal Energy Storage
Types of Thermal Energy Storage
Thermochemical Energy Storage
 Some examples of reactions include decomposition of metal hydrides, oxides,
peroxides, ammoniated salts, carbonates, sulfur trioxide etc.
Properties of thermochemical storage media
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Thermal Energy Storage
Design of Storage System
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
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Thermal Energy Storage
Design of Storage System
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% NaNO3/50 wt% KNO3) 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.
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Thermal Energy Storage
Design of Storage System
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
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Thermal Energy Storage
Design of Storage System
Selection of Storage Material
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
D. Y. Goswami, F. Kreith, J. F. Kreider
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Thermal Energy Storage
Design of Storage System
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.
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Thermal Energy Storage
Design of Storage System
Design of Containment
Examples of PCM containment
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Thermal Energy Storage
Design of Storage System
Design of Containment
Thermal conductivity of potential containment materials
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Design of Storage System
Heat Exchanger Design
 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
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Design of Storage System
Heat Exchanger Design
Pebble bed storage system (left) and TES unit with PCM
encapsulated in tubes
Average void fraction for packed
beds
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Design of Storage System
Heat Exchanger Design
 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
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Design of Storage System
Heat Exchanger Design
Or the mass flow rate of air divided by the minimum free flow area.
D. Y. Goswami, F. Kreith, J. F. Kreider
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Thermal Energy Storage
Design of Storage System
Heat Exchanger Design
To find the pressure drop in packed beds, the following equation developed by Ergun
[10] may be used:
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Thermal Energy Storage
Design of Storage System
Heat Exchanger Design
 Alternately, pressure drop can also be found from figures below
Rocked bed performance map
Schematic of packed bed storage
 For flow across tube bank,
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Thermal Energy Storage
Energy Transport Subsystems
Piping Systems
 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.
 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.
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Piping Systems
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
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 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.
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The other method is called the Effectiveness – NTU method
Effectiveness
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 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:
 For hot fluid, if Ch = Cmin
D. Y. Goswami, F. Kreith, J. F. Kreider
 For cold fluid, if Cc = Cmin
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D. Y. Goswami, F. Kreith, J. F. Kreider
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Thermal Energy Storage
Energy Transport Subsystems
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Use of Heat Exchanger to
isolate a Collector
The heat exchange factor is given by,
Uc
- heat exchanger effectiveness
- Collector loss coefficient
- Mean collector operating temperature
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Piping Systems
 Two parasitic losses occur in pipes – pressure drop and heat loss.
 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.
 Darcy friction factor is defined as
D. Y. Goswami, F. Kreith, J. F. Kreider
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 For engineering calculations, the following figure is used for estimation of friction factor
Friction factor for pipe flow as a function of Re
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Roughness factor
 The viscosity values of some common
heat transfer fluids used in high
temperature solar systems range from 0.2
to 5.0 cp
Properties of common heat transfer fluids
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Properties of pipe insulation for elevated temperatures
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