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1 Introduction
An increasing percentage of renewable energy is being deployed in the total amount of
global energy supply in light of the challenging criteria and scenarios for the reduction
of emissions of greenhouse gases from using fossil fuels. However, the biggest drawback of renewable energy is the intermittence of harvest and conversion, most of which
is quite time-and-climate dependent. To mitigate or work on these issues, various
materials and devices have been designed to store the excess energy in different forms,
such as mechanical storage in flywheels, electrochemical storage in batteries, and thermal energy storage (TES) in phase change materials (PCMs), solids or molten salts.
(Note: Heat is defined as thermal energy in transit, which is a process function in a
thermodynamic system. In this chapter, thermal energy and heat are used as exchangeable terms to which no depiction of a particular process is referred). Figure 1 shows a
brief working process of heat source utilisation with the integration of TES [1–3].
Commonly, the TES systems are classified by storage concept (active or passive)
and storage mechanism (sensible, latent or chemical/thermochemical) [4]. In the
sensible heat storage, sensible TES materials (e.g., steam, oils and molten salts)
undergo no changes in phase over the operating temperature range [1]. Latent heat
storage using phase change materials (PCMs) works in a nearly isothermal way
during melting/solidification or gasification/liquefaction processes whilst storing or
releasing thermal energy [5]. Thermochemical energy storage utilises sorption
materials. For example, silica gel, magnesium sulphate, lithium bromide, lithium
chloride and sodium hydroxide are all capable of absorption or adsorption of water
vapour. Since the sorption process of reactive components is thermally reversible,
high heat storage capacity can be achieved with negligible thermal losses during the
storage period [3].
In addition, the TES system can also be categorised into low, medium and high
temperature TES systems by their operating temperatures. Low-temperature TES
takes advantages of lower rates of off-peak electricity based on a short-term storage
capacity in a day. The storage medium is working at operating temperatures
comparable with the spatial temperature in heating/cooling applications. Chilled
water, phase change materials (PCMs), ice or cryogen (e.g., liquid air and liquid
nitrogen) are common options. Medium-temperature TES usually operates at the
temperature slightly higher than the spatial temperature range in the facilities such
as solar hot water, air heating and solar heating/cooling units in buildings and
G. Z. Chen (*)
Department of Chemical and Environmental Engineering, Faculty of Science and
Engineering, University of Nottingham Ningbo China, Ningbo, People’s Republic of China
Energy Engineering Research Group, Faculty of Science and Engineering, University of
Nottingham Ningbo China, Ningbo, People’s Republic of China
Department of Chemical and Environmental Engineering, Faculty of Engineering,
University of Nottingham, Nottingham, UK
Advanced Materials Research Group, Faculty of Engineering, University of Nottingham,
Nottingham, UK
e-mail: george.chen@nottingham.ac.uk
X. Guo et al.
1 Introduction
An increasing percentage of renewable energy is being deployed in the total amount of
global energy supply in light of the challenging criteria and scenarios for the reduction
of emissions of greenhouse gases from using fossil fuels. However, the biggest drawback of renewable energy is the intermittence of harvest and conversion, most of which
is quite time-and-climate dependent. To mitigate or work on these issues, various
materials and devices have been designed to store the excess energy in different forms,
such as mechanical storage in flywheels, electrochemical storage in batteries, and thermal energy storage (TES) in phase change materials (PCMs), solids or molten salts.
(Note: Heat is defined as thermal energy in transit, which is a process function in a
thermodynamic system. In this chapter, thermal energy and heat are used as exchangeable terms to which no depiction of a particular process is referred). Figure 1 shows a
brief working process of heat source utilisation with the integration of TES [1–3].
Commonly, the TES systems are classified by storage concept (active or passive)
and storage mechanism (sensible, latent or chemical/thermochemical) [4]. In the
sensible heat storage, sensible TES materials (e.g., steam, oils and molten salts)
undergo no changes in phase over the operating temperature range [1]. Latent heat
storage using phase change materials (PCMs) works in a nearly isothermal way
during melting/solidification or gasification/liquefaction processes whilst storing or
releasing thermal energy [5]. Thermochemical energy storage utilises sorption
materials. For example, silica gel, magnesium sulphate, lithium bromide, lithium
chloride and sodium hydroxide are all capable of absorption or adsorption of water
vapour. Since the sorption process of reactive components is thermally reversible,
high heat storage capacity can be achieved with negligible thermal losses during the
storage period [3].
In addition, the TES system can also be categorised into low, medium and high
temperature TES systems by their operating temperatures. Low-temperature TES
takes advantages of lower rates of off-peak electricity based on a short-term storage
capacity in a day. The storage medium is working at operating temperatures
comparable with the spatial temperature in heating/cooling applications. Chilled
water, phase change materials (PCMs), ice or cryogen (e.g., liquid air and liquid
nitrogen) are common options. Medium-temperature TES usually operates at the
temperature slightly higher than the spatial temperature range in the facilities such
as solar hot water, air heating and solar heating/cooling units in buildings and
G. Z. Chen (*)
Department of Chemical and Environmental Engineering, Faculty of Science and
Engineering, University of Nottingham Ningbo China, Ningbo, People’s Republic of China
Energy Engineering Research Group, Faculty of Science and Engineering, University of
Nottingham Ningbo China, Ningbo, People’s Republic of China
Department of Chemical and Environmental Engineering, Faculty of Engineering,
University of Nottingham, Nottingham, UK
Advanced Materials Research Group, Faculty of Engineering, University of Nottingham,
Nottingham, UK
e-mail: george.chen@nottingham.ac.uk
X. Guo et al.