**1. Introduction**

Thermal energy storage (TES) systems can be employed for both heating and cooling applications. TES is a process of storing heat from various sources like waste heat or solar thermal applications or electricity used at off-peak rates or can also be used in cooling applications. The heat transfer fluid (HTF) at low temperature is stored and used in peak hours of heating TES. The estimated market by the research study of the global thermal energy storage market was 4,281.6 Million USD in 2019 and is anticipated to grow upto USD 8558.34 Million by 2026. An annual compound rate of growth of the thermal energy storage market is going to be 10.4% from 2019 to 2027 as reported.

Various types of new and innovative TES are available and used for various purposes as follows:

1.Cooking and Greenhouse

2.Building passive heating and cooling from stable PCMs


The TES can be combined with waste heat recovery or solar thermal heating or cooling applications. In heating, electric-resistance efficiencies (near 100% on an energy basis) combined with lower off-peak electric rates can produce heating at a fraction of the cost of conventional systems. One of the applications in cold countries is to produce heat at night with air heating using electrical resistance heater and stored in such storage media like ceramic bricks in insulated containers. The heat produced due to electric off-peak rates in the night is advantageous and generally 33–75% cheaper than the peak rates. On demand of the space heating, the stored heat is transferred to the room in the peak hours.

Another important application is the Concentrating Solar Power (CSP) plants which are important solar energy technologies where sunlight is converted to hightemperature heat by concentrating it in mirrors and lenses from 25 to 3000 times of the solar light intensity. If heat transfer fluids like sodium are used, heat at high temperatures (3000<sup>0</sup> C) can be retrieved. Sunlight being an intermittent source of energy that can produce heat helps to fill the gap between the energy demand and supply. A thermal energy storage is designed based on the diurnal, seasonal or annual storage needs and integrated into the system to sustain full or partial load operation.

This thermal energy can be stored at any particular temperature by using sensible or latent heat or both sensible and latent heat and can also be designed based on the application which can always be used in tropical countries due to the availability of solar radiation. A line diagram of a CSP with a two-tank TES taken from Ref. [1] is depicted in **Figure 1** where the line-concentrating solar panels concentrate the light and generate high-quality heat, which heats the heat transfer fluid. The heat transfer fluid in turn heats the incoming steam in the super heater. This two-tank molten salt TES system used in CSP is a technology commercially employed.

#### **1.1 Objectives of thermal storage**

The main objective of thermal storage is to deal with the amount of heat being stored at a particular temperature based on the application. The disparity of energy when supplied and consumed along with the necessity to store the surplus energy which would or otherwise go to waste including the shifting peak energy or power demand suggests the use of thermal energy storage for various areas of application such as space heating, hot water and air conditioning. Thermal energy storage facilitates superior and efficient use of the sources of fluctuating energies by harmonising the supply and demand of energy.

The methods used for thermal energy storage apply for less than 8 hours in northern winters, and the economical supply of the required energy becomes limited when compared to the costs of the combined solar collector and thermal energy storage system in these winters. An alternate heating source is taken as a backup for

*Technology in Design of Heat Exchangers for Thermal Energy Storage DOI: http://dx.doi.org/10.5772/intechopen.108462*

#### **Figure 1.**

*Line diagram for a two-tank thermal energy storage CSP system.*

extended sunless periods but it limits [2] the use of solar energy further. However, an economically acceptable storage system will take advantage of low off-peak electricity prices, which enhances the benefits in thermal energy storage.

#### **1.2 Types of thermal energy storage**

Three types of TES systems are categorised as sensible heat thermal energy storage, latent heat thermal energy storage and thermochemical energy storage [1, 3].

#### *1.2.1 Sensible thermal energy storage*

Storing of sensible energy due to the virtue of increase or decrease of temperature for a storage material is called sensible thermal energy storage. Air, water, rock, brick and concrete are a few sensible heat storage materials. Based on each material's advantages and disadvantages, it is preferred subject to the specific heat capacity including volume occupied by the TES [3]. To quantify sensible thermal energy stored, the following equation is given as follows:

$$Q = m.Cp.\Delta T\tag{1}$$

*Q* represents the total quantity of heat stored or saved in the sensible heat material (J), *m* represents the mass of the sensible heat material used (kg), *Cp* represents the specific heat of the stored material used (J/ kg K) and *ΔT* means the rise or fall of temperature difference measured (K). All mediums like solid liquid and gas can store sensible heat.

The TES materials are listed in **Table 1**, referred from Ref. [4], they have a high thermal capacity, are obtainable in profuse and found to be low priced. To select a suitable sensible heat storage material, the properties required have higher thermal


#### **Table 1.**

*Typical materials used in sensible heat TES storage.*

conductivity, higher specific heat, lower density, lower vapour pressure, higher diffusivity, complementary to the storage tank materials and chemical stability. Sometimes sensible heat also needs high fluidity to carry the heat from one place to another, such as water and oil, which is also mentioned as the HTF.

#### *1.2.2 Latent heat thermal energy storage*

Latent heat TES utilises the change of phase of latent heat TES material. The transition of phase change is utilised to heat up or retrieve heat from the latent heat material by melting or solidification from solid to liquid or vice versa respectively. On melting, large amounts of latent heat are transferred at a consistently constant temperature to the latent heat material; on solidification of the material the stored heat is released. The materials utilised here are called latent heat thermal energy storage (LHTES) materials and are named as phase change materials (PCM). The quantity of heat stored can be quantified in phase change material, as shown in Eq. (2)

$$Q = m.L \tag{2}$$

*Q* represents the heat stored for the latent heat storage material used (J), *m* represents the mass of latent heat material used (kg), whereas *L* represents the enthalpy of the phase change (J/kg). Water is being utilised most commonly as ice for cold storage. There are other materials which are listed in **Table 2** are referred from Ref. [4]

Though numerous materials have been reviewed for PCM, only a small number of latent heat materials have been practically used, mostly due to corrosion, subcooling, separation of phase, and low conductivity of heat, longer-term cyclic stability. So generally, PCMs are chosen depending upon the suitable temperature, enthalpy required for melting, accessibility and price.

The comparison between the different hot storage media as shown in Ref. [4], the operating temperatures can be in different ranges. The specific heat and cost are less

*Technology in Design of Heat Exchangers for Thermal Energy Storage DOI: http://dx.doi.org/10.5772/intechopen.108462*


#### **Table 2.**

*The melting temperatures and latent heat enthalpies of the different materials.*

for the sensible heat storage materials, however, the latent heat observed in PCM's is higher and is available at constant temperature.

#### *1.2.3 Thermochemical energy storage*

The thermochemical form of energy is stored and generated once a high-energy thermochemical reaction is utilised to store up energy. Products of the resultant reactions and the heat are separately stored during the forward reaction. In the reversed reaction, the stored heat is retrieved. Thus, a reversible reaction only could be utilised for this method of heat stored.

There are two types of thermochemical energy storage, mainly bifurcated into thermochemical reactions and sorption systems. Preferred high intensity of thermal energy storage density, along with cyclic reversibility, is the prerequisite of any chemical reaction. The thermochemical energy transformation has improved the efficiency of performance than the methods adopted by physical nature. It is difficult to search for the proper chemical reaction, which is reversible, for the energy source utilised. In chemical reactions, reversibility is an important behaviour of materials required.

Higher temperatures of greater than 400°C and with higher enthalpy in the range of 80–180 kJ/mol are the properties of thermochemical materials (TCM) used for the thermochemical reactions. The reactants– products are stored separately from the heat of reactions; the TCM systems are useful as seasonal storage systems.

For example, Zeolites and Silica gels are commonly used adsorbents. To characterise these storage materials, the probable temperature lifts, the breakthrough curves, energy density and the thermal coefficient of the absorbent volume performance are to be understood as also illustrated in **Figure 2** referred from Ref. [4].

The volumetric storage capacities are illustrated in **Figure 3** in addition to a schematic comparison. Water being easily available is selected as a representation of the sensible heat storage materials taken from [1]. There are no strict limitations on the coloured region boundaries of PCMs and TCMs. It is a gross illustration of the overall

**Figure 3.** *Storage capacities of various PCM.*

trend, and appreciably other TCMs and PCMs are available at high temperatures. A broader understanding of the available PCM with higher temperatures of melting is visualised in the following **Figure 4** taken from Ref. [5].

#### **2. Design criteria**

The thermal energy storage system consists of multiple components like the heat exchanger based on the phase change materials, the pumps, solar panels, insulations, storage tanks, etc. Each component has a different design criteria based on the aspects *Technology in Design of Heat Exchangers for Thermal Energy Storage DOI: http://dx.doi.org/10.5772/intechopen.108462*

**Figure 4.** *Existing PCMs melting temperature vs. phase change enthalpy.*

such as available heat content, flow rates, and applications. The aim of the design criteria is to maximise and optimise the heat transfer with the most effective costing using available resources and components designed for it. The established design criteria for the heat exchanger components to achieve their targets are discussed in the next section is referred from Ref. [2].

### **2.1 Selection criteria for thermal energy storage system**

In CSP plants or any process industries, the TES system depicts an important part in the stability of generation and power supply to be met with energy demand; nevertheless there are only few TES plants with high temperature, tested using thermal energy storage and have a lot of scope for research. Thermal energy storage systems must meet the following criteria:

