5. Materials for thermal energy storage

Thermal energy storage materials are very specific in terms of physical and thermal properties for the best performance of the CSP plants. These materials are generally categorized into three, namely, sensible storage, latent storage, and thermochemical storage. Explanation of these categories is given in the subsequent sections. There are few properties of TES materials which are common for all materials. Energy storage density is very basic thing that defines the size of the TES tanks as well as associated cost with it. The higher is the energy storage density of a material, the less of its quantity is required to store a specific amount of thermal energy [14]. Similar is the case of mass density of the material. In the case of latent heat and thermochemical storage, equivalent terms are heat of fusion of the material and heat of reaction of the materials, respectively [14]. Thermal conductivity and operating temperatures are very important parameters in defining the overall efficiency and performance of the system. If a material is a good thermal conductor, it takes less time during charging and discharging. In case thermal conductivity is low, conductivity enhancers, nanofillers, and enhanced contact surface geometries are utilized for better results. In general, the materials should be inexpensive, readily and widely available, less corrosive, and less hazardous to the environment and to human health [14].

#### 5.1 Sensible TES storage materials

These materials store heat in the form of rise in temperature. The property of the material that is associated with this phenomenon is the heat capacity. A material with higher heat capacity is good for energy storage. Examples of such materials are sand, rocks, concrete, etc. [18]. The amount of thermal energy stored in TES materials as sensible storage can be calculated using Eq. (23):

$$Q\_T = V \times \rho \times \mathbf{C} \times \Delta T \tag{23}$$

where V is the volume, ρ is the average density, C is the specific heat capacity of the sensible energy storage material, and ΔT is the difference of temperature from initial to final stage.

#### 5.2 Latent heat storage

In these types of materials, energy is stored in the form of change of phase. This phase change may be in solid-liquid, liquid-gas, solid-gas, and solid-solid [19]. Commonly known materials lie in the categories of liquid-gas and solid-liquid phase change materials for CSP applications. Water-steam is an example of the former type, and binary salt is an example of the latter type. For latent heat storage materials, parameter of interest is the phase transition temperatures and latent heat of fusion during this phase transitions. It is highly desirable that materials are stable within a long temperature range, and its solidification temperature is as low as possible. The reason behind the low preferred solidification temperature is that it should not be deposited as a solid in the circulation pipes in active systems. The

Figure 6. (a) PCM with respect to storage capacity and (b) PCM with respect to heat of fusion [21].

amount of energy in this type of material is the sum of sensible energy storage from initial temperature to the final temperature and the energy storage during phase transition as a latent heat [18, 19]. Energy stored as a latent heat of fusion can be calculated using Eq. (24):

$$Q\_{\text{latent}} = V \times \rho \times L \tag{24}$$

where L is the latent heat of fusion of the material.

#### 5.3 Thermochemical energy storage

This type of energy storage is based on the chemistry of endothermicexothermic reversible reactions. Surplus heat energy is used to initiate a reaction which is highly endothermic. During charging, the heat is taken by the reactants, and due to reactions occurrence, the reactants are converted into products. These products are stored for days, weeks, and seasons. Interestingly, the storage is at ambient conditions, and energy losses in this storage are minimal. During discharging, these products are converted back to the reactants with the release of huge amounts of heat. That heat is transported to the thermodynamic cycle [20]. The reversible CaO/CaCO3 carbonation reaction (CaL) is one of the most promising since CaO natural precursors are affordable and earth-abundant. However, CaO particles progressively deactivate due to sintering-induced morphological changes during repeated carbonation and calcinations cycles.

Figure 6(a) is the representation of different types of materials based on the heat storage capacity [21]. As it is evident from the figure, thermochemical materials (TCM) possess the most storage density in the range of 170–600 kWh/m<sup>2</sup> . Energy storage density of latent heat storage called PCM comes lower than TCM ranging from 70 to 250 kWh/m2 . Sensible storage materials are the lowest in terms of energy storage density. The only advantage in sensible storage is the absence of degradation and corrosion and very low cost. Figure 6(b) is the classification of TES materials based on the melting points [21]. This analysis gives an indication about the selection of materials for specific ranges of melting points.

## 6. Characteristics of thermal fluids

According to the 2050 vision of the International Energy Agency (IEA), energy production share by CSP is 630Gwe. Keeping in view the high future targets,

Advances in Concentrated Solar Power: A Perspective of Heat Transfer DOI: http://dx.doi.org/10.5772/intechopen.84575

scientists and researchers are working on different designs of CSP. Among all parameters, thermal fluid is a key component because overall performance of the CSP is dependent on the thermal energy. Thermal fluid is the transport material that carries thermal energy from solar receiver/hot storage and delivers it to the thermodynamic cycle [22]. In the context of thermal fluids, required characteristics are:


Thermal fluids are categorized into two classes based on the behavior of the materials. The classification of the thermal fluids is represented in Figure 7.

In certain cases, thermal property enhancer nanoparticles agglomerate and form clusters after a limited operational life. This agglomeration of nanoparticle declines the performance of thermal fluids. A quaternary salt is developed recently with low melting point (85.4°C), wide operating range (600°C), reduced risk of blockage, and less corrosive effect with the system [23]. Correlation of heat transfer with nanofluid is described in the subsequent section.

#### 6.1 Heat transfer with nanofluids

Thermal characteristics of nanofluids are different than solid-liquid mixtures as these fluids contain suspended particles (metallic or nonmetallic) in the liquid base. Heat transport properties are altered because of the suspended ultra-fine particles [24]. Generally, the volume content of these particles is below 10% in the fluid. Addition of nanoparticles increases thermal conductivity and heat transport properties of the fluid as compare to the pure fluid. For instance, Xuan and Li reported an increase in the thermal conductivity ratio from 1.24 to 1.78 with the increase in particles from 2.5 to 7.5% [25]. The change in properties of the fluid is dependent on the particle shape, dimensions, quantity, and characteristics. However, micrometerand millimeter-sized particles are reported to settle down quickly producing clogs in the channels, eroding pipelines, and causing huge pressure drop [26]. Heat

Figure 7. Classification of thermal fluids.

transfer correlations, fundamentals, and theory can be read through the literature presented in [24, 27, 28].
