Energy becomes the basic requirement for various ranges of applications, and it is a time variable. Energy storage is essential when energy is abundantly available. The need for energy storage is to conserve energy, and it can be used whenever necessary. Thermal energy storage is defined as the temporary holding of thermal energy in the form of hot or cold substances for later utilization.

The thermal energy system deals with the storage of energy by cooling, heating, melting, solidifying, or vaporizing a material, and the thermal energy becomes available when the process is reversed.

In the charging process, the surplus energy is stored. In the discharging process, the stored energy is utilized in the demand period. For example, surplus solar energy is stored during charging time, and that stored energy is utilized for off-sunshine hours.

Thermal Energy Storage (TES) comprises sensible storage systems, latent heat storage, and thermochemical heat storage. Sensible heat storage (SHS) systems are widely used for low-temperature thermal applications without the change of phase. However, SHS systems require a large storage volume for a small temperature cycle. The latent heat storage (LHS) systems stand out due to high storage density and nearly isothermal phase change. The thermal energy storage materials used for LHS systems are also known as phase change materials (PCM).

Sensible Heat: 

In sensible heat storage, the temperature of the material varies and does not undergo any phase transformation during charging or discharging cycles, whereas thermal energy is stored based on the specific heat capacity of the material. The amount of energy stored by the material is given by the following equation.

 

 

Latent Heat: 

In a latent heat storage system, the process of storing and retrieving the thermal energy is based on the latent heat of fusion, where the storage medium undergoes a phase transformation at a constant temperature. As the source temperature increases, the chemical bonds of the PCM material break up, which leads to the transformation from one phase to another. Energy stored in a latent heat storage medium is given by the following equation.

Thermo-chemical Heat: 

Thermal energy can also be stored using reversible chemical reactions. Thermal energy is stored and retrieved by breaking and reforming of molecular bonds through reversible chemical reactions. In the simplest case, a reactant X is transformed in the products Y and Z in an endothermic reaction during the charging step. Y and Z compounds are normally stored separately. The discharge then accounts for the reversible process, in which Y and Z are recombined, releasing thermal energy. The stored heat (Q) is proportional to the molar reaction enthalpy (ΔHr), the number of moles of one of the products (nB) and the conversion achieved (X) and it was calculated by the following equation. 

The thermal energy storage materials used for LHS systems are also known as phase change materials (PCM). These PCMs can be categorized as organic (paraffins and fatty acids), inorganic (salt hydrates and metallic) and eutectic combination of organic and/or inorganic materials. A detailed classification of PCM for latent heat storage applications is shown in following figure.

Proper utilization of the latent heat storage unit mainly depends on the properties of the PCM. Desirable properties of PCM for successful utilization of thermal energy are high latent heat, high thermal conductivity, high specific heat, highly reliable, large thermal cycle, non-corrosive and non-toxic.

A phase change material which contains carbon atom is known as organic PCM. PCM’s are available for a wide range of temperatures which are stable till 300˚C.  Inorganic PCMs are materials which consist of salt hydrates, nitrates and metallic’s. Inorganic PCM can also be used for higher temperatures up to 1500˚C. Eutectic PCMs are mixture of two or more compounds at a particular percentage of composition. The compounds can be of any combination like organic-organic, inorganic–inorganic and organic–inorganic.

Organic PCMs have been widely used due to their low price, high heat storage capacity and various phase change temperatures, good thermal stability, self-nucleating properties, no phase segregation, non-reactivity, non-corrosivity and non-toxicity. Limitations are low thermal conductivity, variation in thermo-physical properties of PCMs under extended cycles, phase segregation, subcooling and sometimes flammable. Methods of enhancing thermal and physical properties are achieved by incorporating nanomaterials, encapsulation, shape stabilization, inserting fins and foils.

Case Study:

1. Nanoparticles to Enhance Melting Performance of Phase Change Materials for Thermal Energy Storage

A solar collector system integrated with a thermal energy storage unit containing phase change materials was studied. The integration of an energy storage system ensures the stability and flexibility of renewable-energy-based heating and cooling systems because it mitigates the intermittency of solar energy. A solar energy heating system containing a PCM-based thermal energy storage unit, which contains the combination of PCM and heat transfer fluid (HTF) in a triple concentric-tube heat exchanger. Aluminium oxide nanoparticles are employed to enhance the PCM melting process, which improves the performance of thermal energy storage in the solar heating systems. 

At the earliest stage of PCM melting, spontaneous convection owing to the buoyancy effect dominates the flow’s behaviour. High natural convection at the base of the annular tube pulls the liquid–solid boundary lower by moving the molten PCM upwards from the side. The inclusion of Al2O3 nanoparticles at a concentration of 3 percent by volume improves PCM melting performance by shortening PCM melting time by roughly 15 percent.

The liquid fraction of PCM with and without nanoparticles during the melting process was shown in the figure. The addition of Al2O3 nanoparticles significantly enhances the melting performance of PCM during the initial stage. At t=40 min, the thickness of the melted PCM composite around the hot outer wall was found to be twice that of the pure PCM. High natural convection at the base of the annular tube pulls the liquid–solid boundary lower by moving the molten PCM upwards from the side. This shows that including the Al2O3 nanoparticles in the PCM significantly enhances the PCM melting performance.