Latent heat thermal storage with renewable energy: A short review

Energy storage is one of the important parts of renewable energies. The continuous increase in building's energy consumption is the main driving force behind the growing interest in using latent heat storage materials in building applications. Consequently, latent heat storage materials have been introduced into cement-based composites to store thermal energy and reduce energy loss in constructions. Latent heat storage material reinforced cement-based composites have the function of self-adjusting the indoor temperature of constructions, so as to improve the indoor thermal comfort. Temperature difference over 6 degc has been reported between the cement-based composites filled with latent heat storage materials and those without admixtures.

Energy can be stored in several ways such as mechanical (e.g., compressed air, flywheel, etc.), electrical, electrochemical (e.g., batteries), chemical (e.g., fuels), and thermal energy storage. Among all these energy storage methods the thermal energy storage has many advantages because it is done by changing the internal energy of materials, such as sensible heat, chemical heat, latent heat, or a combination of them. Among the all-above-mentioned heat storage systems, the latent heat storage system is particularly remarkable. One of the main reasons is its ability to store a large amount of energy at an isothermal process. As the process is isothermal at the melting point, the material can be picked to have the desired temperature range.

A latent heat storage material is a substance that releases/absorbs sufficient energy at phase transition to provide useful heat/cooling.

Generally, the transition will be from one of the first two fundamental states of matter - solid and liquid - to the other. The phase transition may also be between non-classical states of matter, such as the conformity of crystals, where the material goes from conforming to one crystalline structure to conforming to another, which may be a higher or lower energy state. In this context, latent heat storage material based cooling is a very attractive process of thermal control, considering the advantages, such as high latent heat storage capacity, small volume change in the phase transformation, high specific heat capacity to provide additional sensible heat storage, chemical stability and no degradation for a large number of cycles, a phase transition occurs within the desired operating temperature range of the system, high nucleation rate to avoid supercooling of the liquid phase, high thermal conductivity to assist the absorption and release of energy in the storage system, high density, no corrosiveness, non-flammable, nontoxicity, relatively low cost, and high availability

Latent heat storage materials act as a battery for heat energy because they absorb heat energy as they melt and can be “recharged” by cooling them until they crystallize and give the stored energy back to the environment. They can store and release heat energy thousands of times without a change in thermal properties

 There are several types of phase change material available, there are three main types: organic (paraffin and nonparaffin), inorganic (salt hydrates and metallic alloys), and eutectic (mixture of two or more latent heat storage materials organic, inorganic, and both).

Two examples of latent heat storage materials

Organic latent heat storage material: Paraffin

Paraffin waxes usually consist of straight-chain n-alkanes, CH3-(CH2)n-CH3. These materials can store and release a large amount of heat during liquid-solid phase changes. They have an extremely large latent heat storage capacity of around 200–250 kJ kg?1 Paraffin waxes also have a broad range of melting points, with thermal stability at around 250°C. They exhibit little to no phase separation during frequent phase changes. They are chemically inert, long-lasting, non-corrosive, odorless, cheap, easily accessible, ecologically harmless, and non-toxic.

Inorganic latent heat storage material: Salt hydrate

Salt hydrates are a category of inorganic salts that contain one or multiple water molecules such that the resulting crystalline solid has a chemical formula of AB·nH2O. They are among the cheapest latent heat storage materials, with the average material cost ranging from $0.13 to $0.46/kg for calcium chloride CaCl2·6H2O. Furthermore, they show good latent heat values and high thermal conductivity due to their higher density, combine a wide melting temperature range of 5–130°C with a fixed value for phase transition temperature, are not flammable, and are biodegradable and recyclable.

Market size and growth potential

The market size was valued at around USD 1.9 billion in 2019 and the projected growth rate of over 17.4% from 2020 to 2026. In the construction industry, latent heat storage systems are used for solar water heating, space heating/cooling, and waste heat recovery systems, among others.

 Credit: Google