Thermal energy storage (TES) is a technology based on stocking thermal energy by heating or cooling a storage medium to enable use of the stored energy at later times for heating and cooling applications and power generation. Thermal energy storage systems can help balance energy demand and supply by storing energy at off-peak periods and enabling its use during high-peak energy demand periods, they also contribute to reducing CO2 emissions and costs, while increasing overall efficiency of energy systems.
TES systems are mainly employed in buildings and industrial processes and are particularly important for electricity storage in combination with concentrating solar power (CSP) plants.
There are three main categories of TES systems, Sensible (specific) Heat Storage (SHS), Latent Heat Storage (phase change materials PCM) and Thermo-Chemical Storage (TCS). (Source: irena.org)
Sensible Heat Storage (SHS)
Sensible heat storage is the simplest method for thermal energy storage, it is based on heating or cooling a liquid or solid storage medium in order to store and release thermal energy for later applications. Solid materials that can be used are concrete and castable ceramics that are characterized with their low price and good thermal conductivty as well as sand and rocks. Whereas for liquid materials, a variety of fluids have been tested to transport heat, including water, air, oil, sodium and molten salts. But water remains the most largely used material in sensible heat storage systems with the best compromise between cost, heat storage capacity, density and environmental impact.
Although this method of heat storage is currently less efficient for heat storage, it is the least complicated compared with latent or chemical heat storage processes, and it is inexpensive. (Source: sciencedirect.com, thermal-engineering.org)
From a ‘’thermodynamics’’ point of view, SHS systems utilize the heat capacity and the change in temperature of the storage medium during the process of charging and discharging, as the following equation:
The amount of heat stored depends thus on the specific heat of the medium, the temperature change, and the amount of storage material. (Source: mdpi.com, thermal-engineering.org)
Virous technological concepts for thermal energy storage are used, amongst which the following can be mentioned:
Underground thermal energy storage (UTES)
Underground thermal energy storage is mostly used for seasonal heat/cold storage. The main utilized concepts are Aquifer thermal energy storage (ATES), Borehole thermal energy storage (BTES), Cavern thermal energy storage (CTES), Ducts in soil and Pit storage, as shown in the following figure. (Source: sintef.no)
Above ground water tank storage
The most common use of water tanks above ground in Europe is in connection with solar collectors for production of warm water for space heating and/or tap water. The use of hot-water tanks is a well-established technology for thermal energy storage that serves the purpose of energy saving in water heating systems via solar energy and via co-generation (heat and power) energy supply systems. Water tank storage systems are considered to be a cost-effective storage option. (Source: sintef.no)
The following figure illustrates a storage water tanks for small-scale solar collectors.
Latent Heat Storage (phase change materials PCM)
Latent heat storage systems utilize the energy absorbed or released during the isothermal phase change of materials that are called ‘’phase change materials, PCM’’, the change of phase can either be a solid/liquid or a solid/solid process; the most known and employed PCM is water, used as ice for cold storage.
Phase change materials melt and absorb energy corresponding to the latent heat of the material when the temperature increases above the melting point. Whereas, when the temperature decreases below the material’s melting point, the PCM solidifies (freezes) and the latent heat is released, thus the heat is mainly stored in the phase-change process (at a quite constant temperature) and it is directly linked to the latent heat of the substance.
Storing thermal energy through LHS systems using PCMs has proven to be a more effective method for storing thermal energy compared to SHS systems that have a lower storing capacity. (Source: mdpi.com, sintef.no, irena.org)
The following figure compares the achievable storage capacity at a given temperature difference for a storage medium with and without phase change. (Sensible heat storage vs Latent heat storage).
The LHS stored capacity can be calculated using the following equation: (Source: mdpi.com)
The amount of stored heat can be calculated using the following equation: (Source: sciencedirect.com)
Thermal properties of some PCMs are given in the following table.
Thermal properties of some PCMs (Source: irena.org)
Thermo-Chemical Storage (TCS)
Thermo-chemical storage systems (TCS) use thermo-chemical materials (TCM) that store and release heat through reversible endothermic/exothermic reactions. As other energy storage processes, the TCS system can be divided into three steps: Charging, Storage and discharging step.
During the charging process, heat is applied to the material AB, resulting in its separation into two parts A+B as the following reaction.
The resulting reaction products can be easily separated and stored until the discharge process is required. During the discharging process, the two parts A + B are mixed at adequate pressure and temperature conditions, and energy is released. (Source: mdpi.com)
This process is illustrated below.
Thermo-chemical reactions also include adsorption reactions that consist of the adhesion of a substance onto the surface of a solid or a liquid. These reactions can be used to store heat and cold, as well as to control humidity. Thermo-chemical adsorption reactions have high storage capacity, and thus allow thermal energy transportation. (Source: mdpi.com)
The storage systems based on chemical reactions have negligible losses but a sensible heat storage dissipates the stored heat to the environment and need to be prevented by isolation. (Source: celsius.co.kr)
Thermo-chemical storage materials have the highest storage capacity of all thermal storage methods. Some materials may even approach the storage density of biomass. Solid silica gel has a storage capacity that is up to about four times higher than that of water. (Source: celsius.co.kr)
The following table gives the typical parameters of TES systems.
Thermal energy storage data
According to PRNEWSWIRE, the global Thermal Energy Storage (TES) market size is forecast to reach USD 6019.7 million by 2027, from USD 3817.1 million in 2020, at a CAGR of 6.3% during 2021-2027. (Source: prnewswire.com)
According to Thermal Energy Storage Market Size Research Report [2020–2026], the global Thermal Energy Storage Market in 2019 was approximately USD 4,281.6 Million. The market is expected to grow at a CAGR of 10.4% and is anticipated to reach around USD 8558.34 Million by 2026. (Source: globalnewswire.com)
According to ResearchAndMarkets report on Molten Salt Thermal Energy Storage Market – Forecasts from 2021 to 2026, the global molten salt thermal energy storage market is expected to grow at a CAGR of 15.65% reaching a market size of $1,743.663 million in 2026, up from $629.969 million in 2019. (Source: smart-energy.com)
