Thermal Energy Storage for Solar Power: Maximizing Efficiency and Output
Solar energy has become an important part in the world’s transition to renewable energy and provides a clean, abundant, and sustainable substitute for fossil fuels. However, with intermittency and availability of sunlight practically during the day, and disappearing at night, or through clouds, this has led to the energy enduring a challenge in its broader acceptance. This limitation is being overcome by storing excess energy during sunny hours when the sunshine is maximum, and discharging it when otherwise solar input is low or absent, be it nighttime or under cloudy skies. Thermal Energy Storage (TES) generates more efficient, reliable, and usable solar energy possible by decoupling energy generation from demand, especially in Concentrated Solar Power (CSP) plants.
TES systems indirectly store thermal energy by a diverse (heat or cold) medium from which heat can later be released for electricity generation, heating, or cooling. An effective TES system will be characterized by heat capacity of the storage medium, the ability of the system to store energy over long or short periods of time, heat transferability of the system, and further, operational temperature range. This elaborates on three TES principles practically: sensible heat storage, latent heat storage, and thermochemical heat storage. These three technologies function over a range of temperatures for different periods and hence may fulfill the needs of several energy systems.
Sensible Heat Storage- This method is based on raising or lowering the temperature of a medium (liquid or solid) such as water, sand, molten salts, graphite, or rocks in order to store and later use that thermal energy for applications between low temperature and very high temperature. Sensible heat commonly fills the role for TES, thus bringing about commercial practice ranging from the residential to the industrial scale. The storage temperatures for this process range from less than 0 to over 2400 °C, with their application durations ranging from a few minutes to a few months in cases such as low-temperature storage that utilizes underground water storage.
Latent Heat Storage- It utilizes a phase-change material that absorbs and stores thermal energy at a constant temperature during off-peak hours by melting and releasing the stored thermal energy during peak demand time as it solidifies. It is able to store heat at temperatures ranging from <0 °C and up to 1600 °C for a duration of hours to days.
Thermochemical Storage- The thermochemical storage works in two ways: chemical reactions and sorption processes. In the first case, energy is retained as the heat of reaction of reversible reactions; in the latter case, thermal energy is stored by physically bonding (adsorption) or being dissolved (absorption) by the working medium. TCS technologies can store thermal energy at temperature ranges of < 0 °C up to about 900 °C, usually for the duration of hours to days and at potentially months.
Furthermore, as TES solution allows the preservation of heat for later use, for instance, in on-demand electricity or in industrial processes. Concentrating solar-thermal power plants utilize TES as peakers, baseloads, or continuous sources of solar industrial process heat; thus, making them flexible by offsetting or replacing conventional fuel combustion.
Moreover, the TES mechanisms can be integrated with concentrating solar power systems. In CSPs, solar radiation focused onto a receiver generates the heat required for subsequent high-temperature applications. Unlike photovoltaic systems, which convert sunlight into electricity directly, CSP uses the thermal energy to be stored and can be converted to electricity using steam or gas turbines. Further, TES allows the operation of CSP as dispatchable power sources that can provide electricity elsewhere than when the sun shines. There is growing innovation in the TES mechanism utilized for solar power generation. For instance, in January 2025, CST Researchers of Khalifa University of Science and Technology developed a perovskite blend to enhance the performance of solar thermochemistry and redox cycleability for lowering solar fuels and thermal energy storage costs, which was revealed at the 30th SolarPACES Conference.
Moreover, Noor Energy 1 added 400 MW this 2023, increasing the capacity of the world’s largest CSP plant in the United Arab Emirates, thus bringing the total global installed CSP capacity to 6.7 GW, where 6.3 GW was accounted for in the previous year i.e. 2022, as stated in the Renewables 2024 Global Status Report collection, Renewables in Energy Supply data. Moreover, as of the end of 2023, China has reported 40 new CSP projects at different construction and commissioning stages.
Increase in Concentrated Solar Thermal Power (CSP) Installed Capacity Globally, in Gw, in 2022 and 2023
Renewables 2024 Global Status Report collection, Renewables in Energy Supply
Additionally, TES is progressively combined with PV beyond CSP, particularly in hybrid forms, such as powering resistive heaters with excess PV electricity to charge thermal storage media like sand or concrete, releasing heat later on for district heating or industrial use.
In September 2024, researchers at the National Renewable Energy Laboratory (NREL) demonstrated a new long-duration thermal energy storage technology, sand as the storage medium, for 100 hours. The ultimate goal is to provide storage at the cost of pumped hydro, with a Levelized Cost of Storage (LCOS) of $0.05/kWh. It heats the sand using a series of heating elements, much like a toaster, so it can later discharge heat or power for 100 hours.
Furthermore, the ongoing research deals with advanced TES materials and technologies that can use or withstand higher temperatures while still keeping cost low. In addition, improving the efficiency and durability of TES systems is also of utmost importance in the growing research by diverse organizations, and market players.
In August 2024, the US Department of Energy funded a demonstration of GeoTES (Geologic Thermal Energy Storage), a novel climate technology that stores concentrated solar heat for long durations. A full-scale validation is to be undertaken at a 5-acre site near Bakersfield, California, with parabolic trough solar collectors gathering the sun’s energy every day, which will enable the technology to supply energy for 40 days of 24-hour periods or 80 consecutive nights at a time for testing and operation at a depleted oil reservoir underground.
Similarly, Synhelion invented the technology to produce solar fuels day and night in 2024. This minimizes the production of CO2 and greatly helps the transportation industry to be net-zero. The proprietary technology of the company developed solar fuels and uses high-temperature thermochemical processes to convert solar energy into renewable fuels through “Sun-to-Liquid.” As production is not around for 24 hours, the solar energy storage is important for optimized and cost-effective production and provides thermal energy storage solutions for solar energy storage.
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