Thermal energy storage
09-01-2023 | Posted by Joaquín Martí
The world is now fully aware of the serious consequences of burning fossil fuels. This is triggering all sorts of attempts to replace them with other energy sources. Currently the more promising ones, which are experiencing enormous growth, are wind and solar, both abundant, clean, and renewable sources; they jointly represent about two-thirds of the generating capacity added globally over recent years.
But even in the best locations, the wind doesn’t blow all the time, nor with the same intensity. And the sun only shines part of the time. This intermittency exacerbates the problem of balancing supply and demand. And the obvious answer is energy storage but the methodology to achieve it is less clear.
Indeed, there is a need to store energy for a few hours, for a few days, or for longer periods. The technologies involved vary; each one of them has its own characteristics, making them more or less suitable for each application.
Long-duration energy storage (LDES) covers any technology that can store energy for periods above ten hours, as defined by the US Department of Energy. Of the world’s current LDES capacity, 94% is provided by pumped hydro installations, which use surplus energy to pump water to a higher-elevation reservoir and recover the energy by sending the water back down through a turbine, with a typical round-trip energy efficiency of about 70-80%.
Another technology, much less used but with some interesting characteristics, is thermal energy storage. For one thing, you may notice that the energy involved in changing the temperature of water by 100ºC is more than two orders of magnitude greater than that used to change its elevation by 400 meters. Hence, the energy storage capacity per litre of water is much greater, with clear advantages regarding space requirements.
The energy efficiency of thermal storage is normally very high, about 95%, so long as the stored energy is used directly for heating; but it must go through the corresponding thermodynamic cycle if used for generating electricity, which reduces the efficiency to around 50-70%.
The materials used for storing the energy, which must be kept in suitably insulated tanks or containers, can be of different types. There are examples using water in places like Finland and Denmark, sand in Berlin, and molten salts at numerous solar energy installations. All of these are fairly innocuous materials, without any particular associated hazards, unlike typical battery materials.
The stored heat may have many different origins: wind and solar energies, cogeneration plants burning fossil fuels or biomass, heat-only boiler stations, geothermal heating, heat pumps, central solar heating, waste heat from factories and industrial facilities, and nuclear power electricity generation.
It may also have different destinations. Heat represents about 50% of the global final energy demand, split roughly equally between industry and buildings. Most of that heat is currently being provided by fossil fuels. As we walk away from those, we need to replace them with renewable energy, which makes storage an obvious requirement.
Principia has experience in various pumped hydro installations, as well as in thermal storage using molten salts at several solar plants, and hot- water tanks for thermal energy storage for district heating. If you have a need along any of those lines, it pays to check with an expert.
