### Seasonal thermal storage
Figure 2 shows a heat storage facility built in Finland by Polar Night Energy [20; 24; 25; 26; 27]. When electricity is cheap or over-produced, the sand is heated using appropriate heaters. In the summer, these heaters are powered by energy from renewable sources, such as photovoltaic panels. Thanks to this, the solar energy collected in summer can be used to heat houses in the winter. The stored heat can also be used for high-temperature industrial processes. The heat storage has a heating power of 100 kW and energy capacity of 8 MWh [28; 29; 30]. The Finnish company Power Night Energy is not the only one interested in using sand as a heat storage material. Currently, the Italian company Magaldi Green Energy has the largest number of patents related to sand batteries. The company's solutions include both short- and long-term heat storage systems with sand bed fluidization. Fluidization increases the transfer of thermal energy when charging and discharging storage facilities. However, the Finnish company was the first to build its installation using its own technologies and may become a pioneer in the future in this segment of the energy market [31; 32]. The undoubted advantage of the new Power Night Energy solution is its small carbon footprint. Greenhouse gas emissions are limited to the construction phase. The operation  
Figure 2: Polar Night Energy’s sand battery is a large-scale high-temperature thermal energy storage that uses sand or sand-like materials as its storage medium. It stores energy in sand as heat. It has three main purposes: to store excess wind and solar energy; to balance the electrical grid, and to produce heat and power without burning. [Illustration by [PERSON]/Polar Night Energy]. of a sand heat storage facility itself exposes the environment to negligible CO2 emissions--provided that the stored heat comes from renewable sources. Sand storage is available for sale to large entities, such as energy and industrial companies or operators of residential and commercial buildings. There are no sand heat accumulators for individual use yet. The price for building 1 kWh of heat capacity reaches EUR 10 [33]. There are the following advantages of sand heat accumulators:  
* High heat capacity compared to, e.g., water; sand can be heated to temperatures well above the boiling point of water. Sand-based heat storage facilities can store several times more energy than water storage facilities of the same volume. This saves space and facilitates versatile use in many industrial processes;
* Fuel versatility--heat storage is not sensitive to the size of sand grains. Another example of the use of sand, water, and carbon dioxide is the solution from Echogen. This solution includes two tanks. One is filled with hot sand and the other is a cold tank filled with cold water or ice. Using an electrically powered heat pump, thermal energy is stored in the sand. When energy is needed, the heat from the sand travels through a heat engine to generate electricity. The Echogen energy storage project is distinguished by the use of supercritical carbon dioxide, which serves in this technology as a heat carrier or as the so-called working fluid [34,35]. Surplus electricity from renewable energy sources is used at the US National Renewable Energy Laboratory (NREL) for testing the heat storage using the silica sand. The heat storage facility, with a maximum capacity of 26,000 MWh, is in the final phase of testing there. The sand in this storage facility can be heated to a temperature of 1200 \({}^{\circ}\)C. During the energy demand, heat is used to heat and pressurize the working gas used to drive the turbines and rotate the generator. Using a heat exchanger, this energy can also be used to heat buildings, providing an alternative to heating based on coal or natural gas [28].