Last month, operation began at Polar Night Energy Oy’s (Tampere, Finland; www.polarnightenergy.com) first commercial sand-based thermal-energy storage system. The thermal storage system, which is located at the Vatajankoski power plant, is now producing low-emission district heating to the city of Kankaanpää in Western Finland. The unit has a heating power of 100 kW and an energy-storage capacity of 8 MWh.
The storage system is an insulated steel tank (4-m wide and 7-m tall) that contains 100 tons of sand, and a network of pipes. Additional equipment, such as automation components, valves and fan and a heat exchanger (or steam generator), are located outside. The operating principle is very simple: electricity is converted to heat by an electrical-resistance heater that heats air in a closed-loop piping system. When the hot air is circulated through the storage, the heat is transferred to the sand. The sand can be heated to temperatures of 500–600°C, and stored for long periods of time. The storage is unloaded by blowing cold air through the piping, and then the heated air can be used to convert water to process steam or to heat district-heating water in an air-to-water heat exchanger.
The use of sand is said to be an effective solution for reducing CO2 emissions. According to a 2020 report by Mission Innovation (http://mission-innovation.net), sand-based high-temperature energy storage could save more than 100 million metric tons of CO2 equivalents (CO2e). Saved emissions would be around 3% of the current emissions of the whole E.U., the company says.