A huge insulated pile of stones is heated with excess electricity at Trimet Aluminium SE in Altenwerder. This gigantic battery stores excess electricity temporarily thanks to a gargantuan hot-air dryer which heats the stones to 600 degrees Celsius. The well-insulated stones store the heat - or 30 megawatt hours of electricity - for several days. When electricity is needed, the heat is extracted and its steam is used to drive a turbine, which in turn drives a 1.5-megawatt generator. The plant then produces electricity for up to 24 hours and can supply 1,500 households or charge around 50 electric cars.
Charging the thermal battery takes about six hours and the temperature loss is only about 15 degrees Celsius. The test facility is about the size of an Olympic swimming pool. The concrete structure is 22 metres in length, eleven metres in width, eleven metres in height and contains around 1,000 tonnes of volcanic rock.