Salt, Concrete and Compressed Air: Storage Systems Deliver Energy on Demand

Solar and wind power stations rarely conform to demand from electricity customers. Therefore, low priced and efficient energy storage systems are the key towards a reliable supply of electricity produced from regenerative sources. Researchers from the German Aerospace Centre (DLR) develop thermal and compressed air energy storage for the future supply with energy.

“We envision three application fields for our storage technologies”, says Dr. Rainer Tamme, Department Head at the DLR Institute of Technical Thermodynamics in Stuttgart. With the help of thermal energy storage, conventional and steam turbine power plants with combined heat and power could be operated more flexibly and hence more efficiently. Air pressed into subterranean cavities under high pressure can be used to smooth production peaks from wind parks. And ultimately thermal energy storage safeguards the electricity production of solar power stations for many hours at night.

Such thermal energy storage already today solves the storage problem at the first solar-thermal power stations. For instance, at the plants Andasol 1 and 2 near the city of Granada in the south of Spain more than 28,000 tons of liquid salts – a mixture of potassium nitrate and sodium nitrate – are heated to some 400 degrees Celsius during the day. The salts store the sun’s warmth efficiently enough in order to power the steam turbines for up to seven hours after sunset. “However, such liquid salts have the disadvantage of causing high costs and turning solid at 220 degrees and hence become useless”, says Tamme.

On the Stuttgart site and at the Plataforma Solar de Almería in Spain, the DLR scientists are already working on a cheaper alternative: Concrete, which can be heated to up to 400 degrees. “The firm blocks can on occasion also cool down without taking damage”, says storage expert Dörte Laing. In principle, this technology is available already and waits to be adopted by the industry.

By contrast, so-called latentheat storage (LHS) units based on phase change materials still have to pass their field test. Again, these consist of many tons of salt, or to be precise, sodium nitrate. Yet in contrast to the liquid salts, these amounts are permitted to change phase, that is, solidify. The thermal conductivity indeed is so much reduced that it is difficult to liquefy the salt again. But numerous aluminium ribs across the salt reservoir can counterbalance this disadvantage. How well this principle for thermal energy storage actually works has to be proven this year at a reservoir with 14 tons of nitrate salt at a power station run by the Spanish electricity supplier Endesa in Carboneras. “This is currently the largest latent heat storage unit of the world and we expect results by the end of the year”, says Laing.

These thermal reservoirs may be useful:  In order not to lose surplus kilowatt hours during phases of strong wind, they are to power air compression pumps. These press compressed air into subterranean cavities such as, for instance, salt caverns. Once the demand for electricity rises, the air can escape again and set the generators’ turbines in motion. In cooperation with RWE, General Electric (GE) and Ed. Züblin AG, the DLR is working on the development of these technologies. The “adiabatic compressed air energy storage for electricity supply”, short Adele, is to provide intermediary storage for five hours for the power production of up to 40 wind turbines.

However, the compressing and release of compressed air is not a trivial matter. During compression, the air heats up to above 600 degrees. Conversely, it cools down when escaping. A high degree of efficiency of the compressed air energy storage of around 70 percent can only be achieved when the compression heat is also stored and later can be used to warm up the escaping air. And it is precisely there, where “Adele” will profit from the DLR’s great expertise in the field of thermal energy storage.