Concentrated solar power (CSP) involves the use of heliostats to concentrate the sun’s heat and then channel it into a turbine to generate electricity. There are a number of CSP systems available, including towers, parabolic troughs and compact linear fresnels. The heat that these systems produce can either be used immediately for electricity generation, or stored using a storage medium.

While a number of storage mediums are currently being explored – such as molten salt, solids, ammonia, oil and water – presently molten salt is the only commercially viable option. Other storage mediums mostly exist in research and development stages, or are unable to offer substantial storage periods.

Salt storage in Spain

Molten salt storage has been commercially proven since 2008, when the 50 megawatt (MW) Andasol-1 parabolic trough plant, located near Guadix in the province of Granada in Spain, began power production with 7.5 hours of molten salt storage. As of May 2011, six similar 50 MW plants, each with 7.5 hours of molten salt storage have come online in Spain.

“In Spain there has been quite a boom in the uptake of CSP," says Rebecca Dunn from the Australian National University's Solar Thermal Group. "At the moment there are 27 CSP plants that have been built since Spain’s feed-in tariff scheme started 2007. Half of those plants have storage, and all of that storage is molten salt storage. One plant has six hours of storage, the 19.9 MW Gemasolar tower plant has 15 hours of storage and all of the others, which are parabolic trough plants, have 7.5 hours of storage.”

Parabolic trough plants function by using the sun’s energy to heat a product similar to engine oil in the trough, which is then sent to a receiver where the oil transfers its heat to the molten salt for storage.

“Most of the plants with parabolic trough technology have 7.5 hours of storage, and these are 50 MW plants. The reason they’re 50 MW is because that was the capacity limit set by the Spanish government when they introduced the feed-in tariff, but the 7.5 hours originally was what was deemed profitable for the parabolic trough plants,” explains Ms Dunn.

There are three plants in Spain that use pressurised water as a storage medium, instead of molten salt. They function by making steam which is then put under pressure under a tank so it turns to water, which can then be bled off. This type of storage, however, can only last for half an hour to an hour, so is appropriate for temporary cloud coverage only.

Other storage research

As mentioned, there are a number of alternative storage mediums to molten salt that are presently in prototype and research and development stages. While some of those are related to molten salt storage, others rely on completely different mediums.

“The parabolic trough plants all use oil in their receivers, however, there is a prototype project currently being carried out in Italy known as the Archimedes Solar Power Plant, and there they’re trying to heat salt directly in the parabolic trough tube. That’s still in the prototype stage and they’ve still got quite a lot of challenges ahead of them because you can have problems with salt freezing in all of that tubing,” notes Ms Dunn.

In Spain, SENER Engineering and Systems is working on a single tank system to reduce construction costs, as presently all molten salt systems have two tanks, a hot and a cold tank.

Work is also happening in the field of thermochemical storage, which involves the storing of heat through chemical bonds, in products such as ammonia and hydroxides, although this type of storage is the least developed.