Energy question of the week: Can solar power be stored?
Solar power stations generate electricity only when it is sunny. So they do not exactly have a great reputation as reliable power providers. But this disadvantage can be overcome with efficient forms of low-cost heat storage. Many ideas are currently being tested, and some of these are even in use. How do storage facilities go about of storing solar power?
Solar power electricity could be stored in powerful batteries. However, efficient batteries are simply far too expensive to be used cost-effectively on a large scale. It is better to store the heat of solar radiation in liquids, solids or air. The energy could be used at night or on cloudy days to heat airflows to drive power-generating plant.
Liquid salts retain heat for many hours
Liquid salts, heated to almost 400 degrees Celsius, are now being used in the first solar thermal power stations to solve the storage problem. For example, in Granada, a city in southern Spain, more than 28 000 tons of liquid salts are heated up during daylight hours at the Andasol 1 and 2 solar-thermal power plants. These salts store solar heat efficiently enough to drive steam turbines for up to seven hours. A mixture of potassium and sodium nitrate is employed.
"Liquid salts of this kind do of course come with the disadvantage that they cause high costs and solidify and become useless at 220 degrees," says Rainer Tamme from the DLR Institute of Technical Thermodynamics in Stuttgart. DLR researchers are therefore working on alternatives. In a test facility, they have already demonstrated that even simple concrete is capable of storing heat for several hours. "In principle, the technology is already available and simply needs to be taken up by the industry," states Tamme.
Heat storage in concrete – a pilot plant built in collaboration with the company Züblin at DLR in Stuttgart. Photo: DLR. Top image: Heat storage in salt at the Andasol 1 solar power plant. Source: Solar Millennium.
Sand trickling through a hot airflow
If a solar thermal power station is used to heat air instead of thermal transport liquids to several hundred degrees, other storage media become feasible. Sand is undergoing trials. It can be heated in a stream of hot air. Very promising tests are running in Jülich. The first pilot project with an air-sand heat exchanger and a power rating of 15 kilowatts was completed recently. Independently, DLR researchers are working on compressed air storage units in which solar power is used to compress air in subterranean caverns. At night, this air is able to flow back out of its storage units to drive generators.
Looking further into the future, concepts involving thermochemical heat storage units are currently on the drawing board. For example, calcium hydroxide can be split by heat into water and calcium. These substances react with one another to release the heat, with very low losses. However, before a first test storage unit can be built, the researchers will need to devise a suitable process technology.
Storage units increase power supply security
With the exception of liquid salts, none of these many storage methods has reached market readiness, but they do represent tremendous potential. Progressively larger pilot plants will be able to investigate techniques in years to come. Depending on the medium being heated – oil or air – practical technologies may soon follow which will help to turn solar power into a more reliable source of electricity.
The DLR Energy question of the week in 'The future of energy' Year of Science
The Federal Ministry of Education and Research (BMBF) has given the Year of Science 2010 the motto 'The future of energy'. For this reason the science journalist Jan Oliver Löfken will this year answer a question on the subject of energy in his blog each week. Do you have a question about how our energy supply might look in the future? Or do you want to know, for example, how a wave power plant works and how it can efficiently generate electricity? Then send us your question by email. Science journalist Jan Oliver Löfken will investigate the answers and publish them each week in this blog.