Energy question of the week: What is the best way to harness solar energy?
On average, sunlight illuminates every single square metre of the Earth with 1340 watts of power. Measured on human timescales, this energy source is infinite; it warms our planet, enables plants to grow and is the engine driving the winds and weather. But which technology is best able to harness the power of sunlight?
Solar cells currently hold the record for converting the energy of sunlight into electrical power, with efficiency ratings greater than 40 percent. However, these photovoltaic cells are simply too expensive for use in large solar power stations. Standard commercial modules for large photovoltaic power stations are only gradually approaching the 20 percent threshold. With efficiency ratings of between 20 and 25 percent, solar thermal power stations are now sufficiently mature for dozens of these plants to be built or planned around the globe. Solar thermal power stations will also provide the backbone for power generation in the Sahara as part of the Desertec Industrial Initiative, where DLR researchers are currently hard at work on the scientific fundamentals. Solar energy can be stored as heat in these power stations, which means that they can generate electricity on demand and also after sunset.
More than 100 gigawatts of power from solar thermal power stations by 2020
Several scenarios for the future provision of power assume that within the next decade, solar thermal power stations with an aggregate power generation capacity of 100 to 200 gigawatts will be up and running. First in the plan is a focus on parabolic-trough power stations, in which curved mirrors direct sunlight to centrally-mounted absorber tubes. The concentrated rays of sunlight then heat oil circulating through these tubes to a temperature of up to 400 degrees Celsius. The oil then flows into a heat exchanger and transfers this heat to water. The steam generated in this process drives a generator via a turbine.
Parabolic trough at the Plataforma Solar de Almería. Credit: DLR/Steur. Above: Heliostat of a solar tower power plant. Credit: DLR/Steur.
As mature as this new technology is, we are still a long way from discovering its full potential. "At present, we are working on ways of moving away from oil, which is comparatively expensive," says Christoph Richter from the DLR Institute of Technical Thermodynamics (Institut für Technische Thermodynamik) and facility manager at the DLR Solar Research site in Almería, southern Spain. At the Plataforma Solar de Almería, he and his colleagues are testing a process that enables the evaporation of water directly inside the absorber tubes and then to be circulated under high pressure. This makes it possible to achieve higher temperatures, up to 500 degrees. This is one way to further increase the efficiency of solar power stations and to produce cheaper solar power. After some promising results, Richter and his colleagues are currently planning to build a five-megawatt test plant.
Hydrogen from a solar tower
Along with their work on parabolic troughs, DLR researchers are also driving forward the concept of solar tower power stations. This concept involves hundreds of mirrors, arranged in a semicircle, directing sunlight at a ceramic absorber block that is located centrally in the upper section of a tower. With temperatures in excess of 1000 degrees Celsius, and in a manner similar to the parabolic trough system, steam is produced and can be used to turn a generator turbine. “With these higher temperatures, tower power stations are able to achieve higher efficiencies,” says Richter.
Having said that, solar tower power stations are suited to more than simple power generation. "With this tremendous heat from concentrated sunlight, hydrogen can be obtained," explains Richter. For example, as part of the EU's HYDROSOL I and II projects, water molecules can be broken down into their constituent elements of hydrogen and oxygen at temperatures between 800 and 1200 degrees Celsius. This process is not yet suitable for commercial exploitation, but it does point the way towards a future in which solar thermal power stations not only generate electricity but also produce hydrogen, an energy source that is suitable for storage.
DLR solar research at the Almería site
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.