Power from the Tower
solar thermal power plant is under construction in Israel’s Negev Desert, which will use thousands of mirrors to focus concentrated sunlight onto the highest solar energy tower in the world. German researchers are also working to perfect the technology.
A glittering sea is forming in the Negev Desert in Israel – a sea of mirrors – 55,000 of them, to be precise. And right in the center of this tissue of light, engineers are building an imposing 240-m tower which, when complete, will be the highest solar energy tower in the world. The Ashalim Tower, named after a nearby town, is scheduled to go into operation in 2017.
The tower and its attendant fleet of light collectors are designed to harvest sunlight. The facility covers an area of 1 million square meters. The orientation of each “heliostat” is automatically controlled to track the position of the Sun, and reflect the incoming radiation onto a boiler at the top of the tower. The steam created at a peak temperature to nearly 700°C is used to drive a turbine located at the base of the tower, which generates electricity. The facility will produce 120 MW of electricity – enough to power 120,000 households – without concomitant carbon emissions. A conventional power plant with the same capacity would generate approximately 100,000 tons of greenhouse gases annually.
However, night falls on the Negev too. And to keep the desert cold at bay, Ashalim’s customers need power when it’s dark. But the Sun is so bountiful, and the design of the solar facility so clever, that the heat harvested during the day is sufficient to keep its thermal storage units supplied for the night hours. When the Sun appears again at dawn, the heat exchanger is already well warmed up, so the plant can readily meet the early morning demand.
Solar power plants such as that under construction in Ashalim are in operation in Morocco, Spain, South Africa and in California. Ivanpah, in the Mojave Desert, is the site of what is currently the world’s highest (137 m) operative solar energy tower. Weather conditions in Germany mean that such solar towers would not be economically competitive here. Nevertheless, there is enough sunshine to enable the potential of the technology to be explored, improved and developed further.
For example, the Institute for Solar Research at the German Aerospace Center (DLR) in Jülich, some 60 km west of Cologne, maintains a pilot plant for this purpose. The facility basically follows the same design as that in Israel, but on a much smaller scale. The plant in Jülich occupies an area of around 10 hectares, and uses over 2000 mirrors to collect sunlight and direct it onto the top of a 60-m tower, where the radiation is converted into heat. This in turn is used to heat ambient air to a temperature of nearly 700°C. The resulting steam is used to drive a turbine, which in turns powers a generator that produces electricity. The radiation reflected by the mirrors can also redirected – wholly or in part – to an experimental station in a research complex located halfway up the tower.
At the Institute for Solar Research, Prof. Bernhard Hoffschmidt and his colleagues are studying ways of enhancing the efficiency of the energy tower approach, so as to reduce the cost of the energy produced by such solar-based thermal generators. Large-scale plants of this type achieve an efficiency level of around 20%, and this can undoubtedly be improved. The Jülich engineers are now working on optimizing the air circulation in the tower, boosting the efficiency of thermal storage and refining sensor technology for fully automatic operation of the facility.
“Today’s alternative technologies are still more costly than many conventional energy systems based on fossil fuels like coal,” Bernhard Hoffschmidt explains. But over the past 5 years, production costs per units of electricity have fallen by 30%. According to the latest studies, large-scale tower systems should be able in the medium term to bring the cost of electricity down to 10 cents per kilowatt-hour. Hoffschmidt is therefore convinced that “in the long term, power plants of this type will play a central role in an energy market in which much of the electricity on offer is generated without simultaneous emission of CO2.