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What is the difference between solar heat and photovoltaics? Which technology will become more established?



Every year, 5.4 x 1024 joules (1.5 x 1,018 kWh) reach the Earth’s surface from the sun. That’s over 10,000 times more than the global demand for energy. But we have made little use of all this energy so far.

Concentrated solar power (CSP) and photovoltaics are two different technologies that allow us to capture the sun’s energy and turn it into something useful.

As a type of solar thermal technology, CSP involves transforming sunlight into heat by using mirrors to concentrate the rays. The heat energy is then used to produce steam that drives a turbine – just like in a conventional power plant. The advantage with this process is that we can store the heat and still generate electricity at times when the sun isn’t shining. Today’s CSP plants currently achieve efficiencies of 15 to 25 percent.

Photovoltaics uses solar cells that immediately transform sunlight into electrical current. When sunlight hits the cells, it separates positive and negative charge carriers, which in turn produces direct current. The biggest problem with photovoltaics is that we have no effective storage solutions at the moment. The most common types of cells are made of crystalline silicon, a semiconductor doped with impurity atoms. Modern silicon cells can already deliver efficiencies of 25 percent.


Photo: DLR

Both technologies are currently the focus of intensive research – for example at the Helmholtz-Zentrum Berlin (HZB) and at the DLR Institute of Solar Research, which operates the Jülich solar tower power plant. The researchers’ main aim is to make the systems more efficient so that the costs of generating electricity with them come down.

Both technologies will be around for a long time to come – and maybe other technologies will join them. DLR researchers estimate that if CSP plants covered just one percent of North Africa’s desert, they could meet the whole world’s energy needs. But that would present the problem of transporting the energy as electricity. Photovoltaic systems offer smaller, decentralised solutions. If Germany covered every industrial wasteland and every roof of every home with the systems by 2050, they could supply 25 percent of its energy.

Dorothee Bürkle, energy technology editor at DLR, answered this question.