BERLIN - Winegrower Barbara Banke of the Kendall-Jackson Wine Estates in California doesn’t just love the sun because it ripens grapes. The sunnier it is, the lower the winery’s electricity and gas bills are. And those savings are possible because of the large mirrors mounted on the football-field-sized tin roof of the winery’s production facility that for a year now have been concentrating sunlight on a narrow band of thin silicon solar cells.
These cells deliver electricity – but that’s not all. A tubular system on the back of the cells has a mixture of water and glycol flowing through that cools the silicon modules and heats a water cistern to a good 60° Celsius. The winery needs more than five million liters of hot water per year to clean the tanks in which the wine is stored. The hybrid installation’s photovoltaics also supply lighting and air conditioning. "We’re saving so much energy we expect to pay off the installation in three years," says Banke.
The hybrid installation using both photovoltaic and solar thermal technology comes from Cogenra in California, which has installed solar systems at the University of Arizona and Facebook’s new headquarters in Menlo Park among others.
The two-in-one solution is highly efficient. "Pure silicon photovoltaics use 15% to 20% of sunlight. The rest is lost heat,” product manager Mani Thothadri explains. "If on the other hand you collect and use the heat, then overall efficiency per surface goes up to 75%."
Israel’s Zenith Solar company also claims a similar rate of overall efficiency. It uses even bigger mirror systems and expensive but efficiently stacked solar cells that react to the different colors of sunlight. For three years, one of their installations has been providing a kibbutz in Israel with electricity and heat. Zenith has also announced that it will be providing the Chinese province of Gansu with two 10-megawatt installations.
The Pike Research marketing research institute in Boulder, Colorado, is predicting quick growth for solar hybrids. According to their estimates, by 2022, solar hybrids will be providing energy for 13.5 million households around the world.
Improving electrical output
But putting futuristic small-sized power stations like these on German roofs doesn’t make as much sense. Systems that concentrate sunlight with mirrors are mostly suitable in areas where there is a lot of direct sunlight. So researchers at the Institute for Solar Energy Research (ISFH) in Hameln, Lower Saxony, have been testing hybrid models that work like the Cogenra systems – except without the mirrors. Essentially, these are classic photovoltaic modules cooled by liquid circulated on the back.
However in places where there is relatively little sun, like Germany, using the lukewarm waste heat for heating or hot water is not an option with the ISFH system. "But the hybrid installation can, for example, help to warm the air in a public indoor pool, and our tests have also shown that it can provide extra heat for a geo-thermal heat pump," explains ISFH engineer Erik Bertram.
Using the hybrid with the pump can improve electrical output because photovoltaic cells work better when they remain cool, which is why the module they tested produced 5% more current than usual and the heat pump needed 10% less electricity. Bertram, however, readily admits that the hybrid isn’t yet at the point where it would make sense to mass-produce.
Researchers at the Fraunhofer Institute for Solar Energy Systems (ISE) in Freiburg, Germany, prefer a different combination concept. They are developing a module that combines solar cells and heat absorbers in a well-insulated metal box that can produce a heating-compatible 60° Celsius even in Germany. On the solar side the module is covered by a coated sheet of glass that lets a lot of light in and no heat out.
This subtle heat trap can reduce electricity yield, since with every further degree this sinks by nearly half a percent. That is best observable during the summer months when the heating is off and no heat is drawn from the solar collector. "Generally the temperature in the modules then rises to between 40° Celsius and 60° Celsius," says ISE researcher Gerhard Stryi-Hipp. This means that efficiency sinks from a typical 15% at 25° Celsius to 12.5% – i.e. by a sixth. "That’s why in warm areas it’s worth considering adding a solar thermal air conditioning system that converts surplus heat," he says.
Security concerns raised by some – the electricity and the fluid are very close together in the hybrid modules – do not worry the ISE team because "while it is difficult to find materials that conduct heat well and insulate electric current reliably, it is doable," Stryi-Hipp says. His team mainly works with special synthetic foils, but is also testing lacquers and electrolyzed oxidized aluminum. The researchers aim to have their concept ready for serial production in three years.
"Buyers probably won’t save money on these solutions," says Stryi-Hipp. "But for the same money on the same roof surface they can produce more usable energy."
