FAQ: A concentrated power boost for solar energy

Using very different designs, several companies are focusing the sun's energy onto solar cells to lower the cost of solar electricity. Photos: More solar bang for the buck

Concentrating solar power, which has been around for decades, is one of the most promising techniques being tried today to make solar electricity more cost effective.

The concept is simply to focus light in order to boost electricity output. But there's a wide disparity in the types of solar concentrators being built, from utility-scale solar thermal projects to specialized photovoltaic solar panels that could one day go on a homeowner's roof.

In this FAQ, we will specifically discuss concentrating photovoltaics, a design being pursued by a number of solar companies seeking to lower the cost per kilowatt the sun can deliver.

What are the primary forms of solar concentrators?
Solar concentrators use lenses, mirrors, parabolic dishes or other optics to concentrate energy from the sun. Very often, they have a mechanism so that these devices track the path of the sun during the day. In solar thermal applications, troughs or large mirrors amplify sunlight to create heat, which heats a liquid or gas that turns turbines to make electricity. Solar thermal is used for large-scale power plants operated by utilities, usually in the desert. After a 16-year hiatus, companies are opening up new plants or contemplating new ones in the southwestern U.S., India, southern Europe and North Africa.

This same technique is also being pursued in conjunction with photovoltaic solar cells, which convert light to electricity. Among concentrating photovoltaic companies, there is a wide range of approaches. There are systems designed for utilities' central power stations, mounted concentrators that can go on the roof of an office building, and those that are the same size as traditional solar panels.

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Why is there interest in concentrating photovoltaics?
Three words: the solar constant. The sun radiates about a kilowatt of energy per square meter on the surface of Earth, according to B.J. Stanberry, CEO of HelioVolt. There are 2.6 million square meters in a square mile. Thus, every square mile gets about 2.6 gigawatts. It's a number that just can't be increased.

Concentrators essentially try to artificially increase the constant by virtually expanding the size of solar cell with mirrors or lenses. The quality of the concentrator is rated by how much solar real estate it can cram onto a solar cell without creating things like shadows or interfering with other solar cells.

One number you hear a lot is how many suns a concentrator replicates. GreenVolts, which is commercializing technology licensed from Lawrence Livermore National Laboratory, has a concentrator it says can deliver the equivalent of the energy of 625 suns to a solar cell.

Why not just improve solar cells?
That's also being done. Without concentration, the efficiency of commonly used solar cells made from silicon tops out at around 22 percent. Physics says that crystalline silicon PV cells will top out at around .

High-efficiency cells, historically used for satellites or spacecraft and made from different layers of materials, can exceed 40 percent efficiency or more, but this pushes up the price. Focusing more light onto cells makes them more productive.

The relatively high cost of photovoltaic material--the most common being silicon, which is in short supply--represents a significant cost to an overall solar power device. Using concentration, manufacturers are looking to lower the overall cost per kilowatt-hour of a solar power purchase. People often believe that since most solar cells are made of silicon, panel manufacturers inherit Moore's Law, which stipulates that the performance of microprocessors double every 24 months. But the same dynamics don't play out, solar industry executives say. Instead, the solar industry is focused exclusively on cost and making solar power competitive with traditional fossil fuel-based power generation. That's why many companies, including a number of start-ups, are trying to concentrate solar power, along with thin-film solar cells made from other materials, and lowering their manufacturing costs.

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