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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.

OK, concentrating light onto solar cells means more power output. But does that mean it's more cost-effective?
Not necessarily. Concentrating photovoltaic systems often require lenses or mirrors to focus the light onto solar cells. To maximize the amount of light they receive, high-concentration systems can be mounted and need a motor so that the cells track the movement of the sun over the course of the day. So although manufacturers may be saving money on solar cells, the additional equipment can raise the price.

But solar concentrators stand to benefit from the incremental improvements in solar cell efficiency. Brad Hines, the chief technology officer of solar concentrator company Soliant Energy, says that concentrators are becoming more cost-effective as cell efficiency climbs. A company can build a concentrator that's cost-competitive with traditional solar panels when cells are 16 or 17 percent efficient, he figures. Once manufacturers start using cells with efficiency above 18 percent, there's a significant cost advantage. "When I make a concentrator, it costs me the same to build the frame and the tracking system and optics regardless of the solar cell I put in," Hines said. The same basic math holds true with high-efficiency cells, even though they are more expensive, he says.

So what do these solar concentrators that use photovoltaic cells look like?
Designs vary greatly (see a few of them ). One could segment the industry into high-, mid- and low-concentration categories.

An example of a high-concentration company is SolFocus, a venture spun off from Xerox's Palo Alto Research Center. The company is using a series of curved lenses that focus light on high-efficiency triple-junction (i.e. multi-material) cells. The company's "honeycomb" structure places 16 of these dishes on a flat panel, which is mounted on a pole. This design magnifies light by 500 times normal sunlight, according to the company. There are a number of companies pursuing this basic design of concentrating solar arrays, essentially large assemblies with several panels. When scaled up into many very large arrays mounted on the ground, they can be used for multi-megawatt power plants. Or, one or a few arrays can be used on the rooftop of a commercial building to supplement their power consumption.

Energy Innovations is another company targeting the flat commercial rooftop business, but with a different design. It, too, uses high concentration--on the order of 800 times--and high-efficiency cells with its Sunflower product. It has a tracking system that can follow the sun's altitude and azimuth (its angle on the horizon) and a specially designed mounting system meant to keep a low profile on the roof.

Makers of low-concentration panels are satisfied with much lower magnification--as low as two or three times concentration. Companies pursuing this general track are Soliant Energy, Solaria and Silicon Valley Solar, which recently acquired NuEdison. Although they have different optical techniques for directing sunlight, the end product is meant to have the same shape and size as traditional solar panels. That should make it easier for installers and distributors familiar with solar panels able to work with these products without any special training or mounting equipment.

What are the tradeoffs of this approach?
Magnifying light many times, of course, creates a lot of heat, which lowers the efficiency of solar cells. As a result, high-concentration manufacturers often use specialized cooling systems. In addition, large-scale systems with several mounted arrays can be big civil engineering projects. As noted, all the additional equipment and engineering involved in building an entire system that tracks the sun during the day can raise the overall cost of the system, even if manufacturers are being thrifty with solar cells.

Very large concentrating photovoltaic arrays, which could be used for a medium-size power plant, are designed mainly for very sunny environments like the desert of the southwestern United States, according to solar industry executives.

Because they align with the sun very closely, a concentrating photovoltaic power plant will not perform as well on cloudy days, whereas a power field with hundreds of traditional flat-plate solar panels could still generate a significant amount of electricity, said Nancy Hartsoch, vice president of marketing at CPV start-up SolFocus.

"Put us in the Mojave Desert and we'll significantly outperform flat-plate photovoltaics. Put us in a power field in Germany and we won't," she said.

Because concentrating photovoltaic is a relatively new technology, Hartsoch expects that utilities will use a combination of flat-plate solar panels with concentrating photovoltaic arrays in the near term.

Are these concentrating photovoltaic systems commercially available?
Many of these systems are being tested now with utilities and commercial customers. A number of vendors have promised commercial availability of their systems late this year or next year. It doesn't appear that panels aimed at residential rooftops will be coming in the near future.

How will things look a few years from now?
With so much investment and engineering being poured into concentrating solar power, it is likely to endure once products are commercially available. What is less clear is which designs will win out. Solar industry executives expect that different approaches will find their market niches, such as smaller power plants for utilities looking to boost the amount of renewable energy they produce to meet government mandates. For on-site power generation, rather than centralized power plants, the success of commercial customers will help sort out the winners and losers, as they represent the mass market.

Apart from design, one of the major factors of success is manufacturing processes, argues Suvi Sharma, CEO of Solaria. With so much competition among solar companies--as well as other forms of electricity generation--the cost-effectiveness of the end product will hinge heavily on a provider's scale and operational efficiency, he said.

"Not every Silicon Valley company getting funding will be standing five years from now, but there will be some great successes," said Sharma. "Once things get more cost sensitive and commoditized, there will be a weeding-out's very important to bridge the gap from a technology development phase to mass production."

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