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Publication Date: 12/1/2009
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Solar Energy Comes in Many Flavors
Every available inch of rooftop and backyard space is used in this solar installation at a gas station/convenience store in Chico, CA.

The U.S. rooftop solar industry is, at the moment, famously in a slump. When business picks up again, the U.S. will be in a position — perhaps — to begin to catch up with Europe in rooftop solar power generation.

Europe has the advantage of very generous governmental incentives for homeowners to install solar. As Raju Yenamandra, head of sales for North America for SolarWorld USA explains, "If you have a roof (in Europe), you have a business opportunity to make money — you generate your electricity and you sell all of it to the utility at 50 (Euro) cents, and then you buy what you need from the utility, and you only pay 20 cents."

Incentive Programs
Currently California has the most generous and forward-looking incentive program in the U.S. The California program is so successful that about 60 percent of all residential and commercial installations in the U.S. are now in the Golden State. When the overall financial climate improves, other states are likely to become more involved, and solar installations are likely to grow rapidly.

The assumption is that the cost of solar installations will steadily decline, while the cost of grid electricity will increase, probably at about 6 percent per year. At some point the two lines will cross, and installing solar will confer an immediate advantage.

When installations do start to boom, consumers will begin to find out that not all solar panels are equal. Most solar panels are a standard 2 x 4 feet (61 x 122cm) in size and look more or less the same, but in fact, there are dozens of different flavors — meaning differences in design, materials, and efficiency.

All solar panels convert photons (incoming light) into electricity, but numerous materials can be used to make the conversion. Steve Chadima, vice president for External Affairs for SunTech America, notes that panels are made out of either crystalline silicon or one of several thin-film materials such as Cadmium Telluride (CdTe) or Copper Indium Gallium Selenide (usually abbreviated as CIGS). Even within each of these materials, there are many variations from manufacturer to manufacturer.

Crystalline Silicon
Crystalline silicon, Chadima notes, is better known in a manufacturing sense because it has been used in semiconductors for half a century. But even here there are variations: monocrystalline silicon (relatively costly), polycrystalline silicon (cheaper) and even amorphous silicon.

The many materials differ in their efficiency — the percentage of incoming energy that is converted into electricity. Overall, the range of efficiencies is probably from about 6 to about 20 percent. But more is not always better. First Solar, for example, uses a Cadmium Telluride thin-film. Lisa Morse of First Solar notes that their modules are not as efficient as some competing modules, but the production cost for CdTe is much lower. "A few years ago we were manufacturing our solar panels at $3 a watt," she observes, "and as of last quarter we were manufacturing them at 93 cents a watt"..if you take the cost of the modules and cost of the mounting structures, it's still less expensive than using higher efficiency modules."

In Europe, First Solar sells mainly rooftop systems, but in the U.S. is sales are chiefly to power companies who want solar farms. The low-cost approach recently paid off handsomely when First Solar entered into an agreement with China to install a 2-gigawatt solar power field that, at 25 square miles, will be slightly larger than the island of Manhattan.

Steve Chadima points out some of the diverse developments in the solar arena:

  • Amorphous silicon solar panels are relatively new and have rather low efficiency around 6 percent, but are inexpensive to produce. Other technologies such as CdTe started out with low efficiencies (7 percent to 8 percent for CdTe), but gained efficiency through development; First Solar's CdTe is now around 10.5 percent. So amorphous silicon may become an additional low-cost choice.
  • Chadima's own company (SunTech America) has a new technology known as Pluto that uses a novel design to boost efficiency. Metal lines running across a solar cell collect the electrons, but these lines also shade some percentage of the cell area and thus reduce efficiency. Using patents licensed from the University of New South Wales in Australia, Sun Tech uses much thinner wires (for less shading) and places them closer together. Since some electrons get lost on their way to the nearest wire, closer placement captures more electrons and raises efficiency. The efficiency of the technology is 16.5 percent using polycrystalline silicon, and 19 percent using monocrystalline silicon.
  • In ski country, you may have noticed metal roofs that have ridges spaced about every 18 inches. The idea is to make it easy for snow to slide off. UniSolar makes a thin-film material that can be rolled out between the ridges, thus converting the roof into a power source while still letting the snow slide off.
  • Chadima's company also makes an innovative glass-on-glass thin film solar product that is about 6 percent efficient. It can be used in place of conventional glazing on office buildings, with the result that the whole side of the building is turned into a power generator. The huge area compensates for the relatively low efficiency. "It comes in three different degrees of opacity, and so you can have either light sunshade, medium sunshade, or dark sunshade," Chadima says. From inside the building, an observer has the impression of looking through sunglasses.
"Put it on the south side or the west side of a building and you'll get at least half the day's worth of sun out of it," he explains. 

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