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Concentrating sunlight onto solar arrays has made big gains in efficiency and drops in cost thanks to recent technological breakthroughs. Here's how this large-scale solar system works.
With four silver sheets hung from horizontal arms and attached to a stout mast, MegaWatt Solar's trademark solar system brings to mind a square-rigged schooner of old. But instead of catching the wind, the sheets—actually parabolic mirrors—reflect sunlight onto a thin band of photovoltaic cells suspended in front of them. MegaWatt Solar, based in Hillsborough, N.C., is one of dozens of companies around the globe who are counting on a new generation of concentrating solar photovoltaics (CPV) to provide renewable electricity on a utility scale at a price competitive with conventional sources.
All photovoltaic (PV) systems use solar cells made of a semiconductor material to capture sunlight and convert it to electricity. Most are flat-plate systems that are stationary and involve no concentration of sunlight. Thin film systems do the same. The most expensive component of these PV systems is the cell, so if you can use less of them to produce the same amount of energy, you are dollars ahead. One way to do that is to concentrate sunlight on the cell. CPV systems designed with parabolic reflectors concentrate sunlight on the order of 20 times (20x). Systems using magnifying lenses that can achieve concentrations of up to 1000x.
Parabolic reflectors are also used for concentrated solar thermal power, such as the Solar Energy Generating Systems built in the Mojave Desert in the 1980s. These systems heat fluid in a pipe, which in turn produces steam to drive an electric turbine. CPV systems convert sunlight directly into electricity with much greater efficiency and, theoretically, far less up front and maintenance costs.
CPV currently represents a small fraction, about 0.1 percent, of installed PV capacity around the world. Flat-plate systems, with their simple, well-proven design, are better suited to the small, off-grid applications that have dominated the markets to date. But recent mandates that electric utilities generate a certain fraction of their output from renewable sources, combined with breakthroughs in CPV technology, are opening doors for this industry.
The biggest advances are being made in solar-cell efficiency. Traditional PV systems use silicon cells that capture only one band of sunlight and convert it to electricity with about 20 percent efficiency. CPV systems use multi-junction cells that, layered like a cake, capture three bands of light. These have achieved efficiencies of 41 percent and hold the promise of 50 percent or higher. Made of materials such as germanium and gallium arsenide, multi-junction cells are more expensive than silicon, but in CPV systems far fewer are needed to capture the same amount of light.
Improvements in optics and an ingenious use of materials are also raising efficiencies and lowering costs. Megawatt Solar is taking a low-cost approach, manufacturing its 20x parabolic reflectors out of Dibond, a material used to make advertising signs. SolFocus, of Mountain View, Calif., has gone high-tech with its optics, using a unique combination of mirrors and lenses to achieve a concentration of 500x.
Motors and controls that allow the solar array to track the sun have also improved. Software programs have been developed to keep the array pointed at the sun even when it has gone behind clouds or over the horizon. Stronger materials and better designs give the latest arrays a higher wind tolerance than earlier systems.
Because lenses or reflectors on CPV systems can only focus a direct beam of light, not diffuse light, they are best suited for areas like the southwestern U.S., where peak electrical demand occurs on sunny afternoons. "CPV is already cost-effective in Southern California at times of peak demand," says Bob Cart, founder of San Francisco–based GreenVolts, which has a contract with Pacific Gas & Electric Company to provide 4670 megawatt-hours per of electricity, roughly the annual demand of 1000 homes.
Despite its recent allure, CPV faces serious challenges. Because the industry is young and demand for its products low, manufacturing costs remain high. Lowering prices requires automation; however, investors are reluctant to put big money into a technology for which there is limited field experience.
"The problem is greatest for the many newcomer CPV companies that have no real-world deployment experience to back up their claims," says Terry Peterson, solar power consultant to the Electric Power Research Institute and the U.S. Department of Energy. "Prudent investors know the odds are that these systems will require design adjustments in the first several incarnations … and this invariably drives up costs, sometimes dramatically."
Sarah Kurtz, principal scientist for the National Renewable Energy Laboratory, remains optimistic. Kurtz just returned from CPV-6, the International Conference on CPV held in Freiberg, Germany. "System efficiencies are edging up, a few companies are reporting installation of automated assembly lines, and several are reporting months or years of field data," she says. "CPV has the potential to capture a significant fraction of the market, but will require a few more years before we can see large increases in deployment."
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