Given the immense size of the sun—its diameter is greater than a hundred earths lined up—you'd think that solar power would be similarly huge in the energy business. But it's the cost of solar energy that's sizeable—not its popularity. This is because the sun's potent energy is not only widely dispersed by the time it reaches the earth, it's also inefficiently converted into electricity.

Fortunately, this could change because of work being conducted by Israeli researchers, who are taking the simple concept of concentration—a feat easily performed by any person wielding a magnifying glass on a sunny day—to a new level. The scientists are focusing sunlight into a small space to boost the amount of available energy and to reduce the price per watt.

To do this, researchers at the Jacob Blaustein Institute for Desert Research of Ben-Gurion University of the Negev in Israel are using mirrors, fiber optics and photovoltaic cells.

They have developed a parabolic mirror that is less than 20 centimeters on a side. It directs reflected light onto a tiny, flat reflector that in turn focuses it down a 20-meter-long strand of optical fiber. From the other end of the fiber emerges a powerful laser-like beam. "We're taking the sunlight and concentrating it close to the fundamental limits, what it is at the surface of the sun," says professor Jeffrey Gordon of the Blaustein Institute's department of energy and environmental physics.

This light-concentrating device could even one day replace laser surgical instruments used in sunny areas. It could be mass-produced for as little as $1,000 a pop—much more affordable than current lasers used in medicine, which run in the low six-figure range. So far, the solar-powered device has been tested by veterinarians.

Indeed, the idea of concentrating solar power has already proven effective. In fact, solar furnaces—for example, the facility managed by the U.S. Department of Energy's National Renewable Energy Laboratory in Golden, Colorado—have demonstrated the value of focusing energy on a large scale. The Colorado installation employs 32 square meters of mirrors to concentrate solar energy by up to 50,000 times, creating a 10-kilowatt heat source. The furnace has been used to help apply coatings on metals and ceramics, fabricate nanoscale materials and detoxify hazardous substances.

The High Cost of Solar Energy

Concentrating sunlight is a good idea because by the time solar radiation gets to earth, it is spread out over a sphere with a radius of about 150 million kilometers. Moreover, many solar power-converting devices—especially photovoltaic (PV) cells, which generate electricity—waste a lot of energy. In fact, the world's top mark for photovoltaic efficiency, achieved by BP Solar International, is only 18.3%. At that rate, one kilowatt of solar power would generate a little over 180 watts of electricity—just enough to power a standard PC and monitor.

This wide distribution and inefficient transformation of solar power accounts for its high cost. The price tag for converting an average household to solar ranges from $10,000 to $40,000, says BP Solar, a division of global petroleum company BP plc and a major manufacturer of solar energy equipment. And large-scale solar electricity plants average $3 million to $4 million in costs per megawatt of capacity, while a fossil fuel-fired plant only costs between $1.5 million and $1.7 million per megawatt.

Combining Concentrators & Photovoltaic Cells

Solar cells developed with current technology "have reached a plateau," says Roland Winston, a University of Chicago physics professor who researches optics for focusing sunlight. Fortunately, Gordon and his colleagues at Ben-Gurion University don't expect this to be a problem. Even if the efficiency of conversion doesn't improve, they believe they can make the system more effective just by focusing more solar energy from a wider area—something that they plan to do by teaming a photovoltaic cell with their mirror concentrator.

By boosting the amount of power sent to even a wasteful cell, the researchers believe they can increase electrical output. While the net generation of power using a concentrator might be equivalent to placing solar cells over a larger area, the overall cost would be lower because fewer costly photovoltaic devices would have to be utilized.

The Ben-Gurion researchers have been collaborating with a team from Drexel University on a U.S. Department of Energy-funded project to combine the concentrator with a high-efficiency photovoltaic cell located on the other end of the fiber optic strand. Using this configuration, the scientists have raised a typical cell's electrical output from 2 watts to 3 watts, and they are confident that they can boost it to 4. The Drexel team is trying out a one-kilowatt mini-dish power plant, which is expected to post a conversion efficiency rate above 20%. Meanwhile, other groups, such as Spectrolab, a subsidiary of Boeing, are also developing combinations of concentrators and solar cells.

The union of photovoltaic cells and concentrators is also promising because it can produce devices that may be mass-produced. "You stamp (the combined PV cell and concentrator) out," says Gordon. "Everyone is identical to the next." The cells could then be linked, generating nearly any amount of power, at least in places that get lots of sunshine. If the wattage produced per photovoltaic cell can be successfully doubled, then a solar power plant would require only half as many cells and become nearly as affordable to run as a traditional coal- or petroleum-stoked facility. And with a fuel supply that's free, solar plants could finally be the next big thing in energy.

Source: Concentrating on Solar
Erik Sherman
Technology Review, May 30, 2003
http://www.technologyreview.com/articles/wo_sherman053003.asp