Concentrating on Solar

With a diameter bigger than a hundred earths laid out in a line, the sun is big-really big. Solar power advocates thought that their technologies would be the correspondingly next big thing in the energy business. Instead, the costs are mammoth. Research suggests that one practical solution to the solar conundrum is concentration. Focusing sunlight into a tiny space can increase the amount of available energy and drive down the  price per watt. In other words, to get big, the trick might be to think small.

In this case, small is a parabolic mirror a little less than 20 centimeters on a side, developed by researchers at the Jacob Blaustein Institute for Desert Research of Ben-Gurion University of the Negev in Sede Boqer, Israel. The mirror focuses the reflected light to a small, flat reflector 12 centimeters away that, in turn, directs it down a 20 meter-long strand of optical fiber. Out the other end of the fiber comes an intense stream of heat that can cut like a knife-or a laser. “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. Although just a prototype, in sunnier climes it could become a solar-powered mobile replacement for laser surgical instruments at a cost of as little as $1,000 in mass production. That compares favorably to the low six-figure price tag of the conventionally powered lasers now used in medicine. Experiments have progressed from dead animal tissue to tests by veterinarians.

The concept of concentrating solar power is as old as the first time a child grasped a magnifying glass and focused it on a leaf. And solar furnaces, like the one operated by the U.S. Department of Energy’s National Renewable Energy Laboratory in Golden, CO, have shown the effectiveness of concentrating energy on a large scale. That installation uses 32 square meters of mirrors to concentrate solar radiation by as much as 50,000 times, forming a ten-kilowatt heat source. Researchers have used that furnace to help deposit coatings on metals and ceramics, produce nanoscale materials, and detoxify hazardous wastes.

In a sense, concentrators restore sunlight to something a little closer to its original intensity. While the sun emits enormous energy, by the time it reaches the earth, that radiation is dispersed over a sphere with a radius of about 150 million kilometers. Moreover, many uses of solar energy, particularly conversion into electricity through photovoltaic (PV) cells, are highly inefficient: the world record for photovoltaic efficiency, currently claimed by BP Solar International, stands at a mere 18.3 percent. At that conversion rate, the kilowatt of solar power that, on average, strikes every square meter of sunlit land would produce just over 180 watts of electricity-barely enough to run a typical PC and monitor.

Solar cells built with existing technology “have reached a plateau,” says Roland Winston, a University of Chicago physics professor who studies optics for concentrating sunlight. However, even if the efficiency of light-to-electricity conversion remains level, Gordon and his colleagues at Ben-Gurion University believe that they can raise the effectiveness of the system simply by funneling more solar energy from a wider area by combining a photovoltaic cell with their mirror concentrator. Increase the amount of power delivered onto even an inefficient cell, and electrical output will similarly increase.  Although the net generation of power with a concentrator might be the same as by placing solar cells over a bigger area, the overall cost would be less because fewer of the expensive photovoltaic devices are needed.

The Ben-Gurion scientists have been working with a team from Drexel University on a U.S. Department of Energy-sponsored project to marry the concentrator to a high-efficiency photovoltaic cell sitting at the other end of the fiber optic strand. Using this setup, the researchers have improved a typical cell’s electrical output from 2 watts up to 3 watts-and they believe that they will be able to achieve 4 watts. The Drexel group is working to test a one-kilowatt mini-dish power plant with an expected conversion efficiency exceeding 20 percent. Others are also working on combining concentrators with solar cells. Spectrolab, a Boeing subsidiary and an industrial partner of the National Renewable Energy Lab, has driven a triple-junction solar cell, which uses a stack of three PV cells, to 34 percent efficiency under concentrated sunlight.

What makes the combination of photovoltaic cells and concentrators additionally interesting is that the resulting devices might lend themselves to mass production. “You stamp [the combined PV cell and concentrator] out,” says Gordon. “Everyone is identical to the next.”  The cells could then be connected, providing virtually any amount of power, at least in regions with plentiful sunshine. And if the wattage generated  per photovoltaic cell can indeed be doubled, then a solar power plant would need only half as many cells-bringing its costs much closer to those of conventional coal- or petroleum-stoked facility. Add a fuel supply that costs nothing, and a focus on the small might deliver big savings.