There are at least a dozen major ways to turn sunlight into electricity, but one of the more interesting is using a field of mirrors to focus the sun’s energy on a “power tower” where the heat is captured and used later to spin a turbine and turn a generator. As I have previously written, two companies are now planning to build such systems in the desert Southwest with hardware that will store the heat for a rainy day or for the period right after sunset when power demand is still high.
But now comes a new player with a different concept: build the tower, but on a smaller, simpler scale, and skip the storage in favor of using using biogas ornatural gas to power the system after dark.
The new player, Aora, has two systems running, one on a kibbutz in southern Israel and the other in Almeira, Spain. It hopes to announce soon that it will be building several more in Arizona.
The basic unit planned there is an array of about 50 mirrors focused on a tower that rises 115 feet. It produces 100 kilowatts of electricity, which is enough to run about 100 window air conditioners.
The competition is hundreds of times larger. The two companies already in business in this field in the United States are BrightSource, which also has Israeli roots, and SolarReserve. Their towers are hundreds of feet high and surrounded by thousands of mirrors, and they produce power at the level of tens or hundreds of megawatts. (A megawatt is approximately the level of energy used by a Super Wal-Mart, and a big coal plant generates about 600 megawatts.)
For example, BrightSource has a power plant in Coalinga, Calif., that produces 13 megawatts of electricity with a 327-foot tower. It is building another project with 500-foot towers and has plans for another with 700-foot towers. These would be surrounded by thousands of heliostats, or devices equipped with mirrors that redirect the sun.
The giant towers heat water into steam or heat molten salt that will be used to boil steam; the steam flows through a turbine that converts the heat energy to rotational movement that drives a generator. Plants running on coal or nuclear energy do the same thing with the steam they make.
Aora, on the other hand, heats ordinary air that then expands and spins a gas turbine that resembles a jet engine. During periods when there is no sun, the system injects a hydrocarbon fuel, which can be ordinary natural gas, methane from a landfill, propane or a variety of other substances to expand the gas and spin the turbine.
One advantage of using air instead of steam is that the air can simply be vented, whereas the steam has to be converted back to water for reuse. “Where you have the best sunlight, you have the least amount of water’’ available locally, Aora’s chief executive, Zev Rosenzweig, noted. His design does require water for washing the mirrors, however, and if the outdoor air temperature is high, it needs some water to cool the air and make it more dense before it can be compressed by heat to drive the turbine.
For solar, the big issue is the early evening: “Solar bows out at 5 p.m., Mr. Rosenzweig noted. But that’s when peak demand begins — people come home, turn on the air conditioner full blast, put dinner in the oven or microwave and turn on television sets in the ensuing hours, he said. Rather than building a system with storage, which requires an oversize gathering system to take in more sun than is required for instant generation, Aora decided that it was easier to equip its system to burn natural gas, using most of the original hardware.
While the system has potential for further growth, one question is what to do with the hot air that is expelled from the turbine. There is not quite enough of it to boil water to make steam for electricity, but it could have a variety of uses, including driving an air-conditioning system. In that sense, a small modular design has advantages over a mega-project. In the California desert, there is a market for electricity, which can be easily shipped, but there is not much market for waste heat.