A potential breakthrough in harnessing the Sun’s energy

Palo Alto, California-based Ausra will employ compact linear Fresnel reflectors - flat mirrors that deliver the same focus - to heat water at a planned 177-megawatt solar thermal plant named Carrizo Plains in central California by next year. Already, the company opened a 5-megawatt demonstration plant last October near Bakersfield, Calif.

Nor is the technology confined to the southwestern US: Florida Power & Light will build a 75-megawatt solar thermal trough plant north of Miami. Hurricanes are not a major concern; Ausra’s chief scientist and founder, David Mills, notes that in testing their mirrors have withstood winds of more than 91 miles per hour.

But the most promising technology is one using molten salts, as it overcomes one of the chief traditional drawbacks of solar energy generation - that when the sun sets, the lights go out. The Andasol power plant uses more than 28,000 metric tons of sodium and potassium nitrates to store some of the sun’s heat for use at night or on a rainy day. The molten salts are stored in enormous hot and cold vats, able to be employed on command to soak up extra heat or drive the generation of electricity.

“The turbine is running more hours every day because we have storage and we have the possibility to plan our electricity production,” said Sven Moormann, a spokesman for Solar Millennium, the German company building Andasol.

Abengoa Solar and Arizona Public Services are now using the molten salts technology in portions of the Solana - or “sunny place” - power plant, located 70 miles southwest of Phoenix on nearly 2,000 acres of land. The plant will ultimately produce enough electricity to power 70,000 Arizona homes.

“One of the great things about molten salt technology is that you can get more energy out of the same facility,” says Barbara Lockwood, manager for renewable energy at Arizona Public Services.

But molten salts don’t have to be just used for storage, as they are at Andasol and will be at Solana. They can also be used directly as a fluid in solar thermal power plants that operate at a much higher temperature, replacing the synthetic oil or water used in power towers. In this variation of the solar thermal technology, large fields of mirrors concentrate the sun’s heat on a central tower that glows with intense light.

Such plants operate at more than 540C - closer to the temperatures employed at a coal-fired power plant - and therefore can use the salts directly as a heating medium. At night, when temperatures begin to drop, the cooling salts that have already transferred their heat to drive a turbine simply drain to the bottom of the tower, where they are stored in tanks, ready to be heated again the next sunny day.

Cheaper power towers that do not employ the molten salts are also being built: an 11-megawatt power tower that employs steam directly - built by Abengoa - now operates outside Seville, Spain. Southern California Edison has contracted for 1,300 megawatts of such direct steam solar thermal power towers with developer BrightSource.

Some utility-scale solar thermal projects have provoked opposition due to the large land area occupied by the arrays in Bakersfield, Los Angeles, San Diego, and elsewhere in California. But proponents of solar thermal power argue that the benefits of the carbon-free technology far outweigh any local impact.

“We’re not going to solve the [climate change] problem without putting large-scale concentrated solar facilities in the American Southwest,” SolarReserve’s Murphy says. “It doesn’t take that big a footprint to make a pretty big impact.”

In addition, there is another way to use this technology for capturing the sun’s heat - cleaning up existing fossil fuel-fired power plants or other operations that burn a lot of CO2-emitting fossil fuels.

By employing the mirrors of a solar thermal array to pre-heat steam, the amount of natural gas, oil, or coal that must be burned can be reduced. In fact, Ausra’s first installation boosted the efficiency of a coal-fired power plant in Australia by providing 9 megawatts of steam to the 2,000-megawatt Liddell Power Station. The company also hopes to work with some California oil producers that currently inject steam - generated by burning natural gas - into the old reservoirs to enable more oil to be pumped to the surface. Ausra argues that it can generate the same steam without any CO2 emissions by employing its solar thermal technology.

The Electric Power Research Institute, a utility-funded consortium, will study the potential of the technology to reduce fossil fuel-burning at power plants in Arizona, New Mexico, Nevada, and North Carolina.

“People need to look at this as a hedge against fossil fuel prices,” says Murphy. “You could start deploying a new type of power plant. We used to burn coal and natural gas - now we can use the sun to make steam.”