Making hay while the sun shines

Making hay while the sun shines

In the high desert of southern Spain, not far from Granada, the Mediterranean sun bounces off large arrays of precisely curved mirrors that cover an area as large as 70 soccer fields. These parabolic troughs follow the arc of the sun as it moves across the sky, concentrating the sun’s rays onto pipes filled with a synthetic oil that can be heated to 750 degrees Fahrenheit. That oil is used to boil water to power steam turbines, or to pump excess heat into vats of salts, turning them a molten, lava-like consistency.

Molten salts

The salts are fertilisers — a mix of sodium and potassium nitrate — but they represent a significant advance in the decades-old technology of solar thermal power production, which has traditionally used mirrors to heat water or oil to generate electricity-producing steam. Now, engineers can use the molten salts to store the heat from solar radiation hours after the sun goes down and then release it at will to drive turbines. That means solar thermal power can be used to generate electricity nearly round-the-clock.
The plant in southern Spain, known as Andasol 1, provides 50 megawatts of power, enough electricity to supply 50,000 to 60,000 homes year-round. When the entire Andasol complex is completed in 2011, it is expected to generate enough electricity to power 150,000 households — about 600,000 people.
In the face of mounting concern about climate change, developing alternatives to coal and natural gas combustion has taken on a new urgency, and the construction of utility-scale solar thermal power plants in deserts and arid areas is looking like an increasingly promising option.
Researchers at the German Aerospace Center have estimated that 16,000 square kilometres of solar thermal power plants in North Africa — paired with a new infrastructure of high-voltage, direct-current transmission lines — could provide enough electricity for all of Europe. And scientists have estimated that constructing solar thermal power plants on less than one per cent of the world’s deserts — an area roughly the size of Austria — could meet the entire world’s energy needs.
Photovoltaics — semiconductor panels that convert sunlight to electricity — deliver power at roughly 40 cents per kilowatt-hour, while conventional solar thermal power plants can do so for around 13 cents per kilowatt hour, according to the U S National Renewable Energy Laboratory. This is only marginally more expensive than the average U S price for coal-generated electricity in 2008 of 11 cents per kilowatt hour. The cutting-edge technology of using molten salts to store solar-generated heat is considerably more expensive, but experts expect that price to fall steadily as the technology improves and is mass-produced.

Solar power

Roughly 612,000 megawatt-hours of electricity from the sun were produced in 2007, according to the Energy Information Administration (EIA), and solar thermal collectors sufficient to cover more than 15 million square feet were shipped and ready for installation that year — more than double the amount in 1998.
In the United States, some 3,100 megawatts of solar thermal power are planned by 2012, and capacity worldwide is expected to reach 6,400 megawatts within 3 years — roughly 14 times the current amount. Traditionally, solar thermal power plants have been built two ways — using trough-like mirrors to focus the sun’s heat on water or oil in nearby pipes, or using mirrors to focus solar radiation on a central spot, such as a liquid-filled “power tower.”
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.

Solar energy in tanks

Such plants operate at more than 1,000 degrees Fahrenheit — 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 operates outside Seville, Spain. Southern California Edison has contracted for 1,300 megawatts of such direct steam solar thermal power towers with developer BrightSource.
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.

(Yale Environment 360, part of Guardian Environment Network)