Indirect Dry Cooling System

Michael Hogan, Power Programme Director for the European Climate Foundation, has written an indepth article for the Climate Progress weblog. His guest post details various approaches to low-water-use and high-efficiency when implementing utility-scale, solar thermal systems for electric power generation. As After Gutenberg readers might know already, the largest solar installation in the world is a solar thermal electric power plant in the Nevada desert.

As a senior executive of InterGen, Michael Hogan oversaw deployment in the late 1990s of an indirect dry cooling system. The particular Heller system was part of a 2,400 MW gas-fired combined cycle plant in Adapazari, Turkey and still is the world’s largest installation of an indirect dry cooling system. “[It] continues to work extremely well,” says Hogan.

Water usage is a critical factor since since large-scale solar thermoelectric plants generally will be located in desert regions, e.g., Australia and MENA (Middle East – North Africa), in addition to the Southwestern U.S.. “Indirect dry cooling (also known as “Heller” systems) will be a crucial enabling technology” in Hogan’s opinion because they “can reduce water consumption… by 97% with minimal performance impact.” Solar thermal developers have long experience in certain regions with Heller systems, reports Hogan, “due to their higher efficiency, smaller footprints, quieter operation, lower maintenance, higher availability, and more flexible site layout.”

Hogan also opines that the feature, which most worked against Heller systems in US fossil plant applications, i.e., the visual impact of the tall cooling towers, “should be far less of an issue in remote desert sites, especially with solar power tower complexes where the central towers will likely be of similar height.”

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jcwinnie

Posted 2009-5-7 at 11:10 am | Permalink

David Biello writes that use of molten salts for thermal storage is a promising technology, “as it overcomes one of the chief traditional drawbacks of solar energy generation — that when the sun sets, the lights go out.”

Andasol 1 is a utility-scale, solar thermoelectric plant that employs molten salt thermal storage. The plant is located in southern Spain and began operating last November. Biello notes that it “now provides 50 megawatts of power, enough electricity to supply 50,000 to 60,000 homes year-round.”

Andasol 2 will come online later this summer, with Andasol 3 already under construction. 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.

Image: Solar Millenium
“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.”

“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 super-heated 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.”

“The salts are just fertilizers — 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 many 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 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 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.

After Gutenberg