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Groups store renewable energy to use on rainy days

July 14, 2007 - McClatchy Newspapers

WASHINGTON - Scientists and engineers are struggling to find ways around a major obstacle to the growth of renewable energy: the fact that inexhaustible sources of energy, such as the sun and the wind, are undependable.

Solar power doesn't work at night or on cloudy days. Wind is notoriously fickle, often dying down in the late afternoon just as electricity demand is peaking.

This on-and-off variability is a serious problem, because many people who worry about global warming hope that clean, nonpolluting renewables will reduce the demand for fossil fuels such as coal and oil. More than 20 states have passed laws requiring utilities to generate 15 to 20 percent or more of their electricity from nonfossil fuel sources in the next two decades. Congress is considering similar proposals.

A proposed solution to the reliability problem is to store extra energy that's produced while the sun is shining or the wind is blowing. Energy can be turned into heat, and the heat preserved in a tank of liquid or salt or even a block of concrete for use later when it's needed.

Supporters say effective storage systems could shrink the cost of renewable energy, lower pollution and reduce the need to import oil.

Energy storage is becoming a hot topic as utility companies and government laboratories experiment with various technologies to temporarily stockpile surplus power. Many hurdles remain, however, and costs need to be lowered drastically.

The Department of Energy is researching ways to store energy at solar power plants that use thousands of mirrors to concentrate the sun's rays on pipes filled with oil. The oil, heated to 750 degrees Fahrenheit, turns water into steam, which drives an electric power generator.

In one design from the Sandia National Lab-oratory in Albuquerque, N.M., excess heat is channeled into tanks of molten salt - a mixture of sodium, potassium and nitrogen that melts at 430 degrees Fahrenheit - where it can be stored for up to a week. The stored heat then can be transferred to a "heat exchanger" to boil water to make steam to run a generator at night or whenever necessary. Several power plants under construction in Spain plan to use this concept.

Another approach being tested at the University of Stuttgart in Germany would run pipes of fluid heated by the sun through a solid block of concrete. The concrete holds the heat for later use. To recover it, cold fluid is passed through the pipes, picking up heat on the way.

Various technologies also are being developed to store power from the wind for use when it's not blowing.

The Iowa Association of Municipal Utilities is planning to use power generated by windmills to compress air and pump it into a cavern 2,000 feet below the ground near Fort Dodge. When needed, the pressurized air would be released and mixed with natural gas to drive a generator.

Small compressed-air storage systems already are operating in McIntosh, Ala., and Huntorf, Germany.


Updated: 2016/06/30

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