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, since 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 decade or two. Congress is considering similar proposals.
A proposed solution to the reliability problem is to store up 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 stockpile surplus power temporarily. Many hurdles remain, however, and costs need to be lowered drastically.
Daniel Arvizu, the director of the Energy Department's National Renewable Energy Laboratory, told a congressional hearing last month that the lack of an effective way to store wind and solar energy is a major drawback.
"Storage has continued to be the Achilles' heel of renewable energy, and progress is slow,'' he said.
Herbert Hayden, solar technology coordinator for the Arizona Public Service Co. in Phoenix, noted that solar power doesn't match hourly demand for electricity.
"Solar electricity production reaches its peak levels sometime in the midafternoon and tails off significantly in the early evening as the sun lowers on the horizon,'' Hayden said. Energy storage "has the potential to bridge the gap between maximum generation and peak demand.''
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 up to 750 degrees Fahrenheit, turns water into steam, which drives an electric power generator.
In one design from the Sandia National Laboratory 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.
Another way to store wind energy is in giant metal flywheels that spin at speeds up to 50,000 revolutions per minute. The flywheels gain speed as they absorb energy from a set of windmills. Later, when their momentum is converted into energy to spin an electric generator, the wheels slow down. This process can be recycled repeatedly. Such systems are in use in the Azores and on an island off the coast of Japan.
A third method stores wind-generated electricity in large batteries made of sodium and sulfur. The Tokyo Electric Power Co. is installing such a 30-megawatt battery system in a 100-megawatt wind farm. A megawatt, or 1 million watts, is enough power to serve 300 average homes.
A problem with renewable-energy storage systems is they're still too expensive to compete with fossil fuels. But an aggressive research and development program could make stored solar power competitive with coal — currently the cheapest but dirtiest fuel — by 2020, the Energy Department's Arvizu said.
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