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,
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
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
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.