- The sun does not shine at night, and the wind does not always blow; methods to store large amounts of energy for downtimes are needed to make widespread solar and wind power more practical.
- Some utility companies already use excess solar or wind power to pump water to uphill reservoirs, where it can later fall to turn turbines; this pumped-hydro approach could be installed in many more locations.
- Other viable energy storage solutions include facilities that compress air into large underground caverns, that heat fluids or molten salts that later create steam to turn turbines, or that can charge advanced batteries. These methods require breakthroughs to make them more efficient so they can compete on price with the cost of electricity from traditional power plants.
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To see the big obstacle confronting renewable energy, look at Denmark. The small nation has some of the world’s largest wind farms. Yet because consumer demand for electricity is often lowest when the winds blow hardest, Denmark has to sell its overflow of electrons to neighboring countries for pennies—only to buy energy back when demand rises, at much higher prices. As a result, Danish consumers pay some of the highest electricity rates on the planet.
Utilities in Texas and California face a similar mismatch between supply and demand; they sometimes have to pay customers to take energy from their windmills and solar farms. On paper, wind and sun could supply the U.S. and some other countries with all the electricity they require. In practice, however, both sources are too erratic to supply more than about 20 percent of a region’s total energy capacity, according to the U.S. Department of Energy. Beyond that point, balancing supply and demand becomes too difficult. What are needed are cheap and efficient ways of storing power, to be tapped later, that is generated when winds are howling and the sun is beating down.