More than a third of the electricity in the western United States could come from wind and solar power without installing significant amounts of backup power. And most of this expansion of renewable energy could be done without installing new interstate transmission lines, according to a new study from the National Renewable Energy Laboratory (NREL) in Golden, CO. But the study says increasing the amount of renewables on the grid will require smart planning and cooperation between utilities.

The NREL findings provide a strong counterargument to the idea that the existing power grid is insufficient to handle increasing amounts of renewable power. As California and other states require utilities to use renewable sources for significant fractions of their electricity, some experts have warned that measures to account for the variability of wind and solar power could be costly. At the extreme, they speculated, every megawatt of wind installed could require a megawatt of readily available conventional power in case the wind stopped blowing. But the NREL findings, like other recent studies, suggest that the costs could be minimal, especially in the West.

"The studies are showing the costs are a lot lower than what people thought they were going to be," says Daniel Brooks, project manager for power delivery and utilization at the Electric Power Research Institute. Even if wind farms had to pay for the necessary grid upgrades and backup power themselves, they could still sell electricity at competitive rates, he says.

NREL considered a scenario in which 30 percent of the total electricity produced in a year in western states comes from wind turbines and 5 percent comes from solar power--mostly from solar thermal plants that generate power by concentrating sunlight to produce high temperatures and steam. The researchers assumed the solar thermal plants would have some form of heat storage, although not all planned plants do. The study used detailed data about wind speeds, solar irradiance, and the operation of the electrical grid. GE Energy researchers commissioned by NREL then used the data to simulate the impact of various scenarios for wind and solar power use.

The researchers found that one way to keep the number of new backup power plants to a minimum is to expand the geographical area that renewable energy is gathered from, says Debra Lew, the NREL project manager in charge of the study. If utilities can call on wind farms and solar power from several states, rather than just from the local area, a drop in wind in one area is likely to be offset by an increase in wind elsewhere, and solar panels shaded by clouds in one area will be offset by others in sunny areas.

That makes it far less necessary to have conventional power plants standing by to make up for drops in power. The NREL study estimated that drawing only on local resources would increase variability on the grid by a factor of 50. That's "a huge increase," Lew says, too big for a local utility to balance using backup power and other resources. If you aggregate resources over several states, the increase is less than a factor of two.

Increasing cooperation among utilities can also give each better access to reserve generating capacity that can absorb this variability, Brooks says. A utility in Arizona could draw on coal plants in Wyoming to make up for a drop in solar power.