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.
There will still be times during the year when poor weather forecasts and high demand lead to drops in power that are too big to compensate with supplemental power plants. But according to NREL’s models, this will happen rarely–just 89 hours out of the year, or 1 percent of the time. For such a small amount of time, it’s not economical to build backup generators to make up the difference. But NREL found that a strategy called demand response could keep the grid from collapsing under the pressure. It’s already been used in Texas, for example, to make up for sudden drops in wind power.
In demand response, utilities send out signals to customers–typically businesses–asking them to cut their power use, in return for favorable electricity rates. That cuts demand enough to balance the grid. “It’s effective and a lot less expensive” than building backup power plants, Lew says.
Distributing renewable energy among several states would not require extensive new interstate transmission lines at first, Lew says, because in the West there is already enough transmission available–if operators are given ready access to spare capacity. No new interstate transmission will be needed to accommodate wind farms that supply up to 20 percent of the electricity in a year and solar power that provides up to 3 percent, although there will need to be additional power lines to convey power locally from new renewable power plants, which might be in a remote area.
This is in contrast to the eastern United States, where long-distance transmission would likely need to be built to carry power from the Midwest, where it is windy but electricity demand is low, to cities on the coast, where demand is high, she says. In the West, reaching 30 percent wind and 5 percent solar will require more transmission, but this could easily be built in the time it would take to reach such high levels of renewable energy.
In addition to sharing power over large areas, for example, utilities could significantly reduce variability by changing their scheduling practices. Utilities now use algorithms to predict demand a day ahead and then schedule hourly changes to power generation. These changes on the hour, when a utility might cut down the amount of power a coal plant generates in anticipation of lower demand, actually introduce some instability into the grid. Scheduling changes more frequently could smooth this out, and allow more accurate coordination with renewable energy resources. Scheduling more often isn’t a technical challenge, although it could require software upgrades–it’s mostly a matter of changing practice, Brooks says.
But utilities, notoriously slow to change, are showing signs of responsiveness in the face of renewable energy mandates, Brooks says. The need is urgent because some areas of the country already rely on relatively large amounts of wind power, at least part of the time. In some places in Texas, wind power, at times, can account for a quarter of the power on the grid, although it still only accounts for just over 6 percent of the total electricity in a year. In the Bonneville Power Administration in the Northwest, during some short periods of time, half of the power being generated comes from wind. “Utilities see more and more solar and wind coming on line and they’re scared that they can’t manage this on their own,” Lew says.
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