Q: Can wind energy by stored? How much wind or solar power does it take to produce the equivalent of a gallon of gas? How does that compare to five-10 years ago and to projections two or three years out? Is there a way to compare efficiencies of carbon-based fuels to solar and wind?
Asked by Mark Kalkus, ’84, JD ’88, Littleton, Colo.
A brisk breeze blows across the 49 megawatt Guyang Damao wind farm in Inner Mongolia, China. In the distance, a herd of wild horses stampedes past the rapidly-spinning turbines. The 30-mile per hour winds are powerful—powerful enough to light almost a quarter million homes. At the same time, the power plant's chief engineer tells of how light the previous day's winds had been—and how little electricity had been produced. And how those quarter million homes were not wind-powered. Fossil fuel plants ramped up, and Inner Mongolia's electricity-associated carbon footprint suddenly became much larger.
The intermittent, or sporadic, nature of wind, sun and other renewable energy sources is one of the more problematic aspects of sustainable energy generation. In the current United States electrical grid system, there is little capacity to store energy when the wind isn't blowing or the sun isn't shining. However, as Stanford professor and director of sustainable energy education Karl Knapp, '81, MS '82, PhD '99, notes, storage is not necessary if you're connected to the grid. Wind power is abundant during the night; solar power is abundant during the day; and hydropower and geothermal, able to produce power at any time, are complementary sources of power. If one renewable power source is not producing electricity at a given time, another one will be able to.
A smarter, modernized grid would have the potential to incorporate much greater proportions of renewables into the U.S. electricity system. "A solution to improve wind power reliability is interconnected wind farms," says Stanford civil and environmental engineering professor Mark Jacobson, '87, MS '88, in an article on expanding wind's presence in the nation's overall energy mix. If more power and transmission lines connect more power plants, then wind from one state where the wind is blowing will be able to power homes in another state where the wind has stopped, as it is rare that wind stops everywhere at once.
As far as comparing end-use efficiencies—or the energy service output per energy input—between carbon-based and renewable energy sources, there isn't a metric of any merit to compare them, says Knapp. For example, an efficient coal power plant might have an end-use efficiency of 40 percent, meaning that 40 percent of the heat given off by combusted coal is utilized in electricity generation. Wind turbines also have an efficiency of 40 percent, but the remaining 60 percent isn't a carbon-intensive, dirty resource essentially going to waste—it's clean wind energy that just wasn't ever harnessed. Wind will blow whether or not there are turbines to profit from the resource, but coal will only be burned if there's a coal power plant. Thus, comparing fossil fuels to renewable efficiencies is like comparing apples to windows.
However, efficiency of different kind—using less energy in the first place—is perhaps the most valuable energy resource we have. As the United States modernizes it's electrical grid, emphasizing efficiency as a renewable energy resource will reduce energy demand and reduce the climate change-causing emissions that spew into the atmosphere.
Lauren Kubiak, '10, expects to receive her master's degree in earth systems in 2011.
