Energize Weekly, March 3, 2018
Wind and solar generation could meet up to 80 percent of U.S. electricity demand—but going beyond that toward all-renewable electricity could require trillions of dollars of investment in transmission, generation and storage, according to a new study.
Researchers from the Carnegie Institution for Science, the California Institute of Technology and the University of California, Irvine (UCI) analyzed 36 years of hourly wind and solar data for the continental U.S. to calculate the potential electricity generation and identify weaknesses and reliability issues.
They found that wind and photovoltaic solar could generate a significant portion of the nation’s electricity needs—between 65 percent and 80 percent—especially when coupled with 12 hours of battery storage. The paper was published last week in the journal Energy & Environmental Science.
“The fact that we could get 80 percent of our power from wind and solar alone is really encouraging,” Steven Davis, UCI Earth system science professor and co-author of the study, said in a statement. “Five years ago, many people doubted that these resources could account for more than 20 or 30 percent.”
Still, Davis said, “The sun sets, and the wind doesn’t always blow. If we want a reliable power system based on these resources, how do we deal with their daily and seasonal changes?”
The data showed that over time and space, solar and wind varied by season, with solar summer generation 3.7 times the winter minimum and wind generation peaking in the spring and falling by as much as a factor of 3.5 during the summer.
“Combining solar and wind generation can alleviate some of the seasonal deficiencies of each, but the generation complementarity is limited by the large variability in the wind resource on a wide range of time scales, as well as by the substantial difference in the amplitude and seasonal cycles of the solar resource relative to either wind or electricity demand,” the study said.
To reach 80 percent reliability in a “solar heavy” mix of wind and solar generation would require energy storage to overcome the daily solar cycle. Without any storage, the analysis calculates that solar could meet 48 percent of hourly electrical demand, with 12-hour batteries that could rise to as much as 85 percent.
The variations in wind, however, are so great that a “wind heavy” portfolio would require a continental-scale transmission system to take advantage of the geographic diversity of wind. “Generating wind energy over larger ‘aggregation areas’ is most effective at reducing the hourly and daily energy production variability,” the study said.
One advantage that wind has is a higher capacity factor—the efficiency of turning the natural resource into electricity—than solar. This results in “lower total installed capacity and potentially less investment, depending on the capital cost differences between solar and wind generation systems,” according to the analysis.
To get to 100 percent reliable, renewable generation would require several weeks of energy storage capacity or the installation of backup wind and solar generation above what is needed to meet peak demand to deal with seasonal cycles and unpredictable weather, the analysis said.
This is a potentially expensive approach. It would require “a distinct combination of infrastructure and future investments, and it may be unnecessary (and economically inefficient) to extensively pursue both large-scale storage and long-distance transmission,” the study said.
One proposal for a U.S.-wide transmission system with 21,000 miles of lines was estimated to cost $410 billion. To produce enough battery storage for a nationwide renewable energy system would cost $1 trillion, the researchers estimated.
In 2012, the National Renewable Energy Laboratory did an analysis that concluded that the U.S. could reach 80 percent renewable generation by 2050 and that it would take doubling the pace for installing renewable energy sources and then doubling it again and somewhere between $320 billion and $1 trillion—on top of a baseline $4 trillion in investments.
“Our work indicates that low-carbon-emission power sources will be needed to complement what we can harvest from the wind and sun until storage and transmission capabilities are up to the job,” Ken Caldeira, Carnegie Institution for Science researcher and paper co-author, said in a statement. “Options could include nuclear and hydroelectric power generation, as well as managing demand.”
