Adding to the emerging consensus that it is economically feasible to supply close to four-fifths of U.S. power demand from renewable energy sources — but nearly impossible to get to 100% — a group of high-profile energy and climate scientists have released a study of the geophysical limits to high penetrations of renewables.
"Assuming minimal excess generation, lossless transmission, and no other generation sources, the analysis indicates that wind-heavy or solar-heavy U.S.-scale power generation portfolios could in principle provide [about] 80% of recent total annual U.S. electricity demand," the authors concluded.
The finding should bolster clean-energy advocates who argue that the decarbonization of the power sector, already underway, can be dramatically accelerated without damaging the U.S. economy. There is a long way to go: As of 2016, renewable sources accounted for less than 16% of net power generation in the United States.

The authors of the paper, published on Feb. 27 in the journal Energy & Environmental Science, include prominent scientists such as Ken Caldeira of the Carnegie Institution for Science, Nathan Lewis of the California Institute of Technology and University of California-Irvine professor Steven Davis. Their research marks the latest salvo in a contentious argument over the physical and economic limits to renewables penetration in the electricity system.
The new study contradicts the work of Mark Jacobson, a professor of civil and environmental engineering at Stanford and the director of the university's Atmosphere and Energy Program, who in 2014 unveiled a 50-state roadmap for getting 100% of U.S. electricity from renewables.
"[T]o reliably meet 100% of total annual electricity demand, seasonal cycles and unpredictable weather events require several weeks' worth of energy storage and/or the installation of much more capacity of solar and wind power than is routinely necessary to meet peak demand," the authors of the new report stated. Storing enough energy to meet several weeks of U.S. demand would cost trillions of dollars, they noted.
36 years of weather
The key difference in the new study and Jacobson's work, said Caldeira in an email, centers on energy storage: "If you have enough energy storage you can match any intermittent generation source with variable demand, and that is exactly what Jacobson and colleagues did." To get close to 100% renewables, though, "you either need to build a whole lot more solar and wind capacity, or you need enough battery sufficient to power the United States for a month so — and almost nobody thinks batteries will ever get cheap enough to do that."
Earlier studies from the National Oceanic and Atmospheric Administration and from the National Renewable Energy Laboratory have reached similar conclusions. "[R]enewable electricity generation from technologies that are commercially available today, in combination with a more flexible electric system, is more than adequate to supply 80% of total U.S. electricity generation in 2050 while meeting electricity demand on an hourly basis in every region of the United States," concluded NREL’s "Renewable Electricity Futures Study" in 2012.
For the new study, the researchers compared electricity demand over a long time period to the resources available to meet it. They analyzed U.S. weather data at hourly intervals over 36 years, from 1980 to 2015, and matched that to the variable demand over the same period. The result is a relatively simple model that outlines the geophysical limits of wind and solar resources, without extravagant assumptions about energy storage.
"I think what makes the study exciting is that our conclusions don't rely on assumptions about this or that technology or cost," Davis said in an email. "Rather, we're looking at patterns of sun and wind over 36 years, and the results describe the fundamental challenge mother nature has laid out for us."
Billions in investment
The challenge is still formidable. Building a flexible electric system based mostly on renewables would require hundreds of billions of dollars in investment in not only storage technology, but also long-distance transmission lines. Storing just 12 hours worth of U.S. electricity demand would require about 150 years of output from Tesla's Gigafactory, Davis points out. And continent-scale high-voltage transmission lines, which would be critical to bring wind power from the Great Plains to the population centers of the coasts, have proven expensive and difficult to obtain regulatory approval and financing for.
"Key [technological] advances will be making those things cheaper, particularly storage," added Davis.
The study carries a few key policy implications. One is, simply, that it's possible: the belief that the absolute limit for renewable penetration lies somewhere south of 50% is not supported by the study. The second is that some always-on baseload generation capacity, either from low-carbon natural gas or zero-carbon nuclear plants, will likely still be required beyond mid-century.
The third is that, when it comes to the national power grid, realism must trump ambition.
"Policy makers would be well-advised to consider the data and trade-offs that result from this type of multi-decadal, continental-length resource availability analysis before adopting policies and/or mandates for a 100% wind/solar grid, especially over small geographic regions and with reliability as a requirement for a suitable, stable electricity grid," Lewis said.
