To be cost-effective, any source of power has to produce more energy than it consumes. Oil companies would hardly turn a profit, for example, if extracting a barrel of oil required the energy output of a second barrel of oil.

The same holds true of renewable energy sources like wind and solar. Though renewable projects pay the lion's share of both their energetic and financial costs up front, they still have to recover those costs over a lifetime of service and continue to produce value if they are to yield a net-energy surplus.

Thanks to dramatic improvements in the manufacture of both wind and solar technologies, that appears to be more than possible. A new study published in the journal Energy & Environmental Science finds that wind and solar not only produce enough power to be energetically sustainable but could support grid-scale energy storage as well.

"What we're saying is that theoretically, it is now theoretically possible to have this perfect world that's just based on wind and solar," said Charles Barnhart, a postdoctoral scholar with the Global Climate and Energy Project at Stanford University and a co-author of the study. Rather than using existing "stock" fuels like fossil fuels, he said, renewables put out enough excess energy to fuel their own expansion.

While the majority of deployed renewables yield surplus benefits, those benefits aren't necessarily equal, said Sally Benson, a professor of energy resources engineering at Stanford University and co-author of the study, in a release from the institution.

"Within a few months, a wind turbine generates enough electricity to pay back all of the energy it took to build it," she said. "But some photovoltaics have an energy payback time of almost two years."

Storage must be a growth industry
The picture quickly gets more complicated when you add the fact that wind and solar power alone can't provide consistent, uninterrupted power output. One option to smooth that intermittency is grid-scale storage, but all storage systems -- be they pumped hydro, chemical batteries or compressed air -- come with their own energy costs.

The Stanford report focuses on this combination of renewable energy and storage infrastructure. Even with the addition of grid-scale storage, they found, there is enough surplus wind power in the United States today to support 72 hours of energy storage.

Solar energy, which enjoys a slimmer margin of surplus, could sustain about 24 hours of backup energy storage, they found.

"Our analysis shows that today's wind industry, even with a large amount of grid-scale storage, is energetically sustainable," said Michael Dale, a research associate at Stanford and lead author of the report, in the release. "We found that the solar industry can also achieve sustainable storage capacity by reducing the amount of energy that goes into making solar photovoltaics."

The energy needed to expand the existing renewable fleet depends on the speed of the expansion, of course. The more new wind turbines, solar panels and batteries enter the grid, the more their energy costs.

Existing onshore wind power could sustain global wind capacity's current growth rate, at 100 percent, and still maintain an energy surplus, the authors found.

How much excess energy would be available to the public after providing for the costs of expanded renewables and accompanying storage depends on many factors, Barnhart said. Further declines in the energy costs of manufacturing renewables and greater energy efficiency would grow the net energy surplus.

Better batteries would likewise increase the surplus, particularly over longer time scales, he said. "The amount of surplus available to society is widely different depending on the technology you use. Not all storage technology is created equal. We're finding that in some cases the cheapest batteries also have the lowest energy costs, but we've also seen that in many other cases, they're not."

Low-cost, long-lived technologies, like pumped hydro, often yield a greater surplus over time than cheap lead acid batteries, which must be replaced after only a short life span, he said.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500