Tuesday, September 10, 2013

Calculating Energy Required To Store Wind, Solar Power Efficiently

Lee Rannals for redOrbit.com – Your Universe Online
Stanford University scientists, publishing a paper in the journal Energy and Environmental Science, calculated the energy required to store wind and solar power on the electrical grid to determine the total amount of fuel and electricity required to build and operate storage technologies.
“We looked at batteries and other promising technologies for storing solar and wind energy on the electrical grid,” said Charles Barnhart, the lead author of the study and a postdoctoral scholar at Stanford’s Global Climate and Energy Project (GCEP). “We found that when you factor in the energetic costs, grid-scale batteries make sense for storing surplus solar energy, but not for wind.”
The overall energetic cost of wind turbines is lower than for conventional solar panels, which require lots of energy, primarily fossil fuels, to build. However, they found that curtailing wind power reduces the energy return on investment by 10 percent, while storing surplus wind-generated electricity in batteries results in even greater reductions.
“Ideally, the energetic cost of curtailing a resource should at least equal the amount of energy it cost to store it,” said GCEP postdoctoral scholar Michael Dale, a co-author of the study. “That’s the case for photovoltaics, but for wind farms, the energetic cost of curtailment is much lower than it is for batteries. Therefore, it would actually be more energetically efficient to shut down a wind turbine than to store the surplus electricity it generates.”
Essentially, Dale said that this would be like spending $100 on a safe to protect a $10 watch.
“Likewise, it’s not sensible to build energetically expensive batteries for an energetically cheap resource like wind, but it does make sense for photovoltaic systems, which require lots of energy to produce,” the researcher said in a press release.
Barnhart said that increasing the cycle life of a battery would be the most effective way to improve its energetic performance. However, these batteries must be able to endure 10,000 to 18,000 cycles, compared to conventional lithium-ion which can endure about 6,000 charge-discharge cycles.
“Storing energy consumes energy, and curtailing energy wastes it,” Barnhart said. “In either case, the result is a reduction in the overall energy return on investment.”
A better alternative to batteries would be hydroelectric storage. This system would have an energy return on investment 10 times better than conventional batteries, but engineers would run into geologic and environmental constraints on where pumped hydro can be deployed.
“Policymakers and investors need to consider the energetic cost as well as the financial cost of new technologies,” Dale said. “If economics is the sole focus, then less expensive technologies that require significant amounts of energy for their manufacture, maintenance and replacement might win out – even if they ultimately increase greenhouse gas emissions and negate the long-term benefits of implementing wind and solar power.”
Co-author Sally Benson, the director of GCEP and a professor of energy resources engineering, said the team’s goal is to understand what is needed to build a scalable low-carbon energy system.
“Energy return on investment is one of those metrics that sheds light on potential roadblocks. Hopefully this study will provide a performance target to guide future research on grid-scale energy storage,” Benson said.