Building storage costly, says study

A key problem to wind energy development in the US is that the electrical grid has virtually no storage capacity, so grid operators can't stockpile surplus clean energy and deliver it at night, or when the wind isn't blowing. To provide more flexibility in managing the grid, researchers have begun developing new batteries and other large-scale storage devices. But the fossil fuel required to build these technologies could negate some of the environmental benefits of installing new solar and wind farms, according to Stanford University scientists.

"We calculated how much energy it will cost society to build storage on future power grids that are heavily supplied by renewable resources," said Charles Barnhart, a postdoctoral fellow at Stanford's Global Climate and Energy Project (GCEP) and lead author of the study. "It turns out that that grid storage is energetically expensive, and some technologies, like lead-acid batteries, will require more energy to build and maintain than others."

The results are published in a recent online edition of the journal Energy & Environmental Science.

Most of the electricity produced in the United States comes from coal- and natural gas-fired power plants. Only about 3 percent is generated from wind, solar, hydroelectric and other renewable sources. The Stanford study considers a future U.S. grid where up to 80 percent of the electricity comes from renewables.

Wind and solar power show great potential as low-carbon sources of electricity, but they depend on the weather, said co-author Sally Benson, a research professor of energy resource engineering at Stanford and the director of GCEP.

The total storage capacity of the U.S. grid is less than 1 percent, according to Barnhart. What little capacity there is comes from pumped hydroelectric storage, a clean, renewable technology. Here's how it works: When demand is low, surplus electricity is used to pump water to a reservoir behind a dam. When demand is high, the water is released through turbines that generate electricity.

For the Stanford study, Barnhart and Benson compared the amount of energy required to build a pumped hydro facility with the energetic cost of producing five promising battery technologies: lead-acid, lithium-ion, sodium-sulfur, vanadium-redox and zinc-bromine. The data revealed that all five batteries have high embodied-energy costs compared with pumped hydroelectric storage.

After determining the embodied energy required to build each storage technology, the next step was to calculate the energetic cost of maintaining the technology over a 30-year timescale. To quantify the long-term energetic costs, the team came up with a new mathematical formula they dubbed ESOI, or energy stored on investment. The higher the ESOI value, the better the storage technology is energetically.

A pumped hydro facility has an ESOI value of 210. It can store 210 times more energy over its lifetime than the amount of energy that was required to build it. The five battery technologies fared much worse. Lithium-ion batteries were the best performers, with an ESOI value of 10. Lead-acid batteries had an ESOI value of 2, the lowest in the study.

To reduce a battery's long-term energetic costs, one way is to improve its cycle life -- that is, increase the number of times the battery can charge and discharge energy over its lifetime. None of the conventional battery technologies featured in the study has reached that level. Lithium-ion is the best at 6,000 cycles, while lead-acid technology is at the bottom, achieving a mere 700 cycles.

They also calculated the material costs of building these grid-scale storage technologies. And found that the material constraints aren't as limiting as the energetic constraints. It appears that there are plenty of materials in the Earth to build energy storage. There are exceptions, such as cobalt, which is used in some lithium-ion technologies, and vanadium, the key component of vanadium-redox flow batteries.