Stanford study quantifies energetic costs of grid-scale energy storage over time; current batteries the worst performers; the need to improve cycle life by 3-10x
|A plot of ESOI for 7 potential grid-scale energy storage technologies. Credit: Barnhart and Benson, 2013. Click to enlarge.|
A new study by Charles J. Barnhart and Sally M. Benson from Stanford University and Stanford’s Global Climate and Energy Project (GCEP) has quantified the energetic costs of 7 different grid-scale energy storage technologies over time. Using a new metric—“Energy Stored on Invested, ESOI”—they concluded that batteries were the worst performers, while compressed air energy storage (CAES) performed the best, followed by pumped hydro storage (PHS). Their results are published in the RSC journal Energy & Environmental Science.
As the percentage of electricity supply from wind and solar increases, grid operators will need to employ strategies and technologies, including energy storage, to balance supply with demand given the intermittency of the renewable supply. The Stanford study considered a future US grid where up to 80% of the electricity comes from renewables.
Only about 3% currently is generated from wind, solar, hydroelectric and other renewable sources, with most of the electricity produced in the United States currently coming from coal- and natural gas-fired power plants, followed by nuclear according to data from the US Energy Information Administration (EIA) .
They quantified energy and material resource requirements for currently available energy storage technologies: lithium ion (Li-ion), sodium sulfur (NaS) and lead-acid (PbA) batteries; vanadium redox (VRB) and zinc-bromine (ZnBr) flow batteries; and geologic pumped hydroelectric storage (PHS) and compressed air energy storage (CAES).
The current total energy storage capacity of the US grid is less than 1%, according to Barnhart. What little capacity there is comes from pumped hydroelectric storage, which works by pumping water to a reservoir behind a dam when electricity demand is low. When demand is high, the water is released through turbines that generate electricity.
By introducing new concepts, including energy stored on invested (ESOI), we map research avenues that could expedite the development and deployment of grid-scale energy storage. ESOI incorporates several storage attributes instead of isolated properties, like efficiency or energy density.
Calculations indicate that electrochemical storage technologies will impinge on global energy supplies for scale up—PHS and CAES are less energy intensive by 100 fold. Using ESOI we show that an increase in electrochemical storage cycle life by tenfold would greatly relax energetic constraints for grid-storage and improve cost competitiveness. We find that annual material resource production places tight limits on Li-ion, VRB and PHS development and loose limits on NaS and CAES.
This analysis indicates that energy storage could provide some grid flexibility but its build up will require decades. Reducing financial cost is not sufficient for creating a scalable energy storage infrastructure. Most importantly, for grid integrated storage, cycle life must be improved to improve the scalability of battery technologies. As a result of the constraints on energy storage described here, increasing grid flexibility as the penetration of renewable power generation increases will require employing several additional techniques including demand-side management, flexible generation from base-load facilities and natural gas firming.—Barnhart and Benson
The first step in the study was to calculate the cradle-to-gate embodied energy—the total amount of energy required to build and deliver the technology—from the extraction of raw materials, such as lithium and lead, to the manufacture and installation of the finished device.
To determine the amount of energy required to build each of the five battery technologies, the authors used data collected by Argonne National Laboratory and other sources. The data revealed that all five battery technologies 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, Barnhart and Benson came up with a new mathematical formula they dubbed ESOI, or energy stored on investment.
ESOI is the amount of energy that can be stored by a technology, divided by the amount of energy required to build that technology. The higher the ESOI value, the better the storage technology is energetically.—Charles Barnhart
The results showed that CAES had the highest value: 240. In other words, CAES can store 240 times more energy over its lifetime than the amount of energy that was required to build it. (CAES works by pumping air at very high pressure into a massive cavern or aquifer, then releasing the compressed air through a turbine to generate electricity on demand.) PHS followed at 210.
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.
The best way to reduce a battery’s long-term energetic costs would be to improve its cycle life, the Barnhart said. Pumped hydro storage can achieve more than 25,000 cycles; 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.
The most effective way a storage technology can become less energy-intensive over time is to increase its cycle life.Most battery research today focuses on improving the storage or power capacity. These qualities are very important for electric vehicles and portable electronics, but not for storing energy on the grid. Based on our ESOI calculations, grid-scale battery research should focus on extending cycle life by a factor of 3 to 10.—Sally Benson
In addition to energetic costs, Barnhart and Benson also calculated the material costs of building these grid-scale storage technologies. In general, they found that the material constraints aren’t as limiting as the energetic constraints. However, PHS has a different type of challenge—the number of geologic locations conducive to pumped hydro is dwindling, and those that remain have environmental sensitivities, Barnhart noted.
A primary goal of the study was to encourage the development of practical technologies that lower greenhouse emissions and curb global warming, Barnhart said. Coal- and natural gas-fired power plants are responsible for at least a third of those emissions.
I would like our study to be a call to arms for increasing the cycle life of electrical energy storage. It’s really a basic conservative principal: The longer something lasts, the less energy you’re going to use.—Charles Barnhart
The study was supported by GCEP and its sponsors: ExxonMobil, GE, Schlumberger and DuPont.
Charles J. Barnhart and Sally M. Benson (2013) On the importance of reducing the energetic and material demands of electrical energy storage. Energy Environ. Sci., doi: 10.1039/C3EE24040A