As California lawmakers consider a package of bills aimed at increasing the production of clean energy, a major question arises: How would we store all this new power?
Storage is a vital issue because while the state can create plenty of energy through solar, wind and hydro power, there must be ways to effectively stockpile it for use overnight or on calm, overcast days or when the waterways dwindle.
A project commissioned by the California Energy Commission and led by UC Merced electrical engineering Professor Sarah Kurtz aims at evaluating solutions for long-duration energy storage.
Researchers examined various scenarios to better understand the value of long-duration energy storage to meet California's goal to reach net-zero emissions from retail electricity sales by 2045. The study also explored duration, cost and other attributes of future energy storage.
The project also studied how energy policy and the state's power grid's conditions may affect the need, benefits and desired attributes of energy storage. Researchers wanted to find solutions that would be practical and relatively inexpensive to implement. They leveraged existing modeling tools and modified them.
Solar accounts for 27% of the electricity generated in California. Historically, the state turned to natural gas in the overnight hours, But this year, battery storage provided enough energy to meet peak demand in the early evening.
"During times of high demand (like the state's recent heat wave) and during cloudy times, having storage that holds enough energy to provide power to the grid for a longer time (long-duration energy storage) will be useful," Kurtz said. "The study showed that the majority of the storage should be designed to last eight hours, with some storage designed to last 100 hours to serve during the most challenging demands for power.
"It's an exciting time to see which technologies will come together to provide a reliable, sustainable energy system."
The most efficient system for storing energy now is lithium batteries, which have been dropping in price. Researchers advised utilities and homeowners investing in solar energy to consider including lithium batteries to extend the use of power collected during the day into the evenings and through the night. Though many current lithium batteries last for four hours, the study found an eight-hour battery would be ideal to serve electricity needs overnight.
In addition to storing solar energy, the report advocated rethinking when electricity should be used the most. Electricity providers historically have structured rates to discourage people from using power during times of peak demand, currently 4 -9 p.m.
"In the future, the focus should be to shift electricity use to daytime," Kurtz said. Thinking about shifting electricity use toward daytime (maybe 9 a.m. to 3 p.m.) instead of away from peak hours will be a change in mindset for our utilities, but an important change."
This shift can present a low-cost opportunity to reduce the need for energy storage while still working to expand storage capacities. For example, Kurtz said, many electric vehicles are programmed to charge after midnight. But it would make more sense to charge them during the day in California when solar energy is abundant.
"The study predicts that daytime charging of electric vehicles could reduce the required storage power for charging electric vehicles in the electric grid by half compared to nighttime charging, leading to substantial savings in grid-level storage capital costs," said Farzan ZareAfifi, a Ph.D. candidate in Kurtz' research group. "Therefore, it would be beneficial to install more charging stations at workplaces, enabling owners to charge their cars while they work at times when solar power is abundant."
When fixed arrays are used, south-facing solar panels can help to balance seasonal supply with demand. Although east- and west-facing arrays effectively reduce the storage needed to go through the night, a study explained that south-facing solar arrays better align with seasonal energy demands, reducing the reliance on long-duration energy storage compared to east-west-facing solar panel orientation.
California aims for 7.6 gigawatts of offshore wind by 2035 as the first step toward fulfilling the state's long-term plan of getting 25 gigawatts of wind energy online by 2045. A gigawatt can sustain up to 300,000 homes in the United States a year, according to the Department of Energy. The high wind integration may change the storage landscape in the grid. Zabir Mahmud, another Ph.D. candidate in Kurtz' lab, noted, "As the proportion of wind energy increases, the optimal storage duration is likely to shift from 8 hours to 24-36 hours."
"A part of the study shows that the 100 hours of storage will be more valuable in the future in terms of the cost target relative to the 4-hour Li battery," recent UC Merced graduate Mahmoud Abido said. For low storage efficiencies (around 50%), the cost targets for both 8-hour and 100-hour storage are almost the same in 2030. For higher efficiencies (around 80%) the 100-hour duration has a higher cost target than the 8-hour duration especially in 2045. Thus, the 100-hours duration will be more attractive to the investors in the future.
Abido graduated this summer and moved to Texas just in time for Hurricane Beryl. He said some of that long-duration energy storage would have come in handy at his house when the power went off for six days.
"There's quite a buzz about long-duration energy storage and it was an honor to be funded to work on such an important project," Kurtz said. She added that those involved in clean energy hope the pace of regulation catches up with the pace of innovation, a topic included in some earlier research she helped publish. "It's a serious challenge to plan for tomorrow, when technology is changing so fast, but our regulatory process advances at a snail's pace."
