At an industrial site in Ticino, Switzerland, a 20-story tower raises 35-ton composite blocks on winches using renewable energy from the electric grid. Then, after dark, when solar panels no longer collect energy, the computer-controlled winches lower the blocks, spinning generators and feeding energy back into the grid without using fossil fuels.
This commercial-scale demonstration plant, built by Energy Vault in 2020, has successfully demonstrated the technologies for the company’s gravity storage system, designed to replace peaker power plants powered by fossil fuels. Such replacements are still needed despite the rise of renewable energy.
“The increased amount of solar and wind on the grid is creating this very significant need for storage to shift those electrons into hours of the day when people are at home,” explains Vanessa Witte, senior analyst for energy storage at Wood Mackenzie.
The need for grid-scale storage
To go completely carbon-free—and avoid the potentially thorny issues of nuclear energy—for power generation, building more solar and wind farms isn’t enough. Cities, towns, businesses, and homes also need ways to store energy for when the sun doesn’t shine, and the wind doesn’t blow. That requires grid-scale energy storage.
Grid-scale chemical batteries already exist, but, according to Witte, they don’t offer long enough duration, since they are currently only economical for four to eight hours. “The cost has come down dramatically,” she says. “But it’s still an expensive asset, especially in comparison to solar and wind, whose prices are on par with fossil fuel.”
The increased amount of solar and wind on the grid is creating this very significant need for storage to shift those electrons into hours of the day when people are at home.
—Vanessa Witte, senior analyst for energy storage, Wood Mackenzie
She also cites their other challenges to widespread adoption: the risk of thermal runaway for lithium-ion batteries, i.e., overheating and fire risk, sourcing the comparatively rare raw materials needed to build them in quantity, and degradation.
But other grid storage systems in development promise to overcome these limitations.
A vault for energy
Robert Piconi, CEO and co-founder of Energy Vault, sees gravity-based storage as the most promising lithium-ion battery alternative for the grid. “We wanted something that could get to market quickly,” he says. “That meant we couldn’t rely on chemistry roadmaps of five to 10 years or science projects. It also meant we couldn’t have some rare metal that existed in only three countries.”
Gravity storage fits the bill, he says, and has a long pedigree in pumped hydro storage systems. Such systems pump water to the top of a hill or mountain and store it until it’s released to turn generators when needed.
Piconi and his team elected not to use concrete in the blocks their system raises and lowers because of the building material’s significant carbon footprint. Instead, they decided to make their blocks out of available materials at the site of their plants, starting with the earth excavated during construction. “So our only [capital expenditure] is the brick machine, which we transport to the site,” he says.
Now that they’ve refined the technology, including the power electronics and software needed to precisely regulate the raising and lowering of blocks, Piconi and company are forging ahead. Next up: a deal in progress for a 100-megawatt-hour system in Asia. If all goes well, the company plans to bring the new system online by the end of the year.
Power from mine shafts
Gravitricity, based in Scotland, also intends to put gravity-based storage on the grid, but its approach is slightly different. Instead of building up, its design calls for drilling down.
Managing Director Charlie Blair says drilling deep shafts reduces the structural load on its system, allowing it to store more energy and release it over longer periods. “If you want to store significant amounts of energy, you need both a lot of height and a lot of weight,” Blair says. How much of each? Hundreds of meters and thousands of tons, according to Blair.
“Anything can be done, but can you do it cost-effectively?” he asks of building above-ground structures to handle the required weight at a significant height. “We believe not.” On the other hand, “If you’re going down, you sink your hole, but you don’t have to reinforce it.”
Gravitricity completed a 250-kilowatt demonstrator plant in Edinburgh last year to prove the viability of its technology. In February, it received a £912,000 ($1.1 million) grant from the U.K. government to build a larger, four-megawatt-hour prototype plant. Blair says the first commercial-scale plant out of the gate will be a system built into an abandoned mine shaft in one of several sites the company has located in central Europe.
Power from air and other sources
Highview Power, also based in the U.K., connected a 15-megawatt-hour liquid air battery to the grid outside Manchester, England, in 2018. The system uses grid power to compress air until it liquefies. Stored in insulated tanks, the air gets released to expand when needed, driving turbines to generate electricity. The company is building a 250-megawatt-hour plant, also near Manchester, and has raised more than $145 million to further its plans.
Other concepts use sodium-ion battery chemistry, flywheels, and even molten salt, among other alternatives to the current standard in lithium-ion batteries. All aim to make renewable energy true replacements for fossil fuels for power generation while lowering costs and overcoming the limitations of lithium-ion batteries.
These and other concepts are gaining traction and investment as the world barrels closer to the 1.5 degrees of average global temperature increase that the U.N.’s Intergovernmental Panel on Climate Change (IPCC) says is the limit to avoid major catastrophe. The IPCC’s Sixth Assessment Report, which was published in April, says emissions must peak by 2025 and drop by at least 43% by 2030.
“It’s now or never if we want to limit global warming to 1.5° C (2.7° F),” IPCC Working Group III Co-Chair Jim Skea said in a press release accompanying the report.
Innovators say novel grid storage can be part of the solution.
Lead image: Energy Vault’s Commercial Demonstration Unit in Switzerland. Courtesy of Energy Vault.