Energy storage is recognized as an essential ingredient in the move to emissions-free electricity, but given the variability of wind and solar output, we need a technology to help balance the ups and downs. Lithium­-ion (Li-ion) batteries have become the go-to technology, and adoption rates seem to break new records each quarter. But this technology is best suited for durations of 4–8 hours, and the grid needs support across a larger range of time. That’s why the long-duration storage market, with claims of storing power up to 100 hours, or even seasonally, has become the next growth target for energy investors.

According to the American Clean Power Association (ACP), the United States installed 8 gigawatts (GW) of capacity in 2023, reaching a total of 17 GW, almost doubling the nation’s capacity. More than 4 GW of new storage was added in Q4 2023, with grid-scale installations more than doubling Q3’s performance. ACP and research partner Wood Mackenzie anticipate total U.S. storage capacity to hit 59 GW by the end of 2028.

But even as utilities buy more battery systems, there is growing recognition that today’s technology leaves gaps in electricity supply portfolios. Li-ion approaches work well during the day’s peak early-­morning and early-evening shoulder periods, where they can step in to support renewable supplies ramping up and slowing down. They also can be called on for much shorter­-duration service to maintain steady voltage levels across a distribution grid. But longer-term scenarios, like an extended weather event or output shortfall, need a different solution.

Energy innovators have come up with a number of approaches to help fill those production gaps. Analysts with the consulting firm Guidehouse have outlined four categories for describing these options:

Mechanical, which converts electricity into mechanical energy. This includes strategies such as pumped hydro, which uses excess electricity to pump water to a higher-­elevation reservoir, from which it’s released to drive a traditional hydropower turbine. This approach accounts for 96% of all U.S. utility-scale energy storage, according to the Department of Energy.

Electrochemical, which essentially refers to batteries. There are several nonlithium electrochemical options developed for long-duration uses, including zinc-air and iron-air technologies.

Chemical, in which electricity is used to create a chemical reaction—like using electrolysis to create hydrogen—with a resulting product that can drive a turbine or otherwise generate electricity.

Thermal, which uses electricity to create storable heat that can then be used to create electricity through turbines or other means.

Emerging strategies

Some of these processes are quite complex, such as developing electrolysis at grid-scale for hydrogen manufacturing. Others are deceptively simple. Swiss manufacturer Energy Vault, for example, has developed a system that stores mechanical energy through the process of lifting 30-ton blocks. When backup electricity is needed, the blocks, manufactured from waste material such as coal ash, are retrieved and lowered to the ground in a controlled manner, releasing kinetic energy that can drive generation.

In essence, this is similar to pumped hydro approaches, but it offers some big advantages. First, it’s much easier to site, as pumped hydro requires access to a water reservoir at an elevation high enough to be useful. Additionally, the Energy Vault strategy is inherently scalable—to store more energy, the company just adds more blocks and storage modules. That level of adaptability to changing energy requirements just isn’t possible with pumped hydro. 

The company has completed one installation in China, with several others now underway, and it’s also signed an agreement with a Washington utility to “serve multi gigawatt-hour long-duration energy storage requirements,” according to a project release.

Energy Vault is additionally developing what it says is an ultra-long-duration energy-­storage microgrid system using green hydrogen—that is, hydrogen that’s been electrolyzed using renewable energy—for Pacific Gas & Electric in Southern California. It’s also involved in more traditional Li-ion battery projects in California and Nevada.

As this company’s background illustrates, long-duration remains a challenging market because of its unique economics—shorter­-duration projects are simply easier to pencil out, financially. Many utilities are now seeing how an increase in variable renewable resources causes strain during specific periods, but fewer need the kind of long-­duration solutions yet. 

Long-duration storage is a kind of insurance policy for a carbon-free grid. As wind and solar, which already meet 16% of U.S. demand, continue to grow, utilities and their regulators are likely to want equally sustainable solutions as long-term backups.

Header image: Energy Vault’s Gravity Vault system uses excess generation to lift and store 30-ton composite blocks. It recaptures that electricity as the blocks are returned to ground level.

energy vault