Long duration storage: What you need to know

As states and cities throughout the country embrace the renewable energy transition, setting and committing to 100 percent clean energy targets, one crucial gap remains in ultimately meeting those targets: how do you stretch the production from renewable resources like wind and solar to keep powering the country even when the sun isn't shining or the wind isn't blowing? Enter long-duration storage, the promising broad category of technologies that can fill this gap.

Most commercially available energy storage systems at the residential or commercial scale are shorter-duration solutions: they are designed to provide power for 2 to 6 hours at a time. (Note: Home batteries can continue providing electricity for more extended periods–such as from sunset to sunrise–but only if you draw less than their max power over that time.)

Long-duration storage describes the category of storage solutions that can consistently discharge at their max power (or close to it) for an extended period: instead of six hours of discharge, we're talking about six days of output.

Importantly, long-duration storage differs from long-term storage: long duration describes the time a battery can consistently discharge, while long-term–or seasonal–storage describes how long a battery can store energy before it must be used. In other words, it's the difference between keeping energy to provide power consistently for six straight days and doing so to provide power six months from now.

Some technologies may be able to provide both services, and both types of storage will ultimately be necessary for the clean energy transition. However, in this article, we're focusing just on long duration and not on long-term storage.

Long-duration storage helps fill the gaps when renewable energy isn't producing power. At the moment, if you have solar panels, during a day or week when it's cloudy where you live, you can pull electricity from the grid instead of from your rooftop panels. The power provided by the grid is likely from a form of fossil fuel generation–mostly coal or natural gas-fired power plants. But in a future where we phase out fossil fuel-fired power plants, something else will need to fill the gap to keep providing a consistent, reliable source of energy when renewable energy isn't producing as much. This is precisely the role that long-duration storage promises to play.

Conversations around how we'll transition to 100 percent renewable or clean energy often end up mired in the question of: "Well, what happens when the wind stops blowing or the sun stops shining?" With long-duration storage, that's much less of a concern. What's more, our weather forecasting is robust enough that we can predict reasonably high certainty where the wind will be stronger and weaker and where the sun will be shining more than elsewhere. And with a completely renewable future, we'll have enough distributed energy resources–like solar!–spread out across a wide enough geographical area to smooth the impacts of local weather conditions.

For more information about why long-duration storage is essential, check out this article from the Department of Energy's ARPA-E program.

There are a few primary forms of long-duration storage at the moment:

• Pumped hydro storage: Perhaps the oldest, most well-understood form of storage in general, pumped hydro storage plants pump water uphill into a reservoir when electricity prices are low and then release the water back downhill to run through turbines to produce electricity when prices are high, or the grid needs it. The best-known pumped hydro storage facility is actually at Niagara Falls!

• Other gravity-based storage: Instead of pumping water uphill, some companies are experimenting with other gravity-based, long-duration storage solutions and, for instance, using a mechanical process to raise a heavy object high in the air, where it will stay until energy is needed on the grid. When you release the heavy object, as gravity pulls it back down, you can have the object move turbines to create electricity.

• Compressed air energy storage (CAES): Another twist on pumped hydro storage is compressed air storage. This technology sends highly pressurized air into a vault (built or natural, like large caves) and continues pumping the air into the vault until it's super compressed. Releasing the "valve" to the compressed air works like letting air out of a balloon to spin turbines and create electricity.

• Flow batteries: This type of storage is closer to the technology used by solar batteries you may install at your home. These are chemical-based, instead of mechanical-based, storage solutions.

Long-duration storage is on the cutting edge of technological innovations. As more news emerges from–and more investment pours into–the industry, we'll keep tabs on recent news updates here. Here's the long-duration storage news you need to know about:

• Energy Vault receives $110 million in funding to scale gravity-based storage: This unique, innovative idea takes pumped hydro storage to the next level. Effectively, Energy Vault stacks thousands of heavy concrete bricks to store energy and then unstacks them to release/utilize the power.

• Form Energy to develop a 1 megawatt, 150-hour duration battery for Minnesota utility: The Somerville-based, Breakthrough Energy Ventures-backed startup makes its first foray into commercializing its long-duration storage technology. The first deal, for a 1 MW, 150-hour duration battery with Minnesota's Great River Energy, is a significant milestone for the industry, and yet still far from Form Energy's vision of what their technology will eventually be capable of. Great River Energy can now reduce its reliance on coal plants in favor of wind + storage.

• Form Energy receives $200 million in funding and announces its iron-air-exchange battery: The battery will be used in the Great River Energy project, will deliver electricity for 100 hours, and will cost less than $20 per kilowatt-hour–a significant reduction in price from lithium-ion batteries.

Energy storage installations of all shapes and sizes are taking off in the US, especially at the residential level. EnergySage has all your questions answered, from our explainer on how batteries work to our head-to-head comparison of the top batteries available to homeowners today. Register for an account on EnergySage to receive free solar + storage quotes for your home or business to get in on the action. Let us know if you're interested in receiving storage quotes in your preferences, and we'll do the rest!