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.
What is long duration storage?
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 longer periods of time–such as all the way 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 of time: instead of six hours of discharge, we’re talking about six days of output.
Importantly, long duration storage is different from long term storage: long duration describes the length of time a battery can consistently discharge for, 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 storing 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.
Why is long duration storage necessary?
Long duration storage helps fill in 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 from the grid is likely from a form of fossil fuel generation–mostly likely either 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 exactly the role that long-duration storage promises to play.
A lot of the time, conversations around how we’ll transition to 100 percent renewable or clean energy 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 to a reasonably high degree of 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 that we can smooth the impacts of local weather conditions.
For more information about why long duration storage is important, check out this article form the Department of Energy’s ARPA-E program.
Forms of long duration storage
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 down hill 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. 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 then 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 (either built or natural, like large caves), and continues to pump 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.
Keeping tabs on recent long duration storage news
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 is a pretty unique, innovative idea that 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 energy.
- 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 the commercialization of their long duration storage technology. The first deal, for a 1 MW, 150-hour duration battery with Minnesota’s Great River Energy is a major 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 is now able to 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.
Interested in storage for your home? Learn more on EnergySage!
Energy storage installations of all shapes and sizes are taking off in the US right now, especially at the residential level. EnergySage has all of your questions answered, from our explainer on how batteries work to our head-to-head comparison of the top batteries available to homeowners today. To get in on the action, register for an account on EnergySage to receive free solar + storage quotes for your home or business. Just let us know you’re interested in receiving storage quotes in your preferences, and we’ll do the rest!
