When reading about energy storage you may come across terms like long-term storage, seasonal storage, diurnal storage, or long-duration storage. Long-term storage can include seasonal energy storage, which can shift delivery of power to a different time of year. Diurnal storage can shift power delivery over a few days. And, long-duration storage is particularly important for the power grid’s transformation to clean energy and what I’m focusing on here.
Long-duration refers to the amount of time a power system can discharge electricity. That is to say, once a battery is fully charged, the duration is equal to the number of hours that it can deliver power at a certain power capacity. This is different from long-term storage, which refers to the amount of time a system can store energy before discharging it.
As large amounts of wind and solar resources are connected to the grid, long-duration energy storage could prevent curtailment of renewable energy sources resources during periods of excess generation. Curtailment occurs when the transmission grid is overloaded and unable to absorb all the clean and affordable electricity being produced. This results in a deliberate reduction of electricity output. Energy storage can alleviate curtailment by facilitating the efficient use of clean energy resources so that extra production can be stored and used when it’s most needed. As renewable deployment increases, the grid flexibility provided by long-duration energy storage will become more relevant and useful.
Long-duration storage could also offer greater grid flexibility because it can store large amounts of energy. A long-duration storage system can charge when electricity demand is low and discharge later when it is most needed. When transmission systems need costly upgrades, energy storage can be deployed to help with these services instead. Longer duration systems can further extend the life of transmission equipment by operating more frequently and for longer periods. As extreme weather events and outages become longer and more frequent, long duration discharges could better accommodate turbulent grid conditions and offer greater resiliency.
Long-duration energy storage is assumed to have full capacity value since it could discharge for up to a day. The capacity contribution of a resource determines how much that resource counts towards resource adequacy requirements. The marginal effective load carrying capacity (ELCC) for storage under 12-16 hours would likely still decline over the course of increasing storage deployment, but not as rapidly as short-duration storage. (Read this post by my colleague Mark Specht for a full rundown on what ELCC is and why it matters.)
How long is long-duration?
There is no single definition of “long duration”, but according to the National Renewable Energy Laboratory (NREL), the most commonly cited number is 10+ hours. NREL also states that context around application is important when discussing what is meant by “long duration.” For example, a 6-hour battery might be able to provide firm capacity–the ability to meet peak demand and cover any other adverse conditions like blackouts–in some situations, while in others, a storage system with 100 hours of duration might be more necessary.
Just like short-duration energy storage, long-duration energy storage technologies come in many forms and chemistries. The most common types are thermal, electrochemical, and mechanical. Because long-duration energy storage is gaining a lot of recent attention, the landscape of technologies is constantly changing.
Can’t grid operators and utilities just pick the best long-duration energy storage technology that is readily available and use that?
Yes and no.
There are tradeoffs with different technologies. Pumped hydro storage, which has been around for a long time, has relatively good efficiency and isn’t as expensive as some other options, but there are limits to where we can build this type of system and environmental impacts to consider. (A huge dam probably won’t be appearing in the middle of Times Square any time soon!)
An electrochemical system like a metal-anode battery has fewer siting limits, but right now it’s expensive and hasn’t been deployed as much as older technologies. As technology providers continue to test their long-duration energy storage systems in different scenarios we’ll likely see some “weeding out” of the technologies that don’t keep up. (Read this blog post by my colleague Guillermo Pereira to understand how important pilot projects can be.)
Having a variety of long-duration energy storage options is helpful
One advantage of having a power grid with different long-duration energy storage options is that it allows for a more diverse supply chain, potentially alleviating some supply constraints that arise when only sourcing for one specific chemistry.
Lithium-ion batteries are currently a hot topic of conversation because of the huge demand for the materials that make it up like lithium, nickel, and sometimes cobalt. This growing demand plus the fact that these materials are finite, create a market for supplies that becomes constrained. With this in mind, further development of long-duration energy storage should consider what other options are out there that use abundant materials and result in a sustainable supply chain. It is important to explore the landscape of technologies available in order to best meet the grid’s needs and make the transition to renewable energy.
A quick snapshot of energy storage, using some of NREL’s data, shows us that 12-hour pumped-hydro storage has dominated the U.S. storage market for a long time. Over time, more batteries of varying sizes have come online. As the need for storage increases, longer duration options are deployed. By 2050, NREL expects around 9.5 gigawatts of 10-hour battery storage to be deployed. That’s enough to power over 7 million homes for, well, 10 hours!
Challenges ahead for long-duration energy storage
This type of energy storage can sound like an ideal solution for many of our grid needs. Because renewable sources like solar and wind are not always available at night or when the wind isn’t blowing, renewables are often critiqued for how well they can (or can’t) be dispatched—that is, for their ability to be turned on and off to supply power on demand. With energy storage, the energy harnessed by renewable resources can be stored and used on demand. But right now, long-duration energy storage is not yet the magic bullet we wish we had.
As we saw above, many promising long-duration energy storage technologies are still emerging and maturing and are not yet commercially available. What this usually means is that, for the time being, they are expensive and may lack confidence from investors, developers, or utilities in real-world scenarios. Regulatory bodies like state public utility commissions may hesitate to approve projects that take on a large cost with technologies that have yet to be field-tested as much as some other alternatives.
In addition to that, energy storage is not a resource that has been very well-defined across energy entities like utilities, regional transmission organizations (RTOs), or the power industry in general. Energy storage is not treated the same as solar or wind, and without standardized definitions and understanding of its value, it can be tricky to figure out exactly how to use it.
Long-duration energy storage is not just a shiny and exciting discussion topic. It’s a resource that can help usher in very significant amounts of reliable and resilient clean energy for our planet. It can work together with renewable energy to deliver power when we need it most, and conserve it when we have plenty of sun and wind to go around. As energy demand grows across sectors of our economy, it’s crucial that we understand the key role long-duration energy storage can play in reducing the energy industry’s reliance on fossil fuels and meet our needs with solutions that are better for our communities and our planet.