Here at Volts, I recently spent a week … OK, a month writing about batteries, which store energy for electronic devices, electric vehicles, and, at least for short periods of time (four to six hours), the power grid.Lithium-ion batteries are extremely good at those tasks — and they’re getting better, and cheaper, all the time.But here’s the thing: a net-zero-carbon grid is going to need storage that lasts a lot longer than six hours. It’s going to need durations of up to 100, 300, 500 hours or more, and it’s going to need them cheap. Lithium-ion batteries just aren’t going to work for that. What will work? Good question! No one really knows yet. Whatever it is will require substantial research, development, and scaling, and possibly some good geographical luck. We can’t know yet what technology or technologies might win that race, but we do have a good sense of what they need to accomplish, thanks to some new research in Nature Energy from a team at MIT (along with Jesse Jenkins, who used to be at MIT but is now at Princeton). Their findings on long-duration energy storage (LDES) are daunting and somewhat deflationary. In a nutshell: LDES needs to get extremely cheap before it will play a substantial role in a clean grid — cheaper than almost any candidate technology today, and cheaper than any geographically unconstrained technology is likely to get any time soon. Let’s start with some background on the need for LDES.Renewables on the grid need firmingThe cheapest large-scale renewable energy sources are wind and solar, but wind and solar are variable. They come and go with the sun and the wind, and as you may have heard, the sun is not always shining and the wind is not always blowing. We cannot turn them on or off, up or down.The supply of wind and solar energy will not always match the “demand curve,” i.e., the level of electricity demand throughout the day. As more and more wind and solar are added to the grid, there’s more and more need for flexible resources that can fill the gaps when supply doesn’t match demand.Today, those gaps are overwhelmingly filled by natural gas power plants, which are 100 percent “firm” in that they can be turned on at will and run as long as necessary. As the grid is decarbonized, however, fossil fuel plants (at least those without carbon capture) will be phased out of the electricity system and wind and solar penetration will increase. As that happens, other resources will be needed to firm the system. There are four basic options.Transmission: connecting larger geographical areas raises the chances that sun or wind will be available somewhere within them.Low-carbon (“clean”) firm generation: the MIT team’s paper lists “nuclear, fossil fuels with carbon capture and storage (CCS), bioenergy, geothermal, or hydrogen and other fuels produced from low-carbon processes.” (The first three items are anathema to some climate activists, but they may end up being necessary.)Negative emissions technologies (NETs): technologies that permanently bury carbon dioxide can offset emissions from firm fossil fuel plants. Long-duration energy storage (LDES): technologies that can store enough energy, for long enough, to displace firm generation. (Note: natural gas power plants are much cheaper than any of these options, save perhaps some transmission. There will be no market or pressure for any of them unless there are policies that require reduced greenhouse gases.)NETs are likely to stay expensive and transmission can only do so much, so the real fight is likely to be between clean firm generation and LDES. The new research is an extremely detailed modeling look at the role LDES might play in a decarbonized energy grid — how much clean firm generation it might displace and how much it might reduce energy system costs.The LDES “design space”The researchers took an interesting and (to me at least) somewhat novel approach. The problem with trying to study LDES is that a bunch of incredibly heterogenous technolo