Carbon-free energy: Is the answer blowing in the wind? Perhaps, but the wind doesn’t always blow, nor does the sun always shine. The energy generated by wind and solar power is intermittent, meaning that the generated electricity goes up and down according to the weather.

Because storage can enable renewables to meet changing demand, we often assume the technology is inherently green — that is, by adding storage and renewables to the grid, we reduce greenhouse gas emissions. It’s similar to noticing that computers can provide education and productivity benefits, and then assuming that everything a person does on a computer is educational or productive. For both energy storage and computers, it depends on how you use it.

In our analysis, we found that adding storage can, for some grids, increase carbon emissions. While counterintuitive at first glance, this result makes sense when one considers how electricity grids are operated. Broadly speaking, the entire U.S. grid is operated as a set of regional sub-grids that cover the U.S. like a patchwork quilt.

So, how can grid planners achieve the promise of a happy marriage between storage and renewables, assuming that they have to live in the same house with crusty old Uncle Coal?

One possibility is that, even with storage operating to maximize profit, adding enough wind and solar to the grid could counteract the effect of coal. With enough excess renewable energy, storage in any form — batteries or water reservoirs, for example — would preferentially use solar and wind because they are the cheapest sources when the supply of power exceeds the demand. Storage would still be shifting coal power from night to day, but enabling renewables more would be enough to make up for the extra emissions.

Read the full article about whether energy storage can clean up the electric grid at The Conversation.