More research shows that batteries aren't necessarily helping decarbonize the grid. In many cases, according to a team from the Rochester Institute of Technology, batteries are making it dirtier.

The team, led by Naga Srujana Goteti of RIT’s Golisano Institute for Sustainability, found batteries installed on the Midcontinent Independent System Operator (MISO) grid led to a net increase in carbon emissions, by increasing coal consumption.

The finding is not entirely surprising, since coal is one of the major generation sources on the MISO grid. But in New York, where coal is a minor component of the generation mix, batteries could still result in increased emissions if gas prices are high.

“With current low natural-gas prices, adding storage slightly reduces carbon emissions in New York, while increasing them in the Midcontinent ISO,” states the study.

“Storage increases carbon emissions when it enables a high-emissions generator, such as a coal plant, to substitute for a cleaner plant, such as natural gas.”

The study unpacks the complex relationship between emissions, generation mixes and pricing.

Because the batteries under study are set up to charge when electricity prices fall and discharge when the price rises, they increase consumption of whatever generation source is in use when energy demand is low.

In a coal-based grid such as MISO, for example, charging batteries at night creates extra electricity demand that will mostly be met by coal generation even if the wind is blowing.

This coal-based energy then gets released during peak demand periods, often displacing lower-carbon natural gas.

As a result, “in a coal-based grid, storage increases emissions even if you charge from renewables,” Goteti said, unless excess renewable generation is being curtailed and can be used for charging instead.

Depending on fuel pricing, this can hold true even on a grid like that managed by the New York Independent System Operator, where generation is predominantly carbon-free.

Only about 3 percent of New York’s electricity comes from coal, which means emissions from battery storage are usually a result of charging on natural gas.

But if the price of gas were to go up to $5 per million Btu, from the $2.60 level at the time of the study, coal would be dispatched first and could end up being used for battery-charging, leading to increased emissions overall.

The researchers were able to calculate roughly how much renewable energy would be needed on a grid for batteries to start cutting emissions. In MISO, storage would not be carbon-neutral until the share of wind and solar reached 18 percent.

As with New York, this level is highly sensitive to the price of natural gas. If gas were to go up to $5 per million Btu in MISO, the system operator would need to get 35 percent of its generation from renewables before storage could become carbon-neutral.

Such a percentage of renewable generation is likely to be at least a decade away, the study commented.

Currently, MISO has a negligible amount of solar generation; around 7.6 percent of the operator’s electricity comes from wind and a further 3.3 percent comes from a mix of other sources, including hydro and biomass but also diesel.

“The effect of natural-gas price leaves the economic and environmental effects of storage at odds,” concludes the report. “We have found that storage-induced emissions that are zero or negative depend on the currently-low natural gas prices.

“However, storage providing energy arbitrage only makes financial sense if natural gas becomes more expensive, in which case energy storage will induce greater use of coal generation, increasing system emissions.”

While the research may be sobering for those who assume batteries automatically cut emissions by mopping up renewable generation, Goteti and her colleagues also discovered that the picture could change radically with carbon taxes.

“A carbon price on emissions from generators would shift operation to make energy storage carbon-neutral even with current wind and solar capacities,” said the study.

At the end of the day, said Daniel Finn-Foley, senior energy storage analyst at GTM Research, “it depends on why you're installing storage. If you are adding storage in the MISO region then you're aiming to maximize revenue, not limit greenhouse gas emissions reductions.

“Operating storage to provide another less-economic service, such as capturing curtailed wind or shifting solar energy to times of peak use, would yield much different results.”

In January of last year, researchers showed that residential battery storage could be a significant source of emissions if not paired directly with renewable energy.