How Energy Storage Can Cut Peaker-Plant Carbon for the Clean Power Plan

The Clean Power Plan cites energy storage as an enabling technology, one that could help utilities and grid operators store, shift and smooth the increasing amounts of intermittent wind and solar that will be needed to meet the Obama administration’s ambitious greenhouse-gas reduction goals.

But AES Energy Storage says grid-scale batteries don’t need to wait for that green power revolution to start contributing to carbon-cutting. There’s a big, fat target on the grid already -- simple-cycle natural gas “peaker” plants that exist only to serve the grid’s brief moments of peak demand.

These peaker plants emit significantly more carbon dioxide and nitrous oxide than combined cycle power plants, which make up the vast majority of existing and planned gas-fired generation today. And because peaker plants usually run for only a few hours at a time, they could potentially be replaced by energy storage that can stack up several hours of capacity -- and last for decades like power plants do.

The Energy Information Administration (EIA) estimates the country will need about 26,000 megawatts of peak capacity over the next two decades to meet demand. That makes for a huge potential market for energy storage that can fit the grid’s reliability and cost requirements, Kiran Kumaraswamy, market development director at AES Energy Storage, wrote in a blog post.

Adding batteries to combined-cycle plants, in particular, allows those plants to start serving peaker functions, improves their “capacity factor” -- a measure of how often they’re generating power in sync with the system’s supply-demand needs -- and cut carbon in the bargain, he said. Here’s a comparison of the two types of power plants, in terms of emissions rates per megawatt-hour of power produced.

And here’s how AES calculates that storing an hour of combined-cycle energy and replacing an hour of simple-cycle energy would yield roughly a one-third reduction in carbon emissions.

Most of the world’s energy storage isn’t built to provide this kind of workhorse capability for the grid. Mid-Atlantic grid operator PJM is the country’s biggest market for energy storage, with some 150 megawatts of fast-responding assets now on-line and hundreds more megawatts being planned. But the vast majority of those systems are serving the short-duration frequency regulation market, which requires batteries to quickly charge and discharge, not continually pump power for hours at a time.

Lithium-ion batteries in particular can face accelerated breakdown and loss of efficiency if asked to undergo charge-discharge cycles of more than an hour or two over significant periods of time. Longer-duration batteries are available, but they tend to be limited in supply (sodium-sulfur batteries) or still in the pilot and early deployment stage (flow batteries, aqueous zinc, liquid metal, etc.).

Even so, AES Energy Storage, a subsidiary of power giant AES Corp., is building a 100-megawatt, 400-megawatt-hour lithium-ion battery complex for Southern California Edison meant to be a peaker replacement. The system will store power and inject it into the grid to help meet the region’s late afternoon demand spikes and peak capacity requirements.

AES Energy Storage is also building a 20-megawatt system for sister company Indianapolis Power & Light, meant to serve as a multi-hour capacity resource, and it has other similar projects planned, company president John Zahurancik told me in a recent interview. That stored power could come from cheap overnight wind power or midday solar energy, which comes with a carbon footprint of zero. But it could also come from combined-cycle gas plants like those AES operates around the country.

The fact that batteries can both absorb and inject energy make them even more flexible than a power plant, which can only generate power, AES notes. That's why it classifies its 20-megawatt Indianapolis project as having the "equivalent of 40 MW of flexible capacity" for the grid. For example, when grid demand falls, power plants may be asked to curtail or ramp down their production, which reduces their efficiency. With batteries, those plants could just store their own power and stay running at peak output.

Of course, all this is presuming that grid-scale energy storage can achieve the 90-percent-and-up efficiencies being promised by today’s leading lithium-ion battery manufacturers and storage developers. Some points may need to be subtracted for the energy required to cool the batteries, convert their direct current to grid-ready alternating current (and vice versa for storing grid power), and other operations -- a measure known as “station power” in the utility industry. Just how these kinds of issues will be resolved in the Clean Power Plan is still an open question.