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Energy Storage Does Not Always Make the Electric Grid Cleaner

Energy storage can help grids use more wind and solar power, but it does not always reduce carbon emissions, write Naga Srujana Goteti, Eric Hittinger and Eric Williams of the University of Rochester. In some cases, adding storage actually even increases carbon emissions. This happens when consumption is shifted to periods when coal power is used more. Article courtesy The Conversation.

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.

But the output from the electricity grid must be controllable to match the second-by-second changing demand from consumers. So the intermittency of wind and solar power is an operational challenge for the electricity system.

Energy storage is a widely acknowledged solution to the problem of intermittent renewables. The idea is that storage charges up when the wind is blowing, or the sun is shining, then discharges later when the energy is needed. Storage for the grid can be a chemical battery like those we use in electronic devices, but it can also take the form of pumping water up a hill to a reservoir and generating electricity when letting it flow back down, or storing and discharging compressed air in an underground cavern.

Motivated by a view that storage is a “green” technology, governments are increasingly promoting utility-scale and distributed energy storage. For example, in November 2017, New York Gov. Andrew Cuomo signed a bill mandating targets for storage adoption by 2030. Other states with similar policies are Oregon, Massachusetts, California and Maryland. Companies like Tesla also have been branding storage systems as clean technologies.

But do large storage systems lower emissions in our current grids? In a recent study, we found this isn’t necessarily the case – a reflection of how complex the electricity system can be.

The role of coal

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 the Midwest, we found that adding a storage capacity of 3 gigawatts, enough power to supply roughly 500,000 U.S. homes, raises carbon emissions an equivalent of adding 6,700 cars per year to the road.

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.

Energy storage has no smokestack emissions like coal or natural gas power plants. But new storage affects the operation of other power plants on the grid, resulting in an increase or a decrease in carbon emissions depending on the type of power plants supplying electricity for that region.

In most cases, storage systems in the U.S. operate to maximize profit. To do this, storage “buys low and sells high.” Electricity is typically cheap at night when demand is low, and more expensive in the daytime, especially when people are getting home from work and turning on a bunch of appliances. So storage system operators tend to buy at night and sell during the day. The net effect of storage on emissions thus depends on what kind of generators are used to meet new demand at night versus the day.

In grids with a lot of coal power – Midwestern, Western and Southern states rely heavily on coal – the coal plants are typically used to meet small changes in demand at night. Natural gas plants tend to work during the day to meet peak demand. In these electricity grids, storage tends to charge up with coal power at night, displacing natural gas power during the day.

Coal power is a dirtier source of electricity than natural gas, with about twice the carbon emissions for every unit of electricity produced. Therefore, in places where new storage means more coal and less natural gas generation, storage will increase total carbon emissions from the grid.

In the Midwest, we found that adding a storage capacity of 3 gigawatts, enough power to supply roughly 500,000 U.S. homes, raises carbon emissions an equivalent of adding 6,700 cars per year to the road. And as more storage is added, the carbon emissions increase.

While a national carbon tax does not look likely in the near future, there are others paths to ensuring green outcomes from storage

On the other hand, we found that in New York, a state with very little coal power, adding storage reduces carbon emissions. The Midwest is currently the dirtiest electricity grid in the U.S., and New York is one of the cleanest, so other regions would fall somewhere in between.

Not always easy being green

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.

Storage will always help us to use more of our low-cost electricity sources. The question is whether that is coal, nuclear or renewables

We studied this and found that for the Midwest grid there is a turning point when wind and solar reach about 18 percent of total generating capacity: At that point, adding storage starts to decrease rather than increase emissions. The current adoption level is 10 percent, so it would take some time before storage in the Midwest reduces emissions.

Another option is to change how storage is operated. With a modest price on carbon, for example, the cost of different generators would shift so that storage charges less often from coal plants, reducing emissions even in the coal-heavy Midwest grid.

While a national carbon tax does not look likely in the near future, there are others paths to ensuring green outcomes from storage. For example, states can put in place policies that encourage more use of zero-carbon resources instead of coal.

Regardless, storage will always help us to use more of our low-cost electricity sources. The question is whether that is coal, nuclear or renewables.

By  and 

Eric Williams is Assocatiote Professor of Sustainability at Rochester Institute of Technology. Eric Hittinger is Assistant Professor of Public Policy at the same institute. Naga Srujana Goteti is a PhD Student in Energy and Sustainability at the same institute. This article was first published on The Conversation and is republished here with permission from the authors and publisher.

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Content Discussion

Bob Meinetz's picture
Bob Meinetz on February 13, 2018

Naga, you miss an important piece of the puzzle with storage: resistance losses.

No chemical battery is 100% efficient. Less energy comes out, than goes in, due to resistance losses dissipated as heat within the battery. The best lithium-ion batteries are 90-95% efficient (5-10% wasted); lead-acid batteries commonly used in microgrid systems are 80-85% efficient (15-20% wasted).

Stored energy coming out of a battery is thus ~10% “dirtier” than what goes in. So though it might be useful for energy companies to avoid the cost of added generation capacity (energy generated during periods of low demand can be distributed during periods of high demand), there’s nothing “green” about grid energy storage.

Roger Arnold's picture
Roger Arnold on February 13, 2018

I question this result. It’s not really about storage, it’s about a very specific tradeoff between coal-fired and gas-fired dispatchable generation in particular regions.

For the result to hold (storage leading to increased carbon emissions), there must be

a) an economic advantage to the utility in coal-fired generation, which
b) they can’t exploit due to constraints of ramp rates during the day.

Their coal-fired assets would presumably be unable to accommodate the high ramp rates and reversals that high PV penetration imposes on partly cloudy days. That would force them to use gas or diesel fueled peaking units. Presumably simple combustion turbines or IC gensets. Those are both relatively inefficient but perhaps still having lower carbon emissions than the type of older coal-fired assets that can’t handle daytime ramp rates and short-term cycling.

Storage could slow the ramp rates and eliminate most transient slope reversals in the demand curve. That would allow the utility to utilize their coal-fired assets more than fast-response peaking units. However, it would also allow them to utilize high efficiency CCGT’s — if they had them.

Or, of course, nuclear.

Jesper Antonsson's picture
Jesper Antonsson on February 13, 2018

The emissions required to construct the batteries are probably in the same ballpark as the resistance losses for high-CF usage (and much worse for more long-term storage), so it really adds up.

James Hopf's picture
James Hopf on February 15, 2018

I can grasp the concept how storage could result in more coal use in some markets, as it allows them to operate less “flexible” coal plants in place of more flexible gas plants. All this being due to the fact that coal is *slightly* cheaper than natural gas, right now, that being the entire problem.

The author suggests introducing more intermittent renewables onto these grids (by govt. mandate, or large subsidy?) as a solution to this problem. But is that the most economic solution? To me, this is one more example of how what’s needed are market-based, technology-neutral policies to reduce CO2 emissions and air pollution, where all means of reduction receive equal financial incentive, and the market decides how to respond.

As gas is only slightly more expensive than coal, it seems clear that giving gas a small financial advantage over coal (through a small subsidy or, better yet, carbon tax) would be the least expensive approach. It would be far less expensive, and require much less market intervention (mandates, subsidies, etc..) than forcing more renewables onto the grid. Incentivizing coal-to-gas switching remains the least expensive means of emissions reduction in general, and only small market interventions would have a huge impact. A small financial incentive to use gas vs. coal would eliminate the potential adverse storage effect discussed in this article.

There are other ways that energy storage could make the grid cleaner, given that proper disincentives to pollute were in place. Energy storage could help prevent nuclear plant closures (by lessening the impacts of intermittent renewables on their economics, e.g., negative pricing, etc..). Storage could also allow, and encourage, the use of more efficient combined cycle gas units, in place of simple turbines, due to the reduction of sharp swings in needed generation (ramp rates).

BTW, if one thinks that actual CO2 taxes are too difficult politically, there are other ways to achieve a similar result. We could alter dispatching rules so that environmental costs are included in the determination of “lowest cost” suppliers that are dispatched first. That way, gas plants would still be dispatched before coal, even if the raw variable cost was somewhat higher. I’ve written about this…..

http://ansnuclearcafe.org/2011/09/28/the-dispatch-queue-%E2%80%93-an-alternative-means-of-accounting-for-external-costs/#sthash.MeTX2p1i.dpbs