Batteries have a dirty secret/Vox

This is the complicated side of building a sustainable future.  We know the many benefits of clean energy, and we know renewables need some storage to make them really user friendly to the grid.  But, nothing is perfect.  Here we see the "dirty side" of battery storage.

Despite this data, the net gain of moving energy local and using storage as part of a microgrid is far better than fossil fuel and the long delivery systems we depend on today.  

Energy storage (batteries and other ways of storing electricity, like pumped water, compressed air, or molten salt) has generally been hailed as a “green” technology, key to enabling more renewable energy and reducing greenhouse gas emissions.

But energy storage has a dirty secret. The way it’s typically used in the US today, it enables more fossil-fueled energy and higher carbon emissions. Emissions are higher today than they would have been if no storage had ever been deployed in the US.

This is not intrinsic to the technology, by any means. If deployed strategically, energy storage can do all the things boosters say, making the grid more flexible, unlocking renewable energy, and reducing emissions.

But only if it is deployed strategically, which it generally hasn’t been.

In and of itself, energy storage is neither clean nor dirty — it is neutral, as likely to boost the revenue of fossil fuel plants as it is to help clean energy. If policymakers want to use it as a tool to enable clean energy, they need to be conscious of its characteristics and smarter about its deployment.

Why energy storage increases emissions

There is a growing body of scholarly research around energy storage; the key paper on its emission effects is by the Rochester Institute of Technology’s Eric Hittinger and Carnegie Mellon’s Inês Azevedo, in Environmental Science & Technology.

Modeling energy mixes and energy prices across the country, Hittinger and Azevedo determine that the deployment of energy storage increases emissions almost everywhere in the US today. Yikes.

By way of background, it’s important to understand that while energy storage can provide a wide array of services to the grid (more on that later), these days it is primarily used for energy arbitrage — storing energy when it is cheap (usually at night) and discharging it when it is more valuable (usually during the day). So it’s energy arbitrage that Hittinger and Azevedo model.

There are two reasons why energy storage deployed for the purpose of arbitrage increases emissions:

1) Storage increases the value of the energy sources it draws from (a source that can store some of its energy can generate more) and decreases the value of the energy sources it competes against when discharging. If the energy sources it draws from are more carbon-intensive than the energy sources it competes against, then it will have the effect of increasing the carbon intensity of the overall power mix.

Say a battery bank absorbs cheap energy being produced by coal plants overnight and then discharges it in the day, competing with natural gas combined-cycle (NGCC) plants. The net effect will be to favor coal against natural gas, thus increasing net emissions.

2) Every bit of energy stored also represents a bit of energy lost. The “round-trip efficiency” of energy storage — the amount of energy it releases relative to the amount put in — ranges, depending on the technology, from around 40 to 90 percent.

Let’s take, for representative purposes, 80 percent, a relatively optimistic assumption for the efficiency of lithium-ion batteries. For every 1 megawatt-hour put in, 0.80 megawatt-hours comes out.

That means, if it is stored along the way, getting 1 MWh to the customer requires generating 1.25 MWh. The more energy that gets stored, the more generation has to increase to compensate for the round-trip losses.

If the generation that increases to compensate for the losses is more carbon-intensive than the energy that storage displaces, net emissions nudge up.

Even when a battery stores zero-emissions renewable energy, it is not increasing or decreasing total generation; it’s just moving it around (unless the renewables would otherwise have been curtailed; see below). If coal steps in to cover for the renewable energy that is stored, but it displaces natural gas when it’s discharged, it still might increase net carbon emissions.