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Coal fueled half of US electricity generation just a little over a decade ago, but its market share has been collapsing ever since. My colleague Emma Spellman, one of our 2019 UCS Schneider Sustainable Energy Fellows, took a closer look at the data and generated some great new perspectives on that downward trajectory. Here are her thoughts and visuals:



America’s dirtiest fuel is on the decline. The need to transition fossil fuels to renewable sources of energy is not new; in fact, it’s on its way. Policy, market development, and technological progress play pivotal roles in this transition.

While these components continue to change and progress, it’s important to recognize how far we’ve come to help us keep the momentum going.

A graphic in a recent Guardian article skillfully illustrates these concepts in showing Britain’s quick transition away from coal. In the graphic, which shows the percentage of coal in their electricity mix each day over several years, coal-heavy days in 2012 give way to the first coal-free days by 2017, and a predominance of coal-free days this summer—including its first “coal-free fortnight.”

Indigo rising: transition in the Northeast and Great Plains

Inspired by the Guardian graphic, I set out to replicate this gradient approach for the US.

Unlike Britain, the US has multiple electrical grids, and those are separated into regions. Many are operated by regional transmission organizations (RTOs) or independent system operators (ISOs). And I found the right data for a couple of those—ISO-NE (New England) and SPP (the Southwest Power Pool, stretching from Oklahoma and parts of Texas and New Mexico to the Canadian border).

ISO-New England. New England states were early champions in clean energy policy and energy efficiency, and have been making a move away from coal for years. And when I graphed the ISO-NE data for generation in the region over the last dozen years, that transition showed quite clearly—in black and white and indigo.

In my graphic, each row corresponds to a year and each stripe within that row corresponds to a day. Pure black indicates the maximum during these years (in ISO-NE’s case, about 21% of that day’s generation coming from coal). Lower percentages are linearly scaled to some shade of gray. Indigo indicates days with zero coal generation within ISO-NE.

There is a clear gradient from 2008, with each year becoming lighter and lighter, and coal-free days beginning to pop up in 2012. In 2018, New England saw indigo for 108 days— 29.6% of the year, and over five times as many days as in 2013, just five years earlier.

Southwest Power Pool. SPP shows a different version of the transition away from coal. The speed of transition may not be on par with its northeastern counterpart, but the direction is right.

To start, it’s scaled to 77%, thus starting at a much larger ratio of coal to total generation for the maximum days. Unfortunately, there are also no indigo days. In fact, the minimum in the time frame observed is 24% (higher than ISO New England’s maximum).

However, there is still lightening as the years go by, which, for a historically coal-dependent region, is definitely promising. Days in which coal accounted for more than half of the region’s generation dropped to 55 in 2018 (15% of days) from 357 in 2014 (as in, virtually every day of the year). And coal’s average contribution over those years dropped to 42% from 60%.

Looking Forward

While these are two important case studies, I would love to continue to look at other regions as well. Unfortunately, some regions’ data wasn’t publicly available, some only went a couple years back, while others had varying formats between each year or even each month. However, the difficulties in data retrieval don’t negate the importance of the analysis.

While the transition to renewables from dirty sources of energy is clearly not over, these graphics prove that we’re going in the right direction when it comes to coal, verifying past analyses by UCS and others.

Obviously, these graphics point towards the question: What’s taking coal’s place? We have to be aware of what fuel types we’re moving to as transitions like these happen.

Natural gas is an important example: While it burns more cleanly than coal, it is certainly not the standard we’re reaching for (and some regions’ ramped-up reliance brings challenges of its own).

Fortunately, our rapid increase in using renewable energy sources like wind and solar, plus energy efficiency, has been another way we’ve decreased our reliance on coal. Wind power’s portion in the Southwest Power Pool in spring, for example, more than quadrupled between 2011 and 2018, rising to 27% from 6%.

A decline in coal is notable and as the trend continues. We must keep our sights on policy and technology to allow for clean energy to fill the gaps.

Credit: Emma Spellman, Union of Concerned Scientists

Credit: Emma Spellman, Union of Concerned Scientists

Posted in: Energy Tags: Clean Energy Momentum, coal, coal power



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