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Just Add Water: How Retired Coal Assets Could Help Power A Low-Carbon Future

This article is more than 4 years old.

How can states meet aggressive low-carbon electricity targets over the next two to three decades? Energy leaders from across western North America chewed on this question a few weeks ago at the Western Energy & Water Forum in Steamboat Springs, Colorado.

Renewable energy is clutch to meeting low-carbon goals, and its variability and intermittency present two major opportunities for innovation. To meet demand at any point in time, we need a greater diversity of supply resources—and that requires expanding the functionality and geography of the western grid. And to better calibrate the timing of supply and demand, we’ve got to have better energy storage systems.

These challenges are yielding some fascinating potential solutions—including repurposing coal assets.

For example, as discussions took place in Steamboat Springs, the ink was drying on Federal Energy Regulatory Commission (FERC) approval of a preliminary permit application by Daybreak Energy to build a 2.2 gigawatt pumped hydro energy storage facility near the Arizona-Utah border. Its goal? To address both grid connection and energy storage needs in the region.

Using solar power generated at mid-day, this operation would pump water out of Lake Powell into a newly constructed elevated reservoir. Later the facility would release the water, energizing turbines that generate electricity to meet late-day and evening peak demand. Best of all? The new facility would utilize existing transmission line capacity freed up by the November 2019 closing of the nearby Navajo Generating Station, once the largest coal-fired power plant west of the Mississippi River.  

I previously wrote about the Navajo facility, which is located on land belonging to the Navajo and Hopi tribes, following its owners’ decision to close it. While some considered the closure a devastating blow to employees and tribal budgets, others have seen it as an opportunity to prioritize environmental stewardship and other tribal values. Plans for a large pumped hydro facility to take the Navajo Generating Station’s place may provide an opportunity to reboot its economic and environmental legacy.          

Large-Scale Storage… But Is It Large Enough?

The proposed Navajo Energy Storage Station (NESS) addresses two major challenges of deep decarbonization: procuring energy storage at scale and transmitting electricity from the often-remote areas where large-scale renewable energy is produced to the population centers where it is used.       

At 2.2 gigawatts, the pumped hydro facility could produce roughly as much electricity when operating as two nuclear power plants can. The key qualifier here is “when operating.” Unlike solar and wind power, pumped hydro can be dispatched when needed to meet load. However, like solar and wind, it is intermittent because it is being powered by temporarily stored water rather than a steady supply of fuel.

The good news? The intermittency is complementary to solar. It is expected that the Navajo pumped storage unit will be able to dispatch those 2.2 gigawatts for up to ten hours at a time. Depending on the intensity of the sun, the time of year, and the load profile, ten hours of storage could make the entire facility something closer to a 24-hour operation, with obvious advantages for reliability and grid stability.

NESS would also add about 10 percent to the existing 23 GW of pumped hydro capacity in the United States, which now accounts for about 95 percent of all utility-scale energy storage in use.

By contrast, it would take at least 8.2 million lithium-ion battery cells covering more than 60 acres to provide the same amount of power, at a roughly 50 percent higher cost. And the batteries would need to be replaced much sooner than a pumped hydro facility. While batteries are great for many energy applications, they can’t match pumped hydro in energy capacity, durability, and cost for grid-level bulk storage.

That said, we’re going to need much, much more utility-scale storage than we have today if we’re going to meet aggressive low-carbon targets. How much more? According to a 2018 study published in Energy & Environmental Science, the United States can meet 80 percent of its electricity demand through wind and solar with about 5.4 TWh of energy storage dispensed on a daily basis. This would require a 20-fold increase in U.S. pumped storage—about 250 pumped storage projects the size of Navajo… or 150 years of output from the Tesla Gigafactory in Nevada.

It’s a big challenge. And NESS, as compelling a case as it may be, only scratches the surface.

Unicorn Or Beta?  

The NESS proposal is remarkable, combining a vast water source, enormous solar energy potential and a pre-existing transmission infrastructure that can already deliver more than 2 gigawatts of power to large metropolitan areas like Los Angeles, Phoenix, and Las Vegas. It is a near-perfect mix of conditions for a solution that can integrate a lot more renewable power into the western grid in the next decade and beyond. But if it’s successful, how much opportunity is there for replication elsewhere?

A few similar proposed projects are also on the books.

Another large (1.5 gigawatt) Daybreak Energy pumped hydro facility planned near Hoover Dam in Arizona will also take advantage of a large water source, abundant renewable energy potential, and existing transmission infrastructure.

TD Energy Park, LLC has applied to build a closed-loop pumped storage hydro project near Two Dot, Montana. The proposed project would tap into two nearby existing 500 kV transmission lines that were built to deliver electricity generated at the four-unit Colstrip coal-powered plant. Colstrip Units 1 and 2 (1,480 MW) shut down in January, with Units 3 and 4 (740 MW) facing financial pressure to retire early.  Two Dot and Colstrip are 200 miles apart—in contrast to NESS, which is essentially next door to the closed Navajo plant. But each proposed hydro project has access to now-underutilized transmission lines that can be hard to site and expensive to build from scratch.     

Meanwhile, back in Arizona, the proposed Sacaton Energy Storage Project southeast of Phoenix would consist of a 150 MW pumped hydro storage facility and a 100 MW solar plant next to an abandoned open pit mine used to extract copper, molybdenum, silver, and gold.

What these examples have in common is that they avoid the new damming of rivers, depletion of groundwater, or violation of sacred cultural sites—all of which are restricted for new pumped hydro. But those constraints may ultimately bind a significant scaling-up of new pumped hydro capacity. 

Different Horses For Different Courses

It’s understandable that, facing the complexities of the energy transition, we might hanker after a silver-bullet solution to the storage conundrum. But it’s realistic to pursue a diversified approach.

Batteries work well for applications like electric vehicles, behind-the-meter storage, and short-term frequency regulation and balancing at the grid level. And they will likely dominate transportation and commercial and residential energy storage markets for some time.

But grid-scale accommodation of zero-carbon energy necessitates storage with higher energy capacity, longer lifetimes, better economies of scale, and greater adaptability to local conditions.

My prediction? The prospect of reviving assets used by retired coal generation units for pumped hydro energy storage at significant scale is definitely appealing, but its deployment is likely to be limited. So the industry will need to assemble a portfolio of  assets across a range of technologies to integrate renewables into the grid at scale. Bulk storage provides one set of options but so does dispatchable zero-carbon electricity (e.g., geothermal power, hydrogen from renewables, small modular nuclear, and economical carbon capture and storage). In some cases, the best grid-scale storage solution may be none at all.

[This article benefitted from background research, discussions, and editorial feedback by Will Niver and Braden Welborn of the Duke University Energy Initiative]

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