Batteries get hyped, but pumped hydro provides the vast majority of long-term energy storage essential for renewable power – here’s how it works

Andrew Blakers, Professor of Engineering, Australian National University, Matthew Stocks, Research Fellow, ANU College of Engineering and Computer Science, Australian National University, and Bin Lu, Research Fellow, Australian National University

<span class="caption">The U.S. has thousands of lakes and reservoirs that could be paired for pumped hydro storage without the need for rivers.</span> <span class="attribution"><a class=
The U.S. has thousands of lakes and reservoirs that could be paired for pumped hydro storage without the need for rivers. Ollo via Getty Images

To cut U.S. greenhouse gas emissions in half within a decade, the Biden administration’s goal, the U.S. is going to need a lot more solar and wind power generation, and lots of cheap energy storage.

Wind and solar power vary over the course of a day, so energy storage is essential to provide a continuous flow of electricity. But today’s batteries are typically quite small and store enough energy for only a few hours of electricity. To rely more on wind and solar power, the U.S. will need more overnight and longer-term storage as well.

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While battery innovations get a lot of attention, there’s a simple, proven long-term storage technique that’s been used in the U.S. since the 1920s.

It’s called pumped hydro energy storage. It involves pumping water uphill from one reservoir to another at a higher elevation for storage, then, when power is needed, releasing the water to flow downhill through turbines, generating electricity on its way to the lower reservoir.

Illustration of two open- and closed-loop hydro storage systems. Closed-loop systems use two reservoirs rather than running water.
Two types of pumped-storage hydropower; one doesn’t require a river. NREL

Pumped hydro storage is often overlooked in the U.S. because of concern about hydropower’s impact on rivers. But what many people don’t realize is that most of the best hydro storage sites aren’t on rivers at all.

We created a world atlas of potential sites for closed-looped pumped hydro – systems that don’t include a river – and found 35,000 paired sites in the U.S. with good potential. While many of these sites, which we located by satellite, are in rugged terrain and may be unsuitable for geological, hydrological, economic, environmental or social reasons, we estimate that only a few hundred sites are needed to support a 100% renewable U.S. electricity system.

Why wind and solar need long-term storage

To function properly, power grids must be able to match the incoming electricity supply to electricity demand in real time or they risk shortages or overloads.

There are several techniques that grid managers can use to keep that balance with variable sources like wind and solar. These include sharing power across large regions via interstate high-voltage transmission lines, managing demand – and using energy storage.

Aerial view of a pumped hydro project's two reservoirs and solar array on a dry landscape
The Kidston pumped hydro project in Australia uses an old gold mine for reservoirs. Genex Power

Batteries deployed in homes, power stations and electric vehicles are preferred for energy storage times up to a few hours. They’re adept at managing the rise of solar power midday when the sun is overhead and releasing it when power demand peaks in the evenings.

Pumped hydro, on the other hand, allows for larger and longer storage than batteries, and that is essential in a wind- and solar-dominated electricity system. It is also cheaper for overnight and longer-term storage.

Off-river pumped hydro energy storage

In 2021, the U.S. had 43 operating pumped hydro plants with a total generating capacity of about 22 gigawatts and an energy storage capacity of 553 gigawatt-hours. They make up 93% of utility-scale storage in the country. Globally, pumped hydro’s share of energy storage is even higher – about 99% of energy storage volume.

Pump hydro projects can be controversialparticularly when they involve dams on rivers that flood the land to create new reservoirs and can affect ecosystems.

Creating closed-loop systems that use pairs of existing lakes or reservoirs instead of rivers would avoid the need for new dams. A project planned in Bell County, Kentucky, for example, uses an old coal strip mine. Little additional land is needed except for transmission lines.

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