Pumped hydro supplies the great bulk of the long-term energy storage required for renewable energy, and here’s how it works

20th January, 2022.      //   Technology  // 

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To achieve the Biden administration’s goal of halving US greenhouse gas emissions in a decade, the US will need a lot more solar and wind power generation, as well as a lot of cheap energy storage. Because the amount of energy generated by wind and solar power fluctuates throughout the day, energy storage is required to ensure a constant supply of electricity. Today’s batteries, on the other hand, are usually quite small and only hold enough power for a few hours of use. The United States will require more overnight and long-term storage in order to increase its reliance on wind and solar power. While battery technology receives a lot of attention, there’s a long-term storage method that has been employed in the United States since the 1920s.

Two types of pumped-storage hydropower; one doesn’t require a river

Two types of pumped-storage hydropower; one doesn’t require a river

While battery technology receives a lot of attention, there’s a long-term storage method that has been employed in the United States since the 1920s.

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

Because of concerns about the impact of hydropower on rivers in the United States, pumped hydro storage is often disregarded. Many people are unaware, however, that the majority of the best hydro storage sites aren’t even on rivers.

There are 35,000 paired sites in the United States with good potential, according to our globe atlas of possible locations for closed-looped pumped hydro — systems that don’t include a river. We estimate that only a few hundred sites are needed to support a 100 percent renewable U.S. electricity system, despite the fact that many of these sites, which we found by satellite, are in rugged terrain and may be unsuitable for geological, hydrological, economic, environmental, or social reasons.

Why wind and solar need long-term storage

The Kidston pumped hydro project in Australia uses an old gold mine for reservoirs.

The Kidston pumped hydro project in Australia uses an old gold mine for reservoirs.

Power grids must be able to match incoming electrical supply to demand in real time in order to work correctly, or they risk running out of power or overloading. With fluctuating sources like wind and solar, grid management can utilize a variety of approaches to retain the balance. Managing demand, exchanging power across broad areas via interstate high-voltage transmission lines, and implementing energy storage are just a few of them.

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 controversial, particularly when they involve dams on rivers that flood 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.

An off-river pumped hydro system comprises a pair of reservoirs spaced several miles apart with an altitude difference of 200-800 meters (about 650-2,600 feet) and connected with pipes or tunnels. The reservoirs can be new or use old mining sites or existing lakes or reservoirs.

On sunny or windy days, water is pumped to the upper reservoir. At night, the water flows back down through the turbines to recover the stored energy.

A pair of 250-acre reservoirs with an altitude difference of 600 meters (1,969 feet) and 20-meter depth (65 feet) can store 24 gigawatt-hours of energy, meaning the system could supply 1 gigawatt of power for 24 hours, enough for a city of a million people.

The water can cycle between upper and lower reservoirs for a hundred years or more. Evaporation suppressors – small objects floating on the water to trap humid air – can help reduce water evaporation. In all, the amount of water needed to support a 100% renewable electricity system is about 3 liters per person per day, equivalent to 20 seconds of a morning shower. This is one-tenth of the water evaporated per person per day in the cooling systems of U.S. fossil fuel power stations.

Storage to support 100% renewables
Little pumped storage has been built in the U.S. in recent years because there hasn’t been much need, but that’s changing.

In 2020, about three-quarters of all new power capacity built was either solar photovoltaics or wind power. Their costs have been falling, making them cheaper to build in many areas than fossil fuels.

Australia is installing solar and wind three times faster per capita than the U.S. and is already facing the need for mass storage. It has two systems under construction that are designed to have more energy storage than all the utility batteries in the world put together; another dozen are under serious consideration. None involve new dams on rivers. The annual operating cost is low, and the working fluid is water rather than battery chemicals.

Shifting electricity to renewable energy and then electrifying vehicles and heating can eliminate most human-caused greenhouse gas emissions. The U.S. has vast potential for off-river pumped hydro storage to help this happen, and it will need it as wind and solar power expand.

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