Gravitricity generates electricity by dropping an iron weight down a shaft.

Gravitricity/Peter Dibdin

EDINBURGH, U.K.—Alongside the chilly, steel-gray water of the docks here stands what looks like a naked, four-story elevator shaft—except in place of the elevator is a green, 50-ton iron weight, suspended by steel cables. Little by little, electric motors hoist the weight halfway up the shaft; it is now a giant, gravity-powered battery, storing potential energy that can be released when needed. And that moment is now: With a metallic moan, the weight inches back down the shaft. Reversing direction, the motors become electric generators, sending up to 250 kilowatts of power back to the grid. For peak power, the weight can descend in 11 seconds—but for testing purposes, it moves just a few meters at “creep speed,” says Douglas Hitchcock, project engineer at Scottish startup Gravitricity.  

The company announced this week that its small-scale demonstrator is now operational, capable of switching between drawing energy from the grid and sending it back in a matter of seconds. The design offers an alternative to the chemical batteries that dominate the global energy storage market—a market that is growing hand in hand with renewable power, which needs to bank energy when the Sun shines or the wind blows, and release it when the grid faces high demand.

Gravitricity is one of a handful of gravity-based energy storage companies attempting to improve on an old idea: pumped hydroelectric power storage. Engineers would dam up a reservoir on a hill, pump water to it at times of low demand (usually at night), and release it to generate electricity. But the systems require specific terrain, expensive infrastructure, and planning approval that is increasingly hard to come by. These days, banking energy usually means hooking up renewable power to giant batteries.

Yet gravity-based storage has some distinct advantages, says Oliver Schmidt, a clean energy consultant and visiting researcher at Imperial College London. Lithium-ion batteries, the technology of choice for utility-scale energy storage, can charge and discharge only so many times before losing capacity—usually within a few years. But the components of gravity storage—winches, steel cables, and heavy weights—can hold up well for decades. “It’s mechanical engineering stuff,” Schmidt says. “It’s relatively cheap.” And whereas mining the minerals for lithium-ion batteries brings environmental and human rights problems and recycling the batteries is hard, a bucket of iron has a much lighter footprint, says Miles Franklin, Gravitricity’s lead engineer.

Using Gravitricity’s own cost and performance estimates, Schmidt compiled a 2019 report for the company showing that all told—including construction, running costs, and maintenance—gravity storage can be cheaper than lithium-ion batteries. For a 25-year project, he estimates that Gravitricity would cost $171 for each megawatt-hour. Jessika Trancik, an energy storage researcher at the Massachusetts Institute of Technology, says that number is aspirational and still needs to be supported with field data. But Schmidt’s calculation of the lifetime cost per megawatt-hour for lithium-ion batteries, $367, is more than twice as much. Flow batteries, a promising grid-scale technology that stores charge in large tanks of liquid electrolyte, come in at $274 per megawatt-hour.

Other gravity-based storage companies have their own twists on the technology. The idea behind California-based Gravity Power is just a small step away from pumped hydro: It uses renewable energy to pump water under a heavy piston and lift it. When power is needed, the piston weight is released, forcing the water through a hydroelectric generator. The German company New Energy Let’s Go uses a similar design. Switzerland-based Energy Vault wants to use a multiarmed crane with motors-cum-generators to stack and disassemble a 120-meter-tall tower made of hundreds of 35-ton bricks, like a Tower of Babel that rises and falls with the vagaries of energy demand. Gravitricity keeps it simple, with its clusters of elevatorlike shafts. It plans to scale up to 500-ton weights, which would require mammoth foundations for a tower—so the best place to put a full-scale system is underground, says business development manager Ruth Apps. The company is scoping out disused mines in the Czech Republic, Poland, and South Africa for its first commercial projects.

The technology is still “incredibly immature,” Schmidt cautions, and although battery prices continue to drop, the gravity companies have made little progress. Energy Vault, probably the leader of the pack, announced in 2019 it had raised $110 million in investment, and plans to start commercial developments this year. But like all storage technologies, gravity-based storage will flounder if climate regulations don’t create incentives for carbon-free energy, says Rebecca Willis, an environment policy researcher at Lancaster University. In most places, she says, small natural gas plants that can be easily turned on and off remain the cheapest way to cope with fluctuations in demand.

With a small staff of 14 and just £3 million in investment, Gravitricity has no illusions about the hurdles ahead. As they put the demonstrator through its paces, they have already turned up unexpected difficulties—like the torques exerted by the steel cables, which, like any rope, want to untwist as they lift the weight. After working through these teething issues, the company by 2023 plans to build a full-scale plant—with the heavier weights and a shaft nearly 1 kilometer deep that could produce up to 4 megawatts of peak power.

Ghosts of an ailing industry surround Gravitricity’s test site: A supply vessel serving North Sea oil rigs slumbers nearby. But on a clear day, one can make out Scotland’s energy future: the wind turbines that dot the horizon less than 20 kilometers from here. Two of Gravitricity’s recent hires come from the oil and gas sector, Apps says. And mining communities are thrilled at the prospect of turning their heritage to something new: “They see the idea of having a second life.”

Source: https://www.sciencemag.org/news/2021/04/gravity-based-batteries-try-beat-their-chemical-cousins-winches-weights-and-mine-shafts

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