Gravity is hard to escape. So why not put it to good use? Researchers have now proposed a new concept to store energy using gravity that could turn skyscrapers into giant batteries.
The idea piggybacks on existing elevators and empty spaces in high-rise buildings. Renewable energy, the researchers suggest, would be used to carry a heavy solid mass up to the top of a building, effectively storing it as potential energy. As the mass comes down, it would drive a generator that produces electricity. Given the stock of tall buildings around the world, such systems could store anywhere from 30 to 300 gigawatt-hours of energy, they report in the journal Energy.
As renewable energy surges globally, the need for low-cost, long-lasting energy storage as an alternative to batteries is increasing. Gravity energy storage is one such novel concept that is being tested around the world. A handful of startups are developing systems that rely on using cranes or existing mine shafts to lift and drop heavy masses for storing and releasing energy.
The elevator system has a key benefit that the storage capacity is already out there, and situated exactly where the stored energy is needed. There are over 18 million elevators in operation around the world, and many spend a significant amount of time sitting empty and idle. During this time, and when extra renewable energy is available, the researchers propose, elevators can lift heavy objects like large containers of wet sand up to the top. Autonomous trailer robots could lift and transport the containers.
Then, when power is needed, the heavy containers can be dropped down, and the kinetic energy harvested using a regenerative braking system. Regenerative braking recovers some kinetic energy from a moving vehicle or object that would otherwise become heat and instead turns it into electricity. It is used commonly in electric and hybrid-electric vehicles.
“This is a realistic alternative because it utilizes existing infrastructure to provide a secondary service, energy storage,” says Julian Hunt of the International Institute of Applied Systems Analysis in Austria and lead author of the paper, who also recently proposed the idea of running electric trucks downhill to store energy. “The technology is important because it allows energy to be stored in a decentralized way close to where the electricity is consumed in an urban setting.”
For implementation, a building has to meet two key requirements: empty spaces at the top and bottom to store the containers, and an elevator with a regenerative braking system. While only a few newer buildings have regenerative braking elevators today, Hunt says, they are expected to go into more buildings around the world. Aging elevators that require replacement could also implement regenerative braking systems.
“If the building is at least fifty meters high, has a regenerative braking system, and space to store weight on the top and bottom of the building, can be implemented on a large scale,” he says.
The approach also has the advantage of low cost, ranging from 20 to 120 USD per kilowatt-hour. That cost is competitive, or better than batteries, but even then, Hunt clarifies, elevator energy storage would not compete with batteries in most cases. Battery costs are falling rapidly, and batteries are good at quickly providing power to meet a spike in demand that the grid cannot meet. Elevator storage, on the other hand, would be ideally suited for balancing out supply and demand on a weekly basis.
The next step for the research is to implement the system in an existing building, he says, and to estimate the overall efficiency and operational costs of the system.
Source: Julian David Hunt et al. Lift Energy Storage Technology: A solution for decentralized urban energy storage. Energy, 2022.
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