Have you ever gazed up at a city skyline and wondered whether those giant towers might be able to do more than store offices and apartments? With the drive toward cleaner, more efficient energy, researchers and engineers are beginning to look to skyscrapers as a tapped-out resource. The unexpected answer is in elevators—devices that typically spend a lot of time idle, waiting for the next rider.
As cities continue to grow and clean energy like wind and solar becomes more popular, the problem of how to store electricity is becoming more important. Traditionally, batteries and pumped hydro storage have been the go-to solutions, but they are expensive and are limited by geography. What if the solution was right under our noses, just built into the buildings we already have?
It is the new concept from the scientists at the International Institute for Applied Systems Analysis (IIASA), Vienna, which defines the idea as the Lift Energy Storage System or LEST. The method is to convert high-rise elevator systems into huge batteries, which are energy storage and release devices by using the power of nature: a vacuum and weights are lifted and lowered to absorb and provide energy.
The mechanism is surprisingly simple. When there is abundant renewable energy—say, a sunny afternoon or a windy evening—elevators employ it to haul heavy containers loaded with heavy material like wet sand. Raising these masses to the upper levels stores the energy as potential gravity. When electricity is in high demand later, the containers are brought down, and the regenerative braking of the elevator captures that motion as electricity for the grid.
The IIASA researchers have made detailed models demonstrating how current elevators can be fitted with minimal investment. The system would be based on automatic robots to upload and download the weights, holding them in vacant areas at the ceiling and ground floor, or even in empty apartments and corridors.
Contemporary elevators already employ effective motors and regenerative braking, which recovers energy that otherwise would be lost as heat during descent. Indeed, contemporary high-performance elevators can achieve efficiencies of about 92 percent under optimal conditions.
In order to get it all done efficiently without inconveniencing passengers, the proposal involves clever algorithms and small autonomous robots. The robots would move weights in silence, stopping work whenever a person enters the elevator. The system would run primarily during off-peak hours, such as in the middle of the night, when elevators have fewer people.
One of the strongest features of LEST is price. Based on building height, the cost of storage ranges from $21 to $128 per kilowatt-hour. That’s not only comparable to, but in certain instances lower than, current lithium-ion batteries, which in 2020 cost an estimated $345 per kWh and will not fall below $100 per kWh for decades.
Worldwide, the possibilities are staggering. If sufficient high-rises were converted to this system, researchers estimate that between 30 and 300 gigawatt-hours of storage can be generated. At the top end, that is equivalent to powering New York City for a month.
Naturally, it’s not a sea of smooth sailing. Not all buildings are designed structurally to support huge weights on higher levels. Prime space at the top and bottom of skyscrapers typically goes to tenants, not shipping containers. Retrofitting existing elevators with regenerative systems is also expensive, and the space vs. rental income trade-off will be a large factor for building owners to consider.
There are also engineering challenges. Floors must be able to support the weight safely, and city officials or grid operators must be persuaded that it’s worth using. Incentives will be crucial to get owners to sign up.
The Future of Vertical Energy Storage in Cities
As cities expand and renewable energy expands further, the potential to use skyscrapers as giant gravity batteries is beginning to attract attention. Thanks to smarter elevators, self-driving robots, and more efficient energy management systems, this once-conceptual solution is becoming a reality.
