Solar and wind power are now the cheapest sources of electricity in most of the world. (Solar power dropped in cost by a staggering 89% over the last decade.) But one of the reasons that it’s challenging to switch fully to renewable energy is the problem of intermittency: The sun doesn’t always shine and the wind doesn’t always blow, and it’s still relatively expensive to use batteries to store energy.
A U.K.-based startup called RheEnergise is working on a new way to store energy, making use of gravity on hills. It’s similar to existing hydropower plants that pump water up mountains when energy is cheap, and then release it when needed to spin turbines and generate more electricity. But while building a huge dam can take a decade or even longer and makes economic sense only on large, steep hills, the new system can be built on smaller hills, making it feasible in more areas. It also has less impact on the environment and can be built faster.
Instead of using water, the new system pumps a fluid that’s two and a half times denser, which means that it can store the same amount of energy on a smaller hill. The technology could potentially be put in abandoned mines that are now ready for new uses. The fluid travels up and down in pipes that can be put underground so they aren’t visible in more pristine areas.
Unlike a large hydro plant, it could be scaled down to store only the amount of energy produced by a typical solar farm or wind plant. “As the energy grid is transitioning to accommodate these distributed generation technologies, you also need a distributed storage technology, so you store it more locally to where it’s been produced,” says Stephen Crosher, chief executive of RheEnergise. “Then as you scale down the projects, rather than taking decades to build them, you can build projects in nine months, for a small one, and maybe 15 months for a large project.” Some other startups are working on other alternatives for gravity-based energy storage, such as Energy Vault, a company that stores renewable energy by lifting bricks into towers.
While lithium-ion batteries can make sense when energy only has to be stored for a couple of hours, the new technology would be cheaper when the energy has to be stored for several hours before it’s used. Depending on the cost of renewable energy in a particular area, the total cost of the renewables plus the new storage system could already be competitive with energy from a new gas-fired power plant, Crosher says.
The company recently mapped out 9,500 sites in the U.K. with hills that could accommodate the technology. It’s now raising funds for a pilot plant. “Commercializing this should be a rapid process for us, because our supply chain exists—we can take our IP to existing manufacturers and say, ‘You know how to build this,'” Crosher says. “We can theoretically go within months of receiving an order to actually be delivering equipment to a site.”