It’s already cheaper to build a new solar or wind farm than a coal plant. But when the sun isn’t shining and the wind isn’t blowing, renewable electricity can still be fairly expensive to store–even though the cost of batteries is dropping. If the world shifted to 100% renewable electricity right now, we might pay more on electric bills.

A new solution that uses basic physics could cut the cost of storage in half, or by as much as 80% over the total life of the system. It makes it possible for renewable power to be cheaper than fossil fuels all day, every day of the year, everywhere. “Our solution, for the first time, will enable the world to achieve this,” says Robert Piconi, CEO and cofounder of Energy Vault, the startup that developed the new system. Tata Power, the giant Indian electric utility, will be the first customer.

Energy Vault, based in California and Switzerland, took inspiration from the way that some dams store energy–hydro plants pump water uphill when energy demand is low, and then produce energy by turning turbines as the water flows back down. The system works, but only in places where dams can physically be built; dams also harm fish, force people to relocate, and can burst and flood villages.

Like dams, the new solution–a massive tower, roughly the height of a 35-story building–relies on gravity. But it doesn’t require water. When a wind or solar farm makes more energy than the grid needs, an automatic crane on the battery uses the extra electricity to lift a giant brick, weighing 35 metric tons, up to the top of the tower. “When that tower’s stacked, that’s all potential energy,” says Piconi. When the grid needs power, the crane automatically lowers a brick, using the kinetic energy to charge a generator.

All of this happens almost immediately. “We can have a millisecond response time,” he says. The system’s software takes signals from the grid to automatically control the cranes, which carefully raise and lower the giant bricks while taking into account wind and weather. The cranes lower the bricks at exactly the speed needed to provide electricity continuously.

It’s cheaper than building giant lithium-ion batteries, like the huge batteries that Tesla has installed in Australia and elsewhere. In part, that’s because the bricks can be made from cement that would normally be wasted. “These materials we’re using are actually materials that you’d have to landfill,” says Piconi. In California, for example, a construction site with concrete debris has to pay as much as $55 a cubic yard to get rid of it. Unlike lithium batteries, building the system doesn’t require a specialized multimillion-dollar factory; the autonomous crane comes from another manufacturer. Mining lithium also uses huge amounts of water and risks toxic leaks.

In a small town near its headquarters in Switzerland, Energy Vault built a small prototype of the device–72 feet tall, instead of the usual 393. (The system also works at a small scale, but the company is focused on the largest market, utility-scale customers; it’s also less efficient and less disruptive in terms of cost at a smaller scale.) The company is now beginning to build its first units for customers around the world. It’s also in talks with some customers who have been considering constructing huge new dams. “We can do that at a quarter of the cost, without the environmental problems, and have something that would deliver more on the performance side,” says Piconi.