What goes up must come down. This principle applies to most things in our current gravitational setup — college tuition being a conspicuous exception — and it could provide a significant boost to green energy initiatives, too.

A California-based company called Advanced Rail Energy Storage (ARES) is using the power of gravity to help renewable energy utilities compete with coal and gas. The idea is to help solve the perennial problem of energy storage. Because wind and solar installations can't always generate energy on demand — sometimes it's cloudy and the air is still — green utilities need a reliable method of storing surplus energy.

There are several ways to do this using high-tech industrial batteries, flywheels, or hydroelectric facilities, but these approaches tend to be expensive and complicated.

ARES's solution? Run some old trains up and down a hill.

The company harnesses the power of potential and kinetic energy to help utilities add and subtract to the energy grid as needed. When the wind or solar farm is producing excess energy, that power is shuttled over to the adjacent ARES facility. The surplus energy is used to power repurposed electric locomotives, which in turn haul enormously heavy railroad cars to the top of a hill.

When less energy is being produced but more is needed for the grid, the railroad cars roll back down, turning potential energy back into kinetic energy by powering onboard generators with the force of their descent. The technique is similar to the regenerative braking system that is used in electric and hybrid vehicles, which routes deceleration energy to the vehicle’s battery.

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The system is also similar to existing hydroelectric (“pumped hydro”) solutions that essentially do the same things with water — pumping water uphill and capturing downhill flow. A benefit of the rail energy storage solution is that it doesn't need to be near a large source of water. That’s good for wind and solar installations, which are often located in remote areas.

It's cheaper, too. Ares contends that its rail energy solution costs about half as much as competing energy storage solutions, and has less of an environmental impact.