Grid-scale battery based on train cars and gravity

A California start-up named Advanced Rail Energy Storage (ARES) has a clever idea for storing electrical power at the most extreme scales, e.g. those of the power grid itself. It's a battery of sorts. The scheme include a really big hill and a few railroad cars. Energy to be stored in the system is first used to pull the rail cars via electric locomotives to the top of the hill, where it persists as potential energy. So long as the cars are at the top of the hill, the initial energy expended to get them up there remains trapped within the system. To recover that energy, the cars are simply lowered down the hill, turning built-in motor-generators in the process. This power is collected and then returned to the grid. The system is able to recover 80 percent of the power that it takes in, which is better than pumped-storage hydro (due to evaporation) or most batteries.

Earlier this month, ARES won approval from the Bureau of Land Management for a lease of 156 acres in southern Nevada featuring some train tracks on a hill and connectivity to the greater western US power grid. The 50 megawatts facility is not huge (enough to power 15,000 or so homes) but ARES imagines large regional facilities capable of 2 to 3 gigawatts in the future.