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THE easiest way to squirrel electricity away in times of plenty, for use when it is scarce, is to pump water uphill with it. Such pumped storage is widely employed where local geography and hydrology permit, but it does need two basins, at different heights, to act as reservoirs, and a supply of water to fill them. At least one of the basins is likely to have to be artificial. The two must be connected by a tunnel that lets water flow between them. And the tunnel must house turbines attached to electrical devices that can do double duty—as motors to turn the turbine blades when they are pushing water from the lower reservoir to the upper one, and as generators when the blades are rotated in the opposite direction by an aqueous downrush after the upper sluices are opened.



Where geography does not favour pumped storage, though, the search is on for alternatives. These range from giant batteries, via caverns filled with compressed air, to huge flywheels made of carbon-fibre composites. But one firm looking into the matter eschews all these. It has stuck with the logic of pumped storage, which is to move large amounts of matter up and down hills. The difference is that in its case the matter is solid.

The firm in question calls itself ARES, which stands for Advanced Rail Energy Storage. A more apt figure from Greek mythology than the god of war, though, might be Sisyphus, who was condemned by the gods to push a rock to the top of a mountain, only for it to roll back down again so that he had to repeat the punishment for eternity. ARES does indeed push rocks uphill, only to let them roll down again.

The rocks stand in for the water in a pumped-storage system. They are carried up- and downhill by a train that is thus the equivalent of the turbines. The track the train runs on is equivalent to the tunnel. And the motors that drive the train act, like the electrical kit of a pumped-storage turbine, as generators when they run in reverse as the train rolls backwards downhill, pulled by gravity.



ARES built a prototype of this arrangement in 2013, near a wind farm in Tehachapi, California. Linking a storage system with an intermittent source of supply such as a wind farm is useful, because it can be employed to bolster the farm’s output when the wind is not strong enough, a process called load balancing.



The prototype proved the principle, and now the company has bigger plans. In March it received approval from America’s Bureau of Land Management to lease land to build a track near Pahrump, Nevada. This would run larger trains than those at Tehachapi, and these would carry their rocks in concrete boxes, rather than loose. Once at the top of the track, the boxes would be raised by jacks built into the wagons carrying them, rotated and then lowered back down onto supports on either side of the track, so that they straddled the track above the height of a train, like bridges. Freed of their burdens, the trains would then run back downhill to fetch more loads. When the time came to generate power, the process would be reversed.



The hill ARES has chosen has a gradient of about 8%. The track itself is just under 9km (about 5½ miles) long. The company estimates that its proposed system will be able to store 12.5 MWh of energy, and deliver it back to the grid at a rate of up to 50MW. That is still small compared with pumped storage (the Dinorwig facility in Britain, for example, has a capacity of 10.8GWh and a maximum output of 1.8GW), but ARES’s engineers think it is enough to make commercial sense, at least in principle. And if principle turns to practice, it can be enlarged.



Such a Sisyphean solution is unlikely to beat pumped storage in places where that is possible, but in parched landscapes like Nevada’s it has every chance of doing so. And, since deserts often host power stations that rely on the renewable but intermittent fuel of sunlight, this might give it quite a comfortable niche in a world where using fossil fuels to generate electricity is increasingly frowned on. At the moment, ARES’s plan is simply to draw power from the grid when it is cheap and sell it back when it is expensive. But the logical end of the line for such a railway is as a load-balancer for local solar-power stations.