THE problem with wind power is that is cannot always be relied upon. The wind—and other transient, environmental energy sources such as solar—must either be used when it is harvested or stored expensively in batteries or specially designed hydroelectric schemes that use the resulting energy to pump water uphill. Alternatives would be extremely welcome. Alexander Slocum, of the Massachusetts Institute of Technology, thinks he has one. Observing that the fashion among wind-power fans is to build turbines out at sea, where the wind blows strongest, he proposes a pumped-storage system that uses seawater.

Dr Slocum's scheme involves anchoring a hexagonal array of hollow, 31-metre-diameter concrete spheres to the ocean floor at a depth of approximately 350 metres. Floating turbines would be tethered to these spheres and surplus power from these turbines, generated during periods of high wind and low electrical demand, would be used to pump water out of the spheres, evacuating the central chamber. When the wind faltered or the lights went back on, water forced into the central chamber by the pressure of the surrounding ocean would pass through a turbine and generate electricity. Each sphere would provide a five megawatt turbine with four hours of storage capacity.

An all-concrete structure would simplify the manufacturing process and allow experience garnered exploiting the oil fields of the North Sea to be used, says Dr Slocum. This knowledge, he thinks, should allow the structures to have lifetimes of about four decades. The load-smoothing effect of the system will also allow the transmission lines linking the turbines with the shore to be cheaper (because they will not have to cope with peak capacity) and deliver power that, being better matched to demand, is worth more. In combination, he thinks, these things will bring the price of offshore wind power into line with that of its onshore cousin.

They will also make an electricity grid supplied primarily by renewable sources a technically feasible proposition by reassuring customers that the lights will never go off. His system would, for example, make it possible for a bank of several hundred thousand offshore turbines dotted along the 800km of America's eastern seaboard from Maine to New Jersey to meet the electricity needs of the eastern United States as reliably as existing coal-based generators. It would not be as cheap as coal, of course. Nothing is. But it would eliminate the argument that “alternative” is not merely expensive, but also unreliable.