Remorseless German and Austrian boffins have a cunning new plan which could be good news for cutting down on fossil fuel use: they can make "synthetic natural gas" using electric power, water and carbon dioxide.

"Our demonstration system in Stuttgart splits water using electrolysis. The result is hydrogen and oxygen," explains Dr Michael Specht of the Zentrum für Sonnenenergie- und Wasserstoff-Forschung (Solar energy and Waterstuff [Hydrogen] Research centre - ZSW).

"A chemical reaction of hydrogen with carbon dioxide [then] generates methane – and that is nothing other than natural gas, produced synthetically."

The process is thought to scale up well, with Specht and his colleagues planning to fire up a "double-digit megawatt range" unit by 2012.

The kit is intended to solve one of the great problems of renewable power supplies - wind, solar and tidal - which is that their electric output is variable and uncontrolled, bearing no relation to demand at the time. Storing electricity on a large scale can be done by pumping water uphill for later use in hydropower turbines, but this is expensive even where suitable geography exists.

Even with the present, fairly low fractions of wind and solar power seen on the linked grids of Germany and adjacent countries, sudden pulses of renewables output often result in negative electricity prices as renewables operators pay to get their power onto the grid (and so claim the subsidies which are the principal source of their revenues).

"Surplus wind and solar energy can be stored in this manner. During times of high wind speeds, wind turbines generate more power than is currently needed. This surplus energy is being more frequently reflected at the power exchange market through negative electricity prices," confirms Specht's colleague Dr Michael Sterner.

This is not a good situation for renewables, but scope for expansion of pumped storage at reasonable cost is limited in Germany. Hence the plan to store surplus electricity as gas, rather than by lifting water.

At the moment, pumped storage has better efficiency - 70 per cent or better of the electricity so stashed can be recovered, compared to the ZSW synthi-natural gas figure of "more than 60 per cent".

"In our opinion, this is definitely better than a total loss," says Specht, modestly.

The synthi-gas plan has other things going for it, however, which pumped storage doesn't offer. First off you can store a lot more power, as massive gas storage is already available within the gas grid - to the tune of 200 terawatt-hours in Germany, apparently. That's enough to cover months of present-day German gas demand, or alternatively to generate more than two months' German electricity assuming the gas is used in efficient combined-cycle turbine generators.

Thus Germany could potentially store enough renewable juice in gas form to last it through a days-long midwinter calm of the sort which feasible amounts of pumped storage could probably never cope with.

For nations like Germany and the UK which make massive use of gas in the home and industry, too, supplies of natural gas made sustainably are potentially a secure option compared to Russian imports, though they could never compete on price at the moment.