The article ‘Novel offshore floating system’ (August 25) had a description of a concept for large-scale offshore renewable energy storage.

The point of the device was said to be “solving” the very real problem of supply-demand mismatch when renewable sources, most of which have a variable output, have to be matched with grid demand. Wind (electrical) energy surplus at a given time is stored in a “hydro-pneumatic accumulator”.

Two parameters are not mentioned: the efficiency of storage and, on the assumption that the energy will eventually be sent to the grid, the efficiency of reconversion.

That apart, there may be a simpler and, in view of fairly near-future developments in transport, more rewarding form of ‘storage’: using the ‘surplus’ electricity to electrolyse water to produce hydrogen, for use in fuel cells now coming into favour to power clean transport and also in static applications. Mechanics and process efficiencies are well-known and shore-based electrolysers can have gas storage and dispensers alongside. Our small size also means we would not need an extensive hydrogen distribution network.

Despite much interest in fuel cells in the first years of this century, there was a pause after 2005. One fuel cell car on the market is the Toyota Mirai (above), about to undergo extensive trials in California, where there is a skeleton hydrogen distribution network.

Hyundai are evaluating their iX35 fuel cell car in a 50-vehicle car hire scheme in Munich but the primary fuel in this case is ethanol, from which the hydrogen for the solid oxide fuel cell is produced on board.

In both cases, range is reckoned to be about 600 kilometres, better than that of battery-powered electric cars on the market. Of course, this latter case does not provide a ‘storage system’ for surplus RE electricity.