There is truth in Professor Underwood’s assertion (Letters, 16 May) that nothing can surpass the “round trip” efficiency of lithium-ion batteries from, for example, solar input to final user’s output. But in focusing on this undoubted advantage he omits the overriding issue of energy storage at very much larger scales. It is this concern which has driven the Institution of Mechanical Engineers (not in the “gas industry’s pocket”, by the way) to take a serious look at hydrogen, which used to be a substantial component of our former “town gas”, derived rather filthily from coal, but which can now can be derived very cleanly from solar and wind power, directed through the water-splitting magic of modern electrolytic machinery. Such renewably generated hydrogen could supply energy storage at scales many times beyond which even the largest battery systems could attain, and could do so both in the UK and in diverse economies throughout the world. Batteries will always be needed for specific uses, but in order to displace the carbon-laden fossil fuels which now imperil climate, ocean and the whole biosphere something rather different must be adopted – something storable at all scales, transmissible, fully functional as a fuel, and climate-neutral. Only hydrogen fills this particular bill.

Mike Koefman

Director, Planet Hydrogen, Manchester

• Professor Underwood correctly asserts that the efficiency of a Li-ion and heat pump system in terms of heat generation is far better than electrolysing water to make hydrogen. But the purpose of storing hydrogen was, it seems, to smooth out the supply of electricity from renewables in dead periods, not generate heat per se. Electricity generation is usually provided by turbines which are driven by steam at high temperatures. I may be wrong, but I thought heat pumps did not generally reach much above 80C and would not be suitable for electricity generation.

There is a strong case for both. It is not difficult to imagine we could create a hydrogen storage system for electricity generation, preferably away from populated areas. Individual houses might well benefit from local storage and heat pump systems, but that raises two important questions: where will the supply of lithium and batteries needed come from (which is also infrastructure, I suggest), and will heat pumps operate from the air or ground? Recycling heat from the atmosphere seems appropriate (cooling the outside while warming the inside) but ground-based heat pumps may lead to unexpected consequences (frozen water pipes or cracks) when many might be employed. Surely it is time for joined-up thinking between government, organisations such as IMechE, universities and the public to plan our future energy strategy, not leave it, as usual, to those with vested interests.

John Ellis

Tavistock, Devon

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