On 16 September 1910 the Canadian inventor Reginald A Fessenden, who is best known for his work on radio technology, published an article in the journal The Electrician about energy storage. “The problem of the commercial utilization, for the production of power, of the energy of solar radiation, the wind and other intermittent natural sources is a double one,” he wrote. “The energy of the sources must first be charged so as to be suitable in form, it must next be stored so as to be available in time.”

As we attempt to lessen the impact of climate change, Fessenden’s comments remain just as pertinent today – if not more so. That’s because many countries are reducing their reliance on fossil fuels by boosting renewable-energy output, whether solar, wind or hydro. And given that so many European nations rely on fossil fuels from Russia, this trend towards green energy is likely to speed up following the invasion of Ukraine that began in late February, having already resulted in a raft of economic sanctions and measures against Russia’s economy.

We can expect that future electric power generation in the UK will be based on variable renewables. This will be primarily wind and solar, possibly supported by nuclear. Unlike fossil fuel and nuclear generation, such renewables will, by their very nature, often produce less power than required – an extreme case being the lack of solar electricity generated at night. One of the biggest challenges for renewables will therefore be to deal with the periods when “the Sun doesn’t shine and the wind doesn’t blow”.

This situation is beautifully captured by the German word Dunkelflaute (meaning dark doldrums). It is critical that we store enough renewable electrical energy that has been produced during periods of excess generation – such as those during favourable wind conditions – for the inevitable Dunkelflaute periods that follow. But this is far from easy. And thanks to detailed studies on future electricity storage requirements and cost, we know it is not cheap either.

We will need adequate excess renewable generation capacity pre-Dunkelflaute to ensure that stored electricity is available over any such period. On a cold winter’s day in the UK, for example, the country requires at least 40 GW of electricity, which equates to about 1 terawatt-hour (TWh). If half of that comes from variable renewables, then on a challenging Dunkelflaute day we will need to have stored 0.5 TWh – assuming that the other 50% is composed of non-renewable sources of gas, nuclear and biomass.

The situation is starker still for a period of 10 successive Dunkelflaute days – a not-uncommon situation in a typical British winter – where we would need some 5 TWh of battery storage. To get an idea of the price tag, we know that the energy company InterGen is currently building a 1 GWh lithium-ion battery-storage facility at DP World London Gateway, a new port on the Thames Estuary in south-east England. It will cost about £300m to build, so a simple extrapolation would mean that having a 5 TWh capacity would be £1.5 trillion. If we depended entirely on renewable electricity, the corresponding battery storage cost would be £3 trillion. This is clearly unfeasible, so what else could we do?

Keeping the lights on

Lithium-ion technology has remained the benchmark for batteries since its discovery four decades ago. Some hope that battery costs will continue to drop – as they have done for the last two decades – or that a new, cheaper kind of battery technology will surely emerge over the next few years. However, there is a worldwide shortage of lithium for building battery storage at scale, while cobalt mining – the material that provides a stabilizing effect in lithium-ion batteries – comes at a heavy environmental price.

Another possibility for storage is hydrogen, which is produced by electrolysis from excess renewable energy generation. It can be converted into electricity through fuel cells or internal combustion engines and can also be used for a range of industrial processes. There are several major hydrogen projects under way in the UK, but it is hard to directly compare the cost of hydrogen with other large-scale storage technologies given the unknown costs of associated conversion technologies and the diverse range of applications.

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It is possible, however, to make some qualitative estimates for Dunkelflaute events. If we assume that the storage mechanism is hydrogen and it is converted into electricity by a combination of fuel cells and internal combustion engines with an overall efficiency of 50%, then we would require 10 TWh of stored energy if we are to generate half our electricity from renewables. Using a value of 50 kilowatt hours per kilogram for electrolyser efficiency, that’s equivalent to about 200 million kilograms of hydrogen. So, at a cost of £2 per kilogram that would be about £400m. If we assume an equivalent cost for the process of converting hydrogen to electricity, then the cost of storage and generation in each 10-day Dunkelflaute period would be about £800m. If there are two periods annually and the electrolysers and engines last for 30 years, the total cost would be about £50bn.

Net-zero carbon targets require almost all energy to be provided by renewable electricity. It is essential therefore that we find ways of storing renewable energy during Dunkelflaute periods, but this rather obvious issue is all too often neglected by governments around the world. The costs of either battery storage or energy storage via hydrogen are huge – and even if the costs of batteries can be reduced, big questions about the space, security and safety of such storage installations remain. Decisions are urgently required about the way forward since electricity storage must evolve alongside plans for variable renewable energy.