Energy Storage:  Horses for courses.

By Prof Seamus Garvey, Neville Rieger Professor of Dynamics, University of Nottingham.

Net Zero is achievable, affordable and essential – for the UK and for most other countries. A small minority (Mino henceforth, for brevity) still sees fit to question those three facts. The arguments upon which Mino bases its objections weaken by the day.  Two points trotted out repeatedly by Mino relate to offshore wind in particular: (1) the energy produced by windfarms in the UK so far has been moderately expensive and (2) the wind resource does not always coincide with the occurrence of high energy demand. Point #1 is valid – but Mino extrapolates incorrectly from this point to conclude that offshore wind must necessarily be expensive in future. The strongest evidence that wind power and solar power deliver low cost per unit of energy collected comes from the ever-increasing occurrence of cases where hard-nosed businesses are installing solar panels and wind turbines to supply their energy needs. This report is a good case-in-point. Point #2 is commonly misunderstood. Of course, no fixed mixture of solar panels and wind turbines can magically deliver output energy exactly matching demand. Mino is correct to assert that IF we were to attempt to use lithium-ion batteries to provide all of the energy storage suitable for reconciling supply and demand in future zero-carbon energy system, the costs would be unacceptable. In energy storage, like everything else, there are horses for courses.

A report entitled Large-scale Electricity Storagewas published in September 2023 by the Royal Society (R-S). This examined whether a Net Zero UK electricity system UK could be achieved affordably using energy storage to provide all flexibility. It was recognised that spreading power between countries via interconnectors does help. Similarly, managing demand can help – but both of these measures have limitations. Prof. Sir Chris Llewellyn Smith (a former Director-General of CERN) led the R-S report development and commanded a formidable team of experts in the field of clean energy to get the best available evidence. The R-S report revealed some striking findings:

• To understand the problem properly, it is necessary to model several decades of data for both demand and supply characteristics. The R-S work used 37 years and the Met Office recommended 60 or more. No other substantive report had taken this long view.
• Both wind power and solar power are strongly seasonal and by mixing them optimally, we get a system in which net supply (averaged over a few weeks) varies approximately like net demand. That optimum calls for a mix of energy from wind:solar of about 80:20
• The energy collected from a fleet of wind turbines in a year has strong variation. In a future Net Zero energy system, this year-to-year variability drives a realisation that substantial amounts of energy might be stored for several years at a time.
• In a cost-optimal Net Zero UK, several tens of TWh of electricity storage are called for. It is realistic for the UK to implement most of this storage using hydrogen stored in salt caverns. As well as being rich in wind resource, the UK is fortunate also to have large deposits of bedded salt suitable for caverns.
• The costs of implementing storage using hydrogen stored in caverns is about 0.5% of the cost of lithium batteries for the same output electrical energy. The salt caverns would credibly last a century or two. Implementing, say, 40TWh of such storage might cost us ~£30bn – much less than one year of present UK energy costs.

Perhaps predictably, Mino sought to discredit the report and challenged various aspects without any success and with several striking instances of desperation. An assertion around how much energy would be required to realise 1kWh of electricity storage capacity was incorrect by a factor of 2,400!

Mino is absolutely right about one thing though: the UK taxpayer and energy consumer should expect clear and quantitative explanations behind choices made on our behalf.

It is beyond doubt that the future Net Zero UK will draw much of its primary energy input from wind power. Agglomerating renewable energy supply and electricity demand over the country and then scaling generation and storage such that they can meet demand, we find that >90% of all energy coming from energy storage would emerge in periods between 6 and 200 hours. The R-S report found we can operate a Net Zero UK using hydrogen in caverns for storage and achieving costs well below £100/MWh. Cost reductions of 5%-10% are possible if we blend in compressed air, liquid air or pumped thermal energy storage technologies. Then the spend on these technologies would be approximately the same as the spend on the hydrogen technologies and roughly 65% of the energy emerging from storage would come from these. Our energy storage needs and opportunities will be driven mainly by wind power characteristics. The solutions we choose should match those. The nature of resources in the UK determines the course we will take towards decarbonising power. We should be careful to select the appropriate energy storage horses to compete on that course.