Sustainable, Affordable and Viable Compressed Air Energy Storage (SAVE-CAES)

Project SAVE-CAES is all about developing large-scale long-duration energy storage that will enable the UK to be powered largely (and possibly completely) from renewables. That energy storage must be affordable, sustainable and large-scale. Compressed air energy storage (CAES) has the potential to meet all these critically-important criteria. Developing such storage is probably the biggest single challenge standing in the way of "Net-Zero" for the UK by 2050.

Offshore wind around the UK is a remarkable resource for a future zero-carbon UK electricity system. If we were to exploit all of the area that could feasibly be imagined, UK offshore wind could produce about 2000TWh of electrical energy every year - more than 5 times greater than the amount of electricity we presently consume in one year. Electricity usage will increase, of course, between now and 2050 - possibly increasing from ~350TWh each year to ~1000TWh annually. However, it is perfectly feasible that we can generate all of this electricity from wind.

Solar power will also play a key role in powering the Net Zero UK but there are straightforward reasons why this will provide only about 20% of our power in the future. The strongest of those has to do with seasonality: solar on an average day in mid Summer is 9 times higher than on an average day in mid Winter, however our energy demand in Winter is higher than that in Summer. Happily the wind is also seasonal and it typically delivers 2.3 times more energy on an average mid-Winter day than it does on an average mid-Summer day. Nuclear power will also have some role. Opinions differ on how substantial that role will be but that is not very important for the purposes of understanding or justifying this research proposal.

The key problem with having a country powered largely from inflexible low-carbon sources is that demand and supply must be matched and demand is relatively "inelastic". This means that proportionately small changes in the cost of electricity have very small influence on how much electricity that is consumed. Quantitative assessments of how much we will be paying for our electrical energy by 2050 suggest that less than half will be made up of the direct cost of generating the actual units of electrical energy. The larger cost will be connected with providing the flexibility - the ability to match up supply and demand. Different researchers predict different proportions, but the consensus is that flexibility costs will be the dominant ones.

CAES is one of the most promising sets of options available in the UK for storing very large quantities of (wind or solar) energy over periods of tens of hours - possibly up to 100 hours. CAES has the potential to combine good performance (upwards of 70% round-trip efficiency) with relatively low costs (<£2/kWh). There are two different grid-scale energy storage plant which store compressed air in the world - one at Huntorf in Germany and the other in McIntosh, Alabama - however, these plants also store fossil fuel. Many commentators make the serious mistake of extrapolating from these to estimate what CAES can possibly do. Project SAVE-CAES sets out to apply fundamental engineering science to determine what a well-designed CAES plant without fossil fuel addition could possibly do.

SAVE-CAES is a project filled with novelty. Pressurised air will be stored in salt caverns that are either offshore or at the coast. The project will explore the use of isobaric storage of the pressurised air and the management of concentrated brine (salt-water) for pressure regulation. It will also explore ultra-high-pressure air storage (for best value per cubic metre of cavern). It will also explore the potential for exploiting relatively mild geo-thermal heat during the re-expansion of the air and the possibility that some wind turbines might be deployed directly as last-stage compressors for charging the energy stores.