Tony Roulstone and Stan Zachary – Written evidence (LES0010)



Tony Roulstone. Department of Engineering, University of Cambridge.

Stan Zachary. Heriot-Watt University and University of Edinburgh.


In support of the Royal Society’s report on long duration energy storage, we have been studying this topic for the last 4 years and have published our results in the recent The Royal Society Report[1] as well as conference papers and academic papers including [Cosgrove, Roulstone & Zachary (2023)][2]. These two main sources support our responses to your questions 1-4, which are given below.



1. How much medium- and long-duration energy storage will be needed to reach the Government’s goal of a fully decarbonised power grid by 2035 and net zero by 2050, and by when will it need to be ready?


Renewable energy systems that exceed energy shares of about 50% lead to mistiming of supply and demand. This leads to large gaps in supply which could in principle be filled by CCGT & CCS, or by a suitable mixture of energy storage technologies. These low or zero carbon supplies have the scale to meet in the need in a way that other technologies do not.


If long duration energy storage has durations longer than a few hours, our modelling shows that GB would require about 70 TWh of Medium and Long duration storage for a net-zero energy system in 2050 with the power and size values in Table 1. These store powers and sizes are for 600 TWh of demand in line with the 2019 Climate Change Committee report - which is now viewed as central estimate for 2050.


Table 1. Store sizes and powers optimised for reliability- fully renewable system with 30% overcapacity[3]

Store periods



Short - a few hours

20 GW

100 GWh

Medium - week

60 GW

4.8 TWh

Long - seasons & years

60 GW

66 TWh


These sizes are optimised for system reliability and capital cost. They depend on the solar/wind mix being close to 20/80 and there being 30% generation overcapacity. For different solar/wind mixes and lower levels of overcapacity the energy storage requirements would be larger.


For a more mixed power system with baseload supplies such as nuclear or bio-energy supply, store sizes and powers would be somewhat lower - for 30% baseload a total of 45 TWh of storage of Medium and Long duration storage would be needed.


In 2035, when the power system is planned to be fully decarbonised storage needs would be at a least have these amounts.


Even with optimisation such a system, though it seems feasible in the UK1, will be of unprecedented size – a hundred times the size and power of anything built to date:

Li-ion battery 0.75 GW: 3 GWh, CAES 0.5 GW: 3.0 GWh, Hydrogen store 82 GWh


Most of the energy storage is required to address, seasonal differences in supply and demand and more importantly extreme weather events such as extended periods of low wind in winter that cannot be offset by higher solar output. Such events are infrequent but are not evenly distributed in time. Therefore they require very large energy stores or similar high power complementary (zero-carbon) supplies.


These energy storage effects can be studied only analysing decades of weather data. Any shorter periods of analysis, either short-snap-shots or single years of data as used by many economic models could result in underestimating the need for energy storage. Our analysis used 37 years of hourly weather data. It provides similar results to US and German studies that also used several decades of weather data.


The timing of provision of this very large amount of energy storage is determined by the growth in demand as other sectors in the economy decarbonise by transferring demand to electricity. Also, by the retirement of flexible supplies such as CCGT, that currently complement variations in solar and wind supply. Government plans are for unabated fossil-fuelled generation, including CCGT, to be closed by 2035, energy storage or high power CCGT & CCS dedicated to the complementary (low utilisation) supply role will be required by then.



2. How sensitive is the amount of storage needed to assumptions about the future balance of supply and demand on the grid?


Different demand assumptions for 2050 range from 500-750 TWh. As a basis for scoping the problem, the energy storage requirements could be scaled in proportion to these demand values.


The provision of zero-carbon baseload power has the effect of reducing the scale of the wind and solar systems and therefore the range of the power and energy fluctuations experienced by the Grid. Nuclear power, bio-energy and CCGT & CCS (operated at high levels of utilisation) could in principle provide this baseload supply, though it is likely that all of these would have somewhat higher energy costs than solar and wind (plus their necessary back-up) supply. Nevertheless, a proportion of baseload supply would provide some diversity of supply and in times of very low wind, would ensure a minimum level of power was available. These supply technologies can in principle operate flexibly complementing variable renewable supplies. Their high capital costs would render this approach uneconomic.


In comparing renewable costs with baseload supply costs, the extra costs of making intermittent renewables reliable and the higher grid costs of transporting and balancing the renewable power would need to be taken into account. Though these extra costs have significant uncertainties, it is likely that they would at least double the stand-alone energy costs of solar plus wind supply.


Interconnectors with Europe will play an important part in future renewable energy system management. They spread the times of peak demand and they even out, to some degree, the variations in renewable supply - both solar and wind. One of key issues with a Grid that is dependent on a high share of renewables is the very high variable power requirements, more than 70 GW (for 70% renewable share). Current plans are for the UK to have 18 GW of interconnectors with Europe and scope for expansion to 30 GW. Even at this higher level, interconnectors could not always meet the missing supply of a largely renewable energy system.


Further, for UK extreme weather events which are also likely to affect many countries in Northern Europe at similar times, there may be questions in future about whether the extra supply would be available when called for.


Similarly, demand management will be important for an energy system that is largely supplied by solar and wind. Experts who gave evidence to the Royal Society’s Working Group indicated that demand side response could reduce demand up to 20 GW for several hours. In more extreme circumstances industrial demand could be cut further, by shutting down operations and production.


Though important, demand side response will not obviate the need for large and higher power energy storage and for much long term mismatches in supply and demand.


Though our studies used past weather (1980-2017), there is a view that Climate Change could in the future makes things worse. Evidence provided to the Working Group from the Met Office, based on their modelling of Climate Change, indicates that the changes in UK weather to 2050 are not larger than those experienced in the past decades - such as the weather used for our studies. There is uncertainty on this forecast both about future behaviour and about models used so far into the future. Nevertheless, the Met Office considers these uncertainties lower than the variability of the historical weather.


Profiles of future demand will be different from today. They will include new demands such as EV and heat pumps and perhaps increased used of air-conditioning as well as new demands from industry. In our work we used an hourly model of future demand that was prepared for the CCC report in 2019. It provides an estimate of the demand for a single year. More than one version of this model was used. Not included in our models, there are feedbacks between weather and demand. For example cold winter weather and hence higher heating demand is correlated with low wind speeds. Some work has been done by UCL and IC that indicates this effect could increase the amount of energy storage required, but by less than 10%.



3. Which technologies can scale up to play a major role in storage?


Energy storage technologies are covered in some detail in the Royal Society’s Report. There are many energy storage technologies, some developed an in use and others at a much more early stage of development. They can be split into three groups by their means of storage and similarly by their economics:


Large quantities of Li-ion batteries are being produced globally for the EV market with resulting falling cost. It is likely that the UK will need several tens of GWh of short term storage, for which Li-ion batteries are the natural choice, until other batteries demonstrate much lower storage costs.


Compressed Air Energy Storage (CAES) has been demonstrated in the US and in Germany at hundreds of MW scale. These first generation projects have lower efficiency (~50%) because they do not capture and re-use the heat of compression. Higher efficiency ~70% is possible with heat capture (adiabatic CAES), but this needs to be demonstrated. Its lower energy storage costs of CAES are dependent on the use of large underground high pressure caverns, similar to those proposed for hydrogen.


Hydrogen energy storage uses existing technologies of hydrolysis and power conversion. Its economics depends critically on the use of large high pressure underground salt caverns. Such caverns have been used for storing hydrogen for many years in the US and the UK. There are suitable salt formations in East Yorkshire and detailed studies support the idea that storing many tens of TWh of hydrogen in the UK is technically feasible.


There are a range of costs estimates for hydrogen systems based on data extrapolated from existing large hydrogen stores and detailed studies of planned future systems[4]. Power related costs for hydrogen are high because they include: large electrolysers, compressors, thermal stores and turbines or fuel cells, which have not been deployed at scale. The cost of storage volume is much lower than the alternatives - potentially a hundred times lower than mature battery costs. The immaturity of hydrogen energy storage technology means the uncertainty of their significant.


Except for batteries systems which are well characterised, there is large range between current future costs of storage. These differences are due to the small amount of experience building such systems. Also, the costs are affected by the scale of installation, both the processing equipment and importantly the cost of storing compressed air or hydrogen. Low storage cases require very large caverns (each storing up to 3,000 tn of gas), located up to 1000 m below the surface, operating at high pressure (in excess of 150 bar).



4. What policy support is currently in place to support deployment of storage technologies? Is it sufficient to support deployment at scale?


Policy support and markets for medium and long duration energy storage are nascent topics. The scale of investment required - many tens of billions of pounds - and their role as a back-up system for the Grid, make market mechanisms problematic for medium and long duration energy stores.


Energy storage requires high powers which will be used infrequently and also require very large stores, some of which may not be called upon for several years. Because their utilisation is low, the cost of energy supplied to back-up solar and wind will be high (£200-600/MWh). Though these energy costs are high because they only affect part of the overall supply, their costs are can be combined with solar, wind and any other supply costs into a system cost of renewables made reliable. Energy storage costs are probably lower than CCGT & CCS systems operating in the same back-up low utilisation mode.


It is very hard to see how a conventional revenue based private sector business case can be constructed for such Grid back-up systems. Some form of capacity mechanism or public-private partnership to guarantee payments over many years, or direct investment will be required.


Our work has shown that several different types of energy store technologies will be required for an optimal system. These different stores will need to be coordinated in their operation so that energy is retained and made available when required for future extreme weather events. As well as coordination of operation, some form of forecasting and scheduling would reduce the overall costs and would ensure grid reliability. It seems this will be part of the role of ESO because both the transmission grid and storage need to be managed together as an integrated system in order to keep the electricity system balanced. Also, energy storage will significantly affect Ofgem as the energy economic regulator ensuring that these essential back-up system can be financed. Energy companies as the suppliers and distributors of power will need to provide funds, build and operate these large items of national infrastructure.


The current method of ensuring energy security makes sure that the maximum forecast demand is covered by supply power capacity that will be available, plus a small margin. This is about providing enough power (GW). When the power system is dominated by solar and wind supply the issue will be the provision of enough back-up energy (GWh) to cover the expected power deficit and its duration. This view of system reliability is completely new and will require some organisation to assume the responsibility for ensuring sufficient back-up supplies of energy are built, charged and are available. This will require new reliability targets as well as new system adequacy responsibilities.


Concluding remarks


The launch of the Royal Society’s Report on Long Duration Energy Storage demonstrates that the future energy system dependent on solar and wind energy need to be complemented by energy storage. The scale of energy storage that will be required is of a completely different order from the Battery Energy Storage Systems that are being install today in the UK or elsewhere. These very large requirements are a direct consequence of the commitment to net-zero by 2050 and the linked policies of de-carbonisation of the economy that include banning fossil fuel for transport from 2030 and power by 2035.


Once the scale of the need to balance variable solar and wind supply is accepted the Government should make clear the responsibilities for meeting the need. Once the responsibilities are defined, a plan, the means, the mechanisms and the resources for delivering energy storage can be developed. The plan will also addresses the important issues of supply chain, funding mechanisms, markets and skills.


Energy storage is both integral to net-zero for the UK and urgent to complement the roll-out of solar and wind energy and the retirement of fossil fuel generation over the next 15 years. Action on energy storage is required now.


11 September 2023



[1] Large-scale electricity storage/ Issued: September 2023 DES6851_. ISBN: 978-1-78252-666-7 © The Royal Society

[2] Cosgrove, Roulstone & Zachary (2023) Intermittency and periodicity in net-zero renewable energy systems with storage. Renewable Energy 212 (2023) 299–307

[3] Zachary (2021). Scheduling and dimensioning of heterogeneous energy stores, with applications to future GB storage needs. Nov 21.

[4] Roulstone (2022). UK Multi-year Renewable Energy Systems with Storage - Cost Investigation. University of Cambridge, April 2022.