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Large-scale electricity storage2 LARGE-SCALE ELECTRICITY STORAGE Large-scale electricity storage Issued September 2023 DES8702 ISBN 978-1-78252-666-7 The Royal Society The text of this work is licensed under the terms of the Creative Commons Attribution License which permits unrestricted use, provided the original author and source are credited. The license is available at creativecommons.org/licenses/by/4.0 Images are not covered by this license. This report can be viewed online at royalsociety.org/electricity-storage Cover image iStock.com / Bjoern Wylezich.LARGE-SCALE ELECTRICITY STORAGE 3 Contents Executive summary 5 Major conclusions 5 Modelling the need for storage 6 Storage technologies 6 Average cost of electricity with all large-scale storage provided by hydrogen 7 Addition of other types of store 7 Market and governance issues 7 Caveats and avenues for further work 7 Chapter one Introduction 9 1.1 Scope of this report 9 1.2 Supply and demand in a net zero context 9 1.3 Storage 11 1.4 Cost considerations 15 Chapter two Electricity demand and supply in the net zero era 16 2.1 Introduction 16 2.2 Future electricity demand in Great Britain 17 2.3 Weather, wind and sun 17 2.4 Matching demand and direct wind and solar supply 19 2.5 Residual demand, energy and power 23 2.6 Generating costs 27 2.7 Demand management 28 Chapter three Modelling the need for storage 29 3.1 Introduction 29 3.2 Modelling and costing with a single type of store 29 3.3 Modelling and costing with several types of store 32 Chapter four Green hydrogen and ammonia as storage media 34 4.1 Introduction 34 4.2 Hydrogen and ammonia production 34 4.3 Transport 38 4.4 Storage 38 4.5 Electricity generation 41 4.6 Safety 44 4.7 Climate impact 44 Chapter five Non-chemical and thermal energy storage 45 5.1 Advanced compressed air energy storage ACAES 45 5.2 Thermal and pumped thermal energy storage 48 5.3 Thermochemical heat storage 49 5.4 Liquid air energy storage LAES 50 5.5 Gravitational storage 50 5.6 Storage to provide heat 514 LARGE-SCALE ELECTRICITY STORAGE Chapter six Synthetic fuels for long-term energy storage 52 6.1 Electro-fuels 52 6.2 Liquid organic hydrogen carriers LOHCs 52 Chapter seven Electrochemical and novel chemical storage 54 7.1 Electrochemical storage 54 7.2 Novel chemical storage 59 Chapter eight Powering Great Britain with wind plus solar energy and storage 60 8.1 Technology choices 60 8.2 Additional costs 60 8.3 Provision of all flexible power by a single type of store 63 8.4 Multiple types of store 67 8.5 Use of natural gas with CCS 70 8.6 Possible uses and value of surplus electricity 72 8.7 Contingencies against periods of low supply 72 8.8 Different levels of demand 73 8.9 Other studies of the cost of storage in Great Britain 74 Chapter nine The Grid, electricity markets and coordination 75 9.1 The grid 75 9.2 Markets issues 75 9.3 Possible reforms 76 Chapter ten Conclusions, further steps and opportunities 78 10.1 Conclusions 78 10.2 Further steps 81 10.3 Demonstrators, deployment and opportunities 83 Annexes Annex A Glossary and abbreviations 84 Annex B Contents of supplementary information 89 Acknowledgements 92LARGE-SCALE ELECTRICITY STORAGE 5 Ex ECuTI v E Su MMARY Executive summary a Northern Ireland is excluded from the study as its electricity grid is integrated with that of the Republic of Ireland. b This is the thermal energy content of the stored energy expressed in terms of the Lower Heating Value – see the Glossary. The UK Government has a stated ambition to decarbonise the electricity system by 2035 and is committed to reaching net zero by 2050. As Great Britain’s electricity supply is decarbonised, an increasing fraction will be provided by wind and solar energy because they are the cheapest form of low-carbon generation. Wind and solar supply vary on time scales ranging from seconds to decades. However high the average level of supply might be, there will be times when wind and solar generation is close to zero and periods when there is enough to meet part of but not all demand, as well as times when it exceeds demand. To ensure that demand is always met, the volatile wind and solar generated electricity that is fed directly into the grid must be complemented by other flexible low-carbon sources, and / or using excess wind and solar energy that has been stored. The excess could be stored in a variety of ways, for example electrochemically in batteries, gravitationally by pumping water into dams, mechanically by compressing air, chemically by making hydrogen, or as heat. This report considers the use of large-scale electricity storage when power is supplied predominantly by wind and solar. It draws on studies from around the world but is focussed on the need for large-scale electrical energy storage in Great Britain a GB and how, and at what cost, storage needs might best be met. Major conclusions In 2050 Great Britain’s demand for electricity could be met by wind and solar energy supported by large-scale storage. The cost of complementing direct wind and solar supply with storage compares very favourably with the cost of low-carbon alternatives. Further, storage has the potential to provide greater energy security. Wind supply can vary over time scales of decades and tens of TWhs of very long- duration storage will be needed. The scale is over 1000 times that currently provided by pumped hydro in the UK, and far more than could conceivably be provided by conventional batteries. Meeting the need for long-duration storage will require very low cost per unit energy stored. In GB, the leading candidate is storage of hydrogen in solution-mined salt caverns, for which GB has a more than adequate potential, albeit not widely distributed. The fall-back option, which would be significantly more expensive, is ammonia. The demand for electricity in GB in 2050 is assumed to be 570 TWh/year in most of this report. In principle it could all be met by wind and solar supply supported by hydrogen, and some small-scale storage that can respond rapidly, which is needed to ensure the stability of the transmission grid. With the report’s central assumptions, this would require a hydrogen storage capacity ranging from around 60 to 100 TWh b depending on the level of wind and solar supply. The average cost of electricity that is available to meet demand varies very little over this range as the rising cost of wind and solar supply is offset by the decreasing cost of the storage that is needed. 6 LARGE-SCALE ELECTRICITY STORAGE Ex ECuTI v E Su MMARY Although some hydrogen or ammonia storage will be needed, it is quite likely that a portfolio of different types of storage would lower the average cost of electricity. Including steady nuclear ‘baseload’ supply would increase costs, unless the cost of nuclear is near or below the bottom of the range of projections made by the Department for Business, Energy and Industrial Strategy BEIS and / or the costs of storage are near the top of the range of estimates in this report. The addition of bioenergy with carbon capture and storage generation BECCS would lower the cost if it attracts a carbon credit of order 100 / tonne CO 2 saved or more, but it could not provide GB with more than 50 TWh/year without imports of biomass. Using natural gas generation equipped with carbon capture and storage CCS to provide flexibility, instead of storage, would lead to unacceptable emissions of CO 2 and methane, and also to higher costs. Used as baseload, it would only lower costs appreciably if added in amounts that would lead to unacceptable emissions; the future price of natural gas is lower than expected; and storage costs are high. Using a combination of storage and gas plus CCS to provide the flexibility required to match wind and solar supply could lower costs significantly, without necessarily leading to unacceptable emissions. Whether it would lower costs depends on the costs of storage, wind and solar power, and gas plus CCS, the price of gas and the carbon price. It would not remove the need for large-scale long-term storage, although it would reduce the required scales of storage and wind plus solar supply. While it would provide diversity, it would expose GB’s electricity costs to fluctuations in the price of gas, and increasing reliance on imports as GB’s gas reserves decline. Modelling the need for storage To quantify the need for large-scale energy storage, an hour-by-hour model of wind and solar supply was compared with an hour-by- hour model of future electricity demand. The models were based on real weather data in the 37 years 1980 to 2016 and an assumed demand of 570 TWh/year. Thirty-seven years is not enough to provide a full sample of rare weather events which can seriously affect the supply of wind-generated electricity. Contingency is added to allow for this, and for the possible effects of climate change. Studies based on less than several decades of weather data are liable to very seriously underestimate the need for storage, and overestimate the need for other sources of flexible supply. These under/overestimates are especially large in studies that look only at individual years rather than sequences of years or examine selected periods of high stress. Storage technologies The contents of stores with large capital costs per unit of energy stored have to be cycled frequently in order to recover the investment. The storage technologies considered in this report can be grouped into three categories according to the typical time in which their contents must be cycled 1. Minutes to hours conventional non-flow batteries; 2. Days to weeks flow batteries, advanced compressed air energy storage, Carnot batteries, pumped thermal storage, pumped hydro, liquid air energy storage; or 3. Months or years synthetic fuels, ammonia, hydrogen. Stores in category one are generally more efficient than those in two, which are more efficient than those in three. Higher efficiency can compensate for higher costs depending on how the stores are used. LARGE-SCALE ELECTRICITY STORAGE 7 Ex ECuTI v E Su MMARY Average cost of electricity with all large-scale storage provided by hydrogen A case in which all demand is met by wind and solar energy supported by hydrogen storage, plus 15 GW of batteries used to stabilise the grid, was analysed and used as a benchmark against which the other options were assessed. The average cost of electricity fed into the grid, was calculated with a range of assumptions for the 2050 cost of storage and of solar and wind generated electricity. In 2021 prices it ranges from 52/MWh – with the low assumptions for the costs of storage and wind plus solar power 30/MWh and a 5 discount rate; to 92/MWh – with the high assumptions for the costs of storage and wind plus solar power 45/MWh and a 10 discount rate. The overall average cost is dominated by the cost of the wind and solar supply. The average cost of electricity would be at least 5/MWh higher if all storage were provided only by ammonia. It appears very unlikely that any other form of storage could meet all needs on its own. For comparison in 2010 – 2020, the wholesale price of electricity hovered around 46/MWh, but it was more than 200/MWh during most of 2022. Addition of other types of store Advanced compressed air energy storage ACAES was studied in detail as an exemplar of stores in the second category identified above. A combination of ACAES with hydrogen storage provides the benefits of the greater efficiency of the former and the lower storage cost of the latter. The costs and efficiencies of large ACAES systems are poorly known. However, for a wide range of assumptions, it was found that combining ACAES with hydrogen would be likely to lower the cost relative to that found with hydrogen alone by up to 5, or possibly more, although this is not assured. When they are optimally combined, the capacity of ACAES is much smaller than that of the hydrogen store, but ACAES delivers more energy because it is cycled more frequently. Adding other types of store to hydrogen and ACAES could lower the cost further. Market and governance issues The cost of electricity provided by storage will be many times the cost of wind and solar supply that is fed directly into the grid. Building the storage needed to provide this expensive but essential electricity will take large financial investments and time. While price differentials in wholesale and balancing markets may incentivise the construction of significant amounts of short-term storage, new mechanisms, including forms of guarantees, will be needed to make investment in large- scale, long-duration storage attractive. To contain storage costs, generators and owners of storage will have to cooperate to an unprecedented degree in scheduling charging and dispatch of energy from different types of store. Ensuring this cooperation is likely to require radical reforms. Caveats and avenues for further work This report is focussed on the large-scale storage that will be needed in 2050 in GB. While the possible roles of nuclear and of gas plus CCS are considered, the modelling does not take account of continuing contributions from burning waste and biomass, hydropower and interconnectors, or the relative locations of supply, storage, and demand, and their implications for the grid. The design and implementation of procedures for scheduling the use of a mixture of different types of store together with other flexible supply need to be studied further. More work is also needed on the long-term variability of wind and solar supply and the need for contingency. The need for hydrogen for large-scale electricity storage should be studied together with other uses for green hydrogen. This would almost certainly reveal systems benefits that would lower costs.8 LARGE-SCALE ELECTRICITY STORAGE Ex ECuTI v E Su MMARY The underlying assumptions on the cost of storage and of providing wind and solar power should be underpinned by detailed engineering estimates. Meanwhile, it should be stressed that the cost estimates in the report, which are in 2021 prices, are obviously sensitive to increases in commodity prices and other forms of inflation, and depend critically on estimates of the future cost of wind and solar power. Constructing the large number of hydrogen storage caverns that this report finds will be needed to complement high levels of wind and solar supply by 2050 will be challenging but appears possible. GB will need large-scale energy storage to complement high levels of wind and solar power. No low-carbon sources can do so at a comparable cost. Construction of the large- scale hydrogen storage that will be needed should begin now. More details and background information are provided in supplementary information available at royalsociety.org/electricity-storage. This includes a description of unpublish