I was asked by a member of the House of Lords Environment and Climate Change Committee to prepare some notes on ultra long duration energy storage (ULDES). I provided the following document to the Committee in early November 2026.

Background assumptions

1, By ultra-long duration energy storage (ULDES) I mean storage for weeks and months. I believe that any UK policy should look to store many tens of terawatt hours (TWh). The storage medium should be convertible into electricity at very short notice. I am assuming that the UK gradually switches to the use of near-100% electricity rather than fossil fuels over the next 25 years, making ULDES a central part of energy policy.

2, The UK does not have a clear plan for the development of ULDES. In my view, expressed in books and other media, hydrogen stored in underground caverns is likely to be the predominant storage medium by 2050. A small fraction of ULDES needs will also be provided by stored heat.

3, The predominant response to the idea that the UK should use hydrogen as the main storage medium for energy has been scepticism. This is because that the conversion of surplus electricity to hydrogen and eventually back to power is relatively inefficient. Most types of electrolysers only convert about 70% of the energy in electricity into H2 although this will rise. A gas fired power station will then loss a further 40% of the hydrogen energy in the conversion back to electricity. At best, the round trip efficiency will probably only rise to a net 50% unless a large external source of heat is available to make the electrolysis approach more efficient.[1] (Efficiency would be improved if the heat from electrolysers could be used to fuel heat networks while the heat from the hydrogen turbines is employed to run solid oxide electrolysers).

4, Other countries are developing ULDES policies more rapidly and hydrogen is an important element of their approaches. It may be worth noting that very few countries see a large role for gas + CCS. I believe this policy choice is correct because ULDES using hydrogen will probably be cheaper and easier to implement. Below I refer to the plans of Germany and the Netherlands as examples which the UK could copy. Additionally, the UK could directly link to the large hydrogen networks in construction in these countries to assist in minimising the cost of ULDES.

Germany

5, German plans encompass hydrogen production, storage, transport and finally use in CCGT power stations for conversion back into electricity. Additionally, the country is actively exploring formal import and export arrangements. In addition to what is written below, I can provide more detailed costs and projections of use if required.

6, Hydrogen production. Germany’s 2020 strategy, updated in mid 2023, looks for up to 10 GW of electrolyser for the production of green hydrogen. This is substantially greater than the UK and a many times multiple of today’s capacity here. Much of Germany’s green hydrogen production will be devoted to serving the plastics, aviation and shipping fuel industries but some is intended for ULDES. If the 2030 target is achieved, and the electrolysers operate for 4,000 hours a year, a total of about 40 TWh of electricity will be used to make up to 30 TWh of green hydrogen. (As reference point, the current German consumption of hydrogen is about 60 TWh but the government also has plans for significant importation of H2 in the future).

7, Hydrogen storage. Germany intends to use salt caverns for hydrogen storage. The north of the country has exceptional access to the salt strata underneath northern Europe although the UK also possesses substantial capacity. A salt cavern for hydrogen is made by washing out salt from the underground layer creating a spheroidal upright empty shape. The process of creating a new cavern can take several years but the country intends initially to convert existing natural gas salt caverns to hydrogen. This might provide roughly 35 TWh of storage and Germany intends to create total hydrogen storage of up to 100 TWh by 2045. For reference, it may be useful to mention that the UK has stored small amounts of hydrogen in salt caverns for several decades.

8, Hydrogen transport. The country plans a network of over 9,000 km of pipeline by 2032. About 60% of this will be converted natural gas pipelines and the rest will be newly constructed. Some observers have expressed concerns about leakage of the very small molecules of H2 compared to the much natural gas. This means that pipelines will need to be carefully engineered but many countries around the world, including the UK, already move hydrogen in this way. Germany plans a very substantial maximum input and output rate of 101 GW into the network and 87 GW out. As a comment, Germany’s current maximum electricity usage is about 75 GW so the network could probably provide over 50% of required power in most circumstances when wind and solar are under-performing.

German opened the initial 500 km of the network in north-west of the country and two other places this year. For example, the north-western pipeline now takes hydrogen from the electrolyser at RWE’s Lingen plant to chemical industry customers 120 km away.

9, Conversion back into electricity. A few days ago, the Germany authorities announced support for a new generation of gas-fired power stations that will initially operate using natural gas but can switch to hydrogen. The total capacity is just over 5 GW. RWE was one of the main winners of the auction with over 2 GW supported. RWE is already actively researching the use of 100% hydrogen in a trial with a 34 MW Kawasaki turbine at Lingen. On November 6th 2026, RWE wrote about this experiment: ‘As the share of wind and solar power in electricity generation grows, it is becoming increasingly important to secure reliable, flexible capacity that can ensure consistent electricity supply when renewable energy sources cannot generate sufficient volumes. Flexible, hydrogen-compatible gas-fired power stations are therefore being designed to provide backup when wind and solar plants do not generate enough electricity over a prolonged period’.

The Netherlands

10, Firstly, I need to mention that some existing Netherlands hydrogen subsidy schemes have not been immediately successful. Costs have been higher than expected and, in contrast to Germany, uptake has been slower than predicted. Nevertheless, the country is continuing to push for the growth of the hydrogen industry and sees an important role of H2 as a key means to balance electricity supply and demand. TenneT, the Netherlands electricity network operator, recently wrote: ‘Hydrogen can contribute to security of supply of the electricity system by making the energy supply less dependent on weather and seasonal influences’.

11, Hydrogen production. The Netherlands government has stated an ambition to have 3-4 GW of electrolyser capacity by 2030. (Per unit of GNP, this is a more ambitious target than Germany). The early projects are in the industrial areas near Rotterdam and are sited for closeness to customers, including oil refineries but also because of easy access to offshore electricity coming onshore. ULDES is also an objective, made more urgent by the need to have a means of storing excess wind power from the Dutch North Sea. Indeed some wind farms will have electrolysers installed offshore for this purpose.

12, Hydrogen storage. Gasunie, the Netherlands gas network operator is building hydrogen storage in four salt caverns in the north of the country as a pilot product. It plans to complete the first cavern in 2031. The total capacity of the four caverns will be just under 1 TWh of hydrogen. In addition, Gasunie expects to use the capacity of the pipelines it owns to ‘line pack’ hydrogen. That is, when large amounts of H2 are available, the gas is pumped into the pipeline network to a higher pressure, meaning that more energy is stored in the pipes themselves. This is already done with natural gas in the UK and elsewhere.

The Netherlands geological survey body is estimating the amount and suitability of salt cavern storage and also the use of depleted gas reservoirs for holding large amounts of hydrogen for ULDES.

13, Hydrogen transport. The network operator intends to convert a large part of the country’s natural gas network into hydrogen pipelines by 2032. This is made easier by the gradual running down of the natural gas wells in the Netherlands. A network of about 1,200 km is planned of which about half will be new hydrogen pipelines and the rest converted from natural gas lines.

14, Conversion back into electricity. Power stations in the Netherlands are being prepared for the switch to being fuelled by hydrogen. For example, French company Engie says its priorities include a 5 year target to ‘Modernize existing power plants to prepare for the arrival of hydrogen’. A current project is to adapt its 900 MW Maxima power station to run on 50% hydrogen. This upgrade will be completed in 2027. The 1410 MW RWE plant at Eemshaven has also been designed, with some future modifications, to run on hydrogen.

International trade in hydrogen via pipeline

15, Hydrogen can be stored across Europe and the proposed networks of pipelines, such as those in Germany and the Netherlands can bring the gas to countries with a shortage of renewable energy supplies, whether temporary or long-term. Among other benefits this will mean the amount of hydrogen storage for ULDES in any particular country will be lower than if the country were isolated from the rest of Europe.

16, Both the Netherlands and Germany emphasise the importance of connections across Europe and there are detailed but earlier stage plans for a network across the continent. Germany assumes it will have to import large quantities of hydrogen to meet national needs and is progressing hydrogen pipelines from North Africa. Its gas transmission operator, Gascade, is also working with the UK’s National Gas on plans to link the two countries with a hydrogen pipeline. That will make both states more resilient. The 2023 Royal Society report on energy storage assumed no hydrogen trading or electricity imports when it suggested a target of 100 TWh of ULDES. Good pipeline links with Europe would tend to reduce this number.

Conclusion

17, In both Germany and the Netherlands, as well as other European countries, ULDES is presumed to be predominantly provided by hydrogen. While technical challenges remain, the feasibility of this route is broadly accepted in other countries. The UK should engage with its neighbours to explore how we could work together to secure adequate ULDES at the lowest possible price using the increasingly frequent periods when UK renewables output exceeds national electricity needs.

Chris Goodall, chris@carboncommentary.com, +44 (0)7767 386696. November 2026.

[1] I am talking specifically about Solid Oxide (SOEC) electrolysers, which can deliver 90% efficiency if high temperature heat is available.