Arup – Written evidence (LES0031)


Arup are a multi-disciplinary sustainable development firm of 18,000 experts working across 140 countries. Arup provide design, engineering, architecture and advisory services across the built environment, with a purpose to shape a better world through a commitment to environmental, social and governance priorities.


Arup is a thought leader in exploring and articulating hydrogen’s role in the energy system as the UK transitions to net zero carbon emissions by 2050. Arup are the Hydrogen Technical Adviser to the Department for Energy Security and Net Zero, have published a number of studies exploring hydrogen’s role in the future energy economy for UK and devolved governments, and support major development projects in hydrogen production, transport, use and storage.


The inquiry sought evidence on ten questions and sub-bullet points as numbered in the headings below. The following questions are answered through the perspective of using underground hydrogen storage as a primary method of long duration energy storage. Other technologies exist and may be better suited for shorter-term energy storage, however, the use cases and discussion of those technologies is beyond the remit of this written evidence.


A comprehensive answer is provided for question 3, allowing for presentation of context and strategic use cases for each underground hydrogen storage method, and is referred to in later questions. A reference list is provided at the end of the document.


Executive Summary

  1. Underground hydrogen storage will play a key role in the future decarbonised energy system, as both an energy carrier and long duration energy storage. A range of geological technologies are available that allow for short, fast cyclic storage over a period of days/weeks through to slow discharge grid balancing over seasons/years.
  2. A significant up-scale of underground hydrogen storage capacity is required to reach the anticipated capacity requirements for Net Zero. The UK risks falling behind European competitors and missing the window of opportunity for timely development.
  3. Technology readiness levels for grid-scale cyclic storage are in prototype phase, and technical, social and economic questions remain. Pilot studies alongside commercial development are advised to de-risk future projects and incentivise investment, as well as testing public attitudes towards new technology.
  4. A diversity of underground hydrogen storage technologies will be required for a functional energy system, including lined rock caverns, salt caverns and depleted gas fields/aquifers. Each technology has a unique system benefit in a robust energy system.


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?

No evidence provided.


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

No evidence provided.


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

Hydrogen, as power-to-gas, may be viewed as a multi-purpose tool in addition to an energy storage vector, and will be a key enabler in a NetZero energy system by supplying energy feedstock to a range of hard to decarbonise sectors, such as industrial processes, transport (shipping and aviation) and heating (industrial and domestic). UK government has stated an ambition of delivering 10 GW of hydrogen production [1], with electrolysis (or green hydrogen) being a key component of this target. To scale with production commitments in addition to long duration energy storage functions, underground hydrogen storage will play a major role in the future energy system. According to the National Grid’s Future Energy Scenarios (FES) [2], the UK will need between 12 and 51 TWh of hydrogen storage by 2050, compared to the current c. 25 TWh of natural gas stored [3]. Currently, the UK has 0.025 TWh of hydrogen stored underground [4], showing that a large upscale of storage is required by 2035 and subsequently to 2050. To achieve the storage capacity targets, allow for system responsiveness and create system resilience, a diversity of underground storage technologies is required; such as lined rock caverns for MWh, salt caverns for GWh and depleted gas fields/aquifers for TWh scales of energy.


Several methods exist for storing hydrogen, including mechanical approaches and chemical compounds. Aboveground storage, in surface tanks, is commonly used but is vastly uneconomic to scale to grid requirements and has a large aboveground footprint. Studies suggest that underground hydrogen storage is the most economic method of storing hydrogen at a scale required for grid balancing, due to the efficiency of the system and economies of scale [5]. Each method of underground storage has its advantages and disadvantages and are expected to play a role in the hydrogen economy. For the purposes of this question, only underground hydrogen storage technologies have been considered, although ammonia, metal hydride and liquified hydrogen may be expected to play a role in smaller local use cases.


Figure 1 below [6], provides a summary of technology readiness levels (TRL) of the key underground hydrogen storage technologies. Notably, other than static storage of hydrogen in salt caverns, which will meet a small proportion of the UK’s storage demands, all of the technologies are at TRL 5 to 6 i.e. prototype stage and are based on global prototype testing. Equally, it is important to state that each geological storage technology has its own unique technical challenges that need to be locally tested and demonstrated. Unlike engineered and man-made technology, the geological concepts may not simply be reproduced and commercialised at large-scale without extensive site-specific assessments. A more detailed summary of each storage technology is provided below in descending order of TRL.


Salt caverns are already in widespread use for natural gas storage around the world, and as hydrogen storage onshore in the UK and US, typically over seasonal timescales. As static storage, salt caverns are considered commercially operational. However, use in grid-utility and fast cycling is still in prototype phase. Typically, caverns in salt are of greater volume and are constructed deeper than lined rock caverns, enabling for higher operating pressures, and therefore, allowing for more hydrogen to be stored (GWh-scale). Because of this, salt caverns favour storage for weeks/months. Of note, suitable salt may only be found in select locations. Also, significant aboveground facilities may be required during construction and operation. Depending on local geological conditions this may include large volumes of brine storage, discharge, and processing facilities, which may limit wide-spread deployment across onshore UK.


Lined rock caverns are shallow (<100 m), hollow, mined spaces. A prototype technology that has been used in the hydrocarbon industry since the 1950’s, lined rock caverns are able to be constructed in almost any geological rock type, are not geographically constrained unlike other technologies and allow for fast cycling and reduced volume of cushion gas. Hydrogen has never been stored in lined rock caverns; however, a hydrogen pilot is being deployed in Sweden in the HyBrit project. The discharge duration and storage capacity of lined rock caverns are relatively small and favour balancing supply and demand profiles over a weekly duration.


Depleted gas fields/aquifers may be used to store hydrogen in the pore spaces of porous rocks. Town gas and blended hydrogen-methane gas have been stored at prototype and commercially viable projects. Theoretically, gas fields may provide TWh-scale storage over inter-seasonal/yearly timescales and have equivalent capacity to hundreds of caverns. Significant laboratory testing has proven porous rocks are a viable and scalable storage technology, however, pure hydrogen has not been stored in a porous rock at scale before and significant techno-socio-economic barriers remain to be overcome. The technology readiness for pure hydrogen is therefore low, and requires significant testing, including a pilot study and in situ testing, before widespread deployment. Because of the scale of storage capacity and the slow discharge time, this technology is most suited for inter-seasonal and inter-year balancing, system resilience and energy security.

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Figure 1. Overview of Technical Readiness Levels for different underground hydrogen storage technologies according to the IEA TRL framework [6]



Storage Capacity

Discharge Time

Relative Cost per Kg of Hydrogen

System Role

Technology Readiness Level


Lined Rock Cavern




Local storage, local supplier/customer demand, electricity grid peaking supply


Not geologically constrained, hard to scale, limited supply chain.

Salt Cavern




Regional storage, local supplier/customer demand, electricity grid peaking supply


Geologically constrained, significant above ground footprint, environmental considerations during construction.

Depleted Gas Field/ Aquifers




National storage asset, system resilience, energy security


May repurpose old infrastructure, Just Transition for hydrocarbon sector.

Table 1. Overview of storage technologies listing their function and technology readiness level according to IEA TRL framework.

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

The 2021 Hydrogen Strategy [1] recognised that as the hydrogen economy develops, there will be a need for hydrogen storage when supply does not align with the demand for hydrogen. The government has provided small-scale support through the Net Zero Hydrogen Fund Hydrogen Business Model allowing for producers to manage local variation in supply/demand to offtakers. This support is designed for above-ground storage in compressed tanks, and is not adequate for the deployment of large scale long duration storage by late 2020’s and beyond.


The UK Government provided a commitment in the British Energy Security Strategy [7] to design a new business model for hydrogen transportation and storage by 2025. Recently, a consultation and minded position on hydrogen storage was published by DESNZ [8], which recognised the scale and pace of delivery required to implement underground hydrogen storage coevally with the wider hydrogen economy. The minded position is for a focus on geological storage of hydrogen, with minimum technology readiness required for eligibility and a cap and floor funding mechanism. How the business model will incentivise a diversity of storage options, including technology with a low readiness level, while rapidly expanding storage capacity remains to be seen.


Arup support the development of the Hydrogen Storage Business Model and will work closely with DESNZ on this issue as part of our Hydrogen Technical Advisory role. However, without specific detail and strong stakeholder feedback, it is difficult to understand whether the business model will be able to deliver the rapid and urgent infrastructural development needed. Investigation and scrutiny are needed to hold the business model accountable to creating a fair and equitable competition between storage technologies. Also of note, should the Hydrogen Storage Business Model be implemented by 2025 and accept initial applications by c. 2026, then that leaves a very short timescale for storage developments to be deployed by current grid decarbonisation targets in 2035 [9].


No evidence provided.


No evidence provided.


The challenge in up-scaling hydrogen storage is staggering. Even the lowest National Grid Future Energy Scenario [2] estimate of 18 TWh will require the equivalent of hundreds of salt caverns, a construction feat that is not technically or commercially likely to be delivered on Net Zero timescales. Instead, a diversity of storage options will be required, with a large reliance on depleted gas fields/aquifers to fulfil the majority of storage capacity. To help facilitate the upscale of storage on time, it is likely that the public sector will need to incentivise investment and de-risk projects through business model support, supportive regulatory frameworks, and government support pilots projects to garner wider societal acceptance. This must happen alongside and in tandem commercial development, perhaps replicating the role out of CCUS-technology in parallel with cluster development.



5. How well developed is the UK industry across different storage technologies, such as hydrogen or redox flow batteries? How does the UK compare to global competitors in these industries?

Underground hydrogen storage is a fast-developing technology, and no precedent of being used for fast cycling, grid-scale long duration energy storage. The UK, alongside the US, has a competitive advantage having already stored hydrogen in three salt caverns in Teesside since the 1970’s for industrial processes. These caverns, however, have been used for static storage, and not operated under fast cycling likely needed for grid-utilisation. Significant upscale and pilot programmes, specifically for depleted gas fields and porous rock storage, are required to prove the technical, socio and economic cases for hydrogen storage.


In the UK research landscape, energy storage, hydrogen and alternative energy vectors are identified as an ‘area of investment and support’ with Engineering and Physical Sciences Research Council (EPSRC). However, the UK risks falling behind as other nations look to significantly increase investment in research and development and pilot projects to demonstrate the viability of a range of underground hydrogen storage technologies (Fig. 2). Several pilot sites are being developed across Europe, to investigate specific knowledge gaps associated with storing hydrogen underground in porous media and salt caverns. Whilst these projects are valuable for collaboration and knowledge exchange, the energy system needs a dedicated UK-based research and innovation facility as well as pilot studies specific to the needs and challenges of the UK's hydrogen storage infrastructure.


The UK is developing the HyKeuper project in Cheshire, storing 1.2 TWh by later this decade, and Aldbrough at Humberside storing of 0.3 TWh by 2028. Teesside’s caverns are also considering conversion to GWh-scales of hydrogen storage and anecdotal evidence suggests they are oversubscribed. Other potential projects include Centrica’s Rough gas field converting to hydrogen, potentially offering 10’s of TWh of storage.

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Figure 2. Locations of know projects where underground hydrogen storage is being actively tested, demonstrated or implemented. Most projects are located in Europe, specifically in France and Germany for salt caverns and Austria and Hungary for aquifers. Most projects are in a preliminary stage, with isolated pilot studies being developed.



Underground hydrogen storage has ongoing research raising the technology readiness level by conducting laboratory experimentation and modelling, e.g., universities of Edinburgh, Heriot-Watt, Bath, and the British Geological Survey. However, despite the competitive research advantage of British universities, converting R&D into business is lagging. Of note, the technical case has to a large extent been taken to the limit of what can be achieved in the laboratory, and the major next steps are the socio-economic cases, and technical in situ experiments. The next logical step, therefore, is to develop field-scale demonstrator projects, which will require government support. Arup are advocating for a combined consortium of research and investment to develop a pilot scheme/s that will showcase the technical capability and economic case for hydrogen storage.



6. Beyond the cost of deploying long-duration energy storage, what major barriers exist to its successful scale up (e.g. the availability of a skilled workforce, the ability to construct the necessary infrastructure on time, or safety concerns around new technologies)?

Underground hydrogen storage has key technological and socio-economic barriers that need to be overcome before widespread deployment. Geoscientific questions remain, such as the integrity and safety of storage infrastructure, managing the permeability of storage formations to minimize hydrogen leakage. Economic questions also remain, such as the availability of the supply chain, top-side market constraints, and optimising storage management and efficiency while considering economic viability. Environmental impacts must be mitigated, such as the construction impacts associated with the development and operation of salt caverns (e.g., brine water disposal). The hydrogen village trials of Cheshire and Teesside, with the Whitby community effectively rejecting hydrogen show that social acceptance and community understanding is key; how accepting are wider society to onshore hydrogen storage and will they inhibit wider deployment? Finally, considering the timeline to construct a new salt cavern or repurpose a depleted gas field might take in excess of 8 years each, imminent Net Zero deadlines require the initiation and deployment of these infrastructure as soon as possible. To successful answer these questions and to de-risk investment into hydrogen storage, the UK needs a dedicated research and innovation facility, replicating the CCUS model, as well as pilot studies to prove depleted gas field/aquifer technology. These will meet the specific challenges of the UK's hydrogen storage infrastructure.


The development of underground hydrogen storage infrastructure to meet the scale required, will take many decades and requires a long-term view. For perspective, the construction time for a single new salt cavern may take between 5 and 10 years, and it is likely the UK will need 100’s such caverns alongside alternative underground storage technologies. Given the low TRL of many of the alternative geological storage technologies and current lack of supportive planning and regulatory frameworks, it is expected that, without significant intervention the development of the necessary storage infrastructure will take between 10 to 20 years, putting storage on the critical path to the UK achieving net zero and energy security. This leaves the window of opportunity for acting, testing and incentivising hydrogen storage investment as rapidly closing.



7. What steps should the Government take now to ensure this storage can come online later in the current decade?


Grid-scale (TWh) hydrogen storage is unlikely to be delivered later in this current decade as the lead time to construct new caverns and repurpose gas fields takes a multi-decade view. To ensure the delivery of underground hydrogen storage at the pace, scale and function required the government must be cognisant that a diversity of underground storage options will be needed. Lined rock caverns, salt caverns and depleted gas fields will all need to be explored, tested and developed. To create a route to market, de-risk future investment and share learnings from project findings in a UK-based system, open access pilot studies are required for each method. This has been recommended in the IEA report, Task42 [6], and is underway in European competitors, such as France, Germany and Austria. It is possible to take learnings from other countries, but the geological, societal and economic problems faced in the UK are unique and must be tested by a UK-specific pilot scheme.



[1] BEIS, UK Hydrogen Strategy, August 2021, Department for Business, Energy and Industrial Strategy. [2] National Grid ESO, Future Energy Scenario, 2023. [3] Grant Wilson, 2023, The role of energy storage in the low carbon energy system, HyStorPor Final Event. Underlying data are from National Grid ESO and National Gas, Elexon and DESNZ. [4] BEIS, Hydrogen Transport and Storage Analyytical Annex, Sept 2022, Department for Business, Energy and Industrial Strategy. [5] Abdin, Z., Khalilpour, K. and Catchpole, K., 2022. Projecting the levelized cost of large scale hydrogen storage for stationary applications. Energy Conversion and Management, 270, p.116241. [6] IEA, Technology Monitoring Report. 2023. Hydrogen TCP-Task 42, International Energy Agency. [7] BEIS, British Energy Security Strategy, April 2022, Department for Business, Energy and Industrial Strategy. [8] DESNZ, Hydrogen transport and storage infrastructure: government response to consultation, 2023, Department for Energy Security and Net Zero. [9] BEIS, Net Zero Strategy: Build Back Greener, October 2021. Business, Energy and Industrial Strategy.


11 September 2023