The need for hydrogen storage
A significant barrier to the ramp-up of the hydrogen industry, particularly green hydrogen, is the challenge of developing storage facilities with the desired capacity. Although hydrogen gas has a larger mass-specific energy density, it has a lower volumetric density when compared to petrochemical fuels. This means that larger volumes are required to store the same amount of fuel energy when under the same pressure and temperature conditions as fuels such as natural gas.
Green hydrogen production will make use of renewable energy sources and low-demand peak grid supplies to generate low-cost hydrogen. This will result in conditions where the production of hydrogen will not match the usage requirements of the industry. This disparity creates the need for buffer storage, from hours and days to weeks and months, to ensure a consistent supply to end users. This is unlike other petrochemicals such as natural gas, which can be extracted from underground natural storage reserves better matched with the demand profile, meaning less buffer storage is required.
The challenge that hydrogen storage presents is building safe, economically viable solutions with sufficient volumes in the required locations. Not only does hydrogen have a lower volumetric density, it is also characterised by high flammability, small molecular size leading to permeation through materials, and it causes hydrogen embrittlement, which leads to material degradation. There are limited storage solutions for gaseous hydrogen that currently exist, namely salt caverns, porous reservoirs, lined rock caverns, buried pipe, and above-ground pressure vessels, all of which have limitations. Gravitricity’s H2 FlexiStore underground storage solution, utilising lined rock shafts, has the potential to combine the benefits of the alternative options, while offering a unique medium-volume storage capacity.
Existing hydrogen storage technologies
Salt caverns
Salt caverns have been used for the storage of large volumes of hydrogen since the early 1970s for the chemical industry, with half of the UK’s hydrogen storage capacity (30 TWh) currently stored in salt caverns in Teeside, North East England. Their storage capacity can range from hundreds to thousands of tonnes of hydrogen, with a typical storage pressure of approximately 200 bar at 20°C. They are created by dissolving salt in deep salt beds and domes by the circulation of hot water through wells that have been drilled into the salt formation, a process that takes years to complete.
Rock salt has material properties that make it very effective in supporting significant pressures with gas tightness that provides an acceptably low leak rate. To maintain the structural integrity of a salt cavern, cushion gas is required to maintain outward pressure on the cavity so that it doesn’t collapse. Typically, this is around 30% of the total stored gas. The cushion gas also serves to maintain a minimum flow rate and pressure outlet as required by the end user.
Salt caverns are a very attractive option for storing high volumes of hydrogen, as despite the high construction costs, the resulting levelised cost of storage (LCOS) is very low. There are, however, significant drawbacks to this method. Due to the solution mining that is required to create the cavern, the lead time can be from 5 to 10 years from the outset of the project, making it unsuitable for rapid deployment. A cavern requires sufficient rock salt deposits, which often doesn’t align with where the storage is required, meaning it is heavily geographically constrained. Due to the mineral lining of the cavern, the hydrogen is exposed to contaminants, which reduces its purity. The presence of microbes underground can also lead to hydrogen loss, among other negative effects. Finally, the use of salt caverns is typically economically viable only for large volumes of hydrogen due to the high initial costs.
Porous reservoirs
A second underground storage option, which is less technologically developed, is porous reservoirs such as depleted oil and gas reservoirs and aquifer formations. The main principle of this method is that the porous media that previously stored a different liquid or gas medium is injected with high-pressure hydrogen. Due to the size of oil and gas reservoirs, which are typically larger than salt caverns, their potential to store vast quantities of hydrogen is great. However, this method presents additional challenges beyond those discussed for salt caverns, including higher contamination levels, the formation of corrosive gases, and permeability changes due to geochemical interactions. Ongoing research into porous reservoirs is being conducted, but it will be some time before commercial projects come to fruition.
Lined rock caverns
A third option for underground storage is lined rock caverns, with one existing for natural gas in Skallen, Sweden. This method uses an impermeable lining, such as steel, inside a concrete layer, which exerts the internal pressure of the gas on the surrounding rock formation. The example in Sweden has a diameter of 35 meters, a height of 51 meters, and a maximum storage pressure of 200 bar. If used for hydrogen storage, this would provide a capacity in the region of hundreds of tonnes. Among the benefits of this system is less geographical constraint compared to the previously mentioned options, although it does require exclusively hard rock conditions. The LCOS for this method is also higher than the previous options due to the increased material requirements for the concrete and steel lining, as well as the mobilisation and excavation of the cavern and the tunnel network down to the cavern to facilitate the excavation and monitoring.
Pipe storage
The final underground method is pipe storage, which has been used for the storage of natural gas since the late 1960s. In this method, long lengths of pipe are buried a few metres below the surface. While this method is technically feasible for hydrogen storage, it is more expensive compared to its use for natural gas. The occurrence of hydrogen embrittlement, which degrades the mechanical properties of materials, necessitates the use of higher-performance materials with larger design safety factors leading to higher overall costs. Compared to the other underground storage methods, the LCOS and the surface area requirement for useful volumes are much greater. Due to the large surface area requirement and high LCOS, this method is typically only suitable for small storage volumes up to 10–15 tonnes.
A similar option, which is used heavily in the natural gas industry but only suitable for pipe networks, is line packing. This involves increasing the pressure within the pipe network to store gas within the network itself rather than using bespoke storage. This does, however, require a very extensive pipe network, which takes a significant amount of time to develop, and it may be more suitable for hydrogen gas blends rather than pure hydrogen.
Pressure vessels
Pressure vessels are the primary above-ground method for hydrogen storage, with tanks typically made from steel or composites. The working pressures typically range from 30 to 900 bar, depending on the application, which requires significant energy to compress the hydrogen up to the higher rates. The decades-long use of pressure vessels for gas storage across various industrial applications means the technology is very well established, making them suitable for rapid deployment in the hydrogen sector. However, due to manufacturing constraints on the dimensions and wall thickness, the storage capacity of a single vessel is relatively small – in the range of tens to hundreds of kilograms. Despite the low storage capacity, pressure vessels are expensive due to the amount of high-cost material used and the high energy requirements for compression. Due to the lower storage capacity, their main uses are for tube trailers, filling stations, small buffer storage, and other applications with lower storage demands, such as passenger car fuel tanks. The total storage for a single application using a collection of vessels is usually up to 10–15 tonnes. Any greater leads to increasingly high LCOS due to the extra ancillary equipment required. Compared to underground pipe storage, above-ground pressure vessels are more prevalent, potentially due to easier access for maintenance and inspection, similar costs, all while offering similar benefits.
Medium storage requirement
As mentioned earlier, above-ground pressure vessels or buried pipe are suitable for storage volumes up to a maximum of 10 to 15 tonnes, while underground options can store hundreds to thousands of tonnes. If an end user requires a medium storage volume between these two options, they must accept significant costs due either to high material costs for above-ground pressure vessels or a sub-optimum underground storage facility – feasible only if they are located in proximity to the required geological conditions. A comparison of these options is illustrated in Figure 1.
The system being developed by Gravitricity is lined rock shafts (LRS), which is similar to the lined rock cavern method but provides an economically viable medium capacity system with a lower LCOS, reduced deployment times, and increased geological flexibility. The intended storage capacity of the LRS is between 15 and 100 tonnes, and multiple shafts can be co-located to provide even larger storage volumes.
The specific need for medium storage volumes within the range targeted by the LRS method can be shown by predicted usage requirements of many of the target end users. These include chemical and high-temperature industrial uses, heavy transport and shipping, and power generation. The buffer storage required for large renewable energy assets also presents the need for medium storage volumes, to account for intermittent production and potential low-output events such as low-wind periods.
Lined rock shafts
LRS, the technology being developed by Gravitricity, is a method of high-pressure hydrogen storage that uses the surrounding rock mass to support the gas pressure, minimising the amount of containment material required. A bespoke shaft is sunk for each case, with a range of shaft dimensions available, providing a range of different storage capacities. After the shaft has been sunk, a series of lining materials are installed within, the combination of which contain the hydrogen within a gas-tight layer, whilst transferring the load from the internal pressure to the rock mass. An overburden above the containment vessel is installed, which prevents uplift due to the internal pressure. This overburden also has the function of increasing the safety of the stored hydrogen compared to above-ground methods due to it being stored at a reasonable depth below the surface. The height of the overburden is in the region of tens of metres. Hydrogen will then be injected and withdrawn from the shaft on a daily, weekly, or monthly basis, depending on the user’s requirements. The maximum operating pressures will reach approximately 220 bar, with a minimum pressure of approximately 30 bar. The remaining hydrogen will serve as a cushion gas, providing the same function as that described for salt caverns. Should access be required to the system for inspection and maintenance, the hydrogen could be evacuated, and remote monitoring and instrumentation used. The LRS method could store up to 100 tonnes of hydrogen per shaft. Co-located shafts also provide the opportunity for significantly higher storage volumes while still maintaining a small surface footprint.
The main driver of the low LCOS of an LRS is the reduction in the amount of material required to contain the hydrogen pressures, as the rock mass fulfils this function. This reduces the steel tonnage by a factor of 3–5 per unit mass of hydrogen. Therefore, the LCOS of LRS storage is not as low as that of salt cavern storage, it remains much lower than above-ground methods for medium storage volumes. As the LRS technology is proven and shaft sinking costs are predicted to decrease over time, the LCOS of LRS storage will likely decrease. The costs to transport the hydrogen will also be much lower compared to salt caverns, which are geographically constrained, requiring hydrogen to be transported either by vehicles or pipelines, thereby adding to the LCOS. The potential storage costs are illustrated in Figure 2.
As previously indicated, the small surface footprint of a LRS is a significant advantage compared to the area that would be required for above-ground pressure vessels or buried pipe storage. The space required would be primarily determined by the auxiliary system equipment associated with the LRS, such as compressors, pipework, control systems, and more.
A key risk when working with hydrogen is leakage due to its small molecular size, which makes it easier for the gas to permeate through materials or through interfaces and seals. When hydrogen is contained underground, any potential permeation through the lining will be either contained or dispersed by the surrounding rock mass. By contrast, leaks in typical above-ground vessels or pipe storage pose a greater risk due to the higher number of interfaces and connections. If any leaks occur in such systems, the gas is not contained or dispersed by the rock mass.
The purity of hydrogen is maintained within an LRS due to the gas-tight lining. This is a significant advantage compared to large-capacity underground storage methods, where the hydrogen is exposed to impurities that degrade the purity of the gas. Therefore, hydrogen stored in an LRS would be suitable for all end-use cases, provided that gas of the required purity is injected into the system.
As previously discussed, a major barrier for the use of salt caverns and porous reservoirs is the requirement for specific geological conditions, which may not be available where hydrogen storage is required. LRS, however, doesn’t require as specific geological conditions, making it much less geographically constrained. This means that a system could be installed exactly where it is needed, whether within an industrial cluster, at a renewable energy asset or for an off-grid network
A key benefit of an LRS is the speed of deployment. It is estimated that the system can be constructed within two years, which is significantly quicker than salt caverns. Given the required rapid acceleration of the hydrogen industry and the urgent need for storage to support the proposed hydrogen projects, LRS systems will play a critical role in supporting this planned growth.
The future hydrogen storage landscape
The use of hydrogen will play an essential role in the UK’s target path to net zero by 2050. The targets for production rates set by governments in the UK and Europe are significant and continually increasing, and planned hydrogen projects of all scales are rapidly rising in number. Storage plays a critical role in facilitating the growth of the hydrogen industry.
Small above-ground pressure vessels will be needed for a wide range of uses, while salt caverns and porous reservoirs will enable vast reserves to provide seasonal storage and energy security. Medium storage volumes will be crucial in supporting the industry by providing a midpoint solution between these two methods – one that is economically competitive, safe, scalable and geographically unconstrained.
The LRS technology that Gravitricity is developing will be a key part of the hydrogen infrastructure system of the future.
