Underground hydrogen storage in salt caverns or depleted reservoirs presents a land-efficient solution compared to surface storage methods, though it still requires certain above-ground infrastructure. The surface footprint is primarily determined by the compression stations, monitoring equipment, and access roads, while the bulk of storage occurs below ground.

Salt caverns, created through solution mining, require a surface area typically ranging between 1 to 5 hectares per cavern, depending on depth and diameter. The above-ground facilities include wellheads, gas treatment units, and compression stations, which may occupy an additional 0.5 to 2 hectares. Monitoring equipment, such as pressure sensors and leak detection systems, is distributed across the site but does not significantly increase the footprint. Access roads and safety buffers add to the spatial requirements, though these are often shared among multiple caverns in a storage field.

Depleted oil or gas reservoirs have a similar subsurface advantage but may require more extensive surface infrastructure due to pre-existing wells and the need for additional integrity monitoring. The surface footprint for a depleted reservoir site can range from 2 to 10 hectares, depending on the number of wells and the scale of compression and purification systems. Unlike salt caverns, these sites may need more frequent maintenance and monitoring due to geological heterogeneity.

In contrast, surface storage methods such as compressed gas or liquid hydrogen tanks demand significantly more land. A large-scale above-ground hydrogen storage facility with multiple tanks, safety buffers, and auxiliary systems can occupy 10 to 50 hectares or more, depending on capacity. Metal hydride and chemical storage systems also require substantial space for material handling and thermal management.

Land rehabilitation after decommissioning underground storage sites is generally straightforward. Salt cavern sites may require minimal restoration, particularly if the land was previously industrial. Depleted reservoirs often revert to their pre-use state, though well-plugging and soil remediation may be necessary if contaminants were present. Surface storage facilities, once decommissioned, leave a more pronounced impact, with foundations and impermeable surfaces requiring removal or repurposing.

The choice between underground and surface storage depends on land availability, geological suitability, and project scale. Underground options excel in minimizing surface disruption while providing large-scale capacity, making them preferable in regions with land constraints. Surface methods, while more flexible in siting, impose greater long-term land use commitments.

Compression stations are a critical component of underground hydrogen storage, typically housing multi-stage compressors, cooling systems, and electrical infrastructure. These facilities are compact but require careful placement to minimize energy losses and ensure safety. A single compression station for a salt cavern facility may occupy 0.2 to 0.5 hectares, with larger installations needed for depleted reservoirs due to higher gas volumes.

Monitoring systems for underground storage include surface and subsurface sensors, often connected to centralized control rooms. These systems are designed to detect leaks, measure pressure changes, and ensure structural integrity. The land impact is minimal, as most monitoring equipment is integrated into existing infrastructure or placed along wellheads.

Safety buffers around underground storage sites are mandated by regulations, typically extending 100 to 300 meters from critical infrastructure. These zones restrict development but allow for low-impact land uses such as agriculture or solar farms. In contrast, surface storage facilities require larger exclusion zones due to higher risks of leaks or combustion, further increasing their land footprint.

The spatial efficiency of underground storage becomes more pronounced at scale. A salt cavern field with ten caverns may use only 10 to 20 hectares above ground while storing gigawatt-hours of energy. An equivalent surface facility would require at least five times more land, not accounting for additional safety margins.

Post-decommissioning, salt cavern sites can be repurposed for other industrial uses or returned to natural states with minimal intervention. Depleted reservoirs may require more extensive remediation, particularly if hydrocarbons were previously extracted. Surface storage sites often leave lasting alterations, such as compacted soil or residual infrastructure, necessitating more intensive rehabilitation efforts.

In summary, underground hydrogen storage in salt caverns or depleted reservoirs offers a land-efficient alternative to surface methods, with most infrastructure concentrated below ground. The above-ground footprint is dominated by compression and monitoring systems, which are compact compared to the sprawling requirements of tank-based storage. Land rehabilitation is simpler for underground sites, reinforcing their sustainability advantages in large-scale hydrogen deployment.