Hydrogen storage with gravel and pipes in lakes and reservoirs

Climate change is projected to have substantial economic, social, and environmental impacts worldwide. Currently, the leading solutions for hydrogen storage are in salt caverns, and depleted natural gas reservoirs. However, the required geological formations are limited to certain regions. To increase alternatives for hydrogen storage, this paper proposes storing hydrogen in pipes filled with gravel in lakes, hydropower, and pumped hydro storage reservoirs. Hydrogen is insoluble in water, non-toxic, and does not threaten aquatic life. Results show the levelized cost of hydrogen storage to be 0.17 USD kg−1 at 200 m depth, which is competitive with other large scale hydrogen storage options. Storing hydrogen in lakes, hydropower, and pumped hydro storage reservoirs increases the alternatives for storing hydrogen and might support the development of a hydrogen economy in the future. The global potential for hydrogen storage in reservoirs and lakes is 3 and 12 PWh, respectively. Hydrogen storage in lakes and reservoirs can support the development of a hydrogen economy in the future by providing abundant and cheap hydrogen storage.


Ocean
Lake/reservoir Advantages -Great depths, result in high storage pressures and lower storage costs.-Colder temperatures in the deep ocean.
-Large space availability.
-In case of an accident, the gas dissipation will be fast.-Possibility of storing gases from off-shore operations, such as hydrogen production with off-shore wind power.-Explosion hazards in floating storage facilities can be avoided.
-Freshwater increases the lifetime of system components.
-No currents at the bottom of the lake or reservoir.-Possibility of storing green hydrogen produced from hydropower, wind, or solar power.-Low risk of sabotage of the tank.
-Short distance from the bottom of the reservoir to the land.-Existing infrastructure surrounding hydropower reservoirs.-Explosion hazards in aboveground storage tanks can be avoided.

Disadvantages
-Salinity of the water increases corrosion.
-Strong currents in the bottom of the oceans.-Distance from the deep ocean to the coast.-High risk of sabotage of the tank.
-Stormy weather can impact construction and operation and increase the -Relatively low depth, which results in medium storage pressures and higher storage costs.-Appropriate locations to install the storage tanks might be restricted due to the bathymetry of the lake or reservoir.-In case of an accident, the gas dissipation will take longer.Depending on the gas, it maintenance costs of the system.-High costs of offshore operations.
can have severe impacts on the lake or reservoir environment.

Storing other gases in lakes and reservoirs
Other gases can be stored in the same way.Supplementary Table 2 presents some gases that can be stored in lakes and reservoirs.Methane, ethane, and propane can form hydrides at certain temperatures and pressures.The formation of hydrates will impede the gas from flowing out of the tank.Ethane, propane, and carbon dioxide are stored as a liquid in the tank, significantly reducing the tank volume required to store the same mass when compared to storing these elements in a gaseous state.Gases with high solubility in water cannot be stored using the approach proposed in this paper because a high share of the gas will be lost as the water leaves the tank.Also, toxic gases such as ammonia and carbon monoxide cannot be stored with the proposed method because they will impact aquatic life.For instance, 1 mg L -1 of ammonia in water can kill fish at 0.5 mg L -1 1 .

Supplementary Table 2
Gases that can be stored in lakes and reservoirs.

Gas
Solubility in water at 0 o C (mg kg -1 ) 2 Maybe with membrane and small quantities to control toxicity if no hydrate formed (i.e., pressure lower than 1.7 bar at 0 o C and 5.4 bar at 5.3 o C, at higher temperatures, the pressure increases significantly) 5 .

49.5
Maybe with membrane and at small quantities to control reservoir acidity and if no hydrate formed 6 .

Hybrid hydrogen and compressed air energy storage
An interesting possibility to increase the energy storage flexibility of hydrogen storage in lakes and reservoirs is to use the tanks to store hydrogen in seasonal cycles.Once the hydrogen is extracted from some of the tanks, these tanks can be used to store compressed air (CAES) and provide energy storage in hourly, daily and weekly cycles.The tanks need to be completely emptied of hydrogen before they can be used to store compressed air, and they would be completely emptied of air before it is used to store hydrogen.Hydrogen and compressed air pipelines would be required to connect the compressors on the land with the tanks at the bottom of the lake or reservoir.Comparing hydrogen and compressed air storage, compressed air stores 1% of the energy stored within the hydrogen.However, if this tank is filled and discharged 200 times a year, it will generate two times more electricity than if the tank was filled with hydrogen once per year.To make the plant a hybrid hydrogen and compressed air energy storage facility, additional compressors/decompressors are required to increase the installed power capacity for the plant and allow the tanks to be filled with and emptied of compressed air in daily and weekly cycles.We suggest using isothermal compressor/decompressors with higher efficiencies than other compressor technologies 7,8 .Supplementary Table 3 presents a comparison of using the storage tanks for hydrogen and compressed air energy storage.

Supplementary Table 3
Comparison between hydrogen and compressed air energy storage.

Plant details
* The fuel cell operates at 35% capacity factor.*** Assuming only 50% of the tanks store compressed air and 15% storage capacity.****Assuming only 50% of the tanks store compressed air and 20% storage capacity. *