Cleaner energy resources are the dire need of the hour and this is a known fact. While scientists and experts across the planet are striving hard to reduce our reliance on fossil fuels, our energy needs have never faced a downfall- thanks to rapid industrialization and urbanization. Although renewable resources like solar, wind, and hydro-electric power are the most popular alternatives, these are seasonal energy sources and the energy production from the same will not be similar all around the year. The fluctuations in production hence cannot always meet the energy demand of the population, and this makes the renewable energy sources not completely reliable.
Solar Production v/s Demand of the same in a year
What and How H2 is produced?
Now, this is where Hydrogen- the first element of the periodic table comes to the spotlight with a solution. Being a gas, hydrogen fuel can very well cater to our energy needs and is produced from techniques including Thermochemical, Solar-Water splitting, electrolytic and biological processes. While the production of this cleaner energy source leaves a carbon footprint of about 830 million tonnes in the form of CO2 annually, the result being a zero-emission fuel is what makes H2’s future bright.
Storage of H2 – the million-dollar question:
Having almost cleared the need and methods of producing hydrogen fuel, we will be looking at an area that is usually not given much thought about and that is the storage of H2. As already mentioned, for time being let us consider hydrogen as an alternative to renewable resources which is utilized when the energy demand increases drastically. While producing the fuel in the nick of time is obviously undoable, sufficient storage of H2 anticipating the demand is the best choice. Like Natural Gas, Hydrogen is also compressed before storing to achieve lower volume and also because liquid hydrogen demands a 64% higher amount of energy for storage than its compressed gaseous counterpart.
Storage tanks v/s Geological landforms:
Compressed Hydrogen can be stored in surface storage vessels (like steel composite concrete vessels and in wind turbine towers) or in geological landforms like (salt caverns, depleted O&G reservoirs, and aquifers). Nevertheless, unlike the underground geological landforms which offer huge storage capacity owing to their sheer scale, the storage tanks which can range in size from a small bottle to a huge tank require high amounts of pressure to store an appreciable amount of H2 in it. Since these storage tanks are usually constructed on the surface, the pressure conditions in these tanks need to be artificially stimulated and thereby mount huge upfront costs when compared to their geological storage counterpart.
H2 storage prices in Geological Landforms v/s Storage Vessels (in $/kg)
The above is a table comparing the prices of Hydrogen storage in Geological landforms and Storage Vessels at different pressure conditions. It is visible from the table that it's about 218 times cheaper to store the same amount of hydrogen in Geological landforms than in storage vessels.
Is geological storage truly a better option?
Like any other storage option geological storage too has its pros and cons. From the erosion of pipelines to the tedious task of injecting the gas and maintaining it at apt pressure conditions, geological storage has its limitations. However, the important prerequisite is the availability of the suitable landform itself.
While most of the Depleted O&G Reservoirs have already met all the requirements for a suitable Underground Hydrogen Storage (UHS) system, the presence of unrecoverable remnant fluids in it makes it both a boon and a bane. This is because the presence of remnant fluids like oil and gas satisfies the cushion gas need for efficient storage of H2 in the reservoir, chances of contamination of H2 by the same is also high. This is the reason why Aquifers too aren’t favorable underground landforms when it comes to hydrogen storage.
Salt Caverns- the best UHS System?
The problem of Hydrogen contamination in Depleted Oil & Gas reservoirs and aquifers leaves us to the next big suitable subsurface landform- salt caverns. Unlike the other two landforms, the problem of contamination can be prevented in these dome-like structures formed due to the upliftment of salt deposits and it is also found that about 98% of its storage efficiency can be used to store Hydrogen here. The reason behind its relatively expensive nature when compared to its other two counterparts is due to the process of salt removing or leaching that must be done before storing to ensure that the contamination of the gas is unheard of at least here.
Suitable Conditions of UHS:
As per Stefan Iglauer, the maximum amount of H2 can be stored at a depth of 1100m beneath the Earth’s surface and the capacity gradually decreases up until 3700 m depth beyond which the wettability of the gas increases as it percolates through the rocks and hence cannot be permanently immobilized. Conclusively it is found that suitable landforms formed at 1km depth can store up to 2.0 Mt of H2. Comparing this 2 MT storage capacity of Salt Caverns with the currently available storage tanks which can store about 800 kg of H2 in it, it is visible that geological landforms have a clear upper hand at least when it comes to storage capacity.
Future of UHS:
With demands for Hydrogen fuel estimated to grow at 5.48 % annually and the need for a suitable storage system of the same at 5.8% annually, the field of Underground Hydrogen Storage systems indeed has a bright scope. Moreover, to meet the large-scale needs of Industries, there is an imminent need to level up the storage capacity of H2 and by exploring suitable geological landforms across the globe, the estimated industrial need of 1200 kT/ year in 2050 can be met.