Liquid ammonia stored in an underground salt cavern can provide 300 times the energy capacity of gaseous ammonia stored in a facility of the same volume. Such was the conclusion made by researchers from the University of Alberta in Canada, who have evaluated the feasibility of large-scale underground storage of ammonia as a hydrogen carrier. Their calculations suggest that a salt cavern with a volume of 500,000 cubic meters could hold a supply of liquid ammonia with a usable energy capacity of more than 1.6 TWh, whereas storing ammonia in a gaseous state would only yield a capacity of some 5.25 GWh.
Ammonia has long been considered a promising hydrogen carrier, as it is much easier to store in large volumes. In order to liquefy hydrogen, it needs to be cooled to about −253°C, whereas ammonia liquefies at −33.4°C under atmospheric pressure; its volumetric energy density in the liquid state is about 3.5 kWh per liter. This is why surplus electricity from solar and wind power plants could be used, inter alia, to produce hydrogen so as to then convert it into ammonia and store it for months, using it directly as fuel or converting it back into hydrogen when needed.
For this type of storage, the researchers propose using salt caverns, large underground cavities created by dissolving salt deposits in water. Saliferous rock is virtually impermeable and is already being used to store natural gas and other energy resources. The scientists simulated the operation of salt storage facilities at depths ranging from 500 m to 1,500 m, accounting for temperature increases of about 2.5°C for every 100 m of depth. The calculations helped identify the pressure and depth ranges where ammonia remains liquid, while the internal cavern pressure is sufficient to ensure structural stability without being high enough to cause rock failure. The most reliable conditions are found at depths greater than 500 m.
The researchers also compared three options for ammonia storage. In the gaseous state, its energy capacity proved too low. Although storing a mixture of liquid and gaseous ammonia increases the cavern’s capacity to some 850 GWh, the liquid starts to boil and cool during extraction, causing pressure fluctuations and complicating pump operation. The most effective approach turned out to be the one keeping the ammonia in a liquid state: this makes it possible to use over 95% of the cavern’s volume, resulting in a usable energy capacity of about 1,615 GWh.
Another challenge is concerned with maintaining pressure during the injection and extraction of ammonia. While brine is typically used to displace liquid products in salt caverns, ammonia is highly soluble in water, reaching concentrations of up to 36% by mass in solution. For this reason, the scientists propose using nitrogen as a compensating medium. Since nitrogen is non-flammable, relatively inexpensive and lighter than liquid ammonia, it forms a gas cushion above the liquid, thereby maintaining pressure within the cavern. However, a small amount of nitrogen dissolves in the ammonia, which can reduce the purity of the recovered product to about 96%. While this level of impurity is acceptable for gas turbines and solid-oxide fuel cells, further purification will be required for the subsequent production of high-purity hydrogen.
The biggest technical challenge is the brine that always remains at the bottom of the cavern. The scientists calculate that if the brine layer has a volume of 20,000–50,000 cubic meters, it could dissolve 3,400–8,500 tons of ammonia, or 2–17% of the total reserves depending on the size of the storage facility. Once the brine becomes saturated, further losses should substantially decrease; however, the resulting solution will be highly alkaline and capable of corroding the steel components of the well. One possible solution suggested by the scientists is to create an additional buffer layer, like silicone or mineral oil, between the ammonia and the brine. At the same time, simulations have shown that ammonia interacts relatively weakly with the salt and other minerals commonly found in these caverns.
This is why salt caverns could potentially serve as large-scale seasonal storage facilities for energy in the form of ammonia: surplus renewable energy could be used to produce ammonia, and the resulting energy resource could be stored underground for months to be later used to generate electricity or produce hydrogen.
