Using solid air to transport liquid hydrogen

Hydrogen is set to become a useful way to store and transport renewable energy, but the gas itself is hard to store and transport.

A new paper in the International Journal of Hydrogen Energy has proposed a new method of keeping hydrogen gas stable: use extraordinarily cold, solid air.

One of the most promising ways to move hydrogen around is to cool it until the gas condenses and becomes a liquid.

But hydrogen condenses at -253°C, much colder than other supercooled liquids that often get carried around, like liquid nitrogen (-196°C) or liquefied natural gas (-160°C).

Cooling hydrogen to this temperature is a very energy-intensive process: it needs about 13 kWh per kilogram of hydrogen to do, or about 30% of the energy the hydrogen is storing in the first place.

An international team of researchers has proposed that using solid air could be a way around the problem.

They’ve called the process “Solid Air Hydrogen Liquefication,” or SAHL.

At -253°C, both nitrogen and oxygen – the two main components of air – are so cold that they freeze and become solid. (Nitrogen’s melting point is -210°C, while oxygen’s is -219°C).

Turning hydrogen from a liquid back into a gas – regasification – cools things down. The researchers propose that regasifying hydrogen could be a simple way to make solid air, which could then be purified to solid oxygen and nitrogen.

The solid air could then be transported back to the source of hydrogen gas, and used to liquefy the hydrogen there.

Diagram showing green hydrogen-solid air process
The solid air-liquid hydrogen chain. Green hydrogen is liquefied with solid air, then transported to its destination where it’s turned back into a gas, creating solid air in the process. The solid air is then shipped back to the hydrogen source. Credit: Hunt et al.

“Using solid air as a medium for recycling cooling energy across the hydrogen liquefaction supply chain can reduce the cost and energy consumption for transporting hydrogen between continents,” says lead author Julian Hunt, a researcher at the International Institute for Applied Systems Analysis (IIASA).

“This would increase the viability of a global hydrogen economy in the future and increase the number of hydrogen suppliers for energy-demanding regions, such as China, Europe, and Japan. The possibility of selling hydrogen could result in a further expansion of solar and wind power in developing countries, contributing to their economies.”

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While there would inevitably be some energy lost throughout the process, the researchers calculate that using solid nitrogen could reduce the energy needed to liquefy hydrogen by 25.4%.

Using solid oxygen would be slightly more efficient, decreasing energy needs by 27.3%, but the researchers believe that solid nitrogen is a better idea because it doesn’t explode.

They point out that the pure oxygen could be used to make hydrogen combustion more energy-efficient, or help with carbon capture and storage.

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