The challenges of storing hydrogen                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  

The subject of hydrogen and, in particular, green hydrogen has been the focus of many discussions in the renewable energy sector over the last year - due largely to rising gas & oil prices in 2022, seeing a flurry of activity surrounding investment in the production of green hydrogen.

However, less discussed are the challenges of storing (and transporting) hydrogen - challenges that arguably hinder its widespread production and use.

Current ways of storing hydrogen

There are currently a number of ways to store hydrogen -

• Liquefaction

• Chemisorption; metal hydride - this gives off heat when ‘charging’ and requires heat to discharge H2, hydrogen bicarbonate salt

• Physisorption; palladium, polyaniline where H2 is adsorbed onto the surface

• Compression; gas grid and salt caverns

• Ammonia; hydrogen is used to synthesise ammonia, the ammonia is then cracked to release the hydrogen - ammonia is able to be stored as a solution in water which allows a greater density for transportation

Where liquefaction is the only hydrogen storage technology where ‘boil off’ is an issue.

The challenges of storing hydrogen

When stored as a gas, hydrogen requires a vast amount of pressure to compress it due to its low density - making the volume of hydrogen much larger than other hydrocarbons (up to 4x larger in fact).  

Hydrogen as a cryogenic liquid must be stored at an incredible -253 degrees Celsius in dedicated, thermally insulated tanks. Within this, no insulation is ideal and, as such, parasitic (natural) heat transfer from the surrounding environment permeates these tanks and heats up the hydrogen inside, which sees tank pressure increase and gaseous hydrogen vented , in order to maintain pressure levels.

This evaporation of hydrogen is called ‘boil-off gas’ and is considered to be unavoidable; the rate of this fluctuates between 0.1 and 5% loss per day, depending on the insulation of the tanks.

Although these may seem like small figures, when you consider the capacity of a cryogenic tank, just 1% loss could equate to losing a third of the stored hydrogen within just 1 month of storage.

What’s more, the low temperatures and high pressure of storing a low density element such as hydrogen poses challenges - not only to the storage of it but also to the transportation of it; as transporting hydrogen is particularly expensive, in part due to the fact the infrastructure to transport and distribute hydrogen needs upgrading and creating - the infrastructure simply doesn’t exist as we need it to.

Essentially, storing hydrogen and then transporting (and distributing) hydrogen is currently inefficient, making green hydrogen production and storage particularly expensive.  

So, what’s the solution?

There are a number of possible solutions to the challenges faced when storing hydrogen, some of which are still being worked on.

For example, materials-based hydrogen storage is an alternative to compression and liquefaction - seeing solids and liquids that are chemically able to absorb hydrogen used to bind it.

And, as recently as October 2022, Interesting Engineering released an article on new research that suggests the solution to the ‘hydrogen storage problem’ could be salts.

A major advantage of this storage method, reportedly, is that it is reversible and therefore allows the salts to be reused.

However, a disadvantage of this method is the use of precious metals as catalysts and the production of carbon dioxide as a byproduct of the process.

Use in the gas network

It has been said that hydrogen could replace 20% of natural gas in the grid from this year; with all 5 of Britain’s gas providers reportedly preparing for it. And using hydrogen created from renewables - i.e. green hydrogen - in this way is an ideal solution; as adding hydrogen to the gas mix would also enable the country’s gas-fired power plants to use hydrogen to generate cleaner electricity.

Alongside this, the introduction of hydrogen would reduce carbon emissions by the equivalent of 2.5 million cars per year without the need for domestic heating systems or cooking appliances to change.

It’s also worth noting that the only commercially available storage technology (or method) is compression, specifically to the gas grid.

In short, hydrogen is an incredibly promising and exciting emerging market with advancements being made every day, with the production of green hydrogen even more so - hydrogen’s use in the gas network as early as this year being a strong indicator of its power and potential.

Storage (and transport), however, is the biggest barrier we’re yet to confidently overcome with an environmentally and economically conscious solution; such a solution still requires research and investment.

That being said, perhaps the solution doesn’t involve focusing on the storage of hydrogen? Perhaps, instead, there’s merit in shifting focus to the production of green hydrogen made from renewables, used as an energy carrier to add to the mix of gas supplied to the gas grid?