hydrogen-powered aircraft

Affordable green hydrogen is seen as one of the holy grails of the energy transition: The silver bullet for decarbonising tough-to-electrify bastions of fossil fuel use and a useful tool in many other cases.

But even as renewable electricity prices fall further, electrolyser economics improve, and the cost of green hydrogen production approaches that of grey hydrogen, even as people opt for distributed or centralised green hydrogen systems for economic and environmental reasons… then what?

What exactly can you do with your freshly-electrolysed green hydrogen? What should we do?

We assume (and hope) that if you’ve come as far as producing it, you will already have your answer. Green hydrogen isn’t the solution to all energy problems. But it is for many, so we believe it’s time to talk – about the practical areas of application for green hydrogen, from flames to fuel cells, fertiliser and factories.

The fuel cell factor

Fuel cells are the all-rounder of green hydrogen applications – once you’ve created H2 from renewable electricity and stored or transported it, you may well need to convert its energy back into electricity. 

Fuel cells sandwich negative and positive electrodes around an electrolyte solution. In hydrogen fuel cells, a catalyst at the negative electrode separates hydrogen molecules into protons and electrons. The electrons take an external route to the positive electrode, creating electricity flow, and reunite with the protons (and oxygen) to produce hydrogen fuel cells’ only other emissions: water and heat.

By essentially flipping the function of hydrogen electrolysers, fuel cells make themselves very useful indeed, for example, in hydrogen fuel cell vehicles like Toyota’s answer to Tesla – the Mirai – the Hyperion hypercar, heavy vehicles like hydrogen trucks from Hyundai, iLint trains from Alstom, experimental watercraft, and aircraft, as demonstrated by ZeroAvia and Germany’s HY4 Project.

But even if hydrogen fuel cells’ use in mobility is perhaps the most high-profile application, they also find their place in residential and industrial developments where stored hydrogen made from excess power can be reconverted into electricity at times of higher demand. More on that later.

Hydrogen combustion engines

Although fuel cells coupled with electric motors are two to three times more efficient than internal combustion engines running on gasoline, it’s also possible to use hydrogen in the same way as petrol. It is increasingly possible for combustion engines to be retrofitted to instead use H2, allowing the life of a vehicle, machine or even gas power plant turbines to be extended without fossil fuels.

So yes, it’s possible but is it worth it? Well, H2 retrofitted combustion engines emit zero CO2, which is great from a climate standpoint, but they still emit high levels of NOx (nitrogen oxides) if not run carefully. While questions about efficiency and safety remain as different retrofit engines will have differing reactions to hydrogen combustion, such challenges are being overcome in emerging H2-native engines.

While hydrogen combustion engines could make sense as a temporary solution, just burning the hydrogen usually isn’t as efficient as using it in a fuel cell. So burning H2 might not be worth it unless you can’t yet afford to replace your combustion engine machines – or if you’re launching a rocket.

Are you actually launching a rocket? Then it can be a great idea: Liquid hydrogen burns with extreme intensity, has low molecular weight and, emitting only NOx, is the lowest emission rocket fuel.

More combustion – heating with hydrogen

While we’re on the topic of burning hydrogen, heating is an obvious contender. For a start, hydrogen has the potential to be blended into and distributed via existing natural gas infrastructure at concentrations of up to 20%, lowering emissions for gas-dependent heating and creating green hydrogen demand.

This is a tempting solution for older residences that aren’t suitable for electrified heating options like heat pumps or which want to use existing infrastructure. However, using gas mixes with up to or more than 20% H2 can be problematic both for older gas infrastructure and existing burners.

Specialised heating systems designed for hydrogen combustion – such as the hydrogen combustion boilers trialled in Rozenburg, the Netherlands – can help, and NOx must also be considered.

Looking beyond the residential sector, industrial needs for low-to-high grade industrial heat can also be met by green hydrogen burners where electrical heating is not appropriate and hydrogen is available. But H2 can do much more than go up in flames.

The pull of hydrogen-derived fuels

Hydrogen, the simplest molecule in the universe, is also an important component of countless compounds, chemicals and products, with hydrogen-derived fuels currently of great interest.

The two main contenders for power-to-gas production from green hydrogen are green ammonia and green methane. Although you may well be asking yourself, why create green gas from green gas?

Green ammonia, for a start, is one of the most energy-dense substances we can create from renewable electricity, is much more compact and easier to transport than hydrogen, and the infrastructure and procedures for dealing with it are already well-established. Moreover, it can also be used directly in fuel cells, combustion engines or converted back into hydrogen, which makes it extremely interesting as a clean alternative fuel for the shipping industry, with first trials underway.

Green methane arises from green H2 combined with carbon dioxide extracted from the air to create a renewable version of the fossil fuel. Although pyrolysis of this gas could avoid CO2 emissions entirely, most green methane would be carbon neutral, releasing only as much CO2 as used in making it.

Chemical building block

It’s clear that the chemical use of hydrogen goes far beyond power-to-gas – one all-to-obvious example being green ammonia-based fertiliser, a use case with huge potential since 80% of all ammonia (the world’s second most heavily-produced commodity chemical) is processed into fertiliser.

Beyond this, hydrogen also enjoys wide use in the production of chemicals and intermediates including methanol, aldehydes and higher alcohols, as well as use in creating hydrogenated fats and oils.

All of these areas could benefit from use of green hydrogen – and the chemical properties that make it useful in the production of commodity chemicals also lead to considerable industrial applications.

Industry – protect, reduce, carry, refine

Hydrogen finds a myriad of applications in industries of all sizes and shapes. It’s poised to find its way into the energy-intensive metallurgy industry as a reduction agent, where it has the chance to decarbonise iron and steel production by replacing the carbon-based ‘coke’ currently used as fuel and a reduction agent, which is responsible for vast amounts of CO2 emissions.

Its other many industrial applications include use as an inerting or protective gas in float glass production, for generator cooling or corrosion protection in power plants, etching and use as a carrier gas for semiconductors and other electronics, nitrogen purification and research applications.

Perhaps the one industrial application where its use might raise the most eyebrows is in oil refining, which today makes up a big chunk of H2 use. Although replacing this with green H2 could slightly decrease oil’s greenhouse gas impact, it pales in comparison to simply transitioning away from oil.

The storage solution

One final hydrogen application that ties together all of the above is its utilisation as a storage medium. The increased availability and dropping price of both modular decentralised electrolysers and centralised large-scale electrolysers means green H2 is in some cases already a feasible energy storage solution.

Hydrogen storage has considerable advantages over storing electricity in batteries. Especially for mid-to-long-term storage solutions, the advantages include low cost of storage capacity (basically just the tanks!) and the fact that the stored energy doesn’t discharge over time. This must be balanced with the fact that H2 may require compression for space-efficient storage. In stationary applications, metal hydride hydrogen storages could also provide a safe, low-pressure and compact solution once costs are reduced a bit further. For transport, it can make sense to store hydrogen in the most compact forms possible, for example cryogenic storage in liquid form – or, in an emerging solution – by converting it into denser, easily stored and hydrogen-rich green ammonia.

Green H2 storage opens doors – whether it’s for short or long-term storage of renewable energy (e.g., storage of residential solar to use overnight or over the winter), for power producers to retain the value of curtailed energy when power prices go negative, for peak shaving, or for preparation of green hydrogen stock that can be transported or stockpiled for any fuel, industrial or chemical uses.

Are there priority hydrogen applications?

There are endless possibilities for green hydrogen use but what are the priorities, given that green hydrogen production capacity is currently limited and still relatively expensive? Industry, long-term energy storage and heavy transport may seem to be the best candidates, and some energy pundits are already urging us to adopt green hydrogen first where it is most needed.

But this is a rhetorical question. The green hydrogen industry is still in its infancy. However, it will grow faster than many of us can imagine and give us the tools to solve climate change. We may not even have seen the killer application for hydrogen yet (ever heard of hydrogen being fed to bacteria to produce protein?). It is not up to us to pick winners for hydrogen applications; we are sure there will be many, with profound implications that will bring disruptive change to society. But it is up to us to take the first steps in making them a reality.