As 120 global leaders meet in Glasgow for COP26 we take a look at a potential game-changer in the race to find alternatives to fossil fuels.
Sitting at the number-one spot on the periodic table as well as the list of most abundant elements in the universe, hydrogen has a habit of topping charts. To add to these accolades, in recent years hydrogen has been making a push towards the top of the ever-growing list of climate-related buzzwords.
The hype around hydrogen as an alternative energy carrier to traditional fossil fuels is by no means unfounded. As a zero-carbon fuel, it has the potential to power all sorts of industries without contributing to the rising levels of atmospheric carbon dioxide and the resultant rising global temperatures. In fact, it is speculated that human civilisation could move towards an economy relying on entirely green energy with hydrogen fuel delivering a substantial part of it – a so-called "hydrogen economy".
Over the last couple of years, governments around the world have been setting out their intentions to integrate hydrogen into their energy systems as a clean replacement for fossil fuels. The UK recently joined the party, releasing in August 2021 its highly-anticipated first "Hydrogen Strategy", detailing its aim to develop a world-leading hydrogen economy as a key part of its journey to net-zero emissions by 2050.
There are substantial limitations associated with hydrogen as an energy carrier that have so far prevented it from being a mainstream energy source. Nowadays around 80% of global energy consumption is still accounted for by fossil fuels, with the remainder being mostly nuclear, and electricity from renewables such as hydropower, wind power and solar power. But continued innovation in hydrogen technology, as well as growing commitment from governments to reaching zero carbon emissions, could mean that the hydrogen economy is not quite as distant as it seems.
Hydrogen as a fuel
Hydrogen acts as a fuel similarly to coal, oil or natural gas - it reacts with oxygen to release energy that can be harnessed. In practice, the reaction of hydrogen with oxygen may take the form of combustion to release heat, or an electrochemical reaction in a hydrogen fuel cell to produce an electric current.
But while the combustion of carbon-based fossil fuels releases carbon dioxide, a major contributor to the greenhouse effect, the only material product of the reaction of hydrogen and oxygen is water (or water vapour). While water vapour is itself technically a greenhouse gas, its contribution to global warming is minimal because of its relatively short-lived persistence in the atmosphere in comparison to carbon dioxide.
The energy-releasing reaction of hydrogen with oxygen is free of any harmful emissions. However, naturally-occurring pure hydrogen is extremely scarce on Earth, so hydrogen for use as a fuel has to be obtained by splitting molecules of naturally occurring hydrogen-containing compounds, with an input of energy. Hydrogen is therefore an "energy carrier" rather than a primary energy source, and its manufacturing process must also be considered when determining whether it truly is a zero-carbon fuel.
Manufactured hydrogen goes by different names depending on its production source.
By far the most common is "grey hydrogen", which is made by steam-methane reforming (SMR) using natural gas and steam as feedstocks. Grey hydrogen plants use the SMR reaction followed by some further processing to produce high-purity hydrogen gas along with carbon dioxide as a by-product. Clearly, the use of non-renewable natural gas and the release of carbon dioxide into the atmosphere make grey hydrogen an environmentally-unfriendly fuel.
The SMR process can be combined with carbon capture technology, whereby the resultant carbon dioxide is prevented from entering the atmosphere and instead transported to long-term storage, usually in the form of underground geological formations. Hydrogen produced in this way is termed "blue hydrogen", and although it represents an improvement on the grey variety from an emissions perspective, it still requires non-renewable natural gas as a feedstock. Besides, a process entirely free of carbon would be preferred.
The holy grail of manmade hydrogen is the "green" variety. To obtain it, a process called "electrolysis" is performed using naturally-occurring water as a hydrogen source. This is essentially the reverse of the process undergone in a hydrogen fuel cell - an electric current is applied to water in an electrolyser, causing the release of hydrogen and oxygen. Water is, of course, a renewable source - when the produced hydrogen is used as a fuel it will reproduce the water. For the electrolysis process to be truly carbon-free (and for the hydrogen to qualify as "green"), the electric current must have itself been derived from a renewable source such as wind power.
Green hydrogen is therefore, as the name would suggest, a completely green fuel. From the initial production of electricity, to its use in an electrolyser, to the reaction of the resultant hydrogen to release usable energy, not a single molecule of carbon dioxide needs to be released to the atmosphere.
If the hydrogen economy is to be truly realised, green hydrogen will be taking centre stage. But where green hydrogen is currently lacking is in its cost-competitiveness - it is currently two-to-three times more expensive to produce than the reasonably sustainable blue hydrogen, due to the price of renewable electricity as well as the significant capital and operating costs of electrolysers. Innovations in green hydrogen production technology – in particular making electrolysers cheaper, smaller and more energy-efficient – will be absolutely key to enabling the green hydrogen economy.
What do we do with the hydrogen?
Knowing that hydrogen is an energy carrier that can be made and exploited in a completely green way, the question turns to where it can (and should) be applied.
The push for decarbonisation has so far largely been a story of renewable electricity – that is, using renewables such as wind, solar power and hydropower to supply the electrical grids that bring power to homes, streets and cities. Renewables nowadays account for almost 30% of the global share of electricity generation, but some notable drawbacks have prevented it from surpassing the still-dominant coal- and natural gas-fired power plants. Although we can expect the cost of renewables to fall as technology advances, an electrical grid supplied entirely by renewables is not really feasible as long as they continue substantially to rely on unpredictable weather conditions.
With this in mind, a key aspect of the hydrogen economy is using hydrogen to decarbonise areas that have been difficult to electrify.
Fuel cell vehicles harness the clean reaction of hydrogen and oxygen in an electrochemical cell to produce a continuous electric current, which in turn powers a vehicle's on-board electric motor.
Fuel cell vehicles are, of course, emission-free and represent a green alternative to petrol cars. Since hitting the market they have been in competition with battery-powered electric vehicles, which have historically held a commercial advantage because battery charging stations have been easier to install than hydrogen refuelling stations. Fuel cell vehicles do however have notable benefits over electric vehicles in terms of their range and speed of refuelling, which come into their own in particular in heavy transportation.
In the trucking industry, for instance, a power source is needed that can propel heavy loads over long distances while minimising idle refuelling time. Batteries become a lot less feasible the bigger vehicles get and the longer they need to travel between stops. Increasing the range of a battery-powered vehicle requires piling on more and more heavy, space-consuming batteries, making the vehicle impractically heavy and lacking in space for cargo. Increasing the range of a fuel cell vehicle in principle does not require adding more fuel cells, just a bigger hydrogen tank, and as hydrogen is far more energy-dense than even the best batteries this comes at a much smaller premium of weight and space. It also allows for refuelling in a matter of minutes as opposed to several hours of battery recharging. Put another way, in order to supply energy down a wire to an electric vehicle at the same rate as it can be supplied to a hydrogen fuel cell vehicle by pouring hydrogen into its fuel tank, the wire would have to be a foot thick.
With investment in our hydrogen refuelling infrastructure, fuel cell vehicles could certainly hit the mainstream in personal transportation in the coming years. Meanwhile, in the shorter term it could well be the solution to decarbonisation of the still emission-intensive heavy transportation industry. We can expect to see fuel cell-powered trucks, trains and ships in common use in the not-so-distant future.
Renewable energy storage
Another use of hydrogen is as a means by which to store energy produced by renewables at times when demand does not meet supply. Renewable electricity is largely dependent on uncontrollable factors such as the weather, meaning that at times where the amount being generated exceeds the demand of the area it supplies, that energy goes to waste unless stored somewhere.
Batteries are one way of storing surplus energy from renewables, but this is only economically feasible for short-term storage. Instead, surplus electricity from renewables on a particularly windy or sunny day can be diverted to an electrolyser to make green hydrogen, which can then be stored for long periods in underground salt mines, for example. Underground storage of hydrogen in the long term is far cheaper than the use of batteries to store an equivalent amount of energy. The hydrogen can then be accessed whenever it is needed, such as when energy demand exceeds supply from renewables, and burned cleanly in a power plant.
Use in heavy industry
Hydrogen is already used in significant quantities in industrial processes. The biggest hydrogen consumers nowadays are oil refineries, which use hydrogen to reduce sulphur content in petroleum, and ammonia and methanol plants which both take in hydrogen as a feedstock to produce their respective final products. These processes have historically used grey hydrogen as it is the cheapest to produce, and largely continue to do so. By weaning these processes off grey hydrogen and onto the green variety, perhaps via blue hydrogen as an intermediary, the sustainability of these processes could be dramatically improved.
In steel production, the reduction of iron ore is traditionally achieved using carbon monoxide obtained by burning the carbon-based fuel "coke". Iron ore can also be reduced by hydrogen with no carbon emissions, making the steel industry another potential target area for decarbonisation using green hydrogen.
Integration in the gas grid
The lack of hydrogen infrastructure to deliver it safely and efficiently from producers to consumers is one of the greatest barriers to its adoption as a primary fuel. In lieu of a purpose-built hydrogen pipeline system, which will take billions in investment, it has been proposed in the meantime to integrate hydrogen into the already existing natural gas grid. Hydrogen can be burned by end appliances along with the natural gas - the more hydrogen that is integrated with the natural gas, the greater the reduction in associated carbon emissions.
It has been demonstrated in the UK that so-called hydrogen "blending" is possible – natural gas networks can handle blends of up to 20% hydrogen by volume without any significant modification to existing infrastructure. Beyond the 20% threshold however, current domestic appliances such as boilers will need to be changed out for appliances compatible with a higher mix of hydrogen. Pipelines themselves would also need modification or replacement if hydrogen content were to increase, as existing metal or polyethylene pipes can be weakened by embrittlement if exposed to high concentrations of hydrogen.
In the long term, immense investments may need to be made to achieve a grid system suitable for carrying 100% hydrogen. But hydrogen blending in existing gas pipelines is a viable strategy at least in the short term for decarbonising our energy network.
The fuel of the future?
We have known for a couple of centuries that hydrogen can be used as a fuel, and the concept of a "hydrogen economy" for providing the world with clean sustainable energy has been around for several decades. Up to now, the sheer cost of producing hydrogen cleanly, storing it safely and integrating it into our energy systems has confined the hydrogen economy to the periphery of human attention. Fossil fuels have always been, and continue to be, cheap and convenient, and the activation energy required to transition to an economy based on an entirely different fuel has simply been too high to surpass thus far.
As a result of the continued innovation and research from those who have continued to believe in the hydrogen economy, the mountain of problems associated with it has been crumbling at an ever-increasing rate. We know how to make green hydrogen, and the cost of doing so can be expected to plummet as we get better at it. We understand the areas where hydrogen can be most effectively and viably employed to achieve decarbonisation. Perhaps most importantly, people appreciate more than ever the gravity of our environmental situation, and the urgency with which we have to revolutionise our energy landscape to address it.
If there ever was a time to start taking hydrogen seriously as a true fuel of the future, now is the time.
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