With decarbonisation options limited for some applications, Tim Hartley looks at some of the practicalities of green hydrogen as a substitute fuel. In particular, comparing some of the containment methods for hydrogen which determine the cost and scale of systems required to convert to this renewable alternative.
Why convert in the first place?
With the issue of climate change and our impact on the planet pushing further to the front of the public’s consciousness, legislation and consumer pressure is driving industry to demonstrate green credentials.
The UK government announced in March 2021 that it intends to use economics as one of the primary drivers for industry to achieve its 2050 net zero targets which it made into law in 2019. The foremost lever amongst these is ‘Carbon Pricing’.
Following Brexit, the UK has decided to continue to use an emissions credit system very similar to that of the EU, with the total amount of emissions from heavily polluting industries restricted within a budget.
It’s predicted to cost industry around £400m annually until the end of 2023 when the fourth emissions budget begins, and the restrictions become even tighter . As subsequent budgets shrink as the 2050 target approaches, the cost of emitting will continue to rise, meaning that investing in green technologies will provide significant economic benefit in addition to improving customer perception of a company.
With this in mind, the primary concern in reducing emissions for many industries is to find the most effective route to decarbonisation. One option, which has gained much popularity and support, is green hydrogen.
The main attraction of hydrogen is that when it’s recombined with oxygen, either through combustion or in a fuel cell, energy is released with the only product being water – with no tailpipe greenhouse gas emissions. Hydrogen has therefore been investigated as an alternative to fossil fuels for many years. The Carbon Pricing drivers mean that the technology is now reaching a stage where it’s becoming both a technically feasible and an economic choice for an increasing number of applications.
The UK government has stated in its recent ten-point plan: “[our] aim is for the UK to develop 5GW of low carbon hydrogen production capacity by 2030”. This will be backed up by £240m worth of investment into hydrogen technologies, with 20% of the 5GW supply estimated to be available by 2025.
There are three main advantages that hydrogen has over current battery technologies: energy density, refill time and retrofitting. Hydrogen has an extremely high gravimetric energy density (excluding containers) – higher than that of petrol and diesel. This is a large advantage in applications where weight is a concern, such as transportation.
Hydrogen can also be used in a more comparable way to conventional transport fuels, physically loading up a tank very rapidly for readily available use. It also has the capability for retrofitting in certain applications. If the capital expenditure of new equipment or infrastructure is a barrier, or the current equipment is essential to a business, it’s sometimes possible to convert this equipment to burn hydrogen instead of fossil fuels.
What are the challenges?
Whilst there are obvious benefits to using hydrogen, there are also challenges to be overcome that will depend on how hydrogen is to be used. The primary consideration when designing for hydrogen is the containment method. Whilst hydrogen has a very high energy density per unit weight, it is gas at standard temperatures. This means that it takes up much more space than petrol or batteries if left in its natural state. Additionally, as hydrogen is a very small molecule, it presents additional challenges containing the gas without leaks.
There are several methods to circumvent this such as extreme compression, reducing the temperature until the hydrogen becomes a liquid or storing within another compound. These in turn have their own considerations and different challenges.
Solution 1: Compressed hydrogen
Compressed hydrogen has predominantly been investigated for the automotive industry where the desire is to achieve a volumetric energy density comparable to that of conventional fuels. The hydrogen is transported in bulk containers between 300-500 bar as the infrastructure is not yet in place in most areas. It then needs to be compressed again at the point of dispense, usually to around 700 bar to achieve the best energy density. Around 5-20% of the hydrogen’s usable energy can be lost to compression.
To create a vessel that is able to withstand these high pressures requires expensive materials. Production has focussed on achieving this for containers around 6L in volume, equivalent in vehicle range to a conventional tank.
To use this type of storage would require significant investment into tanks that could be used for other applications. Currently, if a larger vehicle such as a truck or a ship converts to hydrogen, standard practice is to use an array of smaller tanks, further reducing the volumetric energy density.
It should also be noted that the increase in pressure is accompanied by an increase in the size and weight of the container to withstand the pressure. Therefore, the benefits of the higher compression must be weighed against the higher costs and compression energy.
Solution 2: Liquid hydrogen
Liquid hydrogen is a more popular option for storing hydrogen due to its higher volumetric energy density and better scalability of container past a certain size. This form requires high levels of tank insulation but lends itself to larger containers as the surface area to volume ratio decreases with container size leading to lower heat loss per unit of stored hydrogen for larger tanks of the same shape.
One of the current main inhibitors with this technology is the high energy requirement to liquefy the hydrogen. It can require between 30-40% of the hydrogen’s usable energy to achieve the 20K storage temperature that is required.
Solution 3: Storage in other compounds or materials
Other methods of storing hydrogen involve storing it in metal organic frameworks (MOF) or other compounds; either liquid organic hydrogen carriers (LOHC) or small molecules such as ammonia or methanol. MOFs show much promise for hydrogen storage, but are still in the research and development stage with minimal commercial options currently available.
Additionally, for both methanol and LOHC there needs to be an extraction of the hydrogen from the compound beforehand, using a significant amount of the stored energy or, for methanol, capture of the carbon following the utilisation of the fuel. For ammonia, one of the biggest concerns is sourcing enough green ammonia for the desired application, despite its promising characteristics. Even though it has moderately high energy density compared to some other forms of hydrogen storage, it’s still half that of gasoline, meaning that if a conversion were made, there would be a compromise with tank space.
How to decide between the different solutions?
These options have advantages and disadvantages that are highly dependent on the application intended and are affected by factors such as infrastructure, availability of components or availability of raw materials. The energy density of the hydrogen also impacts the distribution, with the financial and energy costs tied closely with how easily hydrogen can be transported.
Without pipelines to transport the hydrogen or suitable infrastructure in place to move it over long distances, there is a heavy reliance on fossil fuels, such as gas trucks. This then makes the location of the hydrogen source very important. Investment in distributed sources rather than a more centralised model is one way to circumvent this issue, but with this comes reduction in efficiencies and higher initial investment costs and infrastructure requirements.
Hydrogen presents a unique opportunity to fill a gap that will soon be vacated by conventional fossil fuels. There are many aspects to consider, with storage being one of the more crucial for deciding if hydrogen is the correct clean fuel to convert to. Whilst some aspects of this technology require advancements to become truly competitive with the fuels that they are replacing, these advancements are being funded and supported by multiple governments who strongly believe and are investing in hydrogen to be a key component of the transition to clean energy.
With large economies such as China promoting ambitious targets for hydrogen vehicles and production, the question of a hydrogen economy moves from an ‘if’ to a ‘when’.
If your company is interested in transitioning to renewable technologies or would like to investigate potential options available to minimise the impact of the change to renewables, please contact us at email@example.com.
Tim is a consultant at 42 Technology with experience in conceptual design, development, testing and prototyping. He has worked on many different products from a range of industries, from surgical devices to food and beverage handling systems, drawing in knowledge from other sectors to achieve the client’s goals.
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