Hydrogen has use as a fuel in many applications. It can achieve this in a number of ways, either by direct combustion, electrochemical oxidation or conversion into different fuel, which may be easier to transport and store.
Hydrogen for electricity generation
Fuel cells are essentially electrolysers operated in reverse mode. They combine hydrogen and oxygen from air to generate electricity. This is an electrochemical conversion process and thus
avoids mechanical work from thermal energy, which is typical of combustion technologies, leading to much higher efficiencies. In analogy with electrolyser technologies, several fuel cell technologies exist, which are characterised by their ion conducting electrolyte. Depending on this, the technologies vary in operating temperature, materials selection and fuel capability. PEM (proton exchange membrane) fuel cells are currently the fuel cell of choice for transport applications, due to their relatively light weight, resilience to vibration and low operating temperature (60 – 80°C). Their electrolytes are polymer based (perfluorosulfonic acid) and can suffer from mechanical failure, particularly upon ageing. Electrodes contain platinum catalyst which require high purity hydrogen [1]. [2] to avoid contamination rendering the fuel cells/stacks inoperable. The image shows HEXIS Galileo 1000N 1 kWe CHP system installed at the University of St Andrews. The system utilises a solid oxide fuel cell stack.
Other fuel cell types include alkaline, phosphoric acid and solid oxide and these are typically aimed at stationary power production. Alkaline fuel cells operate at a similar temperature as PEM fuel cells, but can use cheaper electrode materials, due to the alkaline nature of the electrolyte. This also means they do not suffer as much from high electrode overpotentials, resulting in higher electrical efficiencies. Challenges are the corrosive liquid electrolyte which is prone to ‘poisoning’ by CO2, leading to carbonate formation and electrode deactivation. They therefore require either a CO2 scrubber when operating in ambient air, or pure oxygen as the oxidant, adding to system complexity and cost. Phosphoric acid and solid oxide fuel cell operate at higher temperatures, making them more tolerant towards fuel impurities. A major challenge for PAFC is the highly corrosive electrolyte. SOFC are ceramic cells which operate at 500 – 1000°C, allowing the use of cheaper electrode catalysts, such as nickel. The high operating temperatures however requires careful selection of materials to ensure mechanical and chemical compatibility and thus long-term stability. Lowering the operating temperature allows for higher electrical efficiencies, whilst imposing fewer thermal strains on the system. The higher operating temperatures as compared to other fuel cell technologies, however, offers the potential of combined heat and power (CHP) generation, allowing for overall system efficiencies up to 90%.
Scotland has particular research strengths in materials development for high temperature solid state fuel cell technologies. On a system level, fuel cells need to be integrated into power systems, e.g. an automotive powertrain. Arcola Energy is one of the UK’s leading system integrators for mobile applications and are based in Dundee. They are involved in a number of Scottish projects, aimed at delivering Fuel Cell Electric double deck buses, refuse collection vehicles and a demonstrator train. In such integrated systems, understanding duty cycles of different vehicles is key in optimally designing aspects of the vehicles, such as battery and fuel cell size, as well as onboard hydrogen storage capacity. Researchers in Glasgow University have been working on developing modelling and simulation tools in this field. The role of fuel cells in stationary applications has thus far mostly been as backup power supply or delivering power to poorly connected areas, electrically, but with access to gas supply.
Hydrogen for heat
Domestic and commercial heating is one of the main culprits responsible for CO2 emissions in the UK. Replacing natural gas by hydrogen is seen as one option to decarbonise heating, requiring an upgrade to boilers to be compatible with burning hydrogen. A trial project is underway in Fife, H100, aiming to install hydrogen boilers and heat 300 homes in Levenmouth with 100% hydrogen. Other alternative technologies will additionally be required and potentially more suited to decarbonise heating, such as ground source heat pumps, but hydrogen will likely be part of the mix. The previously mentioned fuel cell powered CHP units are also likely to contribute to decarbonising heating, and their modular design up to hundreds of kilowatts allows for use in commercial as well as domestic settings of various sizes.
Hydrogen for synthetic fuels and ammonia
For the most demanding transport and mobile applications, such as aviation and shipping, it is likely a fuel with a higher volumetric density than hydrogen is required. Liquid fuels, such as synthetic fuels and ammonia are the most likely candidates to fulfil this role, although across the globe some are pursuing liquid hydrogen as fuel too. Synthetic fuels can be synthesised using a mixture of carbon monoxide and hydrogen and performing Fischer-Tropsch reactions, yielding hydrocarbon of varying length, depending on process conditions and reactor types. Innovation in re-using carbon dioxide to be the feedstock for carbon monoxide through electrolysis is ongoing. When both carbon monoxide and hydrogen are produced by electrolysis/renewable energy, these can be feedstock for green synthetic fuels. Similarly, ammonia may be synthesised by combining hydrogen and nitrogen and when the hydrogen is green, this offers potential for green ammonia. Ammonia can be liquefied at relatively mild conditions and is thus a viable option as a hydrogen containing energy carrier. The established method of ammonia production is through the Haber-Bosch process, which itself is highly energy intense, due to the need for elevated reaction temperatures and pressures. Research activity is ongoing to find a process with milder conditions, such as electrochemical conversion. Ammonia can either be re-converted to hydrogen or potentially utilised directly as a fuel.
[1] https://www.sae.org/standards/content/j2719_201109/
[2] https://www.iso.org/obp/ui/#iso:std:iso:14687:ed-1:v1:en