The fundamental reasons for considering the adoption of hydrogen as a fuel, industrial feedstock and energy storage medium are presented. Hydrogen production methods are outlined, with reference to the colour prefixes used to describe different types of hydrogen. The relative greenhouse gas emissions and economics of green and blue hydrogen production are considered for achieving a ‘net zero’ climate-neutral energy system by 2050. In general, it appears that green hydrogen will soon be cheaper than blue hydrogen due to the falling costs of renewable electricity and electrolysers, then cheaper than grey hydrogen, and in the long term potentially cheaper than natural gas.
This article explores the importance of renewable hydrogen in achieving a ‘climate neutral’ energy system, which will require a large amount of renewable electricity and a very large amount of renewable hydrogen. Because of the fundamental need for energy storage to match supply and demand, a two-carrier approach needs to be adopted, where both electricity and hydrogen are derived from renewable energy. National hydrogen strategies, electrolyser deployment plans and the actions required by governments to overcome the current policy vacuum are discussed. It is recommended that a cross-sector ‘green electrons and green molecules’ strategy is taken, and that policies are developed urgently for advancing the adoption of renewable hydrogen.
The use of any fuel depletes the oxygen content of the atmosphere, with one exception: hydrogen produced from water. Water electrolysis liberates oxygen from water in the precise stoichiometric ratio required to oxidise (and hence release energy from) the co-produced hydrogen. As a commercial fuel production process, electrolysis is unique in providing the oxidant as well as the fuel; electrolytic oxygen can thereby replenish the consumption of atmospheric oxygen due to hydrogen use. Furthermore, the amount of water consumed during electrolysis is reproduced when the hydrogen is oxidised. So the use of electrolysers and electrolytic hydrogen does not affect global oxygen and water resources: ‘green’ hydrogen may thus be described as the only oxygen and water balanced fuel. Conversely, the use of hydrogen derived from fossil fuels (with or without carbon capture and storage, CCS) depletes the oxygen resource and increases water vapour emissions to the atmosphere, which enhances the rate of global warming. Therefore, a worldwide multi-TW deployment of electrolysers could provide very substantial amounts of hydrogen for the energy system, and oxygen for the global ecosystem. This should be done in combination with other measures for combatting oxygen depletion (such as reducing combustion, increasing forestation, and reducing nutrient inputs to the ocean from sewage and agriculture). In this way the long-term objective should be to stabilise, or even increase slightly, the concentrations of atmospheric and aquatic oxygen, and possibly speed up the decay of atmospheric methane. Clearly the production-and-use of hydrogen derived from fossil fuels contravenes this objective, and should cease without delay.
In addition to fuel cell applications, water electrolysis affords several opportunities for producing high-grade heat by utilising electrolytic hydrogen and oxygen in combustion applications. This can reduce or eliminate the atmospheric emissions associated with the combustion of conventional fuels. Importantly, net-zero heat can be produced if the electrolyser is powered by renewable electricity, whereby the output gases may be termed green hydrogen and green oxygen. This paper provides an overview of prospective applications for electrolysers in the heat sector by considering three general implementation pathways based on how the electrolytic oxygen is used.
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