25-05-2023 | Posted by Principia
Climate change is forcing us to look for alternative energy sources and carriers to reduce carbon emissions. In this endeavour, hydrogen promises to take a progressively increasing role.
Hydrogen is the most abundant chemical substance in the universe, constituting roughly 75% of all normal matter (excluding dark matter and dark energy). Stars such as the Sun are mainly composed of hydrogen in the plasma state. However, in the Earth’s crust, it only represents 0.14% by weight, mostly in the form of water and organic compounds. Although molecular hydrogen barely exists in nature, it can be produced from various sources and it constitutes an energy carrier that can be used to store, move, and deliver energy.
And why is hydrogen potentially useful in the context of sustainable energy? Because it can be burned to produce heat or combined with oxygen in fuel cells to generate electricity directly, with water being the only emission at the point of usage. On a mass basis, hydrogen has a stupendous energy content of 120 MJ/kg. Just look at what powers rocket engines to go to the Moon.
The problems are producing, storing, and distributing the hydrogen. Its overall lifecycle emissions depend on how it is produced. Nearly all the world’s current supply of hydrogen comes from fossil fuels. The main method is steam methane reforming, with hydrogen being produced in a high-temperature reaction between steam and methane. Producing one tonne of hydrogen through this process emits 6.6–9.3 tonnes of carbon dioxide. While carbon capture and storage can remove a large fraction of these emissions, the overall carbon footprint of hydrogen from natural gas is difficult to assess, in part because of emissions created in the production of the natural gas itself.
Water can also be separated into oxygen and hydrogen by electrolysis. Electricity is used to split the water molecules, producing sustainable hydrogen provided the electricity was generated sustainably. This process is currently more expensive than creating hydrogen from methane and the efficiency of energy conversion is inherently low. But hydrogen can be produced when there is a surplus of renewable electricity, then stored and used to generate heat or to re-generate electricity.
The low density of hydrogen gas complicates its efficient storage. Density can be increased by high pressures, but then one must deal with the problems of the high pressures. Hydrogen can also be liquefied, though this requires cooling it below 20 K at atmospheric pressure or, at any pressure, at least below its critical point of 33 K. And even then, the density is still very low: note that a cubic metre of water contains 111 kg of hydrogen while one of liquid hydrogen only has 71 kg.
Innovation in electrolysers could make large-scale production of hydrogen from electricity more competitive. And hydrogen could play a significant role in decarbonising energy systems where it would be very difficult to replace fossil fuels with electricity. Hydrogen fuel can produce the intense heat required for industrial production of steel, cement, glass, and chemicals. For steelmaking, hydrogen can function as a clean energy carrier and simultaneously as a low-carbon catalyst replacing coal-derived coke. In transportation, hydrogen would burn relatively cleanly, with some NOx emissions, but without carbon emissions. The disadvantages of hydrogen as an energy carrier include high costs of storage and distribution due to its explosivity, its large volume compared to other fuels, and its tendency to make pipes brittle.
Currently the largest users of hydrogen in the world are industries such as oil refining, and production of ammonia and methanol. For the time being, its use as an energy carrier remains marginal, but this may not be true for long.