section_path: Applications/Energy carrier
section_name: Energy carrier
paragraph_text: Energy carrier
  
  Elemental hydrogen is widely discussed in the context of energy as an energy carrier with potential to help to decarbonize economies and mitigate greenhouse gas emissions. This therefore requires hydrogen to be produced cleanly in quantities to be supplied in sectors and applications where cheaper and more energy-efficient mitigation alternatives are limited. These include heavy industry and long-distance transport. Hydrogen is a carrier of energy rather than an energy resource, because there is no naturally occurring source of hydrogen in useful quantities.
  
  Hydrogen can be deployed as an energy source in fuel cells to produce electricity or via combustion to generate heat. When hydrogen is consumed in fuel cells, the only emission at the point of use is water vapor. Combustion of hydrogen can lead to the thermal formation of harmful nitrogen oxides. The overall lifecycle emissions of hydrogen depend on how it is produced. Nearly all the world's current supply of hydrogen is created from fossil fuels. The main method is steam methane reforming, in which hydrogen is produced from a chemical reaction between steam and methane, the main component of natural gas. Producing one tonne of hydrogen through this process emits 6.6–9.3 tonnes of carbon dioxide. While carbon capture and storage (CCS) could remove a large fraction of these emissions, the overall carbon footprint of hydrogen from natural gas is difficult to assess as of 2021, in part because of emissions (including vented and fugitive methane) created in the production of the natural gas itself.
  
  Electricity can be used to split water molecules, producing sustainable hydrogen, provided the electricity was generated sustainably. However, this electrolysis process is currently more expensive than creating hydrogen from methane without CCS and the efficiency of energy conversion is inherently low. Hydrogen can be produced when there is a surplus of variable renewable electricity, then stored and used to generate heat or to re-generate electricity. Hydrogen created through electrolysis using renewable energy is commonly referred to as " green hydrogen ". It can be further transformed into synthetic fuels such as ammonia and methanol.
  
  Innovation in hydrogen electrolyzers could make large-scale production of hydrogen from electricity more cost-competitive. There is potential for hydrogen produced this way to play a significant role in decarbonizing energy systems where there are challenges and limitations to replacing fossil fuels with direct use of electricity.
  
  Hydrogen fuel can produce the intense heat required for industrial production of steel, cement, glass, and chemicals, thus contributing to the decarbonisation of industry alongside other technologies, such as electric arc furnaces for steelmaking. However, it is likely to play a larger role in providing industrial feedstock for cleaner production of ammonia and organic chemicals. For example, in steelmaking, hydrogen could function as a clean energy carrier and also as a low-carbon catalyst, replacing coal-derived coke. Hydrogen used to decarbonise transportation is likely to find its largest applications in shipping, aviation and, to a lesser extent, heavy goods vehicles, through the use of hydrogen-derived synthetic fuels such as ammonia and methanol and fuel cell technology. For light-duty vehicles including cars, hydrogen is far behind other alternative fuel vehicles, especially compared with the rate of adoption of battery electric vehicles, and may not play a significant role in future.
  
  Disadvantages of hydrogen as an energy carrier include high costs of storage and distribution due to hydrogen's explosivity, its large volume compared to other fuels, and its tendency to make pipes brittle.