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Hydrogen vehicle
A hydrogen vehicle is a vehicle that uses hydrogen to move. Hydrogen vehicles include some road vehicles, rail vehicles, space rockets, forklifts, ships and aircraft. Motive power is generated by converting the chemical energy of hydrogen to mechanical energy, either by reacting hydrogen with oxygen in a fuel cell to power electric motors or, less commonly, by hydrogen internal combustion.
Hydrogen burns cleaner than fuels such as gasoline or methane but is more difficult to store and transport because of the small size of the molecule. As of the 2020s hydrogen light duty vehicles, including passenger cars, have been sold in small numbers due to competition with battery electric vehicles. As of 2021[update], there were two models of hydrogen cars publicly available in select markets: the Toyota Mirai (2014–), the first commercially produced dedicated fuel cell electric vehicle (FCEV), and the Hyundai Nexo (2018–). The Honda CR-V e:FCEV became available, for lease only, in very limited quantities in 2024.
As of 2019, 98% of hydrogen is produced by steam methane reforming, which emits carbon dioxide. It can be produced by electrolysis of water, or by thermochemical or pyrolytic means using renewable feedstocks, but the processes are currently expensive. Various technologies are being developed that aim to deliver costs low enough, and quantities great enough, to compete with hydrogen production using natural gas.
Vehicles running on hydrogen technology benefit from a long range on a single refuelling, but are subject to several drawbacks including high carbon emissions when hydrogen is produced from natural gas, capital cost burden, high energy inputs in production and transportation, low energy content per unit volume at ambient conditions, production and compression of hydrogen, and the investment required to build refuelling infrastructure around the world to dispense hydrogen. In addition, leaked hydrogen is an invisible, highly flammable gas and has a global warming effect 11.6 times stronger than CO2.
The rationale for hydrogen vehicles lies in their potential to reduce reliance on fossil fuels, associated greenhouse gas emissions and localised air pollution from transportation. This would require hydrogen to be produced cleanly, for use in sectors and applications where cheaper and more energy efficient mitigation alternatives are limited.
Many large rockets use liquid hydrogen as fuel, with liquid oxygen as an oxidizer (LH2/LOX). An advantage of hydrogen rocket fuel is the high effective exhaust velocity compared to kerosene/LOX or UDMH/NTO engines. According to the Tsiolkovsky rocket equation, a rocket with higher exhaust velocity uses less propellant to accelerate. Also the energy density of hydrogen is greater than any other fuel. LH2/LOX also yields the greatest efficiency in relation to the amount of propellant consumed, of any known rocket propellant.
A disadvantage of LH2/LOX engines is the low density and low cryogenic temperature required to maintain of hydrogen as a liquid, which means bigger and insulated and thus heavier fuel tanks are needed compared to methane, although methane is more polluting. Another disadvantage is the poor storability of LH2/LOX-powered rockets: Due to the constant hydrogen boil-off, the rocket must be fueled shortly before launch, which makes cryogenic engines unsuitable for ICBMs and other rocket applications with the need for short launch preparations.[citation needed] For first stages, dense fuelled rockets in studies may show a small advantage, due to the smaller vehicle size and lower air drag.
LH2/LOX were also used in the Space Shuttle to run the fuel cells that power the electrical systems. The byproduct of the fuel cell is water, which is used for drinking and other applications that require water in space.
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Hydrogen vehicle
A hydrogen vehicle is a vehicle that uses hydrogen to move. Hydrogen vehicles include some road vehicles, rail vehicles, space rockets, forklifts, ships and aircraft. Motive power is generated by converting the chemical energy of hydrogen to mechanical energy, either by reacting hydrogen with oxygen in a fuel cell to power electric motors or, less commonly, by hydrogen internal combustion.
Hydrogen burns cleaner than fuels such as gasoline or methane but is more difficult to store and transport because of the small size of the molecule. As of the 2020s hydrogen light duty vehicles, including passenger cars, have been sold in small numbers due to competition with battery electric vehicles. As of 2021[update], there were two models of hydrogen cars publicly available in select markets: the Toyota Mirai (2014–), the first commercially produced dedicated fuel cell electric vehicle (FCEV), and the Hyundai Nexo (2018–). The Honda CR-V e:FCEV became available, for lease only, in very limited quantities in 2024.
As of 2019, 98% of hydrogen is produced by steam methane reforming, which emits carbon dioxide. It can be produced by electrolysis of water, or by thermochemical or pyrolytic means using renewable feedstocks, but the processes are currently expensive. Various technologies are being developed that aim to deliver costs low enough, and quantities great enough, to compete with hydrogen production using natural gas.
Vehicles running on hydrogen technology benefit from a long range on a single refuelling, but are subject to several drawbacks including high carbon emissions when hydrogen is produced from natural gas, capital cost burden, high energy inputs in production and transportation, low energy content per unit volume at ambient conditions, production and compression of hydrogen, and the investment required to build refuelling infrastructure around the world to dispense hydrogen. In addition, leaked hydrogen is an invisible, highly flammable gas and has a global warming effect 11.6 times stronger than CO2.
The rationale for hydrogen vehicles lies in their potential to reduce reliance on fossil fuels, associated greenhouse gas emissions and localised air pollution from transportation. This would require hydrogen to be produced cleanly, for use in sectors and applications where cheaper and more energy efficient mitigation alternatives are limited.
Many large rockets use liquid hydrogen as fuel, with liquid oxygen as an oxidizer (LH2/LOX). An advantage of hydrogen rocket fuel is the high effective exhaust velocity compared to kerosene/LOX or UDMH/NTO engines. According to the Tsiolkovsky rocket equation, a rocket with higher exhaust velocity uses less propellant to accelerate. Also the energy density of hydrogen is greater than any other fuel. LH2/LOX also yields the greatest efficiency in relation to the amount of propellant consumed, of any known rocket propellant.
A disadvantage of LH2/LOX engines is the low density and low cryogenic temperature required to maintain of hydrogen as a liquid, which means bigger and insulated and thus heavier fuel tanks are needed compared to methane, although methane is more polluting. Another disadvantage is the poor storability of LH2/LOX-powered rockets: Due to the constant hydrogen boil-off, the rocket must be fueled shortly before launch, which makes cryogenic engines unsuitable for ICBMs and other rocket applications with the need for short launch preparations.[citation needed] For first stages, dense fuelled rockets in studies may show a small advantage, due to the smaller vehicle size and lower air drag.
LH2/LOX were also used in the Space Shuttle to run the fuel cells that power the electrical systems. The byproduct of the fuel cell is water, which is used for drinking and other applications that require water in space.
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