Just like coal, oil, natural gas and electricity, hydrogen is also an energy carrier – it contains energy that can be released, for example through combustion. Hydrogen has a high energy density of around 33 kWh per kilogram, almost three times the value of petrol. In practice, however, this high value is rather disappointing because the density of hydrogen is very low. The energy density of one liter of liquid hydrogen at extremely low temperatures of less than -250 °C is therefore a factor of three lower than that of one liter of petrol. And a liter of gaseous compressed hydrogen at a pressure of several tens or hundreds of bars can contain even less energy than liquid hydrogen. And this is independent of the weight of the hydrogen storage tank with a sufficiently cold-insulating or pressure-resistant metal shell.
Natural gas, oil and coal can be ‘simply’ mined from the ground, where this mining of course costs some energy. Hydrogen in its elemental form is abundant on earth, for instance as one of the building blocks chemically bound in the water molecule. Molecular gaseous or liquid hydrogen, on the other hand, is does hardly occur freely in nature. The most common way to produce hydrogen is in a chemical way by having hydrocarbons such as natural gas react with steam at high temperatures. Hydrogen is thus released from natural gas, but the greenhouse gas carbon dioxide is also produced.
This means that hydrogen still has a fossil origin. An ideal way to generate ‘clean’ hydrogen is to pass an electric current through water, thus electrolysing the water into the gases hydrogen and oxygen. Preferably, this electric current has a non-fossil origin, and is generated, for example, by converting sunlight into useful energy with a solar cell or by harvesting the power of the wind with a wind turbine. These gases hydrogen and oxygen can be stored separately or supplied to a fuel cell that produces water again, as well as electricity that can be used meaningfully – for example to power an electric motor.
By the way: in a fossil-free future, the existing infrastructure of natural gas pipelines can be used to transport hydrogen. When countries that use central heating systems to heat their homes want to move away from natural gas as a fuel, hydrogen might be a substitute. After all, the entire pipeline system is already in the ground, right up to the house. Pipeline materials such as steel and polyethylene are chemically resistant to hydrogen and are ‘hydrogen-tight’, and gas pipelines made from these materials can transport hydrogen just as well as natural gas. Whereas 1 liter of natural gas, depending on its composition, generates 30 to 40 kJ of combustion heat, for hydrogen gas this is slightly more than 10 kJ per liter. Therefore, to have the same amount of heating energy available, more powerful pumps100 will be needed to pump three times the amount of hydrogen through the pipelines. The central-heating boiler will also have to be slightly modified for a different fuel/air ratio. The transition from natural gas to hydrogen can be done gradually, by adding (increasingly) more hydrogen gas to the (increasingly smaller quantities of) natural gas in the pipelines.
Actually, the main use of hydrogen is not for energy applications, but is in the chemical industry. More than half of all synthesised hydrogen finds its application in the industrial preparation of fertiliser via the Haber-Bosch synthesis route. Hydrogen gas (H2) and nitrogen gas (N2) react under high pressure of about 200 bar and a temperature of about 450 °C at the surface of iron particles to form ammonia (NH3), an important raw material for nitrogen-based fertilisers such as ammonium nitrate and urea. A large part of the fertiliser production is thus based on natural gas.
Although nitrogen gas (N2) is abundant in the air and gets in direct contact with plants, they cannot use it as nitrogen supply. This is because the gas is very inert due to the strong triple bond between the two nitrogen atoms. In order to be able to produce the nitrogen compounds they need themselves – such as proteins – plants need nitrogen in a reactive form. They obtain this usable form of nitrogen from nitrates (NO3–) and ammonium (NH4+) contained in (artificial) fertilisers.