Hydrogen is a gas which consists of molecules with two atoms (H2). Hydrogen molecules are energy rich compared to most of the other hydrogen compounds, especially Water (H2O). Usually energy is set free when hydrogen binds with other elements. The difference in energy is the reason for us being able to store energy with hydrogen. The following example with water makes it clear:
Water+ Energy → Hydrogen + Oxygen
Hydrogen + Oxygen → Energy + Water
The release of energy when hydrogen binds with other elements is the reason that molecular hydrogen can be found in the environment only in fractional quantities. Hydrogen is thus not a primary energy carrier like coal, natural gas or crude oil, but a secondary energy carrier, like electricity, which must be produced from hydrogen-rich compounds in conversion processes.
Conventional techniques for molecular hydrogen production mostly base on thermic separation of hydrogen from natural gas (methane) and other fossil hydrocarbons. Hydrogen production from biomass or through electricity deriving from renewable energy sources is more environmental friendly, though.
The common technique for hydrogen production with electricity is electrolysis of water. Water is being split into hydrogen and oxygen and by that the electrical energy is converted into chemical energy. There is, however, a loss of ca. 35% in the form of heat if it is not used downstream for other purposes.
Elektrical Energy + Water → Hydrogen + Oxygen + Heat
The energy caught in the hydrogen can be utilized again by combustion in engines or turbines or by "reversing" it in fuel cells.
In fuel cells electricity is produced directly. Further products are water and, again, heat in the form of steam, but no fumes.
Hydrogen + Oxygen → Electrical Energy + Water + Heat
Despite the losses of conversion - for both conversion steps together this could sum up to ca. 50 % - this kind of energy storage can be sensible in many cases. The decoupling of hydrogen production and usage creates new ways of integration of renewable energy sources like wind and sun into the existing energy supply structures.
For the stationary use of hydrogen a large proportion of the waste heat can be used in cogeneration plants. With electrolysis a use of the waste heat is theoretically also possible, but not yet field-tested.
Hydrogen has a high energy density per mass:
- 1 kg contains as much energy as 3 kg of petrol (33.33 kWh/kg hydrogen)
Hydrogen is, however, a very light gas. Therefore, its energy density per volume is rather low:
- about 12 cubic meter of uncompressed hydrogen only contain as much usable energy as 1 litre of petrol
By compression or liquification of hydrogen or by means of <a href> metal hydrides energy densities near that of petrol can be achieved.
- 1 litre of liquid hydrogen equals about 0.27 litre of petrol
Storage tanks for hydrogen are much heavier than those for petrol or diesel, though.
Hydrogen has clear advantages over conventional forms of electricity storage (e.g. batteries or pumped-storage hydroelectric plants):
- storage volumes from a few grams to several thousand tons are possible
- the option to choose between different storage solutions (pressurised or liquefied hydrogen, metal hydrides), depending on the purpose
- loss free storage over a prolonged period of time in the case of pressure and hydride storages
For the distribution of hydrogen there are also several options: transport through pipelines or by road is being used even today. In the future ships could also be used for transportation.
Hydrogen is multifunctional and serves not only as a stationary electricity storage, but also as fuel for cars and buses.
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