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Production of hydrogen The secondary energy carrier hydrogen, like the secondary energy carrier electricity, can be produced from all forms of energy :
Of the not entirely complete list, only the first two processes are of technical and economic importance. Only these will be explained in more detail.
water electrolysis When discussing the production of hydrogen in connection with the energy industry, the production of hydrogen by means of electrolysis is usually meant. The background is the storage of excess electricity in the form of hydrogen. Therefore, the principle of electrolysis should be briefly presented here. |
Electrolysis is the reverse process to the fuel cell . In principle, it can be the same construction. During electrolysis, water and electricity must be supplied. With the fuel cell, hydrogen and oxygen (air) must be supplied. A fuel cell produces electricity and water. Both cell types can be built as large or as small as you like. In the case of fuel cells, sizes in the kW scale are preferred, and in the case of electrolysers, sizes in the MW scale are preferred |
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Principle of the fuel cell If you immerse 2 electrodes in an electrolyte and supply hydrogen and oxygen (air) to the electrodes, current begins to flow from around 1V. Salt solutions, bases or acids are suitable as electrolytes. Shown here is an acidic (proton conducting) plastic membrane that requires porous electrodes |
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Principle of water electrolysis If you immerse 2 electrodes in an electrolyte, the splitting of water into hydrogen and oxygen begins at a voltage of 1.23 V. At 1.48V the cell operates at 100% energy efficiency. At higher voltages, the cell heats up. The supplied energy is then not only bound to hydrogen, but is partly released as heat. Salt solutions, bases or acids are suitable as electrolytes. Shown here is an acidic (proton conducting) plastic membrane that requires porous electrodes |
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Hydrogen economy with electrolysis hydrogen Hydrogen from electricity is always more expensive than the electricity from which it was produced. This statement can often be read: It is nonsensical to produce gas from electricity with losses that is then converted back into electricity with losses. That's true when hydrogen is used in a niche and all that matters is electricity. This is different in a hydrogen economy with a systemic surplus of electricity, also because the infrastructure for hydrogen is simpler and in a heat-driven energy economy, electricity is provided without losses. The cost-effective distribution of the hydrogen alone would justify electrolysis. Economically, the situation today is that electricity from fossil power plants is not competitive with electricity from renewable sources. We only pay part of these electricity costs from fossil power plants via the electricity bill. We pay the social costs of power generation (experts say external costs) as health insurance contributions, illness and death (70,000 deaths per year from particulate matter in Germany alone) or shift the costs to future generations (CO2). We pay another part of the costs with tax deductions, because the subsidies for fossil fuels from the federal budget alone are higher than all the costs caused by the EEG. |
hydrogen from biomass Hydrogen can also be chemically obtained directly from biomass. This happens by reacting with water vapor at 800°C to 1000°C. This thermochemical reaction is called steam reforming. The chemical model equations are: C 6 H 9 O 4 + 8 H 2 O = 6 CO 2 + 12.5 H 2 (allothermic) C 6 H 9 O 4 + 1.095 O 2 + 5.81 H 2 O = 6 CO 2 + 10.31 H 2 (autothermic) It is an endothermic reaction in which no heat is released, but heat must be supplied for the reaction to be possible in the first place. This means that all the energy that is supplied comes out bound to hydrogen at the end. If energy is supplied from outside (e.g. electric heating), one speaks of an allothermic reaction process. If the required heat is generated by partial combustion of the biomass, this is referred to as an autothermal reaction. This procedure requires oxygen. The efficiencies of both methods are the same. The equations given above are gross equations. In practice, synthesis gas is first generated at approx. 850°C, which still contains carbon monoxide (CO). This is reacted at temperatures of 200°C to 400°C with steam in the presence of a catalyst in so-called shift reactors to form CO2 and H2. |
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