2.2 Renewable Energies

Overview

Electricity from renewable energy sources is, along with water, the central raw material for the production of green hydrogen. In addition, the most important technologies used for this purpose will be briefly presented.
As the figure below shows, all renewable energy flows or energy sources that can be converted into electrical energy come from one of three origins. These three basic origins are geothermal heat, solar energy and the gravity between the celestial bodies, basically between the moon, the earth and the sun. The sun's radiation energy is not only an energy stream itself but is also converted into various other renewable energy flows and energy carriers through a variety of natural processes inside the earth's atmosphere. Examples of this are wind energy and biomass. In case of biomass, the radiation energy of the sun is converted into a usable energy carrier by the photosynthesis of plant organisms. Wind is an energy stream that occurs due to spatial differences in the atmospheric air pressure distribution, which, in turn, result from variations in solar radiation.

Renewable energy sources, natural conversions and selected technical utilisation options by Kaltschmitt et al. (2020), Fabian Carels (CC BY-SA)

These different renewable energy streams can be used to generate electrical energy. A variety of different technologies, which are shown in the blue boxes in the figure, are used for this purpose. On a global level, the following technologies are of particular importance for the generation of renewable electricity:

Photovoltaic systems
Solar thermal power plants
Wind energy turbines
Hydropower plants
Biomass power plants

In some regions of the world, hydropower is already being used on a large scale to generate electricity. In Norway, for example, the share of hydropower in the total electricity mix is over 90 %. In Brazil, the electricity supply is also largely based on hydropower. Here, its share is about two thirds. However, the use of hydropower is often linked to considerable interventions in natural cycles. Rivers have to be dammed up, which on the one hand significantly changes their dynamics and on the other hand, can lead to the flooding of large areas. Additionally, in many regions, the usable potential for generating electricity from hydropower has already been largely exploited – for example in Germany. For these reasons, the use of electricity from hydropower is probably not an option for large-scale production of green hydrogen. The same is true for the use of biomass. As already explained, land-use conflicts, for example with food production, limit the extended use of biomass for energy production.

Therefore, the use of renewable electricity from photovoltaic systems, as well as wind turbines, will most likely be the main options for producing large quantities of green hydrogen. The basics of these technologies are briefly described in the following.