Solar chemistry

With the help of solar energy , chemical reactions can be set in motion. These can be photochemical or solar thermal reactions.

Photochemical detox

Since the sunlight does not have enough UV rays to disinfect waste water or exhaust gases, a suitable catalyst must also be used. The European Union funds research projects that aim to develop such photocatalysts. The aim is to generate singlet oxygen, which is a short-lived, high-energy state of the O ₂ molecule. In this state, oxygen is a powerful oxidizing agent .

Solar thermal hydrogen production

With the solar thermal hydrogen production one tries to produce hydrogen for a possible future hydrogen economy. The focus is not only on the use of hydrogen, but also on further processing into methanol . In principle, the processes can be carried out with or without the addition of fossil fuels. There are thereby different ways, all of which are not yet ready for the market and compared to the hydrogen production z. B. from natural gas by means of steam reforming are much more expensive.

Solar thermolysis

In test facilities it has been possible to split water (H ₂ O) directly into hydrogen (H ₂ ) and oxygen (O ₂ ). Since temperatures are necessary for direct water splitting , the so-called solar thermolysis, which cannot yet be generated with solar thermal systems or whose handling is difficult to control due to the load on the material and the separation of the two product gases is not easy, thermochemical ones are often used Circular processes .

Thermochemical cycle processes

In this process, a metal oxide is heated in a reactor that is heated directly by concentrated irradiation, thereby splitting off oxygen. The reaction is:

${\displaystyle \mathrm {M_{x}O_{y}\longrightarrow x\ M+{\frac {y}{2}}\ O_{2}} }$

If the reduced metal is diverted and water vapor is supplied , it oxidizes and breaks the oxygen from the water vapor and the desired hydrogen can be captured. The pairs Zn / ZnO and Fe ₃ O ₄ / FeO are preferred for this process . With this method, process efficiencies of up to 40 percent are theoretically possible.

Fuel upgrading

The third important application is fuel upgrading. Conventional fossil fuels such as coal , methane and by-products of the oil refinery as well as biomass are processed into higher-quality hydrogen. Thermal cracking is ideal for this:

${\displaystyle \mathrm {C_{x}H_{y}\longrightarrow x\ C+{\frac {y}{2}}\ H_{2}} }$

These reactions are highly endothermic . The remaining carbon is not released into the environment in the form of carbon dioxide . Applied to methane, this reaction results in a lower electricity output than if the methane were burned directly in a combined cycle power plant .

The second method is solar steam reforming .

${\displaystyle \mathrm {C_{x}H_{y}+x\ H_{2}O\longrightarrow x\ CO+({\frac {y}{2}}+x)\ H_{2}} }$

If coal is chosen as the raw material , one speaks of solar gasification . These reactions are also strongly endothermic. However, the necessary heat is not generated by burning part of the fuel, as usual , but by solar heat. For example, 48% of all hydrogen today is made from methane. About 30% of the methane has to be burned to drive the endothermic reaction. Due to the solar steam reforming is not a CO ₂ produced -neutral hydrogen, but the CO ₂ significantly reduces emissions.

Web links

• Sunlight for chemical reactions
• Paul Scherrer Institute
• Solar thermochemistry
• Vistaverde article on solar chemistry at DLR (see below)
• Institute for Solar Research at the German Aerospace Center (DLR)

literature

• Manfred Becker, Karl-Heinz Funken (ed.): Solar chemical technology. Solar chemistry colloquium June 12th and 13th 1989 in Cologne-Porz Conference reports and evaluations . tape 1 : Basics of solar chemistry . Springer, Berlin / Heidelberg 1989, ISBN 978-3-540-51336-0 .
• Manfred Becker, Karl-Heinz Funken (ed.): Solar chemical technology. Solar chemistry colloquium June 12th and 13th, 1989 in Cologne-Porz. Conference reports and evaluations . tape 2 : Solar detoxification of problem waste . Springer, Berlin / Heidelberg 1989, ISBN 978-3-642-83842-2 , doi : 10.1007 / 978-3-642-83842-2 .