Beyond electricity generation, concentrated solar energy can provide heat to chemical reactions. In this context, especially redox systems are being investigated at the Institute of Solar Research. The goal of this research is to store solar heat chemically and to produce solar fuels.
What are redox systems?
A redox system is a material or a chemical substance which can change its state between being reduced (oxygen deficient) and oxidized (oxygen rich). Cyclic processes can exploit this redox property. The scheme below shows a two-step redox cycle which is applied to split water with metal oxides (MO).
In the high temperature step (reduction), the redox material releases oxygen (O2). A typical temperature range of this reaction is 1200 to 1500 degrees Celsius. As this reaction is endothermic, it requires heat, which is introduced as concentrated solar radiation. In the second reaction (oxidation), which typically takes place between 600 and 1000 degrees Celsius, the reduced metal oxide Mred is oxidized by water steam or carbon dioxide (CO2). The gases release oxygen which is incorporated into the metal. The products of this reaction are hydrogen (H2) and carbon monoxide (CO) , so-called solar fuels. These fuels can be used in fuel cells or as base materials in chemical plants. It is also possible to further process these gaseous fuels in order to produce liquid fuels.
Such redox systems thus enable long term storage of solar energy in a transportable fuel. This is an important feature to decouple the energy use from the local and temporal availability of solar energy. Hence, solar fuels can be produced in regions with good solar resources and subsequently be transported to regions with less irradiation.
Current research activities
One focus of current research is to develop and optimize the redox material. A promising candidate is ceria CeO2, which can take up and release oxygen in large quantities. The material properties can be further improved by incorporating other elements (doping). For example, doping can influence the oxygen uptake characteristics of a material. Doped materials also often have different colors as shown in the photograph below.
The goal of the research work is to increase the process efficiency by improving the activity of the redox material. Furthermore, the long term stability shall be improved and the reaction speed shall be increased. All these research activities are being conducted in parallel in one building – the CeraStorE® - and in close cooperation with scientists from the institute of material research. The thermochemistry lab provides the required infrastructure.
As an alternative to using redox systems based on metal oxides, we also investigate processes which realize the endothermic splitting of sulfuric acid to sulfur dioxide. Sulfur dioxide can for example be electrolyzed in aqueous solution to produce solar hydrogen, which goes along with efficiency improvement compared to conventional water electrolysis.
Next to the topic of solar fuel production, the application of redox systems for flexible storage of solar heat has drawn more and more attention in recent years. In this case, the oxidation is realized with air instead of water or carbon dioxide. The oxygen present in air is used as the oxidizing agent. During oxidation, heat is released. This heat has been stored previously in the material during the endothermic reduction step. In such a configuration, redox systems can be used as thermal heat storage in CSP plants.
In a very novel approach, redox systems are being investigated without focusing on heat storage, but on removing oxygen from air. Then, mainly nitrogen is left as a product which can be applied to produce ammonia together with renewably generated hydrogen. Ammonia is the main precursor of fertilizers. Hence, this approach aims at a sustainable and CO2-neutral fertilizer production based on water and air, which are converted using solely solar energy. This visionary idea was awarded first prize in the DLR competition of visions in 2014.