Criteria for the Efficiency, Stability, and Capacity of Abiotic Photochemical Solar Energy Storage Systems

The utilization of simple photochemical reactions for the storage of solar energy in the form of chemical energy in energy‐rich products has often been considered in the further development and improvement of e. g. simple thermosolar techniques. The hitherto proposed criteria for the qualification of an abiotic photochemical system are, however, mostly of a qualitative nature, so a mutal comparison of the systems is not precise enough. In this article it is shown how a useful correlation on the basis of time‐independent experimental data can be achieved and how, from the viewpoint of photochemistry, a comparative classification of known reactions is possible. The following reactions are compared: the [2 + 2]‐photocycloadditions of norbornadiene, dimethyl 2,3‐norbornadienedicarboxylate, and dicyclopentadienone, the photoisomerization of trans‐ to cis‐diacetylindigo, the photodissociation of nitrosyl chloride as well as a photocatalytic redox reaction. The quantity of material required and storage efficiency are by far the most favorable in the case of trans‐diacetylindigo. The main disadvantage of the latter however, is that the energy‐rich cis‐from rapidly reverts to the stable trans‐form at elevated temperatures. Harnessing of solar energy for production of “fuel” is possible in principle. For example, (1) norbornadiene, or NOCl are reversibly transformed on illumination into heigh‐energy products whose stored energy can be recovered as heat. Newly developed criteria permit quantitative comparison of systems with regard to their suitability as storable and transportable “fuels”.