Nature solved the problem of supplying energy through solar energy by means of the photosystem found in green plants. In this system, sunlight is converted into energy-rich compounds. Scientists from the Max Planck Society were the first to elucidate the structure of a light-driven enzyme system, which is that of the photosynthetic reaction centre in purple bacteria.
Hartmut Michel, Robert Huber and Johann Deisenhofer, the latter of whom worked at the Howard Hughes Medical Institute, were awarded the Nobel Prize for chemistry in 1988 for this work. Many aspects of the functioning of the photosystem of green plants are now also understood; a crucial breakthrough in the understanding of the central Mn4 cluster of photosystem II was recently achieved at the Max Planck Institute for Bioinorganic Chemistry. Although this research provides the blueprint for a system enabling the use of sunlight for the generation of energy, unfortunately we are still a long way from being able to copy this photosystem in the form of simple robust model systems, or from finding a different form of photocatalytic solar energy conversion. If we could succeed in converting solar energy into energy-rich molecules and storing it, we would make significant progress towards the resolution of our energy problems. The corresponding approaches are still far from ready for technological development and fundamental questions remain to be answered. These include the question as to how light-collecting molecules, so-called antenna systems, can be efficiently coupled to other molecules which mediate the transfer of electrons, and hence split water into its components hydrogen and oxygen.
The stability of such systems is also of fundamental importance. The plant photosystem is only intact for around 20 minutes on average when operational. After this, ingenious cellular repair systems must intervene to ensure that photosynthesis does not grind to a halt. Technology still largely lacks analogous autonomous repair mechanisms, the importance of which far exceeds energy generation. Irrespective of the way in which we will utilise solar energy in the long term without the detour via fossil energy sources – that is whether by direct imitation in the fusion process, the biological conversion of solar radiation, the photo-catalytic splitting of water or using photovoltaics – it will also be necessary to store and distribute energy differently than we do today. Hydrocarbons in the form of petrol, diesel, kerosene and heating oil are the main forms of energy storage and transportation currently used. Electrical energy must be generated at precisely the scale on which it is consumed; the storage of large volumes of electrical energy is not possible today.