The Transition to a Sun-Powered World: Status, Perspectives, Bottlenecks

The energy transition from fossil fuels to renewables is already ongoing, but it will be a long, complex and difficult process to carry out, since the energy system is a gigantic and complex machine spread all over the world. The starting point to outline a possible scenario for this transition is to quantify the unsustainable differences in the availability of per capita energy across the planet, setting 2.8 toe/y as a desirable average target to reach, with related consequences. Key renewable energy production data will be presented, which show the remarkable growth of solar electricity technologies and indicate that crystalline silicon PV and wind turbines are the workhorse of the first wave of renewable energy deployment on the TW scale around the globe. The other PV market alternatives (CIGS, CdTe), along with other less mature options under intensive research, such as DSSC, OPV and perovskite cells, will be briefly presented. The status and perspectives of solar concentrating options (CPV and CSP) will be also discussed. As far as fuels are concerned, the situation is significantly more complex because making chemicals with sunshine is far more complicated than generating electric current. The prime solar artificial fuel is molecular hydrogen, which is characterized by an excellent combination of chemical and physical properties. The routes to make it via solar energy (PEC cells, DSPEC cells, PV electrolyzers) and then synthetic liquid fuels are presented. The interconversion between electricity and hydrogen, two energy carriers directly produced by sunlight, will be a key tool to distribute renewable energies with the highest flexibility. However, the full integration of solar hydrogen in the world energy system will not be achieved soon. The overall discussion takes into account two parameters and concepts that are often neglected in the scientific energy debate: the EROI (Energy Return On Investment) and the fact that the energy transition will not be limited by the availability of photons, but by the availability of natural resources - particularly minerals - which are needed to manufacture energy converters and storage devices on a multi-TW scale. The future solar-powered civilization will be based on energy technologies with relatively low EROI, in the context of an increased global competition for natural resources. Ultimately, this may reveal the physical and energy limits of economic growth.