Many ways towards 'solar fuel': quantitative analysis of the most promising strategies and the main challenges during scale-up

Future global needs for liquid energy carriers, commodity chemicals and renewable materials should no longer be covered by exploration of fossilized carbon deposits. Therefore, processes are urgently needed that can replace this source of carbon for the production of these materials. The alternative route of production most often referred to is via their synthesis from CO2 (and water), using the (free) energy of sunlight. This process has been intensely studied, particularly during the past decade, and has resulted in a wide range of proposed solutions. However, with the ultimate constraint that a limited surface area will be available on our planet to catch the necessary photons, the picture is emerging showing that three approaches turn out to be most promising to achieve commercial production of this range of products. Interestingly, they all exploit living cells to facilitate formation of essential, select, carbon-carbon bonds. In one approach, photovoltaic cells provide electricity to generate hydrogen that can be used for lithoautotrophy (or: 'chemosynthesis') in organisms like Cupriavidus or Clostridium. An alternative approach is to use solar-driven (i.e. large-surface area) photobioreactors for the growth of engineered cyanobacteria, to carry out 'direct conversion' of CO2 into products like ethanol, iso-butanol, lactic acid, etc. In a hybrid derivative of these two approaches renewable (solar) electricity may be converted into monochromatic light of similar to 650 nm that is optimal to drive photosynthesis in cyanobacterial photobioreactors, equipped with internal LED illumination. Here we discuss strengths and weaknesses of these three approaches, analyse the range of products for which proof-of-principle production has been demonstrated, and compare a selection of such studies with respect to efficiency and productivity of the CO2-to-product conversion. As for all approaches large-scale application is crucial, we also discuss the pitfalls and limitations of their scale-up.