solar energy, CO2 fixation, photosynthesis, bioplastic, bio-oil
Producing fuels and materials from solar energy and CO2 is a clean sustainable solution to the arising fossil energy consumption and CO2 emission. This paper presents a high intensity process that can run continuously at a much higher efficiency than plants and algae. The core facility is an artificial photosynthetic system, consisting of photovoltaic assembly, water electrolyzer, and dark fermenter. The system decouples the intermittent solar energy capture from the slow biological CO2 fixation. An autolithotrophic bacterium is hired to fix CO2 with H2 and O2 derived from water electrolysis. The overall efficiency from solar to cell mass energy ranges from 6.3% to 8.9%. Since the solar energy is stored in H2, the H2-based CO2 fixation is not restricted by the intermittency of insolation and can be operated continuously in dark conditions to a high cell density (> 20 g/L). A kinetic study reveals that the specific growth rate is about 3 times higher than that of the fastest growing microalgae. About 50% of cell mass formed from CO2 fixation is polyhydroxybutyrate or PHB, containing 50% more energy than starch. The biopolyester has the similar mechanical properties of polypropylene and can also be liquified into bio-oil.