Solar-energy-driven bioethanol production from carbohydrates for transportation applications

As one of the best alternatives to petroleum, the production of bioethanol has increased since 1990, with a sharp increase from the year 2000 onwards. Bioethanol also offers an attractive alternative as a fuel in low-temperature fuel cells, as it can be produced in large quantities from agricultural waste and biomass. Currently, global ethanol is produced mainly from sugar and starch feedstock. Successful utilization of solar energy which is renewable, abundant, and inexpensive, for bioethanol production from biomass, has the potential to solve the fuel shortage problem. Solar energy provides an important alternative energy source, even if only a portion of this energy is harnessed for heating applications. Our work focuses on using solar thermal energy for bioreaction leading to ethanol production. A solar reactor was developed to perform the conversion of starch (in a batch process) and glucose (in a continuous flow system) to bioethanol by heating the reactor using solar irradiation. Aqueous starch solution (5 wt%) was charged into the reactor bed loaded with baker’s yeast (Saccharomyces cerevisiae) and enzymes, resulting in the conversion of starch into ethanol in a single-step process, yielding 0.5 M, 25 mL ethanol/day. A significant amount of ethanol corresponding to 84% of the theoretical yield was obtained after two months. The process was scaled up to 15 wt% starch, producing 1.3 M ethanol, which was demonstrated as a potential and sustainable fuel for direct ethanol fuel cells (DEFCs) (310 mAmgPt -1, 0.75 V). Additionally, the secondary metabolite glycerol was fully reduced to 1,3-propanediol, which is the first example of a fungal strain that converts glycerol in situ to a value-added product. The batch process of bioethanol production was further developed to a continuous-flow process. When aqueous glucose solutions of 10, 20, 30, and 40 wt% were fed into the reactor, high ethanol yields (91, 86, 89, and 88% of the theoretical yield, respectively) were obtained, indicating the atom efficiency of the process. No loss was observed in the activity of the yeast even after two months of continuous operation of the process. The ethanol produced from 20 wt% glucose feed (2 M) was demonstrated as a potential fuel for DEFCs with current and power density values as high as 700 mA/cm2 and 330 mW/cm² at a modest open circuit voltage of 1.65 V. Productive utilization of solar energy for driving the fermentation reaction as well as the special design of the reactor that facilitates in situ separation of ethanol from the fermentation broth, make the current process economically feasible and environmentally friendly, and therefore industrially appealing and adoptable.