ABSTRACT
This study investigates the net energy balance of algal bioenergy production, using nutrients present in the dairy manure. Algal bioenergy production system includes dairy waste treatment through anaerobic digestion, uses of solubilized nutrients for algal growth, uses of algal biomass for biodiesel production, and then recovery of nutrients and energy from the residual biomass through anaerobic digestion. The useful energy outputs from the algal bioenergy production include methane and biodiesel. An attempt has been made to minimize the energy burdens of algal bioenergy production by using constraint optimization, such that highest net energy gain was possible. For optimization purpose, an energy input-output function was developed for the production of bioenergy through different processes, as described before. The net energy of a process was defined as the difference between the sum of total energy required for the process and the useful energy output from the process. The simulation was carried out for one year, taking residence time of completing one cycle of bioenergy production as 45 days. Net energy ratio (NER) was varied from 1.91 to 2.39 for different scenarios considered in our study. For the highest NER scenario, net energy value for one year of algal bioenergy production was estimated at 3184.861 GJ (Giga-Joule), where the total input was 3260.541 GJ and the output was 6445.402 GJ. Drying of algal biomass was the most energy-intensive process in the algal bioenergy production, contributing about 40% to 60% of the total input, depending on the scenario.
