Energy efficiency analysis of outdoor standalone photovoltaic-powered photobioreactors coproducing lipid-rich algal biomass and electricity

The need for thermal regulation in microalgal photobioreactors is a significant impediment to their large-scale adoption. The energy costs associated with thermal regulation alone can easily result in a negative energy balance. Self-sustaining photovoltaic powered photobioreactors that do not require cooling systems provide an opportunity to maximize biomass productivity, generate local electricity, reduce thermal regulation requirements, and significantly improve the energy balance of the system. Net energy analysis of a spectrally-selective, insulated-glazed photovoltaic photobioreactor (IGP) with an integrated capability for renewable electricity generation used to cultivate Nannochloropsis sp. without freshwater-based cooling resulted in a net energy ratio of 2.96, a figure comparable to agricultural bio-oil crops such as Jatropha and soybean. Experimental data from pilot-scale operation of this novel photobioreactor producing Nannochloropsis biomass under outdoor conditions was extrapolated to a 1-ha IGP installation. Annual biomass productivity reached 66.0-tons dry weight ha(-1), equivalent to overall energy output of 1696.2 GJ ha(-1). The integrated semi-transparent photovoltaic panels generated an additional 1126.8 GJ ha(-1) yr(-1) (313.0 MWh ha(-1) yr(-1)). Energy demands from plant building materials, machinery, fertilizers, plant operations, and biomass harvesting constituted total energy input with a combined value of 707.3 GJ ha(-1) yr(-1). Comparison with a conventional photobioreactor requiring passive evaporative cooling showed novel photobioreactor had a 73% greater net energy ratio. Nannochloropsis cultivation in IGP system ensured co-production of lipid and protein of 34.7 and 25.7-tons ha(-1) yr(-1), respectively. These results suggest that this novel photobioreactor could be a viable and sustainable biomass production technology for mass microalgal cultivation.