The net energy ratio of microalgae biofuels production based on correlated cultivation parameters in flat plate photobioreactors

Abstract

The net energy ratio (NER) of microalgae biofuels depends fundamentally on assumptions of operational energy demand and biomass yield. However, for the NER calculation, it has not previously been considered that these parameters are related. This study derives a correlation between these values from published experimental data on flat plate photobioreactors and uses it to calculate the NER. The found correlation is adapted to outdoor conditions, and the energy demand throughout the life cycle for cultivation and biogas production is added. Results show that an asymptotic correlation between the aeration rate and the photosynthetic efficiency hampers net energy output. Maximising the biomass output requires a disproportionately high aeration rate, and thus operational energy demand. Best NER is obtained at low aeration rates, however the biomethane yield at these rates cannot compensate for the total energy demand throughout the life cycle. A NER of 3.6 is calculated. Sensitivity analysis on further technological improvements, such as more efficient energy supply, reduces the NER to 1.8. A comparison of assumptions from seven previous life cycle analyses demonstrates that NER below one results from incomplete system boundaries, or unrelated parameters for operational energy demand and biomass yield. Our findings help to explain contradictory results in previous studies, and will improve the quality of future NER analyses.