Energy Balance of Biogas Production from Microalgae: Development of an Energy and Mass Balance Model

The paper describes the construction of a mechanistic energy balance model for the production of biogas from anaerobic digestion of microalgal biomass grown in raceways, based on simple principles and taking into account growth, harvesting and energy extraction. The model compares operational energy inputs with the calorific value of the output biomass in terms of the energy return on operational energy invested (EROOI). Initial results indicate that production of microalgal biogas will require: a) Favourable climatic conditions. The production of microalgal biofuel in the UK would be energetically challenging at best. b) Achievement of ‘reasonable yields’ equivalent to ~3% photosynthetic efficiency (25 g m^-2 day^-1). c) Low or no cost and embodied energy sources of CO2 and nutrients from flue gas and wastewater. d) Mesophilic rather than thermophilic digestion. e) Adequate conversion of the organic carbon to biogas (≥ 60%). The model itself provides a powerful assessment tool both for comparison of alternative options and potentially for benchmarking real schemes.