A model was developed for any PBR, based on mixing requirement per unit of biomass production rather than constant amount of mixing energy. The model assumes constant biomass concentration throughout the culture time which means the volume of the culture would increase over time; the mixing energy will also increase over time according to the volume of the culture. Such incremental energy supply (IES) consumes much less energy compared to constant energy supply (CES); higher the culture time in the PBR, more is the savings in the IES compared to CES. In addition to mixing energy, light energy can also be applied using IES scheme. The model was validated with the algae Nannochloropsis sp.; 44% of the energy input of CES was saved by adopting IES with equal biomass productivities for a culture period of 60 h. (C) 2010 Elsevier Ltd. All rights reserved.