This study provides design and operational guidelines for achieving maximum biomass productivity in outdoor photobioreactors (PBRs). Detailed simulations of coupled light transfer and growth kinetics of microalgae were performed for open ponds, vertical flat-plate, and tubular PBRs operated in batch mode and exposed to time-dependent collimated and diffuse solar irradiance. The temporal evolution of microalgae concentration was predicted by accounting for light saturation, photoinhibition, and respiration. Three-dimensional spectral light transfer simulations of collimated and diffuse solar radiation in the PBRs were performed at different times of the day. The green microalgae Chlamydomonas reinhardtii was used for illustration purposes. The study demonstrated that the daily productivity per unit of illuminated surface area for PBRs operated in batch mode was identical and depended uniquely on the ratio X-0/a where X-0 is the initial microalgae concentration and a is the illuminated surface area per unit volume of PBR. A maximum daily productivity of about 0.045 kg/m(2)/day was achieved for X-0/a = 0.035 kg/m(2). Remarkably, similar results were obtained with experimental data and other simulation results based on different models reported in the literature, for different microorganisms and PBRs operated in continuous mode. The PBR optical thickness, represented by X-0/a, constitutes a convenient parameter for designing (via a) and operating (via X-0) these PBRs to achieve their maximum performance. (C) 2013 Elsevier Ltd. All rights reserved.