Temperature-tolerant Chlorella sorokiniana was cultivated in a 51-L column photobioreactor with a 1.1 m(2) illuminated area. The reactor was operated outdoors under tropical meteorological conditions (Singapore) without controlling temperature and the culture was mixed at a power input of 7.5 W/m(3) by sparging CO(2) -enriched air at 1.2 L/min (gas hold-up of 0.02). Biomass productivity averaged 10 ± 2.2 g/m(2) -day over six batch studies, yielding an average photosynthetic efficiency (PE) of 4.8 ± 0.5% of the total solar radiation (P = 0.05, N = 6). This demonstrates that temperature-tolerant microalgae can be cultivated at high PE under a mixing input sevenfold to ninefold lower than current operational guidelines (50-70 W/m(3)) and without the need for temperature control (the culture broth temperature reached 41 °C during operation). In this study, the PE value was determined based on the amount of solar radiation actually reaching the algae and this amount was estimated using a mathematical model fed with onsite solar irradiance data. This determination was found to be particularly sensitive to the value of the atmospheric diffusion coefficient, which generated a significant uncertainty in the PE calculation. The use of the mathematical model, however, confirmed that the vertical reactor geometry supported efficient photosynthesis by reducing the duration and intensity of photoinhibition events. The model also revealed that all three components of direct, diffuse, and reflected solar radiation were quantitatively important for the vertical column photobioreactor, accounting for 14%, 65%, and 21% of the total solar radiation reaching the culture, respectively. The accurate prediction of the discrete components of solar radiation reaching the algae as a function of climatic, geographic, and design parameters is therefore crucial to optimize the individual reactor geometry and the layout/spacing between the individual reactors in a reactor farm.