A mathematical model of the lumped parameter type was developed in order to analyze the water temperature in bioreactors for outdoor biomass production. A photobioreactor (PBR) water tank was employed to properly validate the model outcome by means of the experimental data and the main environmental parameters such as solar radiation, relative humidity, ambient temperature and wind velocity. Solar radiation control is achieved by means of shading the water tank. The model allows modifying the aspect ratio of the bioreactor and studying the temperature variations, one of the variables of major concern in the growth of biomass, for different mesh shading and for any given day along the year. Hence, geometry selection and reactor design, coupled with the design of the shading device, can be chosen to reduce conventional energy usage and capitalize thermal exchange with the environment. Hence, optimization of bioreactor design can be attained once the optimal conditions for commercial production are established for a particular biomass production. (C) 2007 Elsevier Ltd. All rights reserved.