A novel mathematical model to simulate the growth of microalgae in the solar collector of recirculating helical photobioreactors (BIOCOIL) is proposed. The dynamic-spatial behavior of the nutrient species in liquid phase (CO2, nitrates and orthophosphates) and the primary photosynthesis product (O-2) are quantitatively described through the proposed model where the simulation of cell growth, proliferation and its distribution within the tubular section of the helical photobioreactor is improved with respect to the existing literature by properly taking into account suitable population balances. Light intensity distribution within the culture medium is also quantitatively described. Model results and the literature experimental data in terms of dry biomass content and its distribution within the photobioreactor tube have been successfully compared, thus demonstrating the validity of the proposed model as well as its predictive capability. (C) 2009 Elsevier B.V. All rights reserved.