A vertical concentric-tube airlift photobioreactor (ALP) was used to cultivate Phaeodactylum tricornutum UTEX 640 in outdoor continuous mode during the summer. A mathematical model is developed to estimate the irradiance profile and average irradiance inside the culture, and hence, to compare the biomass production capability of the airlift device with a horizontal-loop tubular photobioreactor (HLTP) located at the same place as the ALP. The maximum biomass productivities were similar in both photobioreactors in spite of the higher light availability in HLTP; thus, the photosynthetic efficiency was higher in ALP. This behavior was attributed to photoinhibition in HLTP and the negative effects of an inappropriate light-dark cycling.