We estimate the energy content, the operational energy inputs, and the net energy ratio (NER) of an industrial tubular photobioreactor used for the photosynthetic production of H-2 by microalgae. The NER is the dimensionless ratio of the H-2 chemical energy output (higher heating value) to the total primary energy input into the system. The calculated H-2 output of the photobioreactor is based on a range of algal photosynthetic H-2 generation efficiencies, and on the application of standard theory for tubular solar collectors. For small diameter reactors ( <= 50 mm) the mixing energy is large, and can reduce the NER by a third or more, compared to bigger diameters. For a tubular photobioreactor, low-density polyethylene (LDPE) film and glass have significantly higher NERs than rigid polymers such as polymethyl methylacrylate (acrylic). Using a hypothetical improved microalgal H-2 generation efficiency of 5%, an NER similar to 6 can be obtained for LDPE film and for glass. For mechanical and assembly reasons LDPE film is the material of choice. These results show that photobiological hydrogen could be a viable H-2 generation technology, if tight constraints on energy inputs are met. (c) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.