Materials, geometry, and net energy ratio of tubular photobioreactors for microalgal hydrogen production

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 H2 by microalgae. The calculated H2 output of the photobioreactor is based on a range of algal photosynthetic H2 generation efficiencies, and on the application of standard theory for tubular solar collectors. Small diameter reactors have a low NER as the mixing energy becomes large. 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 H2 generation efficiency of 5 %, a NER ~ 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 photobiohydrogen could be a viable H2 generation technology.