Materials, operational energy inputs, and net energy ratio for photobiological 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 NER is the dimensionless ratio of the H₂ chemical energy output (higher heating value) to the total primary energy input into the system. 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. 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 H₂ generation efficiency of 5%, an 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 photobiological hydrogen could be a viable H₂ generation technology, if tight constraints on energy inputs are met.