Mechanistic Modeling of Broth Temperature in Outdoor Photobioreactors

This study presents the first mechanistic model describing broth temperature in column photobioreactors as a function of static (location, reactor geometry) and dynamic (light irradiance, air temperature, wind velocity) parameters. Based on a heat balance on the liquid phase the model predicted temperature in a pneumatically agitated column photobioreactor (1 m(2) illuminated area, 0.19 m internal diameter, 50 L gas-free cultivation broth) operated outdoor in Singapore to an accuracy of 2.4 degrees C at the 95% confidence interval over the entire data set used (104 measurements from 7 different batches). Solar radiation (0 to 200 W) and air convection (-30 to 50 W) were the main contributors to broth temperature change. The model predicted broth temperature above 40 degrees C will be reached during summer months in the same photobioreactor operated in California, a value well over the maximum temperature tolerated by most commercial algae species. Accordingly, 18 000 and 5500 GJ year(-1) ha(-1) of heat energy must be removed to maintain broth temperature at or below 25 and 35 degrees C, respectively, assuming a reactor density of one reactor per square meter. Clearly, the significant issue of temperature control must be addressed when evaluating the technical feasibility, costs, and sustainability of large-scale algae production.