Transient analysis and performance studies of two tubular photobioreactors for outdoor culture of Spirulina

A mathematical model to make a transient thermal analysis and to estimate the incident solar energy for two designs of tubular photobioreactor installed outdoors is presented here. In the first photobioreactor design the tubes were arranged in one plane, whereas in the second the tubes were arranged in two planes. The model was validated by comparing the experimental data and predicted values of culture temperature. Both the input solar energy and culture temperature in a tubular photobioreactor may be predicted with a reasonable degree of accuracy by employing the model. The performance of the two photobioreactors for mass culture of Spirulina was also studied in relation to their design and culture temperature. The average biomass yield obtained in one-plane and two-plane photobioreactors were (dry weight) 23.7 g m-2 day-1 and 27.8 g m-2 day-1 respectively. Such biomass yields corresponded to a volumetric productivity of (dry weight) 0.466 g litre-1 day-1 in the one-plane reactor and 1.5 g litre-1 day-1 in the two-plane reactor. We further observed that biomass yield could be increased by about 21% when the culture temperature was maintained at the optimal value of 35°C compared to another culture in which temperature changed according to the ambient temperature from 20 to 39°C during the day.