A novel tubular photobioreactor immersed in open waters for passive temperature control and operated with the microalga Tetradesmus obliquus

Abstract

Temperature control of outdoor photobioreactors is mandatory for stable microalgal growth rates. The large necessity of water and energy, however, imposes economic and ecologic drawbacks. A novel floating tubular photobioreactor, placed horizontally on a channel outdoors is presented in this study. It is equipped with a technology allowing variable immersion of the transparent parts into the water for passive temperature control. The photobioreactor was operated in three subsequent cycles with the microalga Tetradesmus obliquus . The horizontal immersion of 50 % of the photobioreactor tubes at high ambient temperatures up to 38.9 °C prevented the photobioreactor from temperature peaks above 33.5 °C without any energy input for cooling. While operating the photobioreactor without cooling through immersion the culture temperature exceeded the ambient temperature constantly during afternoon hours. The realised cultivations were stable and productivities from 161 mg L −1 day −1 to 230 mg L −1 day −1 were recorded. The harvested biomass contained up to 49.8 ± 0.8 % DW protein and 21.0 ± 2.0 % DW lipids with a high fraction of α -linolenic acid. • An immersible photobioreactor technology was designed and tested on open waters. • Immersion of photobioreactor into surface water shows significant tempering effect. • Stable cultivation of T. obliquus shows practicability of the photobioreactor. • Promising energy- and land-saving alternative for microalgae cultivation.