A closed solar photobioreactor for cultivation of microalgae under supra-high irradiance: basic design and performance

An account is given of the setting up and use of a novel type of closed tubular photobioreactor at the Academic and University Centre in Nove Hrady, Czech Republic. This "penthouse-roof" photobioreactor was based on solar concentrators (linear Fresnel lenses) mounted in a climate-controlled greenhouse on top of the laboratory complex combining features of indoor and outdoor cultivation units. The dual-purpose system was designed for algal biomass production in temperate climate zone under well-controlled cultivation conditions and with surplus solar energy being used for heating service water. The system was used to study the strategy of microalgal acclimation to supra-high solar irradiance, with values as much as 3.5 times the ambient value, making the approach unique. The cultivation system proved to be fully functional with sufficient mixing and cooling, efficient oxygen stripping and light tracking. Experimental results (measurement of the maximum photochemical yield of PSII and non-photochemical quenching) showed that the cyanobacterium Spirulina (= Arthrospira) platensis cultivated under sufficient turbulence and biomass density was able to acclimate to irradiance values as high as 7 mmol photon m^–2 s^–1. The optimal biomass concentration of Spirulina cultures in September ranged between 1.2 to 2.2 g L^–1, which resulted in a net productivity of about 0.5 g L^–1 d^–1 corresponding to a biomass yield of 32.5 g m^–2 d^–1 (based on the minimum illuminated surface area of the photobioreactor).