Multifactorial comparison of photobioreactor geometries in parallel microalgae cultivations

Abstract

Efficient photosynthetic biomass production in a high rate pond (HRP) or a photobioreactor (PBR) represents the first step of microalgae platforms for the production of renewable fuels, animal feeds and a diverse range of high value products. This study analyses the interplay between solar energy input, ambient temperature and system surface area to volume (SA:V) ratio in terms of photosynthetic performance (yield, areal and volumetric productivity, photon conversion efficiency). Ten pilot scale trials were conducted under subtropical conditions using 2 microalgae strains (Chlorella sorokiniana and Chlorella sp.) in 5 different cultivation system geometries: HRPs, flat panel PBRs (0.75m and 1.5m high) and tubular PBRs (0.74m and 1.49m high). The evaluation of culture temperature and biomass productivity response to solar irradiance in the five production systems suggests that the optimal SA:V ratio range lies between 43–73m2m−3 for C. sorokiniana in non-cooled systems regardless of system geometry under the conditions tested. The overall photosynthetic performance at higher SA:V ratios was improved for Chlorella sp. using temperature regulation. The highest observed daily photon conversion efficiency (PCE) was 4.44% (based on illuminated PBR surface area and total solar spectrum) in the high flat panel PBR using C. sorokiniana (40.8gm−2d−1, 0.23gL−1d−1). The highest achieved mean PCE (based on illuminated PBR surface area and total solar spectrum) was 2.5% in the low tubular PBR with Chlorella sp. (24.9gm−2d−1, 0.43gL−1d−1). The trial data provides important design principles to help fast track systems optimisation for near optimal sub-tropical conditions.