The biomass productivity of Scenedesmus obliquus was investigated outdoors during all seasons in solar tracked flat panel photobioreactors (PBR) to evaluate key parameters for process optimization. CO2 was supplied by flue gas from an attached combined block heat and power plant. Waste heat from the power plant was used to heat the culture during winter. The parameters pH, CO2, and inorganic salt concentrations were automatically adjusted to nonlimiting levels. The optimum biomass concentration increased directly with the photosynthetic active radiation (PAR) from 3 to 5 g dry weight (DW) L−1 for a low PAR of 10 mol photons m−2 day−1 and high PAR of 40–60 mol photons m−2 day−1, respectively. The annual average biomass yield (photosynthetic efficiency) was 0.4 ± 0.5 g DW mol−1 photons. However, biomass yields of 1.5 g DW mol−1 photons close to the theoretical maximum were obtained at low PAR. The productivity (including the night biomass losses) ranged during all seasons from −5 up to 30 g DW m−2 day−1 with a mean productivity of 9 ± 7 g DW m−2 day−1. Low night temperatures of the culture medium and elevated day temperatures to the species-specific optimum increased the productivity. Thus, continuous regulation of the biomass concentration and the culture temperature with regard to the fluctuating weather conditions is essential for process optimization of outdoor microalgal production systems in temperate climates.