title: Comparison of four outdoor pilot-scale photobioreactors.
abstract: BACKGROUND
  Microalgae are a potential source of sustainable commodities of fuels, chemicals and food and feed additives. The current high production costs, as a result of the low areal productivities, limit the application of microalgae in industry. A first step is determining how the different production system designs relate to each other under identical climate conditions. The productivity and photosynthetic efficiency of Nannochloropsis sp. CCAP 211/78 cultivated in four different outdoor continuously operated pilot-scale photobioreactors under the same climatological conditions were compared. The optimal dilution rate was determined for each photobioreactor by operation of the different photobioreactors at different dilution rates.
  
  
  RESULTS
  In vertical photobioreactors, higher areal productivities and photosynthetic efficiencies, 19-24 g m(-2) day(-1) and 2.4-4.2 %, respectively, were found in comparison to the horizontal systems; 12-15 g m(-2) day(-1) and 1.5-1.8 %. The higher ground areal productivity in the vertical systems could be explained by light dilution in combination with a higher light capture. In the raceway pond low productivities were obtained, due to the long optical path in this system. Areal productivities in all systems increased with increasing photon flux densities up to a photon flux density of 30 mol m(-2) day(-1). Photosynthetic efficiencies remained constant in all systems with increasing photon flux densities. The highest photosynthetic efficiencies obtained were; 4.2 % for the vertical tubular photobioreactor, 3.8 % for the flat panel reactor, 1.8 % for the horizontal tubular reactor, and 1.5 % for the open raceway pond.
  
  
  CONCLUSIONS
  Vertical photobioreactors resulted in higher areal productivities than horizontal photobioreactors because of the lower incident photon flux densities on the reactor surface. The flat panel photobioreactor resulted, among the vertical photobioreactors studied, in the highest average photosynthetic efficiency, areal and volumetric productivities due to the short optical path. Photobioreactor light interception should be further optimized to maximize ground areal productivity and photosynthetic efficiency.
authors.affiliation: Bioprocess Engineering, AlgaePARC, Wageningen University, P.O. Box 16, 6700 AA Wageningen, The Netherlands ; Bioprocess Engineering, AlgaePARC, Wageningen University, P.O. Box 8129, 6721 NG Bennekom, The Netherlands.
authors.forename: Jeroen H
authors.identifier: 0000-0002-7022-2552
authors.initials: JH
authors.lastname: de Vree
authors.affiliation: Bioprocess Engineering, AlgaePARC, Wageningen University, P.O. Box 16, 6700 AA Wageningen, The Netherlands.
authors.forename: Rouke
authors.initials: R
authors.lastname: Bosma
authors.affiliation: Bioprocess Engineering, AlgaePARC, Wageningen University, P.O. Box 16, 6700 AA Wageningen, The Netherlands.
authors.forename: Marcel
authors.initials: M
authors.lastname: Janssen
authors.affiliation: Food and Biobased Research, AlgaePARC, Wageningen UR, P.O. Box 17, 6700 AA Wageningen, The Netherlands.
authors.forename: Maria J
authors.initials: MJ
authors.lastname: Barbosa
authors.affiliation: Bioprocess Engineering, AlgaePARC, Wageningen University, P.O. Box 16, 6700 AA Wageningen, The Netherlands ; Biosciences and Aquaculture, Nordland University, 8049 Bodø, Norway.
authors.forename: René H
authors.initials: RH
authors.lastname: Wijffels
keywords: Areal productivity; Microalgae; Nannochloropsis sp.; Outdoor; Photobioreactors; Photosynthetic efficiency; Pilot-scale