MODELING OF BIOMASS PRODUCTIVITY IN TUBULAR PHOTOBIOREACTORS FOR MICROALGAL CULTURES - EFFECTS OF DILUTION RATE, TUBE DIAMETER, AND SOLAR IRRADIANCE

A macromodel is developed for estimating the year-long biomass product ivity of outdoor cultures of microalga in tubular photobioreactors. Th e model evaluates the solar irradiance on the culture surface as a fun ction of day of the year and the geographic location. in a second step , the geometry of the system is taken into account in estimating the a verage irradiance to which the cells are exposed. Finally, the growth rate is estimated as a function of irradiance, taking into account pho toinhibition and photolimitation. The model interconnects solar irradi ance (an environmental variable), tube diameter (a design variable), a nd dilution rate (an operating variable). Continuous cultures in two d ifferent tubular photobioreactors were analyzed using the macromodel. The biomass productivity ranged from 0.50 to 2.04 g L-1 d(-1), and fro m 1.08 to 2.76 g L-1 d(-1), for the larger and the smaller tube diamet er photobioreactors, respectively. The quantum yield ranged from 1.1 t o 2.2 g E-1; the higher the incident solar radiation, the lower the qu antum yield. Simultaneous photolimitation and photoinhibition of outdo or cultures was observed. The model reproduced the experimental result s with less than 20% error. If photoinhibition was neglected, and a gr owth model that considered only photolimitation was used to fit the da ta, the error increased to 45%, thus reflecting the inadequacy of prev ious outdoor growth models that disregard photoinhibition. (C) 1998 Jo hn Wiley & Sons, Inc.