EFFICIENCY OF SUNLIGHT UTILIZATION - TUBULAR VERSUS FLAT PHOTOBIOREACTORS

The light saturation effect imposes a serious limitation on the effici ency with which solar energy can be utilized in outdoor algal cultures . One solution proposed to reduce the intensity of incident solar radi ation and overcome the light saturation effect is ''spatial dilution o f light'' (i.e., distribution of the impinging photon flux on a greate r photosynthetic surface area), but consistent experimental data suppo rting a significant positive influence of spatial light dilution on th e productivity and the photosynthetic efficiency of outdoor algal cult ures have never been reported. We used a coiled tubular reactor and co mpared a near-horizontal straight tubular reactor and a near-horizonta l flat panel in outdoor cultivation of the cyanobacterium Arthrospira (Spirulina) platensis under defined operating conditions for optimum p roductivity. The photosynthetic efficiency achieved in the tubular sys tems was significantly higher because their curved surface ''diluted'' the impinging solar radiation and thus reduced the light saturation e ffect. This interpretation was supported by the results of experiments carried out in the laboratory under continuous artificial illuminatio n using both a flat and a curved chamber reactor. The study also showe d that, when the effect of light saturation is eliminated or reduced, productivity and solar irradiance are linearly correlated even at very high diurnal irradiance values, and supported findings that outdoor a lgal cultures are light-limited even during bright summer days. It was also observed that, besides improving the photosynthetic efficiency o f the culture, spatial dilution of light also leads to higher growth r ates and lowers the cellular content of accessory pigments; that is, i t reduces mutual shading in the culture. The inadequacy of using volum etric productivity as the sole criterion for comparing reactors of dif ferent surface-to-volume ratio and of the areal productivity for evalu ating the performance of elevated photobioreactors operated outdoors i s stressed; it is furthermore suggested that the photosynthetic effici ency achieved by the cu Itu re also be calculated to provide a suitabl e parameter for comparison of different algal cultivation systems oper ated under similar climatic conditions. (C) 1998 John Wiley & Sons, In c.