Efficiency of Sunlight Utilization: Tubular Versus Flat Photobioreactors, excerpts
Mario R. Tredici, Graziella Chini Zittelli
The importance of the light saturation effect in outdoor algal cultivation was recognized very early. Burlew (1953), in the introduction to the famous book Algal Culture: From Laboratory to Pilot Plant, referred to this phenomenon as the challenge of light saturation, and suggested two different strategies to overcome it: turbulent mixing and spatial light dilution. Spatial dilution as a means to overcome the light saturation effect and limit damages due to strong light is a strategy commonly adopted by phototrophs in nature, especially among the higher plants which, being fixed in space, are much more endowed than microalgae to cope with excessive light. Whereas phytoplankton in natural waters essentially adopt avoidance strategies (van Liere and Walsby, 1982), plants have developed a very complex array of responses to light of excessive intensity (Long et al., 1994; Powles, 1984). Among these, the reduction of the chloroplast surface area exposed to strong light plays a relevant role.
This strategy is implemented through rapid movements of chloroplasts within the cells, by changing the leaf angle, through leaf flutter, or, more efficiently, by adopting a special canopy architecture that distributes the impinging PPF as uniformly as possible over the leaves and minimizes the fraction of leaves that are exposed to PPFD levels above saturation (Nobel et al., 1993; Powles, 1984). Particularly relevant to the objective of this study is the consideration that the leaf area index (i.e., the leaf area per unit land area) is often much greater than 1 (e.g., some coniferous canopies support leaf area indexes higher than 15). This means that plant leaves usually do not receive orthogonal rays of light and a moderate PPFD of a few hundred micromoles of photons per sq. meter per second can occur at the top of the canopy even with full sunlight overhead (Nobel et al., 1993).
Unlike plants and phyto-plankton, microalgae cultivated outdoors in artificial basins or photobioreactors experience a rather unnatural and stressful situation, because turbulence forces the cells to move back and forth along the "dense" profile of the culture and subjects them to short-term fluctuations in light intensity from full sunlight to complete darkness. When these shade-adapted cells are brought to the surface by mixing and ex-posed to high PPFD, they achieve lower photosynthetic efficiencies than they are potentially able to achieve and can even suffer severe damage (photoinhibition). Artificial algal cultures, like terrestrial plants, thus have to compromise between maximizing light interception to attain maximum volumetric productivity and reducing excessive light to achieve high light conversion efficiency.
The experiments described in this article demonstrate that dilution of excessive light, achieved through a particular arrangement or shape of the reactor, can lead to significantly higher light conversion efficiency and, consequently, to higher productivity under both artificial and natural illumination. It is expected that the higher the light intensity or the higher the dilution factor, the higher the beneficial effect of spatial light dilution.