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For the design of a large field of vertical flat plate photobioreactors (PBRs), the effect of four design parameters-initial biomass concentration, optical path length, spacing, and orientation of PBRs-on the biochemical composition and productivity of Chlorella zofingiensis was investigated. A two-stage batch process was assumed in which inoculum is generated under nitrogen-sufficient conditions, followed by accumulation of lipids and carbohydrates in nitrogen-deplete conditions. For nitrogen-deplete conditions, productivity was the most sensitive to initial biomass concentration, as it affects the light availability to individual cells in the culture. An initial areal cell concentration of 50 g m(-2) inoculated into 3.8-cm optical path PBR resulted in the maximum production of lipids (2.42 ± 0.02 g m(-2) day(-1)) and carbohydrates (3.23 ± 0.21 g m(-2) day(-1)). Productivity was less sensitive to optical path length. Optical path lengths of 4.8 and 8.4 cm resulted in similar areal productivities (biomass, carbohydrate, and lipid) that were 20 % higher than a 2.4-cm optical path length. Under nitrogen-sufficient conditions, biomass productivity was 48 % higher in PBRs facing north-south during the winter compared to east-west, but orientation had little influence on biomass productivity during the spring and summer despite large differences in insolation. An optimal spacing could not be determined based on growth alone because a tradeoff was observed in which volumetric and PBR productivity increased as space between PBRs increased, but land productivity decreased.

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The combination of a 100% increase in diesel fuel prices since 2002 and a new photobioreactor technology has renewed interest in producing biodiesel, a direct petroleum diesel fuel substitute, from microalgae. A new photobioreactor technology in which the microalgae are grown on vertically suspended membranes promises to increase algal productivity per acre ten-fold compared to microalgae ponds, and 400-fold compared to soybeans. This paper describes the general photobioreactor concept and assesses the economic viability of such technology given the current crude oil prospects. The majority of the data necessary for assessment are obtained from published articles, with experimental results providing the remaining necessary information. Analysis results indicate that the photobioreactor would need to be constructed and operate on the order of dollars per square foot per year.