hours per ha per day for a mixing velocity of 30 centimeters per second (Benemann, 1986). Other estimates of power requirements for large ponds (for example, Cyanotech’s 2,900 m2 ponds mentioned earlier) range from about 1,200-3,700 W/ha for mixing velocities of 20 to 30 centimeters per second (Pedroni et al., 2001; Frank et al., 2011). A raceway pond of 85 m2 that uses an air-lift pump for circulation has a power consumption of 195 W based on a compressor efficiency of 70 percent and an air demand of 120 liters per second. Ponds in Chile and Brazil have used motor-driven drag boards as an alternative to paddle wheels; the energy requirement was reported to be only 20 percent of the energy needed for a comparable agitation with paddle wheels (Becker, 1994). Laws et al. (1983) introduced the concept of foils that create circular vortices to effectively mix the pond suspension from top to bottom. This is the type of agitation device that Algenol uses in its plastic and covered photobioreactor design (Chance et al., 2011b; see also Chapter 3).

2.2.3 Photobioreactors

Mass cultivation of microalgal species that lack pronounced environmentally selective advantages might require the use of photobioreactors (Milledge, 2011). Photobioreactors are transparent containers or vessels designed to have reduced light path to enhance the amount of available light to the algal cells, and the cultures within are continuously mixed to enhance nutrient distribution and gas exchange. Photobioreactors for microalgae production have an optimal thickness of about 2-4 centimeters (Borowitzka, 1999). The tubular and the flat-plate are the two most common types of microalgal photobioreactors.

All photobioreactors have large surface to volume ratio (SVR). Because of their widespread availability, tubes long have been used as a basic photobioreactor material. The geometric configurations of tubular photobioreactors span a wide range from straight horizontal, straight vertical, helical, to triangular configurations (Figures 2-5, 2-6). One of the world’s largest photobioreactor facilities is in a greenhouse in Klotze, Germany. This facility consists of straight horizontal tubes stacked in vertical fence-like arrays (Figure 2-7). The facility has a total volume of 700 cubic meters (m3), occupies a total land area of 10,000 m2, and produces 35-41 grams dry weight per m2 per day or 120-140 dry tonnes per year. Algae wall adhesion, biofouling, large pressure drop, and gradients in pH, dissolved oxygen, or CO2 can occur along the tube length. These factors are potential disadvantages of tubular photobioreactors (Chen et al., 2009), which might be resolved by innovative engineering designs.

Flat-plate (or flat-panel) photobioreactors are transparent rectangular containers (usually vertical or inclined) with a light path of 1-30 centimeters (Figure 2-8). Flat-plate photobioreactors mix substrate by vigorous air sparging from the bottom.

Productivities of algal biomass in photobioreactors vary with the type of geometric configuration used and the algal species grown (Table 2-3). Many novel production systems have been designed and currently are being developed and tested. The new production systems aim to lower construction and maintenance costs close to those of open-pond systems and maintain the high, stable productivity and reduced contamination risk of closed photobioreactors. These systems include the Solix, ACCORDION, Algenol, and the National Aeronautics and Space Administration’s (NASA) Offshore Membrane Enclosure for Growing Algae (OMEGA), and Photon8’s traveling wave system.

The Solix photobioreactor is an elongated (low height-to-length ratio) flat-panel photobioreactor made of plastics. It is designed to bridge the gap from flask to open raceway pond by serving as a controlled-environment test bed or as an algae inoculum scale-up device (Figure 2-9; Solix Biofuels, 2011). The Solix photobioreactor allows for open-pond deployment by using the water as a thermal regulator for open-air field applications. Air sparging for aeration and mixing occurs along the full length of each panel.



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