nutrients and exposes algal cells to sunlight and CO2. A velocity of 10-20 centimeters per second (cm/s) prevents algal cells from depositing and settling (Shen et al., 2009). Higher velocities are preferred, but a velocity greater than 30 cm/s could consume too much energy to be economically viable (Sheehan et al., 1998). Earthrise Nutritional, LLC, in California, and Cyanotech Corporation, in Hawaii, have some of the largest algal open ponds lined with plastic liners. Earthrise maintains 30 production ponds each about 5,000 m2 and a series of research ponds (1,000 m2, 200 m2, and 50 m2) (Earthrise Nutritional, LLC, 2009b). Cyanotech has more than 60 ponds, each of which is about 2,900 m2 (Lorenz, 2002; Enay, 2011). The depth of these ponds varies from 30 to 40 centimeters. For raceway ponds, a cell concentration of up to 1 gram dry weight per liter can be achieved, and productivities of 10 to 25 grams dry weight per m2 per day have been reported (Shen et al., 2009). Table 2-2 shows algal productivities for open systems, which vary widely depending on numerous factors, including the type of open system and the algal species grown. Although a productivity of 50 to 60 g dry weight per m2 per day is possible with open systems, achieving even 10 to 20 g dry weight per m2 per day in large-scale systems is difficult on an annual basis because of operational conditions and seasonal variations in temperature and sunlight intensity (Shen et al., 2009).

In a raceway pond of 100 m2, a paddle wheel driven by an electric motor has a power demand of 600 watts (W) (Becker, 1994). The overall energy requirement for paddle wheels in a pond with a roughness coefficient of 0.025 has been estimated at 20 kilowatt hour (kWh) per ha per day for a mixing velocity of 15 centimeters per second and 160 kilowatt

TABLE 2-2 Microalgae Productivities in Open Ponds


Pond Type Volume (L) Microalgal Species Areal Productivity (g DW/m2/d) Volumetric Productivity (g DW/L/d) Reference

Circular

1,960

Chlorella spp.
Scenedesmus spp.

1.61–16.47
2.43–13.52

0.02–0.16
0.03–0.13

Kanazawa et al. (1958)
Kanazawa et al. (1958)

Circular

 

Oscillatoria

15

 

Sheehan et al. (1998)

Sloped (cascade)

1,970

Chlorella spp.

25

10

Lee (2001)

Slope

1,990

Scenesdesmus obliquus

24.8

 

Becker (1994)

Raceway

 

Spirulina (Arthrospira)

9–13

 

Olguín et al. (2003)

Raceway

282

Spirulina platensis

14.47

0.183

Pushparaj et al. (1997)

Raceway

300

Anabaena spp.

9.4–23.5

0.031–0.078

Moreno et al. (2003)

Raceway

135,000

Spirulina (Arthrospira) spp.

2–17

0.006–0.07

Jiménez et al. (2003)

Raceway

 

Dunaliella salina

1.6–3.5

 

García-González et al. (2003)

Raceway

750

Spirulina platensis

15–27

0.06–0.18

Richmond et al. (1990)

Raceway

4,150

Phaeodactylum tricornutum

2.4–11.3

0.0028–0.13

Laws et al. (1988)

Hybrid system (open ponds and closed photobioreactors)

 

unknown

30 (anticipated)

 

Phycal (2011)

Raceway (proprietary lined “Super Trough System”)

 

Cyanobacteria spp.

15.36 (anticipated)

 

Phyco BioSciences, Inc. (Cloud, 2011a,b)


SOURCE: Adapted from Chen et al. (2009).



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