The second year of this project emphasized the use of “flashing light to enhance algal mass culture production” (Laws, 1982; see also Laws et al. 1983). The basic idea was that a “foil array” in the pond culture would generate a vortex that would create organized mixing in the ponds, expected to result in exposure of the cells to regular dark-light cycles (Figure III. B.2.). Based on data in the literature, this effect would be predicted to increase overall productivity. These a priori arguments were not supported by the algal physiological literature (the flashing light productivity enhancements are observed at much shorter time constants), and neither were the hydraulic arguments plausible (organized mixing would be seen only in a small fraction of the pond volume). However, the key issue here is not the theory but the actual experimental results.

From November 1981 to January 1982, an average productivity of only about 3.3 g/m2/d was recorded for the 50-m2-flume reactor, a very low value for Hawaii, even in winter. After installation of the foils, productivities, from February to March 1982, increased to about 11 g/m2/d. This increase was attributed to the effect of the foils, though lack of a control did not allow isolation of this variable from other effects. Five-day running mean average photosynthetic efficiencies (PAR) are shown in Figure III. B.3. The author stated that productivity could be doubled with semi-continuous operations. One observation was infestation of the culture by algal predators, which could have been one reason for the rather large variability in productivities observed during this operation (Figure III. B.3.). However, day-to-day variability in productivities is a fact of outdoor pond microalgae cultivation, even in the best of cases.

Heat exchanger

Drain box

FV—«

Lilt box

Recirculation and

airlift

LOW PRESSURE OVER

FLOW»

HIGH PRESSURE

UNDER

CROSSOVER

VORTEX

Figure III. B.2. Hawaii ARPs with mixing foils

a. (Top). Schematic of the 48-m2 flume, showing heat exchangers, lift box, drain box and airlift mixer.

b. (Bottom) Schematic of mixing resulting from foils inserted in the shallow flumes.

(Source: Laws 1982.)