Microalgal Cultivation Reactor Systems - Biotechnological Applications of Microalgae - page 55

across the reactor volume by rotating cells between the light and dark phases of the
reactor.
In addition to individual reactor design, the configuration of multiple reactor units
can be designed for optimal light distribution. For example, placing plate reactors
very close together dilutes strong light, which leads to an increase in photosynthetic
efficiency. Overlapping tubular systems can also be used to dilute strong sunlight.
However, the benefits of high photosynthetic efficiency may be offset by the increased
cost of reactor hardware (Richmond, 2000).
The use of internal illumination can remove some of the surface-area-to-volume
constraint on bioreactor design (Ugwu et al., 2008). Both natural and artificial light
sources could be utilized, either using optic fibers to distribute solar energy inside
the PBR or placing waterproof artificial illumination internally. Artificial lighting
has the advantage that it can be used to supplement the light supply at night or dur-
ing cloudy days. However, it adds to the operational cost and energy input; therefore,
higher biomass yields are crucial (Ugwu et al., 2008; Kumar et al., 2010).
5.2.2 t eMperature
Along with light intensity, temperature is one of the most difficult parameters to
optimize in large-scale outdoor culture systems. Fluctuations in temperature, both
daily and seasonally, can lead to significant decreases in productivity. The optimal
growth temperature for microalgae is species specific, but often in the region of 20°C
to 30°C (Chisti, 2008). Many algal species can tolerate temperatures of up to 15°C
lower than their optimum, with reduced growth rates, but a temperature of only a few
degrees higher than optimal can lead to cell death (Mata et al., 2010). The net effi-
ciency of photosynthesis declines at high temperature as the rate of respiration rises
significantly, while the increased flux through the Calvin cycle is moderate. This
effect is worsened by the fact that CO 2 becomes less soluble at elevated temperatures,
more rapidly than O 2 ( P ul z, 20 01).
Low seasonal, morning, and evening temperatures can lead to significant losses
in productivity, although low nighttime temperatures are potentially advantageous
due to a reduction in the respiration rate. As much as 25% of the biomass produced
during daylight hours can be lost at night due to respiration (Chisti, 2007). Cool
nighttime temperatures can minimize this loss.
Closed reactor systems almost always require some form of temperature control.
They often suffer from overheating during hot days when temperatures inside the
reactor can reach in excess of 50°C. Heat exchangers or evaporative water-cooling
systems may be employed to counteract this (Mata et al., 2010). The culture system
can also be placed inside a greenhouse, or contacted with water to minimize tem-
perature fluctuations (Chisti, 2007). Closed PBRs are sometimes floated, either
whole or just the solar collector, in a temperature-modulating water bath. Double-
walled reactors with part of the liquid volume used for heating and cooling have
been devised (Ugwu et al., 2008), although all such modifications add to the cost of
production.
There is a relationship between temperature and light availability. Exposure to
a rapid increase in light intensity when the temperature is below optimum (as occurs