Photobioreactors
145
confirmed, saying that OAD is shifted towards lower values with increasing
surface/volume ratio. Therefore, high cell densities are not desirable in cases of
long dark phases.
Lee [155] proposed heterotrophic growth for Chlorella in order to avoid
non-productive night periods. Oxygen-consuming mixotrophic growth during
the day would be a further effect of sugar addition releasing the cultures from
symptoms of toxic O2-concentrations. Ogawa and Aiwa 1-156] demonstrated
that, compared with Chlorella vulgaris which was cultivated at autotrophic
conditions, the specific growth rate is two times higher at mixotrophic conditions and is in range with heterotrophic growth, up to concentrations of 10 g
glucose 1-1. Yokoi et al. [157] reported that high density cultivation up to
30 g d.wt. 1-1 is possible by the use of carbon hydrate feed, even with higher
plant cells (Oryza sativa). Beside the high microalgal cell concentrations, the
content of polyunsaturated fatty acids could be increased three-fold in heterotrophic cultures [158]. A variety of algal strains is able to utilize organic carbon
sources [159]. The main problem is to find less expensive carbon sources for
enhancing the economic production of algae. Organic wastes and animal
manure were tested for their potential to yield phototrophic biomass [160,
162, 163].
The fact that both continuous and intermittant lighting result in stable
culture productivities prevents a definite comparison of closed reactors to
outdoor systems. For a more appropriate evaluation of the performance of
photobioreactors, independently of their design or arrangement, it is suggested
to consider the photosynthetic efficiency (PE) or the growth yield Y, as g dry
biomass per mole absorbed photons [98]. That only makes sense if cultivation
units for maximum biomass production are considered, because both PE and
Y are dependent on the quantum yield which is much higher at low radiant
energy corresponding to poor growth. Nevertheless the economic importance of
energy conversion at artificially illuminated reactors is magnified with increasing scale up. In Table 2 growth yield values YPAR on the basis of absorbed
photosynthetically active radiation are summarized for several open and closed
systems, which were all operated at high irradiance levels. Though depending on
cultivated species, this bioenergetic consideration allows for a rough estimation
of the energy conversion performance.
It seems that outdoor systems are exhausted by values of 0.5 g E- 1 corresponding to an areal productivity of 25-30 g m- 2 d- 1. Considerations of light
intensive summer periods and sophisticated cultivation techniques, e.g. the
tubular reactor of Lee et al. [94] promise a possible doubling of productivity.
Most of the tubular indoor reactors merely compensate the natural dark cycle.
An outstanding exception is represented by the results of Pirt et al. [99] with
3.18 gE-1 the highest conversion rate of all the studies examined. Common
laboratory cultivation units and plate type reactors are in range or slightly
succeed tubular performance. However, through the use of light distribution
units, like side light optics within the reactors, Year increased approximately
fourfold with regard to outdoor results. Photons emitted by internal light
145
confirmed, saying that OAD is shifted towards lower values with increasing
surface/volume ratio. Therefore, high cell densities are not desirable in cases of
long dark phases.
Lee [155] proposed heterotrophic growth for Chlorella in order to avoid
non-productive night periods. Oxygen-consuming mixotrophic growth during
the day would be a further effect of sugar addition releasing the cultures from
symptoms of toxic O2-concentrations. Ogawa and Aiwa 1-156] demonstrated
that, compared with Chlorella vulgaris which was cultivated at autotrophic
conditions, the specific growth rate is two times higher at mixotrophic conditions and is in range with heterotrophic growth, up to concentrations of 10 g
glucose 1-1. Yokoi et al. [157] reported that high density cultivation up to
30 g d.wt. 1-1 is possible by the use of carbon hydrate feed, even with higher
plant cells (Oryza sativa). Beside the high microalgal cell concentrations, the
content of polyunsaturated fatty acids could be increased three-fold in heterotrophic cultures [158]. A variety of algal strains is able to utilize organic carbon
sources [159]. The main problem is to find less expensive carbon sources for
enhancing the economic production of algae. Organic wastes and animal
manure were tested for their potential to yield phototrophic biomass [160,
162, 163].
The fact that both continuous and intermittant lighting result in stable
culture productivities prevents a definite comparison of closed reactors to
outdoor systems. For a more appropriate evaluation of the performance of
photobioreactors, independently of their design or arrangement, it is suggested
to consider the photosynthetic efficiency (PE) or the growth yield Y, as g dry
biomass per mole absorbed photons [98]. That only makes sense if cultivation
units for maximum biomass production are considered, because both PE and
Y are dependent on the quantum yield which is much higher at low radiant
energy corresponding to poor growth. Nevertheless the economic importance of
energy conversion at artificially illuminated reactors is magnified with increasing scale up. In Table 2 growth yield values YPAR on the basis of absorbed
photosynthetically active radiation are summarized for several open and closed
systems, which were all operated at high irradiance levels. Though depending on
cultivated species, this bioenergetic consideration allows for a rough estimation
of the energy conversion performance.
It seems that outdoor systems are exhausted by values of 0.5 g E- 1 corresponding to an areal productivity of 25-30 g m- 2 d- 1. Considerations of light
intensive summer periods and sophisticated cultivation techniques, e.g. the
tubular reactor of Lee et al. [94] promise a possible doubling of productivity.
Most of the tubular indoor reactors merely compensate the natural dark cycle.
An outstanding exception is represented by the results of Pirt et al. [99] with
3.18 gE-1 the highest conversion rate of all the studies examined. Common
laboratory cultivation units and plate type reactors are in range or slightly
succeed tubular performance. However, through the use of light distribution
units, like side light optics within the reactors, Year increased approximately
fourfold with regard to outdoor results. Photons emitted by internal light