Photobioreactors
147
sources are effectively absorbed and transformed into biomass energy. This is in
agreement with latest data published. A set of cultivation experiments demonstrated good energy utilization with uniform light distribution at moderate
quanta fluxes 1-122]; beside technical characteristics of gas exchange rate and
quality of mixing, productivity is simultaneously reciprocal to layer thickness
and proportional to the surface/volume ratio [163].
Lee and Low 1-164] added some interesting aspects by short-time bioenergetic investigation of outdoor Chlorella cultures. During the morning hours,
YPAR reached values up to 5 g E- 1 with a fiat, horizontally arranged 10 1 reactor.
At midday, growth yield decreased to approximately 1 g E-1 by photoinhibition, and failed to increase to values of 2 g E- 1 during the rest of the day. These
effects strongly suggest that cells are primarily limited by their inability to
respond properly to the daily cycle of varying illumination.
With respect to an overall evaluation of reactor performance, the absolute
upper limit for volumetric productivities of microalgae seems to be confined to
the range of 5-10 g 1-1 d- 1. According to the growth yield potentials, it would
require an input of PAR of at least 20 ~tE 1-1 s- 1 (ca. 4 W 1-1). Thus it became
apparent that light limitation remains the main technological challenge for
designing industrial photobioreactors.
6 Conclusions
Today, commercial algal mass production is mainly performed in open ponds.
However, as was discussed in this article, area intensive cultivation reactors
are limited by a variety of natural restrictions and are technologically exhausted
by the efforts of the last decades. Using minimal investment efforts, each
improvement in mixing and illumination of the phototrophic cells extensively
increased the operational cost. This is the crux of microalgal biotechnology:
the price for microalgal biomass has to be reduced by every means. This could
be done either by increasing areal productivity or by the isolation of novel
products which could justify the high technical expense. New developments
of closed systems are directed to the high-value product market. Industrial
photobioreactors like the Biocoil or the plate-type PBR move slowly into
this market because they combine reliable production of high biomass quality
and simple maintenance with inexpensive technology. Nevertheless their photosynthetic efficiency is far from being optimized and their installation is
still capital intensive. Further efforts should be made to apply inexpensive
materials to arrangements running automatically at low operational costs.
Ultrathin layers and immobilization of algae on inexpensive material could be
a very interesting alternative for suspension culture, since this configuration
promotes the main factor which induces quick algal growth - nutrition and light
supply, respectively.
147
sources are effectively absorbed and transformed into biomass energy. This is in
agreement with latest data published. A set of cultivation experiments demonstrated good energy utilization with uniform light distribution at moderate
quanta fluxes 1-122]; beside technical characteristics of gas exchange rate and
quality of mixing, productivity is simultaneously reciprocal to layer thickness
and proportional to the surface/volume ratio [163].
Lee and Low 1-164] added some interesting aspects by short-time bioenergetic investigation of outdoor Chlorella cultures. During the morning hours,
YPAR reached values up to 5 g E- 1 with a fiat, horizontally arranged 10 1 reactor.
At midday, growth yield decreased to approximately 1 g E-1 by photoinhibition, and failed to increase to values of 2 g E- 1 during the rest of the day. These
effects strongly suggest that cells are primarily limited by their inability to
respond properly to the daily cycle of varying illumination.
With respect to an overall evaluation of reactor performance, the absolute
upper limit for volumetric productivities of microalgae seems to be confined to
the range of 5-10 g 1-1 d- 1. According to the growth yield potentials, it would
require an input of PAR of at least 20 ~tE 1-1 s- 1 (ca. 4 W 1-1). Thus it became
apparent that light limitation remains the main technological challenge for
designing industrial photobioreactors.
6 Conclusions
Today, commercial algal mass production is mainly performed in open ponds.
However, as was discussed in this article, area intensive cultivation reactors
are limited by a variety of natural restrictions and are technologically exhausted
by the efforts of the last decades. Using minimal investment efforts, each
improvement in mixing and illumination of the phototrophic cells extensively
increased the operational cost. This is the crux of microalgal biotechnology:
the price for microalgal biomass has to be reduced by every means. This could
be done either by increasing areal productivity or by the isolation of novel
products which could justify the high technical expense. New developments
of closed systems are directed to the high-value product market. Industrial
photobioreactors like the Biocoil or the plate-type PBR move slowly into
this market because they combine reliable production of high biomass quality
and simple maintenance with inexpensive technology. Nevertheless their photosynthetic efficiency is far from being optimized and their installation is
still capital intensive. Further efforts should be made to apply inexpensive
materials to arrangements running automatically at low operational costs.
Ultrathin layers and immobilization of algae on inexpensive material could be
a very interesting alternative for suspension culture, since this configuration
promotes the main factor which induces quick algal growth - nutrition and light
supply, respectively.