132
o. Pulz and K. Scheibenbogen
Compared with fluorescent lamps, incandescent and discharge lamps produce a significantly lighter emitting surface, frequently with increased IR-emittance; in most cases this requires the external installation of the fluorescent
material. Due to the small sizes concerned, they are suited very well to a focussed
light transfer via light-guiding systems with optical fibres. Light yield and colour
spectra vary significantly from type to type. The sodium high pressure lamps
known from street lighting produces a very high conversion of electrical power
to visible light (> 25%o), but are limited in the discontinuous spectrum to the
yellow-orange range. Despite this, some researchers have achieved good results
with this type of lamps [27, 61].
Of the commercial lamps, metal vapour and xenon high-pressure lamps have
the highest light intensity and are used for sunlight simulation because of the
well-balanced light distribution. During optimized pulsating operation xenon
flash-light lamps can reach an efficiency of 50-80% [62]. However, for a corresponding scale-up the instrumental expenditure (operating equipment, cooling,
etc.) will add significantly to the costs; therefore, this type of lamps is not
described in the literature for plants exceeding the laboratory scale.
Light emitting or laser diodes could be an interesting prospect [63] because
they can convert electrical energy into radiation energy at up to 80% efficiency.
Lee and Palsson [64] have reached very high cell densities with Chlorella with
the aid of a densely packed array of monochromatic red LEDs (680 nm).
However, prices and complexity of the required control electronics do not allow
for a commercial application.
Another alternative is currently developed by the U.S. company Fusion. In
spherical glass beads sulfur atoms are excited by microwaves to light emission.
The efficiency in VIS amounts to approximately 30%, interfering UV and
IR-radiation being emitted only to a very small extent. Commercial application
is to be expected in the next few years [65].
By direct use of fluorescent colours in the algal suspension and in the reactor
jacket, light spectra were modified so that their photosynthetical utilization
became more effective [26]. However, the high toxicity and low stability of the
colours were a handicap; nevertheless, yield increases up to 20% were achieved
for a short time.
4 Problems of Cultivating Phototrophic Microorganisms
The basic ability of photoautotrophic microorganisms to utilize light quanta in
the visual range as an energy source for metabolism is handicapped by high
population densities. As against heterotrophic microorganisms, such as yeasts
or bacteria where the distribution of the organic molecules as energy carriers by
mixing is solved technologically, the supply of photons is dependent on surface
area. Light dissipation in the cell suspension is realized by numerous quantum
o. Pulz and K. Scheibenbogen
Compared with fluorescent lamps, incandescent and discharge lamps produce a significantly lighter emitting surface, frequently with increased IR-emittance; in most cases this requires the external installation of the fluorescent
material. Due to the small sizes concerned, they are suited very well to a focussed
light transfer via light-guiding systems with optical fibres. Light yield and colour
spectra vary significantly from type to type. The sodium high pressure lamps
known from street lighting produces a very high conversion of electrical power
to visible light (> 25%o), but are limited in the discontinuous spectrum to the
yellow-orange range. Despite this, some researchers have achieved good results
with this type of lamps [27, 61].
Of the commercial lamps, metal vapour and xenon high-pressure lamps have
the highest light intensity and are used for sunlight simulation because of the
well-balanced light distribution. During optimized pulsating operation xenon
flash-light lamps can reach an efficiency of 50-80% [62]. However, for a corresponding scale-up the instrumental expenditure (operating equipment, cooling,
etc.) will add significantly to the costs; therefore, this type of lamps is not
described in the literature for plants exceeding the laboratory scale.
Light emitting or laser diodes could be an interesting prospect [63] because
they can convert electrical energy into radiation energy at up to 80% efficiency.
Lee and Palsson [64] have reached very high cell densities with Chlorella with
the aid of a densely packed array of monochromatic red LEDs (680 nm).
However, prices and complexity of the required control electronics do not allow
for a commercial application.
Another alternative is currently developed by the U.S. company Fusion. In
spherical glass beads sulfur atoms are excited by microwaves to light emission.
The efficiency in VIS amounts to approximately 30%, interfering UV and
IR-radiation being emitted only to a very small extent. Commercial application
is to be expected in the next few years [65].
By direct use of fluorescent colours in the algal suspension and in the reactor
jacket, light spectra were modified so that their photosynthetical utilization
became more effective [26]. However, the high toxicity and low stability of the
colours were a handicap; nevertheless, yield increases up to 20% were achieved
for a short time.
4 Problems of Cultivating Phototrophic Microorganisms
The basic ability of photoautotrophic microorganisms to utilize light quanta in
the visual range as an energy source for metabolism is handicapped by high
population densities. As against heterotrophic microorganisms, such as yeasts
or bacteria where the distribution of the organic molecules as energy carriers by
mixing is solved technologically, the supply of photons is dependent on surface
area. Light dissipation in the cell suspension is realized by numerous quantum