Algae have the fastest growth rate and are among the most efficient
plants for oxygen and food production. It has been amply demonstrated by
Myers ([ref.179]) and other workers that _Chlorella_ can be used in a
closed ecological system to maintain animals such as mice and a monkey.
The use of algae for supplying O₂ and food, and for removing CO₂ and
odors has been considered by many authors for use in spacecraft, space
platforms, and for establishing bases on the Moon or Mars.
Estimates of total efficiency are based on extrapolated laboratory data
and vary widely, since many different types of data have been used as a
basis for these estimates.
The respired air containing about 4-5 percent CO₂ is bubbled into the
_Chlorella_ culture, at either atmospheric or increased pressure. Air
containing a high percentage of oxygen and saturated with moisture is
released from the algal system.
The use of algae for several purposes might require from one to three
separate algal systems. For food production, _Chlorella_ produces 50
percent protein and 50 percent lipids in high-nitrogen media. In
low-nitrogen media, it produces 85 percent lipids. Proper choice of
_Chlorella_ strains and media will produce not only the necessary
calories but also the necessary specific nutrients required. Certain
strains are more effective in O₂ production, and others in the use of
urine and other wastes.
Some of the early estimates, using _Chlorella_ grown at 25° C, for
supplying these requirements for a single man in space include the
following: 168 kg of algal suspension ([ref.179]), 200 kg of algal
suspension and 50 kg of equipment including pumps (refs. [ref.180] and
[ref.181]), and 100 kg of algal suspension and 50 cubic feet for
equipment and gas exchange ([ref.182]). Using the blue-green alga
_Synechocystis_, 600 kg of algal suspension would be required, according
to Gafford and Craft. These estimates are based on preliminary studies,
are quite high, and are not of real practical value.
Other studies have indicated an extremely efficient algal system which
offers a real potential for a practical and effective gas exchanger
([ref.183]). A thermophilic strain of _Chlorella_ with an optimum growth
temperature of 39° C and an optimum temperature for photosynthesis of
about 40° C can increase its cell mass 10 000-fold per day. When
operating at one-half maximum efficiency, this alga produces 100 times
its cell volume of oxygen per hour. Burk et al. ([ref.183]) state:
"Future engineering development should lead to a space requirement, per
adult person, of no more than 3 to 5 cubic feet of algal culture,
equipment, and instrumentation for adequate purification of air." The
requirements of this system would require additional energy in the form
of light and of small amounts of nitrogenous and mineral material for
the algae. The light source used by Burk et al. ([ref.183]) is a
tungsten filament quartz lamp the size of a pencil, which has a long
life, produces a luminous flux 5-10 times greater than sunlight on
Earth, and operates at a 10-12 percent light efficiency.
Research is being carried out on algal regenerative systems by about 40
or 50 laboratories in the United States. NASA is supporting several
basic studies on photosynthesis, the physiology of algae, and
engineering pilot-plant development. Much of the research on algae is
being supported by the Air Force.
Most algal studies have been carried out in small units and the data
obtained have been used as a basis for extrapolating logistic values for
the use of these organisms in manned space vehicles. Myers ([ref.179])
has shown that the quantity of algae necessary to support a man (with an
assumed O₂ requirement of 625 liters per day) would yield about 600-700
grams dry weight of new cells per day. If algal growth in mass cultures
could be maintained in a steady-state concentration of 2.5 gram dry
weight per liter with such a growth rate as to yield 10 grams weight per
liter per day, the volume of algal culture would be 60-70 liters and the
total mass of the system would approximate 200-250 pounds.
Using an 8-liter system, Ward et al. ([ref.176]) have produced algal
concentrations of 5-7 grams of dry algae per liter with a
high-temperature algal strain. The maximum growth rate observed with the
culture was 0.375 gram dry weight per liter per hour, or 9 grams dry
weight per liter per day. This was accomplished by using 1-centimeter
layers of culture and a light intensity of 8000 foot-candles. The
culture system consisted of a rectangular plastic chamber having an area
of 0.5 square meter and illuminated on each side to an intensity of 4000
foot-candles (cool-white). To produce 25 liters of oxygen per hour, an
area of 8.3 square meters (85 square feet) would be required.
The major problem in large-scale production of algae is that of
illumination. Conversion of electricity to light has an efficiency of
only 10 to 20 percent. In addition, the maximum efficiency of light
utilization by _Chlorella_ algae lies in the range of 18-22 percent.
This results in a maximum efficiency of only 4 percent for
photosynthetic systems. Another problem involved in conversion of
electricity to light is the production of heat which has to be removed
even with thermophilic algae. With a human demand of 600 liters of
oxygen per day, the minimum electrical requirement becomes 4 kW. No
large-scale culture has yet been managed at anything close to this
minimum figure.
Another problem is the poor penetration of light into concentrated
cultures of algae. This necessitates construction of large tanks of only
about ¼-inch thickness. This results frequently in fouling of the
surfaces of the tank by algae and makes the removal of the excess algae
difficult. Production of 1 liter of oxygen results in the production of
1 gram dry weight of algae. Although a small amount of CO is produced by
some algae, it can probably be removed by catalytic oxidation. Other
problems include mutation and genetic drift of the algae and the
necessity for maintaining bacteria-free cultures. There are also
difficulties in maintaining a sterile culture if urine is to be used as
a nitrogen source. While there is a potential for using algae as food,
more research is required before it can be determined what quantity and
methods of processing can be used. Research and development on algae is
much greater than on both the higher plants and the
electrolysis-_Hydrogenomonas_ systems together.
The difference between the photosynthetic and
electrolysis-chemosynthetic systems is the way electrical energy is made
available to the organisms. In the photosynthetic system, electrical
energy is converted to light which the algae or plants transform into
chemical energy. In the chemosynthetic process, electrical energy is
transformed into the chemical energy of hydrogen gas which is used by
the bacteria. Both organisms use the chemical energy available to them
to synthesize cell material with similar degrees of efficiency. The
problem is to make the conversion of electricity to available chemical
energy as efficient as possible.
In photosynthetic systems much energy is lost in the conversion of
electricity to light, a process only 10-20 percent efficient at best.
When this is combined with the loss from the inefficient use of light by
plants, an overall efficiency of about 4 percent is obtained. In the
electrolysis-_Hydrogenomonas_ system, the two steps are very efficient.
Electrolysis cells can operate at up to 85 percent efficiency and the
overall efficiency can be up to seven times that of a photosynthetic
system.