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.