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.
ELECTROLYSIS-HYDROGENOMONAS SYSTEM
Electrolysis is carried out in a closed unit containing an electrolyte (KOH solution) with an anode and a cathode. These cells produce a maximum yield (60-80 percent or more) in gas production per unit of power consumption. According to Dole and Tamplin ([ref.184]), a unit capable of producing enough oxygen to sustain one man would be highly reliable, weigh approximately 18 kg, and require a power input of 0.25 kW.
One approach to zero-gravity operation is to rotate the electrolysis cell as described by Clifford and McCallum ([ref.185]) and Clifford and Faust ([ref.186]). The smallest known electrolysis cell under development uses this artificial gravity to separate oxygen from the anode and electrolyte, while the dry hydrogen gas permeates through the foil cathode, fabricated from palladium-silver alloy. This electrolysis cell, which would provide breathing oxygen for three men, has a volume of 1.4 liters, weighs 4.5 kg, and requires 0.67 kW, excluding auxiliary equipment, and has an efficiency of 84 percent.
The chemosynthetic conversion is carried out by the hydrogen bacteria. By the oxidation of molecular hydrogen, supplied from the electrolysis of water, energy is made available for biosynthesis. The generation of this "biological energy" is mediated by the stable enzyme hydrogenase which is present in the bacteria. On the average, the oxidation of 4 moles of H2 is required for the conversion of 1 mole of CO2 (the hourly production of a man). The removal of this amount of CO2 would thus require the cleavage of 4 moles of water. In addition, to supply oxygen for human respiration (at a rate of 1 mole of O2 per hour) the cleavage of two additional moles of water is required. Therefore, the chemosynthetic regeneration and human respiration together would require, on the average, the splitting of 6 moles of water per hour.
The material balance for electrolysis, biosynthesis, and human metabolism, with gram molecular weights in parentheses, are shown in equations (1) to (3), respectively:
6H2O ———————> 3O2 +6H2
(108) ———————> (96) + (12) (1)
The bacterial synthesis requires 6 moles of H2, 2 moles of O2, and 1 mole of CO2 (from the astronaut), as shown in equation 2:
6H2 + 2O2 + CO2 ———————> CH2O + 5H2O
(12) + (64) + (44) ———————> (30) + (90) (2)
The respiration of the astronaut requires 1 "food" mole (CH2O) representing about 120 kcal, and 1 mole of O2, as shown in equation 3: