Electrochemical CO2 and O2 Separation for Crew and Plant Environments 921319

For long-duration space explorations such as the advanced manned missions to the moon and Mars, fully optimized environmental conditions and control systems are essential. This approach will not only maximize the efficiencies of the crew and other systems, but also minimize the requirements for power, weight, volume and expendables. Life Systems, working with the National Aeronautics and Space Administration-Johnson Space Center, has been investigating ways to apply various physical, chemical and electrochemical methods for this purpose.

This paper presents a description of a closed ecosystem concept that includes electrochemical CO₂ and O₂ separators and a moisture condenser/separator for maintaining CO₂, O₂ and humidity levels in the crew and plant habitats at their respective optimal conditions. This concept was developed as a part of the Advanced Electrochemical CO₂ Removal Process Study program sponsored by NASA-JSC. The key processes of this concept are aqueous electrolyte-based electrochemical CO₂ and O₂ separations. The principles and cell characteristics of these electrochemical gas separation processes are described in this paper. Also presented in this paper are the descriptions of test hardware and the test results of the Electrochemical CO₂ Separator (ECS) and the Electrochemical O₂ Separator (EOS), and the combination of the ECS and the EOS. These tests were conducted at Life Systems, with system requirements as defined by NASA-JSC. Future plans for this program, pending funding availability, include expansion of the modules to a one person-day level followed with parametric/endurance testing at Life Systems and NASA-JSC.

Much of the research in advanced long-duration manned missions has been in the use of higher plants with the emphasis on the effects of the environment on the growth of the plants.^{(1,2,3,4,5,6,7)}∗ Many attempts have been made to define environmental conditions which will maximize the crop productivity of several higher plants. However, not much research has been conducted in conjunction with the above on the concepts for providing optimal environmental conditions for separate crew and plant habitats with devices which can maintain the habitats at their respective optimal conditions at minimum power, weight, volume and expendables requirements.

For extended duration missions such as the human explorations of the moon and Mars where resupply of consumables is either infrequent or difficult, various bioregenerative life support systems using higher plants may be used. This approach will not only reduce the amount of materials to be stored aboard the spacecraft, but also provide an Earth-like living environment. However, due to the inherently slow nature of bioregenerative mechanisms, plant systems require large volume, weight, power, etc. Since plants generally grow faster in environments where carbon dioxide partial pressure (pCO₂), temperature, light and humidity levels are different from the levels optimal for human beings, two separate environments may be necessary. An environment optimized for plant biological functions such as photosynthesis and respiration will result in a more efficient bioregenerative system requiring less weight, power, volume and expendables. Listed below are the critical atmospheric environmental parameters which are known to affect the growth of plants:

• Light - Photosynthesis occurs only when there is sufficient energy from light available.
• Temperature - Stomata are temperature sensitive. At both low and high temperatures stomatal closure may occur. As temperature increases, CO₂ uptake increases to a maximum and then decreases with further temperature increases.
• Humidity - Lack of moisture (water stress) causes a decrease in photosynthesis. This is mostly due to poor plant hydration, which can cause stomatal closure. This will restrict CO₂ uptake and reduce photosynthesis.
• Carbon Dioxide - The optimal CO₂ level is several times higher than terrestrial levels with a fall-off in photosynthesis with both CO₂ deficiency and over-abundance. A deficiency limits CO₂ uptake by unavailability, and overabundance limits CO₂ uptake by causing stomatal closure.

Aqueous electrolyte-based electrochemical gas separation technologies, such as the Electrochemical CO₂ Separation and the Electrochemical O₂ Separation processes, in combination with a physical moisture separator and appropriate temperature control, can be used effectively in creating these separate optimal atmospheric environments in CO₂, O₂, humidity and temperature levels for both habitats (i.e., optimal pCO₂ level and higher temperature and relative humidity for the plant habitat than for the crew habitat). The main advantages in using this combined ECS/EOS system for this purpose are:

• No expendables required.
• Flexible process control using electrochemical cell current, temperature and air flow rate.
• Capacity can be easily increased by adding electrochemical cells.