Ionic Liquid-based CO2 Control of Plant Growth Chamber Atmospheres

Control of carbon dioxide (CO2) remains important for human spaceflight. Long-term continuous exposure to elevated CO2 concentrations (> 2,000 ppm (0.2 kPa)) has been hypothesized to adversely affect crew performance and contribute to crew physiological issues (headaches, vision impairment, and intracranial pressure). Open plant chambers such as the Russian Lada and NASA's Veggie unit have utilized the cabin CO2 to sustain photosynthesis, however dynamic changes in CO2 levels can adversely affect gas exchange and plant growth as the plants adapt to the new CO2 levels. A closed plant growth chamber with independent control of CO2 can establish conditions ideal for photosynthesis and thereby reduce stress on the plants. In this paper, we describe a CO2 control system that uses an ionic liquid membrane contactor for selective transport of CO2. A blower circulates the plant growth chamber atmosphere through the contactor. Red, blue, and green light-emitting diodes illuminate plants which dynamically alter the CO2 level within the growth chamber; plants remove CO2 during the light cycle and respire CO2 when it's dark. When the plant growth chamber CO2 level is low, the control system scrubs CO2 from a simulated CO2-laden cabin atmosphere and delivers it to the plant growth chamber to maintain the desired set point. If the chamber CO2 level is higher than the set point, then the control system reverses the direction of transport by scrubbing CO2 from the plant growth chamber atmosphere and rejecting this CO2 to the cabin. A model was developed with the V-HAB virtual habitat modeling and simulation tool to characterize CO2 control over a broad range of operating conditions and demonstrate feasibility for control between 1,000 and 2,000 ppm (0.1 and 0.2 kPa).