Membrane-Based Humidity Control in Microgravity: A Comparison of Membrane Materials and Design Equations

A microgravity dehumidification system for plant growth experiments requires the generation of no free-liquid condensate and the recovery of water for reuse. In membrane dehumidification, the membrane is a barrier between the humid air phase and a liquid coolant water. The coolant water temperature combined with a trans-membrane pressure differential establishes a water flux from the humid air into the coolant water. Building on the work of others, we directly compared hydrophilic and hydrophobic membranes for humidity control. Hydrophobic membranes did not meet the required operational parameters. In a direct comparison of the hydrophilic membranes, cellulose ester membranes were superior to metal and ceramic membranes in the categories of condensation flux per surface area, ease of start-up and stability. However, cellulose ester membranes were inferior to metal membranes in one significant category, longevity/durability. Cellulose ester membranes broke down with operational lifetimes of hours to days. Our work established that surface science principles control the relative performances of membrane materials. In addition, we clarified design equations for operational parameters such as trans-membrane pressure differential. Finally, we developed design models to take advantage of membranes to decouple humidity control from temperature control, i.e. dehumidification at bulk air temperatures above the dew point. This technology has several potential benefits related to earth environmental issues including the minimization of airborne pathogen release and higher energy efficiency in air conditioning equipment. Armed with our study results; we designed, constructed, and flew on Space Shuttle Missions STS-77 and STS-88 a membrane-based dehumidification system for a plant growth chamber. During the design and construction phase, we also gained engineering insights leading to the development of an innovative rigid-membrane water saturation method which is superior to conventional methods.

National Aeronautics and Space Administration Grant NAGW-1197 supported this work.