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Porous plate dehumidifiers (PPD) and porous tube nutrient delivery systems (PTNDS) are designed to provide a means for accurate environmental control, and also allow for two-phase flow separation in microgravity through surface tension. The technological challenges associated with these systems arise from the requirement to accurately measure and control the very small pressures that typically occur within and across the porous media. On-orbit automated priming or filling of the system in the absence of gravity may be necessary. Several porous plate dehumidifiers and porous tube nutrient delivery systems have been tested and evaluated, and experimental results for engineering design are presented.

Accurate root zone moisture control in microgravity plant growth systems is problematic. With gravity, excess water drains along a vertical gradient, and water recovery is easily accomplished. In microgravity, the distribution of water is less predictable and can easily lead to flooding, as well as anoxia. Microgravity water delivery systems range from solidified agar, water-saturated foams, soils and hydroponics soil surrogates including matrix-free porous tube delivery systems. Surface tension and wetting along the root substrate provides the means for adequate and uniform water distribution. Reliable active soil moisture sensors for an automated microgravity water delivery system currently do not exist. Surrogate parameters such as water delivery pressure have been less successful.

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.