(694d) Analysis, Modeling and Simulation of a Gravity-Independent Porous Membrane Condenser Cooled by a Thermoelectric-Driven Heat Pump

Dehumidification of air in astronaut living quarters and space wastewater treatment systems that involve evaporation or distillation require a microgravity-compatible condenser for recovering liquid water. The porous membrane condensing heat exchanger (PMCHX) provides gravity-independent condensation and phase separation in a single step. No free liquid is produced since the condensate is confined by capillary forces in the porous membrane. Design equations for the PMCHX in microgravity were derived from a theoretical analysis of the principles governing its operation. A PMCHX driven by a thermoelectric (TE) heat pump was fabricated as part of a closed air-loop drying system for astronaut cabin waste. The PMCHX was designed to condense water vapor from the hot, moist air leaving the drying chamber while recovering some of the latent enthalpy to heat the air leaving the condenser before it returns to the drying chamber. Experiments on the PMCHX gave the effect of moist air flow rate, air temperature, and temperature difference between the hot and cold side of the TE on condensate production and thermodynamic efficiency. Results showed that the compact PMCHX system is capable of producing 2 L/day of condensate with a COP of 2.5. A computational model for the condenser system based on the conservation equations for momentum, energy, and moisture applied to the air and porous membrane phases together with the TE heat pump equations has been developed. The resulting differential equations were solved numerically by the finite element method. Good agreement between the simulation and experiment was observed. The model was used to predict the system performance and energy cost per unit of recovered water for a range of operating conditions with different values of cold-side plate temperature, air flowrate, humidity, and temperature.