(718p) Modeling of a Closed-Loop Heat Pump Dryer for Cabin Solid Waste on Long-Duration Space Missions

During long-term space missions, astronauts generate wet trash, including food residues, moist hygiene wipes and wet paper towels. This waste gives rise to two problems: first, the generation of odors and potential health hazards by microbial growth; second, the diversion of water from the available recovery loops. We have designed and built a prototype of an energy-efficient closed air-loop heat-pump dryer to remove water from wet material. The equipment is designed to stop microbial activity by both pasteurization and desiccation, and to recover humidity from the closed air loop in a gravity-independent Porous Membrane Condensing Heat Exchanger. A computational model of the system consisting of the dryer, condenser, and heat recovery modules has been developed. This is based on the conservation equations for energy and moisture applied to the air and solid phases and its formulation includes the unique waste characteristic of having both dry and wet solids. It also incorporates the heat and mass transfer coefficients for the system that have been measured. The experimentally determined moisture sorption equilibrium relationship for the wet material is also used. The resulting system of differential equations is solved by the finite-volume method as implemented by the commercial software COMSOL Multiphysics. The model is used to predict the system performance and energy cost per unit of recovered water under a range of operating conditions including air temperature, air flowrate, and initial moisture content of the trash.