Closed Air Loop Dryer for Waste Processing in Space
CALS Impact Statement
We are developing a drying system to recover water from cabin trash, water-reprocessing brines, and other wet solid wastes generated on board a spacecraft. The system is designed to operate at zero to full gravity, to contain odors, dusts and micro-organisms, to require a minimum of crew time for operation and maintenance and to improve energy efficiency over conventional "once-through" drying or lyophilization.
In space, who takes out the trash? No one - and that`s exactly the problem. Wet trash spoils, creating odor problems and health hazards for the crew, and water in space trash is not currently recycled for crew use. We are developing a drying system for space trash and for the brine byproduct of current spaceflight water reprocessing systems. The dryer will dry and stabilize these wastes and recover their water content while minimizing crew involvement and while retaining odors, bacteria and dust within the system. Using a heat pump to heat the drying air and condense water vapor in a closed air loop produces better energy efficiency than once-through drying with separate air heating and condensation. The population directly affected by this work is a small one - astronauts on long term missions such as a lunar outpost or Mars transit - but the technology we are developing should spin off improvements in the energy efficiency of consumer items like condensing clothes dryers and food dehydrators, and in temperature/humidity control for ultra-clean environments such as integrated circuit manufacturing rooms, hospital operating rooms, and sickrooms for immunocompromised patients and burn patients.
See 2004 report for earlier results. A new condenser with significantly lower pressure drop and higher condensing surface/volume ratio than its predecessor was designed and built. The testbed was built and tested using soybeans (a well characterized material) as a model system. In consultation with NASA engineers new concepts for wick-based evaporators, and evaporator operation, were proposed, with the goals of preventing mold overgrowth on wicks, enabling the crew to change out spent wicks without directly handling contaminated wick materials, and recovering dry salt particles from wicks to extend wick life. Permanent stabilization of food wastes requires a water activity below 0.6 for complete suppression of microbial growth. Since this level of dryness is difficult to attain, target water activity levels sufficient to stabilize contaminated food materials for 90 and 180 day periods must be determined, where "stability" is defined operationally as absence of offensive odor, airborne mold spores, and free water. We have established cultures of xerophilic molds and yeasts to be combined with other environmental contaminants in ordinary trash (e.g. soil, human waste, dust, saliva) as a worst case system for inoculation into food materials for drying and storage studies. A system of desiccators with saturated salt solutions was built to establish known levels of water activity. Storage studies will begin in early 2006.
No relevant changes have yet been implemented. The Cornell/Orbitec dryer system is one of several candidates for further development for a manned lunar or Mars mission; so far it appears to be the most energy-efficient of those proposed. Heat pump dehumidification drying is an established terrestrial technology used in "ventless" condensing laundry dryers and for drying food, timber, and biomaterials. Available commercial systems are gravity dependent and/or contain components or materials unacceptable for space applications. Uncertainties in space drying include drying time, distribution of air through nonuniformly packed solids, control of solids movement inside the dryer, dust control in the moist air stream, biofilm on the condenser, the effect of small damp regions on the storage stability of dried product, and the compatibility of condensate with existing liquid waste processing systems. DRYER technologies developed for space applications may find use in commercial terrestrial systems as OEM components to improve performance, power, efficiency, and operating life.