Phase Change Water Recovery Techniques: Vapor Compression Distillation and Thermoelectric/Membrane Concepts

Two techniques are being considered for the recovery of water from urine and from other waste waters as part of regenerative life support a-board the projected Space Station. One is the Vapor Compression Distillation (VCD) process while the other is based on a thermoelectric/membrane evaporation concept. Both use a phase change recovery process, and both try to minimize the energy input per unit mass of water recovered by using the heat released from condensation as the heat of evaporation for steam generation, i.e., operate at a Coefficient of Performance (COP) of greater than unity. The VCD process compresses steam to achieve the desired temperature differential to transfer the heat of condensation to the evaporation site, while the thermoelectric/membrane concept uses the Peltier effect. This paper describes a projected Water Reclamation System (WRS) of the Space Station's Environmental Control/Life Support System (ECLSS), in general, and defines in detail the two phase change concepts culminating in a comparison of these competing technologies. The analyses are based on current Space Station ground rules and requirements. The subsystem schematics, configurations and their operation are defined, including sizing of components, to form the basis of the comparison. Total equivalent weights for each subsystem, based on fixed hardware weight and power and heat rejection penalties are presented. The VCD process has a clear advantage over the thermoelectric/membrane concept even when operating at 305 K (90 F) versus 339 K (150 F). The thermoelectric concept is 26% heavier, has a 56% higher total equivalent weight and needs over twice the energy per unit mass of water recovered. The data and results presented and conclusions drawn are based on test experience at Life Systems and on information available in the literature.