Experimental studies of a hygroscopic condenser-evaporator heat exchanger based on concentration differences of vertically falling films

Abstract

A concentration-driven power cycle motivated by differences in vapor partial pressures (boiling point rise) and latent heats of brine and water is studied. The condensation of relatively low-pressure, low-temperature vapor occurs on the free interface of a relatively hot falling film of a hygroscopic salt solution due to the reduced vapor pressure of the brine. The heat released is transferred to the evaporating/cooling water film on the other side of a vertical plate separating the brine and water films. The process is maintained because the latent heat of condensation on the brine film is higher than the latent heat of evaporation of pure water. The condensation driving force is the difference between the partial pressure of condensing water vapor and that of water in the brine solution. The simultaneous mass and heat transfer mechanisms associated with this nonisothermal absorption can occur even against an opposing thermal driving force in the condensing vapor phase. Complementing earlier studies by the same authors, a vertical film-type condenser-evaporator heat exchanger is considered. The experimental study deals with the effects of the various parameters involved in this rather unique process and the mechanisms that control them. The experimental results prove the potential of operating this new heat transfer modality and provide the background for the theoretical determination of the optimal performance of this direct-contact power cycle.