Variable Pressure Humidification Dehumidification Desalination System

Nature uses air as a carrier gas to desalinate seawater through evaporation and rain. Several investigators have previously studied desalination cycles based on carrier gas processes. However, single pressure carrier gas cycles suffer from low energy recovery and hence low performance. Here we discuss a novel carrier gas cycle which operates under varied pressure. This cycle operates the evaporation process under a reduced pressure and the condensation process at an elevated pressure to enhance energy recovery. The pressure is varied by using a mechanical compressor. This cycle has been found to be several times as efficient as the existing carrier gas cycles. In this paper, the salient features of this cycle are analyzed in an on-design sense by defining a component effectiveness for the simultaneous heat and mass exchange components and an isentropic efficiency for the compressor and the expander. Based on this study, ways to improve the cycle are proposed. The possibility of using a throttle valve instead of an expander and the effect this would have on the overall performance is reported. Comparison of the new desalination cycle with existing ones is also performed in terms of specific work consumption.