Impact of extraction on a humidification-dehumidification desalination system

Humidification-dehumidification desalination systems obtain higher water output per unit energy input when the entropy generation of the system is minimized. Past studies have shown that entropy production is lowered when the system's humidifier and dehumidifier have nearly equal rates of enthalpy flow for both air and water streams. In this study, we examine the effect of extracting water or air from one component and injecting into the other, with the aim of bringing the enthalpy rates into balance. We consider the effect of extraction or injection on standalone heat and mass exchange devices. Similar to cases without extraction, it is found that entropy generation is reduced when a component's modified heat capacity rate ratio is brought closer to unity. We then consider the entire system with one or more extractions at various mass flow rates. The results show that it is possible to significantly improve the cycle performance, but that performance can be degraded in some cases. The effect of injection or extraction was greatest for components with high effectiveness and low terminal temperature difference. While multiple extractions resulted in higher performance gains, the impact of a single extraction was greatest with subsequent extractions yielding diminishing returns. (C) 2012 Elsevier B.V. All rights reserved.