Saturation curve analysis of humidification-dehumidification desalination systems and analysis of reversible ejector performance

Abstract

The limitations upon the efficiency of humidification-dehumidification (HDH) desalination systems are investigated. Secondly, ejector technologies are analyzed as a means of powering desalination systems thermally. Thermal desalination systems offer advantages over membrane technologies in terms of robustness. They are less prone to scaling and fouling and capable of treating feed water of higher temperatures. One major drawback of thermal desalination is the high energy consumption, particularly for small scale systems. This work is motivated by the need for drastically improving the efficiency of thermal desalination technologies, particularly those on scales of 1 to 1000 m3 per day. Given multiple liquid and gas streams and simultaneous heat and mass transfer, the limits on the performance of HDH systems are not easily recognisable. By considering the saturation curve for moist air, a methodology is proposed to graphically understand these limits. A pinch point methodology is employed to evaluate the impact of heat and mass exchanger size upon performance. The impact of salinity upon the design methodology is described. As the pinch point temperature and concentration differences decrease, the gained output ratio (GOR) is shown to increase and recovery ratio is shown also to increase. There is shown to be a critical pinch point temperature difference beyond which there is no advantage to having multiple stages, each with different liquid to dry air mass flow ratios. Finally, the GOR is shown to increase as the temperature range of the cycle decreases. Little attention is given in the literature to the definition of an ejector efficiency. Here, the entrainment ratio of real devices is compared to the reversible entrainment ratio and denoted the reversible entrainment ratio efficiency. The definition of different performance metrics for ejectors are analyzed and compared. Graphical illustrations are provided to support intuitive understanding of these metrics. The performance metrics are applied to existing experimental data. The behaviour of reversible ejectors is investigated. Analytical equations are also formulated for re-versible ideal-gas ejectors. For general air-air and steam-steam ejectors, the exergetic efficiency [eta]x is found to be very close in numerical value to the reversible entrainment ratio efficiency, [eta]/RER.