Performance limits of zero and single extraction humidification-dehumidification desalination systems

Given simultaneous heat and mass transfer and a multiplicity of possible temperature and flow configurations, the optimization of humidification-dehumidification desalination systems is complex. In literature, this optimization has been tackled by considering moist air to follow the saturation curve in the humidifier and dehumidifier of a closed air water heated cycle. Under similar conditions and the same pinch point temperature differences, energy recovery was shown to improve with an increasing number of stages. In the present work, the limits upon the energy recovery and the water recovery (product water per unit of feed) of closed air water heated cycles are investigated. This is done by considering heat and mass exchangers to be sufficiently large to provide zero pinch point temperature and concentration differences with in the humidifier and dehumidifier. For cycles operating with a feed temperature of 25°C and a top air temperature of 70°C, GOR is limited to approximately 3.5 without extractions (i.e. single stage system) and 14 with a single extraction (i.e. dual stage system) while RR is limited to approximately 7% without extractions and 11% with a single extraction. GOR increases and RR decreases as the temperature range of the cycle decreases, i.e. as the feed temperature increases or the top air temperature decreases. A single extraction is shown to be useful only when heat and mass exchangers are large in size. In addition, the effects of salinity and the validity of ideal gas assumptions upon the modeling of HDH systems are discussed.