Indirect mechanical heat pump assisted humidification‐dehumidification desalination systems

Abstract

In this study, four new indirect heat pump assisted mechanical vapor compression humidification-dehumidification (HDH) systems are proposed and their superiorities over the reference system are demonstrated from thermodynamics and thermoeconomics viewpoints. The proposed models are configured based on an HDH unit and a simple cascade heat pump, an HDH unit and a heat pump with an ejector, an HDH unit and a cascade heat pump with an ejector, and an HDH unit and a vapor injection heat pump. Although employing a heat pump with cascade and ejector configurations improved gain-output-ratio (GOR) and specific power consumption (SPC) values in comparison with the base system, the performance metrics of such layouts are still inferior to those of the HDH unit coupled with the vapor injection heat pump. It is found that the desalination unit with vapor injection mechanism has the highest GOR and the lowest SPC values of 9.22 and 71.29 kWh/m3, respectively, under constant electrical power of 31.26 kW. Overall, proposal of the cascade heat pump, heat pump with an ejector, cascade heat pump with an ejector, and a vapor injection heat pump instead of the conventional heat pump improved GOR by 4.83%, 10.03%, 11.52%, and 14.25%, respectively. Meantime, the SPC of the HDH unit coupled with a simple heat pump decreased by 4.6%, 9.1%, 10.26%, and 12.46% when a cascade heat pump, a heat pump with an ejector, a cascade heat pump with an ejector, and a vapor injection heat pump were used instead of the conventional heat pump, respectively. Also, it is found that a maximum improvement of 14.13% can be achieved in exergy efficiency with a cost penalty of only 1.59 $/m3. Also, it was found that with the use of a simple cascade heat pump instead of the simple heat pump, the exergy efficiency can be enhanced by 4.9%, while the unit cost of the distilled water (UCDW) was degraded by 51.32%. Likewise, the HDH unit coupled with the cascade heat pump with an ejector was resulted in high UCDW compared to the base system, while its exergy efficiency was not enhanced so significantly. Variation of exergy efficiency versus the intermediate temperature had a maximum value of 1.93% (for the HDH unit coupled with a simple cascade heat pump) and 2.05% (for the HDH unit coupled with a cascade heat pump with an ejector) at Tint = 315 K. Moreover, it was found that the UCDW can be decreased with the rise of the intermediate temperature and cascade heat exchanger temperature difference as the fresh water cost rate decreases as a result, while the exergy efficiency can be increased with the rise of the cascade heat exchanger temperature difference.