Performance enhancement of a conventional multi-effect desalination (MED) system by heat pump cycles

Abstract

Multi-effect desalination (MED) systems are highly promising for high salinity seawaters at large scales, and hence are used broadly in desalination plants. Despite the fact that the MED systems can be driven by various renewable or waste heat energies, many of the available renewable technologies are expensive in some parts of the globe. Hence, proposing and developing high-efficient mechanical-based MED units can be an encouraging alternative. In pursuance of this objective, two innovative mechanical-driven MED units are devised, simulated and the results are compared with those of the conventional MED-MVC (mechanical vapor compression) unit. For this aim, absorption-compression heat pump (ACHP) and vapor compression heat pump (VCHP) systems are used for integrating with the MED unit. Under a constant input power of 1845 kW it is found that proposing the MED-VCHP system instead of the conventional MED-MVC system improves gain-output-ratio (GOR), performance ratio (PR), freshwater rate, specific work consumption (SWC), exergy efficiency, exergy destruction rate, and exergy of loss by 1%, 12.86%, 12.64%, 11.45%, 10.78%, 3.3%, and 10.86% respectively. However, the unit cost of distilled water (UCDW) of the MED-VCHP system is around 6.7% higher than that of the MED-MVC system. Also, the MED-VCHP system uses less exergy of fuel than the MED-MVC system due to its high seawater exergy. Furthermore, it is found that one can decrease the overall exergy loss rate from 145.4 kW to 129.6 kW when the MED-VCHP system is used instead of the MED-MVC system.