Theoretical analysis of heat pump assisted air gap membrane distillation

Authors: Martijn Bindels, Bart Nelemans

Summary: Three configurations for heat pumps for simultaneously heating and cooling of vacuum assisted air gap membrane distillation (V-AGMD) are proposed and theoretically analysed. The benefit of using heat pumps is that local heating is not needed. The first configuration is the current state of the art a heat pump is used for the heating and cooling of V-AGMD. In the second configuration, a heat pump is connected to a V-AGMD module where the membrane and condenser channel have a constant temperature. The second configuration has a total distillate production of 3825 l/m2/h, but has a specific electrical energy consumption (SEEC) ranging between 16 and 107 kWhel/m3. The SEEC of the first configuration ranges between 9 and 22 kWhel/m3, which is higher than the thermodynamic limit. In the third configuration, a zero liquid discharge setup was proposed based on the first configuration. Results indicate that 26.7 kWhel/m3 is needed to raise the 35 g/l feed water to 250 g/l, which is only slightly higher than the 25 kWh el/m3 of mechanical vapour compression. Ammonia was used as a refrigerant in the calculations, making results applicable to real setups. In conclusion, heat pumps can provide heating and cooling, making them a promising solution.

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