Energy efficient configuration of Membrane Distillation units for brackish water desalination using exergy analysis

Abstract

Membrane Distillation (MD) became a promising technology for water desalination. MD is energy-driven process and with the limitation and cost of energy resources it is imperative to examine improving the energy effectiveness of such units. For this purpose, Exergy analysis is applied to a validated model of typical MD module to identify the sources of inefficiencies. Reject brine is found to be an origin for exergy losses when the energy of the geothermal feed is not fully utilized. In this case, feeding the reject brine to sequential MD units connected in series can capitalize the production rate and energy utilization. In fact, the exergy losses are reduced from 70% to 30%. The enhanced energy efficiency made the exiting warm permeate responsible for another exergy losses if disposed to the environment. Repeated recycling of the outlet permeates to successive MD vessels connected in parallel improves the process performance. This cascaded structure of the desalination process converts most of the available exergy in the geothermal source into work to produce additional fresh water and dispose the salty brine and warm permeate at very low exergy levels.