2022 Articles

Energy efficiency analysis of membrane distillation for thermally regenerative salinity gradient power technologies

Brogioli, Doriano; Yip, Ngai Yin

The abundant resource of low-temperature heat sources (≈80–120 °C), e.g., low-concentration solar collectors or shallow geothermal wells, hold huge potential to be a renewable energy supply. A proposed method for harnessing such low-grade heat is to use the heat to distill a solution and then utilize the produced concentration difference to generate work, by means of salinity gradient power (SGP) technologies. In this application, the energy efficiency of the distillation process (ratio between produced mixing free energy and consumed heat) becomes a fundamental performance parameter. This study systematically analyzes the energy efficiency of direct contact membrane distillation (DCMD). This technique is often billed as an emergent membrane innovation that can use low-temperature thermal sources and additionally has the advantages of compactness and relatively simple implementation. However, we show that DCMD always has lower efficiencies than traditional vacuum distillation to achieve the same separation. In particular, the efficiency is smaller at high concentrations, which is desired for the distillation–SGP approach of energy conversion. The main source of entropy production in membrane distillation is the unavoidable thermal conduction across the membrane. Alternatively, vacuum membrane distillation could be a better candidate to regenerate the salinity gradient in low-grade heat utilization with distillation–SGP.