Energetic and exergetic analyses of a closed-loop pressure retarded membrane distillation (PRMD) for low-grade thermal energy utilization and freshwater production

Abstract

Low-grade thermal energy is abundant but hard to utilize because of the low energy availability. Recently, research on low-grade thermal energy has been conducted to enhance the energy availability. One of these approaches, pressure-retarded membrane distillation (PRMD), has been proposed. It was designed by combining the operating principles of membrane distillation and pressure-retarded osmosis. In this paper, a closed-loop PRMD is proposed to include the structure of thermo-osmotic energy conversion (TOEC) to enable simultaneous desalination and power generation. Mathematical modeling and energetic/exergetic analyses are performed through MATLAB, to evaluate the feasibility of new process. In addition, a sensitivity analysis is conducted to analyze the energy efficiency of the process. The results reveal that the closed-loop PRMD system requires the lowest heat input (1.74 kW/m2) and increases the electrical energy production (1.91 W/m2) to achieve the same water permeation, compared with existing processes. In addition, unlike the TOEC process, water and electrical energy can be produced simultaneously. Moreover, the exergy destruction of the closed-loop PRMD is observed to be the lowest. Thus, the proposed closed-loop PRMD is a promising solution for desalination and energy harvesting systems from low-grade thermal energy.