Harvesting net power and desalinating water by pressure-retarded membrane distillation

Pressure-retarded membrane distillation (PRMD) can convert low-grade heat to useful work by harvesting the pressure energy of the condensation liquid on the cold side. In this study, a new type of PRMD system for combined freshwater and power production was proposed. For this configuration, the thermal energy of the phase change is transported to the interface mainly by conduction rather than convection, which significantly reduces the pump power loss of the liquid flow. In addition, it also utilizes a multistage structure to regenerate heat with low pump power loss. The experimental results showed that, for a module with a heating area of 1.0 m2, this system can produce 188 L of freshwater and 27.8 kJ of power each day when operating between 80°C and 40°C. The water and power densities of the PRMD configuration would be affected by saline ions in the feed liquid, air resistance in the evaporation chamber, membrane wetting, and membrane compaction. The experimental and molecular dynamics simulation results indicated that a higher temperature difference or working temperature will significantly improve the desalination and power generation rates because of the increased mass transfer driving force of the vapor gradient. This study proved that it is possible for a PRMD system to simultaneously obtain net power and freshwater using low-grade heat as the only energy input. Nevertheless, the liquid supply, working pressure, and membrane properties should be improved to achieve better performance.