Energy Conversion And Water Desalination Based On The Pressure-Increasing And Transmembrane Phase Changes

Natural and industrial activities continue to produce abundant low-grade heat,therefore,it is of great significance to develop efficient methods of utilizing low-grade heat.Recent studies have shown that it can convert low-grade heat to hydraulic energy and distill brine synchronously by using the two pressure-increasing and transmembrane phase change processes of capillary evaporation or thermo-osmosis.In order to address the problems of the studies,such as low power density,large heat transfer loss,large flow loss,and difficulties with heat recovery,this dissertation proposed three systems of power production or combined power and water production.And the performance and mechanisms of these systems were studied through theory,simulation,and experiment.First,in terms of negative pressure capillary evaporation based on hydrophilic nanoporous membranes,a capillary evaporation combining power and water production technology was proposed.A molecular dynamics model was constructed based on a carbon nanotube membrane with a pore size of approximately 2.46 nm.Simulation showed that,at a heating temperature of 400 K and a working pressure of 400 bar,it obtained a desalination flux of about 3.6 g/（cm²·s）,a power density of about 170 W/cm²,and a heat-work efficiency of about 1.95%,which preliminarily revealed the development potential of the technology.Then,a capillary evaporation heat-work conversion experimental system was built based on a nylon filter membrane with a pore size of approximately 100 nm.The experiment showed that the external non-condensable gas can infiltrate into the system by the partial defects of the nanoporous membrane,which led to the reduction of the working pressure and mass flux,and affected the power output and stability of the system.At a heating temperature of 85℃and a working pressure of 0.10 bar,it obtained a power density of about 77 m W/m² and a heat-work conversion efficiency of about 1.6×10^-6.In addition,in terms of positive pressure thermo-osmosis based on hydrophobic nanoporous membranes,a self-pumping and stacking thermo-osmotic energy conversion system which has low flow loss and is easy to heat recovery was proposed.The feasibility of the technology was verified by using a composited membrane with a small pore sublayer of 100 nm pore size.It was experimentally found that the total heat-work conversion efficiency increased approximately linearly with the total stage.And the evolution and accumulation of non-condensable gas in the evaporation chamber may influent the stability of the system.At a heating temperature of 80°C and a working pressure of 1.0 bar,the 5-stage system achieved a total power density of about 0.8 W/m²and a total heat-work efficiency of approximately 4.0×10^-5.Furthermore,a theoretical model describing the stacking thermo-osmotic energy conversion technology was built,and it proved that reducing the air partial pressure,optimizing the membrane layer structure,and optimizing the thickness parameters are its improvement methods.Calculations showed that if employing composite membranes with a pore size of 20 nm,an optimized system with 32 stacking stages can obtain a total heat-work efficiency of about 3.79%and a total power density of approximately 43.4W/m² at a heating temperature of 80°C and a working pressure of 50 bar.Finally,a stack thermo-osmosis combined power and water production technology was proposed,which has the advantage of low flow loss and high water quality.The experiment results showed the water production rate and heating power decrease with the increase of working pressure,which should be related to the increase of transmembrane phase change resistance and reversed transmembrane leakage.At the conditions of a heating temperature of 80°C and a working pressure of 1.5 bar,a permeator stack with a flux size of one square meter can simultaneously produce about 188 L high-quality fresh water and about 27.8 k J power per day.In conclusion,for the pressure-increasing and transmembrane phase change processes of capillary evaporation and thermo-osmosis,this dissertation had further demonstrated their development potential in low-grade heat conversion and water desalination by theory,simulation,and experiment.Especially,the invention of the regenerative system had further improved their development framework.