Modeling And Multi-objective Optimization Of Low-grade Thermal-driven Membrane Distillation System

With the increasing global demand for freshwater resources,about 3.5 billion people will face the plight of water shortage by 2025.The current water supply situation in China is also not optimistic,and the per capita water availability is only a quarter of the world’s per capita level.The development of desalination technology is the primary means of coping with water shortages.However,most of the current desalination technologies worldwide are driven by fossil fuels.Mainstream desalination technologies,such as reverse osmosis,need to consume much high-grade electricity as an energy-intensive process.The vast consumption of fossil fuels and carbon emissions are crucial factors limiting the development of desalination technology.Membrane distillation is an emerging desalination technology that can utilize low-grade heat sources such as industrial waste heat and renewable energy;hence it has the potential to desalinate seawater sustainably and is now widely used.In this thesis,a mathematical model based on an equation-oriented method is built to analyze the thermal economy of the membrane distillation system driven by three low-grade heat sources:supercritical CO₂ Brayton cycle waste heat,geothermal energy,and solar energy.Meanwhile,a whole life cycle evaluation framework is built to assess the MD systems’environmental impact.These works provide a theoretical basis for designing and applying low-grade thermal energy-driven membrane distillation technology.The thermodynamic model of the membrane distillation process is established,and its accuracy is verified.Thermo-economy optimization models of low-grade thermal energy-driven membrane distillation desalination systems are constructed.The results show that the waste heat-driven membrane distillation scenario has the smallest water production cost of0.589$/m~3 among the three heat source scenarios,showing a cost advantage compared with mainstream desalination technologies,while the annual water production can reach462.3×10~3 m~3,which is about one-fourth of the average water production of desalination plants in the world.The electricity cost of pumping and the cost associated with the MD module are the main factors that lead to changes in water production costs when the demand for water produced by the system changes.A framework for the full life-cycle evaluation of MD systems from the construction stage to the decommissioning stage is developed.The results show that the waste heat-and solar energy-powered desalination scenarios lead to a global warming potential of2.321 kg CO₂-Eq/m~3 and 6.326 kg CO₂-Eq/m~3,respectively.They have environmental advantages compared to those of conventional thermal desalination technologies.Meanwhile,due to electricity consumption for pumping,the impact of the operation stage is more significant than the other stages in most of the evaluation indicators in all three heat source scenarios.In terms of endpoint indicators,the environmental score of the waste heat scenario is 0.160 Pt/m~3,which is more environmentally friendly than the other two renewable energy scenarios.The multi-objective optimization analysis shows that all three heat source scenarios have conflicting economic and environmental objectives,which can be traded off to make the MD system have better economic performance while achieving lower environmental emissions.