Experimental Investigation And Thermodynamic Analysis Of Hollow Fiber Membrane-based Desalination System Driven By Solar Energy And Heat Pump

Water is the source of human life.If water is scarce,life will be threatened.97.5%of the water resources on the earth are seawater or bitter brine,but they cannot be directly used by humans.The freshwater resources that can be used only account for 0.25%of the global water resources.In recent decades,global freshwater resources have been decreasing,global freshwater resources are declining,and human demand for freshwater is growing.If we can use seawater desalination technology to turn these water resources into fresh water resources,this will undoubtedly solve the global shortage of fresh water.The desalination technology is mainly divided into thermal method and membrane method.Later,a new type of desalination technology,humidification-dehumidification desalination,was developed.This method is based on membrane distillation.It combines the advantages of both thermal and membrane methods(using heat to drive the system and membrane to filter seawater),humidification-dehumidification desalination technology is expected to replace the traditional desalination method.Compared with other desalination technologies,such as multi-stage flashing,membrane distillation can be driven by low-grade energy sources such as solar energy,air energy,etc.,and operating temperature and pressure are low(usually operating at atmospheric pressure,operating temperature is 60-80?),it is a new energy-saving and environmentally friendly seawater desalination technology.This paper mainly uses solar energy and heat pump complementary drive membrane humidification-dehumidification desalination technology for experimental research.The main advantages and innovations of this system are as follows:(1)The operating temperature of the system is generally lower than 80?.The system mainly uses clean energy solar energy and part of the air to provide energy for the seawater desalination system,thus reducing the oil and coal.The consumption of fossil fuels reduces pollution to the soil atmosphere;(2)The hollow fiber membrane module is used as a humidifier instead of the traditional direct contact humidification tower.Compared with the traditional humidification tower,the membrane humidifier realizes the indirect heat and mass transfer of air and solution,avoiding the problem of droplet entrainment in the traditional humidification tower,so the quality of fresh water produced by the membrane humidifier is higher.The main work of this thesis consists of the following aspects:(1)According to the requirements of seawater desalination membrane,the hydrophobic hollow fiber membrane was selected as the membrane module base material,and the membrane module humidifier was fabricated using the modified PVDF hollow fiber membrane independently developed by the research group.The hollow fiber membrane tube bundle adopts an equilateral triangle arrangement,and simulates seawater with a solution of 3.5%NaCl,the seawater walks through the tube and the air goes through the shell.The two heat and mass transfer are carried out in a cross-flow manner.The membrane area of the humidifier is 1 m~2and the packing density is 25%.(2)A hollow fiber membrane seawater desalination system(MHDH)with solar and heat pump complementary drive was designed and built.The whole system mainly includes three parts:solar collector module,heat pump module and seawater desalination module.The main energy supply time of solar collector module is 8:00-19:00,the working time of the heat pump module is 19:00-21:00,and the working time of the whole desalination system is 8:00-23:00.In the system experiment,the effects of parameters such as inlet air flow,inlet seawater flow rate and packing density of different membrane modules on system performance were studied.Under the condition of inlet air volume of 30 m~3/h and seawater flow rate of 200 L/h,the system's total high-purity freshwater production is 24.15 kg,its electrical conductivity is lower than 12?S/cm,and the system's unit volume water production and electricity consumption is14.57 kWh.The system's COP and electric power COP_E are 0.565 and 47.77 The collector efficiency of the solar collector at 11 o'clock noon has reached 0.52,and the heating COP of the air source heat pump has a maximum of 2.48.(3)During the period from 19:00 to 23:00,the desalination system driven by the residual heat of the water tank and using the heat pump was analyzed.It was found that the final output of fresh water was obtained when the seawater desalination system was driven by the waste heat of the water tank.For 19.01 kg,the system COP and COP_E are 0.514 and 37.60 respectively,and the system energy consumption per unit volume is 18.47 kWh/m~3.After the introduction of the heat pump,the total fresh water production of the system is 24.15 kg,the system COP and COP_E are 0.565 and 47.77,respectively,and the system energy consumption per unit volume is14.57 kWh/m~3.It shows that the performance of the system is greatly improved after the introduction of the auxiliary energy heat pump.(4)By using the first law and the second law to analyze the MHDH seawater desalination system driven by solar energy and heat pump,the experimental data shows that:Solar energy accounts for 80.55%of total input energy,electrical energy accounts for 14.21%,and water tanks account for 5.24%.In terms of energy output,only 55.34%of the energy is effectively utilized to produce fresh water,and the rest of the energy is dissipated during system operation.At 13 o'clock noon,the utilization exergy efficiency of the solar collector is about 32.71%.Most of the energy is collected by the hot water storage tank,and only part of it is output from the water tank to the desalination module for fresh water production.Analysis of the whole system found that the membrane component exergy damage accounted for 65.12%of the solar seawater desalination module damage,indicating that the membrane module humidifier lost most of the energy,so the membrane module humidifier needs to be optimized.