Investigation Of Heat And Mass Transfer And Performance Of A Solar Energy Driven Membrane-based Desalination System Based On Hollow Fiber Membrane Modules

In recent years,the demand for fresh water resources has been increasing.Meanwhile,about 97.5%of the water is sea water which contains large amounts of salt.As a result,desalination of sea water becomes the solution to overcome water shortage.Desalination can be accomplished by a number of techniques.Of those technologies,membrane distillation(MD),an emerging membrane separation technology,has been known since the late 1960s.Compared with the traditional method such as Multi Stage Flash(MSF)and Reverse Osmosis(RO),MD can work well at lower operating temperatures(60-80°C)and hydrostatic pressures,which means that it can be powered by low-grade solar energy.Accordingly,MD claims to be a cost-effective and environmental-friendly alternative to the existing commercial desalination technologies.The newly proposed membrane-based air humidification-dehumidification desalination(MHDD)method is a desalination technology based on membrane distillation.Its main advantage lies in:(1)this method uses a hollow fiber membrane module,to replace the traditional packed columns to realize humidification.The saline water and the air are in a non-direct contact with each other,therefore the problem of liquid solution droplets cross-over which is usually encountered in traditional packed column humidification technologies,is addressed.In this way,high purity water can be obtained.(2)the operating temperature is less than 80°C.In the process,low grade energy such as solar energy can perform well.To date,however,on the one hand,flow characteristics and convective heat and mass transfer mechanism in the hollow fiber membrane bundles for desalination are the key parameters for performance analysis of such membrane-based desalination system,which need to be deeply revealed.On the other hand,little attention has been paid to the thermodynamic performance and economic evaluation of such solar energy powered MHDD system.In order to solve the above problems,this study is mainly carried out in the following aspects:(1)A solar energy driven MHDD system is designed and constructed.The system mainly includes a solar heating unit and a desalination unit.It can provide high purity drinkable water(The electric conductivity of the produced water is less than 12?S/cm).A hollow fiber membrane module is employed as the humidifier.Humidification is realized in a non-direct contact manner.For 0.59 m~2 membrane area,the accumulated water production is 15.27 kg/d.The consumption of electric energy is 19.23 kWh/m~3.Solar energy accounts for 92.0%of the total energy consumption.The COP of the whole system is about 0.75.(2)Laminar flow and conjugate heat and mass transfer in a cross-flow hollow fiber membrane bundle for desalination are investigated on a periodic computational cell when the air Reynolds number ranges from 50 to 300.The friction factors,Nusselt numbers and Sherwood numbers are obtained.For the saline water stream,the concentration boundary layer develops much slower than the thermal boundary layer.For the air stream,rising packing fractions can improve the Nusselt numbers and Sherwood numbers but with the expense of increasing the friction factors.(3)The air flow tends to become turbulent due to the successive disturbances from the numerous fine fibers when the Reynolds numbers ranges from 350 to 600.The turbulence is transitional and the flow can be considered as two-dimensional turbulence.The Low Re k-?model shows a better agreement with the experiments than the STD k-?model in the tested Re_a range from 350 to 600.Heat and mass transfer increase mainly by decreasing transverse pitches,rather than by decreasing longitudinal pitches.(4)A 3D low Re k-?turbulence model is adopted for the calculation of fluid flow and heat and mass transfer when the air Reynolds numbers ranges from 650 to 900.The new proposed 3D low Re k-?model is compared to previous 2D turbulence and laminar models.The 3D laminar model can be simplified into a 2D laminar model.However for turbulent flow,the normal velocity non-uniformity is very large and thus the turbulent flow field is rather irregular and chaotic,which typically show three-dimensional flow characteristics.(5)A mathematical model for the whole system simulation is developed and validated.The effects of various parameters including the structural parameters of solar heating unit,operating conditions of the desalination unit and the packing fraction of the membrane module,on system performance are examined.It indicates that the feasible operating parameters investigated are:hot saline water flow rate,236 L/h for per unit area of membrane;air flow rate,25 m~3/h for per unit area of membrane;module packing faction,30%.An economic analysis on the whole system reveals that the final cost for water production is about 16.88$/m~3.The system needs a low maintenance cost.