Performance Research On Multi-effect Solar Seawater Desalination Device Based On Falling Film Evaporation

With the continuous advancement of industrialization and the continuous growth of population,the demand for safe and sufficient freshwater resources is increasing.However,the industrial seawater desalination system can’t meet the requirements of distributed small freshwater preparation because of its high energy consumption,high requirements for seawater pretreatment and supporting facilities,reliance on municipal pipe networks to transport water,and high construction and operation costs.The areas lacking fresh water are often distributed with salt water and abundant solar energy.Using advanced solar thermal utilization technology to desalt and desalinate salt water can be one of the effective ways to alleviate the " bottleneck " of freshwater shortage.However,the problems of low efficiency and high cost still hinder the large-scale application of solar seawater desalination devices.Therefore,a multi-effect solar seawater desalination device based on falling film evaporation is designed in this work,which has the characteristics of small space liquid film evaporation,multiple uses of water vapor condensation latent heat,short mass transfer distance of binary mixed gas,small heat transfer thermal resistance,and efficient coupling of solar concentrating temperature and seawater desalination temperature.Based on the classical thermodynamics and heat transfer theory,the heat and mass transfer mechanism of the gaswater binary mixed gas in the annular closed small space was analyzed,and the calculation correlation of the heat transfer coefficient of the binary mixed gas was obtained.The factors affecting the water production performance and thermal performance of the multi-effect solar seawater desalination device were studied in the laboratory.The methods and ways to strengthen the heat and mass transfer process in the annular closed evaporation and condensation chamber were explored.According to the structural characteristics of the multi-effect solar seawater desalination device with falling film evaporation,the heat and mass transfer process of binary mixed gas in the annular closed small space was analyzed.Based on the relationship between evaporative mass transfer,convective heat transfer,and radiative heat transfer,the calculation formula of convective heat transfer coefficient hc of binary mixed gas in the multi-effect solar seawater desalination device was obtained.The theoretical water production rate m was calculated according to the formula proposed in this work and was compared with the test data.The key factors affecting the liquid film evaporative mass transfer,convective heat transfer,and radiative heat transfer in the annular closed small space were clarified.This work provides a theoretical basis for the following test research and application and provides a reference for the design of multi-effect solar seawater desalination devices.The test system was built in the laboratory.The influence of inlet water flow on the water production rate of the single-effect solar seawater desalination device under rated input power was tested.The performance ratio(PR)and gained output ratio(GOR)of the singleeffect solar seawater desalination device under different input power conditions were compared and analyzed.The effects of the use of mirror condensation cover and non-mirror condensation cover on the water production performance and thermal performance of the single-effect solar seawater desalination device under rated operating temperature conditions were explored.The results show that when the input power was 200 W and the inlet flow rate was 0.4 kg/h,the water production rate and the PR of the single-effect solar seawater desalination device were 0.188 kg/h and 0.60,respectively.Compared with the inlet flow rate of 0.2 kg/h and 0.3 kg/h,the water production rate and the PR increase by 0.011 kg/h,0.008 kg/h,and 0.03,0.02,respectively.When the inlet water flow was 0.4 kg/h and the input power was 180 W,the PR of the single-effect solar seawater desalination device was0.65,which was 0.5 and 0.1 higher than when the input power was 200 W and 220 W,respectively.By using the mirror condenser cover,the proportion of radiative heat transfer in the annular closed small space can be reduced,and the proportion of evaporative mass transfer can be enhanced.When the inlet water flow was 0.4 kg/h and the input power was200 W,the maximum water production rate of the device using the mirror condenser cover was 0.254 kg/h,which was 12 % higher than that of the device using the non-mirror condenser cover.When the input power was 180 W,the water production rate,the steadystate operating temperature,and the PR of the double-effect solar seawater desalination device were 0.334 kg/h,65.9 °C,and 1.18 respectively.Under the same input power condition,compared with the single-effect solar seawater desalination device,the water production rate of the double-effect solar seawater desalination device was increased by 83.2 %and the PR was increased by 0.53.In order to enhance the heat and mass transfer of the binary mixed gas in the device,the high-purity helium(99.99 %)was filled in the double-effect solar seawater desalination device.This work studied the influence mechanism of the gas physical parameters on the water production performance of the double-effect solar seawater desalination device.The results showed that the water production rate of the double-effect solar seawater desalination device filled with helium was 0.587 kg/h when the inlet water flow is 0.9 kg/h and the operating temperature was 80 °C,which was 40.3 % higher than that when the device was filled with air.The negative pressure operation can alleviate the obstruction of the non-condensable gas to the heat and mass transfer of the binary mixed gas in the device and reduce the liquid film vaporization temperature.In this work,the effects of the operating pressure on the water production rate and thermal performance of the double-effect solar seawater desalination device were studied by adjusting the operating pressure.The results show that when the operating temperature was 80 °C and the operating pressure was 60 k Pa,the steady-state water production rate of the double-effect solar seawater desalination device was 0.508 kg/h,which was 10.9 % higher than that when the operating pressure was 80 k Pa.When the operating temperature was 80 °C and the operating pressure was 60 k Pa,the condensing temperature of the first effect was 2.8 °C,which was 1.4 °C higher than that when the operating pressure was 80 k Pa.The condensing temperature of the second effect was 2.5 °C higher than that when the operating pressure was 80 k Pa.The variation of the condensing temperature with operating pressure further explains why the water production rate increases with the decrease of operating pressure.