| Obtaining drinkable water from seawater by desalination is one of the significant ways to mitigate the water scarcity crisis faced by mankind in the 21st century.Thermal desalination with solar energy as the energy source owns advantages of simplicity in maintenance and easiness in miniaturization and modularization,making it more suitable for meeting small-scale freshwater demand in areas with inadequate infrastructure.Water productivity is low for a traditional solar still,which can be increased by reducing the pressure in the distillation chamber.With a focus on tubular solar stills,experiments were firstly carried out to test the productivity of single-and multi-effect stills,and to analyze the effect of operating pressure on important parameters like evaporation and condensation temperatures,etc.Visualization and numerical research were then conducted to further clarify the convective diffusion in the chamber.A model was developed finally to predict the productivity for tubular solar stills.The main contents and key progress are summarized as follows:Single-and multi-effect tubular solar distillation systems were designed and fabricated.System performance was tested indoors and outdoors.For the indoor test,a thermostatic water bath or an electric heating rod was used to heat the water stored in the still,to study the productivity and system temperatures under stable heat source conditions.Outdoor tests were carried out in Chengdu and Xichang on a three-effect tubular solar distillation system,where a solar collector was employed as the heat source.The results showed that for the single-effect solar still,the water production rate under the operating pressure(pop)of 40 k Pa absolute pressure with water temperature in the range of 50–90°C can be increased by 8%to 32%,compared with that under atmospheric pressure.The performance ratio（PR）can be increased by up to 0.18.For outdoor tests,the daily water productivity of the three-effect tubular distillation system under natural air cooling reached 7.2 kg at pop=40 k Pa with a corresponding PR of 1.45 which is 0.68 higher than that at normal pressure.At pop=20k Pa,on the contrary,the yielding was not increased compared with the case of pop=40k Pa,but with a reduction of 40%,and PR was reduced by 0.5.This is because the vacuum operation reduces the difference between the evaporation and condensation temperatures in all effects.When the temperature difference approaches 0°C,the evaporation and condensation in the still are inhibited,leading to the deterioration of the yielding performance.An appropriate way of heat dissipation should be employed to avoid this situation.With water immersion cooling,the daily yielding and PR reached 10.7 kg and 1.87,respectively.The cost of the freshwater production is 83.2yuan/ton under vacuum operation,which is reduced by about 50%of that under normal pressure.The solar still under vacuum condition is more competitive economically compared with other small-scale solar desalination devices proposed in recent years.Visualized experiment with smoke as the tracer as well as CFD simulation was carried out to investigate the flow,heat and mass transfer in the distillation chamber under reduced pressure.Clarifications were achieved for both the mechanism of performance enhancement by vacuum operation and the influence of geometric parameters.The results showed that the humid air moved in the form of vortices in the still.The reduced pressure accelerated the rotation of the vortices,so the evaporation and transportation process were intensified.At pop=40 k Pa,the horizontal velocity near the evaporation surface was increased by 50%,and the mass transfer rate of water vapor was doubled compared with that at 101 k Pa.Besides,a 30%increase in the maximum velocity of vapor from the evaporation surface to the condensation surface was achieved.Flow transition phenomenon was found as the result of the disturbance caused by the water trough edge when the water depth in the trough was changed.When thermal Rayleigh number（Ra T）exceeds 105,the decrease in water storage depth might destabilize the humid air flow.Two unsteady-state flow regimes were observed which are referred to as periodic flow and chaotic flow.A local peak value of water production rate under the periodic oscillating flow state is obtained at pop=80 k Pa.A correlation to calculate the mass transfer coefficient for small distillation chambers was fitted against the experimental data under different operating pressures.And a theoretical heat and mass transfer model was established for a multi-effect solar still under vacuum operating condition,by which the water productivity was predicted for vacuum-operation solar still under the irradiation conditions of an island.Analysis was conducted to clarify the influence of operating conditions,such as cooling method,etc.The model was verified against experimental data from other pieces of literature,showing a well prediction accuracy.The deviation of predicted productivity is less than21%.The maximum water production of single-and three-effect stills coupled with a1-m2 solar collector were 3.3 and 10.7 kg/d,respectively.For the single-effect still,the water temperature in the storage trough would be decreased if heat dissipation of the shell-side were enhanced.Therefore,increasing the wind speed or adopting water immersion cooling will reduce the water production.As for the three-effect still,although cooling enhancement would decrease the water temperature and the yielding of the third effect,the yielding of the first and second effects can be increased.Therefore,for the three-effect still the above-mentioned cooling enhancement is beneficial to improving the water production performance.The optimal wind velocity and cooling water temperature were 7 m/s and 15°C,respectively. |
