Performance study of a passive vertical multiple-effect diffusion solar still directly heated by parabolic concentrator

This paper presents a passive vertical multiple-effect diffusion (VMED) solar still integrated with a parabolic concentrator, which utilizes concentrated sunlight to directly produce freshwater. The VMED solar still comprised a vertical solar absorber and several partitions arranged parallel and attached with hydrophilic wicks. The high temperature gradient produced by the parabolic concentrator in the still can effectively improve the water productivity. The structure of the parabolic concentrator was designed, and its optical simulation was performed. The energy balance equation was established to analyze the heat- and mass-transfer process. To evaluate the productivity and efficiency of the still, a small prototype was fabricated and tested under different conditions. Indoor steady-state experiment results showed that using a parabolic concentrator could improve the six-effect VMED still efficiency by 60% and increase the temperature gradient in the still to 56.9 °C. With increased irradiance from 500 W/m² to 900 W/m², the freshwater yield of the six-effect still improved from 1.27 kg/m²/h to 2.64 kg/m²/h, and the corresponding gain-output ratio increased from 1.72 to 2.31. Outdoor-experiment results showed that the obtained daily yield was 5.3 kg/m²/day at an average solar radiation of 603.7 W/m². Performance and economic analyses indicate that the designed VMED still was suitable for application in remote offshore areas.