This paper describes a comparison between our first model of a Tubular Solar Still (TSS) and a second one. The first model was designed by our research group using a vinyl chloride sheet as a transparent tubular cover. Since then, we have designed the second model that has improved the assembly, economy and maintenance over the first model. In the second model, a polythene film was adopted as the cover. As a result, the cover weight and cost of the second model were noticeably reduced and the durability was distinctly increased. A set of laboratory experiments was carried out using a special technique to investigate the evaporation, condensation and distilled water production performance independently and simultaneously on the second model. It was revealed that the hourly evaporation, condensation and production fluxes were proportional to the humid air temperature and relative humidity fraction. Consequently, an empirical equation was proposed based on this relation to predict the hourly production flux. The applicability of this equation was examined from the comparison with field experiments in Fukui, Japan and in Ras Al Khaimah, UAE. It was seen that the calculated results had a good agreement with the field data.

In this study, attempts were made to provide a group of complete heat and mass transfer correlations, and to propose a new heat and mass transfer model for a Tubular Solar Still (TSS) by taking account of thermal properties of the humid air inside the still. We developed a new experimental technique for directly measuring the evaporation rate from the saline water surface in the TSS and evaluated the evaporative mass transfer coefficient. Indoor experiments were conducted to investigate the production performance and the heat and mass transfer coefficients of the TSS. It was found that i) the heat and mass transfer coefficients, h cha , h cw , h cdha , h ew can be expressed as functions of the temperature difference between the saline water and the cover, and ii) the conventional formula available for a basin type still overestimates the convective heat transfer coefficient, h cw for the TSS. Furthermore, in this study, the long-wave radiation interaction between the water surface and the tubular cover was shown theoretically.We also proposed a new heat and mass transfer model of TSS. The mass balance of water vapor inside the still was formulized for the first time. The proposed model was successful in the calculation of humid air properties (temperature, vapor density and relative humidity) and condensation flux on the tubular cover inner surface besides the saline water temperature, tubular cover temperature, trough temperature and evaporation flux. The validity of the model was evaluated from the comparison with field experiments in Fukui, Japan and in Hamuraniyah, UAE. It was seen that the calculated results also had a good agreement with the field data.

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Growing freshwater scarcity in arid and remote areas usually is caused by the growth of population, urbanization and industrialization and the rather limited natural resources of potable water. In coastal belt of southwest region of Bangladesh, groundwater contains high salinity as well as excessive arsenic contamination. Though pond sand filter and rainwater harvesting system are usually used to meet the freshwater demand throughout the year in this area, along with these, household-based solar desalination could be a good source of drinking water in this region. In this study, a low-cost Tubular Solar Still (TSS) was designed using locally available materials to meet such demand. It was consisted of a tubular frame (0.00275 m thick helical GI wire, 1.0 m long and 0.2 m in diameter) and a rectangular tray (1.0 m × 0.16 m × 0.05 m) covered with black polythene paper. The collection bottle was kept in an insulation box. The field experiment on the constructed TSS was carried out on the rooftop of Civil Engineering Department building from June 25, 2011, to June 20, 2012. Daily distilled output from the TSS was collected approximately 2 h after sunset. The maximum daily production rate was found as 3.83 l/m 2 in May 2012. The production cost of distilled water is estimated as 0.39 BDT/L or 0.0046 USD/L. The application of this process can fulfill the demand of freshwater for drinking purpose for single household in coastal areas.

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