Evaporation phenomenon inside a solar still: from water surface to humid air

Solar stills of different designs have been proposed and investigated with a view to get greater distillate output (Murase et al., 2006). Solar stills are usually classified into two categories: a single-effect type and a multi-effect type that reuses wasted latent heat from condensation (Fath, 1998; Toyama et al., 1990). The integration between a solar collector and a still is classified into passive and active stills (Tiwari & Noor, 1996; Kumar & Tiwari; 1998). Single-effect passive stills are composed of convectional basin, diffusion, wick and membrane types (Murase et al., 2000; Korngold et al., 1996). The varieties of a still with cover cooling (Abu-Arabi et al., 2002; Abu-Hijleh et al., 1996) and a still with a multi-effect type basin (Tanaka et al., 2000) have been studied. A basin-type solar still is the most common among conventional solar stills (Chaibi, 2000; Nafey et al., 2000; Hongfei et al., 2002; Paul, 2002; Al-Karaghouli & Alnaser, 2004; Tiwari & Tiwari, 2008). A small experimental Tubular Solar Still (TSS) was constructed to determine the factors affecting the nocturnal production of solar stills (Tleimat & Howe, 1966). Furthermore, a detailed analysis of this TSS of any dimensions for predicting its nocturnal productivity was presented (Tiwari & Kumar, 1988). They (Tleimat & Howe, 1966; Tiwari & Kumar, 1988) mainly focused on the theoretical analysis of the nocturnal production of TSS. A simple transient analysis of a tubular multiwick solar still was presented by Kumar and Anand (1992). This TSS (Tleimat & Howe, 1966; Tiwari & Kumar, 1988; Kumar & Anand, 1992) is made of heavy glass and cannot be made easily in remote areas. The cost of glass is quite high as well (Ahsan et al., 2010). When water supply is cut off due to natural disasters (tsunamis, tornados, hurricanes, earthquakes, landslides, etc.) or unexpected accidents, a lightweight compact still, which is made of cheap and locally acquired materials, would be reasonable and practical. The second model of the TSS was, therefore, designed to meet these requirements and to improve some of the limitations of the basin-type still and of the TSS made of glass. Since the cover material (a vinyl chloride sheet) is a little heavy and cannot form into an ideal size easily (Islam, 2006; Fukuhara & Islam, 2006; Islam et al., 2005; Islam et al., 2007a), a polythene film was adopted as a cheap new material for the cover. Consequently, the cover weight and the cost of the second model were noticeably reduced and the durability was distinctly increased. These improvements also can help to assemble and to maintenance the second model of TSS easily for sustainable use (Ahsan et al., 2010). A complete numerical analysis on TSS has been presented by Ahsan & Fukuhara, 2008; Ahsan, 2009; Ahsan & Fukuhara, 2009; Ahsan & Fukuhara, 2010a, 2010b. Many researchers (Chaibi, 2000; Clark, 1990; Cooper, 1969; Dunkle, 1961; Hongfei et al., 2002; Malik et al., 1982; Shawaqfeh & Farid, 1995) have focused their research on conventional basin type stills rather than other types such as tubular still. Most of the heat and mass transfer models of the solar still have been described using temperature and vapor pressure on the water surface and still cover, without noting the presence of intermediate medium, i.e. humid air (Dunkle, 1961; Kumar & Anand, 1992; Tiwari & Kumar, 1988). Nagai et al. (2011) and Islam et al. (2007b), however, found that the relative humidity of the humid air is definitely not saturated in the daytime. Islam (2006) formulated the evaporation in the TSS based on the humid air temperature and on the relative humidity in addition to the water temperature and obtained an empirical equation of the evaporative mass transfer coefficient. Since the empirical equation does not have a theoretical background, it is still not known whether it can be used, when the trough size (width or length) is changed (Ahsan & Fukuhara, 2008). In this chapter, a comparison of the evaporation and distilled water production between the first model and second one is described. Additionally, this chapter aims to present the theoretical formulation of a model for the evaporation in a TSS by dimensional analysis.