where g = gravitational constant, = 9.81 m/s2.
(3 = coefficient of volumetric expansion of fluid (1/K).
AT = temperature difference between surface and fluid (K). ^ = dynamic viscosity of fluid (kg/m-s). v = kinetic viscosity of fluid (m2/s). cp = specific heat of fluid ((J/kg-K).
By using Eqs. (8.26) and (8.27), the hourly distillate output per square meter from a distiller unit (m w) is given by mw = 360^ = 0.0163(Pw - Pg )
v where Lv = latent heat of vaporization (kJ/kg), or
It should be noted that, in the preceding equation, the product GrPr is known as the Rayleigh number, Ra. The constants C and n are calculated by regression analysis for known hourly distillate output (Dunkle, 1961), water and condensing cover temperatures, and design parameters for any shape and size of solar stills (Kumar and Tiwari, 1996).
According to Tiwari (2002), the instantaneous efficiency of a distiller unit is given as hcw(TW - Tg )
Simplifying this equation, we can write
where Tw0 = temperature of basin water at t = 0 (°C).
The preceding equation describes the characteristic curve of a solar still in terms of the solar still efficiency factor (F'), effective transmittance-absorptance product (orf and overall heat loss coefficient (UL) (Tiwari and Noor, 1996).
A detailed analysis of the equations of ^ justifies that the overall top loss coefficient (UL) should be maximum for faster evaporation, which results in higher distillate output.
The meteorological parameters—wind velocity, solar radiation, sky temperature, ambient temperature, salt concentration, algae formation on water, and mineral layers on the basin liner—affect significantly the performance of solar stills (Garg and Mann, 1976). For better performance of a conventional solar still, the following modifications were suggested by various researchers:
• Reducing the bottom loss coefficient.
• Reducing the water depth in a basin-multiwick solar still.
• Using internal and external condensers.
• Using the back wall with cotton cloth.
• Using an energy storage element.
• Using a multiwick solar still.
• Condensing cover cooling.
• Using an inclined solar still.
• Increasing the evaporative area.
About a 10-15% change in the overall daily yield of solar stills due to variations in climatic and operational parameters within the expected range has been observed.
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