Solar Stills
Limitations of Solar Stills
The single most important factor in the performance of a solar still is the amount of radiation it receives. More water can be distilled when a greater amount of energy is received. Thus, solar stills produce less distilled water in winter than in summer. In winter, there are fewer sunny days. The lower height of the sun above the horizon keeps daytime temperatures cooler and the air drier. The success of a solar still depends on both heat and humidity, both of which are less generally in winter, depending on the still's location relative to the Equator in the Northern Hemisphere. Mediating the poor wintertime performance of solar stills is the demand for drinking water, which is generally half as much compared to summertime demand.
According to various experimental and mathematical model results, best performance of the solar still is achieved when the following conditions are satisfied: high intensity of insulation, full insulation, and minimum wind velocity. Under these conditions, a maximum solar still efficiency of approximately 50% is obtained. Although the intensity of the insulation has a proportional effect on the productivity of the solar still system, the type of insulation greatly impacts the still performance. A light insulation causes a reduction of 14.5% in the efficiency obtained with full insulation. The wind effect is much more limited. The increase of wind velocity from zero to 3.6 mph yielded a slight reduction (2%) in the still performance.
Repeated tests have shown that the higher the operating temperature of the still, assuming insulation is equal, correlates with higher efficiency. For example, with each 6 degrees Celsius increase in ambient temperature, a still's output increases 7-8%. The application of this finding means that a still operating in a hot desert climate will produce typically as much as one-third more water than the same unit would produce in a cooler climate.
Production of distilled water is also associated with the thermal efficiency of the still itself. Efficiency may range from 30 to 60%, depending on the construction of the still, the amount and type of insulation used, ambient temperatures, wind velocity, and the availability of solar energy. Thermal losses for a typical vary seasonally. To better understand, we can refer to the following table:
