A solar still is a device that utilizes solar energy to purify water through a process of evaporation and condensation. This technology captures the sun’s heat to vaporize impure water, leaving contaminants behind, and then collects the resulting clean water vapor as it condenses on a cooler surface. The amount of purified water a solar still can produce is highly variable and depends on the still’s physical construction and the surrounding environmental conditions.
Typical Daily Water Production Rates
The standard unit for measuring a solar still’s performance is liters per square meter per day (L/m²/day). For a conventional, passive single-basin solar still, the daily water production typically falls within a broad range. Under good solar conditions, a well-designed basic still generally yields between 2 and 5 L/m²/day.
This typical output represents the yield from a relatively simple setup. Productivity is directly linked to the solar-to-freshwater conversion efficiency, which often ranges between 30% and 45% for passive stills. More advanced designs, such as those that incorporate thermal energy storage materials or multiple condensation stages, can significantly exceed this range, sometimes demonstrating yields as high as 6.5 to 7.35 L/m²/day.
The productivity of the still is a function of the temperature difference achieved between the evaporating water and the condensing cover. A greater temperature differential drives a faster rate of evaporation and subsequent condensation, resulting in a higher water yield. Variations in still design, like the addition of external reflectors or internal wicks, are aimed at maximizing this temperature difference.
How Still Design Impacts Output
The physical design and material choices of a solar still have a direct influence on its daily output. The total collection surface area of the basin is a primary factor, as a larger area intercepts more solar radiation, providing more energy for evaporation. However, simply increasing the area without optimizing other elements may lead to diminishing returns.
The insulation surrounding the still basin is important for minimizing heat loss to the ground and surrounding air. Effective thermal insulation, such as a layer up to 60 millimeters thick, ensures that the absorbed solar energy is retained in the water, increasing its temperature and the evaporation rate. To maximize solar energy absorption, the basin liner should be a dark color, typically black, which has a higher absorptivity.
The condensing cover material and its angle are also factors in still performance. Glass is generally superior to plastic because of its higher transmissivity of solar radiation and better durability. The angle of the cover must be steep enough to allow the condensed freshwater droplets to run off quickly into the collection trough. A shallower water depth in the basin also enhances productivity by allowing the water to heat up faster.
The Role of Environmental Conditions
External environmental factors play a significant role in determining the final daily water yield. The intensity of solar irradiance is the most influential meteorological parameter. Higher solar intensity means more energy is available to drive the evaporation process, resulting in a proportional increase in distillate production.
Ambient air temperature also affects still performance by influencing the temperature differential between the water and the cover. A higher surrounding temperature contributes to a warmer still body, which helps maintain the elevated water temperature needed for efficient evaporation. This means that solar stills generally perform better in hotter climates or during the summer months.
Wind speed across the condensing cover can have a dual effect on still productivity. Mild air movement can slightly increase the convective heat loss from the cover, keeping its temperature lower and increasing the temperature difference needed for condensation. However, high wind speed can cause excessive cooling of the cover, leading to rapid heat loss from the entire system and reducing the overall efficiency. Factors like cloud cover and dust accumulation on the glass cover also negatively impact the still’s output.