Solar Still

a water-distilling device whose energy source is solar radiation.

The most commonly used solar stills are based on the principle of the “hot box” (seeSOLAR HEATER). Such stills are simple in design, are comparatively inexpensive, and do not require maintenance by skilled personnel. This type of still (Figure 1) is made in the form of a thermally insulated basin whose interior is blackened. The basin contains the salt water that is to be distilled. The upper part of the still is covered by a transparent material, such as glass, a polymeric film, or organic glass. The sun’s rays pass through the transparent cover and heat the water, thereby causing it to evaporate. The temperature of the transparent cover is close to that of the surrounding air. When the water vapor comes into contact with the cover, it condenses on the cover’s inside surface. The condensed fresh water then trickles down into collecting troughs. The solar still is usually oriented toward the south. In choosing the optimal slope of the still’s transparent surface, the sun’s altitude above the horizon is taken into account, as is the need to permit the condensate to flow into the collecting troughs. The amount of fresh water produced by such a still depends primarily on the intensity of the solar radiation and on the airtightness of the device; the output of the still is 3–5 liters/m²-day.

Figure 1. Schematic of a solar still of the hot-box type: (1) basin containing salt water, (2) air-vapor mixture, (3) transparent cover, (4) condensate, (5) heat-insulating wall of box; the arrows indicate the sun’s rays

Solar stills have found application in areas where fresh water is in short supply but adequate amounts of salt water (for example, seawater) are available. Inflatable solar stills have been used successfully throughout the world by the crews of aircraft and oceangoing vessels in emergency situations on the open sea.