solar water distillation: introduction
1: Introduction | 2: Energy Exchanges
Solar distillation uses solar radiation to purify water, whether for human consumption or for other uses such as in lead-acid batteries, etc. With zero energy costs and zero emissions, solar distillation has a long history and has been the subject of some research and innovation, but generally speaking it is a less familiar application of solar energy with plenty of room for innovation and improvement. Contrary to common misconception, water does not need to boil in order to evaporate, and in most climates will evaporate if left standing in open sunlight. The purpose of a solar still is to speed up the evaporation process, then to capture the pure evaporated water produced and drain it off for use.
The simplest type of solar still is the basin still, a natural convection still consisting of a shallow black basin of some type covered by a sloping transparent surface of glass or plastic. Slow evaporation occurs on the water surface, due to heating by solar radiation absorbed by the bottom of the basin. The sloping cover is cooled by the atmosphere, so pure water condenses on it and runs down into small collection gutters along its lower edges. The majority of larger operating solar stills are of the basin type. Such stills are relatively cheap and easy to make and maintain, the main requirement being sufficient surface area.
The still illustrated in Figures 1 and 2 is a typical large-scale basin-type still constructed in Florida in the late 1950's and described by Lof (1961). It produced about 3.8L/m2 per day in full sunshine, with production being fairly uniform over a 24hr period. Lof suggests that the design features distinguishing this still from earlier designs are a deeper water layer (approximately 30cm), and construction on the ground rather than elevated on a frame, which enabled conservation of heat from the bottom of the basin using the thermal mass of the earth.
Fig 1: Basin-type solar still (after Lof 1961).
Fig 2: Detail, basin-type solar still (after Lof 1961).