Solar distillation for developing countries: an engineering and economic evaluation

Abstract

Good water in sufficient quantities is one of the most important resources of concern to humanity. It supports life on earth and determines the pattern of human development. Unfortunately, most of the earth's vast reserves of water are either too brackish or too saline to be of potable use by humans and livestock. The grow Lrq imbalance between the supply and the demand of fresh water point to the inevitable fact that man has to turn to the treatment of the vast mineralized water reserves to augment that portion of the reserves that goes through the hydrological cycle and is naturally fresh water. conventional water treatment methods rely on a large array of expensive machinery and complex technologies which few developing countries can afford. They also require large capital energy input. Most developing countries are, however, blessed with abundant sunshine which can, in theory, be harnessed as a substitute for the conventional and increasingly costly fossil fuels. In solar distillation, the natural hydrological cycle is copied on a small scale. Brackish or saline water is held in a pan enclosed by a transparent cover. The sun's radiation which enters the enclosure evaporates the water. The vapour rises and condenses on the underside of the cover. The (vi) distillate is drained off for use as fresh water. The device in which the distillation process takes place is a Solar Still. The objectives of the research is to study the various parameters that influence the performance of a Solar Still and to investigate how the input variables interact with one another with a view to optimising Still design and performance. The study also investigates the economic feasibility of solar distillation in compare on with alternative meals of producing fresh water in developing countries. Four model stills are constructed from locally available materials. The effects of solar radiatJon, wind velocity, water depth, cover slope and of the temperatures of water, vapor, glass cover and ambient air on still productivity are studied ~ The economic analysis compares the cost of solar distillation with the cost of slow - sand filtration and the average - cost rural water supply over a 20 year period of productive life. The results of the study indicate that due to the thermodynamic deficiencies of the single - effect solar still, solar distillation is an (vii) extortionately costly source of fresh water. Consequently, its application is limited to the situations where alternative processes are either physically or technically impracticabJc. Perhaps, the most significant result of the study is the establishment of a relationship between the orientation of the collector surface and the collector slope that maximises solar radiati0P collection. The relationship is: tan 8 = Cos (as - aw ) where B is the cover slope, as is the solar azimuth angJ.e, and aw is the wall azimuth anqle. A few areas are cited for further investigation.