Solar distillation

These solar distillers are popular in remote communities along the Texas—Mexico border where the waters from the Rio Grande basin are saline and tainted by agricultural chemicals. 

Design and Operating Principles in Solar Distillation Basins... 

Growing scarcity of fresh water in many places in this country and the rest of the world has stimulated the development of several potentially useful processes for saline water demineralization. Because fresh water is such a cheap commodity, these processes must demonstrate the maximum conceivable economy to compete with even the most expensive natural fresh water sources. Nearly all of these methods require considerable energy, either as heat or as electric power. Since this is a large cost item in these processes, solar distillation offers substantial operating economies, but at the expense of large investment requirement. 

Minimization of construction cost has therefore been a prime objective in the development of solar distillation. Probably the most promising method for its accomplishment is the combining of all three primary elements in a distillation process—i.e., heat supply facility, evaporator, and condenser—into a single piece of very simple equipment. Such a unit is the basin-type solar distillation plant (4). But the simplicity of this equipment ceases with its general form, and over-all operation of so many functions makes the physical processes of energy and mass transfer highly complex. 

This paper describes and explains the various energy- and water-transfer processes taking place in the basin-type solar distiller, shows their relative significance in affecting performance, and indicates the factors which may be altered for over-all improvement and economy. 

Two forms of the basin-type solar distiller are shown in Figures 1 and 2. Each of these is subject to numerous minor design variations. The configuration of the glass-covered still shown is an improved form of a 1-acre brackish water distillation plant built in Chile in 1872 (3). 

The numerous uncontrollable weather variables, the variety of heat transfer processes occurring, and the transient nature of the operation make solar distillation a much more complicated process than it first seems, particularly in so far as its design and predicted performance are concerned. 

From such information, the performance of a proposed solar distiller installation can be anticipated and the area requirements for a specified water production rate determined. 

Figure 4. Predicted average performance of deep-basin solar distiller in San Diego area...

An analysis of the solar distillation process shows that performance is remarkably insensitive to all variables except solar radiation rate. As atmospheric temperature changes, basin and cover temperatures move similarly, so that their difference remains... 

In view of the several possibilities for reducing energy losses, solar distiller yields might be substantially improved. Decrease of radiation and convection losses to half their present levels, along with the use of low-reflection cover surfaces, would result in about 50% increase in summer production and roughly double the winter per-... 

The support and cooperation of the Office of Saline Water, U. S. Department of the Interior, in a solar distillation program, a portion of which is the subject of this paper, are gratefully acknowledged. 

Construction methods and performance data are presented on solar sea water stills under evaluation at the Florida Solar Distillation Research Station. Three stills representing two basic designs have been constructed and operated there a 2500-sq.-foot glass-covered deep-basin still and 2300- and 500-sq.-foot air-supported plastic stills. Other types of stills are being developed for future construction and field evaluation. 

Among the methods of saline water conversion being investigated by the Department of the Interior s Office of Saline Water is direct solar distillation. One phase of the study of solar distillation includes the field evaluation of stills of promising design, which is being carried out for the Office of Saline Water by Battelle Memorial Institute. 

The objectives of the research program are to obtain realistic engineering and economic data on solar distillation plants and to develop methods of improving their initial and operating costs. Field tests are being conducted at a research station in northern Florida, where three solar stills have been constructed and others are planned to be built. The stills are extensively instrumented, so that heat losses and other performance data may be accurately determined. 

With respect to future activity in the field of solar distillation, three basic categories of solar stills can be visualized permanent, semipermanent, and expendable. It is not possible to single out any one type as the best, because each has advantages in particular applications which may range from small remote domestic installations to plants covering many acres and serving municipalities, industry, and agriculture. 

Nobe (28) is investigating photovoltaic effects in metal single-crystal electrolyte systems and also the effect of stress. A few experimental results are presented on electromotive force vs. stress for steel-NaCl solutions and on photovoltaic potentials vs. light intensity for copper in distilled waters and in NaCl solutions. The effect of light might have some importance in solar-distillation plants. 

Research on specific distillation problems at the Israel Institute of Technology is in its first stages and is not reported here. This work deals with heat transfer in flash distillation units with particular reference to annuli. Older work on solar distillation and ion exchange demineralization of moderately brackish waters was done at the Weizmann Institute of Science (14). 

According to the American Water Works Association, more than 12,500 desalination plants in 120 countries were in operation in 2004 60% of these plants are in the Middle East. Two methods used to purify seawater are reverse osmosis and solar distillation. 

Distilled Water and Desalination. Tap water commonly contains dissolved salts that contribute to its overall hardness. These are removed through distillation to provide distilled water for use in automobile batteries and radiators, steam irons, and other applications where pure water is beneficial. Specialized stills fed with continuously flowing tap water are used to prepare distilled water in chemistry laboratories however, deionized water is becoming an increasingly popular and more convenient alternative. Seawater can be similarly distilled to provide potable drinking water. Desalination of seawater is especially important in arid and semi-arid regions of the Middle East, where the abundant sunshine is used in solar distillation facilities. 

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