A practical basin-type solar still

1965 Conference Paper

A Practical Basin-Type Solar Still J. W. Bloemer, J. A. Eibling, J. R. Irwin Research Engineer

Chief

Project Leader

and

George O. G. L6f

Consulting Chemical Engineer, Denver, Colo.

Thermal Systems Division, Battelle Memorial Institute, Columbus, Ohio

A b a s i n - t y p e solar still has been developed t h a t is practical and e c o n o m i c a l to use i n s o m e parts o f t h e world. Materials of c o n s t r u c t i o n and performance characteristics, as d e t e r m i n e d from extensive field t e s t i n g and from analytical and laboratory i n v e s t i g a t i o n s , are discussed. These studies indicate t h a t t h e primary factor affecting still productivity, i n a d d i t i o n to solar radiation, is basin d e p t h . Of t h e m a n y still designs and materials evaluated, o n e basic design and c o m b i n a t i o n of materials appears m o s t p r o m i s i n g . An Office o f Saline Water report giving details o n t h e c o n s t r u c t i o n o f s u c h a still has been p u b l i s h e d . This paper describes t h a t still briefly. An e c o n o m i c c o m p a r i s o n o f vapor-compression, flash-distillat i o n , and b a s i n - t y p e solar stills shows t h a t t h e solar still produces water at t h e lowest cost w h e n p l a n t capacity is below 50,000 gallons per day.

HE CONVERSION of saline water to fresh water has been shown to be practical in a variety of situations where naturally available fresh water supplies need to be supplemented. At least 100 conversion plants already are in operation. The Office of Saline Water, U. S. Department of the Interior, has had a major role in bringing the seawater conversion field to the present state-of-the-art. One of the processes being investigated under OSW sponsorship is single-effect distillation using unconcentrated solar energy. This particular investigation has been in progress since 1958 under a research contract with Battelle Institute. The objectives are to obtain realistic data on solar-still operation and costs and to apply these data in designs of practical solar stills. ]n working toward these objectives it has been necessary to study in detail the influence of the various design, operating, and environmental variables on operating efficiency. The program has included analysis of field-test data, theoretical studies, and operation of a laboratory still. Information on methods of reducing construction, operating, and maintenance

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Presented at ~he Solar Energy Society Conference, Phoenix, Arizona, March 15-17, 1965. Vol. 9, No. ~, 1965

costs and increasing still life was obtained in the construction and operation of pilot-plant units. The pilot-plant solar stills were built at the Solar Distillation Research Station 1, 2 near D a y t o n a Beach, Florida. One of the most promising of these, the basin-type still, which is described in this paper, is shown in Fig. 1. The still consists essentially of a basin of saline water covered with glass. Solar radiation passes through the glass cover and heats the water in the basin. Part of the water evaporates and condenses on the underside of the cool cover. The pure condensate runs down the sloped cover to a collection lrough and then flows out of the still. The concentrated brine is drained from the basin as required to prevent crystallization. Still Performance Analysis Performance of the still has been determined from field-test data acquired at the Florida station, supplemented b y computer studies 3-5, and the data obtained with an electrically heated laboratory still. In an actual still, most of the solar radiation passes through the water in the basin and is absorbed by the basin liner, the water then being heated by the warm liner. Thus, in the laboratory still it was possible to simulate solar still operation by heating the water from below with a blanket-type electric heater. It was observed in the Florida stills, that although there was considerable circulation of the air-water vapor mixture across the width of the bays, which were about 8 ft wide, there was negligible circulation or mixing along the length of the bay. Therefore, the laboratory-still basin was made 4 ft by 8 ft to represent a 4 ft slice h~)m one of the bays of the Florida stills. The still was constl~cted such that design and operating parameters of basin depth, cover slope, cover-to-brine distance, outside air temperature, outside wind speed, energy input rate, and thermal insulation beneath the basin could be varied independently. Some trials were conducted under steady-state conditions and others were conducted with variable energy input to simulate solar heating. The effect of solar-radiation intensity oil the pro197

Fro. 1.

other hand, shallow basins are somewhat more expensive to construct and require more frequent filling and flushing so that an economic optimum depth must be determined. Selection of depth should be based upon consideration of local soil conditions, grading costs, productivity, and labor costs for the refilling operation.

ductivity of the still when operated with a basin depth of 12 inches and with no thermal insulation under the basin is shown in Fig. 2. This curve was established from field-test data and was duplicated quite closely by the laboratory still. The effect of basin depth and insulation as established by the laboratory still is given in Fig. 3. Decreasing the basin depth was beneficial in each case studied, although the effect for an uninsulated still in a region of high solar-radiation intensity is almost negligible. The use of insulation under the basin increases productivity about 15 percent for a 2-inch-deep basin and about 20 to 30 percent for a 1-inch-deep basin. Any insulation used under a glass-covered basin still must be capable of supporting the load of the still structure. Low-cost loose materials, such as sawdust, which can be used in an air-supported plastic still, are not suitable. Therefore, it appears that the use of insulation will prove to be of marginal economic interest. It is quite evident that the lowest possible water depth is desired for maximum productivity. On the 0.14

R e c o m m e n d e d Still C o n s t r u c t i o n A variety of construction methods and materials have been evaluated at the Florida station. Both glass and inflated-plastic films have been used for covers. The basins, which were constructed directly on the ground, were lined either with plastic films, butyl-rubber sheeting, asphaltic paving, or prefabricated asphaltic mat. For the structural components such as the cover supports and walls, both concrete and wood were tried. Distillate troughs of copper, ~luminum, stainless steel, and plastic were evaluated. From this experience, one basic design and certain materials have been selected. The details of construc-

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tion of this preferred basin still have been published in an OSW report 8 which is intended to be a guide for construction of such a still. Only the principal features are shown in the illustrations presented here. A section view of the still is given in Fig. 4. A basin depth of 2 inches is shown in this sketch. It is expected that for many locations this may be a desirable depth. However, the basin depth can be varied to suit particular conditions or locations. Appropriately sized concrete blocks would be required for the beam supports. Details of the lower glass support and seals are shown in Fig. 5. An identical concrete beam is used for the upper glass support. A photograph taken before the glass was installed, of a similar still built for a basin depth of 12 inches, is reproduced in Fig. 6. This view shows the precast concrete beam supports, pedestals, and pads. The basin liner can be prefabricated of either asphaltic mat or butyl rubber, the selection being affected mainly by the relative shipping and installation costs of the two materials. The distillate troughs can be made inexpensively of a continuous strip of 0.005-inch-thick stainless steel, hand-formed in place. Single-strength glass has been found to be satisfactory for the cover. Since the OSW report s was published, two simple design changes have been made that improve the still. First, instead of placing the 8-by-8-inch pedestal blocks on asphalt pads or directly on the liner, it is now recommended that they be supported by 2-by-16-by-16-inch concrete slabs as shown in Figs. 4 and 6. Second, it is recommended that the space between adjacent glass panes be sealed with a butyl-rubber sealant or the equivalent, rather than asphaltic cement, because the rubber sealant adheres better to the glass. Asphaltic cement is still suggested, however, for use in sealing the glass to the concrete beams at the upper and lower edges. With these minor changes, a basin still should operate for 20 years with little maintenance and operating labor required. Table 1 presents the estimated material costs and labor requirements for constructing the basin still. The material costs are representative of U. S. prices but,

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because of the nature of these materials, costs probably will not vary substantially throughout the world. However, because hourly labor rates vary greatly, labor requirements are given in terms of man-hours. The liner and glass costs in Table 1 include generous allowances for shipping and wastage during construction. Economic Considerations

Solar distillation, if a basin still is used, has a number of economic characteristics that are different from the other seawater conversion processes: 1--Unit construction cost is not affected appreciably by still size. 2--Power requirements are negligible. 3--The still is constructed on-site using unskilled or semi-skilled labor 4--Operation and maintenance can be handled by people with little technical training. 5--Materials of construction are durable and readily available. 6--The still design is essentially modular; capacity of an existing still can be increased by any desired increment with practically no cost penalty. Solar distillation should have excellent possibilities in a situation where these characteristics are important considerations and where there is adequate solar radio-

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TABLE 1--TYPICAL SOLAR STILL COSTS WITHIN THE UNITED STATES Item

Layout, grading, compacting, and soil sterilization Asphalt mat liner Concrete blocks Precast concrete beams Glass and asphaltic cement Distillate trough materials Miscellaneous piping and pumps Storage tank Totals

Material, $/I,000 sq ft of Basin Area

Labor, ManHours/1,O00 sq ft of Basin Area

10

20

80 60 190 210 50 70 50

40 10 25 25 5 5 5

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tion and a need for relatively small quantities of fresh water. Remarkably, m a n y regions of the world present this situation, including the developing countries where in many cases energy costs are high, labor costs are low, and populations are not highly concentrated. A comparison of the costs of water produced by solar distillation and by two commercially successful conversion processes, vapor-compression and multipleeffect flash distillation, is made in Fig. 7. In preparing Fig. 7 it was assumed that the solarradiation intensity averages 2000 Btu per square foot per day and local labor costs $0.50 per man-hour. The prices of fuel and power indicated on the curves are for units of one million Btu and one kwhr. Capital costs and energy requirements of the vapor-compression and flash-distillation plants were based on recent estimates of the manufacturers/ Solar-still costs were based on Table 1, assuming a still with a 2-inch-deep mfinsulated basin. In all cases, the estimates include the costs of installation of the plants, feedwater supply, productwater storage, construction interest, and operating and maintenance labor. A 20-year amortization period with 4-percent interest was assumed for all plants. The curves of Fig. 7 indicate clearly that, for the conditions assumed, solar distillation is preferred when fresh water quantities of 50,000 gallons per day or less are required. The product water cost of about $3 a 1000 gallons may appear to be high by conventional standards, but there are many places in the world where drinking water, often of marginal or very poor

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quality, is now supplied at considerably higher cost. In these localities solar distillation is an economically feasible method of producing fresh water.

BIBLIOGRAPHY 1. Bloemer, J. W., Collins, R. A., and Eibling, J. A., "Study and Evaluation of Solar Sea-Water Stills", Office of Saline Water Research and Development Progress Report No. 50, PB171934 (September, 1961). 2. Bloemer, J. W., and Eibling, J. A., "A Progress Report on Evaluation of Solar Sea Water Stills", Paper No. 61-WA296 presented at ASME Winter Annual Meeting, November 26-December 1, 1961, New York, New York. 3. LSf, G. O. G., Eibling, J. A., and Bloemer, J. W., "Energy Balances in Solar Distillers", AIChE Journal, 7, No. 4 (December, 1961). 4. Bloemer, J. W., Eibling, J. A., Irwin, J. R., and LSf, G. O. G., "Analog Computer Simulation of Solar Still Operation", Paper No. 63-WA-313presented at ASME Winter Annual Meeting, November 17-22, 1963, Philadelphia, Pennsylvania. 5. L5f, G. O. G., "Application of Theoretical Principles in Improving the Performance of Basin-Type Solar Distillers", Presented at U.N. Conference on New Sources of Energy, Paper No. E/CONF. 35/S/77 (May 25, 1961). 6. Bloemer, J. W., Irwin, J. R., and Eibling, J. A., "Design of a Basin-Type Solar Still", Office of Saline Water Research and Development Progress Report No. 112, PB181697 (June, 1964), available from U.S. Department of Commerce, Office of Technical Services, Washington, D.C. 7. "Water Desalination in Developing Countries", United Nations Department of Economic and Social Affairs (1964) U.N. Sales No. 64.II.B.5.

Solar Energy