PRODUCTION OF DISTILLED WATER FROM SALINE WATER FOR DRINKING AND IRRIGATION OF SOPHISTICATED PLANTS USING LOW GRADE SOLAR ENERGY

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1 Proceedings of the National Conference on Trends and Advances in Mechanical Engineering, YMCA Institute of Engineering, Faridabad, Haryana. Dec 9-10, PRODUCTION OF DISTILLED WATER FROM SALINE WATER FOR DRINKING AND IRRIGATION OF SOPHISTICATED PLANTS USING LOW GRADE SOLAR ENERGY Anurag Mudgal 1, V. Mudgal 2, B.P.Singh 3 1 Department of Mechanical Engineering, MIT, Moradabad, U.P 2 Department of primary education, Moradabad, U.P 3 Department of Mechanical Engineering, MIT, Moradabad, U.P 1 Phone: , Fax: , 1 E mail: anurag_mudgal@yahoo.com, anurag.mudgal@gmail.com Abstract There is an important need for clean, pure drinking water in many developing countries. Often water sources are brackish (i.e. contain dissolved salts) and/or contain harmful bacteria and therefore cannot be used for drinking. In addition, there are many coastal locations where seawater is abundant but potable water is not available. Pure water is also useful for batteries and in hospitals or schools. Distillation is one of many processes that can be used for water purification. This requires an energy input, as heat, solar radiation can be the source of energy. In this process, water is evaporated, thus separating water vapour from dissolved matter, which is condensed as pure water. Sunlight is one of several forms of heat energy that can be used to power that process. Sunlight has the advantage of zero fuel cost but it requires more space (for its collection) and generally more costly equipment. To dispel a common belief, it is not necessary to boil water to distill it. Simply elevating its temperature, short of boiling, will adequately increase the evaporation rate. In fact, although vigorous boiling hastens the distillation process it also can force unwanted residue into the distillate, defeating purification. Furthermore, to boil water with sunlight requires more costly apparatus than is needed to distill it a little more slowly without boiling. Many levels of purification can be achieved with this process, depending upon the intended application. Sterilized water for medical uses requires a different process than that used to make drinking water. Purification of water heavy in dissolved salts differs from purification of water that has been dirtied by other chemicals or suspended solids. The present cost of solar-distilled drinking water is several times that of water provided by most municipal utilities, but it costs less energy-wise. On the other hand, solar-distilled water is much less expensive than bottled water purchased in the stores. For people concerned about the quality of their municipally-supplied drinking water and unhappy with other methods of additional purification available to them, solar distillation of tap water or brackish groundwater can be a pleasant, energy-efficient option. Solar distillation systems can be small or large. They are designed either to serve the needs of a single family, producing from ½ to 3 gallons of drinking water a day on the average, or to produce much greater amounts for an entire neighborhood or village. In some parts of the world the scarcity of fresh water is partially overcome by covering shallow salt water basins with glass in greenhouse-like structures. These solar energy distilling plants are relatively inexpensive, low-technology systems, especially useful where the need for small plants exists. Solar distillation of potable water from saline (salty) water has been practiced for many years in tropical and sub-tropical regions where fresh water is scare. However, where fresh water is plentiful and energy rates are moderate, the most cost-effective method has been to pump and purify. Although India has a wealth of fresh water, growing demand and rising pollution levels demand more and more energy to pump and purify it. Critical seasonal water shortages are occurring with increasing frequency in some parts of the country. Also, natural fresh water often cannot be diverted for direct human consumption without substantial environmental damage. It can also be used for irrigation of sophisticated plants especially in arid and high saline region. The economic feasibility of solar desalination of ocean water will, therefore, improve considerably as energy costs continue to escalate and population pressure exerts more stress on available fresh water supplies. There are several acceptable designs for small solar stills for the individual family; however, there is still much room for innovation and improvement. Key Words: Desalination, Solar, Heat Recovery, Shallow, Stills 1

2 1.0 Basic Principles The basic concept of using solar energy to obtain drinkable fresh water from salty, brackish or contaminated water is really quite simple. Water left in an open container in the backyard will evaporate into the air. The purpose of a solar still is to capture this evaporated (or distilled) water by condensing it onto a cool surface, using solar energy to accelerate the evaporation (Figure 1). Figure 1 The rate of evaporation can be accelerated by increasing the water temperature and the area of water in contact with the air. A wide, shallow pan painted black makes an ideal vessel for the water. It should probably be baked in the sun for a while before it is used in order to free the paint of any volatile toxicants which might otherwise evaporate and condense along with the drinking water. The pan is painted black (or some other dark color) to maximize the amount of solar energy absorbed. It should also be wide and shallow to increase the surface area, assuming the availability of a substance with good solar absorbing properties and durability in heated salt water. To capture and condense the evaporated fresh water, it needs some kind of surface close to the heated salt water which is several degrees cooler than the water. A means is then needed to carry this fresh water to a storage tank or vessel. The evaporating pan usually is covered by a sheet of clear glass or translucent plastic (to allow sunlight to reach the water) which is tilted at a slight angle to let the fresh water that condenses on its underside trickle down to a collecting trough. The glass also holds the heat inside. Comparing this with a reverse osmosis system, costing several hundred dollars, and giving good water ( ppm TDS). Solar distilled water runs about 1 ppm TDS (or less), with an initial cost of Rs /m², depending upon size. RO systems need to have membranes replaced. Stills don t. 2.0 Design Objectives For an Efficient Solar Still For high efficiency the solar still should maintain: 1. a high feed (undistilled) water temperature 2. a large temperature difference between feed water and condensing surface 3. low vapour leakage. 2

3 Proceedings of the National Conference on Trends and Advances in Mechanical Engineering, YMCA Institute of Engineering, Faridabad, Haryana. Dec 9-10, A high feed water temperature can be achieved if: 1. a high proportion of incoming radiation is absorbed by the feed water as heat. Hence low absorption glazing and a good radiation absorbing surface are required 2. heat losses from the floor and walls are kept low 3. the water is shallow so there is not so much to heat. A large temperature difference can be achieved if: 1. the condensing surface absorbs little or none of the incoming radiation 2. condensing water dissipates heat which must be removed rapidly from the condensing surface by, for example, a second flow of water or air, or by condensing at night. 3.0 Design Types and Their Performances Single-basin stills have been much studied and their behaviour is well understood. Efficiencies of 25% are typical. Daily output as a function of solar irradiation is greatest in the early evening when the feed water is still hot but when outside temperatures are falling. Material selection is very important. The cover can be either glass or plastic. Glass is considered to be best for most long-term applications, whereas a plastic (such as polyethylene) can be used for short-term use. Sand concrete or waterproofed concrete are considered best for the basin of a long-life still if it is to be manufactured on-site, but for factory-manufactured stills, prefabricated ferro-concrete is a suitable material. Multiple-effect basin stills have two or more compartments. The condensing surface of the lower compartment is the floor of the upper compartment. The heat given off by the condensing vapour provides energy to vaporize the feed water above. Efficiency is therefore greater than for a single-basin still typically being 35% or more but the cost and complexity are correspondingly higher. 3.1 Wick Stills In a wick still, the feed water flows slowly through a porous, radiation-absorbing pad (the wick). Two advantages are claimed over basin stills. First, the wick can be tilted so that the feed water presents a better angle to the sun (reducing reflection and presenting a large effective area). Second, less feed water is in the still at any time and so the water is heated more quickly and to a higher temperature. Simple wick stills are more efficient than basin stills and some designs are claimed to cost less than a basin still of the same output. 3.2 Emergency Still To provide emergency drinking water on land, a very simple still can be made. It makes use of the moisture in the earth. All that is required is a plastic cover, a bowl or bucket, and a pebble. 3.3 Hybrid designs There are a number of ways in which solar stills can usefully be combined with another function of technology. Three examples are given: Rainwater collection: By adding an external gutter, the still cover can be used for rainwater collection to supplement the solar still output. Greenhouse-solar still: The roof of a greenhouse can be used as the cover of a still. Supplementary heating: Waste heat from an engine or the condenser of a refrigerator can be used as an additional energy input. 3.4 Output of a Solar Still An approximate method of estimating the output of a solar still is given by: Q= ExGxA/ 2.3 where: Q= daily output of distilled water (liters/day) E= overall efficiency, for solar systems taken as 30% 3

4 G= daily global solar irradiation (MJ/m²) A= aperture area of the still i.e., the plan areas for a simple basin still (m²) For calculating approximate output of a solar still same formula is given by (Hislop, 1992) as: Q = 1.6e Al d Where Q = daily output of distilled water, litres/day e = overall system efficiency, typical 30% I d = average daily solar radiation, kwh/day A = aperture area of the still, m 2 In a typical country the average, daily, global solar irradiation is typically 18.0 MJ/m² (5 kwh/m²). A simple basin still operates at an overall efficiency of about 30%. Hence the output per square metre of area is: Daily output = 0.30 x 18.0 x 1/ 2.3 = 2.3 litres (per square metre) The yearly output of a solar still is often therefore referred to as approximately one cubic metre per square metre. Figure showing position of sun in winter and summer 4.0 Experimental Set Up Fabricated At MIT, Moradabad In developing countries the minimum requirement for distilled water is 5 litres per person per day for cooking and drinking. Therefore a little more than 2m² of still area is needed for each person to serve. Keeping in mind this typical requirement the plan area of the basin still is kept as 24 ft 2 i.e. approximately 2.3 m 2 by taking dimensions as 6 ft. x 4 ft. An inclination of 10 0 is maintained to collect the condensed distilled water at inside of the basin. Arrangements have been made to collect the rain water falls over the glass sheet. The glass sheet is placed in three parts by providing two supports in between, so that only one third of the total glass has to be changed in case of any breakage. To minimize the heat loss by conduction the basin chamber is separated from supporting frame introducing wood strips in between frame and the chamber. Further the chamber has been fully insulated at the bottom and walls using glass wool from the outside to minimize any possibility of heat loss by convection. The floor of the chamber is painted black to absorb the maximum radiation and glass side supports are painted silver to manage significant temperature difference between bottom and the glass. Height of the basin is kept 10 cm and 31 cm respectively at both the ends. 4.1 Results Total plan basin area available, A = 2.3 m 2 Global solar irradiation is typically, G = 18.0 MJ/m² (5 kwh/m²) Overall efficiency, E = 30% Daily output = 0.30 x 18.0 x 2.3/ 2.3 = 5.4 litres 4

5 Proceedings of the National Conference on Trends and Advances in Mechanical Engineering, YMCA Institute of Engineering, Faridabad, Haryana. Dec 9-10, Economics Solar energy can be used to separate pure water from most of the natural contaminants, such as dissolved solids (salts) and particles (dirt and algae). Solar distillation is most economically effective when sunlight is allowed to pass through a transparent cover and into a black evaporating pan with little or no concentration of the sun's rays. A reasonable production rate would be about 3-4 liter of water per day per square meter (10.7 square feet) of still area. If it costs about Rs Rs.3000 per square meter to build the still and if this water is worth roughly Rs.200 per 1000 liters, the still should pay for itself in 2,000 to 3,000 days, or 6 to 8 years. As the value of solardistilled water increases, the payback time shrinks. If one values this water at 20 paisa per liter, about what distilled water costs at the supermarket, then the payback time is only days. Clearly, solar distillation of water is not quite competitive with utility-supplied drinking water in, but it is highly competitive with bottled water. Rising energy prices seem bound to create an early market for small manufactured solar water distilling units. In a few more years, large-scale solar distillation may also become economically viable for utility use. The cost of pure water produced depends on: 1. the cost of making the still 2. the cost of the land 3. the life of the still 4. operating costs 5. cost of the feed water 6. the discount rate adopted 7. the amount of water produced. The cost of a solar still is normally Rs /m². The price of land will normally be a small proportion of this in rural areas, but may be prohibitive in towns and cities. The life of a glass still is usually taken as 20 to 30 years but operating costs can be large especially to replace broken glass. Performance varies between tropical locations but not significantly. An average output of lt/m²/day is typical, that is, about 1m³/m²/year. The cost of a solar still fabricated is Rs. 6000/- If plant runs 25 days in a month, it will provide 125 lts per 5 lts per day In a year plat is capable to produce 125 x 12 = 1500 lts of pure distilled water In this way if cost of distilled water is assumed to be as low as Rs. 4 per lt, even then brake even comes in 12 months. And it will serve for free in the number of years to come at very nominal maintenance cost. 6.0 Water Quality Obtained By Solar Distillation In principle, the water from a solar still should be quite pure. The slow distillation process allows only pure water to evaporate from the pan and collect on the cover, leaving all particulate contaminants behind. Since a clean glass cover plate and storage vessel should produce no contaminants, the catch basin, or trough, remains as the potential source of direct contamination. (If the design allows for catchment of rain, air pollutants in the rain could also be a form of contamination). The catch trough should be made of material unlikely to degrade water flowing through it, even at the moderately elevated temperatures which might be encountered. PVC (polyvinyl chloride) plastic plumbing pipe is commonly available at relatively low cost. Since vinyl chloride has been identified as a carcinogen potentially harmful to workers in plants manufacturing PVC products, one should be very careful about using this material in a drinking water system. Secondary potential sources of contamination include materials present in the air inside the distiller, and in the lining or coating of the evaporating pan, which might somehow find their way into the water condensing on the underside of the cover glass. There is little available information on this complex subject. It is possible that a chemical in the feed water (or in the still itself) which evaporates along with the water could condense on the underside of the cover and be carried into the catch basin. There are several ways to minimize contamination from the materials in the still itself. Preconditioning of the distiller by "baking" it under the sun for several days may be sufficient to drive off most volatiles. Non-volatile 5

6 materials left behind in the concentrate may be discarded. Avoiding use of materials containing known toxicants is another way to ensure condensate water purity. With care in design and operation, the solar still should, therefore, be capable of producing good drinking water free of cancer-causing pollutants and other harmful substances water that is colorless, odorless and, unfortunately, tasteless. When the minerals common to drinking water are removed, taste goes, too. One flavor recommendation is to add small amounts of minerals or salts to the distilled water maybe a good idea, since the minerals found in water may be healthful. Lost minerals also can be replaced by trickling the distilled water through a bed of marble chips. 7.0 Conclusion For building a solar still, a good design should be capable of producing l/2 to 1 gallon of fresh water per day for each square meter (10.7 square feet) of still area. Material costs generally do not exceed about Rs /- per square meter for large solar distillation plants. For smaller, backyard models, the material costs are likely to be somewhat higher. The per-gallon cost of solar-distilled water can be calculated as follows: (a) estimate the usable lifetime of the still; (b) add up all the costs of construction, repair and maintenance (including labor) over its lifetime; and (c) divide that figure by the still's total expected lifetime output in gallons (or liters). Such a cost estimate is only approximate since there are large uncertainties in both the lifetime and the yield estimates. However, as times change, water prices rise. The quality of "city water" is deteriorating in many parts of India and some people are buying expensive water filters or drinking only bottled water. Consequently, a more favorable evaluation of solar-produced fresh water costs would involve a comparison with bottled drinking water prices. Thus the solar-distilled water costs much less than bottled water and somewhat more than utility-supplied water. Small solar stills capable of producing pure drinking water even for as much as 20 to 30 paisa per liter might find many buyers who are unhappy with the quality of the water they are presently getting. As the cost of purifying polluted groundwater and delivering it to the home continues to rise, the solar distillation market should continue to grow significantly especially if someone comes up with a unit that produces good drinking water at a reasonable price. It can also be used for irrigation of sophisticated plants and medicated crops especially in arid and high saline region. References 1. Hislop, D, 1992, Energy Options An introduction to small-scale renewable energy technologies. Intermediate Technology Publications, UK. 2. Karekezi, S and Ranja, T, 1997, Renewable Energy Technology in Africa. Africa Energy Policy Research Network (AFEPREN), Biddles Ltd. UK. 3. Kudish, A I, 1991, Water Desalination. In Solar Energy in Agriculture, Energy in World Agriculture, 4, Edited by Parker, F. (pp ) New York, USA. 4. Reddy, T A, 1987, the design and sizing of active thermal systems. Oxford Science Publications, New York. 5. Kudish, A.I., 1991, Water Desalination. In Solar Energy in Agriculture-Energy in World Agriculture, Parker, B.F. (Ed), Vol. 4, pp Elsevier Science Publishers B.V., Amsterdam. 6. Malik, M.A.S., Tiwari, G.N., Kumar A. and Sodha, M.S., 1982 Solar Distillation, Pergamon Press, Oxford. 7. Mink, G. Aboabbound, M.M. and Karmazsin, E Solar still of improved efficiency. In Proceedings of ISES Solar World Congress, Budapest, p