The Solar Water Still Challenge

Fred C. Jensen

The world land area affected by questionable drinking water sources is staggering in size. According to the World Health Organization, about 1.2 billion people do not have safe clean water sources. This should not be allowed to continue. A possible solution is to use a solar water still to distill water on site. The problem is the current cost of $200 to $250 US dollars, when many of these families only earn a few dollars per day. The alternatives, such as the yearly expense of pumping well water or bulk water delivery can cost even more. With few alternatives, those affected are subjected to poor health and early death.

The intent of this paper is to challenge the world’s designers to come up with a viable solar water still to help these people. The factors that make a good drinking water solar still design are mostly material selection as related to cost and service longevity. We will attempt to show some of these design factors.  A design schematic for a basic solar still is shown in Figure 1. A solar still functions under the general principle of the "greenhouse effect". The incoming energy of the sun strikes the glass panel of an airtight box filled with a shallow pool of water that covers a black basin liner. Heat is generated by the sun’s energy absorbed in the water and the black liner. This energy evaporates moisture from the surface and that moisture in turn condenses on the cooler underside of the glass panel. Reference 3 states that 95% of the sun’s spectral energy radiates between the range of 0.3 to 2.7 micrometers (mm). The sun is the classic black body radiating at 5780-degree K (10400 degree R). Glass has the property of a low absorbance for solar radiation primarily at the short wavelengths in this spectrum and allows the solar energy to pass right through. Whereas the water and black basin liner inside the still are near ambient temperature and their emissions are at the long wavelengths in this spectrum. The glass traps the energy because of its high absorbance or poor transmission at the long wavelengths.  The first thought might be to use clear plastic panels or plastic film sheets as a cost based substitute for glass. However these materials do not have this selective wavelength property at the same efficiencies as glass. Plastics degrade in sunlight at rates depending on the grade and thickness. Plastics are not as hard as glass and wind blown sand can blast and scratch the surface.  Looking at Figure 1, a few design points become immediately apparent. First look at the angle of the glass. The single angle shown in the figure is more efficient then a double-angle or double sloped still. By double slope we mean the glass is shaped with a peaked roof in the middle. The result is a double slope will always have half of the glass area in the shade, which lowers the efficiency. The original designers may have tried to match the different angles of morning and afternoon rays; however it is far better to have a low slope angle of less then 10 degrees and catch the mean path of the sun through the day. A five-degree slope of the glass panel is shown in the figure. Secondly, the major efficiency driver for a solar still is the temperature difference between the underside of the glass and the upper water surface. Absolute water temperature in the still is driven by the heat, absorbed in the water and the black basin liner. Therefore it is important to keep the mass of the water to be heated low by limiting the water depth. This is done in Figure 1 with an overflow weir (exit hole) to control water depth. Thirdly, as a practical matter, the heated volume of the airtight box should be kept relatively small but not so small as to make the conductive heat path dominate. The glass should be tempered to resist hailstorms and the intrusion of animals; ordinary window glass that is not tempered cannot be used. Also the glass must be extremely clean on the underside so that the condensing vapor forms as a film rather than droplets which would fall back into the brine; therefore a high quality glass surface finish is required.  
It is probably best to design smaller modular solar still units to supply water for typical single family systems. There are multiple reasons for this. Smaller components are more manageable for unskilled persons with no specialized equipment when assembling or maintaining. A smaller stack of glass panels is easier to carry up a mountainside or down a jungle trail. A good size small generic still is one that could be made from three equal squares of 3-mm thick tempered glass. Three square panes at about 0.91 meter on a side would result in a 2.74 meter long still giving 2.49 square meters of surface area.  Reference 1 recommends 0.5 square meters per person along the US Mexican boarder for production of drinking water. This is based on a production rate of about 0.8 liters per sun hour per square meter. Our 2.49 square meters solar still would than produce 12 liters (3.18 gallons) in 6 solar-hours.

A further design improvement on the generic solar still is to introduce the water as a film over the outside of the glass surface. This cools the surface, slightly preheats the feedwater, cleans the outer surface, introduces water as an automatic fill function and reduces convection losses due to wind. Additionally the water film itself lets more solar radiation into the still due to the difference in the sun’s absorptivity and emissivity of glass with a water film as opposed to glass without a water film. Just feeding the water as shown in the figure can increase the efficiency by 5 to 10%. A simplification would be to forgo this increase in efficiency and fill the still by hand once or twice a day. A modern well-designed solar still with all improvements in place can have conversion efficiencies over 50%.   The purpose of distilling water is to get rid of the salts, minerals and bacteria. These contaminates do not evaporate along with the water. Ordinary table salt, for example, does not turn into vapor until it gets over 1400 degrees C, so it remains in the brine when the water evaporates. If we take our production rate of 0.8 liter/hr-m2, this would consume 0.5 cm of the original feedwater charge of 1.5-cm depth in our example still. If the still is flushed out each day through the drain, the brine may still be usable for other household purposes.
Still interior temperatures can approach 95 degrees C (200 degrees F). This makes the seal, basin liner design and material selection critical.   If the glass seal leaks one drop carrying microorganisms into the still, this would contaminate the entire water output. Choosing the wrong liner material could leech plastic compounds into the water. It appears that a sun shielded silicone rubber seal is the best material for the glass seal along with a FDA approved black silicone membrane for the liner.
As a design-manufacturing goal, we estimate a price of $55 to $73 per unit plus shipping. We base the estimate on the assumption that some families could pay $0.15 to $0.20 USD per day for clean water. Assuming the frame is built from local materials, a do-it-yourself still kit could be as simple as three glass panels, several tubes of silicone sealer, black plastic liner, feed-water manifold, collection trough, bug screen mesh, some drain tubing and a drain valve. The design suggestions offered have the potential of providing a household appliance that could supply drinking water for many years.

1. “Ten Years of Solar Distillation Application along the US-Mexico Boarder”, Solar World Congress International Solar Energy Society, Orlando, Florida, August 11, 2005. by Robert E. Foster, New Mexico State University, William Amos, SolAqua, Inc., El Paso, Texas, Sharon Eby, El Paso Solar Energy Association, El Paso, Texas 79926

2. “Understanding Solar Stills”, TP# 37: 9/85, by Horace McCracken and Joel Gordes, Published by: VITA, Appropedia, the sustainability wiki. 

3. “Thermal Radiation Heat Transfer”, 2nd Ed, by Robert Siegel & John Howell, McGraw-Hill Book Company, 1981, Pages 153 to 157.  

Fred C. Jensen, PE, MSME is director of engineering at Patriot Engineering Company, which has been providing mechanical engineering solutions since 1979.

Solar Still