Inexpensive pure water from the sea or elsewhere
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Much of the world lacks clean water

Whether due to insufficient quantity or quality, much of the world is beset by lack of water.

An innovative response

Watershine introduces a simple, elegant, inexpensive concept for the distillation and delivery of water: a tubular solar still patented in the USA. Seawater introduced to the tube can be distilled into pure water (0.1 parts per thousand (ppt) salinity). Because distillation removes radionuclides (possibly excepting radon), toxic chemicals, heavy metals, bacteria, viruses, parasites and other contaminants, the still can also be used for ablating agents of water-related disease. The still can simultaneously perform two important functions, distillation and transportation. Because the solar energy input is free and because water usually needs to be transported anyway after treatment, the still should be extremely favorable economically relative to methods that treat water at a plant and then deliver it. This solar-driven method is also environmentally favorable relative to methods that use nonrenewable energy (most desalination plants are very fossil-fuel intensive). With this concept, the treatment plant is the delivery pipe.

Designs

Figures 1 through 3 are cross-sectional views of several possible designs for the core of the technology, each with one or two elevated troughs to catch and channel the distillate.

U-shaped design in cross-section
Figure 1: Cross-section of a tubular still with a U-shaped roof and a single elevated trough.

Cross-section of tube with rounded troughs
Figure 2: Cross-section of a tubular still with rounded elevated side troughs.

Cross-section of tube with v-shaped troughs
Figure 3: Cross-section of a tubular still with V-shaped elevated side troughs.

In any of the designs, undistilled water introduced into the lower part of the tube, when heated, will evaporate, condense and drop into the elevated trough or troughs, thus creating one or two elevated distilled fractions. The tube may be tilted or drained slightly to induce flow. The apparatus may be appropriate for separating or purifying not only seawater or saline water but wastewater or water containing substances other than salt, such as suspended solids, algae or gasoline. A substance more volatile than water, such as gasoline, however, under properly managed conditions could preferentially evaporate and be brought to fill the elevated trough or troughs, achieving separation from now purer water below. Volatile components, which can harm air quality, can be captured within the tube.

A prototype

To evaluate the effectiveness of this concept, a specialty glass fabricating and scientific testing company built and tested a still using the design shown in Figure 2 rather than either of those shown in Figures 1 or 3 primarily because it was thought easier to make than the other designs. The completed borosilicate glass prototype measured 45 inches (1143 mm) long by 6 inches (150 mm) in outer diameter with a wall thickness of 0.125 inch (3.2 mm). Each of the elevated troughs measured approximately 0.75 inch (19 mm) in outer diameter. The prototype has a collector area of one-half the surface area of an open-ended cylinder:

collector surface area = 0.5 (2 x π x radius x height)
collector surface area = 0.5 ( 2 x π x 3 inches x 45 inches)
collector surface area = 424 in2 (= 0.27 m2, approximately 1/4 of a square meter)

This counts only the bottom half of the tube as collector, based on analogy with flat-plate box stills for which the upper glass plate is discounted as collector area. Figures 4 and 5 give two views of the prototype (the insulation and absorber plate used in the second configuration are not shown).

Oblique view of the tube
Figure 4: An oblique view of the prototype. Compare with Figure 2.
Axial view of tube and cover
Figure 5: An almost-axial view of the prototype. An end cap is in the foreground to the right. Compare with Figure 2.

Preliminary testing

The preliminary testing was conducted in San Diego and approximately sixty miles farther east in Jacumba Hot Springs, California. The prototype was tested in two different configurations. The initial configuration had neither insulation nor black absorber plate whereas the second configuration, to increase absorption of solar radiation and the internal temperature, employed both. Testing of the first configuration included evaluation of the distillation of seawater drawn from the Pacific Ocean at Torrey Pines State Beach. Because the preliminary testing in the first configuration established the qualitative effectiveness of the device (see results below), tap water was used for testing in the second configuration as the focus of the evaluation shifted from the purity of the distillation to the rate of the distillation.

Results of preliminary testing

Jacumba1 San Diego1 San Diego2
Initial salinity 30.7 ppt 5 ppt tap water
Final salinity 0.1 ppt 0.1 ppt -
Max air temp 45º C 31º C -
Max water temp 50º C 39º C 32º C
Distillation rate 4-5 ml/hr 5 ml/hr 8 ml/hr


1Configuration 1: no insulation, no black absorber plate.
2Configuration 2: insulation, black absorber plate.

The seawater used in the test at Jacumba Hot Springs was distilled to very high purity fresh water - 0.1 parts per thousand (ppt) salinity - and the testing company remarked on the distillate's excellent taste. The testing configuration also had a noticeable effect: despite cooler, cloudier weather, the prototype distilled faster with the absorber plate and insulation than it did without the absorber plate and insulation. The preliminary tests indicate that the prototype (collector surface area of approximately one-quarter of a square meter) could be technologically competitive with existing solar stills which typically produce one liter of fresh water per day per quarter square meter of collector area (Peter H. Gleick, The World's Water 2001-2002: The Biennial Report on Freshwater Resources, 2000, Island Press, Washington, D.C., p. 105). But existing solar stills typically have limited collector area whereas this tubular still might extend for tens, hundreds or even thousands of kilometers and have an enormous collector area. So even if conventional stills are more efficient per unit area of the collector, the tubular still, owing to greater size, and perhaps much, much greater size, may be more effective in delivering a desired volume of water. Also, while the insulation and absorber plate were used to hasten evaporation, a low pressure atmosphere in the tube might further improve evaporation. It also may be easy to enhance condensation. For example, the tube could be run through a sand dune to cool its upper surface on which condensation occurs. The tube might also be improved via special coatings (e.g., to improve transmissivity of radiation into the tube or to prevent its escape). Many other improvements that should boost the distillation rate are possible.

Why distillation is remarkable

Distillation extracts purified water from the water source, leaving in the unpurified stream radionuclides (possibly excepting radon), salt, toxic chemicals, heavy metals, bacteria, viruses, parasites and other contaminants that are harmful to humans, other animals and plants.

What makes this approach so innovative?

Photo of commercial salt production
Figure 6: Commercial salt production.

Patent protection

The United States Patent & Trademark Office awarded patent 6,342,127 to protect this property.