Solar Still for Desalinization

Introduction and History

While solar stills have been available for thousands of years, they have never been thought to be “efficient” or “cost effective” and have been ignored for their utility as such. The contention of this document is that technological advances require a new evaluation of time honored wisdom in terms of the commercial feasibility of the solar still as a useful and marketable product.

The simplest still is a box with a translucent sheet covering it. Water is placed in the box, evaporates because of the sun’s energy entering through the sheet, condenses out on the highest point (the sheet), runs down the sheet and is captured, the contaminants remaining behind in the un-evaporated water in the bottom of the box.

The still proposed here is somewhat more sophisticated. It can be manufactured with many off-the-shelf items and can be easily repaired and maintained by unskilled operators. In its simplest form, it is a sealed pot suspended over a concave mirror with a cooling coil to condense the evaporant, much like the old moon shiner’s still but using the Sun for a heat source.

Basic Design - With Manual Operation

(Proof of Concept)

The pot should be a pressure vessel with fittings for inlet, outlet and pressure relief valves on top, a nonstick, non-permeable inner surface, a black outer surface and a rounded or cone shaped bottom with a flush valve in the center.

Considerations for the inner surface:

Ÿ Nonstick, non-corroding with regard to salt or sulfuric acid

Ÿ Non-permeable with regard to volatiles (i.e. Petroleum products, esters)

Ÿ Wear resistant to sand, rocks, shells, coral, etc.

Ÿ Resistant to decomposition, cracking or de-bonding as a result of rapid temperature changes from several hundred degrees Fahrenheit to temperatures below the freezing point of salt water.

The flush valve should be capable of passing objects 150% larger than the inlet valve will pass. It should empty into a vented container capable of containing boiling water.

All valves should be replaceable with simple tools and have the same characteristics as the inner surface of the pot with the addition of being capable of being operated at the extreme temperatures mentioned above and under pressure.

The outlet valve should be fitted with a cooling coil that can consist of anything from copper tubing to a car radiator with the provision that it would not add any contaminants to the water condensed. The cooling coil should be placed below the mirror to shade it from the Sun as this will be the coolest location and add convective heat back into the system. The cooling coil should empty into a vented reservoir which can be drained or replaced as it becomes full. The reservoir should be capable of containing boiling water and be non-permeable to the volatiles mentioned above.

The pot is suspended at the focus of the concave mirror with the cooling coil and valves positioned outside the focus on extended pipes. The supports need to be built with screw jacks in the legs to allow the pot to be leveled on uneven or shifting surfaces. Spring gauges should be built into the pot supports to aid in determining when the pot is level and measure the water level in the pot by weight.

The mirror should be constructed of triangular segments that can be manipulated into place by one, or at most, two individuals on a frame that can be leveled in the same manner as the pot. The segments should be composed of a flexible material that is resistant to UV and heat with a mirror surface that can be either repainted or re-polished.

Operation

Salt or contaminated water is pumped into the pot through the inlet valve with the outlet valve open and flush valve shut until the pot is 2/3 full. The inlet valve is shut and the pot is allowed to heat over the mirror until the weight of the pot and water has decreased by approximately 90% of the weight of the water. The flush valve is opened, allowing the brine or contaminated water to be evacuated (this may be facilitated by opening the inlet valve for a short period if the residue is too concentrated to flush freely). The flush valve is shut and the entire process is repeated.

All of the surfaces that come into contact with water will eventually scale. Two accesses must be built in so that a de-scaler may be introduced to the system before the inlet and outlet valves. Also, a larger access must be provided to allow scouring and resurfacing of the pot.

This basic model of the still could be sold as a kit, with tools included. It could be used for operation in primitive conditions for either water purification or desalinization.

It is suggested that all components use metric measurements and coarse threads.

Water Intake

Salt

Since the intake piping for the still will probably run across areas where a salt water spill would be undesirable, we need to consider how to make the probability of leaks low. Other piping problems include a buildup of flora and fauna on the inside of the pipe, susceptibility to freezing and corrosion. I suggest a double intake pipe, the inner core being composed of a material that resists the buildup of sea life and can be scraped clean. The outer pipe being the weather shield and buffer against anything from bulldozers to bullets with an insulating gel between the two pipes which hardens on contact with air or water and to absorb shock and temperature changes. Both pipes must expand and contract at about the same rate and be fitted together in such a fashion that they will not separate or rupture at joints between sections. Since corrosion is always a problem in the presence of salt water, it may be advisable to have sections of pipe with an access for inserting zincs and anti-fouling agents. As an enhancement, a larger access could be added for the insertion of a tube crawler that scrapes the inside of the pipe and pops to the surface of the sea with a radio transponder for later pickup.

Most bodies of salt water have at least some wave action which stirs up sand and silt and does nasty things to man-made structures. To break the surf line, I suggest that a trench be dug from a spot well back from the beach to a point beyond the debris stirred up by the waves and the intake pipes be buried. The intake point should be away from shipping and oil rigs since the still will mix petroleum products with it’s fresh water output since they evaporate at lower temperatures than water. A self cleaning sand/silt trap must also be built into the intake point to prevent wear on the pumps up stream and constriction of the pipes.

Pumping stations would need to be placed at appropriate levels along the pipeline as it climbed to the still. These should be run by electric motors when possible and have a fairly long life even though they may ingest the occasional frond of kelp or sea shell. Again I recommend a selection of equipment that is easily repaired with simple tools or easily replaced with off-the-shelf equipment.

Contaminated Water

The problems with contaminated water may be somewhat different than salt water. If the contaminant is petroleum or any other substance that evaporates before or close to the same temperature as water then a second process will have to be used after the still to remove this contaminant. The double wall piping would not be necessary if the water was only being purified and was not contaminated with something harmful to the environment if spilled. But the corrosive properties of the contaminant could force a different material to be used in the intake piping. The contaminant would also have an effect on the anti-fouling agent used. Since the source of the contaminated water may not be as plentiful as the sea, it would be a good idea to have pumps that could switch themselves off if blocked or above water at the intake point. An automatic re-priming function would be a good idea as well. Once again, a self cleaning sand/silt trap should be built into the intake.

Outputs

Water

While the temperatures involved in evaporating the water output by the still may be an effective means of killing biologicals contained in the input water, there is nothing to guard against reintroduction of biologicals in the cooling coils or output reservoir. For water which will be consumed, it is recommended that it be treated the same way any other drinking water is treated. It is further recommended that kits be made available/sold for treating the output water when the still will be used in a primitive location. To reiterate the statement made earlier, the output will be hot, collect it in a vessel that can withstand boiling water.

Brine

When desalinating salt water, the last 10% of the input water by weight will be brine. When this is emptied from the pot via the flush valve, it should be collected as it will poison the soil around the still. This brine may be dumped into a large (4M x 4M) pan (sold as an add-on) and allowed to dry. The sea salt residue left after drying can be marketed as it is in many places around the world. Another alternative would be to add it to existing salt mines.

Sludge

When purifying contaminated water, the last 10% of the input water by weight will be sludge. If it is dried as the brine above, it may be useful as fertilizer or mined for the chemicals contained.

Power

The steam generated during evaporation could be used to turn a turbine in the output line that is connected to and electrical generator, thus providing power to aid in running pumps or other electrical devices. It is recommended that a power storage option be offered with the turbine generator so that power can be stored for startup in the mornings.

Enhancements

Vacuum

Since water will boil faster if the surface pressure is lowered, add a vacuum pump and connection to the pot and replace the output valve with a check valve so that a vacuum may be drawn either while the pot is filling or after it is filled. The output valve would be replaced by a check valve that would only open if there were positive pressure in the pot.

Preheating

Construct a cooling coil with alternating tubes that carried the output condensate in one and the inlet water in the other. This would cool the condensate faster, making the cooling coil smaller and preheat the inlet water so that it would boil faster.

Servos and Sensors

In the less primitive model, all operations could be automated and monitored at a central site. The inlet and flush valves would be operated by servos when a sensor indicated it was time to flush and refill the pot. The mirror frame could be mounted on rollers and driven by two servos so that the mirror was always pointed at the Sun and rotated around the pot. Sensors could be used to feed a computer with inputs that could detect when a valve opened or closed at the wrong time or pressures were too high or low. These enhancements would be useful when controlling a field of stills providing water to a city like Los Angeles. Any of these electronic enhancements could be duplicated by mechanical devices for automatic operations in remote locations.

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