Fresh Water from the Sea -- entirely using solar distillation & PVs for pumping Horace McCracken Practically any seacoast and many desert areas can be made inhabitable by using sunshine to pump and purify water. A 10 gallon/day solar still now purifies sea water for drinking, cooking and other household needs for a residence near George Town, Exuma, in the Bahamas. Solar energy does the pumping, purification, and controls seawater feed to the stills. Solar Distillation This system uses three solar distiller pans 33" wide by 10'3" long. They are insulated, lined with a special corrosion-resistant membrane. The pans contain salt water, about 3/4 of an inch deep. Tempered glass covers are cemented over the pans. Sunlight shines through the glass cover, warming the salt water in the pan. Pure water evaporates, leaving the salts, minerals, fertilizers, etc. in the pan, still dissolved in the brine. In the solar still, the glass cover traps the water vapor which then condenses on the underside of the glass (which is cooled by the outside air). The cover is tilted, so that the condensed water runs into a trough, where it flows into a storage tank. The process is exactly Mother Nature's method of getting fresh water into the clouds from oceans, lakes, swamps, etc. All the water you have ever consumed has already been solar distilled a few thousand times around the hydrologic cycle. The still is filled once daily, at night or in the morning, with at least twice the amount of water that was distilled the preceding day. There is an overflow fitting at the opposite end, so that the extra water runs out, keeping the salts from building up in the still. In this installation, the overflow from one still feeds the next one and the total amount of feed was set at 40 gallons per night. On this tiny island, the overflow is diverted back to the sea. In cities, when the still is operating on tap water, the overflow may be used to water plants. For this installation, we built the distillation pans on the site, from locally available materials. Complete stills, ready to be set on a level support, are also available. A photovoltaic panel powers the pump filling the salt water reservoir. The water comes from a well about 10 feet deep in limestone rock. The well's bottom is about a foot below sea level. Some rain water mixes in, but it is mostly sea water, well filtered by the 50' or so of sand between the sea and the well. Good filtration is imperative to keep the pump running dependably. Dropping a hose into the sea with a screen over it is not adequate. The sea water is pumped up hill via 300 feet of pipe, to a reservoir about 25 feet above sea level. Feeding sea water into the still when the sun is shining would substantially reduce production because the heated water would be flushed from the still, So the PV pumped sea water is held in reserve in a shallow reservoir. A special solar actuated valve (invented and manufactured by McCracken Solar), stays closed all day and then it lets water flow into the stills about an hour after the sun goes down. This slow response time prevents emptying the still when a passing cloud goes by. The reservoir was built with a black impervious membrane liner and a glass cover, so that the stored sea water is also solar heated during the day, about 60¡ warmer than the outside air. This pre-heated water also contributes to evaporation within the still, increasing the day's yield by Å10%. All parts of this system, except the working parts of the pump, are designed to last for 20 years. The pump has been chosen for simple, inexpensive, and infrequent maintenance. In this installation, the pump runs for only about an hour a day, so its parts may last for years before replacement. Solar Still Performance Operation of the still is totally automatic. It requires no routine maintenance and has no routine operating costs. The rated production of the still is an estimated annual average and is not exact, as the amount of sunshine can vary widely. These stills produce more in hot climates than in cold ones, more at low latitudes than high, and more in summer than in winter. At the 23¡ North latitude of the central Bahamas, the estimated average production of this installation in June will be 15 gallons per day, down to about 5 in mid-winter. In higher latitudes, addition of a mirror to the rear of each still increases winter production. The still also functions in freezing climates. The still itself is entirely unharmed by freezing, any number of times, but exposed water lines must be insulated. The still's production is greatly diminished or ceases in very cold weather, i.e., below freezing during the day. Use a larger distilled water reservoir to store up excess production from summer and fall for winter use. In addition to leaving salts, minerals, and other dissolved substances behind, the evaporated water also leaves bacteria in the pan. The evaporated water is sterile and does not contain dead bacteria. Fertilizers, pesticides, and other organic materials are largely left behind by this evaporation process. The distilled water produced by solar distillation is of very high quality, normally better than that sold in bottles as distilled water. It routinely tests lower than one part per million total dissolved solids. It is also aerated, as it condenses in the presence of air inside the still, and it tastes delicious. Solar Still Cost The cost of a solar distillation system will vary widely, due to size and site-specific circumstances. A residential system, like the one described here, will cost several thousand dollars. This project was designed and constructed by Horace McCracken, long a pioneer in solar distillation. McCracken Solar Company can be reached at 329 West Carlos, Alturas, CA 96101 or call 916-233-3175.