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Dávila's goal was an inexpensive solar-operated system that would give unlimited boiled drinking water, already cooled to room temperature, straight from a tap any time of day. Furthermore, the device had to work under cloudy skies. Using an oxidized copper formula already developed, Gaviotas solar collectors already heated water to 120 degrees Fahrenheit under diffused light. Increasing their normal operating temperature just 10 percent would eliminate many unwanted microbes. From there, the Gaviotas engineers calculated that they would need to raise the water to full boiling temperature for at least two minutes to kill all pathogens.
They accomplished this with a very efficient heat exchanger. As untreated water was pumped into the solar panel, it traveled through one chamber of a double copper pipe. At the same time, water that was already boiled was flowing in the opposite direction through the pipe's other chamber, toward a reservoir tank connected to the tap. When the hot and cold water passed each other with just a thin copper membrane between them, the boiled water cooled down and the "raw" water warmed up—the heat exchange. Once inside the solar panel, the untreated—but now preheated—water's temperature rose quickly; from there, it only needed a little push from direct sunlight to boil. Whenever a burst of sunshine brought the temperature to boiling, pressure forced the steam that formed through a one-way valve into an upper tank. From there, it condensed back into water, which flowed down through the heat exchanger to the faucet tap. Using a one-meter-square solar collector as its heat source, the kettle needed only one minute of direct sunlight to make water start to boil and pass through the one-way heat valve. Because the upper tank couldn't fill unless direct sunlight actually pushed purified water vapor through the valve, any water reaching the tap was always trustworthy. The storage capacity was great enough that, even allowing for days when the sun never broke through, the kettle delivered about eight gallons daily of pure drinking water—more than enough for an average family—only two degrees warmer than when it left the ground.