In a blog entitled "Capillary Pumping" I was hoping that capillary tubing with inside diameter 0.1 millimeter would lift sea water up 10 cm to wet the surface of a solar-heated evaporator. I bought some glass tubing and cut off a 10 cm length, stuck one end in distilled water, and waited for water to come out the top end. Nada. Nunca. The water didn't even get part way up the tube. I bought a great book entitled "Capillarity and Wetting Phenomena - Drops, Bubbles, Pearls, Waves" which explained that water attracts contamination. Agnes Pockels, experimenting in her kitchen about 100 years ago, found that it was necessary to "scrape" the water surface in order for it to exhibit the theoretical surface tension of 72 mN/m. Even if the water had risen to the top end of the tube, surface tension of the water film covering the exit hole might have prevented the water from exiting and wetting the evaporator surface.
Now think about bird poop and other stuff covering up the capillary exit, and we obviously need a different design. A flat sheet of black plastic welded to the bottom of a rectangular frame of plastic tubing would float just below the water surface. An array of 3 mm diameter holes would insure that the top surface of the sheet would always be wet. There is not much thermal insulation between the solar-heated surface water and the sea below - just the thickness of the plastic sheet. This is compromised by the constant exchange of water through the holes. Box ribbing on the under side of the plate would shield the plate from convection uderneath the plate and would also add stiffening to the plate. I had originally thought of a basic float one meter square but decided to reduce it to 0.5 meter square. This should allow the tubing of the frame to be rigid enough without having to add braces. The buoyancy of the frame would be insured against damage by plugging the tubing at the corners of the frame to compartmentalize it. Plastic loops would be built into the corner plugs. A wire connector with four prongs would be dropped through the corner loops of four evaporator panels and be captivated by some sort of do-hicky that will allow movement to conform to the sea surface.
We obviously need some drawings to explain all this. The July 6 LICN meeting came at the right time for me, and I will have to gain enough proficiency to do an adequate job. I will also have to make some realistic estimates of thermal insulation between the evaporating water surface and the ocean below. I have been assuming no thermal conduction, but this cannot be true for an evaporator tray that is continuously awash. At least the constant swishing should clean away bird poop and other debris.
The purpose of solar-heated evaporator rafts is to enhance the moisture content of air coming from the sea and passing over mountains, where it rises, expands, and cools below the dew point. Moisture condenses on cloud condensation nuclei (CCN). These droplets or ice particles are supposed to combine and get heavy enough to fall as precipitation. This has been failing, perhaps because there are too many CCN due to dust storms, open-pit blasting of coal, and other causes. The condensate particles are too small to coalesce and fall. Drought now exists at high altitude, for example the Lake Victoria area, the Ethiopian highlands, North African mountains, Mongolia, Australian Alps, and U.S. West. If evaporator rafts can increase humidity and yield bigger condensate particles, precipitation might be increased.
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