We have a serious drought in the U.S., but I don't have a good understanding of the North American monsoon wind patterns. There is also serious drought on the the entire Indian Subcontinent and the Indian summer monsoon is simpler to describe. When Spring arrives, the land heats up, heating the air above it. This air rises and is replaced by air that comes in from the Arabian Sea which lies to the west of the Indian peninsula. This air contains moisture that evaporates from the water. The Western Ghats mountain range runs along the western edge of the Indian peninsula and the incoming air must rise up to cross the mountains. This air encounters lower pressure at the higher altitude, so it expands, cools adiabatically, and if it cools down to the dew point, moisture condenses out on cloud condensation nuclei (CCN). CCN are aerosol particles which can be dust particles, sea salt crystals, organic matter, soot, etc. If the water droplets are large enough, they may coalesce and form droplets that are heavy enough to precipitate. My understanding of cloud physics is deficient, so I will oversimplify the reason why the Indian summer monsoon has not produced sufficient rain: too little moisture distributed on too many CCN. The drought is not only on the coast. The winds flow over the entire subcontinent, reaching the Himalayas.
My oversimplification leads to the conclusion that increasing the humidity of the monsoon winds might help to produce rain. Evaporation of water from the sea is not a very efficient process. The solar radiation is mostly in the visible range and high frequency infrared band just below the visible frequency band. Depending on the clarity, which is influenced by the concentration of phytoplankton, the sunlight can penetrate to considerable depth, which means that a considerable volume of water is heated. This results in a negligible temperature rise, which means that very few water molecules will acquire sufficient energy to leave the liquid and enter the air stream. By covering a large area of the coastal waters with dark-colored solar absorber plates that are floating a fraction of an inch below the water surface, we can dissipate the incoming solar energy in the absorber surface, which heats the thin layer of overlying water and achieves a useful temperature rise. The water, being heated from below, will rise from the absorber surface and spread out at the air/water boundary. However, if a very large area is covered by solar absorber plates, the loss of warm water is confined to the edges of the evaporator raft, and the (hopefully) small gaps between the individual absorber plates.
We can assign some dimensions in order to visualize the complete evaporator raft and its deployment and retrieval. Picture a hollow plastic tube 20 feet long sealed water tight and filled with plastic foam to insure that it will float even if damaged. Eleven ropes, each 60 feet long have their ends looped around this tube, which I will call a spreader bar. The other ends of the ropes are respectively looped around a second spreader bar. Individual solar absorber plates 6 inches wide by about 2 feet long have rings at their ends which are threaded over the ropes to maintain a 2 foot separation between the ropes. This creates a section of evaporator raft 20 ft by 60 ft with 1200 individual absorber plates. Another set of 11 ropes is also looped around the second spreader bar. Another 1200 absorber plates are threaded onto these ropes and the loose ends of the new ropes are respectively looped around a third spreader bar. The foregoing process is repeated to form a string of raft sections, each 20 ft by 60 ft.
The spreader bars serve as hinge pins, so that with a suitable twin-boom handling crane, the 60 ft sections can be folded into a 62 ft long by 22 ft wide cargo hold of a work boat. I will describe the work boat and manor of deployment and retrieval in future posts.
I have made some simple test absorbers and temperature measurements. A neighbor threw out a supply of oak strips .25 inch thick and 1.5 inch wide, which I salvaged. I made one absorber with 3 strips 15 inches long and another with 4 strips. The strips are nailed to cross pieces at the ends. The area between the cross pieces is covered with tar paper to absorb solar energy. (I had the tar paper, whereas all my black paint is dried up.) I used twenty-penny nails to make the combination of tar paper, oak strips, and nails negatively buoyant. They would sink to the bottom, were it not for the incremental displacement of the cross strips. The number of nails is adjusted to maintain about an eighth of an inch of water above the tar paper absorber.
The absorbers fit snuggly into a nice rectangular steel tank. First measurements showed that after a few hours of sunshine the entire volume of water was at a uniform temperature. I never thought that the solar heated water at the top surface would transfer heat to the tank so rapidly that the steel would conduct it to the tank bottom to drive natural convection. I substituted a plastic kitty litter pan and found that the entire volume was still being heated. Then I noticed that my nails were dropped into holes drilled in the wood and extended down almost to the bottom of the shallow pan, once again destroying the stratification. Now I have the nails horizontal, wired to the bottoms of the absorbers. But I think the kitty litter pan is too shallow. I will probably make a new absorber to fit into a plastic dish pan, which is deeper. Stumble forward.
Great article...Plastic Tubing can be of great help. It does not allow rust to come on the pipe as water gets evaporated with sun and air. Plastic tubing increases the life span of the pipes.
Posted by: tubing caps | August 31, 2012 at 03:59 AM