Carl Schwab asked what depth of water I planned to have above the solar energy absorber plates of the evaporator rafts. The simple answer is as little as possible, because the water is rough enough in San Francisco Bay or San Diego Bay to wash over the raft and require solar heating of a new batch of cold water. A small layer of water will heat fast and get the evaporation going again. I had a depth of 1/8 inch in mind. Another reason for a small depth is that the absorber plate and cross pieces are made of wood which we hope is thoroughly sealed and painted black. This includes sealing all pre-drilled holes and treating the parts as gently as possible to avoid surface damage. But it's a rough environment, and repeated dunking and drying off, cooling and heating, might produce cracks that let water into the wood and reduce its buoyancy. So initially floating as high as possible would keep the raft functioning longer. Once the cross pieces are completely submerged, the raft will sink.
But I have no guarantee that the absorber plate will remain parallel to the water surface so that it is always completely wet. A dry surface will not evaporate water and wood is not a good heat conductor. The cross pieces that are above the water surface will be solar heated and evaporate water that splashes on them. There are synthetic fiber plastic ropes that have neutral buoyancy. If we use these as stringers, we can use the bottom metal plates to provide extra weight to adjust the floating depth. Enlarging the bottom plates will keep the center of gravity low and encourage level floating. The density of the metal chosen is an important parameter.
I originally wanted to clamp the deployed rafts together to form one continuous raft surface so that solar heated water from some parts of the large surface would wash over other raft sections in a disturbed sea. This would require less reheating of new sea water. However there are advantages to leaving open ocean space between deployed raft sections. This avoids the awkward operation of having to clamp the sides of the raft sections together and allows workboat access to all raft sections for cleaning and repairs. The width of each section would be six absorbers adding up to 9 feet plus the space required by the 7 stringer ropes and the wires that enclose them. The length of each section would be determined by how much raft could be wound up in a roll and deployed from a workboat. Obviously, the width of the roll will be the smaller dimension. Therefore we would expect the cold water to be washing in across the outer ropes. The raised cross pieces might block some of this cross flow.
If you were hoping for a simple, definite answer to Carl's question, sorry to disappoint. Abandon all hope, ye who enter here.
Dick----I think your comments about cross flow are valid and preventing cross flow will improve the evaporation efficiency significantly. For example if the top surface of the rectangular area was gridded with a 1/8" high separator grid, the surface can have considerable rocking motion without the water running off and continuing to evaporate. If these areas are say 2"x2" then they could have 1/16" of water depth and the evaporation efficiency will grow dramatically. Just a thought.
Carl Schwab
Posted by: Carl Schwab | March 02, 2014 at 08:19 AM