Evaporator Raft Construction Details - Dick LaRosa

I think I have figured out how to attach individual solar absorber plates to stringer cables (or ropes) so that large rafts can be assembled, rolled up, and deployed from workboats. The individual absorber plates are hardwood 6 inches wide and 18 inches long, with the grain parallel to the long dimension. The thickness is tentatively 0.25 inches. A thickness of 0.375 inches would be stronger, but the additional buoyancy might make it more difficult to weigh the plate down so that it floats just below the water surface. Cross pieces perhaps one inch square and 6 inches long would be fastened across the absorber plate at each end and would protrude through the water surface to stabilize the floating depth. If some force pulls the absorber plate down, the additional displacement will provide an upward force to maintain the absorber at the correct depth. The idea is for the absorber plate, painted black, to heat the thin layer of water above the plate to facilitate evaporation. The cross pieces will also be painted black so they will attain a high surface temperature and evaporate water that splashes on them.

The absorber plate and cross piece are sandwiched between a top and bottom plate made of a metal that, together with its surface treatment, will survive in sea water. Exactly what is unknown to me at this time. But it will have to be compatible with two hefty flat-headed rivets that hold a pair of wires against the bottom plate. The wires have round loops bent in their respective ends that fit around the rivets. The rivets pass through predrilled holes in the metal and wood parts of the sandwich, and the top end of each rivet is upset by a rotating forming tool that squeezes the wires, metal plates, and wood pieces together. The metal plates have bent edges with notches to position the wires. The wires are bent upward at a right angle far enough from the absorber plate to allow free passage of a stringer rope. The wire spacing at the left end of the absorber is different from the spacing at the right end, so that one pair of wires fits between the wires of the neighbor. A whole row of plates, making up the entire width of the raft, is laid down and the stringer ropes are dropped between the nested, upward pointing wires.

There are two right-angle wire bending operations performed by multiple tools simultaneously on all the wires. The first operation bends the top ends of all the wires toward their respective absorber plates. The second operation bends all wires so that they fall into the notches of their respective top plates with the first bend hooked over the wood cross piece to resist forces tending to pull the absorbers away from each other. I originally thought there would be six rows of absorber plates strung on seven stringers, giving a raft width of nine feet plus the spaces to captivate the stringer ropes. Tooling is required for bending 24 interleaved wires in opposite directions with the ability to retract the tools and get ready for the next set of absorbers. I have visions of short pins with beveled flanges so that the wires don't slip off. Also we need a series of floating clamps to hold the plates in position during the second bending operation. The clamps and bending tools must float because there are cumulative tolerances in the six-plate width.

Considering all the problems and unknowns, I don't have any urge to go to more than six rows of absorbers with a total width slightly more than nine feet. I need to learn about metals in sea water, metal working, tooling, fixturing, and whatever. I don't even know the language to describe what is needed. But I'm a lot better off than when I didn't have the foggiest notion of how to assemble a complete raft.

Comments

[Carl Schwab]

Reply to Dick LaRosa---
This phrase is taken from you write up---
“The absorber plate and cross piece are sandwiched between a top and bottom plate made of a metal that, together with its surface treatment, will survive in sea water. Exactly what is unknown to me at this time?”
Doesn’t the top metal plate block the sunlight from the top of the absorber plate? What am I missing?
Car Schwab

[Dick LaRosa]

In answer to Carl Schwab's question, the top and bottom metal plates are only about the width of the wood crosspieces. The solar absorber between the crosspieces is not shaded. The bottom metal plate is needed to provide a solid unyielding surface to press the wire loop against. Wood is too soft. A round flat washer might work, but the plate takes care of two wires, and its bent-down edge can have slots to point the wires in the correct direction. The top plate provides a good backing against which to roll out the end of the rivet. You could just peen it, but I think it is better to drill a small hole in the end and heat it up and spread it out with a rotating tool pressing on it. I have seen eyelets and hollow rivets rolled out this way and they don't split because they are heated and softened. Might have to clamp the rivet head to keep it from spinning. This operation is done on the individual solar absorber, so it can be done in a drill press.

I don't know what metal or surface treatment to use. I think bronze might work, but I'm guessing it is too expensive. The wires, rivets, and backing plates have to be compatible. Maybe some kind of steel or hard aluminum alloy?

[Carl Schwab]

Dick-----What depth of water do you plan over the blackened 6" x 18" surface? How accurately will you try to maintain it?
Carl Schwab