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The dripwall delta uses a dark wall or fence north of a container bed to collect solar heat. Water drips down the wall and takes a lot of the heat with it and down into the soil. The "Delta" up top has slots in it to distribute the water drip. Holes block quickly, slots fill up pretty evenly with muck from the water so the maintain the came drip quantity for a long time. My test dripwall is 18 inches high and leans to the back. I have tested lots of materials and slate, charred planks, sheet metal and galvanize all work well. Mine does a maximum of about 50 watts heat transfer in sunshine. However, when I tweak it some more, it could do a lot more. Perhaps in the region of 200 watts.