Just been thinking about your idea. Radiators are quite bulky and hold large volumes of water but only a cirtan amount of the water will come into contact with the sun warmed metal. With this in mind, this would be my solution. I would have thought that using a similar system (Glass box with none reflective backing to hold the heat) but instead of using a rad, use a length of micro bore gas pipe, say 8mm dia, coiled, but spread out. this would improve the surface area of the water in contact with hot metal and as copper conducts heat better rather than an aluminum rad, this would improve the e