Sure, I'll email you. The aluminum foil is cheap, and I found some anodized foil that is really dark for great absorption. Anodized black foil is much better t

Sure, I'll email you. The aluminum foil is cheap, and I found some anodized foil that is really dark for great absorption. Anodized black foil is much better than painting foil black. I don't believe the total BTUs would be any higher if one replaced the thin foil with a 1" sheet of aluminum. An aluminum sheet with fins on the backside for heat dissipation might help, but I still think the total heat would be the same because the amount of energy hitting the surface is the same. It is surface area and temperature of that surface that counts and we don't need fans because of convection. If you take an infrared thermometer and take a reading off a baseboard heater you will see 140-180F temps radiating off the casing and the metal fins inside that are put there to increase surface area. These flat panel aluminum foil based collectors work the same way. When light/heat energy enters a room through the window some of it turns to heat and some is reflected back out the window. That is why you can see objects from the other side. Seeing those objects is how we lose our heat. These collectors let us capture and convert a much higher percentage of the energy coming into the window already. If you look at one of these collectors through the window from the outside it appears invisible almost, they are hard to see, nothingness from which precious free energy is harnessed and consumed and not allowed to escape.