Solar Water Heater, Plastic or Metal?
Question:
Thin black plastic or black metal pipes? Maybe ever painted matt black copper piping?
Answer:
The answer is it depends on the exact type of plastic or metal and the thermal characteristics of each.
Generally speaking, metal has a higher thermal conductivity than, say, standard plastic or PVC piping. PVC pipe is a great insulator and doesn't handle thermal stress well. Some types of plastic - especially cheap black plastic, will melt or stretch when heated.
Their are some plastics that will work as well or better than metal, but the generally cost much more.
Copper pipe works well, but if you paint it black, you will want to use a thin layer of flat black paint that isn't enamel or latex - these will act as an insulator and prevent the copper pipe from absorbing as much heat as a unpainted pipe. The pain also tends to come off quickly as the copper pipe expands and contracts (thermal stresses). Paintin it probably isn't worth the expense and effort.
Your best bet is to use a plain but tarnished copper pipe and make sure the heat box traps as much light and heat as possible. Use a type of reflective glazed glass that allows light in, but not out. Seal the box tightly and insultate the heck out of it.
Two other things you can do from to increase efficiency will little extra cost:
1. Use a Fresnel lens and/or reflectors to gather and concentrate more light into the heat box.
2. Beyond insulating the heat box to retain heat, add a thermal absorption layer to the box itself. Putting - for example, a 1/2 think piece of blackened steel easily found in junkyards to act as an additional heat sink can cheaply increase the ability of the box to soak up even more heat, retain it longer, and can help to stabilize the box from high winds. The steel or other plate acts as a thermal ballest, reducing the thermal stresses by preventing a fast cool-down.
I took a copper coil the size of the inside of a 55 gal metal drum and put it in the drum, allowing for external connections to the coil. Fill the drum full of sand and put the lid on it. Paint the drum and lid black and place it in the sun. Very efficient and it will heat water even after the sun goes down.
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