EDIT: oh, to keep the water unfrozen, gotcha. Never heard of one but we use a dog bowl heater under ours. Needs electricity out to the coop but works really well.
This question has been brought up before and no one really has an answer. Anyone out there an engineer? A solar water heater design would make someone a million bucks.
I have electric & a water heater in my main coop, but will need something to keep water unfrozen in my tractor. It is a regular little coop on wheels, 2 ft off the ground, but the floor is insulated don't know how much that will help). I use a 1 gallon plastic waterer for my 12 bantams & am planning on trying the Snuggle Safe pet warmer to keep the water unfrozen for at least part of the day. It is a microwaveable disc for keeping pets warm. I think the disc is 8 inches in diameter, so I have to measure the base of the waterer and see how it compares. I don't have any idea if this will work at all, just throwing it out there for you.