www.builditsolar.com is the best database resource on the web. Excellent plans and such. I only wish the forum were as spectacular as this one -- in fact, it doesn't have one. Look under solar heater, space.
It converts solar radiation into heat so, in fact, it DOES work when the sky is overcast, because radiation penetrates clouds. Ever gotten a sunburn on a cloudy day?! But you're right -- certainly not as well when it's sunny. I've noticed that around here on the most bitter cold days, it tends to be sunny. At night?
You don't need a fan. It will help, but you don't need it. If you are just building the window box kind the hot air will "thermosyphon." In other words, the hot air in the box will rise into the coop and that will therefore suck the cooler air back into the collectorl. The window box style heaters are split down the middle w/ the cool air entering the bottom of the window, running down the bottom of the collector, getting pulled onto the top (sunny) side of the collector, heated, and then vented back through the same window. Now if you're trying to heat a massive area, a fan will help, but you're not trying to cook the birds -- just take the edge off -- right?
You don't want "thermal mass" (rocks, metal, soapstone, etc.) INSIDE the collector -- you want it inside the coop. So put a box of rocks where the heat enters your coop and those rocks will radiate the heat gently for a few hours. Water also works to store heat.
I still don't buy that the cans are doing a thing. Once that energy enters the glass and hits the flat black metal of the collector, that energy is converted to heat. Mash the cans flat, nail them to the back of the collector, paint them black, and now the air is being heated. You can buy commercial solar collectors and they don't have cans or tubes -- just a box. If you had double pane glass (I do) it would be fine. But again, I have no expertise -- just trying to think this through. I'll try and post a picture of my pilot window
here.
(here you can see the top and the hole in the foam/foil board) It's just taped on)
(and here, if you tip your head sideways, is the back. When I adapt this for the coop I'll make another "layer" on the back (nicely insulated) so air from the coop can enter the holes on the bottom and get heated to the top)
By the way, I was getting over a 60 degree increase, from cold outside air to heated top -- in the coop the warmed air will continue to warm and soon the chickens will be drinking tropical drinks and doing the limbo under the roosts.