One season-extending upgrade I've added to the new seedling greenhouse is a solar heat absorber. The idea is to replace part of the south wall with something that will primarily heat the inside air. I used a completely passive design - this system requires no power to run, other than direct solar heat gain from the sun. This basic design was popularized during the energy crisis in the 70s, so it is easy to find reference material to help design it.
I started out by building the south knee wall "inside out" - that is, I tacked the plywood on to the inside of the framing, instead of the outside. This creates a cavity where the absorber will be built. In the picture below, you can see the cavity as well as some circular ducts I drilled out for cold air in (at the bottom) and warm air out (at the top).
Next, I added two layers of black aluminum window screen, separated by a small spacer. It is held in place with an additional spacer strip. The screen is attached to the outside of the framing on the top, and to the inside below, so that the screen is actually held at an angle in the cavity. The purpose of the angled fastening is to force the air though the screen as it flows through the system. Cold air enters through the lower ducts (outside of the screen) and is forced back through the screen on its way up towards the upper ducts. Since the black window screen heats up when the sun shines on it, natural convection keeps the air flowing during the day. When the sun goes down, the convection shuts down and the collector acts much more like an insulated wall than like a window.
Finally, the cavity is glazed with corrugated pvc. I chose this material for a couple of reasons. FIrst of all, it is just about the cheapest solid glazing available. The corrugations create more turbulent flow in the absorber which is supposed to increase efficiency. Lastly, I was not sure about using the brittler, more expensive polycarbonate where it might be exposed to huge temperature variations.
Here is the view from the inside. I've just covered the ducts with scraps of screening. Mainly this is to provide a backstop for the backdraft dampers, but I also felt like it might be necessary to keep the mice and other varmints out of the space.
And here is the system in action! The backdraft dampers are thin pieces of polyethylene plastic taped up above the ducts. At night (or on a very cloudy day), there is not enough airflow to push the plastic out so reverse flow is prevented (in which air would be cooled by the collector).
The day I took this photo was nice and sunny but with weak winter sun. Still, a steady stream of hot air pushed the dampers open. The temperature was probably in the 120 degree F range (although I didn't measure exactly).
There are many more sophisticated designs out there, but I put this together without really adding any new pieces to the plan with the exception of window screen and glazing. I'm pretty happy with the results so far - we'll see how it does in the early spring when I really need the extra heat. Next up: a solution for how to store heat in the structure overnight.