A little idea I was playing around with is making a solar air heater for next winter. Usually they force air through a box that is essentially a solar oven. But a lot of heat is lost on the glass and even through the walls of these devices. So, they end up using special glazing or double walled glass to add insulation value. Also, they can actually add insulation to the walls as well.
But what if you used air itself as the insulator.
I'm not a huge fan of solar photovoltaic due to its price. Although I have 3 of them, each 120 watts. They can be made cheaper by building them yourself though. But using the sun's heat is usually cheap and efficient. A typical PV panel is about 15% efficient, but solar hot water or air heater is about 50% to 70%.
Basically it is just plywood walls and that transparent sheeting (2 feet by 8 feet). There is a 8 foot length of flat black stove pipe run through the middle. And the walls are covered in shiny mylar or aluminum foil. Each pipe would get sun on every square inch and the pipe is super insulated by the hot air surrounding it. Assuming this is at least 50% efficient, then this would be about 1,000 watts of power or about 3,400 btu per hour. You can string these together and get more power. You would want to use a squirrel cage fan that has some power to push the air if you have more than 2 or 3 of these though.
Notice too, that the glazing is at a 45 degree angle. If you put it on a roof that has a 45 degree angle, then the glazing will be at 90 degrees. This would be great for the low winter sun. Most houses have a more shallow angle, like maybe 30 degrees. Still, that would be 75 degrees, great for the low winter sun.
Also, if you put it on the ground and have a higher exit and low input, you would have a thermosiphon system. The colder air at floor level would "fall" outside and into the solar heater bank. Then as it heats up it rises into the house through another pipe connection. You would need flaps on it at night so it shuts down air flow at night, otherwise it would become a cooler instead of a heater.
I'd like to test this sometime, we'll see.