Low-tech solar panel
Here's how it works ...
A sheet of glass is placed in front of a panel of darkish stone, with a gap of approx. 1-2 cm between them. The sun shines through the glass and heats the stone panel. Cold air from inside the room passes through the two circular holes at the bottom of the panel and is heated as it flows up through the gap between the glass and the stone. The warmer air then flows back into the room through two vents at the top of the panel. In the summer, if the warm air isn't needed, the two vents are closed on the inside and opened on the outside, so the warmer air is released. No energy needed (apart from the sun).
