As winter is drawing in, I thought I would take a look at passive solar heating.
This may be over simplistic, rather than insulate an external 'box' and pump warm air in (as with the blackened soda can idea) I decided to just have an alluminium sheet painted matt (flat) black behind the double glazed window.
The panel absorbs the solar radiation and then simply radiates any heat into the room - well thats how I see it

Admitedly the amount of heat is small, but if every south facing window in a home had a similar 'panel' it may help some.
Thought I would toss the idea out for discussion, It's very cheap..

