Briefly the principle is to absorb the sun's energy in black polyester felt which then warms air recirculated from the room by a thermostatically controlled fan.
This system is new to me but it appears to be popular in the DIY market in USA. The 20 page description and instructions (pdf) are of a DIY collector which is a 1 metre square box mounted on an outside wall and costing only $185US plus your time in construction. It is claimed to have the potential to save the user 50 gallons of heating oil per annum and gives a pay back time of months only.
We live in a 3 bed semi and spend £600pa on 300 gallons of oil per year. We might save £100 per year and pay back our investment in 7 months! The disadvantage would appear to be aesthetic. A roof level installation could be larger but would not work without ducts through the house to bring the heat downstairs. We could replace one of our patio doors with a box between October and March when we would have our heating on and do not use the patio too often, but is it worth the hassle?.
It would be great to know what installations have been successful in USA, UK or elsewhere.