Solar hot air collector, again

Im getting excited about this solar hot air collector idea, again. I found this book at our office: Sun, Wind and Light, Architectural Design Strategies. Its got over a hundred strategies for designing or retrofitting buildings with all kinds of passive green strategies that use the sun, wind, and light of your specific area. I like the mantra, "Passive buildings, active occupants," which encourages people to use buildings to temper their spaces, and not be so dependent upon "active" mechanical components, like fossil-fuel fired heating and cooling systems, or even super expensive photovoltaic solar panels. The simplest way to think about this would be to open the windows on a nice day rather than use the air conditioner. Or, turn off the electric lights and open the shades to allow daylight in, especially on the south face in the winter. Anyway, Im thinking about installing a rooftop unit like this one on our house instead of smaller window units in the previous post:

The basic premise of the panel is there is a glass / plexiglass face, with a thin air space, and a corrugated steel backing, painted black. In the winter, cold air enters the lower intake, is super heated in the sealed air chamber by the sun, and exits at the top, pulled by an inexpensive fan, through ductwork (or not) into the house. Because heat rises, you need to get that free hot air down to the first floor, so I would hook ours into our existing ductwork system, then it would rise throughout the house naturally. You can even hook these up to thermostats, so they turn on and off. All told, I'm told you can build them for between $100 and $500......free heat for about 25 years after that.