Windowbox Solar Air Heater
This is a demonstration model of a solar windowbox air heater.
You can build it from a sheet of insulation board, some clear glass or plastic, and a furnace air filter painted black.
When the sun shines, the black absorber inside the insulated windowbox gets hot, heating air which rises into the room behind the window, and drawing cooler air from inside that room past the absorber in a thermal loop that lasts until the sun goes down.
At night, the cold air sinks to the bottom of the windowbox and closes off this thermal loop so there is no additional night time heat loss.
Increase the efficiency of the windowbox solar air heater with a solar electric, photovoltaic, PV fan.
This demonstration model uses a repurposed solar car window ventilator. Only when the sun hits the PV panel does the fan move air, an automatic solar circuit thermostatic control.
The windowbox solar air heater is a supplemental heater for one room with a south-facing window and can be modified to provide increased ventilation and cooling in the summer as Edward Sylvester Morse's 1881 solar air heater design did.
Cost for a 2 foot by 3 foot insulation board solar windowbox air heater is around $100. PV fan car vents are available for as little as $5. One recently built windowbox has heated air by at least 20º Fahrenheit, from 65 to 85º, and will do so all winter.