As the sun rays pass through the glass, the dark surface on the panel absorbs the solar energy and warms the air above it. The warm air rises and leaves the top of the unit as cooler air is drawn in from the bottom. The insulation barrier separates the two chambers and allows the natural movement of the convection current.
Power is not required for these heaters. The natural upwards movement of hot air leaving the top will draw in cooler air at the bottom. As long as there's a temperature difference between the two chambers, air will continue to circulate through the heater.
These units are most effective on south facing windows & roofs. Despite the outside air temperature, the sun will heat the dark surface of the panels. The temperature increase of the warmed air will depend on the amount of surface area of the panel and how direct the sun is shining on the unit. The larger the unit and the more direct the sun, the hotter the air will be. You should place your unit in a location with the most amount of sun and least amount of shadows as possible.
Since it would be most advantageous to use this free heat in winter, build the unit resting at an angle to maximize the winter sun. Typically, the ideal angle to position a south-facing unit is your latitude + 10 degrees. You may have to test for the optimum angle in your location. In the summer, remove this portable unit from the window or close off the indoor vents to avoid adding unnecessary heat from entering the room.
This unit efficiently draws cool air from the bottom and returns the same air warmed by the thermal panel. A closed system requires separate upper and lower chambers. A modification on this hot air solar collector unit is to have an open system which only has one chamber that draws in cool outside air. However, in winter, the cold outside air may be too low in temperature for the thermal panel to sufficiently warm. It's also open to the outside thus allowing insects and other critters indoors.