A south facing window in any Passive Solar House design can be optimized by using a flat black metal shade to create a solar space heating panel out of any window. The flat black metal type of shade will absorb the radiant solar heat of the sun with more efficiency than the floor of the room, and it can be kept closed day or night and not effect its performance, acting to close off the window day or night, while not blocking the radiant heat.
The shade will heat up while the air in contact with it will travel upwards as it is heated, creating hot air convection currents. Heavy insulated drapes closed over the shade will create a cavity where the air will enter behind this cavity near the floor and travel up due to air convection principles and exit near the ceiling as heated air.
Adjustment of the shade slats to make the shade flat with respect to the window and blocking the light, will help with the degree of heat absorption, while leaving a little bit of opening between each slat allows for air currents to pass between them effecting a better amount of heat removal, hence the shade acts as your solar space heating panel heat exchanger.
In Passive Home designs, the home owner closes the drapes at night for privacy reasons and must remember to get up early enough to open them in the morning to allow the radiant heat of the sun into the home. Using this method you can forget having to bother with this and keep them closed during the day but still acquire the radiant solar heat for use in your home. In effect you have turned you window into a passive solar space heating panel, while no one actually realizes that this is how it functions.
In the summer time you raise the black metal shade so that no solar radiation can reach it, and you may keep the drapes closed in the summer, furthermore the south facing wall will have to have some sort of solar shading added to it on the exterior.