This is a very simple project proving that you can have free heat for just pennies up front. A bit of cardboard or styrofoam and some aluminum foil taped to it. Place it into your window and you get free solar heat. No moving parts and no maintenance.
Take any type of rigid material and cut it to fit tightly into your window frame on the inside. Then cover it with aluminum foil on one side. Cut a slot in the top and a slot on the bottom for air flow. Paint it all black on one side. Put the heater into a south facing window and now you have free heat when the sun shines.
If you use styrofoam and cover the window fully, you also have an insulated window to reduce home heat loss at night.