This is the latest version of a poster I've been using at energy shows for 30 years.
A South-Facing Window Is Already a Solar Collector
Any window that sees direct sunlight is a solar collector. You can learn how to use that free energy to make your home more comfortable and more secure.
Caulk and seal the window against drafts.
Install storm windows on the exterior, interior, or both.
Cover the window at night to prevent conduction, convection, and radiation heat loss.
A valence above the window will prevent night-time drafts and a curtain will reduce condensation.
A sunny window can double as a greenhouse for starting seedlings or growing house plants.
Add an aquarium and you can grow fish.
Expand the solar space below, above, or beside the window with a windowbox solar collector.
A south-facing window is already a solar collector. Learn how to use it.