Heliostat – it’s a fancy name for a simple idea:
Shine a big mirror in your window on a cold day, and you get transmitted heat, as well as reflected light. Build the mirror so it tracks the sun’s arc, and you get that heat and light all day long!
Some folks up in Massachusetts have invented a DIY model they call the Sunflower.
They say it’s “the first affordable heliostat designed for ease of use for the homeowner”. (@ $399.-) And they claim you only need a Phillips screwdriver to set it up.
I’ve got a big north-facing window that would make a nice target for a heliostat.