Last year I started fiddling with passive heating and whipped up one of these window box heaters.
I found several flaws with my design and have since learned how to improve the efficiency greatly.
For this test, I just leaned it on the sill of the kitchen window (the most northerly facing window) in the sun.
It’s about 15C today and this little box is pumping 26C while it’s in the sun!
My (not really passive) version has a two small solar panels that I rescued from somewhere last year that power a small DC fan and a temperature switch.
So, how to improve on this design?
Firstly, the temperature switch is redundant, mostly over-engineering for its own sake. The inlet and outlet holes were cut so the box would work in a window that we’ve found to be better closed and insulated in winter. They’ll have to be redone so the inlet is on the inside of the kitchen as well.
I’m hoping that, by having both inlet and outlet at different places in the kitchen, I can get a flow started and, after a while, be heating already warmed air and this build up the heat faster.
The other thing that should improve efficiency in a big way is to use a glass cover instead of perspex. I’m hoping that the characteristics of glass will heat things up more.
It’s all experimentation, but now that so know the theory is sound and have some ideas on improving what we’ve got, I hope to get a better design working on the three of the windows that get the most winter sun.