I'm honestly kinda surprised that nobody has tried that (at least, my Google Fu failed me in finding someone that did).
I just bought my first home and noticed something. My outdoor coil for my heat pump sits right behind my house. Due to it's location, it's always in the shade and the temperature around it is much lower than the temperature in the sun. I was wonder (how stupid this idea is) if I were to put some sort of reflectors so that the sun hits the outdoor coil thereby warming it up and putting that heat in the house, would it be worth it? I already have an vague idea of what I need to do but figured I'd get some advice before trying it out.