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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.

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    To be effective you would have to have an arrangement for the reflector to track the sun. It would be only marginally effective and a lot of trouble. – Jim Stewart Jan 7 '18 at 14:18
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    You're not going to get much from direct incident radiation. Your whole outside unit probably has about 4 square feet of surface area that could be exposed. You might be able to collect 200W under absolutely perfect conditions (perfectly absorbing receiver, 24 hour full sunlight, etc). Compare this to the heating capacity of the unit (my small heat pump moves (nominally) 6740W). So you may gain 3%. In practice, imperfect conditions definitely reduce you to less than half of this, perhaps realistically more like 10% or less - ie, a .3% efficiency improvement. Yea, it's not zero, but ... – Jean-Paul Calderone Jan 7 '18 at 14:27
  • Probably not worth the effort, but you could set up a solar water heater that warmed the pump's incoming air. Actually, almost certainly not worth the effort. (Interesting idea, though.) – Daniel Griscom Jan 8 '18 at 0:01
  • Geo thermal units do something similar in both summer and winter - the ground temp at 3 feet down is roughly a constant 68 degrees Fahrenheit. – Ken Jan 8 '18 at 8:36
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In some ways, you're not far off from a system I've been working on for a while. Instead of reflecting sunlight, however, which, as others have mentioned, wouldn't really be effective, you could consider other forms of heating, such as a solar thermal array that heats water, or a cogenerating PV array that provides both hot water and electricity.

Of course, you're better off with a ground-source heat pump and utilizing any available sunlight for electricity, but what you're describing is typically referred to as solar gain, whether passive or active, and is a component in green building design.

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