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The condenser fins are manufactured very close together, so there's no sunlight that makes it through the fins to warrant using a mirror on the back of the unit. I remember this unit was in an insulated box with a glass front.
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Those fins are basiclly a heat sink to help pull heat away from the pipes, I don't know if it would work the other way, but worth a try for those who have the materials and time.
as far as using a reflective back or black back behind the pipes, definitely try both. You might want to consider some form of magnifying glass. or several. there pretty cheap in the dollar store market in the states.
or a little more expensive magnifier is a cosmetic mirror you can find in the womans makeup section of a department store. 15x, 10x 7x, and 5x.
I helped a daughter make a solar stove to cook a hotdog. with a small cardboard box lined completly inside with aluminum used a hanger to angle the cosmetic mirror into the box. her hot dog was burnt in 3 minutes on a party cloudy day with a drizzle of rain.