Heating the cold exhaust on heat pump
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.