Physics question

>
> Wouldn't it make sense, if indeed a SPS is of a solar thermal
> design, to incorporate the waste heat generated from the dc-to-
> microwave energy conversion process back into the solar thermal
> cycle rather than radiate it into space.
>

No, not really.

That waste heat is low grade, low temperature. Heat engine efficiency
increases as you increase the temperature difference.

You would get more energy by putting the engine in sunlight than by trying to
use the same engine to capture PV waste heat.

>
> In your opinion, if you accept the premise of the solar thermal
> option, wouldn't the radiatiant heat feeback loop seriously diminish
> the radiator size on an SPS?
>

Not by much.