It doesn't have to be that big. A Fresnel lens about 1.13 meters in diameter would deliver roughly 900 kilowatts of power to a spacecraft orbiting Earth. I'm still not sure how big the parabolic mirror array orbiting Earth would have to be, because of the intense heat. It would certainly have to be double the area, to reduce the temperature below the melting point of aluminum. But still, even if the mirrors survived the heat collector element and the working fluid would simply explode if 4.5 megawatts per square meter were focussed on it.

So a heat collector element would not be useful. Rather, the whole array could function as a heat absorber *without* focussing the light at all. A carbon-black heat absorber riddled with heat-exchange tubes might be the simplest thing imaginable. And CHEAP too! For this application, the carbon-black target could be as small as the Fresnel lens. The challenge would be targeting accuracy, with a time-lag of 8.31 minutes between the Sun and Earth.

A rectangular photovoltaic array, or a solar concentrator, of similar power orbiting Earth would have to be about 58 meters wide. The size of the structures for greater power would be a tremendously difficult problem for stability and control. In fact those systems might eat up alot of the power produced just keeping it stable.

Now I'm doing alot of hand-waving, cause I don't have time at the moment to do the calculations, but I'll get around to eventually, cause it seems like a potentially very fruitful idea.