It's only, 20 mW, or .02 watts/m^2, or just one tenth what you stated. Solar flux averages out to less than 850 W/m^2 because of night, clouds, and the poles, but it's still well over 100 W/m^2.

If we took the heat coming up from the entire US (9.8M km^2), the sustainable thermal power would about 200GW thermal. Given a 20% conversion efficiency, we would get 40GW electric.

I'm also afraid of what cooling such massive volumes of rock could do to the geosphere. It will cause the rock to become less plastic and to shrink (thermal contraction). If done on a large enough scale, could this cause large scale subsidence (from denser rock sinking) or earthquakes (from thermal stresses and from subsidence)? Could it slow down the movement of the plate in that area by making it less fluid, causing stresses to build up.