I have to agree that dc transmission makes sense for the long distance "backbone" of the grid.
AC was easier in Edison's day, because they did not know how to make generators and motors as efficient as they do today, and of course, they
had no solid state technology at all. You need to send power via high-voltage lines to reduce I**2xR losses, but back then, transformers were needed to convert lo-v to hi-v and back, and transformers don't work for dc (sorry, kids, transformers are not what you think they are).

My initial point was based on the idea that ground covered with solar cells will absorb more energy from the sun than bare desert ground, and the cells will not radiate that heat back out into space at night, rather, that heat will stay on earth, and end up as waste heat from all the motors etc that are runs by all this energy. It's exactly the same problem as the greenhouse effect, except that we do get some use out of it.
MD