Lunar Ring

The Japanese construction firm Shimizu is suggesting building a 250-mile wide ring of solar panels around the lunar equator to harness solar energy as the main power source of an advanced human civilization.

Shimizu is known for coming up with such big ideas; it's a way to publicize the firm.  Collecting solar energy in space and beaming it into the electric power grid, however, is an option to power an advanced society over the long term.