You have both the sun and the sand in the Sahara desert. So use the energy from the sun to power a plant that generates solar panels from sand. Use these solar panels to build more such panel generating plants, and so on.....
 
A Japanese scientist is taking this idea and running with it, in collaboration with the Algerian government.
 
Here is a you-tube video on the idea:
 
http://www.youtube.com/watch?v=UdO6T1TIDzQ
 
A CBS news article:
 
http://www.cbsnews.com/stories/2010/12/01/tech/main7108204.shtml
 
The claim is that such a scheme could provide 50% of the world's energy needs by 2050. The article is thin on numbers, but I see no obvious holes in the basic scalability of the idea, other than the long gestation period of the pay-back.