AES Corp., an Arlington, Va.-based company that has grown into the world's largest independent power producer, has struck a $500 million deal with the Ugandan government to build a massive dam near Bujagali that would greatly expand power capacity for the country, where fewer than 5 percent of the 22 million residents have electricity. While constructing a 100-foot-high wall of concrete across the Nile is certainly a technical feat, the business deal is encountering even bigger cultural and environmental challenges.