CHASING THE SUN: But there is a big if in this scenario of urban transformation. It will happen only if Exelon receives the generous loan guarantees for renewable-energy projects promised in this year’s federal stimulus bill–funds that in this case would cover 80 percent of the project’s costs. Barely viable with the loan guarantees and a handful of other federal and state subsidies, the $60 million solar plant would not be possible without such government support. . . . Even with the federal