ow he thinks otherwise. Leuschen isn't turning his back on coal, which generates more than half the U.S. electricity supply. But his company, together with Florida Power & Light Co. (FPL ), owns one of the world's largest solar-power plants, in California's Mojave Desert. Investors in Riverstone's renewable-energy fund have been told to expect annual returns of 15% or so and perhaps much more, based largely on the fact that most alternative-energy producers are able to enter into long-term contracts with utilities, some lasting 15 to 20 years.