The Tennessee Valley Authority could be generating more "alternative" energy for less cost, if only the public wasn't so enamored with wind and solar power. Methane gas, formed as human and animal waste or garbage decays, produces more power dollar for dollar. It's half as expensive as wind power and a tenth the cost of solar power, according to TVA figures. But, coming from a stinky mess, it lacks appeal to the rate-paying public. TVA depends on ratepayers' choosing to pay extra to help fund alternative energy sources. "From a marketing viewpoint, it's hard to promote," said Jim Keiffer, TVA senior vice president of marketing. ... That's why TVA's program, Green Power Switch, available through distributors including Nashville Electric Service, requires that at least half the energy it creates come from the favorites: solar and wind.