Scientific American: Although the environmental benefits of burning less fossil fuel by using renewable sources of energy–such as geothermal, hydropower, solar and wind–are clear, there’s been a serious roadblock in their adoption: cost per kilowatt-hour. That barrier may be opening, however–at least for one of these sources. Two recent reports, among others, suggest that geothermal may actually be cheaper than every other source, including coal. Geothermal power plants work by pumping hot water from deep …