In making the case for a rapid conversion away from heavily polluting energy sources like coal and nuclear power to cleaner generation, renewable energy advocates often confront the argument that their scheme is impossible due to the intermittent nature of sun and wind. But a groundbreaking study out of North Carolina challenges that conventional wisdom: It suggests that backup generation requirements would be modest for a system based largely on solar and wind power, combined with efficiency, hydroelectric power, and other renewable sources like landfill gas.