NRG Energy, one of the United States' most coal-dependent utilities, on Monday signed a deal with California startup eSolar to develop solar power plants. The agreement calls for NRG to invest $10 million in Pasadena-based eSolar for the right to use the startup's technology to develop and operate three solar power projects in California and the Southwest that would generate 500 megawatts of greenhouse gas-free electricity. NRG ranks as one of the nation's dirtiest utilities, spewing 70 million tons of carbon dioxide annually from its coal-fired power plants. eSolar will use fields of mirrors to heat water to create steam that drives electricity-generating turbines.