50 Percent in 2007 ^ Huge solar power station planned for Portugal ^ a b c d " The Future of Geothermal Energy " (PDF). MIT . http://geothermal.inel.gov/publications/future_of_geothermal_energy.pdf . Retrieved 2007-02-07 .   ^ a b c Fridleifsson,, Ingvar B.; Bertani, Ruggero; Huenges, Ernst; Lund, John W.; Ragnarsson, Arni; Rybach, Ladislaus