**Acknowledgements**

This work was supported in part by the "Ministerio de Ciencia y Innovación" (ENE2009-13106) and in part by the "Junta de Comunidades de Castilla-La Mancha" (PEII10-0171-1803), both projects co-financed with FEDER funds.

This work was also supported by the U.S. Department of Energy under Contract No. DE-AC36-08-GO28308 with the National Renewable Energy Laboratory.
