**7. Acknowledgment**

The authors would like to thank the Spanish Government for the partial funding of this work within the framework of research projects ENE2005-08256-C02-02 and ENE2008- 06687-C02-02. Pablo Dolado would specially like to thank the former Spanish Ministry of Education and Science for his FPI grant associated with the research project. The authors also wish to thank the company CIAT for the support given in the early stages of the experimental work. Special thanks are extended to Mr. Miguel Zamora, CIAT R&D Manager, for his collaboration.
