1. National Renewable Energy Laboratory, 15523 Denver West Parkway, Golden, Colorado 80401-3393, United States
2. Department of Industrial Engineering, University of Florence, Viale Morgagni, 40-44, 50134 Florence, Italy
3. Johnson Matthey Technology Centre, P. O. Box
1, Belasis Avenue, Billingham, Cleveland TS23 1LB, United Kingdom