1. e-mail:
2. Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN 55455
3. Institute of Mechanical Engineering, EPFL Lausanne 1015, Switzerland; Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720
4. Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL 32611
5. Energy Research Center, Vorarlberg University of Applied Sciences, Dornbirn 6850, Austria
6. Department of Mechanical and Process Engineering, ETH Zurich, Zurich 8092, Switzerland; Solar Technology Laboratory, Paul Scherrer Institute, Villigen 5232, Switzerland