1. Department of Engineering, University of Cambridge, Cambridge CB2 1PZ, UK
2. Bauhaus-Universität Weimar, 99421 Weimar, Germany
3. Department of Engineering Sciences and Applied Mathematics, Northwestern University, Evanston, IL 60208-3125, USA
4. Renewable Energy Systems Ltd, Beaufort Court, Egg Farm Lane, Kings Langley WD4 8LR, UK