[1] Univ London Imperial Coll Sci Technol & Med, London SW7 2AZ, England
[2] Univ London Imperial Coll Sci Technol & Med, DTI Ctr Distributed Generat & Sustainable Elect E, London SW7 2AZ, England
[3] Univ Manchester, Elect Energy & Power Syst Grp, Manchester M13 9PL, Lancs, England