1. School of Mathematics and Physics, North China Electric Power University, Beinong Road 2, Huilongguan, Beijing 102206, P. R. China
2. School of New Energy, North China Electric Power University, Beinong Road 2, Huilongguan, Beijing 102206, P. R. China
3. Institute of Clusters and Low Dimensional Nanomaterials, North China Electric Power University, Beinong Road 2, Huilongguan, Beijing 102206, P. R. China
4. Department of Power Engineering, School of Energy, Power and Mechanical Engineering, North China Electric Power University, Baoding 071003, China