Evaluation and development of empirical models for estimating daily and monthly mean daily diffuse horizontal solar radiation for different climatic regions of China

The diffuse solar radiation is required for estimating global solar radiation on inclined surface or for estimating beam solar radiation for concentrating photovoltaic applications. In the present study, the accuracy and suitability of 72 existing and developed empirical models in nine categories are evaluated for estimating diffuse horizontal solar radiation in different climatic regions of China, i.e. the arid desert of northwest China (NWC), the Qinghai-Tibetan Plateau (QTP), the (semi-)humid cold-temperate northeast China (NEC), the semi-humid warm-temperate north China (NC) and the humid (sub-)tropical central and south China (CSC). For this purpose, diffuse solar radiation and other meteorological data during 1966-2000 and 2001-2015 from 10 weather stations were used for model calibration and validation, respectively. The results show that the sunshine duration-based models performed similarly to the clearness index-correlated ones on a daily basis, while they were more accurate on a monthly mean daily basis. The diffuse fraction-correlated models performed similarly to models based on the diffuse coefficient in NWC and QTP but better in NEC, NC and CSC. Models with both sunshine duration and global solar radiation as input parameters outperformed single variable-based models. The incorporation of average temperature and relative humidity further improved the prediction accuracy of diffuse solar radiation. The best performing model differed among the five studied climatic regions and also at the two time scales. The proposed models VIII-3, IV-3, IV-3, IV-3 and the existing model III-2 were the best-performing models in NWC, QTP, NEC, NC and CSC on a daily basis, respectively. The best performance on a monthly mean daily basis was obtained by the proposed models IV-3, VIII-3, VII-7, VIII-5 and IV-5, respectively. The different categories of models reviewed and developed in the study can be of great interest to researchers and managers in the design and applications of solar thermal/photovoltaic systems.