I. Introduction
Energy is the basis and driving force of sustainable economic and social development. In recent years, the energy crisis and climate warming have become global problems. As an inexhaustible, clean and environmentally friendly new energy source, solar energy is one of the most reliable alternatives to widely used fossil fuels. The solar photovoltaic sector is expected to have the largest installed capacity (8,519 GW) by 2050, generating about 25% of total electricity demand [1]. However, the power output of a solar PV system depends on the variation of solar irradiance, which is affected by a variety of conditions such as seasonality and geography [2]. Global horizontal irradiance (GHI) has a linear relationship with the power output of PV cells, therefore, accurate GHI forecasts are important for grid planning, operation and management.