Prediction of daily global solar radiation and air temperature using six machine learning algorithms; a case of 27 European countries

The prediction of global solar radiation in a region is of great importance as it provides investors and politicians with more detailed knowledge about the solar resource of that region, which can be very beneficial for largescale solar energy development.In this sense, the main objective of this study is to predict the daily global solar radiation data of 27 cities (Brussels, Paris, Lisbon, Madrid…), located in 27 countries, which have mostly different solar radiation distributions in Europe.In this research, six different machine-learning algorithms (Linear model (LM), Decision Tree (DT), Support Vector Machine (SVM), Deep Learning (DL), Random Forest (RF) and Gradient Boosted Trees (GBT)) are used.In the training of these algorithms, daily air temperature(Ta), wind speed(Va), relative humidity(RH) and solar radiation of these cities are used.The data is supplied from the Meteonorm tool and cover the last years grouped in two periods (1960-1990; 2000-2019).To decide on the success of these algorithms, four different statistical metrics (Average Relative Error (ARE), Average absolute Error (AAE), Root Mean Squared Error (RMSE), and R 2 (R-Squared)) are discussed in the study.In addition, the forecasting of air temperature and global solar radiation of these cities in 2050 and 2100 were made using three of the most recent Intergovernmental Panel on Climate Change (IPCC) scenarios (RCP2.6;RCP 4.5, and RCP 8.5).The results show that ARE, R, 2 and RMSE values of all algorithms are ranging from 0.114 to 6.321, from 0.382 to 0.985, from 0.145 to 2.126 MJ/m 2 , respectively.By analysing all the algorithms, it is noticed that the Decision tree exhibited the worst result in terms of R, 2 and RMSE metrics.Among the six prediction algorithms, the DL was recognized as the only algorithm that exceeded the t-critical value (The t-critical value is the cutoff between retaining or rejecting the null hypothesis).Globally, all the six machine learning algorithms used in this research can be applied to predict the daily global solar radiation data with good accuracy.Despite this, the SVM model is the best model among all the six models used.It is followed by the DL, LM, GB, RF and DT, respectively.