A comprehensive study of renewable energy sources: Classifications, challenges and suggestions

Renewable energy (RE) is the key element of sustainable, environmentally friendly, and cost-effective electricity generation. An official report by International Energy Agency (IEA) states that the demand on fossil fuel usage to generate electricity has started to decrease since year 2019, along with the rise of RE usage to supply global energy demands. Researches on RE technologies are continuously growing in order to enhance the performance of RE generation, especially in term of energy conversion efficiency. The aim of this review paper is to understand and study further the current RE technologies such as solar energy, hydro energy, wind energy, bioenergy, geothermal energy, and hydrogen energy. Several hybrid RE technologies have been also studied and compared, to improve the overall performance of RE in generating electricity. Lastly, suggestions are provided for the purpose to solve and overcome the challenges and limitations of RE technologies in terms of economy, technical, and energy conversion efficiency.