Hybrid renewable energy systems: state of the art and trends

In the last decades, the energy consumption increases due to several factors such as the economic and population growth and the climate change. Both the rapid depletion of the fossil fuels and the rising concern on the environmental issues make the renewable energy sources (RESs), e.g. solar, wind, etc., a promising alternative with the aim to produce low-carbon heat and electricity. Within this energy transition scenario, several countries, by promoting new government policies, make efforts to encourage their diffusion. Such sources allow achieving multiple environmental and financial goals, e.g. the reduction of the greenhouse effect, lower costs, high efficiency and the opportunity to provide electricity to remote areas. For these reasons, the scientific literature focuses on supporting hybrid renewable energy systems (HRESs). Such systems integrate one or more RESs and a backup source, e.g. diesel, oil, natural gas, etc. or the electric grid, and they often include an energy storage (ES) system. HRESs join the strengths of each power source and they try to reduce the impact of their weaknesses. The aim of this paper is to revise the literature on HRESs for power generation, identifying the main technologies and facilities for both on-grid and offgrid applications, considering different operative and geo-climate scenarios and different users. The scope, i.e. research question, is to categorise the developed models and approaches by considering different KPIs identifying feasibility limits, best practices and suitable directions for future research.