I. Introduction
In recent decades, a significant increase in energy demand has been observed worldwide. This phenomenon, in conjunction with the increase in the greenhouse effect generated by the use of fossil fuels, has prompted the energy market to move towards the search for and effective integration of new, clean and cheap energy sources and to adopt energy-saving policies. The technological revolution that is taking place has mainly focused on optimising the use of traditional energy sources [1] and developing renewable energy sources (photovoltaic, solar thermal, wind, biomass, etc.) [2]-[5].