Renewable energy is the collective name for a number of energy resources
available to man on Earth. Their conversion has always played an important
role for the inhabitants of the planet, and apart from a period of negligible
length – relative to evolutionary and historical time scales – the renewable
energy sources have been the only ones accessible to mankind.
Yet the study of renewable energy resources, their origin and conversion,
may at present be characterised as an emerging science. During the past fifty
years of scientific and technological revolution, much more effort has been
devoted to the extraction and utilisation of non-renewable energy resources
(fuels), than to the renewable ones. Only very recently have funds been
made available to re-establish renewable energy research and development,
and it is still unclear whether the technologies based on renewable energy
sources will become able to constitute the backbone of future energy supply
systems.