This first chapter attempts to provide answers to basic questions on the nature of photovoltaic (PV) energy; on what it produces, at what cost and for what applications; and on the type of equipment needed in particular cases. All these points are dealt with in greater depth in the chapters that follow. In more general terms, we will consider the role of PV energy in sustainable development, for today this is a major challenge that can affect many of our choices.

In Chapter 2, certain PV materials such as crystalline and amorphous silicon have been described. Other materials apart from silicon can, however, be used. We consider here their nature, properties and potential uses. There are two major families of PV materials: Solid crystalline materials, Solid thin films (on a substrate). We also cover other technologies, in particular Gratzel-type dye-sensitised solar cells, polymer solar cells, cells containing small organic molecules and photoelectric chemical sensors. These technologies, while promising in terms of cost and adapt ability, have yet to show their competitiveness against the traditional technologies. Most have stability problems and have low efficiency (<5%). This is why, despite much promise, they have only had a few recent commercial applications.ι The material most widely used in solar PV is silicon, a type ΓV semiconductor. It is tetravalent, meaning one atom of silicon can bind with four other atoms of the same type. Solar silicon is either crystalline or amorphous. In the amorphous state, it is used as a thin film with thicknesses of around 1 μm and above, deposited on a backing, while crystalline cells are solid and around 0.1-0.2 mm thick. Other semiconductors used are types III-V like gallium arsenide (rare and expensive) and thin films like CdTe (cadmium telluride) and CIS (copper indium selenide).

Stand-alone PV generators are the oldest historically. They first appeared in the 1970s as solar panels mounted on satellites. These were soon followed by applications for rural pumping and electrification in developing countries, then domestic and technical stand-alone applications in industrialised countries. The reader will perhaps be surprised by the size of this chapter, which is disproportionate to the share of stand-alone in PV systems today. The length is justified by the fact that the stand-alone solar system is not just a PV generator but also includes energy storage, regulators and appliances specially developed for this purpose. Designing a stand-alone system often requires reviewing energy consumption and optimising it so as to only produce the essential energy necessary for the function to be supplied. In addition, the applications described in this chapter are highly diversified: stand-alone energy generation is attractive in a wide variety of circumstances, which is best to identify and deal with on a case-by-case basis.