Solar energy is generated by harnessing the energy of the sun. Some solar systems simply harness or use the heat of the sun to heat water or dry clothes. Other solar systems use a more complex system composed of photovoltaic cells (PV), constructed using pieces of silicon doped with either phosphorous or boron, to convert the sun's energy to thermal or electric energy. PV energy generation is a zero-emissions process; it generates no pollution or noise. Since the sun is the source of energy, solar energy is a completely renewable and sustainable energy source. According to a report by the European Photovoltaic Industry Association, solar energy could provide more than a quarter of total projected global energy needs by 2040 if resources were provided to facilitate its production and use.