Solar Cells
Solar cells convert light energy to electrical energy using photovoltaic
means. The basic cell consists of two layers of semiconductor material.
When light strikes the cell, many electrons gain enough energy to cross from
one layer to the other to create a dc voltage.
Solar energy has a number of practical applications. Figure 2–19, for
example, shows an array of solar panels supplying power to a commercial ac
network. In remote areas, solar panels are used to power communications
systems and irrigation pumps. In space, they are used to power satellites. In
everyday life, they are used to power hand-held calculators.
DC Generators
Direct current (dc) generators, which convert mechanical energy to electrical
energy, are another source of dc. They create voltage by means of a coil of
wire rotated through a magnetic field. Their principle of operation is similar
to that of ac generators (discussed in Chapter 15).