Over the centuries a wide array of devices and systems
has been developed for this purpose. Some of these
energy converters are quite simple. The early windmills,
for example, transformed the kinetic energy of wind into
mechanical energy for pumping water and grinding
grain. Other energy-conversion systems are decidedly
more complex, particularly those that take raw energy
from fossil fuels and nuclear fuels to generate electrical
power. Systems of this kind require multiple steps or
processes in which energy undergoes a whole series of
transformations through various intermediate forms.

Many of the energy converters widely used today
involve the transformation of thermal energy into
electrical energy. The efficiency of such systems is,
however, subject to fundamental limitations, as
dictated by the laws of thermodynamics and other
scientific principles. In recent years, considerable
attention has been devoted to certain direct energy-
conversion devices, notably solar cells and fuel cells,
that bypass the intermediate step of conversion to heat
energy in electrical power generation.