FUEL CELLS—ENERGY FOR A POWER-HUNGRY WORLD - Chemistry: Notable Research and Discoveries - page 147

available energy is converted into work, or in other words, how little is
wasted. For example, an engine with 50 percent efficiency converts half
of the available energy into work.
Engineers can reduce some of the heat loss by careful design, but
can a heat engine ever be designed to prevent all heat loss? This is a
question addressed by thermodynamics, the study of heat and energy.
(he word thermodynamics comes from a Greek term, thermē, meaning
“heat.”) The answer, much to the dismay of engineers, is no. No heat
engine, no matter how it is designed or what kind of fuel it burns, can
ever be 100 percent efficient.
This limitation, imposed by a scientific law called the second law of
thermodynamics, can be difficult to understand. It involves a concept
known as entropy, which can be thought of as a measure of disorder. En-
tropy must increase in natural processes; in other words, processes natu-
rally go from order to disorder (as observed by anyone who has bought a
shiny new bicycle or automobile and watched it fade, corrode, break down,
and finally fall apart—usually just after the warranty expires). The second
law of thermodynamics requires a heat engine to vent some heat into the
environment, thereby raising entropy. This loss is unavoidable, and a heat
engine will not operate without it. No one will ever buy a car powered by a
gasoline engine that does not exhaust, and lose, some of its heat.
The good news for fuel cells is that they run on a different process.
Fuel cells are not exempt from scientific laws, but their manner of ener-
gy conversion is electrochemical rather than thermal. The maximum ef-
ficiency for the electrochemical processes in fuel cells is higher than for
the internal combustion engines that power many automobiles today.
But to reach these maximum efficiencies, researchers and engineers
must find the optimal design. Even the most modern internal combus-
tion engine does not operate at its highest possible efficiency—addi-
tional heat is lost through the parts of the engine, for example, which
is not required by thermodynamics but occurs because engineers can-
not find a suitable material that will prevent the loss. (Such a material
should be a thermal insulator, which would prevent heat from escap-
ing. The problem is that engines require material that is strong and able
to withstand high temperatures, and material having these properties
tend to be metals or alloys—which are thermal conductors, not insula-
tors.) Fuel cells also need careful design considerations or they will fail
to reach their optimal efficiency.