Green Chemistry and the Ten Commandments

a steam turbine as shown in Figure 6.7. If the inlet temperature is 850 K and the outlet
temperature is 330 K, substitution into the Carnot equation gives a maximum theoretical
efficiency of 61%. An inability to introduce all the steam at the highest temperature
combined with friction losses of energy reduce the energy conversion efficiency of most
modern steam turbines to just below 50%. Since only about 80% of the chemical energy
used to raise steam by combustion of fossil fuel in a boiler is actually transferred to water
to produce steam, the net efficiency for conversion of chemical energy in fossil fuels to
mechanical energy to produce electricity is about 40%. Fortunately, essentially all the
mechanical energy in a rotating turbine can be converted to electricity in the generator
to which it is connected, so the overall efficiency of conversion of fossil fuel chemical
energy to electricity is about 40%. The conversion of nuclear energy to mechanical energy
in a reactor-powered steam turbine is only about 30% because reactor peak temperatures
are limited for safety reasons.

Superheated

steam in, T 1

Cooler steam

out, T 2

Turbine vanes

Rotating shaft connected

to electrical generator

Figure 6.7. A steam turbine in which superheated steam drives vanes attached to a shaft, thus converting
heat energy in steam to mechanical energy. The rapidly rotating shaft is usually attached to an electrical
generator to generate electricity.

Another example of the application of the Carnot equation is provided by the internal
combustion piston engine shown in Figure 6.8. A complete cycle of this engine consists
of two downward and two upward strokes of the piston. As the piston moves downward
during the first stroke, air or an air/fuel mixture is drawn into the cylinder through the
open intake valve. This gas is then compressed to a very small volume by the upward
movement of the piston during the next compression stroke, during which both valves
are closed. Near the top of the compression stroke, fuel is injected, if it were not taken
in as a mixture with air by the intake stroke. The fuel/air mixture is ignited at the end
of the compression stroke, and the sudden formation of very hot, high-pressure exhaust
gas drives the piston downward in the power stroke, which is the one shown in Figure
6.8. Next, the exhaust valve is opened, and the exhaust gases are expelled as the piston
moves upward, at which point the entire 4-step cycle is ready to start again. An amazing
number of these cycles occur each second for each piston in an engine. If you have a
tachometer on your car you may notice that it registers around 3000 rpm (revolutions per
minute) at cruising speed. That means that each piston performs an up and down motion
50 times each second and a total of 25 complete cycles each second!

146 Green Chemistry, 2nd ed