Any engineer is annoyed by having to throw energy away. Why is energy
ever wasted? The Second Law of Thermodynamics guides us here. Experience
has shown that heat cannot be transformed into another form of energy
with 100% efficiency. We can’t explain Nature’s idiosyncracies, but we
have to live with them. So, we accept the fact that every engine that
starts out with heat must ultimately waste some of that energy (Figure
3).
[Illustration: Figure 3 _A typical heat engine showing heat input,
useful power output, and the unavoidable waste heat that must be
rejected to the environment. A pressure-volume diagram is shown
underneath for a closed gas-turbine cycle. Circled numbers
correspond. The energy produced is represented by the shaded area.
Similar diagrams can be made for all heat engines as an aid in
studying their performance._]
A TYPICAL HEAT ENGINE
HEAT IN
HEAT SOURCE
REACTOR, BOILER
ELECTRICITY OUT
ENERGY CONVERTER
PUMP
FLUID PIPE
RADIATOR
WASTE HEAT OUT
PRESSURE-VOLUME DIAGRAM
HEAT IN
ENERGY OUT
GAS PRESSURE
WASTE HEAT OUT
GAS VOLUME
Direct conversion devices are no exception. Consequently, every
thermoelectric element or thermionic converter will have to provide for
the disposition of waste heat. The designer will try, however, to make
the engine efficiency high so that the waste heat will be small. Figure
4 shows the extensive waste heat radiator on a SNAP 50 power plant
planned for deep space missions.
[Illustration: Figure 4 _Model of SNAP 50 power plant planned for
deep space missions showing extensive waste heat radiator. The
system will provide 300 to 1000 kilowatts of electrical power._]