You Can’t Even Break Even
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]).
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.
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.