1. LECTURES IN THERMODYNAMICS Claus Borgnakke CHAPTER 5
For the 8th Edition of:
Fundamentals of Thermodynamics
Claus Borgnakke, Richard Sonntag
John Wiley & Sons, 2013

2. Chapter 5 Heat Engines and Refrigerators
The Second Law of Thermodynamics
The Reversible and Irreversible Process
The Carnot Cycle
The Temperature Scales
The Ideal and Real Machines
Typical Devices and Applications

3. Heat Engines and Refrigerators
Historical development of the second law.
Possibility of running heat engines and refrigerators.
Cyclic Machines-Energy Conversion Devices
Heat engines:
To produce work from an energy input of heat.
Steam power plant
Engine (gasoline, diesel)
Gas Turbine
Heat Pumps:
To move energy as heat from lower T to higher T
Refrigerator/Freezer (purpose cooling)
Heat pump (purpose heating)
Air-conditioner (cooling or heating)

4. Heat Engines
Work is net WHE = WT - WP

5. Heat Engines

6. The Refrigerator

7. The Refrigerator

8. The Second Law of Thermodynamics
The statements of Kelvin-Planck and Clausius
A heat engine cannot convert QH to W 100% Heat cannot be moved from TL to TH with W = 0
Conclusion: Conclusion:
There must be a QL rejected so There must be some Win > 0 so
W = QH – QL QH = Win + QL
η = W / QH < β = QH /Win < ∞
Notice energy equation does not say these are impossible

9. The Equivalence of Kelvin-Planck to Clausius
Assume a refrigerator can run with W = 0 which is a violation of Clausius, then run a heat engine to have the same QL. The combination shown as a CV is a heat engine exchanging heat with only one reservoir (QL net = 0) and some work output, which is a violation of Kelvin Planck.

10. Perpetual Motion Machines
A perpetual-motion machine of the first kind:
Creates work from nothing or creates energy, violating the energy equation (1st Law)
A perpetual-motion machine of the second kind:
The machine makes an energy conversion that violates the second law.
A perpetual-motion machine of the third kind:
The machine has no friction and produces no work. (Example: Vibrations in a molecule)
A perpetual-motion machine of the second kind:

11. The Second law of Thermodynamics

12. The Reversible and Irreversible Processes
A reversible process:
A process can take place and then be reversed to return the system to its original state and leave the surroundings unchanged. The latter is the most crucial point.
A process can happen and be reversed as
shown. However the surroundings in this case
must supply the work and absorb the heat
transfer. Those two terms are not equivalent.
A near reversible process can take place as
shown. Typically this places certain restrictions
on the process that may be undesirable in a
practical application.

13. Effects Making Processes Irreversible
We illustrate what a reversible process is by showing processes that are not reversible. These are called irreversible processes and common in most practical processes.
Friction:
Sliding something against a surface requires a work input to overcome friction. Work is converted to internal energy leading to a warmer surface.
Example: brake pads on a car.
In-elastic deformation:
Bending a piece of metal giving a permanent deformation turns work into internal energy. This is internal friction called plastic deformation (unlike a spring action which is elastic, the work input is stored as potential energy in the spring).
Current through an Ohmic resistor:
A current through a resistance dissipates power which heats the resistor. You cannot recover the electrical energy 100%.

14. Effects Making Processes Irreversible
We illustrate what a reversible process is by showing processes that are not reversible.
These are called irreversible processes and common in most practical processes.
Unrestrained Expansion:
A process can happen and be reversed as shown. However the surroundings in this case must supply the work and absorb the heat transfer. Those two terms are not equivalent. Flow through a valve is similar, there a pressure is “lost”.
Heat Transfer over a finite temperature difference:
Heat transferred from a higher T to a lower T domain cannot be brought back to the higher T domain without work input (a heat pump). In the limit of a differential “dT” the heat transfer approaches a reversible process. This becomes a very slow process, recall
𝑄 = CA ∆T
Mixing:
After mixing two gases a complicated process with work input is required to separate the gases again.

15. The Second Law of Thermodynamics

16. The Carnot Cycle Heat engine in reverse is a refrigerator,
Q’s and W’s change sign.

17. Two Propositions Regarding Carnot Cycle Efficiency

18. The Thermodynamic Temperature Scale

19. The Ideal Versus Real Machines
TL = 293 K
Temperatures and efficiencies are approximate numbers. If you look at combined cycles (co-generation of heat and work) efficiency can be 60%.
The nuclear power plant has low efficiency due to low high T for safety, it could be improved if a natural gas burner is added to superheat steam.

20. The Ideal Versus Real Machines
βHP
βref
Ref: TL
HP: TH
TL = 293 K
TH = 293 K
Notice the T’s are selected from what the refrigeration cycle typically will have to be at not the reservoir T’s (otherwise T high = T room or T atm)

21. The Ideal Versus Real Machines

22. The Ideal Versus Real Machines
A/C in cooling mode

23. Real Heat Exchangers
The sign of ∆T depend on the direction of the heat transfer. This gives the heat engine less ∆T for its cycle and it requires a larger ∆T for the heat pump cycle making it harder (the compressor must push to higher T).

24. The Ideal Versus Real Machines

25. The Ideal Versus Real Machines

26. Processes limited by first law (energy eq.)

27. Processes limited by second law

28. Some Historical Event Related to Thermodynamics