1. ENGR 2213 Thermodynamics F. C. Lai School of Aerospace and Mechanical Engineering University of Oklahoma

2. Lesson from First Law

3. Second Law of Thermodynamics The first law of thermodynamics requires that energy be conserved during a process, but place no restriction on the direction of a process. Soda 4 ºC Q Q Satisfying the first law does not guarantee that a process will actually occur.

4. Second Law of Thermodynamics The inadequacy of the first law to identify whether a process can take place is remedied by introducing the second law of thermodynamics. A process will not occur unless it satisfies both the first and second laws of thermodynamics. The second law asserts that 1. Processes occur in a certain direction. 2. Energy has quality as well as quantity.

5. Second Law of Thermodynamics A hypothetical body with a relatively large thermal energy capacity that can supply or absorb finite amount of energy as heat without undergoing any change in temperature. Source A reservoir that supplies energy in the form of heat Thermal Energy Reservoir Sink A reservoir that absorbs energy in the form of heat

6. Second Law of Thermodynamics Devices that are used to convert heat to work. Characteristics of Heat Engines 1. They receive heat from a high-temperature source. Heat Engines 2. They convert part of this heat to work. 3. They reject the remaining waste heat to a low- temperature sink. 4. They operate on a cycle.

7. Second Law of Thermodynamics High-temperature Reservoir at T H Low-temperature Reservoir at T L QHQH QLQL W HE W = Q H – Q L

8. Second Law of Thermodynamics Thermal Efficiency Performance < 100 % Automobile Engine 20% Diesel Engine 30% Gas Turbine 30% Steam Power Plant 40%

9. Example 1 Heat is transferred to a heat engine from a furnace at a rate of 80 MW. If the rate of waste heat rejection to a nearby river is 50 MW, determine the net power output and the thermal efficiency.

10. Second Law of Thermodynamics It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce an equivalent amount of work. No heat engine can have a thermal efficiency of 100% Kelvin-Planck Statement The impossibility of having 100% efficiency heat engine is not due to friction or other dissipative effects.

11. Second Law of Thermodynamics Devices that are used to transfer heat from low- temperature medium to high-temperature one. Refrigerators/Heat Pumps Like heat engines, they are cyclic devices. Refrigerators and heat pumps operate on the same cycle but differ in their objectives. Refrigerators – maintain the refrigerated space at a low temperature. Heat pumps – maintain the heated space at a high temperature.

12. Second Law of Thermodynamics High-temperature Reservoir at T H Low-temperature Reservoir at T L QHQH QLQL W Ref Q L = Q H - W Objective

13. Second Law of Thermodynamics High-temperature Reservoir at T H Low-temperature Reservoir at T L QHQH QLQL W HP Q H = W + Q L Objective

14. Second Law of Thermodynamics Coefficient of Performance (COP) Performance Refrigerators > 1Heat Pumps

15. Second Law of Thermodynamics Energy Efficient Rating (EER) Air-conditioners The amount of heat removed from the cooled space in BTU’s for 1 Watt-hour of electricity consumed. 1 Wh = 3.412 BTU EER = 3.412 COP R Most air conditioners have an EER between 8 and 12.

16. Example 2 The food compartment of a refrigerator is maintained at 4 ºC by removing heat from it at a rate of 360 kJ/min. If the required power input is 2 kW, determine the COP and the rate of heat discharged.

17. Second Law of Thermodynamics It is impossible to construct a device that operates on a cycle and produce no effect other than the transfer of heat from a low-temperature body to a high-temperature body. Clausius Statement Equivalence of the two statements A violation of one statement leads to the violation of the other statement.

18. Equivalence of the Two Statements High-temperature Reservoir at T H Low-temperature Reservoir at T L Q H + Q L QLQL W = Q H RefHE QHQH Net Q IN = Q L Net Q OUT = Q L HE + Ref

19. Equivalence of the Two Statements High-temperature Reservoir at T H Low-temperature Reservoir at T L QHQH QLQL QLQL HERef QLQL Net Q IN = Q H - Q L HE + Ref W = Q H – Q L Net W = Q H - Q L