Introduction to the 2nd Law of Thermodynamics: Heat Engines & Thermal Reservoirs

1. ChemE 260 Introduction to the2nd Law of ThermodynamicsHeat Engines & Thermal Reservoirs Dr. William Baratuci Senior Lecturer Chemical Engineering Department University of Washington TCD 6: A & BCB 5: 1 - 3 April 25, 2005

2. 1st Law & Spontaneity • 1st Law: Energy is neither created nor destroyed • Places no restriction on the direction that energy flows spontaneously • Imagine a cup of water rejecting 100 kJ to the surrounding air and freezing solid. • Imagine a cup of water absorbing 100 kJ from the surrounding air and boiling. • We need another law to help us understand why these things do not happen spontaneously. • Spontaneity • Unbalanced forces tend to drive the state of a system towards an equilibrium state • We can harness these unbalanced driving forces to do work for us. • The greater the unbalanced driving force, the greater the potential to do work. Baratuci ChemE 260 April 25, 2005

3. Hot Reservoir Cycle Cold Reservoir Thermal Reservoirs & Cycles • Thermal Reservoirs • Bodies than can exchange an infinite amount of heat, but the temperature of the thermal reservoir never changes. • Heat Sink: Reservoir that absorbs heat • Heat Source: Reservoir that puts out heat • Types of Thermodynamic Cycles • Power Cycle • Purpose: produce WS, Input: QH, Waste: QC • Refrigeration Cycle • Purpose: produce QC, Input: WS, Waste: QH • Heat Pump Cycle • Purpose: produce QH, Input: WS, Waste: QC Baratuci ChemE 260 April 25, 2005

4. Hot Reservoir QH HE WHE QC Cold Reservoir Power Cycles Produce Work • 1st Law: IN = OUT • Thermal Efficiency of a Power Cycle Baratuci ChemE 260 April 25, 2005

5. Pump Hot Reservoir QH Boiler Wturb 2 3 Wpump Turbine 1 4 Condenser QC Cold Reservoir Vapor Power Cycle Components • Note, in the tie-fighter sign convention : Subcooled Liquid at PHi Sat’d Vapor at PHi Sat’d Liquid at PLow Sat’d Mixture at PLow , high quality Baratuci ChemE 260 April 25, 2005

6. Hot Reservoir QH Ref WRef QC Cold Reservoir Refrigeration Cycles • 1st Law: IN = OUT • Coefficient of Performance of a Refrigeration Cycle Baratuci ChemE 260 April 25, 2005

7. Hot Reservoir Subcooled Liquid at PHi QH Superheated Vapor at PHi 4 3 Condenser WRef Expansion Valve Compressor Evaporator 1 2 Sat’d Mixture, at Plow , low quality Sat’d Vapor at PLow QC Cold Reservoir Vapor Refrigeration Cycle • The working fluid that flows through the four processes is called a refrigerant. • The turbine has been replaced by an expansion valve because • Expansion valves are less expensive • The turbine in a vapor refrigeration cycle produces very little work. Baratuci ChemE 260 April 25, 2005

8. Hot Reservoir QH HP WHP QC Cold Reservoir Heat Pump Cycles • 1st Law: IN = OUT • Coefficient of Performance of a Refrigeration Cycle Baratuci ChemE 260 April 25, 2005

9. Hot Reservoir Subcooled Liquid at PHi QH Superheated Vapor at PHi 4 3 Condenser WRef Expansion Valve Compressor Evaporator 1 2 Sat’d Mixture, at Plow , low quality Sat’d Vapor at PLow QC Cold Reservoir Heat Pump Cycle • The working fluid that flows through the four processes is called a refrigerant. • The turbine has been replaced by an expansion valve because • Expansion valves are less expensive • The turbine in a vapor heat pump cycle produces very little work. Baratuci ChemE 260 April 25, 2005

10. Next Class … • The 2nd Law of Thermodynamics • Clausius Statement of the 2nd Law • Kelvin-Planck Statement of the 2nd Law • Perpetual Motion Machines Baratuci ChemE 260 April 25, 2005

11. Example Problem • Home Heat Pump Performance • A heat pump provides 75 MJ / h to a house. If the compressor requires an electrical energy input of 4 kW, calculate the COP • If electricity costs $0.08 per kW-h and the heat pump operates 100 hours per month, how much money does the homeowner save by using the heat pump instead of an electrical resistance heater ? Baratuci ChemE 260 April 25, 2005