ChemE 260 Introduction to the 2nd Law of Thermodynamics Heat Engines

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ChemE 260 Introduction to the 2nd 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
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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
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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
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Power Cycles Produce Work

• 1st Law IN OUT
• Thermal Efficiency of a Power Cycle

Baratuci ChemE 260 April 25, 2005
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Vapor Power Cycle Components

• Note, in the tie-fighter sign convention

Subcooled Liquid at PHi
Satd Vapor at PHi
Satd Liquid at PLow
Satd Mixture at PLow , high quality
Baratuci ChemE 260 April 25, 2005
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Refrigeration Cycles

• 1st Law IN OUT
• Coefficient of Performance of a Refrigeration
  Cycle

Baratuci ChemE 260 April 25, 2005
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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• The turbine has been replaced by an expansion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
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Heat Pump Cycles

• 1st Law IN OUT
• Coefficient of Performance of a Refrigeration
  Cycle

Baratuci ChemE 260 April 25, 2005
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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
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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
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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