Transcript ChemE 260 - Thermodynamics

ChemE 260
Introduction to the 2nd Law of Thermodynamics
Heat Engines & Thermal Reservoirs
Dr. William Baratuci
Senior Lecturer
Chemical Engineering Department
University of Washington
TCD 6: A & B
CB 5: 1 - 3
April 25, 2005
st
1
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
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
Baratuci
ChemE 260
April 25, 2005
• Purpose: produce QH, Input: WS, Waste: QC
Hot Reservoir
Cycle
Cold Reservoir
Power Cycles Produce Work
• 1st Law:
IN = OUT
QH  QC  WHE
Hot Reservoir
QH
• Thermal Efficiency of a Power Cycle
HE
QC
Desired Result WHE


Required Input Q H
Q H  QC

QH
Baratuci
ChemE 260
April 25, 2005
WHE
Cold Reservoir
QC
  1
QH
Vapor Power Cycle Components
• Note, in the tie-fighter
sign convention :
WHE  Wturb  Wpump
Hot Reservoir
Subcooled
Liquid at PHi
Wpump
2
Sat’d Vapor
at PHi
QH
Boiler
Wturb
3
Turbine
Pump
1
4
Sat’d Liquid
at PLow
Baratuci
ChemE 260
April 25, 2005
Condenser
QC
Cold Reservoir
Sat’d Mixture
at PLow , high quality
Refrigeration Cycles
• 1st Law:
IN = OUT
Hot Reservoir
QH
QH  QC  WRef
Ref
• Coefficient of Performance of a
Refrigeration Cycle
 Re f
QC
Desired Result
 COPR 

Required Input WRe f
 Re f
QC
 COPR 
Q H  QC
Baratuci
ChemE 260
April 25, 2005
WRef
QC
Cold Reservoir
 Re f  COPR 
1
QH
1
QC
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• The turbine has been
replaced by an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
Hot Reservoir
Subcooled
Liquid at PHi
4
QH
3
Superheated
Vapor at PHi
Condenser
Expansion
Valve
WRef
Compressor
1
Sat’d Mixture, at
Plow , low quality
Evaporator
QC
Cold Reservoir
2
Sat’d Vapor
at PLow
Heat Pump Cycles
• 1st Law:
IN = OUT
Hot Reservoir
QH
QH  QC  WHP
HP
• Coefficient of Performance of a
Refrigeration Cycle
 HP  COPHP
QH
Desired Result


Required Input WHP
 HP  COPHP
QH

Q H  QC
Baratuci
ChemE 260
April 25, 2005
QC
WHP
Cold Reservoir
 HP  COPHP
1

QC
1
QH
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
Hot Reservoir
Subcooled
Liquid at PHi
4
QH
3
Superheated
Vapor at PHi
Condenser
Expansion
Valve
WRef
Compressor
1
Sat’d Mixture, at
Plow , low quality
Evaporator
QC
Cold Reservoir
2
Sat’d Vapor
at PLow
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
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