Refrigerators

1
Refrigerators

• Physics 313
• Professor Lee Carkner
• Lecture 15

2
Limits on Engines

• Engines convert heat into work and waste heat
• Experiments show that the waste heat cannot be
  reduced to zero
• Second Law of Thermodynamics
• It is impossible to convert heat completely into
  work
• An engine cannot have 100 efficiency

3
1st and 2nd Laws

• Converting heat completely into work does not
  violate the 1st law
• You cannot get more out than you put in
• The second law is an independent statement
• You cannot get as much out as you put in

4
Refrigerators

• An engine is a device that produces work by
  moving heat from high to low temperature
• A refrigerator is a device that uses work to move
  heat from low to high temperature

5
How a Refrigerator Works

• Fluid flows through the cold chamber and
  evaporates, adding heat QL to the fluid from the
  chamber
• The fluid is pumped into the hot chamber and
  compressed, adding work W
• The fluid condenses releasing heat QH
• QL W QH

6
Refrigerator Cycle
Compressor (work W)
Gas
QL
QH
High Pressure
Low Pressure
Heat removed from fridge by evaporation
Heat added to room by condensation
Liquid
Expansion Valve
7
Refrigerator Performance

• The equivalent of efficiency for a refrigerator
  is the coefficient of performance K
• K QL/W
• K QL/(QH-QL)
• Unlike efficiency, K can be greater than 1
• Real K for refrigerators 5
• Real K for air conditioners 2-3

8
Air Conditioner

• An air conditioner is a refrigerator that has
  your house as the cold chamber and the outdoors
  as the hot chamber
• Air conditioners also have condensers that dry
  out the air
• Dryer air feels more comfortable

9
Heat Pump

• The heat removed from the inside of a
  refrigerator is ejected into the kitchen
• Refrigerators heat the house
• A refrigerator that has the cold chamber as the
  outdoors and the hot chamber as the house is
  called a heat pump
• Many heat pumps can be reversed in summer to
  function as air conditioners

10
Refrigerators and the Second Law

• You cannot move heat from low to high temperature
  without the addition of work
• K cannot be infinite
• You need to do work on the coolant in order for
  it to release the heat

11
Statements of the Second Law

• Kelvin-Planck Statement
• It is impossible for any device which operates in
  a cycle to convert heat completely to work
• Clausius Statement
• It is impossible for any device that operates in
  a cycle to transfer heat from a low to high
  temperature reservoir with no other effect

12
Equivalence

• The Kelvin-Planck statement and the Clausius
  statement are equivalent
• One implies the other
• For example
• A 100 efficient engine connected to a high T
  reservoir powering a refrigerator cooling a low T
  reservoir to the same high T reservoir
• The violation of the Kelvin-Planck statement by
  the engine will cause a violation of the Clausius
  statement by the refrigerator

13
Engines and Refrigerators

• Efficiency
• h W/QH (QH-QL)/QH 1 - (QL/QH)
• Can rewrite using
• Q CDT
• Adiabatic relations from chapter 5
• Coefficient of performance
• K QL/W QL/(QH-QL)
• There are no perfect refrigerators or engines