Refrigerators

1
Refrigerators
2
Reversing the Flow

• Heat flow through an engine was used to generate
  work.
• Work can also be used to create a net flow of
  heat.
• That flow can reverse the trend of the second law.

An engine takes a temperature difference to
create work.
A refrigerator or heat pump takes work to push
heat against the temperature difference.
3
Refrigerator

• Thermodynamically a refrigerator pumps heat from
  a small cold region into a large hot region.
• The work is supplied by a motor.
• Air conditioners are a type of refrigerator.

4
Refrigeration

• The motor forces gas at high pressure into a
  condenser
• The condenser allows the gas to cool exhaust heat
  to the outside.
• The cool gas goes into the unit and draws heat
  from the inside.

5
Performance

• The second law prohibits a perfect refrigerator.
• A coefficient of performance (Kr) compares the
  work to the low temperature side.

6
Heat Pump

• A heat pump is similar to a refrigerator.
• Work is used to move heat from a low temperature
  to a high temperature.
• A heat pump has the small volume at high
  temperature.
• Refrigerator is the opposite
• Heat pumps can become air conditioners in the
  summer.

7
Heat Pump Performance

• Heat pumps deliver heat to the high temperature
• Refrigerators took heat from the low temperature.
• A coefficient of performance (Kp) is different
  for heat pumps.
• Compare the high temperature side to the work.

8
Comfort Zone

• A heat pump has Kp 3.0 and is rated to do work
  at 1500 W.
• How much heat can it add to a room per second?
• What is its Kr as an air conditioner in the
  summer?

• Find the heat directly from the definition.
• QH Kp W
• QH 3.0 (1500 W)(1 s) 4500 J
• Use conservation to get QL.
• QL QH - W
• QL 3000 J
• Kr QL / W 2.0

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