Heat pumps

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Heat pumps
Air conditioners Heat pumps Poker odds
Rosencrantz I say - look at this!
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Mechanisms for passive heat transfer
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Poker entropy
Hand w microstates ln w Royal
Flush 4 1.39 Straight Flush 36 3.58 Four of
a Kind 624 6.44 Full House 3,744 8.23 Flush
5,108 8.54 Straight 10,200 9.23 3 of a
kind 54,912 10.91 2 pairs 123,552 11.72 1
pair 1,098,240 13.91 no pairs 1,302,540 14.08
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Active heat transfer

• Can use heat flow to get work done
• and vice versa
• Heat engine creates
• work from heat flow
• Heat pump uses
• work to move heat

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Why heat goes where it goes
DS must be positive!
Hot
The change in entropy
DSt DSC DSH
YES
Cold
If QC QH then DS gt 0!
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On the other hand
Hot
QH
DSt DSC DSH
NO
QC
If QC QH then DS lt 0
Cold
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Otto or four-stroke cycle named for inventor
Nikolaus Otto.
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Varoomingness
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(No Transcript)
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Heat pumps ACs

• Evaporator located in room air
• transfers heat from room air to fluid
• Condenser located in outside air
• transfers heat from fluid to outside air
• Compressor located in outside air
• does work on fluid and creates entropy

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Evaporator
Heat exchanger made from a long metal
pipe
Fluid approaches evaporator as a high pressure
liquid near room temperature
A constriction reduces the fluids pressure
Fluid enters evaporator as a low pressure liquid
near room temperature
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Evaporator, pt II
Fluid evaporates at low pressure
Absorbs energy like sweat dooes on a hot day in
front of a fan breaking bonds takes energy from
surroundings
So fluid cools, and evaporator pipe cools and air
around it cools
Cold fluid leaves the cold side and heat is
removed from that side of the system
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Compressor
Piston compresses fluid
Piston does work so heat is added
As in Otto cycle, compressed gas is hotter
Hot gas is now at high pressure
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Condenser
Fluid condenses at heat flows out of it into
surroundings
Gas is hot and condensed and as it cools energy
is lost so bonds can re-form
So fluid condenses back to liquid state
KE that was in gas molecules is now thermal
energy in the surroundings so are warmed.
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Heat pump or AC?
Heat Pump

• Heat pump has cold part outside
• and warm part inside
• AC has cold part inside
• and warm part outside
• Else they are the same
• Many central heating homes have whole unit
  outside and
• insulated airways/fluid pipes to transfer heat
  in both directions

Cold outside
Warm Inside
Air Conditioner
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Dont turn that key!

• Car engines especially are the cause of CO, CO2,
  NO, etc gases
• These in turn are linked to temperature
  variations called ice ages in the past
• Perhaps worse in the
• futures

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Dont touch that key
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Take home messages

• Energy flow is constrained by conservation of
  energy
• (i.e. The 1st Law)
• Energy flow costs energy and increases entropy
• (The 2nd Law)