More physics of the home: Heat and Air Conditioning

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More physics of the home Heatand Air
Conditioning

• Physics of Modern Devices
• February 9, 2009

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U.S. Energy consumption

• Approximately 1/4 of the energy used in this
  country goes into heating and cooling of
  buildings.
• In the residential sector, an average of 50 of
  this energy is used for heating/cooling homes.

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Heat

• Heat flows from a hotter object to a colder
  object.
• The rules governing the movement of heat
• THERMODYNAMICS

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Observations about Air Conditioners

• They cool room air on hot days
• They emit hot air from their outside vents
• They consume lots of electric power
• They are less efficient on hotter days
• Some can be reversed so that they heat houses

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5 Questions about Air Conditioners

• Why doesnt heat flow from cold to hot?
• Why does an air conditioner need electricity?
• How does an air conditioner cool room air?
• What role does the electricity play?
• How does an air conditioner heat outdoor air?

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Question 1

• Why doesnt heat flow from cold to hot?
• Does such heat flow violate the laws of motion?
• Does such heat flow violate some other laws?

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Laws Governing Heat Flow

• The four laws of thermodynamics
• are the rules governing thermal energy flow
• and establish the relationships between
• disordered (thermal) energy and ordered energy
• heat and work

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0th Law of Thermodynamics

• The law about thermal equilibrium
• If two objects are in thermal equilibrium with a
  third object, then they are in thermal
  equilibrium with each other.

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1st Law of Thermodynamics

• The law about conservation of energy
• Change in internal energy equals heat in minus
  work out
• where
• Internal energy (?U) thermal stored energies
• Heat in (Q) heat transferred into object
• Work out (W) external work done by object

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Order versus Disorder

• Converting ordered energy into thermal energy
• involves events that are likely to occur
• is easy to accomplish and often happens
• Converting thermal energy into ordered energy
• involves events that are unlikely to occur
• is hard to accomplish and effectively never
  happens
• Statistically, ordered always becomes disordered

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Entropy

• Entropy is the measure of an objects disorder
• Includes both thermal and structural disorders
• An isolated systems entropy never decreases
• But entropy can move or be transferred

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2nd Law of Thermodynamics

• The law about disorder (entropy)
• Entropy of a thermally isolated system never
  decreases

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3rd Law of Thermodynamics

• The law about entropy and temperature
• An objects entropy approaches zero as its
  temperature approaches absolute zero

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• "Now, in the second law of thermodynamics..."

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More on the 2nd Law

• According to the 2nd Law
• Entropy of a thermally isolated system cant
  decrease
• But entropy can be redistributed within the
  system
• Part of the system can become hotter while
  another part becomes colder!
• Exporting entropy is like throwing out trash!

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Natural Heat Flow

• One unit of thermal energy is more disordering to
  a cold object than to a hot object
• When heat flows from hot object to cold object,
• the hot objects entropy decreases
• and the cold objects entropy increases,
• so the overall entropy of the system increases
• and total energy is conserved
• Laws of motion and thermodynamics satisfied

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Unnatural Heat Flow

• When heat flows from cold object to hot object,
• the cold objects entropy decreases,
• and the hot objects entropy increases
• so the overall entropy of the system decreases
• although total energy is conserved
• The 2nd law of thermodynamics is violated
• To save 2nd law, we need more entropy!
• Ordered energy must become disordered energy!

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Demonstration

• Thermo-electric converter
• Two small electric motors are connected to a
  device which takes a thermal difference and
  converts it into a voltage.
• Transfers heat between two metals using
  electricity.
• Work depends on reservoirs of different
  temperature.

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Another Demonstration

• Stirling Engine http//www.stirlingengine.com/faq
  
• Gas expands when heated, and contracts when
  cooled. Stirling engines move the gas from the
  hot side of the engine, where it expands, to the
  cold side, where it contracts.
• When there is a temperature difference between
  upper displacer space and lower displacer space,
  the engine pressure is changed by the movement of
  the displacer.
• The pressure increases when the displacer is
  located in the upper part of the cylinder (and
  most of the air is on the hot lower side).
• The pressure decreases when the displacer is
  moved to the lower part of the cylinder.
• The displacer only moves the air back and forth
  from the hot side to the cold side.

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Refrigerator / Heat Pump

• A refrigerator (heat pump)
• Cyclically removes heat from a cold system and,
  by doing work, delivers it to a warmer one.
• Carnot cycle ideal (reversible) heat engine

Work consumed
Heat removed from cold object
Heat added to hot object
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A problem

• A steam engine takes steam from the boiler at
  200o C and exhausts directly into the air at
  100oC. What is its maximum possible efficiency?
• Actual efficiencies are lower because energy is
  lost to friction, turbulence, etc.
• For example, theoretical efficiencies for an
  ordinary automobile are 56 but practical
  considerations reduce this to 25.

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Another problem

• A heat pump can heat a house by drawing heat from
  the outside, doing some work, and discharging
  heat inside the house. The outside temperature
  is -10oC and the interior is kept at 22oC. It is
  necessary to deliver heat to the interior at the
  rate of 16kW to make up for the normal heat
  losses. At what minimum rate must energy be
  supplied to the heat pump?

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Answer
By using the heat pump as a refrigerator to cool
the outdoors, you can deliver 16kW to the inside
of the house but only pay for 1.7kW to run the
pump! This is a thermodynamic bargain!
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Question 2

• Why does an air conditioner need electricity?

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Heat Machines

• Air conditioners
• use work to transfer heat from cold to hot
• are a type of heat pump
• Automobiles
• use flow of heat from hot to cold to do work
• are a type of heat engine

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Air conditioners (Part 1)

• An air conditioner
• moves heat from cold room air to hot outside air
• moves heat against its natural flow
• must convert ordered energy into disordered
  energy
• so as not to decrease the worlds total entropy!
• uses a working fluid to transfer heat
• This fluid absorbs heat from cool room air
• This fluid releases heat to warm outside air

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Air conditioners (Part 2)

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

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Question 3

• How does an air conditioner cool room air?

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The Evaporator (Part 1)

• The evaporator is a long, wide metal pipe
• pipe is heat exchanger between air and working
  fluid
• The working fluid
• arrives as a high pressure, room temperature
  liquid
• but loses pressure passing through a constriction
• and enters the evaporator as a low pressure
  liquid
• Loss of pressure destabilizes the liquid phase
• The liquid working fluid begins to evaporate!

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The Evaporator (Part 2)

• Working fluid evaporates in the evaporator
• Fluid absorbs thermal energy while evaporating,
• so it transforms into a cold gas
• Heat flows from the hot room air to the cold gas
• Working fluid leaves the evaporator
• as a low density gas near room temperature
• and carries away some of the rooms thermal
  energy
• Heat has left the room!

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Question 4

• What role does the electricity play?

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The Compressor

• The compressor increases density of a gas
• Working fluid
• arrives as a low density gas near room
  temperature,
• has work done on it by the compressor,
• and experiences a rise in temperature as a
  result.
• Working fluid leaves the compressor
• as a hot, high density gas
• and carries away electric energy as thermal
  energy
• Ordered energy has become disordered energy!

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Question 5

• How does an air conditioner heat outdoor air?

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The Condenser (Part 1)

• The condenser is a long, narrow metal pipe
• pipe is heat exchanger between air and working
  fluid
• The working fluid
• arrives as a hot, high density gas
• but begins to lose heat to the cooler outdoor air
• Loss of heat destabilizes the gaseous phase
• The gaseous working fluid begins to condense!

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The Condenser (Part 2)

• Working fluid condenses in the condenser
• Fluid releases thermal energy while condensing,
• so it transforms into a hot liquid
• and even more heat flows from fluid into outside
  air
• Working fluid leaves the condenser
• as high-pressure room-temperature liquid
• having released some of the rooms thermal energy
• Heat has reached the outside air!

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Air Conditioner Overview

• Evaporator located in room air
• transfers heat from room air to working fluid
• Compressor located in outside air
• does work on fluid, so working fluid gets hotter
• Condenser located in outside air
• transfers heat from working fluid to outside air,
• including thermal energy extracted from inside
  air
• and thermal energy added by compressor

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BTUs

• Most air conditioners have their capacity rated
  in British thermal units (BTU). Generally
  speaking, a BTU is the amount of heat required to
  raise the temperature of one pound (0.45 kg) of
  water 1 degree Fahrenheit (0.56 degrees Celsius).
  Specifically, 1 BTU equals 1,055 joules. In
  heating and cooling terms, 1 "ton" equals 12,000
  BTU.
• A typical window air conditioner might be rated
  at 10,000 BTU. For comparison, a typical
  2,000-square-foot (185.8 m2) house might have a
  5-ton (60,000-BTU) air conditioning system,
  implying that you might need perhaps 30 BTU per
  square foot.

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EER

• The energy efficiency rating (EER) of an air
  conditioner is its BTU rating over its wattage.
• For example, if a 10,000-BTU air conditioner
  consumes 1,200 watts, its EER is 8.3 (10,000
  BTU/1,200 watts).
• Obviously, you would like the EER to be as high
  as possible, but normally a higher EER is
  accompanied by a higher price.

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A problem

• How much heat is required to raise the
  temperature of 4 gal of water from 60F to 100F?
  Answer in BTUs.

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Another problem

• Let's say that you have a choice between two
  10,000-BTU units. One has an EER of 8.3 and
  consumes 1,200 watts, and the other has an EER of
  10 and consumes 1,000 watts. Let's also say that
  the price difference is 100. What is the
  payback for the more expensive unit?
• Let's say that you plan to use the air
  conditioner in the summer (four months a year)
  and it will be operating about six hours a day.
  Let's also imagine that the cost in your area is
  0.10/kWh.

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Answer
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Summary aboutHeat and Air Conditioners

• They pump heat from cold to hot
• They dont violate thermodynamics
• They convert ordered energy to thermal energy
• HW3 due Monday, Feb. 16 (Airplanes)
• Next time solar power/energy