<< Chapter < Page Chapter >> Page >
  • Describe the use of heat engines in heat pumps and refrigerators.
  • Demonstrate how a heat pump works to warm an interior space.
  • Explain the differences between heat pumps and refrigerators.
  • Calculate a heat pump’s coefficient of performance.
Photograph of various expensive refrigerators displayed in a home appliance store.
Almost every home contains a refrigerator. Most people don’t realize they are also sharing their homes with a heat pump. (credit: Id1337x, Wikimedia Commons)

Heat pumps, air conditioners, and refrigerators utilize heat transfer from cold to hot. They are heat engines run backward. We say backward, rather than reverse, because except for Carnot engines, all heat engines, though they can be run backward, cannot truly be reversed. Heat transfer occurs from a cold reservoir Q c size 12{Q rSub { size 8{c} } } {} and into a hot one. This requires work input W size 12{W} {} , which is also converted to heat transfer. Thus the heat transfer to the hot reservoir is Q h = Q c + W size 12{Q rSub { size 8{h} } =Q rSub { size 8{c} } +W} {} . (Note that Q h size 12{Q rSub { size 8{h} } } {} , Q c size 12{Q rSub { size 8{c} } } {} , and W size 12{W} {} are positive, with their directions indicated on schematics rather than by sign.) A heat pump’s mission is for heat transfer Q h size 12{Q rSub { size 8{h} } } {} to occur into a warm environment, such as a home in the winter. The mission of air conditioners and refrigerators is for heat transfer Q c size 12{Q rSub { size 8{c} } } {} to occur from a cool environment, such as chilling a room or keeping food at lower temperatures than the environment. (Actually, a heat pump can be used both to heat and cool a space. It is essentially an air conditioner and a heating unit all in one. In this section we will concentrate on its heating mode.)

Part a of the figure shows a heat pump, drawn as a circle. Work W, indicated by a bold orange arrow, is put in to to the pump to transfer heat Q sub c, indicated by a bold orange arrow, out of a cold temperature reservoir T sub c, drawn as a blue rectangle, and pumps heat Q sub h, indicated by a larger bold orange arrow, into high temperature reservoir T sub h. Part b of the figure shows a P V diagram for a Carnot cycle. The pressure P is along the Y axis and the volume V is along the X axis. The graph shows a complete cycle A D C B A. The path begins at point A, then it drops sharply down and slightly to the right until point D. This is marked as an adiabatic expansion. Then the curve drops down more gradually, still to the right, from point D to point C. This is marked as an isotherm at temperature T sub c, during which heat Q sub c enters the system. The curve then rises from point C to point B along the direction opposite to that of A D. This is an adiabatic compression. The last part of the curve rises up from point B back to A. This is marked as an isotherm at temperature T sub h, during which heat Q sub h leaves the system. The path D C is lower than path B A. Heat entering and leaving the system is indicated by bold orange arrows, with Q sub h larger than Q sub c.
Heat pumps, air conditioners, and refrigerators are heat engines operated backward. The one shown here is based on a Carnot (reversible) engine. (a) Schematic diagram showing heat transfer from a cold reservoir to a warm reservoir with a heat pump. The directions of W size 12{W} {} , Q h size 12{Q rSub { size 8{h} } } {} , and Q c size 12{Q rSub { size 8{c} } } {} are opposite what they would be in a heat engine. (b) PV size 12{ ital "PV"} {} diagram for a Carnot cycle similar to that in [link] but reversed, following path ADCBA. The area inside the loop is negative, meaning there is a net work input. There is heat transfer Q c size 12{Q rSub { size 8{c} } } {} into the system from a cold reservoir along path DC, and heat transfer Q h size 12{Q rSub { size 8{h} } } {} out of the system into a hot reservoir along path BA.

Heat pumps

The great advantage of using a heat pump to keep your home warm, rather than just burning fuel, is that a heat pump supplies Q h = Q c + W size 12{Q rSub { size 8{h} } =Q rSub { size 8{c} } +W} {} . Heat transfer is from the outside air, even at a temperature below freezing, to the indoor space. You only pay for W size 12{W} {} , and you get an additional heat transfer of Q c size 12{Q rSub { size 8{c} } } {} from the outside at no cost; in many cases, at least twice as much energy is transferred to the heated space as is used to run the heat pump. When you burn fuel to keep warm, you pay for all of it. The disadvantage is that the work input (required by the second law of thermodynamics) is sometimes more expensive than simply burning fuel, especially if the work is done by electrical energy.

The basic components of a heat pump in its heating mode are shown in [link] . A working fluid such as a non-CFC refrigerant is used. In the outdoor coils (the evaporator), heat transfer Q c size 12{Q rSub { size 8{c} } } {} occurs to the working fluid from the cold outdoor air, turning it into a gas.

Questions & Answers

Calculate the work done by an 85.0-kg man who pushes a crate 4.00 m up along a ramp that makes an angle of 20.0º20.0º with the horizontal. (See [link] .) He exerts a force of 500 N on the crate parallel to the ramp and moves at a constant speed. Be certain to include the work he does on the crate an
Collins Reply
What is thermal heat all about
Abel Reply
why uniform circular motion is called a periodic motion?.
Boniface Reply
when a train start from A & it returns at same station A . what is its acceleration?
Mwdan Reply
what is distance of A to B of the stations and what is the time taken to reach B from A
the information provided is not enough
Hmmmm maybe the question is logical
where are the parameters for calculation
there is enough information to calculate an AVERAGE acceleration
mistake, there is enough information to calculate an average velocity
what is the unit of momentum
wha are the types of radioactivity ?
Worku Reply
what are the types of radioactivity
what is static friction
Golu Reply
It is the opposite of kinetic friction
static fiction is friction between two surfaces in contact an none of sliding over on another, while Kinetic friction is friction between sliding surfaces in contact.
I don't get it,if it's static then there will be no friction.
It means that static friction is that friction that most be overcome before a body can move
static friction is a force that keeps an object from moving, and it's the opposite of kinetic friction.
It is a force a body must overcome in order for the body to move.
If a particle accelerator explodes what happens
why we see the edge effect in case of the field lines of capacitor?
what is wave
Muhammed Reply
what is force
force is something which is responsible for the object to change its position
more technically it is the product of mass of an object and Acceleration produced in it
wave is disturbance in any medium
energy is distributed in any medium through particles of medium.
If a particle accelerator explodes what happens
Eboh Reply
we have to first figure out .... wats a particle accelerator first
What is surface tension
Subi Reply
The resistive force of surface.
Who can tutor me on simple harmonic motion
yusuf Reply
on both a string and peldulum?
Do you have a chit-chat contact
I dont have social media but i do have an email?
Which is
Where are you chatting from
I don't understand the basics of this group
teach him SHM init
Simple harmonic motion
how.an.equipotential.line is two dimension and equipotential surface is three dimension ?
syed Reply
definition of mass of conversion
umezurike Reply
Force equals mass time acceleration. Weight is a force and it can replace force in the equation. The acceleration would be gravity, which is an acceleration. To change from weight to mass divide by gravity (9.8 m/s^2).
how many subject is in physics
Adeshina Reply
the write question should be " How many Topics are in O- Level Physics, or other branches of physics.
how many topic are in physics
Praise what level are you
If u are doing a levels in your first year you do AS topics therefore you do 5 big topic i.e particles radiation, waves and optics, mechanics,materials, electricity. After that you do A level topics like Specific Harmonic motion circular motion astrophysics depends really
Yeah basics of physics prin8
Heat nd Co for a level
yh I need someone to explain something im tryna solve . I'll send the question if u down for it
Tamdy Reply
a ripple tank experiment a vibrating plane is used to generate wrinkles in the water .if the distance between two successive point is 3.5cm and the wave travel a distance of 31.5cm find the frequency of the vibration
please send the answer
the range of objects and phenomena studied in physics is
Bethel Reply
I don't know please give the answer
Practice Key Terms 2

Get the best College physics course in your pocket!

Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'College physics' conversation and receive update notifications?