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Air conditioners and refrigerators

Air conditioners and refrigerators are designed to cool something down in a warm environment. As with heat pumps, work input is required for heat transfer from cold to hot, and this is expensive. The quality of air conditioners and refrigerators is judged by how much heat transfer Q c size 12{Q rSub { size 8{c} } } {} occurs from a cold environment compared with how much work input W size 12{W} {} is required. What is considered the benefit in a heat pump is considered waste heat in a refrigerator. We thus define the coefficient of performance     (COP ref ) size 12{ ital "COP" rSub { size 8{"ref"} } } {} of an air conditioner or refrigerator to be

COP ref = Q c W . size 12{ ital "COP" rSub { size 8{ ital "ref"} } = { {Q rSub { size 8{c} } } over {W} } "." } {}

Noting again that Q h = Q c + W size 12{Q rSub { size 8{h} } =Q rSub { size 8{c} } +W} {} , we can see that an air conditioner will have a lower coefficient of performance than a heat pump, because COP hp = Q h / W size 12{ ital "COP" rSub { size 8{"hp"} } =Q rSub { size 8{h} } /W} {} and Q h size 12{Q rSub { size 8{h} } } {} is greater than Q c size 12{Q rSub { size 8{c} } } {} . In this module’s Problems and Exercises, you will show that

COP ref = COP hp 1 size 12{ ital "COP" rSub { size 8{"ref"} } = ital "COP" rSub { size 8{"hp"} } - 1} {}

for a heat engine used as either an air conditioner or a heat pump operating between the same two temperatures. Real air conditioners and refrigerators typically do remarkably well, having values of COP ref size 12{ ital "COP" rSub { size 8{"ref"} } } {} ranging from 2 to 6. These numbers are better than the COP hp size 12{ ital "COP" rSub { size 8{"hp"} } } {} values for the heat pumps mentioned above, because the temperature differences are smaller, but they are less than those for Carnot engines operating between the same two temperatures.

A type of COP size 12{ ital "COP"} {} rating system called the “energy efficiency rating” ( EER size 12{ ital "EER"} {} ) has been developed. This rating is an example where non-SI units are still used and relevant to consumers. To make it easier for the consumer, Australia, Canada, New Zealand, and the U.S. use an Energy Star Rating out of 5 stars—the more stars, the more energy efficient the appliance. EER s size 12{ ital "EER"} {} are expressed in mixed units of British thermal units (Btu) per hour of heating or cooling divided by the power input in watts. Room air conditioners are readily available with EER s size 12{ ital "EER"} {} ranging from 6 to 12. Although not the same as the COP s size 12{ ital "COP"} {} just described, these EER s size 12{ ital "EER"} {} are good for comparison purposes—the greater the EER size 12{ ital "EER"} {} , the cheaper an air conditioner is to operate (but the higher its purchase price is likely to be).

The EER { ital "EER"s} {} of an air conditioner or refrigerator can be expressed as

EER = Q c / t 1 W / t 2 , { ital "EER"= { {Q rSub { {c} } /t rSub { {1} } } over {W/t rSub { size 8{2} } } } ,} {}

where Q c {Q rSub { {c} } } {} is the amount of heat transfer from a cold environment in British thermal units, t 1 {Q rSub { {c} } } {} is time in hours, W {W} {} is the work input in joules, and t 2 is time in seconds.

Problem-solving strategies for thermodynamics

  1. Examine the situation to determine whether heat, work, or internal energy are involved . Look for any system where the primary methods of transferring energy are heat and work. Heat engines, heat pumps, refrigerators, and air conditioners are examples of such systems.
  2. Identify the system of interest and draw a labeled diagram of the system showing energy flow.
  3. Identify exactly what needs to be determined in the problem (identify the unknowns) . A written list is useful. Maximum efficiency means a Carnot engine is involved. Efficiency is not the same as the coefficient of performance.
  4. Make a list of what is given or can be inferred from the problem as stated (identify the knowns) . Be sure to distinguish heat transfer into a system from heat transfer out of the system, as well as work input from work output. In many situations, it is useful to determine the type of process, such as isothermal or adiabatic.
  5. Solve the appropriate equation for the quantity to be determined (the unknown).
  6. Substitute the known quantities along with their units into the appropriate equation and obtain numerical solutions complete with units.
  7. Check the answer to see if it is reasonable: Does it make sense? For example, efficiency is always less than 1, whereas coefficients of performance are greater than 1.

Questions & Answers

what is variations in raman spectra for nanomaterials
Jyoti Reply
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
Damian
yes that's correct
Professor
I think
Professor
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
Rafiq
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
Damian
How we are making nano material?
LITNING Reply
what is a peer
LITNING Reply
What is meant by 'nano scale'?
LITNING Reply
What is STMs full form?
LITNING
scanning tunneling microscope
Sahil
how nano science is used for hydrophobicity
Santosh
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
what is differents between GO and RGO?
Mahi
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
Rafiq
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
Renato
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
Adin
why?
Adin
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
research.net
kanaga
sciencedirect big data base
Ernesto
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
Daniel
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, College physics ii. OpenStax CNX. Nov 29, 2012 Download for free at http://legacy.cnx.org/content/col11458/1.2
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