When is a refrigerator a heater? Making machines more efficient.
When is a refrigerator a heater? Making machines more efficient.
Praveen Ravi
G 11
Physics essay
A refrigerator is a cooling device meant to keep substances at temperatures lower than the outside temperature, as low as 2 to 3 °C. A refrigerator works on the basic principle of heat transfer like a heat engine. While a heat engine takes in heat from a higher temperature, expends some of it for doing work and rejects the remaining as exhaust at a much lower temperature, a refrigerator does quite the converse. It takes in heat from a body making it cold, has work done upon it and rejects the heat at a much higher temperature (Fig.1; Appendix). A heat pump as the name suggests, is a device which can pump heat in or out from an enclosure like a room. It is very similar to that of a refrigerator, the only difference being that it takes in heat from a cold reservoir like the outside air and passes it to a room at a higher, suitable temperature. It can also operate vice versa by introducing a valve where the process of cooling or heating can be adjusted. Theoretically, a refrigerator should be able to work like a heat pump. However, it has its own possibilities and limitations which shall be discussed in this essay.
A schematic diagram of a refrigerator is shown in the Appendix (Fig.2). The working of a refrigerator comprises of four major steps namely - evaporation, compression, condensation and expansion. A chemical called a refrigerant is used in this process to remove the heat from the refrigerator. Refrigerants must have certain specific properties - they must be easily liquefiable, they must have low specific heat capacities, they must be non-toxic, non-flammable and non reactive with foodstuff etc. The first refrigerants used were ammonia, ether and chemogene (a mixture of petrol ether and naphtha). However, nearly all were toxic, flammable and reactive. Thomas Midgley made a new species of compounds from fluorine called Chloro-Fluoro Carbons (CFC)1. They were non-toxic, non flammable and removed large quantities of heat from the refrigerator and were ideal refrigerants.
The refrigerant is contained in the refrigerant tank in a cold liquid state. First, it goes to the evaporator. The evaporator is a snake like pipe which is in close contact with the refrigerator cabin. The refrigerant absorbs this heat cooling the cabin. It uses it as latent heat and changes from cold liquid to cold vapour without any change in temperature. Thus, the evaporation is isothermal.
The cold vapour then goes to the compressor. The compressor is of a big volume with a piston attached to it. Electrical energy is used to move the piston. The cold vapour is compressed suddenly increasing the pressure and decreasing the volume rapidly. Because of the sudden compression, there is no time for the heat generated in the vapour to flow out and temperature increases. Thus, this process is adiabatic. Since work is done on the gas, W < 0. The cold vapour now changes into very hot vapour.
The hot vapour then flows into the condenser. ...
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The cold vapour then goes to the compressor. The compressor is of a big volume with a piston attached to it. Electrical energy is used to move the piston. The cold vapour is compressed suddenly increasing the pressure and decreasing the volume rapidly. Because of the sudden compression, there is no time for the heat generated in the vapour to flow out and temperature increases. Thus, this process is adiabatic. Since work is done on the gas, W < 0. The cold vapour now changes into very hot vapour.
The hot vapour then flows into the condenser. The condenser is a vertical grating of coils located at the back of the refrigerator. This is quite the opposite of what happens in the evaporator. The hot vapour condenses and gives away its thermal energy in the form of latent heat. It changes from hot vapour to hot liquid at the same temperature, making the process isothermal. The heat is given out from the back to the surroundings.
Hot liquid from the condenser passes through an expansion valve. This is a chamber of large volume but with a series of narrow, twisted tubes before it. The hot liquid passes through the loops and turns and becomes hotter and an increase in pressure. It then rapidly expands into the large volume. Pressure decreases rapidly, volume increases rapidly and temperature drops. The hot liquid turns into cold liquid. Because of all three changes, this process is adiabatic. Work is done by the gas so W > 0.
A heat pump, as the name implies, moves heat from one place to another, usually used in homes. In winter, it moves in heat from outside to inside the house and in summer, moves heat from inside to outside. This dual nature of a heat pump can be done with a valve which can reverse the direction of the transfer of heat. Thus, heat pumps can act as heaters as well as air conditioners.2
Most heat pumps are air-source pumps; they use air to get heat into the house. Water-source heat pumps also exist which get heat from an outside source of water, usually well water. Heat pumps can also use refrigeration techniques to cool or warm a house. A diagram of how a heat pump can use refrigeration techniques to warm or cool a house is shown in the Appendix (Fig. 3 and 4). As it can be seen, the process is fairly similar to that of a standard refrigerator, the only difference being that the interior is the house and the exterior is the air.3
Analysing the heat transfer in a refrigerator, a refrigerator could also, theoretically, be made to function as a heat pump. The heat that is given out in the third phase after condensation could be captured and delivered to the interior of a house through a pump. Similarly, introducing a valve, the vice versa should also be possible. However, the second process would seem rather difficult since in condensation, heat is given out of the refrigerator by the vapour. When cooling the house, the heat would have to be given to the condenser. This would change the liquid refrigerant back to vapour upsetting the refrigeration cycle. However, this process could theoretically be prevented if while cooling the house, the heat is supplied not to the condenser but to a place which needs heat to be given in. the most suitable place is when the hot vapour is moving from the compressor to the condenser. Giving in heat will raise its temperature further. It should not upset the refrigeration process.
It is necessary to see that the above mentioned process is theoretical to a large extent. The first objective of warming the house is not very difficult since a pump can be attached to the condenser and the heat rejected can be supplied to the house, suitable regulating the flow of heat. However, the second objective is far more difficult to achieve because it requires a modification of the framework of a refrigerator. It requires an additional, alternate way from the pump supplying the heat to the house to the vapour when it is going from the compressor to the condenser only when the house is being cooled. This requires a fairly large amount of money and modification in the building of a refrigerator. Present refrigerators do not have this modification because the aim of a refrigerator is to cool stuff kept in it; making it function like a heat pump is an additional theory that can make the refrigerator serve for a double purpose. It is, though, not impossible to make the refrigerator function like a heat pump to warm the house; it should rather be called a one way heat pump since heat transfer in only one direction will take place.
Another fact to be noted is that despite the heat given out from the condenser, the temperature of the heat is grossly insufficient to warm a house of moderate size. This can be seen by placing our hands behind a refrigerator. The air coming out is warm, no doubt, but if thought practically to use this heat to warm a room, the idea would seem rather absurd because the heat, is in no way, sufficient. Despite this, to use this heat, one would further have to heat the air using another source of energy and then convey this heat to the house. Instead of following this complicated process, we can buy an external heater itself. Thus, it can be seen that the functioning of a refrigerator as a heat pump is very, very theoretical.
The fact that the use of a refrigerator as heat pump is not suitable practically leads us to the question why? Why isn't it possible? This leads us to the concept of efficiency. Efficiency, as the name implies, can best be described as how "good" a machine is, how well it does its work. The scientific definition is "The ratio of the useful energy delivered by a dynamic system (such as machine, engine, or motor) to the energy supplied to it over the same period or cycle of operation."4 If an amount Q of energy is expended and an amount W of work is obtained, then the efficiency is W/Q. Efficiency is often expressed as a percentage; it gives a clearer idea of how good and efficient the machine is. A good and efficient machine should waste as little heat as possible and do maximum work. 100% efficiency is not possible because there is always some way, where the machine will waste some heat. There are reasons for this - "perfect" insulation of the system is not possible, machine parts will absorb some energy in the form of heat etc.
In the case of a refrigerator, the concept of efficiency is slightly modified because of its nature of working. As mentioned before, it takes heat from a colder body and releases it to a hotter body. Thus, the work done will be the heat it removes from the refrigerator cabin. For a refrigerator, efficiency is called Coefficient of Performance (COP). It is "the ratio of the amount of heat removed at the lower temperature to the work put into the system".5
Let heat removed be or = Qlow
work done
Heat given out = Qhigh
at high temperature
Heat supplied = Qhigh - Qlow
COP = W
Q
= Qlow
Qhigh - Qlow
For a heat pump as well, the efficiency is called the COP. In this case, the work done by the heat pump will be the heat given or removed from the house and the energy supplied will be the heat supplied or removed from the house minus the heat taken or given to the surrounding.
Let heat removed/given = Qhouse
house
Heat given/removed = Qoutside
surrounding
Heat supplied = Qhouse - Qoutside
COP = W
Q
= Qhouse
Qhouse - Qoutside
In conclusion, a refrigerator will be a bad heat pump because its aim, objective is to cool stuff kept inside. To behave like a heat pump will require certain modifications mentioned before. In the present world, such technology is not available and is also not worth it. It is highly possible in the years to come, refrigerators be used for double purposes of cooling and heating. They would no doubt be different in structure to the refrigerators we use today. Restricting ourselves to the present situation, it is highly absurd to think about using refrigerators as heat pumps.
Bibliography
. Examples of Heat Pump and Refrigerator (2005). http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/ThermLaw2/HPandRefer.html (updated 4th April 2005, accessed 4th April 2005)
2. Hypertextbook (2005), Refrigerators. http://hypertextbook.com/physics/thermal/refrigerators/ (updated 4th April 2005, accessed 4th April 2005)
3. Physics202 (2005), Carnot Engines, Heat Pumps and Refrigerators. http://oak.cats.ohiou.edu/~piccard/phys202/carnot/carnot.html (updated 4th April 2005, accessed 4th April 2005)
Appendix
(Fig. 1)6
(Fig. 2)7
(Fig.3)8
Heating with a heat pump
(Fig.4)9
Cooling with a heat pump
Trane (2005) Refrigerant History. http://www.trane.com/commercial/issues/environmental/cfc6b.asp (updated 4th April 2005, accessed 4th April 2005)
2 The Energy Outlet (2005), Heat Pumps - How does a heat pump work. http://energyoutlet.com/res/heatpump/how.html (updates 4th April 2005, accessed 4th April 2005)
3 The Energy Outlet (2005), Heat Pumps - How does a heat pump work. http://energyoutlet.com/res/heatpump/how.html (updates 4th April 2005, accessed 4th April 2005)
4 US DOE Energy Efficiency and Renewable Energy (EERE), Efficiency. www.eere.energy.gov/ (updated 4th April 2005, accessed 4th April 2005)
5 Heat Engines (2005). http://electron9.phys.utk.edu/phys136d/modules/m3/heatpump.htm (updated 4th April 2005, accessed 4th April 2005)
6 The open door website, Thermal Physics - Heat Pumps. http://www.saburchill.com/physics/chapters/0126.html (updated 4th April 2005, accessed 4th April 2005)
7 UNIFIED ENGINEERING September 1996, Refrigeration cycles. http://web.mit.edu/16.unified/www/FALL/thermodynamics/chapter_5.htm (updated 4th April 2005, accessed 4th April 2005)
8 The Energy Outlet, Heat Pumps - Pumping heat uphill. http://energyoutlet.com/res/heatpump/pumping.html (updated 4th April 2005, accessed 4th April 2005)
9 The Energy Outlet, Heat Pumps - Pumping heat uphill. http://energyoutlet.com/res/heatpump/pumping.html (updated 4th April 2005, accessed 4th April 2005)
