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What is Heat Pump How does it work Definition
Heat Pump How does it work. In general, a heat pump is a device that transfers heat energy from a heat source to a heat sinkwarmer inside of the house. Thermal Engineering
Heat Pump, Refrigerator, Air Conditioner basic principle of operation
The termheat pumpis usually reserved for a device that can heat a house in winter by using an electric motor that does work W to take heatQcoldfrom the outside at low temperature and delivers heatQhotto the warmer inside of the house.
The operating principle ofrefrigerators,air conditioners, andheat pumpsisthe sameand it is just thereverseof aheat engine. In general, aheat pumpis a device that transfers heat energy from aheat sourceto a heat sink, but in this case the transfer occurs in the opposite direction of spontaneous heat transfer byabsorbing heatfrom acold spaceandreleasing itto awarmer one. As diagrammed in the figure, by doing external work W, heat is taken from a low-temperature region (heat source) and a greater amount of heat is exhausted at a higher temperature (heat sink).
The most widely used thermodynamic cycle or method for heating, air-conditioning, refrigerators and heat pumps is thevapor compression cycle.What is Heat in Thermodynamics
Whileinternal energyrefers to the total energy of all the molecules within the object,heatis the amount of energyflowingfrom one body to another spontaneously due to their temperature difference.Heatis a form of energy, but it isenergy in transit. Heat is not a property of a system. However, the transfer of energy as heat occurs at the molecular level as a result of atemperature difference.
Consider ablock of metalat high temperature, that consists of atoms that are oscillating intensely around their average positions.At low temperatures, the atoms continue to oscillate, but withless intensity. If a hotter block of metal is put in contact with a cooler block, the intensely oscillating atoms at the edge of the hotter block gives off its kinetic energy to the less oscillating atoms at the edge of the cool block. In this case there isenergy transferbetween these two blocks andheat flowsfrom the hotter to the cooler block by this random vibrations.
In general, when two objects are brought intothermal contact,heat will flowbetween themuntilthey come intoequilibriumwith each other. When atemperature differencedoes exist heat flows spontaneouslyfrom the warmer system to the colder system. Heat transfer occurs byconductionor bythermal radiation. When theflow of heat stops, they are said to be at thesame temperature. They are then said to be inthermal equilibrium.Vapor-compression Cycle Vapor-compression RefrigerationVapor-compression cycle Thermodynamic cycle of heat pumps.
Thevapor-compressionuses a circulating liquid refrigerant as the medium (usuallyR134a) which absorbs and removes heat from the space to be cooled and subsequently rejects that heat elsewhere. The figure depicts a typical, single-stagevapor-compression system. The typical vapor-compression system consist of four components:CompressorCondenserExpansion valve(also called a throttle valve)Evaporator
In an ideal vapor-compression cycle, the system executing the cycle undergoes a series of four processes: one isentropic (reversible adiabatic) process, one throttling process alternated with two isobaric processes:
(compression in the piston compressor) A circulating refrigerant such as R134a enters a compressor as low-pressure vapor at or slightly below the temperature of the refrigerator interior. The gaseous medium is compressed adiabatically from state 1 to state 2 by piston compressor (or by centrifugal pumps) to a relatively high pressure and temperature. The surroundings do work on the gas, increasing its internal energy (temperature) and compressing it (increasing its pressure). On the other hand the entropy remains unchanged. The work required for the compressor is given by
Isobaric heat rejection(in a condenser)
The superheated vapor travels under pressure through coils or tubes that make up the condenser. In this phase the refrigerant passes through the condenser, where the refrigerant condenses and there is heat transfer from the refrigerant to the cooler surroundings. The net heat rejected is given by
As the refrigerant leaves the condenser, it is still under pressure but is now only slightly above room temperature.
(expansion in an expansion valve) The refrigerant at state 3 enters the expansion valve and expands to the evaporator pressure. This process is usually modeled as a throttling process for which enthalpy remains constant.
The sudden decrease in pressure results in explosive-like flash evaporation of a portion (typically about half) of the liquid. The latent heat absorbed by this flash evaporation is drawn mostly from adjacent still-liquid refrigerant, a phenomenon known as
) The cold and partially vaporized refrigerant continues through the coils or tubes of the evaporator unit. In this phase (between state 4 and state 1) there is a constant-pressure heat transfer to the liquid medium from an external source, since the chamber is open to flow in and out. As the refrigerant passes through the evaporator, heat transfer from the refrigerated space results in the vaporization of the refrigerant. The net heat added is given by
During a vapor-compression cycle, work is done on the fluid by the pumps between states 1 and 2 (isentropic compression). There is no work is done by the fluid since between stages 3 and 4 the process is isenthalpic. The working fluid in a vapor-compression cycle follows a closed loop and is reused constantly.R134a Refrigerant - PropertiesR134a, chemically1,1,1,2-tetrafluoroethane(CF3CH2F)is a non-flammable gas used primarily as a high-temperature refrigerant for domestic refrigeration and automobile air conditioners. It is safe for normal handling as it is non-toxic, non-flammable and non-corrosive. It is now being used as a replacement for the more environmentally harmful R-12 refrigerant in the area of centrifugal, rotary screw, scroll and reciprocating compressors.R134a refrigerant table of parametersReversible Heat Pumps
Reversible heat pumpswork in either direction to provideheatingorair conditioning(cooling) to the internal space. They employ areversing valveto reverse the flow of refrigerant from the compressor through the condenser and evaporation coils.
Inheating mode, heat pumps are three to four times more effective at heating (i.e. they can have COP = 4) than simple electrical resistance heaters using the same amount of electricity. Typically installed cost for a heat pump is about 20 times greater than for resistance heaters. In heating mode, the outdoor coil is an evaporator, while the indoor is a condenser.
Incooling mode, the flow is reversed and the outdoor coil is a condenser, while the indoor is an evaporator. In heating mode, the outdoor coil is an evaporator, while the indoor is a condenser. The COP for cooling mode is less than for heating mode, because the work done by compressor is utilized only during the heating mode.Coefficient of Performance Heat Pump, Refrigerator, Air Conditioner
In general, thethermal efficiency,th, of anyheat engineas the ratio of theworkit does,W, to theheatinput at the high temperature, QH.
Thethermal efficiency,th, represents the fraction ofheat,QH, that is convertedto work.
But inheat pumpsandrefrigerators, the work is not an output. For a refrigeration or heat pumps, thermal efficiency indicates the extent to which the energy added by work is converted to net heat output. From an economic point of view, thebest refrigeration cycleis one thatremoves the greatest amount of heatfrom the inside of the refrigerator (cold reservoir) for the least expenditure of mechanical work or electric energy. The relevant ratio is therefore the larger this ratio, the better the refrigerator. We call this ratio thecoefficient of performance, denoted byCOP.
Thecoefficient of performance,COP,is defined also for heat pumps, but at this point we follow the net heat added to the hot reservoir. TheCOPusually exceeds 1, especially in heat pumps, because, instead of just converting work to heat, itpumps additional heatfrom a heat source to where the heat is required.
In general,COPis highly dependent on operating conditions, especially absolute temperature and relative temperature between heat sink and system.Coefficient of Performance Refrigerator, Air Conditioner
Thecoefficient of performance,COP, of arefrigeratoris defined as the heat removed from the cold reservoir Qcold, (i.e. inside a refrigerator) divided by the work W done to remove the heat (i.e. the work done by the compressor).
As can be seen, the better (more efficient) the refrigerator is when more heatQcoldcan be removed from the inside of the refrigerator for a given amount of work. Since thefirst law of thermodynamicsmust be valid also in this case (Qcold+ W = Qhot), we can rewrite the above equation:
For an ideal refrigerator (without losses and irreversibilities) can be derived that:
These formulas are applied also for anair conditioner, which works very much like a refrigerator.
On the other hand, the COP for heating and cooling are different.Coefficient of Performance Heat Pump
For heating, theCOPis the ratio of the heat added to the system (hot reservoir). Using the first law of thermodynamics define COP also as the heat removed from the cold reservoir plus the input work to the input work.
For an ideal heat pump (without losses and irreversibilities) can be derived that:
Note that, these equations must use anabsolute temperature scale(Tcold, Thot) and it is only atheoretical maximum efficiency. According to the above formula, the maximum achievable COP for Thot= 35 C (308 K) and Tcold= 0 C (273 K) would be 8.8. But in reality the best systems are around 4.5.
As can be seen, the COP of a heat pump system can be improved by reducing thetemperature difference(Thot Tcold). Therefore, reducing the output temperature (Thot) is very efficient, but requires very efficient heat transfer from heat pump system to surroundings (i.e. use of piped floor). An increase in the input temperature (Tcold) means, for example, an oversized ground source of heat.Example Heat Pump Heating and Air Conditioning
Areversible heat pumphas a coefficient of performance,COP = 3.0, when operated in theheating mode. Its compressor consumes1500 Wof electric energy.Calculate the amount of heat (Qhot) the heat pump can add to a room?If theheat pumpwere turned to thecooling mode(i.e. to act as an air conditioner in the summer), what would you expect itscoefficient of performanceto be? Assume all else stays the same and neglect all other losses.
the amount of heat the heat pump can add to a room is equal to:
Qhot= COPheatingx W = 3 x 1500 =4500 W or 4500 J/s
In case of thecooling mode, the heat pump (air conditioner) with1500 Wmotor can take heatQcoldfrom inside the house and then dump Qhot= 4500 W to the hot outside. Using the first law of thermodynamics, which states:
we obtain the heat,Qcold= 3000 W. From the definition:COPcooling= 3000/1500 =2.
Note that, in this example we have many assumptions. For example, we assumed that the temperature difference (Thot Tcold) is the same for both modes. But we have swapped reservoirs, without any impact on COP. It is only an illustrative example.Reverse Brayton Cycle Brayton Refrigeration Cyclereverse Brayton cycle
In general, theBrayton cycledescribes the workings of aconstant-pressure heat engine. Today,modern gas turbine enginesandairbreathing jet enginesare also a constant-pressure heat engines
ABrayton cyclethat is driven in reverse direction is known as thereverse Brayton cycle. Its purpose is to move heat from colder to hotter body, rather than produce work. In compliance with the second law of thermodynamics,heat cannot spontaneously flowfrom cold system to hot system without external work being performed on the system. Heat can flow from colder to hotter body, butonly when forced by an external work. This is exactly what refrigerators and heat pumps accomplish. These are driven by electric motors requiring work from their surroundings to operate. One of possible cycles is a reverse Brayton cycle, which is similar to the ordinary Brayton cycle but it is driven in reverse, via net work input. This cycle is also known as the gas refrigeration cycle or Bell Coleman cycle. This type of cycle is widely used in jet aircrafts for air conditioning systems using air from the engine compressors. It is also widely used in the LNG industry where the largest reverse Brayton cycle is for subcooling LNG using 86 MW of power from a gas turbine-driven compressor and nitrogen refrigerant.References:Nuclear and Reactor Physics:J. R. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed., Addison-Wesley, Reading, MA (1983).J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.W. M. Stacey, Nuclear Reactor Physics, John Wiley & Sons, 2001, ISBN: 0- 471-39127-1.Glasstone, Sesonske. Nuclear Reactor Engineering: Reactor Systems Engineering, Springer; 4th edition, 1994, ISBN: 17W.S.C. Williams. Nuclear and Particle Physics. Clarendon Press; 1 edition, 1991, ISBN: 67Kenneth S. Krane. Introductory Nuclear Physics, 3rd Edition, Wiley, 1987, ISBN: 33G.R.Keepin. Physics of Nuclear Kinetics. Addison-Wesley Pub. Co; 1st edition, 1965Robert Reed Burn, Introduction to Nuclear Reactor Operation, 1988.U.S. Department of Energy, Nuclear Physics and Reactor Theory. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.
Advanced Reactor Physics:K. O. Ott, W. A. Bezella, Introductory Nuclear Reactor Statics, American Nuclear Society, Revised edition (1989), 1989, ISBN: 0-894-48033-2.K. O. Ott, R. J. Neuhold, Introductory Nuclear Reactor Dynamics, American Nuclear Society, 1985, ISBN: 0-894-48029-4.D. L. Hetrick, Dynamics of Nuclear Reactors, American Nuclear Society, 1993, ISBN: 0-894-48453-2.E. E. Lewis, W. F. Miller, Computational Methods of Neutron Transport, American Nuclear Society, 1993, ISBN: 0-894-48452-4.
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