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tory general what links here related changes upload file permanent link page information cite this page get shortened url switch to legacy parser print export download as pdf printable version in other projects wikidata item appearance move to sidebar hide from wikipedia the free encyclopedia for details of practical heat pumps see heat pump mathematical models of heat pumps and refrigeration thermodynamics the classical carnot heat engine branches classical statistical chemical quantum thermodynamics equilibrium non equilibrium laws zeroth first second third systems closed system open system isolated system state equation of state ideal gas real gas state of matter phase matter equilibrium control volume instruments processes isobaric isochoric isothermal adiabatic isentropic isenthalpic quasistatic polytropic free expansion reversibility irreversibility endoreversibility cycles heat engines heat pumps thermal efficiency system properties note conjugate variables in italics property diagrams intensive and extensive properties process functions work heat functions of state temperature entropy introduction pressure volume chemical potential particle number vapor quality reduced properties material properties property databases specific heat capacity c displaystyle c t displaystyle t s displaystyle partial s n displaystyle n t displaystyle partial t compressibility β displaystyle beta 1 displaystyle 1 v displaystyle partial v v displaystyle v p displaystyle partial p thermal expansion α displaystyle alpha 1 displaystyle 1 v displaystyle partial v v displaystyle v t displaystyle partial t equations carnot s theorem clausius theorem fundamental relation ideal gas law maxwell relations onsager reciprocal relations bridgman s equations table of thermodynamic equations potentials free energy free entropy internal energy u s v displaystyle u s v enthalpy h s p u p v displaystyle h s p u pv helmholtz free energy a t v u t s displaystyle a t v u ts gibbs free energy g t p h t s displaystyle g t p h ts history culture history general entropy gas laws perpetual motion machines philosophy entropy and time entropy and life brownian ratchet maxwell s demon heat death paradox loschmidt s paradox synergetics theories caloric theory vis viva living force mechanical equivalent of heat motive power key publications an inquiry concerning the source friction on the equilibrium of heterogeneous substances reflections on the motive power of fire timelines thermodynamics heat engines art education maxwell s thermodynamic surface entropy as energy dispersal scientists bernoulli boltzmann bridgman callen carathéodory carnot clapeyron clausius de donder duhem gibbs von helmholtz joule kelvin lewis massieu maxwell von mayer nernst onsager planck rankine smeaton stahl tait thompson van der waals waterston other nucleation self assembly self organization category v t e thermodynamic heat pump cycles or refrigeration cycles are the conceptual and mathematical models for heat pump air conditioning and refrigeration systems a heat pump is a mechanical system that transmits heat from one location the source at a certain temperature to another location the sink or heat sink at a higher temperature 1 thus a heat pump may be thought of as a heater if the objective is to warm the heat sink as when warming the inside of a home on a cold day or a refrigerator or cooler if the objective is to cool the heat source as in the normal operation of a freezer the operating principles in both cases are the same 2 energy is used to move heat from a colder place to a warmer place thermodynamic cycles edit according to the second law of thermodynamics heat cannot spontaneously flow from a colder location to a hotter area mechanical work is required to achieve this 3 an air conditioner requires work to cool a living space moving heat from the interior being cooled the heat source to the outdoors the heat sink similarly a refrigerator moves heat from inside the cold icebox the heat source to the warmer room temperature air of the kitchen the heat sink the operating principle of an ideal heat engine was described mathematically using the carnot cycle by sadi carnot in 1824 an ideal refrigerator or heat pump can be thought of as an ideal heat engine that is operating in a reverse carnot cycle 4 heat pump cycles and refrigeration cycles can be classified as vapor compression vapor absorption gas cycle or stirling cycle types vapor compression cycle edit main article vapor compression refrigeration vapor compression refrigeration 5 for comparison a simple stylized diagram of a heat pump s vapor compression refrigeration cycle 1 condenser 2 expansion valve 3 evaporator 4 compressor note that this diagram is flipped vertically and horizontally compared to the previous one 6 temperature entropy diagram of the vapor compression cycle the vapor compression cycle is used by many refrigeration air conditioning and other cooling applications and also within heat pump for heating applications there are two heat exchangers one being the condenser which is hotter and releases heat and the other being the evaporator which is colder and accepts heat for applications which need to operate in both heating and cooling modes a reversing valve is used to switch the roles of these two heat exchangers at the start of the thermodynamic cycle the refrigerant enters the compressor as a low pressure and low temperature saturated vapor in heat pumps this refrigerant is typically r32 refrigerant or r290 refrigerant then the pressure is increased and the refrigerant leaves as a higher temperature and higher pressure superheated gas this hot pressurised gas then passes through the condenser where it releases heat to the surroundings as it cools and condenses completely the cooler high pressure liquid next passes through the expansion valve throttle valve which reduces the pressure abruptly causing the temperature to drop dramatically 7 the cold low pressure mixture of liquid and vapor next travels through the evaporator where it vaporizes completely as it accepts heat from the surroundings before returning to the compressor as a low pressure low temperature gas to start the cycle again 8 some simpler applications with fixed operating temperatures such as a domestic refrigerator may use a fixed speed compressor and fixed aperture expansion valve applications that need to operate at a high coefficient of performance in very varied conditions as is the case with heat pumps where external temperatures and internal heat demand vary considerably through the seasons typically use a variable speed inverter compressor and an adjustable expansion valve to control the pressures of the cycle more accurately citation needed the above discussion is based on the ideal vapor compression refrigeration cycle and does not take into account real world effects like frictional pressure drop in the system slight thermodynamic irreversibility during the compression of the refrigerant vapor or non ideal gas behavior if any 4 vapor absorption cycle edit main article absorption refrigerator in the early years of the twentieth century the vapor absorption cycle using water ammonia systems was popular and widely used but after the development of the vapor compression cycle it lost much of its importance because of its low coefficient of performance about one fifth of that of the vapor compression cycle nowadays the vapor absorption cycle is used only where heat is more readily available than electricity such as industrial waste heat solar thermal energy by solar collectors or off the grid refrigeration in recreational vehicles the absorption cycle is similar to the compression cycle but depends on the partial pressure of the refrigerant vapor in the absorption system the compressor is replaced by an absorber and a generator the absorber dissolves the refrigerant in a suitable liquid dilute solution and therefore the dilute solution becomes a strong solution in the generator on heat addition the temperature increases and with it the partial pressure of the refrigerant vapor is released from the strong solution however the generator requires a heat source which would consume energy unless waste heat is used in an absorption refrigerator a suitable combination of refrigerant and absorbent is used the most common combinations are ammonia refrigerant and water absorbent and water refrigerant and lithium bromide absorbent absorption refrigeration systems can be powered by combustion of fossil fuels e g coal oil natural gas etc or renewable energy e g waste heat recovery biomass combustion or solar energy gas cycle edit this section does not cite any sources please help improve this section by adding citations to reliable sources unsourced material may be challenged and removed june 2020 learn how and when to remove this message when the working fluid is a gas that is compressed and expanded but does not change phase the refrigeration cycle is called a gas cycle air is most often this working fluid as there is no condensation and evaporation intended in a gas cycle components corresponding to the condenser and evaporator in a vapor compression cycle are the hot and cold gas to gas heat exchangers for given extreme temperatures a gas cycle may be less efficient than a vapor compression cycle because the gas cycle works on the reverse brayton cycle instead of the reverse rankine cycle as such the working fluid never receives or rejects heat at constant temperature in the gas cycle the refrigeration effect is equal to the product of the specific heat of the gas and the rise in temperature of the gas in the low temperature side therefore for the same cooling load gas refrigeration cycle machines require a larger mass flow rate which in turn increases their size because of their lower efficiency and larger bulk air cycle coolers are not often applied in terrestrial refrigeration the air cycle machine is very common however on gas turbine powered jet airliners since compressed air is readily available from the engines compressor sections these jet aircraft s cooling and ventilation units also serve the purpose of heating and pressurizing the aircraft cabin stirling engine edit main article stirling engine the stirling cycle heat engine 9 can be driven in reverse using a mechanical energy input to drive heat transfer in a reversed direction i e a heat pump or refrigerator there are several design configurations for such devices that can be built several such setups require rotary or sliding seals which can introduce difficult tradeoffs between frictional losses and refrigerant leakage reversed carnot cycle edit the carnot cycle which has a quantum equivalent 10 is reversible so the four processes that comprise it two isothermal and two isentropic can also be reversed when a carnot cycle runs in reverse it is called a reverse carnot cycle a refrigerator or heat pump that acts according to the reversed carnot cycle is called a carnot refrigerator or carnot heat pump respectively in the first stage of this cycle the refrigerant absorbs heat isothermally from a low temperature source t l in the amount q l next the refrigerant is compressed isentropically adiabatically without heat transfer and its temperature rises to that of the high temperature source t h then at this high temperature the refrigerant isothermally rejects heat in the amount q h 0 negative according to the sign convention for heat lost by the system lastly the refrigerant expands isentropically until its temperature falls to that of the low temperature source t l 2 absorption compression heat pump edit main article absorption compression heat pump an absorption compression heat pump achp is a device that integrate an electric compressor in an absorption heat pump in some cases this is obtained by combining a vapor compression heat pump and an absorption heat pump it is also referred to as a hybrid heat pump 11 which is however a broader field thanks to this integration the device can obtain cooling and heating effects using both thermal and electrical energy sources 12 13 this type of systems is well coupled with cogeneration systems where both heat and electricity are produced depending on the configuration the system can maximise heating and cooling production from a given amount of fuel or can improve the temperature hence the quality of waste heat from other processes 14 this second use is the most studied one and has been applied to several industrial applications 15 coefficient of performance edit main article coefficient of performance the merit of a refrigerator or heat pump is given by a parameter called the coefficient of performance cop the equation is c o p q w n e t i n displaystyle rm cop frac q w_ net in where q displaystyle q is the useful heat given off or taken up by the system w n e t i n displaystyle w_ net in is the net work done on the system in one cycle the detailed cop of a refrigerator is given by the following equation c o p r desired output required input cooling effect work input q l w net in displaystyle rm cop_ r frac text desired output text required input frac text cooling effect text work input frac q_ l w_ text net in the cop of a heat pump sometimes referred to as coefficient of amplification coa is given by the following equations where the first law of thermodynamics w n e t i n q l q h δ c y c l e u 0 displaystyle w_ net in q_ l q_ h delta _ cycle u 0 and q h q h displaystyle q_ h q_ h was used in one of the last steps c o p h p desired output required input heating effect work input q h w net in w n e t i n q l w net in 1 q l w net in displaystyle rm cop_ hp frac text desired output text required input frac text heating effect text work input frac q_ h w_ text net in frac w_ net in q_ l w_ text net in 1 frac q_ l w_ text net in both the cop of a refrigerator and a heat pump can be greater than one combining these two equations results in c o p h p 1 c o p r displaystyle rm cop_ hp 1 rm cop_ r for fixed values of q h and q l this implies that cop hp will be greater than one because cop r will be a positive quantity in a worst case scenario the heat pump will supply as much energy as it consumes making it act as a resistance heater for carnot refrigerators and heat pumps the cop can be expressed in terms of temperatures c o p r c a r n o t t l t h t l 1 t h t l 1 displaystyle rm cop_ r carnot frac t_ l t_ h t_ l frac 1 frac t_ h t_ l 1 c o p h p c a r n o t t h t h t l 1 1 t l t h displaystyle rm cop_ hp carnot frac t_ h t_ h t_ l frac 1 1 frac t_ l t_ h these are the upper limits for the cop of any system operating between t l and t h references edit the systems and equipment volume of the ashrae handbook ashrae incorporated atlanta georgia 2004 1 2 cengel yunus a boles michael a 2008 thermodynamics an engineering approach 6th ed mcgraw hill isbn 978 0 07 330537 0 fundamentals of engineering thermodynamics by howell and buckius mcgraw hi...