Thermodynamic heat pump cycles or refrigeration cycles are the conceptual
and mathematical models for heat pumps and refrigerators. A heat pump is a
machine or device that moves heat from one location at a lower temperature to
another location at a higher temperature using mechanical work or a
high-temperature heat source. Thus a heat pump may be thought of as a
"heater" if the objective is to warm the heat sink, or a "refrigerator" if the
objective is to cool the heat source. In either case, the operating principles
are identical. Heat is moved from a cold place to a warm place.
Thermodynamic cycles According to the second law of
thermodynamics heat cannot spontaneously flow from a colder location to a hotter
area; work is required to achieve this. An air conditioner requires work to cool
a living space, moving heat from the cooler interior to the warmer outdoors.
Similarly, a refrigerator moves heat from inside the cold icebox to the
warmer room-temperature air of the kitchen. The operating principle of the
refrigeration cycle was described mathematically by Sadi Carnot in 1824 as
a heat engine. A heat pump can be thought of as heat engine which is
operating in reverse. Heat pump and refrigeration cycles can
be classified as vapor compression, vapor absorption, gas cycle, or Stirling
cycle types. = Vapor-compression cycle=
The vapor-compression cycle is used in most household refrigerators as well as
in many large commercial and industrial refrigeration systems. Figure 1 provides
a schematic diagram of the components of a typical vapor-compression
refrigeration system. The thermodynamics of the cycle can be
analysed on a diagram as shown in Figure 2. In this cycle, a circulating
refrigerant such as Freon enters the compressor as a vapor. The vapor is
compressed at constant entropy and exits the compressor superheated. The
superheated vapor travels through the condenser which first cools and removes
the superheat and then condenses the vapor into a liquid by removing
additional heat at constant pressure and temperature. The liquid refrigerant goes
through the expansion valve where its pressure abruptly decreases, causing
flash evaporation and auto-refrigeration of, typically, less than half of the
liquid. That results in a mixture of liquid and
vapor at a lower temperature and pressure. The cold liquid-vapor mixture
then travels through the evaporator coil or tubes and is completely vaporized by
cooling the warm air being blown by a fan across the evaporator coil or tubes.
The resulting refrigerant vapor returns to the compressor inlet to complete the
thermodynamic cycle. 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. = Vapor absorption cycle=
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. Nowadays, the vapor
absorption cycle is used only where heat is more readily available than
electricity, such as waste heat provided by solar collectors, or off-the-grid
refrigeration in recreational vehicles. The absorption cycle is similar to the
compression cycle, except for the method of raising the pressure of the
refrigerant vapor. In the absorption system, the compressor is replaced by an
absorber which dissolves the refrigerant in a suitable liquid, a liquid pump
which raises the pressure and a generator which, on heat addition,
drives off the refrigerant vapor from the high-pressure liquid. Some work is
required by the liquid pump but, for a given quantity of refrigerant, it is
much smaller than needed by the compressor in the vapor compression
cycle. In an absorption refrigerator, a suitable combination of refrigerant and
absorbent is used. The most common combinations are ammonia and water, and
water and lithium bromide. = Gas cycle=
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=
The Stirling cycle heat engine can be driven in reverse, using a mechanical
energy input to drive heat transfer in a reversed direction. 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.
= Comparison with combined heat and power=
A heat pump may be compared with a combined heat and power unit, in that
for a condensing steam plant, as it switches to produced heat, then
electrical power is lost or becomes unavailable, just as the power used in a
heat pump becomes unavailable. Typically for every unit of power lost, then about
6 units of heat are made available at about 90 °C. Thus CHP has an effective
COP compared to a heat pump of 6. It is noteworthy that the unit for the CHP is
lost at the high voltage network and therefore incurs no losses, whereas the
heat pump unit is lost at the low voltage part of the network and incurs
on average a 6% loss. Because the losses are proportional to the square of the
current, during peak periods losses are much higher than this and it is likely
that widespread i.e. city wide application of heat pumps would cause
overloading of the distribution and transmission grids unless they are
substantially reinforced. = Reversed Carnot cycle=
Since the Carnot cycle is a reversible cycle, the four processes that comprise
it, two isothermal and two isentropic process, can all be reversed as well.
When this happens, it is called a reversed Carnot cycle. A refrigerator or
heat pump that acts on the reversed Carnot cycle is called a Carnot
refrigerator and Carnot heat pump respectively. In the first stage of this
cycle, the refrigerant absorbs heat isothermally from a low-temperature
source, TL, in the amount QL. Next, the refrigerant is isentropically compressed
and the temperature rises to the high-temperature source, TH. Then at
this high temperature, the refrigerant rejects heat isothermally in the amount
QH. Also during this stage, the refrigerant changes from a saturated
vapor to a saturated liquid in the condenser. Lastly, the refrigerant
expands isentropically where the temperature drops back to the
low-temperature source, TL. Coefficient of performance
The efficiency of a refrigerator or heat pump is given by a parameter called the
coefficient of performance. The COP of a refrigerator is given by
the following equation: COP = Desired Output/Required Input =
Cooling Effect/Work Input = QL/Wnet,in The COP of a heat pump is given by the
following equation: COP = Desired Output/Required Input =
Heating Effect/Work Input = QH/Wnet,in Both the COP of a refrigerator and a
heat pump can be greater than one. Combining these two equations results
in: COPHP = COPR + 1 for fixed values of QH
and QL This implies that COPHP will be greater
than one because COPR 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. However, in reality, as in home heating, some of QH is lost
to the outside air through piping, insulation, etc., thus making the COPHP
drop below unity when the outside air temperature is too low. Therefore, the
system used to heat houses uses fuel. For an ideal refrigeration cycle:
COP = TL/(TH-TL) For an ideal heat pump cycle:
COP = TH/(TH-TL) For Carnot refrigerators and heat pumps,
COP is expressed in terms of temperatures:
COPR,Carnot = 1TL) - 1) COPHP,Carnot = 1/(1 -)
References Notes
Turns, Stephen. Thermodynamics: Concepts and Applications. Cambridge University
Press. p. 756. ISBN 0-521-85042-8. Dincer, Ibrahim. Refrigeration Systems
and Applications. John Wiley and Sons. p. 598. ISBN 0-471-62351-2.
Whitman, Bill. Refrigeration and Air conditioning Technology. Delmar.
External links "The Basic Refrigeration Cycle"