This document describes an experiment to enhance the performance of a vapor compression refrigeration system using a thermoelectric module. The system uses LPG as the refrigerant. A thermoelectric module is used to provide subcooling of the refrigerant after condensation. Performance metrics like COP, refrigeration effect, and heat rejected by the condenser are calculated and compared at different system loads with and without subcooling. The results show that subcooling of 3°C using the thermoelectric module improves all the performance metrics and reduces compressor power consumption. This demonstrates that thermoelectric modules can enhance vapor compression refrigeration system performance.

1. Performance Enhancement of Vapour
Compression Refrigeration System by
Using Thermoelectric Module
E.B. Priyanka1
, S.Thangavel2
, Yokesh Chakravarthi3
,Shibiraj.S4
1
Asst.Professor, Dept of Mechatronics, Sri Krishna College of Engineerng & Technology, Coimbatore.
2
Asst.Professor, Dept of Mechatronics, Kongu Engineering College, Perundurai.
3,4
UG Scholar, Dept of Mechatronics, Sri Krishna College of Engineering & Technology, Coimbatore.
Email id: priyankaeb@skcet.ac.in, thangavelmts@kongu.ac.in
ABSTRACT: Vapour compression technology has been available to the world more than a century ago. Due to
Ozone Depletion Potential (ODP), Global Warming Potential (GWP), R-134a, R-12 & R-22 are to be replaced by
alternate refrigerants like LPG. It is the mixture of Propane and Isobutane (each 50%). This is having less value
of ODP and GWP. In this work LPG operated Vapour Compression Refrigeration System (VCRS) performance
is analyzed at various loads like 2.5lit, 5lit, 7.5lit and 10lit. Subcooling is the concept used to enhance the
performance of the VCRS. Thermo Electric Component (TEC) is the device selected for producing subcooling in
the VCRS. It is located after the condensation process. Significant improvements in COP, Refrigeration effect,
Heat rejected by condenser are achieved and also deduction in the power consumption by the compressor is
achieved. The above analysis is carried out at 3o
C of subcooling.
KEYWORDS: Vapour Compression, VCRS, TEC, COP, Subcooling.
I. INTRODUCTION
1.1 Refrigeration
Refrigeration is a process in which work is done to move heat from one location to another. This work is
traditionally done by mechanical work, but can also be done by magnetism, laser or other means. Refrigeration
has many applications, including, but not limited to: household refrigerators, industrial freezers, cryogenics, air
conditioning, and heat pumps.
1.2Methods of refrigeration
1.2.1 Non-cyclic refrigeration
In non-cyclic refrigeration, cooling is accomplished by melting ice or by subliming dry ice (frozen carbon
dioxide). These methods are used for small-scale refrigeration such as in laboratories and workshops, or in
portable coolers.Ice owes its effectiveness as a cooling agent to its melting point of 0 °C at sea level. To melt, ice
must absorb 333.55 kJ/kg of heat. Solid carbon dioxide has no liquid phase at normal atmospheric pressure, and
sublimes directly from the solid to vapor phase at a temperature of -78.5 °C, and is effective for maintaining
products at low temperatures during sublimation.
1.2.2 Cyclic refrigeration
This consists of a refrigeration cycle, where heat is removed from a low-temperature space or source and rejected
to a high-temperature sink with the help of external work, and its inverse, the thermodynamic power cycle. A
refrigeration cycle describes the changes that take place in the refrigerant as it alternately absorbs and rejects heat
as it circulates through a refrigerator. It is also applied to Heating, Ventilation, Air Conditioning & Refrigeration
(HVACR) work, when describing the "process" of refrigerant flow through an HVACR unit, whether it is a
packaged or split system.Heat naturally flows from hot to cold. The most common types of refrigeration systems
use the reverse-Rankine vapor-compression refrigeration cycle, although absorption heat pumps are used in a
minority of applications.Cyclic refrigeration can be classified as:
1. Vapor cycle
2. Gas cycle
1.2.2.1 Vapor-compression refrigeration
Vapor-compression refrigeration is one of the many refrigeration cycles available for use.The vapor-compression
uses a circulating liquid refrigerant as the medium which absorbs and removes heat from the space to be cooled
and subsequently rejects that heat elsewhere. Fig.1 represents single-stage vapor-compression system.
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2. Fig.1 Vapour Compression Refrigeration System
All such systems have four components: a compressor, a condenser, a thermal expansion valve (also called a
throttle valve), and an evaporator. Circulating refrigerant enters the compressor in the thermodynamic state known
as a saturated vapor and is compressed to a higher pressure, resulting in a higher temperature as well. The hot,
compressed vapor is then in the thermodynamic state known as a superheated vapor and it is at a temperature and
pressure at which it can be condensed with typically available cooling water or cooling air. That hot vapor is
routed through a condenser where it is cooled and condensed into a liquid by flowing through a coil or tubes with
cool water or cool air flowing across the coil or tubes. A fan circulates the warm air in the enclosed space across
the coil or tubes carrying the cold refrigerant liquid and vapor mixture. That warm air evaporates the liquid part
of the cold refrigerant mixture. At the same time, the circulating air is cooled and thus lowers the temperature of
the enclosed space to the desired temperature. The evaporator is where the circulating refrigerant absorbs and
removes heat which is subsequently rejected in the condenser and transferred elsewhere by the water or air used
in the condenser.To complete the refrigeration cycle, the refrigerant vapor from the evaporator is again a saturated
vapor and is routed back into the compressor.VCRS processes are plotted in T-S and p-h plots shown in Fig-2 and
Fig-3. Various thermodynamic processes are given below.
Fig.2 Temperature–Entropy diagram
VCRS processes
 Process 1-2: Isentropic compression of vapour refrigerant
 Process 2-3: Vapour superheat removed in condenser
 Process 3-4: Vapour converted into liquid in condenser
 Process 4-5: Liquid flashes into liquid + vapour across expansion valve
 Process 5-1: Liquid + vapour converted to all vapour in evaporator
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3. Fig.3 Pressure- Enthalpy diagram
1.3 Evaporator
The evaporator is the component of a refrigeration system in which heat is removed from air, water or any other
body required to be cooled by the evaporating refrigerant.Evaporators used in this experiment are direct expansion
evaporators. In direct expansion evaporators a part of the heat transfer surface is used for superheating the vapour.
A capillary tube is used in conjunction with a direct expansion valve. To facilitate the return of oil to the
compressor, direct expansion evaporators are fed from the top by a capillary tube.
1.4 Expansion valve
The expansion valve used here is a capillary tube. The capillary tube is a fixed restriction type device. It is long
and narrow tube connecting the condenser directly to the evaporator. The pressure drop through the capillary tube
is happens due to the friction, due to the fluid viscosity, resulting in frictional pressure drop and acceleration, due
to the flashing of the liquid refrigerant into vapour, resulting in momentum pressure drop.The cumulative pressure
drop must be equal to the difference in pressure at the two ends of the tube.
Fig.4 Expansion valve
1.5 Compressor
Compressors are often described as being open, hermetic, or semi-hermetic, to describe how the compressor and
motor drive is situated in relation to the gas or vapour being compressed. The industry name of the compressor
which is used in the system is hermetically sealed compressor.A compressor is the heart of the vapour compression
system. It pumps and circulates refrigerants through the system just as the heart pumps and circulates blood
through the body.
Fig.5 hermetically sealed compressor
Fig.5 represents a diagram of hermetically sealed compressor. In hermetic compressors, the compressor and motor
driving the compressor are integrated, and operate within the pressurized gas envelope of the system. The motor
is designed to operate and be cooled by the gas or vapour being compressed.The hermetic uses a one-piece welded
steel casing that cannot be opened for repair; if the hermetic fails it is simply replaced with an entire new unit
1.6 Condenser
The type of condenser used in the experiment is air cooled condenser. In air cooled condenser, heat is removed
by air using either natural or forced circulation. In this project case the condenser has forced air circulation. The
condensers are made of steel, copper, aluminium tubing provided with fins to improve air side heat transfer.Air
cooled condensers show in Fig.6 are used only in small capacity machines, such as refrigerators and small water
coolers.
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4. Fig. 6 Air cooled condenser
1.7Refrigerants
A refrigerant is defined as any substance that absorbs heat through expansion or vaporization and loses it through
condensation in a refrigeration system. Refrigerant include those working medium that passed through the cycle
of evaporation, recovery, compression, condensation and liquefaction. These substances absorb heat at one place
at low temperature level and reject the same at some other place having higher temperature and pressure.
1.7.1 LPG
Liquefied Petroleum Gas (LPG) is a flammable mixture of hydrocarbon gases used as a fuel in heating appliances
and vehicles. It is increasingly used as an aerosol propellant and a refrigerant, replacing chlorofluorocarbons in
an effort to reduce damage to the ozone layer. When specifically used as a vehicle fuel it is often referred to as
auto gas.Blended of pure, dry "isopropane" (R-290a) and isobutane (R-600a) have ozone depletion potential of 0
and very low global warming potential of 8 and can serve as a functional replacement for R-12, R-22, R-134a,
and other chlorofluorocarbon or hydro fluorocarbon refrigerants in conventional stationary refrigeration and air
conditioning systems.
1.7.2 Units of refrigeration
The units of refrigeration are always a unit of power. Domestic and commercial refrigerators may be rated in kJ/s
of cooling. For commercial and industrial refrigeration systems most of the world uses the kilowatt (kW) as the
basic unit refrigeration. Typically, commercial and industrial refrigeration systems are rated in tons of
refrigeration (TR). One TR is defined as the energy removal rate that will freeze one short ton of water at 0 °C in
one day.1 tonne of refrigeration = 3.876 kW
1.8 Alternative cooling technologies
Different alternative cooling technologies are
1. Magnetic refrigeration
2. Thermoelectric refrigeration
1.8.1 Magnetic refrigeration
Magnetic refrigeration, or adiabatic demagnetization, is a cooling technology based on the magneto caloric effect,
an intrinsic property of magnetic solids. The refrigerant is often a paramagnetic salt, such as cerium magnesium
nitrate. The active magnetic dipoles in this case are those of the electron shells of the paramagnetic atoms.A strong
magnetic field is applied to the refrigerant, forcing its various magnetic dipoles to align and putting these degrees
of freedom of the refrigerant into a state of lowered entropy. A heat sink then absorbs the heat released by the
refrigerant due to its loss of entropy. Thermal contact with the heat sink is then broken so that the system is
insulated, and the magnetic field is switched off. This increases the heat capacity of the refrigerant, thus decreasing
its temperature below the temperature of the heat sink.
1.8.2 Thermoelectric refrigeration
Thermoelectric cooling uses the Peltier effect to create a heat flux between the junctions of two different types of
materials. This effect is commonly used in camping and portable coolers and for cooling electronic components
and small instruments.
1.8.2.1 Thermoelectric cooling
N-type semiconductor serially connected to form a junction to which a DC current is applied which results in the
flow of electrons which thereby results in heating up of one junction and cooling of the other. Thermoelectric
cooling uses the Peltier effect to create a heat flux between the junctions of two different types of materials. A
Peltier cooler, heater, or thermoelectric heat pump is a solid-state active heat pump which transfers heat from one
side of the device to the other side against the temperature gradient (from cold to hot), with consumption of
electrical energy. Such an instrument is also called a Peltier device, Peltier heat pump, solid state refrigerator, or
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5. thermoelectric cooler (TEC). Therefore it can be used either for heating or for cooling (refrigeration), although in
practice the main application is cooling. It can also be used as a temperature controller that either heats or cools.
Fig.7 Thermoelectric module with inside view
From Fig.7 show the internal arrangement of p-type and n-type semiconductors. It is visible from the diagram
how the DC voltage is applied. This technology is far less commonly applied to refrigeration than vapor-
compression refrigeration. The main advantages of a Peltier cooler (compared to a vapor-compression
refrigerator) are its lack of moving parts or circulating liquid, and its small size and flexible shape (form factor).
1.8.2.2 Performance
Thermoelectric junctions are generally only around 5–10% as efficient as the ideal refrigerator (Carnot cycle),
compared with 40–60% achieved by conventional compression cycle systems (reverse Rankine systems using
compression/expansion). Due to the relatively low efficiency, thermoelectric cooling is generally only used in
environments where the solid state nature (no moving parts, maintenance-free, compact size) outweighs pure
efficiency.Peltier (thermoelectric) cooler performance is a function of ambient temperature, hot and cold side heat
exchanger (heat sink) performance, thermal load, Peltier module (thermopile) geometry, and Peltier electrical
parameters.
II. EXPERIMENTAL SETUP
Pressure is measured in the unit bar. Pressure of the refrigerant before and after compression, after condensation
and after expansion is measured. Similarly temperatures of the refrigerant before and after compression, after
condensation, after subcooling and after expansion are measured in degree Celsius. Since the experimental setup
is unalterable due to cost wise and safety wise reasons we decided to integrate the thermoelectric module into the
existing system. It is accomplished by using a water jacket and a cooling set up. The water which is passed through
the water jacket is cooled using the thermoelectric module using a cooling set up. The thermoelectric module in
the cooling setup is used to cool the water. Then that cooled water is passed through the water jacket to provide
the subcooling. After passing through the water jacket the water which was used to provide the subcooling is
removed. Thus a continuous flow of cooled water is used provide to the water jacket.
Fig.8 Vapour compression system with thermoelectric module attached
Fig.8 shows the schematic diagram of the experimental setup. The main parts of the experimental setup are the
following
 A 10 liter water tank to store water which will be cooled
 Evaporator coil surrounding the water tank
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6.  A hermitically sealed compressor
 Air cooled condenser
 Expansion valve (capillary tube)
 Pressure gauges to measure the pressure of the refrigerant at different points of the system
 Temperature gauge (digital) to measure the water temperature and refrigerant temperature at
different points of the system
 DC rectifier to modulate the direct current to the thermoelectric module
 A thermoelectric module
III. RESULTS AND DISCUSSIONS
3.1 Graph for subcooling of 3o
C
A calculation and comparison of the different performance characteristics like COP, refrigeration effect, heat
rejected by the condenser and power consumption by the compressor is calculated and tabulated. Enthalpy values
of the refrigerant corresponding to its temperature at different part of the system are used to calculate the
performance characteristics. From the calculated values it could be understood that the subcooling has improved
the performance characteristics of the vapour compression refrigeration system. Comparative tables and there
corresponding graphs are given below. Below the graph the percentage of improvement in the properties are stated
and explained.
Table. 1 COP at various load conditions with subcooling of 3o
C
COP at various load conditions with subcooling of 3o
C
Water
temperatu
re in
degree
celcius
2.5 liters of water 5 liters of water 7.5 liters of water 10 liters of water
Without
subcoolin
g
With
subcoolin
g
Without
subcoolin
g
With
subcoolin
g
Without
subcoolin
g
With
subcoolin
g
Without
subcoolin
g
With
subcoolin
g
6 6.27 6.56 6.38 7.02 6.22 6.50 6.39 7.09
8 6.34 6.89 6.57 6.97 6.17 6.50 6.39 7.18
10 6.16 6.55 6.70 7.45 6.58 6.94 6.39 7.04
12 6.39 6.89 7.27 7.88 6.98 7.60 7.18 7.54
14 7.06 7.30 7.27 7.96 7.04 7.78 7.04 7.96
16 7.29 7.45 7.45 8.14 7.48 7.84 6.91 8.02
18 7.34 7.46 7.69 8.57 7.83 8.22 7.37 7.96
20 7.40 7.70 8.39 9.17 7.77 8.22 7.93 8.20
22 7.40 7.70 8.94 9.33 8.35 9.03 8.20 8.86
24 7.58 7.78 9.20 10.25 8.56 9.26 8.26 9.00
26 8.16 8.50 9.56 10.67 9.27 9.78 8.85 9.92
28 8.72 8.93 10.55 11.86 9.82 10.06 9.54 9.99
Fig.9 Water temperature vs COP (2.5 liters)
COP
Water temperature in degree celcius
without subcooling
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7. From the figure 9 it could be learned that with 3o
C subcooling the COP of the vapour compression system
increased by 4.2%.
Fig.10 Water temperature vs COP (5 liters)
From the figure 10 it could be learned that with 3o
C subcooling the COP of the vapour compression system
increased by6.3%.
Fig.11 Water temperature vs Power consumption by compressor (10 liters)
From the figure 11 it could be learned that with 3o
C subcooling the power consumption by compressor has
decreased by 5.2%.
IV. CONCLUSIONS
From the analysis it is found that subcooling enhances the performance of VCRS. For 3oC and 4oC of subcooling
the Refrigeration effect, COP, Heat rejected by the condenser is improved. The Power consumption by the
compressor is decreased when subcooling is incorporated with the system. The experiment is carried out at various
loads in the evaporator like 2.5 lit, 5 lit, 7.5 lit, 10 lit. The various performance characteristics are calculated and
show that there is an enhancement in the performance. The various improvements in performance characteristics
are given for 3o
C subcooling.
1) With 3o
C subcooling using chilled water the
(a) For 10 liters of water
1. COP increased by 7.9%
2. Refrigeration effect increased by 2.1%
3. Heat rejected in condenser increased by 1.2%
4. Power consumption in compressor decreased by 5.2%
(b) For 7.5 liters of water
1. COP increased by 8.7%
2. Refrigeration effect increased by 1.7%
3. Heat rejected in condenser increased by 0.9%
4. Power consumption in compressor decreased by 6.0%
(c) For 5 liters of water
1. COP increased by 6.3%
2. Refrigeration effect increased by 2.3%
3. Heat rejected in condenser increased by 1.7%
4. Power consumption in compressor decreased by 3.6%
(d) For 2.5 liters of water
COP
Water temperature in degree celcius
without subcooling
Power
consumptionby
compressorin
kJ/kg
Water temperature in degree celcius
with out subcooling
with subcooling of 3 degree celcius
with subcooling of 4 degree celcius
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8. 1. COP increased by 4.2%
2. Refrigeration effect increased by 2.4%
3. Heat rejected in condenser increased by 1.9%
4. Power consumption in compressor decreased by 1.7%
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