Energy and Exergy Analysis of Eco Friendly Refrigerants based
Domestic Refrigerator System AbstractIn this paper energy and
exergy performance of different refrigerants like CFC12, HFC134a,
HFC143a, HFC152a, HC290, HC600a and HO1270 used in vapour
compression refrigeration system have been compared. Vapour
compression refrigeration systems exergetic modelling has been
presented. The effects of evaporator and condenser temperature on
the coefficient of performance, exergy flow destruction, and
entropy generation in the major components of refrigerator has been
studied and presented. It has been found from the study,
refrigerant HC600a is the best based on second law
analysis.Keywords Exergetic efficiency; Heat exchanger
effectiveness; Subcooling; Superheating; Ozone depletion Potential;
Global warming potential. NomenclatureEXw=Work done by or on the
system second law=Second law efficiencyi=Exergy at inlet m=mass
flow rateo=Exergy at outlet I= IrreversibilitySgen=Entropy
generation = Rate of heat transfer in condenserWc=Rate of energy =
rate of heat transfer in evaporatorT0=Ambient temperature
Tk=Temperature of heat source or sink IntroductionThe domestic
refrigerator is an appliance that is found in almost all households
for storing food, vegetables, beverages and much more. Domestic
refrigerators were among the first appliances to be targeted for
energy efficiency improvements, as it was the largest energy user
at homes. As against the general perception that global warming
impact is caused due to refrigerant leakage, it is interesting to
note that 80% of it happens actually because of energy consumption.
Overall the refrigeration accounts for about 15% of the total
electricity consumed in the world, which in turn is mostly produced
by burning of fossil fuels (coal, oil and gas). Fossil fuels gives
rise to strongly elevated greenhouse gas emission as well as
environmental damage, both of which have extreme impacts on the
respective ecosystems. Unlike fossil fuels, renewable energies are
conventional energy resources, which are inexhaustible so we can
switch to renewable energy resources. In the present scenario
commercial enterprises, residential estates and private households
all over the world are increasingly in need of reliable,
affordable, and environment friendly energy to fulfill their demand
for electricity, heat, cooling and mobility. Renewable energies can
make a considerable contribution to fulfill this demand, both in
private households as well as in commercial enterprises. To
ascertain how much energy is required by the system and how
efficiently it is utilizing that energy, it is necessary to perform
exact analysis. And exergy analysis is the means to perform exact
analysis. Exergy analysis seems to be an effectual method using the
conservation of mass and conservation of energy principle in
proximity with second law of thermodynamics for the design and
analysis of energy systems. It is an acceptable technique for
promoting the goal of more efficient energy resource use and
ascertaining the locations, types and true magnitudes of waste and
losses. It is an efficient technique divulging whether or not and
by how much it is feasible to design more efficient energy systems
by reducing the inefficiencies of the system. The damage caused by
CFCs was discovered bySherry Rowland andMario Molinawho, after
hearing a lecture on the subject of Lovelock's work in 1974
(Ozonecell, 2013). (Sentence incomplete)To prevent environment from
impact of these refrigerants many treaties came into action. First
one was Montreal Protocol according to which production of such
kind of substances that deplete the ozone layer must be phased out.
Then Kyoto Protocol was negotiated in December 1997, at the city
Kyoto, Japan and came into force on February 16th, 2005. Its
objective was to reduce greenhouse gases emissions. The properties
of the refrigerants which are taken into consideration are given in
table 1.Table 1 Refrigerants properties (Ozonecell,
2013)RefrigerantsOzone depletion potential(ODP)Global warming
potential(GWP)
HFC134a01300
HFC143a04470
HFC152a0120
HC290020
HC600a020
HO1270020
CFC12110900
It has been observed that energy analysis is the most common
analysis of refrigeration system. This approach fails to make out
the real energetic losses in refrigeration cycle (Arora and
Kaushik, 2008).Exergy analysis provide the improved and deeper
insight view into the process (Saidur, 2007).Chen & Prasad
(1999) did the simulation of a vapour-compression refrigeration
cycle and compared the performance of HFC134a and CFC12. Sencan et
al. (2005) has performed computer based energy and exergy analysis
and observed the effect of subcooling and superheating on the
vapour compression refrigeration cycle containing R134a, R407C and
R410A. Mani & Selladurai (2008) compared the performance of
R290/R600a refrigerant mixture as drop-in replacement of CFC12 and
HFC134a in vapour compression refrigeration cycle. Arora &
Kaushik (2008) presents a detailed exergy analysis of an actual
vapour compression refrigeration (VCR) cycle and concluded that
R507A is a better substitute to R502 than R404A. Mohanraj et al.
(2008) discussed the possibility of using R152a and hydrocarbon
refrigerants (such as R290, R1270, R600a, and R600) as alternative
of R134a in domestic refrigerator. Padilla et al. (2010) calculated
the exergy and compared the performance of domestic refrigerator
based on R12 with the retrofitted zeotropic mixture of R413A. The
purpose of this study is to find out the best refrigerant on the
basis of four parameters i.e. GWP, ODP, first law analysis and
second law analysis. The thermophysical properties of selected
refrigerants are given in table 2. Table 2 Physical properties of
Refrigerants
RefrigerantChemical formulaMolecular mass(g/mol)Normal boiling
point(C)Tc(C)Pc(bar)
CFC12CF2CL2120.9-29.811241.15
HFC134aCF3CH2F102.03-26.1101.140.64
HFC143aCH3CF384.04-47.272.937.8
HFC152aCH3CHF266.05-24.0113.345.2
HC290C3H844.1-42.196.842.5
HC600a(CH3)3 CH58.12-11.7313536.6
HO1270C3H6
42.09-47.791.946.1
Modelling of Vapour compression refrigeration systemDomestic
refrigerator is based on vapour compression refrigeration cycle.
The schematic diagram of vapour compression cycle is shown in
fig.1. The T-s chart of vapour compression cycle is shown in fig.2.
For steady state flow, the exergy balance can be given by ignoring
kinetic and potential energy changes
EXw=Qk+-T0Sgen (Cengel, 2012) (1) Exergy balance equation is
employed for various components of VCR system.
Exergy balance for compressorT0Sgen= m1 (2-3) + Wc (2)
I destroyed= T0Sgen= m1T0 (S3-S2) (3)
second law =1- (I destroyed/ Wc) (4)
Figure 1: Vapour Compression Refrigeration cycle
Figure 2: Temperature Entropy (T-s) diagram of Vapour
Compression CycleExergy balance for condenserT0Sgen=m3 ( 3- 4) -
0/k) Qk (5)
I destroyed= T0Sgen= (m1 (h3-h4)-T0 (m3 (S3-S4)) - 0/k) Qk (6)
Exergy balance for heat exchangerT0Sgen=m4 ( 4- 5)-m1 ( 2- 1)
(7)
I destroyed= T0Sgen= (m4 (h4-h5)-m1 (h2-h1))-T0 (m4 (S4-S5)-m1
(S2-S1)) (8)
second law =1- (I destroyed/ m4 ( 4- 5) (9)
Exergy balance for Throttle valveT0Sgen= m5( 5- 6) (10)
I destroyed= T0Sgen= m5 ((h5-h6)-T0 (S5-S6) (11)
Exergy balance for evaporatorI destroyed= T0Sgen= (m6 (h1-h6)-T0
(m6 (S1-S6)) - 0/k) Qk (12)
Energy analysis (Ballaney, 2012) COP = (h1-h6) *F*iso*g /
(h3-h2) (13)
Exergy analysis (Cengel, 2012) second law=COP/(T1/(T4-T1))
(14)Energy analysisThe objective of first law analysis is to
predict the variation of COP with evaporator temperature, condenser
temperature, subcooling temperature, superheating temperature and
effectiveness of liquid suction heat exchanger. The operating
temperature range for domestic refrigerator is taken from 248K to
328K (Arora, 2000). Assumptions observed during this work are
summarized in table 3.Table 3 Assumptions for domestic
refrigeratorm(kg/s)1(Constant)
T0(K)278K
Te(K)216K to 263K
Tc(K)318K to 338K
TSub(K)5K
TSuper(K)5K
Effectiveness0.8
F0.8
c0.85
motor0.9
The COP variation is shown in figures 9, 10, 11, 12 and 13. COP
decreases with rise in condensing temperature as work done by
compressor increases, HO1270 shows the maximum drop in COP and
CFC12 shows the minimum.
Figure 9: Variation of COP with condenser temperature
The evaporator temperature is varied from 216 K to 263 K. COP
increases with rise in evaporator temperature as pressure ratio
decreases which cause reduction in compressor work. HC600a shows
the highest COP among the refrigerants, closely followed by
HFC134a.
Figure 10: Variation of COP with evaporator temperature
The subcooling temperature is varied from 256.2 K to 260.2 K.
COP is showing decreasing trend with subcooling temperature.
Maximum drop in COP is seen in HO1270.
Figure 11: Variation of COP with sub cooling temperature
The superheating temperature is varied from 248.2 K to 253.2 K.
COP increases with superheating as it increases the refrigeration
effect. Maximum COP is observed in HC600a.
Figure 12: Variation of COP with superheating temperature
The effectiveness of heat exchanger is varied from 0.6 to 1. COP
increases with effectiveness, maximum increase is shown by
HO1270.
Figure 13: Variation of COP with liquid suction heat exchanger
effectiveness
We have also studied the exergy destruction in each component
with selected refrigerants. The irreversibility variations of
compressor, condenser, heat exchanger, expansion valve and
evaporator with selected refrigerants are presented in fig.4, 5, 6,
7 and 8 respectively. From the graphs we can observe that CFC12 is
having the lowest exergy destruction as compared to other
refrigerants and HO1270 is highly irreversible in all components
except heat exchanger.
Figure 4: Variation of Irreversibility in compressor with
selected refrigerants
Figure 5: Variation of irreversibility in condenser with
selected refrigerants
Figure 6: Variation of irreversibility in heat exchanger with
selected refrigerants
Figure 7: Variation of irreversibility in throttle valve with
selected refrigerants
Figure 8: Variation of Irreversibility in evaporator with
selected refrigerantsExergy AnalysisThe second law analysis is
based on an abstract idea of exergy. It can be defined as
quantification of work potential or quality of different forms of
energy relative to environmental state. When exergy analysis is
applied to a system it describes all loses in the system components
and in the whole system. The exergy method is a corelatively new
evaluation technique in which the basis of assessment of
thermodynamic loses follows from the second law rather than the
first law of thermodynamics. Our aim of exergy analysis is to
specify the variation of exergetic efficiency with evaporator
temperature, condenser temperature, subcooling and superheating
temperature and effectiveness of liquid suction heat exchanger.The
figures 14, 15, 16, 17 and 18 shows the variation of exergetic
efficiency. Exergetic efficiency decreases with increase in
evaporator temperature. As evaporator temperature increases, COP
increase causing minimum exergy intake to perform the given work.
HC600a is showing highest exergetic efficiency.
Figure 14: Variation of Exergetic efficiency with evaporator
temperature
The condenser temperature is varied from 318 K to 338 K.
Exergetic efficiency increases with increase in condenser
temperature. HC600a is showing maximum exergetic efficiency.
Figure 15: Variation of Exergetic efficiency with condenser
temperatureThe subcooling temperature is varied from 256.2 K to
260.2 K. Exergetic efficiency is decreasing with subcooling and
highest is given by HC600a.
Figure 16: Variation of Exergetic efficiency with sub cooling
temperatureThere is slight decrease in exergetic efficiency with
superheating. The temperature is varied from 248.2 K to 253.2 K.
With increase of degree of superheating the total exergy loss
increases due to which second law efficiency decreases.
Figure 17: Variation of Exergetic efficiency with superheating
temperaturesThe effectiveness of liquid suction heat exchanger is
varied from 0.6 to 1. Exergetic efficiency increases with the
effectiveness of heat exchanger. HO1270 is showing maximum
exergetic efficiency.
Figure 18: Variation of Exergetic efficiency with heat exchanger
effectiveness
ConclusionThe performance of theoretical Vapour compression
refrigeration cycle was assessed by both energetic and exergetic
COPs, with the latter providing good guidance for system
improvement. It is concluded that HC600a can be good drop-in
replacement of HFC134a, as HC600a is having low value of global
warming potential (GWP) as compared to HFC134a. For HC600a the
coefficient of performance and exergetic efficiency is found to be
maximum among the selected refrigerants. From the irreversibility
viewpoint, worst component is condenser and worst refrigerant is
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