International Forum for Green Asia 2020

The exergy investigation of a R410a mechanical vapor compression chiller forresidential coolingUthpala Perera1,2, Nobuo Takata1, Takahiko Miyazaki1,3, Yukihiro Higashi3 and Kyaw Thu1,31Interdisciplinary Graduate School of Engineering, Kyushu University
2
Green Asia Education Center, Kyushu University
Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan,
3
Reasearch Center for Next Generation Refrigerant Properties (NEXT-RP)
I2CNER-WPI, Kyushu University, JapanAbstract
The high global warming potential refrigerant R410a is being phased out under the KigaliAmendment. However, it is still a common refrigerant in residential units for most developing
nations. Thus to provide an insight into the potential areas of improvement for the coolingsystem and for future comparisons with environmentally friendly refrigerants, the first andsecond laws of thermodynamics is used to analyze a 2.0 kW chiller. The optimum charge isdetermined to be 0.84 kg. Exergy analysis highlights the high irreversibilities in the compressor.
The exergetic efficiency illustrates the effects of evaporator temperature and reiterates thededuction of the optimum charge amount and areas for improvement.
1. Introduction
The need for cooling has been
increasing[1] since the introduction of thecommercial air conditioner in the early
1900s[2]. A report by the International Energy
Agency (IEA) shows that the increase indemand for cooling will result in a threefold
increase in global energy use for coolingcompared to the 2016 level[3]. The main
drivers of this increase are the increased
global temperatures resulting from global
warming and climate change[4] which have
led to frequent unbearable heat waves
through many countries, the increase inliving standards of people especially indeveloping nations and the creation of urban
heat island effects due to the rise inurbanization. Unsurprisingly, the largest
contributors to the increased growth of thecooling industry is seen in Asia, with China,
India and Indonesia collectively resulting inhalf of the projected cooling energy use in
2050[3]. When the cooling demand is broken
down into the commercial and residential
sectors, it is clear that the largest contribution
is caused by the latter, in both the number ofunits and the overall energy consumption[3].
A further look at the breakdown of theparticular types of cooling systems usedreveals that mini-split types and multi-split
type air conditioners occupy around 77% ofthe market[3]. These split type units haveoften been using R410a as the mainrefrigerant since moving away from R22[5]which possessed high ozone depletion
potential (ODP). The use of R410a iscurrently being controlled by the Kigali
Amendment and many fluorine gas
regulations[14][15] across the globe due to its
high global warming potential (GWP). In
many countries including the EuropeanUnion[6][7], Japan[8][9] and several othernations[10] which fall under the Group 5[11] of
the Montreal Protocol[12], still rely on R410aeven with the phase out of this refrigerant
eminent under the Kigali Amendment[13].
Since many countries are either still in thetransition phase or still have several years till
the strict control of R410a occurs, there is anecessity to explore alternatives to this high
GWP refrigerant. Several key studies related
to the use of R410a have already been
published by Fukuda et al.[16][17] and Thu etal.[18] regarding the heating applications
which are of importance to countries athigher latitudes. Chen W.[19] has usedsimulations to compare the performance andenvironmental impact of converting11

International Forum for Green Asia 2020
residential air conditioners larger than 3.0
kW to R410a from R22. Fukuda et al.[20] hasalso looked at the cooling applications of
R410a with respect to the cycle performance,
by using the internal measurements of therefrigerant side properties. This method isoften more applicable to scientific studies
rather than practical and industrialevaluations. The authors have also havestudied the cooling conditions as per the airconditioning, heating and refrigeration
institute (AHRI) standards and also exploredthe exergy analysis of the system which waslimited to 1.6 kW of cooling[21]. The use ofexergy analysis has been proven to beimportant in finding the key areas ofimprovement for a chiller or other systems.
Exergy investigations can offer insight into
the component level losses and room for
improvement as pointed out by the authors’previous work[21] and have since then also be
used to show the limits of improvement
which can be achieved for components understudies by Morusuk et al.[22] and also been
applied to simulations in research by Bilgen
et al.[23].
However, for the case of small capacity
split units which often caters to a single room,
the system analysis for R410a under regular
operating conditions has not been carried out.
Thus a 2 kW chiller is operated at varied
cooling capacities and variable charge
amounts to determine its performance
characteristics and also evaluate its exergetic
performance. The external circuit or waterside properties and power consumption areused in order to easily measure theperformance of the system as a whole, whilst
more in-depth analysis has employed internalmeasurements to provide a more holistic
evaluation. Utilizing the first law ofthermodynamics the basic performance isanalyzed and then the second law ofthermodynamics is employed for a look into
the exergy destruction at the component level
as well as the system exergetic efficiency
which are presented herein.

The experimental apparatus used for theexperiments is a 2 kW rated chiller withvariable speed compressor and chilled waterand cooling water systems (see Fig. 1). Thisapparatus has been previously described inliterature[18][21]. The inverter power to thecompressor can be recorded, whilst thetemperatures and pressures at the inlets andoutlets of the heat exchangers both in thewater side and refrigerant side, as well as thesuction and discharge points of thecompressor and inlet and outlet of theexpansion device together with the mass flow
rates are recorded. The required conditions
for the cooling mode are shown in Table 1.
Under these conditions the cooling capacity
can be varied between 1.4 kW and 2.4 kW,
whilst the R410a charge amount is varied
between 0.74 kg and 0.90 kg.
2.2. Equations
The analysis of the mechanical vapor
compression (MVC) cycle is carried out
based on the first and second laws ofthermodynamics. The refrigerant properties
when required are determined using Refprop
10.0[24] and the analysis of data has been
carried out using Mathematica[25] software.
Utilizing the first law, the energy balance isfirst calculated to ascertain the validity of theresults to follow. This is carried out in asimilar method as presented in previous
works[21][26]. Secondly the basic indicator ofperformance for a chiller, the coefficient ofperformance (COP) of the system isdetermined based on Eq. 1, where the cooling
effect of the chiller is measured using thechilled water side properties, whilst thepower input into the system is directly read
off the power meter of the compressor. The
employment of external measurements
provides an easy way to compare thisperformance with other chillers and evenother refrigerants, which is essential in thecase of R410a which should be phased out ofuse in the near future as per international
treaties and local laws in many countries.

2. Experimental Methodology
2.1. Experimental Appartus
12International Forum for Green Asia 2020
COP =m! CH ´ c p ´ (TCH , In - TCH ,Out )PComp

As seen in Fig. 3, the exergy destruction
rate is lowest for the newly determined
optimum charge of 0.84 kg. This reinforces
the previous result. To further explore thereasons for the reducing exergy destruction
when the optimum charge is reached and theincrease when it is overcharged orundercharged, the component level exergy
destruction will be investigated in the nextsection.

(1)

Moving on to the second law analysis,
the exergy destruction is calculated based on
the dead state temperature and pressure of T0= 298.15 K and P0 = 101.325 kPa. The
general form of the exergy destruction
equation applied is shown in Eq. 2-3, whilst
more detailed expressions for eachcomponent of the cycle can be referred to inpreviously published work[21][26].
n æT ödV öæE! D = å ç1 - o ÷Q! j + çW!cv + po cv ÷ + å m! i e f i - å m! e e f e ,ç T ÷dt ø ièj =1 èej øe f = ( hi - h0 ) - T0 ( si - s0 ) .

3.3. Component Exergy Destruction at 2.0kWTo compare the exergy destruction
behavior for the design load of 2.0 kW, thelowest, optimum and largest charge amountsof R410a are analyzed and presented in Fig.
4. It is possible to see that the exergy
destruction within the condenser increases as
the charge amount is increased. This could becaused by the reduced contact surface area
within the heat exchangers since more liquid
would accumulate at the end and theresistance to flow caused by this, which isindicated by the increased subcool
temperatures in Fig. 2 as well. The expansion
valve shows the opposite trend with reduced
exergy destruction rates. This could be a
result of the reduced mass flow rates in orderto maintain the constant cooling load. The
evaporator shows the smallest exergy
destruction during the optimum charge
conditions. This is possibly due to the factthat the evaporator is working at its designed
conditions and is exchanging maximum
amount of heat per refrigerant kg. As for thecompressor the high exergy destruction at
both under and overcharge conditions could
be a result of inadequate refrigerant andreduced compressor performance at very low
frequencies, respectively. It is noted that thecompressor is the largest contributor to theexergy destruction in all cases.

(2)
(3)

The exergy efficiency ( h Ex ) is shown as
follows in Eq. 4,
æ T0
ö
- 1÷ .
çT÷è EvapøhEx = COP ç(4)

3. Results and Analysis
3.1. Optimum Charge
The optimum charge for this systemneeds to be determined by evaluating its COP
at different charge amounts. Since the system
is designed for 2.0 kW, the respective COPs
from 0.74 kg to 0.90 kg are shown in Fig. 2.
The dome like behavior indicates that thecharge of 0.84 kg of R410a provides theoptimum behavior or highest COP for thecurrent system. Thus 0.84 kg is determined tobe the optimum charge amount. In the case ofovercharging the system, the COP drops
rapidly, whilst when undercharged the drop is
not significant till it differs by more than 0.1
kg. This suggests that care must be taken notto increase the overall charge of a system in
the hopes of expecting improved
performance and that as long as therefrigerant levels does not drop largely fromthe optimum, an adequate performance canbe obtained.

3.4. Exergy Efficiency at 2.0 kW
The exergy efficiency represents theirreversibilities or exergy destruction and its
effects on the overall refrigeration cycle. It isclearly seen that the evaporator andcompressor have the largest effect on this3.2. Exergy Destruction at 2.0 kW
13

International Forum for Green Asia 2020
since Eq. 4 shows its dependence on COP.
However, the slight variations in theevaporator temperature results in the shiftingof the exergy efficiency towards a morepositive value as is the case of 0.77 kg (seeFig. 5). However, the optimum charge
amount of 0.84 kg takes the 2nd highest
exergetic efficiency, signifying its high COP
and reiterates its deduction.

[7]
[8]

[9]
[10]

4. Conclusion
The analysis of a 2.0 kW chiller, hasallowed the determination of the optimum
amount of charge of R410a as 0.84 kg andalso allowed the exploration of the exergetic
parameters. The exergy destruction exhibitedthat the compressor was the largest
contributor to the irreversibilities within thesystem and that the total exergy destruction
was the lowest at 0.84 kg. Over charging or
under charging refrigerant will cause
increased losses and contribute to the lower
efficiency of each component. The condenser
was the only component which showed
increasing exergy destruction with increased
charge amount. Finally, the exergetic
efficiency showed the resultant exergy
destructions and their effects on the overall
performance of the chiller system, whilst any
increase in evaporator temperature can cause
positive exergetic efficiencies.

[11]
[12]
[13]

[14]
[15]
[16]
[17]
[18]
[19]
[20]

Acknowledgment
The authors are grateful to the funding
provided by the Kyushu University Green
Asia program for the doctoral funding ofUthpala Perera and to the collaborative effort
on the experiments by Mr. K. Takezato and
Mr. S. Senba.

[21]
[22]
[23]
[24]

Reference[1]
A. Vaughan, Guard. 2018.
[2]
P. Lester, Dep. Energy 2015.
[3]
The Future of Cooling Opportunities forEnergy- Efficient Air Conditioning, 2018.
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B.J. van Ruijven, E. De Cian, I. Sue Wing, Nat.
Commun. 2019, 10, 1.
[5]J.M. Calm, P.A. Domanski, EcoLibrium n.d.,no. 10, 18.
[6]
B. Gschrey, J. Kleinschmidt, S. Barrault,
Briefing Paper : HFCs and HFC Alternatives

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in Split Air Conditioning Systems, 2020.
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H. YAMADA, Efforts to Reduce Greenhouse
Gas Emissions for Air Conditioning andRefrigeration Products, 2018.
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M. Seki, The Latest Developments under theMontreal Protocol and the Kigali Amendment,
2017.
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Protocol on Substances That Deplete theOzone Layer, Nairobi, Kenya 2019.
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The Amendment to the Montreal Protocol
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F-Gas Regulation Japan. Minist. Environ.
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S. Koyama, Int. Refrig. Air Cond. Conf. 2016.
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K. Thu, K. Takezato, N. Takata, T. Miyazaki,
Y. Higashi, Appl. Therm. Eng. 2021, 182,115952.
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S. Fukuda, H. Kojima, C. Kondou, N. Takata,
S. Koyama, Sci. Technol. Built Environ. 2016,
22, 1118.
C.U.A. Perera, N. Takata, T. Miyazaki, Y.
Higashi, B.B. Saha, K. Thu, Heat Transf. Eng.
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Vapour Compression Chiller at ElevatedChilled Water Outlet Conditions, 2018.

Email: perera.c.845@s.kyushu-u.ac.jp

14International Forum for Green Asia 2020
Table 1. Experimental conditions for the 2.0 kW chiller.
Parameters
Inlet

OutletEvaporator chilled water temperature (°C)

2010Condenser cooling water temperature (°C)

3045

Superheat (K)4±1

Figure 1. The schematic of the mechanical vapor
compression chiller.

Figure 3. Total exergy destruction at 2.0 kW forvaried R410a charge amounts.

Figure 5. The exergy efficiency versus subcool at
2.0 kW for varied R410a charge amounts.

15

Figure 2. The COP versus subcool at 2.0 kW forvaried charges (0.74 kg – 0.90 kg).

Figure 4. Component exergy destruction at 2.0
kW for (a) 0.74 kg, (b) 0.84 kg and (c) 0.90 kg.

International Forum for Green Asia 202016