九州大学大学院総合理工学府報告第 39 巻 第 2 号 7-13 頁 平成 30 年 2 月

Engineering Sciences Reports, Kyushu University
Vol. 39, No. 2, pp. 7-13, FEB, 2018

Exergy Analysis of a Vapour Compression Chiller at
Elevated Chilled Water Outlet Conditions
Colombatantirige Uthpala Amoda PERERA*1,*2 Bidyut Baran SAHA*3,*4 Takahiko
MIYAZAKI*2 Shigeru KOYAMA*2,*3 and Kyaw THU*1,*2,††E-mail of corresponding author: kyaw.thu.813@m.kyushu-u.ac.jp(Received January 18, 2018, accepted January 19, 2018)

In the pursuit of efficient energy use, decoupling the sensible and latent loads of a Mechanical
Vapour Compression Chiller system with the use of a Dehumidifier has been of great interest tosociety. Focusing purely on the sensible load allows the supply chilled water to be elevated totemperatures above the standard of 7°C. This paper focuses on using prior research on an elevatedchilled water chiller system to carry out an exergy analysis, in order to identify the contribution ofeach and individual component to the total exergy destruction as well as the exergetic efficiency.
Varying the temperature of the chilled water outlet conditions between 7-17°C provides valuable
insight into how the total exergy destruction increases at a rate of 1.2% per degree Celsius of chilledwater supply whilst the exergetic efficiency decreases at a rate of 1.4% per degree Celsius. The
largest contributor to the exergy destruction is identified to be the compressor, whilst theevaporator and condenser show similar exergy destructions and the expansion valve has the lowest
exergy destruction. These findings highlight the irreversibilities occurring within the system when
the operation conditions of the chilled water are varied above the standard.
Key words: Chiller, Exergy, Sensible Cooling, Second Law Efficiency

Nomenclature
𝐶𝐶𝐶𝐶𝐶𝐶
coefficient of Performance [-]𝑞𝑞𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸
cooling effect [kW]𝑚𝑚̇𝐶𝐶ℎchilled water flow rate [kg/s]
𝐶𝐶𝑝𝑝specific heat capacity [kJ/kgK]
𝑇𝑇𝐶𝐶ℎ,𝐼𝐼𝐼𝐼
chilled water inlet temperature [°C]
𝑇𝑇𝐶𝐶ℎ,𝑂𝑂𝑂𝑂𝑂𝑂 chilled water outlet temperature [°C]
𝑞𝑞𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶
condenser heat rejection [kW]𝑚𝑚̇𝐶𝐶𝐶𝐶
cooling water mass flow [kg/s]
𝑇𝑇𝐶𝐶𝐶𝐶,𝐼𝐼𝐼𝐼
cooling water inlet temperature [°C]
𝑇𝑇𝐶𝐶𝐶𝐶,𝑂𝑂𝑂𝑂𝑂𝑂 cooling water outlet temperature [°C]
𝐸𝐸𝑐𝑐𝑐𝑐exergy of control volume [kW]𝑡𝑡time [s]𝑇𝑇0dead state temperature [°C]𝑇𝑇𝑗𝑗
temperature of ith component [°C]̇
𝑄𝑄𝑗𝑗
heat transfer of ith component [kW]̇
𝑊𝑊𝑣𝑣
shaft work [kW]𝑝𝑝0dead state pressure [bar]*1 Kyushu University Program for Leading Graduate
School, Green Asia Education Center*2 Interdisciplinary Graduate School of Engineering
Sciences*3 International Institute for Carbon-Neutral Energy
Research (WPI-I2CNER)*4 Mechanical Engineering Department, Kyushu
University

𝑉𝑉𝑐𝑐𝑐𝑐
𝑚𝑚̇
𝑒𝑒𝑓𝑓
𝐸𝐸̇𝐷𝐷
ℎ
𝑠𝑠
𝜂𝜂𝐸𝐸𝐸𝐸
𝑃𝑃𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶

volume of control volume [m3]mass flow rate [kg/s]
flow exergy [kW]exergy destruction rate [kW]specific enthalpy [kJ/kg]
specific entropy [kJ/kgK]
exergetic efficiency [%]compressor power [kW]Subscripts
In
inlet
Out
outletEvap
evaporator
Cond
condenser
Compcompressor
Exp
expansion valveChchilled waterCw
cooling waterSucsuctionDischdischarge
1. Introduction
Attaining comfort conditions for habiting
indoor environments is of major importance inExergy Analysis of a Vapour Compression Chiller at Elevated Chilled Water Outlet Condition

everyday life. In most tropical countries andduring summer seasons, the warmer outside
air temperatures require to be conditioned inorder to attain cooler temperatures and lower
relative humidity values within indoor spaces.
This has boosted the demand and use of air
conditioners,
especiallywithintropicalcountries1). In the case of many office buildings
and industrial applications, chillers with water
as the secondary working fluid are utilized dueto the ease in the distribution of chilled water
within the building. However, chillers are very
energy intensive and can consume up to 50% of
a building's electrical energy demand2).
Therefore, they contribute indirectly to adverse
effects such as global warming which humanity
is currently trying to control. Since the basic
invention of the chiller by Willis Carrier in 1902,
both sensible and latent loads have beenhandled together3). This dual load controls thetemperature of the air as well as the relative
humidity or water content, both of which areconsidered to be important factors indetermining thermal comfort4). In order tohandle both sensible and latent loads, chilledwater is required to be cooled down totemperatures below the dew point of theoutdoor air, which removes moisture whilst
simultaneously cooling the supply air. Coolingthe chilled water to such low temperatures inorder to handle the latent load of the air
requires more energy than in the case of puresensible load5).
With the emergence of new dehumidification
technology, there is a possibility of decoupling
the sensible and latent loads. Research carriedout by Oh et al. have shown that there is great
promise in using dehumidifier technology withsolid desiccants in order to control water
vapour concentrations of air6). There has also
been the application of liquid desiccant coolingwhich have been studied by Peng et al. using
exergy analysis7). Wang et al. have also usedmodel based optimization for a liquid desiccantsystemwhich
havehelpedfurther
improvement in possible operating conditions8).
Much of these findings are summarized by Janiet al. where details regarding the use of solid
desiccants and their benefits are reiterated9). It
has also been shown that there exist many
alternative
waysofassisting
thisdehumidification process, such as the use ofsolar assisted dehumidification in order to treat
fresh air10). Thus with the possibility ofutilizing dehumidification systems withchillers, the cooling load can be decoupled suchthat the chiller handles only the sensible loadwhilst the dehumidifier handles the latent load.
To provide an idea on the benefits as well as
feasibility of adapting such a process, an
important analytical tool is exergy analysis. In
system analyses, exergy technique has beenutilized often in order to provide an accurate
and realistic representation of the efficiency ofa process as well as its deviations from the ideal
conditions11). These deviations are a result of
irreversibilities within the system, which areimportant to both engineers and designers forassessing the improvement and optimization ofmachines and processes. It is quite commonly
seen that exergy analysis is applied by many
researches, such as in the case of comparing
alternative refrigerants for replacing ones withhigher Global Warming Potential (GWP) or
Ozone Depletion Potential (ODP) in domestic
refrigeration systems12.14). These studies
justified their findings using exergy analysis
since it allows the performance of therefrigeration systems to be compared even if
the capacities and refrigerant used are
different. Adsorption chillers and coolingsystems have also been analysed due to theirability to utilize waste heat or solar thermalenergy and further reduction in theenvironmental impact15-18). Ahamed et al.
carried out a review on the use of exergy
analysisofvapourcompression(VC)
refrigeration systems which points out thatfactors such as evaporator temperature andcondenser temperature affect the overall
exergetic performance of the chiller19). It is alsomentioned that the use of alternativerefrigerants and utilizing nanofluids as
lubricants can reduce the exergy loses in somecomponents.
Although exergy analysis is an important
method which is utilized for the study ofRefrigeration Systems, Adsorption Chillers andVapour Compression (VC) Chillers, there has
been little work which explores the exergetic
relation of operating Mechanical Vapor
Compression (MVC) Chillers at elevated chilledwater temperatures above the standard of 7°C.
Therefore, based on experimental works
carried out by Thu et. al. regarding a MVC
chiller at elevated chilled water temperatures,a second law exergetic analysis is presented inthis paper20).
2. Experiment
2.1 Experimental Method
A water cooled Mechanical Vapour Chiller

Exergy Analysis of a Vapour Compression Chiller at Elevated Chilled Water Outlet Condition

with a capacity of 10kW is employed as theexperimental set up20). In order to make thegathereddataandpresented
results
internationally comparable, AHRI Standard
551/59121) is utilized. AHRI Standard 551/591
requires that the supply chilled water bemaintained at 7°C with a mass flow rate of
0.0478 L/(s-kW) whilst the cooling watersupplied to the condenser is maintained at
30°C22). The chilled water supply which iselevated above the standard of 7°C, is raised up
to 17°C, whilst the cooling water supply isvaried between 28-32°C. The operation
conditions of full load and part load are alsostudied with a temperature difference of both
4°C and 5°C being assigned to the cooling liquid.
In order to achieve these required set points,
the chiller system incorporated mixing tanks,
which allowed both chilled water, cooling waterfrom the cooling tower and from the condenser
to be mixed in required rates using PID
controllers for the valves. The measuring
instruments for temperature and flow volume
allowed the accuracy of the system to be
maintained at ±0.3°C and ±0.45 LPM
respectively. During the data collection, thesystem was allowed to reach steady state
conditions for at least 45 minutes and thesuperheat conditions were maintained at 6.5±0.5°C and sub-cooling at 3.5±0.5°C.
2.2 Exergy
In order to calculate the cooling capacity ofthe evaporator and the heat rejection of thecondenser the secondary fluid or chilled waterand cooling water is utilized by applying anenergy balance to the heat exchangers20).
𝑞𝑞𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸 = 𝑚𝑚̇𝐶𝐶ℎ × 𝐶𝐶𝑝𝑝 × (𝑇𝑇𝐶𝐶ℎ,𝐼𝐼𝐼𝐼 − 𝑇𝑇𝐶𝐶ℎ,𝑂𝑂𝑂𝑂𝑂𝑂 )𝑞𝑞𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 = 𝑚𝑚̇𝐶𝐶𝐶𝐶 × 𝐶𝐶𝑝𝑝 × (𝑇𝑇𝐶𝐶𝐶𝐶,𝑂𝑂𝑂𝑂𝑂𝑂 − 𝑇𝑇𝐶𝐶𝐶𝐶,𝐼𝐼𝐼𝐼 )

(1)
(2)

Energy conservation is applied for the heatexchangers and any heat leaks are neglected.
The compressor power input is directly
recorded from a power meter which measuresthe electrical power usage of the compressor
through the duration of the experiment.
The exergy analysis of this MVC Chiller iscarried out with the aid of p-h diagram andderived equations for individual exergy
destruction within each component. Figure 1
displays the p-h diagram of both 7°C and 17°C
Chilled Water Outlet Temperature (TChOut)
conditions at Condenser Inlet Temperature(TCondIn) 32°C. The displayed points from 1-9

represent the green colour 7°C cycle whilst the
points a-i represent the purple colour 17°C
cycle;
1/𝑎𝑎 = 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆
2/𝑏𝑏 = 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷ℎ𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎
3/𝑐𝑐 = 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼
4/𝑑𝑑 = 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆
5/𝑒𝑒 = 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆
6/𝑓𝑓 = 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂
7/𝑔𝑔 = 𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸 𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉 𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼
8/ℎ = 𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸 𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼
9/𝑖𝑖 = 𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆

Fig. 1 p-h Diagram of Chiller at 7°C & 17°C TChOut at 32°C

TCondIn

It is also noted that data for a constantambient condition or Condenser Inlet
Temperature (TCondIn) 32°C with varying TChOut
7-17°C will be solely utilized for the analysis in
this paper, since the mechanical vapour
compression chillers are often operated and areconsidered to be more beneficial during higher
ambient temperatures in order to achieve
comfort conditions for occupants within indoor
spaces. The exergy analysis will be based on thefollowing equations, Eqn .3 to Eqn. 9. With theassumption of steady state conditions, the rateof exergy change within the control volume iszero and heat leaks are also assumed to be zero.
Exergy Balance Equation𝑛𝑛

𝑑𝑑𝑑𝑑𝑐𝑐𝑐𝑐
𝑇𝑇0
𝑑𝑑𝑑𝑑𝑐𝑐𝑐𝑐= � �1 − � 𝑄𝑄̇𝑗𝑗 + �𝑊𝑊̇𝑣𝑣 + 𝑝𝑝0
�
𝑑𝑑𝑑𝑑
𝑇𝑇𝑗𝑗
𝑑𝑑𝑑𝑑
𝑗𝑗=1

+ � 𝑚𝑚̇ 𝚤𝚤 𝑒𝑒𝑓𝑓𝑓𝑓 − � 𝑚𝑚̇ 𝑜𝑜 𝑒𝑒𝑓𝑓𝑓𝑓 − 𝐸𝐸̇𝐷𝐷
𝑖𝑖

𝑒𝑒Flow Exergy
𝑒𝑒𝑓𝑓 = (ℎ𝑖𝑖 − ℎ0 ) − 𝑇𝑇0 (𝑠𝑠𝑖𝑖 − 𝑠𝑠0 )Evaporator

(3)

(4)

Exergy Analysis of a Vapour Compression Chiller at Elevated Chilled Water Outlet Condition(5)

Condenser𝐸𝐸̇𝐷𝐷 = 𝑚𝑚̇𝑟𝑟𝑟𝑟𝑟𝑟 (𝑒𝑒𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓ℎ − 𝑒𝑒𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓,𝑜𝑜𝑜𝑜𝑜𝑜 ) − 𝑚𝑚̇𝐶𝐶𝐶𝐶 (𝑒𝑒𝑓𝑓𝑓𝑓𝑓𝑓,𝑖𝑖𝑖𝑖− 𝑒𝑒𝑓𝑓𝑓𝑓𝑓𝑓,𝑜𝑜𝑜𝑜𝑜𝑜 )(6)Compressor𝐸𝐸̇𝐷𝐷 = 𝑃𝑃𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 + 𝑚𝑚̇𝑟𝑟𝑟𝑟𝑟𝑟 (𝑒𝑒𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 − 𝑒𝑒𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓ℎ )(7)𝐸𝐸̇𝐷𝐷 = 𝑚𝑚̇𝑟𝑟𝑟𝑟𝑟𝑟 (𝑒𝑒𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓,𝑜𝑜𝑜𝑜𝑜𝑜 − 𝑒𝑒𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓,𝑖𝑖𝑖𝑖 )(8)Expansion Valve

Exergetic Efficiency

𝜂𝜂𝐸𝐸𝐸𝐸 = 𝐶𝐶𝐶𝐶𝐶𝐶 �

𝑇𝑇0

𝑇𝑇𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸− 1�

Exergy Destruction [kW]

0.7
0.6

Evap Ed
Cond Ed

Fig. 2Comp Ed
Exp Ed7
Fig. 3

12

TChOut [°C]17

Exergy destruction vs. TChOut at 32°C TCondIn
for compressor and expansion valve

The compressor and expansion valve which
involve no heat transfers are affected by thechange in the compression ratio which isclearly seen in Fig. 1 as the difference between
the condenser and evaporator pressuressignified by the decrease in the vertical span
between the two horizontals of 6-5-4-3 and 8-91 to the smaller gap of f-e-d-c and h-i-a. The
reduced compression ratio at the elevated
TChOut, will result in lesser exergy destruction
within the compressor due to the lower work
input, and lower pressure drop when passing
through the expansion valve will also result ina similar observation. Respectively, the exergy
destruction rates reduce by 0.6% per °C of
TChOut and 0.4% per °C of TChOut within theCompressor and Expansion Valve.
3.1.2 Exergetic Efficiency
The exergetic efficiency of the system isshown in Fig. 4. It is observed that theefficiency decreases with increasing chilledwater outlet temperature at a rate of 1.4% per°C of TChOut.

0.5
0.5
0.4
0.4
0.3
7

1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0

(9)

3.1 Results and Analysis
3.1.1 Exergy Destruction
With the increase in TChOut, there are mainly
two characteristics visible as shown in Fig. 2
and Fig. 3. The two heat exchangers(evaporator and the condenser) show anincreasing exergy destruction trend, whilst thecompressor and expansion device show adecreasing trend. Both the evaporator andcondenser are the heat exchanging componentsof this system and with increasing TChOut, theyalso exhibit increased quantities of heatexchanged20). Thus the major contributor to theincreased exergy destroyed would be thisincreased rate of cooling effect at theevaporator and heat dissipation rate at thecondenser.

0.6

water temperature, the p-h diagram (Fig. 1)
shows the evaporator pressure increase from 89-1 to h-i-a, which would also result in thelarger exergy destruction than at lower
evaporator pressures. The evaporator shows an
exergy destruction increase rate of 4.1% per °Cof TChOut, whilst the condenser increases at a
rate of 5.0% per °C of TChOut.

Exergy Destruction [kW]𝐸𝐸̇𝐷𝐷 = 𝑚𝑚̇𝑟𝑟𝑟𝑟𝑟𝑟 (𝑒𝑒𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓,𝐼𝐼𝐼𝐼 − 𝑒𝑒𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 ) − 𝑚𝑚̇𝐶𝐶ℎ (𝑒𝑒𝑓𝑓𝑓𝑓ℎ,𝑖𝑖𝑖𝑖− 𝑒𝑒𝑓𝑓𝑓𝑓ℎ,𝑜𝑜𝑜𝑜𝑜𝑜 )12

TChOut [°C]17

Exergy destruction vs. TChOut at 32°C TCondIn forevaporator and condenser

Furthermore, with the increase in the chilled0.24190.23470.24800.24380.26410.25590.27130.2646

0.27700.3

0.28680.4

0.2771

0.5

0.2825

0.6

0.2
0.1
0
7
8
9
10
11
12
13
14
15
15.6
16
17

Exergetic Efficeincy [-]Exergy Analysis of a Vapour Compression Chiller at Elevated Chilled Water Outlet Condition

TChOut [°C]

Exergetic efficiency vs. TChOut at 32°C TCondIn

Fig. 4This trend is expected as the chilled watertemperature is raised, the pressure of theevaporator increases. As the evaporator
temperature reaches T0, the exergetic efficiency
will reach zero.
3.1.3

Component and Total Exergy
Destruction
It would also be beneficial to ascertain how
the components rank with respect to the total
exergy destruction within the system. In Fig. 5
the exergy destruction values for each of thefour components are presented at 7°C and 17°C
TChOut.

Exergy Destruction [kW]Evap Ed

3.0

Cond Ed

Compressor Ed

2.5
2.0

0.270

1.5

1.178

0.0

Fig. 512%

52%

Expansion Ed

10%

Exergy destruction of individual components
vs. TChOut at 32°C TCondIn

The compressor clearly shows larger values incomparison to the rest of the components with
1.178 kW at 7°C and 1.090 kW at 17°C. The
evaporator and condenser have almost equalvalues in terms of exergy destruction with
0.426 kW and 0.410 kW at 7°C and 0.617 kW
and 0.624 kW at 17°C. The lowest exergy
destruction occurs within the expansion valve
with 0.270 kW at 7°C and 0.254 kW at 17°C.
Figure 6 presents the percentage values forexergy destruction within each individual

Cond
Comp
Exp

24%

Evap
Cond

42%

24%

Comp
Exp

0.617
17

Evap

Componenet Exergy Destruction
Percentage :TChOut = 17°C and TCondIn = 32°C

0.624

TChOut [°C]

19%
18%

1.090

0.410
0.426
7

Component Exergy Destruction Percentage:TChOut = 7°C and TCondIn = 32°C

0.254

1.0
0.5

component, which further helps visualized theimpact each component has on the overallsystem exergy destruction. The compressor
which has the largest exergy destruction with
52% and 42% at TChOut 7°C and 17°C
respectively shows a decreasing trend withincreasing TChOut. The evaporator andcondenser show almost equal amounts ofexergy destruction varying between 18-24%
which increase with elevated TChOut. The
expansion device has the lowest exergy
destruction values between 10-12% of thewhole system with a decreasing trend as TChOutis increased. Hence the compressor is the major
contributor to the exergy destruction of thesystem, followed by the heat exchangers whichshow increasing exergy destruction with therate of heat exchange.

Fig. 6Component exergy destruction percentage at

TChOut = 7°C & 17°C at 32°C TCondIn

The overall exergy destruction or the total
exergy destruction of the system increases at a
rate of 1.2% per °C with increasing TChOut asdisplayed in Fig. 7. This highlights the increasein irreversibilities of the system which results
in a reduction of the exergetic efficiency at a
rate of 1.4% per °C of TChOut. Even though the
COP and the cooling capacity of the system isstated to have increased by Thu et al. at rates
of 3.5% and 4% per °C increase in chilled watertemperature20), there still lies space for
improvement of the overall system andcomponents such as the compressor which

Exergy Analysis of a Vapour Compression Chiller at Elevated Chilled Water Outlet Condition

2.7
2.5
2.4
2.3
2.2

References

2.1

Fig. 7

4.

temperatures, which would help reduce thelarge effect chillers have on the environment.
Acknowledgments
The authors gratefully acknowledge the
Kyushu University Program for Leading
Graduate School, Green Asia Education Centerfor their support.

Total Ed

2.6

7
8
9
10
11
12
13
14
15
15.6
16
17

Exergy Destruction [kW]result in large irreversibilities and hence
exergy destruction.

TChOut [°C]

Total Exergy Destruction vs. TChOut at 32°C

TCondIn

Summary and Conclusions
The exergy analysis carried out in this paper
provides insight into the irreversibilities
associated with the operation of a mechanicalvapour compression (MVC) chiller at elevatedchilled water temperatures when thecondenser inlet cooling water is maintained at
32°C. Though initial findings by Thu et al. haveshown the increase in COP and performance ofthe chiller at elevated chilled watertemperatures and highlighted the possibility ofusing a MVC chiller coupled with adehumidifier, the exergy destruction within thesystem also increases at a rate of 1.2% per
degree Celsius of supply chilled watertemperature, whilst the exergetic efficiency
reduces at a rate of 1.4% per degree Celsius.
The compressor which contributes between 4252% of the total exergy destruction andrequires the most attention in order to lessen
the total exergy destruction of this system. This
would help the system reach a maximum
conversion of work potential during its coolingprocess whilst using the concept of handlingsensible and latent loads separately forpractical applications. The heat exchangers
also show an increasing trend of exergy
destruction with increasing heat transfer loads,
which should be of interest to designers since
this would highlight the possibility of including
further measures to reduce the irreversibilities
within these components. This study brings
forth a better understanding to the individual
efficiencies of the components of a MVC chiller
system and highlights that there is a possibility
to improve both the components individuallyand the system as a whole in order to lessen the
exergy destruction at elevated chilled water

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