chapter 3 experimental study of a chiller...

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CHAPTER 3

EXPERIMENTAL STUDY OF A CHILLER UNIT

INTEGRATED WITH COOL THERMAL ENERGY

STORAGE SYSTEM

An experimental investigation to study the performance of a chiller

unit integrated with encapsulated phase change material (PCM)-based cool

thermal energy storage is carried out. A series of charging experiments are

conducted to evaluate the performance of chiller unit integrated with/without

storage system. The experimental set-up and the performance results are

reported in this chapter.

3.1 EXPERIMENTAL SET-UP

A schematic diagram of the experimental apparatus designed and

constructed is shown in Figure 3.1 and the photographic view is shown in

Figure 3.2. The experimental set-up consists of two parts, cool thermal energy

storage tank and a vapour compression refrigeration system. A vertical

storage tank is integrated with the evaporator of the vapour compression

refrigeration system. The evaporator coils are designed in such a way that

they are completely immersed in the heat transfer fluid. The heat transfer fluid

returning from the storage tank is made to fall on the evaporator by using a

distribution pipe. In the evaporator, cold energy is transferred to the heat

transfer fluid and a temperature controller is attached to it, which can

maintain the evaporator tank temperature at any desired constant temperature

between 0ºC to – 20°C during the charging process. The evaporator tank is

28

V1

V2

5

7

8

T

P

F

1

A

P P

T

M

TC

T

4

3

6

V3

P

2

T P

Figure 3.1 Schematic diagram of experimental set-up

1. Compressor, 2. Condenser, 3. Expansion valve, 4. Evaporator tank, 5. Vertical

CTES tank, 6. Rotameter, 7. Data acquisition system, 8. Personalcomputer,

A-Accumulator, M-Fan motor, F-Filter and drier, T-Temperature measurement,

P-Pressure measurement, TC-Temperature controller, P-Pump, V1, V2,

V3-Valves

insulated with polyurethane foams (PUF) of 50 mm thickness. An aqueous

solution of 30 wt. % ethylene glycol is used as the heat transfer fluid and

R134a is used as a refrigerant. The main components of chiller unit are shown

in Figure 3.3 and the properties of R134a are given in Table 3.1. A cylindrical

storage tank with a diameter of 240 mm and 500 mm length, made of 3 mm

thick stainless steel plate with a design pressure of 5 bar installed in a vertical

manner is shown in Figure 3.4.

29

Figure 3.2 Photographic view of the experimental set-up

1. Chiller unit, 2. Vertical storage tank, 3. Horizontal storage tank,

4. Flow meter, 5. Acrylic tank, 6. Energy meters, 7. Electric heater,

8. Data acquisition and control system, 9. Pentium-4 PC

30

Figure 3.3 Photographic view of chiller unit

1. Evaporator tank, 2. Condenser, 3. R134a – compressor,

4. Thermostatic expansion valve, 5. HTF circulation pump

31

Figure 3.4 Sectional views of CTES tank

HTF Inlet T(in)

HTF Outlet T(out)

Out

let f

or

ther

moc

oupl

e

A A’

PCM

fille

d no

dule

s

Fixt

ure

for

ther

moc

oupl

es

TP5

TP3

TP1

T5

T3

T4

T2

T1

T10 T11

T9 T8

T6 T7

T7

T6

T11

T10 T9 T8

Section at A – A’

1 6 7f1

3 8 9f3

5 10 11f5

T +T +TT =3

T +T +TT =3

T +T +TT =3

32

The tank is provided with an upper opening for loading and

unloading the PCM spherical capsules. The tank is insulated with

polyurethane foam of 50 mm thickness. The phase change material is filled in

the spherical capsules of 48 mm diameter. The PCM capsules are made of

high density polyethylene material. The maximum number of spherical

capsules in the tank is 250. The PCM inside each capsule is 50 g of distilled

water with heterogeneous nucleation agents. Adding nucleation agents

initiates the freezing of water at its melting temperature. The PCM capsules

are filled with 90% volume to prevent the capsules from cracking due to

thermal expansion during the phase change process. The process of filling of

PCM capsules are shown in Figures 3.5 and 3.6. The thermo physical

properties of HTF and PCM are given in Table 3.2.

Table 3.1 Properties of R134a (CF3CH2F)

Mol. Wt.

Cr.Temp. ºC Boil. Point (at 1 bar)

Density kg/m3 (at -25ºC)

ODP GWP

102.3 101.1 -26.16 5.50(v)

1371.0(l) 0 0.27

Table 3.2 Thermo physical properties of HTF (30 wt.% Ethylene

glycol) and PCM (water)

Properties HTF PCM

Density (kg m-3) 1056 1000 (l), 920 (s)

Specific heat (kJkg-1 K-1) 3.65 4.186 (l), 2.01 (s)

Thermal conductivity (Wm-1 K-1) 0.485 0.566 (l), 2.22 (s)

Freezing point (ºC) -17.8 0

Latent heat (kJkg-1) - 333.6

33

Figure 3.5 Photographic view of drilling in spherical capsules 1. Polyethylene spherical capsule, 2. Micro drilling machine, 3. 1.5 mm drill pit

Figure 3.6 Photographic view of filling the PCM in capsules

1. Distilled water with bio-additive, 2. Filled PCM capsule

34

In the top and bottom of the cylindrical storage tank, perforated

distributor plates are provided to achieve uniform flow distribution. A

rotameter with an accuracy ± 0.5% is installed in the flow line between the

evaporator and storage tank to measure the flow rate of the aqueous ethylene

glycol. In the refrigeration circuit six RTD (PT100) sensors with ± 0.5°C

accuracy were fixed to measure the temperature at various points. Pressure

gauges with ± 0.25 % accuracy were also provided suitably. The locations of

temperature and pressure measurement are shown in Figure 3.1. To measure

the compressor and heater power input two energy metres are provided. The

temperature of the HTF and the PCM temperature are measured using k-type

thermocouples (with 1.5 mm OD). Five thermocouples are positioned axially

with 100 mm interval and six of them are positioned at radial location and an

interval of 60 mm. A fixture has been provided at the centre to hold the

thermocouples in position. Three thermocouples inserted into the spherical

capsules measure the temperatures of PCM. Also two thermocouples are

provided at inlet and outlet to measure the temperature of the HTF entering

and leaving the storage tank. All the thermocouples and RTDs are connected

with a data acquisition system (AI 8000+). The data acquisition and control

system and a personal computer (Pentium 4) are used for data recording and

storage.

3.2 EXPERIMENTS

A series of charging experiments are performed by varying the

HTF inlet at various temperatures between -2ºC and -15°C. The temperature

of the refrigerant in the chiller circuit at various locations, the temperature of

the HTF in the storage tank and PCM inside the spherical capsules are

recorded continuously using data acquisition system. The charging process

ends when the outlet temperature of HTF approaches nearly equal to the inlet

HTF temperature. The inlet and outlet pressures of the compressor and

35

evaporator are recorded continuously with 10 minutes interval during the

experimentation. The energy metre initial and final readings are noted to

calculate the cumulative energy used during the charging process. The

experiments are repeated for various condensing temperatures and also

varying the porosity at three different values (ε = 36%, 49% and 61%). The

porosity signifies the volume fraction of the HTF in the storage tank. In order

to evaluate the energy loss from the insulated cylindrical cool storage tank to

the surrounding, the overall heat loss coefficient is evaluated through the heat

gain experiment.

3.3 EVALUATION OF OVERALL HEAT LOSS COEFFICIENT

In order to evaluate the energy loss from the insulated cylindrical cool

storage tank to the surrounding, the overall heat loss coefficient is evaluated

through the following heat gain experiment. The average temperature of the

heat transfer fluid in the storage tank is maintained initially at -10 ºC. The

HTF temperature increases with respect to time due to the influence of higher

surrounding temperature. The temperature of heat transfer fluid inside the

storage tank and room temperature are recorded continuously. The final

temperature of HTF is noted after ten hours to evaluate the overall heat loss

coefficient. Figure 3.7 shows the temperature variation of HTF and room

temperature for ten hours.

The average overall heat transfer coefficient of the surface of the

storage tank can be determined as

( )st f fdTUA LMTD m cdt

(3.1)

36

0 100 200 300 400 500 600

-10

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

Time in minutes

HTF

Tem

pera

ture

o C

HTF temp.

0

5

10

15

20

25

30

35

Roo

m te

mp.

o C

Room temp.

where ( ) ( )

ln

fi sur f sur

fi sur

f sur

T T T TLMTD

T TT T

(3.2)

where Tfi denotes the average initial temperature of HTF, Tsur is the

surrounding temperature and Tf is the final HTF temperature. From the

experiment it is evaluated that the average overall heat loss coefficient (U) of

the cylindrical storage tank with the surrounding is 0.5978 W/m2°C.

Figure 3.7 Variation of temperature with time during heat gain

experiment

37

The maximum cool energy stored Qmax, in the cool thermal energy

storage (CTES) tank can be expressed as the sum of the energy storage in the

HTF and PCM capsules.

max HTF PCMQ Q Q (3.3)

The cold energy stored in the heat transfer fluid is given by

( )HTF f f i fQ m c T T (3.4)

The cold energy stored in the spherical capsules is given by

[ ( 0 ) (0 ) ]PCM P P i PS fQ m c T c T (3.5)

where Ti denotes the initial temperature and Tf denotes the final temperature

during charging process, mf is the mass of heat transfer fluid, mP the total

mass of the PCM inside spherical capsules, cf is the specific heat of heat

transfer fluid, cP, cPS are the specific heat of phase change material at liquid

and solid state respectively and λ is the latent heat of freezing of phase change

material.

3.4 PERFORMANCE EVALUATION PARAMETER

The specific energy consumption (SEC) of the chiller unit with and

without storage system defined as below is used as a parameter for evaluation.

.

max

( / )( )

3.52

Compwithout storage

st

PSEC kW TR

Q UA Tt

(3.6)

38

The compressor power (PComp) is the ratio of total electrical energy consumed

during charging process in kWh to the time for complete charging (Δt) in

second. The overall heat loss coefficient (U) of the storage tank and Qmax are

calculated using the equation (3.1) and (3.3).

max

( / )

3.52

Comp pumpwith storage

P PSEC kW TR

Qt

(3.7)

where Ppump is the power utilized to circulate the HTF between the evaporator

and the storage tank during the charging process.

3.5 ERROR ANALYSIS

The errors associated with various primary experimental

measurements and the calculations of performance parameters are detailed in

Appendix 3. The summary of estimated uncertainties is given in Table 3.3.

Table 3.3 Summary of estimated uncertainties

Parameters Uncertainty (%) Temperature 0.39 Length and Diameter 0.05 Time 1.66 Flow rate 0.5 Porosity 0.32 Energy stored 0.89 Compressor power 0.33 SEC without storage 2.57 SEC with storage 1.86

39

3.6 RESULTS AND DISCUSSION

The experimental results showing the effect of the inlet temperature

of heat transfer fluid and the performance parameters, namely, average rate of

charging, energy stored, specific energy consumption of the chiller with and

without storage system are reported in this section.

3.6.1 Effect of HTF inlet temperature

The results obtained from experimental investigation for various

inlet heat transfer fluid temperature under different condenser temperature are

presented to analyze the performance of the integration of cool thermal

storage system with the refrigeration system. The temperature histories of the

HTF and PCM in the storage tank for various HTF inlet temperatures with a

mass flow rate of 12 kg/min are studied and discussed. In all the figures the

temperature of heat transfer fluid and the phase change material are shown for

three different axial locations (bottom, centre and top) of the storage tank.

Figures 3.8 and 3.9 correspond to HTF and PCM temperatures for a HTF inlet

temperature of –2°C. Similarly figures are also drawn for HTF inlet

temperatures of -3, -5, -8 and -10°C respectively in Figures 3.10 to 3.17.

It is seen from Figures 3.8 and 3.9 that both the HTF and PCM

have reached the thermal equilibrium within a short period. The small

temperature gradient between the HTF and PCM is not sufficient for the

occurrence of ice nucleation and hence freezing does not occur and only

sensible cool energy is stored in the storage system. When the HTF inlet

temperature is maintained at -3°C (Figure 3.11) the PCM temperature has

reached nearly -3°C after 50 minutes and remains in the sub-cooled state for a

long period of time. After 200 minutes the freezing has been initiated in the

PCM capsules at the bottom portion of the tank. Suddenly the temperature of

40

the PCM is increased to its freezing temperature as the sub-cooled energy

available in the PCM capsules are used for the initial dendritic ice formation.

The freezing continues by rejecting its latent heat to the surrounding HTF at a

very low rate as the temperature potential difference available between HTF

and PCM is very low. The freezing has been initiated in the middle portion of

the tank only after 330 minutes and it is not initiated in the top portion of the

tank even after 400 minutes where the experimental trial is stopped.

When the HTF inlet temperature is maintained at -5°C

(Figures 3.12 and 3.13) both the HTF and PCM temperature in the storage

tank are brought down below 0°C (freezing temperature of the PCM) within a

short period of time. A short duration after the start of the process, a small

increase in temperature is observed in the HTF in the bottom portion of the

storage tank (Figure 3.12). This is due to sudden large heat removal from the

PCM as the freezing is initiated in the inner surface of the PCM capsules with

high temperature gradient. As the HTF temperature increases the temperature

gradient between the PCM and HTF decreases and the HTF in the bottom

portion of the storage tank attains an equilibrium condition after a small time

interval i.e., the heat absorbed from the PCM and the cool energy absorbed

from the incoming HTF are maintained at a constant level (Figure 3.13). A

sudden increase in temperature of the PCM is also observed when the freezing

is initiated by dendritic ice formation at 60 minutes, 90 minutes and 125

minutes respectively at bottom, centre and top portion of the storage tank. The

freezing process continues at constant temperature till the PCM rejects its

total latent heat. Then the PCM temperature decreases till it attains

equilibrium with the HTF inlet temperature. Similar trends are observed when

the HTF inlet temperatures are maintained at -8 and -10°C (Figures 3.14

to 3.17). The complete freezing time for the HTF inlet temperatures of -5°C,

-8°C and -10°C are approximately 425, 250 and 190 minutes respectively.

41

0 50 100 150 200 250 300 350 400 450 500-15

-10

-5

0

5

10

15

20

25

Flow rate = 12 kg/min

Tf1 Tf3 Tf5

HTF

Tem

pera

ture

o C

Time in minutes

0 50 100 150 200 250 300 350 400 450 500-15

-10

-5

0

5

10

15

20

25


TP1 TP3 TP5

PCM

Tem

pera

ture

o C

Time in min

Figure 3.8 Variation of HTF temperature with time for Tf,in = -2°C

Figure 3.9 Variation of PCM temperature at centre of capsules with

time for Tf,in = -2°C

42

0 50 100 150 200 250 300 350 400-15

-10

-5

0

5

10

15

20

25


Tf1 Tf3 Tf5

HTF

Tem

pera

ture

o C

Time in minutes

0 50 100 150 200 250 300 350 400-15

-10

-5

0

5

10

15

20

25


TP1 TP3 TP5

PCM

Tem

pera

ture

o C

Time in minutes




43

0 50 100 150 200 250 300 350 400 450 500-15

-10

-5

0

5

10

15

20

25


Tf1 Tf3 Tf5

HTF

Tem

pera

ture

o C

Time in minutes

0 50 100 150 200 250 300 350 400 450-15

-10

-5

0

5

10

15

20

25


TP1 TP3 TP5

PCM

Tem

pera

ture

o C

Time in min


Figure 3.13 Variation of PCM temperature at center of capsules with


44

0 50 100 150 200 250-15

-10

-5

0

5

10

15

20

25


Tf1 Tf3 Tf5

HTF

Tem

pera

ture

o C

Time in minutes

0 50 100 150 200 250

-10

-5

0

5

10

15

20

25


TP1 TP3 TP5

PCM

Tem

pera

ture

o C

Time in minutes


Figure 3.15 Variation of PCM temperature at center of capsules with


45

0 50 100 150 200-15

-10

-5

0

5

10

15

20

25


Tf1 Tf3 Tf5

HTF

Tem

pera

ture

o C

Time in minutes

0 25 50 75 100 125 150 175 200-15

-10

-5

0

5

10

15

20

25


TP1 TP3 TP5

PCM

Tem

pera

ture

o C

Time in minutes




46

3.6.2 Energy stored in CTES at different HTF inlet temperature

Porosity determines the quantity of cool thermal energy that can be stored and it is varied by changing the number of PCM capsules in the storage tank. Figure 3.18 shows the variation of cool energy stored with time at

different inlet HTF temperature for porosity ε = 36% and .

m f = 12 kg/min.

The influence of inlet HTF temperature during charging on cool thermal energy storage has been studied by decreasing inlet HTF temperature from -2°C to -15°C without changing the flow rate and porosity. As in the case of -2°C, the energy is stored only in the form of sensible cooling in the HTF and

PCM. Therefore, the accumulated cool storage does not increase any more while it attains the maximum amount of sensible cooling (2082 kJ) within 150

min and remains constant. For -3°C the cool storage curve rises continuously even after 450 min which shows that the PCM capsules are undergoing the charging process. This is due to small temperature gradient between the HTF and freezing temperature of PCM is not sufficient for the heat flow for the

completion of freezing. For -5°C HTF inlet temperature, the charging has been completed at 450 min and maximum energy charged is about 7267 kJ.

The complete charging curve has two parts, the initial part with a large slope represents the sensible cooling and the later part with a small slope represents the latent cool storage and sensible cooling in frozen PCM. It is observed that when the inlet HTF temperature decreases the time required for complete

charging decreases as high thermal potential difference is available for the heat flow. The maximum cool energy stored for various HTF inlet temperatures of -8°C, -10°C and -15°C are approximately 7455 kJ, 7581 kJ and 7895 kJ respectively. The time for complete charging

with a HTF inlet temperature of -8°C, -10°C and -15°C are 350 min, 250 min and 200 min respectively. It is observed that when the HTF inlet temperature

is maintained below -5 ºC the charging time decreases and energy charged is slightly increased. However production of low temperature HTF is very expensive that reduces the COP of the refrigeration system.

47

0 50 100 150 200 250 300 350 400 4500

1x103

2x103

3x103

4x103

5x103

6x103

7x103

8x103

9x103

= 36%

Tf,inoC

-2 -3 -5 -8 -10 -15

Ene

rgy

stor

ed k

J

Time in minutes

Figure 3.18 Variation of cool energy stored with time at different inlet

HTF temperature for = 36%

Figures 3.19 and 3.20 show the variation of cool energy stored with

time at different inlet HTF temperature for ε = 49% and ε = 61% respectively.

It is observed from the figures that for -2°C, the energy is stored only as

sensible cooling even at higher porosities (49% and 61%) due to lower

thermal gradient between HTF and PCM capsules. It is seen from Figure 3.19

that when the HTF inlet temperature is maintained at -3°C, the maximum

possible energy (5760 kJ) is stored within 400 min and at 61% porosity

(Figure 3.20) charging has been completed at 375 min and maximum energy

stored is 5134 kJ. For -5°C, when porosity is increased from 36% to 49% and

61% the charging time is reduced by 22% and 29% respectively and energy

stored is decreased by 14% and 29% respectively. Similarly when the porosity

is increased from 49% to 61%, the time required for complete charging is

reduced to 12.55, 14.3% and 16.6% for HTF inlet temperature of

-8°C, -10°C and -15°C respectively. This is due to increase in porosity that

48

0 50 100 150 200 250 300 350 400 4500

1x103

2x103

3x103

4x103

5x103

6x103

7x103

8x103 =49%

Tf,inoC

-2 -3 -5 -8 -10 -15

Ener

gy s

tore

d kJ

Time in minutes

0 50 100 150 200 250 300 350 400 4500

1x103

2x103

3x103

4x103

5x103

6x103

7x103

8x103

= 61%

Tf,in oC

B C D E F G

Ene

rgy

stor

ed k

J

Time in minutes





49

would result in an increased HTF passage and lower mass of PCM capsules in

the entire storage tank. However, the higher porosity value decreases the

amount of energy stored for a given volume of storage tank and this causes to

increase the size of the storage tank.

3.6.3 Energy stored in PCM and HTF

Figure 3.21 shows the variation of energy stored in PCM and HTF

with HTF inlet temperature for different values of porosity. It is found that the

energy stored in HTF is gradually increasing with decrease in HTF inlet

temperature and the value is higher at higher porosity. When the HTF inlet

temperature is maintained above -3°C, the energy stored in PCM is very low

at all porosity values due to the absence of latent cool energy storage process.

However, when the HTF inlet temperature is maintained below -3ºC the

amount of energy stored in PCM is very considerable. It is also observed that

for the HTF inlet temperature of -5 to -15°C, the energy stored in the PCM

does not vary much as the amount of energy stored during sensible cooling in

solid PCM is negligible compared to the latent cooling process. Hence, for the

application where the requirement is above 0 °C, the HTF inlet temperature to

the storage system may be restricted around -4°C to avoid the high production

cost at lower temperature, which is again subjected to a constraint arising

from the heat exchange process. The amount of latent cool energy stored is

nearly 7, 5.5 and 4 times higher than the sensible heat stored for ε = 36%,

49% and 61% respectively for the HTF inlet temperature of -4°C. While

calculating the quantity of sensible cool energy stored a temperature gradient

of 20°C is considered.

In practical applications, since the actual temperature drop is much

less than 20°C, the ratio of useful latent cool energy stored to the useful

sensible cool energy stored will be much higher and hence the size of the

50

-15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 00

1x103

2x103

3x103

4x103

5x103

6x103

7x103

8x103

9x103 HTF PCM = 36% = 36% = 49% = 49% = 61% = 61%

Ener

gy st

ored

kJ

HTF Temperature oC

latent cool energy storage system requirement will be nearly 3 to 6 times less

than the chilled water storage system depending on the porosity and the

application.

Figure 3.21 Variation of energy stored in PCM and HTF with HTF inlet

temperature at different porosity

3.6.4 Average rate of charging and time for complete charging

Figure 3.22 shows the variation of average rate of charging and

time for complete charging with HTF inlet temperature for different porosity

values. It is observed that the average rate of charging increases when the

HTF inlet temperature decreases for all porosity values and it is higher for

higher porosity values. The time for complete charging decreases with

decrease in HTF inlet temperature for all porosity values and it is lesser for

higher porosity values. It is seen from the figure that the increase in rate of

charging or decrease in time for complete charging is higher as it decreases

from 0 to -8°C. Further reducing the HTF inlet temperature does not lead to

51

-15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -20.0

0.2

0.4

0.6

0.8

1.0

Temperarture oC

Rate

of C

harg

ing

kJ/s

ec

= 36% = 49% = 61%

0

100

200

300

400

500

600

700

Tim

e in

min

utes

= 36% = 49% = 61%

the corresponding increase in the rate of charging or decrease in time for

complete charging.

Hence for the application where the required heat transfer cannot be

achieved with the given surface area, the HTF inlet temperature can be

brought down to -8°C. Very low HTF inlet temperature is not augmenting the

heat transfer appreciably and also making the production cost very expensive.

It is also seen from the figure that the porosity has certain influence on the

rate of charging. At lower porosity value during the initial charging process,

the energy that is being absorbed by the PCM is more than that supplied from

the inlet to the storage tank. Hence the temperature of the HTF in the storage

tank increases and it reduces the temperature difference between the HTF and

the PCM, which in turn reduces the heat transfer.

Figure 3.22 Variation of average charging rate and time for complete

charging with HTF inlet temperature at different porosity

52

3.6.5 Specific energy consumption (SEC)

The variations of SEC with HTF inlet temperature for two different

condensing temperatures for the chiller unit with and without storage system

are shown in Figures 3.23 to 3.25 for porosities ε = 36%, 49% and 61%

respectively. It is seen from Figure 3.23 that the SEC of the chiller with and

without storage system increases with decrease in HTF inlet temperature and

it is higher at higher condensing temperature. While comparing the SEC of

the chiller unit with and without storage a 6% to 20% increase in SEC is

observed and this percentage increase is higher at higher HTF temperature. It

is due to the fact that when the HTF inlet temperature is high the time

duration for complete charging is higher that increases the heat loss from the

storage tank and the pumping power for the circulation of HTF. It is also

calculated from the results that 1°C decrease in HTF temperature increases

the SEC between 3% and 4% and 1 ºC increase in condensing temperature

increases the SEC between 2.75% to 3.25%. The above said trends are also

seen for other porosities shown in Figures 3.24 and 3.25. Further, it is seen

from the figures that when the porosity is decreased the SEC increases, which

is already explained in the previous section that the lower porosity decreases

the charging rate that in turn increases the SEC.

53

-15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -11.0

1.5

2.0

2.5

3.0

3.5

4.0

without storage with storage

TC = 38 oC TC = 38 oC

TC = 54 oC TC = 54 oC

SEC

kW

/TR

HTF Temperature oC

-15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -11.00

1.25

1.50

1.75

2.00

2.25

2.50

2.75

3.00 without storage with storage

TC = 38 oC TC = 38 oC

TC = 54 oC TC = 54 oC

SEC

kW

/TR

HTF Temperature oC

Figure 3.23 Variation of SEC with HTF inlet temperature at different

condensing temperature for ε =36% during charging


condensing temperature for ε = 49% during charging

54

-15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -21.00

1.25

1.50

1.75

2.00

2.25

2.50

2.75

3.00 without storage with storage

TC = 38 oC TC = 38 oC

TC = 54 oC TC = 54 oC

SEC

kW

/TR

HTF Temperature oC


condensing temperature for ε = 61% during charging

When the chiller is integrated with storage system for any

application, the freezing temperature of the selected PCM should be as high

as possible which depends on the process-cooling requirement to achieve

higher COP or lower SEC. Under any circumstances the chiller has to be

operated at a temperature lesser than the system without storage to achieve the

required temperature potential difference for the heat transfer. This may

reduce the performance of the chiller plant and the energy consumption is

higher at lower operating temperature. However, when the system is

integrated with storage system, the cooling load can be shifted to the night

hours where lower SEC is possible due to lower condensing temperature. This

benefit is not applicable to the system with water-cooled condenser, as the

condensing temperature has no influence due to the ambient condition. In

addition the storage system has the following advantages.

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(i) The CTES system can be charged during the night hours and

the stored energy can be retrieved during the daytime, hence

the tariff difference during peak hours and off-peak hours can

be exploited.

(ii) Chiller plant can be operated always under full load condition

and hence the efficiency of the system is high.

(iii) Reducing the monthly demand charges for building air

conditioning and industrial refrigeration applications. Further

there are additional savings through reduced size of

refrigeration system components and piping, which resulted in

significant saving in capital and operating cost.

chapter 3 experimental study of a chiller...

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