i n t e rn a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 8 9 3e9 0 1

www. i ifi i r .org

ava i lab le at www.sc iencedi rec t . com

journa l homepage : www.e lsev ier . com/ loca te / i j r e f r ig

Performance characteristics of a simultaneous coolingand heating multi-heat pump at partial load conditions

Youngju Joo, Hoon Kang, Jae Hwan Ahn, Mooyeon Lee, Yongchan Kim*

Department of Mechanical Engineering, Korea University, Anam-Dong, Sungbuk-Ku, Seoul 136-713, Republic of Korea

a r t i c l e i n f o

Article history:

Received 14 October 2010

Received in revised form

29 November 2010

Accepted 26 December 2010

Available online 9 January 2011

Keywords:

Heat pump

Heat recovery

Variable speed

Electronic expansion valve

R410A

* Corresponding author. Tel.: þ82 2 3290 336E-mail address: yongckim@korea.ac.kr (Y

0140-7007/$ e see front matter ª 2011 Elsevdoi:10.1016/j.ijrefrig.2010.12.025

a b s t r a c t

A simultaneous cooling and heating multi-heat pump can improve thermal comfort and

energy efficiency through heat recovery at partial load conditions. The performance of

a simultaneous heating and cooling multi-heat pump with four indoor units was measured

by varying the compressor speed, EEV opening, and fan speed at full and partial load

conditions in five operating modes. In the cooling-only and heating-only modes, the

heating and cooling capacities were properly controlled by varying the compressor speed.

However, in the cooling-main and heating-main modes under partial load conditions,

a large imbalance between the cooling and heating capacities was observed even though

the compressor speed was optimized. This capacity imbalance under partial load condi-

tions was optimized by adjusting the EEV openings in the mode change unit and the

outdoor unit. In addition, in the entire-heat recovery mode, rating compressor speed ratios

were proposed under the full and partial load conditions.

ª 2011 Elsevier Ltd and IIR. All rights reserved.

Caracteristiques de la performance de plusieurs pompes achaleur assurant le chauffage et le refroidissementsimultanes, fonctionnant en charge partielle

Mots cles : Pompe a chaleur ; Recuperation de chaleur ; Vitesse variable ; Detendeur electronique ; R410A

1. Introduction

In recent years, multi-heat pumps (or variable refrigerant flow

systems) have been widely applied in residential and commer-

cial buildings to improve building energy efficiency. Compared

to conventional central air-conditioning systems, multi-heat

pumps can achieve precise temperature control of each indi-

vidual room without air and water distribution circuits and

6; fax: þ82 2 921 5439.. Kim).ier Ltd and IIR. All rights

ducts (Xia et al., 2004). Generally, a conventional multi-heat

pump operates in a single mode (heating-only mode or

cooling-onlymode) at the same time. However, a simultaneous

heating and cooling multi-heat pump can provide heating and

cooling for different zones at the same time by using a variable

compressor, an electronic expansion device (EEV), and a mode

change unit (MCU). The MCU distributes the refrigerant to the

indoor units (IDUs) in five operating modes: cooling-only,

reserved.

Nomenclature

COP coefficient of performance

CR compression ratio

CSR compressor speed ratio (%)

EEV electronic expansion valve

FSR fan speed ratio (%)

IDU indoor unit

m mass flow rate (kg h�1)

MCU mode change unit

ODU outdoor unit

P pressure (kPa)

q capacity (W)

RH relative humidity (%)

W power consumption (W)

FEEV EEV opening (%)

Subscripts

c cooling

comp compressor

d compressor discharge

f fan

h heating

s compressor suction

t total

i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 8 9 3e9 0 1894

heating-only, cooling-main, heating-main, and entire-heat

recovery. The simultaneous cooling and heating multi-heat

pump can improve thermal comfort and energy efficiency

through heat recovery at partial load conditions.

Many researchers have investigated the performance of

a multi-heat pump in relation to the compressor speed and

EEV opening. Choi and Kim (2003) measured the perfor-

mance of a two-room multi-heat pump by varying the

compressor frequency and EEV opening. Kwon et al. (2004)

investigated the performance of a two-room heat pump

with a constant speed compressor. They found that the

lowest driving voltage of the compressor was approximately

75% of the rating voltage. Park et al. (2001) carried out

performance analysis for a multi air-conditioner using an

EEV and a variable speed compressor in a two-room envi-

ronment. Hu and Yang (2005) investigated the performance

of a multi air-conditioner using a digital scroll compressor.

Sarkar et al. (2004) studied the performance characteristics

of a simultaneous cooling and water heating system that

used CO2. Kang et al. (2009) measured the performance of

a simultaneous heating and cooling multi-heat pump in the

five operating modes, but they did not discuss the perfor-

mance characteristics of the multi-heat pump at partial load

conditions.

For a simultaneous heating and cooling multi-heat pump,

precise control of refrigerant distribution into each IDU is

required for each operating mode. Especially, at partial load

conditions, the control of refrigerant flow should be carefully

carried out to overcome performance degradation of the

simultaneous heating and cooling multi-heat pump system.

However, there are hardly any studies in the open literature

on the performance of the simultaneous heating and cooling

multi-heat pump under partial load conditions.

The objective of this study is to investigate the performance

characteristics of a simultaneous cooling and heating multi-

heat pump under partial load conditions. The performance of

the simultaneous heating and cooling multi-heat pump with

four IDUswasmeasured by varying the compressor speed, fan

speed, and EEV openings in the MCU and ODU under full and

partial load conditions in the five operatingmodes. The effects

of the compressor speed on system performance were inves-

tigated to achieve the designed capacity for each operating

mode under partial load conditions. In addition, the effects of

the EEV openings and fan speed on system performance were

discussed to optimize the cooling andheating capacities under

partial load conditions in the cooling-main and heating-main

modes.

2. Experimental setup and test procedure

2.1. Experimental setup

Fig. 1(a and b) shows schematic diagrams of the experimental

setup in the cooling-main and heating-main modes.

A simultaneous heating and cooling multi-heat pump using

R410Awas designed to have a cooling capacity of 8.0 kW in the

cooling-only mode. The experimental setup consisted of an

outdoor unit (ODU), an MCU, and four IDUs. The ODU con-

sisted of a blushless direct current (BLDC) type rotary

compressor, an oil separator, a liquidegas separator, an

accumulator, a finned-tube heat exchanger, and an EEV. Each

IDU consisted of a finned-tube heat exchanger and an EEV.

The heat exchangers for the IDUs and ODU used micro-fin

tubes and slit-fins. Each IDU had a cooling capacity of 2.15 kW

at an evaporating temperature of 7.2 �C and an air flow rate of

6.0 m3 min�1. The outdoor heat exchanger had a condensing

capacity of 11.34 kW at a condensing temperature of 54.4 �Cand an air flow rate of 37.5 m3 min�1.

An MCU, consisting of header pipes, branch pipes, and

solenoid valves, was installed between the ODU and IDUs. The

solenoid valves controlled the refrigerant flow path entering

into each IDU, which changed the system operating mode. An

EEV, consisting of a stepping motor and a needle valve, was

adopted for optimum control of the refrigerant flow rate. The

orifice diameters for the EEVs were 1.4 mm for the IDUs and

1.8 mm for the ODU.

Table 1 shows the operational status of each IDU in the

five operating modes: the cooling-only, heating-only, cooling-

main, heating-main, and entire-heat recovery modes. In the

cooling-only mode, “0H4C” implies that all four IDUs operate

as cooling coils under full load conditions, while “0H3C”

indicates that three IDUs operate as cooling coils under

partial load conditions. Therefore, in the 0H3C mode, one IDU

does not work at all through the closure of the EEV in the

IDU under no-operation. In the heating-only mode, “4H0C”

implies that all four IDUs operate as heating coils under full

P

Accumulator

V A W

4-Way

valve

P

T

Comp.

P

Oil Separator

T

ODU

Heat exchanger

TP

MCU

IDU (No.2)

IDU (No.3)

IDU (No.4)

C

H

C

H

C

H

IDU EEV(4)

( 1.4)

IDU EEV(3)

( 1.4)

IDU EEV(2)

( 1.4)

P2

IDU (No.1)

C

HT1

IDU EEV(1)

( 1.4)

T2

P3

T3

Pressure

regulator

T1 P2 T2

T3

P3

T1

T3

P3

T1

T3

P3

ODU

EEV

( 1.8)

D

D

M

M

Mass flow meter

(Heating)

Mass flow meter

(Cooling)

D

ODU IDU

TP TP

Suction line

Suction line

Bypass

line

Liquid line

MCU

EEV

Solenoid

valve

Cooling

Cooling

Cooling

Heating

High pressure

Low pressure

a

b

Cooling-main mode (1H3C)

P

Accumulator

V A W

4-Way

valve

P

T

Comp.

P

Oil Separator

T

ODU

Heat exchanger

TP

MCU

IDU (No.2)

IDU (No.3)

IDU (No.4)

C

H

C

H

C

H

IDU EEV(4)

( 1.4)

IDU EEV(3)

( 1.4)

IDU EEV(2)

( 1.4)

P2

IDU (No.1)

C

HT1

IDU EEV(1)

( 1.4)

T2

P2 T2

P2 T2

P3

T3

Pressure

regulator

T1 P2 T2

T3

P3

T1

T3

P3

T1 P2

T3

P3

ODU

EEV

( 1.8)

D

D

M

M

Mass flow meter

(Heating)

Mass flow meter

(Cooling)

DODU IDU

TP TP

Suction line

Suction line

Bypass

line

Liquid line

MCU

EEV

Solenoid

valve

High pressure

Cooling

Heating

Heating

Heating

Low pressure

Heating-main mode (3H1C)

Φ

Φ

Φ

Φ

Φ

Φ

Φ

Φ

Φ

Φ

T2

P2 T2

Fig. 1 e Schematic diagram of the experimental setup.

i n t e rn a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 8 9 3e9 0 1 895

Table 1 e Operational status of each IDU in the five operating modes.

Operating mode Symbol IDU #1 IDU #2 IDU #3 IDU #4 Load condition

Cooling-only 0H4C Cooling mode Cooling mode Cooling mode Cooling mode Full load

0H3C Cooling mode Cooling mode Cooling mode No-operation Partial load

Cooling-main 1H3C Cooling mode Heating mode Cooling mode Cooling mode Full load

1H2C Cooling mode Heating mode Cooling mode No-operation Partial load

Heating-only 4H0C Heating mode Heating mode Heating mode Heating mode Full load

3H0C Heating mode Heating mode Heating mode No-operation Partial load

Heating-main 3H1C Heating mode Cooling mode Heating mode Heating mode Full load

2H1C Heating mode Cooling mode Heating mode No-operation Partial load

Entire-heat recovery 2H2C Heating mode Cooling mode Heating mode Cooling mode Full load

1H1C Heating mode Cooling mode No-operation No-operation Partial load

i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 8 9 3e9 0 1896

load conditions, while “3H0C” indicates that only three IDUs

operate as heating coils under partial load conditions.

In the cooling-main mode, “1H3C” denotes that there are

one heating-operated IDU and three cooling-operated IDUs

under full load conditions. In the 1H3C mode, as shown in

Fig. 1(a), the refrigerant flow exiting the compressor splits into

twopaths: themainpath through theODUandcooling-operated

IDUs, and the sub-path through the heating-operated IDU.

Finally, the bypassing refrigerant flow through the sub-path

merges into the main path at the inlet of the cooling-operated

IDUs. At partial load conditions in the cooling-main mode

(1H2C), oneof thecooling-operated IDUsisshutoffbyclosing the

EEV in the IDU. In the heating-main mode (3H1C), as shown in

Fig. 1(b), the refrigerant flow exiting the compressor passes the

heating-operated IDUs and then splits into two paths: themain

path through the ODU and the sub-path through the cooling-

operated IDU. Finally, the refrigerant flow through the sub-path

merges into the main path at the inlet of the accumulator. In

these operatingmodes, the partial heat absorbed by the cooling-

operated IDUs is reused forheating; this is referred to as theheat

recovery operation. In addition, the outdoor heat exchanger is

usedasacondenser inthecooling-onlyandcooling-mainmodes

and as an evaporator in the heating-only and heating-main

modes.

In the entire-heat recovery mode, the number of cooling-

operated IDUs is the same as that of heating-operated IDUs.

Since there is no refrigerant flow to the outdoor heat

exchanger, all the heat absorbed from the cooling-operated

IDUs is reused for heating. “2H2C” indicates that two IDUs

operate as heating coils and two IDUs operate as cooling coils

under full load conditions, while “1H1C” denotes that only two

IDUs operate as heating and cooling coils, respectively, under

partial load conditions.

Table 2 e Test conditions for each operating mode.

Operating mode IDU forcooling

IDU forheating

ODU

Cooling-only 27 �C, 47% RH e 35 �C, 40% RH

Cooling-main 27 �C, 47% RH 20 �C, 59% RH 35 �C, 40% RH

Heating-main 27 �C, 47% RH 20 �C, 59% RH 7 �C, 87% RH

Heating-only e 20 �C, 59% RH 7 �C, 87% RH

Entire-heat

recovery

27 �C, 47% RH 20 �C, 59% RH e

2.2. Measuring equipment and test procedure

The ODU was installed in a psychrometric chamber, and each

IDU was installed in separate air handling units that could

control the temperature and humidity entering the unit.

Pressure transducers and thermocouples were installed at the

inlet and outlet of each component to measure the pressure

and temperature, respectively. A resistance temperature

detect sensor (PT 100 U) was used to measure the dry and wet

bulb temperatures of air to an accuracy of �0.15 �C. The air

flow rate of the indoor heat exchanger wasmeasured by using

a nozzle with a diameter of 76.2 mm. The pressure difference

across the nozzle was measured by using an electronic

differential pressure transducer with an accuracy of �0.25%.

The power consumption of the multi-heat pump was

measured by using an electronic power meter with an accu-

racy of �0.2%.

The performance of the simultaneous heating and cooling

multi-heat pump was measured in the five operating modes

under full and partial load conditions (Table 1). The refrigerant

charge amount affects the system performance at each

operating mode. Before the performance tests, the optimum

refrigerant charge was determined based on the cooling-only

mode because the system performance in the cooling-only

mode was more sensitive to the refrigerant charge than in the

heating-onlymode. Table 2 shows the test conditions for each

operating mode (ARI 210/240, 1986; ISO/DIS 15042, 2005). The

indoor air temperature and relative humidity in the cooling-

operated unit were set at 27.0 �C and 47%, respectively, while

those in the heating-operated unit were set at 20 �C and 59%,

respectively. The outdoor air temperature and relative

humidity in the cooling mode were set at 35.0 �C and 40%,

respectively, while those in the heating mode were set at

7.0 �C and 87%, respectively. During the tests, the compressor

frequency varied from 30% to 100% of the rated value of 50 Hz.

The EEV openings in the IDUs were adjusted to maintain the

superheat at 5 �C. During the baseline tests, the EEV openings

in the MCU and ODU were fixed at 100%. However, during the

optimization tests, the EEV openings in the MCU and ODU

varied from 20% to 100%.

The air-side cooling and heating capacities were deter-

mined by utilizing the air enthalpymethod (ASHRAE Standard

116, 1993) based on both the air flow rate and enthalpy

difference across the heat exchanger. The air flow rate was

determined by measuring the pressure difference between

the inlet and outlet of the IDU. The COP of the system was

Compressor speed ratio, CSR (%)

30 40 50 60 70 80 90 100

)h

/g

k(

ti

nu

re

pe

ta

rw

ol

fs

sa

me

ga

re

vA

0

10

20

30

40

50

60

70

80

90

100

1H3C

1H2C

Compressor speed ratio, CSR (%)

30 40 50 60 70 80 90 100 110

)h

/g

k(

ti

nu

re

pe

ta

rw

ol

fs

sa

me

ga

re

vA 0

10

20

30

40

50

60

70

80

90

100

Cooling IDU Heating IDU

0H4C

0H3C

1H3C

1H2C

a b

Fig. 3 e Variation of the average mass flow rate per unit

with the CSR in the cooling-only and cooling-main modes.

i n t e rn a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 8 9 3e9 0 1 897

calculated by Eq. (1). As shown in Eq. (2), the power

consumption includes the compressor and fan power. The

estimated uncertainties of the cooling and heating capacities,

and COP of the systemwere to be approximately 2.93%, 2.41%,

and 3.61%, respectively, by single-sample analysis according

to ASHRAE Guideline 2 (1986).

COP ¼ �qt;c þ qt;h

��Wt (1)

Wt ¼ Wcomp þWf (2)

3. Results and discussion

3.1. Performance characteristics at partial loadconditions

Fig. 2(a) compares the average cooling capacities per unit

under full load conditions (0H4C, 1H3C) with those under

partial load conditions (0H3C, 1H2C), respectively, in the

cooling-only and cooling-main modes. The average cooling

capacity per unit was evaluated by the ratio of the total cooling

capacity of the cooling-operated IDUs to the number of

cooling-operated IDUs, and vice versa for the average heating

capacity per unit. The compressor speed ratio (CSR) was

defined as the ratio of the actual compressor speed to the

reference value of 3500 rpm. The cooling capacity per unit in

the cooling-only (0H4C, 0H3C) and cooling-main (1H3C, 1H2C)

modes increasedwith an increase in the CSR because the total

mass flow rate in the cooling-operated IDUs increased.

Generally, the average cooling capacity per unit is strongly

dependent on the number of the cooling-operated IDUs

because themass flow rate per cooling-operated IDU increases

with a decrease in the number of cooling-operated IDUs, as

shown in Fig. 3(a). Therefore, the average cooling capacities

per unit in the 0H3C and 1H2Cmodes (partial load conditions)

were higher than those in the 0H4C and 1H3Cmodes (full load

conditions), respectively, atmediumand high CSRs because of

a lower evaporating pressure and higher mass flow rate per

cooling-operated IDU at partial load conditions. However, at

low CSRs, the average cooling capacities per unit in the 0H4C,

0H3C, and 1H3Cmodeswere very similar due to an insufficient

Compressor speed ratio, CSR (%)

30 40 50 60 70 80 90 100

)W

(ti

nu

re

py

ti

ca

pa

cg

ni

lo

oc

eg

ar

ev

A

500

1000

1500

2000

2500

3000

3500

Cooling capacity Heating capacity

0H4C

0H3C

1H3C

1H2C

Compressor speed ratio, CSR (%)

30 40 50 60 70 80 90 100

)W

(t

in

ur

ep

yt

ic

ap

ac

gn

it

ae

he

ga

re

vA

500

1000

1500

2000

2500

3000

3500

1H3C

1H2C

a b

Fig. 2 e Variation of the average cooling and heating

capacities per unit with the CSR in the cooling-only and

cooling-main modes.

mass flow rate in each IDU as compared with the required

value for stable system operation. In addition, for the cooling-

only mode, the difference in the average cooling capacity per

unit between the 0H3C and 0H4C modes became larger with

an increase in the CSR because themass flow rate per cooling-

operated IDU at partial load conditions increasedmore rapidly

than that at full load conditions, as shown in Fig. 3(a). It is

interesting that the 1H3C and 0H3C modes showed similar

average cooling capacities per unit at all CSRs due to the same

number of cooling-operated IDUs. Therefore, the CSR for each

operating mode should be properly controlled to obtain the

designed cooling capacity for each cooling-operated IDU. In

this study, the rating CSRwas approximately 80% for the 0H3C

and 1H3C modes, and 50% for the 1H2C mode to obtain the

designed cooling capacity of 2026 W.

Fig. 2(b) compares the heating capacity per unit in the 1H3C

mode under full load conditions with that in the 1H2C mode

under partial load conditions. As expected, the heating

capacity per unit increased with an increase in the CSR.

However, at all CSRs, the heating capacity per unit in the 1H3C

modewashigher than that in the1H2Cmode.As thenumberof

cooling-operated IDUs decreased by one (1H3C / 1H2C), the

condensingpressuredecreasedandthemassflowrate through

the bypass line entering into theheating-operated IDUbecame

lower under partial load conditions (1H2C) because of the

higher mass flow rate in the main path through the cooling-

operated IDUs, as shown in Fig. 3(b). Therefore, for the 1H2C

mode under partial load conditions, there was a large imbal-

ance between theheating and cooling capacities at a givenCSR

and EEV opening in the MCU. At the CSR of 100%, the average

cooling capacity per unit was 61.3% higher than that of the

ratedvalue,but theheatingcapacityperunitwas19.0%smaller

than thatof the ratedvalue. Inorder tobalance thesecapacities

under partial load conditions, it is necessary to increase the

mass flow rate through the heating-operated IDU by reducing

the EEV opening in the MCU at the optimized CSR.

Fig. 4 shows theCOPaccording to theCSR in the cooling-only

(0H4C, 0H3C) and cooling-main (1H3C, 1H2C) modes. For all

modes, the COP decreased with an increase in the CSR because

the increase inthecompressorworkwasgreater thanthat in the

total capacity. For the cooling-only mode, the COP in the 0H3C

mode was on average 13.4% lower than that in the 0H4C mode

Compressor speed ratio, CSR (%)

30 40 50 60 70 80 90 100

PO

C

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

0H4C

0H3C

1H3C

1H2C

Fig. 4 e Variation of the COP with the CSR in the cooling-

only and cooling-main modes.

Compressor speed ratio, CSR (%)

)h

/g

k(

ti

nu

re

pe

ta

rw

ol

fs

sa

me

ga

re

vA

0

10

20

30

40

50

60

70

80

90

100

Heating IDU Cooling IDU

4H0C

3H0C

3H1C

2H1C

Compressor speed ratio, CSR (%)

20 30 40 50 60 70 80 90 100 20 30 40 50 60 70 80 90 100

)h

/g

k(

ti

nu

re

pe

ta

rw

ol

fs

sa

me

ga

re

vA

10

20

30

40

50

60

70

80

90

100

3H1C

2H1C

a b

Fig. 6 e Variation of the average mass flow rate per unit

with the CSR in the heating-only and heating-main modes.

i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 8 9 3e9 0 1898

due to performance degradation under partial load conditions.

However, the COPs in the 0H4C and 0H3Cmodes at each rating

CSR (0H4C¼ 100%, 0H3C¼ 80%)were 3.23and3.42, respectively,

due to lower rating CSRs in the 0H3C mode. At partial load

conditions, the ratingCSRcanbe reducedtosatisfy thedesigned

cooling capacity, resulting in decreased condensing pressure

andpowerconsumption.For thecooling-mainmode, theCOP in

the 1H3Cmodewas higher than that in the 1H2Cmode because

of a larger imbalance between the heating and cooling capac-

ities per unit in the 1H2Cmode, as shown in Fig. 2. However, the

COPs in the 1H3C and 1H2C modes at each rating CSR

(1H3C¼ 80%, 1H2C¼ 50%) were 4.78 and 5.56, respectively, due

to a lower rating CSR in the 1H2Cmode.

Fig. 5(a) compares the average heating capacities per unit at

full load conditions (4H0C, 3H1C) with those at partial load

conditions (3H0C, 2H1C), respectively, in the heating-only and

heating-mainmodes. The average heating capacity per unit in

the heating-only (4H0C, 3H0C) and heating-main (3H1C, 2H1C)

modes increasedwith an increase in the CSR because the total

mass flow rate to the heating-operated IDUs increased. As

shown in Fig. 6(a), themass flow rate per heating-operated IDU

Compressor speed ratio, CSR (%)

20 30 40 50 60 70 80 90 100

)W

(t

in

ur

ep

yt

ic

ap

ac

gni

ta

eh

eg

ar

ev

A

0

500

1000

1500

2000

2500

3000

3500

4000

Heating capacity Cooling capacity

4H0C

3H0C

3H1C

2H1C

Compressor speed ratio, CSR (%)

20 30 40 50 60 70 80 90 100

ap

ac

gn

il

oo

ce

ga

re

vA

)W

(t

in

ur

ep

yt

ic

0

500

1000

1500

2000

2500

3000

3500

4000

3H1C

2H1C

a b

Fig. 5 e Variation of the average cooling and heating

capacities per unit with the CSR in the heating-only and

heating-main modes.

increases with a reduction in the number of heating-operated

IDUs. Therefore, the average heating capacities per unit in the

3H0C and 2H1C modes (partial load conditions) were higher

than those in the 4H0C and 3H1Cmodes (full load conditions),

respectively, at medium and high CSRs because of higher

condensing pressure andmass flow rate per heating-operated

IDU at partial load conditions. In addition, the average heating

capacity per unit in the 3H1C mode was slightly higher than

that in the 3H0Cmode at high CSRs due to slightly highermass

flow rate through the heating-operated IDU in the 3H1Cmode,

as shown in Fig. 6(a). At low CSRs, the average heating

capacities per unit in the 4H0C, 3H0C, and 3H1C modes were

very similar due to an insufficient mass flow rate in each IDU.

In addition, the difference in the average heating capacity per

unit between the partial and full load conditions became

larger with an increase in the CSR because the mass flow rate

per heating-operated IDU increased more rapidly at partial

load conditions than at full load conditions. In order to obtain

the designed average heating capacity per unit in the heating-

only and heating-main modes, it is necessary to control the

CSR. In this study, the rating CSR was approximately 80% for

the 3H0C and 3H1C modes, and 50% for the 2H1C mode to

obtain the designed heating capacity of 2026 W.

Fig. 5(b) compares the cooling capacity per unit in the 3H1C

mode under full load conditions with that in the 2H1C mode

under partial load conditions. The cooling capacity per unit in

the heating-main (3H1C, 2H1C) mode increased with an

increase in the CSR because the total mass flow rate to the

cooling-operated IDUs increased. However, at all CSRs, the

cooling capacity per unit in the 2H1C mode was higher than

that in the 3H1C mode because of a higher mass flow rate

through the cooling-operated IDU in the 2H1C mode with

a decreased number of heating-operated IDUs, as shown in

Fig. 6(b). It should be noted that the cooling capacity per unit in

the 3H1Cmode did not satisfy the designed cooling capacity of

2026 W. As shown in Fig. 1(b), in the heating-main mode

(3H1C, 2H1C), the refrigerant flowexiting the heating-operated

IDUs splits into the cooling-operated IDU and the ODU. In this

study, the evaporating pressure in the cooling-operated IDU

was controlled by increasing the EEV opening in the cooling-

operated IDU at the full EEV opening in the ODU. Therefore, in

order to increase the cooling capacity per unit in the 3H1C

Compressor speed ratio, CSR (%)

20 30 40 50 60 70 80 90 100

PO

C

2.0

3.0

4.0

5.0

6.0

7.0

8.0

4H0C

3H0C

3H1C

2H1C

Fig. 7 e Variation of the COP with the CSR in the heating-

only and heating-main modes.

i n t e rn a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 8 9 3e9 0 1 899

mode, the mass flow rate in the cooling-operated IDU should

be increased by reducing the EEV opening in the ODU.

Fig. 7 shows the COP according to the CSR in the heating-

only (4H0C, 3H0C) and heating-main (3H1C, 2H1C) modes. The

COP in these modes decreased with an increase in the CSR

because the increase in the compressor workwas greater than

that in the total capacity. For the heating-only mode, the COP

in the 3H0Cmodewas on average 14.2% lower than that in the

4H0C mode due to performance degradation under partial

load conditions. However, the COPs in the 4H0C and 3H0C

modes at each rating CSR (4H0C ¼ 100%, 3H0C ¼ 80%) were

3.33 and 3.38, respectively, due to a lower rating CSR in the

3H0C mode. For the heating-main mode, the COP in the 2H1C

mode was higher than that in the 3H1C mode because of

a larger imbalance between the heating and cooling capacities

per unit in the 3H1Cmode, as shown in Fig. 5. The COPs in the

3H1C and 2H1C modes at each rating CSR (3H1C ¼ 80%,

2H1C ¼ 50%) were 4.44 and 5.60, respectively, due to a lower

rating CSR in the 2H1C mode.

Fig. 8 shows the average cooling and heating capacities per

unit and COP according to the CSR in the 2H2C and 1H1C

modes. For the entire-heat recovery mode, the EEV in the

Compressor speed ratio, CSR (%)

20 30 40 50 60 70 80 90

)W

(t

in

ur

ep

yt

ic

ap

ac

eg

ar

ev

A

0

500

1000

1500

2000

2500

3000

3500

4000

PO

C

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

Cooling capacity (2H2C)

Heating capacity (2H2C)

Cooling capacity (1H1C)

Heating capacity (1H1C)

COP (2H2C)

COP (1H1C)

Fig. 8 e Variation of the average capacities per unit and the

COP with the CSR in the entire-heat recovery mode.

heating-operated IDUs was fully opened. It should be noted

that the limiting values of the CSRs in the 1H1C and 2H2C

modes were 60% and 90%, respectively. As the CSRs increased

beyond these limits, the condensing pressure became so high

that the compressor did not work properly. Generally, in the

entire-heat recovery mode, the average cooling and heating

capacities per unit increasedwith an increase in theCSR,while

the COP decreased. The heating and cooling capacities per unit

in the 1H1Cmodewere higher than those in the 2H2Cmode at

a given CSR because of higher condensing pressure and mass

flow rate per unit in the 1H1C mode. In order to achieve the

designed heating capacity per unit, the rating CSR was 50% in

the 2H2C mode, and 30% in the 1H1C mode. At the optimized

CSRs, the COPs for the 2H2C and 1H1C modes were 6.81 and

5.98, respectively, due to heat recovery without operating the

outdoor heat exchanger. In addition, for each operatingmode,

the imbalance between the average cooling and heating

capacities per unit increased with an increase in the CSR

because the heat transfer area in the cooling-operated IDUs

was the sameas that in theheating-operated IDUs. It should be

noted that the condenser capacity should be higher than the

evaporator capacity to have a balanced refrigeration cycle.

3.2. Performance optimization at partial load conditions

Based on the results of the baseline tests, a large imbalance

between the cooling and heating capacitieswas observed in the

cooling-main and heating-main modes even though the CSR

was optimized. It became more severe under partial load

conditions. Therefore, the cooling and heating capacities under

partial load conditions were optimized by controlling the EEV

opening in the MCU, fan speed and EEV opening in the ODU.

Fig. 9 shows the average cooling and heating capacities per

unit and COP in the cooling-main mode under partial load

conditions (1H2C) according to the EEV opening in the MCU at

the rating CSR of 50%. As the EEV opening in the MCU

decreased, the average cooling capacity per unit remained

nearly constant, while the average heating capacity per unit

increased because the bypassing mass flow rate entering into

the heating-operated IDU increased. As the EEV opening in the

MCU decreased, the flow resistance in the main path through

the cooling-operated IDUs increased, resulting in the increase

φEEV

of MCU (%)

0 20 40 60 80 100

)W

(ti

nu

re

py

ti

ca

pa

ce

ga

re

vA

500

1000

1500

2000

2500

3000

PO

C

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

Cooling capacity (1H2C)

Heating capacity (1H2C)

COP (1H2C) CSR in 1H2C = 50%

Fig. 9 e Variation of the average capacities per unit and the

COP with the EEV opening of the MCU in the 1H2C mode.

Fan speed ratio, FSR (%)

0 20 40 60 80 100

)W

(t

in

ur

ep

yt

ic

ap

ac

eg

ar

ev

A

500

1000

1500

2000

2500

3000

CO

P

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

Cooling capacity (1H2C)

Heating capacity (1H2C)

COP (1H2C)

CSR in 1H2C = 50%

Fig. 10 e Variation of the average capacities per unit and

the COP with the FSR in the 1H2C mode.

i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 8 9 3e9 0 1900

of the mass flow rate through the heating-operated IDU. As

shown in Fig. 2, at a given CSR, the imbalance in themass flow

rate between themain path and the bypass line becameworse

under partial load conditions. Therefore, in the cooling-main

mode, a decrease in the EEV opening in the MCU can reduce

the imbalance between the cooling and heating capacities

under partial load conditions by increasing the condensing

temperature in the heating-operated IDU. As the EEV opening

in the MCU decreased from 100% to 20%, the COP in the 1H2C

mode increased by 6.0%.

Fig. 10 shows the average heating and cooling capacities

per unit and COP according to the fan speed ratio (FSR) in the

cooling-main mode under partial load conditions (1H2C).

The FSR was defined as the ratio of the actual fan speed to the

reference value of 960 rpm. As the FSR in the ODU decreased,

the average heating capacity per unit increased due to an

increase in the condensing pressure, while the average cool-

ing capacity per unit decreased. Therefore, by decreasing the

FSR (to 22.5%), the average cooling and heating capacities per

unit can be brought to the designed value of 2026W. However,

the COP decreased by 12.7% with a decrease in the FSR from

100% to 22.5% because the power consumption increasedwith

a higher compression ratio.

20 40 60 80 100

)W

(

ti

nu

re

p

yt

ic

ap

ac

eg

ar

ev

A

0

500

1000

1500

2000

2500

3000

3500

4000

PO

C

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

Cooling capacity (3H1C)

Heating capacity (3H1C)

Cooling capacity (2H1C)

Heating capacity (2H1C)

COP (3H1C)

COP (2H1C)

EEV of ODU (%)

CSR in 3H1C = 80%

CSR in 2H1C = 50%

Fig. 11 e Variation of the average capacities per unit and

the COP with the EEV opening of the ODU in the 3H1C and

2H1C modes.

Fig. 11 shows the average cooling andheating capacities per

unit and COP according to the EEV opening in the ODU in the

3H1Cand 2H1Cmodes at CSRs of 80%and50%, respectively. As

the EEV opening in the ODU decreased, both the average

cooling and heating capacities per unit increased. The mass

flow rate through the cooling-operated IDU increasedwith the

decrease of the EEV opening in the ODU due to an increase of

the flow resistance in the main path through the ODU. In

addition, the totalmassflowrate through thesystemincreased

because of an increase in the temperature at the compressor

inlet. However, the increase in the cooling capacity per unit

was larger than that in the heating capacity per unit. The COP

increasedwithadecrease in theEEVopening in theODU.As the

EEV opening in theODUdecreased from 100% to 20%, the COPs

in the 3H1C and 2H1C modes increased by 24.2% and 14.8%,

respectively. The optimized COP in the 2H1C mode at an

averagecapacityof 2026Wwas6.27at theODUeEEVopeningof

30%. Therefore, in the heating-mainmode, the optimization of

the CSR and EEV opening in the ODU can provide balanced

cooling and heating capacities and enhance the COP. Espe-

cially, in order to increase the cooling capacity, the control of

the EEVopening in theODUwasmore effective than that of the

CSR due to the COP degradation with an increase in the CSR.

4. Conclusions

The performance of a simultaneous heating and cooling

multi-heat pumpwith four IDUs wasmeasured by varying the

CSR, EEV opening, and FSR at full and partial load conditions.

The COPs and average cooling and heating capacities at full

load conditions were compared with those at partial load

conditions in each operating mode. For all operating modes,

the rating CSRs were strongly dependent on the number of

principal IDUs in the main path. The rating CSR for the

designed capacity was 100% for the 0H4C and 4H0C modes,

80% for the 0H3C, 3H0C, 3H1C, and 1H3C modes, and 50% for

the 2H1C and 1H2C modes. For the cooling-only and heating-

only modes (0H4C, 0H3C, 4H0C, 3H0C), the average heating

and cooling capacities per unit were properly controlled by

varying the CSR. However, for the cooling-main and heating-

main modes under partial load conditions (1H2C, 2H1C), there

was a large imbalance between the cooling and heating

capacities even though the CSR was optimized. Therefore, in

order to obtain the designed capacity in the cooling-main

(1H2C) and heating-main (2H1C) modes, the mass flow rate

through the IDU in the sub-path was controlled by adjusting

the EEV opening in the MCU and ODU, respectively, at the

optimized CSR. In the 1H2C mode, the average heating

capacity per unit increasedwith a decrease in the EEV opening

in the MCU, while the average cooling capacity per unit

remained nearly constant, satisfying the designed heating

capacity and yielding a higher COP. In the 2H1C mode, both

the average cooling and heating capacities per unit increased

with a decrease in the EEV opening in the ODU, yielding

a higher COP and better capacity balance. In addition, the

average heating capacity per unit increasedwith a decrease in

the FSR,while the average cooling capacity per unit decreased,

providing a better capacity balance between heating and

cooling. However, the COP decreased by 12.7%with a decrease

i n t e rn a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 8 9 3e9 0 1 901

in the FSR from 100% to 22.5%. In the entire-heat recovery

mode, the average cooling and heating capacities per unit

increased with an increase in the CSR, while the COP

decreased. The rating CSR for the designed heating capacity

was 50% in the 2H2C mode and 30% in the 1H1C mode. At the

optimized CSRs, the COPs for the 2H2C and 1H1C modes were

6.81 and 5.98, respectively, due to heat recovery.

Acknowledgments

This research was sponsored by the Korea Institute of Energy

and Resources Technology Evaluation and Planning (Grant No.

2008NBLHME0900002008).

r e f e r e n c e s

ARI 210/240, 1986. Unitary Air-conditioning and Air-source HeatPump Equipment. ARI, Arlington, VA.

ASHRAE Guideline 2, 1986. Engineering Analysis of ExperimentalData. ASHRAE, Atlanta, GA, USA.

ASHRAE Standard 116, 1993. Methods of Testing for SeasonalEfficiency of Unitary Air-conditioner and Heat Pumps.ASHRAE, Atlanta, GA, USA.

Choi, J.M., Kim, Y., 2003. Capacity modulation of an inverter-driven multi-air conditioner using electronic expansionvalves. Energy 28, 141e155.

Hu, S.C., Yang, R.H., 2005. Development and testing of a multi-type air conditioner without using AC inverters. EnergyConver. Manage. 46, 373e383.

ISO/DIS 15042, 2005. Multi-split Air Conditioners and Air-to-airHeat Pumps Testing and Rating for Performance.

Kang, H., Joo, Y., Chung, H., Kim, Y., Choi, J., 2009. Experimentalstudy on the performance of a simultaneous heating andcooling multi-heat pump with variation of operation mode.Int. J. Refrigeration 32, 1452e1459.

Kwon, Y.C., Kwon, J.T., Jeong, J.H., Lee, S.J., Kim, D.H., 2004.Performance of a 2 room multi-heat pump with a constantspeed compressor. Int. J. Air-cond. Refrigeration 12, 184e191.

Park, Y.C., Kim, Y., Min, M.K., 2001. Performance analysis ona multi-type inverter air conditioner. Energy Conver. Manage.42, 1607e1621.

Sarkar, J., Bhattacharayya, S., Gopa, M.R., 2004. Optimization ofa transcritical CO2 heat pump cycle for simultaneous coolingand heating applications. Int. J. Refrigeration 27, 830e838.

Xia, J., Winady, E., Georges, B., Lebrun, J., 2004. Experimentalanalysis of the performances of variables refrigerant flowsystems. Build. Serv. Eng. Res. Technol. 25, 17e23.