theoretical and experimental studies of water injection scroll compressor

www.elsevier.com/locate/enconman

Energy Conversion and Management 46 (2005) 1379–1392

Theoretical and experimental studies of water injectionscroll compressor in automotive fuel cell systems

Yuanyang Zhao *, Liansheng Li, Huagen Wu, Pengcheng Shu

National Engineering Research Center of Fluid Machinery and Compressors,

School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, PR China

Received 16 April 2004; accepted 21 August 2004

Available online 6 October 2004

Abstract

A water injection scroll compressor to supply clean compressed air to an automotive fuel cell system is

researched. The water is used as both the lubricant and coolant in the compressor. A thermodynamic model

of the water injection scroll compressor considering leakage and heat exchange for use with an automotive

fuel cell system was developed using the conservation of energy and mass equations and the equation of

state. The results show that the scroll compressor has nearly isothermal compression when injecting waterin it. Increasing the compressor rotation speed increases the discharge loss and the volumetric efficiency of

the scroll compressor. The difference between the calculated power and the isothermal power increases as

the compressor rotation speed rises, which means the efficiency of the compressor decreases. Increasing the

flow rate of water injected increases the indicated isothermal efficiency and decreases the discharge temper-

ature. Under the condition studied, the mass flow rate of water has the greatest effect on the discharge

temperature.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: Scroll compressor; Water injection; Fuel cell

0196-8904/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.enconman.2004.08.006

* Corresponding author. Tel.: +86 29 82675398/2675391; fax: +86 29 83237910/82663792.

E-mail address: [email protected] (Y. Zhao).

mailto:[email protected]

Nomenclature

A areac specific heatC flow coefficientd moisture contenth specific enthalpym massn rotation speedN powerP pressureQ quantity of heat exchanger latent heat of vaporizationRg gas constantT temperatureV volumev specific volumeW power

Greek letterse pressure ratioh orbiting angleg efficiencyj specific heat ratioq density

Subscripts0, 1, 2 state pointsa airc control volumecr critical pointd dischargel leakage, liquidli leakage inlo leakage outm mechanicali flow into control volumeis isothermalo flow out of control volumes suctionv vaporw water

1380 Y. Zhao et al. / Energy Conversion and Management 46 (2005) 1379–1392

Y. Zhao et al. / Energy Conversion and Management 46 (2005) 1379–1392 1381

1. Introduction

The increase of global energy consumption and pollutant emissions has been forcing automo-bile manufacturers to look for future power systems and alternative drive concepts to improve thequality of the urban environment and save energy. Nowadays, car manufacturers have presentedalternative drive systems such as electric, hybrid, or proton exchange membrane (PEM) fuel cellsystems. Among these systems, fuel cell systems seem to have the highest potential to competewith the internal combustion engine, and the fuel cell is also one of the most effective hydrogenenergy applications [1]. A PEM fuel cell automotive engine system is composed of many subsys-tems, fuel and air supply, cooling, energy management, controller, electric system and the fuel cellitself. The air supply system is an important part of PEM fuel cell systems, while the compressor isthe core of the air supply system.In PEM fuel cell systems, air is normally used for the cathode of the fuel cell. Because the par-

tial pressure of oxygen in air is lower than that of the pure gas, compressed air is supplied. Onlywith the higher pressure of compressed air can the aims be obtained such as high efficiency, powerdensity, good dynamic performance and compact dimensions of the system.Scroll compressors have been widely used because of their compactness, high efficiency, low

vibration and noise level and excellent running reliability, and they have been developed foruse in the automotive fuel cell system [2,3].In PEM fuel cell systems, any oil contamination will result in serious performance degradation

of the systems. Hence, some measures have to be taken to avoid oil leakage into the air stream,either by fitting sealing systems or by absolutely avoiding the use of oil or oil containing lubri-cants. In this paper, water is used as both the lubricant and the coolant.

Fig. 1. Schematic of water injection scroll compressor.


2. Structure of the prototype of scroll air compressor

Fig. 1 shows the structure of the prototype scroll compressor. There are two water inlets on thewall of the compressor shell. Part of the water is injected to the compressor suction chamberthrough water inlet 1 (shown in Fig. 1) and then flows into the suction chamber directly. The otherwater is injected to part of the compressor drive mechanism through water inlet 2. The secondpart of the water injected into the compressor lubricates and cools the drive mechanism beforeit flows into the compressor suction chamber where it is mixed with the first part of the water in-jected. It absorbs the heat from the air in the compression chamber, and it is finally dischargedfrom the discharge hole.

3. Water injection scroll compressor modeling

3.1. Governing equations

The process of the gas state change in the compression chamber conforms to the following laws:energy conservation and mass conservation and the equation of state. Selecting one chamber ofthe scroll compressor as the control volume, the assumptions for the simulation model are asfollows:

(1) The gas properties in the control volume are uniform.(2) The changes of gravitational and kinetic energy are negligible.(3) The processes of the gas flowing in or out of the scroll compressor are adiabatic steady flow.(4) Heat transfer between the compressor wrap and the working fluid is negligible.(5) The volumes of the induction chamber and exhaust chamber are infinite, and thus, the gas

pulsation is neglected.

Within the control volume, the simultaneous differential equations of pressure, mass and tem-perature with respect to the orbiting angle can be deduced from the conservation of energy andmass equations and the equation of state. The specific expressions are as follows [4–6]:Equation for conservation of energy:

dpdh

¼1v

ohov

� �T

� oh=oTð Þv � op=ovð ÞTop=oTð Þv

� �dvdh

� 1

V c

X dmi

dhhi � hð Þ

� �þ dQdh

� �

1� 1

v� oh=oTð Þvop=oTð Þv

ð1Þ

Equation for conservation of mass:

dmdh

¼ dmi

dh� dmo

dhþ dmli

dh� dmlo

dhð2Þ

Equation of state:

dTdh

¼ dpdh

þ dV c

dh� dmdh

ð3Þ


In Eq. (1), the item dmi means the gas mass flowing into the control volume, which includes thesuction gas and the gas leakage into the control volume.

3.2. Mass flow leakage

There are two kinds of leakages in scroll compressors (shown in Fig. 2). One leakage is calledradial leakage and is caused by the gap between the bottoms or the top plate and the scrolls. Theother is called tangential leakage and is caused by the gap between the flanks of the two scrolls.These leakages result in a decrease of the compressor volumetric efficiency, and at the same time,the compression work is increased because the gas leaks from the high pressure chamber to thelow pressure chamber.In order to predict the performance of the scroll compressor with water injection, the leakage

gas should be calculated. To simplify the simulation model, the leakages are determined by apply-ing the equation of one dimensional compressible flow in a nozzle with an assumption of an isen-tropic process. The equation of leakage rate is shown as follows [7]:

dml

dt¼ C

Av1

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2j

j � 1P 1v1 e

2j � e

jþ1j

rð4Þ

In Eq. (4), the item e is defined by following section.Here, the critical pressure ratio ecr is defined as following:

ecr ¼2

j þ 1

� � jj�1

ð5Þ

Fig. 2. Leakage diagrammatic sketch.


Hence, e can be expressed as Eq. (6)

e ¼ P2P1

for P 2P 1

P ecr

e ¼ ecr for P 2P 16 ecr

ð6Þ

In Eq. (4), the area A is the leakage area of the scroll compressor, which is a function of thescroll design parameters and the compressor clearances [8]. In Eqs. (4) and (6), P1 is the high pres-sure and P2 is the low pressure, and the leakage gas flows from the high pressure district P1 to thelow pressure district P2.

3.3. Heat exchange

As the water is injected into the scroll compressor chamber, it absorbs heat from the wet air inthe compressor. As the air is compressed, the temperature of the air increases. Water injectioninhibits the increase of the gas temperature, and thus, the compression process is approximatelyisothermal, which is the most highly efficient compression process.The process of heat exchange in the compressor between the air and water is complex, and in

this model, the heat exchange was assumed to be a steady state process.

3.3.1. Suction passageHeat is exchanged between the suction air and injection water in this passage. Here, we assume

the heat exchange is complete between the air and water, which means that the wet air is saturatedand has the same temperature as the water before compression. Considering this assumption, theheat exchange in this process can be presented in two parts.One part is the heating of the water caused by the mechanical loss of the scroll compressor,

which can be expressed as Eq. (7), in which Tw is the water temperature after heated by themechanical loss.

1� gmð ÞW ¼ mwcw T 0 � T wð Þ ð7Þ
The other part is the heat exchange between the heated water and suction air. In this process, the
balance of heat between the water and air is reached, and the following equation can be derived:

macp T 2 � T 1ð Þ ¼ mwcw T w � T 2ð Þ ð8Þ
Here, using the assumption that the wet air is saturated after heat exchange with the water, this
process can be taken as an adiabatic saturated process. Using the law of energy conservation,Eq. (9) can be obtained.

h1 þ ðd2 � d1ÞhL2 ¼ h2 ð9Þ
where h1 and h2 are the enthalpies of the wet air and d1 and d2 are the moisture contents of the wetair.Eq. (9) can also be expressed as
d1ðhv1 � hL2Þ ¼ cpðT 2 � T 1Þ þ d2ðhv2 � hL2Þ ð10Þ
where hv1 and hv2 are the enthalpies of the saturated vapor and hL1 and hL2 are the enthalpiesof the saturation water, respectively.


From the iterative calculation of Eqs. (7)–(10), the state parameters of the suction gas and thewater can be obtained where temperature T2 is the temperature of the water and wet air beforeflowing into the compressor suction chamber.

3.3.2. Compression process

In this process, we assume that the wet air is always in the saturated state and its temperatureis the same as that of the water. Hence, the following equations can be obtained:

dQ1 ¼ ðmiþ1 � miÞ � r ð11Þ

dQ2 ¼ ðT i � T iþ1Þ � cw � mi ð12Þ

dQ ¼ dQ1 þ dQ2 ð13Þ
Here, m is the water mass in the control volume and subscripts i and i+1 are the step numbers
in the simulation process.

3.4. Extra power caused by the water injection

When water is injected into the scroll compressor, extra power is required to compress the watervapor, and the rise of the water pressure with the increase of the wet air pressure also needs power.The pressure of the water in the compressor chamber rises with the compression of the wet air.

According to the theory of phase equilibrium, the water has the same pressure as the air in thecompressor chamber, and in this process, the compressor is like a water pump. The power tothe water can be calculated by

W 1 ¼_mw

qðP d � P sÞ ð14Þ

where _mw is the water mass flow rate.The wet air in the scroll compressor chamber is composed of dry air and the vapor evaporated

from the water. Because the amount of vapor in the wet air varies with the process of compres-sion, the power to compress the vapor can be calculated by Eq. (15) in the simulation.

W 2 ¼n60

XV iðP iþ1 � P iÞ ð15Þ

where Vi is the volume of vapor in the compressor chamber.Thus, the extra power required by the water injection can be obtained as follows:

W w ¼ W 1 þ W 2 ð16Þ

3.5. Numerical method

Taking into account the mass flow leakage and the heat exchange, the differential respect to theorbiting angle at a given frequency.After solving these differential equations, we can obtain the stateparameters of the wet gas in any working condition. From these parameters, some macroeconomic


parameters of the scroll compressor can be calculated, such as volumetric efficiency, power, isother-mal efficiency and so on.

4. Experimental investigation

To verify the simulation model, the scroll compressor with water injection was tested in a com-pressor experimental rig. The configuration of the test scroll compressor was the same as that usedin the simulation. The main parameters of the prototype scroll compressor are listed in Table 1.The operational conditions of the compressor in the experiment are shown in Table 2. The per-

formance of the scroll compressor was recorded with varying speed from 1000 to 3500rpm with a500rpm interval and varying water flow rate from 60 to 105kg/h with a 15kg/h interval.Fig. 3 shows the experimental system. The air is filtered before entering the scroll compressor

and then is compressed with the water. The mixture of water and air flows into the water–airseparator after discharge from the compressor. Finally, the water is re-injected into the scrollcompressor after being cooled and filtered in the water cooler and filter.All temperatures in the system were monitored using T-type thermocouples with an uncertainty

of ±0.2�C. The compressor power input was monitored using a power meter whose accuracy waswithin 0.1% of the reading. The pressures were monitored using pressure sensors, and their overallerror is less than ± 0.3%. The water mass flow rate was measured by the flow meter with an uncer-tainty of ±0.2% of the reading.The frequency changer is used in the test system to obtain variable speed performance of the

water injected scroll compressor. The water mass flow rate can be adjusted by the controllingvalve and measured by the flow meter installed on the injected water pipe.

Table 1

Main parameters of the scroll compressor

Delivery (m3/min) 3.0

Radius of basic circle (mm) 6.048

Scroll pitch (mm) 38.0

Scroll width (mm) 7.0

Scroll height (mm) 60.0

Scroll turns 2.25

Table 2

Operational conditions of the scroll compressor

Suction pressure (kPa) 97.2 (local atmospheric pressure)

Discharge pressure (kPa) 200.0

Suction temperature (�C) 26.0

Speed of compressor (rpm) 1000; 1500; 2000; 2500; 3000; 3500

Water mass flow rate (kg/h) 60; 75; 90; 105

Fig. 3. Water injection scroll compressor test system.


Because the water injection is maintained by system pressure, the absolute pressure in thewater–air separator was set to be higher than 0.13MPa in the experiment. Thus, the reliabilityof the running system can be obtained.

5. Results and discussion

5.1. Effects of rotation speed of scroll compressor

When the speed of the scroll compressor is varied, its performance varies. Using the simulationmodel, the pressure–volume diagram of the compressor control volume was calculated at thespeeds of 1000, 3000 and 5000rpm, respectively. These results are shown in Fig. 4. Here, Vth isthe maximal suction volume, i.e., the theoretical design volume, and the water mass flow rate is60kg/h.As the speed of the scroll compressor decreases, the leakage masses both in and out of the con-

trol volume will increase, but the leakage in mass is larger than the leakage out mass during thestarting stage of compression, so the rate of pressure increase is higher at the lower speed. At thefinal stage of the compression process, the increment of pressure variation is greater at the higherspeed because the mass leakage in is larger than the leakage out. From Fig. 4, it also can be seenthat the discharge loss increases with the increase of rotation speed in the discharge process.Fig. 5 shows the comparisons of the predicted and measured volumetric efficiencies of the scroll

compressor when the water mass flow rate is 60kg/h. It can be seen that the volumetric efficiency ofthe scroll compressor increases with rotation speed of the scroll compressor, but the increase ratedecreases. The results of calculation and experiment compare reasonably well. The calculated vol-umetric efficiency was higher than the measured value with a mean deviation of 0.95%. The max-imum deviation of the calculated volumetric efficiency was approximately 1.43% at 1000rpm,which might have resulted from some deviations in the prediction of leakage by neglecting thewater effects.

1000 2000 3000 4000 50000.75

0.80

0.85

0.90

0.95

1.00

Vo

lum

etri

c ef

fice

ncy

Rotation speed (rpm)

Cal.

Exp.

Fig. 5. Volumetric efficiency of compressor.

0.0 0.2 0.4 0.6 0.8 1.0 1.2

100

120

140

160

180

200

220

Pre

ssu

re(k

Pa)

Volume ratio (V/Vth)

1000 rpm3000 rpm5000 rpm

Fig. 4. Pressure–volume diagram at rotation speeds.


Fig. 6 shows the compressor shaft power at different rotation speeds when the water mass flowrate is 60kg/h. Here, the isothermal power is obtained from Eq. (17).

N is ¼ maRgT s � lnP d

P s

=gm ð17Þ

The calculated compressor power shows good agreement with the measured value with a meandeviation of 7.28%. The maximum deviation of the calculated value was 10.51% at 3500rpm,which might have resulted from some deviations in the prediction of mechanical loss by neglectingthe water effects. However, the deviation decreased with the reduction of speed.The isothermal power is the least power for all kinds of compression processes. The difference

between the actual power and the isothermal power shows the degree to which the compressionprocess deviates from isothermal compression. From Fig. 6, it can be seen that the difference be-tween the actual and isothermal power increases when the compressor rotation speed increases.

1000 2000 3000 4000 5000

1000

2000

3000

4000

5000

Po

wer

(W

)

Rotation speed (rpm)

Cal.IsothermalExp.

Fig. 6. Comparison of compressor powers.


This means that the efficiency of the compressor is higher at low rotation speed than that at highspeed.

5.2. Effects of injected water mass on performance of the compressor

From the calculations of the simulation model, the relationship between the pressure–volumediagram and the mass of injection water is shown in Fig. 7 and, at the same time, compared withthe ideal processes, adiabatic and isothermal processes. Here, the predicted data was calculatedfor the speed of 3000rpm.For the injection water mass of the research, the simulated processes approached the isothermal

process very well, which is shown in Fig. 7. When the water injection mass increases, the degree ofthe approach to the isothermal process increases. From Fig. 7, it also can be seen that the pressureof an adiabatic process is the biggest and that of an isothermal process is the least among all the

0.0 0.2 0.4 0.6 0.8 1.0

100

150

200

250

Pre

ssu

re (

kPa)

Volume ratio (V/Vth)

Injection water mass: 40 kg/h100 kg/hAdiabaticIsothermalPd

vd

Fig. 7. Pressure–volume diagram for different injection water mass.

0 20 40 60 80 1000.90

0.92

0.94

0.96

0.98

1.00

Iso

ther

mal

ind

icat

ed e

ffic

ien

cy

Injection water mass flow rate (kg/h)

Rotation speed:3000 rpm5000 rpm

Fig. 8. Change of isothermal indicated efficiency.


processes at the end of compression (Vd shown if Fig. 7). Hence, at the same discharge pressurePd, the isothermal process discharge loss is the least and that of the adiabatic process is the most.Fig. 8 shows the predicted isothermal indicated efficiency of the water injection scroll compres-

sor from the simulation model. Here, the isothermal indicated efficiency is defined as

gis ¼ maRgT s � lnP d

P s

=Ni ð18Þ

where Ni is the actual indicated power and is calculated by the simulated data

Ni ¼n60

ZV dp ð19Þ

The value of the isothermal indicated efficiency presents the degree to which the process ap-proaches the isothermal process. This value rises with decreasing rotation speed or/and increasingflow rate of injection water mass.Fig. 9 shows that the discharge temperature decreases as the injection water mass flow rate in-

crease. Here, the measured and calculated temperatures are obtained for the compressor speed of3000rpm. The mean error between the measured and the calculated temperature is 5.41%, and thecalculated data is larger than that of the experiment. One reason is that the heat transfer betweenthe wet air and the compressor shell is not considered in the simulation model.

5.3. The extra power required by the water injection

Table 3 shows the predicted extra power required by the water injection. The following pre-dicted data was calculated with the water mass flow rate of 60kg/h. It can be seen from this thatthe power to the water is 0.275% of the total indicated power when the speed is 1000rpm and0.049% when the speed is 5000rpm. The power to compress the water vapor is less than 2.5%of the total indicated power at all calculated speeds. Thus, the total extra power required bythe water injection is less than 3% of the compressor indicated power.

40 60 80 100

314

316

318

320

322

324

326

Dis

char

ge

tem

per

atu

re (

K)

Injection water mass flow rate (kg/h)

Cal.

Exp.

Fig. 9. The effect of water mass on discharge temperature.

Table 3

Extra power required by the water

Speed (rpm) 1000 3000 5000

Indicated power (W) 639.89 2111.20 3607.94

Power to the water (W) 1.76 1.76 1.76

Power compress vapor (W) 10.81 49.02 87.27


6. Conclusions

On the basis of the conservation of energy and mass equations and the equation of state, themathematical model of a water injection scroll compressor used in the automotive fuel cell systemwas developed. The leakage and heat exchange between the air and water were taken into account.The predictions agree well with the measured results.The results of simulation and experiment show that the scroll compressor has nearly isothermal

compression with water injection. Increasing the compressor rotation speed increases the dis-charge loss and increases the volumetric efficiency of the scroll compressor. The calculated volu-metric efficiency was higher than the measured value with a mean deviation of 0.95%. Thecompressor power has a linear relationship with the speed. The difference between the calculatedpower and isothermal power increases with speed, which means the efficiency of the compressordecreases. Increasing the flow rate of water injection mass increases the isothermal indicated effi-ciency and decreases the discharge temperature. The extra power required by the water injection isless than 3% of the total compressor indicated power under the speeds studied. Under the condi-tions studied, the water flow rate has the greatest effect on the discharge temperature.

Acknowledgements

The work described in the paper is funded by the key Research Project Fund of the ChineseMinistry of Education (grant no. 104211) and the Doctoral Foundation of Xi�an JiaotongUniversity (DFXJTU 2003-6).


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theoretical and experimental studies of water injection scroll compressor

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