influence from the rotating speed of the windward axial fans on the performance of an air-cooled...

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Applied Thermal Engineering 65 (2014) 14e23

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Applied Thermal Engineering

journal homepage: www.elsevier .com/locate/apthermeng

Influence from the rotating speed of the windward axial fanson the performance of an air-cooled power plant

Weifeng He a,*, Yiping Dai b, Dong Han a, Chen Yue a, Wenhao Pu a

a Jiangsu Province Key Laboratory of Aerospace Power Systems, Nanjing, Jiangsu 210016, ChinabXi’an Jiaotong University, Xi’an, Shaanxi 710049, China

h i g h l i g h t s

� Fan array performance at different rotating speeds of the windward fans is considered.� Power plant characteristics at different rotating speeds are investigated.� Beneficial net power is calculated to show the promotion effect from increasing rotating speed.� Thermal efficiency of the closed Rankine cycle is calculated based on the obtained results.

a r t i c l e i n f o

Article history:Received 24 April 2013Accepted 28 December 2013Available online 4 January 2014

Keywords:Air-cooled steam condenserMeteorological conditionsRotating speedFan consumptionThermal efficiency

* Corresponding author. College of Energy & Poweratory of Aerospace Power Systems, No. 29 Yudao StrChina. Tel./fax: þ86 (0)2584892201.

E-mail address: [email protected] (W.

1359-4311/$ e see front matter � 2014 Elsevier Ltd.http://dx.doi.org/10.1016/j.applthermaleng.2013.12.06

a b s t r a c t

The performance of the fan array in the air-cooled steam condensers (ACSCs), which consumes a largeamount of the power from the generator, always affects the characteristics of the whole power plantunder the effect of the meteorological conditions. As a result, it is significant to optimize the fan mode toobtain a maximum operation efficiency of the air-cooled thermal system. In the paper, a 2 � 600 MWpower plant is modeled to investigate the influence from the fan rotating speed on the performance ofthe fan array as well as the whole air-cooled power plants. It is found that the promotion of the fanrotating speed of the windward fans improves the performance of the fan array and the heat transfercharacteristics of the exchangers in the air-cooled steam condenser, and the beneficial net power inconsideration of the fan consumption is also raised effectively with an improved thermal efficiency of theclosed Rankine cycle.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Air-cooled power plants are always applied to reject waste heatfrom the turbine exhaust in the dry areas due to the characteristicsof water conservation, and the related heat rejection is carried outcontinuously by the ambient air through the outside of the finnedtube exchangers. Based on the property of the cooling medium,forced draught devices should be installed to enlarge the flow rateof the ambient air. Taking a 600 MW air-cooled power plant forexample, there are generally 56 axial fans, which consists of a fanarray, and a large amount of the electricity from the generator willbe consumed by the axial fan array.

The performance of the fan in the air-cooled steam condenseris seriously influenced under the effect of wind conditions due to

r, Jiangsu Province Key Labo-eet, Nanjing, Jiangsu 210016,

He).

All rights reserved.8

the flow distortion under the fan inlet [1e3], and then the char-acteristics of the whole power plant will change, responding to theflow rate alteration of the cooling medium. As a result, the air-cooled power plant always operates off the design points espe-cially at strong wind and high ambient temperature. Therefore, itis very significant to investigate the influence principles from thewind conditions on performance of the power plant. As early as1995, an air-cooled steam condenser was modeled [4], and therelevant effect from the ambient conditions was numericallystudied. The performance difference from different types of heatexchangers as well as the arrangement of the power plants wasevaluated. In view of the fine prediction of the power plant per-formance, the superiority of the new applied CFD method wasvalidated. Gao et al. [5] proposed a simplified model with a cubicfluid zone as the air-cooled steam condenser cell, and the effectfrom the wind conditions as well as the height of the air-cooledsteam condenser on the relevant condenser performance wasanalyzed. Furthermore, Yang et al. [6] established the physical andmathematical models of the air-side fluid and heat flow in the air-

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Nomenclature

Roman symbolsC inertial resistance coefficient (m�1)C1, C2, Cm constants in turbulence equationscp specific heat (J kg�1 K�1)e volumetric effectivenessH height (m)k turbulent kinetic energy (m2 s�2)n fan rotating speed (rpm)N fan power consumption (kW)p pressure (Pa)P power (MW)Pr Prandtl numberS source term (N m�3)t time (s)T temperature (K)v velocity (m s�1)x,y,z coordinates (m), steam quality

Greek lettersa blade installation angle (�)b thermal expansion coefficient (K�1), wind angle (�)r density (kg m�3)m dynamic viscosity (kg m�1 s�1)3 turbulent kinetic energy dissipation rate (m2 s�3), heat

transfer effectivenesss Prandtl number for k, 3and Tf general variableG diffusion coefficient

Subscriptsd designm measureop operations steamt turbulencew wind

W. He et al. / Applied Thermal Engineering 65 (2014) 14e23 15

cooled condensers at various wind conditions, and the radiatortype was introduced to model the flow and heat transfer charac-teristics of the fin-tube bundles. The volumetric flow rate as wellas the inlet air temperature and heat rejection for different air-cooled steam condensers as a whole, condenser cells and fin-tube bundles were obtained. Based on the complicated heattransfer phenomenon with phase change, user defined function(UDF) method based on the actual steam properties was firstapplied to simulate the condensation of the turbine exhaust in thefinned tube heat exchangers [7,8], and the numerical model closedto the true status was used to analyze the wind influence on thepower plant performance.

Van Rooyen and Kroger [9] found that the performance of theedge fans toward the inflow is especially sensitive to the windconditions, and the characteristics of the air-cooled steamcondenser are influenced as the wind speed increases. As a result,it is verified that increasing the rotating speed of the edge fans inthe fan array is effective to improve the hot air recirculation by Liuet al. [10]. However, the effect from the adjustment of the rotatingspeed for the windward fans on the performance of the fan arrayas well as the whole power plant is neglected. In the currentinvestigation, based on the serious performance reduction of thewindward fans from the wind conditions, the effect from therotating speed of the windward fans is investigated by using theexisting numerical model [7]. The user defined function (UDF)based on the actual steam property is applied to simulate thecondensation of the turbine exhaust in the finned tube ex-changers, and the pressure jump model and porous media arerespectively used to simulate the dynamic performance of theaxial fan and the resistance characteristics of the exchangers. Theperformance of the fan array and the heat exchangers at tworotating speeds is obtained, and the corresponding stable backpressures are predicted since the appearance of the imbalancebetween the heat rejection of the turbine exhaust and the heattransfer capacity. Finally, the net power and the closed Rankinecycle efficiency of the air-cooled power plant are calculated tovalidate the benefit after increasing the rotating speed of thewindward fans. The analysis results provide the references toimprove the fan mode for a much better comprehensive perfor-mance in the air-cooled power plant especially at extreme windconditions.

2. Numerical model and computing method

2.1. Numerical model

The influence from increasing rotating speed of the windwardfans on the performance of the air-cooled power plant at designwindangle is studiedbasedon thenumericalmodel of a2�600MWpower plant [7], which is located in the north of China in ShaanxiProvince, and the layout of the fan array is presented in Fig. 1. It isseen that there are 112 axial fans in the air-cooled fan array, and 22green fans are marked as the windward fans at the design windangle, b ¼ 22. 5�. Heat rejection from the turbine exhaust is carriedout by the ambient air from the forced draught axial fans.

2.2. Validation of the numerical model

Grid independence test is achieved based on a very small devi-ation of the heat transfer rate from the three set grids of the nu-merical model. Moreover, the performance of the air-cooled steamcondenser at design wind conditions, which is the prevailing windangle, b¼ 22. 5�, and thewind speed, vw¼ 3m/s, is compared to thefive typical design cases, including turbine rated load (TRL), turbineheat acceptance (THA), turbine maximum continues rate (TMCR),valve wide open (VWO) and choking pressure (CP). The consistencyof the fan array performance as well as the condenser heat transferrate in Fig. 2 shows the rationality of the applied model.

2.3. Computing method and boundary conditions

The phenomenon of fluid flow is dominated by the conservationlaws including mass, momentum, energy and extra turbulencetransportation, and governing equations are the mathematical ex-pressions of the conservation laws. The general governing equationis as follows [11]:

vðrfÞvt

þ vðrvxfÞvx

þ v�rvyf

�vy

þ vðrvzfÞvz

¼ v

vx

�Gvf

vx

�þ v

vy

�Gvf

vy

�þ v

vz

�Gvf

vz

�þ S (1)

Fig. 1. Schematic diagram of the fan array as well as the windward fans at the prevailing wind angle.

W. He et al. / Applied Thermal Engineering 65 (2014) 14e2316

where f stands for different variables, G the corresponding gener-alized diffusion coefficient and S the source term.

For the continuity equation:

f ¼ 1; G ¼ 0; S ¼ 0

For the momentum equation:

x-direction

f ¼ vx; G ¼ meff ¼ mþ mt; S ¼ �vpvx þ v

vx

�meff

vvxvx

�þ v

vy

�meff

vvyvx

�þ v

vz

�meff

vvzvx

�þ Fx

y-direction

f ¼ vy; G ¼ meff ¼ mþ mt; S ¼ �vpvy þ v

vx

�meff

vvxvy

�þ v

vy

�meff

vvyvy

�þ v

vz

�meff

vvzvy

�þ Fy

z-direction

f ¼ vz; G ¼ meff ¼ mþ mt; S ¼ �vpvz þ v

vx

�meff

vvxvz

�þ v

vy

�meff

vvyvz

�þ v

vz

�meff

vvzvz

�þ Fz

9>>>>>>>>>>>>>=>>>>>>>>>>>>>;

For the energy equation:

f ¼ T ; G ¼ m=Pr þ mt=sT

For k equation:

f ¼ k; G ¼ mþ mt=sk; S ¼ Gk � r 3þ Gb

For 3equation:

f ¼ 3; G ¼ mþ mt=s 3; S ¼ 3

kðC1Gk � C2r 3þ C1C3GbÞ

where m is viscosity, mt the turbulent viscosity, and meff effectiveturbulent viscosity.

mt ¼ rCmk2

3(2)

Gb represents the generation of turbulent kinetic energy due tothe buoyancy effect.

Gb ¼ bgmtsT

vTvz

(3)

where b is the thermal expansion coefficient, and it can beexpressed as:

b ¼ �1r

vr

vT(4)

Gk is a term which generates because of turbulence kinetic en-ergy k caused by the gradient of the average velocity.

Gk ¼ mt

(2

"�vvx

�2

þ�vvy

�2

þ�vvz

�2#þ�vvx þ vvy

�2

vx vy vz vy vx

þ�vvxvz

þ vvzvx

�2þ�vvyvz

þ vvzvy

�2)

(5)

According to the recommended value, the empirical constantswhich appear in the turbulent equations are assigned the valuesshown in Table 1.

Finite volume method is used to discretize the governingequations with the SIMPLE algorithm for a steady-state solutionduring the numerical simulation. The structure of the fluid domain,500 m � 1000 m � 300 m, is shown in Fig. 3. The prevailing windangle, b¼ 22. 5�, and fivewind speeds, are taken into considerationto investigate the promotion effect from increasing the rotatingspeed of the windward fans. The ground is regarded as a no-slipwall with the ambient temperature. Two velocity inlets are desig-nated for entrances of the fluid domain at the prevailing windangle, and the opposite surfaces of the inlets are outlets with astatic pressure. The exponential wind speed distribution isdemonstrated according to the statement in VGB-131Me [12] asfollows:

vw ¼ vm

�Hw

Hm

�0:2

(6)

where vm is the value of thewind speed with themeasuring height,Hm, and vw is thewind speed above the distributing tube at a heightof Hw according to the guide from VGB-131Me.

3. Results and discussion

The rapid development of the air-cooled power plants in the dryareas is completely attributed to the shortage of the water

Fig. 3. Boundary conditions of the computing fluid domain at design wind conditionsin the numerical model.

TRL THA TMCR VWO CP0

10000

20000

30000

40000

50000

60000 Design values Simulation results

Fan

flow

rate

/m3 ·s

-1

Design cases

(a) Volumetric flow rate of the fan array

TRL THA TMCR VWO CP0

200

400

600

800

1000

1200

1400

1600

1800 Design values Simulaltion results

Hea

t tra

nsfe

r rat

e/M

W

Design cases

(b) Heat transfer rate of the condenser

Fig. 2. Comparison between the simulation results and the values at five typical designconditions. (a) Volumetric flow rate of the fan array; (b) Heat transfer rate of thecondenser.


resources, and the layout of the forced draught devices is deter-mined by the characteristics of the cooling medium, which has amuch smaller specific heat capacity and a higher dry bulb tem-perature compared to water in the general water-cooled powerplants. It is obvious that the fan driven equipments will consume alarge amount of the generated electricity, and finally the net power,which is the difference of the output power from the turbine andthe fan array consumption, will be reduced significantly. In thepaper, the performance of the air-cooled steam condenser as wellas the power plant is investigated at design wind conditions fordifferent fan rotating speeds of the windward fans. The parametersof the exhaust at TRL case in Table 2 are considered during the

Table 1Values of the constants in the turbulence equations.

Cm C1 C2 sT sk s 3

0.09 1.44 1.92 0.85 1.0 1.3

simulation, while the ambient temperature is 306 K, and theambient pressure is 101,325 Pa.

3.1. Performance of the fan array as well as the whole air-cooledpower plant

During the design of the air-cooled power plants, the fan modeincluding the rotating speed and the blade angle depends on theheat rejection capacity of the turbine exhaust in the finned tubeexchangers. According to the fan theory, the axial fan will operatealong the characteristic curve, which varies with the blade instal-lation angle, a, and rotating speed, n. In the current paper, the in-fluence from the rotating speed on the power plant performance isanalyzed.

The characteristics curve of the axial fan at off design rotatingspeed can be obtained from that of the design conditions based onthe similar principles. The relationship among the fan flow rate,pressure difference and the power consumption with the rotatingspeed is shown in Eqs. (7)e(9) [13]:

VVd

¼ l3nnd

(7)

ppd

¼ r

rdl2�nnd

�2

(8)

NNd

¼ r

rdl5�nnd

�3

(9)

where V is volumetric flow rate through the axial fan when thevalue of the rotating speed is n, and p is the pressure rise, N the

Table 2Steam parameters flowing into the air-cooled steam condenser under TRL case.

Item ms (kg/s) ps (kPa) x (%) Dps (Pa)

Value 738.5 29 96.4 820


power consumption, l the linear size ratio of the two comparedfans. In view of the same axial fan with different rotating speedsinvolved in the comparison and the incompressibility of theambient air, Eqs. (7)e(9) can be simplified to Eqs. (10)e(12) [13]:

VVd

¼ nnd

(10)

ppd

¼�nnd

�2

(11)

NNd

¼�nnd

�3

(12)

After the similarity analysis of the axial fan in the air-cooledsteam condenser, the characteristics curve at the maximum fanrotating speed, n ¼ 107%nd is obtained in Fig. 4, and the corre-sponding performance of the fan array as well as the whole powerplant will be numerically investigated.

Volumetric effectiveness is applied to represent the actualoperation performance of the axial fan, which is defined as the ratioof the calculated flow rate and the design value in Eq. (7).

e ¼ Vc

Vd(13)

where e is the volumetric effectiveness and Vc is the actual calcu-lated fan flow rate, Vd the design flow rate of the axial fan in thepaper, Vd ¼ 467 m3 s�1. The average volumetric effectiveness of thefan row and column can also be obtained based on the expressionof the single fan.

The average effectiveness of the fan row with sixteen axial fanswith different wind speeds at two prescribed rotating speeds of thewindward fans is presented in Fig. 5. It is observed that the averagevolumetric effectiveness of the windward fan row rises signifi-cantly under the effect of the increased fan rotating speed, whilethe internal fan rows is not so sensitive as the windward fan rowexcept the leeward row at vm ¼ 2 m s�1 with a relatively higher riseamplitude. The promotion effect on the performance of the wind-ward fan row varies with the raised effectiveness from 0.09,42.3 m3 s�1 per fan, at vm ¼ 2 m s�1 to 0.10, 46.4 m3 s�1 per fan, atvm ¼ 10 m s�1. As a result, it is concluded that elevating the fan

200 300 400 500 600 700

60

80

100

120

140

160

180

Design rotating speed107% design rotating speed

Pres

sure

diff

eren

ce/P

a

Fan flow rate/m3·s-1

Fig. 4. Characteristic curve of the axial fan at two designated blade angles.

rotating speed of the windward fans is effective for the perfor-mance of the windward fan row, and the corresponding risingamplitude increases with the wind speed.

The performance of the fan column at n ¼ 107%nd is alsocompared to the design conditions under the effect of differentwind speeds in Fig. 6. It is seen that the raised amplitude of thevolumetric effectiveness for the windward fan column increasesfrom 0.07 at vm ¼ 2 m s�1, 34.5 m3 s�1 per fan to 0.09 atvm ¼ 10 m s�1, 40.0 m3 s�1 per fan. Different from the effectivenessof the fan row, the average performance of the internal fan columnis also raised significantly after the adjustment of the rotating speedfor the windward fans.

The influence from the fan rotating speed on the fan row andcolumn effectiveness is illustrated respectively above, and theanalysis demonstrates the local influence on the fan array perfor-mance. The corresponding flow rate of the whole fan array atdifferent wind speeds for the two fan rotating speeds is also shownin Fig. 7. Obviously, the total flow rate of the fan array will alsoincrease as a result of the local positive influence on the fan row andcolumns from the elevated fan rotating speed, and the total flowrate rises from 53,608.8 m3 s�1 and 44,143.3 m3 s�1 at vm¼ 2m s�1

and vm ¼ 10 m s�1 to 54,392.5 m3 s�1 and 44,956.3 m3 s�1. It isvalidated that the promotion effect from the fan rotating speed isvery significant based on the comparison of the fan array perfor-mance at the prescribed wind speeds.

The waste heat from the turbine exhaust in the exchangers iscarried out by the ambient air from the forced draught fans. It iswell known that the variation of the wind condition willcontribute to the performance alteration of the steam condenseras well as the whole power plant because the flow rate of thecooling medium is seriously influenced by the wind conditions.Therefore, the performance of the power plant will also changeresulting from the increased rotating speed of the windward fansdue to the imbalance of the heat rejection and the actual heattransfer capacity between the ambient air and the turbineexhaust. The heat transfer rate and the stable predicted backpressure at different wind speeds for the two rotating speeds arepresented in Fig. 8. It is seen that the heat transfer rate rises from1694.0 MW at vm ¼ 2 m s�1 and 1462.9 MW at vm ¼ 10 m s�1 to1714.0 MW and 1489.0 MW respectively, and the correspondingrising amplitude is 20 MW at vm ¼ 2 m s�1 and 26.1 MW atvm ¼ 10 m s�1. As a result of the increased heat transfer capacity,the thermal system will respond to the difference of the heatrejection and the heat transfer rate until a new equilibrium backpressure appears. The stable back pressure at different windspeeds is predicted for the two prescribed rotating speeds basedon the user defined function method, which is programmed ac-cording to the actual steam properties to simulating thecondensation of the turbine exhaust in the finned tube ex-changers. Finally, the stable back pressure is effectively decreasedfrom 27.3 kPa at vm ¼ 2 m s�1 and 47.5 kPa at vm ¼ 10 m s�1 to25.9 kPa and 44.9 kPa respectively.

3.2. Analysis for the net power of the air-cooled power plant

A large amount of the power from the generator is consumeddue to the forced draught fans in the condenser, and it is significantto optimize the net power between the axial fan consumption andthe output power from the generator. Based on the relationshipbetween the flow rate and the power consumption, the poweralteration of the axial fan can be obtained from Eq. (12) after theadjustment of the rotating speed in Fig. 9.

After increasing the rotating speed of the windward fans, therunning points of all the axial fans in the array will change due tothe interaction among the fans, and the output power of the

1 2 3 4 5 6 7

0.80

0.85

0.90

0.95

1.00

1.05

1.10

1.15


Vol

umet

ric e

ffec

tiven

ess/

%

Fan row1 2 3 4 5 6 7

0.6

0.7

0.8

0.9

1.0

1.1


Vol

umet

ric e

ffec

tiven

ess/

%

Fan row(a) vm=2m·s-1 (b) vm=4m·s-1

1 2 3 4 5 6 70.5

0.6

0.7

0.8

0.9

1.0

1.1


Vol

umet

ric e

ffect

iven

ess/

%

Fan row(c) vm=6m·s-1

1 2 3 4 5 6 7

0.5

0.6

0.7

0.8

0.9

1.0

1.1


Vol

umet

ric e

ffect

iven

ess /

%

Fan row

1 2 3 4 5 6 70.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1


Vol

umet

ric e

ffect

iven

ess /

%

Fan row

(d) vm=8m·s-1

vm=10m·s-1(e)

Fig. 5. Volumetric effectiveness of the fan row with different wind speeds at two designated rotating speeds. (a) vm ¼ 2 m s�1; (b) vm ¼ 4 m s�1; (c) vm ¼ 6 m s�1; (d) vm ¼ 8 m s�1;(e) vm ¼ 10 m s�1.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 160.90

0.92

0.94

0.96

0.98

1.00

1.02

1.04

1.06

1.08


Vol

umet

ric e

ffect

iven

ess/

%

Fan column1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

0.84

0.86

0.88

0.90

0.92

0.94

0.96

0.98

1.00

1.02

1.04


Vol

umet

ric e

ffec

tiven

ess/

%

Fan column

(a) vm=2m·s-1 (b) vm=4m·s-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

0.82

0.84

0.86

0.88

0.90

0.92

0.94

0.96

Design rotating speed107% design rotating speedV

olum

etric

eff

ectiv

enes

s/%

Fan column

(c) vm=6m·s-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

0.78

0.80

0.82

0.84

0.86

0.88

0.90

0.92

0.94

Design rotating speed107% design rotating speedVol

umet

ric e

ffect

iven

ess/

%

Fan column

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 160.74

0.76

0.78

0.80

0.82

0.84

0.86

0.88

0.90

0.92

0.94


Vol

umet

ric e

ffec

tiven

ess/

%

Fan column

(d) vm=8m·s-1

(e) vm=10m·s-1

Fig. 6. Volumetric effectiveness of the fan column with different wind speeds at two designated rotating speeds. (a) vm ¼ 2 m s�1; (b) vm ¼ 4 m s�1; (c) vm ¼ 6 m s�1; (d)vm ¼ 8 m s�1; (e) vm ¼ 10 m s�1.


2 4 6 8 10

44000

46000

48000

50000

52000

54000


Vol

umet

ric fl

ow ra

te/m

3 ·s-1

Wind speed/m·s-1

Fig. 7. Fan array flow rate at different wind speeds for the two rotating speeds.

200 300 400 500 600 70050

55

60

65

70

75

80

85


Pow

er/k

W

Fan flow rate/m3·s-1

Fig. 9. Power of axial fan related to the pressure rise and the flow rate with therotating speeds.


generator will also respond to the alteration of the fan runningmode. As a result, the net power from the difference of the fanpower consumption and the output power is calculated to judgethe benefit after the adjustment, and the difference of the outputpower and the power consumption can be expressed as:

DP ¼ Pn¼107%nd� Pn¼nd (14)

DN ¼ Nn¼107%nd� Nn¼nd (15)

where P is the output power of the generator and N is the powerconsumption of the fan array. After the iterative simulation basedon the parameters at TRL conditions, the stable back pressures ofthe power plant corresponding to different wind speeds at thetwo rotating speeds are numerically predicted, and the outputpower from the generator can be calculated according to therelation between the output power and the heat load of the

2 4 6 8 10

1450

1500

1550

1600

1650

1700


Hea

t tra

nsfe

r rat

e/M

W

Wind speed/m·s-1

(a) Heat transfer rate

Fig. 8. Heat transfer rate as well as the predicted back pressure with the wind speed

power plant in Fig. 10 supplied by Shanxi electric power researchinstitute. Moreover, the power consumption of the fan array canbe calculated according to the fitted equation from the curve inFig. 9. After the difference of the output power and the powerconsumption is obtained, the beneficial net power of power plantfrom the rotating speed alteration, DPnet, can be calculated asfollows:

DPnet ¼ DP � DN=1000 (16)

Fig. 11 presents the beneficial net power of the power plantfrom increasing the rotating speed. It is found that the outputpower difference from the generator rises gradually with theincreasing wind speeds from 12.41 MW to 19.33 MW, and thecorresponding amplitude of the fan consumption power in-creases from 453.47 kW to 498.22 kW. Based on the alterationtrend of the output power and the fan consumption, the final

2 4 6 8 10

25

30

35

40

45

50


Bac

k pr

essu

re/k

Pa

Wind speed/m·s-1

(b) Predicted back pressure

for the two rotating speeds. (a) Heat transfer rate; (b) Predicted back pressure.

Fig. 11. Net power from the difference of the out power and fan consumption power.

Fig. 12. Power vs the heat rate of the power plant at different back pressures.

2 4 6 8 1040.0

40.5

41.0

41.5

42.0

42.5

43.0

43.5


Ther

mal

cyc

le e

ffici

ency

/%

Wind speed/m·s-1

Fig. 13. Thermal efficiency of the closed Rankine cycle at different wind speeds for thetwo rotating speeds.

350 400 450 500 550 600 650400

500

600

700

800

900

1000

Exha

ust h

eat l

oad/

MW

Output power/MW

ps=8kPaps=15kPaps=30kPaps=45kPa

Fig. 10. Power vs the heat load of the turbine exhaust at different back pressures.


beneficial net power of the power plant also rises from 11.96 MWto 18.83 MW. It is concluded that increasing the fan rotatingspeed will improve the performance of the whole power plantdespite of the increased power consumption of the fan array.Furthermore, the corresponding promotion effect from theadjustment of the rotating speed is more significant for highwind speed conditions since the performance of the power plantis seriously influenced, which is demonstrated in the previousinvestigations.

3.3. Influence of the fan rotating speed on the closed Rankine cycleefficiency

In an air-cooled power plant, turbine exhaust in the finnedtube exchangers condenses when the ambient air flow outside,and the condensate enters the thermal cycle directly. Therefore,the closed Rankine cycle results in the characteristics of waterconservation. Generally speaking, the efficiency of the cycle is themost important index to characterize the economy of the ther-mal system, and the relationship between the steam turbinepower and the relevant heat rate at different back pressures isillustrated in Fig. 12 to calculate the total heat absorption of thepower plant.

Finally, the thermal efficiency of the closed Rankine cycle atdifferent wind speeds is obtained, and the related results for thetwo fan rotating speeds are shown in Fig. 13. It is seen that thethermal efficiency of the closed Rankine cycle drops significantlywith the increasing wind speed. Taking the design case forexample, the efficiency value decreases from 43.1% at vm ¼ 2m s�1

and 40.5% at vm ¼ 10 m s�1. Moreover, the thermal efficiency isreally raised after the elevation of the fan rotating speed fromn ¼ nd to n ¼ 107%nd, and the promotion effect at higher windspeeds seems more efficient compared to the low wind speedcondition.

4. Conclusions

The current investigation studies the characteristics of a2 � 600 MW power plant at prevailing wind angle and differentwind speeds. The influence from the rotating speed of the wind-ward axial fans on the performance of the fan array as well as thewhole condenser is demonstrated comprehensively. The net power


of the power plant and the thermal efficiency of the closed Rankinecycle are respectively analyzed to show the promotion effect afterthe rotating speed adjustment.

The performance of the windward fans is effectively raisedafter increasing the rotating speeds. Under the driving effectfrom the windward fans on the other fans, the flow rate of thefan array also rises significantly. As a result, an increased heattransfer rate as well as the lower back pressures of the powerplant are obtained. Finally, the benefit from the rotating speedalteration of the windward fans is validated through theimprovement of the net power and the closed Rankine cycle ef-ficiency. It is also found that the promotion effect is more effi-cient at higher wind speeds due to the serious reduction of thepower plant performance at such wind conditions. The obtainedresults are significant to improve the efficiency of the thermalsystem and provide the references to optimize the running modeduring the extreme weather period.

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influence from the rotating speed of the windward axial fans on the performance of an air-cooled...

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