ASHRAE LV-11-027-2011 Optimization of the Cooling Tower Condenser Water Leaving Temperature Using a Component-Based Model.pdf

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1、934 ASHRAE TransactionsABSTRACTThis study investigates the optimization of the coolingtower condenser-water leaving temperature using a compo-nent-based model. This model consists of a chiller, acondenser-water pump, and two cooling towers. The chiller ismodeled with a Gordon-Ng model for vapor-comp

2、ressionchillers with variable-condenser flow. The cooling tower issimulated with an effectiveness-NTU model. The pump poweris calculated from the pump flow rate, pump head, and effi-ciency. This optimization problem is formulated as that of mini-mizing the total power of the chiller, pump, and fans

3、byselecting an optimal cooling tower condenser-water leavingtemperature at given weather conditions, chiller load, chilled-water leaving temperature, and condense-water flow rate. Themodel is applied in an example chiller CW system, and a gener-alized reduced gradient solver is used to search the op

4、timalcooling tower approach setpoint. Simulation results show thatthe optimal cooling tower approach reset schedule can beapproximated with two straight lines. Significant energysavings could be achieved if compared with the scenario witha constant cooling tower condenser-water leaving tempera-ture.

5、 Further simulations show that the chilled-water leavingtemperature, chiller part-load ratio, and the climate zones theplant locates in have a minor effect on the optimal approachreset schedule. A higher condenser-water flow rate per coolington leads to a higher optimal cooling tower approach, but t

6、hiseffect can be neglected for a system with a constant CW flowrate. The approach setpoint reset schedule that yields optimalcontrol depends on the performance characteristics of thechiller and the cooling tower. INTRODUCTIONA condenser water (CW) loop consists of chillers,condenser-water pumps (CWP

7、), and cooling towers (CT). Theelectricity consumption of these components accounts for themajority of total electricity consumption in a chiller plant. Fora water-cooled chiller system, it is typically designed aroundentering condenser-water temperatures of 85F (29.4C) witha nominal CW flow of 3.0

8、gallon per minute (gpm) per ton(0.1937 m3/h per kW cooling) and a 10F (5.6C) range(Furlong and Morrison 2005). However, most of the time, thesystem could be operated under nondesign load and weatherconditions. How to optimize the operation of the condenser-water loop is of great interest.Supervisory

9、 control is typically applied in the chillerplant. The CWP control is dedicated to the chiller control toprovide relatively constant flow for individual chillers. It ismore and more popular to apply variable-speed devices (VSD)to cooling tower fans to reduce their cycling frequency andallow better t

10、emperature control for any given chiller load andweather conditions. The CT condenser-water leaving temper-ature (CWLT) setpoint is maintained by modulating the CTfan speed. A dead band for the CWLT setpoint is adopted toavoid fan cycling. Braun and Diderrich (1990) demonstratedthat feedback control

11、 for cooling tower fans could be elimi-nated by using an open-loop supervisory control strategy. Thisstrategy requires only measuring chiller loading to specify thecontrol and is inherently stable.Optimization of the cooling tower CWLT setpoint isintensively studied by some researchers. This setpoin

12、t and theCWP flow rate are the main inputs that are directly related tothe optimization of the condenser side. Some engineers keepthe setpoint at the lower limit at any time to minimize chillerOptimization of the Cooling Tower Condenser Water Leaving Temperature Using a Component-Based ModelZhiqin Z

13、hang, PhD Hui Li, PhDStudent Member ASHRAE Associate Member ASHRAEWilliam D. Turner, PhD, PE Song Deng, PE Member ASHRAEZhiqin Zhang is a PhD student in the Department of Mechanical Engineering and a graduate research assistant in the Energy Systems Labo-ratory, Hui Li is a post-doctorate and Song D

14、eng is an associate director in the Energy Systems Laboratory, and William D. Turner is a profes-sor in the Department of Mechanical Engineering, Texas A (a) I-P, (b) SI.Table 2. Hour Number in Each DB and WB Bin for Houston, TXWet-Bulb Temperature, F (C)Dry Bulb, F (C)17(8)24 (4)30(1)36(2)43(6)49(9

15、)55(13)62(17)68(20)74(23)81(27)21 (6) 5 7 29 (2) 26 66 37 (3) 2 118 227 25 46 (8) 4 147 395 78 54 (12) 18 158 503 272 62 (17) 19 202 432 681 18 70 (21) 42 152 380 915 239 78 (26) 48 111 467 1476 7387 (31) 13 154 644 22895 (35) 29 258 108103 (39) 1 14 5(a)(b)all2011 ASHRAE 939humid). The Typical Mete

16、orological Year 3 (TMY3) hourlyweather data (NREL 2008) are used to generate a two-dimen-sion bin. One dimension is DB temperature and the other isWB temperature. In each bin, the hour number is counted andthe average DB and WB temperatures in each bin are calcu-lated. These data are used as inputs

17、for the CW loop simulationprogram. Table 2 shows an 11 by 11 bin for the TMY3 weatherdata of Houston, TX. For example, for the bin of the DB is54F and the WB is 49F, the total hour number is 503. In thisstudy, to achieve a higher accuracy, the DB is divided into 46bins and the WB is divided into 39

18、bins.OPTIMIZATION RESULTSOptimal Cooling Tower Approach TemperatureIn this simulation, the chiller part-load ratio is 80%(4400 ton 15,474 kW), and the chiller ChW leaving temper-ature is 36.0F (2.2C). The chiller CW flow rate is10,000 gpm (2271 m3/h). The system is located in Houston,TX, and it is a

19、 VSD-equipped fan. These are the default condi-tions for the following analysis. Figure 4 shows the optimalcooling tower CWLT setpoint versus the ambient WB temper-ature, and a strong linear correlation can be observed. Thisrelationship can be approximated with two straight lines toform a near-optim

20、al fan control.(17)When the ambient WB is lower than 47.0F (8.3C), thetower CWLT is controlled at 55.0F (12.8C) to meet thelower limit of the chiller. When the WB is higher than 47F(8.3C), a higher WB temperature leads to a lower optimaltower CWLT. The slope and the intercept of the optimal resetFig

21、ure 4 Optimal CT approach temperature versus ambient WB temperature; (a) I-P, (b) SI.(a) (b)Figure 5 Cooling tower approach temperatures under various fan control strategies; (a) I-P, (b) SI.(a) (b)TAppTwb 55 if Twb47F+=TApp0.1325Twb 13.56 if Twb47F+=940 ASHRAE Transactionsschedule could be changed

22、with many factors, which will bediscussed one by one in the following sections.Energy Savings PotentialFigure 5 shows the simulated cooling tower approachversus the ambient WB temperature for different CT controlstrategies. For the scenario with 70.0F (21.1C) CWLTsetpoint, if the ambient WB is highe

23、r than 66.0F (18.9C), thefan speed reaches 100% and the approach setpoint cannot bemaintained. For the scenario with 4.0F (2.2C) constantapproach temperature, if the ambient WB temperature is lowerthan 68.0F (20.0C), the fan is running at full speed and theactual approach is higher than 4.0F (2.2C).

24、 The scenario of controlling the cooling tower CWLTsetpoint at 70F (21.1C) is used as the baseline. The annualtotal electricity consumptions of the chiller, cooling towerfans, and CW pump are simulated. Another six CT fan controlstrategies are simulated and the energy savings percentages foreach str

25、ategy are shown in Table 3. The optimal control canreduce the chiller power consumption by 5.8%, but consume19.7% more of the fan power. The total electricity energysavings are 4.1%. The total power savings for the near-optimalcontrol are very close to that for the optimal control. If thecooling tow

26、er approach is 1F (0.6C) higher than the optimalvalue, less electricity is consumed by the fan but more isconsumed by the chiller. The change of the annual total powersavings is 0.3%. If a constant approach setpoint of 8.0F(4.4C) is selected, the annual energy savings is 2.6%, whichis 1.5% or 426,63

27、7 kWh per year less than the savings of theoptimal control. The energy consumption with a constantapproach setpoint of 4.0F (2.2C) is almost equal to that withthe optimal control, which means that the operation with alower constant approach temperature is closer to the optimaloperation. In other wor

28、ds, it is preferred to run cooling towerfans at a higher speed. This is consistent with the conclusiondrawn by Braun et al. (1989).Figure 6 Optimal CT approach temperature under different CW flow rates; (a) I-P, (b) SI.(a) (b)Table 3. Annual Electricity Consumption Change under Different CT Control

29、StrategiesCT ControlCHLR Power, kWhCT Fan Power, kWhCW Pump Power, kWhTotal Power, kWhBaselineCT CWLT = 70F (21C) 24,611,417 1,325,921 2,245,375 28,182,713 TApp,sp = Optimal 5.8% 19.7% 0.0% 4.1%TApp,sp = Near-optimal 5.7% 18.6% 0.0% 4.1%Energy savings percentageTApp,sp = Optimal+1F (0.6C) 3.7% 12.8%

30、 0.0% 3.8%TApp,sp = 4F (2.2C) 5.8% 20.8% 0.0% 4.1%TApp,sp = 6F (3.3C) 4.1% 1.3% 0.0% 3.6%TApp,sp = 8F (4.4C) 2.0% 18.3% 0.0% 2.6%2011 ASHRAE 941Condenser-Water Flow RateBased on the velocity limits of condenser water passingthrough the condenser, the acceptable range of varying CWflow rate would be

31、50%100% of the nominal flow rate. Thechiller load is 80% (4400 ton 15,474 kW), and thecondenser-water flow is varying at 9000 gpm (2044 m3/h),10,000 gpm (2271 m3/h), 11,000 gpm (2498 m3/h), and12,000 gpm (2726 m3/h). Figure 6 shows the scatter plots ofthe optimal CT approach as a function of the amb

32、ient WBunder different CW flow rates. The slope of each plot is almostthe same, but a higher CW flow rate leads to a slightly higheroptimal CT approach temperature. The optimal CT approachdifference for these various CW flow rates is around 1.0F2.0F (0.6C1.1C). In Table 3, it is shown that, if theap

33、proach setpoint is 1F (0.6C) higher than the optimal value,the total energy consumption increases 0.3%. Considering thatthe CW flow rate is controlled at a constant value or in anarrow range, this difference could be neglected in designingan optimal cooling tower approach setpoint reset schedule. Ch

34、iller Part-Load RatioTo test the relation between the optimal CWLT resetschedule and the chiller PLR, the optimal cooling towerapproach setpoints when the chiller part-load ratio is 100%,80%, 60%, and 40% are plotted against the ambient WBtemperature in Figure 7. For 100% PLR, when the WB isbetween

35、45.0F (7.2C) and 60.0F (15.6C), the fan speedreaches 100% and the simulated setpoint is higher than theoptimal approach setpoint. A higher PLR leads to a lower opti-mal approach setpoint. These four scatter plots are overlappedwith each other in most areas, and the differences at the certainWB tempe

36、ratures is within 1.0F. Consequently, a sameFigure 7 Optimal CT approach temperature under different chiller loads; (a) I-P, (b) SI.(a) (b)Figure 8 Optimal CT approach temperature under different chiller ChW leaving temperatures; (a) I-P, (b) SI.(a) (b)942 ASHRAE Transactionsapproach optimal reset s

37、chedule can be applied at differentchiller PLRs.Chilled-Water Leaving TemperatureThe chiller chilled-water leaving temperature plays asignificant effect on the chiller performance. In practicality, itis typically reset based on the weather conditions or chillerload. Figure 8 shows the simulated opti

38、mal CT approachtemperature under different chilled-water leaving tempera-tures. The four scatter plots are overlapped with each other,which indicates that the chiller chilled-water leaving temper-ature has no effect on the optimal reset schedule. Chiller and Tower PerformanceThe chiller sensitivity

39、factor is defined as the incrementalincrease in chiller power for each degree increase incondenser-water temperature as a fraction of the power(ASHRAE 2003). A large sensitivity factor means the chillerpower is very sensitive to the CT control, favoring operationat higher airflow rates or lower cool

40、ing tower approach. Thisvalue can be obtained by calculating the derivative of Equa-tion 10 to chiller condenser-water entering temperature. Atypical factor is between 0.01 and 0.03 per F (0.02 and 0.06per C). For this particular chiller, it is 0.02 per F (0.04 per C),which indicates the chiller is

41、sensitive to the condenser-waterentering temperature. This explains why a lower than typicalapproach is selected for optimization.The tower performance is determined by two coefficients,c and n, which are empirical constants specific to a particulartower design. A lower c indicates that the heat tra

42、nsfer area issmaller for the airflow, leading to a higher airflow rate for thesame CWLT. Figure 9 shows the optimal cooling towerapproach temperature under different cooling tower coeffi-cients of c. If the coefficient c decreases, the tower heat dissi-pation capacity drops and more airflow is requi

43、red to achievethe same cooling tower CWLT. The plot indicates that a highercooling tower approach will make the system optimal. TheFigure 9 Optimal CT approach temperature under different CT coefficients; (a) I-P, (b) SI.(a) (b)(a) (b)Figure 10 Optimal CT approach temperature in different climate zo

44、nes; (a) I-P, (b) SI.2011 ASHRAE 943tower performance plays a significant effect on the optimalresults.Climate ZonesThe scatter plots of the optimal CT approach tempera-tures at six typical climate zones are shown in Figure 10. Thechiller is loaded at 80% PLR, the chilled-water leavingtemperature is

45、 36F (2.2C), and the condenser-water flowrate is 10,000 gpm (2271 m3/h). Except for the plots forDenver, these plots are overlapped with each other and asignificant correlation can be observed between the optimalcooling tower CWLT setpoint and the ambient WB tempera-ture. The atmospheric pressure in

46、 Denver is 0.824 atm and itis cool and dry. It is easier for water to evaporate. For the sameairflow rate or fan power, a lower tower CWLT can beachieved. SUMMARY AND CONCLUSIONResetting the cooling tower CWLT is one of the mostpopular measures to improve the performance of a chillerplant. It plays

47、opposite effects on the efficiencies of the chillerand cooling tower. An optimal value exists for specific oper-ating conditions to minimize the power consumption of thechiller, cooling tower, and CW pump. This paper introduces the optimization of the coolingtower CWLT using a component-based model.

48、 The model isapplied in an example chiller CW system and the coolingtower approach temperature setpoint is optimized to minimizethe total power of the chiller, pump, and fans at given weatherconditions, chiller load, chilled-water leaving temperature,and condenser-water flow rate. Simulation results

49、 show thatthe optimal cooling tower approach setpoint reset schedulecan be approximated with two straight lines. Significantenergy savings could be achieved if compared with thescenario with a constant cooling tower CWLT. Further simu-lations show that chiller PLR, chiller ChW leaving tempera-ture, and climate zones the plant locates in play minor effectson the coefficients of the optimal CWLT reset schedule. Ahigher condenser-water flow rate per cooling ton l