Proceedings: ISHVAC 007 Using Annual Building Energy Analysis for the Sizing of Cooling Tower for Optimal Chiller-Cooling Tower Energy Performance Chia-Wei Liu, Yew-Khoy Chuah Institute of Mechanical and Electrical Engineering, National Taipei University of Technology, Taipei 10608, Taiwan China yhtsai@ntut.edu.tw ABSTRACT Lower condenser water temperature is an important factor for water chiller energy performance. In contrary to the usual consideration of trade off between cooling tower fan power and chiller power consumption, this study investigated the effects of oversized cooling tower on the total chiller-cooling tower energy performance. A dynamic building energy method was used in this study to compare the annual energy performance with this strategy using an office building under typical climatic conditions of tropical and subtropical regions. It has been found that oversizing cooling tower would result in ever decreasing total annual energy consumption for chillers and cooling towers as a whole. These results were obtained with different control methods used for the cooling tower fans. Moreover, this rate of decrease in total energy consumption is steepest at 10% oversize of cooling tower at standard conditions. Discussion is presented on the analysis for understanding the principles behind the results thus obtained. KEYWORDS Cooling tower, Chiller energy consumption, Cooling tower energy consumption, Building energy INTRODUCTION Chillers constitute the largest power consumption item of all the air-conditioning equipment, usually take up 60% of the power demand. Therefore, increasing the operating efficiency of the chiller is very important for building energy saving. Most chiller manufacturers provide energy efficiency index for their chillers such as the coefficient of performance (COP) at full load conditions. However, a water chiller usually operates at part load, and usually with condenser water temperature different from standard test conditions. Therefore, evaluation of the actual energy use should consider the cooling tower performance due to year round weather conditions. Liu and Chuah (006) analyzed for three cities in Taiwan, the effects of condenser water temperature control on annual energy saving with different tower fan control methods. Stout and Leach (00) presented a model for simplified analysis on the design and control of cooling tower and condensing water systems to save energy. The control of the condenser water temperature is important to increase the chiller energy efficiency in operation (Crowther and Furlong 004). It has been known that lower condenser water temperature could be achieved at the expense of higher cooling tower energy. This study analyzed the effects on the lower condenser water supply temperature by the oversizing of cooling tower. This study was done with different cooling water temperature control methods. The advantages and energy saving potential of these methods are presented and discussed. SIMULATION This study used the VisualDOE (Eley Associates 001) dynamic building energy program for hourly building energy analysis in one year duration. Annual hourly weather conditions at three different cities were considered in the analysis. Then, the annual total energy consumption by the chillers and the cooling tower fans were analyzed for various cases and are discussed in terms of energy saving. Electric chiller component model The VisualDOE dynamic building energy program is based on DOE-.1E. It can be used for simulating the hourly power consumption of water chillers and cooling tower fans (Winkelmann et al. 1993). Chiller and cooling tower power consumption can be modeled for operation under different conditions (Hydeman et al 00). In DOE-.1E, three regression curves are used to represent the performance of a chiller at part-load and off-design conditions as follows: CAPFT: A curve that represents the available capacity as a function of evaporator and condenser temperatures. EIRFT: A curve that represents the full-load efficiency as a function of evaporator and condenser temperatures. EIRFPLR: A curve that represents the efficiency as a function of part-load ratio. - 43 -
Proceedings: ISHVAC 007 The chilled water and the condenser water fluid temperatures are used as indicators of refrigerant pressures respectively in the evaporator and the condenser in CAPFT and EIRFT as shown below in Eqs. (1)-(4). CAPFT a b t d t EIRFT a b t d t EIRFPLR c t e t c t e t f t f t t t (1) () a b PLR c PLR (3) Q PLR (4) Q CAPFT rated By solving Eqs. (1)-(4), the power consumption of the water chiller can be determined at various chiller operating conditions using Eq (5). P Prated CAPFT EIRFT EIRFPLR Cooling tower component model (5) Two regression curves are used to represent the cooling tower performance at part-load and offdesign conditions : GPMRA: A curve that represents the capacity variable as a function of range and approach. GPMWB: A curve that represents the capacity variable as a function of GPMRA and outdoor wetbulb temperature. These two regression curves are combined to obtain the capacity as a function of three variables, and shown below in equations (6) to (11). t A t R tcwr t (6) t A t towb (7) a b tr c tr d GPMRA (8) e GPMRA f tr GPMRA GPMRA d f t R ( d f t ) d t owb R e e t 4 e ( a b t GPMWB a b GPMRA c GPMRA owb R t ) (9) f GPMRA t (10) tr q available qrated GPMWB (11) 10 This study used three types of cooling tower fan modulations, i.e., one-speed fan (on-off), two-speed fan for 1/ capacity (100%-50%-off) and variable A owb speed fan (ASHRAE 1999 and 001). Figure 1 shows the performance of the three fan modulation methods at different fan flows. Fan power (%) 100 80 60 40 0 0 (on-off) (100%-50%-off) Variable-speed fan (VFD) 0 0 40 60 80 100 Fan flow (%) Figure 1: Cooling tower fan modulations. The coefficients of the chiller and the cooling tower regression curves used are shown in Table 1. Table 1: Chiller and cooling tower curve coefficients a b c CAPFT -1.7404 0.099-0.000067 EIRFT 3.11750-0.10936 0.001389 EIRFPLR 0.903 0.313387 0.46371 GPMRA -.8889 0.1667954-0.014105 GPMWB 0.605314-0.0355454 0.0080408 d e f CAPFT 0.048054-0.00091-0.000106 EIRFT 0.00375 0.00015-0.000375 GPMRA 0.0333 0.185601 0.45187 GPMWB 0.08606.4970E-4 0.0049086 Building energy simulation An office building with 10 floors was used as the building cooling case in this study. Table shows the chiller and cooling tower system parameters at design conditions. Figure shows the building configuration and load planing parameters. Typical weather types in this study are represented by cities located in tropical and subtropical regions, namely, Taipei, Hongkong and Singapore. Figure 3 shows the annual wet-bulb temperature profiles of these three cities. - 44 -
Proceedings: ISHVAC 007 Table : Building chiller and cooling tower system parameters. Building Type Office Floor Area 193750 ft (18000 m ) Floor Height 13.1 ft (4 m) Number of Floor 10 Schedule 9:00 am to 6:00 pm Summer Design 78 (5.6 ) Winter Design 64.4 (18 ) Number and Cooling Tower Fan Performance 1 at 0.017 bhp/rt Cooling Tower Range 10 (5.6 ) Cooling Tower Approach 7 (3.9 ) Cooling Tower Rating WB 78 (6.6 ) Number and Chiller Performance 1 at 0.6356 kw/rt Chiller Water Temperature 44 (6.7 ) Condenser Water Temperature 85 (9.4 ) Location Taipei Hongkong Singapore Chiller Capacity (Calculated) 57 RT 593 RT 664 RT Cooling Tower Capacity (Calculated) 68 RT 70 RT 786 RT Construction: Exterior wall: 3.94 W/m K Interior floor: 3.37 W/m K Roof: 0.75 W/m K Partition:.84 W/m K Occupancy: 10 m /person Light: 0 W/m Equipment: 0 W/m OA requirement: 8.5 L/s*person Opening: Fenestration 30% Glazing: 3 W/m K SC: 0.96 Figure : Building configuration and load planing parameters. Wet-bulb (C o ) 8 4 0 16 1 1 3 4 5 6 7 8 9 10 11 1 Month Taipei HongKong Singapore Figure 3: Annual wet-bulb profiles. RESULTS AND DISCUSSION Cooling tower and chiller energy performance Cooling towers of various sizes were first used in the analysis of the variation of fan power with condenser water supply. Figure 4 shows for July in Taipei (at wet-bulb temperature of 78 (5.6 )) the variation of fan power ratio at different condenser water temperature for different sizes of cooling tower. It is shown in Figure 4 that the use of larger cooling towers could result in lower fan power ratio. The fan power ratio is compared with cooling tower fan power at 100% load at design conditions. For the range of condenser water supply temperature analyzed, larger cooling towers have lower fan power ratio at the same supply temperature, thus lower energy consumption. It is observed in Figure 4 that larger cooling tower can reach smaller approach temperature when fans operating at full load, but relatively have higher power consumption. Figure 5 shows the results of chiller power ratio obtained with different condenser water temperature for different sizing of cooling tower. Chiller power ratio is compared to chiller operating at full load at standard conditions. It can be seen that lower condenser water temperature obtained is the result of smaller approach with larger cooling tower. It also clearly shows that the chiller power decreases with larger sizes of cooling tower, hence higher efficiency can be obtained with larger sizes of cooling tower. - 45 -
Proceedings: ISHVAC 007 Fan power ratio Chiller power ratio 1 0.8 0.6 0.4 0. Approach 10% Design 6 7 8 9 30 31 3 Condenser water supply (C o ) Figure 4: Cooling tower fan performance for the month of July, one-speed fan, Taipei. 0.75 0.74 0.73 110% Desigh Approach = 5.1 o F 140% Desigh Approach = 3.6 o F 6 7 8 9 30 31 3 Condenser water supply (C o ) 100% Desigh Approach = 5.8 o F 10% Desigh Approach = 4.5 o F 130% Desigh Approach = 4 o F 150% Desigh Approach = 3. o F 160% Desigh Approach =.9 o F Figure 5: Chiller performance for the month of July, fan speed at full load, Taipei. Annual energy consumption Three different control methods of condenser water temperature were compared in this study. First is the fixed temperature control at 85 (9.4 ) which is the condenser water temperature for full load operation of chiller. Second is the wet-bulb reset method that reset the condenser water temperature hourly according to the variation of ambient wetbulb temperature. Lastly, the control of condenser water temperature at lowest temperature allowed for the chiller, eg, 66 (18.9 ). In this method, the cooling tower will operate at its capacity to achieve or approach this low temperature. Table 3 to Table 5 are the results the annual energy consumption of chiller and cooling tower due to the different control methods, for the three cities. The case of fixed temperature at 85 (9.4 ) with one speed fan is used as the base case. The one speed fan allows for on/off control. Even using the variable speed control could save no more than 1% of total chiller cooling tower annual energy for the three cities. However, the wet-bulb reset and the low fixed temperature controls result in significant energy saving. Although higher tower fan energy is required, it is more than offset by the decrease in chiller energy consumption. It is noticed that the low temperature setting has the best energy saving. Also for this case, the control method of fan speed matters less. It is also noticed that for Singapore that located in tropical climatic region, energy saving is less significant for the control methods due to the yearly round higher ambient wet bulb temperature. Figure 6 and Figure 7 use Taipei as an example, to analyze the energy consumption of cooling tower and chiller respectively with sizing of cooling tower. It can be seen in Figure 6 that when using fixed temperature 85 (9.4 ) and wet-bulb reset control methods, larger cooling tower results in lower fan energy consumption. This result was obtained as the fan of larger cooling tower operates at partial load in longer duration. However, when the condenser water temperature was set at low temperature 66 (18.9 ), the cooling tower fan would operate at larger or full load for longer duration, thus consumed more energy hencewith. But it can be seen in Figure 7 that when the control of condenser water temperature at low temperature 66 (18.9 ) was used, larger cooling tower would result in lowest condenser water temperature in operation. Therefore, largest saving in chiller energy was obtained. It is thus shown that the control of condenser water temperature is important to the energy saving in chilller. Table 3: Simulation results of chiller-tower performance, Taipei. Chiller Location: Taipei Tower Total Energy Savings Fixed Temp. 85 (9.4 ) One-speed 71576 1406 7768 Baseline Two-speed 71576 8369 73645 0.55% VFD 71576 775 7551 0.70% Wet-bulb Reset One-speed 678896 17360 69656 4.3% Two-speed 678896 1164 690538 5.10% VFD 678896 977 68863 5.37% Fixed Temp. 66 (18.9 ) One-speed 651484 9196 680680 6.46% Two-speed 651484 7531 679015 6.69% VFD 651484 663 678116 6.81% - 46 -
Proceedings: ISHVAC 007 Table 4: Simulation results of chiller-tower performance, Hongkong. Table 5: Simulation results of chiller-tower performance, Singapore. Chiller Location: Singapore Tower Total Energy Savings Fixed Temp. 85 (9.4 ) One-speed 973841 1056 994897 Baseline Two-speed 973841 14630 988471 0.65% VFD 973841 11740 985581 0.94% Wet-bulb Reset One-speed 963549 307 986756 0.8% Two-speed 963549 16996 980545 1.44% VFD 963549 13189 976738 1.83% Fixed Temp. 66 (18.9 ) One-speed 99745 39658 969403.56% Two-speed 99745 39658 969403.56% VFD 99745 39658 969403.56% Cooling tower fans energy consumption 44000 40000 36000 3000 8000 4000 18000 14000 10000 6000 1000 8000 4000 Chiller Location: Hong Kong Tower Total 100 10 140 160 Cooling tower capacity (%) Fixed Temp. 66 o F(18.9 o C) Wet-bulb Reset Fixed Temp. 85 o F(9.4 o C) Energy Savings Fixed Temp. 85 (9.4 ) One-speed 763100 046 783346 Baseline Two-speed 763100 1596 77906 0.55% VFD 763100 14613 777713 0.7% Wet-bulb Reset One-speed 74641 5 749863 4.7% Two-speed 74641 1998 743939 5.03% VFD 74641 17071 74171 5.31% Fixed Temp. 66 (18.9 ) One-speed 6959 37747 733669 6.34% Two-speed 6959 361 73143 6.53% VFD 6959 35351 73173 6.65% Figure 6: Cooling tower energy consumption for oversizing cooling tower, Taipei. Chillers energy consumption 655000 650000 645000 640000 635000 678750 67850 677750 715500 715000 714500 714000 100 10 140 160 Cooling tower capacity (%) Fixed Temp. 66 o F(18.9 o C) Wet-bulb Reset Fixed Temp. 85 o F(9.4 o C) Figure 7: Chiller energy consumption for oversizing cooling tower, Taipei. Figure 8 to Figure 10 show the comparison of energy saving of chiller and cooling tower for the three cities. It can be seen that variable speed fan control has the largest energy saving potential followed by two speed fan control. It is also noticed that 110% of design value has the largest increase in energy saving. The benefits of larger cooling tower become more marginal thereon. It has been noticed that larger cooling tower can result in smaller approach. As the decrease in the approach will become more marginal, the increase in cooling tower performance will becoming smaller, as can be seen in Figure 5. In effects, the condenser water temperature can be lowered very little when the size of the cooling tower is already relatively large. Therefore, the benefits of increasing chiller efficiency become marginal. It can also be seen in the comparison between the three cities in Figure 8 to Figure 10 that Singapore has less energy saving potential for chiller-tower. It was shown in Figure 3 that the ambient wet-bulb in Singapore is relatively high year round. In comparisin Taipei has lower year average ambient wet bulb temperature. This seasonal change in ambient wet-bulb temperature allows for effective condenser water temperature control. It is observed in Figure 10 that for condenser water temperature control set at 18.9, the magnitude of increase in energy efficiency is greatest when cooling tower increase to 110%. However, it is seen in Figure 10 that energy consumption for cooling tower will increase beyond the energy saving of chiller when the size of the cooling tower increase to 140%. This trend is more obvious for Singapore with higher wet-bulb temperature year round. - 47 -
Proceedings: ISHVAC 007 Energy savings (%) 1.6 1. 0.8 1.4 1 0.6 0.8 0.4 0 Fixed Temp. 85 o F(9.4 o C) 10% Design Taipei Hongkong Singapore Baseline Figure 8: Chiller-tower energy saving for different sizing of cooling tower, fixed temp. 85 (9. 4 ) control method. Energy savings (%) 7.4 7 6.6 7. 6.8 6.4 7 6.6 6. Fixed Temp. 66 o F(18.9 o C).8.6 3.8.6 3.8.6 10% Design Taipei Hongkong Singapore Figure 10: Chiller-tower energy saving for different sizing of cooling tower, fixed temp. 66 (18.9 ) control method. 3 Energy savings (%) 6.4 6 5.6 5. 6 5.6 5. 4.8 5. 4.8 4.4 4 Wet-bulb Reset.8.4 1.6.4 1.6 1. 1.8 1.4 1 0.6 10% Design Taipei Hongkong Singapore Figure 9: Chiller-tower energy saving for different sizing of cooling tower, wet-bulb reset control method. CONCLUSION Standard test conditions are often used for selection of cooling tower, and the condenser water supply temperature of 85 (9.4 ) is usually taken as the control temperature. The benefits of lower condenser water temperature often do not attract sufficient attention. This study has found that oversizing of cooling tower can achieve lower energy concumption for chiller and cooling tower as a whole. Moreover, 10% oversizing has the greatest rate of increase in the total energy efficiency of chiller and cooling tower. This increase in energy efficiency is resulted from the higher fan power being offset by the energy saving of chiller power. As the cooling tower over sizing about 40%, the benefits of energy saving becomes marginal as the cooling tower approach is meeting the limit. For the the cases studied for the three cities in Asia, Singapore seemed to have less potential for energy saving by the control methods analyzed here. It is due to the higher ambient wet-bulb temperature year round. It has been found that 6% to 7% of energy saving is possible with proper design and control of cooling tower. ACKNOWLEDGMENT It is acknowledged herewith this study is supported by the National Science Council of Taiwan. - 48 -
Proceedings: ISHVAC 007 REFERENCES [1]ASHRAE. 1999. Handbook of HVAC Applications. pp.40.9~40.13. []ASHRAE. 001. ANSI/ASHRAE/IESNA Standard 90.1, User s Manual. pp.6-71~6-7. [3]Crowther, H. and J. Furlong, 004. Optimizing Chillers & Towers. ASHRAE Journal Vol. 46 Iss. 7 p. 34. [4]Eley Associates. 001. VisualDOE 3.0 Program Documentation. 14 Minna St., SF, California. [5]Hydeman, M. and K.L. Gillespie. 00. Tools and Techniques to Calibrate Electric Chiller Component Models. ASHRAE Transactions Vol. 108. pp.733~741. [6]Hydeman, M., S.T. Taylor and D. Winiarski. 00. Application of Component Models for Standards Development. ASHRAE Transactions Vol. 108. pp.74~750. [7]Liu C.W. and Y.K. Chuah. 006. The Effects of Weather Conditions on the Cooling Tower Performance and the Energy Consumption of Water Chillers in Taiwan. ACRA006 Conference V ol.. pp.589~593. [8]Stout Jr, M.R. and J.W. Leach. 00. Cooling Tower Fan Control for Energy Efficiency. Energy Engineering Vol. 99. NO. 1. pp.9~31. [9]Winkelmann, F.C., B.E. Birdsall, W.F. Buhl, K.L. Ellington, A.E. Erden. 1993. DOE- Supplement version.1e. Lawrence Berkeley Laboratory, Berkeley, California. q rated : Heat rejection capacity from cooling tower at rated condition (MBtu/hr) NOMENCLATURE a,b,c,d,e,f: Regression coefficients t : Condenser water supply temperature ( ) t : Chilled water supply temperature ( ) Q: Capacity of chiller (tons) Q ref : Capacity of chiller at rated condition (tons) PLR: Operating part-load ratio of chiller P: Present power consumption of chiller (kw) P ref : Power consumption of chiller at rated condition (kw) t R : Range ( ) t A : Approach ( ) t cwr : Condenser water return temperature ( ) t : Condenser water supply temperature ( ) t owb : Outdoor wet-bulb temperature ( ) q available : Available heat rejection capacity from cooling tower (MBtu/hr) - 49 -