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1 Optimal Control of Chiller Condenser Sub-cooling, Compressor Speed, Tower Fan and Pump Speeds, and IGV Omer Qureshi, Hassan Javed & Peter Armstrong, June 2013 btrc.masdar.ac.ae

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2 Presentation Outline Introduction SCADA and Heat Balance Analysis Component Models Chiller System Solver Optimization Conclusion and Future Work

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3 Introduction Plant under consideration-(4x2500T). Collection and analysis SCADA Development of sub models for Individual chiller components Validation of model Development of solver- to execute these sub models and predict chiller performance. Optimize the model to produce set of conditions for optimum power consumption.

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4 District Cooling Plant Selected District cool Plant Capacity (4x2500T) Shell and tube Evaporator and Condenser Constant speed single stage centrifugal compressor Capacity control by Pre-rotation vanes Surge control Variable geometry diffuser 2-cell cooling tower each with variable speed fan (Fan diameter: 8m) Variable speed chilled water pump Constant speed condenser water pump

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5 Chiller Unit 1. Maintenance manual of York Chiller(Source: Tabreed)

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6 SCADA & Heat Balance Analysis

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7 Components Models—Chiller Unit Steady-state models based on first principle Inputs Component engineering parameters SCADA Data Simple models, less computation time Four Component models for district cooling plant Evaporator Model----Shell and tube Condenser Model----Shell and tube Centrifugal Compressor Model (Isentropic work + loss Mechanism) Constant speed Variable speed Variable speed pump model

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8 Evaporator Model ENGINEERING PARAMETERS TubesCopper Length of shell6.6 m Tube Pass (water)2 Total no. of tubes1234 Tube Diameter0.75" or 1.905x10 -2 m Tube thickness0.028" or 7.11x10 -4 m Assumptions: No pressure drop considered for refrigerant side Thermal resistance from refrigerant side is neglected.

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9 Evaporator Model Evaporation Superheating Evaporator Two regions for refrigerant were modeled: Evaporation Superheating – NTU Method Single Stream HX for evaporation Crossflow HX for super heating 1 st Pass2 nd Pass

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10 Evaporator Model Equations utilized in Evaporator Model Evaporation Superheating

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11 Evaporator Model Equations utilized in Evaporator Model Equation for regressed length: Equation for temperatures:

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12 Evaporator Model 1. Maintenance manual of York Chiller(Source: Tabreed)

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13 Evaporator Model RMS0.2096 C NRMS0.0319

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14 Condenser Model ENGINEERING PARAMETERS TubesCopper Length of shell6.6 m Tube Pass (water)2 Total no. of tubes937 Sub-cooling Section: Tube Diameter0.75" or 1.905x10 -2 m No. of tubes180 Tube thickness0.028" or 7.11x10 -4 m Tube Surface Area66.78 m 2 Condensation & de-superheating Section: Tube Diameter1" or 2.54x10 -2 m No. of tubes757 Tube thickness0.035" or 8.89x10 -4 m Tube Surface Area376.44 m 2 Assumptions: No pressure drop considered for refrigerant side Thermal resistance from refrigerant side is neglected.

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15 Condensation Sub-cooling Conden- sation De- superheating Condenser Condenser Model Three regions for refrigerant were modeled: Sub-cooling Condensation De-Superheating – NTU Method 1 st Pass2 nd Pass

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16 Equations utilized in Condenser Model 1a. Sub-Cooling Section(First Pass): Condenser Model

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17 1b. Condensation Section (First Pass): Mixing Section: Condenser Model

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18 2a. Condensation Section (Second Pass): 2b. De-superheating Section (Second Pass): Condenser Model

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19 Condenser Model RMS0.0949 C NRMS0.0225

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20 Condenser Model RMS0.6481 C NRMS0.1471

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21 Compressor Model Integral and mathematically most complex part of chiller Constant and variable speed compressor model Non-Dimensional loss model based on Aungier(2000) Assumptions Gear efficiency is taken as 90% Velocity profile is assumed as constant, along the hub and tip The kinetic energy of refrigerant entering the diffuser is completely converted to useful energy Diffuser and IGV losses are not modeled Water flow rate for motor cooling is taken as constant Complex engineering parameters in impeller geometry Centrifugal Compressor Specification Refrigerant R134A Rating (Btuh) 2500 Rating (kW input) 1817 Rating discharge pressure (psig) 162 Rating suction pressure psig) 34 Rating suction temperature (F) 33/34 Impeller diameter (outlet diameter) m 0.7 Impeller hub diameter (inlet diameter) 0.3 Impeller Blade Angle (degree) 45/50

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22 Constant Speed Model Variable speed Model Compressor Model-Inputs and Outputs InputOutput Mass flow rate of refrigerant Inlet and outlet pressure of compressor Inlet and outlet blade and velocity angles of impeller Impeller Inlet and outlet engineering parameters and dimensions Gear efficiency Compressor Power Compressor RPM Pressure at impeller exit Temperature at compressor outlet Pressure drop due to Impeller losses InputOutput IGV Positions Constant RPM Inlet and outlet pressure of compressor Inlet and outlet blade and velocity angles of impeller Impeller Inlet and outlet engineering parameters and dimensions Gear efficiency Compressor Power Pressure at impeller exit Temperature at compressor outlet Pressure drop due to Impeller losses

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23 Validation Constant Speed Compressor Model

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24 Validation Constant Speed Compressor Model RMS108.34 KW NRMS0.1553

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25 Variable Speed Compressor Model Loss Model Calculations

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26 Variable Speed Compressor Model-Benefits/comparison Variable Speed Compressor (KW) Measured Compressor Power (KW) Compressor Power (KW) No. of Observations Operation Conditions: 1.m r (kg/s) 2.P out /P in Power (KW)1504.702 IGV Position44.2

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27 Impeller Loss Model

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28 Variable Speed Compressor Model-losses profile

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29 Effectiveness NTU Method Cooling Tower Model

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30 Assumptions, Specifications and Input/ Output Variables Cooling Tower Model Assumptions Air exiting the tower is saturated with water vapor and is only characterized by its enthalpy Reduction of water flow rate by evaporation is neglected in the energy balance. Mass flow rate is calculated by considering linear proportionality of mass flow rate of air and motor speed. InputsOutputs Wet-bulb temperature Cooling tower supply water temperature Dry-bulb temperature Mass flow rate of water Cooling tower fan/motor speed Cooling tower return water temperature Merkel’s Number Cooling Tower Specifications Rating (RT) 5000 Rating flow rate (GPM) 15300 Rating ambient wet bulb (F) 86 Rating ambient dry bulb (F) 122 Rating entering condenser water temperature (F) 105 Fan diameter and speed (m, RPM) 8/152.6 Air flow rate (CFM)776383

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31 Cooling Tower Model

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32 Pump Model

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33 Pump Model Validation Graph + 5%Error Line

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34 Solver Description Q t,e T w,in,e T w,in,c V e V c dT sh,e Q t,e T w,in,e T w,in,c V e V c dT sh,e

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35 Optimization Optimization performed with two configurations: Chiller Water Flow Optimization Chiller Water Flow And Condenser Water Flow Optimization Objective Function: Minimize total power consumption i.e. compressor power and pump(s) power combined.

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36 Optimization Chiller Water Flow Optimization:

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37 Optimization Chiller Water Flow And Condenser Water Flow Optimization: Qe= 10,000 kW Ve,opt= 0.349 m 3 /s Vc,opt= 0.408 m 3 /s Tw,in,e = 14 C; Tw,in,c = 25 C Vc (m 3 /s) Total Power (KW)

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38 Optimization Chiller Water Flow And Condenser Water Flow Optimization: Qe= 8,000 kW Ve,opt= 0.296 m 3 /s Vc,opt= 0.355 m 3 /s Tw,in,e = 14 C; Tw,in,c = 25 C Vc (m 3 /s) Total Power (KW)

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39 Optimization Chiller Water Flow And Condenser Water Flow Optimization: Qe = 6,000 kW Ve,opt= 0.249 m 3 /s Vc,opt= 0.332 m 3 /s Tw,in,e = 14 C; Tw,in,c = 25 C Vc (m 3 /s) Total Power (KW)

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40 Optimization Chiller Water Flow And Condenser Water Flow Optimization: Qe = 4,000 kW Ve,opt = 0.205 m 3 /s Vc,opt = 0.251 m 3 /s Tw,in,e = 14 C; Tw,in,c = 25 C Vc (m 3 /s) Total Power (KW)

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41 Optimization Chiller Water Flow And Condenser Water Flow Optimization: Tw,in,e = 14 C; Tw,in,c = 25 C

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42 Optimization Chiller Water Flow And Condenser Water Flow Optimization:

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43 Conclusions Variable Speed compressor provide savings of 30-40% Variable speed pump for water circulation play an imperative role in reducing overall power consumption of chiller plant. Modeling of chiller components can be performed with limited engineering information from manufactures.

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44 Future Work More rigorous compressor loss model Transient model for the condenser and evaporator Cooling tower Model Variable Speed condenser pump Investigate the effect of pressure drop and resistance from refrigerant side

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45 Q&A 45

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