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Commissioning the ‘Optimized’ Chilled Water Plant
Janelle H. Griffin, PE, ACP, LEED AP BD+C
AIA Quality Assurance
The Building Commissioning Association is a Registered Provider with The American Institute of Architects Continuing Education Systems (AIA/CES). Credit(s) earned on completion of this program will be reported to AIA/CES for AIA members. Certificates of the Completion for both AIA members and non-AIA members are available upon request.
This program is registered with AIA/CES for continuing professional education. As such, it does not include content that may be deemed or construed to be an approval or endorsement by the AIA of any material of construction or any method or manner of handling, using, distributing, or dealing in any material or product.
Questions related to specific materials, methods, and services will be addressed at the conclusion of this presentation.
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Learning Objectives
1. Identify potential obstacles to achieving desired chilled water plant control strategies
2. Recognize the advantages and disadvantages of various control techniques to optimize chiller plant energy consumption
3. Apply essential elements of a control design that is prescriptive and verifiable
4. Utilize best practices within project teams so the design intent can be implemented, commissioned, and sustained through building occupancy
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• Widespread – Buildings > 50,000 SF
• 46% (by floorspace) Chilled Water for cooling • 36%: Central Chillers• 10%: District Chilled Water
Source: CBECS 2012 Table B41
• State of Industry–simplistic reset strategies universally applied across chilled water systems with widely different configurations, load profiles, and locations
Introduction: Chilled Water System Control
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• Optimal Control minimizes power at each instant of time while meeting the plant load (kW/ton)
• Considers Chillers, Pumps, and Cooling Towers, AHU Fans• Variables: CHWST, CWST, CHW Flow, Staging, CW Flow, …
Optimal Chiller Plant Control Strategies at Off-Design Conditions
Chiller
CWS
Cooling Tower
TCWR
Cooling Load
CHWS
CHWRCHWP
VFD
CWP
VFDT
T
Condenser
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Common Control Sequences – Falling Short
• Chilled Water Supply Temperature Reset -- OAT or CHWRT• Reset CWST on OAWB• Reset CW Flow on OAWB or Chiller Load (%)
Although these strategies seem reasonable, they do not generally minimize operating costs
(2011 ASHRAE Handbook-HVAC Applications)
…simulations seldom indicate a good fit to optimal operation (Optimizing Design & Control Of Chilled Water Plants Part 5: Optimized Control Sequences
Taylor, 2012).
Resetting the CW to a fixed value above OAWB has not proven to provide near optimum results.
(PG&E, 2000 CoolToolsTM Chilled Water Plant Design and Specification Guide)
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Design Development
• Plant Design• Theory of
Operation
Construction Documents
• Controls Design
• Sequences of Operation
Construction
• Controls Submittal
• Pre-Functional Process
• Functional Test Development
Acceptance
• Final Controls Design
• Final Sequences of Operation
• Functional Performance Test
Chilled Water Plant Controls Design to Completion
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Design Development
• Plant Design• Theory of
Operation
Construction Documents
• Controls Design
• Sequences of Operation
Construction
• Controls Submittal
• Pre-Functional Process
• Functional Test Development
Acceptance
• Final Controls Design
• Final Sequences of Operation
• Functional Performance Test
Chilled Water Plant Controls Design to Completion
As-Built Control
Theory of Operation
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Construction Documents
• Controls Design
• Sequences of Operation
Construction
• Controls Submittal
• Pre-Functional Process
• Functional Test Development
Acceptance
• Final Controls Design
• Final Sequences of Operation
• Functional Performance Test
Chilled Water Plant Controls Current Practice
“Optimize”Resets
ProprietaryCreativePartial
Unintended
FPT CriteriaIdentify Mis-interpretation
Operations
PersistenceModification
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Project Example: Cooling Tower ControlDesign
• BAS provider shall provide controls that calculate the optimal tower setpoint at any chiller(s) load and ambient wet bulb
• Bypass valve shall modulate to maintain minimum CWST of 65°F
OAWB
Chiller Load (%)
u1
x2
x1f(x1...xn)
Optimal CWST
65°F Minimum
P-6
P-5
Chiller 1CT-1 CT-2
VFD
VFD
Chiller 2
CWR
CWR
CWS
CWS
CWS CWS
CHWS
CHWSCHWR
CHWR
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• Condenser water supply setpointbased on chiller load ONLY according to AHRI conditions (Accepted)
• Bypass valve modulates to condenser water supply setpoint (Cx Issue; resolved)
Project Example: Cooling Tower Control
• Schedule: optimization control strategy not developed until Acceptance Phase testing
• Expertise Mismatch: Experienced controls programmer not suited to develop strategy based on power, ambient conditions, and load
Actual
45°F55°F65°F75°F85°F95°F
0% 25% 50% 75% 100%
EC
WT
Chiller Load
Setpoint(Tower Fan)
Minimum (Bypass Valve)
Causes
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1. Complexity2. Uniqueness3. Delegated Design4. Expertise Mismatch 5. Persistence
Barriers to Implementation of Optimal Controls
MechanicalDesign
Modeling and Simulation
Control Implementation
CommissioningCx, EBCx
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• Tools to estimate total Plant power at any given operating condition (load, chilled water supply temperature, ambient wet bulb) • Tools to define the optimal operating parameters at any given operating condition • Method to implement them as straight-forward control sequences
Barriers to Implementation of Optimal ControlsConnections Needed
OAWB
Chiller Load (%)
u1
x2
x1f(x1...xn)
Optimal CWST
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Chiller Modeling Options (Public)• BLAST• DOE-2.1• CoolToolsTM Reformulated • Models available for over 150 chillers• Custom Chillers - CoolToolsTM Chiller
Bid and Performance Tool (EDR)
Cooling Tower Modeling (Public)• Model based on Merkel’s Equations
(Merkel 1925) • CoolToolsTM Regression-Based
Model (Benton et al, 2002)
Modeling – Applied to Cooling Tower Control
Goal Estimate Power (kW) at any given operating condition (load, chilled water supply temperature, ambient wet bulb)
Chiller Model Chiller kW = f (load, CHWST, CWT)
Cooling Tower Model Approach= f (Range, OAWB, CW
Flow, Airflow)
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• 100,000 SF Office Space, Washington DC• 200 ton VFD Chiller, CW Pump, CHW Pump, CT Fan• Focus on Cooling Tower Temperature Control • Proposed Sequence of Operation • Review Tradeoff between Chiller and Cooling Tower
Project Example 2:
45
55
65
75
85
95
0 10 20 30 40 50 60 70 80
EC
WT
(°F)
Outside Air Wet Bulb (°F) 15
Chiller Modeling Chiller ModelChiller kW = f (load,
CHWST, CWT)
EIRFTemp(%) = a + b*CHWT + c*CHWT2 + d*CWT + e*CWT2 + f*CHWT*CWT
EIRFPLR (%) = a + b*PLR + c*PLR2 + d*CWT + e*CWT2 + f*PLR*CWT
ChillerCapFT (%) = a + b*CHWT + c*CHWT2 + d*CWT + e*CWT2 + f*CHWT*CWT+ f*CHWT*CWT
Cooling Tower Modeling
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Cooling Tower Model Approach=
f (Range, OAWB, CW Flow %, Airflow %)
…
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Example: 40% Chiller Load, 60°F Wet BulbChiller Model Tower Model
30.0
35.0
40.0
45.0
50.0
55.0
65 70 75 80 83
Pow
er (k
W)
Condenser Water Temperature (deg F)
40% Load, 60F WB
0.0
1.0
2.0
3.0
4.0
5.0
65 70 75 80 83
Pow
er (k
W)
Condenser Water Temperature (deg F)
40% Load, 60F WB
30.0
35.0
40.0
45.0
50.0
55.0
65 70 75 80 83
Pow
er (k
W)
Condenser Water Temperature (deg F)
40% Load, 60F WB
Combined Models
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30.00
35.00
40.00
45.00
50.00
55.00
77 78 80 83
40% Load, 75F WB
30.00
35.00
40.00
45.00
50.00
55.00
73 75 80 83
40% Load, 70F WB
30.00
35.00
40.00
45.00
50.00
55.00
65 70 75 80 83
40% Load, 65F WB
30.00
35.00
40.00
45.00
50.00
55.00
65 70 75 80 83
40% Load, 60F WB
30.00
35.00
40.00
45.00
50.00
55.00
65 70 75 80 83
40% Load, 55F WB
30.00
35.00
40.00
45.00
50.00
55.00
65 70 75 80 83
40% Load, 50F WB
30.00
35.00
40.00
45.00
50.00
55.00
65 70 75 80 83
40% Load, 45F WB
30.00
35.00
40.00
45.00
50.00
55.00
65 70 75 80 83
40% Load, 40F WB
Example: 40% Chiller Load 40F to 75F WB kW vs. CWT
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45
55
65
75
85
95
0 10 20 30 40 50 60 70 80
ECW
T (°
F)
Outside Air Wet Bulb (°F)
Condenser Water Temperature Control: 40% Chiller LoadDesign vs. Analysis-Driven
0
10
20
30
40
50
45
55
65
75
85
95
0 10 20 30 40 50 60 70 80
kW S
avin
gs
ECW
T (°
F)
Outside Air Wet Bulb (°F)
Design Analysis Result
28%
13.5 kW x 1314 run-hours (15%) @ $0.10/kWh $1774 cost savings per year
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• New Construction Building Commissioning
• Quantifying performance gap between optimal and specified
• Performance metrics • Existing Building Commissioning
• Performance Metrics
• Control Development
• Engineering
Applications
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Project Improvements Summary
Design Development
• Documentation of Design Intent
Construction Documents
• Dynamic Optimization or Efficient Control
Construction
• Controls Submittal
Acceptance
• Functional Testing
Specification Review
Delegated engineering
Project-specific Control Strategies
Verification of Part-Load Operation
Carefully review VE Options
Equipment Submittals – Part Load Efficiencies
Sensor Applicability and Commissionability
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• Opportunities to Improve the State of Practice • Project specific analysis and engineering• Resources
• Energy Design Resources www.energydesignresources.com/resources• Chiller Bid and Performance Tool
CoolToolsTM – PG&E / Energy Design Resources• HVAC Simulation Guidebook, Volume I Part 2: “Energy Efficient
Chillers”
• An Improved Cooling Tower Algorithm for the CoolToolsTM Simulation Model (Benton et al. ASHRAE TRANSACTIONS 2002, V. 108, Pt. 1.) Methodology utilized within EnergyPlus Whole Building Simulation Tool
• Central Chilled Water Plants ASHRAE Journal Article Series Steven T. Taylor 2011-2012
Conclusion
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