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.
2
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
3
Introduction: Chilled Water System Control
•
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
4
Optimal Chiller Plant Control
Strategies at Off-Design Conditions
•
•
•
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, …
Cooling
Tower
CWS
CWP
T
T
CWR
Condenser
VFD
Chiller
VFD
CHWP
CHWR
T
CHWS
Cooling
Load
5
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)
6
Chilled Water Plant Controls
Design to Completion
Design
Development
• Plant Design
• Theory of
Operation
Construction
Documents
• Controls
Design
• Sequences of
Operation
Construction
• Controls
Submittal
• PreFunctional
Process
• Functional
Test
Development
Acceptance
• Final Controls
Design
• Final
Sequences of
Operation
• Functional
Performance
Test
7
Chilled Water Plant Controls
Design to Completion
Design
Development
• Plant Design
• Theory of
Operation
Theory of
Operation
Construction
Documents
• Controls
Design
• Sequences of
Operation
Construction
• Controls
Submittal
• PreFunctional
Process
• Functional
Test
Development
Acceptance
• Final Controls
Design
• Final
Sequences of
Operation
• Functional
Performance
Test
As-Built
Control
8
Chilled Water Plant Controls
Current Practice
Construction
Documents
• Controls
Design
• Sequences of
Operation
“Optimize”
Resets
Construction
• Controls
Submittal
• PreFunctional
Process
• Functional
Test
Development
Proprietary
Creative
Partial
Unintended
Acceptance
• Final Controls
Design
• Final
Sequences of
Operation
• Functional
Performance
Test
FPT Criteria
Identify Misinterpretation
Operations
Persistence
Modification
9
Project Example: Cooling Tower Control
Design
CT-1
CT-2
CHWR
CHWS
CWS
P-6
Chiller 1
VFD
CWS
CWS
CWR
P-5
VFD
CHWR
CWS
CHWS
Chiller 2
CWR
• BAS provider shall provide
controls that calculate the
f(x ...x )
x
optimal tower setpoint at any
u
chiller(s) load and ambient wet
OAWB
x
bulb
• Bypass valve shall modulate to
Chiller Load (%)
maintain minimum CWST of
65°F
1
Optimal CWST
n
1
1
2
65°F Minimum
10
Project Example: Cooling Tower Control
Actual
Setpoint
(Tower Fan)
85°F
ECWT
• Condenser water supply setpoint
based on chiller load ONLY according
to AHRI conditions (Accepted)
• Bypass valve modulates to condenser
water supply setpoint (Cx Issue;
resolved)
95°F
75°F
65°F
Minimum
(Bypass Valve)
55°F
45°F
0%
25% 50% 75% 100%
Chiller Load
Causes
• 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
11
Barriers to Implementation of Optimal Controls
1.
2.
3.
4.
5.
Complexity
Uniqueness
Delegated Design
Expertise Mismatch
Persistence
Mechanical
Design
Commissioning
Cx, EBCx
Control
Implementation
Modeling and
Simulation
12
Barriers to Implementation of Optimal Controls
Connections Needed
• 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
x1
OAWB
Optimal CWST
f(x1...xn)
u1
x2
Chiller Load (%)
13
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)
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 Model
Approach= f (Range, OAWB, CW
Flow, Airflow)
Cooling Tower Modeling (Public)
•
Model based on Merkel’s Equations
(Merkel 1925)
•
CoolToolsTM Regression-Based
Model (Benton et al, 2002)
14
Project Example 2:
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
95
85
ECWT (°F)
•
•
•
•
•
75
65
55
45
0
10
20
30
40
50
60
70
80
Outside Air Wet Bulb (°F)
15
Chiller Modeling
Chiller Model
Chiller kW = f (load,
CHWST, CWT)
ChillerCapFT (%) = a + b*CHWT + c*CHWT2 + d*CWT +
e*CWT2 + f*CHWT*CWT+ f*CHWT*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
Cooling Tower Modeling
Cooling Tower Model
Approach=
f (Range, OAWB, CW
Flow %, Airflow %)
…
17
Example: 40% Chiller Load, 60°F Wet Bulb
Chiller Model
Tower Model
40% Load, 60F WB
5.0
50.0
4.0
Power (kW)
55.0
45.0
40.0
35.0
3.0
2.0
1.0
30.0
65
70
75
80
0.0
83
65
Condenser Water Temperature (deg F)
70
75
80
83
Condenser Water Temperature (deg F)
Combined
40% Load, 60F Models
WB
55.0
50.0
Power (kW)
Power (kW)
40% Load, 60F WB
45.0
40.0
35.0
30.0
65
70
75
80
83
Condenser Water Temperature (deg F)
18
Example: 40% Chiller Load 40F to 75F WB kW vs. CWT
40% Load, 45F WB
40% Load, 40F WB
40% Load, 50F WB
55.00
55.00
55.00
50.00
50.00
50.00
45.00
45.00
45.00
40.00
40.00
40.00
35.00
35.00
35.00
30.00
65
70
75
80
30.00
83
65
40% Load, 55F WB
70
75
80
30.00
83
65
40% Load, 60F WB
55.00
55.00
50.00
50.00
50.00
45.00
45.00
45.00
40.00
40.00
40.00
35.00
35.00
35.00
30.00
65
70
75
80
65
83
70
75
80
83
55.00
55.00
50.00
50.00
45.00
45.00
40.00
40.00
35.00
35.00
30.00
75
80
80
83
30.00
65
70
75
80
83
40% Load, 75F WB
40% Load, 70F WB
73
75
40% Load, 65F WB
55.00
30.00
70
83
30.00
77
78
80
83
19
Condenser Water Temperature Control: 40% Chiller Load
95
50
85
40
Design
75
65
kW Savings
ECWT (°F)
Design vs. Analysis-Driven
Analysis
Result
30
20
28%
55
10
45
0
0
10
20
30
40
50
60
70
80
Outside Air Wet Bulb (°F)
13.5 kW x 1314 run-hours (15%) @ $0.10/kWh
$1774 cost savings per year
20
Applications
• New Construction Building Commissioning
• Quantifying performance gap between optimal and specified
• Performance metrics
• Existing Building Commissioning
• Performance Metrics
• Control Development
• Engineering
21
Project Improvements Summary
Design
Development
Construction
Documents
Construction
Acceptance
• Documentation
of Design Intent
• Dynamic
Optimization or
Efficient Control
• Controls
Submittal
• Functional
Testing
Specification
Review
Project-specific
Control
Strategies
Equipment
Submittals – Part
Load Efficiencies
Verification of
Part-Load
Operation
Delegated
engineering
Carefully review
VE Options
Sensor
Applicability and
Commissionability
22
Conclusion
•
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
23
Janelle H. Griffin, PE, ACP, LEED AP BD+C
Project Manager
[email protected]
www.dewberry.com