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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

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)

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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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Janelle H. Griffin, PE, ACP, LEED AP BD+CProject Manager

jgriffin@dewberry.comwww.dewberry.com