A Chilled Water System Analysis Tool for Industrial Assessments

Chiller System Optimization & Energy Efficiency Workshop

September 2003

Presented by Michael Socks

UMass Industrial Assessment Center

The Industrial Assessment Center at UMass-Amherst

• The IAC performs no-cost, on-site energy efficiency, waste reduction, and productivity improvement assessments for small and mid-size manufacturers

• Client Characteristics:1) SIC Code 2000-39992) Annual energy bills of $100,000 to $2,000,0003) Gross annual sales less than $100 million 4) Fewer than 500 employees at the plant site5) No in-house staff to complete a similar assessment

Summary of Operating Cost Reduction Measures

Equipment-based Opportunities• Replace the chiller• Install NG or absorption chillers (Hybrid)• Install HX to recover condenser heat• Store thermal energy for peak use

Summary of Operating Cost Reduction Measures

Control-based Opportunities• Optimize chiller sequence• Raise chilled water temperature setting• Lower condenser cooling water temperature• Use variable speed capacity control• Use 2-speed or VSD control of tower fans• Use VSD control of pump flow• Use free cooling

Summary of Operating Cost Reduction Measures

Load-based Opportunities• Use chilled water efficiently• Distribute chilled water efficiently• Use optimal coil or heat exchanger size/design

Purpose: Reduce the energy consumption of installed chilled water systems

Goal: Create a simple but useful software tool for analyzing potential energy savings in chilled water systems

Program Introduction

Chilled Water System (Water-Cooled)

Condens er

1

Evaporator

Com pres s or

32

4

Expans ionValve

Cooling T ow er

Cooling Load

CHILLER LO O P

HEAT REJ ECT IO N LO O P

HEAT G AIN LO O P

Cooling T ow erP um p

S upplyP um p

Cooling T ow erF an

Chilled Water System (Air-Cooled)

Air-CooledCondens er

1

Evaporator

Com pres s or

32

4

Expans ionValve

Cooling Load

CHILLER LO O P

HEAT G AIN LO O PS upplyP um p

F an

Program Description

Visual Basic Executable Program• User is prompted for system information • Program analyzes the existing system• User is prompted for changes to system• Program analyzes the proposed system• Program presents savings results

Program Overview: Input

Basic System Data:

• Number of chillers (up to 5)• Chilled water supply temperature• Geographic location• Condenser cooling method (water or air)

Program Overview: Input

If chiller condensers are water-cooled:

• Condenser cooling water supply temperature (if constant)

• WB to cooling water temperature differential (if variable)

• Cooling Tower Data (# towers, # cells/tower, motor hp, # motor speeds)

Program Overview: Input

If chiller condensers are air-cooled:

• Cooling air design temperature• Average annual ambient air temperature

(if indoor air is used for cooling)• DB to condenser temperature differential

(if outdoor air is used for cooling)

Program Overview: Input

Pump Data:

• Fixed or variable flow pumping• Flow rate [gpm/ton]• Nominal pump efficiency [%]• Nominal motor efficiency [%]

Program Overview: Input

Chiller Data:

• Chiller compressor type • Chiller capacity • Chiller full load efficiency (if known)• Chiller age

Program Overview: Input

Energy Cost Data:• Average electricity cost [$/kWh]• Average NG cost [$/MMBtu]

System Control Data:• System operating schedule• System loading schedule

Program Overview: Cost ReductionCost Reduction Measures to Consider:

• Increase chilled water supply temperature• Decrease chiller condenser temperature• Upgrade to 2-speed or variable speed tower fan motors • Upgrade to variable speed pump motor control• Replace chillers (use more efficient or NG units)• Replace refrigerant• Install VSD on chiller compressor motor (centrifugal only)• Use free cooling• Sequence chillers

Program Overview: Output

Output Information:

• Annual chiller energy consumption (kWh and/or MMBtu) and cost

• Annual cooling tower energy consumption (kWh) and cost

• Annual pump energy consumption (kWh) and cost

• Total annual energy consumption and cost

Program Overview: Output

Chiller energy may be viewed by:

• Chiller

• Load

Cooling tower energy may be viewed by:

• Wet-bulb temperature group

Pump energy may be viewed by:

• Chiller

ExampleLet’s run an example . . .

• (3) 200 ton water-cooled chillers (centrifugal)• 44 ºF chilled water temperature• Located in Boston, Massachusetts• Condenser cooling water is held constant at 85 ºF• (1) 2-cell tower with 15 hp 2-speed motors• Chilled water flow is constant [2.4 gpm/ton]• Condenser water flow is constant [3.0 gpm/ton]• Electricity is $0.06 per kWh• Operates 24/7 and serves an air-conditioning load• Install VSDs on each chiller compressor motor

Example: Input Screen

Example: Output Screen

Output Summary:

• Chillers: 2,187,676 kWh (92%)

• Tower: 4,768 kWh (< 1%)

• Pumps: 193,934 kWh (8%)

• Total Energy: 2,386,378 kWh Total Cost: $143,183

Example: Operating Cost Reduction Opportunities Screen

Operating Cost Reduction Measure:

• Install a VSD on each Centrifugal Compressor Motor

Example: Savings Screen

Savings Summary:

• 598,797 kWh/yr

• $35,928/yr

Case Study: Application

Manufacturer of laminated circuit boards uses chilled water for process cooling and space conditioning

Process Cooling• Laminating oven cool-down cycle• Plating fluid temperature controlSpace Conditioning• ~ 50,000 ft2 clean rooms• Office and manufacturing floor air conditioning

Case Study: System Specs

Chilled Water System Summary• (2) 250-ton helical rotary chillers (1997)• (2) 350-ton helical rotary chillers (2001)• 45ºF chilled water; 2.4 gpm/ton• 75ºF condenser water; 3.0 gpm/ton• (4) cooling towers; (3) 15-hp fans each (2-speed)• Operates 24/7 year-round• Free cooling is used when possible

Case Study: System Loading

Typical Loading Schedule

• 20% load for 25% of year• 30% load for 25% of year• 40% load for 25% of year• 50% load for 25% of year

Note: These are average system loads.

Individual chiller loading will differ.

Case Study: Results

Case Study: Prediction vs. Actual

Without Using Free Cooling • 3,478,905 kWh actual• 3,436,931 kWh predicted• Difference: 41,974 kWh (-1.2%)

With Free Cooling• 489,054 kWh and $41,570 actual savings• 608,720 kWh and $51,744 predicted savings• Difference: 119,666 kWh and $10,174 (+24%)

Case Study: Other Observations

• Chiller and pumping energy decrease by approximately 22%

• Tower energy increases by approximately 63%

Closing Comments

• The Program IS NOT intended to determine system energy use down to the kWh or MMBtu

• Program IS intended to direct analysis effort toward the most promising cost reduction opportunities

• I need your help to make this program better: 1) Download the program from www.ceere.org 2) E-mail questions, suggestions, errors, etc. to me at

msocks@ecs.umass.edu• Any questions?