FDD for Rooftop Air Conditioning

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Introduction to Load Shifting and Peak Load
Reduction using Building Thermal Mass
Jim Braun Purdue University
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Outline

• Building Thermal Mass Concept
• Strategy Development and Evaluation
• Previous Work
• Objectives of Current Work

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Control of Building Thermal Mass

• Precool at night during off-peak hours
• Adjust daytime setpoints to control discharge
• Cooled structure reduces daytime, on-peak cooling
  loads
• Savings due to
• reduced on-peak energy and demand usage
• improved equipment performance
• night ventilation

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Building Structural Storage Potential
Internal Gains 4 - 8 Watts/sq. ft.
Thermal Capacity 2 - 4 Watts-Hours/sq. ft. -
F
Concrete Floor
0.25 - 1 hours of storage per 1 degree F
temperature change
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Types of Strategies
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Load Effects
Cooling Loads
On-Peak Period
Unoccupied Period
Occupied Period
Time of Day
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Thermal Mass vs. Ice Storage

• No additional costs (Structure already exists!!)
• Charging constraints due to occupant comfort
• Variable storage efficiency due to coupling
  between building and environment

• Initial cost associated with ice tank(s), piping,
  support equipment, installation.
• No direct comfort constraints
• Constant storage efficiency with easily
  determined state of charge

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Strategy Development and Evaluation
Laboratory Testing
Simulation
Controlled Testing Validate Simulations
Demonstrate Savings
Evaluate Maximum Savings Potential
Develop and Evaluate Generic Control Strategies
Field Testing
A Tool to Develop Site-Specific Control Strategies
Implementation Issues
A Tool to Evaluate Field Savings
Evaluate Real-World Savings Potential
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Forward Simulation
use to evaluate savings potential develop
simple control strategies
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Inverse Simulation
use to develop site-specific control
strategies evaluate field savings
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Previous Studies

• Simulation Work
• Up to 30 HVAC energy demand cost savings for
  large commercial buildings (Braun (1990), Synder
  and Newell (1990), Rabl and Norford (1991),
  Andresen and Brandemuehl (1992))
• Cost savings very sensitive to control method,
  system parameters, utility rates, and weather
• Inverse modeling approach for developing and
  evaluating site-specific control strategies
  (Chaturvedi and Braun, 2002)
• Laboratory Testing
• Up to 50 load shifting peak reduction for a
  lightweight internal zone (Conniff (1991), Morris
  et. al (1994))
• Good agreement between measured loads and load
  predictions from TRNSYS building model (Morris
  et. al (1994))
• Load shifting and peak load reduction very
  sensitive to control strategy

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Previous Studies

• Field Testing Large Commercial Buildings
• Small load shifting and peak reduction reported
  by Ruud et al. (1990)
• 100 shedding from 2 pm to 630 pm reported by
  Sukkhbir et al. (1993)
• 25 peak cooling load reduction for side-by-side
  tests from 7 am to 6 pm reported by Keeney and
  Braun (1996)
• Up to 40 HVAC cost savings predicted for large
  commercial building by Braun et. al (2002)
• Field Testing Small Commercial Buildings
• 23 load shifting for small commercial building
  reported Braun et. al (2002)

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Simulated Load Shifting Cost Savings (2-to-1
time-of-day rates)
high sensitivity to building and plant
Heavy Zone Good Part-Load
Daily Cooling Cost Savings
Heavy Zone Flat Part-Load
Light Zone Flat Part-Load
Light Zone Bad Part-Load
Average Daily Temperature (F)
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NIST Laboratory Test Facility
Controlled to emulate internal zone within a
multi-story building
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Demand-Limiting Test Results
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Comfort Results Demand Limiting
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Chicago Field Site Description
1.4 million sq. ft., four 900-ton chillers, west
of Chicago 0.052/kw-hr on-peak (9 am 10
pm) 0.023/kw-hr off-peak, 16.41 per peak kW
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Chicago Field Site
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Demand-Limiting Case Study
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HVAC Energy Cost Case Study

• Created an inverse model from measured data
• Used model to develop evaluate control
  strategies

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Model Validation(HVAC utility costs, July 11 -
August 8, 1997, field site data)
5 difference in utility costs
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3-Month Cost Savings for HVAC
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Objectives of Current Work

• Demonstrate peak load reduction potential in a
  medium size commercial building
• Further develop and validate inverse modeling
  tools
• a tool for developing site-specific strategies
  and evaluating field site savings
• Evaluate peak load reduction potential for a
  small commercial building