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Introduction to Load Shifting and Peak Load Reduction using Building Thermal Mass

Introduction to Load Shifting and Peak Load Reduction using Building Thermal Mass. Jim Braun Purdue University. Outline. Building Thermal Mass Concept Strategy Development and Evaluation Previous Work Objectives of Current Work. Control of Building Thermal Mass.

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Introduction to Load Shifting and Peak Load Reduction using Building Thermal Mass

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  1. Introduction to • Load Shifting and Peak Load Reduction using Building Thermal Mass Jim Braun Purdue University

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

  3. 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

  4. 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

  5. Night Setup Upper Comfort Limit 76 F Load Shifting Zone Temperature Demand Limiting 70 F Lower Comfort Limit On-Peak Period Unoccupied Period Occupied Period Time of Day Types of Strategies

  6. Night Setup Demand Limiting Load Shifting Load Effects Cooling Loads On-Peak Period Unoccupied Period Occupied Period Time of Day

  7. No additional costs (Structure already exists!!) Charging constraints due to occupant comfort Variable storage efficiency due to coupling between building and environment Thermal Mass vs. Ice Storage • Initial cost associated with ice tank(s), piping, support equipment, & installation. • No direct comfort constraints • Constant storage efficiency with easily determined “state of charge”

  8. 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

  9. System Physical Description Weather Data & Schedules Control Strategy Utility Rates Existing Modeling Tool Performance Estimates Forward Simulation * use to evaluate savings potential & develop simple control strategies *

  10. System Training Data System Model Structure Parameter Estimation Model Validation System Test Data Performance Estimates System Model Control Strategy Weather Data & Schedules Utility Rates Inverse Simulation * use to develop site-specific control strategies & evaluate field savings *

  11. 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

  12. 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 6:30 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)

  13. 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)

  14. NIST Laboratory Test Facility Controlled to emulate internal zone within a multi-story building

  15. Demand-Limiting Test Results

  16. Comfort Results – Demand Limiting

  17. 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

  18. Chicago Field Site

  19. Demand-Limiting Case Study

  20. HVAC Energy Cost Case Study • Created an inverse model from measured data • Used model to develop & evaluate control strategies

  21. Model Validation(HVAC utility costs, July 11- August 8, 1997, field site data) ~ 5% difference in utility costs

  22. 3-Month Cost Savings for HVAC

  23. 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

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