Validation of Energy-Efficiency Measures for Operation of Building HVAC Systems

Grant #: 96-10
Principal Investigator: Theodore F. Smith, Professor
Organization: University of Iowa
Student Support: J. Wen
Technical Area: Energy Efficiency

Background and Significance:

The problem addressed by the project is the validation of energy-efficiency measures for the operation and control of building heating, ventilation, and air-conditioning (HVAC) systems. The overall theme of the project is the energy-efficient operation and control of building HVAC systems so that the operating costs are minimized without sacrificing occupant comfort and building functionality. Building HVAC systems consume nearly one-half of all energy used in Iowa, thereby making building energy consumption a significant opportunity to reduce energy use.

Practical considerations and thermodynamic laws limit the amount of insulation, the use of vapor barriers, the types of windows, the reduction of unwanted infiltration and exfiltration of air, the use of “free energy”, and the equipment efficiency. Even when these measures are accounted for, a need still exists to examine the operation and control of the building HVAC system.

The project is based on a system or whole-building approach that examines the HVAC system for energy-efficient operation while recognizing the required comfort conditions of the occupants. Current controllers that operate independently using locally measured variables make operation of the building from a system point-of-view difficult. When the effects of building mass, changing environmental conditions and internal loads, and operational characteristics of HVAC equipment are accounted for, operation of the entire building using a system controller is expected to lead to more efficient use of energy.

A system is commonly evaluated by introducing an efficiency factor that provides guidelines about the design and operation of the system. Building and HVAC designers as well as building operators and owners could use a similar factor to indicate how efficiently energy is being used in a building. For a given building type, along with specifications of the occupancy, environmental conditions, equipment characteristics, as well as energy and comfort costs, the building efficiency factor comes from an optimal control strategy. Because it may not be practical to operate a building using optimal control, other methods, such as adaptive optimal control algorithms, that provide near-optimal control responses and that can be used online in a commercial building energy management and control system must be developed. An adaptive optimal control algorithm that is currently being developed employs system identification, parameter estimation, and self-tuning techniques to identify the unknown system parameters and conditions and to accommodate changes in these parameters and conditions. The adaptive optimal operation strategy provides the near best control with limited knowledge about the system and environment.

Simulation studies on achieving energy efficiency in buildings using adaptive optimal control methods and modifications to current control schemes reveal that building HVAC energy costs can be reduced. These studies also reveal that variable positioning and flexible operation of control devices are responsible for energy-efficient operation. The findings of these studies need validation using experiments for real-world conditions.

The availability of the Iowa Energy Center’s Energy Resource Station (ERS) offers the opportunity to validate the control and operation strategies suggested by the simulation studies as well as to validate other strategies. The investigators provide assistance for performing tests at the ERS.

Project Objectives:

The objective of the project is to validate energy-efficiency measures for operation of building HVAC systems using the test facilities at the ERS. The tasks for the project are to (1) provide assistance for conducting tests at the ERS, (2) validate energy-efficiency measures at the ERS, (3) provide support personnel to assist the ERS, (4) provide deliverables, and (5) participate in the National Building Control Information Program (NBCIP).

Current activities:

The current activities are focused on developing the adaptive-optimal-operation method (AOOM). The AOOM employs online models for building zones and HVAC components and a genetic-algorithm optimizer to predict the system (including the building zones and HVAC components) dynamic behavior and to determine a set of optimal operation variables, including building zone temperature set points, minimum zone air flow rate, and air handling unit (AHU) supply air temperature set point. The online models used in the AOOM include unknown parameters that are estimated using the recursive-least-square with varying forgetting factor method. The AOOM online models have been successfully used to predict the system behavior, such as zone temperatures, zone entering air volumetric flow rate for certain damper positions, and energy consumed by the system for certain operational conditions. The AOOM sends set points to the building energy management and control system and adjusts those set points based on minimizing the energy consumption by the HVAC system while at the same time recognizing the importance of maintaining thermal comfort conditions within the conditioned zones.

The AOOM code is currently undergoing testing and refinements using the test facility at the ERS. Initially, the tests focused on providing heating temperature set points and set points for minimum air flow rates for the test rooms at the ERS. Currently, the A-Test System uses the AOOM whereas the B-Test System is under conventional operation. Some preliminary test results indicate that, by using the AOOM, the A-Test System needs less time to reach the comfort zone temperature during the morning start up period than that required by the B-Test System (see experimental data for the zone temperature are shown in Fig. 1). If this initial finding is substantiated, it may be possible by using the AOOM to begin the morning start-up at a later time. A comparison between the energy consumption and thermal comfort conditions for both systems is planned. Future work involves extending the AOOM tests to provide set points for operation of the AHU.

Project accomplishments:

Some of accomplishments for the project over the past two years are described in the following articles and internal technical reports, copies of which have been furnished to the ERS.