System Identification and Optimal Control for Mixed-Mode Cooling
Student: Henry C. Spindler (Mechanical Engineering)
Advisor: Leslie K. Norford, Professor of Architecture
The majority of commercial buildings today are designed to be mechanically cooled. To make the task of air conditioning buildings simpler, and in some cases more energy efficient, windows are sealed shut, eliminating occupants’ direct access to fresh air. Implementation of an alternative cooling strategy–mixed-mode cooling–is demonstrated in this thesis to yield substantial savings in cooling energy consumption in many U.S. locations.
A mixed-mode cooling strategy is one that relies on several different means of delivering cooling to the occupied space. These different means, or modes, of cooling could include: different forms of natural ventilation through operable windows, ventilation assisted by low-power fans, and mechanical air conditioning.
Three significant contributions are presented in this thesis. A flexible system identification framework was developed that is well-suited to accommodate the unique features of mixed-mode buildings. Further, the effectiveness of this framework was demonstrated on an actual multi-zone, mixed-mode building, with model prediction accuracy shown to exceed that published for other naturally ventilated or mixed-mode buildings, none of which exhibited the complexity of this building. Finally, an efficient algorithm was constructed to optimize control strategies over extended planning horizons using a model-based approach. The algorithm minimizes energy consumption subject to the constraint that indoor temperatures satisfy comfort requirements.
The system identification framework was applied to another mixed-mode building, where it was found that the aspects integral to the modeling framework led to prediction improvements relative to a simple model. Lack of data regarding building apertures precluded the use of the model for control purposes.
An additional contribution was the development of a procedure for extracting building time constants from experimental data in such a way that they are constrained to be physically meaningful.
Ventilation Control Strategies
Principal investigator: Les Norford
Sponsors: MIT Physical Plant, Northeast Utilities and Empire State Electric Energy Research Corporation
Building space-conditioning systems often perform at poor part-load efficiencies because there is limited information feedback from individual offices and because part-load operation has led to large throttling losses. The increased use of microelectronics and power electronics in building control systems offers two benefits for ventilation systems: first, fans can be controlled not by adjusting dampers that throttle flow but by regulating the speed of the motor; and second, by communicating with digital rather than analog flow-regulation dampers in each occupied space, the central fan can be slowed to the speed that minimizes pressure drops across these dampers. A recently completed program tested and analyzed both of these benefits, with the goal of quantifying energy savings and providing to building owners, control manufacturers and electric utilities the information needed to make informed decisions about investing in new technologies. The performance of ventilation systems was monitored in several buildings and models were developed to correlate fan power with airflow and pressure.
Electric Metering and Diagnostics
Principal investigators: Les Norford, Steven Leeb, James Kirtley
Sponsors: Electric Power Research Institute, Empire State Electric Energy Research Corporation and Johnson Controls
Common electric meters are well developed electromechanical devices with little or no intelligence. The electric utility industry requires extensive load survey data to plan for future power generation needs and to prove the efficacy of utility-supported conservation programs. Customers would benefit from the same data, to assess energy usage and to detect and diagnose equipment faults. The Building Technology Program has joined the Laboratory for Electromagnetic and Electronic Systems at MIT to design and develop a meter that can separate loads from measurements made at a single point within a commercial building, to reduce or eliminate the need for expensive submetering of individual pieces of equipment.
Simulation of HVAC System Performance
Principal Investigators: Les Norford, Philip Haves (Loughborough University, U.K.)
Sponsor: American Society of Heating, Refrigerating and Air-Conditioning Engineers
Heating, ventilating, and air- conditioning (HVAC) systems are often poorly controlled. Engineers have not been able to rapidly prototype HVAC systems, in simulation, and assess the performance of existing or innovative control systems, including interactions between individual feedback control loops. MIT and Loughborough University, UK, have joined forces to develop a simulation test-bed for the development and analysis of control systems for a large class of HVAC systems.