Understanding Thermal Stratification in Naturally-Ventilated Buildings
Student: Maria Alejandra Menchaca Brandan – PhD Candidate (’12)
CoolVent , developed in the Buiding Technology Research Group over the last decade, is a simulation tool for early design of buildings that predicts the effects of natural ventilation on the building’s internal temperatures and airflow rates. The tool, the first of its kind, allows simulating time-varying thermal conditions for a typical day of a month (based on weather data), accounting for the effects of thermal mass, and night cooling. CoolVent calculations are based on a multi-zone model with coupled energy and flow equations, and rely on two basic assumptions: uniform temperature distribution in each zone (floor) of the building, and unidirectional air flow through its openings. It has been successfully validated for conditions that match these two assumptions. However, in situations where the thermal stratification of air in a room is too strong, the predictions of CoolVent need to be improved.
My doctoral work focuses on understanding air thermal stratification in a naturally-ventilated room. My goal is being able to predict the strength of air stratification in a room, and the effect that this density gradient has on the temperature of the room’s occupied zone and the air flow in and out of each zone.
Ventilation Shaft Modeling
Student: Stephen Ray, Ph.D. Candidate (’12) in Department of Mechanical Engineering
Advisor: Leon Glicksman
In the United States and most developed countries buildings consume roughly 40% of the nation’s primary energy, a number that is steadily growing. For all US buildings, space cooling and ventilation consume 16% of building energy use. However, in cooling-dominated climates, this percentage is significantly higher. One strategy for decreasing this energy consumption is to use natural ventilation (NV), a passive cooling and ventilating technique that utilizes natural forces like wind or buoyancy differences to bring outside air into the building.
A fundamental limitation of NV is the required climate. No building owner will naturally ventilate his building if outside air temperature or humidity levels are unacceptable for indoor comfort conditions. Thus, a rare few climates allow for a purely naturally ventilated building. This limitation in NV has lead to hybrid ventilation (HV) – a mix of NV and more traditional mechanical heating, ventilation, and air conditioning (HVAC) methods. While HV systems predictably require a larger capital investment than either a NV or mechanical HVAC system, the cost savings of an HV system over its lifetime could conceivably more than pay back the initial investment.
A major gap currently exists in our ability to predict the performance of an HV system – thus its energy and cost savings – when buoyancy-driven flow is present. Airflow network tools exist that predict airflow driven both by wind and buoyancy effects, however the assumptions used to model buoyancy-driven flow are often unrealistic. Such assumptions include a uniform temperature distribution in the ventilation duct, when actually the distribution is highly stratified especially near the duct entrance, or uni-directional flow in the duct, when bi-directional flow is likely due to the presence of large eddies in the flow.
The aim of this research is to deepen the understanding of NV to allow for better modeling in airflow network tools used in building energy modeling software to more confidently predict the energy and cost savings of a hybrid ventilation system.
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.