Utilizing Diffusion Theory to Model Carbon Dioxide Sources for Improved Energy Efficiency and Ventilation Control

Controlling air exchange rates in response to occupancy has the potential to significantly improve building energy efficiency. A current challenge associated with demand control ventilation research is the development of methods to correlate carbon dioxide concentration to occupancy that are accurate and have low computational overheads for more effective ventilation control. Due to false triggering and drift in readings, ventilation systems are often set to go to full air flow when a minimal carbon dioxide concentration is indicated, which results in excess ventilation and lost opportunity for energy savings. This article describes a new method of relating carbon dioxide source density to carbon dioxide concentration at set locations in a room operating in a reduced ventilation energy-saving mode, by utilizing diffusion theory. Recommendations for carbon dioxide monitor placement are made, along with suggested techniques to correlate occupancy patterns with the developed theory.Dual beam infrared carbon dioxide detectors are used to determine concentration levels. This research can lead to improvements in demand control ventilation systems. A diffusion-based approach to determining the distribution of carbon dioxide in a room offers a clear physical picture, a low computation threshold, and an approximate closed analytic solution, which may be used independently or with conventional computation methods to predict indoor occupancy patterns. The theoretical basis of this research involves solving Fick's second law of diffusion in spherical coordinates for a source of carbon dioxide that flows with a constant rate and is located in the center of an impermeable spherical boundary. A spherical diffusion test chamber is used to validate and benchmark the developed theory. The benchmarking includes the use of dispersion coefficients for cases with large flow rates when momentum flux caused by the diffusion of carbon dioxide induces convective currents. The theoretical model is adapted to model a room operating with restricted ventilation in the presence of a known, constant source of carbon dioxide. The room is modeled as a sphere of volume equal to the room, with a dispersion coefficient that is consistent with published values for rooms operating without ventilation. The carbon dioxide flow rates being considered in the test room produce an amount of carbon dioxide comparable to that produced by a person during quiet breathing, also known as the tidal volume. Thus, the developed diffusion model provides a new method to indicate occupancy patterns and enhance the performance of demand control ventilation systems. Results from the developed diffusion model and tests conducted in the test chamber and in two test rooms indicate that it is desirable to place carbon dioxide monitors in the center of walls at a height slightly above the heads of occupants and away from any vents or doors. Methods to log carbon dioxide concentration levels from meters placed according to the previous recommendations are tested. The use of a wireless communications system facilitates collection of the data. By comparing historic carbon dioxide concentrations at individual sensors over similar time periods, it is possible to determine if a meter is malfunctioning or incorrectly calibrated.