Improvements in Demand-Controlled Ventilation to Reduce Energy Use and Improve Indoor Air Quality

Abstract

Buildings have become more airtight and insulated over the last years––a response to the acute need to reduce building energy demands in cold climates. However, to secure satisfactory indoor air quality (IAQ) this has led to increased attention to the need for ventilation solutions that ensure satisfactory renewal of the air. Mechanical ventilation is customarily selected to ensure ventilation rates that satisfy building-code requirements. In non-residential buildings, together with heat recovery, demand-controlled ventilation (DCV) is used to reduce energy needs. With DCV, the ventilation rates are maximized at the room/zone level during periods of full occupancy -usually based on concentration of carbon dioxide (CO2) and/or temperature, and reduced to minimum levels when the room/zone is vacant. CO2 is a proven indicator of bioeffluents, but it is doubtful that it is an appropriate indicator for pollutants that are not directly connected to room occupancy.

Therefore, the main aim of this PhD study was to explore and develop a holistic methodology for improving ventilation control in order to reduce energy use and to improve the IAQ.

The work can be summarized as a step-by-step study aimed at addressing the following questions:

Why is DCV, as performed today, not good enough? Literature reviews and an analysis of correlations with pollutants have revealed that CO2 and temperature are crucial factors in accounting for room occupancy, but they are not satisfactory for addressing the pollutants that occupants do not directly produce. Therefore, additional pollutants need to be measured, and some should be used to control ventilation. What should be measured to improve DCV? Measurements were collected from three offices, four schools, and 21 home offices in order to verify the literature findings and corroborate their application in Norwegian cases. How can these extra parameters be measured affordably? With the development of low-cost sensors (LCSs), the possibility to affordably measure several extra parameters has opened up. However, LCSs suffer from accuracy, precision, and bias problems. Calibrating these sensors had to be performed in a thorough manner. A calibration methodology was developed to handle the data collected, with the calibration experiments designed to allow for autocorrelation of the data. Which parameters should be used to improve the controls logics? Which are the most significant indicators? CO2, temperature, relative humidity, PM2.5, total volatile organic compounds, and formaldehyde were measured. In ventilation control, it is important to use all the significant parameters, but every extra parameter adds a layer of complexity. Two methodologies were used to select the significant parameters: Cross-correlation functions (CCFs) with de-trended time series and indoor/outdoor ratios were used to evaluate the significant parameters. To account for the influence of the building’s characteristics on the pollutant concentrations, the generalized estimation equation method was used. When are the control strategies better? How can these be evaluated? Simulation and measurements were used to evaluate the effects of the different control logics. Co-simulation between CONTAM and EnergyPlus softwares was selected because these simulation programs simultaneously enabled energy use and IAQ analysis. The models developed were validated against measurements. Improved controls on supply airflow rates and the share of recirculated returned air were evaluated. The simulation results were analyzed by looking at annual energy use and key performance indicators. They were evaluated using the share of the simulated time during which a parameter was below or within a defined range.

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In conclusion, the findings of this study provide a workable solution for ventilation control logic improvements that consider the conflicting demands of improving IAQ while reducing energy use. The findings can contribute to the development of more advanced solutions for regulating ventilation that ensure healthier and more productivity-promoting indoor environments and, at the same time, buildings that use less energy.