Modeling on single-sided wind-driven natural ventilation
Buildings use 40% of the total primary energy in the United States, with a significant part of this energy being used for ventilation and cooling. Despite the large amount of energy used in buildings, reports have shown that the indoor air quality (IAQ) and thermal comfort are not satisfactory. The lowered productivity due to the bad IAQ could cause $125 billion loss per year and the sick building syndromes could cause $32 billion direct healthcare costs. Most of the problems related to IAQ are caused by insufficient fresh outdoor air supply or lack of maintenance with traditional mechanical ventilation systems. Natural ventilation is an alternative method to mechanical ventilation to reduce building energy use and improve indoor air quality. Natural ventilation can usually be classified into cross ventilation and single-sided ventilation. Cross ventilation is often favored for its larger air exchange rate than single-sided ventilation. However, in most cases, few buildings can achieve cross-ventilation due to the interior partitions, obstacles, and thicknesses. Therefore, single-sided ventilation is still of great importance in building design. However, the modeling of single-sided ventilation rate is difficult due to the bi-directional flow at the opening and the complex flow around buildings. The first part of this study is to develop a simple empirical model for buildings with simple openings. The model is able to accurately predict the mean ventilation rate and fluctuating ventilation rate caused by the pulsating flow and eddy penetration. This new model calculated the eddy penetration effect in the frequency domain based on Fast Fourier transform. We conducted Computational Fluid Dynamic (CFD) simulations with Large Eddy Simulation (LES) and used experimental data from other researchers to validate the new empirical model. The model predictions were generally within 25% error for simple opening. After we developed the model for simple openings, the second part of the research is to develop models for more complicated openings. In reality, only very few buildings use simple openings in their design, instead, the majority of the buildings use hopper, awning or casement windows. Therefore, based on the newly-developed model, we modified it to predict the ventilation rate for these windows types. In order to understand the flow characteristics around the complex openings, we used the CFD to generate database for various wind conditions. First, we validated the accuracy of the CFD LES model by conducting full-scale outdoor measurements and comparing against the CFD simulations. After validating the LES model, it was used to generate database to develop the semi-empirical models for hopper, awning and casement windows. Finally, the full-scale measured data was also compared with the proposed model predictions to validate the semi-empirical models. The comparison showed that the models were able to predict the ventilation rate generally within 30% error.^ After we developed the models for predicting single-sided, wind-driven ventilation rate, we evaluated the availability of natural ventilation in the future considering the impact of climate change. This research projected the future monthly weather based on HadCM3 Global Circulation Model (GCM) for 2020, 2050 and 2080 for three CO2 emission scenarios. To use the monthly weather data in energy simulation programs, we downscaled the monthly data to hourly data by Morphing method. Then we used the projected data to predict the future cooling and heating energy use in all seven climate zones in the U.S. for various commercial and residential buildings. We also coupled the newly-developed semi-empirical model with EnergyPlus to evaluate the natural ventilation potential in San Diego, San Francisco and Seattle, which are the representations of the typical climates where natural ventilation could be used. The results showed that the impact of climate change varied greatly depending on the geographic locations and building types. Also, the simulations showed that natural ventilation would still be acceptable by 2080 for San Francisco and Seattle even based on the worst case emission scenario, however, for San Diego or regions with warmer summer, natural ventilation could only be used for very limited time each year. Based on our study, the last step of this research is to seek potential approaches to utilize natural ventilation in hotter climates. One major limitation of natural ventilation is that it can only be used when outdoor is cool and may underperform during days with high outdoor temperatures. Mixed-mode cooling that combines natural ventilation and mechanical ventilation has the advantage of natural ventilation and mechanical cooling. To maximize the savings of mix-mode cooling, natural ventilation should be used as much as possible. In order to use natural ventilation mode during temporary hot weather, adequate amount of thermal mass with night cooling strategy would be one potential approach. However, the amount of thermal mass needed to be investigated to achieve cost-effective design. We conducted energy simulations with EnergyPlus to evaluate the impact of thermal mass on mixed-mode cooling energy savings. The results showed that electricity use can be reduced by 6-91% with mixed-mode ventilation compared to traditional mechanical cooling in different climates.^
Qingyan Chen, Purdue University.
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