Simulated Performance of Natural and Hybrid Ventilation Systems in an Office Building

Past research on natural ventilation has revealed that the application of pure natural ventilation systems may be limited in the United States by issues such as climate suitability, humidity control, and reliability. However, hybrid (or mixed-mode) ventilation systems offer the possibility of attaining energy savings in a greater number of buildings and climates through the combination of natural ventilation systems with mechanical equipment. The objective of this study is to investigate the potential energy and indoor environmental performance of natural and hybrid ventilation alternatives in low- to mid-rise US commercial buildings in a variety of US climates. In this effort, the National Institute of Standards and Technology (NIST) reviewed hybrid ventilation approaches and conducted simulations to predict and compare the indoor environmental and energy performance of natural, hybrid, and mechanical systems in an otherwise similar building. Due to the strong interaction of airflow and heat transfer in naturally ventilated buildings, a coupled multi-zone airflow and thermal simulation tool was used to model the systems in a five-story office building in five US cities. Overall, the natural ventilation system performed adequately in San Francisco and Los Angeles, although some tolerance for imperfect thermal and indoor air quality (IAQ) control is required. Natural ventilation system performance was poor in the more challenging climates of Boston, Minneapolis, and Miami due to poor thermal control, unreliable ventilation, or high heating loads. The hybrid ventilation system improved on the performance of the natural ventilation system in all climates, with dramatic improvements in some. Compared to the mechanical system, the hybrid system saved significant amounts of fan energy, reduced cooling loads, or reduced both fan and cooling loads in all climates but often resulted in higher heating loads. Although the hybrid system provided acceptable thermal control, the mechanical system provided more consistent control, as expected. The hybrid ventilation system provided better IAQ control, as indicated by CO2 concentrations, in most but not all cases.