Natural ventilation is a form of air movement through a space without the means of mechanical support. Natural wind driven ventilation makes use of large or multiple windows to allow air to move through the space. Natural buoyancy driven ventilation promotes hot air to rise through the space by design. Natural ventilation strategies are often best incorporated as a hybrid strategy, in which dependence on mechanical ventilation and air-conditioning is reduced by appropriate building design to take advantage of natural ventilation. Technical complexity of natural ventilation varies from the simple act of opening a window to sound absorbing trickle vents that automatically adjust by either mechanically or electronically driven valves in order to provide for a constant air supply, independent of outside wind speed (20). This analysis for the East Campus Project considers natural ventilation as a hybrid system that makes use of mechanical ventilation when the exterior climate is not suitable to provide for the required ventilation rates.
Preliminary factors to consider when assessing the technical feasibility of natural ventilation strategies include suitability of climate, outdoor air quality, suitability of site, and indoor occupant density. Climate wise, the Boston area is a good candidate for hybrid natural ventilation, given its temperate spring and fall climates. However, outdoor air quality may pose an issue on some summer days. Boston is a Clean Air Act non-attainment area with health advisories during the summer. Generally, days of poor air quality coincide with hot days during which natural ventilation may not meet the cooling and ventilation requirements of the building, as higher concentrations of air-borne pollutants are associated with stable, hot air bodies. Thus, for the East Campus Project, air quality should be monitored and the natural ventilation system controlled so as protect the building occupants from poor outdoor air quality. Finally, air flow studies and occupant density assessments should be conducted to complete the initial feasibility assessment of using natural ventilation for the East Campus Project.
Once the decision is made to incorporate natural ventilation into a building design, the architectural and engineering team will ideally consider a number of factors, including building orientation, floor plan, positioning and design of openings, shading, landscape, thermal mass, and building typology, to co-optimize energy consumption, indoor air quality, thermal comfort, building functionality, cost, and aesthetics. When these goals are taken into account in parallel at the birth of a conceptual design, the likelihood of successful co-optimization is far greater. Table 3 above provides descriptions of the design considerations relevant for natural ventilation.
Technical Assessment
In addition to understanding the design considerations for natural ventilation, it is also important to identify the technical risks associated with use of a new strategy. Raue et. al. identify the following risks associated with natural ventilation:
• Localized high temperatures
• All-around high temperatures
• Insufficient ventilation rates
• Drafts
• Variability in occupant control
• Sensation of dry air, and
• Noise transmission through ventilation grills.
Thermal environments found in naturally ventilated buildings are typically more variable and less predictable than those found in air-conditioned buildings, but not necessarily less comfortable.
Comparison of this list with typical complaints regarding comfort levels in air-conditioned spaces shows that these problems are not unique to natural ventilation. However, in many cases, including the Boston climate, natural ventilation is best considered as part of a hybrid strategy – one that combines mechanical HVAC systems with natural ventilation when conditions allow. As a hybrid strategy, naturally ventilated hybrid buildings provide inherent flexibility and redundancy in the space conditioning systems of a building, resulting in potentially longer life and greater adaptability to changing uses. Strategies to consider to obtain the best performance from a building that incorporates hybrid natural ventilation include the following:
• Employ mechanical ventilation before mechanical air-conditioning, using properly positioned fans, which consume one-tenth as much electrical energy as AC systems.
• Cool the building at night, using thermal mass such as concrete or granite.
• Dehumidify the air using desiccant dehumidification, as air often feels better at lower relative humidity, even at a higher temperature.
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