The word “passive” suggests inaction, but the use of passive cooling and ventilation is among the most active ways architects and design teams can make buildings more energy efficient.
“We’re seeing a lot more interest in passive cooling due to energy costs and the drive to be more sustainable,” says Eric Kirkland, director of engineering with SmithGroup’s Phoenix office. “It really speaks to using two natural forces to induce cooling and introduce outdoor air: prevailing wind pressures and the stack effect or warm air rising in taller higher buildings.”
Passive designs need less work from fans and air handlers to push air around and maintain thermal conditions and air quality within spaces to keep building occupants comfortable. According to the Lawrence Berkeley National Laboratory (LBNL) Building Technologies Program, power for mechanical cooling and fans account for more than 20 percent of commercial building electrical consumption in the United States, notes a paper on double-skin façades and passive ventilation.
This can work for existing buildings, says Steve Thomas, an executive with Milwaukee-based Johnson Controls. When outdoor temperature and humidity conditions are favorable, some buildings can virtually shut down mechanical air-handling systems and associated chillers and boilers.
“There are buildings in Europe that operate very effectively without traditional mechanical ventilation,” Thomas adds. “Mixed-mode ventilation combines traditional air-conditioning with passive ventilation to assure a proper level of cooling while still offering substantial energy savings.”
Physics of Air Movement
To design new or retrofit facilities to incorporate passive ventilation, architects and project teams first need to consider the basic physics of air movement:
- First, ambient conditions cause air to heat or cool, and the resulting changes in air pressure induce movement of significant masses of air.
- Second, heated air rises – ideally through HVAC ductwork, atriums, and plenum spaces.
- Third, the hot air is replaced by cooler supply air, including outdoor air.
As an example, Kirkland describes the use of a high atrium with a relief point at the top, which may include a solar-power-driven fan to induce additional draft. Hot air collects or stratifies at the top of the atrium, creating a draft lower in the atrium that draws wind and air through other lower building spaces.
This is simple thermal buoyancy at work, Kirkland explains. “The trick is adjusting the depth of the occupied spaces in relation to the atrium volume and limiting the number of wall partitions to maximize airflow,” he says. Another important design consideration is careful location of places for outside air to enter so that all areas of the building get enough air exchanges. Among the tools used by project teams is computational fluid dynamics (CFD) modeling, which allows a visualization of how air will move under typical conditions.
Outside the building, the shape of the exterior – and even landscape features – should help direct airflow to the inlets, or source points. Hedges and trees can capture breezes, as can berms, building wings, and walls. Some project teams use a wind tunnel to evaluate how their architectural form interacts with prevailing winds.
Design Challenges for Passive Systems
While passive cooling and ventilation designs are uncommon for large commercial buildings, hybrid mixed-mode approaches are effective, according to Dr. Gail Brager, a professor at the Center for the Built Environment, Berkeley, CA. “Mixed-mode cooling strategies can take many forms, but generally will involve an intelligent control strategy and a building envelope that becomes a critical part of the system,” Brager wrote in a recent paper.
That control strategy is likely to be more complex than it would be for traditional control systems. “They rely on more sensors and actuators than traditional systems, especially if operable windows are part of the design,” says Thomas. “The ventilation system design should be tested in a scale mock-up prior to committing to a design, if possible.”
Among the challenges facing design teams are the local climate, as well as solar gain from large glazed wall areas. “All standard practices of how you orient the building are critical,” says SmithGroup’s Kirkland. “Mitigate cooling loads by orienting the building to minimize western exposure, and include a good deal of insulation and a light-colored or reflective roof.” Other tips include reducing hardscape area and raising the occupied floors above the ground plane to capture cooler air and more breezes.
In arid, warmer climates, some passive-ventilation designs will benefit from the use of evaporative cooling, in which a wetted evaporative pad provides a trickle of water through which cross ventilation passes, reducing the dry-bulb air temperature through evaporation. Similar systems are used in greenhouses.
Integrating HVAC and Natural Ventilation
According to the LBNL, commercial buildings in moderate climates with access to unpolluted outdoor air – such as Coastal California, Oregon, and Washington – can most easily take advantage of hybrid passive-cooling strategies. The LBNL describes three basic types of mixed-mode operation (see the illustrations above):
- Contingency: A building system designed as either natural ventilation or conventional HVAC with a provision to convert to the other.
- Zoned: Different conditioning strategies are used simultaneously in different building zones.
- Complementary: Both conventional HVAC and passive ventilation are employed in the same building zones.
Complementary operation is the most common system approach used today, allowing alternating and concurrent use of mechanical and natural ventilation, depending on need. Buildings with operable windows controlled by occupants, which Brager contends are a highly attractive building feature, rely on complementary ventilation systems.
Yet few commercial buildings are designed with operable windows, and the idea of getting occupants to coordinate their use is an obstacle in practice. And, in spite of the benefits of these systems, there are other significant barriers to passive ventilation and cooling. One is the lack of proven projects in North America, says Thomas, although he points to such widely publicized projects as the Chesapeake Bay Foundation headquarters, Gap Inc.’s headquarters, and the Natural Resources Defense Council’s offices. “Other barriers include the fact that we are used to having very consistent temperatures. Thermal ventilation involves the likelihood of temperature fluctuation and windows that open and close automatically,” says Thomas.
On the other hand, says Kirkland, “You have to get people to accept a larger thermal comfort range, too, if we’re going to beat this energy challenge.”
Two other barriers, adds Brager, are energy codes and fire-safety codes. For the latter, the potential for smoke migration in wind-driven or stack-driven ventilation may be seen as too risky for some local authorities. As for the former, today’s energy codes may paradoxically limit the use of low-energy ventilation: California’s Title 24 and even green-building ratings may purposefully or inadvertently limit project designers to working with conventional HVAC systems.
C.C. Sullivan (chris@ccsullivan.com) is a communications consultant and author who specializes in architecture, design, and construction technology.