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Go With the Flow

Interest in mixed-mode buildings, which combine passive ventilation and natural cooling strategies with mechanical ones, heats up some U.S. markets.

By Nancy B. Solomon, AIA

For virtually all of human existence, people have relied on the natural flows of their local environments-the meandering paths of rivers, the orientation of the town to the sun, the prevailing breezes-to shape their patterns of settlement. Similarly, methods of building construction were strongly influenced by the basic principles of fluid dynamics: warm air rises, and cool air falls. That paradigm changed dramatically for the developed world in the 20th century, when technological advances and seemingly unlimited access to cheap energy allowed architects and engineers to design structures that were largely out of sync with their environs. Although hailed as an example of ingenuity and progress, in hindsight this disconnected approach now seems shortsighted. In recent years, concerns about high energy prices, the link between fossil-fuel consumption and global warming, and the health and productivity of occupants with little control over their environment or access to the outdoors has spurred interest in non-mechanical approaches, including natural, or passive, ventilation.
Kirsch Center
Photo © Arup/Cody Andresen
Kirsch Center
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ASHRAE Standard 55 was first released in 1966 to provide guidelines for buildings with centrally controlled HVAC. Unfortunately, the original, lab-based standard was applied too broadly, forcing most designers to stay within this narrow comfort zone.
In 2004, ASHRAE revised Standard 55 to include a broader comfort zone for buildings that take advantage of natural ventilation. This is making it easier for today’s designers to incorporate alternative ventilation and cooling techniques.
Source: U.C. Berkley Center for the Built Environment

While European designers have been on the forefront of the sustainability movement, American architects and engineers are now learning to apply the techniques strategically on this side of the Atlantic. “It’s been evolutionary,” says Dave Deppen, AIA, of Paul Roberts & Partners in Vallejo, California, who worked for six years at Van der Ryn Architects in Sausalito, Calif., serving as the design architect for the naturally ventilated 22,000-square-foot Kirsch Center for Environmental Studies at De Anza College in Cupertino, California. Deppen says, “Over the past few years more clients have expressed interest in energy conservation and user participation,” two characteristics of natural ventilation techniques.

Tapping into Natural Flows

Fresh air is passively brought into a building through pressure differences generated by two physical phenomena: buoyancy and wind. Buoyancy refers to the fact that a column of air differing in temperature from the air around it will either rise or fall until it reaches equilibrium. The stack, or chimney effect, in which heated air will rise vertically, pulling cooler air from outlets below, is a common example of buoyancy. Perhaps less well known in much of the United States is the reverse movement generated by a cool tower: In a hot, dry environment, air moistened at the top of a shaft will be cooled by evaporation and fall, bringing welcome coolness to the area at the base.

Designers can also take advantage of prevailing wind patterns in any number of ways. Probably the simplest approach is through cross ventilation, in which windows are placed on both the windward and leeward sides so that breezes can flow unimpeded across an occupied space. Wing walls can be installed perpendicularly to windows that are at an angle to prevailing winds in order to redirect airflow into the interior. By placing openings on the leeward side of a building, designers can capitalize on the vacuum that naturally occurs when wind blows over and around a structure to draw out stale air from inside.

Fresh air brought in from the outside can serve one or more functions, from providing adequate ventilation for occupants throughout the year and cooling them in the appropriate seasons to flushing and cooling the building itself at night. Executed properly in the right conditions, passive ventilation can improve indoor air quality and thermal comfort while reducing a facility’s reliance on energy derived from fossil fuels. It can also reduce initial construction costs by downsizing the mechanical system and its associated fan noises. Finally, natural ventilation helps occupants reconnect to the outdoors and feel a sense of ownership within their building environment.

New Research, More Flexible Standards

Indicative of the growing awareness of the subject in this country, the American Society of Heating, Refrigerating and Air-Conditioning Engineers revised in 2004 its Standard 55, Thermal Environmental Conditions for Human Occupancy to include an “optional method for determining acceptable thermal conditions in naturally conditioned spaces.” The new provision reflects current research that occupants in passively vented and cooled spaces who are relatively sedentary, free to adjust their clothing, and able to operate the windows are comfortable with a wider range of indoor temperatures than occupants in mechanically cooled spaces.

Despite this new understanding, conditioning a building entirely with natural ventilation is not always feasible. First of all, not all climates lend themselves to natural ventilation alone. In addition, some programming elements require highly controlled environments that can only be achieved through mechanical means. Site-specific security, noise, and pollution concerns are three other potential reasons why at least some parts of a building may not be able to rely completely on passive strategies. Further, many building owners simply feel more comfortable with a backup mechanical system, just in case. To maximize the energy and environmental benefits of passive ventilation without relinquishing attributes that only mechanical cooling can guarantee, an increasing number of U.S. clients have been working with architects and mechanical engineers to create mixed-mode, or hybrid, buildings. In these cases, passive and mechanical strategies are used for ventilation, cooling, or both.

The Center for the Built Environment at the University of California, Berkeley, which has been compiling a worldwide database of mixed-mode buildings (www.cbe.berkeley.edu/mixedmode), has also conducted to date, surveys of more than 41,000 occupants in more than 300 buildings representing mechanically cooled, passively ventilated, and mixed-mode strategies to gauge how people rate the performance of their buildings. According to Gail S. Brager, the center’s associate director, the survey results suggest that, on average, occupants of mixed-mode buildings were more satisfied in terms of thermal comfort and air quality than their counterparts in conventional buildings and even many LEED-rated green buildings that may or may not have natural ventilation options.

Mixed Modes

Designing mixed-mode buildings, like all high-performance structures, requires an integrated multidisciplinary team from the start.  First, the team must understand the regional weather patterns and microclimate of the site, including typical atmospheric conditions, solar orientation, and prevailing winds throughout the year. Next, the designers must consider program functions and the associated comfort criteria required by each. From this, the building program should be organized by the various ventilation and cooling needs. Through site orientation, massing, and detailing, the structure must be designed to minimize unwanted heat gain while maximizing passive ventilation opportunities. The most efficient mechanical equipment should then be specified to handle any cooling needs that exceed the capacity of these passive strategies. Finally, the designers must consider the transparency and ease of use of all operating mechanisms and controls, from the opening of windows and vents to the programming of electronic sensors that govern the interplay between passive and mechanical systems. “Education of the occupants about the performance of the building and operable windows is also critical to ensure that the building achieves its potential for optimizing both energy conservation and thermal comfort,” says Brager.

Professor G.Z. Brown, director of the Energy Studies in Buildings Laboratory at the University of Oregon, stresses the importance of designing for natural ventilation first—getting the right-sized holes in the right places and getting the air to flow through the whole building—before introducing mechanical cooling. “The mechanical system is what consumes energy,” Brown points out, “while the cooling load determines the size of the mechanical cooling system.” So it follows that passive strategies—from orientation and shading devices to natural ventilation—should be the first steps taken to get the greatest reduction in energy costs. When designing for mixed mode, it is also very important to reevaluate any preconceived assumptions about occupancy behavior. Brown has found, for example, that a large portion of the staff in some companies would be very happy to come to (and leave from) work two hours earlier in the hottest months, thereby reducing the peak cooling load in the afternoon and, subsequently, the size requirements for the mechanical system.

There is no one solution for a mixed-mode project. The term has been applied to a whole spectrum of configurations: At one extreme are buildings that have no full-fledged mechanical system at all but merely rely on electrically powered fans to augment the circulation of air that enters the building through operable vents and windows. At the other extreme are projects that have fully ducted mechanical ventilation and cooling systems that can switch on when the passive techniques are not operational for whatever reasons. Brown suggests that a building should be called mixed-mode if natural ventilation serves as the primary method of cooling.

Although the field is so new that there is no formal, agreed-upon nomenclature, the terms zoned, changeover, and concurrent seem to appear frequently in the literature to describe three common configurations. In a zoned project, passive and mechanical strategies occur at the same time but in different parts of the building. According to Peter Alspach, an associate in Arup’s Seattle office, this configuration is climate-restrictive because it assumes that natural ventilation will be able to fully handle portions of the building throughout the year. Such a configuration may be appropriate for buildings that have isolated functions that must maintain stricter standards or for buildings whose site conditions make it impossible to have operable windows on particular elevations.

In a changeover configuration, natural and mechanical conditioning occurs in the same spaces but at different times of the day or year. Here, the air-conditioning serves as a backup whenever natural ventilation cannot meet cooling demands. The advantage of this scenario, continues Alspach, is that it can be done in almost any climate. Of course, projects in some areas will realize greater operational savings than others, depending on the number of days that the building can be adequately serviced by natural ventilation.

“Concurrent” describes the scenario in which both natural and mechanical strategies occur at the same time in the same space. In this case, fresh air is provided throughout the year by operable vents in the building façade. In addition, larger quantities of fresh air can flow through operable windows during the cooling season to reduce the cooling load on the mechanical system. According to Alspach, concurrent mode is suited to a mild climate in which some natural ventilation is always desirable. It would not be appropriate for more extreme climates, where untreated fresh air flowing into the building during very hot or very cold months would work against mechanical conditioning, thereby increasing operational costs.

Who’s in Control?

According to Cole Roberts, a senior consultant at Arup’s San Francisco office and the building-services engineer for the Kirsch Center, hybrid systems can be managed in three basic ways: active, seasonal lockout, and informed occupancy. “Active control” describes an automated system in which a sensor determines a need for a change from one mode to another and, as a result, electronically closes windows and vents while activating the mechanical system (or vice versa). In a seasonal lockout, a facility manager would physically shut and lock windows at the beginning of certain seasons and unlock them at the end of these periods. In the case of informed occupancy, staff is made aware of the building’s various passive and mechanical strategies and typically given a visual cue, such as a green light, to indicate when the windows could or should be opened.           

Roberts explains that the level of control should reflect the culture of the building. “It’s about having good discussions with occupancy groups and their representatives. What will their role be? Do they want the system to manage itself, or do they want to feel a part of the process? And what is the cost implication? Motorized actuators on each window come at a price premium.” This conversation among designers, clients, and users should occur early in the design process, because a poorly managed system can actually waste energy. If someone leaves a manually controlled window open on a very hot and humid day in a changeover configuration, for example, the mechanical air-conditioning will have to work even harder than if the building had been fully sealed. The design should be tailored to the management policy.

At the Kirsch Center, which was completed in 2005, informed occupants and active involvement was a criterion from the start. In fact, the students were so enthusiastic about having a high-performance green building that, through their governing body, they contributed the initial $180,000 for the conceptual design phase and then participated in the design process. According to Julie Phillips, Morgan Family Chair in De Anza College’s Environmental Studies Department, “the students enjoy opening windows, turning on fans, and adjusting the vents at their feet as needed” and are so comfortable in the building that they “don’t want to go home.” 

Deppen sees the Kirsch Center and other mixed-mode projects as part of a transition back to a healthier relationship between occupants and the built environment. “In the past few decades, we have gotten so out of kilter. We didn’t fully lose control in our houses, but we certainly gave it up in our offices.” People are again becoming more aware of the buildings they inhabit and want to regain the ability to effect changes to their personal environments.  “Today, mixed mode is becoming the norm and operable windows, a given,” says Deppen.

This article appeared in the July 2007 print issue of GreenSource Magazine

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