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Double-skin fašades and natural ventilation

Heat extraction double skin fašades | Night-time ventilation | Mixed-mode and natural ventilation

Mixed-mode and natural ventilation 

Conventional office buildings with airtight envelope systems are typically conditioned with mechanical heating, ventilating, and air-conditioning (HVAC) systems. Mechanical HVAC systems maintain fairly constant thermal conditions and can be applied in any geographical location. Since mechanical cooling and fan energy use account for approximately 20% of commercial building electrical consumption in the United States, the concept of integrating passive natural ventilation in conventional air-conditioned buildings has received attention from both the international and U.S. building industry. In addition, users are increasingly interested in measures that can improve indoor air quality via fresh air or free ventilation through windows, in part as a reaction to the problems that result from poorly maintained conventional HVAC systems (e.g., sick building syndrome, Legionnaire's disease, etc.). 

Mixed-mode ventilation refers to a space conditioning approach that combines natural (passive) ventilation with mechanical (active) ventilation and cooling. The system has been used in the United Kingdom over the past 20 years. Only recently has ASHRAE decided to incorporate a new adaptive model for thermal comfort for mixed-mode (or hybrid) ventilation in ASHRAE Standard 55 (Brager et al. 2000). Mixed-mode ventilation is appropriate for the design of new buildings and the retrofit of older, naturally ventilated buildings, where internal loads have increased due to increased occupancy or equipment loads. Commercial buildings in moderate climates with access to unpolluted outdoor air, such as the coastal California, Oregon, and Washington can take advantage of passive cooling strategies by integrating natural ventilation with conventional HVAC systems. 

There are various ways to classify mixed-mode ventilation systems. In the context of high-performance building fašades, mixed-mode ventilation can be classified based on how natural ventilation is provided and the mode of operation. There are three general modes of operation: 

  • Contingency: In this approach, the building is designed either as an air-conditioned building with provisions to convert to natural ventilation or vice versa. This approach is uncommon and is used only in situations where changes in building function are anticipated. 
  • Zoned: Different conditioning strategies are simultaneously used in different zones of the building. For example, an entire building may be naturally ventilated with supplemental mechanical cooling provided only in selected areas. 
  • Complementary: Air-conditioning and natural ventilation are provided in the same zone. This is the most common mixed-mode approach with various operational strategies: 1) alternating operation allows either the mechanical or the natural ventilation system to operate at one time, 2) changeover operation allows either or both systems to operate on a seasonal or daily basis depending on the outdoor air temperature, time of day, occupancy, user command, etc. -- the system adapts to the most effective ventilation solution for the current conditions, and 3) concurrent operation where both systems operate in the same space at the same time (e.g., mechanical ventilation that has operable windows). 

Single-sided, high opening: D = 2H With a single-sided, high level opening, ventilation is generally effective to room depths of up to 10 ft or less than two times the room height.

Single-sided, high and low openings: D = 2.5H With two openings located at the top and bottom level of the window, ventilation can be effective up to 30 ft or less than 2.5 times the room height. The higher window element can be left open for general ventilation while the occupant can maintain control over the lower window(s). 

Cross ventilation: D = 5H When the room has windows on opposite sides, cross ventilation is effective up to 40 ft of the room depth or five times the room height.

Natural ventilation can be introduced in a variety of ways: 1) with operable windows, ventilation can be driven by wind or thermal buoyancy (or stack effect) to ventilate a single side of a building or to cross ventilate the width of a building; 2) stack-induced ventilation uses a variety of exterior openings (windows in addition to ventilation boxes connected to underfloor ducts, structural fins, multi-storey chimneys, roof vents, etc.) to draw in fresh air at a low level and exhaust air at a high level and 3) atria enables one to realize a variant of stack ventilation, where the multi-storey volume created for circulation and social interaction can also be used to ventilate adjacent spaces. 

With single-sided ventilation using operable windows, there are general rules of thumb used to estimate the effective depth of ventilation. With clerestory windows, single-sided ventilation is generally effective up to a room depth of 10 feet, or less than two times the room height. For windows with separate upper and lower openings, ventilation can be effective up to a room depth of 30 feet, or less than 2.5 times the room height. The upper window element can be left open for general ventilation while the lower can be controlled by the occupant. With cross-ventilation, where a zone has windows on opposite sides, ventilation can be effective up to 40 ft of the room width or less than five times the room height. 

The type of window affects the degree of resistance to inflowing air and therefore ventilation potential. Sliders can provide an 100% unobstructed opening while a bottom-hung tipped casement may only provide a 25% unobstructed opening. Screens or mesh used to exclude birds and insects also reduce ventilation potential. Ventilation through a double-skin fašade, as previously discussed, can also occur. Windows may be operated manually or with mechanized arms, similar to those used on HVAC ventilation systems or fire control shutters. To promote user satisfaction, one should allow the automatic control system to be overridden by the occupant. 

For all-glass fašades, solar chimneys are essentially the glazed manifestation of a stack-induced ventilation strategy. A glass, multi-storey vertical chimney (shaft) is located on the south fašade of the building. Operable windows connect to this vertical chimney. Similar to the heat extraction concept described above for double-skin fašades, solar heat gains absorbed within the chimney causes hot air to rise, inducing cross ventilation from the cooler north side of the building. Mechanical ventilation can be used to supplement this ventilation if natural means are insufficient. 

Stack-induced ventilation through atria work using the same principle as a solar chimney but can serve more functions. Atria can be situated in the core of the building or form a single-, double-, or triple-sided, all-glass, multi-storey zone at the exterior of the building. The roof is typically glazed. Atria can be used to provide daylight to adjacent spaces and can act as a thermal buffer during the winter season. 

References 

Brager, G.S., E. Ring, and K. Powell. 2000. Mixed-mode ventilation: HVAC meets Mother Nature. Engineered Systems. May 2000. (online)

CIBSE. 1997. Natural ventilation in non-domestic buildings: CIBSE applications manual AM10: 1997. London: Chartered Institution of Building Services Engineers (CIBSE). 

Ring, E. 2000. Mixed-mode office building: A primer on design and operation of mixed-mode buildings and an analysis of occupant satisfaction in three California mixed-mode office buildings. Thesis (M.S. in Architecture) Berkeley, California: University of California, Berkeley.


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