|
The double-skin façade is an European Union (EU) architectural phenomenon driven
by the aesthetic desire for an all-glass façade and the practical desire to have
natural ventilation for improved indoor air quality without the acoustic and security
constraints of naturally-ventilated single-skin facades.
The foremost benefit cited by design engineers of EU double-skin facades is acoustics.
A second layer of glass placed in front of a conventional façade reduces sound levels
at particularly loud locations, such as airports or high traffic urban areas. Another
cited benefit is that double-skin facades allow renovation of historical buildings
or the renovation of buildings where new zoning ordinances would not allow a new
building to replace the old with the same size due to more stringent height or volume
restrictions.
The second layer of glass provides opportunities for heat recovery during the cold
EU winters and heat extraction during the summer. Shading systems placed within
the interstitial cavity are protected from the weather.
Heat extraction double-skin facades rely on sun shading located in the intermediate
or interstitial space between the exterior glass façade and interior façade to control
solar loads. The concept is similar to exterior shading systems in that solar adiation
loads are blocked before entering the building, except that heat absorbed by the
between-pane shading system is released within the intermediate space, then drawn
off through the exterior skin by natural or mechanical ventilative means. Cooling
load demands on the mechanical plant are diminished with this strategy.
During cooling conditions, the Venetian blinds (or roller shades) cover the full
height of the façade and are tilted to block direct sun. Absorbed solar radiation
is either convected within the intermediate space or re-radiated to the interior
and exterior. Low-emittance coatings on the interior glass façade reduce radiative
heat gains to the interior. If operable, the interior windows are closed. Convection
within the intermediate cavity occurs either through thermal buoyancy or is wind
driven. In some cases, mechanical ventilation is used to extract heat.
The effectiveness of ventilation driven by thermal buoyancy, or stack effect, is
determined by the inlet air temperature, height between the inlet and outlet openings,
size of these openings, degree of flow resistance created by the louver slant angle,
temperature of the louvers and interfacial mixing that may occur at the inlet or
outlet openings if there is no wind.
The position of the Venetian blind within the air cavity affects the rate of the
heat transfer to the interior and amount of thermal stress on the glazing layers.
Placed too close to the interior façade, inadequate air flow around the blind may
occur and conductive and radiative heat transfer to the interior are increased.
The blind should be placed toward the exterior pane with adequate room for air circulation
on both sides. With wind-induced ventilation or high velocity thermal-driven ventilation,
the bottom edge of the blind should be secured to prevent fluttering and noise.
Heat recovery strategies can be implemented using the same construction to reduce
heating load requirements during the winter. This strategy is normally not useful
for the California climate and for commercial buildings, which tend to be cooling-load
dominated year-round. Heat recovery strategies can be used for east- to south-facing
facades to offset early morning start-up loads that occur typically on Mondays or
periods following a holiday but careful engineering is required to avoid overheating
during late morning hours.
During the summer and in the some climates where there is sufficient variation in
diurnal and outdoor temperatures and a good prevailing wind, nighttime ventilation
can be used to cool down the thermal mass of the building interior, reducing air-
conditioning loads. Heat gains generated during the day are absorbed by furnishings,
walls, floors, and other building surfaces then released over a period of time in
proportion to the thermal capacity of the material. Removal of these accumulated
heat loads can be achieved with a variety of cross-ventilation schemes that rely
on wind-induced flow, stack effect, and/or mechanical ventilation.
In recent years, the concept of radiant cooling has been coupled with traditional
across ventilation schemes. For some climates and building types, this strategy
can be used to completely eliminate the need for mechanical air-conditioning. Heavy-weight
thermal mass is strategically located in exposed concrete ceilings.
“Adaptive” thermal comfort is a key concept that must be accepted by the building
owner, facility manager, occupants, and code officials. Interior temperatures are
expected to exceed the limits defined by the ASHRAE Standard 55, which was originally
intended for conventional HVAC applications. Field studies suggest that behavioral
adaptations (changes in clothing level and air velocity, via local fans or operable
windows) and psychological adaptations widen the range of acceptable interior temperatures
– acclimatization or physiological adaptations are unlikely to result in significant
changes (Brager and deDear 2000).
Therefore, occupants of these new buildings who are accustomed to air conditioned
space should be made aware of the design intent of naturally ventilated buildings
so that their expectations for thermal control will be more relaxed.
Double-skin facades have been designed for the purposes of allowing nighttime ventilation,
with the reasons of security and rain protection cited as main advantages. However,
single-skin facades are capable of having a larger proportion of unobstructed operable
windows. The required percentage of facades openness is proportional to the internal
heat load: for milder European limates or northern California coastal climates and
for buildings where daytime solar loads are controlled, such a scheme may be feasible
with a moderate degree of façade openness.
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. 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).
There are various ways to classify mixed-mode ventilation systems.
In the context of high-performance building façades, 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 airconditioned 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 : This is the
most common mixed-mode approach with various operational strategies:
- Alternating operation allows either the mechanical or the natural ventilation system
to operate at one time
- Changeover operation allows either or both systems to operate on a seasonal or daily
basis depending on the outdoor air temperature, time of day, ccupancy, user command,
etc. — the system adapts to the most effective ventilation solution for the current
conditions.
- concurrent operation where both systems operate in the same space at the same time
(e.g., mechanical ventilation that has operable windows).
|