Active doubles, anyone?
Harder to predict are the benefits of
hybrid envelope systems, in which two or more interactive
strategies are combined. Many European architects have integrated
ventilation, shading, and other active technologies into double-wall
facades that serve as primary space conditioners. Unlike Cannons
Occidental Chemical building, early double envelopes had few
moving parts. (Some Europeans use the term active facade
to describe any ventilated double wall, regardless of operability.)
More recent projects feature more edge monkeys:
automated hoppers, vents, and shades.
An extreme example is the philology library
by Foster and Partners at Berlins Free University, completed
last year. The four-story, orblike enclosurewith an
underfloor air plenumis engineered for free cooling
for about seven months of the year using natural ventilation.
A checkerboard cladding of aluminum and glazed panels protects
an inner glass-fiber membrane. Operable panels close during
cold weather, and fresh air is drawn from outside through
the floor cavity and into the envelope void. A concrete internal
structure provides thermal mass and radiant cooling and heating
of recirculated air. The client expects about 35 percent energy
savings over a comparable facility.
images to view them larger
Institute, Arizona State University, Tempe
A large, easterly expanse of windows uses
aluminum louvers that are controlled continuously
by photocells and sun-tracking software. The
design allows occupants to control most of
the louvers in their offices using their PCs,
although at above 8 feet from floor level
the louvers are controlled automatically.
Photography: © Timothy Hursley (middle);
Mark Boisclair (left and right)
Hopefully, performance data will bear
this out. But unlike Fosters 1997 energy-miser Commerzbank
Tower in Frankfurt, most large-scale projects dont document
utility costs. Karl Gertis, a building physics researcher
at the University of Stuttgart, thinks its because they
often miss the target. In the design phase, simulations prove
notoriously unpredictable, he believes. Once built, natural
ventilation often isnt adequate for room air handling
or for maintaining comfortable temperatures. Weak convective
airflows in wall cavities may preclude the use of insect screens
and air filters, too. Last, Gertis cites numerous buildings
designed without mechanical cooling that have failed. Fosters
library stands prepared: On hot days, it leeches supplemental
cooling from an adjacent structure.
For Plantation Place, a large office
development in London, Arup Associates incorporated active
solar shading and occupant-controlled operable ventilation
in its double-skinned cladding design. At their lower levels,
the buildings have a heavy curtain of limestone fins in deference
to the masonry expression of the projects Neoclassical
neighbors. Upper levels, on the other hand, are all glass,
yet those floors can be cooled with only natural ventilation
during much of the year. The outer layer of the 2-foot-deep
double walls comprises a rain-and-wind screen of shingled,
frameless glass panels, angled at 3 degrees, with open joints.
Behind it is a maintenance walkway and solar blinds adjacent
to an inner window wall with operable panels. The two layers
were delivered to the job site as 5-foot modules and prefabricated
on-site into units with integral blinds and catwalks.
To ensure that occupants enjoyed the
benefits of the complex facades, Arup Associates and facade
engineers from Arup planned an unusual daylighting scheme.
In each tenant zone, photosensors were mounted on inner facades
to automatically control the raising and lowering of blinds
based on local conditions. There are reliability questions
for automated daylighting control, admits Arup facade
engineer Neil McClelland. Any design should recognize
that there will be issues and allow for access to the blinds
for cleaning and maintenance. McClelland adds that the
main reason to use automated blind controls is for maximum
transparency, not energy-efficiency.