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By Nadav Malin
Ahh, the night air
Flushing heat out of a building with cool nighttime air is
an energy-saving strategy sometimes used with naturally and
mechanically ventilated buildings. Night flushing is most
effective in climates where hot days are accompanied by cool
nights. These climates also tend to have drier air-which is
important in order to avoid the pitfalls of introducing humid
air into partially air-conditioned spaces.
Night flushing requires some exposed high-mass surfaces that
can store heat during the day and release it at night as the
cooler air moves through. In the Martin Luther King, Jr. Civic
Center in Berkeley, the mass was provided by removing the
dropped ceiling in some spaces to expose the underside of
the concrete floor slab.
Raised access floors are increasingly common in buildings
designed for night flushing. By containing mechanical air
distribution, cable management, and other services under the
floor, these systems make it relatively easy to eliminate
a dropped ceiling entirely, which in turn makes the concrete
slabs above available for thermal mass.
At what price comfort?
Thermal comfort is determined by clothing and activity level,
air temperature, humidity, air speed, and radiant temperature
(the temperature of nearby surfaces). Every individual's allowable
comfort range varies. The American Society of Heating, Refrigeration,
and Air-conditioning Engineers (ASHRAE) and the International
Standards Organization (ISO) have defined narrow allowable
ranges of temperature and humidity for various building types.
Although these ranges were developed for centrally controlled,
air-conditioned buildings, it is often assumed that they should
apply to naturally ventilated buildings as well. Recent research
shows that in naturally ventilated buildings occupants are
comfortable in a wider range of temperature and humidity conditions.
The researchers proposed a separate standard for naturally
ventilated buildings to account for this wider range. This
alternate standard is likely to be included in a future revision
to ASHRAE's standards.
In the York University project, which deals with the thermal
comfort issue by separating the building into zones with differing
temperature set points, the conventionally established comfort
temperature of 70 degrees Fahrenheit will be maintained in
offices, while in lecture halls summer temperatures will be
allowed to climb to 74 to 75 degrees Fahrenheit, and to 81
degrees Fahrenheit in circulation spaces. "Clients must
either hope for stimulating lectures or be willing to dress
appropriately for the season," McGregor says.
| Getting cool help for
architects
Understanding the climate in which one intends
to build is always a good idea, but it is especially
critical for naturally ventilated and mixed-mode
buildings. Architects increasingly rely on computer
software for this help.
One simple but useful tool is Climate Consultant,
developed by Murray Milne and his students at
the University of California at Los Angeles. Climate
Consultant takes standard weather-data files and
displays them in relation to a comfort zone on
a psychrometric chart, which plots temperature
and humidity. Climate Consultant, along with weather
files for many U.S. locations, is available as
a free download from www.aud.ucla.edu/ energy-design-tools.
At the other extreme in terms of complication
is Computational Fluid Dynamics, or CFD. This
software models the amount of airflow through
a building, based on wind pressures and temperature
differences. CFD programs tend to be complex and
require quite a bit of training, so an architect
or engineer is more likely to contract with a
specialist to perform the analyses.
Burt Hill, for example, is working with Brian
Ford, a CFD expert with Brian Ford Associates
in London, for the Pittsburgh Convention Center.
One of the strengths of a good CFD analysis, especially
for buildings with large volumes, is that it can
show how conditions will vary in different portions
of the space. There might be pooling of uncomfortably
hot air near the ceiling, for example, but if
the occupied zone is comfortable, that hot air
is not problematic.
Even good CFD studies with positive conclusions
are not always enough to convince clients, however.
Engineers at Southern California Edison, learning
that all schools are naturally ventilated in San
Diego, commissioned CFD studies of a new elementary
school for Newport Beach, Calif., in hopes of
demonstrating how this low-tech approach can work.
The study showed that comfortable conditions could
be maintained in the occupied zones under all
design conditions, but the school district decided
to go with an air-conditioned, sealed building
anyway. One reason for this choice is that the
facilities staff was concerned that if people
had complaints, there would be no equipment to
adjust or fix.
Interestingly, the CFD studies also showed that
the school district's mandated red-tile roof would
have resulted in slightly less comfortable conditions
than if a heat-reflecting, light-colored roof
were used. N.M.
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