blowing
hot and cold
Modeling the body's reactions provides clues for more efficient climate control.
By N.J. DeVico
What
is the measure of a man? From the point of view of the architect, every
office worker is as interchangeable as a drone. Existing architectural
standards consider each person occupying a building "a two-foot sphere,"
says Edward Arens, professor of architecture and head of the Building
Science Group at the University of California, Berkeley.
"Extending this simplification into practice," Arens said,
"heating and air conditioning systems in most buildings do not
attempt to take the gradients and asymmetries in individuals' microclimates
into account."
Treating every cubicle occupant as if he were a passive sphere has its
consequences. Some workers wind up feeling frozen, while their neighbors
consider themselves baking. And the coarseness of the standard model makes
for inefficient energy use. The more closely engineers can tailor the
heating and cooling systems to individual tastes, the less energy will
be wasted trying to find the one setting that pleases all.
The climate-control situation may change soon. The Berkeley group has
developed a detailed model of human physiology capable of predicting skin
and core temperatures of each major segment of a body exposed to a complex
microclimate, like that found in an office or in the passenger compartment
of a car. The group also developed a model that predicts the thermal sensations
and comfort resulting from those body temperatures.
Once such a model is refined, architects and heating engineers will be
able to find more energy-efficient climate control solutions. And that,
in turn, will reduce one of the largest draws of energy.
According to Arens, 38 percent of the total energy used in the United
States goes into heating, cooling, and lighting buildings. In commercial
buildings, lighting uses a greater proportion of total energy than in
residential buildings, but since America consumes 25 percent of the world's
energy, even small efficiencies in heating and cooling have the potential
to create tremendous savings.
"Comfort is a state of mind," said Ralph Goldman, an adjunct
professor at both Boston University and North Carolina State, and chief
scientist at Comfort Technology Inc. in Framingham, Mass.
The comfort zone is generally considered the 72° to 78°F range,
Goldman said, "but that's only if five other factors are
satisfied." If air moves at 40 feet per minute, the relative humidity
is 40 percent, and the mean radiant temperature equals the air temperature,
an individual will likely perceive comfort. But change air speed 20 feet
per minute, and the result feels like a one-degree change; every 20 percent
of humidity change feels like 1°F; full sun registers as a 13°
difference. Other major influences are the person's metabolic heat
production and clothing.
These results are notably crude. They don't account for differences in
condition across the span of a single body—say, the head and chest
in full sunlight, but damp feet in deep shade. Would such "micro"
microclimates make a noticeable difference?
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| Cold hands, warm heart: Thermal imaging (above and below) reveals temperatures across the body. |
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In the late 1990s and up to the fall of 2003, Arens supervised a doctoral
dissertation by his student Zhang Hui that developed a sensation and comfort
model. Working with 27 students, she individually controlled the temperature
of 20 body sections. Data about body size, blood vessel functioning, core
temperature, skin temperature, skin wetness, and perceived comfort were
collected and analyzed.
"What's comfortable? The highest levels of comfort were
actually attained with contrast," Arens said. "Perception
of comfort depends on the rate of change." While sitting in an
air-conditioned room for an hour might be comfortable on a 90°
day, stepping inside after walking around out there is even more comfortable.
"The ultimate goal," said Arens, "might be to design
heating and ventilation systems that take advantage of the asymmetric
and transient nature of people's surroundings."
To imagine such a system, think about the typical car. Most current car
models let the driver direct the vents to different parts of the body.
"We think buildings should be conditioned like the newest cars,
where occupants can have conditions that satisfy them," Arens said.
An analogous system already exists in buildings for lighting. Some buildings
incorporate task-ambient lighting—overhead light is set to minimal
illumination when an occupant switches a desk light on—that saves
lighting energy and allows the occupant to control lighting levels.
Auto manufacturers interested in finding even more efficient ways to run
air conditioners helped fund Hui's work. Similarly, the solar treatment
of car windows affects comfort inside, and the glazing industry has also
been taking advantage of the model.
A fan unit that blows air from nozzles, for instance, could be installed
in office furnishings and provide cooling, fresh air where the person
wants it. Radiant heating panels could individually target specific body
parts—those wet feet, for example.
Localized control is more energy efficient, and creates a sense of freshness.
"We know from field studies that people tend to find office buildings
stuffy, with insufficient air circulation. That's partly because
the standards limiting drafts are set very low. Mechanical engineers compensate
for low air speeds by setting the air conditioner colder. But moving air
in the right places can cheaply produce a feeling of freshness."
Nothing spurs improvement like customer demand and government regulation.
Drivers want comfort, and regulators want energy efficiency. Since cars
get better fuel economy and produce fewer emissions these days, even though
air conditioners are much improved, the accessory uses a greater percentage
of a car's fuel than in the past, according to Byron Jones, director
of the Engineering Experiment Station at Kansas State University in Manhattan.
So manufacturers focus on improving the efficiency and effectiveness of
environmental control systems. (Since waste engine heat warms interiors,
taken to the extreme, cars may become so efficient that producing enough
heat could present a big challenge.)
"In your home, you use the right glass and insulation to reduce
heating and cooling requirements, then you use the cooled or heated air
that you do have, more effectively. It's the same with cars,"
Jones said. The goal is to deliver more comfort using minimal energy.
"This is where much of our work comes into play."
Kansas state, too, has developed detailed computer models, ones that
describe how the body responds to the environment inside a car. "We
need to understand the nature of the air distribution around passengers,"
Jones said. "We interface our models of the human body to computational
fluid dynamics models—complex models that describe how air moves
and predict air velocity." Subtle changes can make a big difference.
If air directed from the front to back of the car, under the seat, is
interrupted by an object, the dynamics change.
"With this information, we can predict physiological responses,"
Jones said. "Sure, extreme heat or extreme cold makes everyone
miserable, but when you're dealing with comfort, psychological
factors come into play and everybody is different. You can only go so
far with physiological models. We also relate comfort perceptions to physiological
responses."
Common transient thermal conditions—such as getting into a very
cold or very warm car, then activating the environmental control system—complicates
predicting comfort. This complexity surprised Jones. "With steady-state
conditions, people's reactions are pretty predictable, but with
transient conditions, developing accurate and reliable models gets really
challenging," he said.
According to Jones, manufacturers face challenges trying to improve the
environmental conditions. The air must circulate around the whole interior
without feeling like a hurricane to one person. Understanding how people
respond offers clues. For example, passengers who have direct control
of the airflow tolerate more velocity.
Referring to these models speeds up the design cycle. A design concept
that would have taken months to evaluate a few years ago can be evaluated
within a week. Now manufacturers quickly test concepts and design changes
that wouldn't have even been considered before.
N.J. DeVico is a freelance writer based in Titusville,
N.J.