Many
workplaces feature major changes in occupancy over the course of a week. In
academic buildings, hundreds of students may pour in for a lecture, then leave
an hour or two later, while faculty, researchers and staff can enter and exit
in irregular patterns. In commercial structures, workers may come and go en
masse during short time periods during the day. As a result, energy use in
virtually all workspaces can rapidly become inefficient—too large or too small—in
relation to the number of people inside.
Now,
a new study done on Massachusetts Institute of Technology (MIT) buildings
reveals some data that could help designers and building managers, on campuses
or in the commercial sector, optimize energy usage—and suggests a template for
conducting more research on the subject.
The
study, published in Energy and Buildings, examines data from MIT’s
buildings 37 and E52, and finds that while electricity use corresponds to
occupancy fairly well in those spaces, the activity of heating, ventilation,
and air conditioning (HVAC) systems in the buildings does not correlate closely
to occupancy.
“It
can take a huge amount of energy to heat up buildings during the day, and then
suddenly there may be nobody there,” says co-author Carlo Ratti, director of
the Senseable City Laboratory and associate professor of the practice in MIT’s
Department of Urban Studies and Planning (DUSP). “There is significant
potential for improvement and savings, if you could get a more dynamic usage of
energy in this area.”
As
Ratti and the coauthors note, the federal government estimates that commercial
buildings account for about 20% of U. S.
energy consumption, and 12% of U. S. contributions to greenhouse gas emissions;
past studies have shown that commercial buildings could reduce their energy use
20% to 30% by implementing strategies that better match energy use and need.
An energy disconnect
The new study uses data about Wi-Fi connections as a proxy for building
occupancy, a method the researchers believe could be replicated elsewhere at
low cost; while the data does not necessarily reveal an exact population count
inside buildings, it does indicate relative occupancy levels over time. “It’s a
way of trying to make use of information that already exists,” Ratti says.
The
researchers analyzed two very different kinds of buildings. Building 37 at MIT—the
Ronald M. McNair Building, named for the late astronaut
and alumnus (PhD ’77)—houses researchers in astrophysics, aeronautics, and
astronautics in a combination of offices, classrooms, and laboratories. By
contrast, Building E52, the longtime home of MIT’s Department of Economics, is
an Art Deco box standing alone on Memorial
Drive by the Charles River,
with a sizable entrance atrium and a large number of offices inside.
Analyzing
data from all four seasons of 2006, the researchers found that Building E52 has
lower levels of energy use, and that both buildings have a distinctive cyclical “signature” of electricity usage that rises and falls daily. Both buildings use
more steam (for heat) in winter and spring, and more chilled water (for air
conditioning) in summer and fall. But while about two-thirds of the variation
in electricity levels can be accounted for by changing occupancy levels, the
use of the HVAC systems correlated only weakly to occupancy.
In
short, these MIT buildings tended to be heated or cooled over extended periods
of time according to season, but not in a way that optimized the use of energy. “We were not surprised to see this disconnect, but it was good to be able to
quantify it,” says Prudence Robinson, a researcher in the Senseable City Lab and
a co-author of the paper.
In
addition to Ratti, the coauthors of the new paper—titled “ENERNET: Studying the
dynamic relationship between building occupancy and energy consumption”—are
David Lee, an MIT PhD student in DUSP; Claudio Martani, a former visiting PhD
student in DUSP from the Politecnico di Milano in Italy; Rex Britter, an
engineer specializing in fluid dynamics who is currently a research scientist
in the Senseable City Lab; and Robinson.
Experts
on energy use in the workplace say the paper deals with a significant issue in
their field. “This paper is very much in harmony with our recent efforts to
design and modify building infrastructure and systems to match varying
occupancies,” says Jay Phillips, senior director of operations for Harvard University’s Faculty of Arts and
Sciences, who was not involved in this study. He adds: “There is no question
that matching building and equipment schedules with dynamic occupancy patterns
is a significant conservation opportunity.”
‘You want to have a better match’
Part of the “disconnect” between occupancy and energy use, the researchers
acknowledge, may derive from the special needs of academic buildings, which
often house laboratories with around-the-clock heating or refrigeration
demands.
M37
has some spaces like this, although E52 does not. Either way, though, the
nuances of academic buildings make them “an ideal test bed” for thinking about
ways of saving energy, according to Britter. Suppose a building’s largest room
is a lecture hall that is only used for an hour or two a day, but in winter
demands a significant amount of energy in order to become warm. One possible
way of using energy more efficiently would be to surround the lecture hall with
heavily used offices, which can then receive the heat seeping out of the larger
space.
In
such cases, Britter says, “you’ve got to think through the best way of having a
fairly complicated arrangement of offices, so that heat could be distributed
from the lecture hall to more rooms.”
There
are many possible architectural or engineering solutions to this issue, Ratti
notes. The larger point, he says, is that “you want to have a better match. You
can move the people to the energy, in which case the architecture can help a
lot, or you can move the energy to the people, which is more futuristic.” New
sensing-based thermostats, for instance, could regulate temperatures on a more
granular basis within buildings, according to the distribution of people within
them.
Alternately,
Ratti adds, existing spaces can be used for new purposes—such as a once rarely
used large common area in MIT’s Building 9, which now houses its Steam Café,
and thus has chairs and tables occupied throughout the day. “That’s a much more
intensive and better use of the space, with the same embedded energy cost,”
Ratti says.
Ratti
and the Senseable
City researchers are
currently pursuing several other studies that use IT data to track the flow of
people in urban areas, and say they would like to continue to analyze energy
use as part of their ongoing work.
