Energy-Saving Methods for Small Office Buildings Developed
West Lafayette, IN - Engineers have developed a method for
"precooling" small office buildings and reducing energy
consumption during times of peak demand, promising not only
to save money but also to help prevent power failures
during hot summer days.
The method has been shown to reduce the cooling-related
demand for electricity in small office buildings by 30
percent during hours of peak power consumption in California's
sweltering summer climate. Small office buildings represent
the majority of commercial structures, so reducing the
electricity demand for air conditioning in those buildings
could help California prevent power-capacity problems like
those that plagued the state in 2000 and 2001, said James
Braun, a Purdue University professor of mechanical engineering.
The results focus on California because the research was
funded by the California Energy Commission, but the same
demand-saving approach could be tailored to buildings
in any state.
"California officials are especially concerned about
capacity problems in the summertime," said Braun, whose
research is based at Purdue's Ray W. Herrick Laboratories.
Findings will be detailed in three papers to be presented on
Monday (Jan. 23) during the Winter Meeting of the American
Society of Heating, Refrigerating and Air-Conditioning Engineers
in Chicago. Two of the papers were written by Braun and doctoral
student Kyoung-Ho Lee. The other paper was written by
researchers at the Lawrence Berkeley National Laboratory,
a U.S. Department of Energy laboratory managed by the
University of California.
The method works by running air conditioning at cooler-than-normal
settings in the morning and then raising the thermostat to
warmer- than-normal settings in the afternoon, when energy
consumption escalates during hot summer months. Because the
building's mass has been cooled down, it does not require
as much energy for air conditioning during the hottest
time of day, when electricity is most expensive and
in highest demand.
Precooling structures so that it takes less power to cool
buildings during times of peak demand is not a new concept.
But researchers have developed a "control algorithm," or
software that determines the best strategy for changing
thermostat settings in a given building in order to save
the most money. Research has shown that using a thermal
mass control strategy improperly can actually result
in higher energy costs. Factors such as a building's
construction, the design of its air-conditioning system,
number of windows, whether the floors are carpeted,
and other information must be carefully considered to
determine how to best use the method.
"The idea is to set the thermostat at 70 degrees Fahrenheit
for the morning hours, and then you start adjusting that
temperature upwards with a maximum temperature of around
78 during the afternoon hours, " Braun said. "When the
thermostat settings are adjusted in an optimal fashion,
the result is a 25 percent to 30 percent reduction
in peak electrical demand for air conditioning.
"If you couple this reduction in demand with a utility
rate structure that charges more during critical peak
periods, utility costs will drop. Without such a change
in peak rates, though, the actual impact on operating costs
is relatively small, with about $50 in annual savings per
1,000 square feet of building space.
"A good incentive for reducing peak demand would be to
impose a higher peak demand charge for the critical
peak-pricing periods, and if customers reduce their
consumption during these times, they are rewarded with
lower energy costs for the rest of the time."
The recent work at Purdue has been geared toward small
commercial buildings, which use a type of cooling system
called "packaged" air conditioning equipment.
"Small commercial buildings tend to be one to four stories,
but the main distinction is that they use packaged equipment,
" Braun said. "A packaged air conditioner is a cooling system
that is completely assembled in a factory rather than on the
site. An example of a small commercial building might be a
shopping mall, which contains several rooftop air conditioning
units that all have individual thermostat controls, compared
to a system that has one central cooling system that must be
put together on the site."
Researchers at the Berkeley lab performed field demonstrations
and evaluated the human-comfort aspects of different thermostat
adjustment strategies, specifically how cool the temperature
can be reduced in the morning hours and how high it can rise
in the afternoon hours before the building occupants complain.
"We found that you can go down to 70 degrees and people will
not complain," Braun said. "In fact, they won't even notice."
A setting of 70 degrees is about 4 degrees cooler than the
normal setting for that time of day.
"Then, when the critical peak pricing period starts in the
afternoon, you start adjusting that temperature upwards,
going as high as 78," Braun said. "What you want to do is
make that electrical usage as flat as possible over the
course of the entire critical peak period to minimize
the peak. Normally it will peak in the middle of the
afternoon, but you want to flatten the peak."
The findings being presented during the upcoming conference
detail how to achieve the flat power usage for specific
buildings, depending on a structure's characteristics.
"This requires some limited testing for every building,"
Braun said.
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