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
"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
"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.
Posted 19th January 2006