Cutting Cooling Costs at Commericial Buildings

WEST LAFAYETTE, — Research engineers have shown that electricity costs for office buildings can be reduced by up to 40% by running air conditioning overnight.

"Pre-cooling" structures so that it takes less power to cool buildings during times of peak demand is not a new technique. But engineers at Purdue University are the first researchers to create a computer-simulation tool that can be "tuned" to a specific building and used to document the savings that would be realized by using the technique for that specific building. The analysis tool takes into account factors including utility rates and climate, and it can be used to tailor the best pre-cooling strategy for individual buildings.

The pre-cooling technique is especially practical in areas where utility companies are having trouble meeting demands for electricity. In those areas, the price for daytime electricity is much higher than the price charged overnight.

The tool was tested on a four-floor, 1.4 million-square-foot Ameritech Corp. office building in the Chicago-area suburb of Hoffman Estates, Ill. The simulation showed that a pre-cooling technique could reduce electricity costs by as much as 41% during the hottest summer months.

The findings will be detailed in a paper that will appear in October's International Journal of Heating, Ventilating, Air-Conditioning and Refrigerating Research, published by the American Society of Heating, Refrigerating and Air-Conditioning Engineers Inc. The paper was written by James Braun, a Purdue professor of mechanical engineering, and graduate students Kent Montgomery and Nitin Chaturvedi.

The simulation tool also was used to learn how well the technique would work in five cities: Boston, Chicago, Miami, Phoenix and Seattle.

"Significant savings were achieved in all locations except for Seattle," Braun said.

With the exception of Seattle, the utilities in those cities charge considerably more for electricity during peak hours, such as mid-afternoon, than at other times. The difference between peak and off-peak rates ranged from 1.9 times higher in Phoenix to 4.7 times higher in Boston, according to the research paper, which quoted 1999 rates.

The study only considered cooling the building using the air-conditioning system. But even greater savings could be possible in climates in which cool outside air can be brought into the building at night to provide pre-cooling. This option for pre-cooling could have resulted in some savings for Seattle, Braun said.

Utility companies also impose an "on-peak demand" charge, in which the total number of kilowatts consumed during times of peak demand are multiplied by a certain dollar amount. In the case of the Ameritech building, the peak-demand charge was $16.41.

"You take your peak kilowatt draw that occurs throughout the whole month and you multiply that by $16.41," Braun said. "That can really add up for a facility this large."

The higher peak-demand rates are an incentive for customers to conserve power.

"They don't have capacity problems in Seattle, so they don't need the incentive," Braun said.

The conventional cost-cutting approach for commercial buildings is to raise the thermostat settings after workers go home for the day, essentially shutting down the air conditioners while the building is not occupied. However, this approach ignores the "significant thermal storage potential" in many commercial buildings, Braun said.

Sun-heated walls contribute to a building's rising daytime temperature. But pre-cooling the building by running air conditioning overnight helps to control the temperature rise. Because the structure's mass has been cooled down, the building does not require as much energy for cooling during the day, when electricity is most expensive.

"Solar radiation strikes the walls, and then the air is heated by the walls," Braun said. "By cooling the walls, you have reduced those (heat) gains."

The pre-cooling technique is applied in "thermal mass control strategies," in which a building's structure is cooled overnight.

Thermal mass control strategies should be tailored for each building, depending on its size, the climate and other factors. The energy-saving strategy that worked best for the Ameritech building required that the building be cooled overnight to about 67 degrees Fahrenheit. Shortly before employees began arriving for work, the thermostats were turned up to a more conservative temperature, such as 74 degrees.

Thermal mass control strategies are being used by some companies.

"It's being done a little bit, but not in a very organized fashion, and without really understanding how to do it and what the benefits are," Braun said.

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 and regional weather conditions must be carefully considered to determine how to best use a thermal mass control strategy.

The new "thermal mass simulation tool" developed at Purdue is the first method that can be tuned to a specific building's performance and documents the degree of savings that can be realized by the technique. It can be used to pinpoint the exact strategy to use for a particular building.

"With this tool we can do some measurements on a building, and then use those measurements to develop a model," Braun said. "Then, the model can tell you whether it makes sense to use this kind of control in your building.

"The technique is not quite ready for commercialization. The next step is to perform some additional case studies with other buildings, and a graduate student is working on this."