January 17, 2006

Researchers study energy-saving method for small office buildings

WEST LAFAYETTE, Ind. – 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.

Writer: Emil Venere, (765) 494-4709, venere@purdue.edu

Source: James Braun, (765) 494-9157, jbraun@ecn.purdue.edu

Purdue News Service: (765) 494-2096; purduenews@purdue.edu

Note to Journalists: The two Purdue papers will be presented during a symposium called Demand Response Strategies for Building Systems from 10:15 a.m. to 12:15 p.m. on Jan. 23 during the Winter Meeting of the American Society of Heating, Refrigerating and Air-Conditioning Engineers in Chicago. Electronic copies of the research papers are available from Emil Venere, Purdue News Service, at (765) 494-4709, venere@purdue.edu.

This paper describes the development and evaluation of a simple algorithm for limiting peak electrical demand in buildings using building thermal mass. The control algorithm is intended to reduce electrical demand in response to utility critical peak pricing (CPP) signals or similar incentives and involves adjustment of zone temperature setpoints within comfort bounds. The demand-limiting potential and impact on utility costs for CPP rates were evaluated through simulation for representative small commercial buildings throughout California. In general, it is possible to achieve between 1 and 2 W/ft2 (10.76 and 21.53 W/m2) of demand reduction for a four-hour demand-limiting period while keeping zone temperatures between 70°F and 78°F (21.1 C and 25.6 C). This represents a 30 percent to 100 percent reduction in peak electrical requirements for air conditioning depending on the climate. For typical CPP utility rates, the impact of the demand-limiting strategy on operating costs is relatively small, with less than about $50 in annual savings per 1000 ft2 (92.9 m2). Greater incentives may be necessary to encourage customers to respond to critical capacity shortages. Furthermore, CPP super-peak energy rates do not encourage customers to minimize their demand during CPP periods but, rather, encourage them to reduce their integrated energy usage over this period. A better incentive would involve imposing a high peak demand charge for CPP periods.

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