Saving energy in office buildings

Engineers from Purdue University have developed a 'control' algorithm which promises to reduce energy consumption -- and electricity bills -- by as much as 30 percent for small office buildings which represent the majority of commercial structures.

Precooling a structure in the morning before temperature rises has been done before. It later saves energy during times of peak demand and you might even have done it intuitively at home. But now, engineers from Purdue University have developed a 'control' algorithm which promises to reduce energy consumption -- and electricity bills -- by as much as 30 percent for small office buildings which represent the majority of commercial structures. So far, this method has only been tested in California, but the researchers say that their control software could be used anywhere after minor adaptations.

Here is a short introduction by Emil Venere, from Purdue University, about this method.

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 researchers have performed "simulations for combinations of two building types in 16 climate zones in California, which contains hot and dry climates and moderate coastal climates." The map below shows these different climate zones. (Credit: Purdue University).

California climate zones.

So how does this system work?

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.

So the researchers have developed a specific software to check energy efficiencies depending on the buildings.

[Their software] 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.

But even if the method works and permits to save energy, are people living in these buildings affected by these changes of temperature? Not really. Even if the temperature is about 70°F (about 21°C) in the mornings, or about 4 degrees cooler than normal settings, people either are not affected or even don't pay attention.

This research work has been done in collaboration with the the Lawrence Berkeley National Laboratory, which published earlier results in 2004. Here is a link to this paper, "Peak Demand Reduction from Pre-Cooling with Zone Temperature Reset in an Office Building" (PDF format, 15 pages, August 2004).

But the latest research work will be presented next week during the 2006 Winter Meeting of the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), which will be held in Chicago between January 21 and 25, 2006.

Thanks to Emil Venere, I was lucky enough to have access to the two papers that will be presented by the Purdue University engineers, "An Experimental Evaluation of Demand Limiting Using Building Thermal Mass in a Small Commercial Building" and "Assessment of Demand Limiting Using Building Thermal Mass in Small Commercial Buildings" (the above illustration has been extracted from this second document).

Here are some of the conclusions of the first paper.

Demand-limiting strategies that utilize building precooling and afternoon setpoint adjustments were tested in a facility that is representative of small commercial buildings. The results indicate that 30% reductions in peak cooling loads are possible for a five-hour demand-limiting period. However, it is important to vary setpoints following a nonlinear trajectory that is tuned to the facility in order to approach this potential.
Comfort evaluations were performed for the facility during two test phases. The first phase involved aggressive precooling prior to the demand-limiting period and resulted in uncomfortably cool conditions in the morning hours. The second phase involved relatively moderate precooling at 70°F (21.1°C) during the occupied period only, but it utilized a wider range of setpoint adjustments from 70°F (21.1°C) to 78°F (25.6°C) during the demand-limiting period. Occupants were generally satisfied with comfort conditions during the phase 2 testing.

The second paper underlines certain limits of the project.

Only two different small commercial building types were considered in this study. Additional buildings, systems, and utility rates should be considered in order to fully evaluate the potential for demand limiting and the overall economics associated with the control strategy presented in this paper.

Even if some additional work needs to be done, James Braun has a suggestion to further save energy: "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."

Sources: Emil Venere, Purdue University News, January 17, 2006; and various web sites

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