Pre-cooling of office buildings could save money, especially in energy crisis areas

Scientists at Purdue University in Indiana have found that companies’ electricity costs can be reduced by up to 41% in the hottest summer months using the ‘pre-cooling’ method, but they emphasise that this is not the case for all cities in the US, and a company’s cooling scheme has to be carefully planned for each building. The computer tool was tested on a four-floor, 1.4 million-square-foot office building in Illinois, and has also been used to evaluate how well the technique would work in the cities of Boston, Chicago, Miami, Phoenix and Seattle.

“Significant savings were achieved in all locations except for Seattle,” said James Braun, a Purdue professor of mechanical engineering, and one of the researchers on the project. In the remaining four cities, electricity is more expensive during peak hours, often in the afternoon, costing between 1.9 and 4.7 times more using 1999 rates.

Companies often make the mistake of switching off air conditioning when their employees go home at the end of the day in order to save money, says Braun. But this ignores the significant thermal storage potential of a building, he points out. By cooling the whole building down over night, it is less easy for it to be heated up by the sun during the day, which means that although some air conditioning may be required, it will be less than using traditional cooling strategies. “Solar radiation strikes the walls, and then the [internal] air is heated by the walls,” Braun said. “By cooling the walls, you have reduced those [heat] gains.”

However, thermal storage strategies for each building depend on factors such as its size, and the climate of the region, and if applied incorrectly, the result could actually be higher energy bills. This is where Purdue’s new computer simulation tool could be useful. “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 commercialisation,” he added. “The next step is to perform some additional case studies with other buildings, and a graduate student is working on this.”

Although the study only considered cooling the building using the air-conditioning system, even greater savings could be possible in climates where outside air is cool at night and can be brought into the building, says Braun. This option for pre-cooling could even produce some savings for office buildings in Seattle, he said.

The findings of the research will be detailed in the October edition of International Journal of Heating, Ventilating, Air-Conditioning and Refrigerating Research, published by the American Society of Heating, Refrigerating and Air-Conditioning Engineers.