Warehouse supply centers reduce energy costs with thermal destratification

Federated Co-operatives Ltd., a retail cooperative with members throughout western Canada, operates retail shops, fueling stations, building material supply centers and refineries, occupying numerous warehouse spaces to store products. According to Trevor Carlson, environmental and technical services manager at Federated, a fair amount of heat was being wasted at the warehouse ceilings. By bringing this heat downward, he thought, they could decrease the rate at which their buildings were shedding heat through the roofs.

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A potential source of heating energy savings is reducing difference in air temperature between the outdoors and the underside of the roof. Thermal destratification can make the air temperature essentially uniform within the conditioned space, reducing the temperature at the underside of the roof. Estimated energy savings are based on the difference in heat loss through the roof at the temperature differentials before and after destratification.

“In the stratified condition, there will be a significant difference in air temperature between floor level and the underside of the roof,” explains Richard Aynsley, lead aerodynamics engineer, Big Ass Fans.

Destratifying air temperatures in a 300,000 sq.ft. warehouse with large fans saved Federated Cooperatives Ltd. an estimated $19,800 the first year.

Before destratification, the difference between the temperature of air near floor level and at the underside of the roof deck can be as much as 30°F. A critical factor in destratification is that the indoor air is thoroughly mixed to an even temperature, remarks Aynsley. Afterward, the difference between the temperature of air near floor level and at the underside of the roof deck is usually no more than 1°F. Big Ass Fans’ units are designed to provide a low-cost, energy-efficient cooling solution coupled with the ability to significantly reduce energy usage in winter months by destratifying the air within a building. In lieu of reversing the fan, which is common for most small, high-speed fans, large-diameter Big Ass Fans units are designed to simply operate at a slower speed during the winter, eliminating much of the draft associated with fan circulation.

Carlson decided to conduct pilot tests in the 80,000-sq.ft. loading dock area of Federated’s 300,000 sq.ft. warehouse in Saskatoon, Saskatchewan, Canada. He installed five 24 foot-diameter, low-speed Big Ass Fans unit. “After a short period of operation, we noticed multiple benefits,” says Carlson. “The workers in the warehouse actually wanted the temperature decreased in the winter because they were too warm. That was very encouraging for us, as we were able to change the setpoints on the thermostats because the fans were bringing the heat down, keeping workers comfortable while reducing our costs. We looked at our degree-day data and calculated our heating index for the year prior and the year after installing the Big Ass Fans.” Carlson’s findings showed a significant reduction in energy use.

Occupants of existing buildings can obtain fuel use data from their utility companies, while local climate records can provide the average outdoor air temperature during a particular heating season. This data can be used to calculate the heat loss through the building envelope over a given period in the form of a heating index in Btu/ft²/degree day. One degree day is accumulated for each difference of one degree between a day’s average temperature and a reference temperature, typically 65°F.

Carlson tracked data concerning average energy consumption and daily temperature changes from 2007 into 2009. For the Saskatoon facility, the heating index before the installation of the fans was 4.49 Btu/ft²/degree day. The year the fans were installed, consumption decreased to 3.99 Btu/ft²/°F. The following year was the first full year of using the fans, and the heating index was 3.61 Btu/ft²/°F. “We noticed a pretty significant decrease, and it works out to a roughly 10% reduction in natural gas consumption,” says Carlson. “It was a 10% reduction when we saw a 20% increase in natural gas rates. We believe we saved $19,800 in the first year in natural gas consumption as a result of the fans.”