[ Page 5 of 5 ]

New study may boost an old energy-saving technique for HVAC

In many buildings across the U.S., outdoor air is pulled in throughout the day at rates designed to satisfy ventilation requirements for maximum-occupancy conditions, even during times when there are few people in the building (think schools at night or restaurants between lunch and dinner). But a recent study conducted at Purdue University in Indiana has given a shot in the arm to an old strategy for managing energy waste.

For more than a decade, waste from HVAC systems that condition spaces with variable occupancy has been addressed through demand-controlled ventilation (DCV), a strategy that links the amount of outside air drawn in for ventilation to the actual occupancy of the building at any given time, via a network of sensors that use airborne carbon dioxide concentration as a proxy for occupancy levels. DCV has been shown to produce annual energy savings of up to $1 per square foot. Up until now, the high expense and frequent maintenance required for DCV equipment limited the application of this strategy.

But the technology has improved lately. A decade ago, sensors used in DCV systems ranged in cost from $500 to $800 each; now many newer devices cost $200 or less. In addition, some of them remain accurate for 10 to 15 years, substantially reducing the cost of the yearly calibrations that were required for older sensors. Also, many rooftop air conditioners, frequently used in commercial and institutional buildings, come equipped to accommodate sensor inputs, which reduces the amount of labor needed to implement DCV.

DCV uses carbon dioxide monitors and special controls to estimate occupancy and adjust air intake. A study at Purdue concludes that DCV would be financially feasible for several types of buildings. Photography: Courtesy Carrier Corporation

The Purdue study, conducted in 2003 and 2004, highlights these recent improvements in DCV technology and points out new opportunities for energy savings. Jim Braun, professor of mechanical engineering at Purdue University, and his colleague Kevin Mercer, modeled four types of buildings—a restaurant, a retail store, a school, and an office—in two cities in California and three cities outside the state (see table, above). The cities were selected to represent a range of climates for the study, and the modeled buildings varied in size from 5,250 square feet for the restaurant to 80,000 square feet for the retail store. The study compared traditionally operated HVAC systems to those using DCV.

The restaurants and retail stores showed the most potential for savings with DCV, with savings estimated at around 50 percent of the total energy operating cost for HVAC in some cities. Across all the cities and buildings, payback periods ranged from 0.2 to 6.8 years, although 16 of the 20 modeled scenarios yielded a payback of fewer than two years, and 12 yielded a payback of one year or less (see table). The modeling used more conservative numbers for design occupancy than those set forth in the relevant ASHRAE standard for all but office buildings—so it’s likely that payback periods would be even shorter than what the study predicts.

Along with improvements in the DCV hardware itself, new online software tools, available through the Web sites of HVAC manufacturers, make it easier for design teams to determine where DCV can be used. The tools include Carrier’s “Hourly Analysis Program” (www.carrier.com), Honeywell’s “Savings Estimator” (www.honeywell.com), and AirTest’s “CO2 Ventilation Control and Energy Analysis” (www.airtest.com). Each allows users to enter information about a project, such as building type, size, and location. The software takes this information and provides users an analysis of the potential cost-effectiveness of a DCV system—which helps reduce the risk and uncertainty of moving forward with this strategy.

“Our hope is that this research will increase the usage of this effective energy-saving strategy,” said Jim Braun. His hopes have already been realized: Two utility companies, in California and Connecticut, are now using the Purdue study to build programs that will help their customers identify opportunities to implement DCV. In the not-too-distant future, all HVAC systems may be smart enough to know when a building’s empty enough to call it quits.

Peter Criscione

ADVERTISEMENT