by Shane Henson — April 25, 2012—While many commercial building owners know their heating, ventilation and air-conditioning (HVAC) systems drain energy as well as their company’s pocketbooks, not enough have taken the steps needed to improve their HVAC systems because of the perceived difficulty and up-front costs involved. However, U.S. commercial building owners could save an average of 38% on their heating and cooling bills if they installed just a handful of energy efficiency controls that make their HVAC systems more energy efficient, and they could likely recoup their money much more quickly than expected, according to a recent report titled Energy Savings and Economics of Advanced Control Strategies for Packaged Air-Conditioning Units with Gas Heat from the Department of Energy’s (DOE) Pacific Northwest National Laboratory (PNNL).
The PNNL report examines options for improving the efficiency of commercial rooftop systems called packaged HVACs, which combine compressors, fans and heat exchangers into one unit. Packaged HVACs regulate temperatures inside more than 60% of the commercial building floor space in the United States, where commercial buildings consume as much energy as about 90 million typical American homes each year. And about 35% of that is used by HVAC systems, which are often poorly maintained or ignored, causing them to run inefficiently, notes the DOE.
For the report, PNNL considered implementing four different control methods to existing rooftop packaged HVACs:
- Air-side economizers use cool outside air to chill the building instead of creating cool air with the HVAC compressor. Some building codes already require cooling systems to include these, unlike the three other controls examined by the PNNL team.
- Supply fan speed controls slow or speed up the ventilation fan that circulates the building’s air based on whether or not a desired temperature or amount of fresh air has been reached instead of continually running the fan at full speed.
- Cooling capacity controls run the HVAC compressor at different speeds based on need.
- Demand-controlled ventilation slows or speeds up fans and air intake based on carbon dioxide levels inside the building instead of running ventilation fans at a constant rate.
- Supply fan speed controls slow or speed up the ventilation fan that circulates the building’s air based on whether or not a desired temperature or amount of fresh air has been reached instead of continually running the fan at full speed.
The study team tracked the effects of using these methods with a building energy simulation software called EnergyPlus from the DOE. The software created computer simulations that took into account 15 climate zones in 16 major U.S. cities.
The researchers studied four types of commercial buildings: small offices of 5,500 square feet, stand-alone retail buildings of 25,000 square feet, strip malls of 22,500 square feet, and supermarkets of 45,000 square feet. More than 1,400 different simulations estimated the potential savings in electricity used to power fans and cooling compressors, as well as the gas used to produce heat. Energy savings were then translated into dollars and cents.
In general, the researchers found that installing a multi-speed fan control had the greatest impact on energy savings in hot cities such as Miami. And demand-controlled ventilation created the best possible energy savings in colder cities such as Chicago, Duluth and Seattle.
The team reasoned that because ventilation fans generate some heat when they move, slowing fans with multi-speed fan control in hot climates could reduce the amount of chilling needed. And in colder climates, they suspected that demand-controlled ventilation prevents unnecessarily sending warm air outside, which then prompts the HVAC system to create more warm air to maintain desired temperatures inside.
When the research team added up all the numbers, they found the best possible percentage cost savings was 67%, which could occur when all four controls are added to a rooftop packaged HVAC at a small office building in San Francisco. And the minimum percentage cost savings was 28% and could come from adding all four controls to a supermarket in Seattle.
But savings weren’t limited to cash and energy use, say PNNL. The team also found that a substantial amount of carbon emissions could be avoided if HVAC energy efficiency is increased. As many as sixteen 200-MW coal power plants—which generate enough energy to power 3,000 to 4,000 American homes—could sit idle if just half of the nation’s packaged rooftop HVAC units on commercial buildings were retrofitted with controls, the simulations revealed.
Three companies currently manufacturer HVAC controllers, but only one company offers a product with all the control options that resemble the team’s simulations, Katipamula said. To help the manufactures better understand their market, the report also examines potential prices for the controllers and how long it would take for building owners to recoup that cost.
Based on the estimated dollar savings, the team predicted a building owner could recoup his or her investment in a few years. For example, they looked at adding supply fan speed control and demand-controlled ventilation to a supermarket. If that store spends $7,523 to equip its HVAC system, it would see a return in three years, while it would take the same supermarket five years to see a return if the controls had a higher price tag of $12,539.
