The cafeteria is freezing as a solitary worker finishes a late lunch in the cavernous space.
The problem of overly air-conditioned rooms in office buildings is not just one of comfort. It’s a huge waste of energy in commercial buildings, which consume about 36 percent of U.S. electricity. And it’s often caused by heating, ventilating, and air-conditioning (HVAC) systems that are set to provide heating or cooling to a space based on maximum occupancy, regardless of whether it’s full or empty.
But by employing advanced occupancy sensors to measure how many people are in a room at a given time, airflow could be dynamically adjusted based on real-time usage. That could yield energy savings of at least $40,000 a year for the typical large commercial building in most parts of the country, according to researchers at Pacific Northwest National Laboratory (PNNL) in Richland, WA.
As the costs of advanced sensors come down, the payback period on this sort of investment could fit within the five-year window that owners of existing buildings often demand of major energy efficiency improvements, says Guopeng Liu, a senior engineer in the PNNL Building Energy Systems Group. These sensors are an example of the kind of deep energy efficiency improvements that could be made to commercial buildings. And if new models for financing energy efficiency take hold, even deeper improvements could start to make sense financially.
Liu cautions that he hasn’t formally studied the return on investment (ROI) of these advanced sensors yet; that’s the next step. He’s basing this rough ROI estimate on his experience, including leading development of a Department of Energy-sponsored Advanced Energy Retrofit Guide.
Several companies already make occupancy sensors that mount on walls or ceilings and use infrared or ultrasound or even carbon dioxide to detect if someone is in a room. These are connected to other controls and can be set to turn off the lights, and, in more advanced systems, the air conditioning, when no one is using a space.
The PNNL research [PDF] looks at the savings possible from using an advanced infrared occupancy sensor currently in development that effectively counts the number of people entering and exiting a room to determine occupancy in real-time. One company developing these sensors, according to PNNL, is Melexis Technologies, based in Belgium.
Liu says the costs of these devices could come down to the range of $10 to $20 each, though it could easily be twice that much when wiring and other installation costs are added. And a large commercial office—picture a 12-story building with about 500,000 square feet of space—would require at least one sensor in every room or zone served by so-called variable air volume (VAV) equipment.
Present in 84 percent of large commercial buildings built after 1980, VAVs modulate airflow to a specific room or cluster of rooms in response to a thermostat or other control.
To study the potential savings of advanced occupancy sensors, Liu and three other PNNL researchers— team leader Michael Brambley and engineers Jian Zhang and Robert Lutes—started with a hypothetical 500,000-square-foot building built in 1989 and upgraded several times since. Such a building, they reasoned, would represent the energy usage characteristics of the median-age U.S. large office building, which was 23 years in 2012.
Adding common occupancy sensors to turn off lighting and HVAC systems when a room is empty would yield energy savings of at least $20,000 a year in 10 of 15 distinct climate zones across the U.S., the researchers found.
Using advanced occupancy sensors to turn off lighting in empty rooms and throttle back the HVAC system to match actual room usage would cut building energy nearly 18 percent, on average, across the nation, they calculated. The annual savings for the typical building in 13 of 15 climate zones would be $40,000 or more—at least double the savings from common sensors. (Seattle is in the same climate zone as Salem, OR, where a typical building could save $61,200 a year, the researchers say.)
Liu says advanced occupancy sensors could have other applications. In an emergency, for example, they could tell first-responders clearing a building where people are concentrated. They could also have applications for energy demand response—when utility customers voluntarily reduce demand to ease the strain on the electricity grid during periods of peak usage—by enabling building managers to see that a room designed for 20 has only two people working in it, and encourage the pair to relocate to a smaller space so the large room could be shut down entirely.
