Even though most people don’t notice room occupancy sensors, they’re popping up everywhere. The sensors are already present in many offices, mostly to turn lights on or off as workers come and go, or to adjust the airflow based on whether a room is occupied.
But if you’ve ever been the first one to a meeting in a conference room, or stuffed into an over-crowded office, you know that occupancy sensors so far don't provide a fine-tuned system when it comes to ventilation. In large office buildings they usually don’t distinguish between whether there is one person in the room or 100. While that may work for lighting most of the time, it does not work as well for employee comfort nor does it achieve maximum energy efficiency.
There are more advanced sensors being developed that could provide more detailed information to the heating and cooling system. The more sophisticated technology could slash energy use in large, commercial office buildings by an average of 18 percent, according to a new study [pdf] from PNNL. By counting how many people are in a room, the PNNL study found, the sensors would allow heating and ventilation systems to fine-tune their output.
"Technology available today doesn't detect how many people are in a room, and so air flow is at maximum capacity nearly constantly," Guopeng Liu, an engineer at U.S. Department of Energy's Pacific Northwest National Laboratory (PNNL), said in a statement. "That creates a big demand to re-heat the air before it enters the rooms. It takes a lot of energy to keep you comfortable under those circumstances."
The researchers simulated a prototypical large commercial office building. Heating was turned on if the building was below 70 degrees Fahrenheit (21 C; which seems a little aggressive) and cooling was activated when temperatures went above 75 degrees F (24 C). Occupancy patterns were then added based on past studies.
From Fairbanks, Alaska to Miami, Florida every climate zone in the United States they simulated saw an increase in energy efficiency with advanced sensors for lighting and ventilation. Salem, Ore. had the greatest efficiency gains, 23 percent, while Miami had the smallest improvement, 5 percent. Twelve of the 15 cities simulated, mostly the cooler climate zones, had savings of at least 15 percent.
"Using the number of people in a room as a factor in determining the level of air flow offers great promise for saving energy and money," Michael Brambley, staff scientist with the Advanced Buildings Controls Team at PNNL, said in a statement. The study estimates that the energy and money savings might even be conservatively low, because the average building is about 23 years old, while the prototype was modeled using ASHRAE 90.1-2004, a building standard less than 10 years old.
The simulation also considered buildings that already had basic occupancy sensors for lighting and ventilation. The energy savings were still double or triple compared to savings with just the basic sensors.
Another small benefit was that lights would turn off much faster with advanced sensors, usually in seconds instead of 15 to 20 minutes. Because many offices have already upgraded to more efficient lighting, however, the energy savings would likely be minimal.
Although the potential savings for the 408 million square meters of large commercial office space in the United States could be staggering, there are some significant challenges. Most importantly, today's advanced sensors don’t have the control algorithms or interoperability with building control systems needed to dynamically adjust the HVAC system, according to the study authors. In some regions, building codes might have to be adjusted to allow for no ventilation if no one is present. Even with those hurdles, the researchers say the benefit is worth the investment.
"We undertook this study to try to determine if this is a technology worth pursuing vigorously,” said Brambley. “The answer, clearly, is yes.”