Energy-saving CO2 sensor detects building occupants
The new sensor is designed to continuously determine how many people are in a room or building by measuring changes in carbon dioxide concentration. Such an advance, say the researchers, would allow energy savings by largely restricting air conditioning and heating to occupied areas.
“The big picture is that being able to turn off the heating and air conditioning in a sensible way would allow you to save a substantial part of U.S. energy consumption,” says Jeffrey Rhoads, a professor in Purdue’s School of Mechanical Engineering, who leads the project. “To do that you need to be able to tell when a room is occupied and how many people there are.”
One way to determine a building’s changing occupancy, says Rhoads, is to continually measure the concentration of carbon dioxide, which is exhaled by people. To do that, the researchers developed a sensor combining two technologies: carbon dioxide detection and a sensor designed to perform precise measurements to determine how many people are in the room.
“Our approach is to use these two sensors in concert so that you will only do a fine measurement of the concentration of CO2 (carbon dioxide) if someone is present in the room, saving energy in the sensing process,” says Rhoads. “The first sensor detects CO2 and is used as a switch to turn on the second one, and by doing that you save all of the power of running the second one continuously.”
“And this strategy allows us to satisfy some key metrics on power consumption and longevity. It will have to run on very little power and last several years. We don”t want a system that you have to maintain on a yearly or monthly basis.”
The sensor is based on a microelectromechanical system (MEMS) containing vibrating plates coated with a film that absorbs CO2, causing a change in vibration frequency in the presence of the gas. The sensor will be reset by heating the sensor components, driving the CO2 gas out.
“You could think of it as a smoke detector,” says Rhoads. “Except that we are focused on carbon dioxide and not simply detecting; we are also measuring.”
The sensor, say the researchers, will be slightly larger than a postage stamp, cost less than six cents per square footage of building space to operate, and last longer than three years. The project is expected to begin in May, and is funded with a three-year, $1.5 million grant from the DOE’s Advanced Research Projects Agency-Energy through its new Saving Energy Nationwide in Structures with Occupancy Recognition, or SENSOR, program.
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