Occupancy Based Sensors for Energy Management

Light level sensors complement PIR sensors.

Energy management systems rely on occupancy data to make the most of energy savings. In some environments like office buildings, the climate can be controlled based on “time of day” scheduling (using this logic: “We know that this office space will be occupied on weekdays, and unoccupied on weeknights and weekends; therefore, the air conditioning will cool the space more so when occupied, and less so when no one is there”).

In environments with unscheduled occupancy the biggest opportunity for energy reduction is by sensing occupancy. Dormitories, hotels, military barracks, and other similar spaces with unscheduled occupancy, are unoccupied up to 70% of the day. That’s almost 18 hours out of every day!

Using occupancy sensors, we can determine if the space is empty, and then we can allow the temperature to drift. We can cut back on the air conditioning in the summer, and reduce the heat in the winter, since no one is home to benefit from it.

Occupancy sensors come in all manner of shapes, sizes and designs. (Read our white paper-Occupancy Sensors in Hospitality.)

Many rely on passive infrared sensors. These PIR sensors are the very same technology used in yard lights that turn on when someone walks within the line of sight. They also sense heat, which is important when the sensor must sense someone in the space when the occupants are sleeping and near-motionless.

Sleeping Occupants-The Ultimate Sensor Challenge

Sleeping occupants: that’s where light level sensors can be of benefit. Even PIR sensors that are calibrated to extreme sensitivity can benefit from knowing whether it’s day or night.

Pretend you are a guest in a hotel. You turn in at 10:00PM. The thermostat in your room features PIR occupancy sensors but no light level sensors.  You’re exhausted because you’ve run a 10K marathon (bravo!), and you fall instantly asleep and do not move for hours. Your thermostat PIR sensor detects no heat or movement for a set period of time, like 30 minutes. Based on this data, the occupancy sensor determines your room to be unoccupied, and the temperature will drift away from setpoint. You awaken to a hot room in which the a/c has been turned off. You are about as hot and sweaty as you were in mile 8 of your run. And you are NOT pleased. The front desk gets an earful from you.

Now pretend you’re in that very same scenario, except the thermostat features a light level sensor in addition to the heat/motion sensor. Your thermostat detects no heat or movement for that defined 30 minutes just like in the previous scenario, but before allowing the temperature to drift it consults one additional piece of data: light level in the room. If the light level is low, it could indicate nighttime and sleeping guests, and so it extends the set period of time to detect occupancy, allowing extra time to detect heat or movement. After this extended time, if no heat or motion is detected, THEN the temperature is allowed to drift.

Light level sensors are the supporting cast. They’re not the main story, and the story could be told without them but they contribute to an enhanced, holistic, more enriched experience.

If you’re curious about how energy management systems work and want more information, download our white paper, How Energy Management Systems Work.