Occupancy sensors detect whether people are present and turn lights on and off accordingly. Used properly, they can be a cost-effective tool for decreasing the operating time and lumen output, cutting energy consumption up to 50%, and reducing peak demand. However, there are a few issues you should carefully consider before installing an occupancy sensor.
Occupancy sensors are most effective in spaces that are often unoccupied, such as warehouses, corridors, and conference rooms. Open-plan office spaces where one or more people may be moving around throughout the workday aren’t good candidates for this technology. You can also use occupancy sensors to meet codes and standards—including ASHRAE Standard 90.1—that require some form of automatic lighting control for new construction and renovations.
What are the options?
Occupancy sensors use four types of signals to detect the presence of people: infrared radiation (sensors that detect heat), ultrasonic waves (sensors that detect shifts in sound waves), microwaves, and acoustics.
PIR sensors Passive infrared (PIR) sensors are the most common type of occupancy sensor. They’re able to “see” heat emitted by occupants. These sensors are triggered when they detect a change in infrared levels—such as when a warm object moves in or out of a sensor’s range—and are resistant to false triggering. Some PIR sensors have an operating range of up to 35 feet in specific directions in ideal conditions. However, they’re most reliable within a 15-foot range because they’re passive (that is, they only receive signals) and the blind spots between their wedge-shaped sensory patterns get wider with distance.
Ultrasonic sensors Emitting high-frequency sound waves that humans and animals cannot hear, ultrasonic sensors send signals and listen for the reflected sound. Because they’re active instead of passive, ultrasonic sensors are more sensitive and can cover larger areas than PIR sensors can (figure 1). However, they’re more prone to false triggering. Air motion—caused by a person running by a doorway or a ventilator cycling on or off—can set off a poorly located or maladjusted sensor. Curtains, shades, or blinds that move with air currents can also activate ultrasonic sensors.
Microwave sensors Microwave sensors detect occupancy by emitting microwaves and analyzing the reflected waves. They can also detect whether a person is moving toward or away from the sensor. Microwave sensors are versatile, and you can use them in environments that are unsuitable for other sensors, such as high-heat environments. They’re suitable for outdoor use and can cover large areas.
Microwave sensors have three main drawbacks:
- They have high false-alarm rates since slight movements from curtains or blinds activate them.
- They’re expensive to run because they require constant power.
- They work in intervals by sending and receiving signals, instead of working continuously.
Acoustic sensors Acoustic sensors detect people-made noise and mechanical noise related to human activity, such as keyboard tapping, paper shuffling, and photocopying. Unfortunately, these sensors also respond to sounds that have nothing to do with occupancy, such as slammed doors and street noise. They also require high sound levels—higher than a typical quiet office—to activate. Few, if any, sensors use acoustic technology by itself, but some sensors combine PIR with acoustics to increase reliability.
Hybrid sensors Hybrid—or dual-technology—sensors incorporate features of different sensor types in one unit. The most common combination is a PIR sensor with an added ultrasonic sensor, which blends the PIR unit’s resistance to false triggering with the sensitivity of the ultrasonic sensor.
Advanced sensor features Sensor technology is always improving. For example, some new infrared sensors are equipped with double-eye sets to minimize blind spots. Ultrasonic boosters enable a single wall-mounted sensor to cover large or oddly shaped rooms without additional wiring. When connected to a heavy-duty relay, a single boosted ultrasonic sensor can effectively cover several thousand square feet.
Other switches combine occupancy sensors with dimmers to take advantage of the energy-saving potential of both technologies. Line-voltage sensors make it practical to install occupancy sensors on individual fixtures in applications such as high-bay facilities; they’re reliable, cost half as much as previous sensors, and can be installed in half the time.
Finally, new smart sensors use microprocessors to provide motion sensitivity or time adjustment and incorporate analysis of the signals to improve operational reliability. A traditional sensor uses a preset time delay; if the sensor hasn’t detected any occupants, it will shut off after a certain amount of time. However, a smart sensor can learn the activity levels and habits of building occupants, allowing it to adapt the time delay and improve performance. They reduce false on or off triggers and can give an audible or visible indication, warning occupants that lights will be switching off.
How to make the best choice
Monitor lighting-use patterns and occupancy patterns together to estimate how effective occupancy sensors will be in a particular location. You can use that information to figure out how the sensors would reduce lamp-operation hours and then calculate the energy savings.
Gauge occupancy patterns
You can determine lighting usage patterns in various ways. Two of the simplest methods are interviewing custodial and security personnel about their observations and simply monitoring and recording whether lights in different parts of your facility are left on at night or in infrequently used rooms. You can also review the settings on lighting timers or building automation systems if you use them.
A more sophisticated approach involves using a data logger (which counts lighting hours and records the time and duration of use) and then see how the data relates to sensed occupancy. You can place data loggers inconspicuously in rooms and retrieve the information for later analysis. They can be useful for reducing peak demand, as they report when lights are on during the peak demand period.
You can determine occupancy patterns using school and work schedules to establish when classrooms, lecture halls, and offices are likely to be in use. Open-plan offices, lunchrooms, restrooms, and corridors often have well-defined occupancy hours.
A commercial facilities study performed by one West Coast utility found that actual operating hours in halls and lobbies were 50% to 72% higher than estimates based on operating schedules. However, in private areas and conference rooms, operating hours were 29% to 46% lower than the estimates. When estimates are this far off, they could result in significant errors in expected payback periods. Keep in mind seasonal variations in facility operation, as factoring these into calculations can help you avoid incorrectly extrapolating data from one month to a full year.
Occupancy sensors sometimes yield savings that are smaller than expected because facility managers fail to adequately consider utility time-of-use rates and demand charges, improper product selection, unanticipated interactions with other building components, or improper installation.
Measure cost savings A large East Coast utility found that occupancy sensors installed under its rebate program yielded average energy and demand reductions of about 30%. However, any given installation can vary from this average. The US Department of Energy compiled the energy savings in various types of spaces in its study Wireless Occupancy Sensors for Lighting Controls: An Applications Guide for Federal Facility Managers (PDF) (figure 2).
Figure 2: Typical range of savings from occupancy sensors
Evaluate cost-effectiveness Reducing energy consumption doesn’t necessarily lead to an equivalent cost reduction. Sensor manufacturers tend to stress the energy savings associated with their products in promotional activities but don’t typically place as much emphasis on demand and cost reductions. Make sure to evaluate savings projections in the context of your utility rate structures and building-use patterns to get an accurate sense of your expected savings.
A promotional video from one sensor manufacturer states that installing occupancy sensors on a 10-kilowatt lighting circuit in a major New York City building reduced lighting energy consumption by 56%. However, because the reduction in peak lighting demand was only 20%, the cost savings—taking into account the time variable rates and the demand charges involved—were only about 38%.
Determine what kind of sensor you need
Ultrasonic or PIR sensors Ultrasonic sensors can detect small movements but are prone to false triggering. They’re best for covering large areas.
PIR sensors are resistant to false triggering but tend to have blind spots that get larger farther away from the sensor. These sensors are best for small spaces and detection ranges within 15 feet.
Wall versus ceiling mount Wall-mounted sensors are best for smaller rooms such as offices, restrooms, and equipment rooms, where people are likely to be present for only a short time after they walk by the sensor. In an open-plan office or spaces where the lighting load is higher, mount the sensor in the ceiling. You can also find sensors that you can install in corners or on the walls near the ceiling.
Compatibility with other systems Using occupancy sensors outside of their wattage ratings can damage or disable them, so make sure that the circuit wattage is appropriate. Many sensors aren’t designed to handle the larger surge demand of electronic ballasts, and some sensors will fail early if you continuously operate them at their maximum rating.
Install sensors carefully
Sensors are easy to spot, and people might be tempted to adjust, steal, or vandalize them. Position the sensors carefully and train building occupants about their purpose. Effective training techniques include:
- Involving building personnel in planning for the sensors
- Training maintenance personnel and office occupants to keep sensors operational, rather than disconnecting them as problems occur
- Positioning sensors so they “see” only the area intended to be observed—the most common cause of false triggering is incorrect positioning
What’s on the horizon?
You can find new wireless sensors that simplify retrofits and improve sensor effectiveness. For example, a wireless system makes it easier to replace malfunctioning sensors or adjust their positions. These sensors cost more, but they’re still good for applications such as small buildings that haven’t taken advantage of any control technologies; office buildings that frequently reconfigure space for new tenants; and old buildings that are being converted to modern office spaces. Two solar-powered wireless sensors are available: the Leviton WSC15-I0W LevNet RF Wireless Self-Powered Occupancy Sensor, 1500 Square Feet and the Magnum Energy Solutions Wireless Occupancy Sensor (EOSW).
Who are the manufacturers?
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