Logging Occupancy to Optimize Energy Use

by | Apr 9, 2014

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Every occupant in a building creates demand for lighting, ventilation, thermal comfort, and electrical power. Lighting, heating, and cooling unoccupied spaces is a huge source of energy waste in buildings, and many studies have shown that building occupancy profiles have a significant impact on building energy use and operational controls.

Closer alignment of occupancy patterns to building equipment schedules can be an effective low-cost/no-cost energy efficiency strategy leading to more intelligent control of buildings, a better balance between occupant comfort and energy savings, and lower utility bills. This includes, but is not limited to, HVAC temperature set points, lighting schedules, and economizer schedules.

Determining occupancy

Building occupancy is often based on maximum expected occupancy numbers. Average occupancy in most buildings never approaches the maximum, and maximum occupancy typically doesn’t account for widely-shifting occupancy patterns.

Consider a college campus building, for example. Between exam periods, school breaks, and holidays, occupancy can tend to be dynamic and all over the board.

To get closer to actual occupancy information, a variety of approaches are employed. For example, CO2 sensors are used, but since they work by measuring the effluent given off by people, they are generally better suited for determining ventilation needs vs. occupancy patterns. Camera sensor networks can also provide valuable building occupancy information, but they are often complex and pricey – and can be overkill if only occupancy patterns are needed (vs. people counting).

Portable data loggers can be useful tools for tracking building occupancy patterns, and fill the gap between CO2 sensors and expensive camera sensor networks. Occupancy/light loggers are often compact and easy enough to deploy anywhere occupancy pattern information is needed. More important, they take the guesswork out of estimating occupancy by providing hard data.

Occupancy logger basics

Occupancy/light data loggers are battery-powered, matchbox-sized devices that can be easily mounted on ceilings, light fixtures, and stairwells – anywhere data are needed. The loggers typically have two integrated sensors – one to measure room or area occupancy and one to measure light on/off status information. Together, they continuously collect time-stamped data at user-defined intervals.

Once data have been collected, accompanying graphing & analysis software is typically used to read out, plot, and analyze the information. The software typically allows you to combine and compare data, and display measurements in graphs that show profiles over time. Alternatively, tabular data can be viewed as well, or exported to a spreadsheet for more detailed analysis.

Occupancy logging applications

As mentioned earlier, occupancy has a direct impact on everything from lighting efficiency to ventilation. Therefore, it may be important to log occupancy for a variety of reasons.

For example, a facility manager may suspect that adding room occupancy switches would be a good retrofit project within a building. He or she may decide to deploy a number of occupancy/light loggers in rooms where lights are often left on but no one is there. By tracking light use and occupancy patterns in the rooms over a period of time, the facility manager will be able to pinpoint areas that would benefit from occupancy switches.

“Establishing baseline occupancy patterns when evaluating occupancy sensors for lighting controls can be very useful information, and far better than assuming that occupancy sensors will save some X% of the lighting energy use,” explains Mark Stetz, principal of Colorado-based energy consultancy Stetz Consulting.

Stetz also feels that light and occupancy pattern information can help to optimize lighting control schedules.

“These devices can be used to look at occupancy patterns ahead of a lighting control installation and/or to compare lighting patterns to occupancy patterns. Many people are not aware of how their building operates. There are many situations where the facility manager thinks that because people go home at 5:00 pm, the lights are also off. A review of the load data may show that, in fact, the lights are on until 2:00 am, when cleaning crews are active.”

According to Brenden Millstein, founder of San Francisco-based energy consultancy Carbon Lighthouse, occupancy/light data loggers represent a simple approach to tracking building occupancy profiles for lighting audits.

“We use occupancy/light on/off loggers now for all our potential lighting projects,” he said. “Knowing exactly how much each type of retrofit—e.g., lamp only, or fixture, or fixture plus occupancy sensors—would save, and choosing the best option for the client based on data is exactly what we like to do. Having a simple way to do this accurately is fantastic.”

Ventilation is another area that can benefit from occupancy logging. By tracking occupancy patterns around the clock for a few days, you may be able to determine if spaces are under- or over-ventilated based on when they are occupied. The collected data might call for adjusting HVAC set points and economizer schedules.

“We’ve used occupancy and light loggers to track occupancy patterns in some laboratory facilities here in New York,” said Dan Spilman of Energy Miser LLC. “One of the facilities had roughly a dozen individual labs, and we knew that they were running their air handling units 24/7 at full speed, even though the labs were unoccupied a lot of the time. There was little continuous occupancy even during the work day as people would be out working in the field. They might come in during the morning, leave for a good portion of the day, and then stop back in before the day was over.”

By logging occupancy patterns every 15 minutes for two weeks, it became apparent that the facility could reduce their number of air changes per hour and potentially save hundreds of thousands of dollars in the process.

“With labs, you really only need six air changes per hour during occupied times, even though many labs are designed for between eight and fifteen air changes per hour. In general, labs are huge energy users with the amount of fresh air that needs to go through the space.”

Selecting the right logger

Some important factors should be considered when evaluating occupancy loggers for your project. Following are just a few that can help you make the best choice for your application.

1. LCD Display

It’s well worth the peace of mind and time-savings to choose an occupancy and light on/off data logger with an LCD display. Some loggers just have LED light indicators, but with an LCD screen you can, at a glance, confirm successful deployment, signal strength, battery levels, and other levels and settings. You will save time during routine checks to ensure the logger is working properly, and you’ll realize the value of actually capturing the data you need.

2. Logger Accuracy and Range

Occupancy sensors are typically based on two types of technology. A passive infrared (PIR) sensor takes advantage of the fact that all objects radiate infrared light. Though humans can’t see that light, the electronic sensor can, and it designates changes in position of infrared light-radiating objects in its field of view as an occupied state. Ultrasonic sensors actively generate high-frequency sound waves and use the echo received to determine whether a space is occupied or unoccupied. This ultrasonic type of sensor is more energy-intensive than the PIR sensor. Most battery operated data loggers use the PIR technology since ultrasonic technology requires constant power.

In both cases, the sensors have a “cone” of sensitivity that determines their effective range (for example, see illustration of a ceiling-mounted logger in a classroom). The wider the base of the cone (=the farther the distance from the sensor), the larger the occupant’s movement in the space must be for the sensor to log an “occupied” state.

Therefore, it is important that you choose an occupancy/light logger with the sensitivity and range that’s appropriate for the size of the space to be monitored and the activities that take place there. For example, you will need different logger sensitivities for a busy forklift-filled warehouse versus a classroom of seated students.

Discuss your application(s) with the logger manufacturer’s application specialists to ensure you get the right sensor for the job.

3. Speed of Deployment

When choosing an occupancy/light logger, consider the entire deployment and readout process:

  • Logger configuration – Determine start/end logging time and other settings. This is typically done through the logger’s software while the logger is hooked up to a computer, although some functions can be set at the deployment site.
  • Battery check – It’s a good time to check battery strength and, for models that allow it, replace the battery yourself.
  • Mounting – Gather the necessary logger mounting materials (built-in magnets, zip ties, adhesive strips, etc.), as well as any equipment required (ladders, tools, etc.). At the deployment site, point the light sensor (or optional light pipe) toward the target light source while ensuring the occupancy sensor has a clear “view” of the space. If the logger has an LCD screen or LED lights, make sure you can see them from a distance. Secure the data logger in place.
  • Setting the threshold – Occupancy sensors don’t need adjustment, but the threshold must be set for the light on/off sensor. In some models, this can be done conveniently with the push of a button, via the auto-calibrate and signal strength feature.
  • Launch – You may have already set a launch time during configuration; if not, do it manually now.
  • Verification of operation – Check signal strength, battery levels, deployment “on” time, and successful data logging. You may do this routinely throughout the deployment time to ensure all is well, and with loggers with LCD screens, it’s easy to get the information you need.
  • Data download – Once you’ve retrieved the loggers, download the data by connecting to a computer. The fastest models connect via a standard USB cable, and data download takes ten seconds or less.

Bathroom occupancy study

Carbon Lighthouse deployed multiple occupancy/light data loggers across eight men’s and women’s restrooms in a commercial office building. More than 32,000 data points were measured and recorded over a three-week period. The goals of the study were to ascertain the functionality of existing occupancy sensors installed in each bathroom, monitor the general bathroom occupancy and use, and determine if there was an economic and environmental justification for replacing the existing fixtures.

After analyzing the data, Carbon Lighthouse determined that the building should not invest in upgrade fixtures. Although newer fixtures would lower energy costs, the run-hours are not long enough for the energy savings to justify the investment. The low run-hours are due to well-functioning occupancy sensors.

However, replacement of the occupancy sensors in the sixth-floor men’s restroom was recommended. The collected data showed that this particular sensor appeared to be malfunctioning, allowing the lights to be on continuously, even when the restroom was unoccupied for long durations.

Evan Lubofsky is a writer and communications director for Onset.

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