Logging Occupancy to Optimize Energy Use

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Evan Lubofsky, Onset

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.ux-placement

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.

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“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.”

 Learn more about occupancy data loggers at www.onsetcomp.com

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