Personal Comfort Systems Study Collects Data To Better Predict Occupant Thermal Comfort
From ASHRAE Journal Newsletter, July 25, 2017
By Mary Kate McGowan, Associate Editor, News
Some like it hot; some like it cold. And the Center for the Built Environment now has the real-world data to show that its personal comfort systems research may help people better predict occupant thermal comfort.
At the 2017 ASHRAE Annual Conference in Long Beach, Calif., Joyce Kim, Student Member ASHRAE, a Ph.D. candidate at the University of California, Berkeley, and a graduate student researcher at the Center, presented findings from a field study that tested how to use PCS and occupants’ behavior to predict a person’s thermal preference.
The study’s 40 participants reported 96% thermal acceptability and 99% satisfaction with the PCS chairs—battery-powered, mesh-type office chairs with fans and heating strips embedded in the chair’s seat and back panels—at the end of the field study. The real-world data set helps researchers and designers better understand people’s thermal control behavior and comfort preferences, Kim said.
PCS chairs allow people to personalize their thermal comfort, with both cooling and heating systems.
“Our field studies have confirmed the potential energy savings of PCS by expanding temperature setpoints while maintaining or improving occupant comfort,” she said.
The Center began working on a $1.6 million research project supported by the California Energy Commission’s Public Interest Energy Research Buildings Program in September 2013. The research's goal was to create a new occupant-based paradigm for HVAC control and integrate low-energy PCS into HVAC operations that includes integrating the chairs’ data for intelligent comfort management in buildings, according to Kim. The project ended this past March.
FIELD STUDY AND FINDINGS
The Center’s field study spanned three months at a government office building in Redwood City, Calif., last summer. Forty workers participated in the study that required them to use PCS chairs and take daily online surveys about the chair and their thermal comfort.
The study yielded more than 5 million data entries recorded at one-minute intervals from 4,500 survey responses that highlighted several key findings, Kim said.
She said the field study showed:
PCS chair users were more satisfied with their thermal temperature than in a typical office building. The data showed 96% thermal acceptability across all exposed operative temperatures of 20.5°C–24.5°C (69°F–76°F).
People who used the PCS chairs used the heating and cooling functions about 77% of the time they sat in the chair.
Study participants liked the chairs, and daily surveys found a 99% satisfaction rating.
The study’s data helped the Center improve the accuracy of individuals’ comfort predictions by 20%–30% compared to predicted mean vote (PMV) and the adaptive comfort model, according to Kim.
“We also developed comfort profiles based on repeatable patterns observed in the data to better characterize comfort characteristics of PCS users for building design and energy simulations,” she said.
SURPRISES IN THE DATA
Some of the study’s results were not expected.
First, researchers were surprised by how much temperature varied within a variable air volume (VAV) control zone, Kim said. The chairs’ sensors collected indoor environmental data that showed temperatures varied in the same zone as much as 2.9°C (5.2°F) in average temperature.
Depending on where the thermostat is located, temperatures in a local area can be quite different from the thermostat reading, which does not allow the majority of occupants to have thermal comfort, according to Kim.
“The benefit of PCS chairs is that it comes with temperature and relative humidity sensors that monitor local thermal conditions in real time. Hence, integrating PCS chairs into the building's BMS (building management system) can provide more data points to the building’s environmental sensing network and allow more representative temperature control in occupied spaces,” Kim said.
Second, Kim said they did not anticipate finding that the chair users were using the heating capabilities for therapeutic reasons, such as relieving back pain.
“And, these users often had heating and cooling at the same time–to satisfy both therapeutic and comfort needs simultaneously,” she said.
At the end of the three-month study, many of the government workers did not want to give up their PCS chair, Kim said.
ENERGY EFFICIENCY
Not only do the PCS chairs increase occupants’ productivity in a thermally comfortable environment, the PCS chairs use a fraction of the energy HVAC systems require.
The chairs use a maximum of 16 W, while a conventional HVAC system consumes 500 W to 700 W per occupant during occupied hours, on average, Kim said.
Currently, the Center does not yet know how much energy savings the PCS chairs could create because some of the products are not yet available for purchase and have not been tested commercially, she said.
“It’s hard to estimate the payback period of PCS devices since their costs and comfort effects vary among different devices...The degree of energy savings will also depend on how many occupants in the building have PCS and the facilities’ willingness to expand the temperature setpoints,” she said.
The Center does have a license with a manufacturer, and 75 PCS chairs have been installed at the Rocky Mountain Institute's Zero Net Energy building in Colorado.
The Center also has plans to demonstrate how to integrate PCS devices into the HVAC control loop to coordinate comfort management in smart buildings, according to Kim.