Occupant Feedback and Control Behavior with a Newly Developed Personalized Environmental Control System (PECS)

. Personalized Environmental Control Systems (PECS) allow control over indoor environmental quality (IEQ) parameters of the microclimate around individual occupants. The present study reports on the results of human subject experiments evaluating a prototype of PECS. The tested prototype had heating, cooling, and ventilation (air circulation through a filter and an ultraviolet germicidal irradiation component) functions. The objective of the present experiment was to obtain the occupants' subjective responses and physiological parameters such as skin temperature, with and without the use of PECS. The occupants' interaction with the PECS prototype was also observed. Experiments were conducted with 24 university students (12 male and 12 female subjects) over a 5-week period between February and March 2022. Different ambient temperature settings between 18 and 28 °C were tested each week. In each week, subjects participated in two 3-hour sessions, once with PECS and once without it. Subjects with PECS were able to adjust the PECS functions freely throughout the measurements, and the changes they made were recorded in an internal log of the PECS. Subjective responses such as thermal sensation and acceptability were compared with their PECS operation to evaluate the effect of PECS, together with each occupant’s interaction with PECS.


Introduction
Personalized Environmental Control Systems (PECS) enable occupants to adjust the indoor environmental quality (IEQ) of their immediate surroundings according to their individual preferences. Previous studies have revealed differences in the perception of the indoor environment due to variations in factors such as clothing, activity, and preferences in temperatures and air movement, as was summarized by Melikov [1]. A field study conducted by Bauman et al. [2] showed that occupants with PECS were able to tolerate a wider range of temperatures compared to those without it. It was also suggested that the availability of the local control contributed to the difference between the two groups of occupants. According to a literature review conducted by Rawal et al. [3], PECS found in literature were able to provide thermal comfort in the range of 18 -32 °C.
PECS with a ventilation function (i.e., personalized ventilation) provides a ventilation effectiveness higher than that of a mixing ventilation, and can protect occupants from airborne transmission of infectious agents indoors [1]. This characteristic is crucial in mitigating indoor infection risks during pandemics, such as the recent COVID-19 pandemic. The capability of PECS in terms of the thermal and indoor air quality factors makes PECS a viable solution in the context of resiliency towards disruptive events such as heatwaves, power outages and air pollution [4]. * Corresponding author: junshi@dtu.dk Despite its potential and proven benefits, there is a lack of market-available PECS. Therefore, further development and performance evaluation are necessary. This study reports on the evaluation of a newly developed PECS prototype. Human subject experiments were conducted to obtain occupants' subjective and physiological response to the developed prototype. The physiological measurements (e.g., skin temperature, heart rate) were used for the development of a personal comfort model, reported in a separate study [5].

PECS Functions
The development process of the PECS prototypes are described in the authors' previous study [6]. The most recent prototype (PECS v3) was used for the human subject experiment in this study. The PECS comprises a desktop unit and a heating panel under the desk. The desktop unit has a touch screen for control and an air terminal device (ATD) that supplies recirculated air to the breathing zone of the occupant. The recirculated air goes through a filter and an ultraviolet germicidal irradiation (UVGI) component for air cleaning. The head of the ATD is equipped with a Peltier element for active cooling of the recirculated air. Water pipes were installed to remove the waste heat produced by the Peltier element. The pipes connected the ATD and the water tank integrated in the under-desk unit. The heating panel on the under-desk unit was a single flexible panel with a curved surface, which was aimed to provide heating to the thighs, shin, and feet. The heating and ATD could be adjusted in a discrete scale from 0 to 10, and the cooling function could be adjusted in a discrete scale from 0 to 5. The cooling function can be turned on only when the airflow from the ATD is turned on.

Chamber Setup
Measurements were conducted in a climate chamber at the Technical University of Denmark. The chamber has dimensions of 6.0 × 4.7 × 2.5 m (L × W × H). Fresh air supply and room temperature control were done by displacement ventilation. Fig. 1 illustrates the chamber setup. Four workstations were set up in the chamber to mimic an office setting. Two desks had the PECS prototypes installed, and the other two desks without the PECS served as the reference (REF). Wooden floor plates were laid under the workstations to prevent the influence of the supply airflow from the floor on the occupants. Partitions with a height of 1.5 m were installed between the workstations. The control reference and highprecision air temperature sensors were placed in the center of the room. At each workstation, the air and gray-globe temperatures, relative humidity (RH), and air speed were measured in the respective positions indicated in Fig. 1.  Table 1 lists the experimental settings. Measurements were conducted over five weeks between 14 th of February and 19 th of March 2022. Each week had a different setpoint temperature (measured at the control reference temperature in Fig. 1) of 18, 20, 23, 26, or 28 °C. Depending on the room temperature setpoint, subjects were asked to dress up in one of three clothing ensembles, each corresponding to approximately 0.5, 0.75, and 1.0 clo, respectively. A total of 24 university students participated in the study (12 male and 12 female subjects). Overall-healthy non-smokers who had lived in Denmark for more than one year were selected as the subjects. All subjects participated in two sessions every week, one day with PECS and the other day without PECS (REF case).

Experimental Timeline
Fig . 2 shows the experimental timeline of a single session. Each session had a total duration of 180 min. Prior to entering the chambers, subjects filled in the pre questionnaire and put on sensors for physiological measurement. Specifics of the sensors and questionnaire content will be described in the following section. During the first 60 min of measurement, subjects answered questionnaires every 10 min. Short questionnaires were given every other time to ensure enough time to answer the questionnaires. After the first 60 min, full questionnaires were filled in every 20 min until the end of the session. During the entire session, subjects were allowed to do their own work or study on the laptop computer, as long as they remained seated in their seats. After every hour, subjects were instructed to walk up and down a step box (two steps) slowly for 3 min., to prevent the metabolic rate from dropping over time and to maintain an overall stable metabolic rate. Subjects with PECS were allowed to change the PECS settings to their preference throughout the entire session. Table 2 lists the measured parameters in the experiment and the sensor positions. In addition to the environmental sensors shown in Fig. 1, sensors were put on to the subjects to measure their physiological parameters. Wireless temperature sensors were used to measure 13 points of skin temperature for each subject. Chest bands with core temperature and heart rate sensors were used. In addition, a smart watch was also used to measure the heart rate. Table 3 lists the content of each questionnaire that was given to the subjects during the measurements. The pre questionnaire asked for their thermal preference and general health-related questions, such as whether they felt good or bad at that moment, or the time they went to bed and woke up. The clothing they arrived with were also registered. Sick building syndrome (SBS) symptoms such as fatigue, headache, and nose irritation were asked in the pre, full, and post questionnaires.

Sensors and questionnaires
Questionnaires during the measurement (full and short) focused on the thermal response of the subjects, e.g., sensation, acceptability, and preference. In the full questionnaire, questions related to other IEQ factors, namely air quality, lighting, and sound, were asked. In the post questionnaire, subjects were also asked to give feedback to the overall environment and each IEQ factors, in addition to their feedback on the PECS itself.   The results show that significant differences among groups were seen mainly in temperature settings below 23 °C. For female subjects at 18 °C, the use of PECS resulted in a mean TSV closer to neutral with a significance of 0.1%. In all other cases, the differences between PECS and REF cases were non-significant. At 23 °C or lower, the male subjects had a higher TSV compared to females, regardless of the use of PECS.
The TSV in different temperature settings are well aligned with the findings of the thermal manikin measurements that were previously conducted [6]. From the manikin measurements with the same PECS prototype, it was found that the cooling effect was seen in limited areas around the face, with an equivalent temperature difference around 2 -3 K. The heating effect of the PECS was larger, around 2 -5 K at the lower body, with the highest values seen at the thighs. respectively. For each questionnaire during the sessions, subjects responded which body parts felt cold or warm at that time, and whether that sensation was acceptable or unacceptable. Subjects were instructed not to respond to body parts that had a neutral sensation. The warm/cold responses of all subjects were summed and plotted. The ambient temperature settings of 18, 23, and 28 °C were selected for comparison of cold, neutral, and hot ambient conditions. At 18 °C ambient temperature, cold-unacceptable votes were seen most in the extremities, i.e., hands and feet. The use of PECS reduced the cold-unacceptable votes at the feet by approximately 30 votes. In addition, the PECS case had more warm-acceptable votes on the thighs compared to the REF case. For the other lower body parts such as the feet and lower legs, the number of warn-acceptable votes were similar between the PECS and REF cases. This confirms the results of the thermal manikin measurements, where the largest heating effect was seen on the thighs (ca. 5 K; up to 2 K in other lower body parts).

Local thermal sensation vote
At 28 °C ambient temperature, the differences between the REF and PECS cases were less apparent compared to the 18 °C setting. The PECS case had a slightly higher number of warm-acceptable votes overall, compared to the REF case. However, the warmunacceptable votes yielded similar results for the two cases, with slightly lower votes at the head and upper arms for the PECS case. This also confirms the results of the manikin measurements, where the cooling effect was limited (up to 3 K around the face).
At 23 °C ambient temperature, the majority of the votes were "acceptable" votes. In the PECS case, warmacceptable votes were mainly seen on the lower body, which suggests that some subjects were still using the heating function to adjust to their preferences. The number of cold-acceptable votes were similar in most body parts, with higher votes in the feet. In the REF case, the number of warm and cold votes were overall less than other ambient temperature settings, which shows that the room temperature was close to neutral. At 18 °C, most of the subjects were using heating throughout the measurement period. Once turned up, the heating settings were seldom turned down, i.e., they were kept constant. Many subjects also turned on the fan, though at low settings up to around 4. At 20 °C, both the use of heating and the fan decreased. The use of all PECS functions were low at a room temperature setting of 23 °C. There were subjects who used heating, and some used the fan, both with and without cooling. Though the use of PECS was limited at this room temperature, the subjects that used PECS tended to keep their corresponding PECS setting high. At 26 and 28 °C, most subjects that used the cooling function kept it at the maximum setting (5).

Power Use
In addition to the ability to provide improved comfort to the occupants, the power use to achieve higher comfort is equally important from the aspect of energy saving and resiliency [4]. In this section, occupant feedback and power use of PECS were compared. Fig. 7 compares the power use of PECS and the resulting difference in the thermal acceptability vote (TAV). As was done for the thermal sensation vote analysis in section 3.1, the TAV and power use for the first hour of each session was excluded. For the TAV, the two-hour average TAV between the PECS and REF cases were calculated to examine the effect of PECS on the subjects' thermal acceptability. Positive values indicate higher acceptability in the PECS case, and negative values indicate lower acceptability in the PECS case. The total power use over the two hours were calculated in Wh to examine the PECS power use E3S Web of Conferences 396, 01045 (2023) https://doi.org/10.1051/e3sconf/202339601045 IAQVEC2023 associated with the difference in TAV. The base system power (i.e., the power use when all PECS functions are turned off) is included in the power use. As a reference, the PECS power use and the resulting whole-body, manikin-based equivalent temperature is shown in Fig.  8. The power use corresponds to the power use for the specific function, and does not include the base system power use. The measurement conditions and detailed results are presented in a previous study [6]. The heating panel was tested in the room temperature of 18, 20, and 23 °C, and the ATD with and without active cooling was tested at 23, 26, and 28 °C room temperature.
The comparison shows that in lower temperature settings (18 and 20 °C), the TAV tends to be higher with the cost of a higher power use. This is mainly associated with the power use of the heating panel, as many subjects maintained higher settings for the heating panels, as was shown in Fig. 6. At 23 °C room temperature setting, some subjects had higher total power use up to about 450 Wh over the last two hours of measurement, which is also the result of the use of the heating panel. However, the majority of the subjects had lower power use in this room temperature, as the PECS was either turned off or only the ATD fan was being used. In higher ambient temperature settings (26, 28 °C), the total power use mostly ranged between 100 to 300 Wh. Regardless of the power use, both the increase and decrease in thermal acceptability was observed. This is likely due to the lack of cooling power from the ATD (up to 0.3 K cooling of the whole body). A wider range of increased and decreased acceptability was seen in 28 °C, as compared with 26 °C. As was shown in Fig. 8, the  This was also confirmed in the present study, as the increased power use did not necessarily increase the thermal acceptability of most subjects.

Open-ended Feedback
In the full-and post-questionnaires, subjects had the opportunity to provide open-ended feedback to the experiment and the PECS itself. These responses were collected to obtain a qualitative overview of the possible factors that could have affected the subjects' responses to the questionnaire items. This could also provide insight in in the future development of PECS.
Many positive responses were seen in terms of the heating performance of PECS, especially during the 18 °C room temperature setting. However, some subjects still reported cold hands, feet, and back. As the current PECS prototype provides heating mostly to the thighs, the distribution of heat could be further optimized.
In contrary to the heating function, the lack of cooling power was often reported in higher temperature settings of 26 and 28 °C. Many of the subjects did not feel any additional cooling with the cooling function (from the Peltier element). Some also pointed out the limited coverage of the ATD, i.e., the air was supplied to a small area of the face. Feedback was also given to the limited flexibility of the ATD, which may have lowered the cooling effect even more. In terms of the PECS control, overall positive feedback were given. However, some pointed out that while the user interface was intuitive and easy to control, the placement of the control hub made it difficult to control. This may have had an influence on how frequently the occupants interacted with the PECS.

Conclusion
Human subject experiments were conducted with a newly developed PECS prototype to obtain user feedback. Responses from the questionnaire showed that the heating function was able to provide sufficient heating to the occupants. The heating effect was most clearly seen among female subjects at the room temperature setting of 18 °C. Further improvements may be possible by targeting other body parts such as the hands, feet, and back.
A previous thermal manikin study with the same PECS prototype revealed the lack of cooling effect from the ATD, and that the active cooling function was not able to increase the cooling effect despite the increased power use. The issue with the cooling performance was confirmed in the present human subject experiment. As the power use of the fan was lower compared to other functions, it may be more beneficial to supply air to a wider range of the body, rather than implementing an active air-cooling function. Open-ended feedback from the subjects also suggest that the control interface and flexibility of the ATD could also be a limiting factor for the proactive use of PECS.

Acknowledgements
This study was financially supported by Mitsubishi Electric Corporation and by the International Centre for Indoor Environment and Energy (ICIEE), Technical University of Denmark (DTU). The authors would like to thank Nico Henrik Ziersen for his help with preparing the experimental setup.