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A field study on thermal environment and occupant local thermal sensation in offices with cooling ceiling in Zhuhai, China
Energy and Buildings 102 (2015) 277–283
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A field study on thermal environment and occupant local thermal sensation in offices with cooling ceiling in Zhuhai, China Yingdong He, Nianping Li ∗ , Qing Huang College of Civil Engineering, Hunan University, Changsha 410082, Hunan, China
a r t i c l e
i n f o
Article history: Received 6 February 2015 Received in revised form 19 April 2015 Accepted 16 May 2015 Available online 9 June 2015 Keywords: Thermal environment Thermal comfort Local thermal sensation Radiant cooling Cooling ceiling
a b s t r a c t This study aims to investigate thermal environment and occupant local thermal sensation in offices with cooling ceiling. A field survey combining environment measurements and questionnaires was conducted in offices in Zhuhai, China. The obtained results indicated that compared with experimental conditions, occupants in real crowded offices experienced warmer environment because the mean radiant temperature (MRT) was higher than the air temperature due to the shelter effect of indoor furniture etc. MRT also showed higher correlations with both overall thermal sensation (OTS) and overall thermal comfort (OTC) than air temperature because a great part of heat transferred through radiation. Air temperature showed linear correlation with the ceiling temperature when cooling ceiling could fully handle the cooling load, but supplementary air cooling was also necessary for radiant cooling systems in real offices. Moreover, local thermal sensation (LTS) of different body parts showed great differences due to many factors such as clothes and adaptive behaviors, and it significantly influenced both OTS and OTC. Particularly, head, hand, leg and foot had considerable effects on OTS while head and leg showed influences on OTC. In addition, this paper also validated against some important results of previous studies and provided references for future work. © 2015 Elsevier B.V. All rights reserved.
1. Introduction It has been widely recognized that thermal comfort, which attracts many researchers’ attention, is associated with people’s health and productivity. And thermal comfort is a function of air temperature, mean radiant temperature (MRT), humidity, air velocity, clothing insulation, metabolic rate etc. Compared with conventional air cooling system which may lead to complaints of discomfort even in comfort zone [1], radiant cooling systems can provide more chances for energy-saving and offer better thermal environment for long-time-staying occupants due to less noticeable air movement and more uniform distribution of air temperature [1–5]. Some researchers stated that subjects could experience cooler sensation in radiant cooling environment [3,6]. In recent years, various researches have been conducted to evaluate the thermal environment and thermal comfort in rooms with radiant cooling systems. Kim et al. [3] analyzed the performance of a radiant panel system combining manikin and computational fluid dynamics (CFD) simulation. They reported lower MRT and operative temperature
∗ Corresponding author. Tel.: +86 13467504696. E-mail address: [email protected] (N. Li). http://dx.doi.org/10.1016/j.enbuild.2015.05.058 0378-7788/© 2015 Elsevier B.V. All rights reserved.
in radiant cooling environment in comparison with convective cooling environment, and considered it a new approach to energysaving. Catalina et al. [7] simulated the thermal environment of a room equipped with cooling ceiling and found that black globe temperature was lower than air temperature by 0.6–0.9 ◦ C and the vertical temperature difference was less than 1 ◦ C. Meanwhile, air velocity was below 0.15 m/s. Similarly, Corgnati et al. [8] compared simulation results of all-air system and radiant cooling system. A conclusion was drawn that radiant cooling system had advantages of reducing both vertical temperature difference and draft risk. Chiang and Wang [9] reported that cooling ceiling was unable to cool the entire office room without supplementary air cooling. Zhao and Liu [10] studied the application of radiant cooling floor in a large space building exposed to direct solar radiation. The results indicated radiant floor had increased cooling capacity with highintensity solar radiation and was superior to all-air system in terms of energy efficiency and subjective comfort. Besides CFD simulations above, many researchers focused on experimental studies for obtaining more reliable results. Imanari et al. [1] carried out a series of experiments in a meeting room with radiant ceiling panels and compared various characteristics of radiant cooling system with those of all-air system. The radiant cooling system gained more votes for comfort from the subjects. Kitagawa et al. [11] claimed that in radiant cooling environment low air
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velocity produced a tendency of feeling one-scale cooler than still air condition with same SET* . In 1997, Nagano and Mochida [12] undertook 28 experiments in a room installed with cooling ceiling under various conditions with 5 supine subjects and reported a uniform temperature distribution. They also found the skin temperatures of head, hand, instep and shin significantly affected overall thermal sensation (OTS). Another laboratory study conducted by Sakoi et al. [13] in radiant cooling environment showed a positive correlation between comfort level and heat dissipation of head region, while comfort and that of foot region shared a reverse relationship. Further, Schellen et al. [14] found a correlation between local thermal sensation (LTS) and OTS. Later, Schellen et al. [15] explored the influences of different cooling principles on both subjective sensation and physiological responses. However, no significant correlation was found between skin temperature and LTS or OTS, which was conformed with literature [13]. The results of simulations and experiments in previous studies would have been more persuasive if they had been validated by field surveys. However, few field investigations were performed in real radiant cooling environment because of the limited application of radiant cooling system. In 2005, Tian et al. [16] undertook a field survey in a building at the University of Calgary in Canada, measuring environment parameters and collecting subjective responses from 51 occupants in summer. The actual percentage of dissatisfaction was lower than the calculated PPD (predicted percentage of dissatisfied), and the reason was due to the more uniform distribution of temperature and less draft feeling. From the literature research, few studies related to thermal environment and thermal comfort in real radiant cooling rooms were conducted, especially no field survey was conducted in offices with cooling ceiling. Available literatures provide very little information to validate the results of former studies. In addition, some results in previous studies seem contradictory. For example, Catalina et al. [7] declared comfortable thermal environment could be achieved with cooling ceiling, while the results of Chiang and Wang [9] revealed supplementary air cooling was necessary for radiant cooling system. Moreover, some researches in other environment indicated OTS was correlated robustly with LTS [17,18] but no study demonstrated whether similar correlation existed in real radiant cooling environment. Thus more detailed field investigations are in desperate need. This paper aims to study thermal environment and local thermal sensation of occupants in real office rooms with radiant cooling systems. A field investigation was carried out in office rooms with cooling ceilings in two buildings in Zhuhai, China, to measure thermal environment parameters such as air temperature, relative humidity, air velocity, black globe temperature and ceiling temperature, as well as to collect questionnaires from the occupants. And multiple liner regression method was used to analyze the influences of LTS on both OTS and overall thermal comfort (OTC). In addition, this paper confirmed some important results of previous studies and provided reference for future work.
Table 1 Measured parameters and instruments. Parameters
Instruments
Accuracy
Air temperature Relative humidity Air velocity Black globe temperature Ceiling temperature
VELOCICALC-8347 air velocity meter (multiple meter)
±0.3 ◦ C ±3% ±3% ±0.2 ◦ C
TR-102 black globe temperature meter SK-FHRL1810W temperature & heat current meter
±0.2 ◦ C
pipes attached to the ceilings. Because of insulation layers above the ceilings, the system only provided cooling in one direction. Moreover, additional ventilation systems were applied to act as a supply of fresh air for occupants. Thermal environmental parameters, including air temperature, relative humidity, air velocity, black globe temperature and ceiling temperature were measured during the survey, and instruments used are listed in Table 1. MRT was calculated based on the value of black globe temperature, according to ISO standard 7726 [19]. Occupant questionnaires mainly consisted of two parts: (1) personal data (age, gender, inhabit time, clothing, activity etc.), (2) sensation and comfort (OTS, OTC, LTS, humidity sensation and air movement sensation). A discrete seven-point scale for rating thermal sensations offered by ASHRAE Standard 55 [20] was adopted. Besides OTS, occupants were asked to vote for thermal sensation of their body parts, including head, chest, back, abdomen, arm, hand, leg and foot. A six-point scale was also used for rating OTC. Detailed scales are shown in Table 2. Besides, some other factors were also taken into account, e.g. positions of occupants, emotion, decoration and feelings of air quality, acoustics and lighting. Some of them were included in questionnaires or obtained by inquiry, the others were recorded by ourselves when occupants were finishing the questionnaires. In addition, smoking and eating were banned in the office rooms, thus these two factors were not involved in this study. The radiant cooling system was started up 1 h earlier before officers began their work. And field survey was conducted after all officers had been working for 1 h. During the survey, the values of all parameters measured by instruments showed very little fluctuation, thus the thermal environment was considered to be in stable condition. Clothing insulation was calculated according to ASHRAE Standard [20] and the choices of clothes in questionnaires made by respondents. In order to properly determine the metabolic rate, the same method as Tian et al. was used [16], viz. respondents were asked about four periods within that hour (1) 60–30 min, (2) 30–20 min, (3) 20–10 min and (4) 10–0 min prior to finishing questionnaires. The final metabolic rate of a certain person was the average value of these four periods. This paper adopted a same multiple liner regression method as mentioned in [21] to analyze the influence of LTS on both OTS and OTC. According to literature [21], this method can exclude factors which produce multicollinearity or have no significant effects. Additionally, absolute value of local sensation vote was used instead of its true value when analyzing OTC.
2. Method 3. Results and discussion The field survey was conducted in office rooms in two office buildings in August, 2013. Both buildings were located in Zhuhai, Guangdong province, south of China, where the summer climate is characterized by high temperature and high humidity. In summer, the average outdoor temperature is higher than 28 ◦ C and the average relative humidity reaches approximately 80%. The survey was only performed in ordinary employee areas, which covered areas of more than 500 m2 . According to the maintenance staff of air conditioning system, all rooms were installed with cooling ceilings, providing cooling by circulating cool water at 15–22 ◦ C in copper
3.1. Thermal environment Table 3 shows the data of thermal environmental parameters in office areas. The average value of air temperature was in comfort range, while the temperature difference was almost 8 ◦ C. The relative humidity reached a high level beyond the comfort upper limit [20], and MRT was a little higher than air temperature. As for air movement, the average velocity was very low, and it was satisfactory according to ASHRAE Standard [20]. The
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Table 2 Scales for measuring subjective response to thermal environment. Scale point
Thermal sensation
Thermal comfort
Humidity
Air movement
3 2 1 0 −1 −2 −3
Hot Warm Slightly warm Neutral Slightly cool Cool Cold
Unendurable Uncomfortable Slightly uncomfortable Slightly comfortable Comfortable Very comfortable
Very dry Dry Slightly dry Neutral Slightly humid Humid Very humid
Very high High Slightly high Neutral Slightly low Low Very low
Table 3 Parameters of thermal environment. Parameter
Average
Air temperature (◦ C) Relative humidity (%) MRT (◦ C) Air velocity (m/s) Ceiling temperature (◦ C)
25.0 61.4 25.5 0.01 22.1
S.D. 2.3 9.48 2.12 0.01 3.22
Maximum 29.6 79.9 30.1 0.03 28
Minimum 21.9 46.5 22.1 0.00 17.8
S.D.: standard deviation.
ceiling, providing cooling for indoor environment and occupants, were approximately lower than the air temperature by 3 ◦ C in average. As shown in Table 3, the average value of MRT was higher than that of air temperature by 0.5 ◦ C, different from previous studies [7,12,15], where air temperature was usually higher than MRT. This contradiction can be traced back to the differences between practical and experimental conditions. Fig. 1 shows the experimental chamber in the study of Schellen et al. [15]. It was small but not crowded, and each time only one person stayed inside. However, this is not always the case in practical environment. Fig. 2 shows
an example of offices investigated in this study. It is clear that there were plenty of furniture, equipment and occupants thus the offices were much more crowded. In real environment, black globe temperature meters were usually surrounded by furniture, equipment and occupants, thus part of radiant heat came from their surfaces with higher temperature rather than cooling ceilings. Moreover, MRT could be lower than air temperature because the entire enclosure was cooled by radiation [5]. While in practical offices, not only the enclosure but also other surfaces of occupants etc. might not be able to be cooled effectively due to shelter effect of furniture etc. In another word, compared to experimental conditions, occupants surrounded by furniture etc. in crowded offices experienced a warmer environment because of higher MRT. Similarly, the research of Zhao et al. [10] demonstrated the performance of radiant floor cooling was significantly influenced by indoor furniture shelter which decreased cooling capacity and view factors of radiant floor to other surfaces. Maybe the shelter of furniture etc. won’t cause much effect in convective cooling environment, but its influence can be great when radiation is involved because it directly obstructs radiation heat transfer. However, in most previous studies and present design standards, this issue and its potential effects were not fully considered. From Fig. 3, the following linear relationship between ceiling temperature (Tc ) and air temperature (Ta ) was identified: Ta = 0.624Tc + 11.633,
R2 = 0.640.
(1)
The above equation presents the linear correlation based on values of ceiling temperature and air temperature from all measuring points in stable condition, even though some other parameters, e.g. MRT and relative humidity, were not uniform. A very similar tendency was also observed by Nagano and Mochida in experiment [12], while they didn’t analyze the linear correlation. As reported by other researchers [2,3], cooling panels can handle both convective and radiant cooling loads. Since there were only radiant cooling ceilings to provide cooling inside, the air temperature was mainly determined by ceiling temperature. As shown in Fig. 3, for most measuring points, Tc ranged from 17.8–24 ◦ C and the corresponding Ta was lower than 26 ◦ C while a few were beyond 29 ◦ C. These
Fig. 1. Experimental chamber in the study of Schellen et al. [15].
Fig. 2. An example of office rooms investigated in this paper.
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Fig. 3. Relationship between ceiling temperature and air temperature.
Fig. 4. Relationship between occupants’ metabolic rate and clothing insulation.
Table 4 Statistics of age, metabolic rate and clothing insulation of occupants. Items
Average
S.D.
Maximum
Minimum
Age (year) Metabolic rate (met) Clothing insulation (clo)
26.1 1.20 0.58
3.20 0.18 0.10
45 1.60 0.78
22 1.00 0.36
S.D.: standard deviation.
results indicates the cooling capacity of radiant ceilings was insufficient to cool down the entire office areas. In another word, the cooling ceilings were unable to handle the whole indoor cooling load, and this is the main reason that some points exerted great diversity and didn’t match the linear relationship when ceiling temperature was low. It also confirmed the simulation results of Chiang et al. [9], and a similar phenomenon was also reported in literature [3]. To sum up, it is concluded that when cooling ceiling can fully cool down the whole room, a linear relationship between ceiling temperature and air temperature exists, while if cooling load is too high to handle such a correlation will be invalid. In addition, it’s obvious that supplementary air cooling is necessary for cooling ceiling in real environment. During the investigation, condensation was detected on part surfaces of ceilings due to the high humidity. It can hamper occupants’ work. The problems of condensation and insufficient cooling capacity mentioned above can be solved by ameliorating the ventilation systems, and it is alleged that such a combined system possesses superiority of energy saving for rooms with large load [5]. 3.2. Characteristics of subjects In total, 76 subjects participated in this survey, including 30 males and 46 females. All of them were ordinary employees and inhabited in Zhuhai or other places with similar climate for more than 1 year. The detailed information about age, metabolic rate and clothing of respondents is listed in Table 4. Most of occupants in this study were under 30 years old because the survey was only undertaken in ordinary employee areas and most of them were young people. Most occupants had sedentary activity when started to conduct questionnaires, and the results of metabolic rate were close to those reported by Tian et al. (1.18 ± 0.15 met) [16]. The average value of clothing insulation reached about 0.6 clo, in which an extra 0.1 clo from office chairs was included for sedentary occupants. There were no strict rules for employees’ clothing, thus occupants could choose or adjust their clothes at will.
Fig. 5. Subjective vote distribution of overall thermal sensation, overall thermal comfort, humidity and air movement.
Fig. 4 presents the relationship of occupants’ metabolic rate and clothing insulation. Occupants’ metabolic rate correlated well with the clothing insulation (r = −0.572). People wore less when at higher activity level which probably made them feel hot. Therefore, it is obvious that adaptive behavior, namely adjusting clothes, was adopted by the occupants. However, clothes were not only decided by thermal sensation, but also influenced by many other factors like personal taste, thus there was still diversity of clothing at the same activity level. 3.3. Overall sensation 3.3.1. Overall thermal sensation and comfort Fig. 5 summarizes the vote distribution of OTS, OTC, humidity and air movement. As shown in Fig. 5, 7.9% occupants voted for “cold”, 22.4% for “slightly cold”, and 28.9% for “neural”, while 27.6% felt “slightly warm”, 10.5% felt “warm” and 2.6% felt “hot”. There were 6.6% and 50.0% people feeling “comfortable” and “slightly comfortable”, respectively, while 38.2% and 5.3% of people feeling “slightly uncomfortable” and “uncomfortable”, respectively. Nobody voted for “unendurable” or “very comfortable” in this survey. Moreover, 56.6% respondents still felt comfortable even though 28.9% had neutral sensations. This result indicates that people could stay in comfortable condition with non-neutral sensation.
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Fig. 8. Relationship between overall thermal sensation and overall thermal comfort. Fig. 6. Relationship between air temperature and overall thermal sensation.
Fig. 9. Relationship between relative humidity and humidity sensation. Fig. 7. Relationship between MRT and overall thermal sensation.
Figs. 6 and 7 show that OTS was significantly correlated with air temperature and MRT, and the correlation coefficients (r) were 0.735 and 0.760, respectively. Similarly, OTC also correlated well with air temperature and MRT (r were 0.533 and 0.571, respectively). It is clear that MRT, combining effects of radiant heat transfer and air movement, had higher correlations with both OTS and OTC than air temperature. This means that radiant heat transfer exerted greater influence on both OTS and OTC because the air velocity was pretty low and its effect on MRT was negligible. A possible explanation for this was represented by Kim and Kato [3] where total heat from human model removed by radiation significantly increased in radiant cooling environment confronted with all-air case. In addition, radiation takes up a large portion of total heat transfer when cooling ceiling is used with low air velocity, even more than 60% [2]. In conclusion, MRT expressed higher correlations with OTS and OTC arising from the fact that larger portion of heat transfer was achieved through radiation. Similarly, this point was highlighted by Halawa et al. [22] that current standards should fully consider the effect of MRT on thermal comfort. Fig. 8 presents the correlation between OTS and OTC (r = 0.476). Warm sensation intensified uncomfortable feeling, and comfortable feeling existed on cool side. It seems people will still feel comfortable when it is very cold, yet in this survey, only a small portion of people voted for cool side and no one feel cold (see Fig. 5). Therefore, the correlation may be invalid in cold environment and this needs further studies.
3.3.2. Sensations of humidity and air movement With regard to humidity sensation (Fig. 5), only 1.3% and 14.5% respondents felt “humid” and “slightly humid”, respectively; 50.0% voted for “Neutral”, and 34.2% voted for dry side even if the relative humidity was pretty high (Table 3). Fig. 9 presents the relationship between the relative humidity and humidity sensation. Occupants felt more humid with higher relative humidity (r = 0.721). However, this sensation was not very precisely because humidity is hard to be predicted just by feeling. It seems that the occupants wouldn’t feel very humid even when relative humidity was beyond 60%, which was out of the comfort zone specified by ASHRAE Standard [20]. This can be explained by the climate adaptation, because ASHRAE Standard [20] was established on the basis of experimental data obtained in temperate environment from only Americans and Europeans but without subjects from other regions with different climates. The results also indicate that there are different comfort ranges of humidity for people in different regions. As shown in Fig. 5, 56.6% occupants thought the air velocity was very low, 19.7% and 5.3% voted for “low” and “slightly low”, respectively; 17.1% occupants felt neutral while only 1 person considered it “slightly high”. The vote distribution was corresponding with very low air velocity (Table 3). Kitagawa et al. [11] stated that draught could reduce warm sensation when temperature stay unchanged. In this study, most people were far away from draft risk, hence thermal comfort could be ameliorated through improving air velocity, which would be benefit to reduce the energy consumption of HVAC system as well.
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Y. He et al. / Energy and Buildings 102 (2015) 277–283 Table 5 Regressions correlating LTS with both OTS and OTC. Coefficients OTS Head Chest Back Abdomen Arm Hand Leg Foot Sig. of model Adj. R2 *
Fig. 10. Local thermal sensations of different body parts.
Ci LTSi + C0
0.499*
0.290* 0.000* 0.431
P < 0.05.
OTC =
Fig. 10 summarizes the distribution of participant local sensations. Most people hold neutral sensations for chest, arm and abdomen. The percentages of occupants having neutral feelings for head, leg and foot were comparatively lower, while those for hand and back were the lowest. It is showed that head and back were evaluated as the warmest body parts. Meanwhile the coolest body parts went to hand and foot. The differences of local sensation between body parts were caused by many reasons. Human trunk is mainly influenced by the core temperature rather than the ambient environment [23], thus resulting in a stable sensation for abdomen and chest. As for the back, warm sensations probably resulted from the office chairs which reduced the heat dissipation of back. Due to higher skin temperature and better adaptation for cold environment [24,25], head region is sensitive to warm condition rather than cool. According to literature [13], reducing heat dissipation of head region will significantly bring down the comfort level of subjects. In the investigation, most of occupants wore trousers and this could partly explain the less obvious cool sensation for leg regions. Another possible reason for this was the shelter effect of furniture etc. as mentioned in Section 3.1, which could create warmer local environment for legs. However, due to the society acceptance and modesty, adjusting clothes may not help much for reducing the warm sensation of leg, especially for females. In addition, during the survey, it was observed that many people put on long-sleeve clothes when feeling cold. This behavior also contributed to decreasing cold sensation for arm. Thus, in this study, cool sensation for arm was not obvious (less than 10% people thought it was slightly cool), distinguishing from the results of Schellen et al. [15] where participants were asked to wear short-sleeve clothes (see Fig. 1). With regard to hand, adjusting clothes may not bring any impacts because it’s always exposed, thus many occupants (nearly 30%) voted for cold side. In conclusion, the differences of LTS resulted from various factors in real offices. Compared with the results of previous study conducted in aircraft cabin [21], there are still some differences in influence of body parts on OTS which can be attributed to different styles of air conditioning and indoor environment. Thus more investigations are worth to be conducted for LTS in radiant cooling environment in future. Two models obtained through multiple linear regression were expressed below:
0.404* 0.184* 0.180* 0.000* 0.822
OTC
and
3.4. Local thermal sensation
OTS =
0.390*
(2)
Ci LTSi + C0
(3)
where Ci is the coefficient of body part i, C0 is a constant value, LTSi is the LTS of body part i and |LTSi | is the absolute value of LTSi . The data listed in Table 5 presents the influences of thermal sensation of body parts on both OTS and OTC. Head and hand exerted highest influences on OTS, while those of leg and foot were much lower, and for the rest body parts their effects were negligible. When it comes to OTC, only head and leg showed significant influences. Furthermore, local sensation of a certain body part exerted different magnitude influences on OTS and OTC. For example, a difference of 0.1 existed between the coefficients of head for OTS and OTC. In this survey, thermal environment parameters, activity levels and clothing were various. A combined effect of those factors would cover the influence of LTS on overall comfort. Besides, thermal comfort is a function of psychological, physiological and many other factors [26–28], thus it was difficult to be precisely predicted using one or several factors. We consider these main reasons why the value of determination coefficient (R2 ) was not too high when analyzing the influence of LTS on OTC (see Table 5). Complexity in real offices can also explain the deviations in Figs. 6–8. However linear relationship is much more reliable for LTS and OTS (R2 = 0.822) because they are the same type of psychological factor (see Table 5), and similar robust correlations exist in various environment [17,18,21]. In addition, compared with laboratory environment, there are more possible factors which may affect comfort, like working pressure, emotions and even indoor decoration. However, in this survey, many factors investigated, e.g. emotion, decoration, air quality, acoustics and lighting, showed insignificant influence on subjective comfort. And some factors, like circulating water temperature and wall temperature, were not under our control or without access to deep study due to limitations in real environment. For instance, measuring surface temperature of walls or floors was always rejected by occupants for it meant removing furniture or workstations nearby and bring inconvenience to staff. Given this situation, we believe that it would be better to study those potential factors in experimental conditions in future work. Further, our survey was only undertaken in ordinary employee areas and most of respondents were young people, thus the effect of age was not taken into account here. And all offices were located in Zhuhai, China. For other regions, the results might be absolutely different. In addition, available literatures of field study on radiant cooling were very limited for reference, and more field studies are in urgent need.
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4. Conclusions This paper presented the results of a field study on thermal environment and occupant local thermal sensation in offices with cooling ceilings in Zhuhai, China. Thermal environment parameters, e.g. air temperature, relative humidity, black globe temperature, air velocity and ceiling temperature, were measured, meanwhile a questionnaire survey with 76 occupants participating in was conducted in summer. The important conclusions of this study were summarized as follow: (1) Compared with experimental conditions, occupants in real crowded offices experienced warmer environment because MRT was higher than air temperature, which was resulting from the shelter effect of indoor furniture etc. (2) A linear relationship existed between ceiling temperature and air temperature, while this correlation would be weaken if cooling ceiling couldn’t handle all cooling load. Thus supplementary air cooling was necessary for cooling ceiling system. (3) Compared with air temperature, MRT exerted higher correlations with both OTS and OTC because larger portion of heat was transferred through radiation in radiant cooling environment. (4) In real radiant cooling environment, LTS showed great differences for different body parts and the reason was attributed many factors, such as clothes and adaptive behaviors. (5) According to the regression models, LTS of different body parts affected OTS and OTC in different extents. Head and hand showed greatest influences on OTS, followed by leg and foot, and those of the rest body parts were negligible. In terms of OTC, only head and leg revealed significant effects. Due to few available literatures for field study on radiant cooling, conclusions drawn in this paper might not be sufficient to validate all results of previous studies. In addition our survey didn’t deeply study all potential factors that affect thermal sensation and comfort. Therefore, more field investigations on radiant cooling environment and thermal comfort should be conducted in future. Acknowledgements This research was supported by the National Natural Science Foundation of China (Project No. 51178169). The authors would also like to express our gratitude to all respondents for their cooperation. References [1] T. Imanari, T. Omori, K. Bogaki, Thermal comfort and energy consumption of the radiant ceiling panel system: comparison with the conventional all-air system, Energy Build. 30 (2) (1999) 167–175. [2] H.E. Feustel, C. Stetiu, Hydronic radiant cooling-preliminary assessment, Energy Build. 22 (3) (1995) 193–205. [3] T. Kim, S. Kato, S. Murakami, J.-W. Rho, Study on indoor thermal environment of office space controlled by cooling panel system using field measurement and the numerical simulation, Build. Environ. 40 (3) (2005) 301–310. [4] R.A. Memon, S. Chirarattananon, P. Vangtook, Thermal comfort assessment and application of radiant cooling: a case study, Build. Environ. 43 (7) (2008) 1185–1196. [5] A. Novoselac, J. Srebric, A critical review on the performance and design of combined cooled ceiling and displacement ventilation systems, Energy Build. 34 (5) (2002) 497–509.
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[6] M.F. Brunk, Cooling ceilings—an opportunity to reduce energy costs by way of radiant cooling, in: Proceedings of the 1993 Annual Meeting of the American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., June 27–June 30, 1993, ASHRAE, Denver, CO, 1993, pp. 479–487. [7] T. Catalina, J. Virgone, F. Kuznik, Evaluation of thermal comfort using combined CFD and experimentation study in a test room equipped with a cooling ceiling, Build. Environ. 44 (8) (2009) 1740–1750. [8] S.P. Corgnati, M. Perino, G.V. Fracastoro, P.V. Nielsen, Experimental and numerical analysis of air and radiant cooling systems in offices, Build Environ. 44 (4) (2009) 801–806. [9] W.H. Chiang, C.Y. Wang, J.S. Huang, Evaluation of cooling ceiling and mechanical ventilation systems on thermal comfort using CFD study in an office for subtropical region, Build. Environ. 48 (2012) 113–127. [10] K. Zhao, X.H. Liu, Y. Jiang, Application of radiant floor cooling in a large open space building with high-intensity solar radiation, Energy Build. 66 (2013) 246–257. [11] K. Kitagawa, N. Komoda, H. Hayano, S.-i. Tanabe, Effect of humidity and small air movement on thermal comfort under a radiant cooling ceiling by subjective experiments, Energy Build. 30 (2) (1999) 185–193. [12] K. Nagano, T. Mochida, Experiments on thermal environmental design of ceiling radiant cooling for supine human subjects, Build. Environ. 39 (3) (2004) 267–275. [13] T. Sakoi, K. Tsuzuki, S. Kato, R. Ooka, D. Song, S. Zhu, Thermal comfort, skin temperature distribution, and sensible heat loss distribution in the sitting posture in various asymmetric radiant fields, Build. Environ. 42 (12) (2007) 3984–3999. [14] L. Schellen, M.G.L.C. Loomans, M.H. de Wit, B.W. Olesen, W.D.v.M. Lichtenbelt, The influence of local effects on thermal sensation under non-uniform environmental conditions—gender differences in thermophysiology, thermal comfort and productivity during convective and radiant cooling, Physiol. Behav. 107 (2) (2012) 252–261. [15] L. Schellen, M.G.L.C. Loomans, M.H. de Wit, B.W. Olesen, W.D.V.M. Lichtenbelt, Effects of different cooling principles on thermal sensation and physiological responses, Energy Build. 62 (2013) 116–125. [16] Z. Tian, J.A. Love, A field study of occupant thermal comfort and thermal environments with radiant slab cooling, Build. Environ. 43 (10) (2008) 1658–1670. [17] E. Arens, H. Zhang, C. Huizenga, Partial- and whole-body thermal sensation and comfort—Part 1: Uniform environmental conditions, J. Therm. Biol. 31 (1–2) (2006) 53–59. [18] E. Arens, H. Zhang, C. Huizenga, Partial- and whole-body thermal sensation and comfort—Part II: Non-uniform environmental conditions, J. Therm. Biol. 31 (1–2) (2006) 60–66. [19] E. International Standard Organization, ISO 7726: Ergonomics of the Thermal Environment—Instruments for Measuring Physical Quantities, International Standard Organization, Geneva, Switzerland, 2002. [20] ASHRAE, Thermal environmental conditions for human occupancy, in: Standard 55-2004, American Society of Heating, Refrigerating and Air-Conditioning Engineering (ASHRAE), Atlanta, GA, 2004. [21] W.L. Cui, Q. Ouyang, Y.X. Zhu, Field study of thermal environment spatial distribution and passenger local thermal comfort in aircraft cabin, Build. Environ. 80 (2014) 213–220. [22] E. Halawa, J. van Hoof, V. Soebarto, The impacts of the thermal radiation field on thermal comfort, energy consumption and control—a critical overview, Renewable Sustainable Energy Rev. 37 (0) (2014) 907–918. [23] G.-S. Song, J.-H. Lim, T.-K. Ahn, Air conditioner operation behaviour based on students’ skin temperature in a classroom, Appl. Ergon. 43 (1) (2012) 211–216. [24] M. Cabanac, M. White, Heat loss from the upper airways and selective brain cooling in humans, Ann. N.Y. Acad. Sci. 813 (1997) 613–616. [25] C.F. Bulcao, S.M. Frank, S.N. Raja, K.M. Tran, D.S. Goldstein, Relative contribution of core and skin temperatures to thermal comfort in humans, J. Therm. Biol. 25 (1–2) (2000) 147–150. [26] S. Karjalainen, Thermal comfort and use of thermostats in Finnish homes and offices, Build. Environ. 44 (6) (2009) 1237–1245. [27] J. Choi, A. Aziz, V. Loftness, Investigation on the impacts of different genders and ages on satisfaction with thermal environments in office buildings, Build. Environ. 45 (6) (2010) 1529–1535. [28] M. Indraganti, K.D. Rao, Effect of age, gender, economic group and tenure on thermal comfort: A field study in residential buildings in hot and dry climate with seasonal variations, Energy Build. 42 (3) (2010) 273–281.
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A field study on thermal environment and occupant local thermal sensation in offices with cooling ceiling in Zhuhai, China Yingdong He, Nianping Li ∗ , Qing Huang College of Civil Engineering, Hunan University, Changsha 410082, Hunan, China
a r t i c l e
i n f o
Article history: Received 6 February 2015 Received in revised form 19 April 2015 Accepted 16 May 2015 Available online 9 June 2015 Keywords: Thermal environment Thermal comfort Local thermal sensation Radiant cooling Cooling ceiling
a b s t r a c t This study aims to investigate thermal environment and occupant local thermal sensation in offices with cooling ceiling. A field survey combining environment measurements and questionnaires was conducted in offices in Zhuhai, China. The obtained results indicated that compared with experimental conditions, occupants in real crowded offices experienced warmer environment because the mean radiant temperature (MRT) was higher than the air temperature due to the shelter effect of indoor furniture etc. MRT also showed higher correlations with both overall thermal sensation (OTS) and overall thermal comfort (OTC) than air temperature because a great part of heat transferred through radiation. Air temperature showed linear correlation with the ceiling temperature when cooling ceiling could fully handle the cooling load, but supplementary air cooling was also necessary for radiant cooling systems in real offices. Moreover, local thermal sensation (LTS) of different body parts showed great differences due to many factors such as clothes and adaptive behaviors, and it significantly influenced both OTS and OTC. Particularly, head, hand, leg and foot had considerable effects on OTS while head and leg showed influences on OTC. In addition, this paper also validated against some important results of previous studies and provided references for future work. © 2015 Elsevier B.V. All rights reserved.
1. Introduction It has been widely recognized that thermal comfort, which attracts many researchers’ attention, is associated with people’s health and productivity. And thermal comfort is a function of air temperature, mean radiant temperature (MRT), humidity, air velocity, clothing insulation, metabolic rate etc. Compared with conventional air cooling system which may lead to complaints of discomfort even in comfort zone [1], radiant cooling systems can provide more chances for energy-saving and offer better thermal environment for long-time-staying occupants due to less noticeable air movement and more uniform distribution of air temperature [1–5]. Some researchers stated that subjects could experience cooler sensation in radiant cooling environment [3,6]. In recent years, various researches have been conducted to evaluate the thermal environment and thermal comfort in rooms with radiant cooling systems. Kim et al. [3] analyzed the performance of a radiant panel system combining manikin and computational fluid dynamics (CFD) simulation. They reported lower MRT and operative temperature
∗ Corresponding author. Tel.: +86 13467504696. E-mail address: [email protected] (N. Li). http://dx.doi.org/10.1016/j.enbuild.2015.05.058 0378-7788/© 2015 Elsevier B.V. All rights reserved.
in radiant cooling environment in comparison with convective cooling environment, and considered it a new approach to energysaving. Catalina et al. [7] simulated the thermal environment of a room equipped with cooling ceiling and found that black globe temperature was lower than air temperature by 0.6–0.9 ◦ C and the vertical temperature difference was less than 1 ◦ C. Meanwhile, air velocity was below 0.15 m/s. Similarly, Corgnati et al. [8] compared simulation results of all-air system and radiant cooling system. A conclusion was drawn that radiant cooling system had advantages of reducing both vertical temperature difference and draft risk. Chiang and Wang [9] reported that cooling ceiling was unable to cool the entire office room without supplementary air cooling. Zhao and Liu [10] studied the application of radiant cooling floor in a large space building exposed to direct solar radiation. The results indicated radiant floor had increased cooling capacity with highintensity solar radiation and was superior to all-air system in terms of energy efficiency and subjective comfort. Besides CFD simulations above, many researchers focused on experimental studies for obtaining more reliable results. Imanari et al. [1] carried out a series of experiments in a meeting room with radiant ceiling panels and compared various characteristics of radiant cooling system with those of all-air system. The radiant cooling system gained more votes for comfort from the subjects. Kitagawa et al. [11] claimed that in radiant cooling environment low air
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velocity produced a tendency of feeling one-scale cooler than still air condition with same SET* . In 1997, Nagano and Mochida [12] undertook 28 experiments in a room installed with cooling ceiling under various conditions with 5 supine subjects and reported a uniform temperature distribution. They also found the skin temperatures of head, hand, instep and shin significantly affected overall thermal sensation (OTS). Another laboratory study conducted by Sakoi et al. [13] in radiant cooling environment showed a positive correlation between comfort level and heat dissipation of head region, while comfort and that of foot region shared a reverse relationship. Further, Schellen et al. [14] found a correlation between local thermal sensation (LTS) and OTS. Later, Schellen et al. [15] explored the influences of different cooling principles on both subjective sensation and physiological responses. However, no significant correlation was found between skin temperature and LTS or OTS, which was conformed with literature [13]. The results of simulations and experiments in previous studies would have been more persuasive if they had been validated by field surveys. However, few field investigations were performed in real radiant cooling environment because of the limited application of radiant cooling system. In 2005, Tian et al. [16] undertook a field survey in a building at the University of Calgary in Canada, measuring environment parameters and collecting subjective responses from 51 occupants in summer. The actual percentage of dissatisfaction was lower than the calculated PPD (predicted percentage of dissatisfied), and the reason was due to the more uniform distribution of temperature and less draft feeling. From the literature research, few studies related to thermal environment and thermal comfort in real radiant cooling rooms were conducted, especially no field survey was conducted in offices with cooling ceiling. Available literatures provide very little information to validate the results of former studies. In addition, some results in previous studies seem contradictory. For example, Catalina et al. [7] declared comfortable thermal environment could be achieved with cooling ceiling, while the results of Chiang and Wang [9] revealed supplementary air cooling was necessary for radiant cooling system. Moreover, some researches in other environment indicated OTS was correlated robustly with LTS [17,18] but no study demonstrated whether similar correlation existed in real radiant cooling environment. Thus more detailed field investigations are in desperate need. This paper aims to study thermal environment and local thermal sensation of occupants in real office rooms with radiant cooling systems. A field investigation was carried out in office rooms with cooling ceilings in two buildings in Zhuhai, China, to measure thermal environment parameters such as air temperature, relative humidity, air velocity, black globe temperature and ceiling temperature, as well as to collect questionnaires from the occupants. And multiple liner regression method was used to analyze the influences of LTS on both OTS and overall thermal comfort (OTC). In addition, this paper confirmed some important results of previous studies and provided reference for future work.
Table 1 Measured parameters and instruments. Parameters
Instruments
Accuracy
Air temperature Relative humidity Air velocity Black globe temperature Ceiling temperature
VELOCICALC-8347 air velocity meter (multiple meter)
±0.3 ◦ C ±3% ±3% ±0.2 ◦ C
TR-102 black globe temperature meter SK-FHRL1810W temperature & heat current meter
±0.2 ◦ C
pipes attached to the ceilings. Because of insulation layers above the ceilings, the system only provided cooling in one direction. Moreover, additional ventilation systems were applied to act as a supply of fresh air for occupants. Thermal environmental parameters, including air temperature, relative humidity, air velocity, black globe temperature and ceiling temperature were measured during the survey, and instruments used are listed in Table 1. MRT was calculated based on the value of black globe temperature, according to ISO standard 7726 [19]. Occupant questionnaires mainly consisted of two parts: (1) personal data (age, gender, inhabit time, clothing, activity etc.), (2) sensation and comfort (OTS, OTC, LTS, humidity sensation and air movement sensation). A discrete seven-point scale for rating thermal sensations offered by ASHRAE Standard 55 [20] was adopted. Besides OTS, occupants were asked to vote for thermal sensation of their body parts, including head, chest, back, abdomen, arm, hand, leg and foot. A six-point scale was also used for rating OTC. Detailed scales are shown in Table 2. Besides, some other factors were also taken into account, e.g. positions of occupants, emotion, decoration and feelings of air quality, acoustics and lighting. Some of them were included in questionnaires or obtained by inquiry, the others were recorded by ourselves when occupants were finishing the questionnaires. In addition, smoking and eating were banned in the office rooms, thus these two factors were not involved in this study. The radiant cooling system was started up 1 h earlier before officers began their work. And field survey was conducted after all officers had been working for 1 h. During the survey, the values of all parameters measured by instruments showed very little fluctuation, thus the thermal environment was considered to be in stable condition. Clothing insulation was calculated according to ASHRAE Standard [20] and the choices of clothes in questionnaires made by respondents. In order to properly determine the metabolic rate, the same method as Tian et al. was used [16], viz. respondents were asked about four periods within that hour (1) 60–30 min, (2) 30–20 min, (3) 20–10 min and (4) 10–0 min prior to finishing questionnaires. The final metabolic rate of a certain person was the average value of these four periods. This paper adopted a same multiple liner regression method as mentioned in [21] to analyze the influence of LTS on both OTS and OTC. According to literature [21], this method can exclude factors which produce multicollinearity or have no significant effects. Additionally, absolute value of local sensation vote was used instead of its true value when analyzing OTC.
2. Method 3. Results and discussion The field survey was conducted in office rooms in two office buildings in August, 2013. Both buildings were located in Zhuhai, Guangdong province, south of China, where the summer climate is characterized by high temperature and high humidity. In summer, the average outdoor temperature is higher than 28 ◦ C and the average relative humidity reaches approximately 80%. The survey was only performed in ordinary employee areas, which covered areas of more than 500 m2 . According to the maintenance staff of air conditioning system, all rooms were installed with cooling ceilings, providing cooling by circulating cool water at 15–22 ◦ C in copper
3.1. Thermal environment Table 3 shows the data of thermal environmental parameters in office areas. The average value of air temperature was in comfort range, while the temperature difference was almost 8 ◦ C. The relative humidity reached a high level beyond the comfort upper limit [20], and MRT was a little higher than air temperature. As for air movement, the average velocity was very low, and it was satisfactory according to ASHRAE Standard [20]. The
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Table 2 Scales for measuring subjective response to thermal environment. Scale point
Thermal sensation
Thermal comfort
Humidity
Air movement
3 2 1 0 −1 −2 −3
Hot Warm Slightly warm Neutral Slightly cool Cool Cold
Unendurable Uncomfortable Slightly uncomfortable Slightly comfortable Comfortable Very comfortable
Very dry Dry Slightly dry Neutral Slightly humid Humid Very humid
Very high High Slightly high Neutral Slightly low Low Very low
Table 3 Parameters of thermal environment. Parameter
Average
Air temperature (◦ C) Relative humidity (%) MRT (◦ C) Air velocity (m/s) Ceiling temperature (◦ C)
25.0 61.4 25.5 0.01 22.1
S.D. 2.3 9.48 2.12 0.01 3.22
Maximum 29.6 79.9 30.1 0.03 28
Minimum 21.9 46.5 22.1 0.00 17.8
S.D.: standard deviation.
ceiling, providing cooling for indoor environment and occupants, were approximately lower than the air temperature by 3 ◦ C in average. As shown in Table 3, the average value of MRT was higher than that of air temperature by 0.5 ◦ C, different from previous studies [7,12,15], where air temperature was usually higher than MRT. This contradiction can be traced back to the differences between practical and experimental conditions. Fig. 1 shows the experimental chamber in the study of Schellen et al. [15]. It was small but not crowded, and each time only one person stayed inside. However, this is not always the case in practical environment. Fig. 2 shows
an example of offices investigated in this study. It is clear that there were plenty of furniture, equipment and occupants thus the offices were much more crowded. In real environment, black globe temperature meters were usually surrounded by furniture, equipment and occupants, thus part of radiant heat came from their surfaces with higher temperature rather than cooling ceilings. Moreover, MRT could be lower than air temperature because the entire enclosure was cooled by radiation [5]. While in practical offices, not only the enclosure but also other surfaces of occupants etc. might not be able to be cooled effectively due to shelter effect of furniture etc. In another word, compared to experimental conditions, occupants surrounded by furniture etc. in crowded offices experienced a warmer environment because of higher MRT. Similarly, the research of Zhao et al. [10] demonstrated the performance of radiant floor cooling was significantly influenced by indoor furniture shelter which decreased cooling capacity and view factors of radiant floor to other surfaces. Maybe the shelter of furniture etc. won’t cause much effect in convective cooling environment, but its influence can be great when radiation is involved because it directly obstructs radiation heat transfer. However, in most previous studies and present design standards, this issue and its potential effects were not fully considered. From Fig. 3, the following linear relationship between ceiling temperature (Tc ) and air temperature (Ta ) was identified: Ta = 0.624Tc + 11.633,
R2 = 0.640.
(1)
The above equation presents the linear correlation based on values of ceiling temperature and air temperature from all measuring points in stable condition, even though some other parameters, e.g. MRT and relative humidity, were not uniform. A very similar tendency was also observed by Nagano and Mochida in experiment [12], while they didn’t analyze the linear correlation. As reported by other researchers [2,3], cooling panels can handle both convective and radiant cooling loads. Since there were only radiant cooling ceilings to provide cooling inside, the air temperature was mainly determined by ceiling temperature. As shown in Fig. 3, for most measuring points, Tc ranged from 17.8–24 ◦ C and the corresponding Ta was lower than 26 ◦ C while a few were beyond 29 ◦ C. These
Fig. 1. Experimental chamber in the study of Schellen et al. [15].
Fig. 2. An example of office rooms investigated in this paper.
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Fig. 3. Relationship between ceiling temperature and air temperature.
Fig. 4. Relationship between occupants’ metabolic rate and clothing insulation.
Table 4 Statistics of age, metabolic rate and clothing insulation of occupants. Items
Average
S.D.
Maximum
Minimum
Age (year) Metabolic rate (met) Clothing insulation (clo)
26.1 1.20 0.58
3.20 0.18 0.10
45 1.60 0.78
22 1.00 0.36
S.D.: standard deviation.
results indicates the cooling capacity of radiant ceilings was insufficient to cool down the entire office areas. In another word, the cooling ceilings were unable to handle the whole indoor cooling load, and this is the main reason that some points exerted great diversity and didn’t match the linear relationship when ceiling temperature was low. It also confirmed the simulation results of Chiang et al. [9], and a similar phenomenon was also reported in literature [3]. To sum up, it is concluded that when cooling ceiling can fully cool down the whole room, a linear relationship between ceiling temperature and air temperature exists, while if cooling load is too high to handle such a correlation will be invalid. In addition, it’s obvious that supplementary air cooling is necessary for cooling ceiling in real environment. During the investigation, condensation was detected on part surfaces of ceilings due to the high humidity. It can hamper occupants’ work. The problems of condensation and insufficient cooling capacity mentioned above can be solved by ameliorating the ventilation systems, and it is alleged that such a combined system possesses superiority of energy saving for rooms with large load [5]. 3.2. Characteristics of subjects In total, 76 subjects participated in this survey, including 30 males and 46 females. All of them were ordinary employees and inhabited in Zhuhai or other places with similar climate for more than 1 year. The detailed information about age, metabolic rate and clothing of respondents is listed in Table 4. Most of occupants in this study were under 30 years old because the survey was only undertaken in ordinary employee areas and most of them were young people. Most occupants had sedentary activity when started to conduct questionnaires, and the results of metabolic rate were close to those reported by Tian et al. (1.18 ± 0.15 met) [16]. The average value of clothing insulation reached about 0.6 clo, in which an extra 0.1 clo from office chairs was included for sedentary occupants. There were no strict rules for employees’ clothing, thus occupants could choose or adjust their clothes at will.
Fig. 5. Subjective vote distribution of overall thermal sensation, overall thermal comfort, humidity and air movement.
Fig. 4 presents the relationship of occupants’ metabolic rate and clothing insulation. Occupants’ metabolic rate correlated well with the clothing insulation (r = −0.572). People wore less when at higher activity level which probably made them feel hot. Therefore, it is obvious that adaptive behavior, namely adjusting clothes, was adopted by the occupants. However, clothes were not only decided by thermal sensation, but also influenced by many other factors like personal taste, thus there was still diversity of clothing at the same activity level. 3.3. Overall sensation 3.3.1. Overall thermal sensation and comfort Fig. 5 summarizes the vote distribution of OTS, OTC, humidity and air movement. As shown in Fig. 5, 7.9% occupants voted for “cold”, 22.4% for “slightly cold”, and 28.9% for “neural”, while 27.6% felt “slightly warm”, 10.5% felt “warm” and 2.6% felt “hot”. There were 6.6% and 50.0% people feeling “comfortable” and “slightly comfortable”, respectively, while 38.2% and 5.3% of people feeling “slightly uncomfortable” and “uncomfortable”, respectively. Nobody voted for “unendurable” or “very comfortable” in this survey. Moreover, 56.6% respondents still felt comfortable even though 28.9% had neutral sensations. This result indicates that people could stay in comfortable condition with non-neutral sensation.
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Fig. 8. Relationship between overall thermal sensation and overall thermal comfort. Fig. 6. Relationship between air temperature and overall thermal sensation.
Fig. 9. Relationship between relative humidity and humidity sensation. Fig. 7. Relationship between MRT and overall thermal sensation.
Figs. 6 and 7 show that OTS was significantly correlated with air temperature and MRT, and the correlation coefficients (r) were 0.735 and 0.760, respectively. Similarly, OTC also correlated well with air temperature and MRT (r were 0.533 and 0.571, respectively). It is clear that MRT, combining effects of radiant heat transfer and air movement, had higher correlations with both OTS and OTC than air temperature. This means that radiant heat transfer exerted greater influence on both OTS and OTC because the air velocity was pretty low and its effect on MRT was negligible. A possible explanation for this was represented by Kim and Kato [3] where total heat from human model removed by radiation significantly increased in radiant cooling environment confronted with all-air case. In addition, radiation takes up a large portion of total heat transfer when cooling ceiling is used with low air velocity, even more than 60% [2]. In conclusion, MRT expressed higher correlations with OTS and OTC arising from the fact that larger portion of heat transfer was achieved through radiation. Similarly, this point was highlighted by Halawa et al. [22] that current standards should fully consider the effect of MRT on thermal comfort. Fig. 8 presents the correlation between OTS and OTC (r = 0.476). Warm sensation intensified uncomfortable feeling, and comfortable feeling existed on cool side. It seems people will still feel comfortable when it is very cold, yet in this survey, only a small portion of people voted for cool side and no one feel cold (see Fig. 5). Therefore, the correlation may be invalid in cold environment and this needs further studies.
3.3.2. Sensations of humidity and air movement With regard to humidity sensation (Fig. 5), only 1.3% and 14.5% respondents felt “humid” and “slightly humid”, respectively; 50.0% voted for “Neutral”, and 34.2% voted for dry side even if the relative humidity was pretty high (Table 3). Fig. 9 presents the relationship between the relative humidity and humidity sensation. Occupants felt more humid with higher relative humidity (r = 0.721). However, this sensation was not very precisely because humidity is hard to be predicted just by feeling. It seems that the occupants wouldn’t feel very humid even when relative humidity was beyond 60%, which was out of the comfort zone specified by ASHRAE Standard [20]. This can be explained by the climate adaptation, because ASHRAE Standard [20] was established on the basis of experimental data obtained in temperate environment from only Americans and Europeans but without subjects from other regions with different climates. The results also indicate that there are different comfort ranges of humidity for people in different regions. As shown in Fig. 5, 56.6% occupants thought the air velocity was very low, 19.7% and 5.3% voted for “low” and “slightly low”, respectively; 17.1% occupants felt neutral while only 1 person considered it “slightly high”. The vote distribution was corresponding with very low air velocity (Table 3). Kitagawa et al. [11] stated that draught could reduce warm sensation when temperature stay unchanged. In this study, most people were far away from draft risk, hence thermal comfort could be ameliorated through improving air velocity, which would be benefit to reduce the energy consumption of HVAC system as well.
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Y. He et al. / Energy and Buildings 102 (2015) 277–283 Table 5 Regressions correlating LTS with both OTS and OTC. Coefficients OTS Head Chest Back Abdomen Arm Hand Leg Foot Sig. of model Adj. R2 *
Fig. 10. Local thermal sensations of different body parts.
Ci LTSi + C0
0.499*
0.290* 0.000* 0.431
P < 0.05.
OTC =
Fig. 10 summarizes the distribution of participant local sensations. Most people hold neutral sensations for chest, arm and abdomen. The percentages of occupants having neutral feelings for head, leg and foot were comparatively lower, while those for hand and back were the lowest. It is showed that head and back were evaluated as the warmest body parts. Meanwhile the coolest body parts went to hand and foot. The differences of local sensation between body parts were caused by many reasons. Human trunk is mainly influenced by the core temperature rather than the ambient environment [23], thus resulting in a stable sensation for abdomen and chest. As for the back, warm sensations probably resulted from the office chairs which reduced the heat dissipation of back. Due to higher skin temperature and better adaptation for cold environment [24,25], head region is sensitive to warm condition rather than cool. According to literature [13], reducing heat dissipation of head region will significantly bring down the comfort level of subjects. In the investigation, most of occupants wore trousers and this could partly explain the less obvious cool sensation for leg regions. Another possible reason for this was the shelter effect of furniture etc. as mentioned in Section 3.1, which could create warmer local environment for legs. However, due to the society acceptance and modesty, adjusting clothes may not help much for reducing the warm sensation of leg, especially for females. In addition, during the survey, it was observed that many people put on long-sleeve clothes when feeling cold. This behavior also contributed to decreasing cold sensation for arm. Thus, in this study, cool sensation for arm was not obvious (less than 10% people thought it was slightly cool), distinguishing from the results of Schellen et al. [15] where participants were asked to wear short-sleeve clothes (see Fig. 1). With regard to hand, adjusting clothes may not bring any impacts because it’s always exposed, thus many occupants (nearly 30%) voted for cold side. In conclusion, the differences of LTS resulted from various factors in real offices. Compared with the results of previous study conducted in aircraft cabin [21], there are still some differences in influence of body parts on OTS which can be attributed to different styles of air conditioning and indoor environment. Thus more investigations are worth to be conducted for LTS in radiant cooling environment in future. Two models obtained through multiple linear regression were expressed below:
0.404* 0.184* 0.180* 0.000* 0.822
OTC
and
3.4. Local thermal sensation
OTS =
0.390*
(2)
Ci LTSi + C0
(3)
where Ci is the coefficient of body part i, C0 is a constant value, LTSi is the LTS of body part i and |LTSi | is the absolute value of LTSi . The data listed in Table 5 presents the influences of thermal sensation of body parts on both OTS and OTC. Head and hand exerted highest influences on OTS, while those of leg and foot were much lower, and for the rest body parts their effects were negligible. When it comes to OTC, only head and leg showed significant influences. Furthermore, local sensation of a certain body part exerted different magnitude influences on OTS and OTC. For example, a difference of 0.1 existed between the coefficients of head for OTS and OTC. In this survey, thermal environment parameters, activity levels and clothing were various. A combined effect of those factors would cover the influence of LTS on overall comfort. Besides, thermal comfort is a function of psychological, physiological and many other factors [26–28], thus it was difficult to be precisely predicted using one or several factors. We consider these main reasons why the value of determination coefficient (R2 ) was not too high when analyzing the influence of LTS on OTC (see Table 5). Complexity in real offices can also explain the deviations in Figs. 6–8. However linear relationship is much more reliable for LTS and OTS (R2 = 0.822) because they are the same type of psychological factor (see Table 5), and similar robust correlations exist in various environment [17,18,21]. In addition, compared with laboratory environment, there are more possible factors which may affect comfort, like working pressure, emotions and even indoor decoration. However, in this survey, many factors investigated, e.g. emotion, decoration, air quality, acoustics and lighting, showed insignificant influence on subjective comfort. And some factors, like circulating water temperature and wall temperature, were not under our control or without access to deep study due to limitations in real environment. For instance, measuring surface temperature of walls or floors was always rejected by occupants for it meant removing furniture or workstations nearby and bring inconvenience to staff. Given this situation, we believe that it would be better to study those potential factors in experimental conditions in future work. Further, our survey was only undertaken in ordinary employee areas and most of respondents were young people, thus the effect of age was not taken into account here. And all offices were located in Zhuhai, China. For other regions, the results might be absolutely different. In addition, available literatures of field study on radiant cooling were very limited for reference, and more field studies are in urgent need.
Y. He et al. / Energy and Buildings 102 (2015) 277–283
4. Conclusions This paper presented the results of a field study on thermal environment and occupant local thermal sensation in offices with cooling ceilings in Zhuhai, China. Thermal environment parameters, e.g. air temperature, relative humidity, black globe temperature, air velocity and ceiling temperature, were measured, meanwhile a questionnaire survey with 76 occupants participating in was conducted in summer. The important conclusions of this study were summarized as follow: (1) Compared with experimental conditions, occupants in real crowded offices experienced warmer environment because MRT was higher than air temperature, which was resulting from the shelter effect of indoor furniture etc. (2) A linear relationship existed between ceiling temperature and air temperature, while this correlation would be weaken if cooling ceiling couldn’t handle all cooling load. Thus supplementary air cooling was necessary for cooling ceiling system. (3) Compared with air temperature, MRT exerted higher correlations with both OTS and OTC because larger portion of heat was transferred through radiation in radiant cooling environment. (4) In real radiant cooling environment, LTS showed great differences for different body parts and the reason was attributed many factors, such as clothes and adaptive behaviors. (5) According to the regression models, LTS of different body parts affected OTS and OTC in different extents. Head and hand showed greatest influences on OTS, followed by leg and foot, and those of the rest body parts were negligible. In terms of OTC, only head and leg revealed significant effects. Due to few available literatures for field study on radiant cooling, conclusions drawn in this paper might not be sufficient to validate all results of previous studies. In addition our survey didn’t deeply study all potential factors that affect thermal sensation and comfort. Therefore, more field investigations on radiant cooling environment and thermal comfort should be conducted in future. Acknowledgements This research was supported by the National Natural Science Foundation of China (Project No. 51178169). The authors would also like to express our gratitude to all respondents for their cooperation. References [1] T. Imanari, T. Omori, K. Bogaki, Thermal comfort and energy consumption of the radiant ceiling panel system: comparison with the conventional all-air system, Energy Build. 30 (2) (1999) 167–175. [2] H.E. Feustel, C. Stetiu, Hydronic radiant cooling-preliminary assessment, Energy Build. 22 (3) (1995) 193–205. [3] T. Kim, S. Kato, S. Murakami, J.-W. Rho, Study on indoor thermal environment of office space controlled by cooling panel system using field measurement and the numerical simulation, Build. Environ. 40 (3) (2005) 301–310. [4] R.A. Memon, S. Chirarattananon, P. Vangtook, Thermal comfort assessment and application of radiant cooling: a case study, Build. Environ. 43 (7) (2008) 1185–1196. [5] A. Novoselac, J. Srebric, A critical review on the performance and design of combined cooled ceiling and displacement ventilation systems, Energy Build. 34 (5) (2002) 497–509.
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