Effect of natural ventilation on indoor air quality and thermal comfort in dormitory during winter

Accepted Manuscript Effect of natural ventilation on indoor air quality and thermal comfort in dormitory during winter Zhangping Lei, Chuanping Liu, Li Wang, Na Li PII:

S0360-1323(17)30405-5

DOI:

10.1016/j.buildenv.2017.08.051

Reference:

BAE 5076

To appear in:

Building and Environment

Received Date: 24 May 2017 Revised Date:

12 August 2017

Accepted Date: 26 August 2017

Please cite this article as: Lei Z, Liu C, Wang L, Li N, Effect of natural ventilation on indoor air quality and thermal comfort in dormitory during winter, Building and Environment (2017), doi: 10.1016/ j.buildenv.2017.08.051. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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close window 2 0.055m

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Indoor temperature T/

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Air quality, including indoor O2 concentrations (a), CO2 concentrations (b), temperature (c), and relative humidity (d), versus sleep time under different window

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opening areas. With increasing the window opening areas, the indoor O2 concentration gradually increased, the CO2 concentration decreased, and the

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temperature drop met the national heating standard. Approximately 0.055 m2 is the appropriate natural ventilation area (corresponding to 0.036 m3/s natural ventilation

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rate) for dormitories with 10~12.5 m3 per capita space during Beijing’s winter.

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Effect of natural ventilation on indoor air quality and thermal comfort in dormitory during winter

a

School of Energy and Environment Engineering, University of Science and

Technology Beijing, Beijing 100083, China

Beijing Engineering Research Center of Energy Saving and Environmental

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b

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Zhangping Leia, Chuanping Liua, Li Wanga,b, Na Lia

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Protection, Beijing 100083, China

Abstract： ：

This study is to prove that natural ventilation is necessary for students’ sleeping

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even in Beijing’s -9°C winter and explain that opening windows at proper width will decrease the indoor temperature slightly, but obviously reduce the CO2 concentration. The indoor air quality in dormitories was monitored experimentally at different

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ventilation areas (from 0.011 m2 to 0.11 m2), and the thermal comfort and mental state

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of the students after seven hours of sleep were analyzed through a questionnaire survey. Results showed that indoor air quality generally improved with the increase in natural ventilation area, whereas the thermal comfort level gradually declined. Approximately 0.055 m2 is the appropriate natural ventilation area (corresponding to 0.036 m3/s natural ventilation rate) for dormitories with 10~12.5 m3 per capita space during Beijing’s winter. In addition, a model was proposed for window opening to predict indoor air quality and temperature in different natural ventilation areas during 1

ACCEPTED MANUSCRIPT winter nights. With increasing the student number in dormitory, it is necessary to increasing the windows open area, and once the per capita space is less than 6.5 m3, the indoor air quality cannot meet the comfortableness only by windows opening.

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Keywords: natural ventilation; indoor air quality; thermal comfort

1 Introduction

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The dormitories are essential resting places during night sleep for the university

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students. The indoor air quality of the dormitories can directly affect the sleep quality and the next-day performance [1-3]. And the indoor air quality is mainly affected by ventilation rate [4-8]. Poor ventilation can lead to high indoor carbon dioxide (CO2) concentration, which is the key assessment criterion of indoor air quality. The

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standards (62-1989) set by the American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE) [9] recommend a stable concentration level

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of CO2 at 700 ppm above the outdoor levels, and 1,000-1,200 ppm is usually regarded as the acceptable concentration for indoor CO2 concentration. Other indoor

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environmental parameters including indoor oxygen (O2) concentrations, thermal comfort, and relative humidity also play an important role in indoor sleeping environments [10-13].

Kim [14] found that people were exposed to a variety of problems when asleep, related to their sleep environment such as too low or high air temperatures, or relative humidity and high CO2 concentrations. Similar results [15] were found in high-rise residences in Hong Kong that most people felt stuffy because of poor IAQ, and that 2

ACCEPTED MANUSCRIPT approximately 60% of the respondents had experience of waking up during sleep because they felt either cold or warm even if air-conditioners were turned on in their bedrooms. Sekhar’s study in Singapore [16] showed that the overnight build-up of

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CO2 level in a bedroom served by a split system air-conditioning unit can be as high as 2900 ppm, due to lack of ventilation. Although there was no clear evidence to substantiate that sleeping duration decreased with increasing levels of CO2, the

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findings [17,18] did suggest that high levels of CO2 may hinder the duration of sleep.

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The university dormitories in Beijing usually contain four or six-person rooms. The room area ranges from 15 m2 to 30 m2. Students tend to close their room windows to sleep in a warm environment during cold winter. However, closing of windows directly leads to low ventilation, high CO2 concentration. Natural ventilation

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of manual window-openings is proven to be an effective low-cost solution to achieve fresh indoor air and improve air quality [19-22]. However, the indoor temperature decreases after opening windows; therefore, an “indoor air quality–thermal comfort”

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dilemma exists [23,24]. So the optimization of window-openings need to be offered to

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conduct the natural ventilation [25]. Assessment of indoor air thermal comfort relies not only on objective indicators

but also on subjective rating [26,27]. A questionnaire survey can effectively determine the general thermal comfort level [28]. Moreover, predicted mean vote (PMV) is the most widely used thermal comfort index today. ASHRAE [29] adopts the thermal sensation of the PMV index to define the appropriate comfort range. The questionnaire in this study was designed by referring to this method. 3

ACCEPTED MANUSCRIPT This study was conducted in a cold climate to achieve fresh air and improve indoor air quality, which enhance sleep quality through the natural ventilation provided by manual window-openings. The main parameters monitored were indoor

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CO2 and O2 concentrations, temperature, and relative humidity. Thermal comfort levels were determined through a questionnaire survey. The relationship between natural ventilation level and indoor air quality as well as the subjective feelings of the

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occupants were determined. To deal with the “indoor air quality–thermal comfort”

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dilemma, a quality factor, QF, was also proposed to evaluate the general performance of indoor air. 2 Experiment

Experiments were conducted in January, which is usually the coldest month in

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Beijing. The average outdoor temperature in January from 10:00 pm to 7:00 am is about −9 °C. Four dormitories of University of Science and Technology Beijing including four and six-student rooms are used to test (Fig. 1), and the test students

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were 20 to 23 years old. The test dormitories are heated by a finned radiator

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connected with central water heating system, and the natural ventilation is solely based on opening windows manually. The size of the dormitory is listed in Table 1. The aluminum alloy windows of the tested dormitory have a sliding push–pull structure (vertical slide window in Ref. [30]) with 1.1 m height (h) and 1.1 m width. The wind speed and direction outside the window are relatively stable throughout the year because of the layout of the surrounding buildings. Students were in their classrooms before 10:00 pm, and all of the windows of the 4

ACCEPTED MANUSCRIPT dormitory were closed completely. At 10:00 pm, all of the students came back from their classrooms and went to bed after 15 min. The experiments then began. The windows were closed completely or opened manually to a certain width, and kept at

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the fixed position during the whole experimental night. Indoor O2 and CO2 concentrations, temperature, and relative humidity were measured at 1.7 m height over the floor in the room center and recorded once every hour at night. The

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experiment instruments includes OX-100A O2 concentration digital detector (range

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from 0 to 50%, accuracy 0.1%), 7515 IAQ detector to measure CO2 (range from 0 to 5500ppm, accuracy ±50ppm), smart and visual environment detector to measure temperature (range from −20℃ to 60℃, accuracy ±0.5℃) and relative humidity (range from 10% to 95%, accuracy ±3%).

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In this study, the windows opening widths of window 1, 3, 5, 7, and 10 cm were chosen, which corresponded to areas (A) of 0.011, 0.033, 0.055, 0.077, and 0.11 m2, respectively. The outdoor CO2 concentration and temperature was measured by the

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same instrument simultaneously. The outdoor condition changes during the

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experimental period, which brings a certain trouble for the experiments. To eliminate the outdoor environment effects, this study chose the test data when the outdoor environment was similar, that is to say, the outdoor CO2 concentration was around 700 ppm and average outdoor temperature was about -9℃. So the occupants can be regarded as the main factor to affect the indoor environment. The test data are also influenced by the test student’s sleeping condition, although all the experiments are done at similar outdoor environment. To ensure accuracy, the test period for each 5

ACCEPTED MANUSCRIPT room was three nights, and the average value was calculated and used for three-time measurements. The questionnaire in this study was designed based on previous cases in different

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climates [31-34] and filled in by every student in the test dormitory after seven hours of sleep. To eliminate the potential bias related to the students being aware of the experiments, the students completed the questionnaires without knowing which width

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of the window-opening during night. The questionnaire had two key items, namely,

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PMV thermal comfort level and mental state level after sleep. Similar to that in the study of Fanger [35,36], thermal comfort was divided into seven levels. Mental state had five levels in the questionnaire. The detailed classification index is shown in Table 2.

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3 Results and discussion 3.1 Objective air quality

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The variation in indoor O2 and CO2 concentrations, temperature, and relative humidity versus sleep time while the windows were completely closed during winter

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nights is shown in Fig. 2. Fig. 2(a) shows plots of the variation in indoor O2 concentration in the four and six-person rooms versus sleep time. The lowest acceptable indoor O2 concentration plotted in the dotted line is 19.5%. The O2 concentration in the four-person room was generally higher than that in the six-person room. The O2 concentration decreased to 19.5% after 3.3 and 6.5 hours of sleep for the six- and four-person rooms, respectively. Fig. 2(b) shows plots of the variation in indoor CO2 concentration versus sleep time. The highest acceptable indoor CO2 6

ACCEPTED MANUSCRIPT concentration plotted in the dashed-dotted line is 1,000 ppm. The initial indoor CO2 concentration in the six-male room was over 1,895 ppm before sleep. The indoor CO2 concentration increased gradually in the next seven hours and reached 5,150 ppm,

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which is over five times that of the highest acceptable indoor CO2 concentration (1,000 ppm). The values in the other rooms were also over the standard. The indoor temperature in Fig. 1(c) and the relative humidity in Fig. 1(d) slightly increased

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during sleep time.

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Fig. 2 reveals the necessity to open the windows during night sleep, especially in student dormitories. To determine the relation between the window opening area and the corresponding indoor air quality, experiments were conducted in the six-person room, and the experimental data are plotted in Fig. 3. As shown in Fig. 3, a high

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indoor O2 concentration corresponded to the need for a large window opening area. All the O2 levels in the area ranging from 0.011 m2 to 0.11 m2 are acceptable, except for that when the window was closed. When the window opening area was 0.011 m2,

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the O2 and CO2 concentrations presented a subtle change during sleep time. When the

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opening area was 0.033 m2, the final CO2 concentration was still higher than 1,000 ppm. However, when the area increased to 0.055 m2, the corresponding O2 level increased from 20% to 20.6%, the corresponding CO2 level decreased to 1,057 ppm only after four hours of sleep, and the relative humidity met the national criterion (not lower than 30%). The temperature difference over sleep time in Fig. 4 is less than 3 °C with the opening area ranging from 0.011 m2 to 0.055 m2 for the dormitory. Thus, a significant indoor temperature drop did not occur in these cases. However, when the 7

ACCEPTED MANUSCRIPT area reached 0.077 and 0.11 m2, the indoor temperature began with 23 °C and dropped to less than 20 °C after four hours. Furthermore, the corresponding humidity declined to less than 30% in parallel. Therefore, the windows should not be opened by 0.077

0.055 m2, better indoor air quality was achieved. 3.2 Subjective questionnaire results

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and 0.11 m2. Compared with the other conditions, when the window opening area was

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The questionnaire survey results on thermal comfort level versus window

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opening area for the four- and six-person rooms are shown in Fig. 5(a). The PMV level began with 0 and remained over −1.0 when the window opening area ranged from 0 to 0.055 m2. With a PMV comfort range of approximately ±1.0, the students felt comfortable with their thermal environment [37]. However, the PMV level

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decreased to less than −1 when the opening area increased to 0.077 and 0.01 m2. The variation in mental state level versus window opening area is shown in Fig. 5(b).

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Mental state level gradually increased with the opening area. In this case, a conflict emerged between thermal comfort and mental state level, which reflected the air

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quality. To compare the performance under different window opening areas, a quality factor (QF) was applied to evaluate the total performance. The specific expression is as follows:

QF=|Lpmv·S|,

(1)

where Lpmv is the thermal comfort level and S is the mental state level. The calculated results of QF according to the data in Figs. 5(a) and 5(b) are

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ACCEPTED MANUSCRIPT shown in Fig. 6. The peak occurs when the window opening area is approximately 0.055 m2, and the peak indicates that this condition is the suitable choice in comparison with the other conditions. These results are consistent with the conclusion

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in Section 3.1 of this study. 3.3 Mathematical model

To quantify the indoor air quality under different window opening areas, this

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study proposed the following model to predict indoor air temperature as well as O2

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and CO2 concentrations. The windows of the student dormitory had single-side ventilation. According to Refs. [38,39], the volume flow rate (V) is computed as Expression (2). =

ℎ∆ / ,

(2)

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where Cd is the discharge coefficient that is generally taken as 0.6; A is the open area of the window; h is the window height; g is gravitational acceleration; ∆T is the is the

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temperature difference between indoor and outdoor air, ∆T=Tin−Tout; average temperature,

=(Tin−Tout)/2; and Tin and Tout are the indoor and outdoor air

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temperatures, respectively.

The heat loss of a room includes two parts: the heat loss induced by natural

ventilation and heat dissipation from the room’s wall. Q1=cρV∆T+UAwall∆T,

(3)

where c and ρ are the air specific heat capacity and density, respectively; Awall is the wall area of the room; and U is the average heat transfer coefficient of the wall. With natural ventilation, the flow would make O2 and CO2 exchange between indoor and 9

ACCEPTED MANUSCRIPT outdoor air. VO2,1=ζV(CO2,out−CO2,in),

(4)

VCO2,1=ζV(CCO2,out−CCO2,in),

(5)

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where ζ is a scale factor and CO2,in, CO2,out, CCO2,in, and CCO2,out are the O2 and CO2 concentrations of indoor and outdoor air, respectively. The heat generated by sleeping

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students owing to metabolism (Q2) can be expressed as Q2=q2,1Abody+Q2,2,

(6)

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where Abody is the surface area of the naked body; q1 is the heat generated per naked body owing to metabolism, q2,1=40W/m2 for sleeping; and Q2,2 is the heat source from the heating pipe of the municipal system.

(7)

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Q2,2=Afloorq2,2,

where Afloor is the area of the room’s floor and q2,2 is the heating strength. According to the building heating standard of Beijing, q2,2=50W/m2. The surface area of the

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naked body is estimated by student height and weight [40]. Abody=0.202nM0.425H0.725,

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(8)

where n is the number of students, M is the weight of students, and H is the height of students. Based on ASTM D 6245-98[41], the oxygen consumption rate of humans (VO2,2) is calculated as

VO2,2=−165.6nAbodyMs/(0.23RQ+0.77),

(9)

where Ms is the metabolic rate of unit surface area and RQ is the breath coefficient.

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ACCEPTED MANUSCRIPT VCO2,2=RQVO2,2

(10)

According to heat and mass balance, the following expressions can be obtained. (11)

dCO2,in/dt=(VO2,2-VO2,1)/Vroom,

(12)

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dTin/dt=(Q2-Q1)/Croom,

dCCO2,in/dt=(VCO2,2-VCO2,1)/Vroom,

(13)

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where Croom is the thermal capacity and Vroom is the volume of a room.

Before the experiments, students were out of their dormitory, and all the windows

dormitory was in heat equilibrium. Q2,2=UAwall(T0−Tout)

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were closed completely during daytime (V=0). The temperature was stable, and the

(14)

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With the initial indoor temperature (Tin,0), O2 concentration (CO2,0), and CO2 concentration (CCO2,0), the changes after the students slept can be computed according to Expressions (2) to (14). Fig. 7 shows the indoor O2 and CO2 concentrations and

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temperature of the four- and six-male rooms while the windows were closed. The

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computed results are consistent with the experimental results. As the students slept, CO2 decreased and CCO2 and Tin increased. The change rate of the six-male room was larger than that of the four-male room. Fig. 8 shows the indoor O2 and CO2 concentrations and the temperature of the six-male room with different window opening areas. Opening the windows increased the indoor O2 and CO2 concentrations but decreased temperature. When the window opening area was 0.055 m2, the indoor CO2 concentration was less than 1,000 ppm, and the O2 concentration was larger than 11

ACCEPTED MANUSCRIPT 19.5%. Meanwhile, the temperature drop was less than 3 °C. Here, it is considered as the satisfactory performance of the indoor air that CO2 concentration was not lager than 1,000 ppm and temperature was not small than 20 °C.

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In order to meet these requirements, we estimate the acceptable windows opening area with different per capita space, as Fig.9. With increasing the student number in dormitory, it is necessary to increasing the windows open area (natural ventilation), on

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the other hand, the acceptable windows opening area rang becomes small. When the

only by windows opening. 4 Conclusions

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student number is over eight, the indoor air quality cannot meet the comfortableness

This study was conducted in Beijing’s winter to analyze the effect of natural

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ventilation in student dormitories on indoor air quality and thermal comfort. A typical mathematical model was proposed to predict indoor O2 and CO2 concentrations and

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temperature difference ∆T after natural ventilation. The main conclusions of the present work are summarized below, which is suitable for the winter of Beijing or

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similar climate city (outdoor temperature was over -9℃). When the windows opening area was smaller than 0.033 m2 for the experimental

student dormitory (10~12.5 m3 per capita space), corresponding to less than 0.021 m3/s natural ventilation rate, the general CO2 level was still higher than the highest acceptable indoor CO2 concentration (1,000 ppm). When the area exceeded 0.077 m2, corresponding to larger than 0.050 m3/s natural ventilation rate, the indoor temperature decreased to less than 20 °C just after four hours. Furthermore, the 12

ACCEPTED MANUSCRIPT corresponding relative humidity declined to less than 30%, which did not meet the national criterion (not lower than 30%). However, when the window opening area was 0.055 m2, corresponding to 0.036 m3/s natural ventilation rate, all the four monitored

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parameters met the requirements. Meanwhile, the questionnaire results were consistent with the experimental data, and it also indicated that 0.055 m2 is the appropriate natural ventilation area for 10~12.5 m3 per capita space of student

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dormitories in Beijing’s winter.

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According to the mathematical model, with increasing the student number in dormitory, it is necessary to increasing the windows open area (natural ventilation). Once the per capita space is less than 6.5 m3 (student number is over eight in each dormitory), the indoor air quality cannot meet the comfortableness only by windows

Acknowledgements

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opening.

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The authors are grateful for the support of the National Natural Science Foundation of China (Grant No.5105006).

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ACCEPTED MANUSCRIPT [41] ASTM D 6245-98. Guide for using Carbon Dioxide Concentration to Evaluate

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Indoor Air Quality and Ventilation, 1998.

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ACCEPTED MANUSCRIPT Table 1. Size of the tested dormitory Width /m

Height /m

Dormitory volume /m3

Per capita space /m3

Four-student room

6.0

3.0

2.8

50

12.5

Six-student room

8.0

2.8

2.8

63

10.5

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Length /m

Table 2. Classification index of thermal comfort and mental state level after sleep Index/Level

−3

−2

−1

0

Thermal comfort

cold

cool

slightly cool

sleepy

slightly sleepy

2

3

neutral

slightly warm

warm

hot

neutral

slightly sober

sober

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Mental state

1

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24.0 four-male-dormitory four-female-dormitory six-male-dormitory six-female-dormitory

Indoor temperature T/

19.6

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22.5 22.0 21.5 21.0

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Carbon dioxide concentration Cco2/ppm

M AN U 23.0

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Fig. 1. Four and six-student tested dormitories

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Fig. 2. Air quality, including indoor O2 concentrations (a), CO2 concentrations (b), temperature (c), and relative humidity (d), versus sleep time while the windows were completely closed. The dotted line in (a) represents the lowest acceptable indoor O2 concentration, and the dashed-dotted line in (b) represents the highest acceptable indoor CO2 concentration.

ACCEPTED MANUSCRIPT 25 21.3

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close window 2 0.055m

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Fig. 3. Air quality, including indoor O2 concentrations (a), CO2 concentrations (b), temperature (c), and relative humidity (d), versus sleep time under different window opening areas.

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-3

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Fig. 4. Variations in temperature under different window opening areas.

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Fig. 5. Thermal comfort and mental state levels versus window opening area.

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Fig. 6. Quality factor versus window opening area.

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AC C

PMV level LPMV

Six-person-dormitory

0.110

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four-boy-dormitory, Experiment four-boy-dormitory, Simulation six-boy-dormitory, Experiment six-boy-dormitory, Simulation

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Fig. 7. Air quality, including indoor O2 concentrations (a), CO2 concentrations (b), and temperature (c), versus sleep time while the windows were completely closed. Tin,0=21 °C, CO2,0=20%, and CCO2,0=1,690 ppm for the four-male dormitory; Tin,0=22.9 °C, CO2,0=19.7%, and CCO2,0=1,895 ppm for the six-male dormitory. M=65 kg, H=1.72 m, Ms=0.44, ζ=0.2, RQ=0.32, and Croom=16 MJ/°C.

ACCEPTED MANUSCRIPT 21.0

Oxygen concentration C O 2 /%

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Fig. 8. Air quality, including indoor O2 concentrations (a), CO2 concentrations (b), and temperature (c), versus sleep time under different window opening areas. Tin,0=22.9 °C, CO2,0=20%, and CCO2,0=2,070 ppm, M=65 kg, H=1.72 m, Ms=0.44, ζ=0.45,

EP

RQ=0.32, and Croom=16 MJ/°C.

Per capita space V room/n (m 3)

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8.7

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52.2

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Fig.9. Acceptable windows open area. Tin,0=22.9 °C, CO2,0=20%, and CCO2,0=2,070 ppm, M=65 kg, H=1.72 m, Ms=0.44, ζ=0.45, RQ=0.32, and Croom=16 MJ/°C.

ACCEPTED MANUSCRIPT An appropriate natural ventilation area is given in our experimental condition. A model was proposed for proper window opening during winter nights. Indoor air quality generally improved with increasing natural ventilation area.

AC C

EP

TE D

M AN U

SC

RI PT

Thermal comfort level gradually declined with increasing natural ventilation area.