Original Article Journal of the Human-Environment System Vol.19; No 2; 49-55, 2017 Measurement Method for the Solar Absorptance of a Standing Clothed Human Body Shinichi Watanabe 1) and Jin Ishii 2) 1) Department of Architecture, Daido University 40 Hakusui-cho, Minami-ku, Nagoya, 457-8532, Japan Tel: +81-52-612-5571; Fax: +81-52-612-5953 E-mail: nabeshin@daido-it.ac.jp 2) Department of Architecture, Meijo University 1-501 Shiogamaguchi, Tempaku-ku, Nagoya 468-8502, Japan Tel: +81-52-838-2380 E-mail: ishii@meijo-u.ac.jp (received on January 24, 2017, accepted on February 20, 2017) Abstract Solar radiation is one of the most important components of the human energy balance outdoors. To assess outdoor thermal comfort accurately, quantification of the solar absorptance of a clothed human body is needed. A measurement method using subjects lying on a horizontal roof surface was proposed in a previous study. However, this method only provided solar absorptance for the front side of the human body. Therefore, this study aims to establish a method of measurement of the solar absorptance of a standing clothed subject to determine the solar absorptance in any direction. Measurements were performed in September 2012 at Daido University in Nagoya, Japan. Four male and four female Japanese college-aged subjects participated in the experiments. Four clothing ensembles of a black shirt and black trousers (B-B), a black shirt and white trousers (B-W), a white shirt and black trousers (W-B), and a white shirt and white trousers (W-W) were tested for male subjects. Two clothing conditions of a black one-piece dress (Black) and a white one (White) were tested for female subjects. All subjects participated in an additional condition with their casual clothing ensembles. The following findings were obtained through the measurements. For the male subjects, the solar absorptance of the black shirt and black trousers ensemble (B-B) had the maximum value of 0.77. Meanwhile, the white shirt and white trousers ensemble (W-W) provided the minimum solar absorptance of 0.48. For the female subjects, the solar absorptances of the black and white one-piece dresses were 0.74 and 0.44, respectively. The solar absorptances of casual clothing were 0.61 and 0.59 for male and female subjects, respectively. The measurement method using the standing subject used in this study was appropriate for determining the solar absorptances of the clothed human body. Furthermore, this method was able to quantify the solar absorptance in several directions as well as for the front side of the subject. Keywords: solar absorptance; clothed human body; standing posture; solar radiation 1. Introduction Solar radiation is one of the most important components of the human energy balance outdoors. To assess outdoor thermal comfort accurately, quantification of the solar absorptance of a clothed human body is needed. According to the German standard VDI 3787 Part 2 (2008), a value of 0.7 is prescribed as the standard value of the solar absorptance of a clothed human body. Watanabe et al. (2013) reviewed the solar absorptances of garments and fabrics and indicated that solar absorptances are strongly dependent on the color and material. In the literature, the maximum solar absorptance was reported to be 0.97 for a black velvet fabric by Martin (1930). Meanwhile, Shinohara and Tokumoto (1999) found that the minimum value was 0.14 for a white silk. Moreover, few researches provide solar absorptances for the entire body wearing clothing ensembles. Watanabe et al. (2013) indicated that the solar absorptance for the combination of a white shirt and white trousers was 0.38 and that for a black shirt and black trousers was 0.78, while the average solar absorptances of 30 Japanese college-aged male
50 S. Watanabe, J. Ishii subjects wearing their casual clothing in summer, autumn, and winter were 0.66, 0.69, and 0.77, respectively. However, these values represented the solar absorptances only for the front side of the subjects, because the measurements were performed on subjects lying on a horizontal roof surface. During the measurements, the subjects could not move or face in other directions due to the posture limitation. To obtain accurate solar absorptances of the clothed human body, measurements from any direction including the front side should be conducted. Thus, the objectives of this study were 1) to determine the solar absorptances of the standing clothed human body from several directions for the specific black and white clothing ensembles as well as casual clothing, and 2) to confirm the validity of the method of measurement of the solar absorptance of a standing clothed subject. 2. Methods 2.1 Outline of the experiments Measurements were performed on six days, September 5, 6, 10, 12, 13, and 14, in 2012 at Daido University in Nagoya, Japan (35 04' N, 136 54' E). Measurements were performed between 11:00 and 14:30 Japan Standard Time (JST) to avoid low solar altitude conditions. The measurement period and corresponding meteorological data are listed in Table 1. Four male and four female Japanese college-aged subjects participated in the experiments. Table 2 lists information about the subjects physical characteristics. The subject wearing the specific clothing ensemble stood upright in front of a building wall during the measurement. Figure 1 shows the arrangement of the measuring instruments and the subject. A net radiometer (MR-40, EKO) was installed between the subject and the sun to measure the solar radiation on the vertical plane and short wave radiation reflected by the target subject and the surrounding building wall surfaces. The radiometer was installed so that the distance between the center of the subject and the sensor remained at 50 cm, and the height of the sensor was 95 cm above the ground for the male subjects and 86 cm for the female subjects, as shown in Figure 2. A black board (a 1.8 m 1.8 m board covered with black fabric) was placed on the wall to obtain the steady radiative properties as the background during the measurements. Measurements were conducted every 45 for rotation angles from 0 (facing the sun) to 180. To determine the short wave radiation reflected by the building wall, a measurement without the subject was also conducted before and after each measurement. The solar radiation and reflected short wave radiation were automatically recorded every second for a minute under each rotation angle condition. The average of the middle 20 seconds of data was used in the analysis. 2.2 Clothing condition Figure 3 shows the clothing ensembles of the specific black and white garments for the male and female subjects. Four ensembles in combinations of specific black and white garments were tested for the male subjects, that is, a black shirt and black trousers (B-B), a black shirt and white trousers (B-W), a white shirt and black trousers (W-B), and a white shirt and white trousers (W-W). Two clothing conditions of a specific black one-piece dress (Black) and a white one (White) Date Time (JST*) Table 1. Measurement period and corresponding meteorological data Air Temperature C Relative Humidity % Air Velocity m/s Mean Radiant Temperature C Solar Radiation W/m 2 Subject 5-Sep-2012 11:00 12:20 32.9 51 1.44 60.5 741.1 Male-A 6-Sep-2012 11:20 12:27 32.0 53 0.82 39.4 237.9 Male-B 10-Sep-2012 11:10 12:54 32.5 52 1.73 62.2 640.1 Male-C 13:07 14:19 33.1 48 1.74 60.5 543.5 Female-A 12-Sep-2012 12:55 13:57 34.9 34 1.56 73.6 807.2 Male-D 11:35 12:31 32.3 42 1.85 69.7 891.7 Female-B 13-Sep-2012 11:30 12:19 33.0 46 1.43 65.1 837.0 Female-C 14-Sep-2012 12:16 13:19 30.7 58 2.07 50.8 486.1 Female-D *JST stands for Japan Standard Time Table 2. Physical characteristics of the subjects Gender Number Male 4 Female 4 Age years Height cm Weight kg Mean 21.3 165.8 65.5 SD 0.5 3.2 6.6 Mean 21.3 160.0 49.7 SD 0.5 4.0 4.0 Fig. 1. Arrangement of the instruments and the subject with a white shirt and black trousers (W-B).
Measurement Method for the Solar Absorptance of a Standing Clothed Human Body 51 Black board 500 500 135 90 45 180 0 Building wall 500 500 Black board Building wall Clothed subject Short-wave radiometer and Digital camera with fisheye lens 700 Clothed subject N 950 (for male) or 860 (for female) Digital camera with fisheye lens Short-wave radiometer (a) Arrangement plan (b) Section Fig. 2 Arrangement plan and section of the instruments and the subject Fig. 3 Clothing ensembles of the male and female subjects Table 3. Properties of the specific garments Gender Garment Size Color Fabric Weight g Female Sleeveless White Cotton 100% 138 M one-piece dress Black Cotton 100% 138 Male Polo shirt M White Cotton 100% 226 Black Cotton 100% 254 Jeans Waist girth: 82 cm White Cotton 98% + polyester 2% 570 Black Cotton 98% + polyester 2% 610 were tested for the female subjects. These specific garments were obtained on the market and had the properties listed in Table 3. For the male subjects, a black and white polo shirt and jeans were adopted as the upper and lower garments, respectively. Meanwhile, a black or white sleeveless one-piece dress was used for the female subjects. All subjects were asked to participate in an additional measurement while wearing their own casual clothing ensembles. Table 4 indicates the combination of the upper and lower garments that they wore in the measurements. 2.3 View factor View factors between the net radiometer and the subject were derived for each subject in each rotation angle condition from the images captured using a digital camera (Coolpix 880, Nikon) with a fisheye lens (8 mm, f/35, FC-E8, Nikon). After all measurements were completed, the subject moved to a point 70 cm laterally for taking the images. After converting the images into an orthographic projection using the imageprocessing program SPCONV ver. 0.7 (Nagata, 2005), we obtained the ratio of the subject s area to the area of the whole image as a view factor. For the male subjects, the average view factor was 0.405 with a range of 0.288 to 0.521, while for the female subjects, the average was 0.357 with a range of 0.275 to 0.476. 2.4 Calculation of the solar absorptance of the standing clothed human body The solar absorptance of the standing clothed human body was calculated using measured short wave radiation fluxes and the view factor between the radiation sensor and the subject using Equation (1). The
52 S. Watanabe, J. Ishii Table 4 Combination of the upper and lower garments of subjects casual clothing. The color of the garments is shown in parentheses. Subject Upper garment Lower garment Male-A T-shirt (red) Short pants (plaid pattern of red, blue, and white) Male-B Polo shirt (black) Jeans (gray) Male-C T-shirt (light blue) Short pants (dark green) Male-D T-shirt (horizontal stripes of white and blue) Short pants (gray) Female-A Sweater (white) Skirt (light blue) Female-B T-shirt (gray) Jeans (black) Female-C One-piece dress (white) + cardigan (pink) Female-D T-shirt (white) + shirt (plaid pattern of red, deep blue, and white) Short jeans (light blue) + tights (black) solar absorptance was derived by subtracting the solar reflectance of the clothed human body from unity. Therefore, the solar absorptance reported in this study includes the components transmitted through the garments worn by the subjects. αcl = 1- Iref - Iback (1-F) (1), IT F where αcl: solar absorptance of the standing clothed human body, dimensionless IT: total solar radiation on a vertical plane, W/m 2 Iref: short wave radiation reflected by the standing clothed human body and surrounding building walls on a vertical plane, W/m 2 Iback: short wave radiation reflected by the building wall surfaces (without the subject) on a vertical plane, W/m 2 F: view factor between the sensor and the standing clothed human body, dimensionless. 3. Results Figure 4 shows the changes of solar radiation on a vertical plane and reflected solar radiation. The data were recorded with the male subject D for the combination of black shirt and black trousers (B-B) on September 12, as an example. The measurements were conducted every 45 for rotation angles from 0 (facing Fig. 4. Changes of the solar radiation fluxes on the vertical plane and reflected by the target subject and surrounding surfaces for Male-D with the combination of a black shirt and black trousers (B-B). Background shows the period of measurement of solar radiation without the subject to the sun) to 180, as mentioned above. To obtain the solar absorptance of the subjects at each rotation angle, solar radiation reflected by the background, without the subject, was required. In this study, measurements without the subject were conducted before and after each rotation angle measurement. After finishing each measurement, the sensor was covered with a sunshielding box for a moment to separate the data in order to calculate the solar absorptance easily. The solar radiation on a vertical plane was approximately 500 W/ m 2 on average. Short wave radiation reflected by the background, without the subject, was approximately 200 W/m 2 and short wave radiation measured with the subjects was 110-160 W/m 2. Using these data, the solar absorptance of the B-B condition was determined to be 0.857 on average. In this way, the solar absorptance of each subject for each clothing condition was calculated. Figure 5 shows the average solar absorptances of the specific black and white clothing ensembles as well as casual clothing versus the rotation angle of the subject for the male subjects. The B-B condition provided the largest solar absorptance at all rotation angles. Meanwhile, the W-W condition gave the smallest values. The result for the female subjects was the same as that for the male ones, with the black one-piece dress having a higher solar absorptance than the white one (Fig. 6). A one-way ANOVA was conducted to compare the effect of the rotation angle of the subject on solar absorptance at 0, 45, 90, 135, and 180. There were non-significant effects of rotation angle on solar absorptance for the B-B ensemble [F(4, 12) = 1.148, p = 0.381] and casual clothing [F(4, 12) = 2.612, p = 0.089] for the male subjects. Meanwhile, there were statistically significant effects of rotation angle on solar absorptance at the p < 0.05 level for B-W [F (4, 12) = 5.509, p = 0.009], W-B [F (4, 12) = 6.365, p = 0.006], and W-W [F (4, 12) = 7.207, p = 0.003]. Post hoc comparison using the Bonferroni test indicated that the solar absorptances at the rotation angles of 90 and 135 were larger than those at other angles in these clothing conditions. This might be caused by the shape of garment; a half-sleeved polo shirt was used for male subjects in this study. The configuration factors
Measurement Method for the Solar Absorptance of a Standing Clothed Human Body 53 Fig. 6. Average solar absorptances of the specific black onepiece dress, the white one-piece dress, and casual clothing against the rotation angle of the subject for the female subjects Fig. 5. Average solar absorptances of the specific black and white garments as well as casual clothing against the rotation angle of the subject for the male subjects between the sensor and the exposed skin of the arm at these rotation angles were greater than those at other angles. Watanabe et al. (2013) indicated that the whole-body solar absorptance was affected by the radiation properties of the body parts near to the sensor. Hardy (1949) reported that the solar absorptance for darkly pigmented skin is 0.8 and that for lightly pigmented skin is 0.6-0.7. Since the solar absorptance of the subjects skin was larger than that of white fabric in general, solar absorptances of the clothed human body in the side condition (90 and 135 ) were greater those recorded at other angles. In the condition with black garments, there was no difference between the solar absorptance in the side condition and those recorded in other angle conditions, because the short-wave radiation absorptances of the subjects skin and black fabric were almost the same. For the female subjects, a one-way ANOVA showed that the effect of rotation angle of the subject on solar absorptance was not significant for any of the clothing ensembles, that is, the black one-piece dress [F (4, 12) = 0.255, p = 0.902], the white one [F (4, 12) = 0.533, p = 0.715], and casual clothing [F (4, 12) = 2.727, p = 0.080]. The one-piece dresses used in this
54 S. Watanabe, J. Ishii study were sleeveless and above the knee in length, leaving a greater area of exposed skin than the male clothing, as shown in Fig. 3. Therefore, the configuration factors did not differ greatly between the sensor and exposed skin parts at any rotation angle. This might have resulted in the non-significant difference in solar absorptance between rotation angles. 4. Discussion 4.1 Comparison of solar absorptance between standing and lying postures To confirm the validity of the measurement method using standing subjects, we compared the solar absorptances of the specific black and white garments obtained from this study using standing subjects to those from the previous study (Watanabe et al., 2013) using subjects lying down, as shown in Fig. 7. In the previous study, the solar absorptance was obtained only for the front side of the subject, because the subject was asked to adopt a lying-down posture on the horizontal roof. Accordingly, the solar absorptances obtained in this study at the rotation angle of 0 with the subject facing the sun were used for the comparison. The statistical method used to test the significance was a two-way ANOVA with a significance p less than 0.05. There was no significant effect of the subjects postures between standing and lying postures [F (1, 24) = 0.269, p = 0.609]. Figure 8 shows the comparison of solar absorptances of the casual clothing ensembles between this study using standing subjects and the previous study using subjects lying down. In the previous study performed by Watanabe et al. (2013), the solar absorptances of casual clothing were determined using 30 college-aged male subjects in summer, autumn, and Fig. 7. Comparison of solar absorptances of the specific black and white garments between this study, in which subjects adopted a standing posture, and the previous study, in which subjects adopted a lying-down posture Fig. 8. Comparison of solar absorptances of the casual clothing ensembles between this study, in which subjects adopted a standing posture, and the previous study, in which subjects adopted a lying-down posture winter. Since the measurements were performed in September in this study, we compared the present results of the male subjects with those obtained in summer in the previous study. An independent-samples t-test was conducted to compare subjects postures in the standing and lying conditions. There was no significant difference between standing and lying posture conditions [t(3) = 1.556, p = 0.218]. Therefore, we can conclude that the measurement using the standing subjects obtains the appropriate solar absorptance for the specific black and white clothing as well as for the casual clothing ensembles. This study will contribute to an accurate assessment of outdoor thermal comfort, since this method of using a standing subject will be able to quantify the solar absorptance of the clothed human body from any direction. 4.2 Solar absorptances of the specific black and white garment combinations as well as casual clothing Figure 9 shows the solar absorptances of the black and white garment ensembles for the male and female subjects. The data in the figures represent the average values obtained at all rotation angles. The solar absorptance of the B-B condition provided a maximum value of 0.77 for male subjects. Meanwhile, the W-W condition provided the minimum solar absorptance of 0.48. The solar absorptances for the ensembles with different color tops and bottoms were plotted between the values of the B-B and W-W ensembles. Significant differences between the clothing ensembles were found by applying the one-way ANOVA [F (3, 9) = 99.450, p < 0.001]. For female subjects, the solar absorptances of the black and white one-piece dresses were 0.74 and 0.44, respectively. There was a significant difference between the black and white one-piece dresses according to the independent-samples t-test [t(3) = 6.528, p = 0.008]. Therefore, the solar absorptances of the clothed
Measurement Method for the Solar Absorptance of a Standing Clothed Human Body 55 human body depend on the clothing color, as pointed out in the previous study (Watanabe et al., 2013). The maximum values for male and female subjects were 0.77 and 0.74 for both black garments respectively, and these values were larger than the standard value of 0.7 described in VDI 3787 Part 2 (2008). Thus, when occupants wear black clothing in summer, applying the standard value of 0.7 may underestimate the risk of heat stress. Further studies to determine solar absorptances for various clothing ensembles are required in order to achieve an accurate assessment of outdoor thermal comfort. Figure 10 shows the solar absorptances of casual clothing for the male and female subjects. On applying the independent-samples t-test, there was no significant difference between male and female subjects [t(3) = 0.349, p = 0.751]. The average solar absorptance of casual clothing was 0.61 with a standard deviation (SD) of 0.07 for the male subjects. Meanwhile, the solar absorptances of casual clothing for the female subjects ranged from 0.29 (with outer garments of a white onepiece dress and a pink cardigan) to 0.76 (a gray T-shirt and black jeans), and the average value was 0.59 with Fig. 9. Solar absorptances of the specific black and white ensembles for the male and female subjects an SD of 0.21. The SD for the female subjects was greater than that for the male subjects because of the variations in garment shape, color, and material. 5. Conclusions 1) For the male subjects, the solar absorptance of the black shirt and black trousers ensemble (B-B) reached the maximum value of 0.77. Meanwhile, the white shirt and white trousers ensemble (W-W) provided the minimum solar absorptance of 0.48. For the female subjects, the solar absorptances of the black and white one-piece dresses were 0.74 and 0.44, respectively. The solar absorptances with casual clothing were 0.61 and 0.59 for male and female subjects, respectively. 2) The measurement method using the standing subjects used in this study was appropriate for determining the solar absorptances of the clothed human body. Furthermore, this method was able to quantify the solar absorptance in several directions as well as for the front side of the subject. This study will contribute to an accurate evaluation of outdoor thermal comfort. Acknowledgments The authors would like to thank Mr. Yuji Kobayashi for conducting a series of experiments. References Hardy, J. D. (1949) Heat transfer. In: Newburgh LH, editor. Physiology of Heat Regulation and Science of Clothing. Saunders; Philadelphia (PA): 78-108. Martin, C. (1930) Thermal adjustment of man and animals to external conditions. The Lancet 216 (5587): 673-678. Nagata, A. (2005) SPCONV Ver. 0.7. Building simulation resources. Library, application and database, http://news-sv.aij.or.jp/kankyo/s12/resource/ap/ SPCONV/SPCONV.htm. Accessed January 6, 2017. Shinohara, M., Tokumoto, M. (1999) On the heat absorptivity of clothes by solar radiation. Summaries of Technical Papers of Annual Meeting of AIJ, D-2 383-384 (in Japanese). VDI 3787 Part 2 (2008) Environmental meteorology methods for the human biometeorological evaluation of climate and air quality for urban and regional planning at regional level, part I: climate. Watanabe, S., Horikoshi, T., Ishii, J., Tomita, A. (2013) The measurement of the solar absorptance of the clothed human body - the case of Japanese, collegeaged male subjects. Building and Environment 59: 492-500. Fig. 10. Solar absorptances of the subjects casual clothing for the male and female subjects