Journal of Heat Island Institute International Vol.7-2 (2012)
Academic Article
Evaluation of the Solar Reflectance of Cool Materials by On-site Measurement of Deterioration because to Aging
Shinichi Kinoshita*
1, Atsumasa Yoshida and Shintaro Kokawa
*1Osaka Prefecture University,Sakai, Osaka, Japan, Corresponding author email: [email protected]
ABSTRACT
As a strategy for easing the urban heat island (UHI) phenomenon, building exteriors have been constructed using materials with high solar reflectance, and various techniques have been developed to decrease the thermal load of insolation and the thermal storage of the building frame. A method that uses high-reflectance paint is one of these techniques. The reflectance in the near infrared region is high compared with that of general paint of the same color. Recently, construction with this kind of paint has increased. However, it is thought that deterioration due to aging of the material needs to be evaluated before this technique can be further promoted.
This study examined a method for measuring the solar reflectance of a horizontal painted surface using a white and black standard reference board, for which performance is known and validity has been confirmed. The effect of solar reflectance on insolation was evaluated. The results are that solar reflectance tends to decrease as insolation decreases, and that solar reflectance at times of weak insolation is underestimated. For both cool painted surfaces and cool waterproofed surfaces, deterioration due to aging decreases after the first year, and the reflectance performance of the material becomes stable. In addition, solar reflectance can be restored to its original value by washing the surface.
Introduction
In recent years, the urban heat island (UHI) phenomenon whereby air temperatures in urban areas rise more than those in suburban areas in the summer has become a problem in terms of the population’s comfort level, related health concerns, and increased energy consumption. As a strategy for easing the UHI phenomenon and reducing the air conditioning load, building exteriors have been constructed using materials with high solar reflectance. Various techniques have been developed to decrease the thermal load of insolation and the thermal storage of the building frame. A method that uses high-reflectance paint is one of these techniques. The reflectance in the near infrared region is high compared with that of general paint of the same color. Recently, constructions with this kind of paint have increased. However, it is thought that deterioration caused by aging of the material needs to be evaluated before this technique can be further promoted.
The reflection performance of paint was evaluated by multiplying the spectral solar reflectance, which was measured with a spectrometer in the laboratory, with a weighted factor based on the standard solar spectral distribution. The solar spectrum is different from the on-site spectrum. The area surrounding the target area influences on-site measurement, and a device is necessary for obtaining measurements.
This study examined a method for measuring the solar reflectance of a horizontal painted surface using a white and black board, for which performance is known, and validity has been confirmed. This method is called the two-point correction (Murata et al. 2008). It has been confirmed that the spectral distribution of global solar radiation can be measured.
The influence of solar height was examined, and the
recommended measurement condition was clarified. Field measurements were made of the solar reflectance on-site for high reflectance of both a cool painted and a cool waterproofed sheet, based on the aforementioned results. The measurement accuracy and deterioration due to aging were evaluated.
Measurement Solar Reflectance
Solar reflectance (ρm) can be measured as the ratio of reflected solar radiation (
S↑) to global solar radiation ( S↓)
when the object surface is sufficiently wide and uniform, as shown in the following equation:
↓
= ↑
S S
ρm (1) However, the measured surface is generally a restricted area, and the value obtained with this method is influenced by insolation from the surrounding surface. To eliminate this effect, the following measurement was performed. At first, global solar radiation (S↓) and reflected solar radiation (S↑′) were measured above the measured surface area using a net radiometer, which can measure four radiation components:
solar radiation and infrared radiation, each from the upper and lower sides. Apparent reflectance
app -
ρm is given by calculating the ratio of
↑′
S to S↓ same as eq. (1).
↓
↑′
=S S
app -
ρm (2)
Next, a standard reference board, which is white or black board with a known reflectance (ρs), was located under the net radiometer, and the apparent reflectance (
app -
ρs ) of the
condition was measured with global solar radiation (S↓) and reflected solar radiation (S↑′′).
↓
↑′′
=S S
app -
ρs (3) These apparent reflectances can be represented as the following equations by using the geometrical factor (φ) of the standard reference board from the view of the net radiometer, based on the assumption that all the surfaces perfectly diffused reflections.
other m
app -
m φρ (1 φ)ρ
ρ = + − (4)
other s
app -
s φρ (1 φ)ρ
ρ = + − (5)
Where ρm; true reflectance of measuring area, ρother ; reflectance of surrounding area. The true reflectance of measuring area was calculated by eliminating the term of the influence of the surrounding area with equations (4) and (5).
s app - s app - m
m ρ
φ ρ
ρ ρ − +
= (6)
1000 1000
365~700
standard reference board measuring surface
pyranometer pyrgeometer
1000 1000
365~700
1000 1000
365~700
1000 1000 1000
1000
365~700365~700
standard reference board measuring surface
pyranometer pyrgeometer
Figure 1. Measurement apparatus
N
Radiometer
40m
Date logger
& PC
Tile floor Building
Building
N
Radiometer
40m
Date logger
& PC
Tile floor Building
Building
N
Radiometer
40m
Date logger
& PC
Tile floor Building
Building
Measuring Apparatus
Figure 1 shows the setup of the measuring apparatus.
The net radiometer has a pyranometer (wavelength range:
0.305 to 2.800 m) and a pyrgeometer (wavelength range: 5 to
50 m) installed on its upper and lower sides, and simultaneously measures global solar radiation and infrared atmospheric radiation from the sky and reflected solar radiation and infrared radiation from the ground. A standard reference
board was used in two colors: white and black. Each reflectance based on JIS R 3106 (JISC. 2001) was 88.8% and 4.9%, respectively. The size was 100 cm x 100 cm. When the net radiometer was installed at a height of 40 cm, the geometrical factor of the standard reference board from the radiometer was 0.651. Data for the radiation were measured every 10 seconds for 5 to 10 minutes.
Measurement of the Reflectance of the Tile Surface
The reflectance of the tile surface on the roof terrace of Osaka Prefecture University was measured several times from October 2006 to June 2007. Figure 2 shows the shape of the terrace. As shown in the figure, the net radiometer was located near the center of the terrace. The tile surface was wide enough to be almost uniform. Consequently, the reflectance of the tile surface was not evaluated by calibrating
the standard reference board, but was evaluated according to equation (1). Figure 3 shows the influence of the incident angle of insolation for the reflectance of the tile surface. As shown in this figure, the reflectance of the surface tended to be higher as the incident angle increased, and the tendency was significant in the range more than 50 degrees.
Incident angle (deg) R ef le ct an ce ρ
t( % )
Oct.14.2006 Oct.27 Oct.28 Oct.30 Nov.10 Nov.21 Feb.3.2007 Jun.5.2007
36 40 44 48 52 56
0 10 20 30 40 50 60 70 80 90
Figure 3. The influence of incident angle to reflectivity of tile surface
Measurement of Cool Painting Surface Measuring Object and Conditions
Figure 4 shows an object that was painted with several kinds of cool paint and that was located on the steel rooftop of a multi-layered parking lot in Osaka, Japan. The solar reflectance was measured from 10:00 to 16:00 on August 6, 2008. All the cool paints were a uniform light blue. The measuring points were six areas with different cool paints, and one area without paint. The numbers from I to VI in the figure correspond to the surfaces with different kinds of cool paint, and the number VII corresponds to an unpainted surface.
During the measurement process, the height of the net radiometer was 36.5 cm from the surface. The standard
reference boards were white and black. For each cool paint surface, the apparent reflectance for three conditions of standard reference board (i.e., white, black, and without color) were measured rotationally, and the true reflectance was calculated using these data. Measuring time for each apparent reflectance was about 5 minutes, and the interval of data sampling for the global and reflected solar radiation was 5 seconds. Reflectance for areas III and IV was measured three times, and reflectance for the other areas was measured twice.
Ⅰ
Ⅱ
Ⅲ
Ⅳ
Ⅴ
Ⅵ
Ⅶ
31m 38.4m 2.4m
5m
N
Ⅰ
Ⅱ
Ⅲ
Ⅳ
Ⅴ
Ⅵ
Ⅶ
31m 38.4m 2.4m
5m
Ⅰ
Ⅱ
Ⅲ
Ⅳ
Ⅴ
Ⅵ
Ⅶ
Ⅰ
Ⅱ
Ⅲ
Ⅳ
Ⅴ
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Ⅶ
31m 38.4m 2.4m
5m
N
Figure 4. Schematic drawing of the cool paint surface on the rooftop of a parking lot, and areas for measurement
10 20 30 40 50 60
ρ
mw, ρ
mb%
ρmw ρmb
200 400 600 800 100
10 20 30 40 50
Insolation W/m
2ρ
m-app%
I II III
IV V VI VII
Figure 6. Effect of global insolation on instantaneous apparent solar reflectance
0 10 20 30 40 50 60
I II III
ρm %
1st 2nd 1st 2nd 1st 2nd 3rd
Board Threshold (W/m2)
white no white 600 black no black 600 two point 600
0 10 20 30 40 50 60
IV V VI
ρm %
1st 2nd 3rd 1st 2nd 1st 2nd 1st 2nd
VII
Board Threshold (W/m2)
white no white 600 black no black 600 two point 600
Figure 7. Effect of data selection with threshold of insolation on solar reflectance
Results of Measurement and Evaluation of Accuracy Figure 5 shows the results of measuring solar reflectance. In the figure, white and black bars correspond to the reflectance measured with white and black standard reflectance boards, respectively. Solar reflectance of the cool paint surface in areas IV and VI was relatively higher and were greater than 40%. Reflectance in areas I, II, and III was about 35%, and in area V it was about 30%. The solar reflectance of cool paint surfaces was higher than for surfaces without cool paint, which was about 17%. The effect of absorption control for solar radiation was confirmed for each cool paint surface.
The difference in results for the two colors of standard reference boards was not large. The maximum value was about 3% for area II for the first measurement, and in most cases was about 1.5%.
Figure 6 shows the effect of global insolation on apparent solar reflectance without standard reference board, and shows scatter plots of the instantaneous values. As shown in the figure, solar reflectance tends to decrease as insolation decreases; this suggests that reflectance is at times of weak insolation is underestimated. Because it is desirable to measure in as uniform a condition as possible, solar reflectance was evaluated with a threshold value set on insolation and by eliminating data under the threshold. Figure 7 shows the effect of correcting the threshold of insolation when measuring solar reflectance. The threshold is generally set to 600W/m2, and is 500W/m2 in the case of no data greater than 600W/m2. As
Evaluation of Deterioration due to aging
Similar measurements were executed annually, on September 20, 2006 (near the time of painting) and August 7, 2007. Figure 8 shows the past two times of measurement, along with results for 2008. These measurements were made with white standard reference board. The past two times of measurement were not processed with a correction for the threshold of insolation. In the cool paint surface, except for areas IV and VI, deterioration due to aging was found between the measurement taken in 2006 and 2007. In area V, a decrease of approximately 20% in solar reflectance was found. Reduced performance was found in the paint’s reflectance. Staining from the passage of vehicles and deposits of particulate matter were potential causes of deterioration. The change in solar reflectance from 2007 to 2008 was less than that from 2006 to 2007. These results indicate that the reflectance performance of the material becomes stable over time.
0 10 20 30 40 50 60
I II III IV V VI VII
ρ
m%
Board: white '06 Sep. 20 '07 Aug. 07 '08 Aug. 06
Figure 8. Aged deterioration of each cool paint surface
Measurement of Cool Waterproof Sheet Measuring Object and Conditions
Solar reflectance of waterproofed sheet constructed on a rooftop in Osaka was measured on August 7, 2008, and similar measurements were made on February 6 and August 9, 2007. The first measurement on February 6, 2007, was made at the time after execution. Figure 9 shows the schematic drawing of the waterproofed sheet surface on the rooftop and measuring areas. The objects measured were two kinds of waterproofed sheets: a cool waterproofed sheet with highly solar reflectance (“Sheet1”) and a conventional waterproofed sheet (“Sheet2”).
Their reflectance was measured by two net radiometers. In the third measurement on August 7, 2008, a square domain of one side 5 m in the area of Sheet1, inside the dashed line in Figure 9, was washed with a neutral detergent to evaluate the effect of staining on performance deterioration, and the solar reflectance of the domain also was measured. Solar reflectance was evaluated with white and black standard reference boards.
Reflectance was measured several times on each date, and the data were averaged.
Results of Measurement and Evaluation of Deterioration due to Aging
Table 1 shows the results of solar reflectance for waterproofed sheets in each measuring condition. A two-point correction was applied to data for the unwashed area of Sheet1 in the first and third measurements. Measurements were made for the washed area surface in the last condition. As shown in the table, the difference between evaluation methods was not large (i.e., from 1 to 3%). The solar reflectance of Sheet1 was greater than that of Sheet2 in all measuring conditions, and the cool waterproofed sheet maintained a higher solar reflectance performance than did the conventional sheet. Solar reflectance was found to significantly decrease half a year after execution, but hardly changed over the following year. The solar reflectance of Sheet1 was restored to its original value by washing. Thus, it is reasonable to suppose that the decrease of solar reflectance for waterproofed sheet was caused by staining of its surface. Similar deterioration due to aging was found for Sheet2.
Penthouse
Radiometer Data logger & PC
Sheet1 Sheet2
N
20m
20m
Penthouse Penthouse
Radiometer Data logger & PC
Sheet1 Sheet2
N
20m 20m
20m20m
Figure 9. Schematic drawing of waterproof sheet surface of a building rooftop and measuring areas
Table 1. Solar reflectance of waterproof sheets in each measuring condition
Object Method
Solar reflectance (%)
1st 2nd 3rd
Feb.6 2007 Aug.9 2007 Aug.7 2008 Sheet1
(unwashed)
White 59.8 44.9 42.1
Black 59.1 43.4 43.7
2 point correction 59.3 --- 42.9
Sheet1 (washed)
White --- --- 54.0
Black --- --- 55.2
Sheet2 (unwashed)
White 40.3 24.9 25.8
Black 37.4 27.1 27.2
Conclusions
This study examined a method for measuring solar reflectance of horizontal surfaces with high-reflectance paint and with a cool waterproofed sheet, using white and black standard reference boards, for which performance is known and validity has been confirmed. Field measurements were made of the solar reflectance on-site for high reflectance of both a cool painted and a cool waterproofed sheet, based on the aforementioned results. The measurement accuracy and deterioration due to aging were evaluated.
To assess the error factor, the effect of the quantity of insolation on solar reflectance was evaluated. The results were that solar reflectance tends to decrease as insolation decreases; and that solar reflectance at times of weak insolation is underestimated. For both cool painted surfaces and cool waterproofed surfaces, deterioration due to aging decreases after the first year, and the reflectance performance of the material becomes stable. In addition, solar reflectance can be restored to its original value by washing the surface.
Acknowledgments
Daikin Industries, Ltd. which offered the standard reference boards and measured their reflectance, and the people who offered measurement places are sincerely acknowledged.
References
Murata, Y., Sakai, K., Kanamori, H., Takebayashi, H., Matsuo, Y., Moriyama, M., Yoshida A., Nishioka, M., Yano N., Shimizu, R., Miki, K., Murase, T., Akbari, H., 2008.
“A Study on Insolation Reflectivity Measurement of the Cool Painting Surface - A Proposal of Measuring Method by the 2 Point Correction and Accuracy Testing on the Horizontal Surface -” J. Environ. Eng., AIJ, Vol. 73, pp.1209-1216.
JISC (Japanese Industrial Standards Committee). 1998. “JIS R 3106: Testing method on transmittance, reflectance and emittance of flat glasses and evaluation of solar heat gain coefficient”.
(Received Feb 9, 2012, Accepted Oct 10, 2012)
