Evaluation of Solar Reflectance of Cool Materials by On ...
Transcript of Evaluation of Solar Reflectance of Cool Materials by On ...
Evaluation of Solar Reflectance ofCool Materials by On-site Measurement
and its Aged Deterioration
The 2nd International Conference onCountermeasures to Urban Heat Islands
Berkeley, California
S. Kinoshita, A. Yoshida and S. KokawaDept. Mechanical EngineeringOsaka Prefecture University
September 21, 2009
Building Exterior covered with Highly Reflective (Cool) Material
Countermeasure to HeatIsland PhenomenonControl of Solar Absorption toBuilding Structure with CoolMaterial
Decrease in Sensible Heat Transfer towards Outdoor Atmosphere(Decrease of Air Temperature)
Decrease in Cooling Load (Decrease of Anthropogenic Heat)
Study Outline
Establishment of technique for evaluatingRadiative Performance of Cool Materials (Cool Painting and Cool Waterproof Sheet)
On-site Measurement of Solar Reflectance Establishment of Measurement Method Discussion on Measurement Error Factor Evaluation of Aged Deterioration through long term
measurement at the same sites
Solar reflectance is obtained from the ratiobetween irradiated solar energy andreflected solar energy measured byradiometer.
Case (1) Wide Measured Surface
• Total (wavelength)• Diffuse
Measurement of Solar Reflectance
• Measured Site Osaka Prefecture Univ.• Measured Date (2006) October 14, 27, 28 & 30 November 10 & 21 (2007) February 4 & June 5• Measured Height 40cm
Open Terracecovered with Tile
N
Radiometer
20m
40
m
Date logger
& PC
Tile floor
Building
Building
N
Radiometer
20m
40
m
Date logger
& PC
Tile floor
Building
Building
N
Radiometer
20m
40
m
Date logger
& PC
Tile floor
Building
Building
Incident angle (deg)
Reflectance !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
Measured Results
Solar Reflectance increases withincrease in solar incident angle over40 degree.
Specular reflection
Case (2) Narrow Measured Surface
Measurement State
Wavelength Region 305~2800nm
10001000
365~
700
standard reference board
measuring surface
pyranometer
pyrgeometer
10001000
365~
700
10001000
365~
700
1000100010001000
365~
700
365~
700
standard reference board
measuring surface
pyranometer
pyrgeometer
Measured value includesInfluence of Reflectionfrom Surrounding Surface
Use of Standard Reference Boardwith known Solar Reflectance
Measurement apparatus
Measurement Method (A) for Solar ReflectanceRemoval of Reflection Influence from Surrounding Surface by use ofSingle Standard Reference Board (white or black)
: Geometrical factor
+=0.049
Apparent solar reflectance with and without standard board are measured, and realreflectance is calculated by using φ and ρs.
(two points correction method) two standard boards & correction chart
!"!#$%&'!(!!#)!"!&$%&'!(!!&)!"!*$%&'!(!!*)!+,!*$%&'
Observed reflectivity !m-obs (%)
Reflectivity !m (%)
0 20 40 60 80 100
20
40
60
80
100
Standard Plate(Black) = 0.049
withoutStandard Board
Apparent Reflectance (%)
Rea
l Ref
lect
ance
(%)
(JIS R 3106)
Measurement Method (B) for Solar Reflectance
Standard Board(White) = 0.888
(Measured data)
+
+
ρm
ρm-appρm-appρs-app
Measurement Site (1): Cool Painting
!
"#
$
%
&
'
31m
38.4m
2.4m
5m
!
"#
$
%
&
'
!
"#
$
%
&
'
31m
38.4m
2.4m
5m
Outline of Measurement Site (Parking Lot) Measurement Situation
(6 Companies)
Measured Results (1)
Cool Painting (6 Companies)
RegularSurface
0
10
20
30
40
50
60
I II III IV V VI VII
! mw
, !
mb
% !mw !mb
Measured Date: August 6, 2008
Dispersion of measured values areapproximately 1.5%.
Effect of global insolation on instantaneous apparent solar reflectance
200 400 600 800 100010
20
30
40
50
Insolation W/m2
! m-o
bs
%
I II III
IV V VI VII
The solar reflectance tendsto decrease as the insolationdecreases.
The fact suggests thatthe reflectance is underestimatedin the case of measuring in aperiod of weak insolation.
Accuracy of Measurement
0
10
20
30
40
50
60
I II III
! m
%
1st 2nd 1st 2nd 1st 2nd 3rd
Board Threshold (W/m
2)
white no white 600 black no black 600 two point 600
Effect of data selection with threshold of insolation on solar reflectance
Solar reflectances of several cool paint surfaces becomehigher by the data processing.
The difference between methods of measurement is small.
Evaluation of Aged deterioration ofpainted surfaces
2006.9.20 2007.8.7 2008.8.6
Similar measurement are also performed every one yearsince execution of painting.
Evaluation of Aged Deterioration
0
10
20
30
40
50
60
I II III IV V VI VII
! m %
Board: white
'06 Sep. 20 '07 Aug. 07 '08 Aug. 06
DownStain?
In the cool paint surface except IV and VI, remarkable aged deterioration is foundbetween the results of 2006 and 2007.
Compared between the results of 2007 and 2008, the change of solar reflectance duringthe period is smaller than that from 2006 to 2007.
Measurement situation (N→S)
Measured site
Penthouse
Sheet1 Sheet2
N
20m
20mData logger & PC
Radiometer
Washed area
Outline of Measurement Site (Rooftop surface)
Measurement Site (2):Cool Waterproof Sheet
Measured dates Feb. and Aug. 2007, Aug. 2008 (A square area in Sheet1 is washed with neutral detergent.)
Solar reflectance of waterproof sheets in each measuring condition
Solar reflectance significantly decreases for a half year from execution,but it hardly changes in the next one year. The solar reflectance of Sheet1 is restored to around the original value ofexecution by washing.
Measured Results (2)
The decrease of solar reflectance is caused by the effect of deposited stain.
On-site measurements of solar reflectance for coolpainting and water proof sheet can be performed by useof standard reference board.
The precision of reflectance measurement is estimatedas ±1.5%.
The solar incident angle less than 40 degree is preferableas the measurement condition.
The aged deterioration calms down in more than oneyear after execution, and the reflectance performancebecomes stable. In addition, the solar reflectance can berestored to around the original value of execution bywashing stain on the surface.
Summary
Base material
Painted layer
Solar radiationReflection
Transmission
Stain
Need for on-site measurement of radiative properties Transmission from base material (Coating state) Stain on surface Aged deterioration of painted layer
Near InfraredVisible
Wavelength (nm)
Sol
ar Ir
radi
ance
(W・m
2 /µm
)
Wavelength (nm)
(a) Solar Spectrum
Normal Incidence
(b) Reflectance Spectrum of Cool Painting
Cool Painting
General Painting (gray)
(gray)
(white)
(black)
Mon
ochr
omat
ic
Ref
lect
ance
(%)
Measurement Date & Condition
Date : Sep. 20 & Oct. 12 (2006)Height : 40cm,50cm & 70cm
Date:Feb. 6 & Aug. 9 (2007)Item:Total Solar Reflectance & Infrared EmiitanceHeight:40cmStandard Plate : white, black, withoutSampling:10 min. per 20 seconds
Measurement Date & Condition
500 1000 1500 2000 25000
20
40
60
80
100
Sheet1
Sheet2
! nm
"(!
)
%
Reflectivity (%)
JST
No calibration
A (white)
A (black)
B
Sheet1
Sheet2
10 11 12 13 14 1520
30
40
50
60
JST
Reflectivity (%)
A(white)
A(black)
B
Sheet1 (center)
Sheet1 (boundary)
Sheet2
10 11 12 13 14 15 1620
30
40
50
60
Feb. 6 (2007)
Measured Results (2) Solar ReflectanceR
efle
ctan
ce
Ref
lect
ance
Aug. 9 (2007)
Measurement Method of Infrared Emittance
εs (known)Ts (measured)
Ls↑Lm↑
εm (unknown)Tm (measured)
Ls↑ = (εsσTs4+(1- εs) L↓)×φ+Lother↑×(1-φ)
Lm↑ = (εmσTm4+(1- εm) L↓)×φ+Lother↑×(1-φ)
L↓L↓
εm = ・・・
Lother↑Lother↑
Narrow Measured Surface
「Measurement Method of Transmittance,Reflectance and Emittance of Plate Glass」
Monochromatic reflectance spectrum from 300to 2100 nm is measured by spectrometer withintegrated sphere. Solar reflectance isobtained from the measured data multiplied byweight factors. The incident angle is less than15 degree.
JIS R 3106