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