SFERA - Solar Facilities for the European Research...

4th SFERA Summer SchoolDLR

Hornberg, 16th May 2013

Aránzazu Fernández‐Garcí[email protected] Meyen (DR)Florian Sutter (DLR)

Specifications of reflectors based on guidelines and standards

4th SFERA Summer SchoolHornberg, 16th May 2013

Contents

1. Introduction2. Reflectance definitions3. Reflectance models4. Requirements for instrumentation5. Commercial instruments6. Prototypes


Introduction

3

• Goals of standardization of reflectance measurements

‒Relevant information in reported values‒Comparability of reported values, uniformity‒No market disturbance by misleading results‒No penalization of certain manufacturers‒Measurement with off the shelf instruments‒Applicability for anybody, not just well equipped labs‒High accuracy (aim 0.005)


Introduction

4

• What do we need to get this goals

‒ A Guideline that everyone follows‒ Relevant parameters to be reported, which are

flexible to different collector design application‒ Adequate procedures and protocols‒ Adequate instruments to provide high accuracy‒ International agreement!‒ Translation into “official” standards


Introduction

5

Topic of reflectance analysis for

different solar materials first

addressed by R.B. Pettit & others

1970s

A RRT was performed between NREL, CIEMAT & DLR.

SolarPACES paper

Spring 2010

Meeting at PSA to discuss methodology & content of reflectance measurement guideline. Participants: C. Kennedy, A. Fernandez, E.

Lüpfert & S.Meyen. Framework of document

June 2010

1st version interim guideline document

created by DLR, NREL & CIEMAT. Published at SolarPACES homepage

Spring 2011

Received comments & reviews presented at Task III meeting in Granada in by CK

Sep. 2011

• History of current guideline

Received comments & reviews presented at Task III meeting in Marrakech by SM

Sep. 2012

2nd version guideline document created.

Published at SolarPACES homepage

March 2013

2nd RRT organized by ENEA


Introduction

6

• SolarPACES Task III: Solar Technology and AdvancedApplications

• Project: Development of guidelines for standards forconcentrating solar power (CSP) components

• Title: Parameters and method to evaluate the solar reflectance properties of reflector materials for concentrating solar power technology

• Authors (institutions): DLR (Germany), CIEMAT (Spain), ENEA (Italy), NREL (USA), Fraunhofer ISE (Germany)

• Authors (companies): Abengoa Solar (Spain‐USA), Flabeg(Germany), 3M (USA), Alanod (Germany)


Introduction

7

Europe: CEN / TC 192 Glass in buildings /

WG5 Mirrors (Oct. 2012)

SolarPACESTask III

USA: ASTM E44.20 Solar Glass

International: IEC / TC 117 Solar Thermal Electric Plants / AG2

Systems and components(March 2012)

Spain: AENOR / CTN 206 Solar Thermal

Power Systems / GT2 Components

(Beginning 2010)

Germany: DKE / K374 Solar Thermal Facilities

for Electricity Generation(nov 2011)


Contents



Reflectance definitions

• Interaction between light and matter

• Reflectance– Wavelength, λ– Incidence angle, θ– Light polarization unpolarized

9

ρ

τ

α

++ = 1

Law of conservation of energy

i

r

θ

Is a material property and its angular distribution depends on microscopic

surface flatness



• Laws of reflexion (followed by light)

• Hemispherical reflectance, ρh(λ,θ,h)

10

θi First law of reflexion: same planeSecond law of reflexion: θi = θr

θr

Reflexion into the hemisphereFunction of λ and θ


• Diffuse reflectance– Bundle of light rays impinges on an object with a rough or

microscopically structured surface (size≥λ), each individual ray encounters a different surface slope and therefore the law of reflection takes effect for a different θ to the surface normal at this point

– As a result the bundle of light is diffusely scattered in all directions in the plane of incidence


11



• Specular reflectance, ρs(λ,θ,φ), and specularity

12

Reflexion close to the specularFunction of λ, θ and φ



• Specular reflectance function, ρs(λ,θ,φ)

– Hemispherical reflectance– Specular reflectance distribution (beam spread and scattering),

depending on surface roughness and microstructure– Convolution of Gaussians distributions, depending on λ and θKj: weighting factor for the several termsσ: standard deviation of the distribution of reflected light

13

M

j jjhs Kh

12

2

,2exp1,,,,



• Solar‐weighted reflectance

n

iiib

n

iiibi

h

E

ESW

1,

1,)(

)(

14

ASTM G173‐03λ=[280,2500] nmAir Mass AM 1.5



• The proper optical parameter to evaluate the quality of reflectors is the ρs(SW,θ,φ)– SW in the solar spectrum range– As a function of θ– As a function of φ for the angle of interested (application)

• No commercial instruments currently available• Some prototypes and models

15



• Required set of reflectance parameters

16

Spectral hemispherical reflectance, ρh(λ,θ,h)λ=[280,2500] nm, meas θ≤15ºIdeally: meas/model θ ≥15º

Specular reflectance, ρs(λ,θ,φ)At least 3 λ, 0 ≤ φ ≤20 mrad and θ≤15ºIdeally: meas/model as a function of λ, φ and θ

Solar‐weighted reflectanceρh(SW,θ,h) ASTM G173 λ=[280,2500] nm, AM 1.5, θ≤15ºIdeally: ρs(SW,θ,φ) as a function of θ and φ (at least 3)


Contents



• ρs(SW,θ,φ) by Pettit approach (Pettit, 1982)

– Assumptions: ratio between specular and hemispherical is wavelength independent and small σ

– Only valid for reflectors with very good specularity– For example, it can be used in high quality silvered glass

reflectors– Check the valitity by comparing hemispherical and specular

reflectance results for several λ and φ

Reflectance models

),,(),,(),,(),,( hSW

hSW h

meash

measss

18


• ρs(λ,θ,φ) by One Gaussians (Pettit, 1977)

Reflectance models

19


• ρs(λ,θ,φ) by two Gaussians (Pettit, 1977)

Reflectance models

20

σ1 sharpσ2 broader


• ρs(λ,θ,φ) by two Gassians (Gee, 2010)

Reflectance models

21


Reflectance models

22

• ρs(λ,θ,φ) by Gaussians (Heimsath et al., 2010)

– Glass: b is the offset between reflection terms– Aluminum: τ is a parameter of the exponential decrease with

wide angle scattering– Coefficients are obtained with the Fraunhofer ISE experimental

set up


Reflectance models

23

• Off‐normal solar reflectance (Montecchi, 2012)– Equivalent Model Algorithm (EMA)– Inittially proposed for architectural glazings – Based on refractive index calculation and reflector layers study


Reflectance models

24

• Near‐specular reflectance with TIS (Montecchi, 2012)– Total Integrated Scatter (TIS) relashionship (Beckman and

Spizzichino, 1963)

– σφ is the equivalent roughness

– TIS is measured with the ENEA experimental set up– Coefficients A and σ are obtained


Contents



Requirements for instrumentation

26

• Ideal instrument– λ=[280,2500] nm in 5 nm– Controllable that is variable within a wide range and over

small intervals– Measurement at various θ, from near normal to at least 50º or

as a function of θ– Adjustment options to account for different mirror thicknesses

and surface curvature– Absolute measurements, no reference mirror required– Measurement spot size as large as possible, with the option of

reducing size in case of interest– Measurements should be done at the same spot on the sample


27

• Ideal instrument (cont)– Non‐contac measurements to avoid damaging the surface – Portable for field measurements: high battery autonomy,

battery status display, light‐weight, small compact size, easy to handle, robust, and digital data storage

– A coordinate system incorporated in the instrument to identifythe position of defects points and study their evolution overtime

– Small concerning associated type‐Buncertainties– No influence by external stray light– No risk to human health or enviromental hazar involved



28

• Minimun required instrument properties for hemisphericalreflectance, ρh(λ,θ,h)– λ=[280,2500] nm in 5 nm– Measurement at least at near normal incidence (θ <=15º)– Preferably absolute measurements, or with a calibrated reference– If measurements at θ > 15º are possible, the calibration of the

reference mirror must be available for the same θ– If integrating sphere is part of the setup: sphere size needs to be

adequate to minimize errors (diameter>150 mm)



29

• Minimun required instrument properties for specularreflectance– Measurement at several (at least 3) narrow λ– Several and selectable acceptance apertures (0 ≤ ≤ 20 mrad). The

suitable depends on the collector design– Measurement at least at near normal incidence (θ <=15º)– Preferably absolute measurements, or with a calibrated reference– If measurements at θ > 15º are posible, the calibration of the

reference mirror must be available for the same θ– Adjustment options for correcting beam aligment (for different

mirror thicknesses and surface curvature)



Contents



Commercial instruments

• Spectrophotometers

31

Lambda 1050 by Perkin Elmer Cary 5000 by Varian (Agilent)

‒λ = [185,3300] nm‒ Source: Deuterium (UV) + Halogen (VIS+NIR)‒Detectors: PM (UV+Vis)+ PbS (800‐2500) + InGaAs (2500‐3300)‒ Cost: ≈ 68000 €

Spectral Reflectance


• Spectrophotometers Accesories

32

Spectral Hemispherical Reflectance:Integrating Sphere

Spectral Specular Reflectance:URA




• Spectrophotometers (Integrating Sphere)

33

‒ θ = 8 º‒ φ hemispherical‒ λ = [200,2500] nm‒ Detectors: InGaAs + PMT‒Cost: 2900 €



• Spectrophotometers (URA)

34

‒ θ = 8‐65 º‒ φ unknown‒ λ = [190,3100] nm‒ Detectors: PbS and Si‒Cost: 17000 €



• Spectrophotometers: comparison

35

0

10

20

30

40

50

60

70

80

90

100

250 500 750 1000 1250 1500 1750 2000 2250 2500

Ref

lect

ance

Wavelength range (nm)

4-mm silvered glass hem

URA 8º0

10

20

30

40

50

60

70

80

90

100

250 500 750 1000 1250 1500 1750 2000 2250 2500

Ref

lect

ance

Wavelength range (nm)

Aluminium1 hemURA 8º



• Reflectometers

15R‐USB, D&S

36

‒ θ = 15º‒ φ = {3.5,7.5,12.5,23.0} mrad‒ λ = [635,685] nm; 660 nm‒ Source: LED‒ Resolution: 0.001‒ Spot size: 10 mm ‒ Optical aligment‒ Curvature and thinkness‒Time consuming‒ Cost: 16000 €

Monochromatic Specular Reflectance



• Reflectometers

37

15R‐RGB (MWR), D&S

‒ θ = 15º‒ φ = {2.3,3.5,7.5,12.5,23.0} mrad‒ λ = {460,550,650,720} nm‒ Source: white light + filters‒ Resolution: 0.001‒ Spot size: 10 mm‒ Optical alignment‒ Curvature and thickness‒ Cost: 18000




• Reflectometers comparison: 15R and MWR

38

‐0,03

‐0,02

‐0,01

0,00

0,01

0,02

0,03

Δρs=ρs (D

&S, 660

nm) ‐

ρs (M

WR, 650

nm)

Sample

Difference between D&S and MWR specular reflecance

23 mrad

12.5 mrad

7.5 mrad

3.5 mrad

Glass: 0.001±0.001; Innovative: up to 0.016±0.009



• Reflectometers comparison: MWR and spec

39

0,800

0,820

0,840

0,860

0,880

0,900

0,920

0,940

0,960

0,980

1,000

250 500 750 1000 1250 1500 1750 2000 2250 2500

Ref

lect

ance

Wavelength, nm

Comparison hemispherical reflectance and specular reflectance

Glass #1

3.5 mrad

7,5 mrad

12,5 mrad

23 mrad



• Reflectometers comparison: MWR and spec

40

0,600

0,650

0,700

0,750

0,800

0,850

0,900

0,950

1,000

250 500 750 1000 1250 1500 1750 2000 2250 2500

Ref

lect

ance

Wavelength, nm

Comparison hemispherical reflectance and specular reflectance

3.5 mrad

7.5 mrad

12.5 mrad

23 mrad

Aluminium #2



• Reflectometers

Condor, Abengoa

41

‒ θ = n/a‒ φ = 408 mrad‒ λ = {435,525,650,780,940,1050}nm‒ Source: LED‒ Resolution: 0.001‒ Spot size: 1 mm‒ No optical alignment‒ Easy to handle‒ Cost: ≈ 18000 €




• Reflectometers

42

SOC 410 Solar, Surface Optics

Monochromatic Hemispherical and Diffuse Reflectance

‒ θ = 20º‒ φ = 52.4 mrad‒ λ = 7 bands; 330‐2500 nm‒ Source: Tungsten‒ Resolution: n/a‒ Spot size: 6.35 mm‒ No optical alignment‒ Cost: 19000 €



• Reflectometers

CM 700‐d, Konica Minolta

43

Spectral Hemispherical and Diffuse Reflectance

‒ θ = 8º‒ φ ≈ 50 mrad‒ λ = [400‐700] nm‒ Source: Xenon‒ Resolution: 0.0001‒ Spot size: 8 mm‒ No optical alignment‒ No curvature‒ Cost: 10000 €



• Reflectometers

µScan, SMS

44


‒ θ = 25º‒ φ = n/a‒ λ = 670 nm‒ Source: Laser‒ Resolution: 0.001‒ Spot size: 1 mm‒ No optical alignment‒ Small repeatability‒ Cost: ≈14000 €


Contents



Prototypes

46

• Space Resolved Specular Reflectometer (SR)2: DLR

(Sutter et al., 2010)

Aperture wheel

Pinhole

Light source

Camera

Filter wheel / Density filter

Lens 0

Lens 1

Lens 2

Iris

2*Acceptance angle Mirror sample

Housing

‒ θ = 15º‒ φ = {3.5,6.0,12.5} mrad‒ λ = {410,500,656} nm‒ Resolution: 37 px/mm ‒ Spot size: 50 mm ‒ CMOS camera sensor


• Space Resolved Specular Reflectometer (SR)2: DLR– Study of soiling and aging – Average specular reflectance– Detection of corroded area– Reflectance of corrosion spots– Reflectance of non corroded

parts of the surface

Prototypes

47

Exposure Time

0.10 M

0.25 M

0.49 M

0.94 M

(Sutter et al., 2010)

656nm 12.5mrad (D&S*) 12.5mrad(SR)²

Diff.

1.6‐mm silvered glass 96.3% 95.2% ‐1.1%

Aluminum 85.1% 89.2% +4.1%

0.95‐mm silvered glass 96.1% 97.4% +1.3%

4‐mm silvered glass 95.5% 96.2% +0.7%

Fist surface mirror 93.0% 94.6% +1.6%


• Automatic device TraCS (DLR)•Automatic real time measurement of a test mirror with sun spectrum•High time resolution allows for identification of weather influences on soiling•Validated with hand measurement devices

0.5

0.6

0.7

0.8

0.9

1.0

0.5 0.6 0.7 0.8 0.9 1.0Reference cleanliness

TraC

S c

lean

lines

s[Wolfertstetter, 2012]

0.6

0.7

0.8

0.9

1.0

1.1

16-Jun-12 17-Jun-12 18-Jun-12 19-Jun-12 20-Jun-12 21-Ju

clea

nlin

ess

fact

or

Prototypes


Prototypes

• Spectral Specular Reflectometer (SSR): NREL

– High‐speed measurements– Easy to use – removes operator error– Measure s(,) as a function of θ– Flat and curved samples

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Fiber

Light SourceCollimating Lens

MirrorSample

Collection Optics

FiberVariableAperturePinhole

Spectrometer

(Kennedy, 2010)

‒ φ = [2‐50] mrad‒ λ = [300‐2500] nm


Prototypes

50

• ENEA experimental setup

(Montecchi et al., 2012)

‒ θ = 2.9º‒ φ = {3.39,6.56,9.84, 13.97,16.83,19.95} mrad‒ Source: lasers 632.8, 543.5 and 405.5 nm‒ Detector: 150 mm IS


Prototypes

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• Fraunhofer ISE experimental setup

‒ θ = 8‐90º‒ φ = {3.5,6.0,12.5} mrad‒ Source: white light or laser‒ Aperture size: 0.2x20 mm ‒ Detector: photodiode

(Heimsath et al., 2010)


Prototypes

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• Fraunhofer ISE experimental setup

(Heimsath et al., 2010)


Thank you for your attention!!!!!

[email protected]

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