© November 2009 Masdar Institute of Science and Technology. All rights reserved.

Flux Mapping of the Beam Down Solar Thermal Concentrator at

Masdar City

Steven Meyers, Marwan Mokhtar, Peter Armstrong, Matteo ChiesaLaboratory of Energy and Nano-Science (LENS)

Solar Energy Group

Outline Flux Mapping Basics Beam Down Solar Thermal Concentrator Initial Results Forward Optical Model Bidirectional Reflectivity Distribution Function Convolution Model Results Discussion

Page 2

Flux Mapping Basics

SolarPACES 2011 Page 3

• Flux Mapping – A method to determine the distribution and quantity of concentrated solar radiation generated by a CSP facility

•Three Basic Tools• CCD Camera • Diffuse Reflector (Lambertian)• Heat Flux Sensor (HFS)

•CCD Camera –Luminance Map (cd/m2)• 300-750 nm

•HFS – Discrete flux measurement (kW/m2)• 300-3000 nm

•Conversion ratio (kW/cd)•Yields a continuous flux map of (kW/m2) Kaluza and Neumann, 1998

Diffuse Surface

Page 4

•Lambert’s Cosine Law (Lambertian surface)• The measured radiation intensity by an ideal diffusing surface is directly

proportional to the cosine of the angle between the observer’s line of sight and the surface normal (Photometria, 1760).

Source: www.odforce.net

•If the surface obeys Lambert’s Cosine Law, the radiation angle of incidence (AOI) and azimuth are inconsequential

•However, if the surface does not perfectly follow the Law, the reflected radiation to a stationary observer will change based on the radiation AOI and azimuth

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The Beam Down Solar Thermal Concentrator (BDSTC)

Page 5

Central ReflectorStructure

CCD Camera Location

Heliostat Field

Target Receiver

100 200 300 400 500 600 700 800 900

100

200

300

400

500

600

700

800

900

15

HFS Location

N

•Designed by Tokyo Tech, constructed by MES•100 kW/m2 peak flux at a net incident energy of 100 kWt•Flux measurement instrumentation

• Thermally regulated CCD camera (Konika Minolta CS-2000)• Eight in-situ calibrated (Mokhtar et al. 2011) Head Flux Sensors (Medtherm - Gardon,

Schmidt-Boelter)• Diffuse Reflecting Target (sandblasted unglazed tile)

•Goal – Determine the conversion coefficient to generate a flux map (HFS/CCD)

37

WE

SolarPACES 2011

Initial Results

Page 6

AM (blue) PM (red)

1 2 3 4

10

15

20

Air Mass

HFS

/CC

D(W

/Cd)

HFS 1

1 2 3 4

10

15

20

Air Mass

HFS

/CC

D(W

/Cd)

HFS 2

1 2 3 4

10

15

20

Air Mass

HFS

/CC

D(W

/Cd)

HFS 3

1 2 3 4

10

15

20

Air Mass

HFS

/CC

D(W

/Cd)

HFS 4

1 2 3 4

10

15

20

Air Mass

HFS

/CC

D(W

/Cd)

HFS 5

1 2 3 4

10

15

20

Air Mass

HFS

/CC

D(W

/Cd)

HFS 6

1 2 3 4

10

15

20

Air Mass

HFS

/CC

D(W

/Cd)

HFS 7

1 2 3 4

10

15

20

Air Mass

HFS

/CC

D(W

/Cd)

HFS 8

1 2 3 4

10

15

20

Air Mass

HFS

/CC

D(W

/Cd)

HFS 1

1 2 3 4

10

15

20

Air Mass

HFS

/CC

D(W

/Cd)

HFS 2

1 2 3 4

10

15

20

Air Mass

HFS

/CC

D(W

/Cd)

HFS 3

1 2 3 4

10

15

20

Air Mass

HFS

/CC

D(W

/Cd)

HFS 4

1 2 3 4

10

15

20

Air Mass

HFS

/CC

D(W

/Cd)

HFS 5

1 2 3 4

10

15

20

Air Mass

HFS

/CC

D(W

/Cd)

HFS 6

1 2 3 4

10

15

20

Air Mass

HFS

/CC

D(W

/Cd)

HFS 7

1 2 3 4

10

15

20

Air Mass

HFS

/CC

D(W

/Cd)

HFS 8

• Increasing trend due to spectral sensitivity differences between CCD camera and HFS

• (Kaluza and Neumann. 1998, Ulmer et al. 2002)

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1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6

0

5

10

15

20

25

Air Mass

Devi

atio

n (%

)

Spectral Influence on HFS/CCD Ratio

Kaluza and Nuemann - 1998Ulmer et al. - 2002

Beam Down Optics

Page 7

• The 360 degree field of heliostats contribute significantly different radiation quantities over the day

•Not observed in north field dominant towers and dishes due limited changes in cosine loss

•Needed to quantify the changing levels of radiation contribution from each heliostat over the day

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Optical Model Combined heliostat efficiency

Radiation Angle of Incidence on the Receiver

Radiation Azimuth direction

Mean AOI and Azimuth

Assumed Gaussian flux profile (no astigmatism)

Page 8

CCD Camera

HFS

X HFS

Y HFS

Y CR

X CRZ CR

AOI recφrec

CRMirror

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Optical Model

Page 9

-60 -40 -20 0 20 40 60

-150

-100

-50

0

50

100

150

Hour Angle

Dire

ctio

n (D

egre

es) f

rom

Nor

th

Change of Mean Radiation Azimuth

HFS1HFS2HFS3HFS4HFS5HFS6HFS7HFS8

-60 -40 -20 0 20 40 600

0.5

1

1.5

2

2.5

3

3.5

Hour Angle

AO

I fro

m N

orm

al

Change of Mean Radiation AOI

HFS1HFS2HFS3HFS4HFS5HFS6HFS7HFS8

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BRDF• Lambertian assumption?• A Bidirectional Reflectance

Distribution Function (BRDF) was constructed

Page 10

Source: NIST

( ) ( , )[ ( , )]( ) cos( ) ( )cos( )

r r r r ri r

i i i i i i i i

dL dLBRDF fL d L d

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BRDF Results indicated a significant backscatter reflectance,

consistent with a rough diffuse surface (Oren-Nayar model)

Page 11

-30 -20 -10 0 10 20 300.7

0.8

0.9

1

1.1

Theta - Detector

Nor

mal

ized

Ref

lect

ivity

10 AOI Lambertian Test

Measured DataCosine Law

-40 -20 0 20 400.7

0.8

0.9

1

1.1

Theta - Detector

Nor

mal

ized

Ref

lect

ivity

15 AOI Lambertian Test

-40 -20 0 20 400.7

0.8

0.9

1

1.1

Theta - Detector

Nor

mal

ized

Ref

lect

ivity

20 AOI Lambertian Test

-50 0 500.7

0.8

0.9

1

1.1

Theta - Detector

Nor

mal

ized

Ref

lect

ivity

25 AOI Lambertian Test

-60 -40 -20 0 20 40 600.7

0.8

0.9

1

1.1

Theta - Detector

Nor

mal

ized

Ref

lect

ivity

45 AOI Lambertian Test

0 20 40 60 800

0.2

0.4

0.6

0.8

1

1.2

Theta - Detector

Nor

mal

ized

Ref

lect

ivity

Normal AOI Lambertian Test

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BRDF

Page 12

Sun – AfternoonCCD Camera

θ θ

Peak Reflection

Reflected Ray Measured by CCD Camera

South East North West

L L

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Convolution By convoluting the optical

forward model with the BRDF, we can estimate the quantity of light which is measured by the CCD camera (Fmodel )and compare that to predicted light assuming a true Lambertian surface (Flambertian)

Page 13SolarPACES 2011

Results

Page 14

1 2 3 412

14

16

18

Air Mass

HFS/

CCD

HFS1

1 2 3 48

10

12

14

Air Mass

HFS/

CCD

HFS3

1 2 3 410

15

20

Air Mass

HFS/

CCD

HFS5

1 2 3 46

8

10

12

14

Air Mass

HFS/

CCD

HFS7

1 2 3 40.948

0.95

0.952

0.954

0.956

Air Mass

Fm/F

l

HFS1

1 2 3 40.95

0.952

0.954

0.956

0.958

Air Mass

Fm/F

l

HFS3

1 2 3 40.955

0.956

0.957

0.958

Air Mass

Fm/F

l

HFS5

1 2 3 40.95

0.955

0.96

0.965

Air Mass

Fm/F

l

HFS7

Fmodel / (Flambertian) Original Data

West WestEast East

AM (blue) PM (red)

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Discussion The AM/PM trend correlates to the HFS located on the East or

West side of the Diffuse Surface

By applying this methodology to many points across receiver (x,y), compensation for any non-Lambertian reflections allows for proper extraction of the flux levels on the surface and not the light levels reflected to and measured by the CCD camera

A homogeneous Diffuse Surface BRDF is critical to the success of this method

Page 15SolarPACES 2011

𝑘𝑊𝐶𝑑 𝑥 , 𝑦 ,𝜔𝑠

=(𝐻𝐹𝑆𝐶𝐶𝐷 )𝜔 𝑠

∗𝐹𝑚𝑜𝑑𝑒𝑙 (𝑥 , 𝑦 ,𝜔𝑠 )

𝐹𝑙𝑎𝑚𝑏𝑒𝑟𝑡𝑖𝑎𝑛 (𝑥 , 𝑦 ,𝜔𝑠 )

Limitations Assumed Gaussian flux distribution

and no astigmatism Modeled BRDF function valid for

only one location, not entire surface Inconsistent inter-HFS/CCD ratios

due to non-uniform tile surface (poor conditioning) HFS 2 had minimal AM/PM

difference – Measured BRDF was very

Lambertian HFS 5 had significant AM/PM

difference – Measured BRDF showed large back

scattering reflection

Due to non-uniform tile surface, we were unable to generate an accurate flux map

Page 16

1 2 3 4

10

15

20

Air Mass

HFS

/CC

D(W

/Cd)

HFS 1

1 2 3 4

10

15

20

Air Mass

HFS

/CC

D(W

/Cd)

HFS 2

1 2 3 4

10

15

20

Air Mass

HFS

/CC

D(W

/Cd)

HFS 3

1 2 3 4

10

15

20

Air Mass

HFS

/CC

D(W

/Cd)

HFS 4

1 2 3 4

10

15

20

Air Mass

HFS

/CC

D(W

/Cd)

HFS 5

1 2 3 4

10

15

20

Air MassH

FS/C

CD

(W/C

d)

HFS 6

1 2 3 4

10

15

20

Air Mass

HFS

/CC

D(W

/Cd)

HFS 7

1 2 3 4

10

15

20

Air Mass

HFS

/CC

D(W

/Cd)

HFS 8

1 2 3 4

10

15

20

Air Mass

HFS

/CC

D(W

/Cd)

HFS 1

1 2 3 4

10

15

20

Air Mass

HFS

/CC

D(W

/Cd)

HFS 2

1 2 3 4

10

15

20

Air Mass

HFS

/CC

D(W

/Cd)

HFS 3

1 2 3 4

10

15

20

Air Mass

HFS

/CC

D(W

/Cd)

HFS 4

1 2 3 4

10

15

20

Air MassH

FS/C

CD

(W/C

d)

HFS 5

1 2 3 4

10

15

20

Air Mass

HFS

/CC

D(W

/Cd)

HFS 6

1 2 3 4

10

15

20

Air Mass

HFS

/CC

D(W

/Cd)

HFS 7

1 2 3 4

10

15

20

Air Mass

HFS

/CC

D(W

/Cd)

HFS 8

SolarPACES 2011

Conclusion Method can be used to

minimize errors caused by non-ideal Lambertian surfaces

Simple forward optical model and BRDF can provide significant insight into the changing radiation measured by the CCD camera

Allows for less precise (cheaper, more rugged) surfaces to be used for flux analysis, as long as they are homogeneous

Page 17SolarPACES 2011

Thank you

Page 18

Questions?