INFLUENCE OF ARTIFICIAL SOILING ON THE POWER

LOSSES OF DIFFERENT PV TECHNOLOGIES

Jaione Bengoechea Apezteguía(jbapezteguia@cener.com)

SPANISH NATIONAL RENEWABLE ENERGY CENTRE (CENER)

1

INDEX

1 MOTIVATION OF THE STUDY

2 PREPARATION OF SOILED SAMPLES

3 MEASUREMENT OF TRANSMITTANCE (DIRECT AND HEMISPHERICAL)

4 CALCULATION OF THE POWER LOSSES

5 COMPARISON OF POWER LOSSES FOR DIFFERENT PV TECHNOLOGIES

6 VALIDATION WITH MEASUREMENTS

7 CONCLUSIONS

8 FUTURE WORK AND ACKNOWLEDGEMENTS

MOTIVATION OF THE STUDY

Soiling is an important matter, since it directly reduces the amount of light reaching

the solar cell

Very site dependent, but relevant in desert and agricultural environments

The power loss in a PV module due to soiling depends on:

The amount of soiling deposited onto the PV glazing

Inclination, orientation, rainfall, wind, coating…

The soiling characteristics (type, particulate size, shape…)

The type of PV technology

SiO2

OVERVIEW

SOILING OF GLASS SAMPLES:

four soiling materials at three inclinations

(0, 30, and 75)

Measurement of the direct spectral transmittance

Measurement of the hemispherical spectral

transmittance

Calculation of the power losses for concentration

technology

Calculation of the power losses for

crystalline Si, CdTe, and CIS technologies

Comparison of the sensitivity of the four PV technologies with

respect to soiling

SAMPLE PREPARATION

Four different types of artificial soiling were dispersed or dissolved in water

SaltSand

Fine dustCoarse dust

And spray deposited onto flat glass samples placed at three

angles with respect to the horizontal: 03075

030

75 Allowed obtaining a variation in the soiling superficial density (g/m2)

Salt purity of 99.8%Sand round quartz grains, dimensions (100-1000) m

Fine dust (mainly SiO2) particle size < 63 mCoarse dust (63-125) m

Soiling according to Standard HD 478.2.5 S1:1993-Classification of environmental conditions-Part 2: Environmental conditions appearing in nature-Section 5: Dust, sand, salt, mist

Inclination: 0 Inclination: 30 Inclination: 75

11.16 g/m2 2.81 g/m2 0.63 g/m2

2.14 g/m2 0.48 g/m2 0.16 g/m2

20.94 g/m23.36 g/m2 1.41 g/m2

2.04 g/m2 1.25 g/m2 0.74 g/m2

Salt

Sand

Fine dust

Coarse dust

MEASUREMENT OF DIRECT TRANSMITTANCE

Light sources (deuterium and

halogen) Optical fiber

Soiled sample (soiling and

glass) Optical fiber

Acquisition system (grating,

CCDs, electronics)

Only light from a very narrow angle was

detected; the angle of acceptance was of

approximately 0.6, similar to the angle of

acceptance of CPV systems

𝑇,𝑠𝑜𝑖𝑙𝑖𝑛𝑔 =𝑇,𝑔𝑙𝑎𝑠𝑠+𝑠𝑜𝑖𝑙𝑖𝑛𝑔

𝑇,𝑔𝑙𝑎𝑠𝑠

: 300 nm to 1600 nm

Value of direct transmittance used for the calculation of power losses of concentrating

photovoltaics (CPV)

=

Due to soiling inhomogeneity,

the direct transmittance was

measured in 21 positions,

each 5 mm diameter area

Transmittance was averaged

spectrally and spatially

MEASUREMENT OF DIRECT TRANSMITTANCE

0%

20%

40%

60%

80%

100%

300 500 700 900 1100 1300 1500

0%

20%

40%

60%

80%

100%

300 500 700 900 1100 1300 1500

0%

20%

40%

60%

80%

100%

300 500 700 900 1100 1300 1500

0%

20%

40%

60%

80%

100%

300 500 700 900 1100 1300 15000%

20%

40%

60%

80%

100%

300 500 700 900 1100 1300 1500

0%

20%

40%

60%

80%

100%

300 500 700 900 1100 1300 1500

0%

20%

40%

60%

80%

100%

300 500 700 900 1100 1300 1500

0%

20%

40%

60%

80%

100%

300 500 700 900 1100 1300 1500

MEASUREMENT OF HEMISPHERICAL TRANSMITTANCE

Light transmitted in all directions was detected (integrating sphere)

Illumination area was 20 mm diameter; measured only in 12 positions

Measured only for the samples with the highest soiling superficial density

Value of hemispherical transmittance used for the calculation of power losses of

crystalline silicon, CdTe, and CIS technologies

COMPARISON OF DIRECT AND HEMISPHERICAL TRANSMITTANCE

SoilingSample

inclination ()

Soiling

superficial

density

(g/m2)

Average direct

transmittance

(%)

(%)

Average

hemispherical

transmittance(%)

(%)

Salt

0 11.16 92.9 6 96.8 0.7

30 2.81 97.6 1.7

75 0.63 98.6 1.3

Sand

0 2.14 95.8 5.1 99.3 0.4

30 0.48 98.9 0.9

75 0.16 99.0 0.4

Fine

dust

0 20.94 0.0 54.7 34.4

30 3.36 54.7 34.7

75 1.41 70.9 18.0

Coarse

dust

0 2.04 69.9 36.9 93.2 3.9

30 1.25 93.2 9.0

75 0.74 97.1 1.6

*Standard deviation of the 21 or 12 measurements

COMPARISON OF DIRECT AND HEMISPHERICAL TRANSMITTANCE

For increasing soiling superficial density the transmittance decreases (as expected!)

For the same superficial density, the transmittance also depends on the soiling type

The direct transmittance is always lower than the hemispherical

0

20

40

60

80

100

120

0 5 10 15 20 25 30

So

ilin

g a

ve

rag

e t

ran

sm

itta

nc

e (

%)

Soiling superficial density (g/m2)

Salt directSand directFine dust directCoarse dust directSalt hemisph.Sand hemisph.Fine dust hemisph.Coarse dust hemisph

CALCULATION OF POWER LOSSES

𝐽𝑆𝐶 =

=300

=1600

𝑆𝑅 () × 𝐸0() × 𝑇()

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0 500 1000 1500 2000 2500

Spe

ctra

l irr

adia

nce

(Wm

-2 n

m-1

)

Wavelength (nm)

AM1.5G

AM1.5D

JSC is the short-circuit current density

SR is the spectral response

E0 corresponds to the solar spectral irradiance

T is the transmittance

T = 1 without soiling

Assumption: Power losses proportional to short circuit current density

Multijunction solar cells subcell with the lowest photocurrent limits

Multijunction

Crystalline SiCdTeCIS

AM1.5D AM1.5G

Direct Hemispherical

CALCULATION OF POWER LOSSES

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

300 400 500 600 700 800 900 1000 1100 1200

Tran

smit

tan

ce &

EQ

E (%

)

Spe

ctra

l irr

adia

nce

(W

·m–

2·n

m–

1)

-A

M1

.5G

Wavelength (nm)

Spectral Irradiance (AM1.5G)

EQE

Transmittance0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

300 400 500 600 700 800 900 1000 1100 1200

Tran

smit

tan

ce (

%)

∆Js

c (m

A/n

m)

Wavelength (nm)

Jsc Clean Glass

Jsc Fine Dust

Transmittance

Fine dust glass

(20.82 g/cm2)

Application example:

Crystalline

silicon PV cell

Global equivalent transmittance

COMPARISON OF POWER LOSSES

SoilingSoiling superficial

density (g/m2)Technology

Decrease in short

circuit current *(%)

Salt 11.16Crystalline Si, CdTe, CIS 1.4, 1.4, 1.4

CPV 7.1

Sand 2.14Crystalline Si, CdTe, CIS 0.7, 0.7, 0.7

CPV 4.9

Fine dust 20.94Crystalline Si, CdTe, CIS 47.5, 49.3, 47.7

CPV 100.0

Coarse dust 2.04Crystalline Si, CdTe, CIS 7.1, 7.3, 7.1

CPV 29.3

Power losses in CPV are always higher than for crystalline silicon, CdTe, and CIS (for the

same soiling). For crystalline silicon, CdTe, and CIS, losses are very similar

*Decrease calculated with respect to clean sample, (Jclean-Jsoiled)/Jclean

VALIDATION OF CALCULATIONS

Measurements carried out for crystalline silicon solar cells

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Cu

rren

t (A

)

Voltage (V)

I-V curves comparison

Clean glass

Salt (11.16 g/cm2)

Sand (2.14 g/cm2)

Fine dust (20.94 g/cm2)

Coarse dust (2.04 g/cm2)

Sample ISC (A) VOC (V) PM (W) VMP (V) IMP (A) FF (%) ∆ISC (%) ∆PM (%)

Clean glass 7.35 0.618 3.25 0.478 6.80 71.4% - -

Salt (11.16 g/cm2) 7.28 0.617 3.20 0.476 6.73 71.2% -0.9% -1.4%

Sand (2.14 g/cm2) 7.30 0.615 3.20 0.474 6.75 71.2% -0.6% -1.4%

Fine dust (20.94 g/cm2) 3.88 0.599 1.73 0.483 3.58 74.4% -47.2% -46.7%

Coarse dust (2.04 g/cm2) 6.90 0.611 3.02 0.473 6.38 71.5% -6.1% -7.1%

Measured parameters∆ regarding

Clean glass

VALIDATION OF CALCULATIONS

Calculation Measurements

SoilingSoiling superficial

density (g/m2)

Decrease in short

circuit current (%)

Decrease in short

circuit current (%)

Decrease in

power output (%)

Salt 11.16 1.4 0.9 1.4

Sand 2.14 0.7 0.6 1.4

Fine dust 20.94 47.5 47.2 46.7

Coarse dust 2.04 7.1 6.1 7.1

Measurements carried out for crystalline silicon solar cells

CONCLUSIONS

Salt, sand, fine dust, and coarse dust, were spray deposited onto glass samples

placed at different inclinations (0, 30, and 75) provided three soiling superficial

densities, for each soiling type.

Soiling superficial density ranged from 0.16 g/m2 to 20.94 g/m2.

The soiling superficial density was always higher for smaller inclinations.

For increasing soiling superficial density the transmittance decreased.

For the same superficial density, the transmittance also depends on the soiling

type.

The soiling direct transmittance was always lower than the hemispherical

transmittance. This is because light scattered by the soiling particles at angles higher

than 0.6º got lost, whereas this light was detected by the hemispherical set-up.

For the same soiling, the power losses in CPV are always higher than for crystalline

silicon, CdTe, and CIS technologies.

For crystalline Si, CdTe and CIS losses are very similar.

FUTURE WORK AND ACKNOWLEDGEMENT

The influence of abrasion which simulates the effect of a cleaning device or

sandblasting has also been evaluated by means of the same methodology.

Extend the study to the module/installation level.

Study the effect of coatings or self-cleaning patterns in the soiling.

ACKNOWLEDGEMENTS: Ignacio Sánchez, Miguel Murillo, Mikel Ezquer, Ana Rosa Lagunas

This work was financed by the Spanish Ministry of Science and Innovation, current Ministry of Economy and

Competitiveness (inside the INNPACTO call; reference IPT-2011-1468-920000 and co-financed by the ERDF) in its 2011 call

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