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Optical Characterization of PV Glass Coupons and PV Modules Related to Soiling Losses

Optical Characterization of PV Glass Coupons and PV Modules Related to Soiling Losses,
Greg P. Smestad, Ph.D., Sol Ideas Technology Development
December 6th, 2017, 11:35 AM - 12:00 PM
Session 5: Characterization (Chair: Xiaohong Gu, NIST)
Atlas/NIST Workshop on PV Materials Durability
December 5-6, 2017, Gaithersburg, Maryland
National Institute of Standards and Technology, Gaithersburg, Maryland
https://www.nist.gov/el/mssd/agenda

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Optical Characterization of PV Glass Coupons and PV Modules Related to Soiling Losses

  1. 1. Greg P. Smestad, Ph.D. Sol Ideas Technology Development Leonardo Micheli, Ph.D., Na>onal Renewable Energy Laboratory (NREL) Thomas A. Germer, Ph.D., Na>onal Ins>tute of Standards and Technology (NIST) Eduardo F. Fernández, Ph.D. University of Jáen, Spain 12/11/17 Op#cal Characteriza#on of PV Glass Coupons and PV Modules Related to Soiling Losses Session 5: Characterization (Chair: Xiaohong Gu, NIST) December 6th, 2017, 11:35 AM - 12:00 PM
  2. 2. •  Soiling Overview •  Ångström formula •  Experimental Procedure •  Transmission Results •  Coupon vs. PV Module •  Angle of Incidence effects –  Model vs. Experiments •  Conclusions Outline Ambient Particulates (PM) Deposited PM Reduces Transmittance to PV Graphic: M.H. Bergin et al, Duke University, Durham, NC
  3. 3. Ambient Particulates (PM) Deposited PM Reduces Transmittance to PV Graphic: M.H. Bergin et al, Duke University, Durham, NC "Large Reduc>ons in Solar Energy Produc>on Due to Dust and Par>culate Air Pollu>on", Mike H. Bergin, Chinmay Ghoroi, Deepa Dixit, James J. Schauer, and Drew T. Shindell, Environ. Sci. Technol. LeX., 2017.
  4. 4. Soiling: Defini#on Accumula>on of dust, dirt and par>cles on the surface of PV modules. Drop in power output: can be > 50%. Drop in energy yield: 0 to 6% in the U.S. 4
  5. 5. https://earth.nullschool.net/#current/particulates/surface/level/ overlay=pm2.5/ 5 Real time visualization of PM2.5
  6. 6. Defini#on of Par#culate MaBer ø < 10 um ø < 2.5 um Particulate matter (PM): concentration (µg/m3) of solid particles and liquid droplets suspended in 1 m3 of air. PM10 Sources: Crushing or grinding operations Dust stirred up by vehicles on roads PM2.5 Sources: Motor vehicles, power plants, residential wood burning, forest fires, agricultural burning, industrial processes. Source: https://www.airnow.gov/index.cfm?action=aqibasics.particle6
  7. 7. PV EL and Soiling associated with a more constant voltage potential between the external glass surface and the active cell circuit of the module—but lower net current magnitude [17]. a b Fig. 4. Subtractive electroluminescence images taken at 4.1-A forward-bias current on module type A with salt; (a), 73.3% power remaining; (b) without salt, 84.7% power remaining. Degraded areas appear dark. Peter Hacke, et al, Effects of Photovoltaic Module Soiling on Glass Surface Resistance and Potential-Induced Degradation, Conference Paper, NREL/ CP-5J00-64492 December 2015 7
  8. 8. Diagram Courtesy of Al Hicks, NREL8 The Optical Path of the Light (clean vs. soiled)
  9. 9. Particle size is smaller than one- tenth the wavelength of light DIFFUSE (a) RAYLEIGH SCATTERING Particle size is approximately one-fourth the wavelength of light DIFFUSE Particle size is larger than the wavelength of light (b) MIE SCATTERING An Introduction to Solar Radiation, Muhammad Iqbal, Academic Press, New York, 1983, Chapter 6. Atmospheric Scattering 9 can be applied to particles on the glass
  10. 10. β → 0.0 to 0.5 or even higher •  An index representing the amount of aerosols (particles) present •  Wavelength (λ) exponent •  Generally 0.5 to 2.5 (Ångström suggested 1.3) ma is the optical path length Ångström turbidity formula 𝛼=4 for small non-absorbing particles 𝛼=1 for small absorbing particles 𝛼=0 for large particles 10
  11. 11. Spectral impact of soiling: Experiments 11
  12. 12. Experimental procedure -  Seven 4 cm x 4 cm x 3 mm-thick low iron glass coupons shipped to each location. -  Coupon 1 to 6 installed outdoors at zero tilt angle for eight weeks. -  Coupon 0 kept in a dust free container and used to calibrate each spectrophotometer. -  Weekly transmission measurements for coupons 1, 2, 3. -  Daily weather and particulate matter (PM) concentration recorded. -  A dry cleaning is performed by using a microfiber cleaning cloth. •  Coupon 1 cleaned every week, Coupon 2 every four weeks. 12
  13. 13. Aaer 8 weeks (San José, CA) 13
  14. 14. Ångström equation returns in both cases high R2 (≥90%), and low RMSE (<0.3%). Fidng San José and Chennai Data (aaer 8 weeks)
  15. 15. Ångström equation returns in both cases high R2 (≥90%), and low RMSE (<0.3%). …but fitting at low wavelengths can still be improved. Fidng San José and Chennai Data (aaer 8 weeks) 15
  16. 16. First results Absolute direct and hemispherical transmittance of coupon 3 in Golden, CO. Wavelengths between 500 and 1100 nm have been averaged.
  17. 17. Hemispherical transmittance (8 weeks) Hemispherical transmittance in the visible and NIR range of coupon 5 for all the sites (above) and for the low soiling sites (below), referenced to the transmittance of coupon 0. The spectra were measured using a PerkinElmer Lambda 1050 UV/Vis spectrophotometer with a 150 mm integrating sphere at NREL and processed using a local regression technique to remove noise.
  18. 18. Par>cle Area and Hemispherical TransmiXance •  Average particle area was determined by high resolution optical microscopy. •  Percentage of the surface covered by particles was estimated. •  A linear correlation, with R2 higher than 0.99, is found by comparing the percentage area covered by particles to the hemispherical transmission. •  The broadband hemispherical transmission could be directly obtained from the coverage area, independently of location, dust type and composition. •  See the subsequent plot (slide). •  Broadband hemispherical transmittance (300-2500 nm) was measured. 18
  19. 19. Fractional Loss versus Particle Coverage 5% 10% 15% 20% 25% 30% 35% 40% 0% 5% 10% 15% 20% 25% FractionalParticleCoverage Fraction Loss slope = 0.80±0.02 R2 = 0.99 19
  20. 20. The Dust & Rain in Spain falls mainly… Predication: 6-months of outdoor exposure at Jáen, Spain. Estimated weekly evolution of Soiling Ratio index estimated for three PV materials. 20 From the transmission of glass coupons times the solar spectrum times the PV spectral response or QE. There was a Saharan dust storm at week 6 and then rain.
  21. 21. 28 Chapter 2 Fig. 2.10 Geometry for calculation of the absorptivity from a polished (left side) and textured (right side) light absorber. In each case, a summation of multiple reflections must be made in order to calculate the absorptivity. The angle q is measured from the surface normal. These equations can be used to predict the reflectivity, otherwise known as the reflection coefficient, of a solar cell absorber material. The index of n value for Si, for example, is approximately 4 at a wavelength of 600 nm (see Fig. 2.3). Library of Congress Cataloging-in-Publication Data Smestad, Greg P. Optoelectronics of solar cells / by Greg P. Smestad. p. cm.-- (SPIE Press monograph ; PM115) Includes bibliographical references and index. ISBN 0-8194-4440-5 (softcover) 1. Solar cells. 2. Optics. I. Title. II. Series. or soiled Op>cs of a PV Module (right) does not match that of the coupons (lea) or clean glass air
  22. 22. iθ rθ rφ i 180φ = ° rdΩ β Angular Measurements and Model scattering azimuth or “out-of-plane scattering angle” scattering angle incident angle differential solid angle incident azimuth sample rotation Thomas A. Germer, Sensor Science Division, NIST
  23. 23. Describing the scaBer from a delocalized scaBerer The Bidirectional Scattering Distribution Function (BSDF) is the fraction of power scattered per unit projected solid angle. r r 0 i r lim cos P f P θ→Ω Ω = [sr-1] Scattered power [W] Incident power [W] Solid angle [sr] Polar scattering angle Thomas A. Germer, Sensor Science Division, NIST23
  24. 24. IEST-STD-CC 1246E Distribution The cumulative distribution is the number of particles between 1 µm and D. Shown here are the distribution density, dN(D)/ dD, and the distribution density weighted by area and volume. Notice how much the peak appears to change. It is hard to eyeball a peak size in a distribution from an image. 1 10 100 1000 0.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 NormalizedDistributionDensity D / µm N(D) N(D) D2 N(D) D3 Thomas A. Germer, NIST 24
  25. 25. -90° -60° -30° 0° 30° 60° 90° 10- 5 10 - 4 10- 3 10 - 2 10- 1 10 0 101 10 2 103 10 4 105 C7 C3D Control Model BTDF/sr-1 θt The Transmission Data (San José) Thomas Germer, NIST25
  26. 26. Reflectance for collimated incident light Ωr With a lot of experimental geometries, the incident light is close to collimated. Then, the reflectance is r r i i r r i i r r r r r r r r r i i r r ( , ; ) ( , ; , ) cos d d d sin cos ( , ; , ) f f ρ θ φ θ φ θ φ θ φ θ θ φ φ θ θ θ Ω Ω =Ω Ω = ∫ ∫ Program INTEGRATED.MIST calculates the hemispherical reflectance for any BRDF model. Thomas A. Germer, NIST
  27. 27. -90° -60° -30° 0° 30° 60° 90° 10- 4 10 - 3 10 - 2 10- 1 10 0 10 1 102 10 3 10 4 105 2.5° 70° 60°C7 Model BRDF/sr-1 θs 45° The Reflection data Thomas A. Germer, NIST
  28. 28. 0° 30° 60° 90° 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 ρ(θi ,2π) θi specular reflectance of glass IEST-STD-CC 1246E Cleanliness 1000 Predicted Reflectivity vs incidence angle Thomas A. Germer, NIST28
  29. 29. 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0.026 0.027 0.028 0.029 0.030 0.031 0.032 ρ(θi ,2π) λ / µm Calculated Hemispherical Reflectance of Soot Particles Using IEST-STD-CC 1246E Distribution Cleanliness 1000 Thomas Germer, NIST29
  30. 30. Conclusions (Optics of PV soiling) •  Estimating soiling losses using the transmission from glass coupons may not easily translate to knowledge about power losses from PV modules. •  The affect on direct transmission is greater than hemispherical. •  There is a linear correlation between the area covered by particles and the broadband hemispherical transmittance. •  Soiling produces a higher attenuation at shorter wavelengths (Ångström turbidity formula) compared to longer wavelengths. •  The impact of soiling is likely higher on PV materials with larger bandgap (a-Si, CdTe). •  Soiling Losses are certainly a function of input angle.
  31. 31. •  EPSRC SUPERGEN SuperSolar Hub’s “International and industrial engagement fund” for the project “Global investigation on the spectral effects of soiling losses” •  University of Exeter, UK; National Renewable Energy Laboratory (NREL), USA; University of Jáen, Spain; Sol Ideas Technology Development, San José (CA), USA; Tezpur University, Tezpur, India; Robert Gordon University, Aberdeen, UK; South Valley University, Qena, Egypt; British University in Egypt, El Sherouk City, Egypt; BITS Pilani, Dubai Campus, Dubai, UAE; Academy of Scientific and Innovative Research, Chennai, India; CSIR-Central Electronics Engineering Research Institute, Chennai, India; Indian Institute of Technology Madras, Chennai, India. Acknowledgments Greg P. Smestad, Sol Ideas Technology Development smestad@solideas.com — www.solideas.com
  32. 32. The International PV Quality Assurance Task Force (PVQAT, "PV cat”) TG-12. Website: http://www.pvqat.org Soiled Residential PV System of David Bernal in Los Angeles (Photo by Greg Smestad) Greg P. Smestad, Sol Ideas Technology Development smestad@solideas.com — www.solideas.com
  33. 33. Background, Supporting Information Leonardo Micheli, Eduardo F. Fernández, Greg P. Smestad, et al, "A unique global investigation on the spectral effects of soiling losses of PV glass substrates: preliminary results". •  Paper & Poster available •  Presented at ieee-pvsc.org/PVSC44/ •  Download the materials for free here: http://www.solideas.com/projects/pvquality/index.html
  34. 34. SEM (soiling site: San José, CA) SEM image of particles from soiling in San José, CA. Photo: Greg Smestad/NREL, Helio.Moutinho@nrel.gov
  35. 35. 6.2 Attenuation of Direct Solar Radiation 109 / *Οηλ Figure 6.2.2 Attenuation of direct radia- tion through a homogeneous atmosphere. J χ ηλ or Beer's law. According to Bouguer's law, the attenuation of light through a medium is proportional to the distance traversed in the medium and to the local flux of radiation. This simple law applies in most instances (with the exception of absorption by gases), whether the energy is absorbed or scat- tered. Moreover, this law applies to monochromatic radiation only. In terms of this law, 4A = A)nAexp(-/cAm), (6.2.1) where kx is the monochromatic extinction or attenuation coefficient, m is the optical path length, and kxm is the monochromatic extinction optical thickness3 (dimensionless). Bouguer's law - the attenuation of light through a medium is proportional to the distance traversed in the medium. kλ is the monochromatic extinction or attenuation coefficient m is the optical path length kλm is the monochromatic extinction optical thickness particularly useful when one or more of the atte not obey the simple Bouguer's law. The monoch ue to direct radiation can be written as ^ = r L = ΪΊΚΑ, transmittance due to a single process i and τλ is due t ined. The transmittance due to a single process is g τίλ = exp(-feamI). w accurately represents attenuation processes of scAn Introduction to Solar Radiation, Muhammad Iqbal, Academic Press, New York, 1983, Chapter 6. 3 5
  36. 36. Figure 6,6,1 Variation of the horizontal visibility with a and ß. I.OP * 0.8 Id U i 0.6 Z < O 0.4 o £T UJ < 0 . 2 h 0.0 L^^^^^iSm « 0.05 Γ x •" • • J ^.-~ -■» «^* ^«-~·-"""~ l· " ^-^* ^-~—-·""" ψΛ --*·*^ 1 ^>^ „--' / ?s^ / ' ^. L x' ' ^ϊ^Γ X / X ^^<^/ / ' ^ ^ ' '/ ' s ^*^ *' / ' ~ <ί^^ ^ r/ / ^>^^ / ^^/ / ^^ ' ^*— Ϊ / /? /^'^Γ / / ' '^^ / / ' ' %έ^1 / / / Jr / / /^y a = 0.7 ψ /<>' —.-..31 ^* s st '{ySS * I'-*' 1 1 1 t 1 1 1 _L ^***^ ~Ί 0.4 2 . 00.8 1.2 1.8 WAVELENGTH ( p ) Figure 6,6,2 Aerosol spectral transmittance as a function of a and ßm. An Introduction to Solar Radiation, Muhammad Iqbal, Academic Press, New York, 1983, Chapter 6.
  37. 37. First results: Coupon 5 (2 months of outdoor exposure) Broadband hemispherical transmittance (300-2500 nm), average particle area, and percentage of the surface covered by particles, measured at the end of the data collection. Unsoiled PV glass transmittance is 90.4% •  A linear correlation, with R2 higher than 0.99, is found by comparing the percentage area covered by particles to the hemispherical transmission •  The broadband hemispherical transmission could be directly obtained from the covered area, independently of dust type and composition.
  38. 38. Quantum Efficiency (spectral response of PV technologies)

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