Literature Search on Partially Shaded PV Module Operation

Literature search on partially shaded PV module operation

Abstract

A literature search is provided detailing the history and current state of the art for dealing with partially shaded PV module operation.

Introduction

There is continued interest among photovoltaic installers, regulators and owners to obtain accurate information on PV systems operating under shaded or mismatched conditions. Off-nominal operating conditions are sometimes unavoidable, particularly if an otherwise productive solar installation is shaded for only part of the power producing day. This is particularly true in the application of building-integrated PV which often requires the integration of modules with existing structures in sometimes crowded urban environments. In the interest of expanding the number of PV installations worldwide and providing maximum benefit from these systems, it is useful to consider in more detail the power loss from partially shaded PV systems.

Regulatory agencies such as the CEC have a particular interest in obtaining information on shaded PV operation in order to accurately calculate rebate incentives for shaded systems. For incentive programs focused on installed capacity rather than actual kWh production, modeling the PV system and its access to the solar resource is required to obtain the expected performance of the system. Since shading of PV systems disproportionately reduces their power output, the rebate incentive needs to be adjusted accordingly.

Initial studies on shaded solar modules

Many prior studies have been conducted on the effect of shading and mismatch on PV system performance. Even starting with the first practical application of solar modules on the NASA Pioneer 1 mission, there was the understanding that bypass diodes parallel to the solar cell would benefit power production in shaded modules. [[1]] Additional effort was made to produce a simplified model of shaded module operation for future space mission analysis. [[2],[3]]

Other initial studies focused on reliability and performance of modules subjected to reverse-bias and localized hotspot degradation. These studies helped to institute guidelines for application of bypass diodes in multi-cell strings, and module test criteria like in IEC 61215 [[4]], ASTM E2481 [[5]] and UL 1703 [[6]]. This topic will not be discussed in much detail here, soan interested reader can find prior reviews on hotspot degradation and mitigation here: [[7],[8],[9],[10],[11],[12]].

Illumination models

Many illumination models have been considered for a partially shaded module. In these models, it is typically necessary to separately consider the direct and diffuse components of planar irradiance. The most obvious (and disruptive) type of shading on a PV module is the blocking of direct irradiance, which results in a visible shadow on the module. Since the direct component of irradiance comprises the majority of solar energy, this reduces the array output by the most significant amount. Blocking of a portion of diffuse irradiation can also reduce array output, but is not as readily apparent as blocking direct irradiation. Quaschning et al.[[13]] detailed the mathematical basis for a computer simulation, SUNDI which estimates seasonal illumination reduction from azimuth and elevation coordinates of surrounding shade obstructions. The method of Drif et al. [[14]] provides similar quantitative shading results, but for multiple points on the solar array plane, not just at the point at the center of the shading obstruction analysis. Other methods of shading estimation have been conducted with fisheye photographs [[15]] heliodon analysis of scale models [[16]] and multiple-photograph triangulation [[17]]. Correctly calculating incident illumination is the first step in analyzing a shaded solar cell.

Shaded-cell numerical models

An improvement on the JPL shaded-cell model in [2] was put forth by Bishop [[18]] to solve the 5-parameter equivalent circuit model of a shaded solar cell by numerical analysis. This analysis models both forward and reverse bias characteristics of the solar cell under varying illumination conditions, and because of then-recent improvements in computing power was able to solve the coupled equations under partially shaded conditions. The numerical solutions could then be applied to the condition of a series string of cells under inhomogeneous illumination. (Note that reverse bias operation was modeled as a reverse-voltage dependent shunt current) Further refinements in the technique included solving for a two-diode model rather than single-diode model (Quaschning [[19]]) making the analysis more accurate. Typical analysis considers that a shaded cell retains its original cell characteristics, excepting the diode photocurrent which is dependent on incident illumination. A later study investigated additional changes in the single-cell parameters of a shaded cell such as fill factor and shuntresistance [[20],[21]]. Further advances in the numerical study ofshaded solar modules include adapting the analysis to different computing environments, such as MATLAB™ [[22],[23]],utilizing an artificial neural network [[24]] and directly solving the governing equations by use modern mathematical analysis software such as Mathematica™. [[25]] The model of a shaded solar module was also mimicked in an analog circuit with knobs that adjust fill factor, illumination and temperature parameters. [[26]]

Shaded-cell numerical analysishas beenused in modeling several applications. The initial use of these numerical models was to investigate the placement of parallel bypass diodes. [18,19] Another investigation was to conduct a Monte Carlo analysis of mismatches in a typical large PV installation. [[27]] Based on statistical distribution of the peak power currents of the installed modules, the mismatch loss was determined to be less than 1% for this particular system. Several investigations were geared towards finding a more shade-tolerant array configuration through different interconnections or more frequent inverter placement. [[28], ???] Others focused specifically on thin-film amorphous Si modules [[29]] or modules including cells with differing reverse bias characteristics [[30]]. The conclusion of every investigation has been that the shadowing of a single cell reduces the output of the series string by an amount over-proportioned to the size of the shadow, some showing as much as 10x for a singe shaded cell in a long string. [23]

One analysis investigated the topic of system losses due to long-term module degradation of modules (or not?)

Experimental results

Initial experimental results

Individual cell measurements in a shaded module. Reverse characterisitics obtained for each cell for hot-spot mitigation. [[31]]

Shading should be kept to one row of cells if possible. Series connections are more shade susceptible than parallel connections. [[32]]

Statistical data on 180 installations in Japan. Average loss due to shading is 4.1%. [[33]]

Interarray shading at high ground cover ratios is mitigated by low tilt angle. Diffuse light blocking is a non-trivial contribution to shading losses. [[34]]

Assesment of shading losses is more accurate for faraway objects with regular shape. Nearby objects, or foliage changing with the season makes errors in shading estimate upwards of 10%. Module-level microinverters do not significantly reduce the effect of shading on an array. [[35]]

Shaded tracking solar system looking at central-inverter versus higher number of string inverters. String inverters do not outperform the central inverter. [[36]]

Using different sized shadows to estimate peak power losses to a grid-tied a-Si and c-Si system. [[37]] Shadows can result in power losses up to 3x their spatial extent on the panel.

Shading and degradation of a-Si modules. [[38]]

System optimization

Inter-array shading of tracking systems based on ground cover ratio. [[39]]

Shading mitigation strategies

Bypass diodes [[40]]

Backtracking [[41]]

AC modules / microinverters [[42], [43]]

String-level maximum DC/DC power point tracking [[44]]

Improved maximum powerpoint trackers for finding global maximum under shaded conditions. [[45],[46], [47]]

Conclusion

References:

[1]W. Baron, P. Virobik, “Solar array shading and a method of reducing the associated power loss”, NASA technical report serverdocument 19640022825 1964

[2]H. Rauschenbach, “Electrical output of shadowed solar arrays”, IEEE transactions on electron devices 18 vol 8, pp 483-490, 1971. Also available in JPL’s Solar cell array design handbook vol. 1Ch. 9.3.2, NASA technical report server Report Number: JPL-SP-43-38-VOL-1

[3]A. Gupta, A. Milnes, “Effects of shading and defects in solar cell arrays: a simple approach”, IEEE PV specialists conference 1981 p.1111-1116

[4]International Electrotechnical Commission (IEC) 61215, "Crystalline silicon terrestrial photovoltaic (PV) modules -design qualification and type approval", pp. 55-64 (2005)

[5] American Standards of Testing and Measurements (ASTM) E2481, “Standard Test Method for Hot Spot Protection Testing of Photovoltaic Modules” (2006)

[6]Underwriters Laboratory (UL) 1703, “Standard for Flat-Plate Photovoltaic Modules and Panels”, pp 51-57 (2004)

[7] J. Bishop, “Microplasma breakdown and hot-spots in silicon solar cells”, Solar Cells 26 pp. 335-349 1989

[8] G. Bhattacharya, C. Neogy, “Removal of the hot-spot problem in photovoltaic modules and arrays”, Solar Cells 31 pp. 1-12, 1990

[9] E. Molenbroek, D. Waddington, K. Emery, “Hot spot susceptibility and testing of PV modules” 22nd IEEE Photovoltaics specialist conference, 7-11 Oct. 1991

[10] W. Hermann, W. Wiesner, W. Vaassen, “Hot spot investigations on PV modules – new concepts for a test standard and consequences for module design with respect to bypass diodes” 26th IEEE Photovoltaics specialist conference, 1997

[11]A. Woyte, J. Nijs, R. Belmans, “Partial shadowing of PV arrays with different system configurations: literature review and field test results”, Solar Energy 74 pp. 217-233, 2003

[12] G. TamizhMani, S. Sharma, “Hot spot evaluation of photovoltaic modules” Proceedings of the SPIE p. 70480K, 2008

[13]V. Quaschning, R. Hanitsch, “Irradiance calculation on shaded surfaces”, Solar Energy 62 pp. 369-375, 1998

[14]M. Drif, P. Perez, J. Aguilera, J. Aguilar, “A new estimation method of irradiance on a partially shaded PV generator in grid-connected PV systems”, Renewable energy 33 pp. 2048-2056, 2008

[15]T. Tomori, K. Otani, K. Sakuta, K. Kurokawa, “On-site BIPV array shading evaluation tool using stereo-fisheye photographs”, IEEE PV specialist conference 2000, pp.1599-1602

[16]T. Blewett, M. Horne, R. Hill, “Heliodon prediction of shading on BIPV systems”, IEEE PV specialist conference 1997 pp.1393-1396

[17]S. Ike, K. Kurokawa, “Photogrammetric estimation of shading impacts on PV systems”, IEEE PV specialist conference, 2005

[18] J. Bishop, “Computer simulation of the effects of electrical mismatches in photovoltaic cell interconnection circuits”, Solar Cells, 25 pp. 73-89, 1988

[19]V. Quaschning, R. Hanitsch, “Numerical simualtion of current-voltage characteristics of PV systems with shaded solar cells”, Solar Energy 56, pp. 513-520, 1996

[20]V. Quaschning, R. Hanitsch, “Influence of shading on electrical parameters of solar cells”, IEEE PV specialists conference 1996 p.1287-1290, 1996

[21]E. Meyer, E. van Dyk, “The effect of reduced shunt resistance and shading on PV module performance”, 31st IEEE PV specialists conference January, 2005 pp. 1331-1334, 2005

[22]H. Patel and V. Agarwal, “MATLAB based modeling to study the effects of partial shading on PV array characteristics”, IEEE trans. Energy conversion 23 pp. 302-310, 2008

[23]S. Silvestre, A. Chouder, “Effects of shadowing on PV module performance”, Progress in Photovoltaics: res. Appl. 16 pp. 141-149, 2007

[24]E. Karatepe, M. Boztepe, M. Colak, “Development of a suitable model for characterizing photovoltaic arrays with shaded solar cells”, Solar Energy 81 pp. 977-992, 2007

[25]G. Petrone, G. Spagnuolo, M. Vitelli, “Analytical model of mismatched PV fields by means of Lambert W-function”, Solar energy materials & solar cells 91, pp. 1652-1657

[26]H. Nagayoshi, M. Atesh, “Partial shading effect emulation using multi small scale module simulator units”, IEEE PV specialist conference, 2005 pp. 1710-1713, 2005

[27]F. Iannone, G. Noviello, A. Sarno, “Monte Carlo techniques to analyse the electrical mismatch losses in large-scale photovoltaic generators”, Solar Energy 62 pp. 85-92, 1998

[28]N. Kaushika, N. Gautam, “Energy yield simulations of interconnected solar PV arrays”, IEEE trans. Energy conversion 18 pp.127-134, 2003

[29]A. Johansson, R. Gottschalg, D. Infield, “Modelling shading on amorphous silicon single and double junction modules”, 3rd World conference on PV energy conversion May 2003, Osaka, 2003

[30]M. Alonso-Garcia, J. Ruiz, W. Herrmann, “Computer simulation of shading effects in photovoltaic arrays”, Renewable energy 31 pp. 1986-1993, 2005

[31]M. Alonso-Garcia, J. Ruiz, F. Chenlo, “Experimental study of mismatch and shading effects in the IV characterisitic of a PV module”, Solar energy materials & solar cells 90 pp. 329-340, 2006

[32]A. Kovach, J. Schmid, “Determination of energy output losses due to shading of building integrated PV arrays using a raytracing technique”, Solar Energy 57 pp. 117-124, 1996

[33]K. Kurokawa, “Realistic values of various parameters fo PV system design”, Renewable Energy 15 pp.157-164, 1998

[34]M. Van Schalkwijk, A. Kil, T. Van der Weiden, “Dependence of diffuse light blocking on the ground cover ratio for stationary PV arrays”, Solar Energy 61 pp. 381-387, 1997

[35]A. Woyte, J. Nijs, R. Belmans, “Partial shadowing of PV arrays with different system configurations: literature review and field test results”, Solar Energy 74 pp. 217-233, 2003

[36]M. Garcia, J. Maruri, L. Marroyo, E. Lorenzo, M. Perez, “Partial shadowing, MPPT performance and inverter configurations: observations at tracking PV plants”, Progress in Photovoltaics: res. Appl. 16 pp. 529-536, 2008

[37]N. Chaintreuil, F. Barruel, X. Le Pivert, H. Buttin, J. Merten, “Effects of shadow on a grid connected PV system”, Proceedings, 23rd EU PVSEC conference, 2008 p.3417

[38]A. Simmons, D. Infield, “Grid-connected amorphous silicon PV array”, Progress in photovoltaics 4 pp. 381-388, 1996

[39]J. Gordon, H. Wenger, “Central station solar PV systems: field layout, tracker and array geometry sensitivity studies”, Solar Energy 46 pp. 211-217, 1991

[40]W. Baron, P. Virobik, “Solar array shading and a method of reducing the associated power loss”, NASA technical report Document 19640022825 (1964)

[41]D. Panico, P. Garvison, H. Wenger, D. Shugar, “Backtracking: a novel strategy for tracking PV systems”, IEEE PV specialists conference 1991 pp. 668-673, 1991

[42]M. Gross, S. Martin, N. Pearsall, “Estimation of output enhancement of a partially shaded PV array by the use of AC modules”, IEEE PV specialist conference - 1997 pp. 1281-1384, 1997

[43]Y. Kanai, S. Matsumoto, “A novel PV module for severe shade conditions”, IEEE PV specialist conference 2006 pp. 2174-2176, 2006

[44]E. Karatepe, T. Hiyama, M. Boztepe, M. Colak, “Voltage based power compensation system for PV generation under partially shaded insolation conditions”, Energy conversion and management 49 pp. 2307-2316, 2008

[45]R. Bruendlinger, B. Bletterie, M. Milde, H. Oldenkamp, “Maximum power point tracking performance under partially shaded PV array conditions”, 21st European PV energy conference 4-8 Sept. 2006

[46]N. Ahmed, M. Miyatake, “A novel maximum power point tracking for photovoltaic applications under partially shaded insolation conditions”, Electric power systems research 78 pp. 777-784 2008

[47]H. Patel and V. Agarwal, “Maximum power point tracking scheme for PV systems operating under partially shaded conditions”, IEEE trans. Ind. Elec. 55 pp.1689-1698, 2008