Small Photovoltaic Module with Rectangular

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1 Small Photovoltaic Module with Rectangular Cells for Reducing Output Degradation Caused by Spot Dirt Shin-ichi Kobayashi*, Tomonori Iino**, Hironori Kobayashi*, Kazumasa Yamada*, Toshiaki Yachi* *Tokyo University of Science , Kudankita, Chiyoda-ku, Tokyo, 12-73, Japan **NTT FACILITIES, INC. Granparktower, Shibaura, Minato-ku, Tokyo, 18-23, Japan Abstract - This paper reports the output degradation reduction when spot dirt adheres to a small photovoltaic module with rectangular cells. A small photovoltaic module of 13 W was applied to various electric systems. The module consisted of single crystal silicon photovoltaic cells. The effect of the output degradation was studied by changing the aspect ratio of the photovoltaic cell. It was also studied the effect by changing the photovoltaic cell connection applied to the module. Changing the aspect ratio of the photovoltaic cell used for the photovoltaic module results in suppression of the output degradation by 8% or less with 3% of spot dirt on the module area. I. INTRODUCTION Photovoltaic (PV) power supplies that are not evenly distributed by region are used as electric power sources for stand-alone systems [1]. The load on a stand-alone system consists of a small-scale radio base station, a telemeter, and lighting. Now, in the stand-alone PV system, installation with a small photovoltaic module simple substance is increasing by power-saving of electric devices. The output of a photovoltaic module is reduced when installed in an environment with no sunlight. The decrease in the output rate due to vehicle exhaust or particulates in the atmosphere is approximately % [2]. On the other hand, reduction of the acceptance surface area of a cell, by the excrement of a bird or spot dirt such as leaves, greatly reduces the output. For example, an output reduction of 5 has been observed when the spot dirt occupied 3 of the module area of a small 13 W photovoltaic module [3]. However, because the voltage drop with a by-pass diode cannot be permitted when a small module is used alone with all photovoltaic cells connected in series, a different strategy to address output degradation is required. In this study, we examine the effect of using different aspect ratios of the cell shape, and the effect of forming a module output into a large current cell array. Application of the changes and the effect on the output degradation caused by spot dirt are reported.. STAND-ALONE PHOTOVOLTAIC SYSTEM CONSISTING OF A SMALL PHOTOVOLTAIC MODULE Fig. 1 shows the composition of a general stand-alone photovoltaic system. The electrical load is small in this case, the output degradation of the module applied to the standalone photovoltaic system affects stable operation of a system, when what is necessary is just to apply one small photovoltaic module to a system. A decrease in output is caused when the photovoltaic module is installed in an environment without light. Shortage of daylight hours due to weather, a module surface that is covered by dirt, snow, etc., are factors in the reduction of light received. When a module array construction is used for the system, reduction in the amount of power generation of some modules in the array has no overall adverse affect, and smoothing of the amount of power generation can be planned for the whole array. However, the output smoothing by the module array cannot be performed in a system that applies the module simple substance. Fig. 1. Sunlight Photovoltaic module Charge/Load controller + Load Battery The block diagram of a stand-alone PV system for telecommunication equipments /6/$2. )26 IEEE

2 Various materials and structures are used in a photovoltaic cell. For example, silicon single cell open-circuit voltages are typically in the range of.5-.6 V, so that a module should consist of cells connected in series for standard loads[4]. Fig. 2 shows the equivalent circuit of a photovoltaic cell. The equivalent circuit consists of photoelectromotive force Iph, the diode by auto-bias, the in-series resistance R] by the ohmic contact resistance or wiring resistance and the parallel resistance Rsh by leakage current of a junction defect. Since they are miniaturized, the small photovoltaic modules make the in-series connection of cut photovoltaic cells. If, for the output of a photovoltaic module, the module surface is covered with dirt etc., Iph will become small. As for the module output, it will be influenced by Rsh. When all cells are covered and Iph =, the covered cell serves as a resistance component inside the module, and the module output is greatly decreased. A bypass diode Dpass is inserted, as indicated by the dotted line in Fig. 2. Even if the output reduction of an individual photovoltaic cell is avoided by the D'pas,, the voltage that the system requires may not be obtained. The small photovoltaic modules consist of cells of cut silicon wafer after cell-sizing for miniaturization. By cutting a wide cell, this technique is realizable. However, extreme widening is difficult for a silicon wafer cell. If a photovoltaic cell is not made of silicon wafer, but instead of an amorphous silicone, and thin film cells such as CIGS(Copper Indium Gallium Selenium), the flexibility of the cell shape increases and widened cell shapes will be easily realizable. 7 -I (c) Fig. 3. Photovoltaic cell shapes with spot dirt: Aspect ratio 1., aspect ratio.25, (c) aspect ratio.3. \7 7 D'pas" B. Change of the photovoltaic cells connection applied to a module When dirt adheres to arbitrary places in a module, the module output is determined by the dirty cell in the small Fig. 2 Equivalent circuits for a small photovoltaic module: Single cell, connected in series. I. THE PROPOSED REDUCTION METHODS OF OUTPUT DEGRADATION CAUSED BY SPOT DIRT DC/DC A. Change of the photovoltaic cell shape applied to a module The area of the photovoltaic cell used was the same as that shown in Fig. 3, and circular dirt adhered to cells with different aspect ratios of 1.,.25,.3 are shown. The area that is not covered with the dirt spot increases as the cell shape becomes wider. Therefore, output degradation may be improved by widening the shape of the photovoltaic cell used for a module. Fig. 4 Equivalent circuits of a 2 parallel conection module.

3 photovoltaic module that all cells are connected to in series. If some cell arrays are constructed within the module, an output is still possible except from the array to which dirt has adhered. If the number of parallel connections is increased in the cell array of a module in order to reduce output degradation, then the output degradation caused by one dirtied cell is improved. Fig. 4 shows an example of a 2 parallel configuration module. For Fig. 4 and, if spot dirt which covers all unit cells adheres in one of the domains, a portion will cause resistance and a significant drop in the output will result. In order to prevent such an adverse effect, it is necessary to insert an ordiode, as shown in Fig. 4. Because modular output voltage will decline if the number of parallel connections is increased, a step-up converter is required. A block diagram of a system with a step-up DC-DC converter is shown in Fig. 5. Sunlight Photovoltaic module used by the cell which cut the 13mm one-side photovoltaic cell into 1/4. The area of the cut photovoltaic cell is 259mm2. The module consists of 34 cells connected in series. The dirt spot used in the study was circular and its diameter was varied from 5 to 1 mm. The 3 positions of the dirt spot were determined using a random number method. In the simulation, the module output P was determined by the following formula [3]. Cell area without dirt PO = Cel area A. Thefeature ofthe PVmodule with rectangular cells Two small photovoltaic modules were designed with which conventional and different aspect ratios were used as the strategy against output degradation by spot dirt. A total of three modules were set as the simulation target. The three modules with different cell aspect ratios are shown in Fig. 6 and their specifications are given in Table. Since the cell area of each module was identical, the electrical specifications were also the same. max (1) Step-up DC-DC converter Charge/Load controller t Load A cell aspect rate.25 Battery Fig. 5. Block diagram of a system with a step-up DC-DC converter. A cell aspect rate 1. IV. EVALUATION METHODS The electrical characteristics of the small photovoltaic module used in the study are given in Table I [5]. The module area is 378 x 349 mm (131,922 mm2). Each cell is made from single crystal silicon. The module was TABLE I ELECTRICAL CHARACTERISTICS OF THE SMALL PHOTOVOLTAIC MODULE Peak power Pmax 13 W Peak power voltage Vpmax 16.4 V Peak power current Ipmax.79 A Open circuit voltage voc 2.5 V Short circuit current isc.86 A Data at Standard Test Conditions: 1 W/m,2 25C, AM1.5 Fig. 6. Designed PV modules. TABLE (c) A cell aspect rate.3 SPECIFICATIONS OF MODULES WITH CHANGED CELL ASPECT RATIO Type of module (c) A cell aspect ratio Module area (mm2) Cell size 51.5 X X X Cell area (mm2) The number of cells Peak power-pm(w)

B. Thefeature ofthe PVmodule connected in parallel A small photovoltaic module was designed with an increased parallel number of cell arrays as another strategy to address output degradation by spot

4 B. Thefeature ofthe PVmodule connected in parallel A small photovoltaic module was designed with an increased parallel number of cell arrays as another strategy to address output degradation by spot dirt. The module was designed by dividing the original module into two right and left domains, and it was considered as a 17 series-2 parallel (refer to Fig. 7) module. The electrical characteristics of the designed module are given in Table I. With this cell array, the designed module has an equal output to the original module, and the current is doubled. The ordiode of Fig. 4 assumed the Schottky barrier diode. The voltage drop of this Schottky barrier diode was set as.36 V. Circular dirt (6 mm diameter) at 3 places determined by random number is shown in Fig. 8. The spot dirt indicated by the hatched circles shown in the center of a module of Fig. 8 is dirt which influences the output of the cell array for both the right and left (domain of and ) of the module, as shown in Fig. 7. Regarding the dirt over the cell array, the module output calculated from that denominator(cell area) of Eq.(1) was doubled. Fig. 8. Schematic diagram ofthe photovoltaic module with 3 times attack of 6 mm diameter dirt spots. V. RESULTS Effects by changing the photovoltaic cell shape Fig. 9 shows the average output for each aspect ratio when there was spot dirt on the cell area. A cell aspect ratio of.2 or smaller greatly improves the module output. When the ratio of the short side of a cell and the long side is 1:27, this cell aspect ratio is.2. The error bars shown in Fig. 9 is the standard deviation from the average value. The standard deviation is reduced as the aspect ratio becomes small. That is, as the aspect ratio becomes smaller, a reduction in the output degradation is more obvious. A. 1% 9% 8% Fig. 7. The PV module connected in 2-parallel. 7% 6% TABLE I -1c ELECTRICAL CHARACTERISTICS OF PHOTOVOLTAIC MODULES WITH SERIES CONNECTION AND 2 PARALLEL CONNECTION 5% I 4% 3% 2% Series 2-parallel Pmax 13 W 13 W 8.2 V Peak power current Vpmax Ipmax 16.4 V.79 A 1.58 A Open circuit voltage VOC 2.5 V 1.25 V Short circuit current isc.86 A 1.72 A Peak power Peak power voltage % Aspect ratio.8 Fig. 9. PV module output power with 6 mm diameter dirt spots depended on cell aspect ratio.

5 Fig. 1 shows an output power histogram for cells with dirt spots that are the same area as the area of a single cell (6 mm in diameter) for each of the three modules shown in Fig. 6. The mode of output was shifted from 6%, in the case where the aspect ratio is 1., to 9/ when the aspect ratio was.3. The dependence of the photovoltaic module output on the diameter of the spot dirt is shown in Fig. 11. The rate of output reduction to dirt is small for a large module with a small aspect ratio v 1 5 L Aspect ratio 1. 9 Aspect ratio.25 _ M Aspect ratio.3 _ % 2% 3% 4% 5% 6% 7% 8% 9% 1% The rate ofoutput power (/o) Fig. 1. Histograms of PV module output power with 6 mm diameter dirt spots depended on cell aspect ratio. 1% 9% 8% 7% 2-" 6% ; 5% Wo 4% m 3% EH 2% % FM F] n j Diameter ofcircuar dirt(mm) 8 1 Fig. 11. PV module output with cell aspect ratio.3 and 1. dependence on the diameter of the dirt area. B. Effects by changing the photovoltaic cell array The dependence of the photovoltaic module output on the dirt spot area is shown in Fig. 12. Even when there is no dirt for the power loss by the ordiode, the output declines. There is an improvement in the output degradation when the dirt spot diameter is more than the same area as a single cell area. A r/a Fig. 13 shows the relation of the output power to the dirt area fraction of the module. For the module with 2-parallel, there is an effect which reduces the output degradation from 2.7% or more dirt on the module area. When a converter with 9O efficiency is added, it is effective for 4.2% or more dirt on the module area. 1% 9% \ 8% u 7% 2m 6% ' 5% Wo 4% E 3% E 2% % Area ofcircular dirt(mm ) 2 8 Fig. 12. PV module output with series and 2-parallel as a function ofthe dirt spot area. IC-., I-W u C) $:I. '5 $:I. -5 C) 4--. C) u.-o u p- 1% 9% 8% 7% 6% 5% 4% 3% 2% % The dirt spot area/module area.6 Fig. 13. The relationship between the rate PV module of output power with series and 2-parallel and the dirt area fraction in the module. The dependence of the photovoltaic module output on the diameter of the dirt spot area is shown in Fig. 14. The module output decreases even when there is no dirt to cause electric power losses through the diode and converter. When a stepup converter with a conversion efficiency of 9/ is added, the losses from degradation can not be compensated if the diameter of the dirt spot is 8 mm or more. In the case where the converter efficiency is 8%, there is an improvement of output degradation from dirt spots of 86 mm or larger. When the difference of converter efficiency is, the difference of the diameter with the improvement effect of spot dirt is only 6mm.

6 1% [4] Roger Messenger, Jerry Ventre, The PVModule 9 Photovoltaic systems Engineering. Boca Raton: CRC g < ~~~g 8 ~~~~~~~~~~Press, 23, pp % 4 [5] Shell Solar Japan Ltd., Photovoltaic module synthesis 6Ooo O _ ~ ^ X catalog. Tokyo: 23, pp % 4% *Series 3% AWith EfE I% Cnv. a 2 o * With Ef 9% Cnv. i XWithEfE 8%Cnv. % Diameter ofcircular dirt(mm) Fig. 14. PV module output with 9% cnv. and 8% cnv. dependence on the diameter ofthe dirt spot area. VI. CONCLUSIONS In conclusion, change in the aspect ratio of a cell and increase of the current by using a cell array were investigated as methods for compensating for output degradation in a photovoltaic module. * In the case of the module of the same area, output degradation by spot dirt was improved by making the aspect ratio of the cell smaller. The effect of improvement on output degradation becomes larger as the aspect ratio becomes smaller. * If the cell array in a module is changed and the number of parallel connections is increased, degradation from spot dirt may be slightly reduced. In a 2 parallel connection, it was found the reduction of the output degradation when a dirt spot area is more than cell area. * The difference of the converter efficiency used with a parallel connection module does not have any significant influence on reduction of the output degradation caused by spot dirt. ACKNOWLEDGEMENT The funding for this work was provided by the Japan MEXT Grants-in-Aid for scientific research (B), No REFERENCES [1] S. Hashimoto, et al., "A New Stand-alone Hybrid Power System with Wind Generator and Photovoltaic Modules for a Radio Base Station", in Proc. INTELEC, 24, Chicago, pp [2] H. Nakamura, et al., "Evaluation of Losses by Dirt on PV Module surface", in Proc. JSESIJWEA Joint Conference, 1998, Okinawa, Japan, pp [3] S. Kobayashi, et al., "Degradation of Output Characteristics of a Small Photovoltaic Module Due to Dirt Spots", in Proc. INTELEC, 25, Berlin, pp

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