25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain
PERFORMANCE AND PRODUCTIVITY IMPROVEMENTS IN VERY LARGE AREA AMORPHOUS SILICON MODULES Marta Fonrodona1*, Silvia Santos2, Carlos Mata2, Michael Vetter2 and Jordi Andreu1 T-Solar Global S.A., Parc Científic de Barcelona, Baldiri Reixac 4-8, 08028 Barcelona, Spain 2 T-Solar Global S.A., Parque Tecnolóxico de Galicia, Av. Vigo s/n, 32900 San Cibrao das Viñas (Ourense), Spain * corresponding author: +34 93 4031339,
[email protected] 1
ABSTRACT: Enhanced efficiency and cost reduction, both in module manufacturing and in installation, are the main driving forces in the photovoltaic industry. T-Solar has specialized in the fabrication of thin film amorphous silicon modules with sizes up to 5.7 m2. When compared to smaller (Quarter size, 1.43 m2) modules produced with the same technology, the Full size (5.7 m2) modules result in higher stabilized efficiency (production average = 6.6%) due to better exploitation of the module area (3% higher active area ratio) without suffering from any non-homogeneity effects that could be associated to the very large area. The very large area modules are also advantageous in terms of manufacturing costs (savings in material costs) and installation (enhanced balance of system). Cost savings for Full size modules summing up efficiency and material use benefits are of about 15% compared to Quarter size ones. Keywords: amorphous silicon, fabrication, large area, PECVD, PV module
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between Quarter (1.43 m2) and Full (5.7 m2) size modules, focusing especially in the increase in efficiency caused by the increased active area and the possible drawbacks arising from the non-homogeneity that might take place during the various deposition processes over such large areas.
INTRODUCTION
Cost reduction is the main focus of the whole photovoltaic (PV) industry. The expansion of the photovoltaic installed capacity depends on the continuity of the aggressive learning curve of PV technology. The efficiency of the modules is one of the main driving forces in this development, but aspects like material availability, manufacturability and area cost are also highly relevant.
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EXPERIMENTAL
A detailed description of the fabrication of the amorphous silicon (a-Si:H) PV modules at the T-Solar production line can be found elsewhere [1, 2] and is shown in Figure 1. Summarizing, the process starts with a 3.2 mm-thick front glass with a transparent conductive oxide (TCO) layer on top, where the a-Si:H p-i-n structure is deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD). The back contact is sputtered on top of the latter. Cell interconnection within the module is achieved via three laser scribing steps (front TCO, p-i-n structure and back contact) that define the individual cells (106 for the 1.43 m2 Quarter module and 216 for the 5.7 m2 Full size one) and the active area of the module. Next steps comprise edge deletion, soldering of the bus lines, placement and subsequent lamination of the polyvinyl butyral (PVB) foil and the back glass, connection box (JBox) installation, module electrical characterization at the in-line solar simulator and, finally, rail installation for the fixation to the field structure. The 1.43 m2 Quarter modules go through the same pi-n PECVD and sputtering back reflector processes as the 5.7 m2 Full size ones. The design of the laser scribing is, however, different. The Quarter size modules also undergo an extra cutting step.
T-Solar has specialized in the fabrication of thin film amorphous silicon photovoltaic modules with sizes up to 5.7m2 (2.2 m x 2.6 m). When compared to their smaller counterparts, very large area photovoltaic modules present a number of advantages that can be mainly summarized in a lower cost per watt produced due to: - Better exploitation of the module area due to higher ratio of active area (less dead zones). - Being more cost-competitive in terms of production: less material and components required (conductive tape, junction boxes, etc.) and less production steps necessary to reach the final product. - Despite the fact that smaller panels might result more convenient in certain applications where shape, weight, size and facility to handle do play a role, very large area modules are much more suitable for large ground mounted systems, being a significant improvement for cost-effective utility scale PV systems. Large area modules provide enhanced Balance of System (BOS), as the scale economy provided by the size of the modules enables the reduction of the cost for supporting structure, wiring and installation. On the other hand, one of the risks of producing very large modules is associated to the increased nonhomogeneity of the solar cell properties over such a large area that might lead to losses due to thickness mismatch within the individual cells.
Table I summarizes the module area, mean individual cell area and active cell area of the different size panels. The 5.7 m2 (2.2 m x 2.6 m) Full size panels consist of 216 247.5 cm2 cells, what makes a total active area of the panel of 5.35 m2. On the other hand, the 1.43 m2 (1.1 m x 1.3 m) Quarter modules consist of 106 122.2 cm2 cells, making a total active area of 1.30 m2 for these panels. The width of the laser scribes and the dead area between them have been taken into account to evaluate the active
The flexibility in the production process at T-Solar, besides the very large 5.7 m2, also allows the manufacturing of smaller size modules down to 1.43 m2 (1.1 m x 1.3 m, a quarter of the biggest size). This work will provide a comparison in terms of efficiency, manufacturability costs and material use
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25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain
area of the modules.
𝑉𝑐𝑒𝑙𝑙 =
𝑉𝑚𝑜𝑑𝑢𝑙𝑒 𝐼𝑚𝑜𝑑𝑢𝑙𝑒 ; 𝐽𝑐𝑒𝑙𝑙 = 𝑁𝑐𝑒𝑙𝑙𝑠 𝑆𝑐𝑒𝑙𝑙
where Ncells is the number of cells in a module and Scell is the mean individual cell area. The performance of a solar cell can be evaluated from four parameters, the open circuit voltage (V oc), the short circuit current (Isc), the maximum power point voltage (V mpp) and the maximum power point current (Impp). Therefore these four parameters will be the ones analyzed to see if homogeneity somehow affects the performance of our very large area panels. In order to reliably compare the performance of Quarter and Full size modules, a comparable period of Quarter and Full size production at T-Solar has been taken, and the values presented in Table II are the average from the whole production. From the values presented in Table II it can be clearly seen that the mean cell performance is the same for both module sizes, leading to the conclusion that there are no issues associated to the deposition of such a large area in our 5.7 m2 Full size modules. Table II: Average panel and mean cell electrical parameters for Quarter and Full size T-Solar production modules (stabilized values).
Figure 1: The basic fabrication steps of the T-Solar amorphous silicon modules (FEOL = Front End Of Line, BEOL = Back End Of Line).
Full
Quarter
ratio
5.71
1.43
4.007
247.5 216
122.2 106
2.025 2.038
1.30 90.9
4.125 1.030
Ind. Cell area (cm2) Cells per module Active area (m2) % active area
5.35 93.6
3
Isc, Jsc
Vmpp
91.94 V
1.58 A
71.63 V
Impp, Jmpp 1.28 A
Mean cell Full module
0.867 V 187.5 V
12.91 mA/cm2 3.20 A
0.676 V 146.3 V
10.43 mA/cm2 2.58 A
Mean cell
0.868 V
12.94 mA/cm2
0.677 V
10.41 mA/cm2
Table III shows the average power and efficiency of Quarter and Full size T-Solar production modules for the periods considered. Full size modules have an average stabilized power of 377 W, which means a stabilized efficiency of 6.6%. Quarter size panels, on the other hand, have an average stabilized power of 91 W and a stabilized efficiency of 6.4%. The stabilized efficiency histogram for the production of both size modules can be seen in Figure 2. As we have previously demonstrated, the efficiency of both panels at cell level is the same, so the difference in module efficiency is due to the geometry of each module. This difference in performance becomes clear after an analysis of the active and dead areas in both cases.
Table I: Total, active and mean individual cell area for Quarter and Full size modules.
Module area (m2)
Voc Quarter module
RESULTS AND DISCUSSION
Table III: Average stabilized power and efficiency for Quarter and Full size T-Solar production modules.
3.1 Homogeneity and efficiency Associated to the production of very large area modules comes the increased risk of suffering from nonhomogeneity problems in the deposition processes. Such problems might lead to losses due to thickness mismatch within the individual cells throughout the panel. The most adequate to test if the possible higher nonhomogeneity over the larger area has any effects on the performance of the modules is to compare the mean cell performance in both cases. The mean individual cell parameters (voltage, V, and current density, J) can be deduced from the module electrical parameters knowing the number of cells in a module and their area (see Table I) using the following relations:
Full
Quarter
Module Power (W)
377
91
Module Efficiency (%)
6.6
6.4
In module area, a Quarter size module is, as its name indicates, a quarter of its full size counterpart. This ratio is, however, not exactly maintained in terms of active area (that is the total module area minus the dead zones). Dead zones comprise the area trapped in between the laser scribing lines that define the individual cells (see Figure 3) and that left at the edges, framing the active area (edge deletion area). A thorough analysis of the dead areas in each configuration evidences that better surface
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25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain
exploitation is achieved for Full size modules (see Table I for module and active area values). The active area is 5.35 m2 for the Full size module (93.6% of the total module area) whereas it is 1.30 m2 for the Quarter size one (90.9% of the total module area). The higher ratio of active area in the case of the 5.7 m2 module when compared to its Quarter size counterpart is the responsible for the higher efficiency obtained in the very large area modules, as the 3% increase in active area use directly translates in a 3% increase of the average efficiency (6.6% for Full vs. 6.4% for Quarter size modules).
Table IV presents the relative cost of all the components and materials used to manufacture a module as a percentage of the total material cost of a Quarter size module. Those figures evidence that the material costs associated to Full size single junction amorphous silicon modules are about 12% lower than those of Quarter size modules. The main gains in terms of cost are related to junction box, sealing materials and BUS materials. Table IV: Relative cost of all the components and materials used to manufacture a module as a percentage of the total material cost of a Quarter size module. Relative cost Front TCO glass Rails Back glass PVB Junction Box Sealing and potting materials Gases BUS Chemicals Label Sputtering targets Packaging TOTAL
Figure 2: Efficiency histogram for production Full and Quarter size panels. Average values are 6.4% for Quarter modules and 6.6% for Full size ones.
Quarter 27.25% 19.14% 14.70% 13.10% 6.00% 3.89% 6.43% 2.83% 0.20% 0.39% 2.62% 3.44% 100.00%
Full 27.25% 16.70% 13.89% 12.57% 1.50% 0.97% 6.26% 1.32% 0.16% 0.10% 2.62% 4.43% 87.68%
3.3 Balance of System The very large area 5.7 m2 T-Solar Full size module is four times bigger than its Quarter size smaller brother. Therefore, a much smaller amount of 5.7 m2 modules are required in a given installation, yielding to savings in every step of the process. When compared to smaller Quarter size modules, installing Full size 5.7 m2 panels is advantageous in terms of: - More area for power generation. As shown in 3.1, due to better surface exploitation, a Full size module generates 3% more power than four Quarter size ones. - Lower-cost mounting structure and hardware, as for the same surface (one Full vs. four Quarters) half the amount of field belt is required. - Savings in components. Lower amount needed for the same surface: 1/4th less cabling, half the amount of assembly sets -screws, washers, etc- (8 per Full size module vs. 16 per four Quarter size), less electrical connectors (2 per Full size module vs. 8 per four Quarter size), etc. - Faster and easier installation for Full size panels (more stable manipulation with suction pads that also diminishes the amount of breakages).
Figure 3: Detail of the laser scribing lines defining the individual cells. The area left in between is dead area.
3.2 Manufacturability costs and material use Material costs represent between 50% and 60% of the total cost of a single junction thin film silicon module. In those terms there is a significant economy of scale for very large area modules. Even using the same large scale area deposition tools (5.7 m2 substrates) for both Full and Quarter size modules, the production cost of the Full size modules is significantly lower than that of the smaller size ones. In this paper we only consider the main cost benefits of very large area modules that are related to their higher efficiency and the lower material and components use. The cost reduction that comes from manufacturing and tool cost of ownership has not been taken into account (junction box, BUS, cutter and seaming tools go through less workload for Full size than for Quarter size module manufacturing).
In similar terms, comparison between installing 5.7 m2 size modules and current conventional size thin film modules (0.6 m x 1.2 m) by Applied Materials [3] concludes that, assuming the same efficiency in both cases, the 5.7 m2 module enables a 17% overall reduction of installation costs.
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CONCLUSIONS
Enhanced efficiency and cost reduction (both in production and installation) are the driving forces in PV
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25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain
industry. Taking those into consideration, we have shown that very large area 5.7 m2 Full size T-Solar modules overcome their smaller area counterparts, as they result in higher module efficiency due to better exploitation of the active area (6.4% average stabilized efficiency for Quarter modules vs. 6.6% average stabilized efficiency for Full size modules in the analyzed production period) without suffering from extra non-homogeneity issues (same mean cell performance in both cases). We have also shown that, producing 5.7 m2 modules results in a 12% reduction of material costs during production (and that is without taking into account reduction associated to the manufacturing itself). These benefits (efficiency and material use) together provide cost savings of around 15% for Full size modules compared to Quarter size modules. Besides, we have also evidenced that installing very large area modules in the field is also cost advantageous, as it enables the reduction of the costs for supporting structure, wiring and installation.
REFERENCES [1] M. Vetter, C. Mata, J. Andreu, Proceedings 23 rd European Photovoltaic Solar Energy Conference, (2008) 2075-2079 [2] M. Vetter, J.P. Borrajo, C. Mata, J. Andreu, Proceedings 24th European Photovoltaic Solar Energy Conference, (2009) 2408-2411 [3] Applied Materials, SunFab 5.7m2 Panels, Panel Install Brochure, www.appliedmaterials.com
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