ELECTRICAL DESIGN AND LAYOUT OPTIMIZATION ...

32nd European Photovoltaic Solar Energy Conference and Exhibition

ELECTRICAL DESIGN AND LAYOUT OPTIMIZATION OF FLEXIBLE THIN-FILM PHOTOVOLTAIC MODULES

Johannes Hofer, Zoltan Nagy, Arno Schlueter Architecture & Building Systems, Institute of Technology in Architecture, ETH Zurich John-von-Neumann Weg 9, 8093 Zürich, Switzerland

ABSTRACT: Due to the low weight, thinness and the possibility to adapt to non-standard shapes, flexible thin-film photovoltaic (FPV) modules offer new opportunities for building integrated photovoltaics (BIPV). Even though very important for BIPV applications, the performance of FPV modules in partial shading and under curvature has been rarely investigated. In addition, conventional modeling tools can simulate module electrical performance only for planar surfaces. In this contribution, we present a new modeling environment for the calculation of the electricity yield of FPV modules in partial shading and under curvature. We investigate how FPV module design, in terms of cell dimension, interconnection and bypass diode integration, influences module performance in partial shading using experimental measurements and simulation. The methodology is applied in a case study to optimize the design and layout of FPV modules on a doubly curved roof shell structure. Keywords: Thin Film PV Module, Flexible Substrate, Modelling, Building Integrated PV (BIPV), System Performance

1

INTRODUCTION

explained. Then results on FPV module performance in partial shading and under curvature are presented. Finally, the the application of the developed methodology in a case study is demonstrated.

Buildings are responsible for more than one third of total greenhouse gas emissions in Europe [1]. Refurbishment of old buildings, improving energy efficiency and reducing material use over the whole lifecycle are key strategies for the reduction of building related energy use and emissions. In addition, the EU aims for all newly constructed buildings to have close to zero net energy consumption by 2020 [2]. Integrating photovoltaics (PV) in buildings is indispensable for meeting the net zero energy target [3]. In recent years, there has been substantial progress in improving the efficiency and reducing the cost of thin-film PV modules. For example, copper indium gallium selenide (CIGS) modules reach an efficiency of more than 16% today [4,5]. Due to the low weight, its thinness and the possibility to adapt to non-standard shapes, flexible thinfilm PV (FPV) modules offer new opportunities for building integrated photovoltaics (BIPV). Even though very important for BIPV applications, the performance of FPV modules in partial shading and under curvature has been rarely investigated [6]. Typically, partial shading and curvature lead to efficiency degradation due to nonuniform irradiance and electrical mismatch of (a) cells within a module and (b) modules within the PV array. Conventional modeling tools can simulate module irradiance and electrical performance only for planar surfaces. In this contribution, we present a new simulation environment for the calculation of the electricity yield of FPV modules. The developed simulation tool couples irradiation analysis and electrical modeling of FPV modules with high spatial resolution. We investigate how FPV module design, in terms of cell geometrical structure, cell interconnection, and bypass diode integration, influences module performance in partial shading using experimental measurements and simulation. We apply the methodology to a case study of FPV module layout on a double curved roof shell. The developed method can be used to optimize FPV module design and interconnection, including the use of distributed power electronic converters, for the case study building. First, the procedure to simulate module solar irradiance and electrical energy yield for different module designs is

2

METHODOLOGY

The geometry of curved FPV modules is simulated within the Rhinoceros 3D/Grasshopper environment [7,8] using a geometric triangulation method developed for that purpose [9]. It allows us to simulate the layout of FPV modules on arbitrarily curved surfaces and to assess module curvature, twist, and other structural integration parameters. The method is fully parametric so that variations in parameters such as module dimension and orientation can be quickly analyzed. Irradiation on the FPV modules is calculated based on the cumulative sky approach [10,11] as implemented in Ladybug [12] and Radiance [13] utilizing the Perez AllWeather luminance distribution model and EnergyPlus weather files with hourly resolution. In the default case, the sky is divided using the Tregenza scheme. A higher resolution can be achieved with the Reinhart sky patch subdivision [10]. The irradiation data used in this work was exported from the Meteonorm software and is based on long-term measurements of a MeteoSwiss weather station in Zurich, Switzerland. The electric PV model used in this work is suitable to assess the influence of module partial shading and curvature on FPV module current-voltage (I-V) characteristics and follows the approach described in [14]. As shown in Fig. 1, monolithic thin-film cells are discretized into sub-cells to assess the influence of nonuniform irradiance. The sub-cell I-V curve is calculated based on sub-cell irradiance and temperature using the standard equivalent circuit model [15]. Solar radiation analysis and electrical modeling are performed using the same sub-cell level resolution. In case of partial shading, some of the cells are in reverse bias and act as a load. In order to account for this effect, the reverse characteristic of cells is modeled. The module I-V curve is calculated by summing the current of parallel connected sub-cells and subsequently the voltage of series connected cells. Bypass diodes are either integrated between cells in the module or in the module junction box. The array

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32nd European Photovoltaic Solar Energy Conference and Exhibition

performance is calculated based on the I-V curves of modules and their interconnection.

Fig. 3). For an initial assessment presented in this paper, shaded regions were fully shielded from solar irradiation.

Figure 3. Analyzed shading conditions. The module is shaded in distinct steps in lateral and longitudinal direction.

Figure 1. Illustration of a typical FPV module with monolithically interconnected cells. In order to achieve higher resolution in the model, cells are disaggregated into sub-cells. Each sub-cell is modeled using the standard equivalent circuit with a single diode, series and shunt resistance.

For each shading condition, the MPP was inferred from the power-voltage curve of the modules. Fig. 4 compares the measured power loss relative to the unshaded condition as a function of the shaded area for both modules. As shown in Fig. 4a, the power output of module 1 decreases by the same amount as the shaded area in lateral direction increases. For the same type of shading pattern, the power loss of module 2 is higher due to the series connection of cells in lateral direction. If for example 40% of a module is shaded in lateral direction, the decrease in power is ca. 40% for module 1 and 80% for module 2. In the opposite case, with an incremental increase of longitudinal shading, module 1 performs better again due to lower power losses in bypass diodes. The step profile in the power loss of module 1 can be explained by the placement of bypass diodes for every second cell. Fig. 5 shows, that the power loss of module 1 as calculated with the model described above, is in good agreement with the measurement results.

The analysis was performed for two commercial thin-film CIGS (copper indium gallium selenide) modules. The differences in module design are illustrated in Fig. 2. Module 1 consists of 44 cells in series with one integrated bypass diode for every two cells. The cells span the full width of the module. Module 2 consists of 36 cells in series, however, one cell only spans half of the module width and the series connection of cells extends in the lateral module direction. A bypass diode is integrated for every cell.

Figure 2. Cell design and interconnection of two CIGS modules analyzed in this work. 3

RESULTS

3.1 Performance in partial shading In order to assess the influence of module design on the electricity yield in partial shading, the I-V curves of different FPV modules were measured in controlled shading experiments using a PVPM2540C measurement device from PV Engineering. The irradiance in the unshaded region was measured with an external reference cell and module temperature with a thermocouple sensor. The modules were mounted on a building façade and a non-transparent object was moved along the modules to incrementally adjust the shaded area with a step size of one cell in longitudinal and 10% in lateral direction (see

Figure 4. Measured relative power at MPP for two different flexible CIGS modules as a function of the shaded area for shading in (a) lateral and (b) longitudinal module direction.

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32nd European Photovoltaic Solar Energy Conference and Exhibition

the highest module efficiency.

Figure 5. Comparison between modeled and measured power loss in partial shading for module design 1. 3.2 Performance of curved modules To assess the performance of curved FPV modules, module irradiance and efficiency were been simulated for different types of curvature and weather conditions. As an example, Fig. 6 shows the simulated irradiance on a curved FPV module oriented from east to west for one specific hour. To simulate the corresponding IV characteristic and power output of module 1, the resolution in longitudinal direction corresponds to the number of cells of module design 1 (cf. Fig. 2). Module irradiance was coupled with the electrical model described in section 2. In addition to the curved module, a flat and horizontally mounted module was simulated for comparison.

Figure 7. Simulated module irradiance and electrical power at MPP for module design 1 during one clear day in May. Results are shown for the same module in a curved and flat horizontal position.

Figure 6. Simulated irradiance on a curved FPV module during one sunny hour in May for the used weather file of Zurich. The resulting module irradiance and electrical power at MPP during one clear day in May are shown in Fig. 7. While irradiance is very similar for the flat horizontal and curved module, the electrical power output of the curved module is significantly lower in the morning and afternoon. The dependence of power output can be also analyzed in terms of module efficiency as shown in the upper graph of Fig. 8. For flat horizontally oriented modules, the efficiency decreases slightly at solar noon due to module heating. On the other hand, the efficiency of the curved module decreases significantly in the morning and afternoon. The variation of cell irradiance over the day is shown in the bottom graph of Fig. 8. Electrical mismatch between cells due to non-uniform irradiance within the module is the main reason for the observed efficiency loss. Balanced cell irradiance at solar noon coincides with

Figure 8. Top: module efficiency corresponding to the results shown in Fig. 7. Bottom: cell irradiance distribution for the curved module over the day.

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32nd European Photovoltaic Solar Energy Conference and Exhibition

The simulation results have been compared to experimental measurements. For this purpose, a prototype of a substructure with the same geometry of the top surface as the simulated test module has been built (Fig. 9). The I-V curves of the curved module and of a reference module in a flat horizontal position were continuously measured. Simultaneously, five pyranometers measured incident irradiance along the module to infer module efficiency.

orientation of 20° as shown in Fig. 10a, provides the highest energy yield and an acceptable visual appearance.

Figure 9. Illustration of the module holder used to measure the I-V characteristic of an FPV module in a predefined curved shape. Initial measurements were performed during a clear day in May and good agreement between simulated and experimentally measured efficiency has been observed, very similar to the results shown in Fig. 8. 3.3 Building integration The work presented in this paper is applied in the context of the NEST HiLo building [16]. HiLo features several innovations, including a vaulted, ultra-slim roof shell based on a geometrically optimized doubly curved form that integrates solar gains, thermal insulation, and conditioning of the indoor space. The methodology presented in this work is used for the design and layout of FPV modules on the HiLo roof. The FPV modules will be arranged in narrow strips to follow the curved roof geometry. Fig. 10a shows one possible module layout with a panel size of 2m length and 0.6m width, and an orientation of 20° relative to the horizontal. Also shown is solar irradiance on the modules from 11:00 to 12:00 for a clear day in May. The distribution of irradiance between the cells of those modules is shown for module 1-12 in Fig. 10b. Generally, the strong variation of irradiance between cells within a module can lead to electrical mismatch and efficiency loss as shown in the previous section. Different FPV module designs will be investigated that minimize these effects. The approach described in section 3.2, will allow us to build a prototype setup based on the actual roof geometry and to experimentally test FPV module performance. The results can be compared with simulated data to further calibrate the model and optimize module design. The electrical mismatch between different modules within a string can be reduced by using distributed power electronic converters such as DC power optimizers. Knowledge of the FPV module operational performance will inform the selection of those components. Finally, the influence of strip orientation on the number of modules, average irradiance, efficiency and electricity yield has been analyzed. Initial results show that power output is mainly influenced by the number of modules that can be placed on the roof and that average module irradiance is independent of strip orientation. The

Figure 10. (a) Possible PV module layout on the NEST HiLo building and hourly solar insolation. (b) Distribution of irradiance between modules 1-12 and between the cells of each module. 4

CONCLUSIONS

In this work, we presented a novel simulation methodology for the calculation of electricity yield of FPV modules in partial shading and under curvature. We showed that differences in the cell dimension and bypass diode integration of currently available FPV modules have a significant impact on their performance in partial shading. We applied the methodology to a case study of FPV module integration on a doubly curved roof shell and showed how design parameters such as the orientation of FPV module strips influence overall energy yield. The methods developed in this work will inform the design of FPV modules and their layout on the NEST HiLo building.

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32nd European Photovoltaic Solar Energy Conference and Exhibition

ACKNOWLEDGEMENT This research has been financially supported by CTI within the SCCER FEEB&D (CTI.2014.0119) and by the Building Technologies Accelerator program of ClimateKIC. REFERENCES [1] https://ec.europa.eu/energy/en/topics/energyefficiency/buildings, accessed 04.02.2016 [2] Energy Performance of Buildings Directive (2010/31/EU) [3] Scognamiglio A, Garde F. Photovoltaics' architectural and landscape design options for Net Zero Energy Buildings, towards Net Zero Energy Communities: spatial features and outdoor thermal comfort related considerations. Progress in Photovoltaics: Research and Applications, 2014. [4] Nam J, Kang Y, Lee D, Yang J, Kim YS, Mo CB, Park S, Kim D. Achievement of 17.9% efficiency in 30×30 cm2 Cu (In, Ga)(Se, S) 2 solar cell sub‐module by sulfurization after selenization with Cd‐free buffer. Progress in Photovoltaics: Research and Applications, 2015. [5] http://miasole.com/wpcontent/uploads/2015/08/FLEX-02N_Datasheet_1.pdf [6] Sharma P, Duttagupta SP, Agarwal V. A novel approach for maximum power tracking from curved thinfilm solar photovoltaic arrays under changing environmental conditions. IEEE Transactions on Industry Applications, 50(6):4142-51, 2014. [7] Rhinoceros 3D v5. https://www.rhino3d.com/ [8] Grasshopper - algorithmic modeling for rhino. http://www.grasshopper3d.com/ [9] Groenewolt A, Bakker J, Hofer J, Nagy Z, Schlueter A. Methods for modelling, analysis and optimisation of bendable photovoltaic modules on irregularly curved surfaces (in review) [10] Ibarra D, Reinhart CF. Solar availability: a comparison study of six irradiation distribution methods. In Proceedings of the 12th Conference of International Building Performance Simulation Association, Sydney, Australia (Vol. 1416), 2011. [11] Robinson D, Stone A. Irradiation modelling made simple: the cumulative sky approach and its applications. In PLEA Conference, 2004. [12] Roudsari MS, Pak M, Smith A, Gill G. Ladybug: a parametric environmental plugin for grasshopper to help designers create an environmentally conscious design. Proc. of the 13th International IBPSA Conference, Lyon, France, 2013. [13] Ward GJ. The RADIANCE lighting simulation and rendering system. Proc. of the 21st annual conference on Computer graphics and interactive techniques, pp. 459472. ACM, 1994. [14] Hofer J, Groenewolt A, Jayathissa P, Nagy Z, Schlueter A. Parametric analysis and systems design of dynamic photovoltaic shading modules. Energy Science & Engineering, 4(2), 134-152, 2016. [15] Mermoud A, Lejeune T. Performance assessment of a simulation model for PV modules of any available technology. Proc. of the 25th European Photovoltaic Solar Energy Conference, 2010. [16] http://hilo.arch.ethz.ch/

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