High-resolution spatial and electrical modeling for efficient BIPV system design Johannes Hofer, Arno Schlueter Architecture & Building Systems, Institute of Technology in Architecture, ETH Zurich John-von-Neumann Weg 9, 8093 Zürich, Switzerland Corresponding author:
[email protected], Tel +41 44 633 79 19
In recent years, there has been an enormous progress in improving the efficiency and reducing the cost of photovoltaics. In order to satisfy architects and end-user requirements, different technical options exist to improve the aesthetical features of building integrated photovoltaics (BIPV) including colors, textures, patterns, as well as new integration options such as multifunctional components and prefabrication [1]. Moreover, thin-film PV modules offer new opportunities for BIPV due to the low weight, thinness, and the possibility to adapt to nonstandard shapes. It is expected that these advances will further promote the use of PV in buildings, which is highly relevant for realizing the net-zero energy target set by the European Union for newly constructed buildings by 2020 [2]. A key challenge for BIPV system design is partial shading of PV modules due to surrounding objects, irregular terrain, or mutual shading between PV modules. In addition, several new applications such as curved thin-film modules [3] or PV integrated in dynamic shading systems [4-6] result in geometric complexity which cannot be addressed by traditional PV modeling tools focused on flat modules with standardized shapes. In this work, we present a new BIPV modeling framework coupling irradiance and electrical modeling at sub-cell level resolution. It is implemented within a parametric 3D environment enabling high design flexibility and fast alterations of module geometrical and electrical layout. Partial shading and module curvature lead to non-uniform irradiance on PV modules, which in turn induces electrical mismatch. It is shown how PV module layout and electrical design can reduce efficiency losses and improve system reliability. The modeling workflow is illustrated in Fig.1. The geometry of PV modules is simulated within the Rhinoceros software using the parametric modeling plugin Grasshopper. Solar irradiation on the PV modules is calculated based on the cumulative sky approach [7] as implemented in Radiance utilizing the Perez All-Weather luminance distribution model and EnergyPlus weather files with hourly resolution. The electrical model has a high spatial resolution and is suitable to assess the influence of module partial shading and curvature on the current-voltage (IV) characteristic of individual modules and the PV array [6]. The methodology is implemented with different commercial CIGS module designs and used to assess variations of those. As illustrated in Fig. 1, knowledge of PV performance can be used to optimize electrical parameters (e.g. cell structure, interconnection, and bypass diode integration) and system design parameters (module arrangement and dynamic positioning) [3,4,6].
Figure 1. Illustration of the modeling workflow.
The methodology presented in this paper is validated with experimental measurements and applied in the context of the NEST HiLo project (http://hilo.arch.ethz.ch/). HiLo features several innovations, including (a) the Adaptive Solar Façade [4,5], a highly integrated adaptive building façade with dynamic PV shading modules, and (b) flexible thin-film PV modules on a double curved roof structure [3].
As an example Fig. 2a shows a possible layout of flexible PV modules on the HiLo roof and solar insolation during one hour in Zurich at clear sky. The strong variation of irradiance between cells within a module as well as between different modules within a string causes electrical mismatch. Thin-film cells are discretized into sub-cells to assess the influence of non-uniform irradiance (Fig. 2b). Experimental and theoretical analysis has shown that differences in the cell structure and bypass diode integration of currently available flexible CIGS modules have a significant impact on their performance in partial shading and curvature [3]. Different approaches to balance irradiance mismatch are currently investigated, including distributed power electronic converters and a PV module design which is less sensitive to shading and curvature. A prototype of the layout of PV modules on the HiLo roof is currently in planning. The employed CAD software will be used to define the geometry of the real prototype as well as the geometry used in the model. In this way, a direct comparison between model and experiment can be achieved. The method presented in this work is also applied in the context of the Adaptive Solar Façade [4-6].
Figure 2. (a) Possible layout of flexible PV modules on the NEST HiLo roof and simulated solar insolation. (b) Monolithically interconnected thin-film module and discretization of cells into sub-cells to achieve a higher resolution in the model. Each sub-cell is modeled using the standard equivalent circuit with a single diode, series and shunt resistance.
In conclusion, the methodology presented in this work enables calculation of electrical energy yield and system design of complex BIPV applications, such as curved thin-film and dynamic PV shading modules. Generally, the coupling of parametric 3D, high-resolution irradiance and electrical modeling opens new opportunities for design of innovative PV systems. The method is not limited BIPV, but also relevant for other applications such as PV solar tracking and integration of PV in electric vehicles or consumer electronics. 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 Climate-KIC. References [1] Delponte E, Marchi F, Frontini F, Polo L, Cristina S, Fath K, Batey P. BIPV in EU28, from Niche to Mass Market: An Assessment of Current Projects and the Potential for Growth through Product Innovation. Proc. of EU PVSEC 2015 conference, Hamburg, Germany [2] 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. [3] Hofer J, Nagy Z, Schlueter A. Electrical design and layout optimization of flexible thin-film photovoltaic modules. Proc. of EU PVSEC 2016 conference, Munich, Germany [4] Jayathissa P, Schmidli J, Hofer J, Schlueter A. Energy performance of PV modules as adaptive building shading systems. Proc. of EU PVSEC 2016 conference, Munich, Germany [5] Nagy Z, Svetozarevic B, Jayathissa P, Begle M, Hofer J, Lydon G, Willmann A, Schlueter A. The Adaptive Solar Facade: From Concept to Prototypes. Frontiers of Architectural Research 5, 2, 143–156, 2016. [6] Hofer J, Groenewolt A, Jayathissa P, Nagy Z, Schlueter A. Parametric analysis and systems design of dynamic photovoltaic shading modules. Energy Science and Engineering 4, 2, 134–152, 2016. [7] Robinson D, Stone A. Irradiation modelling made simple: the cumulative sky approach and its applications. In PLEA Conference, 2004.
