Publications Search Results

Now showing 1 - 10 of 11
  • Publication
    Performance Analysis of Bifacial PV Modules with Transparent Mesh Backsheet
    ( 2021)
    Jang, J.
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    Pfreundt, A.
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    Mittag, M.
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    Lee, K.
    Due to their transparent rear side, bifacial modules can take advantage of rear side irradiance as opposed to monofacial modules. Glass or transparent backsheets are conventionally used as rear side encapsulation material. To increase coupling gains achieved through internal reflection at the module rear side, a white or reflecting mesh structure can be applied in the areas between the cells on the rear side material. In this study, an existing optical model based on a simplified ray tracing approach is extended to describe the effects achieved though this mesh structure. The model is further integrated into a complete cell-to-module loss and gain analysis. The performance of the mesh backsheet concept is assessed under varying parameters. The impact of mesh reflectance, bifaciality of the cell and width of the mesh compared to the cell spacing are investigated. Losses due to increased module temperature and gains due to internal reflection gains are compared. We confirm that the optimal power gain can be achieved when the width of the mesh is the same as the spacing between the cells. We find that the power gain due to the improved internal reflection outweighs the power loss due to increased module temperature.
  • Publication
    The Challenge of Measuring Busbarless Solar cells and the Impact on Cell-to-Module Losses
    ( 2020)
    Rauer, M.
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    Krieg, A.
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    Pfreundt, A.
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    Mittag, M.
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    Pingel, S.
    The realistic measurement of solar cells is key for the whole PV industry, as accurate information about cell power is one of the most important aspects in solar cell purchase and PV module design. The omission of busbars introduces new challenges to the current-voltage measurement of solar cells, since contact to every single grid finger has to be established with independent current and voltage contacts. It is not just the shadow correction of the measurement unit that needs to be carried out more laboriously, but also the contacting of the front metal grid, which is more critical because of the high resistivity of the grid fingers. The position of the voltage sensing contact and the number of current contacts can thus have a noticeable impact on the measured performance of busbarless solar cells. Measured efficiencies are highly dependent on the contacting schemes used in different measurement systems, as these vary in contact number and sensing configuration. Two different main approaches for measuring busbarless solar cells have evolved, representing either realistic or idealized application of the cells in the module. The pros and cons of both approaches are discussed in detail in this paper. Realistic measurement conditions lead to efficiencies which best predict module performance, but are hard to realize and require knowledge about the subsequent module design. Although not their primary purpose, the use of idealized measurement conditions can make it easier to achieve record cell efficiencies, but with the disadvantage of limited comparability with busbar-based solar cell concepts. Idealized conditions can moreover lead to hidden losses in performance of the solar cells, related to the application in a module, which in turn causes inflated cell-to-module (CTM) losses. If solar cells are bought in terms of $/Wp and modules are sold likewise, the economic implications arising from the different measurement configurations have to be considered. Whichever approach is used for the measurement of busbarless solar cells, full disclosure of the measurement configuration is absolutely essential.
  • Publication
    Impact of Solar Cell Dimensions on Module Power, Efficiency and Cell-To-Module Losses
    ( 2020)
    Mittag, M.
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    Pfreundt, A.
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    Shahid, J.
    We analyze the impact of larger solar cells and cell splitting on module power, efficiency and single gain and loss factors using Cell-To-Module (CTM) analysis. Solar cells from M0 (156.75 mm) to M12 (210 mm) as well as full cells, half and third cut module designs are analyzed for Standard Testing Conditions (STC) and non-STC. We find the modules with larger cells to have a higher module power than modules with smaller cells (up to +77%). The CTMpower-ratio decreases for larger cells (-5%abs) and is higher for split solar cells than for full cells (up to +7.7%abs). Module efficiency increases with cell size if the cells are split (up to +1.1%abs). For full cells significant electrical losses in the solar cell interconnection overcompensate higher active area shares and reduce module efficiency. We calculate the module temperature and find modules with smaller solar cells to be cooler (up to -2.8 K). Also, split cell modules are cooler than full cell modules (up to -1.4 K). The size of the solar cell has a significant impact on the module operation. Modules with smaller or split solar cells perform relatively better at higher irradiance. The impact of irradiance on power output is also relatively smaller. We find modules with M12 solar cells to have the highest power density (W/m²) of all analyzed setups. Splitting of solar cells provides significant benefits for larger solar cells (up to +9.1%). The use of large area full cells should be avoided due to significant CTM-losses.
  • Publication
    Trend Tracking of Efficiency and CTM Ratio of PV Modules
    ( 2020)
    Tummalieh, A.
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    Pfreundt, A.
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    Mittag, M.
    In this study, historical and present PV module concepts are analyzed concerning efficiency, output power and cell-to-module (CTM) ratio by simulating PV modules with different components over the period between 2009 and 2019. Furthermore, the changes occurring in important gain and loss channels within PV modules are illustrated as they are affected by the changes in module design and solar cell performance. Results show that in the last decade, the PV module output power and efficiency exhibit an absolute increase of about +85 Wp and +5.3% corresponding to a yearly rate of about +7.7 Wp and +0.48% respectively. Moreover, gain and loss factors show their impact on the CTM-ratio for both PV module power and efficiency, which also exhibits an absolute increase of about +3.2% and 3.4% respectively. Loss factors in the module such as power loss in cell interconnections show high sensitivity regarding the solar cell output current and the metallization pattern. On the other hand, optical loss and gain factors show a strong dependency on the use of the anti-reflective-coating and the encapsulation and backsheet materials used.
  • Publication
    Energy Yield Modelling of 2D and 3D Curved Photovoltaic Modules
    ( 2020)
    Neven-du Mont, S.
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    Heinrich, M.
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    Pfreundt, A.
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    Kutter, C.
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    Tummalieh, A.
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    Neuhaus, D.-H.
    This paper presents a detailed model to simulate the energy yield potential of various designs of curved PV modules depending on their bending angle, orientation and location. Furthermore, we analyze the effective irradiance incident on the curved cell pattern surface to investigate the current mismatch between cells and strings that results from the deviating level of irradiance reaching the cells. The results of our simulations show that parallel interconnection of strings produces a higher energy yield for most but not all curved module layouts compared to standard series interconnection with three bypass diodes. The energy yield of curved PV modules can be maximized by using an adapted module design that compensates the inhomogeneous irradiance distribution on the curved module surface.
  • Publication
    Cell-to-module Analysis beyond Standard Test Conditions
    ( 2020)
    Pfreundt, A.
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    Shahid, J.
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    Mittag, M.
    Cell-to-module analysis has been an invaluable tool in photovoltaic module design and technology evaluation in the past. Analysis and prediction of module power and efficiency at standard test conditions, and comparing losses and gains between module setups provides a means of virtual prototyping and enables digital module optimization. This toolset has been expanded beyond standard test conditions, to include realistic illumination and ambient conditions. Models are developed to account for these parameters. Additional loss and gain factors are introduced to account for performance changes on the cell level and to attribute changes to different operation parameters.
  • Publication
    Post-Processing Thickness Variation of PV Module Materials and its Impact on Temperature, Mechanical Stress and Power
    ( 2019)
    Pfreundt, A.
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    Yucebas, D.
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    Beinert, A.J.
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    Verissimo Mesquita, L.
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    Pitta Bauermann, L.
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    Romer, P.
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    Mittag, M.
    We measure the thickness of the encapsulation layers in photovoltaic modules using scanning acoustic microscopy and optical microscopic imaging. Based on the measurement data, we analyze the impact of thickness variation on the operating temperature of the module, its peak power and mechanical stresses in the solar cells during lamination and under load testing conditions. Especially in cell-free areas we find an inhomogeneous thickness attributed to a bending of the backsheet, which has a small impact on the backsheet coupling gain. Even though we find significant deviation in the thicknesses of the encapsulation layer of up to 150 mm, the impact of encapsulant thickness under the investigated conditions is small and mainly attributed to changes in operating temperature.
  • Publication
    Techno-Economic Analysis of Half-Cell Modules - the Impact of Half-Cells on Module Power and Costs
    ( 2019)
    Mittag, M.
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    Pfreundt, A.
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    Shahid, J.
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    Wöhrle, N.
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    Neuhaus, D.H.
    Modules using halved cells are a promising development to improve module power and reduce module costs. We perform an analysis of power gains and losses within half-cell modules using the cell-to-module (CTM) methodology and find an increase in internal reflection (backsheet gains) as well as a reduction in electrical losses to be the main influence for a power gain of half-cell modules. The CTM power ratio increases by 2-4% for half-cell modules. We perform a Cost of Ownership (CoO) calculation and find the absolute costs (e) of half-cell modules to be 0.6-1.2% higher than for a comparable full cell reference. The specific costs (e/Wp) of half-cell modules are 0.81.0% lower due to the CTM power gains.
  • Publication
    Thermal Modelling of Photovoltaic Modules in Operation and Production
    ( 2019)
    Mittag, M.
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    Vogt, L.
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    Herzog, C.
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    Pfreundt, A.
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    Shahid, J.
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    Neuhaus, D.H.
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    Wirth, H.
    The temperature of solar cells in photovoltaic modules has a major influence on module power. The module setup, the material structure and the material properties of the module as well as the ambient conditions influence this temperature. These parameters also influence the thermal behavior of the module during the lamination process resulting in a temperature profile through the modules layers. We present a 1-dimensional dynamic model to calculate both the temperature of a solar module in operation as well as during lamination. We analyze the effect of module design (glass-backsheet, glass-glass, full and half cells) as well as bifaciality on the cell temperature during operation. We simulate the lamination process and find the model to be in good agreement with validation measurements. We find significant temperature differences between different module layers.
  • Publication
    Rapid Calculation of the Backsheet Coupling Gain Using Ray Groups
    ( 2018)
    Pfreundt, A.
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    Mittag, M.
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    Heinrich, M.
    ;
    Eitner, U.
    Internal reflections within photovoltaic modules are known to contribute to power gains from cell to module. The module rear cover, usually a white backsheet, is one module component reflecting additional light onto the solar cell. A novel approach to model the effect of backsheet reflectance on the achievable coupling gain in solar modules is presented. Using a discrete ray optics approach, results can be calculated rapidly for arbitrary reflectance distributions using a partition of the emerging rays into groups. The model is fully wavelength resolved, using measured data to model optical material properties. It is therefore suitable for arbitrary material stacks in front of and behind the solar cell with a single diffusely scattering layer. We study the impact of layer thicknesses, incidence angle and distribution function on the coupling gain using the presented approach. Comparison to measurements of the coupling gain using single cell modules shows good agreement with the calculated results.