CFK: a Fresnel–Köhler concentrator with an external confinement cavity

In a concentrator photovoltaic (CPV) module, the solar cell surface reflects a non‐negligible portion of the incoming light, leading to a loss in module efficiency. The Fresnel–Köhler with an external confinement cavity (CFK) is a novel optical CPV concentrator designed to recover this portion of the reflected light. The design is based on an external confinement cavity, an optical element able to redirect the light reflected by the cell surface towards its surface again. Its integration into a CPV module is possible, thanks to the recent invention of advanced Köhler concentrators by LPI. This strategy, based on light recovery, leads to a significant increase in electrical efficiency. We have tested the excellent performance of these cavities by means of integrating one of them into an FK concentrator and manufacturing a proof‐of‐concept prototype. The measured results are outstanding: a relative electrical efficiency and I_sc gains of up to 6% when comparing both with and without cavity designs, and a 33.2% of CFK module electrical efficiency (@T_cell = 25 °C) using a 38.5% nominal efficiency cell (without anti‐reflection coatings on the optics). Copyright © 2015 John Wiley & Sons, Ltd.     

Reflective secondary optical elements for fresnel lens based concentrator modules

This paper reports on the development of secondary optics for concentrator photovoltaic (CPV) modules. It focuses on reflective secondary optics designed for high concentration modules using fresnel lenses as the primary concentrating optics. The development of the secondary optics was guided by the idea of designing an optical element suitable for cost effective mass production. The primary concentrating optics of a CPV module can direct only a limited part of the solar aureole (the immediate surroundings of the sun) onto the solar cell. The same applies to light impinging non‐perpendicular to the optical axis of the module (e.g., due to misalignment of the module). Therefore, one of the main functions of reflective secondary optics is to redirect light onto the solar cell that would otherwise not reach it. In order to analyze the performance of secondary optical elements (“secondaries”), a measurement setup is introduced that measures the angular photocurrent response of a CPV device with highly parallel light. This response is referred to as the “angular acceptance function” (AAF). The AAF is used to estimate the performance of the CPV device under various conditions with differing circumsolar radiation (CSR). A CPV test module was manufactured featuring III–V triple‐junction solar cells, a fresnel lens panel as the primary concentrator optic, and the newly developed reflective secondary. The results of a 6‐month outdoor measurement period are presented and compared to the performance of a reference module as well as to the results of the indoor AAF measurements. Copyright © 2011 John Wiley & Sons, Ltd.     

Development of a mirror‐based LCPV module and test results

The design of a specific low concentration photovoltaic module is described here, with a report of the results of the first experimental tests of its industrial version. The product is a 20× reflective concentrating photovoltaic module based on silicon solar cells. The optics were designed to mount these modules on 2‐axis trackers with angular pointing accuracy of up to about ±4° without significant power loss. The high angular acceptance of the non‐imaging optics permits the collection of a high fraction of the circumsolar light impinging on the module's frontal aperture, providing high direct normal irradiance efficiency in real operative conditions. Many technical features of the product are described here, in which features are the result of 5 years of product development in order to improve performance, reliability and cost issues. Copyright © 2015 John Wiley & Sons, Ltd.     

Performance analysis of a reflective 3D crossed compound parabolic concentrating photovoltaic system for building façade integration

A reflective 3D crossed compound parabolic‐based photovoltaic module (3D CCPC PV) was designed and its electrical and optical performance was analyzed for building integrated photovoltaic applications. A maximum power concentration of 3.0× was achieved compared to similar type of non‐concentrating module. The reduction of the concentration factor from the geometrical concentration of 3.61× for the designed 3D CCPC were due to manufacturing errors, mismatch losses, series resistance losses, and thermal loses. The experimental output was validated by developing a MATLAB simulation code for its electrical performance. Good agreements were observed between experimental and electrical simulation with maximum electrical conversion efficiency of the concentrating system of 14%. The experimental characterization of the optical efficiency was found to show a deviation of 19.4% from the 3D ray tracing simulation efficiency of 94.6% for direct incidence. This deviation is mainly due to the fact that 3D ray tracing simulation does not take the non‐uniform illumination distribution into account. Copyright © 2012 John Wiley & Sons, Ltd.     

Concentration photovoltaic optical system irradiance distribution measurements and its effect on multi‐junction solar cells

This paper proposes an indoor procedure based on charge‐coupled device camera measurements to characterize the non‐uniform light patterns produced by optical systems used in concentration photovoltaic (CPV) systems. These irradiance patterns are reproduced on CPV solar cells for their characterization at concentrated irradiances by using a concentrator cell tester and placing high‐resolution masks over the cells. Measured losses based on the masks method are compared with losses in concentrator optical systems measured by using the Helios 3198 solar simulator for CPV modules. Copyright © 2011 John Wiley & Sons, Ltd.     

Power conversion in concentrating photovoltaic systems: central,string, and micro‐inverters

In this paper, concentrating photovoltaic (CPV) systems coupled with various inverter configurations are modeled, compared, and tested. Because CPV systems use optics to concentrate sunlight onto highly efficient PV cells, the systems are affected not only by mismatches in the I–V characteristics among individual PV cells but also by the electro‐optical mismatches of each concentrator. The best way to minimize power losses by these mismatches is having higher quality controls in aligning at the time of manufacturing and installation. To mitigate the power losses when mismatches are present, electrical components can be considered at the expense of additional cost. The developed models for central, string, and micro‐inverters allow an accurate estimation of power losses in CPV systems and can be used to find an optimum solution for various power conversion schemes on the basis of the given mismatch conditions. Simulation results show that a CPV system with micro‐inverters outperforms a CPV system with conventional inverters. Experimental test results under normal operation validate that power losses in a CPV system can be reduced by more than 5% by using the micro‐inverter scheme. Copyright © 2013 John Wiley & Sons, Ltd.     

A side‐by‐side comparison of CPV module and system performance

A side‐by‐side comparison is made between concentrator photovoltaic module and system direct current aperture efficiency data with a focus on quantifying system performance losses. The individual losses measured/calculated, when combined, are in good agreement with the total loss seen between the module and the system. Results indicate that for the given test period, the largest individual loss of 3.7% relative is due to the baseline performance difference between the individual module and the average for the 200 modules in the system. A basic empirical model is derived based on module spectral performance data and the tabulated losses between the module and the system. The model predicts instantaneous system direct current aperture efficiency with a root mean square error of 2.3% relative. Copyright © 2016 John Wiley & Sons, Ltd.     

YieldOpt,a model to predict the power output and energy yield for concentrating photovoltaic modules

In this work, we discuss three empirical models and introduce one more detailed model named YieldOpt. All models can be used to calculate the power output and energy yield of concentrating photovoltaic (CPV) modules under different ambient conditions. The YieldOpt model combines various modeling approaches: simple model of the atmospheric radiative transfer of sunshine for the spectral irradiance, a finite element method for thermal expansion, ray tracing for the optics, and a SPICE network model for the triple‐junction solar cell. YieldOpt uses a number of constant and variable input parameters, for example, the external quantum efficiency of the cells, the temperature‐dependent spectral optical efficiencies of the optics, the tracking accuracy, the direct normal irradiance, the aerosol optical depth, and the temperature of the lens and the solar cell. To verify the accuracy of the models, the I‐V characteristics of five CPV modules have been measured in a 10‐min interval over a period of 1 year in Freiburg, Germany. Four modules equipped with industrial‐standard lattice‐matched triple‐junction solar cells and one module equipped with metamorphic triple‐junction solar cells are investigated. The higher accuracy of YieldOpt compared with the three empirical models in predicting the power output of all five CPV modules during this period is demonstrated. The energy yield over a period of 1 year was predicted for all five CPV modules with a maximum deviation of 5% by the three empirical models and 3% by YieldOpt. Copyright © 2014 John Wiley & Sons, Ltd.     

Reducing shadowing losses with femtosecond‐laser‐written deflective optical elements in the bulk of EVA encapsulation

A new approach on decreasing the optical shadowing of the solar cell grid fingers is presented. The approach relies on a local change of the optical properties in the bulk of the photovoltaic module encapsulation material ethylene vinyl acetate (EVA). In particular, scattering and diffractive optical elements are locally generated within the volume of cross‐linked EVA encapsulation material by applying a femtosecond‐laser‐writing process. When these optical elements are located above the metal grid fingers, the optical shadowing of these grid fingers can be decreased. In an experimental proof of concept, the optical performance of this approach is demonstrated. The best results obtained so far indicate a decrease in optical shadowing by 17%. The material characteristics of the volume optics were investigated by applying confocal Raman microscopic characterisation, which indicates that the EVA material partially degraded upon the impact of the laser beam and is partly carbonised. Supplementary optical simulations show that the light deflection is caused by diffraction. However, parasitic absorption substantially deteriorates the optical performance of the deflective volume optics. Copyright © 2014 John Wiley & Sons, Ltd.     

Tracking control of high‐concentration photovoltaic systems for minimizing power losses

This paper presents three key factors that cause system mismatches and power losses in high‐concentration photovoltaic (HCPV) systems. The first factor is the I–V mismatch within a module, similar to the manufacturing mismatches in conventional photovoltaic modules. The second factor is the misalignments amongst modules, and the third factor is the tracking control. Unlike in the conventional photovoltaic systems, the second and the third factors in HCPV systems introduce larger electro‐optical mismatches due to narrow acceptance angles. We have developed a model to address these three factors. It allows an accurate estimation of power losses in HCPV systems, which enabled us to propose configurations to reduce power losses without adding additional electrical components to the system. Simulation results show that the power harvest can be increased as much as 8.5% for a system using open‐loop controls by simply increasing the number of strings at the time of calibration. Experimental test results are presented for validation. Copyright © 2012 John Wiley & Sons, Ltd.     

Combined effect of light harvesting strings,anti‐reflective coating,thin glass,and high ultraviolet transmission encapsulant to reduce optical losses in solar modules

Optical losses are a major source for current and power reduction in solar modules. Hence, various improvements aiming at reducing these losses have been suggested. In this work, we have evaluated the effects of anti‐reflective coating, front glass thickness, polyvinyl butyral ultraviolet+ encapsulant, and light harvesting strings on the module performance individually and in combination. The individual and combined contributions were quantified by spectrally resolved optical measurements on the module components and simulations as well as electrical measurements on 1‐cell and 54‐cell modules. Optical gains and their impact on short circuit current are discussed in relation to a maximum current obtained from the solar cells internal quantum efficiency. The results of the electrical characterization are in good agreement with the optical analysis substantiating our approach. They show that a combined, relative current enhancement of 5% can be obtained for an optimized module, which compares to an increase of 1% absolute efficiency. Copyright © 2014 John Wiley & Sons, Ltd.     

Self‐Powered Sensors Enabled by Wide‐Bandgap Perovskite Indoor Photovoltaic Cells

A new approach to ubiquitous sensing for indoor applications is presented, using low‐cost indoor perovskite photovoltaic cells as external power sources for backscatter sensors. Wide‐bandgap perovskite photovoltaic cells for indoor light energy harvesting are presented with the 1.63 and 1.84 eV devices that demonstrate efficiencies of 21% and 18.5%, respectively, under indoor compact fluorescent lighting, with a champion open‐circuit voltage of 0.95 V in a 1.84 eV cell under a light intensity of 0.16 mW cm^?2. Subsequently, a wireless temperature sensor self‐powered by a perovskite indoor light‐harvesting module is demonstrated. Three perovskite photovoltaic cells are connected in series to create a module that produces 14.5 µW output power under 0.16 mW cm^?2 of compact fluorescent illumination with an efficiency of 13.2%. This module is used as an external power source for a battery‐assisted radio‐frequency identification temperature sensor and demonstrates a read range by of 5.1 m while maintaining very high frequency measurements every 1.24 s. The combined indoor perovskite photovoltaic modules and backscatter radio‐frequency sensors are further discussed as a route to ubiquitous sensing in buildings given their potential to be manufactured in an integrated manner at very low cost, their lack of a need for battery replacement, and the high frequency data collection possible.     

Design and realization of transparent solar modules based on luminescent solar concentrators integrating nanostructured photonic crystals

Herein, we present a prototype of a photovoltaic module that combines a luminescent solar concentrator integrating one‐dimensional photonic crystals and in‐plane CuInGaSe₂ (CIGS) solar cells. Highly uniform and wide‐area nanostructured multilayers with photonic crystal properties were deposited by a cost‐efficient and scalable liquid processing amenable to large‐scale fabrication. Their role is to both maximize light absorption in the targeted spectral range, determined by the fluorophore employed, and minimize losses caused by emission at angles within the escape cone of the planar concentrator. From a structural perspective, the porous nature of the layers facilitates the integration with the thermoplastic polymers typically used to encapsulate and seal these modules. Judicious design of the module geometry, as well as of the optical properties of the dielectric mirrors employed, allows optimizing light guiding and hence photovoltaic performance while preserving a great deal of transparency. Optimized in‐plane designs like the one herein proposed are of relevance for building integrated photovoltaics, as ease of fabrication, long‐term stability and improved performance are simultaneously achieved. © 2015 The Authors. Progress in Photovoltaics: Research and Applications published by John Wiley & Sons Ltd.     

A simple ray tracer to compute the optical concentration of photovoltaic modules

In a conventional photovoltaic module, some light that falls between the solar cells is internally reflected onto the cells via the backsheet and the glass–air interface of the module; thus, a module can be considered a static concentrator. We present a simple ray tracer that computes a module's optical concentration as a function of cell separation, cell geometry, and the optical properties of the encapsulants. The ray tracer's primary simplification is to divide the module's backsheet into small pixels and, since the reflection from the backsheet is approximately Lambertian and independent of the incident angle, to sum the intensity of all rays that enter a pixel and treat them as one. The advantage of this pixel approximation is that it makes it simple to simulate curved surfaces—such as the corners of a pseudo‐square solar cell—within short computation times. The results of the simple ray tracer are shown to be consistent with those of a conventional ray tracer and an LBIC experiment. We also apply the ray tracer to a present‐day SunPower module and find that 25% of the photons that fall between the cells are internally reflected onto the cells, which results in an optical concentration of 1·024. Copyright © 2005 John Wiley & Sons, Ltd.     

Losses caused by dispersion of optical parameters and misalignments in PV concentrators

One factor greatly affecting the output power of a photovoltaic concentrator system and not found in conventional flat systems is optical mismatch, caused mainly by dispersion in the efficiencies of the optics and misalignments among collectors and modules. In these systems, there are many factors, besides the electrical performance of the PV modules, acting on the overall efficiency and thus on the power output: reflectivity and shape of the optics, cleanliness, misalignments, etc. They are less known than equivalent topics in flat arrays and usually the data come from medium‐sized prototypes. Now we have the opportunity to study such factors on a big plant where components and the installation have been carried out by industry on a final user site. An analysis of the optical mismatch based on Gaussian distributions of the generated photocurrents will be presented in order to evaluate the power losses. A mathematical model is also proposed to calculate the main moments of the distribution from the experimental V–I curve under concentrated light. Besides, a novel study of the transmission curves of a photovoltaic concentrator has been carried out and will be described throughout the paper. The series/parallel connection of the modules can affect the transmission curve in different ways, depending on the array voltage, and thus this effect must be considered for the definition of the acceptance angle of the system. Copyright © 2005 John Wiley & Sons, Ltd.     

Four‐junction spectral beam‐splitting photovoltaic receiver with high optical efficiency

A spectral beam‐splitting architecture is shown to provide an excellent basis for a four junction photovoltaic receiver with a virtually ideal band gap combination. Spectrally selective beam‐splitters are used to create a very efficient light trap in form of a 45° parallelepiped. The light trap distributes incident radiation onto the different solar cells with an optical efficiency of more then 90%. Highly efficient solar cells including III–V semiconductors and silicon were fabricated and mounted into the light trapping assembly. An integrated characterization of such a receiver including the measurement of quantum efficiency as well as indoor and outdoor I–V measurements is shown. Moreover, the optical loss mechanisms and the optical efficiency of the spectral beam‐splitting approach are discussed. The first experimental setup of the receiver demonstrated an outdoor efficiency of more than 34% under unconcentrated sunlight. Copyright © 2010 John Wiley & Sons, Ltd.     

InGaP//GaAs//c‐Si 3‐junction solar cells employing spectrum‐splitting system

In this work, we practically demonstrated spectrum‐splitting approach for advances in efficiency of photovoltaic cells. Firstly, a‐Si:H//c‐Si 2‐junction configuration was designed, which exhibited 24.4% efficiency with the spectrum splitting at 620 nm. Then, we improved the top cell property by employing InGaP cells instead of the a‐Si:H, resulting in an achievement of efficiency about 28.8%. In addition, we constructed 3‐junction spectrum‐splitting system with two optical splitters, and GaAs solar cells as middle cell. This InGaP//GaAs//c‐Si architecture was found to deliver 30.9% conversion efficiency. Our splitting system includes convex lenses for light concentration about 10 suns, which provided concentrated efficiency exceeding 33.0%. These results suggest that our demonstration of 3‐junction spectrum‐splitting approach can be a promising candidate for highly efficient photovoltaic technologies. Copyright © 2016 John Wiley & Sons, Ltd.     

Record amorphous silicon single‐junction photovoltaic module with 9.1% stabilized conversion efficiency on 1.43 m2

Providing two‐thirds of the total stabilized power of thin‐film tandem MICROMORPH^TM technology, the amorphous junction remains a key element in the quest for higher efficiencies. This paper reports and summarizes a considerable work to achieve a record large‐area amorphous silicon single‐junction photovoltaic module. New hardware has been developed and known process steps have been accurately tuned and combined with new features of cell design. Effort was focused on the deposition of high‐quality and low‐defect a‐Si:H layers that has promoted an improved device stability and resistance against light induced degradation. Efficient light management has been used, and module design has been revised. The word‐record performance reported in this paper for a large‐area (1.43 m²) stabilized module conversion efficiency (total area) was measured and certified by Swiss PV Module Test Center to be 9.1%. Copyright © 2016 John Wiley & Sons, Ltd.     

Characteristics of large‐scale nanohole arrays for thin‐silicon photovoltaics

Nanostructured crystalline silicon is promising for thin‐silicon photovoltaic devices because of reduced material usage and wafer quality constraint. This paper presents the optical and photovoltaic characteristics of silicon nanohole (SiNH) arrays fabricated using polystyrene nanosphere lithography and reactive‐ion etching (RIE) techniques for large‐area processes. A post‐RIE damage removal etching is subsequently introduced to mitigate the surface recombination issues and also suppress the surface reflection due to modifications in the nanohole sidewall profile, resulting in a 19% increase in the power conversion efficiency. We show that the damage removal etching treatment can effectively recover the carrier lifetime and dark current–voltage characteristics of SiNH solar cells to resemble the planar counterpart without RIE damages. Furthermore, the reflectance spectra exhibit broadband and omnidirectional anti‐reflective properties, where an AM1.5 G spectrum‐weighted reflectance achieves 4.7% for SiNH arrays. Finally, a three‐dimensional optical modeling has also been established to investigate the dimension and wafer thickness dependence of light absorption. We conclude that the SiNH arrays reveal great potential for efficient light harvesting in thin‐silicon photovoltaics with a 95% material reduction compared to a typical cell thickness of 200 µm. Copyright © 2012 John Wiley & Sons, Ltd.