• Energy Research

To improve light absorption in organic solar cells, microscale surface-textured light-management (LM) films are applied on top of the front glass substrate. In this study, numerical simulations are employed to determine the optimal texture of the LM films that would result in the highest short-circuit current density of the solar cells in perpendicular, as well as oblique, illumination conditions. Different types of 2-D periodic surface textures are analyzed (pyramidal, parabolic, sinusoidal), and the effects of the period and groove height sizes are investigated. Numerical simulations are based on a model that combines geometric optics and wave optics and, thus, enables simulation of light propagation through the thick microtextured LM film and glass, as well as thin layers of the device, respectively. Results show that parabolic textures are the most advantageous for the solar cells to achieve high performance operating in changing illumination conditions. When properly optimized, they enable over 14% boost of the short-circuit current density in a broad range of illumination incident angles, with the maximum of 22% for perpendicular incidence, with respect to that of the nontextured cell.

In ultra-thin chalcopyrite solar cells and photovoltaic modules, efficient light management is required to increase the photocurrent and to gain in conversion efficiency. In this work we employ optical modelling to investigate different optical approaches and quantify their potential improvements in the short-circuit current density of Cu(In, Ga)Se 2 (CIGS)devices. For structures with an ultra-thin (500 nm)CIGS absorber, we study the improvements related to the introduction of (i)highly reflective metal back reflectors, (ii)internal nano-textures applied to the substrate and (iii)external micro-textures by using a light management foil. In the analysis we use CIGS devices in a PV module configuration, thus, solar cell structure including encapsulation and front glass. A thin Al 2 O 3 layer was considered in the structure at the rear side of CIGS for passivation and diffusion barrier for metal reflectors. We show that not any individual aforementioned approach is sufficient to compensate for the short circuit drop related to ultra-thin absorber, but a combination of a highly reflective back contact and textures (internal or external)is needed to obtain and also exceed the short-circuit current density of a thick (1800 nm)CIGS absorber.

In photovoltaic (PV) systems, energy yield is one of the essential pieces of information to the stakeholders (grid operators, maintenance operators, financial units, etc.). The amount of energy produced by a photovoltaic system in a specific time period depends on the weather conditions, including snow and dust, the actual PV modules’ and inverters’ efficiency and balance-of-system losses. The energy yield can be estimated by using empirical models with accurate input data. However, most of the PV systems do not include on-site high-class measurement devices for irradiance and other weather conditions. For this reason, the use of reanalysis-based or satellite-based data is currently of significant interest in the PV community and combining the data with decomposition and transposition irradiance models, the actual Plane-of-Array operating conditions can be determined. In this paper, we are proposing an efficient and accurate approach for PV output energy modelling by combining a new data filtering procedure and fast machine learning algorithm Light Gradient Boosting Machine (LightGBM). The applicability of the procedure is presented on three levels of irradiance data accuracy (low, medium, and high) depending on the source or modelling used. A new filtering algorithm is proposed to exclude erroneous data due to system failures or unreal weather conditions (i.e., shading, partial snow coverage, reflections, soiling deposition, etc.). The cleaned data is then used to train three empirical models and three machine learning approaches, where we emphasize the advantages of the LightGBM. The experiments are carried out on a 17 kW roof-top PV system installed in Ljubljana, Slovenia, in a temperate climate zone.

The role of a reflecting interlayer in micromorph silicon thin-film solar cells is investigated from the optical point of view. Detailed optical modelling and simulation are used to study the effects of different interlayers on quantum efficiency and short-circuit current of the top, amorphous silicon, and bottom, microcrystalline silicon, solar cell. The role of refractive index of interlayers on quantum efficiency of the top and bottom cell is analysed. Critical issues, such as enhanced total reflection from the solar cell and decreased quantum efficiency of the bottom cell due to interlayer are studied. Besides the single interlayer concept, double and triple interlayer stacks are investigated and improvements in comparison to the single ZnO interlayer are demonstrated. Potential thickness reductions of the top amorphous silicon cell related to different interlayers are presented.

The energy yield of a photovoltaic (PV) system with fixed free-standing PV arrays is affected also by the self-shading effects. The rows of PV modules in arrays may partially shade the PV modules in the rows behind. In this paper the effects of the row distance on the PV system’s energy yield are evaluated. The estimation of the self-shading losses by the irradiation losses simply overestimates the losses; therefore we developed a simulation model to simulate the real energy loss due to shading of the preceding row in a PV system. The model demonstrates that the self-shading energy losses are at commonly used distances between rows from 20 to 40% lower than the irradiation losses at the modules’ bottom considering the shading conditions. The self-shading energy loss is studied in the case of Ljubljana, Slovenia which may refer to the whole Central Europe. To estimate the self-shading losses a technology-and with parameter modifications also location-independent empirical equation based on module-to-cell width ratio was derived and validated.

Abstract To further improve the efficiency of the wafer-based silicon photovoltaic (PV) module, producers are introducing new module designs with cut-cells. Since smaller solar cells might be affected by partial shading even more and earlier than full-size cells, the energy performance simulations of partially shaded modules are crucial. A detailed shading analyses of partially shaded modules with different cut cell designs are presented not only on a single case scenario but on annual energy yield simulations using Spice, where a shading scenario over the whole module by the use of a new 3D shading horizon profile of selected shading objects is calculated. The annual simulations reveal that regardless the module design almost all cells in the module are confronted by reverse bias, which can deteriorate the module performance significantly. Simulation results with three different shading objects on five different module topologies at five locations showed that the best cut-cell module design depends strongly by the micro location and shading objects; however, in general the string of solar cells connected in series should be aligned with the shading shape around noontime as much as possible. A comprehensive annual energy performance evaluation of partially shaded cut-cell modules revealed; that with a correct cell layout of cut-cells in a PV module, the shading losses can be reduced by 30–50% if comparing to the standard PV module design.

The influence of the ethylene-vinyl acetate (EVA) film quality on potential induced degradation was studied on in-house developed mini modules with p -type monocrystalline silicon solar cells. The modules were assembled with EVA films of equivalent qualities, but different ages and exposed to an accelerated test (relative humidity = 85%, T = 60 °C, Vbias = +1000 V). The age of the EVA film was determined from the time we received the EVA film, and opened the sealed enclosure and the time of lamination. After the EVA film was removed from the sealed enclosure, it was kept in a dark place at room temperature. The storage times of the “fresh,” “aged,” and “expired” films were: less than 14 d, around 5 mo, and more than 5 years, respectively. While modules with a “fresh” EVA film exhibit almost no degradation, the modules with the “aged” EVA film degrade very rapidly and severely. Their degradation rate was around 0.2%/d during the 2000 h of damp heat test. We also observed a strong silver line corrosion, which occurs because of the peroxide leftovers in the “aged” EVA films.