PV Modules

Apart from aesthetics, the gain in electrical performance of back-contact solar cells and modules is particularly attractive compared to conventional PV modules. This major benefit results from getting rid of (the majority of ) the metallization at the front, and providing all the cell contacts at the back. An overview is presented here of the different concepts put forward by different institutes and companies around the world for such back-contact modules. The different types of state-of-the-art back-contact cell are first introduced, together with their corresponding contacting and interconnection schemes. Keeping in mind the reference module technology for two-side-contacted cells as a starting point, each module concept is then briefly discussed in terms of technology and level of maturity. Finally, the main technological differences are summarized.

Low-temperature thermal stresses in a manufactured photovoltaic module (PVM) based on crystalline silicon (Si), before the PVM is fastened into a metal frame, are assessed on the basis of a simple, analytical (mathematical), easy-to-use and physically meaningful predictive stress model. The PVM considered comprises the front glass, ethylene vinyl acetate (EVA) encapsulant (with silicon cells embedded into it) and a laminate backsheet. The stresses addressed include normal stresses that act in the cross sections of the constituent materials and determine their short- and long-term reliability, as well as the interfacial (shearing and peeling) stresses that affect the assembly’s ability to withstand delaminations. The interfacial stresses also determine the cohesive strength of the encapsulant material. The calculated data, based on the developed model, indicate that the induced stresses can be rather high, especially the peeling stress at the encapsulant-glass interface, so that the structural integrity of the module might be compromised, unless the appropriate design-for-reliability (DfR) measures, including stress prediction and accelerated stress testing, are taken. The authors are convinced that reliability assurance of a photovoltaic (PV) product cannot be delayed until it is manufactured – such an assurance should be considered and secured, first of all, at the design stage.

Flexible copper-indium-gallium-(di)selenide (CIGS) absorbers offer a wide range of possible applications in rigid as well as flexible and lightweight solar module designs. The main advantage of CIGS in comparison to the well-known flexible module technology based on amorphous silicon is its currently higher efficiency and the promising optimization potential of its efficiency in the future. Because of low cell thicknesses of less than 40µm and the general sensitivity of CIGS to moisture, it is a challenge to develop suitable interconnection and encapsulation technologies that promote long-term reliability of solar modules. Selected aspects of our work in this area will be discussed in this paper.

With current state-of-the-art PV module tests stipulating only a static mechanical load test in accordance with IEC 61215 and IEC 61646 standards, hardly any fatigue stressing is carried out on cells, cell connectors or rigid component parts such as the glass or framing. This paper presents research on dynamic load testing of PV modules and discusses reliability aspects of these essential requirements that must be considered in future standardization work.

Crystalline silicon solar modules installed in the field are exposed to atmospheric conditions and experience stress, which induces a wear-out phenomenon in various parts of the modules and degrades performance over time. The performance eventually reaches a point where the output power falls below an acceptable level. Thermal cycling (TC) and damp heat (DH) are two important reliability tests for estimating infant failures related to materials and the manufacturing process, as well as providing the information on performance degradation with respect to time. In this study, modules composed of 156mm × 156mm multicrystalline silicon cells were subjected to TC and DH tests. By applying acceleration models, such as the Norris-Landzberg model for TC and the Hallberg-Peck model for DH, the minimum guaranteed life was calculated. The electrical and reliability results were interpreted and explained on the basis of the respective models.

Non-destructive methods for measuring photovoltaic modules are discussed in this paper, with the aim of comparing different quality-assurance methods for different module technologies (e.g. crystalline and thin-film technologies: a-Si, CdTe, CIS). For a complete quality control of PV modules, a combination of fast and non-destructive methods was investigated. In particular, camera-based measurements, such as electroluminescence (EL) and infrared (IR) technologies, offer excellent possibilities for determining production failures or defects in solar modules, which cannot be detected by means of standard power measurements. These methods are applied effectively in quality control and development support, and EL is already an important characterization tool in industry and research. Most short circuits reduce the voltage in their surrounding area and appear dark in EL images. However, as this failure is not always critical and apparent, short circuits are only precisely identifiable in combination with IR measurements. Therefore, to quickly detect at high resolution the most common defects in a PV module, a combination of EL and IR measurements is advisable.

This paper presents a novel glue-membrane integrated backsheet specifically for PV modules, which has been designed and fabricated by utilizing a flow-tangent cast roll-to-roll coating process combined with a plasma technique. Polyethylene terephthalate (PET) is adopted as a substrate and is surface activated and etched by atmospheric plasma. Then a special coating formulation containing reactive fluoropolymers is applied to both sides of the PET, followed by thermal curing, resulting in a glue-membrane integrated coating layer with a polyurethane structure. Finally, a monolayer of silane molecules is grafted onto the surface via plasma-enhanced deposition to provide the surface medium with surface energy, rendering excellent long-term adhesion to ethylene vinyl acetate (EVA). Scanning electron microscope (SEM) images have revealed that plasma etching and activation significantly improves compatibility between the PET and the coating layer, resulting in a tight and strong integration between the two. It has also been confirmed by SEM that the obtained novel backsheet integrates the glue layer and the membrane layer perfectly. There is no clear boundary between the two layers, distinguishing the novel backsheet from the conventional layer-by-layer laminated backsheet. The unique glue-membrane integrated structure has already been demonstrated by many practical applications under harsh environmental conditions to have significant advantages over other backsheets regarding delamination, blistering and discoloration. Furthermore, the novel backsheets showed excellent barrier properties, weatherability (85°C, 85% RH, 1000h), mechanical properties and electrical isolation properties. Because it is a promising photovoltaic material, the novel backsheet has already been widely used in China for PV module encapsulation and has obtained extensive praise from customers.

The improved performance and reduced manufacturing costs of photovoltaic (PV) modules that have been achieved in recent years have positioned this technology as an economically attractive renewable electric energy source. In order to verify that this also has a positive impact on energy payback time (EPBT) and carbon footprint, the Energy Research Centre of the Netherlands (ECN) has conducted a life cycle analysis (LCA) for REC Peak Energy-series PV modules produced by Renewable Energy Corporation (REC). The LCA study was based on a full set of actual production data obtained for the first quarter of 2011 from REC’s manufacturing sites. Because REC is an integrated manufacturer, the LCA study includes internal data for the production steps from polysilicon production to module assembly, as well as for all materials and transportation associated with production. ECN used generic figures for installation, operations and recycling together with the REC data to assess the environmental impact indicators. For polysilicon produced in the USA, and for wafers, cells and modules produced in Singapore, an EPBT of 1.2 years was achieved, with a corresponding carbon footprint of 21g CO2-eq/kWh for PV systems located in southern Europe (1700kWh/m2year irradiation). For modules with wafers and cells produced in Norway, the corresponding values were 1.1 years and 18g CO2-eq/kWh. A key contributor in achieving these values is REC’s highly efficient fluidized bed reactor (FBR) process for the production of polysilicon.

This paper presents a new differential scanning calorimetry (DSC) method that allows the determination of the degree, or level, of crosslinking of ethylene-vinyl acetate (EVA) copolymers, including EVA films used as encapsulants for photovoltaic (PV) applications. This method can also determine additional characteristics of EVA, such as its weight per cent (wt %) vinyl acetate (VA) content and its fluidity. The paper describes the procedure and its application to EVA film samples laminated at 145°C, for different lengths of time in an industrial-type laminator for PV modules, as well as to EVA uncrosslinked samples of different composition and fluidity. The scope of the method compared to other characterization methods for the degree of crosslinking of EVA is discussed. An experimental comparison is also made to rheological and gel content methods.

This paper describes the technical concepts and current status of back-contact module technology. A back-contact module has the advantage of a higher conversion efficiency because of less shading of the front of the cell, fewer inactive areas in the module and lower series resistance in the interconnection. Aesthetically, back-contact modules are more attractive than standard modules. Furthermore, module manufacturing is gentler due to there being less cell handling during the process. The two main technical concepts related to back-contact modules – interconnector technology and printed circuit backsheet technology – are discussed in this paper. An overview is given of the production status of current back-contact module manufacturers to also show the significant potential of this technology in economic terms.