Outlier identification in outdoor measurement data - effects of different strategies on the performance descriptors of photovoltaic modules

Outdoor measurement campaigns of PV module performance are normally affected by a relatively large number of outliers. The aim of this paper is to develop a statistically sound approach of obtaining a dataset that allows one to analyse continuously monitored devices. This paper uses ISC as a self-reference parameter to measure the incident irradiance on the module, which largely reduces the error due to spectral and angular effects. The outlier identification procedure is based on statistical distribution analysis of different performance descriptors and it assures 0.99 confidence level and the same skewness for the remaining data. This approach can be applied to whole datasets as well as for data in specific irradiance-temperature bins. The developed methodology will be used to analyze outdoor data from different devices at different locations with reduced uncertainty

The impact of acetic acid corrosion on the front-side contacts and the finger electrodes of c-Si PV cells

Damp-heat stresses typically lead to corrosion related degradation for Ethylene-vinyl acetate (EVA) encapsulated PV modules. The degraded module appears as bright areas along the busbars in electroluminescence (EL) images. The most likely mechanism that causes these bright areas is corrosion at the silver contacts and the finger electrodes at the front surface of the cell, due to acetic acid accumulation. In this work, these mechanisms are investigated by immersion of half encapsulated cells into acetic acid solutions with different concentrations. Dark and light I-V curve measurements and electroluminescence images were conducted during the course of ageing to investigate the power degradation. The chemical changes in materials are identified by optical microscopy images and SEM-EDX analysis

Investigating the degradation of front and rear sides of c-Si PV cells exposed to acetic acid

The goal of this paper is to investigate the reactions responsible for the degradation of the front and rear side of c-Si solar cells that are immersed in acetic acid solutions. The types of degradation are characterised by electrical, optical and chemical methods. The identified degradation modes are compared to those observed in PV cells undergoing damp-heat aging. The degradation of the power output of a PV cell exposed under damp-heat conditions typically is observed after 3000-5000 hours of exposure. The purpose of this work is to research if acetic acid immersion of PV cells can result in the same types of degradation observed for damp-heat aging and how these degradation modes affect their power output. It is observed that acetic acid immersion and damp-heat exposure show a similar mechanism of degradation. This degradation is identified to occur because of reaction of acetic acid with lead, tin and aluminium. Lead and tin migration causes separation of the ribbons and the silver contacts from the cells. Aluminium corrodes severely because of presence of moisture, causing a series resistance increase of the cells

A fast and effective approach to modelling solar energy potential in complex shading environments

A fast and effective model for the computation of solar energy potential in complex shading environments is presented. Accurate calculation and identification of solar energy potential profiles is demonstrated over large areas. Calculation time is exceptionally fast, even on an average specification PC (typically under 1 min per 1 km2). Problems with commonly used low-resolution sky domes that can lead to irradiance calculation errors of ~5% are identified. Ideal placements are easily visually identified from resultant irradiance/irradiation profile images. Image processing techniques for spatially distributed optimization problems are described and an example of energy value optimization is presented by means of individual dwelling demand separation & comparison

Accessing the performance of individual cells of fully encapsulated PV modules using a commercial digital light processing projector

Accessing the electrical parameters of individual cells in fully encapsulated photovoltaic (PV) modules can be a cumbersome and time-consuming procedure. It usually requires mechanical shading, which is achieved by using meshes. This limits the control and variability of shading, as there is always a limited variety of mesh patterns available. In this work digital projection technology is utilised as the light source to achieve this. Partial shading can be applied rapidly and performance parameters of individual cells in fully encapsulated modules can be acquired. This is demonstrated in this work using a custom mini module. Individual cells can be accessed even in the case that bypass diodes are included. Performance information of individual cells acquired with such a system can be used for studying upscaling losses or degradation mechanisms for commercial or research PV modules

Utilising digital light processing and compressed sensing for photo-current mapping of encapsulated photovoltaic modules

Photocurrent mapping can provide useful spatial information about the electrical and optical properties of a photovoltaic (PV) device under actual operating conditions. Although it is a well-established technique for PV cells, direct current mapping measurements of PV modules is impractical and time-consuming to be applied. One has to mechanically shade specific cells of the PV module or destructively access the cell to be measured. In this work, non-destructive, automated current mapping of encapsulated PV modules is demonstrated. A commercial Digital Light Processing (DLP) projector is utilised in order to apply compressive sampling for current mapping of PV modules. This method is non-destructive, cost effective and significantly fewer measurements are needed for acquiring a current map compared to raster scanning methods. When applying compressive sampling, a series of patterns is projected on the sample, the current response is measured for each pattern and the current map is acquired using an optimisation algorithm. Specific shading strategies, voltage bias settings and I-V curve details are investigated for optimised compressive sampling

Degradation study of the peel strength of mini-modules under damp heat condition

This paper presents the degradation study results of adhesion strength between backsheet and encapsulant for a commercial minimodule. A degradation model for the adhesion strength is developed and the activation energy is obtained. Outdoor prediction example is given based on environmental data in Loughborough and Denve

Degradation of adhesion strength within mini-modules during damp-heat exposure

The degradation of adhesion strength between back-sheet and encapsulant due to moisture ingress was investigated for commercial crystalline silicon photovoltaic (PV) mini-modules. The damp-heat test originated from qualification test was carried out at five different temperature and humidity conditions (95oC/85% RH, 85oC/85% RH, 65oC/85% RH, 85oC/65% RH and 85oC/45% RH) to assess the impact of stress levels on test outcomes. The adhesion strength was measured by 90o peel tests, carried out at specified degradation intervals. Several visual defects were observed, including delamination, moisture ingress and bubble formation. The adhesion strength showed a stretched exponential decay with time and significant influences of test conditions was demonstrated. A humidity dose model was proposed by assuming micro-climates seen by the modules, i.e. surface relative humidity of the back-sheet as the driving factor for an Arrhenius based model using temperature as accelerating factor. The correlation between adhesion degradation and humidity dose was investigated and an exponential model was developed to represent this correlation with extracted activation energy (Ea) of 63kJ/mol. This supplies a potential model for the estimation of adhesion strength decay triggered out by humidity in dependence of the humidity conditions

Multi-layer LBIC system for thin film PV module characterisation

Several non-destructive characterisation tools - solar simulator, LBIC, thermography - are used together to investigate the performance of and locate possible defects in TF silicon PV modules of different structures. A special module is investigated where all techniques are compared and good agreement is demonstrated

Investigation of moisture ingress and egress in polymer – Glass laminates for PV encapsulation

The combinations of different and time-varying environmental conditions affect PV modules operating outdoors, both instantaneously and by causing different types of long-term performance degradation and failure modes. Among the most crucial environmental stressors for long-term effects is moisture ingress. Moisture reacts with the back sheet, encapsulant and outer parts of the solar cells. If the encapsulant is the commonly-used ethylene vinyl acetate (EVA), moisture that has entered the laminate reacts with the VA content, producing acetic acid. This reacts with the contacts on the cells, causing corrosion and eventual negative impact on the electrical output. This work investigates the rates and patterns of moisture ingress into and egress from c-Si PV laminates as the external humidity changes, through both simulation and experiment