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

Accuracy assessment of models estimating total irradiance

The accuracy of estimating total in-plane irradiance is investigated for the UK climate. Several models, which differ in essence in an assumed diffuse irradiance distribution at different levels of sky cloudiness, were used to calculate solar irradiance on inclined planes from horizontal irradiance data. The accuracy of this calculation was validated against measured data. It transpires that there is not massive difference between the various methodologies, but on average the Reindl model seems to be slightly better than other methods, followed by Perez et al. model

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

PV module degradation mechanisms under different environmental stress factors

Understanding the degradation behaviours of photovoltaic (PV) devices is critical for optimising its financial viability. The degradation of PV modules is dependent on multiple factors such as installation site, mounting conditions, manufacturing process and module types. This means that in order to understand the long term behaviour of PV modules, one needs to assess the stresses acting on the modules (first two factors) and the module’s response to these. In this paper, the possible degradation mechanisms within a PV module according to different stress factors are discussed and linked to typical module constructions. The relationships between degradation mechanisms and electrical performance are analysed as a first step to predict long term power degradation

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 modelling approach for long-term degradation of thin film silicon photovoltaic modules

This paper introduces a new concept of approach for modelling the ageing behaviour of a-Si PV modules with voltage-dependent photocurrent. The basis is the equivalent circuit of a PV module, specifically the modified single diode model. The parameters are extracted from I-V measurements. Ageing is then analysed by relating these to the environmental stresses seen by the devices. This paper focuses on the behaviour the product of carrier mobility and carrier life time (μτ), since the μτ has been considered to be an important indicator for module degradation of amorphous silicon thin film devices. A fitting approach for determining μτ is discussed and extended to be applied to the outdoor module IV data. Three a-Si modules of the same type operating under different temperature conditions are analysed to identify changes in the μT

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

Optimised inverter sizing in the UK

Optimal inverter sizing strategies in the UK are investigated in this paper, considering both the specific climate and the detailed inverter dynamic characteristics. A model of a PV system linked to an inverter is developed to assess and optimise how the different factors influence the correct sizing of a given PV system. Inverter efficiency changes up to 3% as a funtion of input voltage which needs to be considered in inverter sizing. The environmental data with time resolution higher than 10 minutes is recommended since low frequency data cannot guarantee accurate optimal sizing of inverter. Over-sizing the PV array rated power 10% to 40% with respect to the inverter nominal power appears to achieve the optimisation of inverter sizing in the UK

Modelling of long-term energy yield of photovoltaic devices

Modelling of long-term energy yield of photovoltaic device

Direct Measurement of the Mn(II) Hydration State in Metal Complexes and Metalloproteins through ¹⁷O NMR Line Widths

Here we describe a simple method to estimate the inner-sphere hydration state of the Mn­(II) ion in coordination complexes and metalloproteins. The line width of bulk H₂¹⁷O is measured in the presence and absence of Mn­(II) as a function of temperature, and transverse ¹⁷O relaxivities are calculated. It is demonstrated that the maximum ¹⁷O relaxivity is directly proportional to the number of inner-sphere water ligands (<i>q</i>). Using a combination of literature data and experimental data for 12 Mn­(II) complexes, we show that this method provides accurate estimates of <i>q</i> with an uncertainty of ±0.2 water molecules. The method can be implemented on commercial NMR spectrometers working at fields of 7 T and higher. The hydration number can be obtained for micromolar Mn­(II) concentrations. We show that the technique can be extended to metalloproteins or complex:protein interactions. For example, Mn­(II) binds to the multimetal binding site A on human serum albumin with two inner-sphere water ligands that undergo rapid exchange (1.06 × 10⁸ s^–1 at 37 °C). The possibility of extending this technique to other metal ions such as Gd­(III) is discussed