Investigation of photovoltaic device operation under varying spectral conditions

A unique system for the synchronous measurement of photovoltaic module electrical parameters and solar spectral irradiance is described. Data quality control is addressed in depth and the practices followed to ensure integrity for the ensuing analysis are explained. Irradiance spectra have been characterised with a single-number descriptor, the average photon energy. Unlike previous spectral characterisation work, the figure employed here is independent of artificially imposed references, such as the spectral response of a solar cell. This has allowed it to be used in a full analysis of spectral variation in the UK, with comparison made to a second site of significantly different climate. This simple characterisation has also allowed spectral irradiance measurements to be included for the first time in a thorough investigation of the effects of different environmental factors on photovoltaic device performance in real operating conditions. Discussion is entered on each of the main influences on key device parameters and concludes with a quantification of the principal effects on electrical energy generation. The development of an enhanced spectral irradiance model is described, which can simulate solar spectra under non-ideal weather conditions, as experienced by many locations. In validation against measured data, it is proved a convenient solution to the problem of poor availability of spectral irradiance data for use in photovoltaic performance modelling. The inclusion of spectral effects into photovoltaic device modelling is undertaken and shown to significantly improve existing modelling approaches. Finally, improvements are identified and suggestions made to further the work

Degradation of power output of photovoltaic modules due to accelerated ageing

This research work focusses on a type of indoor accelerated stress test (AST) which is an extension of the damp-heat (DH) test proposed by the international electrotechnical commission in IEC-61215-2, to reproduce the field failures by applying environmental stresses (temperature (T), relative humidity (RH)) to photovoltaic (PV) mini-modules as well as to compare whether the calculated activation energy associated with such degradation modes is consistent with the range of values reported in literature. In addition, the effect of curing time during PV module lamination on the durability of the PV-modules will be evaluated for crystalline silicon (c-Si) PV mini-modules produced from the same materials

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

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

Effect of shading caused by dust on cadmium telluride photovoltaic modules

The effect of dust on Cadmium-Telluride photovoltaic (PV) thin film modules is investigated by the application of a spatial 3 dimensional model developed with the circuit analysis software PSPICE. The effect of dust concentration and tilt angle variation on the PV module’s performance was investigated. The probability of hotspots in different installation positions is investigated. The simulation results showed a reduction in the sample’s performance with increased dust concentration and reduced tilt angle. The variation between cell positions showed that a horizontal orientation of the cells has an increased risk of hotspots with cells with localized lower parallel resistances than cells identified with uniform high parallel resistance

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

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

Improved outdoor monitoring of photovoltaic modules

The Centre for Renewable Energy Systems Technology (CREST) has been operating an outdoor measurement facility for testing photovoltaic (PV) modules since 1998. The facility is used to continuously measure the performance of a range of commercial and prototype modules, by scanning full I-V characteristics every ten minutes with synchronous measurements of ambient and module temperatures and broadband and spectral irradiance. The trend for increasingly high power PV modules and increased demand for channels has precipitated the next stage of development for the CREST system. A number of lessons have been learned from the past system which are indicated in this paper with a thorough system analysis, along with a description of the standard measurement cycle currently in operation. Then follows a full technical description of the new system, including the hardware design and software development

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