Modelling solar flux distributions for fresnel lens CPV systems

A computer model for the simulation of solar flux distribution in the direct and circumsolar regions of the beam irradiation has been created. The model incorporates previous research into circumsolar ratios (CSRs) [1,2]. It is used to demonstrate the importance of realistic solar flux distributions as source inputs in Concentrator Photovoltaic (CPV) simulations. It is shown that the distribution of flux for different circumsolar ratios varies significantly. Such variation will have a considerable effect on the optical image formed at the receiver of a solar concentration system and thus is a necessary consideration in CPV modelling. Flux distributions incident on lenses of various entry apertures are generated and used to investigate the losses in incident flux resulting from tracking errors and CSR variation. It is found that, for a concentrating system with an entry aperture of 0.25°, a 20% loss of net annual incident energy is found with a tracking error of ~0.1°. The same loss is found with tracking errors of ~0.3°, 0.6° and 0.85° with apertures of 0.5°, 0.75° and 1°, respectively

The Performance of Private Equity Funds: Does Diversification Matter?

This paper is the first systematic analysis of the impact of diversification on the performance of private equity funds. A unique data set allows the exact evaluation of diversification across the dimensions financing stages, industries, and countries. Very different levels of diversification can be observed across sample funds. While some funds are highly specialized others are highly diversified. The empirical results show that the rate of return of private equity funds declines with diversification across financing stages, but increases with diversification across industries. Accordingly, the fraction of portfolio companies which have a negative return or return nothing at all, increase with diversification across financing stages. Diversification across countries has no systematic effect on the performance of private equity funds

Modelling the efficiency of terrestrial photovoltaic systems

A computer simulation capable of investigating the interrelationship of module packing densities and module inclination angles and their effects on overall energy yield for a given PV system installation area is presented. It is demonstrated that the simulation is a useful tool in the optimization of proposed system designs, the analysis of electrical performance and, moreover, the prediction of the occurrence of degenerative system effects such as hot-spots. In one case, it is shown that increasing the system height to module spacing ratio from 0.18 to 0.24 results in potentially severe shading effects. Results for Seville (Spain) and Loughborough (UK) are compared. The potential pros and cons of tracking systems are demonstrated, in that elevation only tracking results in an annual irradiance harvest reduction of 0.4% in Loughborough and increase of 3.4% in Seville. Varying module inclination angles shows how significant irradiance losses can occur when static PV arrays are not optimally mounted, reducing the inclination from 40 degrees to zero results in an annual irradiance harvest reduction of ~20% in Seville and ~14% in Loughborough

Spectral irradiance effects on the outdoor performance of photovoltaic modules

The outdoor performance of photovoltaic modules is influenced by spectrum. Even if the irradiance level and the operating temperature is the same, performance difference of photovoltaic modules between the seasons can be increase up to 15% depending on the photovoltaic module type. In this paper, seasonal spectral irradiance effects on the outdoor photovoltaic module performance and previous studies has been summarised thoroughly. The spectrum indicators which are used for spectra characteristics, Useful Fraction and Average Photon Energy are described in detail. This study also indicates spectrum effects on PV performance and outlines the present studies investigating this effect

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

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

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

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