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Analysis of soiling data and its effects on the performance of utility-scale PV plants
Get PDFGjennom de siste årene har solenergi fått et løft politisk, teknologisk og sosialt. Høye elektrisitetskostnader, globale katastrofer og klimaendringer har lagt grunnlaget for satsingen på elektrisk kraftproduksjon fra solenergi. For stor-skala solcelleanlegg er effektiv drift og vedlikehold avgjørende for lønnsomhet. Formålet med denne oppgaven er å finne meningsfull informasjon i tilsmussingsdata fra målestasjoner for stor-skala solcelleanlegg. Forskningsspørsmålet er formulert som følger: Hvordan kan tilsmussingsdata bli filtrert og analysert for å bestemme smuss-situasjonen og dens effekt på ytelsen av et stor-skala solcelleanlegg? Hovedmålet med oppgaven er å finne estimater for daglig tilsmussingsgrad i de forskjellige solcelleparkene. Dette innebærer å utvikle metoder for å bedre differensiere mellom god og dårlig datakvalitet. I tillegg blir forholdet mellom tilsmussingsgrad og korrigert ytelsesgrad i parken undersøkt. Avslutningsvis blir effekten regn har på vasking av panelene undersøkt. Gjennomgående metode i avhandlingen er filtrering og korrigering av datasett, samt diverse statistiske og matematiske analyser sett opp mot eksisterende teori. Hovedfunnene viser daglige tilsmussingsgrader på i snitt, 0, 12 ± 0, 01 %, 0, 135±0, 006% og 0, 047±0, 006% for solcelleanlegg i Sør Amerika, Nord Afrika og Sør Afrika respektivt. I tillegg blir det funnet lite korrelasjon mellom tilsmussingsnivå og ytelsesgrad, noe som tyder på tilstedeværelse av andre uoppdagede ytelsesbegrensende hendelser. Det blir også funnet at daglig nedbør mellom 3, 3mm og 4, 2mm er tilstrekkelig nok til ˚a holde tilsmussingsgraden mellom 1, 0% og 1, 5% i Sør Amerika. For videre forskning vil forbedring av datakvalitet og innsamling av data være en prioritering. I tillegg blir det foreslått en automatiserbar metode som også ville vært interessant å utforske videre. Siden tilsmussing av solcellepaneler kan redusere energiproduksjon betydelig, er videre forskning på dette feltet avgjørende for solcelleteknologiens fremtid.In recent years, solar energy has received a major boost politically, technologically, and socially. High electricity costs, global disasters and climate change has has laid the foundation for a global interest in investments in electric power production from solar energy. For utility-scale photovoltaic systems, efficient operation and maintenance are crucial for profitability. The purpose of this dissertation is to find meaningful information in soiling data from dedicated measuring stations for utility-scale photovoltaic systems. The research question is formulated as: How can soiling station data be filtered and analyzed to determine the soiling situation and its effects on the performance of a utility-scale PV plant? The main goal of this thesis is to find estimates for the daily soiling rates in the various PV plants. This includes developing filters to better differentiate between good and poor data quality. In addition, the relationship between the soiling level and the corrected performance ratio in several plants is examined. Finally, the effect of rainfall panel cleaning is investigated. The general method in the dissertation is filtration and correction of data sets, as well as various statistical and mathematical analyzes, seen in the context of existing theory. The main findings are daily soiling rates of, on average, 0.12 ± 0.01 %, 0.135 ± 0.006% and 0.047 ± 0.006% for utility-scale PV plants in South America, North Africa, and South Africa respectively. In addition, little correlation is found between the soiling level and the corrected performance ratio, which strongly indicates a presence of other performance-limiting events that were undetected. It is also found that daily rainfall between 3.3mm and 4.2mm is sufficient to keep the soiling rate between 1.0% and 1.5% in South America. For further research, improving data quality and collection would be an important priority. In addition, an automatable method is proposed that would also be interesting to explore further. Since the soiling of solar panels can greatly reduce production, further research in this field is crucial for the future of this technology.M-M
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Hydrogen-related defects in hydrothermally grown ZnO studied by Fourier transform infrared spectroscopy
No full textHydrogen is an important impurity in ZnO, and it is believed to act as a shallow donor and to passivate acceptors in the material. H is readily associated with other defects in ZnO, forming complexes with characteristic localized vibrational modes (LVMs). The H-related peaks observed in the IR absorption spectra of ZnO is thus highly dependent on the concentration of other impurities and native defects.
In this work, H-related defects in hydrothermally (HT) grown ZnO single crystals have been investigated by Fourier Transform Infrared Spectroscopy (FTIR), Secondary Ion Mass Spectrometry (SIMS) and four point resistivity measurements.
Due to a high concentration of Li acceptors, the LVM of a OH-Li complex dominates the IR spectra of the as-grown samples. Several other H-related peaks are however also observed. The results presented in this work indicate that the 3577 cm-1 peak associated to the OH-Li defect exhibits a complex annealing dependency, which may be explained by a process involving diffusion and recapture of H. Also, dissociation of the defect occurs at substantially lower temperatures than the previously reported thermal stability of 1200 °C. The absorption cross section of the OH-Li signal has been estimated to be 1.27 x 10^(-17) cm.
A group of IR absorption peaks at 4216, 4240 and 4246 cm-1 have also beenobserved in the IR spectra of as-grown samples. By comparison with SIMS measurements, the previous identification of these peaks as internal electronic transitions of substitutional Ni_(Zn) impurities has been verified. The absorption cross section of the peaks has been found to be 2.91 x 10^(-17) cm.
Several other H-related peaks appeared in the IR spectra recorded after annealing of the samples in H2 and/or D2 atmospheres, caused by diffusion of H/D into the crystals. Two IR absorption peaks at 3303 and 3321 cm-1 were assigned to the LVMs of a defect complex labeled H-X, consisting of two O-H bonds associated to an unknown impurity atom. The 3321 cm-1 mode is oriented along the c-axis of ZnO, while the 3303 cm-1 mode is oriented at an angle with the c-axis. The H-X complex is thermally stable up to ~ 600 °C and the activation energy fordissociation was estimated to be 2.8 eV.
An IR absorption peak at 2783 cm-1 was also observed after hydrogenation. This peak was assigned to the LVM of a defect complex labeled H-Y, involving a single O-H bond oriented at an angle with the c-axis. Also, a pair of peaks at 3347 and 3374 cm-1 were observed in both as-grown and hydrogenated samples after annealing at ~ 500 °C. These peaks were assigned to the LVMs of two O-H bonds associated to the same defect, labeled H-Z. The H-X defect is to our knowledge not reported in the literature. The H-Y and H-Z defects have previously been identified as OH-Ni_(Zn) and (OH)2-Cu_(Zn) complexes, respectively. However, the SIMS and IR absorption data presented in this work indicate that both these assignments should be revisited.
The H2 and/or D2 gas anneals were also followed by a substantial drop in resistivity, which was found to be stable after annealing at 200 °C. The resistivity however increased markedly after subsequent annealing at higher temperatures (>500 °C). The increased carrier density after hydrogenation is presumably caused
by a combination of thermally stable H donors like HO and H passivation of acceptors present in the as grown samples, forming neutral complexes like OH-LiZn
New methods for investigation of surface passivation layers for crystalline silicon solar cells
No full textOne of the main challenges of the c-Si PV industry is the implementation of high quality surface passivation layers. Effective surface passivation is needed in order to avoid large efficiency losses when moving towards thinner silicon wafers and to utilize the full potential of high quality Si material. To obtain a fundamental understanding of the surface recombination mechanisms and their impact on device performance, precise characterization methods and suitable physical models are of high importance.
The main result of this work is the development of a new method for analyzing surface recombination for passivated Si substrates under varying surface band bending conditions: A photoluminescence imaging setup is used to measure the effective minority carrier lifetime of passivated Si wafers while applying an external bias to a metal electrode deposited onto the rear side passivation layer. The experimental measurements have been analyzed using an extended Shockley Read-Hall model with added recombination in the space charge region. This approach is shown to give valuable information about the passivation mechanisms. The measured data can be used to independently determine the fixed charge density under illumination (field-effect passivation) and the surface recombination velocity parameters for electrons and holes (chemical passivation) for a wide range of dielectric passivation layers. The results are in good agreement with both capacitance-voltage measurements and lifetime measurements after deposition of corona charges on the sample surface.
A second major contribution from this work is the development of a modified version of the widely used simulation tool PC1D which runs from a command line, thus allowing for scripted simulations. This modified version of the program has also been used as a basis for a new user interface, which allows for improved visualization, multivariable analysis, optimization, fitting to experimental data and implementation of additional models.
Through this work, several passivation processes have been developed and optimized, and excellent passivation quality has been demonstrated for a stack of a-Si:H and a-SiNx:H deposited by plasma enhanced chemical vapor deposition. Selected passivation layers have also been demonstrated as rear side passivation in a co-planar solar cell test structure with local rear contacts
PC1Dmod 6.2 – Improved Simulation of c-Si Devices with Updates on Device Physics and User Interface
No full textAbstractIn this paper we present a new update to PC1Dmod, which extends the original PC1D program by implementing Fermi-Dirac statistics and a range of state-of-the-art models in order to improve the accuracy of c-Si device simulation. In PC1Dmod 6.2 the list of models is further expanded to include a parameterization of incomplete ionization of dopants (Altermatt et al., J. Appl. Phys.100, 113715, 2006), with parameters for phosphorus, boron, arsenic, gallium and aluminum. The results have been verified against a previous implementation of the model. We show that the inclusion of incomplete ionization is of particular importance for moderately doped surface regions with doping densities in the range ∼5 × 1017 to ∼5×1019 cm-3, resulting in a deviation of up to 15% in the simulated recombination current density and sheet resistance. The effect of incomplete ionization is also shown to be more pronounced in devices made from compensated Si material. Furthermore, the default solar spectrum has also been updated in PC1Dmod 6.2, taking advantage of the increased maximum file size to include a more realistic representation. The generation profiles calculated using PC1Dmod 6.2 together with input from OPAL 2 agree well with results from Sentaurus TCAD for both planar and pyramidally textured surfaces, thus facilitating a more direct comparison between PC1Dmod 6.2 and other simulation programs utilizing standard spectra. The simulation comparison is also extended to the electrical performance of the modeled devices, showing good agreement in the calculated short-circuit current density for a range of planar and textured solar cells with varying emitter doping. Finally, several new output options, including emitter saturation current and injection-dependent lifetime curves have been added, enabling a simpler and more direct way for users to access and plot important device properties
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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