1,720,986 research outputs found
Carnot factor in solar cell efficiencies
No full textWe study the dependence of solar cell efficiency on the illumination factor G, which governs the solid angle subtended by the radiation source at the cell. While the 2?-geometry (or, equivalently, maximum illumination factor G = 1) yields a Carnot factor, this disappears in more general cases when G lies between zero and one and the cell receives both solar and ambient radiation. The Carnot factor also applies to cells with particular angularly selective absorption properties and this can be made obvious even in the more general case, for which a new efficiency formula is found here. This is achieved by a simple algebraic transformation. In this way, the connection between the thermodynamic and statistical mechanical approach to solar cell efficiencies emerges very clearly. The illumination factor is subsequently defined for an arbitrary radiation source and the theory is applied in the case of moonlight
Solar cell thermodynamics including multiple impact ionization and concentration of radiation
No full textThe simultaneous effect of impact ionization and the concentration of radiation on solar cell performance is analysed. For maximum solar concentration and an infinite number of impact ionizations one obtains a maximum cell efficiency (0.845). This corresponds to an optimum reduced driving force qV/Eg = 0.882, given a ratio of Ts/Tp~300/6000 (q,V and Eg are electron electric charge, voltage and bandgap energy, respectively, while Ts and Tp are ambient and sun temperature, respectively). With full concentration the optimum reduced driving force equals the Carnot factor 1-Tc/Tp exactly (Tc is cell temperature). In the case of no illumination, there is a negative driving force. This is related to the coefficient of performance of a Carnot refrigeration engine. In the limit of an infinite number of impact ionizations, the Carnot factor occurs again, this time as an upper bound to the optimum reduced driving force
Thermodynamic optimisation of non-concentrating hybrid solar converters
No full textA model of a hybrid solar converter proposed by Goetzberger et al was used to optimize the overall efficiency of the combined system in two different situations. Firstly, the original semi-empirical models for the solar cell and the thermal engine were used. Secondly, more general theoretical models were proposed for each of the two components of the combined converter. The theoretical model of the solar cell produces results in good agreement with the simple empirical model when the ideal cell with no reflection losses is considered. Both the theoretical model and the empirical model of the thermal engine give similar results in the range of temperatures allowed for the proper usage of a solar cell. Reasonable low optimum temperatures could be reached by using materials with a band gap of less than 2 eV. Some ways of improving the hybrid solar converter model proposed by Goetzberger et al are discussed at the end of the paper
The geometrical factor of spherical radiation sources
No full textThe geometrical factor of a spherical isotropic source of radiation when viewed from a flat cosine
(Lambertian) receiver is , where Ω is the
solid angle subtended by the source, while is the source's zenith angle. Two different
derivations of this result are given in this paper. It holds also for diffuse radiation received
from a hemispherical dome. Geometrical factors can also be defined for non-isotropic sources of
radiation. Taking into account the limb darkening effect leads to solar temperatures of about
6100 K or 6000 K in a better approximation. These are higher than the value derived assuming the
Sun to emit radiation isotropically (5770 K)
Solar energy conversion: list of efficiencies and some theoretical considerations: theoretical considerations part II: results
No full textThe effects of acidification and liming on the mobilization of benzo(a)pyrene (BaP) were studied in a long-term column experiment with a sandy topsoil spiked with this hydrophobic compound at a concentration of 100 mg kg-1. Soil acidification to pH 3.3 initially caused a decreased release of BaP, but in the course of the 14-months study, BaP-concentrations in the leachates surpassed control levels 10-15-fold. This is attributed to DOM-enhanced solubility of BaP, although no correlation with DOC-concentrations was found. Instead, increased DOM-sorptivity is attributed to the increases in DOM-aromaticity and DOM-size which occurs in response to decreasing Ca-concentrations and increasing pH in the solutions. A short term BaP-mobilization in the lime treatment is not related to these parameters but may indicate the participation of heavy metals in DOM-BaP-interactions
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Statistical thermodynamics of bosons in one- and two-level quantum wells
No full textThis paper deals with the thermodynamics of bosons in a three-dimensional square potential of a single- or two-level isolated quantum well. On evaluating the main thermodynamic properties, we find certain expected features: Bose condensation and also a phase transition as a second level is introduced. Its strength depends on the number of particles in the well. The question of energy and entropy additivity and the behaviour of the chemical potential is also discussed
Variations on the Author
Full text link“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
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