1,720,973 research outputs found
Optimal focal length of primary mirrors in Fresnel linear collectors
No full textIn a linear Fresnel plant adopting slightly curved primary mirrors (cylindrical or parabolic), a significant gain in the collected radiation can be achieved using primary mirrors with different focal lengths, dependent on the position of the mirror with respect to the receiver. This work introduces a universal function that provides the optimal focal length of a mirror, given only the mirror's position relative to the receiver and the latitude, for a NS-oriented collector with a flat horizontal effective target. In a solar plant with the focal lengths defined by this function, the efficiency gain with respect to a solar field adopting identical mirrors is estimated in the range 1.5–6%, with the gain increasing if mirror imperfections or tracking errors are present: this means that the regulation of the focal lengths is especially useful in containing the loss of efficiency due to defects. The given rule is tolerant to errors in the focal length regulation (up to 10%). The function can be used as a reference for future projects, or as a starting point for more refined optimizations. © 2017 Elsevier Lt
Optimization of the geometry of Fresnel linear collectors
No full textMethods and results concerning the optical optimization of a linear Fresnel collector are presented. The variables considered in the optimization are the positions, widths and focal lengths of the mirrors; the mirrors can be of variable size and focal length, and they can be nonuniformly spaced. The target function to be optimized is the plant cost divided by the collected solar radiation in a year. The computation of the collected radiation and of its average on the year, and the optimization of the cost/radiation function are carried out via suitable mathematical methods and the choice of a plausible cost function. Four different levels of optimization (uniformly spaced identical mirrors; nonuniformly spaced identical mirrors; mirrors of the same width with uniform spacing and variable focal lengths; and finally a full optimization) are presented, with a discussion of the resulting gain on the target function (i.e. the reduction of the ratio between the plant cost and the collected radiation). The results show that the application of suitable optimization strategies can lead to an estimated gain around 12% with respect to the initial configuration (all mirrors identical and adjacent), and that a full optimization leads to a gain of 4.5% over a simple uniform optimization. This gain is due in large part to the possibility of regulating the focal lengths (the optimization of focals leads to a 2.8% gain over the uniform case), while only a minor improvement (less than 0.4%) is obtained with nonuniformly spaced identical mirrors. © 2016 Elsevier Ltd
Solar balloons as mixed solar-wind power systems
No full textSolar balloons, in which solar radiation is used to heat directly the air in a hot-air flying balloon, can be used as a mixed power system that exploits both solar radiation and high-altitude winds. If the balloon is heated at ground level and then released, the upward drift due to solar energy and the lateral drift due to wind can both be used to generate energy. At high altitude, the balloon is completely deflated (except for a small Helium-filled chamber, which prevents the balloon from falling) and pulled to the ground: the aerodynamical drag is now strongly reduced with respect to the ascent. A production cycle is simulated for balloons of different sizes and in different working conditions, assessing the feasibility of such a system, calculating the energy output and the solar/wind proportion of the produced energy. Simulation results show that such a system can produce energy with non-negligible solar-to-electricity efficiency (up to 5%). Moreover, while this is primarily a solar-based system, wind can yield a significant amount of additional energy for wind velocities greater than 15. m/s. © 2012 Elsevier Ltd
Geometrical Aspects of the Optics of Linear Fresnel Concentrators: A Review
Full text linkLinear Fresnel concentrators (LFR) are widely seen by the scientific community as one of the most promising systems for the production of solar energy via thermal plants or concentrated photovoltaics. The produced energy depends on the optical efficiency of the LFR, which is mainly dictated by the geometry of the plant. For this reason, the analysis of LFR geometry and its effects on optical behavior is a crucial step in the design and optimization of a Fresnel plant. The theoretical and computational tools used to model the optics of a LFR are fundamental in research on energy production. In this review, geometrical aspects of the optics of linear Fresnel concentrators are presented, with a detailed discussion of the parameters required to define the geometry of a plant and of the main optical concepts. After an overview of the literature on the subject, the main part of the review is dedicated to summarising useful formulas and outlining general procedures for optical simulations. These include (i) a ray-tracing procedure to simulate a mirror field, and (ii) a fast quasi-analytical method useful for optimizations and on-the-fly computations
Do we really need a seasonal energy storage? Results for photovoltaic technology in an unfavourable scenario
Full text linkEnergy storage systems play a crucial role in the transition to renewable energy. Short-term storage (STS), e.g., batteries, has a capacity of a few hours, meant to compensate the energy deficit due to day-night cycle or short-term fluctuations. Long-term storage (LTS), e.g., renewable fuels, can compensate seasonal variations. The importance of STS is undisputed; the need for LTS is much more debated. Here we compare two photovoltaic systems, one (A) endowed only with STS, and another (B) equipped also with unlimited LTS, in a scenario unfavourable to (A) because of high seasonal variability of irradiation and high heating load in winter. We show that (A) requires only a moderate oversize of the peak power (about 20%) w.r.t. (B) when both systems are sized to supply 85% of the whole electrifiable load, which includes domestic heating and transport. Therefore, the current lack of clear routes towards grid-scale LTS should not be considered as a reason to delay the transition to renewables
Fast computation of yearly averages of useful quantities for solar engineering
No full textIn solar engineering, many simulations require the computation of averages over a year of quantities such as the efficiency of solar plants. In the case of stationary quantities, i.e., that do not depend on the past history but only on the present conditions, time averages can be replaced by averages over the solar position, which are much faster to compute. Through a suitable choice of coordinates for the Sun position, namely, the hour angle and a declination-equivalent coordinate, the problem can be rewritten as a comparatively simple expression involving the sum of two double integrals; the solution can then be obtained numerically via suitable fast quadrature methods. The average over a year can be computed by transforming the dependence on irradiation, cloudiness, temperature or other instantaneous parameters into two dependences on Sun position – one for winter-spring (ascending declination) and one for summer-winter (descending declination). The proposed method is substantially faster than usual time integration over a year: the speedup factor ranges from 18 to almost 700 in the considered examples. It is especially well-suited to computations that require many yearly averages and could even be unfeasible otherwise. Examples of such applications include multi-parameter optimisations of the configuration of a power plant with the goal of maximizing the overall year production
Almost conformal vacua and confinement
No full textDynamics of confining vacua which appear as deformed superconformal theory with a non-Abelian gauge symmetry, is studied by taking a concrete example of the sextet vacua of N = 2, SU(3) gauge theory with n(f) = 4, with equal quark masses. We show that the low-energy "matter" degrees of freedom of this theory consist of four magnetic monopole doublets of the low-energy effective SU(2) gauge group, one dyon doublet, and one electric doublet. We find a mechanism of cancellation of the beta function, which naturally but nontrivially generalizes that of Argyres-Douglas. Study of our SCFT theory as a limit of six colliding N = I vacua, suggests that the confinement in the present theory occurs in an essentially different manner from those vacua with dynamical abelianization, and involves strongly interacting non-Abelian magnetic monopoles. (C) 2003 Published by Elsevier Science B.V
Ranking nodes in directed networks via continuous-time quantum walks
Full text linkFour new centrality measures for directed networks based on unitary, continuous-time quantum walks (CTQW) in n dimensions—where n is the number of nodes—are presented, tested and discussed. The main idea behind these methods consists in re-casting the classical HITS and PageRank algorithms as eigenvector problems for symmetric matrices, and using these symmetric matrices as Hamiltonians for CTQWs, in order to obtain a unitary evolution operator. The choice of the initial state is also crucial. Two options were tested: a vector with uniform occupation and a vector weighted w.r.t. in- or out-degrees (for authority and hub centrality, respectively). Two methods are based on a HITS-derived Hamiltonian, and two use a PageRank-derived Hamiltonian. Centrality scores for the nodes are defined as the average occupation values. All the methods have been tested on a set of small, simple graphs in order to spot possible evident drawbacks, and then on a larger number of artificially generated larger-sized graphs, in order to draw a comparison with classical HITS and PageRank. Numerical results show that, despite some pathologies found in three of the methods when analyzing small graphs, all the methods are effective in finding the first and top ten nodes in larger graphs. We comment on the results and offer some insight into the good accordance between classical and quantum approaches
Thermal characterization of a cavity receiver for hydrogen production by thermochemical cycles operating at moderate temperatures
No full textThe manganese-ferrite thermochemical cycle developed by ENEA for hydrogen production, whose maximum temperature level lays in the range 750-800. °C, has a high potential for coupling with the solar source using conventional structural materials. As a first step for the on sun feasibility validation of the cycle, an experimental survey of the thermal performance of a receiver-reactor designed by ENEA, to be powered by a solar furnace (1. kW), has been carried out in the absence of a reaction. The temperature distribution over the reactor chamber as a function of solar irradiation has been measured and the thermal inertia of the system has been evaluated. The experimental results confirm that the reactor temperature and inertia are compatible with the manganese-ferrite cycle and other cycles operating at moderate temperatures. In order to set the basis for the evaluation of this and other similar prototypes, a finite element model (FEM) has been developed to describe the thermofluidodynamic behavior of the reactor. Good agreement between calculated and experimental data has been obtained; therefore this model will be improved and extended to describe both the hydrogen and oxygen releasing reactions of the manganese-ferrite cycle, with the aim of optimizing the reactor design. © 2013 Elsevier Ltd
Application of a fixed-receiver Linear Fresnel Reflector in concentrating photovoltaics
No full textAmong the numerous proposals for concentrator design in concentrating photovoltaics, fixed-receiver linear Fresnel reflectors (with a fixed receiver and multiple mirror strips at ground level, rotating on multiple parallel axes) seem to have been quite neglected so far in the literature. In this work we present and discuss properties and possibilities of a fixed-receiver LFR for concentrating photovoltaics, adopting flat mirrors as primary reflectors, integrated with a simple cooling system. Thanks to their technical characteristics, fixed-receiver LFRs are especially well suited for the realization of medium/large scale CPV power plants, in view of the foreseeable use of PV as one of the main electricity sources. The annual performance in a desertic site - inclusive of the energy used for cooling - is computed from a thermal simulation of the system, and it is compared to the performance of a non-concentrating PV system. We show that the use of Fresnel concentrators is currently competitive with non-concentrating systems for medium-to-large plants in suitable locations, and could be an advantageous alternative in the case of a rise in price of PV panels, or as the effect of a price drop in mirrors and Fresnel components due to scale economy. Moreover, CPV Fresnel concentrators do not suffer from drawbacks usually associated with thermal concentrating technologies, such as high working temperatures, the need for large structures and for an external power block, and management complexity. The proposed technology is easily achievable and it is not much more complex to manage than a non-concentrating PV system
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