Comparative study of various models in estimating hourly diffuse solar irradiance

This work presents a comparison among seventeen different proposals for estimating the hourly diffuse fraction of irradiance. Twelve of them are polynomial correlations of different orders, two are based on a logistic function and the three last ones consider the diffuse irradiance values in the previous and posterior hour to that of the calculation. In general, the proposals showing the more favourable statistics indexes are those that consider the process dynamics, as they behave better than the rest of the models even when the polynomial correlations and the logistic function are calibrated for the experimental data used in this work.The models Dirint and BRL are the ones recommended for the data of this study, as they exhibit the highest precision and generate a series of hourly diffuse irradiance values of which the distribution functions are very similar to those of the experimental data.     

Technical and economic considerations on induction motor oversizing

In industry, due to conservative system design, safety factors associated with uncertainty in the load requirements, discrete availability of commercial rated power, and/or load power variation, most three-phase squirrel-cage induction motors are oversized. Besides the extra capital investment, the oversizing of direct-on-line fixed-speed induction motors can lead to a significant efficiency and power factor reduction. However, the part-load efficiency of oversized motors can still be higher than the full-load efficiency of well-sized smaller motors because, in general, the nominal efficiency increases with the rated power. In this paper, an analysis of potential benefits and drawbacks of motor oversizing is carried out. On the basis of the catalogue technical data provided by one of the largest motor manufacturers for IE1-, IE2-, IE3-, and IE4-class four-pole induction motors, the main results of a simulation-based study on the oversizing energy efficiency and cost-effectiveness are presented. A method to estimate the motor efficiency and power factor for any load level using commercial catalogue data is proposed and applied. Some technical issues associated with motor oversizing are also briefly addressed. It is shown that, if the additional reactive energy consumption due to poorer power factor and the slight speed increase are ignored, for IE1-, IE2- and, to a much less extent, IE3-class motors, oversizing can be cost effective for many motor rated powers, resulting in a higher average efficiency and a lower motor lifecycle cost, as well as in an extended motor lifespan. For most IE3- and IE4-class motors, the oversizing is not cost effective because of the lower nominal efficiency gain when moving to a higher oversized rated power. Additionally, the oversizing impact on the motor energy consumption strongly depends on the load profile of the application. When an old motor fails, it will probably be an IE0- or IE1-class equivalent motor, and this situation provides a golden opportunity for replacing the old motor with a properly sized IE3- or IE4-class motor, which offers significantly higher efficiency for a wide range of loads.     

Glycerol steam reforming in a bench scale continuous flow heat recovery reactor

In the present study glycerol was successfully gasified using a diesel engine waste heat recovery system obtaining hydrogen and methane rich gaseous products. The reforming reactor was equipped with a vaporization pre-chamber to ensure uniform reactants distribution and a fixed reaction bed, being mounted in countercurrent flow configuration with the engine combustion gases stream. Accordingly, the reactions were conducted at gradually increased temperature conditions; starting at around 300 °C in the top section of the reaction bed and finishing in a controlled outlet bed temperature of 600–800 °C. When compared to homogeneous temperature reactors, the configuration used here produced a syngas of higher methane and ethylene contents. With regards to the reactor performance, syngas lower heat values of more than 22 MJ/kg were achieved with glycerol feed concentrations within 50–70% and outlet bed temperatures above 700 °C, corresponding to cold gas efficiencies of around 85%. The present results indicate that glycerin can be utilized as a syngas feedstock for steam reforming processes based on waste heat recovery.     

Hydrogen absorption in Pd thin-films

Hydrogen absorption isotherms for Pd thin films were modeled at atomistic scale by Monte Carlo (MC) simulation in the TPμN ensamble and by Molecular Dynamics (MD) simulations at 300 K. The interaction among atoms was modeled by embedded atom method (EAM) potentials. Simulated samples consisted of monocrystalline nanofilms with different thickness (2–8 nm) and two crystallographic surface orientations, (001) and (111). The isotherms were compared to bulk Pd and a few available experimental results. Instead of the plateau corresponding to the α-β PdH equilibrium in the bulk, the isotherms at nano-films show a two-plateaux behavior: a small one corresponding to a surface–subsurface hydride formation, and a larger one for the subsequent bulk hydride formation. This is strongly correlated with the atomic stress distribution induced within the thin film. The equilibrium pressures at the isotherms depend on the thin-film thickness, with pressure being larger for thicker films. The isotherms of the (001) films display lower equilibrium pressures than those for (111) films.     

Fuzzy Online Auto-Tuning for an Industrial PID Controller

This paper presents a fuzzy tuning system for real-time industrial PID （proportional-integral-derivative） controllers. The algorithm set the proportional gain, integral time and derivative time of a classical PID structure according to the set point, error and error derivative of the process, respectively. The tuning of the PID controller is based on a fuzzy inference machine. The set of rules of the fuzzy inference machine was obtained by experts engineering. The system is tested in an austempering process but can be applied in any industrial plant. Besides, an analysis between the response of the process with a PID controller and the system of fuzzy auto-tuning for P1D proposed was made.     

Intrinsic instability of flame–acoustic coupling

This paper shows that a flame can be an intrinsically unstable acoustic element. The finding is clarified in the framework of an acoustic network model, where the flame is described by an acoustic scattering matrix. The instability of the flame acoustic coupling is shown to become dominating in the limit of no acoustic reflections. This is in contrast to classical standing-wave thermoacoustic modes, which originate from the positive feedback loop between system acoustics and the flame. These findings imply that the effectiveness of passive thermoacoustic damping devices is limited by the intrinsic stability properties of the flame.     

Pt-promoted α-Al2O3-supported Ni catalysts: Effect of preparation conditions on oxi-reduction and catalytic properties for hydrogen production by steam reforming of methane

The effect of Pt addition on the oxi-reduction properties of α-Al₂O₃-supported Ni catalysts, with different degrees of interaction between NiO and the α-Al₂O₃ support, was studied using atmospheres of H₂, H₂/H₂O, and CH₄/H₂O. The effect of Pt promotion on the reduction of NiO with H₂ was significant for NiO species that interacted more strongly with the alumina surface, but was much lower when a NiAl₂O₄-like bulk phase was formed. For samples activated with H₂, although metal dispersion decreased with increasing Pt content, the activity was maintained constant by the presence of Pt sites. For samples activated with a CH₄/H₂O mixture, the activity increased with increasing Pt content, due to the higher reducibility of Ni in the Pt-promoted catalysts. The Pt promotion effect was stable; there was no important decrease in the influence of Pt on NiO reduction, even after high temperature re-oxidation of the catalysts.     

Ecological efficiency and thermoeconomic analysis of a cogeneration system at a hospital

This work aims with an approach for cogeneration plants evaluation based on thermoeconomic functional diagram analysis. The second law of thermodynamics is used to develop a methodology to analyse cogeneration systems, based on exergoeconomics evaluation. The thermoeconomic optimisation method developed is applied to allow a better configuration of the cogeneration plant associated to a university hospital. Also ecological efficiency is evaluated. The method was efficient and contributes for thermoeconomics modelling and analysis and can be applied to any sort of thermal system, especially those with combined heat and power in thermal parity.     

Polyetherimide-LaNi5 composite films for hydrogen storage applications

This study investigates the preparation of polyetherimide (PEI) – LaNi₅ composites films for hydrogen storage. Prior to the polymer addition, LaNi₅ was ball-milled at different conditions (250, 350, and 450 RPM) and annealed at 500 °C for 1 h under vacuum. The composites were produced with BM-LaNi₅-350 (PEI/LaNi₅-350) and annealed BM-LaNi₅-350 (PEI/LaNi₅-350-TT). Membranes were successfully produced through solvent casting assisted by an ultrasonic bath. The particles dispersion and the film morphology did not change after hydrogenation cycles. In the H₂ sorption experiments at 43 °C and 20 bar, the films stored H₂ without incubation time; both samples reached a capacity of ~0.6 wt%. The H₂ sorption kinetics of PEI/LaNi₅-350 was comparable to that of BM-LaNi₅-350, whereas PEI/LaNi₅-350-TT presented significantly slower kinetics. LaNi₅ oxidation was hindered by PEI, showing that it can be explored to improve metal hydrides air resistance. The results demonstrated that PEI films filled with LaNi₅ are promising materials for hydrogen storage.