Exergy dynamics of systems in thermal or concentration non-equilibrium

The paper addresses the problem of the existence and quantification of the exergy of non-equilibrium systems. Assuming that both energy and exergy are a priori concepts, the Gibbs "available energy" A is calculated for arbitrary temperature or concentration distributions across the body, with an accuracy that depends only on the information one has of the initial distribution. It is shown that A exponentially relaxes to its equilibrium value, and it is then demonstrated that its value is different from that of the non-equilibrium exergy, the difference depending on the imposed boundary conditions on the system and thus the two quantities are shown to be incommensurable. It is finally argued that all iso-energetic non-equilibrium states can be ranked in terms of their non-equilibrium exergy content, and that each point of the Gibbs plane corresponds therefore to a set of possible initial distributions, each one with its own exergy-decay history. The non-equilibrium exergy is always larger than its equilibrium counterpart and constitutes the "real" total exergy content of the system, i.e., the real maximum work extractable from the initial system. A systematic application of this paradigm may be beneficial for meaningful future applications in the fields of engineering and natural science

Neural networks for small scale ORC optimization

This study concerns a thermodynamic and technical optimization of a small scale Organic Rankine Cycle system for waste heat recovery applications. An Artificial Neural Network (ANN) has been used to develop a thermodynamic model to be used for the maximization of the production of power while keeping the size of the heat exchangers and hence the cost of the plant at its minimum. R1234yf has been selected as the working fluid. The results show that the use of ANN is promising in solving complex nonlinear optimization problems that arise in the field of thermodynamics

Automatic diagnostics and prognostics of energy conversion processes via knowledge-based systems

This paper presents a critical and analytical description of an ongoing research program aimed at the implementation of an expert system capable of monitoring, through an Intelligent Health Control procedure, the instantaneous performance of a cogeneration plant. An application has been tested on a real plant, located on the grounds of the ENEA-Casaccia Energy Laboratories. The expert system, denominated PROMISE as the Italian acronym for PROgnostic and Intelligent Monitoring Expert System, generates, in real time and in a form directly useful to the plant manager, information on the existence and severity of faults, forecasts on the future time history of both detected and likely faults, and suggestions on how to control the problem. The expert procedure, working where and if necessary with the support of a process simulator, derives from real-time data a list of selected performance indicators for each plant component. For a set of faults, pre-defined with the help of the plant operator, proper rules are defined in order to establish whether the component is working correctly; in several instances, since one single failure (symptom) can originate from more than one fault (cause), complex sets of rules expressing the combination of multiple indices have been introduced in the knowledge base as well. Creeping faults are detected by analyzing the trend of the variation of an indicator in a pre-assigned interval of time. Whenever the value of this "discrete time derivative" becomes "high" with respect to a specified limit value, a "latent creeping fault" condition is prognosed. The expert system architecture is based on an object-oriented paradigm. The knowledge base (facts and rules) is clustered: the chunks of knowledge pertain to individual components. A graphic user interface (GUI) allows the user to interrogate PROMISE about its rules, procedures, classes and objects, and about its inference path. The paper also presents the results of some tests conducted on the real plant. © 2004 Elsevier Ltd. All rights reserved

The LETHE (c) (Low Emissions Turbo-Hybrid Engine) city car of the University of Roma 1: Final proposed configuration

A longstanding interest of the Authors' research group at University of Roma Sapienza (UDR1) is the design, development and fielding of a road prototype of a new concept of Hybrid Series vehicle, endowed with a small Gas Turbine set as a thermal engine. This solution offers several advantages with respect to traditional internal combustion engines and even to the existing generation of Hybrid propulsive systems: a reduced engine weight and size, lower emissions, substantially extended range, ease of maintenance, and more efficient braking energy recovery. In the LETHE (c) (Low Emissions Turbo-Hybrid Engine) the GT (gas turbine set) does not directly provide traction, but serves solely as a battery pack recharger. The vehicle is, in all respects, equivalent to a purely electric vehicle, except for the presence of an on-board recharger. Much care was placed in the design phase in the quest for an "optimal" design: first of all, an original method for identifying the most convenient degree of hybridization (ratio of the installed power of the battery pack to that of the GT) was defined and formalized, so that the resulting power balance between the two units satisfies the main design specifications and guarantees a practically acceptable operational life of the battery package while enabling the vehicle to complete a typical city mission (about 25-50 km) in a purely electric mode and without recharge. This paper presents a review of the previous conceptual and design results and describes in detail a possible road prototype configuration (weights, packaging of all units within the body of the vehicle, logic control unit, GT- and electric motor size and power, battery package characteristics). Some discussion is also devoted to the foreseeable impact of the deployment of a LETHE (c) fleet on the mid-range scenario of the Italian urban transportation system. (c) 2013 Elsevier Ltd. All rights reserved

Application of the EGM Method to a LED-Based Spotlight: A Constrained Pseudo-Optimization Design Process Based on the Analysis of the Local Entropy Generation Maps

In this paper, the entropy generation minimization (EGM) method is applied to an industrial heat transfer problem: the forced convective cooling of a LED-based spotlight. The design specification calls for eighteen diodes arranged on a circular copper plate of 35 mm diameter. Every diode dissipates 3 W and the maximum allowedtemperature of the plate is 80 °C. The cooling relies on the forced convection driven by a jet of air impinging on the plate. An initial complex geometry of plate fins is presented and analyzed with a commercial CFD code that computes the entropy generation rate. A pseudo-optimization process is carried out via a successive series of design modifications based on a careful analysis of the entropy generation maps. One of the advantages of the EGM method is that the rationale behind each step of the design process can be justified on a physical basis. It is found that the best performance is attained when the fins are periodically spaced in the radial direction

The concept of the gas turbine-based hybrid vehicle: System, design and configuration issues

The object of this study is a theoretical and experimental analysis of a new hybrid propulsion system for a passenger sedan in which the thermal engine is a small gas turbine set. Some preliminary results on the possibility of replacing the conventional ICE of a hybrid 'series' configuration by a turbogas were presented and discussed in previous papers by the same authors: several possible designs were examined under both a thermodynamic and an operative point of view. This paper presents a summary of the project and constitutes an attempt to put things in a proper engineering perspective: the technical feasibility of the project is assessed via a calculation of the required mission loads, a preliminary design of the most important elements of the propulsive system, the choice of the mission control strategy and the implementation of a numerical system simulator. The experiments that provided a verification for the assumed component efficiencies were carried out, in cooperation with the Research Centre of ENEA-Casaccia, on an ELLIOTT TA-45 group. Our results, though only preliminary, allow for a direct comparison between a GT-hybrid vehicle and a modern diesel car, and indicate that the GT-hybrid may be actually a competitor for the FC-powered vehicle concept. Our 'optimal' configuration is a combination of a 100 kg battery pack and two turbogas set of 5 and 16 kW, respectively. Copyright © 2005 John Wiley & Sons, Ltd

A Note on the "Optimal" Design of Disc-Shaped Heat Exchangers

The continuous quest for improving the performance of heat exchangers, together with evermore stringent volume and weight constraints, especially in enclosed applications (engines, electronic devices), stimulates the search for compact, high-performance units. One of the shapes that emerged from a vast body of research is the disc-shaped heat exchanger, in which the fluid to be heated/cooled flows through radial -often bifurcated- channels inside of a metallic disc. The disc in turn exchanges heat with the heat/cold source (the environment or another body). Several studies have been devoted to the identification of an "optimal shape" of the channels: most of them are based on prime principles, though numerical simulations abound as well. The present paper demonstrates that -for all engineering purposes- there is only one correct design procedure for such a heat exchanger, and that this procedure depends solely on the technical specifications (exchanged thermal power, materials, surface quality): the design in fact reduces to a zero-degree of freedom problem! The argument is described in detail, and it is shown that a proper application of the constraints completely identifies the shape, size and similarity indices of both the disc and the internal channels. Goal of this study is not that of "inventing" a novel heat exchanger design procedure, but that of demonstrating that -in this as in many similar cases-a straight forward application of prime principles and of diligent engineering rules may generate "optimal" designs. Of course, the resulting configurations may be a posteriori tested as to their performance, their irreversibility rates, their compliance with one or the other "techno-economical optimization methods", but it is important to realize that they enjoy a sort of "embedded" optimality

The influence of platform switching on the biomechanical aspects of the implant-abutment system. A three dimensional finite element study

Objective: To evaluate the biomechanical scenario of platform switching geometric implant-abutment configuration relative to standard configurations by means of finite element analysis.Study Design: A 3D Finite Element Analysis (FEA) was performed on 3 different implant-abutment configurations: a 3.8 mm implant with a matching diameter abutment (Standard Control Design, SCD), a 5.5 mm implant with matching diameter abutment (Wider Control Design, WCD), and a 5.5mm implant with a 3.8 mm abutment (Experimental Design, ED). All the different experimental groups were discretized to over 60000 elements and 100000 nodes, and 130N vertical (axial) and 90N horizontal loads were applied on the coronal portion of the abutment. Von Mises stresses were evaluated and maximum and minimum values were acquired for each implantabutment configuration. Results: The load-induced Von Mises stress (maximum to minumum ranges) on the implant ranged from 150 MPa to 58 Pa (SCD); 45 MPa to 55 Pa (WCD); 190 MPa to 64 Pa (ED). The Von Mises stress on the abutment ranged from 150 MPa to 52 MPa (SCD); 70 MPa to 55 MPa (WCD), and 85 MPa to 42 MPa respectively (ED). The maximum stresses transmitted from the implant-abutment system to the cortical and trabecular bone were 67 Pa and 52 MPa (SCD); 54 Pa and 27 MPa (WCD); 64 Pa and 42 MPa (ED), respectively. When the implant body was evaluated for stresses, a substantial decrease in their levels were observed at the threaded implant region due to the diametral mismatch between implant and abutment for the ED configuration. Conclusion: The platform switching configuration led to not only to a relative decrease in stress levels compared to narrow and wide standard configurations, but also to a notable stress field shift from bone towards the implant system, potentially resulting in lower crestal bone overloading. © Medicina Oral S. L

The influence of platform switching on the biomechanical aspects of the implant-abutment system. A three dimensional finite element study

Objective: To evaluate the biomechanical scenario of platform switching geometric implant-abutment configuration relative to standard configurations by means of finite element analysis.Study Design: A 3D Finite Element Analysis (FEA) was performed on 3 different implant-abutment configurations: a 3.8 mm implant with a matching diameter abutment (Standard Control Design, SCD), a 5.5 mm implant with matching diameter abutment (Wider Control Design, WCD), and a 5.5mm implant with a 3.8 mm abutment (Experimental Design, ED). All the different experimental groups were discretized to over 60000 elements and 100000 nodes, and 130N vertical (axial) and 90N horizontal loads were applied on the coronal portion of the abutment. Von Mises stresses were evaluated and maximum and minimum values were acquired for each implantabutment configuration. Results: The load-induced Von Mises stress (maximum to minumum ranges) on the implant ranged from 150 MPa to 58 Pa (SCD); 45 MPa to 55 Pa (WCD); 190 MPa to 64 Pa (ED). The Von Mises stress on the abutment ranged from 150 MPa to 52 MPa (SCD); 70 MPa to 55 MPa (WCD), and 85 MPa to 42 MPa respectively (ED). The maximum stresses transmitted from the implant-abutment system to the cortical and trabecular bone were 67 Pa and 52 MPa (SCD); 54 Pa and 27 MPa (WCD); 64 Pa and 42 MPa (ED), respectively. When the implant body was evaluated for stresses, a substantial decrease in their levels were observed at the threaded implant region due to the diametral mismatch between implant and abutment for the ED configuration. Conclusion: The platform switching configuration led to not only to a relative decrease in stress levels compared to narrow and wide standard configurations, but also to a notable stress field shift from bone towards the implant system, potentially resulting in lower crestal bone overloading. © Medicina Oral S. L

IMECE2008-68339 EXERGY ANALYSIS OF A SOLID OXIDE FUEL CELL-GAS TURBINE HYBRID POWER PLANT

ABSTRACT The paper presents the exergy analysis of a natural gas fuelled energy conversion process consisting of a hybrid solid oxide fuel cell coupled with a gas turbine. The fuel is partly processed in a reformer and then undergoes complete reforming in an internal reforming planar SOFC stack (IRSOFC). The syngas fuels in turn a standard gas turbine cycle that drives the fuel compressor and generates excess shaft power. Extensive heat recovery is enforced both in the Gas Turbine and between the topping SOFC and the bottoming GT. Two different configurations have been simulated and compared on an exergy basis: in the first one, the steam needed to support the external and the internal reforming reactions is completely supplied by an external Heat Recovery Steam Generator (HRSG), while in the second one that steam is mainly obtained by recirculating part of the steam-rich anode outlet stream. The thermodynamic model of the fuel cell system has been developed and implemented into the library of a modular object-oriented Process Simulator, Camel-Pro ® ; then, by means of this simulator, the exergetic performance of the two alternative configurations has been analyzed. A detailed analysis of the exergy destruction at component level is presented, to better assess the distribution of irreversibilities along the process and to gain useful design insight