Table of Contents
Regular Original Research Article
| Entropy Generation Minimization in a Ram-Air Cross-Flow Heat Exchanger |
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Asad Alebrahim, Adrian Bejan |
145-157 |
This paper presents the constrained thermodynamic optimization of a crossflow heat exchanger with ram air on the cold side. The ram-air stream passes through a diffuser before entering the heat exchanger, and exits through a nozzle. This configuration is used in the environmental control systems of aircraft. In the first part of the study the heat exchanger is optimized alone, subject to fixed total volume and volume fraction occupied by solid walls. Optimized geometric features such as the ratio of channel spacings and flow lengths are reported. It is found that the optimized features are relatively insensitive to changes in other physical parameters of the installation. In the second part of the study the entropy generation rate also accounts for the irreversibility due to discharging the ram-air stream into the atmosphere. The optimized geometric features are relatively insensitive to this additional effect, emphasizing the robustness of the thermodynamic optimum. |
| Control System of Substance and Energy Balances of Combined Heat-and-Power Plants Applying the Least Squares Adjustment Method |
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Andrzej Ziebik, Marcin Szega, Henryk Rusinowsk |
159-164 |
The set of substance and energy balance equations of water and steam collectors, together with the balance of boilers and turbines (including the regeneration system) is the base of the control system of the exploitation of a combined heat-and power plant. The initial values of this system are the results of measurements in the course of exploitation. Due to inevitable errors in the measurements the substance balances display discrepancies between the balance of steam and the balance of the feed water, and the calculated technical indices are uncertain. In order to increase the reliability of the results of the technical analysis of exploitation and to co-ordinate the balance equations we may use the least squares adjustment method. It may be applied on the condition that we have a sufficient surplus of measuring data. The number of balance equations must be higher than the number of unknown values. This makes it possible to apply the criterion of least squares warranting a maximum of the reliability function in an n dimensional. space of errors (n - number of measuring data). This method has been applied in the balance systems of the combined heat-and-power generating plant. |
| Opportunities for High-Efficiency Electricity Generation Inclusive of CO2 Capture |
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Giampaolo Manfrida |
165-175 |
Three basic options for advanced power plants, allowing energy conversion inclusive of CO2 capture, are discussed: the semi-closed gas turbine cycle with atmospheric base pressure, the integrated gassifier/combined cycle with pressurised absorption of CO2, and the supercritical semi-closed CO2/H2O cycle with liquid CO2 capture. The merits of the different options are discussed and compared, and improvements to the basic layouts are proposed. The results show that all three solutions have a good potential for application, depending on the size of the plant and on the near or medium-term future perspective. |
| An Approach for the Time-Dependent Thermoeconomic Modeling and Optimization of Energy System Synthesis, Design and Operation Part II: Reliability and Availability |
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Benoît Olsommer, Daniel Favrat, Michael R. von Spakovsky |
177-186 |
Details of the reliability and availability aspects of the thermoeconomic methodology presented in Part I (Olsommer et al., 1999) are given here in Part II of our series of two articles. These system details cannot be forgotten, particularly if the superconfiguration envisioned offers the choice of several technologies for the same function (redundancy) or if there are guarantees of availability which must be met. An original method, which is an extension of the event space method, is presented here for automating the reliability and/or availability calculations of a repairable system chosen from a flexible structure of equipment in active and/or passive redundancy, connected in series and/or parallel. This methodology permits a more realistic evaluation of performance, flows and costs at both the structural (synthesis-design) and operational levels and leads, thus, to more rational decisions on system synthesis, design and operation. Results for the application of this methodology to a waste incineration cogeneration facility with a gas turbine topping cycle are given in Part I (Olsommer et al., 1999). |
| The Influence of Several Major Irreversibilities on the Performance Characteristics of an n-Stage Combined Heat Pump System |
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Jincan Chen, Bihong Lin, Chih Wu |
187-195 |
A universal cycle model of an n-stage combined heat pump system, which includes the irreversibility of finite-rate heat transfer across finite temperature differences, the heat leak loss between the external heat reservoirs, and the irreversibilities inside the working fluid, is established and used to investigate the influence of these irreversibilities on the performance of the system. The coefficient of performance is taken as an objective function for optimization. A general optimum relation is derived for a given specific heating load and a given total heat-transfer area of heat exchangers. The general performance characteristic curves of the system are obtained. The maximum coefficient of performance with non-zero specific heating load is determined. Some key design variables, such as the specific heating load, the temperature ratios of the working fluid in the isothermal processes, the distribution of the heat-transfer areas of heat exchangers, and the total power input of the system, are optimized. The optimal performance of an arbitrary-stage irreversible, endoreversible, and reversible combined heat pump system can be directly derived from the results in this paper. |
ISSN: 2146-1511
