Results 1 - 10 of 518
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[en] In the design and operation of energy intensive systems, the possibility of improving the system's efficiency is very important to explore. The main way of improving efficiency is through thermodynamic analysis and optimization. Methods are universal and make it possible to estimate the fluxes and balances of all energy flows for every element of the system using a common criterion of efficiency. The thermoeconomical approach allows to retain all advantages of exergy method and simultaneously estimate the investment and other monetary costs of a system. In this paper is developed the method of thermoeconomical optimization of a heating system for cottage complex. Example of real optimization is given. (author)
[en] In this study an energy and exergy analysis of a Ceiling-type residential air conditioning (CTRAC) system operating under different climatic conditions have been investigated for provinces within the different geographic regions of Turkey. Primarily, the hourly cooling load capacities of a sample building (Q_e_v_a_p) during the months of April, May, June, July, August and September were determined. The hourly total heat gain of the sample building was determined using the Hourly analysis program (HAP). The Coefficient of performance (COP), exergy efficiency (η) and exergy destruction (Ex_d_e_s_t) values for the whole system and for each component were obtained. The results showed that lower atmospheric temperature (T_a_t_m) influenced the performance of the system and each of its components
[en] The ASST-A refrigeration system at the N15 site of the SSC Laboratory has a guaranteed capacity of 2000 W refrigeration and 20 g/s liquefaction, both at 4.5 K. Parametric tests were conducted for the compressors as well as for the total system. Two test cases were conducted for each of three modes of operation. This paper verifies the design of the ASST-A refrigeration system and compares it with the acceptance tests conducted for one of the test cases. Compressor mapping data and the results of process and exergy analyses are presented. The results show that the system behavior was stable over the wide range of operation. The exergy analysis provides a clear insight into the component and system losses and behavior
[en] The Pelletron-Linac accelerator of the University of Sao Paulo will use the existing electrostatic Pelletron accelerator as an injector for the linear superconducting accelerator (Linac), to increase the acceleration of the particles. The Linac uses a forced flow circulation helium system to promote continuous refrigeration for long periods of time, at temperatures below or equal to 4,9 K. This paper shows the exergetic analysis of the Pelletron-linac refrigerator, identifying the main sources of irreversibilities and evaluating energetic consumption of the system. An exergy-enthalpy diagram for the helium shows the thermodynamic processes that take place in the refrigeration plant and the exergy losses. (author)
[en] The heat pump system with economizer coupled with scroll compressor can operate steadily under the conditions of large temperature differences with high capacity and efficiency, and it has a great potential for application in cold regions. Based on the second law of thermodynamics, the heat pump system with economizer coupled with scroll compressor is exergetically analyzed, and the expressions related to exergy, exergy loss and exergy efficiency are derived. Experimental data for such a heat pump prototype have been obtained and used for the exergy analysis. It is found that compressor is a main bottleneck to improve the energy efficiency of heat pump followed by evaporator and condenser, and the improvement of refrigerating accessories such as filter-dryer and expansion valve is also effective to increase the efficiency of the heat pump. (author)
[en] This work presents a second law analysis efficiency of cogeneration steam turbine unit with backpressure steam turbine. By using the input/product output method is calculated the exergy efficiency of steam turbine unit at four different operational modes. It is obtained the allocation of useful exergy in electricity and released heat to consumers and exergy losses. It is evaluated the impact of referent environment temperature and mass flow from controlled extraction on exergy efficiency.
[en] In this paper, an endoreversible Carnot heat engine with irreversible heat transfer processes is analyzed based on generalized heat transfer law. The applicability of the entropy generation minimization, exergy analyses method, and entransy theory to the analyses is discussed. Three numerical cases are presented. It is shown that the results obtained from the entransy theory are different from those from the entropy generation minimization, which is equivalent to the exergy analyses method. For the first case in which the application preconditions of the entropy generation minimization and entransy loss maximization are satisfied, both smaller entropy generation rate and larger entransy loss rate lead to larger output power. For the second and third cases in which the preconditions are not satisfied, the entropy generation minimization does not lead to the maximum output power, while larger entransy loss rate still leads to larger output power in the third case. For the discussed cases, the concept of entransy dissipation is not applicable for the analyses of output power. The problems in the negative comments on the entransy theory are pointed out and discussed. The related researchers are advised to focus on some new specific application cases to show if the entransy theory is the same as some other theories. (paper)
[en] The purpose of this article is to assess the value of entransy for use in the thermal system engineering domain and in particular for design. The conclusion is that use of entransy is not recommended. This finding is in keeping with increasing uneasiness that has emerged recently in the technical literature about this concept. Throughout this article emphasis is on concise discussions of salient entransy aspects and the presentation is shaped to reach a broad technical audience. Accordingly, because secondary entransy aspects do not play a central role in reaching the above recommendation, they are considered only in passing or deferred. - Highlights: • A methodology for analysis, design, and optimization of thermal systems. • Entransy is not recommended for use in thermal system engineering. • Components of actual thermal systems do not operate ideally on any basis. • Entropy and exergy rest firmly on the second law of thermodynamics. • Entransy arises by analogy
[en] This paper, at first, presents a brief discussion of the concept of exergy. The second part studies the exchange of heat by conduction, convection and radiation as well as the irreversibilities due to the required temperature gradient. It shows the importance of the temperature level on the heat flux and the exergy lost. This analysis results also in conclusions on the fins and the thermal insulation. The third part studies the heat exchangers, in general. The loss of exergy due to the thermal exchange permits a comparison of the thermal value of these apparatus and, as well, shows the influence of the isothermal change of state of a fluid, i.e. in vaporization. Finally, based on the conclusions reached above, different types of heat exchangers used in industrial applications are analysed
[fr]Cet article presente d'abord une discussion breve du concept d'exergie. La seconde partie etudie l'echange de chaleur par conduction, convection et radiation ainsi que les irreversibilites dues au gradient de temperature impose. Il montre l'importance du niveau de temperature sur le flux de chaleur et la perte d'exergie. Cette analyse permet egalement de tirer des conclusions sur les ailettes et sur l'isolation thermique. La troisieme partie etudie les echangeurs de chaleur en general. La perte d'exergie due a l'echange thermique permet une comparaison de la valeur thermique de ces appareils et, egalement, montre l'influence du changement d'etat isotherme d'un fluide, comme en vaporisation. Finalement, sur la base des conclusions atteintes ci-dessus, les differents types d'echangeurs de chaleur utilises dans des applications industrielles sont analyses
[en] An R-152a ejector-jet pump refrigeration cycle and a LiBr-H_2O absorption refrigeration cycle have been integrated with a renewable energy power generator for making a proposed 'novel compact cogeneration cycle'. The exergy analysis of this proposed cycle leads to a possible performance improvement. Nearly 71.12% of the input exergy is destructed due to irreversibilities in the different components. The useful exergy output is around 7.12%. The exhaust exergy lost to the environment is 21.76%, which is lower than the exhaust energy lost 37.6% of the input energy, while the useful energy output is approximately 19.3%. The refrigerants used and the exhaust gas emissions samples are found to be favourable for reducing the global environmental related problems. The results also show that the coupling of the entrainment ratios of the ejector and jet pump has great effect on the exergy and energy efficiency.