Y. Haseli

Unified Approach to Exergy Efficiency, Environmental Impact and Sustainable Development for Standard Thermodynamic Cycles

The exergy efficiency of three standard thermodynamic cycles, i.e., Brayton, Rankine and Otto cycles, are developed and the corresponding analytical equations are derived accordingly. The resultant expressions are applied to typical operating conditions and numerical results are obtained, when the heat of each engine is supplied by burning natural gas as a fuel with 100 percent theoretical air. A common result is the significant effect of the maximum cycle temperature, which causes an increase of exergy efficiency. It is shown that the compression ratio of the Brayton and Otto cycles, as well as the turbine inlet pressure in a steam power plant, raise the exergy efficiency. Moreover, increasing the ambient temperature has a negative influence on the exergy efficiency in the Brayton and Otto cycles, which occurs due to ambient air fed to these systems, thereby decreasing the deviation of the system from ambient conditions and reducing the exergy efficiency. Further findings include an optimal performance point of the Brayton and Rankine cycle, with a high sustainability and exergy efficiency. For instance, at the optimal operating point of the Brayton cycle with a compression ratio of 8 (or 12 for a second case), the exergy efficiency is 73 (60) percent, CO2 emissions is 530 (590) g/kWh and the sustainability index is 3.8 (2.8). The optimal operating point for an example of a Rankine cycle is found to be 50 percent for the exergy efficiency, with 440g/kWh of emitted CO2 and a sustainability index of two.

Exergy Efficiency of Two-Phase Flow in a Shell and Tube Condenser

This study deals with a comprehensive efficiency investigation of a TEMA “E” shell and tube condenser through exergy efficiency as a potential parameter for performance assessment. Exergy analysis of condensation of pure vapor in a mixture of non-condensing gas in a TEMA “E” shell and tube condenser is presented. This analysis is used to evaluate both local exergy efficiency of the system (along the condensation path) and for the entire condenser, i.e., overall exergy efficiency. The numerical results for an industrial condenser with a steam–air mixture and cooling water as working fluids indicate significant effects of temperature differences between the cooling water and the environment on exergy efficiency. Typical predicted cooling water and condensation temperature profiles are illustrated and compared with the corresponding local exergy efficiency profiles, which reveal a direct (inverse) influence of the coolant (condensation) temperature on the exergy efficiency. Further results provide verification of the newly developed exergy efficiency correlation with a set of experimental data.

Formulation of Film Theory Equations for Modeling of Condensation of Steam-Air Mixtures in a Shell and Tube Condenser

Through development of the fundamental equations of Film Theory, condensation of steam in the presence of air in a horizontal counter-current shell and one-path tube condenser is modeled. The interaction between heat and mass transfer and hydrodynamics in the shell-side is taken into consideration. A comparison between the predictions of the model and a set of experimental data available in the archival literature indicates excellent accuracy of the new formulation. The accuracy of the method is further validated by generating profiles of the temperature and pressure drops of the gas flow through the baffles, at various air leakages. Additionally, the effects of air leakage and upstream cooling water temperature are investigated to determine how they influence the total condensation rate, shell-side gas temperature and pressure drops. The results show that the total condensation rate decreases 5% and 20.5% for an air leakage of 1% and 5%, respectively, compared to the situation of pure vapor. Also, increasing the inlet cooling water temperature from 46.5°C to 48.5°C leads to 16.2% reduction in the total condensation rate, i.e., 8.1% per °C. However, this ratio is higher at high temperatures. For example, as the cooling water temperature rises from 50°C to 51°C under identical process conditions, the total condensation rate decreases 11.7% (per °C).© 2008 ASME

Thermodynamic Performance of a Gas Turbine Plant Combined With a Solid Oxide Fuel Cell

This paper undertakes a thermodynamic analysis of a high-temperature solid oxide fuel cell, combined with a conventional recuperative gas turbine. In the analysis the balance equations for mass, energy and exergy for the system as a whole and its components are written, and both energy and exergy efficiencies are studied for comparison purposes. These results are also verified with data available in the literature for typical operating conditions, the predictive model of the system is validated. The energy efficiency of the integrated cycle is obtained to be as high as 60.55% at the optimum compression ratio. These model findings indicate the influence of different parameters on the performance of the cycle and irreversibilities therein, with respect to the exergy destruction rate and/or entropy generation rate. The results show that a higher ambient temperature would lead to lower energy and exergy efficiencies, and lower net specific power. Furthermore, the results indicate that increasing the turbine inlet temperature results in decreasing both the energy and exergy efficiencies of the cycle, whereas it improves the total specific power output. However, an increase in either the turbine inlet temperature or compression ratio leads to a higher rate of irreversibility within the plant. It is shown that the combustor and SOFC contribute predominantly to the total irreversibility of the system; about 60 percent of which takes place in these components at a typical operating condition, with 31.4% for the combustor and 27.9% for the SOFC.© 2008 ASME