Today, the study of thermal systems that take advantage of residual thermal sources in the power generation sector is of great importance to mitigate environmental impact and promote sustainable alternatives in this sector. Among these alternatives, the organic Rankine cycle (ORC) is of great relevance since it allows taking advantage of residual energy sources at low temperatures. This work presents a methodology to evaluate the feasibility of using a refrigerant as a working fluid in an organic Rankine cycle based on an exergetic viability index. As a case study, R134a, R600a, R245fa, and R123 refrigerants were considered. A residual thermal source was used that came from the Hybrid Cycle Plant of the Valley of Mexico. Thermodynamic analysis was performed to determine generated power, thermal efficiency, refrigerant mass flow, pinch point temperature difference, specific steam consumption, unused thermal exergy flow, exergy efficiency, and total heat transfer requirement. The weighted average of the differences between these indicators, the global warming index, and the ozone depletion potential relative to the most favorable indicator corresponded to the definition of the exergetic viability index of the refrigerant. The results indicate that the ORC operating at condensing temperatures of 25, 35, and 45 °C with R245fa shows the highest rate of exergetic viability despite not generating the greatest amount of power and being one of the refrigerants with the highest total heat transfer requirement. Finally, at condensing temperatures above 45 °C, it is observed that R600a is exergetically the most viable refrigerant used in the ORC.
The technology of gas-steam turbine combined cycles is once of the most viable options for the generation of electric power with lower costs of production per kilowatt-hour and environmental, considering that the type of fuel should be gas natural. In the Mexican Republic, the projections of power generation to 2018 through the use of this technology, it establishes the possibility to install new power generation plants with pressures of 100 to 160 bar, and temperatures of 538 to 580°C. Another option it’s the repowering of the thermoelectric plants and cogeneration gas-steam turbines projects. Therefore, in this paper present a thermodynamic analysis of the configurations of combined cycles of one, two and three pressure levels with the objective in order to establish a parametric analysis of the power generation capacity, as well the thermal efficiency and Heat Rate based on the characteristics of the gas and steam turbines, likewise the quality of the last expansion stage has considered as a restriction to delimit the pressure of the steam domes of each level.
ResumenEn este trabajo se presenta el análisis energético y exergético de las centrales de generación de potencia que utilizan las configuraciones de ciclos de vapor ultracríticos, supercríticos, subcríticos regenerativos y los sistemas geotérmicos con dos y tres cámaras flash. Este análisis se realiza principalmente a las turbinas de vapor para analizar la transformación y la degradación de la energía en cada una de las etapas de expansión en función del grado de recalentamiento y de las irreversibilidades generadas. En las etapas de expansión de presión intermedia que se realizan después del primer y segundo recalentamiento se tienen las eficiencias exergéticas más altas. Sin embargo, para el caso del ciclo supercrítico, la máxima eficiencia se tiene en la segunda etapa de expansión de la sección de alta presión. Las menores eficiencias se tienen en los sistemas geotérmicos debido a su condición de baja entalpía, aunque la disponibilidad de la energía se aprovecha en 42%. Palabras AbstractThis paper presents the thermodynamics analysis of the power generation plants that use ultracritical, supercritical, subcritical regenerative steam cycle configurations and geothermal systems with two and three flash steam. This analysis is mainly carried out on the steam turbines to analyze the energy transformation and degradation in each of the expansion stages as function of the degree of overheating and the generated irreversibilities. The greater exergetic efficiencies are obtained in the stages of expansion of intermediate pressure that are done after the first and second reheat. However, for the case of the supercritical cycle the maximum efficiency occurs in the second stage of expansion of the high-pressure section. Geothermal systems have the lowest efficiencies due to their low enthalpy condition, although the availability of energy is used in 42%.
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