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DYNA http://dyna.medellin.unal.edu.co/ Thermodynamic analysis of R134a in an Organic Rankine Cycle for power generation from low temperature sources Análisis termodinámico del R134a en un Ciclo Rankine Orgánico para la generación de energía a partir de fuentes de baja temperatura Abstract Fredy Vélez a, Farid Chejne b & Ana Quijano c a CARTIF Centro Tecnológico, España. frevel@cartif.es b Universidad Nacional de Colombia, sede Medellín, Colombia. fchejne@unal.edu.co c CARTIF Centro Tecnológico, España. anaqui@cartif.es Received: March 26th, 2013. Received in revised form: December 23th, 2013. Accepted: January 14th, 2014 This paper reports the main results of a thermodynamic study realized on the use of a low temperature heat source (150oC as maximum) for power generation through a subcritical Rankine power cycle with R134a as working fluid. The procedure for analyzing the behavior of the proposed cycle consisted of modifying the input pressure, temperature and/or discharge pressure of the turbine with working fluid at conditions of both saturation and overheating. Results show that the efficiency of the cycle for this fluid is a weak function of temperature, i.e., overheating the inlet fluid to the turbine does not cause a significant change in the efficiency. However, when the pressure ratio in the turbine increases, it is much more efficient, and also, as the input temperature to the turbine rises, the efficiency increases more sharply. Furthermore, the effect of adding an internal heat exchanger to the cycle was analyzed, giving as a result a maximum efficiency of 11% and 14% for the basic cycle and with an internal heat exchanger, respectively. Keywords: Energy efficiency; organic Rankine cycle; power generation; waste heat; renewable energy. Resumen Este trabajo presenta los principales resultados del estudio termodinámico realizado sobre el uso de una fuente de calor de baja temperatura (150oC como máximo) para la generación de energía a través de un ciclo Rankine subcrítico con R134a como fluido de trabajo. El procedimiento para analizar el comportamiento del ciclo propuesto consistió en modificar la presión y temperatura de entrada y/o descarga de la turbina, con el fluido de trabajo en condiciones tanto de saturación, como sobrecalentamiento. Como resultado, se puede indicar que la eficiencia del ciclo con este fluido es una débil función de la temperatura, es decir, sobrecalentar el fluido a la entrada de la turbina no causa un cambio significativo en la eficiencia. Sin embargo, cuando la relación de presión en la turbina aumenta, la eficiencia incrementa, y también, conforme la temperatura de entrada a la turbina aumenta, la eficiencia aumenta pronunciadamente. Además, se analizó el efecto de adicionar un intercambiador interno de calor que aumentó los valores de eficiencia obtenidos, dando como resultado, una eficiencia máxima del 11% y 14% para el ciclo básico y con el intercambiador interno de calor, respectivamente. Palabras clave: Eficiencia energética; ciclo Rankine Orgánico; generación de energía; calor residual; energías renovables. 1. Introduction The use of fossil fuels (e.g., oil and coal) as an energy source has many negative environmental impacts, such as the release of pollutants and resource depletion. A high consumption rate of fossil-fuels will result in an increase in environmental pollution during the next century, due to the emission of CO2 and other gases that cause global warming through what is known as the greenhouse effect [1]. In order to reduce CO2 emissions and oil dependency, each country in the world is responsible for improving the quality of its energy sources [1]. One of these improvements is the use of waste heat or low temperature sources (such as some renewables) [2], being the organic Rankine cycle “ORC” a promising technology for their conversion into power [2-5]. The ORC principle of operation is equal to the conventional Rankine cycle, with the difference of using an organic agent as working fluid. However, unlike the conventional Rankine, the change of fluid allows the energy recovery from low enthalpy sources for work or electricity production. Thus, one of the main research lines realized on this issue is the selection of a suitable working fluid due to its great influence in the design of the process [2-4], [6-10]. Depending on the application, the heat source and the temperature level, the fluid must have optimum thermodynamic properties at the lowest possible temperatures and pressures, and also satisfy several criteria such as being economical, nontoxic, nonflammable, environmentally-friendly, allowing a high utilization of the available energy from the heat source. If all these aspects © The authors; licensee Universidad Nacional de Colombia. DYNA 81 (185), pp. 153-159 June, 2014 Medellín. ISSN 0012-7353 Printed, ISSN 2346-2183 OnlinePDF Image | Thermodynamic analysis of R134a in an Organic Rankine Cycle for power generation from low temperature sources Analisis termodinamico del R134a en un Ciclo Rankine Organico para la generacionde energía a partir de fuentes de baja temperatura
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