AVOIDABLE AND UNAVOIDABLE EXERGY ANALYSIS FOR A CASCADE GEOTHERMAL PLANT PRODUCING ELECTRICITY, COOLING AND USEFUL HEAT Víctor M. Ambriz-Díaz1, Carlos Rubio-Maya1*, J. Jesús Pacheco-Ibarra, Hugo C. Gutiérrez Sánchez and Jorge Rangel Arista 1
Facultad de Ingeniería Mecánica, UMSNH. Grupo de Eficiencia Energética y Energías Renovables (GREENER) Morelia, Mich. México Abstract In this paper, an avoidable and unavoidable exergy analysis applied to a plant that uses geothermal energy in the form of a cascade to produce electricity, cold and useful heat is presented. It is assumed that the cascaded geothermal plant is thermally activated by geothermal energy of medium temperature and consists of three thermal levels with one energy transformation technology and one product per level. The first thermal level is coupled to an Organic Rankine Cycle (ORC) producing electricity, the second thermal level is dedicated to a Thermally Activated Refrigeration chiller (TAR) producing a cooling effect to preserve agricultural products at a low temperature (+ 5 ° C) in an storage chamber and to improve the performance of the ORC by keeping constant the condenser temperature. Finally, the third thermal level of the plant provides heat to dehydrator. For the purpose of this study, the plant has been subjected to an exergetic analysis making use of the Fuel-Product concept, obtaining the avoidable and unavoidable parts of the exergy destroyed at each component and on the overall plant. The plant has been modeled working under real, unavoidable and ideal operating parameters, in order to obtain the thermodynamic behavior under different efficiency criteria. The results show that the plant operating under real conditions has the greatest destruction of exergy (124.1 kW) and the lowest energy efficiency (26.97%) as well as the lower production of products (47.47 kW). The results also show that in unavoidable conditions the exergetic behavior of the plant increases, reaching an exergetic efficiency of 29.6%. Finally, the plant reaches its highest exergetic efficiency and production of products operating under ideal conditions having a value of 135.6 kW and 77.57%, respectively. The results obtained through the unavoidable and ideal conditions are very important to have guidelines for future technological improvements in the cascade geothermal plant. Likewise, the results obtained for the destruction of avoidable and unavoidable exergy indicate that the potential components to realize a technological improvement are the ORC and the main heat exchanger. Keywords: Avoidable and unavoidable, exergy analysis, cascade geothermal plant.
1. Introduction Geothermal energy is a type of renewable and abundant source of energy that can be exploited according to the its thermal characteristics [1]. For example, high temperature geothermal energy (T>150 °C) is mainly used for the production of electricity through single flash or double flash plants. Medium temperature geothermal energy (100 - 150 °C) is suitable for the production of electricity through ORC technology and, low temperature geothermal energy (T
