Aspectos clave en la metodología del análisis exergéticoAplicación a procesos de desalación y a ciclos de refrigeración por absorción

Resumen

This work presents an in-depth study of some conceptual aspects of exergy analysis that appear confusing in the literature, which makes their practical application and comparisons between different authors difficult. On the one hand, some theoretical approaches are performed in which an attempt is made to answer the questions: a) How to conveniently define the dead state of a confined system? In exergy analysis, the system is said to be in the dead state when the ambient conditions, normally considered as T0 = 20-25 °C and p0= 1 atm, are reached. However, in certain practical situations, such as a confined system, where mechanical equilibrium with the environment will never be reached, taking the ambient pressure as the dead state pressure may not be the best option. Other possible solutions are analyzed here. b) Does a negative value of the physical exergy make sense? In theory, a negative exergy value does not make sense, but it is not uncommon to find negative physical exergy values in the literature, especially when the system pressure is initially lower than the ambient pressure. Even so, this numerical result seems to contradict what "common sense" defines as exergy: the available energy. This work analyzes this situation and provides alternative explanations. c) Is it correct to keep on presenting exergy only as "maximum theoretical useful work output..."? In Gibbs' original works, the expression "available energy" appears. Subsequently, the definition has been specified as "maximum theoretical useful work output ...". But the reasoning presented here allows us to affirm that the exergy not only represents the "maximum theoretical useful work output..." but could also be represented by the "maximum theoretical useful heat output...", more in agreement with the original definition of "available energy". d) What is the most correct way to state the exergy efficiency of a process? All researchers agree on the definition of the energy efficiency of a process. However, in the definition of exergy efficiency this is not the case. If it were similar to energy efficiency, exergy efficiency could be defined as "the exergy output divided by the exergy resources". However, "input-output" formulation (exergy output divided by exergy input), has gained acceptance. This work discusses the implications and suitability of both formulations for specific industrial processes. On the other hand, this Thesis deals with two practical case studies. In the first one, a complete exergy analysis of a seawater reverse osmosis desalination plant in operation is carried out. The structure of the plant is somewhat complex, due to the successive modifications it has undergone over the years. However, this allows interesting comparisons to be made between the exergy efficiencies of similar devices. Guidance can also be given to plant operators on the devices on which short-term actions should be undertaken to improve their efficiency. The definition of exergy efficiency in reverse osmosis membranes is also discussed, taking into account their purpose, namely, the transformation of mechanical energy into chemical energy. And an extensive and comprehensive comparison with the literature is made. The second case study is related to the exergy analysis of an absorption refrigeration system, using the H2O/LiBr mixture. The system has a single stage and is water-cooled, but the methodology would be equally applicable to multiple stage systems with different heating and refrigeration operation options. Selection of a dead state for confined systems, which need neither to reach mechanical nor chemical equilibrium with the environment, and where, even so, chemical exergy plays an important role, is considered. The study is completed with an in-depth literature comparison. The results obtained confirm that there are some key aspects in the exergy analysis on which more care and attention should be paid when applying this methodology on specific industrial processes and systems: the correct choice of the dead state, the equations used for the calculation of the thermodynamic properties of the substances, including physical and chemical exergy, and the definition of exergy efficiency, so that the studies carried out by different authors can be conveniently and thoroughly compared.