Exergy

This chapter introduces the foundations of the exergy, exergy production cost, and renewability analysis of energy conversion processes. Based on the concept of reversible work, the concept of exergy is derived and the exergy balance is presented as a combination of the energy and entropy balances. Some graphical representations are shown in which it is possible to determine or represent exergy and exergy balances. The exergy efficiency is introduced based on a general definition of efficiency, and the balance of cost is presented as an additional balance equation to be used in the performance analysis of energy systems. A brief discussion on cost partition criteria is presented to aid the analysis of the cost formation processes of the products of energy conversion processes. Finally, the renewability of energy conversion processes is analysed by means of a renewability exergy index that takes into account the type of inputs, renewable or fossil, the wastes, and the destroyed exergy of a given energy conversion process.

The exergy and thermoeconomic analysis of components of power plants, refrigeration and polygeneration systems is presented and discussed to characterize the performance of such systems as well as to determine their products cost formation processes. Based on the general formulation of efficiency, presented in Chap. 2, the expressions of the exergy-based performance parameters of the components of these systems are derived. These concepts are applied to evaluate the electricity cost formation of a combined cycle power plant, and the comparative performance and production costs of steam and electricity of cogeneration plants configurations for chemical and dairy industries. Finally a comparative exergoeconomic study of trigeneration systems to produce electricity, steam, and chilled water is described and discussed.

This chapter deals with the application of exergy analysis to primary and refining petroleum processes due to the important exergy consumption they require. It describes the exergy and thermoeconomic analysis of an offshore platform. The comparative performance of two artificial lift systems is presented and discussed in detail through a scenario of 25 years of operation. A utilities plant of a petroleum refinery is studied in order to characterize its exergy and cost interactions with the hydrocarbons derived production processes, such as the combined distillation, fluid catalytic cracking (FCC), delayed coking, hydrotreating (HDT), hydrogen generation, and sulfur recovery. The hydrogen production in a petroleum refinery to purify diesel oil, based on the steam reforming of natural gas, is analyzed in detail to evaluate its thermodynamic performance as well as to describe the cost formation of the produced hydrogen.

This chapter presents the application of the exergy analysis to assess the industrial acetaldehyde production by ethanol partial oxidation. With the conclusions of such assessment, an improved plant configuration is proposed in order to use more efficiently the required process thermal exergy. A model was developed to predict all relevant thermodynamic properties of the process streams. The vapor phase was considered ideal, while the nonideality on liquid phase was corrected by Wilson equation. The exergy analysis indicates that the irreversibilities are mainly produced in the oxidation reactor (458 kW), the waste heat boiler (118 kW), the acetaldehyde absorption tower (105 kW), and distillation tower (247 kW). The improved proposed configuration decreases the overall irreversibility rate to 925 kW, which represents a reduction of 14.7 %. The modifications were made in the absorption and distillation towers and a new heat exchanger network caused a reduction of 28 % in the thermal load of the plant.

Over the last 500 years, the Brazilian sugarcane industry has evolved from a single product supplier (sugar producer) to an energy enterprise (sugar, alcohol and electricity). Different technological paths were developed in order to improve the energy conversion processes inside the mill. These improvements led mills to first become self-sufficient in energy, and then, sell electricity to the grid. The utilities plant developed were typical steam-based cogeneration systems, using bagasse to produce steam and electricity required by the sugar and alcohol production processes. Hence, considering the mill as a whole, it became a polygeneration plant, using sugarcane to produce sugar, alcohol, and electricity. Yet, many by-products are available (trash, vinasse, filter cake, etc.), and most of them are discarded or used in an inefficient way. This chapter compares, in an exergy basis, current technological paths used in sugarcane mills, with new ones, which can lead to a more renewable use of energy and the by-products of the processes. These technologies aim at converting these low valued by-products into new added value ones. Such technologies include: more efficient steam cycles (such as high pressure and supercritical steam ones) and biomass gasification-based combined cycles.

Liquid biofuels can be produced from a variety of feedstocks and processes. Ethanol and biodiesel production processes based on conventional raw materials are already commercial, but subject to further improvement and optimization. Biofuels production processes using lignocellulosic feedstocks are still in the demonstration phase and require further R&D to increase their production efficiency. Exergy analysis is a primary tool to assess the efficiency and renewability of biofuels production processes from an integrated point of view. In this chapter, an exergy-based comparative analysis of four biofuels production routes are described and discussed. The selected feedstocks are glucose and sugarcane syrups, the fruit and flower stalk of banana tree and palm oil. For each production route, the effect of process variables on the exergy efficiency and the renewability exergy index (presented in Chap. 2) are determined allowing the identification of possible ways to optimize the production of such biofuels. According to the values of the renewability exergy index, ethanol production process using sucrose, amilaceous, or lignocellulosic material cannot be considered renewable, while biodiesel production from palm oil can be considering renewable. The main reason for these conclusions is due to the irreversibilities that take place along the energy conversion processes of these biofuels production routes. These unexpected conclusions highlight that although renewable raw materials are used as feedstocks, the biofuel itself cannot be considered renewable due especially to the exergy destruction of its production process.

A tendency of the commercial aeronautical industry is to develop more efficient aircraft in terms of fuel consumption and direct operational costs. Regarding fuel consumption, some strategies of the aeronautical industry are to use more efficient aerodynamics, lightweight materials, and more efficient engines and systems. The conventional turbo fan engine mainly provides electric power for cabin systems (lights, entertainment, and galleys) and avionics, hydraulic power for flight control systems, and bleed air for ice protection and environmental control systems. More efficient engines and different types of systems architectures, such as more electric systems, are a promise to reduce fuel consumption. In order to compare systems and engine architectures at the same basis, exergy analysis is the true thermodynamic approach that shall be used as a decision tool to aircraft systems and engine design and optimization. This chapter describes applications and a method based on exergy analysis for conception and assessment of aircraft systems. The method can support the design of the complete vehicle as a system and all of its subsystems in a common framework.

This chapter is concerned with the definition and application of exergy indexes to assess the performance of environmental impact mitigation technologies. These parameters are employed to evaluate the performance of air emission treatment systems, procedures to remediate contaminated sites, management of solid wastes, and technologies of wastewater treatment technologies. Initially it is proposed that the cumulative consumption of materials and utilities for construction and operation of environmental treatment systems can be quantified in terms of exergy. In addition, the exergy content of the emissions of a given production process, as well as the exergy of by-products of the treatment process, can be quantified to develop an exergy balance, focused on the treatment process. Thus, a methodology is described to compare different process alternatives in two situations: in the former the main task of the process is to eliminate the exergy content of a given emission, and in the latter the objective of the process is to maximize the utilization of the exergy content of a given emission. The environmental performance of wastewater treatment technologies is conducted by calculating the environmental exergy efficiency and complemented with the determination of the renewability exergy index. This approach is applied to compare three wastewater treatment plants based on biological (aerobic and anaerobic) and physicochemical processes.

This chapter describes two applications of the exergy analysis in order to evaluate the behavior of human body and its systems. In the first one, the exergy destroyed rate and the exergy efficiency of the human body and its systems are determined. The proposed human body thermal model was divided into 15 cylinders with elliptical cross-sections (which leads to realistic dimensions) representing: head, neck, trunk, arms, forearms, hands, thighs, legs, and feet. For each cylinder a combination of the following tissues was considered: skin, fat, muscle, bone, brain, viscera, lung, and heart. In the second study, a model of the human respiratory and thermal system developed to perform the exergy analysis of the human body under physical activities is described. The analysis quantifies the rate at which oxygen is supplied to the lung and transported by the blood to the tissues, and the rate of carbon dioxide elimination.