Relative Exergy Research Articles

The Relative Exergy Destroyed: A New Tool for Process Design and Control

Since the eco-efficiency of all industrial processes/plants has become more and more important, engineers need to find a way to integrate control loop configuration and measurements of eco-efficiency. In this paper, we present the relative exergy destroyed array (REDA), a new tool to measure the relative eco-efficiency of a process. The REDA is a means to compare eco-efficiency of multi input-multi output processes for different combinations of control structures. Although based on steady state information, it is a simple tool for comparing eco-efficiency of a process with several process control structures. The results obtained from the REDA are interpreted and explained with the help of simple case studies. REDA will help guide the process designer to find a quick optimal control design with low operating cost/eco-efficiency.

Component-based exergetic measures of an experimental turboprop/turboshaft engine for propeller aircrafts and helicopters

In this paper, component–based exergetic assessment is presented for an experimental turboprop/turboshaft engine, including comparisons and exergetic performance such as their efficiencies, improvement potentials, exergy destruction rates, relative exergy destructions, fuel depletion ratios, productivity lacks, and fuel and product exergy factors with different power settings and torques. The exergetic assessment of the engine components provided here should be helpful for designing shaft–power aero engines. Results from this study also evaluate effects of the operating parameters on the exergetic performance of the engine components commonly used in regional propeller aircrafts and helicopters.

Exergy analysis and CO2 emission evaluation for steam methane reforming

This paper investigates the industrial production of hydrogen through steam methane reforming (SMR) from both exergy efficiency and CO2 emission aspects. An SMR model is constructed based on a practical flow diagram including desulfurizer, furnace, separation unit and heat exchangers. The influence of reformer temperature (Tr) and steam to carbon (S/C) ratio is analyzed to optimize exergy efficiency and CO2 emission. A clear correlation is obtained between exergy efficiency and CO2 emission. Results also show optimal S/C ratio decreases with Tr. An exergy load distribution analysis which evaluates interactions between the system and its subsystems with parameter variations is employed to find promising directions for efficiency improvement. Results show that the greatest improvement lies in increasing efficiency of furnace without increasing its relative exergy load. Integration of oxygen-enriched combustion (OEC) with SMR is also evaluated. The integration of OEC can increase the system efficiency greatly when the reformer operates above critical point, while in other cases the system efficiency may decrease.

Dynamic Exergetic Cost Analysis of a Space Heating System

The design of heating systems with groundwater source heat pumps (GWHP) is very important for reducing their power consumption. For better design, reasonable analysis of the systems is necessary. In this paper, a dynamic exergy and exergetic cost analysis of a heating system with GWHP is performed in a whole heating season by the use of structural theory of thermoeconomics and the software of TRNSYS. The relative exergy destruction of every component and the exergetic cost of the final product of the system are obtained. The results show that the heat pump has the largest relative exergy destruction under all the working conditions. The terminal unit component has the second largest relative exergy destruction at conditions above 10% load. However, at 10% load, well water transportation component has the second largest relative exergy destruction. The unit exergetic cost of the system final product during the whole heating season is 8.51W/W, similar to the result at 75% load.

Recycle effect on the relative exergy array

Since the eco-efficiencies of all industrial processes/plants have become more and more important, engineers need to find a way to integrate process design, process control and measurements of eco-efficiency. The thermodynamic concept of Exergy can be used to analyze a process in terms of its efficiency. The Relative Exergy Array (REA) that measures both the relative exergetic efficiency and the controllability of a process has been proposed by Montelongo-Luna et al. (2011). The REA can be used for quick comparison between several process/control structure candidates. It is very common that the industrial unit/process includes some recycle loops. The recycle effect on REA is investigated in this paper through two process case studies. Simulation results show that the REA without consideration of the recycle loop may mislead the selection of control configuration. Recycle loop should be considered for selection of control configuration.

Exergetic cost analysis of a space heating system

In order to evaluate and improve the design of space heating systems with groundwater source heat pumps (GWHP), common design practices should be examined. In this paper, a GWHP system with common design is studied. The COP of the heat pump is 3.5 at design condition. The system is divided into five subsystems, and exergetic cost analysis is performed on it based on structural theory of thermoeconomics. The results show that the three largest relative exergy destructions and lowest exergy efficiencies occur in power generation and distribution, heat pump, and terminal unit subsystems with relative exergy destructions of 71.2%, 17.1% and 7.02% and exergy efficiencies of 32.8%, 54.8% and 65.6% respectively. The three subsystems also have the largest increases of unit exergetic costs of 2.04 W/W, 2.15 W/W, and 2.73 W/W respectively. Therefore, designers of GWHP space heating systems should pay close attention to heat pump and terminal unit subsystems, especially to the latter one because of its larger increase of unit exergetic cost. The unit exergetic cost of the system final exergetic product is 7.92 W/W. This value can be used to evaluate the system and compare it with others from the viewpoint of energy conservation.

Degree-days for the exergy analysis of buildings

This paper presents the application of degree-days utilized for the exergy analysis of buildings. The climate is observed as a basis for the exergy analysis of the energy usage of buildings. The definitions and functions of the outdoor temperature distribution density and its cumulative distribution frequency are introduced in the analysis. There are several relative exergy indicators for local climate conditions that are considered in the exergy analysis of the energy usage of buildings. The setting of these parameters allows the thermodynamic evaluation of engineering heating practices in separate locations.

Comparative Study of Two Low CO2 Emission Power Generation System Options With Natural Gas Reforming

Two power plant schemes that reduce CO2 emission and employ natural gas reforming were analyzed and discussed. The first one integrates natural gas reforming technology for efficiency improvement with an oxy-fuel combined power system (OXYF-REF), with water as the main work fluid. The reforming heat is obtained from the available turbine exhaust heat, and the produced syngas is used as fuel with oxygen as the oxidizer. The turbine working fluid can expand down to a vacuum, producing a high-pressure ratio and thus more net work. The second system integrates natural gas reforming in a precombustion decarbonization scheme using chemical absorption technology for the CO2 removal (PCD-REF). The gas turbine is the conventional air-based one with compressor intercooling. Supplementary combustion is employed to elevate the turbine exhaust temperature and thus achieve a much higher methane conversion rate (96.9%). Both systems involve internal heat recuperation from gas turbine exhausts, and particular attention has been paid to the integration of the heat recovery chain to reduce the related exergy destruction. The systems are simulated and their thermal efficiency, overall and component exergy losses, and CO2 removal capacity are compared. The OXYF-REF system has a higher energy efficiency, of 51.4%, and higher CO2 removal, but the product CO2 has lower purity, of 84%. The PCD-REF system has a thermal efficiency of 46%, the captured CO2 is 99% pure, and the CO2 specific emission is 58.5g∕kWh.

Exergoecological analysis of processing of SO2-containing gases from zinc production

AbstractAn energy and technological study of processing SO2-containing metallurgical gases from zinc production to sulfuric acid production using the exergy method of thermodynamic analysis has bean carried out. A real operating installation since 2003 at the KCM-SA, Plovdiv, Bulgaria, has been used as the model scheme. The principal production stages have been analyzed. The absolute and the relative exergy characteristics of the separate stages as well as of the process as a whole have been calculated. The results have been used to evaluate the influence of sulfuric acid production on the environment. The results obtained have been compared with previous studies on similar processes both in zinc and in copper production. The studied installation of sulfuric acid production has good exergoecological indices, defining it as an effective process both in thermotechnical and in ecological aspects.

Exergy Analysis of Combined Cycles Using Latest Generation Gas Turbines

The potential performance of optimized gas-steam combined cycles built around latest-generation gas turbine engines is analyzed, by means of energy/exergy balances. The options here considered are the reheat gas turbine and the H-series with closed-loop steam blade cooling. Simulations of performance were run using a well-tested Modular Code developed at the Department of Energy Engineering of Florence and subsequently improved to include the calculation of exergy destruction of all types (heat transfer, friction, mixing, and chemical irreversibilities). The blade cooling process is analyzed in detail as it is recognized to be of capital importance for performance optimization. The distributions of the relative exergy destruction for the two solutions—both capable of achieving energy/exergy efficiencies in the range of 60 percent—are compared and the potential for improvement is discussed. [S0742-4795(00)00902-9]

Exan™ Pro: Process visualization tool: Increasing your insight into energy conversion and large chemical processes

Exan Pro is a powerful tool for the visualization of process modeling results, which can be obtained using Aspen Plus™ or other flowsheeting packages. Clear Exergy, Energy, Molar, Mass, Volume, Process and Block flow diagrams can be created in an automatic way. In a flow diagram, the width of a stream schematically represents its contents. Exan Pro especially allows you to perform and visualize a complete exergy or energy analysis, which has been calculated using Aspen Plus, by Aspen Tech. Inc., and ExerCom, by Stork Engineers & Contractors b.v. In addition to the visualization using flow diagrams, the results can also be presented using total exergy or energy balance graphs and pies, showing the relative exergy losses. After loading the simulation results, the flowsheet is automatically laid out using specially developed algorithms. The main process flow path is shown in a straight line. Next, the other process paths branch from this main process path. This way the backbone of the flowsheet, is visualized in the form of a Tree. If the process is too complex for complete visualization in one step, the process can be partitioned into sub processes using the partitioning tools. Next, the utilities, which are used by the unit operations, can be assigned to the units, allowing separate modeling of the main process and the utility generation processes. After selecting the desired type of flow diagram, the flow diagram of the (partitioned) flowsheet is automatically send to Visio, a powerful technical drawing package. The flow diagrams can easily be sized, printed, edited and published in high quality, using this package. It can be concluded that clear visualization of process information through flow diagrams can be used as an efficient communication tool during process design. It increases the insight of process engineers into the processes. Weak spots in the simulated processwe quickly recognized and consequently the optimization of the process structure is accelerated. Therefore, process visualization contributes to a more rigorous design of efficient processes.