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Exergy Analysis for Industrial Energy Assessment

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Exergy Analysis for Industrial Energy Assessment EXERGY ANALYSIS FOR INDUSTRIAL ENERGY ASSESSMENT Thesis Submitted to The Department of Mechanical and Aerospace Engineering School of Engineering UNIVERSITY OF DAYTON In partial fulfillment of the requirements for The degree Master of Science in Mechanical Engineering by Wayne Thomas Bader UNIVERSITY OF DAYTON Dayton, Ohio August, 2000 EXERGY ANALYSIS FOR INDUSTRIAL ENERGY ASSESSMENT APPROVED BY: J. Kelly KissockrPrt.D., P.E. Kevin P. Hallinan, Ph.D. Advisory Committee Chairman Committee Member Assistant Professor, Mechanical and Chairman, Professor, Mechanical and Aerospace Engineering Aerospace Engineering jWin J. Schauer,PtT.D. Committee Member Professor, Mechanical and Aerospace Engineering Donald L. Moon, Ph.D. Blake Cherrington, Ph©., P.E. Associate Dean Dean, School of Engineering Graduate Engineering Programs & Research School of Engineering 11 ABSTRACT EXERGY ANALYSIS FOR INDUSTRIAL ENERGY ASSESSMENT Bader, Wayne Thomas University of Dayton, 2000 Advisor: Dr. J. Kelly Kissock Traditional approaches to improving energy efficiency often focus on leak-plugging and housekeeping efforts to avoid wasting energy. In many cases however, the way energy is used, rather than the amount used, is the primary cause of inefficiency. Exergy analysis provides a thermodynamic basis for optimizing energy systems. By considering the quality of energy as well as the quantity, the true thermodynamic efficiencies can be quantified and proper priorities for improvement projects can be established. In spite of its value for optimizing thermodynamic systems, exergy analysis has seen only limited use as a practical tool for industrial energy assessment. In this thesis, the energy and exergy efficiencies of two common industrial processes, air compression and aluminum melting, are developed. Methods for improving the processes are quantitatively discussed. Finally, an application of exergy analysis to analyzing energy information in industrial plants is described. This analysis suggests that in many cases, energy efficiency is an incomplete characterization of the true efficiency of an energy system. In contrast, exergy analysis reveals how and where to make improvements. In addition, the exergy approach can provide information needed to decide whether to improve an existing process or to re-design it. Thus, exergy analysis appears to be an essential component of effective resource utilization in the industrial environment. iii TABLE OF CONTENTS ABSTRACT............................................................................................................................................ iii LIST OF ILLUSTRATIONS................................................................................................................ vi LIST OF TABLES.................................................................................................................................. vii NOMENCLATURE................................................................................................................................ viii CHAPTER I. INTRODUCTION.............................................................................................................................. 1 Energy versus Exergy............................................................................................................. 1 Exergy Content of Energy Forms.......................................................................................... 2 Efficiencies.............................................................................................................................. 5 The Exergy Approach............................................................................................................. 5 II. THERMODYNAMICS OF ENERGY ANALYSIS...................................................................... 6 Gibbs Relations........................................................................................................................ 6 Entropy Change........................................................................................................................ 7 Thermomechanical Exergy Content of a Flow Stream........................................................ 8 Thermochemical Exergy Content of a Flow Stream............................................................ 9 Work of Compression............................................................................................................. 11 Ideal Gas Considerations........................................................................................................ 12 III. THERMOMECHANICAL EXERGY: AIR COMPRESSION PROCESS............................... 15 Energy (First Law) Analysis................................................................................................... 15 Exergy (Second Law) Analysis.............................................................................................. 18 Case Study................................................................................................................................ 21 Operating Parameters.............................................................................................. 22 Case 0: Base System Efficiency............................................................................. 24 Case 1: System Efficiency with Lower Discharge Pressure................................ 25 Case 2: System Efficiency with Reduced Inlet Air Temperature....................... 28 Case 3: System Efficiency when Reclaiming Heat of Compression.................. 30 Case 4: System Efficiency with Reduced Air Demand........................................ 32 Case 5: System Efficiency with Correctly-sized Compressor............................. 33 Case 6: "Best Practices": Combined Effects of Cases 1 through 5.................... 35 Conclusions.............................................................................................................................. 37 iv IV. THERMOCHEMICAL EXERGY: ALUMINUM MELT FURNACE...................................... 40 Combustion Process................................................................................................................. 40 Energy Analysis........................................................................................................ 41 Exergy Analysis........................................................................................................ 43 Case Study................................................................................................................. 46 Energy Analysis......................................................................................... 47 Exergy Analysis......................................................................................... 50 Conclusions................................................................................................................. 53 Melting Process........................................................................................................................ 54 Energy Analysis........................................................................................................ 55 Exergy Analysis........................................................................................................ 55 Case Study................................................................................................................. 56 Energy Analysis......................................................................................... 57 Exergy Analysis......................................................................................... 58 Conclusions................................................................................................................ 60 Melting Heat Exchange............................................................................. 60 Overall Melting Process............................................................................ 60 Exhaust Heat Reclamation....................................................................................................... 62 Heat Transfer Coefficient......................................................................................... 64 Heat Exchanger Analysis......................................................................................... 66 Results of Analysis................................................................................................... 68 Conclusions................................................................................................................ 68 V. ENERGY INFORMATION: ELECTRICAL LOAD FACTOR................................................... 71 Load Factor and Exergy Analysis.......................................................................................... 72 Case Study................................................................................................................................. 73 Analysis of Facility Operation................................................................................ 73 Facility Operating Schedule..................................................................... 73 Reduction in Electrical Consumption...................................................... 74 Billing Error Analysis............................................................................... 74 Verification of Electricity Bills................................................................ 74 Reduction in Electrical Demand.............................................................. 75 Continuous Monitoring
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