2.1.3 Reciprocating internal combustion engine cogeneration systems . ..... destroyed exergy is calculated at a global level from the exergy balance as well as in.
FACULTY OF MECHANICS
Eng. Paula Veronica UNGURESAN
PHD THESIS RESEARCHES CONCERNING THE OPTIMIZATION OF INTERNAL COMBUSTION ENGINES COGENERATION PLANTS, THROUGH EXERGOECONOMIC ANALYSIS OF HEAT EXCHANGERS
ABSTRACT
ADVISOR Prof. PHD. Eng Teodor MADARASAN
2008
TABLE OF CONTENTS
Foreword ..................................................................................................................................................i Symbols and abbreviation.....................................................................................................................3 Theme and layout of the thesis.............................................................................................................7 Chapter. 1 Introduction ........................................................................................................................9 1.1 Cogeneration. Definition and historical development .......................................................9 1.2 Cogeneration in Europe and Romania ...........................................................................11 1.3 The benefits of cogeneration ..........................................................................................12 Chapter. 2 The current state of research .........................................................................................13 2.1 Cogeneration technologies .............................................................................................13 2.1.1 Steam turbine cogeneration systems ....................................................................14 2.1.2 Gas turbine cogeneration systems ........................................................................17 2.1.3 Reciprocating internal combustion engine cogeneration systems ........................19 2.1.4 Combined cycle cogeneration systems .................................................................21 2.2 Performance indices of cogeneration systems ...............................................................22 2.3 Types of heat exchangers used in cogeneration systems .................................. 23 2.3.1 Basic equations for heat exchangers thermal calculus ...................................23 2.3.2 Plate heat exchangers ...........................................................................................26 2.3.3 Shell and tube heat exchangers ............................................................... 27 2.4 Exergy, standard of thermal process quality ...................................................... 28 2.4.1 Exergy transfer with work and heat interaction .....................................................28 2.4.2 Exergy transfer associated with material streams .................................................29 2.4.3 Open system exergy balance ................................................................................34 2.4.4 Specific cogeneration systems irreversibilities ......................................................35 2.4.5 Exergetic efficiency ................................................................................................38 2.5 Exergoeconomics role in thermal process ......................................................................39 2.5.1 Economic analysis .................................................................................................39 2.5.2 Exergoeconomic analysis ......................................................................................42 Chapter. 3 Experimental researches ..................................................................................................47 3.1 Experimental installation .................................................................................................48 3.1.1 Measuring devices .................................................................................................50 3.1.2 Internal combustion engine....................................................................................52 3.1.3 Heat recovery system ............................................................................................53 3.1.4 Electric power generator ........................................................................................54 3.2 Experimental measurements ..........................................................................................55 3.3 Experimental results and analysis ..................................................................................57
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3.3.1 Thermal balances ..................................................................................................58 3.3.2 Exergetic balances.................................................................................................62 Chapter. 4 Mathematical modeling of internal combustion engine cogeneration plants.............69 4.1 Presentation of AEEICMAI program .............................................................................69 4.1.1 “Preliminary data” module......................................................................................70 4.1.2 “Exergetic analysis” module ..................................................................................80 4.2 Exergetic analysis results for cogeneration installation from Thermal Plant No Cluj-Napoca ..............................................................................................................................................87 Chapter. 5 Exergoeconomic analysis of internal combustion engine cogeneration plants ... 101 5.1 Cost calculus.................................................................................................................101 5.2 Exergoeconomics variables calculus ...........................................................................110 Chapter. 6 Exergoeconomics optimization of internal combustion engine cogeneration plants119 6.1 Introduction to exergoeconomic optimization ...............................................................119 6.2 The minimize of the cost of electrical energy produced by studied system .................121 6.3 The minimize of the cost of thermal energy produced by studied system ...................126 Chapter. 7 Personal contributions. General conclusions ..............................................................131 7.1 Personal contributions ..................................................................................................131 7.2 General conclusions .....................................................................................................132 Bibliography........................................................................................................................................135 Annexes...............................................................................................................................................143 Annex A ..............................................................................................................................143 Annex B ..............................................................................................................................161 Published papers................................................................................................................................165 Curriculum vitae .................................................................................................................................177
KEYWORDS: cogeneration, internal combustion engines, exergy analysis, exergoeconomics, optimization
THEME AND LAYOUT OF THE THESIS Approaching the internal combustion engine cogeneration system from exergoeconomic point of view is a vast and challenging field of research (due mainly to the multitude exergoeconomic analysis techniques). The author’s research comprises two main aspects: an experimental one and a theoretical one. The first one aimed on assessing the performances of this type of cogeneration system and underlining the exergy destroyed in every component of the plant and in the whole plant, too. The second one comprises the theoretical researches that are based on a mathematical model which takes into account the whole characteristics aspects of a power plant: the thermodynamic aspects of all processes occurring in this type of installation, the qualitative aspects that are revealed by the exergy analysis, and the economic aspects, pointed by the exergoeconomic analysis. This research requires the conceiving of a calculus program, which allow for changing some of the heat exchangers geometric parameters or other thermophysical parameters, in order to assess the effects of these alterations on the system performances. The thesis is structured in seven chapters, briefly presented below.
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CHAPTER 1 Introduction This chapter presents the cogeneration definition, the history and the development of this technology. There are pointed out the cogeneration advantages, as follows: the increasing efficiency of energy conversion and use, the lower emissions to the environment, the cost savings. There are also presented the main components of a cogeneration system. Also, the presentation reflects the state of cogeneration development in Europe and Romania. It reveals that, in average, the share of cogeneration in the electricity production in Europe is about 12%, with a high variability degree. In the last few years, the cogeneration has developed under both technology and legislation aspects in Romania, too.
CHAPTER 2 The current state of research The current state of research is presented in an exhaustive manner. There are shown the main cogeneration technologies together with the basic configuration, thermodynamic cycle, advantages and disavantages of every system. A main place is dedicated to the performance indices of cogeneration systems. The heat exchangers usually met in cogeneration system are: the plate heat exchangers, used in engine cooling system and the shell and tube heat exchangers for exhaust gases heat recovery. There are exposed the basic equations used for the thermal and fluidodynamic calculus of the above mentioned equipments. Exergy as a quality of energy standard is extensively explained, both the exergy transfer with work and heat interaction and exergy transfer associated with material streams being analysed. The exergetic efficiency is defined by means of terms “product” and “fuel” used in a thermoeconomics sense. In the frame of the exergoeconomic analysis, there are emphasized the major costs related to a “product”: the total capital investment, the fuel costs, and the maintenance and operation costs. In addition, the cost balance is explained and the principles “F” and “P” - that were used in the auxiliary relations written for costs determination - are presented.
CHAPTER 3 Experimental Researches Chapter 3 consists of a detailed exposition of the author’s experimental researches on the cogeneration systems, undertaken at Thermal Plant, no.3 , Cluj-Napoca. This exposition comprises the description and the operating mode of the experimental setup, the presentation of the measuring technique as well as the measurements protocol used for the data acquisition (concerning the parameters used in the research). On the basis of the functional parameters measured in the plant, a complete thermodynamic and exergetic study is performed. The results are presented in tables and graphics like the Sankey and Grasmann diagrams. In order to appreciate the internal engine cogeneration system performance, the performance indices were calculated. While analysing the outcomes, it can be observed that the studied plant has a high performance which is comparable to that of the most performant instalation of this type. Thus, the total efficiency is 85.7%, the power to heat ratio is 62% and the exergetic efficiency is 52%.
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These experimental researches are very useful in numerical modeling of thermal processes developed in the system and for system behaviour for other electrical loads.
CHAPTER 4 Mathematical modeling of internal combustion engine cogeneration plants This chapter shows the algorithm followed by the author in order to accomplish the thermal, fluidodynamic and exergetic calculus. The algorithm was conceived with respect to the mathematical model and was transposed into an original application developed by the author in Engineering Equation Solver (EES) soft.
AEEICMAI PROGRAM PRELIMINARIES DATA MODULE Data introduction
Calculus elements
EXERGETIC ANALYSIS MODULE
SC1 Exergetic analysis
SC2 Exergetic analysis
Engine Exergetic analysis
EXERGOECONOMIC ANALYSIS MODULE Costs calculus
Thermoeconomic variables calculus
EXERGOECONOMIC OPTIMIZATION MODULE Minimizing the cost of electrical energy
Minimizing the cost of thermal energy
Fig. 4.1. AEEICMAI Program structure The first module, entitled “Preliminaries Data” is dedicated to data introduction and to calculus of the functional and thermodynamic parameters that are necessary in exergetic and exergoeconomic studies. The second module is dedicated to the exergy analysis. This analysis is based on some relations used to calculate both the exergy and the destroyed exergy for each system component. It is to be mentioned that the destroyed exergy is calculated at a global level from the exergy balance as well as in
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detail, by taking into account the developed irreversibilities. In order to reveal the influence of some parameters on heat exchangers performance from internal engines cogeneration plant, some studies for heat exchangers are made, studies that involve the modification of the SC1 plates number and of the SC2 tubes length, respectively. In order to appreciate the system performance while operating at other electrical load another study was made by considering the changes made in plates number of engine cooling system heat exchanger and tubes length for exhaust gases heat exchanger. In order to validate the “AEEICMAI” program, proposed by the author, the results obtained by running the application were compared with the experimental outcomes The most important results coming out from comparison are summarized below: •
The exergetic analysis results obtained with the proposed program AEEICMAI, are very close (comparable) to the results of experimental research. Thus, the proposed program can be used for mathematical modeling of the thermal processes in internal combustion engines, in a specific electrical power range.
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The major exergy destroyed rate in cooling system heat exchanger is due to heat transfer at finite temperature difference irreversibility, other destroyed rates being more reduced.
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In exhaust gases heat exchanger, the main exergy destroyed rate is due to irreversibility of heat transfer over a finite temperature difference followed by the exergy destroyed due to pressure losses and due to thermal interaction with the environment.
•
Inside the engine, the exergy destroy rate in combustion process represents more than 97% from total exergy destroyed rate, followed by exergy destroyed rate in mixing process.
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While raising the plates number for heat exchanger SC1, the exergy destroyed rates due to heat transfer over a finite difference temperature and the one due to pressure losses decrease and, as a result, the exergetic efficiency will grow up.
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The results of exergy analysis for different power range reveals that while rising the heat exchanger SC2, length, the exergetic efficiency grow, and the higher values were obtained for 250 kW.
The obtained results led to an amount of information regarding not only the performance of internal combustion engines cogeneration systems but also the influence of geometric parameters on heat exchangers performances.
CHAPTER 5 Exergoeconomics analysis of internal combustion engine cogeneration plants
This chapter comprises important researches on costs formation process and on costs calculus for each „fuel” and „product” in the system. These researches were carried out using the module no3 of AEEICMAI program. Subsequent to the system decomposition into functional zones and to the identification of the „fuel” and the „product” assigned to each zone, the nonexergetic costs are calculated and the auxiliary costing equation are identified, in order to obtain the unknown costs per exergy unit. The chapter also offers some researches regarding the influence of heat exchangers construction on the exergoeconomic aspects for every functional zone. A key role in the thermoeconomic evaluation was dedicated to thermoeconomic variables calculus: the average unit cost of fuel, the average unit cost of product, the cost rate of exergy destruction, the relative cost difference and the exergoeconomic factor. All the thermodinamic variables were studied as a function of plates number for SC1 and tubes lenght for SC2.
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As a result of these researches, we obtained some relations between the essential parameters for the exergoeconomic analysis, relations which were pointed out and used to make some useful technical recommandation for the specialists. We obtained some useful conclusions concerning the construction and the operation of heat exchangers in internal combustion engine cogeneration systems. The higher exergetic efficiency corresponds to heat exchanger for engine cooling system. The cost rate associated with exergy destruction in SC1 heat exchanger diminishes with the plates number growth, because of destroyed exergy rate reduction. The relative difference cost for heat exchanger cooling system presents a minimum, for a number of channels plates of 80, (similar with the one existing in the studied system ) . The exergoeconomic factor grows for both heat exchangers while the plates number of heat exchanger SC1 grows, respectively tubes length grows.
CHAPTER 6 Exergoeconomics optimization of internal combustion engine cogeneration plants
In the first part of this chapter the mathematical model of the specific optimisation problem is configured. The equations which represent the mathematical model and the objective function are presented. The objective function reduces to minimization of the products costs: power and thermal energy, subjected to some constraints. The first subchapter reffers to the minimizing of the electric energy cost carried out by using the submodule entitled „Minimizing electric energy cost” of the AEEICMAI program. On the account of the results obtained, we can draw the curve of cost per exergy unit of power produced, according to engine exergetic efficiency (Fig.6.2.). Similarily, the minimize of the cost of thermical energy produced by the studied system is analised.For this purpose, the system is considered as depending on the two heat exchangers as a whole.In the end, we obtain the variation of cost per exergy unit of thermal energy, depending on exergetic efficiency of the two heat exchangers (Fig. 6.4.). 0.098
0.16 c5
c10 [Euro/kWh]
c5 [Euro/kWh]
c10
0.15
0.096
0.094
0.092
0.14 0.13 0.12 0.11 0.1
0.09
0.09 0.088
0.08
c 5,min η ex, optim
0.086 0.34
0.36
0.38
0.4
0.42
0.44
0.46
0.48
c10,min
0.07 0.5
0.52
0.4
η exergetic,motor
0.45
0.5
ηex,optim 0.55
0.6
0.65
0.7
0.75
η exergetic,schimbatoare
Fig.6.2 The cost per exergy unit of power produced, as a function of engine exergetic efficiency
Fig.6.4 The cost per exergy unit of thermal energy produced, as a function of heat exchangers ensemble exergetic efficiency
Main own contributions in this chapter are: • The minimum cost per exergy unit of power produced is 0.08696 Euro/kWh and it is correlated to an exergy efficiency of engine-generator of 0.45. •
The exergy efficiency of engine-generator for the system studied is around 0.432, so very close to the optimum.
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Similary, the minimum cost per exergy unit of thermal energy is 0.0697 Euro/kWh and it is correlated to an exergy efficiency of heat exchanger ensemble of 0.65.
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The exergetic efficiency of heat exchangers ensemble from system studied is 0.5, to which a cost per exergy unit of 0.09899 Euro/kWh corresponds.
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The researches made in this chapter prove the existence of optimum exergetic efficiency for internal engines cogeneration plants, to which it is to aspire when designing and exploitating them, for minimizing products costs.
These results represent the corollary of the exergoeconomic analysis made in this thesis.
CHAPTER 7 Personal contributions and Conclusions
The chief personal contributions of the author to the research of internal combustion engine cogeneration systems are the following: •
There have been performed ample experimental researches in internal combustion cogeneration system from Thermal Plant No.3 Cluj-Napoca. On the basis of the functional parameters measured in the plant, a complete thermodynamic and exergetic study was performed. The results are presented in tables and graphs as Sankey and Grasmann diagrams.
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The performance indices for the cogeneration system were calculated and compared with similar systems in order to appreciate the system performance.
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The development of the Application Program entitled “Exergoeconomic analysis of internal combustion cogeneration plants”, (AEEICMAI) by use of the soft Engineering Equation Solver (EES), which includes all necessary stages for mathematical modeling for this type of system.
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The study on the influence of geometric parameters on individual components and whole plant performance.
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The exergoeconomic balance equations for each plant component were written and the auxiliary equations were identified (according to principles F and P). Thus, the equation system can be solved.
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The average costs per unit of exergy, cost rates and the exergoeconomic variables for all system physical streams were determinated.
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The mathematical model proposed by Bejan and Tsatsaronis was particularized, in order to minimize the products costs for the studied system.
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The procedure of optimum exergetic efficiency determination, for both engine generator set and heat exchangers ensemble were presented.
All the aforementioned accomplished researches open the way for future research on subjects like Sizing up or redesigning the heat exchangers in order to make possible to operate them close to the optimum exergetic efficiency. Testing other exergoeconomic techniques for obtaining result comparison. Exergetic Cost Theory (ECT); Thermoeconomic Functional Analysis (TFA) ; Structural Analysis Approach (SAA) ; Engineering Functional Analysis (EFA).
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