ABSTRACT
This study applies an advanced exergy analysis to a novel unified biomass-based tri-generation energy system for power generation, heating and cooling with an even simpler cycle configuration. The thermodynamic models which demonstrates the moderate energy and exergy efficiency of the proposed system was conducted using three refrigerants - R245fa, R1234yf, and R1234ze. Variation in the behaviour of the system was established based on several performance index of the system. Additionally, the system’s susceptibility to improvement in exergy efficiency via advanced exergy analysis presented a theoretical framework for the choice of optimum operating variables for this purpose. Models were also developed to provide for the exergoeconomic performance of the system. The results demonstrate that the exergy efficiency of the system is greatly enhanced by virtue of the new Organic Rankine Cycle (ORC) arrangement to include cooling and heating as products. In fact, the incorporation of the cooling arrangement led to an increase in exergy efficiencies in the order of 28.34 %, 22.32 %, and 29.61 % with refrigerants R245fa, R1234yf, and R1234ze in that order. Thus, the system is suitable for direct coupling with a Brayton topping cycle at controlled mass flow rates of flue gas. With any of the three refrigerants, the exogenous exergy destruction is greater than the value of endogenous exergy destruction. Accordingly, the greatest contribution to the exergy destruction rate is from the internal irreversibilities of the system due to the small average exergy efficiency of the plant. With refrigerant R245fa, the total system output diminishes at temperatures in excess of 120 oC, thus establishing a broad optimum temperature of the system at this point at a pressure range of 2.4 to 2.7 MPa. At this condition, the total system output varied from 49.388 kW to 49.70 kW. Consequently, it is optimally feasible to run the system at 2.5 MPa and 120 oC. Furthermore, at the basic operating parameters of the system, the electrical cost from the system due to exergoeconomic analysis is 0.1106 $/kWh, 0.06925 $/kWh, and 0.09813 $/kWh respectively, for refrigerants R245fa, R1234yf, and R1234ze. Accordingly, the least thermoeconomic cost for the system is obtained with R1234yf at 0.06925 $/kWh which corresponds to 24.93 N/kWh, about 10.07 N/kWh less than the national energy tariff, thus demonstrating the feasibility of the proposed energy system.
TABLE OF CONTENTS
Title Page i
Declaration ii
Certification iii
Dedication iv
Acknowledgements v
Table of Content vi
List of Tables vii
List of Figures viii
Nomenclature ix
Abstract x
CHAPTER 1: INTRODUCTION
1.1 Background of Study 1
1.2 Statement of Problem 3
1.3 Aim and Objectives of Study 3
1.4 Scope of Study 4
1.5 Justification of the Study 4
CHAPTER 2: LITERATURE REVIEW
2.1 Biomass Based Energy Conversion Systems 6
2.2 Trigeneration and Multigeneration Energy Systems 8
2.3 Extensive Applications of Organic Rankine Cycles 11
2.4 Application of Low Heat Bottoming Cycles 20
2.5 Exergoeconomic Analysis 25
2.5.1 Cost balance for system components and auxiliary equations 27
2.6 Summary of Literature and Research Gap 28
CHAPTER 3: MATERIALS AND METHODS
3.1 Materials 30
3.2 Methods 30
3.2.1 Unified biomass-based tri-generation energy system 30
3.2.2 Thermodynamic assumptions 34
3.3 Equilibrium Energy Models for the Gasification process 34
3.3.1 Determination of product coefficients of global gasification reaction for wood 36
3.4 Energy Balance for the ORC 39
3.4.1 Evaporator energy balance 40
3.4.2 Domestic water heater energy balance 40
3.4.3 ORC turbine energy balance 40
3.4.4 Heat exchanger energy balance 41
3.4.5 Valve I energy balance 42
3.4.6 Condenser energy balance 42
3.4.7 Valve II, energy balance 42
3.4.8 Evaporator energy balance 43
3.4.9 Condenser energy balance 43
3.4.10 Pump energy balance 43
3.5 Exergy Modelling 44
3.5.1 Evaporator exergy balance 44
3.5.2 Turbine exergy balance 44
3.5.3 Valve I exergy balance 46
3.5.4 Condenser exergy balance 46
3.5.5 Valve II, exergy balance 47
3.5.6 Evaporator exergy balance 47
3.5.7 Heat exchanger exergy balance 47
3.5.8 Condenser exergy balance 48
3.5.9 Pump exergy balance 48
3.6 Advanced Exergy Analysis of the System 49
3.6.1 Advanced exergy methodology via the engineering method 50
3.7 Exergoeconomic Analysis of the System 52
3.7.1 ORC evaporator [1] cost balance 53
3.7.2 ORC turbine cost balance 53
3.7.3 Heat exchanger cost balance 54
3.7.4 ORC condenser II cost 54
3.7.5 ORC pump cost balance 54
3.7.6 ORC valve 1 cost balance 55
3.7.7 ORC condenser [1] cost balance 55
3.7.8 ORC evaporator [2] cost balance 55
3.7.9 Water heater cost balance 56
3.7.10 ORC valve [2] cost balance 56
3.8 Parameter Based Component Cost Rates 56
3.8.1 Purchase and equipment cost of components 58
3.8.2 Cost of ORC condenser 58
3.8.3 Cost of ORC pumps 59
3.8.4 Cost of ORC heat exchanger 59
3.8.5 Evaporator cost 59
CHAPTER 4: RESULTS AND DISCUSSION
4.1 Performance Analysis of the Proposed System 60
4.2 Thermodynamic Performance Index of Operating Refrigerants 60
4.3 State Point Properties in the System 61
4.4 Performance Index at Design Conditions 64
4.5 Results from Advanced Exergy Analysis 67
4.5.1 Endogenous and exogenous exergy computation for the vapour generator 67
4.5.2 Endogenous and exogenous exergy computation for the turbine 69
4.5.3 Endogenous and exogenous exergy computation for the system 69
4.6 Results from Sensitivity Analysis of the System 72
4.6.1 Effect of pinch point temperature on exergy efficiency 73
4.6.2 Effect of pinch point temperature on heating and cooling 75
4.6.3 Effect of turbine inlet pressure on exergy efficiency and turbine output 78
4.7 Effect of Operating Parameters on Component Endogenous and Exogenous Exergy 79
4.8 Optimum Operating Pressure at Fixed Temperatures 82
4.9 Exergoeconomic Analysis of the System 84
4.9.1 Effect of TIT and TIP on cost of electricity 87
4.9.2 Effect of turbine back pressure on cost of electricity 88
4.10 Result Validation 90
CHAPTER 5: CONCLUSION AND RECOMMENDATIONS
5.1 Conclusion 92
5.2 Contribution to knowledge 93
5.3 Possible application of study 94
5.4 Recommendations 94
5.5 Suggestion for further studies 94
References 95
Appendices 103
LIST OF TABLES
3.1 Summary of the Exergy of Fuel, Exergy of Product, Endogenous and Exogenous Parts of Component Exergy Destruction 49
4.1 Physical Properties of the Operating Refrigerants 60
4.2 Environmental and Safety Properties of the Operating Refrigerants 61
4.3 Thermodynamic Properties at the State Points for R245fa 62
4.4 Cycle Thermodynamic Properties at the State Points for R1234ze 63
4.5 Thermodynamic Properties at the State Points for R1234yf 64
4.6 Performance of the System at Standard Conditions 65
4.7 Total Exergy Destruction 66
4.8 Summary of Component Exergy Destruction 67
4.9 Advanced Exergy Results for R245fa 70
4.10 Advanced Exergy Results for R1234yf 70
4.11 Advanced Exergy Results for R1234ze 71
4.12a Effect of Turbine Operating Pressure at 110 oc TIT on Advanced Exergy Analysis for R234fa 80
4.12b Effect of Turbine Operating Pressure at 110 oc TIT on Advanced Exergy Analysis for R234fa 81
4.13 Thermoeconomic Properties at the State Points for R245fa 85
4.14 Cycle Thermoeconomic Properties at the State Points for R1234ze 86
4.15 Thermoeconomic Properties at the State Points for R1234yf 87
4.16 Tabulated Result Validation with Related Studies 91
LIST OF FIGURES
3.1a Biomass Fired ORC Based Energy System 32
3.1b Novel ORC Based Energy System 33
4.1 Evaporator 1 Advanced Exergy Plot [R245fa] 68
4.2 Effect of Pinch Point Temperature on Exergy Efficiency [R245fa] 74
4.3 Effect of Pinch Point Temperature on Exergy Efficiency [R1234yf] 74
4.4 Effect of Pinch Point Temperature on Exergy Efficiency [R1234ze] 75
4.5 Effect of Varying Pinch Point Temperature on Heating and Cooling Loads of the System (R1234ze). 76
4.6 Effect of Varying Pinch Point Temperature on Heating and Cooling Loads of the System (R1234yf). 77
4.7 Effect of Varying Pinch Point Temperature on Heating and Cooling Loads of the System (R245fa). 77
4.8 Effect of Varying Turbine Inlet Pressure on Exergy Efficiency and Turbine Work (R245fa). 79
4.9 Optimum Operating Pressure at Varying TITs for R245fa 82
4.10 Optimum Operating Pressure at Varying TITs for R1234yf 83
4.11 Optimum Operating Pressure at Varying TITs for R1234ze 88
4.12 Effect of TIT and TIP on Cost of Electrical Output 88
4.13 Effect of turbine back pressure on cost of electrical output 89
4.14 Effect of turbine intermediate pressure on cost of electrical output 89
NOMENCLATURE
ACLiBr-water Lithium bromide-water absorption chiller
CPC Compound parabolic concentrators
ENC Efficiency normalization curve
E ̇_(D,k)^EN Endogenous exergy destruction within the kth component
EES Engineering equation solver software
E ̇_(D,COND) Exergy destruction in the condenser
E ̇_(D,valveII) Exergy destruction in valve ii
E ̇_(D,HEX) Exergy destruction in the condenser
E ̇_(D,PUMP) Exergy destruction in the condenser
E ̇_(D,TURB) Exergy destruction in turbine
E ̇_(D,k) Exergy destruction within the kth component
E ̇_(D,others) Exergy destruction in other components
ψ_HEX Exergetic efficiency for heat exchanger
ψ_COND Exergetic efficiency for condenser
E ̇_WT Exergy of turbine work
E ̇_WP Exergy of pump work.
GT Gas turbine
NSGA-II Non-dominated sorting genetic algorithm
ORC Organic rankine cycle
PCMs Phase change materials
SOFC Solid oxide fuel cell
SPECO Specific exergy costing
S/B Steam/biomass mass ratio
E ̇_(F,tot)^ID Total exergy of fuel for ideal system
E ̇_(L,tot)^ID Total exergy loss for ideal system
E ̇_(P,tot) Total exergy of product
VCC Vapour compression cycle
ϕ_(k .) Maintenance factor for each plant component
Ζ ̇. Component cost rate
c_f Cost per unit exergy of fuel
E ̇_f Exergy stream of fuel
N Expected life of a plant component in years;
c_p Cost per unit exergy of product
E ̇_p Exergy stream of product
C ̇_(〖W,〗_ORCP ). The cost of the work done to power the ORC pumps
C ̇_K annual levelised cost
PWF Present worth factor of capital investment
PEC Purchase of equipment cost
SV Salvage value
i Interest rate
CRF Capital recovery factor
LMTD Logarithmic mean temperature difference
A_HE Heat Transfer
U_HE. Heat transfer coefficient
E ̇_(D,VI) Exergy destruction in valve I
W_out Work output
Q_in Heat input
〖ex〗_in Exergy influx
T0 Ambient Temperature
〖ex〗_out Exergy Efflux
P Pressure
P0 Ambient pressure
T Temperature
T0 Ambient temperature
R Universal gas constant
W_P Pump work
Q_evap Evaporator Heat
〖H_f^0〗_wood Heat of formation of wood biomass
∆h Enthalpy Change
∆s Entropy change
E ̇_WT Exergy of turbine work
η_Pump Pump efficiency
m ̇_ Mass flow rate
η_(Therm ) Thermal efficiency
W_GT Turbine Work
Q_cond. Condenser heat
E ̇_(D,TURB) Exergy destruction in turbine
ψ_TURB Exergetic efficiency for the turbine
ψ_VG Exergetic efficiency for the vapour generator
c_p Specific heat capacity at constant pressure
CHAPTER 1
INTRODUCTION
1.1 BACKGROUND OF STUDY
Due to the finite nature of energy resources, concentrated efforts are continuously made to improve on the conversion efficiency of energy systems. Over time, the second law of thermodynamics which culminates in the study of exergy has been used. Exergy analysis is an imperative concept which quantifies the maximum work an energy conversion system can do with respect to the environment where it is resident. It quantifies and recognizes points in a system where thermodynamic inefficiencies are present and offers practical ways to reduce them. Furthermore, exergy analysis can aid the identification of the sources, magnitude and location of the thermodynamic inefficiencies of a system, which can give appropriate information for improving the overall efficiency of the system while focusing in the worst exergy balance elements (Galindo et al., 2016).
Conventional exergy analysis has been studied extensively and applied to some applications (Wang et al., 2016; Fallah et al., 2016; Ehyaei et al., 2012; Özkan et al., 2013). Nevertheless, the conventional exergy analysis is used in the evaluation of the performance of an individual component in a large system, without taking into account interactions among other components. Following this apparent drawback, the concept of advanced exergy analysis was proposed to evaluate energy conversion systems via splitting the exergy into endogenous/exogenous and avoidable/unavoidable parts (Morosuk et al., 2013). By means of the splitting of exergy into unavoidable and avoidable parts, a measure of the potential of improving the efficiency of each component of any considered system can be enhanced. Additionally, splitting component exergy into endogenous and exogenous parts provides information between interactions among several components of a system.
A plethora of research using advanced exergy analysis has been applied to different energy conversion systems. Balli (2017) presented a conventional and advanced exergy analysis of a military aircraft turbojet engine. The key exergy parameters of the engine components were introduced while the exergy destruction rates in the engine components were fragmented into endogenous/exogenous and avoidable/unavoidable parts. Vuckovic et al. (2015) presented the results of an advanced exergy analysis for a real complex industrial plant, with emphasis on the main component from the energy efficiency point of view. Fallah et al. (2016) performed advanced exergy analysis on a Kalina cycle used for enhancing low temperature geothermal system. Wang et al. (2016) performed advanced and conventional exergy analyses of an underwater compressed air energy storage system. Mehrpooya et al, (2016) presented advanced exergy and energy analyses of a hydrocarbon recovery process. In gas turbine systems, Morosuk and Tsatsaronis (2009) presented advanced exergy analysis for chemically reacting systems applied to a simple open gas-turbine system, Söhret et al. (2015) presented advanced exergy analysis of an aircraft gas turbine engine by splitting exergy destructions into endogenous and exogenous parts; inpower plants Petrakopoulou et al. (2012) presented conventional and advanced exergetic analyses applied to a combined cycle power plant, while Wang et al. (2012) did advanced thermodynamic analysis and evaluation of a supercritical power plant. In ORC systems, advanced exergy analysis applied to it, is relatively scanty. Galindo et al. (2016) presented an advanced exergy analysis applied to a bottoming organic rankine cycle attached to an internal combustion engine, also, Nami et al. (2017) performed conventional and advanced exergy analyses of a geothermal driven dual fluid Organic Rankine Cycle (ORC). To the best of my knowledge, advanced exergy analysis applied to biomass fired ORC systems is not in the open domain. Consequently, the current research contributes to knowledge by performing advanced exergy analysis applied to a novel unified biomass-based tri-generation energy system.
1.2 STATEMENT OF PROBLEM
The most common method of biomass energy conversion is direct combustion with coal and provides the ultimate potential for large-scale utilization of biomass energy in the future. Other thermochemical conversion technologies like gasification and pyrolysis are technically feasible and marginally efficient, compared to combustion, for power generation. However, most of these biomass technologies, although potentially feasible, are not yet cost competitive and efficient. Consequently, biomass is not the most favourable options at present. However, increasing political and environmental concerns have impelled the development and utilization of renewable energy. This has resulted to an increase in both the demand and cost of biomass resources. Surely, the paradigm has shifted towards more effective conversion and efficient utilization of biomass resources. With the incorporation of biomass-based ORC applications, effective energy conversion can be achieved. This need is met in this study by the development of a novel biomass-based ORC configuration which can produce power, heating and cooling. The thermodynamic model which demonstrates the intended high energy and exergy efficiency of the proposed system will be conducted using a number of refrigerants. Additionally, advanced exergy methods will be applied to find the interaction of the system components with respect to exergy efficiencies. The results will add to existing body of literature in the application of biomass energy source in power generation, heating and cooling with an even simpler configuration.
1.3 AIM AND OBJECTIVES OF STUDY
The aim of this study is to perform an advanced exergy analysis of a biomass-based tri-generation energy system:
The specific objectives are as follows:
i. Development of thermodynamic modelling and simulation of the proposed system using both energy and exergy approaches.
ii. To quantify the impact of improvements in each component of the system to the general performance of the system using advanced exergy analysis method.
iii. To carry out an economic analysis of the tri-generation energy system.
1.4 SCOPE OF STUDY
The scope of this study includes:
i. Development of all necessary thermodynamic models derived from the first and second laws of thermodynamics.
ii. Analysis of exergy destruction in each component of the system. Furthermore, through the application of advanced exergy analysis, all exergy destruction will be split into endogenous and exogenous parts.
iii. Determination of the exergoeconomic analysis of the system.
iv. Determination of the optimum operating parameters of the system.
1.5 JUSTIFICATION FOR THE STUDY
Different energy sources are available for powering energy conversion systems operating on the Organic Rankine Cycle (ORC). For all energy sources available, the general objective has always been the development of relatively simple energy system with reasonably high performance. In literature a number of such studies are handy.
However, it is not always easy to obtain a robust configuration with good energy and exergy efficiency since the ORC operates at moderate efficiencies lower than the Brayton cycle. To remedy this drawback, several studies are reported in literature. Ahmadi et al. (2013) developed a novel multi-generation system based on a biomass combustor, an organic rankine cycle (ORC), an absorption chiller and a proton exchange membrane electrolyzer to produce hydrogen, and a water heater to produce water for homes. The energy condition of the heat source which powers the vapour absorption system is a comparatively weak refrigerant stream after expansion in the turbine and condensation in the condenser. Consequently, the amount of cooling is severely reduced, resulting in minimal exergy efficiency of the plant at 22 %. To remedy this shortcoming and augment the overall system performance, the proposed cycle incorporates turbine bleeding at a high energy level for cooling, in addition to whole power generation in the turbine as well as heating. Additionally, as contained in Kanu (2017), the details for powering a novel ORC configuration was only limited to specified energy states and quanta which can come from a multiplicity of energy sources. Here, a specified and properly modelled energy source which powers such novel arrangement is proposed in biomass. This will not only enhance efficiency but will result in environmental sustainability consequent upon the utilization of biomass for its operation. This particular enhanced configuration is not in open literature and hence, justifies this research work. Additionally, in ORC systems, advanced exergy analysis applied to it, is relatively scanty. Galindo et al. (2016) presented advanced exergy analysis for a bottoming organic rankine cycle coupled to an internal combustion engine, while very recently, Nami et al. (2017) performed conventional and advanced exergy analyses of a geothermal driven dual fluid organic Rankine cycle (ORC). To the best of my knowledge, advanced exergy analysis applied to biomass fired ORC systems is not in the open domain. Consequently, the study contributes to knowledge by performing advanced exergy analysis applied to a unified biomass-based tri-generation energy system.
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