ABSTARCT
The study investigated the thermodynamic and thermo-environmental analysis of a solar and biomass based trigeneration energy system for power, cooling, and domestic hot water production. The system is powered by municipal waste burnt in an incinerator, and a solar tower for an embedded gas turbine with natural gas as a supplementary fuel. It comprises a simple organic Rankine cycle (ORC) unit for power production, a vapour absorption system (VAS) for refrigeration, and a gas turbine unit energised by a solar tower and minimal supply of natural gas. The objectives were to model and analyse the system from a thermodynamic, exergoeconomic and thermo-environmental perspective. Accordingly, several thermo-environmental indicators were considered including the exergetic utility index (EUI), exergo thermal index (ETI), waste exergy ratio (WER), and sustainability index (SI). The system was simulated using a developed code written in Engineering Equation Solver (EES). The results show that the systems net output, cooling capacity, and the quantity of cooling water is 6.128 MW, 131.1 kW, and, 65.14 kg/s (at 75 oC) respectively. Also, the energy and exergy efficiencies were observed as 66.68 % and 53 %, respectively. Furthermore, the solar tower contributes to reduction of natural gas for firing the system at 633.6 kg/h thereby aiding reduction of carbon emissions to the environment. In terms of exergy destruction, the incinerator and combustion chamber had the highest share of the total in the system. The exergoeconomic analysis suggests little reduction in the investment costs of the gas turbine air compressor, and the incinerator which has nearly hundred per cent exergoeconomic factors. Lastly, the thermo-environmental parameters: EUI, ETI, WER, and SI were recorded as 0.6992, 0.9161, 1.092, and 0.6109, respectively for the entire system. This demonstrates the environmental friendless of the system due to very high ETI stemming from almost complete utilization of the energy stream with minimal discharge temperature to the atmosphere.
TABLE OF CONTENTS
Cover page i
Title page ii
Declaration iii
Dedication iv
Certification v
Acknowledgements vi
Table of contents vii
List of tables ix
List of figures x
Nomenclature xii
Abstract xv
CHAPTER 1: INTRODUCTION 1
1.1 Background to the study 1
1.2 Statement of study 3
1.3 Aim and objectives of study 4
1.4 Scope of study 5
1.5 Justification of the study 5
CHAPTER 2: LITERATURE REVIEW 8
2.1 Trigeneration and multigeneration energy systems 8
2.2 Thermoeconomic and optimization of multigeneration systems 20
2.3 Environmental impact of multigeneration energy systems 26
2.4 Knowledge gap 28
CHAPTER 3: MATERIALS AND METHOD 30
3.1 Materials 30
3.1.1 Working fluid 30
3.2 Methods 31
3.2.1 System description 31
3.2.2 Thermodynamic assumptions 32
3.2.3 System modelling 34
3.2.4 Energy and exergy modelling of the ORC vapour generator 38
3.2.5 Energy and exergy modelling of the ORC turbine 41
3.2.6 Energy and exergy modelling of the ORC condenser 41
3.2.7 Energy and exergy modelling of the ORC pump 42
3.2.8 Environmental analysis of the trigeneration energy system 45
3.2.9 Exergo-economic modelling of the system 48
CHAPTER 4: RESULTS AND DISCUSSION 55
4.1 Thermodynamic simulation of the developed system using both energy and exergy 55
4.1.1 Effect of expansion ratio on turbine back pressure and condenser cooling water flow rate 57
4.1.2 Effect of pinch point temperature on quantity of cooling water and VG stack temperature 58
4.1.3 Effect of circulation ratio on evaporator cooling rate 59
4.1.4 Effect of VAS evaporator temperature on compressor temperature inlet and work 60
4.1.5 Effect of solar irradiance on fuel flow rate 61
4.2 Exergoeconomic analysis of the system 62
4.2.1 Summary of initial investment, monetary flow rate, and levelised capital cost of the system 63
4.2.2 Exergoeconomic parameters of system components 64
4.3 Results from thermo-enviroeconomic impact modelling of the system 66
4.4 Exergo-environmental sustainability of the system 69
4.5 Result validation 72
CHAPTER 5: CONCLUSION AND RECOMMENDATIONS 73
5.1 Conclusion 73
5.1.1 Contribution to knowledge 74
5.2 Recommendations 75
REFERENCES 76
LIST OF TABLES
3.1 Other Design Assumptions of the Plant 33
3.2 Summary of Energy and Exergy Balances 44
3.3 Values for Parameters in Equations (3.62 – 3.65) 46
3.4 Summary of Component Cost and Auxiliary Equations 50
3.5 Summary of Component Cost Functions, Cost of Product and
Cost Fuel 51
4.1 Thermodynamic Data at the State Points 56
4.2 Components Exergy of Fuel, Product, Destruction and Efficiency 57
4.3 Summary of Performance Indices of the Plant 57
4.4 Summary of Initial Investment, Monetary Flow Rate and Levelised Capital Cost Rate 63
4.5 Summary of Exergoeconomic Parameters of the Plant 65
4.6 Result Validation 72
LISTS OF FIGURES
2.1 Trigeneration Plant with Combined SOFC and Organic Rankine Cycle 9
2.2 Proposed Multigeneration Energy System 9
2.3 Solar and Wind Energy Based Multigeneration System 10
2.4 Schematic of the Multi-generation System for the Provision of Heating, Cooling, Electricity, Hydrogen and Hot Water 11
2.5 Sketch of the Developed Multigeneration System 12
2.6 Schematic Diagram of the Solar and Geothermal Energy 12
2.7 Schematic Layout of Tri-generation System Based on the Integration of Power, Cooling and Desalination Thermal Cycles 13
2.8 Schematic Diagram of Solar Based Integrated System for
Multigeneration including Thermoelectric Generator 14
2.9 Schematic for the Geothermal Solar Multigeneration System 15
2.10 Sketch of the Small Trigeneration Plant Supplied by Geothermal and Solar Energies 16
2.11 Schematic Diagram of the Biomass and Solar Energy Integrated Cycle for Multigeneration 16
2.12 Solar Driven Trigeneration System 17
2.13 Schematic of the Multi-generation System 18
2.14 Schematic of the CCHP System 19
2.15 Schematic Diagram of the Solar Multi-generation Energy Production System 20
2.16 Schematic Diagram of Combined Power Plant with Triple-Pressure Levels and one Reheating Stage 21
2.17 Multigeneration Energy System 23
2.18 Schematic of the Polygeneration System for Heating, Cooling, Hot Water and Electricity Generation 24
2.19 Sketch of the Small Trigeneration Plant Supplied by Geothermal and Solar Energies 25
2.20 Solar Driven Trigeneration System 25
2.21 Schematic Diagram of an Integrated Gas, Steam, and Organic Fluid Turbine 27
2.22 Integrated Multi-generation Plant 28
3.1 Schematic Diagram of the Novel Solar Based Trigeneration Energy System 31
4.1 Relationship between Expansion Ratio on Turbine Back Pressure and Condenser Cooling Water Flow Rate 58
4.2 Effect of Pinch Point Temperature on Quantity of Hot Water
VG Stack Temperature 59
4.3 Circulation Ratio Effect on Cooling Rate in the VAS 60
4.4 Effect of VAS Evaporator Temperature on Compressor Work and Temperature Inlet 61
4.5 Relationship between Solar Energy from Tower and Mass Flow Rate of Fuel 62
4.6 Effect of Primary Zone Temperature on Emissions 67
4.7 Effect of Critically High Primary Zone Temperature on Emissions 68
4.8 Effect of Combustion Chamber Inlet Pressure on Emissions 68
4.9 Effect of TIT on Emissions 69
4.10 Exergetic Sustainability Indicators of the System 70
4.11 Effect of GT Overall Pressure Ratio on System Exergetic Sustainability 71
NOMENCLATURE
AC Cross sectional area of glass cover (m2)
ALCC Annualized life cycle cost ($/yr)
BEP Break-even point
Ċ Cost ($)
CRF Capital recovery factor,
Cp Specific heat at capacity (J/kgK)
cv Control volume
d inflation rate
e Exit
Ė Exergy (kW)
ESI Exergetic sustainability index
ETI Exergo thermal index
EUI Exergetic utility index
Es Daily energy demand (kWh/day)
Exd Exergy destruction rate
f Fuel
f Exergoeconomic factor
F_ef Fuel effect factor
H_f^0 Enthalpy of formation (kJ/mol)
g Gravitational force (m/s2)
∆G° Gibbs function of formation
h Specific enthalpy (kJ/kg)
i Input
I Solar radiation in ((W)⁄m^2 )
LCC Life cycle cost ($)
m amount of oxygen per kmol of wood
m ̇ Mass flow (kg/s)
MC moisture content
n Estimated plant life
N Annual operational hours
o Output
p Product
P Pressure (bar)
PECF Purchase of equipment cost function
Q ̇ Heat (KJ)
QAP Annual energy production (kWh/yr)
R Universal gas constant
s Specific entropy ((kJ/kg.K)
SI Sustainability index
T Temperature (K)
T_gasif Gasification temperature (K)
T_pz Primary zone combustion temperature (K)
UEC_MC Cost of conventional electricity supply ($/kWh)
UCOE Unit cost of electricity ($)
v Velocity (m/s)
w amount of water per kmol of wood
W ̇ Work done (KJ)
WER Waste exergy ratio
Wplant Plant Capacity (KW)
YD Exergy destruction ratio
Ż Levelised cost ($)
α Absorptivity of blackened surface
ψ Exergy efficiency (%)
ρ^' Reflectivity of mirror
τ Transmissivity of glass cover
τ Residence time in combustion zone
I Thermal pollution factor
Υ Enviro-thermal conservation factor
ϕ Maintenance factor
T_0 Dead state Temperature
H_DC Heat of the drying chamber
U_∞ Free convective heat transfer coefficient of the ambient air
A_SD Flat surface area of the solar dryer
T_OS Outer surface temperature of the dryer
CHAPTER 1
INTRODUCTION
1.1 BACKGROUND TO THE STUDY
Energy availability and sustainability is an extremely important index in defining the wellbeing of any developed society. It is linked directly with the economic prospect and dynamism of both developed and developing economies of the world (Chen et al., 2020). Because energy sustainability is population dependent, its availability must be sustained and continuously augmented to cater for the needs of the people while also propelling industrialisation. Nonetheless, most of the resources for energy conversion are finite, and continuously involve the burning of fossil fuel which pollute the environment (Johnson et al., 2018; Olivier et al., 2017; Rockström et al., 2017).
Hence, to augment the size of general energy needs while maintaining environmental friendliness, it is pertinent to consider renewable energy resources other than the conventional non-renewable sources which account for about 65 % of present power generation worldwide (Nikhil and Rajesh, 2015). More efficient methods and technologies are required to increase the efficiency and products from existing non-renewable based energy systems which occupy a large portion of global power generation. In this regard, tri- and multi-generation energy systems have been proposed for generating several distinct products using a singular energy source at considerably improved efficiencies. Several configurations of these smart and robust systems have been researched in literature in terms of thermodynamics of operation, economic implications (both thermoeconomic and exergoeconomics), optimization as well as environmental impact assessment.
Although few of these robust systems are non-renewable energy based (Oko and Njoku, 2017; Ahmadi et al., 2013a), most are relatively based on solar renewable energy which is grossly intermittent (Manente, 2016; Hassoun and Dincer, 2015; Suleman et al., 2014; Bellos and Tzivanidis, 2017; Sharifishourabi et al., 2016; Al-Sulaiman et al., 2012; Ozturk and Dincer, 2013). Furthermore, because of the intermittent nature and variation of intensity of most renewable energy sources, like solar irradiance and wind velocity, the multigeneration systems based on these sources have relatively low efficiency and are unavailable during low solar radiation granted, very smart solar powered renewable energy systems have shown relatively high energy and exergy efficiencies and the gross sum of outputs have significantly increased. For instance, Al-Sulaiman et al. (2012) demonstrated a solar driven energy system which produced cooling, power, and heating with an efficiency of about 29.5 %. Similarly, Sharifishourabi et al. (2016) presented a solar powered energy system for power generation, hydrogen production, cooling and drying with an estimated energy and exergy efficiency of 70 % and 53 %, in that order.
In a related study, nanofluids were used to improve the overall performance of a solar powered energy system with only electricity and cooling as products (Bellos and Tzivanidis, 2017). Similar studies of solar powered trigeneration and multigeneration energy systems with improved energy efficiency and number of products are found in recent literature (Ozturk and Dincer, 2013; Buonomano et al., 2015; Al-Ali and Dincer, 2014; Islam et al., 2015). However, because these systems are only solar-powered, their continuous operation is not feasible unless there is provision for energy storage which will add additional cost and adjustment in system design. To remedy this drawback, an additional renewable energy source is required which can be obtained in biomass. This arrangement has been investigated by Khalid et al. (2015) who proposed a novel biomass/solar multigeneration system for power production, cooling, and heating. Although the exergy efficiency of the system was as high as 39.7%, the use of waste biomass flue gas from a turbine to drive three additional turbines is not thermodynamically sustainable due to the low calorific value of wet biomass. Preheating the biomass in a separate unit will greatly enhance the energy content of the exiting syngas since biomass calorific values are directly related with the moisture content (Koroneos and Lykidou, 2011). This consideration of dried municipal waste/solar powered renewable multigeneration energy system is not in the open domain. Therefore, to ensure a sustainable driven solar/biomass fired system, this study proposes the use of municipal waste and solar in a novel energy configuration for power production, cooling and heating. The system is designed to use solar energy as a backup energy resource in a gas turbine so as to minimise the use of natural gas, while municipal waste is dried to fire both an organic rankine cycle and a vapour absorption chiller for power production and cooling, respectively.
1.2 STATEMENT OF PROBLEM
The increasing demand for sustainable energy from available natural resources by current energy conversion technologies and the concern for the impact on the environment due to emission, waste disposal and signs of global warming have brought about the creation of new disciplines that help to understand how to improve the design and operation of energy systems and prevent residues from damaging the environment. Solid waste to energy plants, thermal power plants, and chemical plants are examples of energy systems formed from a set of subsystems or processes. These systems interact with their environment consuming some external resources, which are then transformed into products. The final purpose of this transformation is to increase the economic utility. The production process of a complex energy system can be analysed in terms of its economic profitability and efficiency with respect to resource consumption.
Solid waste conversion to power involves putting up subsystems and processes together which is an expensive technology because of the initial capital costs as well as operation and maintenance costs. Although controlled waste incineration is not a new technology, it has not been used extensively in Nigeria. To reduce the costs of these facilities, heat from the burning waste is used to generate steam and electricity for additional revenue (Owebor et al., 2019). Proper thermoeconomic analysis which applies the concept of cost, originally an economic property, to exergy, is required to determine the cost of fuel, investment, operation and maintenance for the total plant or even individual components. The determination of the efficiency of the individual process of the plant, and the allocation as well as quantification of the irreversibilities done in thermodynamic analysis of the plant, are key elements that determine the viability of a plant.
Thermoeconomic analysis combines economic and thermodynamic analysis by applying the concept of cost, originally an economic property, to exergy. Accordingly, the present research seeks to apply the concept of thermoeconomic and thermodynamic analysis to investigate the prospect of using municipal waste in powering a novel energy system for power generation, cooling, water heating, and drying.
1.3 AIM AND OBJECTIVES OF STUDY
The aim of research is to carry out a thermodynamic and thermo-environmental analysis of a solar and biomass based trigeneration energy system for heating, cooling and power generation. The specific objectives include:
- Thermodynamic modelling and simulation of the developed system using both energy and exergy approaches.
- Determination of the thermoeconomic cost of the system.
- Assessment of the environmental impact of the system using thermo-environmental indicators.
- To perform sensitivity analysis of some of the system parameters.
1.4 SCOPE OF STUDY
The scope of the study is centred on the following areas:
- Thermodynamic modelling and simulation of the developed system using an eco-friendly refrigerant.
- Thermoeconomic analysis of the system and quantification of the operating parameters for optimum cost.
- Thermo-environmental impact analysis of the system using thermo-environmental indicators.
1.5 JUSTIFICATION OF THE STUDY
The past decade has witnessed the proliferation of tri- and multi-generation energy systems for generating several products from single energy resource. This arrangement has greatly enhanced the energy conversion process in terms of efficiency and output, and promises a large of environmental sustainability since few of these robust systems are non-renewable energy based (Oko and Njoku, 2017; Ahmadi et al., 2013a). Interestingly, most of these systems are based on solar renewable energy which can be grossly intermittent depending on the prevailing local weather conditions (Manente, 2016; Hassoun and Dincer, 2015; Suleman et al., 2014; Bellos and Tzivanidis, 2017; Sharifishourabi et al., 2016; Al-Sulaiman et al., 2012; Ozturk and Dincer, 2013). Due to the intermittent nature and variation of intensity of most renewable energy sources, like solar irradiance and wind velocity, the multigeneration systems based on these sources have relatively low efficiency and are unavailable during low solar radiation. Granted, very smart solar powered renewable energy systems have shown relatively high energy and exergy efficiencies and the gross sum of output have significantly increased. For instance, Al-Sulaiman et al., (2012) demonstrated a solar driven energy system which produced cooling, power, and heating with an efficiency of about 29.5 %.
Similarly, Sharifishourabi et al., (2016) presented a solar powered energy system for power generation, hydrogen production, cooling and drying with an estimated energy and exergy efficiency of 70 % and 53 %, in that order. In a related study, nanofluids were used to improve the overall performance of a solar powered energy system with only electricity and cooling as products (Bellos and Tzivanidis, 2017). Similar studies of solar powered trigeneration and multigeneration energy systems with improved energy efficiency and number of products are found in recent literature (Ozturk and Dincer, 2013; Buonomano et al., 2015; Al-Ali and Dincer, 2014; Islam et al., 2015). However, because these systems are only solar powered, their continuous operation are not feasible unless there is provision for energy storage which will add additional cost and adjustment in system design. To remedy this drawback, an additional renewable energy source is required which can be obtained in biomass. This arrangement has been investigated by Khalid et al. (2015) who proposed a novel biomass/solar multigeneration system for power production, cooling, and heating where biomass is directly fed to the combustion chamber. This direct addition severely limits the heating values of the biomass due to high moisture content. Although the exergy efficiency of the system were as high as 39.7%, the use of waste biomass flue gas from a turbine to drive three additional turbines is not thermodynamically sustainable due to the low calorific value of biomass.
To the best of the author’s knowledge, the combination of municipal waste (preheated before combustion) and solar in a trigeneration renewable energy source is not in the open domain. Therefore, to ensure a sustainable driven solar/biomass fired system, this study proposes the use of municipal waste and solar in a novel energy configuration for power production, cooling and heating. The system is designed to use solar energy as a backup energy resource in a gas turbine so as to minimise the use of natural gas, while municipal waste is dried to fire both an organic rankine cycle and a vapour absorption chiller for power production and cooling, respectively.
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