Licenciate Thesis in Energy Technology
The Stirling engine as a part of a hybrid power system
A study of applications in rural areas of Bolivia
LUIS ANTONIO CHOQUE CAMPERO
ISBN 978-91-7873-876-2 TRITA-ITM-AVL 2021:21
NT ON IO C HO QU E C AM PE RO
The Stirling engine as a part of a hybrid power systemK
hybrid power system
A study of applications in rural areas of Bolivia
LUIS ANTONIO CHOQUE CAMPERO
Licenciate Thesis in Energy Technology KTH Royal Institute of Technology Stockholm, Sweden 2021
Academic Dissertation which, with due permission of the KTH Royal Institute of Technology,
is submitted for public defence for the Degree of Licentiate of Engineering on Tuesday 25th May,
at 13:00
TRITA-ITM-AVL 2021:21
Printed by: Universitetsservice US-AB, Sweden 2021
Abstract
Rural electrification in developing countries has become one of the greatest challenges for achieving global access to electricity—one of the United Nation’s sustainable development goals. Governments, international entities and private companies are tasked with improving the quality of life for people and reducing environmentally harmful emissions. Bolivia’s political agenda has been working in coordination with international cooperation organizations, and it has achieved great improvements in access to electricity in recent years. Different strategies and technologies have been used in the various climate scenarios that span Bolivia’s territory. Although more Bolivians have access to electricity than 10 years ago, insufficient knowledge, training, and follow-up from local and national actors (such as power producers, power distributors, and electricity service providers) prevent these solutions from operating as expected.
This study explores the integration of a Stirling engine into a small power production system for use in remote rural areas. The Stirling engine is a well- known technology that can use local fuels to generate power and heat. Here two different hybrid power systems in three case studies are compared: the first system is using photovoltaic (PV) panels, batteries, and diesel engines and the second is using PV panels, batteries, and Stirling engines. In a sustainability analysis the environmental effects, economy, and performances, efficiency and reliability, of the two systems are compared. In addition, the study discusses the maintenance of the Stirling engine in Bolivia rural conditions.
The study began by gathering data from 17 households in different communities, which had just obtained access to electricity. These communities are characterized by different environmental and climate conditions, which allows us to better understand how the systems operate under Bolivia’s varying climate and to consider the state of its economy and technical capacity. With the help of GIS (Geographical Information System) maps, three Bolivian communities were selected: Tirina, Tablani, and El Carmen. Six hybrid power system were simulated for these communities, two dynamic models per community.
The comparison between the two systems shows that Stirling engine hybrid
power system produces at least 7 Tons per year less CO
2emissions than the
Diesel hybrid power system per community. The financial analysis used the
levelized cost of electricity (LCOE) to show the two systems’ cost per kilowatt-hour (in USD). The LCOE of the Stirling system is higher than the diesel engine in the three communities. The net present value was calculated to reflect the costs of the initial investment, as well as maintenance, spare parts, and so on, over the duration of the study. Finally, performance of the two systems was analyzed through a simulated one-day dynamic test of both systems in the three communities. The two systems responded without problem to the communities’ power demands. These power demands have peaks between about 5 kW and 7 kW.
Keywords: Bolivia, hybrid power system, Stirling engine, rural electrification,
dynamic systems, levelized cost of electricity.
Sammanfattning
Landsbygdselektrifiering i utvecklingsländer har blivit en av de största utmaningarna för global tillgång till el - ett av FN: s mål för hållbar utveckling.
Regeringar, internationella enheter och privata företag står inför utmaningen att förbättra livskvaliteten för människor och minska miljöfarliga utsläpp.
Bolivias politiska agenda har handlat om att arbeta samordnat med internationella samarbetsorganisationer och har uppnått stora förbättringar avseende tillgången till el de senaste åren. Olika strategier och tekniker har använts i de olika klimatscenarierna som spänner över Bolivias territorium.
Även om fler bolivianer har tillgång till el än för tio år sedan, förhindrar otillräcklig kunskap, utbildning och uppföljning från lokala och nationella aktörer (som kraftproducenter, kraftdistributörer och elsleverantörer) att dessa lösningar fungerar som förväntat.
Denna studie undersöker integrationen av en Stirling-motor i ett litet kraftproduktionssystem för användning i avlägsna landsbygdsområden.
Stirling-motorn är en välkänd teknik som kan använda lokala bränslen för att generera energitjänster. Här jämförs två olika hybridsystem i tre fallstudier:
det första systemet har solcellspaneler (PV), batterier och dieselmotorer [1]
och det andra har solcellspaneler, batterier och Stirling-motorer. I en hållbarhetsanalys jämförs de två systemens miljöeffekter, kostnader och prestanda - effektivitet och tillförlitlighet. Dessutom diskuterar studien underhållet av Stirling-motorn på landsbygden i Bolivia.
Studien började med insamling av data från 17 hushåll i olika samhällen, som just hade fått tillgång till el,. Dessa samhällen kännetecknas av olika miljö- och klimatförhållanden, vilket gör det möjligt för oss att bättre förstå hur systemen fungerar under Bolivias varierande klimat och att ta hänsyn till läget för dess ekonomi och tekniska kapacitet. Med hjälp av GIS-kartor (Geographical Information System) valdes tre bolivianska samhällen ut: Tirina, Tablani och El Carmen. Sex hybridsystem simulerades för dessa samhällen, två dynamiska modeller per samhälle.
Jämförelsen mellan de två systemen visar att Stirling-motorns
hybridkraftsystem producerar minst 7 ton per år mindre koldioxidutsläpp än
Diesel-hybridkraftsystemet för respektive samhälle. I den finansiella analysen
användes ”levelized cost” för el (LCOE) för att visa de två systemens kostnad
per kilowattimme (i USD). LCOE för Stirling-systemet är högre än
dieselmotorn i de tre samhällena. Nettonuvärdet beräknades för att återspegla
kostnaderna för den initiala investeringen, såväl som underhåll, reservdelar
med mera under studiens varaktighet. Slutligen analyserades de två systemens
prestanda genom ett simulerat dynamiskt test omfattande en dag för
respektive system i de tre samhällena. De två systemen uppfyllde då utan
problem samhällenas effektbehov. Dessa effektbehov har toppar mellan 5
kW och 7 kW.
Acknowledgements
I would like to thank my main supervisor, Prof. Per Lundqvist, for his wise guidance, critical review, and consistently good spirits on my research. I would also want to thank Dr. Lucio Alejo for paving the way for students to follow the research path, starting the sandwich modality in cooperation with Swedish International Development Agency and laying the foundation for energy research labs at UMSS. He gave me the opportunity to participate in the doctoral sandwich modality. This task has continued with Dr. Evelyn Cardozo. She has a key participation in this thesis with her engagement, continuous supervision and trust placed in me. Her strength of will, her enthusiasm and her hope are bringing new and important opportunities to this field of research in Bolivia especially to the UMSS with a new energy center. I greatly appreciate my co-supervisor Dr. Anders Malmquist for his cooperation, support, understanding and patience. His advice, appropriate questions and review comments helped me to finish this thesis. I would like to acknowledge Dr. Jaime Arias Hurtado and Dr. Adhemar Araoz. Their support and help to the work were crucial. I would also want to thank Dr.
Samer Sawalha, Dr. Hatef Madani and Prof. Björn Palm for the support and faith shown in me
I recognize the financial support of the Swedish International Development Agency (SIDA), the Division of Applied Thermodynamics and Refrigeration of the Department of Energy Technology in Royal Institute of Technology and the Energy Department at San Simon University.
It is pleasure to thank all my friends in Stockholm for the support during this time. Their advice support and help made the long journeys went smoothly.
Finally, I owe my deepest gratitude to my family. My mom, Willma, always
showing me the way to improve myself. My dad, Luis, teaching me the
hardworking spirit to help others and my sister, Andrea, cheering me up to
continue. A special thanks to my Uncle Raul; he was the one who introduced
me to the world of engineering
Content
ABSTRACT ... I SAMMANFATTNING ... III ACKNOWLEDGEMENTS ... V CONTENT ... VII LIST OF FIGURES ... XI LIST OF TABLES... XV NOMENCLATURE ... XVII
1 INTRODUCTION: BACKGROUND AND OBJECTIVES ... 1
1.1 Objectives ... 3
1.2 Methodology ... 3
1.3 Structure of the thesis ... 6
1.4 Limitations ... 7
2 THEORETICAL FRAMEWORK: THE STIRLING ENGINE AND HYBRID SYSTEMS ... 9
2.1 The Stirling engine ... 9
2.1.1 Technology Overview: History of the Stirling engine ...9
2.1.2 Configurations ... 12
2.1.3 Advantages and disadvantages of different mechanical configurations ... 17
2.2 Hybrid Power Systems ... 19
2.2 Simulating a Stirling engine hybrid system using HOMER ... 20
2.2.1 Control Strategies ... 21
2.2.2 Components of a hybrid system ... 22
3 BOLIVIA’S ELECTRICITY DEMAND ... 25
3.1 Bolivia ... 25
3.2 Location and population ... 25
3.2.1 Climate ... 26
3.2.2 Economy ... 27
3.2.3 Policies: Bolivia’s Energy and Electricity Situation ... 27
3.2.4 Renewable energy potential ... 30
3.2.5 Renewable energy projects... 31
3.3 Electric power demand in rural Bolivia ... 34
3.3.1 Selected communities: Zones ... 35
3.3.2 Electricity consumption ... 41
3.3.3 Electricity demand profile of the communities ... 45
3.4 Concluding remarks ... 53
4 SELECTION OF THE STIRLING ENGINE AND THE HYBRID POWER SYSTEM COMPONENTS ... 55
4.1 Introduction ... 55
4.1 Stirling Engine ... 56
4.1.1 Stirling engine fuels ... 56
4.1.2 Commercial Stirling engines ... 57
4.1.3 Stirling engine requirements for hybrid systems in rural areas 58 4.1.4 Overhauling and maintenance of the Genoa 01 Stirling engine 60 4.1.5 Concluding remarks ... 61
4.2 Hybrid Power Systems Components ... 62
4.2.1 The Stirling hybrid system ... 62
4.2.2 Diesel system ... 65
4.2.3 Configuration of the Stirling hybrid system ... 66
4.2.4 Environmental, economic, and performance factors: Sustainable analysis ... 66
4.2.5 Environmental ... 68
4.2.6 Economic ... 68
4.2.7 Performance of the system – Dynamic modelling of the components ... 69
4.2.8 Control Strategy ... 76
4.3 Concluding Remarks ... 78
5 RESULTS ... 79
5.1 System configuration ... 79
5.2 Sustainability analysis ... 81
5.2.1 Environmental analysis ... 81
5.2.2 Economic analysis... 82
5.2.3 Electricity performance ... 85
5.3 Concluding remarks ... 92
6 DISCUSSION ... 95
6.1 Bolivia: - General aspect to take into account ... 95
6.2 The Stirling engine ... 96
6.3 Hybrid system ... 96
6.4 Economic analysis ... 97
6.5 Environmental analysis ... 98
6.6 Performance analysis ... 98
7 CONCLUSIONS AND FUTURE WORK ... 101
7.1 Conclusion ... 101
7.2 Future Research ... 102
8 REFERENCES ... 105
ANNEX 1 ... 123
ANNEX 2 ... 131
ANNEX 3 ... 135
ANNEX 4 ... 139
List of Figures
Figure 1 Bolivian population and power grid [25] ...2
Figure 2 Methodology diagram ...4
Figure 3 Tool and methods used for each boundary level of the study ...5
Figure 4 Representation of the ideal thermodynamic cycle. a) p-V Diagram and b) T-s Diagram[43] ... 11
Figure 5 Alpha configuration Stirling engine [47] ... 13
Figure 6 Beta configuration Stirling engine [47] ... 14
Figure 7 Gamma configuration Stirling engine[47] ... 14
Figure 8 Wobble plate or Z-crank [34] ... 15
Figure 9 Rhombic configuration [51] ... 16
Figure 10 Free Piston Stirling Engine ... 16
Figure 11 Typical schematic of a hybrid system in HOMER[71]... 20
Figure 12 HOMER data processing. ... 21
Figure 13 Typical hybrid system configuration (power conversion system PCS) [78] ... 23
Figure 14 Location of Bolivia [82] ... 25
Figure 15 Map of Bolivia’s Geographic Zones ... 26
Figure 16. Bolivia national grid in 2010 [89] and 2016 [85] ... 28
Figure 17 Bolivia Isolated electric Systems[85] ... 29
Figure 18 Map of Bolivia’s renewable energy potential [91] ... 30
Figure 19 Summary of electrical projects using renewable energies in Bolivia ... 31
Figure 20 Guabira sugar facilities[93] and GPOBA project[95] ... 33
Figure 21 Rural villages without electricity or connection to transmission or distribution lines ... 36
Figure 22 Location of El Carmen (Pando Department) with respect to power lines ... 38
Figure 23 Location of Tablani (Chuquisica Department) with respect to the power lines ... 39
Figure 24 Location of Tirina (Potosí Department) with respect to power lines ... 40
Figure 25 El Carmen electricity demand profile a) hourly time step b) minute time step ... 47
Figure 26 El Carmen electricity demand profile with a pelletizer a) hourly time step b) minute time step ... 48
Figure 27 Tablani electricity demand profile a) hourly time step b) minute
time step ... 49
Figure 28 Tablani electricity demand profile with a pelletizer a) hourly time
step b) minute time step ... 50
Figure 29 Tirina electricity demand profile a) hourly time step b) minute time step ... 51
Figure 30 Tirina electricity demand profile with a pelletizer a) hourly time step b) minute time step ... 52
Figure 31 Diagram of Stirling engine compatibility with other power technologies ... 56
Figure 32. Genoa 01 Stirling engine ... 59
Figure 33. Mechanical parts of the Genoa 01 Stirling engine, Gamma type 61 Figure 34 Schematic of a hybrid power system incorporating a Stirling engine ... 63
Figure 35 Schematic for the reference system ... 65
Figure 36 Environmental, economic and performance analysis ... 67
Figure 37 Equivalent circuit model of photovoltaic cell, diagram modified from Duffie [156] ... 69
Figure 38 Diagram of no-linear modelling of the battery [157] ... 70
Figure 39 Modified scheme of diesel generator block diagram [159] ... 71
Figure 40 Stirling engine mass transfer and energy diagram [160] ... 72
Figure 41 Schematic representation of cycle charging operating strategy .... 77
Figure 42 Schematic representation of the load following operating strategy ... 77
Figure 43 Configuration of the hybrid power systems in HOMER: a) Stirling hybrid system b) reference system ... 79
Figure 44 Stirling hybrid and reference system CO
2equivalent emissions for El Carmen, Tablani, and Tirina ... 82
Figure 45 Stirling hybrid and reference system NPC for El Carmen, Tablani, and Tirina ... 83
Figure 46 Stirling hybrid and reference system LCOE for El Carmen, Tablani, and Tirina ... 84
Figure 47 LCOE- Cost of Diesel for systems that are part of the SIN and Cost of average price of diesel in the world ... 85
Figure 48 Daily power demand curve minute-to minute resolution, El Carmen ... 87
Figure 49 Stirling hybrid system, El Carmen. The Stirling engine, batteries and the PV panels dispatch power to meet the daily demand. ... 87
Figure 50 Reference system, El Carmen. The Diesel engine, batteries and the PV panels dispatch powers to meet the daily demand ... 88
Figure 51 Daily power demand curve minute-to minute resolution, Tablani
... 89
Figure 52 Stirling hybrid system, Tablani. The Stirling engine, batteries and
the PV panels dispatch power to meet daily demand ... 89
Figure 53 Reference system, Tablani. The Diesel engine, batteries and the PV
panels dispatch power to meet daily demand ... 89
Figure 54 Daily power demand curve minute-to minute resolution, Tirina 90
Figure 55 Stirling hybrid system, Tirina. The Stirling engine, batteries and PV
panels dispatch power to meet daily demand ... 91
Figure 56 Stirling hybrid system, Tirina. The Stirling engine, batteries and PV
panels dispatch power to meet daily demand ... 91
Figure 57 Capacity factor for the reference system and Stirling hybrid system
... 92
List of Tables
Table 1 Advantages and disadvantages of Stirling engine configuration ... 17
Table 2 Hybrid system operation strategies ... 22
Table 3 Typical hybrid system solutions [79] ... 24
Table 4 Average Temperature in Bolivia Major and Climate Zones [81] .... 27
Table 5 Renewable Electrical Project [91] ... 34
Table 6 Distance to the closest transmission and distribution lines ... 37
Table 7 Power consumption of electric devices used in the selected communities ... 41
Table 8 Electricity demand, Llanos communities - Household Type1 ... 42
Table 9 electricity demand, Llanos communities - Household Type 2 ... 42
Table 10 Electricity demand, Llanos communities - Household Type 3 ... 42
Table 11 Electricity demand, Valley communities - Household Type 1 ... 43
Table 12 Electricity demand, Valley communities - Household Type 2 ... 43
Table 13 Electricity demand, Valley communities - Household Type 3 ... 43
Table 14 Electricity demand, Altiplano communities - Household Type 1 . 44 Table 15 Electricity demand, Altiplano communities - Household Type 2 . 44 Table 16 Electricity demand, Altiplano communities - Household Type 3 . 44 Table 17 Energy consumption for public services ... 45
Table 18 Category of electric load for the electricity demand profile ... 46
Table 19 Commercially available Stirling engine ... 57
Table 20 Specifications of the Genoa 01 Stirling engine ... 64
Table 21 Specifications of Trina TSM-PD14 solar panels ... 64
Table 22 Specification of Victron VRLA 12V deep cycle gel batteries ... 64
Table 23 Specifications for the Red BlueSolar 2.5 inverter ... 65
Table 24 Specifications of the diesel generator ... 66
Table 25 Diesel and Stirling engine emissions ... 68
Table 26 Economic parameters for the diesel and Stirling hybrid systems . 68 Table 27 Inverter equations ... 71
Table 28 Stirling engine dynamic system equation ... 73
Table 29 Stirling engine operation modes ... 75
Table 30 Reference system configurations in El Carmen, Tablani, and Tirina
... 80
Table 31 Stirling engine hybrid system in El Carmen, Tablani, and Tirina . 80
Nomenclature
Abbreviations
𝐴 Exponential zone amplitude [V]
𝑎 Ideality factor of a diode [-]
𝐶 𝑆𝑡 Effective thermal mass [J/K]
𝐶 𝐻𝑋 Total thermal capacitance of the control volume, including the cooling water and encapsulating heat exchangers [J/K]
𝐸 0 Battery constant voltage [V]
𝐸𝑥𝑝(𝑡) Exponential zone voltage [V]
H
DInertia constant of the diesel generator [p.u. s/Hz]
𝑖 ∗ Filtered current [A]
𝑖𝑡 Actual battery charge [Ah]
𝐼 𝐷 Forward-bias diode current [A]
𝐼 𝑃 Current of 𝑅 𝑝 [A]
𝐼 𝑝ℎ Photovoltaic current [A]
𝐼 𝑝𝑣 Current output of the cell [A]
𝐼 0 Reverse saturation current of diode [A]
𝐼 𝑏𝑎𝑡 Battery current [A]
𝐾 Polarization constant [V/Ah]
𝐿𝐻𝑉 𝑓𝑢𝑒𝑙 Lower heating value of the fuel [J/kg]
𝑚̇ 𝑐𝑤 Cooling water flow rate [kg/s]
𝑚̇ 𝑓𝑢𝑒𝑙 Fuel flow rate [kg/s]
𝜂 𝑝,𝑛𝑒𝑡 Net electric efficiency [-]
𝜂 𝑞,𝑔𝑟𝑜𝑠𝑠 Gross electric efficiency [-]
𝑁 𝑐𝑒𝑙 Number of Cells [-]
𝑁 𝑝𝑣_𝑝 Number of Parallel PV panels [-]
𝑁 𝑝𝑣_𝑠 Number of Serial PV panels [-]
𝑃 𝑖𝑛𝑣_𝑎𝑐 Output power, AC [W]
𝑃 𝑖𝑛𝑣_𝑛𝑜𝑚 Nominal Power [W]
𝑃 𝑛𝑒𝑡_
𝑆𝑚Net electrical output in standby mode [W]
𝑃 𝑔𝑑_𝑛𝑜𝑚 Nominal Power of diesel generator [W]
𝑃 𝑔𝑑 Power, diesel generator [W]
𝑄 Battery capacity [Ah]
𝑄 𝑒𝑥𝑝 Exponential Charge [Ah]
𝑞 𝑔𝑒𝑛 Steady-state gross heat generation rate [J/s]
𝑞 𝑔𝑟𝑜𝑠𝑠 Rate of gross thermal input into the system [J/s]
𝑅 𝑠 Serial resistance [Ω]
𝑅 𝑝 Parallel resistance [Ω]
𝑅 𝑏𝑎𝑡 Internal resistance [Ω]
𝑇 𝑎𝑚𝑏 Temperature of the enclosure [K]
𝑇 𝑐𝑤_𝑖 Cooling water inlet temperature [K] [K]
𝑇 𝑆𝑡 Average temperature of the thermal mass [K]
𝑇 𝑆𝑡 Average temperature of the thermal mass [K]
𝑇 𝑐𝑤_𝑜 Cooling water outlet temperature [K]
𝑈 𝑙𝑜𝑠𝑠 Coefficient of heat loss [W/K.m
2]
𝑈 𝐻𝑋 Overall heat transfer coefficient between the control volumes [W/K.m
2]
𝑉 𝑡ℎ Thermal voltage [V]
𝑉 𝑏𝑎𝑡 Battery voltage [V]
𝑉 𝑃𝑣 Voltage output of the cell [V]
Acronyms
IEA International Energy Agency
GIS Geographic Information System
NPC Net Present Cost
CC Cycle Charging
LCOE Levelized Cost of Electricity
LF Load Following
m.a.s.l. Meters above sea level
UN SDG United Nation Sustainable Development Goals
SIN Spanish acronym for National Interconnected Grid
1 Introduction: Background and objectives
One of the principal challenges of the electricity sector is to increase access to electricity without increasing CO
2emissions. Harnessing renewable energy sources makes it possible to reach the goal of access to electricity without harming the environment.
Access to electricity in rural areas reached 77% worldwide in 2018 [1]. In this context, one of the United Nations’ proposed Sustainable Development Goals (UN SDG) is to ensure access to electricity for the entire world population, driven by green technologies [2]. This thesis examines the case of Bolivia, a country that has significantly increased access to electricity over the past 25 years, from 10% to 79% [1]. Access to electricity has also gained strong political support: for example, one article of the political constitution establishes access to electricity as a human right [3]. Even though development in terms of access to electricity has been substantial, Bolivia makes little use of sustainable energy technologies. This becomes clear when we consider that its central plan is to increase electrical service coverage by extending the electrical grid, which is currently powered mainly by fossil fuels [4], despite Bolivia’s great potential to produce electricity using renewable energy [5][6]–[9].
Bolivia has a dispersed population, as seen in Figure 1, and the main electrical grid does not reach all populated areas. Circles represent populated areas, and orange, blue, green, and red lines represent the power grid. Apart from accessibility, the implementation of power systems most consider technological, political, social and legal aspects as well [10]–[15]. This thesis focuses on implementing Stirling engine technology, while remaining aware of these other aspects as well. Thus, implementing new or recycled technologies for sustainable rural access to electricity must pay special attention to these dispersed locations [11]–[13], [16], [17].
The Stirling engine has been studied through the years and has considered to
be a promising technology for rural electrification due to its versatility of fuels,
simple mechanism, low noise, low environmental impact, theoretical high
efficiency and ease of maintenance [10], [18]–[21]. Even though the Stirling
engine has many advantages, the technology is not yet sufficiently mature to
ensure reliable performance in standalone configurations [22]. However, its
use together with other technologies in combined configurations might yield
better results. That is why the Stirling engine, as part of a hybrid power system, could be a beneficial technology for rural electrification. Currently, diesel and gasoline generators, in standalone and hybrid configurations, are used for rural electrification in some developing countries—specifically in the case of hybrid systems that combine fossil fuel generators with solar panels, batteries, and other renewable power sources [23], [24]. Such hybrid systems are still fossil-fuel dependent, however, similar systems that rely solely on green energy sources, without compromising the reliability and the quality of the electrical service, is a goal for the energy systems community. Determining whether the Stirling engine has sufficient technological maturity to be the main generator in a hybrid system for rural electrification is a challenging process. An extensive number of factors must be considered when designing such a system, and these factors must be investigated using various methods and theoretical tools, as well as practical engineering. Examples include technical field visits, mathematical modelling, and technical reviews of such systems. Particular attention should be paid to the Stirling engine as part of an optimal design for hybrid power systems.
Figure 1 Bolivian population and power grid [25]
1.1 Objectives
The overall Objective is to evaluate the potential of using a Stirling engine powered by biomass as part of a small-scale hybrid power system to provide electricity to isolated areas, such as Bolivia’s rural communities. Such a hybrid system integrates the Stirling engine without compromising service quality and ensures the system’s reliability.
The following specific objectives were proposed to achieve the main objective of this work. They individually target the hybrid power system, its location, and Stirling engine technology.
1. Evaluate the dynamic performance and reliability of hybrid systems based on Stirling engines.
2. Analyze the financial and environmental feasibility of the hybrid power system for isolated areas where renewable energy sources are available, considering the case of Bolivia.
3. Assess the level of electric power demand in three typical rural villages in the Bolivian rainforest.
4. Determine the specific design challenges that Stirling engines must address to be part of reliable hybrid systems for isolated areas of Bolivia.
1.2 Methodology
This section presents the methods, models and tools used in this thesis. The
analysis is divided into three boundary levels (Figure 2). Each boundary
addresses one specific objective.
Figure 2 Methodology diagram
The Hybrid power system boundary focuses on the performance and the adequate integration of the system components for proper operation.
The Bolivia boundary analyzes Bolivia’s technological situation, power demand in rural areas, the political situation, and its current power systems.
The Stirling engine boundary emphasizes ideas, concepts, and system design aspects to be considered when integrating the Stirling engine into a hybrid power system for rural Bolivia.
The Case study examines the financial, environmental, and performance of the hybrid power system incorporating the Stirling engine within the context of Bolivia´s technical, technological, social, and environment conditions.
Different methods were used to address each boundary level of the study.
Figure 3 shows the tools and methods used to target each boundary in the thesis.
Hybrid power system boundary
Evaluate the dynamic performance
and reliability of
hybrid systems based on
Stirling engines
Bolivia boundary
Assess the level of electric power demand in three typical rural villages in the Bolivian
rainforest.
Stirling engine boundary
Determine the specific design challenges that Stirling engines must address to be part
of reliable hybrid systems for isolated areas of Bolivia .
Case study
Analyze the financial and environmental feasibility of the hybrid power system for isolated areas where renewable energy sources are available, considering the case
of Bolivia
Figure 3 Tool and methods used for each boundary level of the study The study first analyzes electricity demand in rural communities in the various climate zones of Bolivia. The communities of Tirina, Tablani, and El Carmen were selected. These communities are inhabited by people with differing social and cultural backgrounds, which leads to different energy-load profiles and thus establishes different operational requirements for potential electrical generators: in particular, variation in terms of on-off cycles, required efficiency, and maximum power output. Compiling realistic energy-load profiles required the use of GIS (Geographic Information System) tools, field trips, and surveys.
The climate and load profiles of selected communities determine the sizing of the hybrid power system components. Information on climate zone and system components were used as inputs for a simulation model in HOMER (Hybrid Optimization Model for Electric Renewables), with the aim of determining technologically and financially optimized configurations. To understand how the system components interact, dynamic mathematical models were defined or developed for each system component. These
Hybrid power system boundary
HOMER to design the system configuration
MatLab for dynamic modeling
of the system
Bolivia boundary
Field trips and GIS to collect data from communities
Literature review of Bolivia;s energy policies
Stirling engine boundary
Literature review of the Stirling engine Overhaul of the Stirling engine prototype
Case study
HOMER for the environmetal and economic analysis, yearly.
MatLab for the performance analysis in second-by-second resolution, daily.
components were then used to create a dynamic model of the system, which was then simulated using MATLAB Simulink TM 8.9.
The study is based on the Stirling Genoa 01 engine, because this engine has been previously used in other projects in Bolivia (the Rural Electrification Solutions project at San Simón University). However, the Stirling Genoa 01 engine has experienced various operational problems. For this reason, an additional study was conducted to evaluate one of the most important characteristics of the engine—its simplicity to be maintained. To evaluate this capability, a method for overhauling and corrective maintenance was applied to the Stirling engine prototype. This evaluation took place in Bolivia using local technical resources.
The study concludes by evaluating the engine in terms of financial, environmental, and performance factors. This analysis allows us to compare it to a diesel-based hybrid system to determine its advantages and areas for improvement.
1.3 Structure of the thesis
The thesis is structured according to the objectives in section 1.1.
Chapter 2 contains a literature review of the technologies used in the power system, addressing the classification, application, and development of the Stirling engine. It also defines hybrid power systems and their various configurations and introduces the modeling software used.
Chapter 3 reviews Bolivia´s current electricity scenario, as well as the economic and political situation of the country as they concern the power sector. This chapter also analyzes the electrical needs of Bolivia’s communities and discusses the process of selecting the three communities without access to electricity used in this study.
Chapter 4 has two parts—the Stirling engine boundary and the hybrid system
boundary. The Stirling engine boundary includes a review of the hybrid
system based on Stirling engines, and Stirling engine development, as well as
the current commercial availability of the technology. It then concludes with
the requirements for the Stirling engine to be part of the system, and the
selection of a prototype to be used in this study.
The second part of this chapter describes the mathematical models and data for the components of the hybrid system based on the Stirling engine in comparison with the diesel hybrid system used as a comparative reference.
This information will be used to model the systems, and then the models will be compared in economic, environmental, and performance terms.
Chapter 5 describes the results of the comparison between the Stirling hybrid and the reference system, focusing on performance and economic and environmental impact.
Chapter 6 discusses the results obtained, with focus on the Bolivian context, analyzing the sustainability of the system for solutions in different climate zones.
Chapter 7 shares conclusions from the study and points to future work related to the topic.
1.4 Limitations
The topic of using Stirling engines in hybrid power systems is vast and raises numerous questions. This thesis answers only a few of them. Its assumptions and limitations are presented below to enable better interpretation of the results presented in the following sections and to provide motivation for future research.
- Given the small sample size of households, caution must be exercised. Further data collection would be needed to determine an accuracy of the electricity demand profiles generated.
- The modelled hybrid system was not considered for integration into to the SIN (Sistema Integrado Nacional, Integrated National System)
- The systems’ components take into account technological equipment that can be found in Bolivia now. Because Bolivia’s tech market and supply chain is limited and depends on imports.
- The evaluation of the hybrid system considered the engine to be operating at nominal power output.
- The cost of the equipment, transportation, and import taxes
were not taken into account in the financial analysis.
- The load profile was generated from field data for typical households in the three zone. Further load profile studies are needed to optimize system configuration.
- Solar radiation input was obtained from the Meteonorm 7.3 database, because there is a dearth of timely information and the lack of proper weather stations for energy in Bolivia.
- The maintainability and reliability indicator analysis did not consider the cost and durability of new spare parts for the Stirling engine.
- Only one Stirling engine prototype was tested in the study
of the hybrid power system. More prototypes should be
studied to make a firm conclusion.
2 Theoretical framework: the Stirling engine and hybrid systems
This chapter reviews the concepts of the Stirling engine and hybrid systems. It provides an overview of Stirling engine technology and its applications, thermodynamic cycle, mechanical configurations, and advantages and disadvantages of the Stirling engine. This is followed by an introduction to hybrid electrical power systems and the software used to model them, the HOMER software in particular. This software is widely used to design hybrid power systems. These principles establish the theoretical basis for the study.
2.1 The Stirling engine
Innovative technical solutions will be needed to extend access to electricity in rural areas, including conventional grids, mini-grids, and off-grid solutions.
These solutions should be adapted to the real-world conditions of rural areas, including the locally available energy resources and the technical standards adopted for rural infrastructure. In this context, hybrid power systems offer good capabilities for use in for rural areas, which can be enhanced thanks to the Stirling engine’s simple configurations, flexible fuel options, and ease of maintenance.
2.1.1 Technology Overview: History of the Stirling engine
The earliest Stirling engine—known at the time as a hot air engine—was patented by Robert Stirling in 1816 [26]. He was looking for a safe alternative to the steam engine [27]. This design employed two new concepts for the first time: the regenerator (the economizer heat exchanger) and the closed-cycle air engine. This patented design was a breakthrough at a time when little was understood about thermodynamic concepts (Carnot would not publish the first analysis of heat engines until 1824) [28]. James and Robert Stirling built the first Stirling engine which was designed to generate 2 HP, however it was unable to generate this output as it could only pump water in a quarry before its vessels became overheated and collapsed [29].
The Stirling brothers did make important improvements to their initial design
[30] by sealing the vessel for higher-pressure operation, improving the
regenerator’s heat transfer rate through the use of wire gauze, and developing
a cooling system [31]. However, Stirling engine technology has been slow to
develop.
One of the most successful examples of Stirling engines in the 19
thcentury was the engine used at Dundee Foundry in 1843. The Dundee case demonstrated the technological promise of the Stirling engine. The engine generated 45 HP and had a double acting engine with 16 inch pistons and a 4 ft. stroke [27]. The engine ran the foundry’s machines for 4 years but due to problems with the hot end of the cylinder corrosion, it was ultimately replaced by a more reliable but less efficient steam engine [30]. This case exemplifies the technological state of the Stirling engine, which the industry of that time eventually abandoned due to its unreliability. These technical problems drove Stirling engines into smaller-scale uses, such as driving fans and similar light duties [32]. These drawbacks were even more stark with the development of the internal combustion engine, which offered higher efficiency and better power output than Stirling engines at the time [33].
In the 20
thcentury, the electronic company Philips in Eindhoven began with the development of a small Stirling unit capable to power portable radios sets [34]. This engine generated 16 W of electricity. It was also at this time, at the Philips Corporation, that the engine acquired the name Stirling engine. The upgraded design offered better performance than expected, highlighting the Philips Research Laboratory’s research in this area. From 1937 to 1953, the laboratory developed small units such as the 16 W single-displacer engine, two medium size units—the 500 W engine and a dual-action four cylinder 6 kW engines, and final some large units, such as a 90 HP engine built in 1948 [35]. These units brought the Stirling engine to a high state of technological development [34], with improvements such as heat-resistant materials, better piston lubrication, better regenerator materials, and an innovative crank mechanism. In addition, advances were being made in heat transfer, thermodynamics, and fluid flow theory that allowed the Stirling engine’s power-to-weight ratio to be increased by a factor of 50, and power-per-unit swept volume to improve by a factor of about 125 [35].
Companies, such as Ford, General Motors, United Stirling, and Stirling Thermal Motors as well as research institutes of the U.S. Navy, US Army, and NASA started further research on Stirling engines [36]. Despite all the support and development of the Stirling engine; problems with marketing, cost, and a market dominated by the internal combustion engine ultimately torpedoed the commercial success of the engine.
Despite these drawbacks, the Stirling engine found another niche market.
Thanks to its fuel flexibility and low emissions [37], it was used as an
alternative for electrification, demonstrating its potential as the main generator for small-scale thermal power plants [20], [38]–[41].
Thermodynamic cycle
The Stirling engine works using a closed, regenerative thermodynamic cycle called the Stirling cycle. Its cycle of compression and expansion, with a temperature gradient, includes regenerative heat transfer of the working fluid in the so-called regenerator. The cycle can also work in reverse, consuming work, in applications such as refrigerators or heat pumps [42]. It is helpful here to describe the main principles of the engine’s operation.
Figure 4 Representation of the ideal thermodynamic cycle. a) p-V Diagram and b) T-s Diagram[43]
Figure 4 shows the p-V and T-s diagrams for the ideal Stirling cycle. This figure was adapted from Granryd [43]. The same figure can also be used to explain the Stirling cycle with ideal regeneration.
Figures 4a and 4b illustrate the pressure-volume and temperature entropy diagrams of the ideal Stirling cycle, respectively. These two diagrams show four processes:
(a-b) Isothermal compression process: the working gas is compressed at a constant temperature (Tcold). The isothermal process assumes that the heat increase discharges to the external heat sink and therefore entropy is reduced.
a. b.
(b-c) Isochoric heat-addition process: the working gas starts at minimum volume and temperature and is then heated to a higher temperature (Thot), keeping volume constant. Heating occurs inside the regenerator, with an increase of entropy. The heat is added from the regenerator (q
R)
(c-d) Isothermal expansion: the working gas expands isothermally (Thot) while it is heated. An external heat source produces the necessary heat to keep the temperature constant. This heat is absorbed at an infinite heat transfer rate. The heat input leads to in an increase of entropy.
(d-a) Isochoric cooling: the working gas keeps a constant volume while is cooled to the lower temperature level (Tcold). The regenerator absorbs the heat from the gas (q
R) and therefore entropy is reduced in the cycle.
Note that heat recovery from the regenerator is one of the most critical parameter in determining the efficiency of the Stirling cycle. In Figure 4b, the s,T plot shows that the amount of heat stored during process (d-a) and added during process (b-c) makes a significant contribution to efficiency. Therefore, the efficiency of the regenerator has an important effect on the engine’s thermal efficiency.
The engine’s thermal efficiency also depends on the properties of the working fluid, such as specific heat, thermal conductivity, and density. Hydrogen, nitrogen, helium, and air are the most common working fluids in Stirling engines [44]. The working fluid is a critical mechanical design variable, and engines are designed for one or two working fluids. Helium and hydrogen perform better in Stirling engines because they offer a lower pressure drop and good heat transfer [45]. However the small molecular size of helium means this fluid has leakage problems. Nitrogen and air offer lower heat transfer [46][47] but are more readily available and cheaper.
This cycle can be achieved using various mechanical configurations, although real-world engine behavior deviates from the ideal cycle because its pistons move in a sinusoidal path with a phase angle of around 90 to 110 degrees.
2.1.2 Configurations
The engine is a relatively simple device that has few moving parts. The basic
parts are the piston, the displacer, the cylinder volumes, the heat exchangers,
and the crank mechanism. However, over the years, a variety of Stirling engine
designs have evolved. Designs are most commonly classified according to the mechanical configuration of the pistons and cylinders.
Alpha Stirling Engine
The Alpha configuration consists of two pistons used on either side of the heater, the regenerator, and the cooler. The hot and the cold pistons move uniformly in the same direction to provide constant-volume heating or cooling processes of working fluid. When all the working fluid has been transferred into one cylinder, one piston will be fixed
aand the other piston moves to expand or compress the working fluid, as is shown in Figure 5. The expansion work is performed by the hot piston, while the compression work is performed by the cold piston.
Figure 5 Alpha configuration Stirling engine [47]
Beta Stirling Engine
The Beta configuration has a displacer and a piston (see Figure 6). The engine is designed so that both the displacer and piston are accommodated in the same cylinder. In this design, the compression space consists of the space swept by the underside of the displacer and the top side of the power piston.
The piston and displacer may or may not physically touch, but they are
a
