INSTITUTO TECNOLÓGICO Y DE ESTUDIOS SUPERIORES DE MONTERREY
CAMPUS MONTERREY
DIVISIÓN DE INGENIERÍA Y ARQUITECTURA PROGRAMA DE GRADUADOS EN INGENIERÍA
HYBRID VEHICLE WITH STIRLING ENGINE AND THERMAL ENERGY STORAGE
TESIS
PRESENTADA COMO REQUISITO PARCIAL PARA OBTENER EL GRADO ACADÉMICO DE:
MAESTRO EN CIENCIAS ESPECIALIDAD SISTEMAS DE MANUFACTURA
POR
ERIK WIELAND SEWE
MONTERREY, N. L. MAYO 2010 INSTITUTO TECNOLÓGICO Y DE ESTUDIOS SUPERIORES DE
MONTERREY
CAMPUS MONTERREY
DIVISIÓN DE INGENIERÍA Y
ARQUITECTURA
PROGRAMA DE GRADUADOS EN INGENIERÍA
Los miembros del comité de tesis recomendamos que el presente proyecto de tesis presentado por el Ing. Erik Wieland Sewe sea aceptado como requisito parcial para obtener el grado académico de Maestro en Ciencias con especialidad en:
SISTEMAS DE MANUFACTURA
Comité de tesis:
______
Dr. Noel León Rovira
Asesor
______
Dr. Ciro Ángel Rodríguez González Dr. Humberto Aguayo Téllez
Sinodal Sinodal
APROBADO
______
Dr. Ciro Ángel Rodríguez González
Director del Programa de Maestría en Ciencias
con especialidad en Sistemas de Manufactura
Mayo, 2010
to Alma, you were my motivation for this work Acknowledgements
I would like to thank to …
… Dr. Noel León for making possible this thesis. … Dr. Ciro Rodríguez, Dr. José Huertas and Dr. Humberto Aguayo for her time and advice. … Claudia Gonzales for her excellent work. … My family, for their constant support from Germany. … Luis Diego for friendship and feedback. … Daniel for reviewing the thesis. … All my friends. … ITESM and CONACYT for economic support. Abstract
Advances in electric technology and automotive components make feasible a new kind of hybrid vehicles. The hybrid technology allows gathering advantages from different energy concepts and allows it to eliminate disadvantages which a single energy source or engine concept would present. Hybrid technologies allow using different power sources in one vehicle. Concentrated thermal solar energy is used as primer energy in a vehicle. Basic components are thermal energy storages and Stirling engines. Heat from the sun is gathered in a place outside the car and stored in a tank. The tank is placed in the vehicle and connected to a Stirling engine. A Stirling engine is a device that converts heat energy into mechanical power. The mechanical power is used in a generator to produce electric energy. The vehicle has an electric system with batteries serving as an energy buffer. Physical bases, components and vehicle concepts are described. A market study provides benchmark of Stirling engines and serial hybrid busses. In a case study several options for a possible prototype are revised. For the Circuito Tec service (a service for students living near the university) a detailed analysis is made. With a GPS the driving profile is recorded. A Matlab model uses this data to calculate energy storages sizes and required engine output. In Autodesk Inventor several CAD models are created in order to represent different space concepts. A vehicle with thermal energy storage based on molten glass is compared to conventional and alternative concepts. MSC Adams/car is used to simulate the vehicle dynamics. The influence of a thermal storage tank with the weight of 996kg is observed. Size, weight and price estimation of different thermal-electric vehicles are compared to conventional vehicles.
Keywords Hybrid Vehicle, Stirling Engine, Thermal Energy Storage, Solar Energy
i Table of contents
Table of contents...... ii List of figures...... iv List of tables ...... vi Acronyms ...... vii List of Symbols...... vii Chapter 1. Introduction ...... 1 1.1 Objective ...... 1 1.2 Hypothesis ...... 2 Chapter 2. Theoretical Background ...... 3 2.1 Stirling Engines ...... 3 2.1.1 Engine configurations ...... 3 2.1.2 The Stirling Cycle:...... 5 2.1.3 Alternatives to Stirling Engines...... 5 2.2 Hybrid Vehicles ...... 7 2.2.1 Comparison of different vehicle assemblies...... 10 2.2.2 Vehicles with Stirling Engines – History ...... 11 2.3 Patents ...... 12 2.4 Solar Energy System Design...... 13 2.5 Materials...... 15 2.6 Thermal energy storage tank...... 17 2.7 Heat exchanger...... 17 Chapter 3. Macro vision ...... 18 3.1 Vehicle to Grid...... 18 3.2 Competitors and economic rivals...... 19 3.3 Problems and backup solutions...... 20 Chapter 4. Application: Minibus...... 21 4.1 Market Situation Serial Hybrid Buses ...... 21 4.2 Components...... 24 4.2.1 Market situation: Stirling Engines ...... 24 4.2.2 Cooling system for Stirling engine...... 26 4.2.3 Electric Energy Storage...... 27 4.2.4 Electric engines ...... 27 4.2.5 Air conditioning ...... 28 4.2.6 Electric components ...... 29 4.2.7 Vehicle Concepts...... 29
ii 4.2.8 Expreso Tec ...... 32 4.2.9 Circuito Tec ...... 33 Chapter 5. Modeling and simulation...... 37 5.1 Load Cases...... 37 5.1.1 Drive Cycle...... 37 5.1.2 Vehicle...... 37 5.1.3 Vehicle Dynamics...... 38 5.2 Model...... 39 5.2.1 Energy calculations ...... 40 5.2.2 Configuration...... 41 5.2.3 Simulation Objective ...... 42 5.3 Simulations and Results ...... 43 5.3.1 Matlab...... 43 5.3.2 CAD 3D ...... 49 5.3.3 Vehicle Dynamics...... 49 5.3.4 FEM analysis...... 51 5.3.5 Package dimensions TES ...... 55 Chapter 6. Prototype...... 61 6.1 Different concepts for prototypes ...... 61 6.2 Basic thoughts ...... 61 6.3 Microbus for VIP transport...... 62 6.4 Minibus for student transport...... 62 Chapter 7. Conclusions and Recommendations...... 64 7.1 Conclusions ...... 64 7.2 Recommendations...... 66 7.3 Future Work...... 66 References...... 67 Appendix I – Matlab Program Code ...... 69 Appendix II - Student Transport ITESM ...... 78 Appendix III - Virtual Test Track ...... 79 Appendix IV - Vehicle Comparison ...... 81
iii List of figures
Figure 2.1 Different configurations of Stirling engines [2] ...... 4 Figure 2.2 Free piston Stirling engine [4] and ...... 4 Figure 2.3 Stirling Cycle [3], step 1,2,3 and 4 ...... 5 Figure 2.4 Rupp Hot Air Engine [7] ...... 6 Figure 2.5 Closed Brayton Cycle Engine ...... 6 Figure 2.6 Cutaway View of a Capstone Microturbine Generator [9]...... 7 Figure 2.7 Hybrid propulsion concepts – parallel configuration ...... 8 Figure 2.8 Hybrid propulsion concepts – serial configuration ...... 8 Figure 2.9 Hybrid propulsion concepts mixed configuration ...... 8 Figure 2.10 Power electric motor / Stirling engine ...... 9 Figure 2.11 Efficiency of different vehicle concepts [11]...... 10 Figure 2.12 Hybrid vehicle with Stirling engine; Solar power tower efficiency [1]...... 10 Figure 2.13 GM Stir-Lec I 1969, (http://www.bangshift.com) ...... 11 Figure 2.14 Hybrid Vehicle with Stirling Engine and Thermal Energy Storage...... 12 Figure 2.15 US 7,469,760 B2, US 2008/0121755 A1 ...... 13 Figure 2.16 Solar Radiation in kWh/ (m², year) [11]...... 13 Figure 2.17 Concentration Rate vs. Absorber Temperature [1]...... 14 Figure 2.18 Reflecting Schemes for Concentrating Solar Energy [20] ...... 14 Figure 2.19 Fresnel lens [21]...... 15 Figure 2.20 Virtual Prototype of Solar Energy Storage Tank [25]...... 17 Figure 3.1 Absorption cooling [http://www.dometic-waeco.com/] ...... 18 Figure 3.2 Vehicle 2 Grid [26]...... 19 Figure 3.3 Solar Carport (www.dachscheich.com)...... 19 Figure 4.1 Azure Citibus [31]...... 21 Figure 4.2 Daimler Buses NA - Orion VII Hybrid...... 22 Figure 4.3 EBUS [32] ...... 22 Figure 4.4 Transit Bus Configuration; DesignLine Bus [9] ...... 22 Figure 4.5 Solaris Urbino 18 Hybrid [33] ...... 23 Figure 4.6 Toyota Coaster Hybrid EV[34] ...... 23 Figure 4.7 MB Citaro G Blue Tec Hybrid city bus [30] ...... 24 Figure 4.8 Comparison of Battery Characteristics [38] ...... 27 Figure 4.9 Motoring efficiency map Power Phase 145...... 28 Figure 4.10 Engine Sizes: ICE vs. Stirling Engine ...... 30 Figure 4.11 Routes Expreso Tec...... 32 Figure 4.12 Chassis MB 1219 [40] and Ayco Magno 1040 SC bodywork ...... 32 Figure 4.13 Velocity profile Expreso Tec Cumbres...... 33
iv Figure 4.14 Altitude profile Expreso Tec Cumbres...... 33 Figure 4.15 Mercedes Sprinter 515 CDI, Toyota Hiace...... 34 Figure 4.16 Routes Circuito Tec vs. real data October 2009 ...... 34 Figure 4.17 Use of Circuito Tec Service...... 35 Figure 4.18 Velocity Profile Circuito Tec...... 36 Figure 4.19 Altitude profile Circuito Tec...... 36 Figure 5.1 Distance between two waypoints ...... 37 Figure 5.2 Vehizero serial hybrid truck [43] ...... 38 Figure 5.3 Test Road ADAMS/car ...... 39 Figure 5.4 Cleanergy V161 Stirling Engine [44]...... 41 Figure 5.5 Matlab Macro...... 43 Figure 5.6 Flowchart Matlab Simulation ...... 44 Figure 5.7 Energy Consumption at Constant Speed ...... 45 Figure 5.8 Result Matlab Simulation: SOC ...... 46 Figure 5.9 Actual Power at Wheel ...... 46 Figure 5.10 Actual Power for Selected Data Sets ...... 47 Figure 5.11 EES: 15kWh, 10kWh, 5kWh...... 48 Figure 5.12 5kW Stirling engine and 10kWh EES ...... 49 Figure 5.13 Basic 3D Model...... 49 Figure 5.14 ADAMS/car simulation...... 51 Figure 5.15 FEM simulation results ...... 52 Figure 5.16 Location of Stress Measurements...... 53 Figure 5.17 Stress vs. time ...... 53 Figure 5.18 2600kg vs. 3721kg, point 1 and 2, von Mises stress...... 54 Figure 5.19 2600kg vs. 3721kg, point 4, von Mises stress...... 54 Figure 5.20 Cylindrical TES vs. rectangular TES ...... 55 Figure 5.21 Package Concept, rectangular TES ...... 56 Figure 5.22 Package Concept, rectangular TES: Step 1,2 and 3...... 56 Figure 5.23 Package Concept, rectangular TES: Step 4...... 56 Figure 5.24 Package Concept, rectangular TES: Step 5...... 57 Figure 5.25 Alternative Chassis...... 57 Figure 5.26 TES 3000°C...... 58 Figure 5.27 Electric Vehicle for Circuito Tec – Deep Cycle (left) vs. Li-Ion (right) ...... 58 Figure 5.28 Conventional Serial Hybrid Configuration ...... 59 Figure 5.29 Comparison Vehicle Concepts ...... 60 Figure 6.1 Microbus Expreso Tec...... 62
v List of tables
Table 1.1 Potential of renewable energy [1]...... 1 Table 2.1 Energy Storage ...... 16 Table 4.1 Benchmark Stirling Engines ...... 25 Table 4.2 Stirling Engines: Commercial Applications...... 26 Table 4.3 Vehicles – main characteristics...... 31 Table 4.4 Vehicle Concepts - Energy Storage...... 31 Table 4.5 Statistics Circuito Tec...... 35 Table 5.1 GPS Sample Data...... 37 Table 5.2 Formulas for Energy Calculations [10]...... 40 Table 5.3 Vehicle parameters Vehizero ECCO-C...... 41 Table 5.4 Vehicle weight ...... 42 Table 5.5 Result Matlab Simulation: Energy Consumption...... 45 Table 6.1 Scaling prototypes...... 62 Table 7.1 Actual Vehicle Costs Circuito Tec ...... 64 Table 7.2 V2G economics ...... 65 Table 7.3 Energy Calculations Fresnel Lens ...... 65
vi Acronyms
AC Alternating Current ATV All Terrain Vehicle CAD Computer Aided Design CAE Computer Aided Engineering DC Direct Current DOE Department of Energy (USA) ECE External Combustion Engine EES Electric Energy Storage ESS Energy Storage System FEM Finite element method GM General Motors ICE Internal Combustion Engine ITESM Instituto Tecnológico y de Estudios Superiores de Monterrey PCM Phase Change Material SBI Stirling Biopower Inc. (former: STM) STM Stirling Thermal Motors Inc. (today: SBI) TES Thermal Energy Storage TRIZ The theory of inventor's problem solving V2G Vehicle to Grid
List of Symbols
Af Frontal area cd Air drag coefficient cr Tire rolling coefficient E Energy Ea Energy used for acceleration Eec Electric energy consumption Eair Energy used by air drag Epot Potential energy Er Energy used by rolling resistance Etotal Total energy consumed F Force Fa Force against vehicle acceleration Fair Force against vehicle motion caused by air drag Fpot Force against vehicle motion caused by hill climbing Fr Force against vehicle motion caused by rolling resistance g Gravitational acceleration h Height i Overall gear ratio Iw Moment of inertia of the wheels Ieng Moment of inertia of the engine m Vehicle mass (empty) mmax Maximum vehicle mass P Power Pa Power required to accelerate Pconst Power required to move a vehicle at constant speed rstat Static radius of the tires rdyn Dynamic radius of the tires s Distance t Time
vii Chapter 1. Introduction The present and future situation in energy availability and the increasing environment contamination require drastic changes in the human life if we want to keep our current lifestyle. The crude oil reserves are being diminished and at the same time, the demand for transport is growing day by day, especially in emergent countries traffic is augmenting fast. In first world countries people tend to live at longer distances from their work increasing daily commute between house and work. Nowadays the transportation system is mainly based on gasoline and diesel powered ICE, with all their serious disadvantages: polluting escapes, noise and dependency on oil. This situation justifies putting our best effort in the development of technologies and systems that promise an exit from oil dependency and that promise less contamination. Many alternatives were developed and presented in the past to eliminate the ICE, but still none of them had great success. Advances in electric technology and automotive components make feasible a new kind of hybrid vehicles, using different power sources. The hybrid technology allows gathering advantages from different energy concepts and is able to eliminate disadvantages which a single energy source would present. In actual vehicles as for example the Toyota Hybrid or diesel-electric locomotives this effects can be observed, examples are greater efficiency and brake energy recuperation. Still, the actual technology leaves place for improvements and further investigation, as for example the implementation of renewable energy sources. Depending on location and kind of vehicle, there are different forms of renewable energy. Theoretical Potential Technical Potential (1018J/year) (1018J/year) Solar 2,500,000 600 Wind 100,000 100 Biomass and 4,000 380 Geothermic Hydraulic 287 134 Total 2,604,287 1,088 Table 1.1 Potential of renewable energy [1] Table 1.1 shows the potential of different types of renewable energy, in particular the solar energy has to be mentioned, if we could gather the solar energy of one hour, than we would be able to satisfy the actual global energy consumption of more than one year (340*1018J/year) [1]. It is important to state that there is a great amount of different renewable energy sources. For every location and application the most convenient energy source should be searched. In the case of the city of Monterrey, Mexico, the solar energy offers great opportunities. But one of the challenges is to make the solar energy accessible for the use in vehicles. One possible solution are tanks that store concentrated solar thermal energy. In many investigations for new energy concepts based on renewable energies, presented prototypes are small lightweight cars, with the intention to use small fuel effective engines. In this investigation, the aim is to create a vehicle concept that can be used within the community of the ITESM. Student transportation is a good opportunity to introduce and promote technology. On those bases, the present master thesis presents and further develops a concept for a hybrid vehicle, which uses solar energy as its basic energy source. This investigation is divided in seven chapters; the second consists in the theoretical background in which physical bases, components and vehicle concepts are described. Then, in the third chapter, other aspects of the new proposed technology are mentioned, such as competitors and economic basics. In the fourth chapter, the design process of a vehicle for student transportation is shown, including actual market situation and components. The fifth chapter deals with the evaluation of the minibus in computer simulations, the sixth describes the planning of a prototype. Finally, in the last chapter recommendations and conclusions are exposed.
1.1 Objective The present investigation looks to find new concept for primary energy in a vehicle. It is based on thermal heat storage of solar energy, Stirling engines and the possible use of these components in hybrid vehicles. The First step is to identify actual concepts for hybrid vehicles and evaluate them in reference to the new concept. The Next step are the basics in thermal energy storage (TES), important factors are specific energy, energy density and working temperatures. A short introduction to concentration of solar energy is given. 1 Another section is dedicated to Stirling engines including a benchmark of actual market situation. Finally, different vehicle concepts will be presented and evaluated and joint with the results of the investigation in Stirling engine and thermal energy storage. These concepts indicate possible areas of hybrid thermal vehicles, their benefits but also their restrictions. The Intention of the present work is to find a concept that can be implemented within the ITESM, campus Monterrey. For that reason, local student transportation is observed; properties of actual vehicles are revised. After choosing a case study, the first step is to determine the energy use, sizes for energy storage and engines. Next objective is to find out how does a TES affect to the vehicle dynamics and the changes in comparison to a reference vehicle. Another topic is the rough cost estimation that provides the information about what should be the maximum price for the new concept to be competitive. The objective is to work out the new concept, describing its general properties, benefits and restrictions, without going into too much detail. The detailed vehicle assembly and component connections for example are not included on the scope of the present work and would be part of following investigations.
1.2 Hypothesis It is possible to use thermal energy as primer energy source in vehicles. A thermal-electric hybrid vehicle with a thermal energy storage and Stirling engine can be an alternative to conventional internal combustion engine powered cars. This opens the possibility to use stored thermal solar energy for transportation. Simulations in CAx and math models predicting dynamical behavior and energy consumption of the vehicle are fundamental in the design process.
2 Chapter 2. Theoretical Background This chapter presents the background in which this investigation takes place, most important components and theory are described. Focus is on solar energy, Stirling engines and hybrid vehicle concepts.
2.1 Stirling Engines A Stirling engine is a device that converts heat energy into mechanical power. An external heat source provides thermal energy which is used to heat one side of the engine. The other side is kept cold and the temperature difference is used to drive the engine, the cycle is described in 2.1.2 . The engine is designed so that the working gas is generally compressed in the colder portion of the engine and expanded in the hotter portion resulting in a net conversion of heat into work. An internal regenerative heat exchanger increases the Stirling engine's thermal efficiency. Compared to simpler hot air engines lacking this feature, the colder portion are not connected directly to the hotter portion, the working fluid first has to pass the regenerator, pre- heating respectively pre-cooling the gas. Benefits of Stirling engines • They have a wide application area and can be driven by every possible heat source, by solar or geothermal energy, by fuel combustion, as for example biogas, or by re-using waste heat. • They assembly basic, they have no valves. • They emit less noise than an ICE. • They can be built air-independent, as for example in the use of submarines • Startup is possible at low temperatures, they don’t present problems with frozen oil. Disadvantages • Size: actual Stirling engines need more space than actual ICE providing the same power output. • Costs: due to the small production and especial materials which can resist high temperatures, actual Stirling engines are expensive. • Large temperature differentials are required. • Start-up time: the engine needs to warm up before it can be used. • Change of velocity and power output: the engines output cannot be changed immediately, similar to the start-up it needs to warm up or cool down • Sealings are problematic, especially at the hot side, due to high temperatures and high working gas pressures. Today’s commercial applications are solar power generation, combined heat and power plants and cyrocoolers, that means that the process is inverted and the Stirling engine is driven for cooling. There are plenty of model engines for toys and academic proposes, but it is more difficult to obtain more powerful engines, paragraph 4.2.1 provides a closer look to the actual market situation of Stirling engines > 500We.
2.1.1 Engine configurations There are many possible configurations for Stirling engines, the most common are: Alpha, Beta, Gamma, and free piston Stirling. • Alpha Stirling: An alpha Stirling contains two power pistons in separate cylinders, hot and cold. The hot cylinder is heated by any heat source and the cold cylinder cooled by another medium to create a temperature differential. Both cylinders are connected by a tube system, and may have a regenerator installed. This engine is simple, both cylinders can be connected by one crankshaft but the sealing of the hot cylinder is critic and solutions cause side effects like additional dead space. Another downside is the sealing of two cylinders instead of one single sealing in other configurations. • Beta Stirling The beta engine only counts with one cylinder and therefore is more compact, allows higher compression ratios (less dead space) [2] and as a result, shows an improved power density. A displacer piston serves to shuttle the working gas from the hot to the cold side, it can be designed with a loose fit depending on the
3 configuration. It does not extract any power. A great advantage is the lack of hot moving seals. A negative point is the complicated drive system, for example a rhombic drive, needed to couple the movement of the piston and the displacer.
Figure 2.1 Different configurations of Stirling engines [2] • Gamma Stirling A gamma Stirling is a beta Stirling, but has the power piston mounted in a separate cylinder than the displacer piston. The gas can flow freely between the two cylinders. This configuration produces a lower compression ratio but is mechanically simpler and often used in multi-cylinder Stirling engines [3]. • Free Piston In the free piston configuration, the Stirling engine has no crankshaft but a spring or similar mechanical device, heated on one end and cooled on the other, the expansion and cooling of the gas drives the piston back and forth in the cylinder. The work performed by this piston-motion can be converted into electric energy by a linear alternator. See Figure 2.2. Benefits of this configuration is the simple assembly, it has few moving parts and no rotational movement. One of the critical point is the friction between piston and cylinder. • Other configurations
Figure 2.2 Free piston Stirling engine [4] and Alternative Stirling Engine Configurations [5],[2]
There are more configurations of Stirling engines, Figure 2.2 shows two examples, in the middle a four cylinder free piston alpha engine combined with a gas turbine [5] and at the right hand side, a rotatory piston engine, similar to the well-known Wankel internal combustion engine [2]. 4 2.1.2 The Stirling Cycle: All Stirling Engines have two sections, one of high temperatures and the other of low temperatures. The operating principle is to augment the pressure of the working gas in the hot side of the engine. This is reached by applying heat and resulting gas expansion. On the cold side, heat is dissipated and the working gas contracts. At continuation, the Stirling cycle will be on an alpha Stirling:
Figure 2.3 Stirling Cycle [3], step 1,2,3 and 4 The upper red part shows the hot side of the engine, the blue one is the cold side. • Step 1 In the first step the gas in the hot side has been heated and the force created by gas expansion has moved the piston to the left side. The crankshaft will continue its motion, and starts to push the hot gas towards the cold side. • Step 2 The gas has reached its maximum volume and heat is extracted on the cold side. The piston continues pushing the gas towards the cold side. The working gas starts to contract. • Step 3 Most of the working gas is in the cold side where heat is extracted. The piston is pulled downwards by gas contraction. • Step 4 The working gas has reached the minimum of its volume and restarts the cycle by expanding on the hot side, pushing the piston to the left. The idealized Stirling cycle equals to the hypothetical Carnot cycle, without losses it could reach the highest efficiency attainable by any heat engine. Because of friction, thermal losses and irregularities the idealized process cannot be realized. In most cases, a number of deviations must be accepted which reduce both efficiency and power density [2]: • The piston movement cannot be designed to perform the idealized movement. • The regenerator does not reach 100% efficiency. • Surface designed for better heat transfer does not meet perfectly the requirements of the idealized cycle. • The working fluid has friction in tubes, cylinders etc. • Mechanical friction. • Dead space inside the engine (heat exchangers, regenerator etc.) reduces the compression ratio. • Piston seal leakage and pressure drop across the heat exchangers and regenerator reduce efficiency. In total, these deviations lead to a process significantly different from the ideal process; modern engines achieve approximately 30% efficiency (see Table 4.1).
2.1.3 Alternatives to Stirling Engines Besides the Stirling engines, there are other mechanisms or materials that can convert heat energy into electric energy, for example:
5 • Silicon Nanowire Silicon has the ability to convert heat into electricity by thermoelectric conversion, without any mechanical component. [6] presents investigation in new technologies to increase this ability by as much as 100 times. This is reached by fashioning the material into nanowires with diameters of 10 to 100 nanometers and introducing defects in silicon that slowed the flow of phonons — the acoustic vibrations in the crystal lattice of a material that carry heat. Otherwise the thermal conductivity of silicon would be too high and it would be difficult to create the necessary temperature differential. • Hot Air Engine – Rupp This engine is simpler than a Stirling engine, it has less components, as a difference thisis an open cycle, described in [7]. This engine can run in every position and in both directions, it needs pre-heating and an impulse to start working.
Figure 2.4 Rupp Hot Air Engine [7] • New Stirling / Brayton Engine This concept searches the advantages of Stirling and Brayton engines. A Brayton engine is basically composed of a compressor, a combustor and a turbine. Well-known applications of the Brayton technology are turbines in jet planes. It consists of an open cycle, the gas used in the engine, typically air, is renewed at the beginning of every new cycle, in [8] the function is described: “Initially the gas enters to the compressor where its volume is reduced adiabatically. Later the gas is mixed with some kind of combustible inside the combustor where it ignites producing a change of temperature a constant volume increasing its pressure. The gas passes then through the turbine where the pressure is converted into mechanical energy. Then, the gas is expulsed to the atmosphere where it reduces its temperature until it reaches normal conditions. After that, the cycle begins again.” As the Stirling engine, an engine based on the Brayton cycle, can be driven by many heat sources, such as waste heat, solar thermal energy, amongst others. [8] describes the invention process using TRIZ for a new design that combines the advantages of both cycles: The Brayton engine is changed into a closed cycle, the proposed cycle follows the same phase diagram pattern that the Brayton cycle does, but the heat is applied externally just the same way as it is applied to a Stirling engine.
Figure 2.5 Closed Brayton Cycle Engine In contrast to a Stirling engine, in the closed Brayton cycle the gas flows only in one direction. It is a steady state process, this means that the temperature, pressure and volume in each section remain constant. This engine works with any heat difference between heater and cooler. It has a continuous movement and with solar energy it is possible to obtain electric energy not only without producing any contamination but also without fresh air consumption.
6 Another research aim was to develop an affordable engine concept, for this to happen, engine parts are selected from commercial providers, with preference to components from mass production, for example compressors and turbines used in the automotive industry. This concept is subject of the Mexican patent application number MX/a/2008/012924 “Ciclo mixto de Stirling y Brayton para convertir energía térmica a energía en forma de movimiento rotatorio”. • Microturbine Generator Capstone Turbine Corporation describes the functionality of a microturbine (a small gas turbine) as follows [9]: “Microturbines use a continuous shaft rotation to pull in air, compress it, add fuel for combustion, and use the resulting heated air to drive a turbine wheel. The turbine wheel provides the power to drive the compressor wheel as well as a generator mounted on the same rotating assembly. A recuperator (or air-to-air heat exchanger) is provided to extract energy from the exhaust stream and recycle it to preheat the incoming air to the combustion chamber”.
Figure 2.6 Cutaway View of a Capstone Microturbine Generator [9] Actually, there is a research project with the aim to drive microturbines by solar energy.
2.2 Hybrid Vehicles By their structure, hybrid vehicles can combine the advantages of different energy and engine concepts. With an intelligent structure, it is possible to change drastically the vehicle’s properties; they may exhibit the following advantages [10]: • Reduces fuel consumption • Lowest possible emissions • Noise reduction • Increased operational comfort • Locally emission-free and noise-free operation in sensitive areas Nowadays, the majority of the hybrid cars build in series production are vehicles with ICE and electric engine. The market leader and the best example is the Toyota Prius. Nevertheless, there are many other concepts to combine different engines and energy storages, shortly many new hybrid vehicles will enter the market. Disadvantages are the complicated structure as well as the higher weight and the rising costs. There are three fundamental concepts for the structure of the drive train and the connection of the different engines: parallel hybrids, serial hybrids and mixed hybrids [10]: • Parallel hybrids: In parallel hybrids, both engines are able to drive the wheels simultaneously; this offers the possibility of operation entirely under the power of the first engine, entirely under the power of the second engine, or in combination. Usually this vehicles count with a powerful ICE and a small, limited electric motor.
7
Figure 2.7 Hybrid propulsion concepts – parallel configuration The car can be driven electrically at low speed, for example in city use. Interurban and at higher velocities and larger distances, the car uses the ICE In moments where extra power is needed, for example when climbing hills or when passing other vehicles, both engines combine their power to provide better acceleration and higher speed. Additional improvement potential may be obtained with regenerative braking and shutdown of the ICE: waiting on red lights, and traffic jams. • Serial hybrids:
Figure 2.8 Hybrid propulsion concepts – serial configuration In serial hybrids, the wheels are driven entirely by electric motors, with electrical energy supplied by an electric generator. The battery, as an energy buffer, permits complete decoupling of the engine/generator combination from the drive. In this way, the electricity generation may be set independently from the momentarily driving demands and can be optimized for maximum efficiency or lowest contamination possible. Brake energy can be re-used. All energy saving, however, is diminished because all the output of the first engine must pass through the entire electrical efficiency chain. In this combination, the engine/generator- combination must be powerful enough to satisfy at least the vehicles average energy consumption. The electric motors must meet all the requirements for maximum acceleration and hill climbing with a fully loaded car. This implicates that the electric components installed in serial hybrid vehicles resist larger loads than the ones installed in parallel hybrids. Another approach for serial hybrids is the use of range extenders in electric cars, a small ICE combined with a generator is used to extend the battery range. • Mixed hybrids:
Figure 2.9 Hybrid propulsion concepts mixed configuration Disadvantages of the basic parallel and serial hybrid concepts led to the development of mixed hybrid systems. Mixed hybrids combine properties of the presented concepts; it may be for example a serial hybrid that also has a direct coupling of the first engine to the driven wheels. The term “power split hybrid” is used to describe power flow from the combustion engine reaching the driven wheels through various parallel paths.
8 The most famous example for this concept is the Toyota Prius.The advantage of mixed hybrids is the possibility to combine properties of serial and parallel hybrids, but at the same time increases the system complexity and, more important, the advanced, cost-intensive control. In the present master thesis, the use of Stirling engines and thermal energy storage in hybrid vehicles is proposed. For this application, the most adequate concept is the serial hybrid where the Stirling engine has no direct connection to the driven wheels, so the Stirling engine can work at its best operating point, and the batteries serve as a buffer. Stirling engines downsides as long warming up phase and the retarded change of revolution speed are eliminated. Other benefits are that products designed for hybrid or electric vehicles can be used. The more complicated mixed hybrid configuration is not favorable in this application, in only few situations the Stirling engine could directly drive the wheels. The emphasis of this work is the energy concept and its application in vehicles. The focus is energy supply, storage and transformation, as well as vehicle concepts for different types of use.
Figure 2.10 Power electric motor / Stirling engine Figure 2.10 shows the interaction between electric motor and Stirling engine. The electric motor provides a maximal (short time) power output for vehicle propulsion of Pmax, and supports an average (long time) power output of Pd. The actual vehicle power consumption is described by Pactual, the average value of Pactual must be smaller/equal to the power provided by the Stirling engine PSt. In Figure 2.10 Pactual indicates different driving conditions:
• Brake energy recovery: Pactual < 0, generated energy is transferred to the batteries • Acceleration when overhauling or hill climbing: Pd < Pactual < Pmax, additional energy is provided by the batteries • Moderate driving: 0 < Pactual < Pd, excess energy is stored in the batteries Short distances can be completed only using electric motor and power provided by the Batteries.
9 2.2.1 Comparison of different vehicle assemblies
Figure 2.11 Efficiency of different vehicle concepts [11] Figure 2.11 A and B show the efficiency of conventional vehicles with single ICE and mechanical transmission reached in their best operation point. Most of the energy is lost in heat, about 60%, and the remaining energy for propulsion is about 30%. Diesel engines have better efficiency than gasoline engines (36% to 28%) [12]. In normal drive cycles the efficiency is about 15% to 20% due to idle running engines when traffic stops, braking energy losses and not-optimal operating point of the ICE. Fuel cell powered vehicles (D) lose most part of their energy for supply and use of H2 (60%) [12]. Finally remain 17% for net propulsion in optimal drive cycles and about 13% in typical cycles. Vehicles with energy storage in Li-Ion- batteries (C) offer a significant better efficiency, there is no need to transform energy, and therefore 72%/55% for ideal/typical cycles can be reached.
Figure 2.12 Hybrid vehicle with Stirling engine; Solar power tower efficiency [1]
10 Figure 2.12 (left) shows the complete energy chain for a hybrid vehicle with thermal energy storage and Stirling engine, beginning with solar thermal energy and finishing with net propulsion. Thermal energy is stored in a PCM and transferred by a heat exchanger to a Stirling engine. The Stirling cycle converts about 30% of this energy into electrical power used by an electric system for propulsion. Possible improvements are heat and brake energy recovery. As a reference, in Figure 2.12 (right), the energy chain for a solar power tower is presented. About 19% of the solar energy is converted into electric energy [1].
2.2.2 Vehicles with Stirling Engines – History Years ago several companies worked on alternative vehicle concepts, especially Ford and General Motors presented outstanding concepts. Amongst others, investigations were made to implement a small nuclear reactor in personal vehicles. Nevertheless, none of them passed to series production, many concepts didn’t even reach the prototype status. For the present investigation, the first reference found is from 1969, a General Motor prototype, based on an Opel Kadett body, where a Stirling engine and a electric storage and drive system were built in. Popular Science [13] had the possibility to test drive the car and provides in its report interesting information: The vehicle was planned to reach low emissions, by using a Stirling engine and an electric drive system. Main components were 14 automotive type 12 volt batteries and controls stored under the hood and a Stirling engine in the back. The Stirling engine works at 2800 r.p.m., running quiet. The three-phase induction motor is able to accelerate the vehicle in 10 to 20 seconds to 30m.p.h which is about twice the time of a normal car of that year. Total weight is about 3100 pound, including two passengers. Maximum average speed was 30 m.p.h., maximum speed 55 m.p.h. The maximum distance depended on the size of the gasoline tank. In the driven version the vehicle was able to drive about 150 to 200 miles on gasoline and 15 to 30 miles with only electric power. Start-up time of the car was limited by the Stirling engine and took about 20 seconds.
Figure 2.13 GM Stir-Lec I 1969, (http://www.bangshift.com) In 1971, the Department of Energy (DOE, United States of America) started a research program about the use of Stirling engines in vehicles. Together with other institutions and enterprises, like the NASA, General Motor (GM) and Stirling Thermal Motors Inc. (STM), to mention the most important ones, they achieved to build several prototypes with fuel tanks, Stirling engines and direct power transmission between the engine and the wheels. Technical reports [14], [15], [16], show that these cars had properties similar to conventional cars of that epoch. Still, the Stirling engines were problematic. The last prototype found in this research was a ’95 Chevrolet Lumina, this time a hybrid vehicle with an STM Stirling engine, driving a generator in a series hybrid configuration. The prototype did achieve its emission target, demonstrating half the ultra-low-emission-vehicle (ULEV) standard. On the other hand, this vehicle failed to meet several key requirements: specific shortcomings included lower-than-expected thermal efficiency, high heat rejection requirements, poor specific power, and excessive hydrogen leakage [17]. For this reasons, and additionally the dropping oil prices at the end of the near-east crisis, there were no plans for further development of this concept. Until now, the DOE did not publish further research. SBI
11 (former: STM) continues in the development of Stirling engines opening up new business in Stirling engine power generators. Another important source is [18], a detailed design report for a TES system and its implementation in a compact sized automobile. Materials for storing thermal energy are LiF and NaF/MgF2. Propulsion motor is a Stirling engine, the thermal-electric serial hybrid approach is not described. Total tank weight is about 500kg for approx. 100kWth. 2.3 Patents • History The patent US 5,634,339 describes a turbine adapted to thermal energy storage. Turbine and tank are connected by a working fluid. The thermal storage is recharged by a hot fluid, gasoline or electric energy. The US patent 5,172,784 mentions a hybrid vehicle with external combustion engine, based on liquid combustibles. W. Fopper describes in the documents DE 19,734,733 and US 6,272,856 [19] a method for storing thermal energy and its use in vehicles. Electric energy is used for heat generation. The pending Mexican patent Mx/a/2008/015984 describes a mobile heat-storing device, using molten salt for energy storage. • Patent application MX/A/2009/ 000965 The pending Patent MX/a/2009/00965 named “Sistema Recolectar De Energía Aplicable A Autos Hibridos” describes a system to gather solar energy and the use of this energy in hybrid vehicles. The vehicle’s main component is a thermal energy storage system. The TES is previously filled in fixed places and afterwards installed in the vehicle. A heat exchanger connects the thermal tank with a Stirling engine. An electric propulsion system moves the car. The Stirling engine can be exchanged by any element able to convert heat energy into mechanical energy, as for example a closed Brayton cycle turbine.
Figure 2.14 Hybrid Vehicle with Stirling Engine and Thermal Energy Storage Figure 2.14 shows the vehicle concept. It follows a description of the individual parts and functions: Solar energy (1) is gathered in a fixed place like houses, gas stations or supermarkets, and stored in a thermal tank (4). The tank consists of isolating cover to avoid heat losses and a phase changing material (2) which is able to store thermal energy. In chapter 2.5 a resume of heat storage materials is presented. Once charged, the tank is placed into the car and connected to a heat exchanger (5) which transfers thermal energy to the hot side (6) of the Stirling engine (7). The heat exchanger has a valve to regulate the quantity of heat transferred. By cooling the cold side (8) of the Stirling engine a temperature gradient is created and the Stirling engine starts to produce mechanical energy (9). A generator (10) converts the mechanic energy into electric energy (11). An electric propulsion system (12) can be used to move the car. To improve the efficiency of the vehicle a cooling system is installed which uses the waste heat of the Stirling engine to heat one layer of the tank.
12 The temperature gradient between storage material and ambient air temperature is reduced. Air from the outside of the car (13) is used to cool down the cold side of the Stirling engine (8), the warmed-up air (14) is guided into one layer of the tank (4) and finally leaves the car (15). • Other Patents US 7,469,760 B2 “Hybrid electric vehicles using a Stirling engine”: Dean Kamen claims a personal vehicle for transporting a user over a surface. Components are an external combustion engine, generator for converting the mechanical energy to electrical energy, electric storage system and drive motor in serial hybrid configuration. To generate heat liquid fuel or natural gas is burned.
Figure 2.15 US 7,469,760 B2, US 2008/0121755 A1
US 2008/0121755 A1 “Rankine-Brayton Engine Powered Solar Thermal Aircraft”: This document describes a solar thermal powered aircraft with a Rankine-Brayton hybrid cycle heat engine. It has a thermal battery, preferably containing a lithium-hydride, which is connected by a working fluid to the engine. A solar concentrator, such as a reflective parabolic trough, is movably connected to an optically transparent section of the aircraft body for receiving solar energy. The concentrated solar energy is collected by a heat collection and transport conduit, and heat transported to the thermal battery. US 2009/0313994 A1 “Self-pressurizing Stirling Engine”: Describes an improved Stirling Engine for the patent prior mentioned.
2.4 Solar Energy System Design Solar energy is the most powerful renewable energy source [1].
Figure 2.16 Solar Radiation in kWh/ (m², year) [11] Solar radiation can be expressed by energy per surface and year. In Western Europe (Germany) each year in 1m² about 1000kWh can be collected. In other regions, close to the equator, it can be up to 2500kWh/ m² [1]. For higher energy reception tracking mechanisms can direct surfaces towards the sun. Especial in southern and northern regions this has great effect to the efficiency. In Southern Spain actual photovoltaic panels with 2-axis tracking can gather an annual yield of about 340kWhe/ m² and in the northern Sahara this value increases to more than 400kWhe/ m² [1]. • Gathering Solar Energy The purpose of a solar collector is to intercept and convert a reasonably large fraction of the available solar radiation [20]. Non-concentrating systems work at a temperature below 100°C, to rise the temperature, the following actions can be taken [1]: Modify the absorbing surface in order to reduce the losses by radiation, 13 convection and thermal conduction. Increase of the radiating power on the absorber by focusing and concentrating the incident solar radiation. • Solar Thermal Systems Solar radiation can be concentrated by mirrors or lens to attain higher temperatures [1], see Figure 2.17.
6000 5489 5000
4000 3657 3000 1937 2000 970 1000
absorber temperature °C temperature absorber 426 120 0 1 10 100 1000 10000 100000 concentration ratio
Figure 2.17 Concentration Rate vs. Absorber Temperature [1] For solar thermal systems solar radiation is converted into thermal energy at the desired temperature. It may be stored in a thermal energy storage material and then converted into electricity.
Figure 2.18 Reflecting Schemes for Concentrating Solar Energy [20] • Parabolic trough Parabolic trough concentrates incoming solar radiation onto a line running the length of the trough. A tube (receiver) carrying heat transfer fluid is placed along this line, absorbing concentrated solar radiation and heating the fluid inside. The trough must be tracked in one axis. Because the surface area of the receiver tube is small compared to the trough capturing area (aperture), temperatures up to 400oC can be reached without major heat losses [20]. • Parabolic dish A parabolic dish concentrates the incoming solar radiation to a point where a Stirling engine can be mounted or a material can be heated. Parabolic dishes must be tracked in the two axes to follow the sun during the day [20]. • Central receiver system A central receiver system consists of a large field of independently movable flat mirrors (heliostats) and a receiver located at the top of a tower. Each heliostat moves in the two axes, during day time, to keep the sun's image reflected onto the receiver on the top of the tower. The receiver, typically a vertical bundle of tubes, is heated by the reflected insolation, thereby heating the fluid passing through the tubes [20].
14 • Fresnel Lens Plane Fresnel lenses can act as spherical lenses. By their flat design, they offer space and weight saving, short focal lengths and lower prices. They are widely used for camera field lenses or rear projection screens.
Figure 2.19 Fresnel lens [21] In Mexican patent application MX/a/2008/016474, “Lente concentradora de energía solar por refracción con alta eficiencia”, a Passive Tracking Enhanced Solar Concentrator Device Performed by Fresnel lens is described. The main idea of this design is to create a simple structure able to track the suns trajectory without any movement of the lens. This guides to an effective and low priced system for concentrating solar thermal energy. In practice, computed ideal values for concentration and temperature (see Figure 2.17) cannot be reached because of different natural dissipation factors. The substantial dissipation factors imply [1]: Mirror errors (incomplete reflection or surface defects); absorber losses (reflection, radiation, and convection); orientation errors (tracking, absorber adjustment, and oscillation due to wind load). • Photovoltaic Systems For photovoltaic systems, intercepted solar energy is converted directly into low voltage direct current electricity [20]. They are based on the photoelectric effect which was discovered first in 1893 by Alexander Bequerel. In a solar cell, the production of the energy takes place completely without moving parts and completely without noise. The solar cell does not suffer changes during the energy production process.
2.5 Materials Thermal energy storage materials can be classified into three different types: • Sensible Heat Storage Thermal energy can be stored in a material as sensible heat by raising its temperature. • Latent Heat Storage Thermal energy can be stored in a material by using the energy absorbed or released during the isothermal phase change. • Chemical Reaction Energy Storage Thermal energy can be stored by using the endothermic reaction of the chemicals between different materials. When the process is inversed, the heat is released. In [19], materials are described for thermal energy storage up to 850°C. Reviewing the results, it stands clearly, that the presented materials cannot store sufficient energy per mass and/or volume to be used reasonably in vehicles. This justifies the search for materials, which melt or respectively evaporate at higher temperatures but at the same time offer a better power density. Another option can be materials that have a double phase change in the temperature range of the thermal tank and the Stirling engine. They can store a high amount of energy in a relatively small temperature range. The important characteristics of materials for latent heat storage [22]: • Thermal properties The phase change temperature is fitted to the application, with high change of enthalpy within these temperatures and, in most cases, with high thermal conductivity in both liquid and solid phases. • Physical properties A low density variation, high density is important. Undercooling should be avoided.
15 • Chemical properties Stabile materials should be used with no phase separation. They should be non-toxic, non-flammable, non- polluting and compatible with its container materials. • Economic properties Materials have to be cheap and abundant.
10.00
8.00
6.00
4.00
2.00
0.00 Molten Salt NaCl Molten Glass Boron Nitride Gasoline Battery Li-Ion H2 700bar (800°C) (600 - 1000°C) (2400°C)
specific energy kWh/kg 10.75 0.37 0.66 4.44 0.16 1.67 energy density kWh/l 7.75 1.03 1.77 10.00 0.30 1.11
Table 2.1 Energy Storage In Table 2.1 gasoline and alternative energy storage is compared. The table gives a short overview to actual and possible future energy storage systems. Gasoline has a great specific energy, energy density and it is easy to handle. Tanks for gasoline have to resist forces in case of accidents, but there is no need for thermal insulation. Hydrogen under pressure at 700 bar or at low temperature is lightweight and compact [23], but it is difficult to handle and its production is inefficient [12]. Electric energy storage in these days is relatively heavy and needs a major size to store sufficient energy for the use in vehicles. In addition, many of the batteries contain toxic substances. Best actual technologies are batteries based on Li-Ion [23]. Future technologies with Lithium Titanate are expected to have better specific energy and energy density [23]. Some other possible storage materials not mentioned in the table are compressed air, flywheels [23] and chemical energy storage as for example hydrogen stored in Mg [24]. Last and most important for this work is the thermal energy storage: Boron Nitride at 2400°C is able to store more energy per liter than gasoline fuel. Also its specific energy is high. Still, insulation and security is a problem to be concerned of. In the table there are two more materials which can store thermal energy by phase change, molten salt (NaCl) and molten glass. These materials do not have the high energy density of gasoline, but still they are cheap and available. The glass properties can be designed depending on the needs of the application by changing its chemical composition. The proposed glass is soda-lime-silica (SLS) glass with SiO2 73.08%, MgO 0.18%, CaO 12%, Na2O 13.34%, K2O 0.12%, Al2O3 0.9%. This glass can be obtained from recicled bottles. There are some limitations for the Table 2.1: Gasoline and PCM do not consider tank and insulation. Batteries and hydrogen storages have already the storage containers calculated in energy density. Also, the value of the energy is different: Electric energy stored in EES can be converted directly to mechanical energy with motors at efficiencies above 90%. To convert Gasoline fuel or thermal energy into mechanical energy engines which have lower efficiencies at about 30%, such as Stirling engines or ICEs are required. Finally, the grade of contamination during the whole life cycle of the different energy storage systems is not considered in this evaluation.
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2.6 Thermal energy storage tank The thermal energy storage tank is the element that allows to store and move solar thermal energy.
Figure 2.20 Virtual Prototype of Solar Energy Storage Tank [25] Its Basic function is to store energy in terms of high temperatures. But this implies many other functions and restrictions. The component must be chargeable by concentrated solar energy. It must deliver sufficient thermal energy to the Stirling engine. Cost, manufacture and security are critical points in the design. Due to the high temperatures, complex chemical material properties must be considered. or the use in a vehicle the main criteria are: Size, weight, stored energy and security. Other factors, such as ease to use, and outer temperature must be considered. In a conventional vehicle, engine parts can become hot: Cooling liquid can be heated up to 115°C, oil up to 140°C, and the exhaust can reach temperature above 600°C. In that matter, from the automotive engineering point of view, the outer temperature of the tank could be relatively high. However, the tank has to be moved, after charging it with solar energy. Persons might get in direct contact with surface areas, injuries must be avoided. The European norm EN 563 “Safety of machinery. Temperatures of touchable surfaces. Ergonomics data to establish temperature limit values for hot surfaces” is consulted to define a maximum outer temperature. Touching a powder-coated metal surface has the following maximum temperature without causing skin injuries: 70°C touching max. one second, 51°C touching up to one minute. Based on this data, maximum temperature allowed is defined as 65°C, avoiding injuries by accidental contact. Handles have to be insulated apart.
The design of the TES is out of scope of this work. Prices of 450MXN/kWth [19], 1500MXN/kWth [18] and 700MXN/kWth [L.D. Garcia, Tecnológico de Monterrey, “TANQUE SOLAR DE ALTA TEMPERATURA”] can be found.
2.7 Heat exchanger The heat exchanger connects Stirling engine and thermal energy storage. In its design the main characteristics are: thermal conductivity, weight, size, energy losses and prices. One important element is the regulation of heat flow. A valve must be able to transfer sufficient energy for full function of the engine or must shut down the complete energy transfer if necessary. The heat exchanger can be based on solid materials like copper, transmitting the thermal energy by conduction. Another possibility in the development is the use of heat pipes as a temperature regulator to transmit just a fraction of the stored heat. In MX/a/2009/008186 “Intercambiador de calor para tanque recolector de energía solar” a heat exchanger for solar energy storage tanks is presented.
17 Chapter 3. Macro vision This chapter presents alternatives and concepts for energy use, which are not directly connected with the hybrid vehicle. This includes alternative uses for the TES for air-conditioning and basic thoughts about over- all energy concepts. In a first step, the Fresnel lens, heat exchanger, thermal tank and ECE can be combined to an electric generator for production of distributed electrical energy for homes and industries working 24 hours a day. In this phase, the different components are less delicate in their design; they can be heavier and have less security restrictions. Furthermore, they are not exposed to shocks and vibrations and all components are joined with fixed interfaces. One of the most important benefits of this generator is that it can be designed completely self-sufficient, that means that no resources are needed – apart of solar radiation – to generate electric current. The next step would be to design a mobile generator by creating a box for all components and by making all parts to resist environmental influences such as rain and shocks amongst others. The whole generator could consist of different modules which will be assembled at the point of destination. This results in a grid- independent generator for 24h power supply in remote areas. The thermal energy storage tank can also be used as energy source in refrigeration with processes based on the phenomenon of absorption cooling: In a boiler a concentrated ammonia solution is heated by heat which is extracted by a heat exchanger from the thermal energy storage. The ammonia vaporizes and is translated into a condenser where it liquefies. Supplied with hydrogen, it evaporates and extracts heat from the storage container. The ammonia gas enters the absorber where it is reabsorbed in a weak solution of ammonia. To close the process, the saturated solution flows back to the boiler where the whole cycle starts again. For example WAECO mobile solutions, http://www.dometic-waeco.com/, offers mobile absorption refrigerators which can be connected to the grid or be used with natural gas. The same principle is also applied for air conditioning. It could be used in vehicles with heat stored in TES or by using waste heat of the ICE or the Stirling engine (see 4.2.5 ).
Figure 3.1 Absorption cooling [http://www.dometic-waeco.com/]
3.1 Vehicle to Grid The University of Delaware [26] presents a concept called vehicle to grid, connecting electric and hybrid vehicles to the power grid. Vehicles can communicate with the power grid, recharge their battery when energy is cheap and sell their power at peak time when energy is expensive. This helps for more uniform power plant utilization. Additionally hybrid vehicles can support to cover the peak energy consume during the day. Owners of hybrid vehicles powered by a Stirling engine can connect their car to the power grid when it is parked and earn money with the remaining heat in their tank.
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Figure 3.2 Vehicle 2 Grid [26] In the case of using thermal energy storage and Stirling engines, this concept can help to maximize energy use and over-all efficiency. Assuming a vehicle that is used 5 days a week, consuming only 2/3 of the energy stored in the TES a day. And supposing every day a TES is heated by solar energy and not used thermal energy at the end of a day would be lost if not sold to the grid. We can calculate the following relation: 47.6% (5 days, 66.6%) of the energy is used for vehicle propulsion and 52.4% (2 complete days, 5 days with 33.3%) can be sold.
3.2 Competitors and economic rivals Over-all benchmark must be vehicles based on gasoline fuels. But there is another concept that has to be considered: electric vehicles used with electric energy obtained by solar energy. • Photovoltaic System One actual approach to use solar energy for transportation is solar carports. They have solar cells on the roof converting sunlight into electricity by the photovoltaic effect.
Figure 3.3 Solar Carport (www.dachscheich.com) Research in Germany showed the following pricing: Example is a 11,7kWp photovoltaic solar cell carport for four cars. On a ground surface of 8m * 10m a light metal structure is built, on top solar cells are able to produce 11,700 kWh average electric energy per year. This is about 0.4 kWh/m²/day. Prices are 70,000€, including the complete system, structure, solar cells etc. but without taxes and energy storage system. Energy has to be used directly or sent to the local power grid.
19 • Microgen: Stirling engine, TES and solar collector [27] presents cost estimation for a system with 7.4kWh/day electric power output. It uses the following six key components: (1) Stirling Engine generator (Microgen, 1kW); (2) Solar Collectors (Parabolic trough systems 18m², 30kWh/day, England); (3) Heat transfer system; (4) Thermal Storage (48kWh); (5) Supplementary heat supply; and (6) Control System. The estimation for such a system is between 5,000 and 6,000US$. Average energy generated is 0.4kWhe/m²/day. • Sizing for solar collectors Average solar insolation in Monterrey is 4.8 kWh/m²/day, with highest values in May (6 kWh/m²/day) and lowest insolation levels in December and January (3.3 kWh/m²/day) [28]. Assuming a Fresnel lens with 65% efficiency, a receptor with 80% efficiency, a TES and Stirling engine converting 30% of thermal energy in electric energy we can calculate the following values: 0.7 kWh/m²/day of electric energy average a year, 0.94 kWh/m²/day in summer and 0.51 kWh/m²/day in winter. For Germany (average solar insolation is 2.5kWh/m²/day) a yearly average of 0.39kWh/m²/day of electric energy can be expected, which is similar to the efficiency of photovoltaic systems, but with a more stable, 24h power supply.
3.3 Problems and backup solutions Using solar energy has many advantages, but also presents a great problem: Availability of direct sunlight. As can be seen in Chapter 2, in many places around the world great quantities of solar energy are available. There are cities which have sunny weather and hot temperatures nearly every day, but in rain periods or in the winter they do have clouded days or weeks. For using the hybrid vehicle continuously every day, backup strategies are necessary. Many solutions are possible, the technical and economic feasibility differ depending on the alternative energy source. Mainly, there are the following three strategies: • To heat the TES by a non-solar heat source, such as natural gas o electric energy • To use an alternative heat source inside the vehicle to drive the Stirling engine • To use the hybrid vehicle as an electric vehicle, with a limited range or with additional batteries. Evaluation criteria for the backup solutions are initial price, maintenance price and recharge costs.
20 Chapter 4. Application: Minibus This chapter presents the application for the proposed new vehicle concept. Nowadays, there is no vehicle using stored solar thermal energy. But there are already vehicles with serial hybrid drive trains based on liquid fuels. In addition, there are only few Stirling engines available and in this days there is not one commercial vehicle with an external combustion engine in series production. However, joining these different technologies permits a new vehicle layout. To compare the new vehicle layout against actual vehicles, first actual products are shown. By knowing their properties they provide valuable information in the development of new technologies. Another important fact is that our proposed vehicle will finally be compared by possible clients to actual products and the concept must offer an advantage in at least the most important characteristics. The second step in this chapter is to present different vehicle uses and to work out the best application for the new proposed concept. The actual student transportation service at Tecnológico de Monterrey, campus Monterrey, will be shown. Its aim is to find a possible prototype application with commercial background, which can be used within the ITESM community.
4.1 Market Situation Serial Hybrid Buses There is no company offering mass production of serial hybrid buses for urban transport (2009). Only few companies offer small scale industrial production of hybrid buses, for example as a variation of a conventional bus with ICE. Main reasons are the high prices for electrical components, more weight and less space for passengers. The savings for less energy consumption still do not cover the higher prices of the vehicles. The main market is the United States of America, where the government has several programs to help transit agencies to renew their infrastructure and to invest in green technology. The federal Clean Fuels Grant Program covers 90 percent of the incremental cost of alternative fuel buses, including hybrids. In addition, the Federal Transit Administration (FTA) covers up to 80 percent of the purchase price of a standard diesel bus [29]. Therefore, the price to be paid by transit agencies for a 40-foot diesel-electric hybrid bus of normally $450,000 - $550,000 is only $80,000 compared with $60,000 for a conventional diesel bus (normal price $300,000) [29]. In 2006, there were more than 900 hybrid buses in regular service in more than 40 transit agencies in North America with several hundred ordered for future use. Some of the largest hybrid fleets in the United States include New York City’s fleet of 325 buses with an additional 500 vehicles on order. King County in Seattle operates 214 parallel hybrid buses. Washington DC has placed an order for 100 diesel-electric hybrid buses. In addition, more than 15 states such as California, Connecticut, Indiana, Kentucky, Mississippi, New Mexico, Pennsylvania, Tennessee, and Texas, use hybrid buses in their transit fleets. [29]. In 2009, the Daimler bus brand Orion already has 1,700 hybrid buses in day-by-day operation in North America, which makes it the world market leader for hybrid technology in commercial vehicles [30]. Some of the products offered in the USA: • Azure Dynamics Azure Citibus
Figure 4.1 Azure Citibus [31] Serial hybrid configuration with 150kW electric engine and General Motors Gen III Vortec 4.8L V8 gasoline engine with 270hp [31].
21 • Daimler Buses North America – Orion VII Hybrid:
Figure 4.2 Daimler Buses NA - Orion VII Hybrid Serial hybrid with diesel generator and electric motor, the price is about US$ 514,000. • EBUS
Figure 4.3 EBUS [32] Based on a 6.7m (22 foot) chassis, the EBUS is designed to seat 22 passengers plus standees. There are different engine options, amongst others a serial hybrid configuration with a 30kW micro-turbine. They also offer a 90kW fast charger for the batteries. Top speed is 72km/h. For basic configuration, the hybrid-electric bus starts at $325,000. • DesignLine
Figure 4.4 Transit Bus Configuration; DesignLine Bus [9] 22
DesignLine created a 10.5m long transit bus. It has a weight of about 17,200kg and a 30kW Capstone microturbine. It can be seen for example in New York or in Tokyo. They show comparatively good fuel efficiency and a quite driving mode [9]. In December 2009, DesignLine had received orders for 458 hybrid and alternative fuel buses. Selling prices mentioned in several newspapers differ from 559,000 US$ to 605,000 US$. • Solaris Urbino 18 Hybrid
Figure 4.5 Solaris Urbino 18 Hybrid [33] In various European cities this 3-axis, articulated bus can be seen. It is build by Solaris Bus & Coach S.A., Poland [33]. Length is 18m and up to 161 persons can be transported. It has a mixed hybrid configuration with Allison Ep 50 drivetrain. The diesel engine has 250hp and it counts with two electric motors of each 75kW. The life of a set of NiMH batteries, weighing 410kg, is approximately six years. Price tag is about 470,000€. • Toyota Coaster Hybrid In Japan and Australia in 1997 Toyota introduced the first serial hybrid minibus onto the market: The Coaster Hybrid. It uses a 1.5-liter engine and generator with 25kWe output to constantly generate electricity for the electric motor of 70kW that provides the vehicle’s power. Because the Coaster Hybrid engine operates at a nearly constant rate, its emissions of nitrogen oxide and other harmful gases are lower. Compared to the ordinary Coaster, which runs on a diesel engine, fuel efficiency is 10 percent better and carbon dioxide emissions are 20 percent lower. The vehicle has a maximum speed of 80 km/h and a driving range of between 400 km and 500 km. Its price was more than the double than a conventional Coaster with diesel engine [34].
Figure 4.6 Toyota Coaster Hybrid EV[34] 23 • Mercedes Benz Citaro G BlueTec Hybrid city bus
Figure 4.7 MB Citaro G Blue Tec Hybrid city bus [30] Mercedes-Benz is testing an 18-meter long city bus with diesel generator of 4.8l cylinder capacity (160kW) and four electric wheel hub motors with 320kW total output. Electric power is stored in Lithium-Ion batteries. It is a serial hybrid assembly and planed to be available late 2010.
4.2 Components
4.2.1 Market situation: Stirling Engines Research and development emphasis of Stirling engines is in the United States of America, Germany and Japan. Many companies count with technology and engine prototypes, but few have developed a product and less can offer a product to its customers. There are many small model engines, but engines with considerable electric power output are hard to find. Main applications are the power generating with parabolic reflectors and combined heat- power units. The following list shows some of the enterprises or research centers that provide information of their products in the internet. Products available: • Dieter Viebach, Germany, http://www.geocities.com/viebachstirling/: Gamma Stirling engine, 0.5kWe, was available as construction set based on cast and standardized parts, today only constructions plans in sale. • Whisper Tech Limited, New Zealand, www.whispergen.com: Offer several versions of a 1kWe combined heat and power supply system, running on gasoline fuel, designed for stationary or mobile use. Available in many countries, in America (2010): Canada, USA and Caribbean. • Stirling Biopower, former STM, USA, http://www.stirlingbiopower.com/: Offer a 43kW four piston Stirling engine for distributed energy generation, the engine runs on broad range of gaseous fuels or hot air streams. Temperature range is between 750°C and 950° for bio-fuels. • Stirling DK, Denmark, http://www.stirling.dk/: Offer a 35kWe Stirling engine. Fuel is wood chips, for power plants there is the possibility to combine two o more Stirling engines. • Sunmachine GmbH, Germany, http://www.sunmachine.de/: They sell a 3kWe combined heat and power supply, based on wood chips combustion. Several other applications in development. • Cleanergy AB, Sweden, http://www.cleanergyindustries.com/: Sell a 9kWe Stirling engine with natural gas or solar energy as power sources. The design is based on the former SOLO 161. Technology available but no product in sale for residential o small business use: • Sunpower Inc, USA, http://www.sunpower.com/: They have developed free piston Stirling engines of 7.5kWe up to 35kWe, but it is not available at the moment. Sunpower is searching for potential partners and investors. • Infinia Corporation, USA http://www.infiniacorp.com/: Present free piston Stirling engines. They have developed a dish-style solar concentrator connected with a free piston Stirling of 3kWe, still not available in market for private users. Infinia is searching for potential partners and investors willing to buy engines with the total output of 1MWe or more
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Table 4.1 Benchmark Stirling Engines 25 • SES Stirling Energy Systems, USA, http://stirlingenergy.com/: Have designed a 25kW solar power system, it consists of 11.5m diameter concentrator dish and a 4 cylinder Stirling engine. They are looking for clients buying systems ranging from 1MWe to 1,000MWe or more. • Kockums AB, Sweden, http://www.kockums.se/: Have built a submarine with 75kWe Stirling engine. Its goal was a power supply independent from the air in the submarine. It Used is pure oxygen stored in liquid form and diesel fuel. • BSR Solar Technologies GmbH, Germany, http://www.bsrsolar.com/: Company with the aim to develop systems powered by renewable energies, no product was available. • Stirling Technology Inc., USA, http://www.stirling-tech.com/: Plan to produce a 5hp Stirling for solid bio-fuels, sales are planned from 2010 . • Stirling Systems, Germany and Switzerland, http://www.stirling-systems.ch/: Combined heat and power system, 1kWe, prototypes in field tests, but no product for sale. Vehicles with Stirling engines: • DEKA Research and Development Corporation, USA [35]: Dean Kamen and his team are working in the development of small Stirling engines for remote power supply and water purifying. They present plans to implant the 1kWe engine into a small electric car from THINK Norway [36] as a range extender based on gasoline fuel. • Precer AB, Sweden [37]: On their homepage, they present a two person ATV, with a Stirling engine burning pellets and an electric engine of 12kW or more. The electric motor has 16hp. Total weight is 400kg and it consumes about 1kg of wood pellets in 10 km. There are two major directions in the design and research areas of actual available engines: relatively small engines for combined heat and power production or for the use in solar dishes. And on the other hand engines with more power output for the distributed energy generation based on all kind of bio-fuel and renewable energy sources. See examples in Table 4.2. Most of the engines are designed for application where the weight and volume is not important. MicroGen Stirling Biopower MEC FleXgen G43 Engine Type Free Piston 4 Cylinder Stirling Cycle Energy Source Natural Gas Natural Gas Temperature 250°C to 550°C 750°C to 950°C Output 1kWe, 240V 43kWe 480V Efficiency 27% 28% Dimensions . 40 * 40 * 60 cm 147cm*257cm*86cm Weight 50kg 1606kg Table 4.2 Stirling Engines: Commercial Applications In Table 4.1 a benchmark of available (end of 2009) Stirling engines are provided. In the case of availability, an order can take several months to be considered. In the future, prices are supposed to drop, estimations of about 2,000 US$ per kWe can be found [27]. These prices must also be compared to the competitors: for example, in spring 2010 a Capstone 30kW micro turbine costs about 45,000 to 60,000 US$, depending on the exact configuration.
4.2.2 Cooling system for Stirling engine Stirling engines convert thermal energy into mechanical energy, but they need to maintain low temperatures on the cold side of the engine to keep the temperature difference. The Stirling engine converts about 30% of the heat into electricity. The remaining 70% leave the engine as heat. Powerful cooling is needed. An alternative is to recuperate the waste heat, or using it for air-conditioning. Current vehicles with ICE are operated with cooling liquid of about 80°C, and try to keep the temperature below 100°C. Maximum temperature is 115°C. The cooling systems use circulating water mixed with additives to remove heat from the cylinder heads towards a heat exchanger (radiator) where it is cooled by the air stream. In comparison,
26 Stirling engines need lower temperatures. The cold side of the engine should not be higher than 65°C (depending on the Stirling engine). Lower temperatures are desirable to rise efficiency.
4.2.3 Electric Energy Storage The ideal battery for the use in hybrid vehicles would have the following properties: a high specific power, high specific energy/density, high charge acceptance in recharging long cycle life, low temperature tolerance, minimal exothermic tendencies, and all at low cost. Still, such a battery does not exist yet [38].
Specific Specific Energy Estimated energy power efficiency cost (Wh/kg) (W/kg) (%) Cycle life (US$/kWh) Lead-acid 30-50 150-400 80 500-1000 100-150 Nickel-cadmium 30-80 100-150 75 1000-2000 250-350 Nickel metal-hydride 60-120 200-300 70 1000-2000 250-350 Lithium-polymer 150-200 350 na. 1000 >400 Lition-ion 80-200 200-300 >95 1000-1500 >450 Figure 4.8 Comparison of Battery Characteristics [38] [38] states, that Lithium-ion is the best actual technology for the use in electric and hybrid electric vehicles. Lithium-ion is abundant and chemically superior to other batteries. Limiting factor for cost reduction are expensive cobalt-oxide cathodes with roughly 50% of total cell costs. More than 80% of Li-Ion cells globally are produced in Asia. Self discharge is relatively low at about 10% a month. Operating temperatures are - 20°C up to 60°C. Cycle life is about 1500 charging cycles, which is a great downsize, it means, that in a daily used vehicle the batteries should be changed after approximate five years. The Limiting factor is the integrity of the electrodes. The temperature of the battery has to be monitored carefully. High temperatures of about 250°C can damage the battery or cause accidents. Resumed, the advantages of Li-Ion batteries are [38]: • Highest specific energy and energy density compared to other common battery types • Inherently high voltage allows for superior capacity and specific power • Potential of long cycle life and smaller cells • Significant room for cost reduction with development of future cathodes • No memory effect and low self discharge • Relative abundance of lithium • Are less exposed to metals prices than NiMH Actual disadvantages: • Still more expensive to manufacture than lead-acid, NiMH and other current batteries • Significant safety concerns and public scrutiny - must use external monitoring to maintain cell current and temperature levels • Poor overcharge tolerance requiring careful monitoring • May age when not in use, that means, capacity will degenerate over time • Poor low-temperature performance, a significant issue when used extreme climates. • Still under development - new metals and chemicals are still being researched • Large quantities may be subject to transport restrictions An price research in the internet in 2009 gave prices about US$ 750 per kWh storage capacity, depending on number of batteries bought and specific characteristics. Examples for suppliers are Saft (http://saftbatteries.com) and A123systems Inc. (http://www.a123systems.com/).
4.2.4 Electric engines There are several concepts to drive vehicles by electric motors. Main requirements are high torque at low speeds and high power at high speeds. A wide revolution range is important. High efficiency in the mainly used speed/power combinations is desirable. A high short time over load capacity for boost acceleration and regenerative braking is needed.
27 Depending on the choice of the electric engine, the over-all vehicle efficiency is highly affected. It is important to consider that the efficiency of electric energy use in the proposed concept is basic to bring down size of TES and power needed from Stirling engine. Nevertheless, the scope of this work does not include details of the electric system and a general electric engine will be used. In an optimization process the electric engine is one of the components which can easily improve the vehicle performance. The main variables in the decision process are: type (AC or DC), continuous power, maximum power and torque, torque and speed characteristics, currant and voltage characteristic and type of cooling. For a vehicle built from the scratch electric hub motors should be revised as an option. This type of engine allows new vehicle space concepts. They are compact and allow completely independent wheels. Their disadvantage is their unsprung mass which highly affects to vehicle dynamics. Examples for providers are Electric Motor Works Ltd. (http://www.pmlflightlink.de/) and e-Traction (http://www.e-traction.com/). • Efficiency map Depending on speed (rpm) and power output (kW), the electric motor works on different efficiency.
Figure 4.9 Motoring efficiency map Power Phase 145 Provided by UQM Technologies, Inc (www.uqm.com), Figure 4.9 shows the efficiency map of the SPM218- 143-3 Motor/Generator.
4.2.5 Air conditioning In conventional vehicles, air conditioning works with mechanically driven compressors. Testing several passenger vehicles, the ADAC (Europe's largest automobile club), showed that the extra fuel consumption is about 5% to 20% [39], depending on vehicle type, driving profile and weather. In a Stirling hybrid vehicle, there is no possibility to grab mechanical energy from the Stirling engine. The air conditioning has to work with electrical or thermal energy. To compare these two possibilities, a small calculation is shown, based on the mobile absorption refrigerator Dometic Combicool RC 1700 EGP (www.dometic.com). According to manufacturer information, this cooler spends in 24 hour use either 1.6 kWh on 230 volts, 170 Ah on 12 volts or 187/252 g of gas. This corresponds to 2kWh electric energy on 12V. Natural gas has a heat of combustion of 10 to 14 kWh/kg. As an average estimation, the refrigerator spends 0.225kg * 12kWh/kg = 2.85 kWh of thermal energy a day. To generate 2kWh of electric energy a Stirling engine with net efficiency of 30% needs 6.6kWh of thermal energy. Result: On the one hand we have less than 3kWh of thermal energy using absorption cooling. On the other hand, we have 6kWh of thermal energy, to be converted to electric energy by a Stirling engine. To avoid this double
28 transformation it is convenient to use directly the stored heat in the hybrid vehicle. By this way, the Stirling engine can be downsized. Also, the TES can be divided in a big tank for electric energy generation and a small tank for air conditioning. Another alternative is to use the waste heat of the Stirling engine. It remains to solve the problem that the engine is not always running, which will result in a lack of waste heat for air- conditioning when the Stirling engine is off. In the Matlab model (chapter 5.3.1 ) absorption cooling is already implemented as an option. Nevertheless, this topic implies further research which is not included in the scope of this work. In the case of the first prototype a solution based on standard automotive parts will be used.
4.2.6 Electric components In a conventional vehicle, the ICE delivers mechanical energy to various components like power-steering pump and air conditioning compressor. In an electric vehicle, all this functions depend on the batteries as their unique energy source. In modern vehicles this turns out to be a problem because the number of auxiliary units for comfort and security functions is growing fast. Suspension’s characteristics are controlled by electronic components, entertainment system and navigation are new functions, and electric seats replace manual adjustment. In the presented hybrid concept, all of these components must be electric or manual, there is no mechanical output available from the ECE.
4.2.7 Vehicle Concepts Proposal of this paragraph is to show different vehicle concepts in which the new technology could be applied. Several types of vehicle use will be presented, covering the most common vehicle use cycles. The basis of this analysis is the data for the reference vehicles, presented in Table 4.3. Average consumption is used for rough size estimation of Stirling engine and sizes of energy storages. The vehicles are: • Small City Car This car is planned to be used in the daily life. To go to work, do small trips like shopping, fitness center, and other points near by the living place are the prime reasons for this car. The majority of the time a person uses the car without much baggage. For this reason it can be a small, light vehicle for two persons that consumes few energy and does not need much space. Best examples are the Smart Fortwo or the VW Lupo 3l. It counts with a small electric motor of about 40kWe. During city use the car only runs at low speed and has to stop many times. In the meantime a Stirling engine of 8kWe has sufficient output to fill up the batteries. Average fuel consumption in city: 5.1l/100km. • Family Car (city) Vehicle for up to five persons, for city use with only few times interurban distances. It has relatively high consumption per 100km, but it does not move long distances at the same velocity. A 17kWe Stirling engine and a 90kW electric motor are proposed. 30l of gasoline or its equivalent in other energy storage is sufficient for appropriate use. One of the most famous vehicles of this kind is the VW Passat. • Family Car (interurban) Same vehicle as the family car (city), but with engine and energy storage adequate for greater distances. • Van for Public Transport This vehicle is meant for public transport in big cities, such as student transport. It has a low average speed and can recuperate much brake energy. • Van for Parcel Services Same vehicle as the microbus for public transport, but this time for parcel service between cities. It is important that the vehicle has sufficient energy for one work shift. Speed is higher and few brake energy can be recovered. • Trailer Truck Trailer truck with maximum weight of 40t, mainly highway use with constant, high velocity. Long distance travel with few breaks. Table 4.4 states for every vehicle average consumption (which is the Stirling engine size), range and sizing of storage systems. The estimation gives a first impression of required power and energy. Values change with different using profiles. In Figure 4.10 a benchmark of actual vehicles compared with the prior estimations is shown. Drawing the generator size vs. electric motor size in serial hybrid vehicles guides to an interesting result:
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Figure 4.10 Engine Sizes: ICE vs. Stirling Engine Interpretation of Figure 4.10: A general relation of 2:1 in peak power vs. generator size seems to be adequate. But as it can be seen in the comparison between the DesignLine and the Orion VII autobuses, a great variation from this rule is possible. Both vehicles are similar size and power, but the Orion has six times higher electric power output from the generator. To determine the relation for an especial vehicle it is fundamental to know its exact application. A vehicle in urban use consumes the energy different to a truck on long distance trips. Another point to consider is EES size: bigger batteries con absorb peak energy demand longer and a smaller generator size can be used.
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Table 4.4 Vehicle Concepts - Energy Storage Table 4.3 Vehicles – main characteristics
31 4.2.8 Expreso Tec Tecnológico de Monterrey has a number of shuttle busses called Expreso Tec. They have ten different routes (in 2008) and operate as school buses. Its mission is to offer a save, reliable transport to students and employees covering the most important areas of Monterrey. They pay a fee and can choose the route and time they want to use this service. Aim of this investigation is to obtain data to estimate properties and energy consumption, in order to identify prototypes possibilities.
Figure 4.11 Routes Expreso Tec Normally, city buses of about 35 seats are used. Most common is a Mercedes Benz chassis with an Ayco bodywork. • Chassis and Bodywork
Figure 4.12 Chassis MB 1219 [40] and Ayco Magno 1040 SC bodywork The chassis is equipped with a Mercedes-Benz OM 904 LA-190 engine with 190 hp and torque of 730Nm. The tank can keep up to 204l of diesel, vehicles weight is 4.2t [40]. The bodywork is made by Ayco Autopartes y Componentes S.A. de C.V., with a total of 35 seats. A closer look at the Expreso Tec homepage [41] reveals the main characteristics of this service: In spring semester 2008, there were 10 different routes which cover great part of the city of Monterrey (see Figure 4.11). Garmin MapSource is used for an estimation of distances and drive time for every route. Combined with the schedule, an average profile of distances and moving hours can be set up (see Appendix II). The resulting values are theoretic estimations. In reality the driving profile depends on many factors, as for example hour of the day and schedule of the students. To compare these hypothetical calculations with the real use of the school bus, in September 2009 a unit of the route G2/Cumbres was equipped with a Garmin eTrex Vista HCx handheld GPS. Data sets with position, time, velocity and distance were recorded. Every 15 seconds a point was created. We can observe that the bus does not drive exactly the defined streets. It also makes some extra distance to the depot etc. In 24hours the bus ran 253km at an average speed of 11 km/h. 69l of fuel were consumed.
32 Velocity contribution Velocity vs. time 80
70 > 80km/h
60 71-80km/h
61-70km/h 50 51-60km/h
40 41-50km/h
31-40km/h
Velocity (km/h) Velocity 30 21-30km/h
20 11-20km/h
1-10km/h 10 idle
0 0 500 1000 1500 0 10 20 30 40 50 60 70 time (min) %
Figure 4.13 Velocity profile Expreso Tec Cumbres In Figure 4.13 can be noticed, that most part of the day the vehicle is parking, especial at nighttime. Removing the break and concentrating on the student transport remain 242km in 12 hours at an average speed of 20km/h.
Altitude profile 750
700
650
Altitude (m) 600
550
500 0 500 1000 1500 time (min)
Figure 4.14 Altitude profile Expreso Tec Cumbres Altitude does matter; the highest point is about 740m above sea level, the lowest point at 525m. Every run the vehicle has to climb more than 200m.
4.2.9 Circuito Tec The Tecnológico de Monterrey offers a special shuttle service during nighttime for students living near by the campus. Due to safety problems, two different routes were created to cover most of the streets around the campus. If possible, the driver changes his way to pass by the student’s house. This service is free of charge and can be used by all students. As in the case of the Expreso Tec service, the investigation looks to identify possible prototype applications. Two different types of minibuses are used for this two routes, one of them is a Mercedes Sprinter 515 CDI. 33
Figure 4.15 Mercedes Sprinter 515 CDI, Toyota Hiace This minibus has the maximum weight of 5t, a diesel engine of 110kW, 330Nm and a gas tank of 75l. There are different seat configurations of 15 up to 22 seats. Price is about 900,000 pesos. The other vehicle is a Toyota Hiace, a smaller van for up to 15 persons. It has a 2,7l, 149hp engine and a weight of about 1850kg. More information can be found in [42]. The Circuito Tec personal gives the following costs to operate this vehicle: Price: 350,000MXN, taxes 17,500MXN per year, maintenance 1,800MXN every 10,000km and finally a fuel consumption of 20l/100km. As in the case of the Expreso Tec also for the Circuito Tec theoretic and practical models are compared. In fall semester 2008 there were two different routes for the Circuito Tec, both to the close neighborhood of the campus, see Figure 4.16. Calculated are 220km at about 17km/h, further values can be seen in Appendix II. Still, differences to the real values are expected. Due to the fact that this service depends on the student demand, it only serves the streets that are actually needed. Also in 2009 the service concept was changed: three vehicles recollect students from 6pm to 8am and drive them directly to their home. Distance and driving profile now depends highly on demand and organization between the three drivers. For that reason, in October 2009 the driving profile of one of the units was captured.
Figure 4.16 Routes Circuito Tec vs. real data October 2009 Comparing the theoretic routes and the real path in Figure 4.16 we can observe that this unit serves a lot more streets than planned. Nevertheless total distance is only 117km in this night. After midnight the demand for this service is lower, the vehicle is mainly parked in the campus. Before 0:20am 87.5km were absolved, in the second part of the night only about 30km. Top speed was 93km/h. Talking to the drives it turned out that the demand of the Circuito Tec service depends a lot on day of the week and on several other factors, such as rainy days or exam periods. In spring 2010 more data was collected with the goal to find the average and maximum use. In Table 4.5 the results can be seen:
34 Time Distance Number Datum (total, h) (km) Comments 1 05.10.2009 13 116 2 16.02.2010 14 127 3 17.02.2010 14 104 4 18.02.2010 14 127 5 19.02.2010 14 94 6 20.02.2010 14 76 Friday 7 23.02.2010 14 113 8 09.03.2010 14 136 Exams 9 11.03.2010 14 145 Exams Table 4.5 Statistics Circuito Tec It is worthwhile to take a closer look to this data. Revising the table with the drivers the following conclusions can be made: There are three different patrons: nights with low distances (about 75km), mainly Fridays or days before holidays. Average use is distances about 100 to 120km. And peak use is occurring during exam periods (about 140km and more). In Figure 4.17 examples of these cycles are displayed. The night from Friday to Saturday 20.2. had few demand. The night from Monday to Tuesday 23.2. was an average day and the night from Wednesday to Thursday was during mid-term exams with students leaving the whole night.
35
30
25
20 11.03.2010 23.02.2010 15 20.02.2010
Velocity (m/s) 10
5
0 0 10000 20000 30000 40000 50000 Time (s)
Figure 4.17 Use of Circuito Tec Service We can see that the velocities for one tour campus –> student’s homes –> campus are similar in every cycle, with differences caused by different drivers. What changes is the frequency of the tours, especial after one o’clock. Up to midnight the profile is quite similar, but is the demand in early hours of the morning is different. The drivers say that in their experience the distance can be duplicated between a normal day and a day during exams. The GPS data shows that not the distance is duplicated but the frequency in the second half of their shift. The maximum distance for this service concept can be assumed as 200km a night. The energy stored in the TES, has to be sufficient for this distance. The longest cycle measured will be analyzed and used for sizing of the main components. But it is important to add security factors to the final vehicle set-up to be prepared to days with high demand.
35 Velocity vs. time Velocity contribution 120
100 > 80km/h
71-80km/h
80 61-70km/h
51-60km/h
60 41-50km/h
31-40km/h Velocity (km/h) Velocity 40 21-30km/h
11-20km/h
20 1-10km/h
idle
0 0 100 200 300 400 500 600 700 800 900 0 10 20 30 40 50 60 70 time (min) %
Figure 4.18 Velocity Profile Circuito Tec Most of the time the vehicle is parked, waiting in campus or letting students get on or off. While driving, the speed is mainly in the range of 10 to 40km/h, with peaks to about 100km. Most times the vehicle is moving on small streets. One ore two times in the morning the minibus is going to the medicine school which is located in another campus, meaning higher speed and less stops. Preferred properties for the vehicle can be established as followed: low to zero consumption while parked. High efficiency up to 50km/h, top speed should be at least 80km/h.
Altitude profile 590
580
570
560
550 Altitude (m) Altitude 540
530
520
510 0 100 200 300 400 500 600 700 800 900 time (min)
Figure 4.19 Altitude profile Circuito Tec In the area of the campus there are no hills, so that grade ability is not so important. Some of the altitude difference measured is created by tolerance of the GPS-handheld: It measures the altitude using a barometer. Pressure changes during climatic changes are interpreted as altitude changes. Normally this is a slow process and limited to few meters. Altitude measure remains a valid data for the math model: the tolerance caused by pressure changes is relatively slow, compared to real height difference in streets. • Expreso Tec vs. Circuito Tec Both vehicles are used many hours per day. In both cases the vehicles cover relatively long distances. Nevertheless, it is 100% city use, with relatively low speeds, stop and go and idle times. The Circuito Tec service turns out to be the better application for a first prototype: the vehicle is smaller and more economic. It runs mainly on small streets, with less traffic. Components for the prototype can be obtained at lower costs, especial electric batteries and Stirling engine. Another benefit is the cycle time: the vehicle leaves the campus and about 3 to 9km later it comes back again. Batteries can be recharged in the idle time and a smaller Stirling engine is sufficient. The Circuito Tec is a good application for prototypes, infrastructure would be necessary in only one place and the personal can be capacitated especially. Nevertheless, it is still a bigger challenge than a small city car with an average distance of 30 to 60km per day.
36 Chapter 5. Modeling and simulation In this chapter the computer simulation of the Circuito Tec vehicle exposed in Chapter 4 is described. First, the load case and test cycles are defined. Then the simulation is presented and finally results are discussed. The simulations include: a Matlab model as a tool for estimations of energy use and component sizes: a CAD 3D model for positioning of the main components; a simulation in ADAMS for basic vehicle dynamics.
5.1 Load Cases
5.1.1 Drive Cycle The sizes of the main components can be estimated by simulations of a drive cycle. A correctly defined drive cycle can make the prototype process more effective and economic. In the case of this work the drive cycle has to be defined based on the application Circuito Tec presented in Chapter 4.2.9 . As mentioned before, to obtain the profile for the Circuito Tec application a handheld GPS was used during one shift. Depending on the settings, every defined time or distance a data point is recorded with the following information: Date Altitude Distance Time Velocity Direction Position 22.08.2009 N25 26.617 89 16:02 485 m 551 m 00:00:20 99 km/h 149° wahr W100 09.394 22.08.2009 N25 26.362 90 16:02 487 m 528 m 00:00:20 95 km/h 171° wahr W100 09.224 22.08.2009 N25 26.081 91 16:02 486 m 496 m 00:00:20 89 km/h 161° wahr W100 09.175 22.08.2009 N25 25.828 92 16:03 491 m 367 m 00:00:20 66 km/h 152° wahr W100 09.080 Table 5.1 GPS Sample Data Experiments showed that the time of 15 to 20 seconds between two waypoints is sufficient to determine the exact route taken by the vehicle. Nevertheless for simulation purposes, in curves or fast passages, the error is to high. The Waypoints are connected by straight lines and cut parts of the corners.
Figure 5.1 Distance between two waypoints In Figure 5.1 this effect can be observed. Also velocity differences, accelerating and breaking are not recorded precisely enough. A good compromise between file size and accuracy is a time of 5 seconds for a 14 hour shift, which results in about 10,000 waypoints. Excel files of this size are usable with modern laptops and also Matlab calculation remains quite fast.
5.1.2 Vehicle The vehicle selected for the 3D simulation is the Vehizero ECCO-C serial hybrid truck [43]. Built in Mexico City, it has a 5.2kW ICE and a 19.6kW (51,8kW max.) electric motor. In its original configuration it comes with a small cabin, weights 1600kg and can transport 1000kg payload. Maximum velocity is 100km/h. EES is based on 16 T-145 Plus 6V deep cycle battery. They are provided by Trojan Battery Company, detailed product specifications can be found in http://www.trojanbattery.com/Products/T-145Plus6V.aspx. This lead- acid battery type is designed to deliver a consistent voltage as the battery discharges. It is cheaper and heavier than Li-Ion EES.
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Figure 5.2 Vehizero serial hybrid truck [43] Vehizero clients are enterprises who distribute their products from a central warehouse to small city-shops where they are sold to the end-user. Vehizero states the following benefits for the use of their vehicle in Mexico: Lower taxes and insurance fees; no weekly restriction in Mexico City; less costs, due to energy costs and easy maintenance.
5.1.3 Vehicle Dynamics In the present work, the vehicle runs a relatively well defined route. For that reason, the virtual test circuit is rebuilding a loop starting end and ending in the main entrance of the university, running through some most representative streets in the neighborhood. Doing so, the simulations can be revised easier, simply comparing virtual results against real results obtained by prototypes. In Appendix III a detailed proceeding how to create a complete virtual test track based on GPS data can be found. • Track setup in ADAMS/car To save computing time, it can be reasonable to divide the whole test track in different sections and only simulate the key sections until a good vehicle set-up is found. In our case, cornering and obstacles at relatively low speed are main difficulties. During set-up of the ADAMS model it turned out that a complex road profile extends heavily calculation time and leads to more errors. Within the present work it would be impossible to simulate the complete test track. The setup has to be concentrated on the most important, heaviest load case. On the end of this paragraph the reduced test road will be presented. • Obstacles While moving in the streets near the campus, vehicle velocity has to be reduced for two reasons: because of poor road quality and because of bumps placed in the streets to prevent people from speeding. Most common obstacles are: small, about 10cm high metal half-moons fixed in a row perpendicular to the street; Concrete and asphalt bumps in different forms and heights; Potholes and sometimes improperly set gully covers. The ADMAS 3D road builder permits placing 3D structures on top of the underlying smooth road surface. Some of the available pre-defined structures are: Crown, plank, pothole, ramp, roof, roughness and sweep. Obstacles can be combined. Finally, a test track was created, containing the most important elements (see Figure 5.3). It is a plane road with three different types of obstacles: First a plank, 10cm high, simulating a speed bump. After a right-left curve-combination random road roughness is simulated, on a length of 80m, including another double curve. Finally, a 5cm deep pothole is located on the left road side. This test track contains two different load cases: Pothole and plank are elements widely found in the streets and important for the vehicle setup. Experience with ADAMS/car showed, that a virtual vehicle can work well on a defined test-track, but fails on any other type of obstacle. For this reason the random road roughness was implemented, allowing more security that the vehicle would also work on any other road. Velocity is 20km/h constant speed, simulation length 50 seconds.
38
Figure 5.3 Test Road ADAMS/car
5.2 Model In the evaluation of a vehicle there are several specifications to be met, depending on vehicle type, market and foreseen use. Many of them are defined by regulations, norms and laws, others are essential to satisfy the clients. In the next paragraph the requirements for a vehicle used in the student transport Circuito Tec are presented. • Speed The vehicle is used in a city. Dense traffic, small roads, crossroads and speed reductors slow down the minibus. Low speeds up to 60km are used nearly all the time. In order not to block the traffic in main avenues, maximum speed is defined to 80km/h. • Gradeability Monterrey is surrounded by mountains. Nevertheless, the campus is located in a flat area and gradeability is a minor issue. The vehicle must be able to enter and leave parking decks, but inclination is limited to short distances. • Acceleration The minibus is stopping at many points. It is important that recover cruising speed does not require much time. • Driving range The vehicle must meet the requirements explained in chapter 4.2.9 . In the simulation, the driving profile obtained by GPS is used. • Noise Noise must be same level or less than the conventional minibus. This applies to the interior as also to the exterior. • Pollution A zero pollution vehicle is planned. The vehicle must not emit more than hot air.. • Vehicle Size The interior must be comparative to the actual Circuito Tec. The exterior must be as small as possible, not exceeding the dimensions defined by traffic regulations. • Handling / Comfort The vehicle must be adapted to low speeds, many stops and small streets. Comfort for the driver must be high. For the students the comfort can be lower, but must be adequate for 30 minutes. Air conditioning and noise level is important. • Energy Consumption Low energy consumption is desirable but not primer goal of this investigation. A higher weight and resulting higher energy consumption is acceptable if solar energy is used.
39 • Vehicle Mass At low speeds the vehicle mass is one of the most important parameters for energy consumption. The mass should be as low as possible and not superior to maximum weights allowed by traffic regulations. • Cost Aim of this investigation is to present a concept that is competitive to actual vehicles in terms of costs. This includes purchase prices, maintenance prices and operating costs. This point will be discussed in the last chapter • Propulsion motor requirements, engine / generator requirements and energy storage requirements The results of the following simulations will define sizing of engine, motor and energy storage.
5.2.1 Energy calculations Energy consumption of the vehicle can be calculated based on the basic formulas for the physics of motion.
Force Power Energy
E = P ⋅ t Conversions P = F ⋅ v E = F ⋅ s Rolling P = F ⋅ v Fr = cr ⋅ m ⋅ g r r E = c ⋅ m⋅ g ⋅ s Resistance r r
Air 1 2 1 2 F = c ⋅ A ⋅ ⋅ ρ ⋅ v Pair = Fair ⋅ v E = c ⋅ A ⋅ ⋅ ρ ⋅ v ⋅ s Resistance air d f 2 air air d f 2 air
Grade Fpot = (%inclination) ⋅ m ⋅ g Ppot = Fpot ⋅ v E pot = m ⋅ g ⋅ h resistance
2 IW + i ⋅ I eng 1 2 1 2 meff = m + ⋅ meff ⋅ (v2 − v1 ) E = ⋅ m ⋅ (v − v ) Acceleration r ⋅ r a eff 2 1 resistance stat dyn P = 2 2 a t Fa = meff ⋅ a
Electric Energy Eec = ec ⋅t Consumption
total E total = Er + Eair + E pot + Ea + Eec
Table 5.2 Formulas for Energy Calculations [10] In Table 5.2 the corresponding formulas are presented. Rolling resistance is the result of deformation work on tires and roadway. On surfaced roads, the resistance is mainly deformation work on the tires (flexing) [10]. Aerodynamic drag is the force on the vehicle caused by the relative speed to the air. At higher speeds it is the determining factor and becomes dominant for fuel consumption [10]. Grade resistance is the gravitational force acting on the vehicle. Acceleration resistance is a force created by the inertia of the vehicle and the inertia of the rotating masses. If we neglect rotating masses with small moments of inertia on rotating shafts and in the transmission, and apply constant rotational energy [10], than we can express this effect by calculating an effective mass. Finally, electric devices, such as lights, radio and air conditioning, consume electric energy, independent of the vehicle velocity. These formulas for forces, power and energy allow creating a mathematic model of the vehicle, which allows a calculation of energy consumption. The Matlab program code is found in Appendix I It is important to mention, that the results are approximations. Several factors do change with velocity or during cornering, for example the rolling resistance coefficient. Nevertheless, these changes are relatively small compared to the result. So it is valid to use this model for estimations.
40 5.2.2 Configuration In this paragraph the used variables and its values are presented. • Vehicle Vehizero ECCO-C serial hybrid truck, as presented in 5.1.2 .
Amount Unit vehicle weight, without EES, TES, Stirling engine, incl. persons 2100 kg front surface 4m^2 air drag coefficient 0.32 tire rolling coefficient 0.009 Electric output Stirling engine 9kW Power output electric motor 110 kW efficiency brake energy recovery 0.3 electric energy stored in battery at start 10 kWh maximum electric energy stored in battery 15 kWh efficiency batteries 0.85 thermal energy stored in tank at start 275 kWh maximum thermal energy stored in tank 300 kWh electric energy consumption, including air conditioning 1.2 kW Table 5.3 Vehicle parameters Vehizero ECCO-C • Stirling Engine The estimated size for a Stirling engine in a van used for person transport in cities is 26kW (see Table 4.4). The Circuito Tec vehicle has a lower average velocity and will require less energy per time. For that reason, from Table 4.1 the Cleanergy V161 Stirling engine is selected. It is more economic and more effective than a combination of smaller engines.
Figure 5.4 Cleanergy V161 Stirling Engine [44]
Figure 5.4 shows the basic parameters of the V161 engine, used in a combined heat-power plant. Cleanergy [44] is designing other applications, as for example solar dishes, but they are all based on the same engine. Actually (end 2009), Cleanergy is starting series production of the Stirling engine. The engine has its origin in the Solo Stirling (Germany). The engine weight without generator and box is estimated as 100kg. • Generator The Cleanergy Stirling engine is planed for stationary use, so a heavy generator is used. In the vehicle a small and lightweight generator is needed. An example is the L.M.C. LEM-200 generator (http://www.lmcltd.net/). The D127 motor/generator seems to fit well to the V161 engine, has a weight of 11kg
41 and is way smaller than the original generator. An inverse circuit or an extra electric motor is needed to start the Stirling engine. • Thermal Energy Storage To start the design process, some fundamental properties were defined: The TES must store 300kWh thermal energy at a range of 600°C to 1000°C, sufficient for a shift of the Circuito Tec. The thermal output delivered to the Stirling engine must be 30kW, based on the V161 Stirling engine. 169l of molten glass store 300kWh of thermal energy, with a glass weight of 457kg and a total weight of 996kg [L.D. Garcia, Tecnológico de Monterrey, “TANQUE SOLAR DE ALTA TEMPERATURA”]. • Electric Motor It is not planned to replace the electric engine coming with the Vehizero vehicle. Nevertheless, to obtain a more realistic result the efficiency map of the UQM Technologies, Inc (www.uqm.com) SPM218-143-3 Motor/Generator is taken. This specific data was not available for the Vehizero engine. • Electric Batteries Standard configuration in the Vehizero is 16 T-145 Trojan deep cycle batteries (http://www.trojanbattery.com/Products/T-145Plus6V.aspx) with a total weight of 528kg. It can provide about 15kWh without being recharged (available energy, nominal value is higher, information from Vehizero). • Radiator In the case of first prototypes for Hybcar-ST commercial products from the automotive industries should be used. They can be easily obtained at relatively low costs. Two or more radiators from a mass-produced car will be placed in the experimental prototype. The use of 2 VW Bora Diesel radiators is proposed. • Weight The hybrid vehicle will be heavier than the original Vehizero vehicle. The ICE can be removed, but there are other components to be added: TES with molten glass is 996kg. En extra structure for the TES which protects the tank from torsion and external influences is estimated with 100kg. Stirling engine and generator have a weight of 150kg. 9 Persons (75kg each) sum up to 675kg. The basic Vehizero vehicle is 1600kg and passenger cabin with seats etc. is estimated with 300kg (reference: observation of VW Transporters in different configurations). In total, we can expect a weight about 3,7t for a vehicle full with passengers, compared to 2,6t for the maximum weight of the original Vehizero vehicle. Weight (kg) Vehizero basic vehicle 1600 Passenger cabin 300 Extra Structure 100 removing ICE -100 TES 996 9 persons 675 Stirling + Generator 150 total 3721 Table 5.4 Vehicle weight
5.2.3 Simulation Objective Objective of the different simulations is to get an estimation of the key components. Space for passengers and payload can be determined. Required size of batteries and engines is calculated. The position for the main components, such as Stirling engine, motor, batteries can be found. The influence of forces by new components is determined.
42 5.3 Simulations and Results
5.3.1 Matlab The simulation in Matlab (http://www.mathworks.com) is used to calculate the energy use of the vehicle, based on basic vehicle parameters and on the driving profile. The program uses the physical model described in chapter 5.2.1 . The complete drive cycle is divided into small time sections. For every section a data set is created, using the GPS data and resulting in a complete energy calculation for the data set. The method can be compared with the finite elements method.
Figure 5.5 Matlab Macro To allow easy handling of the Matlab model a small Excel macro is programmed. The input file with nearly all vehicle parameters is linked with a macro running all Matlab simulation. The user gets directly to the results file. The flowchart in Figure 5.6 visualizes the main functions of the Matlab Simulation. Detailed program code with a few remarks is listed in Appendix I. Based on the driving profile presented in 4.2.9 an estimation of energy use and size of main components can be done. Finally, to the calculated values security factors have to be added. Choosing more powerful engines and bigger energy storages permits more variability in vehicle use, but brings up cost and weight. Another point is the accuracy of the mathematic model. A model of this kind always is an approximation and cannot reflect 100% a real vehicle. Simplifications are made and some variables cannot be considered because they are not available. For example the heat loss of the TES is unknown. Other factors are estimated, but for more realistic results should be updated when measured in a prototype or reference vehicle, for example the efficiency of EES.
43 Initialization
Excel file Excel File GPS data Vehicle constants
Data Set: Velocity Import Data Altitude Distance
Calculate Altitude and velocity difference for every data set
Calculate velocity Energy profile calculations
Calculate Calculate consumption at State of Charge constant velocity EES and TES
Results and Statistics
Excel file Result Data
Figure 5.6 Flowchart Matlab Simulation • Results The average velocity is 10km/h, the total altitude difference (altitude end point to altitude starting point) is 41m and the maximum velocity is 106km/h. The total vehicle mass including passengers is calculated as 3636.5kg.
44 Energy consumption for 100km at constant speed 45
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25 Energy (kWh)/100km 20
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10 0 10 20 30 40 50 60 70 80 90 100 Velocity (km/h)
Figure 5.7 Energy Consumption at Constant Speed The first data analyzed is the energy consumption at constant speed (Figure 5.7). At velocities below 10km/h the energy consumption for 100km is high. This is caused by the electric energy spend for air-conditioning, lights, controllers and radio. At high speeds the air drag is the main force that causes consumption. At velocities below 10km/h the energy consumption for 100km is high. This is caused by the electric energy spend for air-conditioning, lights, controllers and radio. At high speeds the air drag is the main force that causes consumption. At velocities below 10km/h the energy consumption for 100km is high. This is caused by the electric energy spend for air-conditioning, lights, controllers and radio. At high speeds the air drag is the main force that causes consumption. Energy (kWh) Comment Total Energy spent (electric) 68.84 without energy recuperation Energy consumed at wheel (electric) 30.96 without energy recuperation Brake Energy recuperated (electric) -3.34 Potential Energy (electric) 1.57 Energy consumed by accelerating (electric) 12.71 Energy consumed by air drag (electric) 3.76 Energy consumed by rolling resistance (electric) 12.92 Energy consumed by electric devices (electric) 16.67 Thermal energy used 214.60 with brake energy recuperation Table 5.5 Result Matlab Simulation: Energy Consumption In Table 5.5 we can observe the calculated energy consumption of the vehicle. The energy consumed at wheel is energy used for propulsion without brake energy recovery. The total (electric) energy spent is the energy consumed used for propulsion, electric system and electric devices. Brake energy recovery is not applied. Finally, the thermal energy used is the complete thermal energy extracted from the TES for one night Circuito Tec service. Brake energy recovery is applied. Air drag and potential energy are relatively low compared to the energy spent to accelerate or consumed by rolling resistance. About 10% of the energy consumed by the electric drive motors can be recuperated by brake energy recovery. Consumption by electric devices is more than 50% of the energy spent to move the car. This can be explained by the fact that the vehicle is idle at about 60% of the time. The total consumption adds up to 215kWh of thermal energy consumed in the whole night. The TES size of 300kWh defined in 5.2.2 is adequate. There are some factors that justify a bigger TES than calculated: The TES will not always be 100% full at the beginning of a shift. Heat losses are not implemented in the Matlab model. Additional distance must be possible.
45 State of Charge: Electric and thermal energy storage systems 100
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Electric Energy Storage 40 Thermal Energy Storage Thermal Energy Storage 2
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20 0 100 200 300 400 500 600 700 800 900 time (min)
Figure 5.8 Result Matlab Simulation: SOC State of charge for TES and EES during the drive cycle: The vehicle consumes electric energy from the EES for propulsion. When the SOC of the batteries goes below 50% of the total capacity the Stirling engine starts and provides electric energy. The remaining energy (no used for propulsion) is stored. When the batteries reach 90% the Stirling engine shuts down. Thermal energy is extracted from the TES while the Stirling engine is working. The model is simplified; trickle charge for better battery performance and thermal loss should be implemented. Thermal Energy Storage 1 is connected to the Stirling engine, TES2 is an option to implement absorption cooling. In the case of the selected driving cycle the Stirling engine works less than 50% of the total time. A smaller Stirling engine would be fine, but was not available during benchmark.
actual Power needed at wheel 250
200
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0 Power at wheel (kW)
-50
-100
-150 0 100 200 300 400 500 600 700 800 900 time (min)
Figure 5.9 Actual Power at Wheel 46 Figure 5.9 displays power of the electric motor vs. time of the night. The few, high peaks are caused by rounding and tolerance of the GPS data. Checking the imported GPS track in Google Earth there is no obvious reason for consumption higher than 100kW. The measurement errors (for example altitude differences) in these data sets lead to impossible results on that few points. It can be concluded that the engine works mainly at less than 50kW, with peaks up to 75kW and few maximum power peaks of about 100kW. The Vehizero electric motor has 70hp (52kW). It will be able to move the vehicle nearly all the time as calculated. Nevertheless it will be limited and works many times at high percentage of its total power, which probably will guide to less efficiency. Also, in other applications than student transport, such as delivery services, tend to drive more aggressive and would require more power. No further information of the motor is available, so that this topic is pending until exact data and efficiency map is available.
(for selected data sets) Actual power consumed for: 70
60 Potential Energy Kinetic Energy Air drag 50 Rolling Resistance Electric devices Brake Energy recuperated 40
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Power (kW) Power 10
0
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-30 15 20 25 30 35 40 45 50 55 time (min) Figure 5.10 Actual Power for Selected Data Sets One feature of the Matlab simulation is the possibility to break down the propulsion power into its constituent parts. For the selected data sets most energy is spent for accelerating. For the complete driving cycle this feature may be restricted in utility, its too much information and gets confusing. But for selected, short cycles it can be useful. The simulation can also be used to test alternative vehicle configurations. The Vehizero vehicle comes with a 15kWh EES and a 5.2kW ICE. The proposed Stirling engine has 9.2kW. An interesting question is the total EES size necessary as a buffer for this special vehicle and application.
47 State of Charge: Electric and thermal energy storage systems State of Charge: Electric and thermal energy storage systems 100 100
90 90
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50 50 State of (%) Charge State of (%) Charge
Electric Energy Storage 40 Thermal Energy Storage 40 Thermal Energy Storage 2
30 30 Electric Energy Storage Thermal Energy Storage Thermal Energy Storage 2 20 20 0 100 200 300 400 500 600 700 800 900 0 100 200 300 400 500 600 700 800 900 time (min) time (min)
State of Charge: Electric and thermal energy storage systems 100
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State of Charge (%) 40 Electric Energy Storage 30 Thermal Energy Storage Thermal Energy Storage 2
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Figure 5.11 EES: 15kWh, 10kWh, 5kWh With 15kWh EES the Stirling engine starts about 4 times a night. With 10kWh it starts about 6 times and with 5kWh about 11 times. It is desirable to have the Stirling engine working continuously at a constant speed, working nearly the whole time. During benchmark of Stirling engine only one suitable engine was found for the Circuito Tec application. But what would happen if we could design our own Stirling and create the ‘perfect’ engine for this application? Figure 5.12 shows the simulation result for a vehicle with 5kW Stirling engine and 10kWh EES. It seems to be a good combination with efficient Stirling engine use. But trips to the medicine school could cause problems: higher speed and longer distances mean more energy consumption in short time. The Stirling engine is not able to deliver that amount of energy and the EES can run out of power. Further simulations, enriched with experimental data can determine the best combination. For the moment, a 9kW Stirling with batteries storing 10kWh is an adequate setup for an experimental vehicle.
48 State of Charge: Electric and thermal energy storage systems 100
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40
30 Electric Energy Storage Thermal Energy Storage Thermal Energy Storage 2 20 0 100 200 300 400 500 600 700 800 900 time (min) Figure 5.12 5kW Stirling engine and 10kWh EES
5.3.2 CAD 3D Autodesk Inventor (http://usa.autodesk.com) is used to construct 3D models of the vehicle. The model in Figure 5.13 is the basic vehicle concept as used in the vehicle dynamics simulation. Chassis, roof, batteries, Stirling engine and TES are created as “black boxes”, e.g. they are of real size, but without further functions. The suspension and steering components are created directly in ADAMS, thus allowing them complete functionality.
Figure 5.13 Basic 3D Model The chassis is made of ‘C’-shaped sections, 10cm high and 15mm thickness. In chapter 5.3.5 further space concepts and details of the main components are presented. The actual model is based on a data sheet and few images, allowing only a rough approximation. When the vehicle is available, reverse engineering will make possible a more detailed, more realistic model.
5.3.3 Vehicle Dynamics MSC Software (http://www.mscsoftware.com/) offers several simulation and FEM programs. One of their products, ADAMS, is software for multibody dynamics and motion analysis. The distribution of loads and forces can be simulated in mechanical systems, in order to improve and optimize the performance. Adams incorporates real physics by simultaneously solving equations for kinematics, statics, quasi-statics, and dynamics.
49 ADAMS/View is a modeling and simulation environment. Before building a physical prototype, the mechanical system can be designed, visualized and improved. ADAMS/Solver computes force and motion behavior of the system. ADAMS/Car is a tool to build and test functional virtual prototypes of complete vehicles and vehicle subsystems. ADAMS/Flex considers component flexibility in a full-system simulation. A quite basic vehicle model is set up in ADAMS/car R3. Purpose is to determine the effect of a 1000kg thermal tank to the vehicle dynamics and the forces to the chassis. In ADAMS/car is based in templates, subsystems and assemblies. A template is the basic element for every component of the vehicle. It defines all the geometry, connectors and joints. The expert user can change every detail of the vehicle in the template builder. Converting a template into a subsystem limits the possibilities of modifications. Restricted geometry changes, variables and mass properties can be modified. Joints and connectors in a subsystem are fixed. By choosing and connecting the corresponding subsystems a full vehicle assembly is created. For every subsystem availability of variables and hardpoints can be set. Hardpoints are basic coordinates which describe the location of the components. An expert user can build a new vehicle by building its own templates or by modifying standard templates. He creates subsystems and defines the details that can be modified by a general user. This user takes the subsystems and redefines hardpoints and variables until the virtual prototype meets the requirements. An un-experimented user can even change the suspension type without knowing well the software. The whole process helps for fast simulations before spending time and money in real prototypes. Nevertheless, in practical use many errors appear, mainly caused by assembly or statically balance problems, so that an expert is needed to fix the problem. At continuation, the main components are described (a detailed description for standard-templates can be found in the help of ADAMS/car). With exception of the front suspension the main components are formed by modified templates provided by MSC. • Rigid Chassis This component represents the base frame of the vehicle, in the case of the present work it is used as a reference for the location of all other components. A flexible chassis is placed in the rigid chassis template in order to make possible force calculations in the chassis. TES, EES and Stirling engine are attached to the flexible chassis by bushings. Weight for passengers, fairing parts etc. is added to the rigid chassis weight. • Front Suspension The front suspension is based on a McPherson design, as it can be found in the get-started-tutorial of ADAMS/car. Values are adapted to the actual vehicle. • Steering The steering subsystem is one of the most delicate components. Wrong defined joints, connectors or variables lead to unpredictable vehicle behavior. In the case of the Vehizero vehicle the steering-shaft geometry was modified to adapt the steering wheel to the new space concept. Running simulations showed that the new shaft geometry disturbed straight line movement of the vehicle. The problem can be solved by creating a new steering template, changing joint types and modify connections to the rigid body. For the present work in the dynamic simulation the exact location of the steering wheel does not matter, it is more a visual problem. For this case the steering wheel location is only corrected in the 3D CAD model and in ADAMS the standard-template is used. But it is highly recommended when the real Vehizero data is available to update this component. Template used: rack and pinion steering system. • Rear Suspension The suspension type used in the Vehizero vehicle is unknown; the information is omitted in the available pictures. For that reason, the trailing arm suspension is chosen. In the ADAMS/car online help the following description can be found: “The trailing arm suspension template is one of the most simple and economical designs for independent suspensions”. • Brakes The standard four wheel disk system is used. • Front and Rear Wheels
50 Tire model PAC89 (provided with ADAMS/car) is used, adapted to new tire size. • Powertrain In the ADAMS/car version R3, only one powertrain template is available: A powertrain based on an ICE engine. The template is adapted to rear wheel drive and graphics were updated to an electric engine. For the selected test-track and load case this is sufficient. The simulation in chapter 5.3.1 shows that the Vehizero electric motor is strong enough for the new hybrid concept. In this stage of the protoype not sufficient information is available to build the electric model. When the ADAMS/car simulation is updated it is recommended to change the engine profile to an electric engine to be able to run longer test-tracks testing the vehicle on changing velocities.
Figure 5.14 ADAMS/car simulation • Model I The first model is created without TES (actually, the TES and its connectors exist, but TES weight is 0kg). The vehicle is used to find an estimation of the original Vehizero setup. Suspension setup is estimated by observing the vehicles behavior on the test-track. Overall vehicle weight is 2600kg including passengers, which is the maximum permitted by Vehizero. • Model II Model II has the same basis vehicle as model I, but now with correct TES weight. It is used to determine if the original Vehizero vehicle is able to support the additional weight or if modifications are necessary. Both models are used on the presented test-track and forces acting over the chassis are saved. Results will be discussed in the next paragraph.
5.3.4 FEM analysis ADAMS/car allows simulating flexible bodies. The module ADAMS/flex is a tool to load FEM-models into ADAMS/car. In ADAMS version 2005 there is also the tool ADAMS/autoflex which is used to create flexible bodies in the ADAMS environment. In newer versions this module is no longer available within the actual license. There are several limitations in the use of flexible bodies, for example automatic upgrade is impossible. The FEM-model has to be rebuild for every geometry change. Another problem for the FEM analysis of the chassis is that the main body of a model is rigid. A lot of work has to be done to properly create a flexible body within the rigid chassis template. At continuation, the basic steps for creating and simulating a FEM model in ADAMS R3 are presented: • FEM model in ADAMS/view 2005 The first step is to import in a new model in ADAMS/view 2005 the solid geometry of the cassis . Preferred file format for the geometry is parasolid. For every location in which the flexible chassis will be connected to components, such as suspension or engine, a construction point has to be created. Furthermore, for every component, an extra body is necessary. The solid chassis and extra components are now connected by rigid joints on the corresponding points. By build -> flexible bodies –> autoflex the mesh-plugin is started. Solid 51 geometry is selected, mesh properties are defined and attachment points created. Material is assigned, in this case standard steel. Result of this operation is a .MNF (model neutral file) file. • Import flexible body in ADAMS/car R3 In the rigid body template the rigid chassis (parasolid file) is imported as a new part. The next step is to replace the rigid chassis by the flexible body (parasolid file) by build –> parts –> flexible body –> new. In every node an interface part has to be created. The interface part is an element used to connect flexible bodies with rigid geometries. Finally, the interface parts are connected with tanks, batteries, suspensions etc. For components within the same template, connectors used are bushings. Bushings allow a defined motion between flexible and rigid bodies, depending on velocity and force. For parts which will be connected in moment of assembly with other subsystems, new bodies have to be created. These bodies have a zero weight, and are used to locate the communicators. A direct connection between interface parts and communicators is impossible. The body template now can be used in the ADAMS/car R3 standard interface. Simulation and post- processing require a powerful computer or long simulation times. The TES is connected on four points to the flexible body. Front and rear suspension at four points each (two per side). EES has three connections, Stirling engine two and the electric propulsion motor has 4. The body, including weight for passengers, seats, fairing parts etc. is connected with 6 bushing to the flexible chassis: two on the outer front side, two in the middle and two on the rear end. • Simulation Results Both models (I: 2600kg, TES without weight; II: 3721kg, TES 996kg) are simulated on the test track presented in 5.1.3 . Critical point on the selected road is the plank were major forces act on the chassis.
Figure 5.15 FEM simulation results In Figure 5.15 vehicle behavior while running over the plank can be observed. Deformation is amplified by factor 25. On the upper picture the front wheels are climbing the plank, on the other picture the rear wheels are passing the plank. Highest stress acts next to the suspension connection points. For further investigation, seven different points on the chassis are selected:
52 3 1
4 2 5 6
7
Figure 5.16 Location of Stress Measurements These points do reflect the most typical load case acting to the chassis. The function Durability (first the Durability plug-in has to be activated) –> compute nodal plots –> is used to select the different nodes and to create the data sets. In the post processing window the data now can be selected. The result for the Model I (vehicle with 2600kg total weight) on the virtual test track can be seen in Figure 5.17.
Figure 5.17 Stress vs. time Highest forces are observed while facing the plank at t = 2s. Between 15s and 20s the vehicle passes the first double curve. From t = 25s to t = 40s the vehicle is on the rough road passage. On t = 44s the vehicle passes the pothole. The extremely high values for point one and two at can be explained by the lack of reinforcement between the front wheels. In the current model the beam is placed about one meter behind the front axis, thus resulting in a high bending moment on the chassis next the connections with the front suspension. By implementing a stabilizer or reinforcement next to the front axis this high stress will disappear. 53 Comparing Model I to Model II we can se a similar behavior:
Figure 5.18 2600kg vs. 3721kg, point 1 and 2, von Mises stress The comportment between the two models is almost the same, but with values about 20% to 25% higher for Model II. Peak value at t = 2.2s is 38.8MPa higher.
Figure 5.19 2600kg vs. 3721kg, point 4, von Mises stress Also for point 4 for Model II a stress level about 20% higher than in Model I. It is interesting to see that the peak value for Model II is about 30MPa lower than the lighter Model I. • Materials In automotive applications high strength steels are used. In the case of the Vehizero vehicle the material is unknown. Lowest standard would be a S335N (ASTM A441) steel [45], with ultimate tensile strength of 485MPa. Keeping in mind fatigue and endurance limit we can expect ¼ of the ultimate tensile strength as a limit for infinite cycle life. A maximum stress of 120MPa should not be exceeded. 54 In several points of the simulated chassis higher stresses are calculated. It is important to rebuild the simulation with the exact chassis dimensions and to revise the exact material used for the Vehizero chassis. • Limitations of the FEM Model The results of the FEM model are affected by several simplifications and missing information: Highest impact has the geometry. The exact geometry is unknown, during this work the Vehizero vehicle was not available, so that the model was estimated by the information provided in few pictures. Also, details of the suspension, especial of the rear suspension were unavailable and had to be estimated. The exact type of the rear suspension is unknown; on the images the rear suspension cannot be identified. Reinforcements, exact shaping of the chassis and details are unknown but indispensable for a correct simulation. Another problem is element size. In the presented simulation the vehicle is simulated with about 90,000 elements, which is the maximum feasible for the available computers. The long, but thin chassis structure only has one element throughout the complete thickness of the ‘C’-shaped beams. Probably this would avoid the elements that have zero stress. It is recommended to implement self-damping in the flexible body. This can best done while creating the .mnf file, but it must be used a program different to ADAMS, as for example Hyperworks. The simulation gives us an idea of vehicle behavior, but with a more detailed CAD model and the proceeding presented the results will be much more detailed and more reliable.
5.3.5 Package dimensions TES A important consideration in the presented vehicle concept is size and location of the energy storage. A TES using molten glass to store 300kWh of thermal energy in the temperature range of 600°C to 1000°C it is a heavy and big component. It has many restrictions in the assembly. It has to be located next to the Stirling engine, with a removable connection to the heat exchanger. The TES has to be easy and fast installed and removed. It must be protected to any damage during normal use and during accidents. It should be located near to the street at the vehicle center to allow acceptable vehicle dynamics. • Cylindrical TES vs. rectangular TES The original TES design is a cylinder of variable height and diameter. A tank of that form and dimension has many disadvantages in package design. In Figure 5.20 we can see a comparison between a cylindrical and a rectangular tank. Volume and weight of the rectangular tank are based on the cylindrical TES.
Figure 5.20 Cylindrical TES vs. rectangular TES
Both tanks use the space available in the middle of the chassis, but in height there is a difference. The round TES is too high to be located within the chassis height and interferes with interior design and available space for seats. The rectangular tank can be located below the interior, allowing flat surface at a height of 80cm in the case of the Vehizero vehicle. 80cm is also the original cargo platform height.
55 A non-cylindrical, especially adapted form is indispensable for a reasonable package design. But there are several limitations to be solved on side of the thermal storage design: Heat transfer within a non-cylindrical tank is way more difficult, a new form of intern heat conductor is needed. Heat loss is greater due to the augmented surface size. Manufacturing will be more difficult. • Package concept for a rectangular TES One of the greatest challenges for the proposed TES is how to make it mobile. It must be exchangeable in order to be heated by a lens outside of the vehicle. The available surface on a vehicle is not big enough to collect sufficient concentrated solar energy during the day. The 300kWh of thermal energy needed for the Circuito Tec use must be 176m² (see 3.2 and 7.1 ). Upper vehicle surface is 10m². For that reason we have to remove a mass of about 1000kg from a place inside the vehicle. The TES is located in the middle of the chassis without direct access from the outside. The only way is to lower the tank and remove it below the vehicle. Without going into detail and with the goal to point out possibilities and restrictions, a solution is presented. The final concept depends on exact vehicle specifications and on restrictions in shaping of the TES. A vehicle built from the scratch would allow better possibilities. By designing a chassis adapted to the TES new ergonomic concept can be implemented. The TES is mounted on an extra frame, which can rotate on an axis on the front of the frame. While mounted, the TES is fixed by four pins, two on the top end, and two on the rear end.
Figure 5.21 Package Concept, rectangular TES The first step is to remove the heat exchanger in direction to the Stirling engine. The second step is to open the rear joint between frame and chassis. The TES is supported by a device which helps to move the tank outside the vehicle. The storage will be lowered on the back side by turning the TES around the front axis of the frame:
Figure 5.22 Package Concept, rectangular TES: Step 1,2 and 3 Step 4: the beam which closes the frame is removed.
Figure 5.23 Package Concept, rectangular TES: Step 4
56 Finally, the TES slides over the frame and can be removed to the rear end of the vehicle:
Figure 5.24 Package Concept, rectangular TES: Step 5 There are several advantages of this concept: The frame protects the thermal energy storage from impacts and gives extra stability to TES and chassis. Due to the rotation point near to the Stirling engine, assembly tolerances remain relatively low on the critical connection to the heat exchanger. The space above the chassis is available completely to passengers and other components. But the disadvantages are serious: Infrastructure outside the vehicle is indispensable: a device to move the TES is required. A special parking lot where the vehicle obtains at least 50cm of additional space between ground and lowest vehicle components must be used. And finally, it is un-ergonomic removing elements below the vehicle: Access is restricted and located near dirty parts. • Alternative Chassis Using a chassis designed for conventional vehicles results in ergonomic problems. It is difficult to exchange the TES. At continuation a variation of the parametric CAD chassis is presented, leading to easier TES replacement concepts. In this case, the chassis is lowered, in order to create a continuous space for the tank from one vehicle side to the other. The TES now can be removed to the side, guided in a structure similar to a drawer, without being disturbed by the frame.
Figure 5.25 Alternative Chassis The lowered chassis allows designing a more comfortable entrance and amplifies interior space. This is important for our Circuito Tec application. Also, the gravity center is low, improving vehicle dynamics. The chassis protects the TES from impacts. TES is 300kWh in molten glass; a 10kWh Li-Ion battery is located in the rear and the 9kW Stirling engine behind the front axis, next to the TES. As mentioned, the chassis is based on the approximated, parametric design of the Vehizero vehicle. There were some restrictions as wheel base and width of the vehicle. One possible area of improvement is to create a chassis which supports the TES above and below, enclosing and protecting the tank from both sides. Wheel hub motors would give additional freedom in design aspects, with slight disadvantages in vehicle dynamics (which in the case of the Circuito Tec wouldn’t be a problem: the vehicle is used in city, without high dynamic requirements). The additional space could be used to locate the Stirling engine in the back of the vehicle and permit this way a lower floor and easier moving inside the minibus. At continuation, a short outlook to possible alternative concepts and solutions is given, based on the actual Vehizero chassis. • Extremely high temperature storage tanks W. Foppe describes in [19] a high temperature accumulator; materials with high evaporation enthalpy are used as storage media. 20l of boron nitride at 3000°C in a cylinder of carbon with the dimensions of 30cm*30cm store about 100kWh of thermal energy. Foppe proposes a thermal insulation with a layer thickness of 30cm. These results in a 100cm*100cm cylinder of 100kg (see Figure 5.26 A).
57 A B C D
Figure 5.26 TES 3000°C We can easily observe that this kind of TES does not fit in our vehicle concept. Additionally, there would be the need for 300kWhth which would be three of the tanks described in [19]. B shows a rectangular storage tank for 300kWh. This would be better than three individual cylinders, but still to big to fit. Compared to the molten glass TES it is relatively lightweight. But it is even bigger. Removing insulation (C and D) we can see that the storage material is compact and lightweight (about 68kg of BN). Without insulation it could be placed on the desired place in the car. The place should be easy to access and safe in the case of accidents. Obviously, without insulation the concept is impossible. So the challenge would be to find a combination of materials that could bring down the temperature from 3000°C to room temperature in less than 15cm. Or, the other way round, design a new vehicle with extra space for the TES. The weight of the TES would be less than 1kg/kWh which is a minor problem compared to the volume. • Electric Vehicle The thermal energy storage also has to be compared to a full electric vehicle. The 300kWh TES and Stirling engine do provide 75kWh of electric energy at 25% efficiency. We do not need any buffer batteries as in a serial hybrid vehicle, because the electric energy is available the whole time. At continuation a rough weight and size estimation will be made. The first case is an economic solution, T-145 deep cycle batteries will replace the thermal system. The amount installed in the Vehizero vehicle is extended by 60kWh to a total of 75kWh. Second estimation is a comparison of the thermal system by State of the Art energy storage: Li-Ion batteries. At continuation, the Vehicle based on the Vehizero truck is illustrated in plug-in electric vehicle configuration:
Figure 5.27 Electric Vehicle for Circuito Tec – Deep Cycle (left) vs. Li-Ion (right)
An enormous benefit of batteries is that they can be located anywhere in the vehicle. They can be separated and put into various places between other components or below seats. For that reason, Figure 5.27 just gives an orientation for total sizes. With more detailed vehicle information individual solutions could be found. The
58 deep cycle batteries consume too much space, they could be located within the chassis, below the interior, but still would be a disturbing factor in the vehicle concept. The Li-Ion batteries could be located without further distortion of the interior space concept. Costs: a T-145 battery costs about 190 US$ (price spring 2010, private end-user price in internet). 80 are needed and sum up to a total cost of about 15,200 US$. For Li-Ion batteries an actual price of 750US$ per kWh is estimated (see chapter 4.2.3 ), which would be about 56,250 US$ for 75kWh. Future price is estimated as >450 US$, which results in a total price of more than 33,750 US$. The calculation does not include any costs or sizes for controllers, cables etc, and neither do the costs reflect the life cycle of batteries or availability problems. • Hybrid vehicle with diesel engine and Li-Ion batteries The thermal hybrid vehicle concept has also to be compared to a conventional serial hybrid:
Figure 5.28 Conventional Serial Hybrid Configuration Diesel fired generators have efficiencies above 40%. To generate 75kWh electric energy about 24l diesel would be necessary. Weight would be about 25kg for diesel and tank (green) and about 75kg for engine and generator (orange). 10kWh of Li-Ion batteries are estimated with 62.5kg (yellow). This assembly is way smaller than the ones presented before, and way lighter. This also results in better vehicle dynamics, and better energy efficiency. • Conclusions In Figure 5.29 different vehicle concepts are compared. Factors are volume, weight, actual price and estimated future price. The data is the sum for energy storages and engines. Price is calculated for ten years. With exception of the TES the prices are selling prices. Vehicle is the presented virtual prototype with 300kWh TES or equivalent energy storage. More detailed data is found in Appendix IV. Vehicle life is estimated with 10 years, which in the case of the Circuito Tec result in about 276.000km total distance. Lead- acid batteries have to be replaced about 2 times within the ten years, Li-Ion EES only once [38]. The life of a high temperature accumulator based on BN at 3000°C is equal to the life of the vehicle [19]. LiF and NaF/MgF2 have an operating life of minimum 10 years [18]. The vehicles are: • Diesel: Reference vehicle for actual used concepts based on gasoline fuels. It has a 100kW diesel engine and tank of 77l which is about the double amount of energy than in the other cases. It is not a hybrid configuration. • Diesel-Hybrid Vehicle in serial hybrid configuration with a 9kW diesel generator, 10kWh EES and electric propulsion. • Electric: Li-Ion Electric vehicle with 75kWh Li-Ion EES. • Electric: Lead-acid Electric vehicle with 75kWh lead-acid (deep cycle) EES. • Stirling, molten glass TES, lead-acid EES Serial hybrid with Stirling engine of 9kW, molten glass TES with 300kWh and 15kWh lead-acid batteries. It coincides with the presented concept. • Stirling, BN TES, Li-Ion EES Serial hybrid configuration with 9kW Stirling engine. TES with BN at 3000°C [19], 10kWh in Li-Ion EES. • Stirling, LiF-TES, Li-Ion EES. inflation-adjusted.
59 Serial hybrid configuration with 9kW Stirling engine. TES with LiF at 850°C [18] rectangular configuration, 10kWh in Li-Ion EES. Price inflation-adjusted. • Stirling, NaF/MgF2-TES, Li-Ion EES Serial hybrid configuration with 9kW Stirling engine. TES with NaF/MgF2 at 850°C [18], 10kWh in Li-Ion EES. Price inflation-adjusted. 3000
cost (1000MXN) 2690 cost future (1000MXN) 2500 weight (kg) size (l)
2000 1772 1724
1500 1410 1251 1203 1134 987
1000 939 877 855 760 698 679 610 610 582 571 519 500 500 463 414 323 306 277 268 223 215 191 149 83 83 0
) ) c c tric on le tri tri tric -I c c ec yc e ec Diesel (Li C l c p E Ele ri )- 2)-el iF F g lect (Dee lass)- (L Diesel-El E c M ri (g ES ES TES(BN)-ElectricT T (NaF/ Elect ES T
Figure 5.29 Comparison Vehicle Concepts Most expensive components are Stirling engine and EES. The conventional propulsion concept is the cheapest one. For a vehicle with molten glass TES the energy storage and propulsion components would cost (future) about 8 times the price of the diesel based concept. From the economic point of view and with actual gasoline prices of 8 MXN/l, it is difficult to justify these additional costs, see Chapter 7. Diesel, Diesel-Electric and Electric (Li-Ion) vehicles are competitive in terms of weight and size. Weight of Li- Ion batteries would be acceptable, but the price is way higher than the other concepts. Lead-acid batteries are way too heavy. For the thermal energy based systems the boron-nitride concept [19] is competitive in terms of weight, it is heavier, but still acceptable. Its size is significantly higher and requires special space solutions. Also the high temperatures used are problematic. Additional obstacle for all TES systems is the fact that the TES prices are manufacturing prices and the other components already are final selling prices.
60 Chapter 6. Prototype In this chapter, possible prototypes are discussed.
6.1 Different concepts for prototypes There are two fundamental concepts for vehicle prototypes: Proof of concept and Proof of business. The first case shall demonstrate the function of the proposed concept; it shows the technical feasibility. The second case is one step ahead; it must also proof economic practicability. It must demonstrate that the proposed technology con compete against hybrid vehicles with ICE. And it must show that all the needed parts and materials are available at reasonable costs. In the following, several possible prototypes are mentioned: • An electrical model car is equipped with a small Stirling engine of approximate 40We and a thermal tank, it could be remote controlled. • An electrical golf kart is equipped with a Stirling engine of approximate 1kWe and thermal energy storage. Conventional vehicles of this size have electric engines of about 2 or 3 kW and can transport 2 up to 6 persons. The Stirling engine of Whisper Tech Limited [46] provides 800We and would be adequate for this application. • Precer AB (Sweden) [37] presents on its homepage an hybrid all terrain vehicle (ATV) with a Stirling engine using wooden pellets as energy source. The pellet storage would be replaced by a thermal tank. • A conventional car or microbus is converted into a prototype. Preferred base vehicle is a hybrid vehicle, so that only the ICE and the fuel tank must be replaced by the Stirling engine and a thermal tank. One possibility is the 9kW Stirling engine from Cleanergy. This vehicle matches with the vehicle described in Chapter 5. • A prototype is build from the scratch. That means that also the structure is designed and developed especial for this application. Frame, interior and all parts are special designed or adapted for this vehicle. In these five concepts, there are clear differences in the price of the prototypes, but also in the technical expenditure and in the final vehicle value. The value in this case is not the amount of money for which the vehicle could be sold. The value is the impression which it creates to the public and in the press. The first proposal has its advantages in the small initial costs, but it cannot create the same impression as a real size car which can transport persons and which shows applied use. Number 2 already can move persons, anyhow, it does not create the same impression as a prototype especial created and optimized for this kind of engine. For proof of business, the requirements are clearly higher, both technically and economically. Conceivable are, amongst others: • A minibus for scholar transportation, with firm basis stations, in which the solar power is collected. • Vehicles, which are used in a closed system, ferries, vehicles on company premises, nature parks, etc. The background is to create in just few points the infrastructure needed for operation of this vehicles. These vehicles must correspond to higher quality and function standards. They must provide similar function as series vehicles, since they are not only used by trained persons, but also by potential customers.
6.2 Basic thoughts As mentioned in part 6.1 , a prototype has several functions. Depending on the foreseen use, a real sized prototype is needed, or it can be scaled to minimize cost and technical obstacles. Table 6.1 shows the effect of scaling a product. While size, range and speed are straight proportional to the original-sized product, power and weight are scaled exponential. Numbers a based on the vehicle presented in 5.1.2 . A prototype at real size implies a great effort in work and money. The prototypes can get heavy and present technical difficulties. Producing a smaller one can bring the costs down. But it is important not to exceed the
61 downsizing process: The prototype must still be able to represent the most important chemical and physical processes. In the case of the thermal energy storage, 1:4 is an adequate sizing for an economic prototype.
scale size range speed power power weight length width height max e. motor Stirling cm km km/h W W kg 1: 1 500 175 200 150 100 52000 9000 3000 1: 2 250 88 100 75 50 6500 3250 375 1: 3 167 58 67 50 33 1926 963 111 1: 4 125 44 50 38 25 813 406 47 1: 5 100 35 40 30 20 416 208 24 1: 10 50 18 20 15 10 52 26 3 1: 15 33 12 13 10 7 15 8 1 1: 20 25 9 10 8 5 7 3 0 Table 6.1 Scaling prototypes
6.3 Microbus for VIP transport Grupo Senda built a microbus based on an elongated Taylor Dunn T-48 electric utility vehicle (http://www.taylor-dunn.com/). Bodywork was done to create a passenger cabin. The electric system remained original.
Figure 6.1 Microbus Expreso Tec The vehicle is mainly used for promotion purposes. Planned future use is VIP transport in ITESM Monterrey campus. For this microbus, a small Stirling engine of max. 1kW electric power output is sufficient. Structural modifications are not foreseen, at least not more than necessary to implement the thermal energy storage. The system can occupy interior space former used for seats. Tank size is estimated with 10l capacity.
6.4 Minibus for student transport In chapter 5.1.2 the Vehizero hybrid vehicle is presented. One unit is ordered by Tecnológico de Monterrey, and delivering is estimated in early summer 2010. It counts with a small cabin for 2 persons and a cargo area. Planned use is student transport in the Circuito Tec service. Several modifications are necessary and can be done in different stages: • Stage I: convert truck into minibus Build a passenger cabin and use the vehicle on gasoline fuel in the Circuito Tec. Comparative data may be obtained: fuel consumption, driving dynamics etc.
62 • Stage II: replace Trojan deep cycle batteries by Li-Ion batteries Actually, the Vehizero vehicle is equipped with more economic, but heavy batteries. In order to reduce weight it is recommended to replace them by compact and lightweight Li-Ion batteries. • Stage III: exchange ICE with Stirling Engine The next step is to exchange the ICE by the Stirling Engine, still running on gasoline fuel or natural gas. • Stage IV: exchange gasoline by thermal energy, tank heated by electricity Stage IV is probably the most challenging one: The gasoline tank is exchanged for TES, the thermal tank has to be adapted to the vehicle structure and connected to the Stirling engine. TES is heated up by electric resistance. • Stage V: adapt Fresnel lens to thermal tank The last step does not change anything in the vehicle structure: The thermal energy storage is no longer heated by electricity. It will be adapted to a Fresnel lens, which concentrates the solar power. The plan is to test the vehicle after Stage I in the regular student transport.
63 Chapter 7. Conclusions and Recommendations
7.1 Conclusions In the present work hybrid vehicle concepts and main components for the use of TES in a minibus are explained. In a case study requirements and properties of a vehicle for student transport are described. The presented solution has severe disadvantages in comparison to a conventional vehicle. It is bigger, heavier and more expensive. Nevertheless, it is still in a margin that would be possible for specially adapted vehicle concepts. And it opens new ways to use solar energy. Actual costs – and reference – for the Circuito Tec vehicle is presented in Table 7.1. The data sources are interviews with the Circuito Tec personal. Amount Unit Price Toyota Hiace 350,000 MXN Taxes (5%/year) 17,500 MXN/year Maintenance 1,800 MXN/1000km Consumption 20 l/100km Price gasoline (4/2010) 8.11 MXN/l
Average km / week 600 km/week Working weeks / year 46 weeks/year Years 10
Total 276,000 km Gasoline cost 447,672 MXN Taxes 175,000 MXN Maintenance 49,680 MXN
Total costs 1,022,352 MXN Table 7.1 Actual Vehicle Costs Circuito Tec The first step to a more environmental friendly vehicle can be a gasoline-electric hybrid vehicle in serial hybrid configuration. The Simulation result for energy consumption was 15.6l/100km which is about 20% less than the conventional vehicle. About 90,000 pesos in ten years in gasoline cost can be saved. The serial hybrid vehicle with replacement batteries for ten years is expected to have an additional cost of 140,000MXN (future: 66,000MXN) compared to the conventional vehicle. It can be noticed, that with the future price of the Li-Ion batteries the investment is economically feasible over ten years of use of the vehicle. But also with actual component prices it can be economic. Lower taxes, rising energy costs and publicity justify the initial investment. For the TES (molten glass) system the price for engines and energy storages is about 500,000MXN higher. Additional costs for lens and infrastructure have to be considered. Without counting these costs, and assuming zero taxes (for an environment friendly vehicle), the system can be profitable . With rising energy costs the results will be more and more favorable for the TES. V2G can bring en extra income. Still, there are still many questions, due to the fact, that the TES has not been constructed yet: What will be the life cycle? How much does security equipment cost? Is it reliable? Will it be possible to heat it with solar energy? Will the real costs be similar to the calculated costs? Is one storage tank enough or will two tanks be needed with additional costs of 100,000MXN? There are several disadvantages in storing thermal energy: security is important, every damage in the storage container can become dangerous. Heat losses will restrict the maximum storage time. In contrast to that, electric energy is versatile can be used for many applications and has already an existing infrastructure. For this reason, the V2G concept is important: The unused thermal energy can be sold. Assuming 2/3 of the thermal energy used on a day during the week for vehicle propulsion and the remaining energy available for V2G we can estimate the income by selling electric energy:
64 Amount Unit TES size 300 kWh days working 230 days/year idle days 135 days/year % of thermal energy used for transportation 66 %/working day
Total thermal energy 109,500 kWh/year Energy thermal used for transportation 45,540 kWh/year Efficiency conversion thermal - electric 30 % Electric energy sold V2G 19,188 kWh/year
Price electric energy (www.cfe.gob.mx) 2.76 MXN/kWh Possible income V2G 52.959 MXN/year Table 7.2 V2G economics
300kWhth are provided every day in a TES. On the weekends 100% of the electric energy can be sold. The calculation is without downtimes for maintenance etc. The price is based on the producing price for electric energy provided on the bills of the Federal Electricity Commission (CFE) in Monterrey, Mexico. Before applying the V2G concept in Mexico there are political problems to be solved. In these days it is not allowed to sell energy to private persons or enterprises (with exception of the CFE). And the CFE is not willing to buy energy from private producers; they establish high requirements and obstacles so that is nearly impossible to sell them energy. Another important point is the efficiency and seasonal dependence of the system that concentrates solar energy: A Fresnel lens big enough to gather 300 kWhth even in winter is estimated with 176m². In summer, this lens is gathering about 600 kWhth. But the exceeding energy cannot be stored in the TES. Amount Unit Fresnel Lens 176 m² Average Thermal Energy 4.83 kWh/m²/day Gathered Thermal Energy 310,279 kWh/year
Average Electric Energy produced 0.7 kWh/m²/day Gathered Electric Energy 44,968 kWh/year Possible Selling Price 124,112 MXN/year
TES size 300 kWh/day Thermal Energy used for TES 109,500 kWh/year Thermal Energy unused 200,779 kWh/year Table 7.3 Energy Calculations Fresnel Lens We can see in Table 7.3 that only 1/3 of the thermal energy gathered by the 176m² lens is stored in the TES. Without a concept that takes advantage of this excess heat the energy would be wasted. A direct conversion of the gathered solar energy to electric energy by a Stirling engine and a storage tank directly connected to the Fresnel lens would be more effective: The electric energy could be created 24h a day. Electric vehicles can be recharged. This concept would need economic and environmental-friendly batteries, with a long life cycle; which actually do not exist. Main problems of TES in vehicles are: • High price estimation compared to conventional gasoline vehicles • Availability of components (especial Stirling engine) • Security, a great amount of high temperature is located in a vehicle, safety concepts must be developed. • Energy density and specific energy of TES • Missing prototypes and practical experience
65
But looking into the future we can expect rising energy prices and a lack of oil. With smart technological solutions in thermal energy storage materials and insulation, the advantages of the thermal-electric serial hybrid concept will justify this kind of new vehicles: • Vehicles without contaminating gases, no restrictions in dense cities, such as Mexico City. • Using energy nearly oil independent • Brake energy can be recuperated • Excess of thermal energy can be sold as electricity (V2G) • Silent use, emits less noise than an internal combustion engine There is still a lot of work to do, but it is a concept that with further research can result in vehicles for specialized use, as for example in the presented Circuito Tec case study. In combination with other vehicles with other renewable energy sources it can help to replace the gasoline fuel based actual vehicles.
7.2 Recommendations For better understanding of physical and chemical processes in Stirling engine, heat exchanger and TES, it is recommended to build small, economic prototypes of about 100 to 1,000W. The aim would be to have a working system, for first practical experience. I would recommend to buy the 3D plans of the Viebach Stirling and to build the engine in Monterrey. Several field reports are available (mostly in German language). The choice of this engine would divide the high initial cost for a Stirling engine in several smaller costs (plans, materials, machining). Also, this engine could be build within several student projects and would possibly benefit the practical experience in classes. The electric power output for the first engine build can be expected at about 200W, with optimization and adaptation to the TES 400 to 500W can be reached. The experience from a test rack of Stirling engine, heat exchanger and TES will be valuable and essential for first mobile applications. In the next step the Viebach Stirling could be used in the vehicle for VIP transport. Alternatives to the Stirling engines must be further revised, in order to find a more economic solution. For simulation and preparation of the Vehizero vehicle for Circuito Tec use there are several indications: The software AVL Advisor could be used for a more detailed vehicle energy consumption. The software has already properties files for the most important vehicle concepts and components. A detailed vehicle simulation and comparison would be possible. For the FEM simulation it is indispensable to do reverse engineering. The vehicle structure must be updated, especial chassis and suspensions. Based on a more realistic model and more advanced TES information the Stirling engine and TES can be implemented and simulated. The information will be much more reliable than a model based on general pictures of the vehicle. Air-conditioning in this case study is a important factor; a great amount of the total energy is used for passenger comfort in a parked vehicle. Smart solutions would allow a smaller Stirling engine and TES. Before using the vehicle in public areas or with passengers, detailed safety concepts are necessary. TES prototypes must be tested on impact, vibration and other environmental influences. It is important to design the TES the way that it will have small, noticeable failures before catastrophic failures (“leak before explode”).
7.3 Future Work One of the basic problems – besides costs – is energy density and specific energy of the TES system. Alternatives in storage and insulation materials promise better performance. As an example: molten glass is a cheap material, it can by obtained by recycling used bottles, but it can only store 0.66kWth/kg (600°C to 1000°C). Lithium [47] is highly reactive, expensive and difficult to handle. But it has the highest specific heat of any solid element and it can store 5.8kWth/kg in its phase change from liquid to gaseous state (1342°C). In a TES we would only need about 50kg for 300kWth. With insulation and structure the total tank weight could be 100kg. But the price is high, about 315,500MXN only for the Lithium. Additional costs would be caused by tank system that can resist Lithium and that divides the TES in many small sub-tanks in order to avoid chemical reaction in the case of accidents. Another approach would be to use a material that has a double phase change within the working temperatures of the Stirling engine. Aim must be to make cost, size and weight of the TES more competitive to gasoline fuels.
66 References [1] K. Molter, “Alternative Energietechniken,” Mai. 2008, S. 195. [2] University of Karlsruhe, “http://www-ifkm.mach.uni-karlsruhe.de/Html-e/Project/Stirling/stirling.html,” 2000. [3] Wikipedia, “http://en.wikipedia.org/wiki/Stirling_engine,” 2009. [4] Infinia Corporation, “http://www.infiniacorp.com/technology/free_piston.php,” 2008. [5] Stirling Engine Configurations, Ohio University, “http://www.ent.ohiou.edu/~urieli/stirling/engines/engines.html,” 2007. [6] N. Savage, “Silicon Nanowires Turn Heat to Electricity,” Jan. 2008. [7] E. Schmidt, “http://www.stirlingmotor.com,” 2009. [8] A. Moreno Lehman und N. León Rovira, “Use of the Triz Methodology in the development of a new Stirling/Brayton engine,” 2008. [9] Capstone Hybrid History.pdf, “http://www.capstoneturbine.com/_docs/Capstone%20Hybrid%20History.pdf,” 2010. [10] H. Braess und U. Seiffert, Handbook Of Automotive Engineering, SAE International, 2005. [11] H. Berger, “Future automobile propulsion technologies,” 2009. [12] H. Berger, “Dem Elektroauto und Photovoltaik gehören die Zukunft,” 2008. [13] J. Dunne, “Test Driving GM´s Hybrid Electric Car,” Popular Science, vol. 193 Nr. 6, 1996, S. 116 -119. [14] R.K. Shaltens, “Results from the Stirling-powered vehicle project: Phase 1,” United States: 1988, S. Pages: (15 p). [15] W. ERNST und R. SHALTENS, “Automotive Stirling Engine Development Project(Final Report),” 1997. [16] N.P. Nightingale, Automotive Stirling engine: Mod II design report, DOE/NASA-0032-28, Mechanical Technology, Inc., Latham, NY (USA), 1986. [17] Department of Energy, Energy Research at DOE: Was It Worth It? Energy Efficiency and Fossil Energy Research 1978 to 2000, NATIONAL ACADEMY PRESS, 2001. [18] D. Morgan, Thermal Energy Storage for the Stirling Engine Powered Automobile (Final Report), US Department of Energy, 1979. [19] W. Foppe, “US Patent 6,272,856 Method for storing energy in the form of thermal energy by means of high-temperature accumulators,” 2001. [20] W.B. Stine und M. Geyer, Power From The Sun, 2001. [21] Nihon Tokusho Kongaku Jushi Co. Ltd., “http://www.ntkj.co.jp/product_fresnel_en.html,” 2009. [22] B. Zalba, J.M. Mari´n, L.F. Cabeza, und H. Mehling, “Review on thermal energy storage with phase change: materials, heat transfer analysis and applications,” Applied Thermal Engineering, vol. 23, 2003, S. 251-283. [23] D. Oertel, “Energiespeicher–Stand und Perspektiven,” 2008, S. 168. [24] BSR Solar Technologies, “http://www.bsrsolar.com/core1-3.php3,” 2008. [25] L. Garcia-Espinosa und N. León Rovira, “Solar Energy Storage Tank,” 2009. [26] University of Delaware, “http://www.udel.edu/V2G/index.html,” 2009. [27] S. Welty, D. Clark, und A. Simms, “Assessment of a Commercially Available Stirling Engine for use in Solar Powered Distributed Energy Applications.,” 2009. [28] C. Whitlock, NASA Surface meteorology and Solar Energy, 2000.
67 [29] Environmental and Energy Study Institute, “Hybrid buses - Costs and Benefits,” 2007. [30] Daimler AG, “http://media.daimler.com,” 2009. [31] Azure Dynamics, “http://www.azuredynamics.com,” 2009. [32] EBUS, INC., “Hybrid-Electric Bus Specification,” 2008. [33] Solaris Bus & Coach S.A., “https://www.solarisbus.pl/,” 2009. [34] Toyota Japan, “http://www.toyota.co.jp,” 2009. [35] DEKA Research and Development Corporation, “http://www.dekaresearch.com/,” 2009. [36] Think Norway, “http://www.think.no/,” 2009. [37] Precer AB, “www.precer.com,” 2009. [38] CLSA Group, “Electric vehicles Special report,” 2008. [39] ADAC, “http://www1.adac.de/Auto_Motorrad/Technik_Zubehoer/Mehrverbrauch_Klimaanlage/default.asp,” 2009. [40] Mercedes Benz de Mexico , “http://www.mb-mexico.com.mx,” 2009. [41] Tecnológico de Monterrey, “http://expresotec.mty.itesm.mx/web/index.html,” 2009. [42] Toyota Mexico, http://www.toyota.com.mx/, 2010. [43] JAZZ VEHIZERO, S.A. de C.V., “http://vehizero.com/,” 2009. [44] Cleanergy AB, “http://www.cleanergyindustries.com/.” [45] H. Wegmann und P. Gerster, “Schweißtechnische Verarbeitung und Anwendung hochfester Baustähle im Nutzfahrzeugbau,” 2008. [46] Whisper Tech Limited, “www.whispergen.com,” 2009. [47] Chemical Fact Sheet Lithium, “http://www.speclab.com/elements/lithium.htm,” 2010. [48] Geographic coordinate system - Wikipedia, the free encyclopedia, “http://en.wikipedia.org/wiki/Geographic_coordinate_system.”
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Appendix I – Matlab Program Code • Main Program This Matlab file is opened by a batch program and contains folder and file definitions. It is used to run subroutines.
% function [] = HYBCAR_ST_02 ()
% Mathematical Model of a hybrid vehicle with Stirling engine and thermal % energy storage % HYBCAR-ST % Version: Circuito Tec; Mercedes Benz Sprinter 515
% Erik Sewe % Tecnológico de Monterrey clc; clear all; close all;
% Definition of files XLSinputfile = 'C:\Dokumente und Einstellungen\Erik\Desktop\tesis\modelo matematico\Matlab\GPStrack_CircuitoTec FIRST.xls'; XLSoutputfile = 'C:\Dokumente und Einstellungen\Erik\Desktop\tesis\modelo matematico\Matlab\MatlabOUTPUT_CircuitoTec FIRST.xls'; XLSvehicleconstants = 'C:\Dokumente und Einstellungen\Erik\Desktop\tesis\modelo matematico\Matlab\DataSheet_CircuitoTec.xls';
% Import Data GPSdataXLS=xlsread([XLSinputfile]); n = max(GPSdataXLS(:,1))-1; %Number of data sets
% Definition of Constants [m,vmax,Af,cd,cr,Pst,nst,Pem,nem,nbrake,Wbattery_start,Wbattery_max,nbattery,Wth ermal_start,Wthermal_max,Wthermal_air,nthermal,ec,eth,g,rho_air,t,j] = Vehicle_Constants(XLSvehicleconstants);
%Calculate speed, distance, difference in altitude and velocity [ntvs,ntvs_text] = ntvs(n,t,GPSdataXLS);
% Velocity profile [vprofile, vprofile_text]= Velocity_Statistics(ntvs,n);
% Consumption at const. Velocity [E_consumption, E_consumption_text] = Consumption_constant_velocity(cd,Af,rho_air,cr,m,g,ec);
% Energy calculations [Energy,Epot_total,Ea_total,Eair_total,Er_total,Eec_total,Ebrake_total,Ewheel_to tal,Ebattery,Econsumed_total,Energy_text,E_aircond] = Energy_calculations (n,ntvs,m,nbrake,g,cd,Af,rho_air,cr,ec,eth,t,nbattery,vmax);
% State of charge battery and thermal energy storage [SOC,j,SOC_text] = SOC_calculations (n,Wbattery_start,Ebattery,E_aircond,Wbattery_max,j,Wthermal_start,Pst,t,ntherma l,Wthermal_max,Wthermal_air);
69 % Results [s_total,v_average,hdiff_total,v_max,Wthermal_total,ResultText1,ResultData1,Resu ltText2,ResultData2] = Result_calculations (n,ntvs,SOC,Econsumed_total,Ewheel_total,Ebrake_total,Epot_total,Ea_total,Eair_t otal,Er_total,Eec_total);
%Write to data file WriteData(XLSoutputfile, ResultData1,ResultText1,ResultData2,ResultText2,ntvs_text,Energy_text,SOC_text,n tvs,Energy,SOC,E_consumption,E_aircond, vprofile, vprofile_text,t,m); quit;
• Definition of Constants In this subroutine Matlab reads basic vehicle properties from an Excel file and performs some basic calculations, for example total vehicle mass. function [m,vmax,Af,cd,cr,Pst,nst,Pem,nem,nbrake,Wbattery_start,Wbattery_max,nbattery,Wth ermal_start,Wthermal_max,Wthermal_air,nthermal,ec,eth,g,rho_air,t,j] = Vehicle_Constants(XLSvehicleconstants)
% Definition of Vehicle Constants constants = ddeinit('excel',XLSvehicleconstants);
% Vehicle Mass m_empty = ddereq(constants,'r7c3'); %weight vehicle in kg % Driving dynamics vmax = ddereq(constants,'r9c3') *1/3.6; %maximum velocity in m/s, defined by user profile % Aerodynamic properties, www.mercedes-benz.de Af = ddereq(constants,'r11c3'); %front surface in m^2, cd = ddereq(constants,'r12c3'); %air drag coefficient % Rolling resistance, cr = ddereq(constants,'r14c3'); %Tyre rolling coefficient cr=0.9% % Stirling engine Pst = ddereq(constants,'r17c3') *1000; %Electric output Stirling engine in W / kg*m^2/s^3 nst = ddereq(constants,'r18c3'); %efficiency factor Stirling engine nm_st = 40 /1000; % weight factor Stirling engine kg/W nm_st1 = 60; % inicial weight in kg % Electric motor Pem = ddereq(constants,'r19c3') *1000; %Power output electric motor in W / kg*m^2/s^3 nem = ddereq(constants,'r20c3'); %efficiency factor electric motor nbrake = ddereq(constants,'r21c3'); %efficiency brake energy recovery % Electric Energy Storage Wbattery_start = ddereq(constants,'r23c3') *1000*3600; %electric energy stored in battery at start in kg*m^2/s^2 Wbattery_max = ddereq(constants,'r24c3') *1000*3600; %maximum electric energy stored in battery in kg*m^2/s^2 nbattery = ddereq(constants,'r25c3'); %efficiency from "power output battery" to (incl.) electric motor nmbattery = 6.25 /(1000*3600); % specific energy Li-Ion in kg / (kg*m^2/s^2) % Thermal Energy Storage Wthermal_start = ddereq(constants,'r26c3') *1000*3600; %thermal energy stored in tank at start in kg*m^2/s^2
70 Wthermal_max = ddereq(constants,'r27c3') *1000*3600; %maximum thermal energy stored in tank in kg*m^2/s^2 nthermal = ddereq(constants,'r28c3'); %efficiency from "thermal energy" to "electric energy at battery" nm_thermal = 1.49 /(1000*3600); % specific energy molten glass in kg/(kg*m^2/s^2) % nm_thermal = 4.2 % specific energy molten salt in kg/ (kg*m^2/s^2) nm_thermal1 = 546; % weight tank structure in kg
% Others ec = ddereq(constants,'r31c3') *1000; %electric energy consumption in W / kg*m^2/s^3 eth = ddereq(constants,'r33c3') *1000; %thermal energy consumption for air conditioning in W / kg*m^2/s^3 Wthermal_air = ddereq(constants,'r29c3') *1000*3600; % thermal energy stored in tank for air conditioning in kg*m^2/s^2 % Physics g = 9.81; %gravitational acceleration in m/s^2 rho_air = 1.2; %air density in kg/m^3 % Program Constants t = ddereq(constants,'r36c3'); %time between two data sets in s j = ddereq(constants,'r37c3'); %Stirling engine off at start of the program
% mass calculation m_battery = Wbattery_max*nmbattery; m_stirling = Pst * nm_st + nm_st1; m_thermal = (Wthermal_max + Wthermal_air)* nm_thermal + nm_thermal1; m = m_empty+m_battery+m_stirling+m_thermal;
• Calculate speed, distance, difference in altitude and velocity Imports drive cycle. function [ntvs,ntvs_text] = ntvs(n,t,GPSdataXLS)
%Calculate speed, distance, difference in altitude and velocity ntvs_text = {'Number','Time (s)','Velocity (m/s)','Distance (m)',' Altitude','Difference Altitude(m)','Difference Velocity (m/s)'}; for i = 1:n %Loop all data ntvs(i,1) = i; %data set number ntvs(i,2) = i*t; %time from start on, in s ntvs(i,3) = GPSdataXLS(i,8)/3.6; %velocity, in m/s if ntvs(i,3) < 1 % delete GPS tolerance in idle ntvs(i,3) = 0; end ntvs(i,4) = GPSdataXLS(i,6); %distance, in m ntvs(i,5) = GPSdataXLS(i,3);
if i==1 %first data set ntvs(i,6)=0; %Altitude difference ntvs(i,7)=0; %Velocity difference else if ntvs(i,3)== 0; ntvs(i,6) = 0; else ntvs(i,6)=(GPSdataXLS(i,3)-GPSdataXLS(i-1,3)); %altitude - altitude one data set before end ntvs(i,7)=(ntvs(i,3)-ntvs(i-1,3)); %speed difference end
71 end
• Velocity profile Creates velocity statistics. function[vprofile,vprofile_text]=Velocity_Statistics(ntvs,n);
%This function calculates the velocity statistics vprofile = zeros(1,10); vprofile_text = {'idle','1-10km/h','11-20km/h','21-30km/h','31-40km/h','41- 50km/h','51-60km/h','61-70km/h','71-80km/h','> 80km/h'}; for i = 1:n %Loop all data if ntvs(i,3)* 3.6 < 1; %velocity, in m/s vprofile(1,1)= vprofile(1,1)+ 1; elseif (ntvs(i,3)* 3.6 >= 1) && (ntvs(i,3)* 3.6 <= 10) vprofile(1,2) = vprofile(1,2)+1; elseif (ntvs(i,3)* 3.6 > 10) && (ntvs(i,3)* 3.6 <= 20) vprofile(1,3) = vprofile(1,3)+1; elseif (ntvs(i,3)* 3.6 > 20) && (ntvs(i,3)* 3.6 <= 30) vprofile(1,4) = vprofile(1,4)+1; elseif (ntvs(i,3)* 3.6 > 30) && (ntvs(i,3)* 3.6 <= 40) vprofile(1,5) = vprofile(1,5)+1; elseif (ntvs(i,3)* 3.6 > 40) && (ntvs(i,3)* 3.6 <= 50) vprofile(1,6) = vprofile(1,6)+1; elseif (ntvs(i,3)* 3.6 > 50) && (ntvs(i,3)* 3.6 <= 60) vprofile(1,7) = vprofile(1,7)+1; elseif (ntvs(i,3)* 3.6 > 60) && (ntvs(i,3)* 3.6 <= 70) vprofile(1,8) = vprofile(1,8)+1; elseif (ntvs(i,3)* 3.6 > 70) && (ntvs(i,3)* 3.6 <= 80) vprofile(1,9) = vprofile(1,9)+1; elseif (ntvs(i,3)* 3.6 > 80) vprofile(1,10) = vprofile(1,10)+1; end end vprofile = (vprofile/n)*100; % normalized
• Consumption at const. Velocity This subroutine creates the energy consumption for constant speeds. function[E_consumption, E_consumption_text]=Consumption_constant_velocity(cd,Af,rho_air,cr,m,g,ec);
% Consumption at const. Velocity % http://www.chemie.fu-berlin.de/chemistry/general/kfz-energetisch.html E_consumption_text = {'v (km/h)', ' ', ' ', 'Air drag', 'Rolling Resistance', 'Electric devices', 'Energy at Wheel'}; for i=1:100 v_const(i) = i/3.6; % velocity s_const = 100000; % distance 100km t_const(i) = s_const/i; E_consumption(i,1)=i; % velocity (km/h) E_consumption(i,2)=0; % potential energy = const E_consumption(i,3)=0; % kinetic energy = const E_consumption(i,4)=cd*Af*(rho_air/2)*(v_const(i)^2)*s_const ; %air drag E_consumption(i,5)=cr*m*g*(s_const); %rolling resistance E_consumption(i,6)=ec*t_const(i) ;
72 E_consumption(i,7)= E_consumption(i,2)+E_consumption(i,3)+E_consumption(i,4)+E_consumption(i,5)+E_co nsumption(i,6); % end
• Energy calculations Actual energy consumption, detailed in friction, air drag etc. function [Energy,Epot_total,Ea_total,Eair_total,Er_total,Eec_total,Ebrake_total,Ewheel_to tal,Ebattery,Econsumed_total,Energy_text, E_aircond] = Energy_calculations (n,ntvs,m,nbrake,g,cd,Af,rho_air,cr,ec, eth,t,nbattery,vmax) % Energy calculations
Energy_text = {'Number', 'Potential Energy', 'Kinetic Energy', 'Air drag', 'Rolling Resistance', 'Electric devices', 'Energy at Wheel', 'Brake Energy recuperated'};
% Calculate Energy used for acceleration / recovered while braking Iw=17.21; %AVL ADVISOR, P205/65R15 i=0.1; % 8000rpm electric engine = 96km/h vehicle Ieng=0.05; %AVL ADVISOR, Honda 49kW (50kg) engine rstat=0.3; rdyn=0.32; meff=m+((Iw+i^2*Ieng)/(rstat*rdyn)); %Braess, Handbook of Automotive Engineering for i = 1:n Energy(i,1)=i; if ntvs(i,7)> 0 %acceleration Energy(i,3)=0.5*meff*(ntvs(i,7)^2); else %braking Energy(i,8)=-nbrake*0.5*meff*(ntvs(i,7)^2); end end % Calculate potential energy Energy(:,2)=m*g*ntvs(:,6); %Potential energy, hdiff: change of altitude from n-1 to n % Calculate Energy against the air drag for i = 1:n Energy(i,4)=cd*Af*(rho_air/2)*(ntvs(i,3)^2)*ntvs(i,4) ; end % Calculate Energy against rolling resistance for i = 1:n Energy(i,5)=cr*m*g*(ntvs(i,4)); end % Calculate Energy used for lights, radio, air conditioning for i = 1:n Energy(i,6)=ec*t ; end % Calculate TOTAL Energy Epot_total=sum(Energy(:,2)); Ea_total=sum(Energy(:,3)); Eair_total=sum(Energy(:,4)); Er_total=sum(Energy(:,5)); Eec_total=sum(Energy(:,6)); Ebrake_total=sum(Energy(:,8));
Energy(:,7) = Energy(:,2) + Energy(:,3) + Energy(:,4) + Energy(:,5); %Energy consumed at wheel, without recuperation Ewheel_total = sum(Energy(:,7)); %Energy consumed at the wheel in the complete cycle
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[Ebattery] = Generator_UQM_PowerPhase145(Energy,ntvs,vmax,n,t); % Ebattery = Energy(:,7) / nbattery; %Energy taken from batteries
Ebattery_total=sum(Ebattery(:,1)); Econsumed_total=Ebattery_total+Ebrake_total+Eec_total;
% Calculate Energy for air conditioning E_aircond = 0; for i = 1:n if i==1 E_aircond(i,1) = eth*t; else E_aircond(i,1) = E_aircond(i-1) + eth*t ; end end
• State of charge battery and thermal energy storage Creates SOC for TES and EES, switches Stirling engine on or off. function [SOC,j,SOC_text] = SOC_calculations (n,Wbattery_start,Ebattery,E_aircond,Wbattery_max,j,Wthermal_start,Pst,t,ntherma l,Wthermal_max,Wthermal_air)
% State of charge battery and thermal energy storage
SOC_text = {'Number', 'SOC EES', 'SOC EES in %', 'SOC TES', 'SOC TES in %','Stirling on/off', 'SOC TES air cond.', 'SOC TES air cond. in %'}; tSTs = 300; %Time for Stirling engine to Start tSTe = 300; %Time for Stirling engine to Shut down for i = 1:n SOC(i,1)=i; if i == 1 %initialization SOC(i,2)=Wbattery_start; else SOC(i,2)= SOC(i-1,2)-Ebattery(i,1); end
if SOC(i,2)> Wbattery_max %Battery cannot be overcharged SOC(i,2)= Wbattery_max; end
SOC(i,3) = (100/Wbattery_max*SOC(i,2)); %SOC in %
if SOC(i,3)< 50 %at low SOC Stirling engine starts to work if j == 2 % Stirling engine was OFF before j = 0; end elseif SOC(i,3) > 90 %at high SOC Stirling engine shuts down if j==1% Stirling engine was ON before j=3; end end
if i == 1 %initialization SOC(i,4)=Wthermal_start; elseif j < 1.5 %if Stirling engine working than thermal energy consumption SOC(i,4) = SOC(i-1,4)-Pst*t*j/nthermal; SOC(i,2)= SOC(i,2)+ (Pst*t)*j;
74 j=j+t/tSTs; % tST = time to full working Stirling engine if j > 1 % Stirling engine working 100% j=1; end else % Stirling engine off j=j-t/tSTe; if j < 2 % Stirling engine completely off j=2; end SOC(i,4) = SOC(i-1,4)-(Pst*t/nthermal)*(j-2); SOC(i,2)= SOC(i,2)+ (Pst*t)*(j-2); end
SOC(i,3) = (100/Wbattery_max*SOC(i,2)); %SOC EES in % SOC(i,5) = (100/Wthermal_max*SOC(i,4)); %SOC TES in % SOC(i,6)=j; %write if Stirling engine is running or not
SOC(i,7)= Wthermal_air - E_aircond(i,1); %TES2 SOC(i,8)= (100/Wthermal_air*SOC(i,7)); %TES2 in % end
• Results Calculation of final results, for example total distance or average speed. function [s_total,v_average,hdiff_total,v_max,Wthermal_total,ResultText1,ResultData1,Resu ltText2,ResultData2] = Result_calculations (n,ntvs,SOC,Econsumed_total,Ewheel_total,Ebrake_total,Epot_total,Ea_total,Eair_t otal,Er_total,Eec_total)
% Calculation of results s_total=sum(ntvs(:,4)); %distance total, in m v_average=s_total/ntvs(n,2); %average speed, in m/s hdiff_total=sum(ntvs(:,6)); %total difference in altitude in m v_max=max(ntvs(:,3)); %Max. velocity, in m/s Wthermal_total=SOC(1,4)-SOC(n,4); %thermal energy used ResultText1=({'Total distance (km)'; 'Average velocity (km/h)'; 'Total altitude difference (m)'; 'Maximum Velocity (km/h)'}); ResultData1=[s_total/1000; v_average*3.6; hdiff_total; v_max*3.6]; ResultText2={'Total Energy spent (kWh)';'Energy consumed at Wheel (kWh)';'Brake Energy recuperated (kWh)';'Potential Energy (kWh)';'Energy consumed by accelerating (kWh)';'Energy consumed by air drag (kWh)';'Energy consumed by roling resistance (kWh)';'Energy consumed by electric devices (kWh)';'Thermal energy used (kWh)'}; ResultData2=[Econsumed_total/3600000;Ewheel_total/3600000;Ebrake_total/3600000;E pot_total/3600000;Ea_total/3600000;Eair_total/3600000;Er_total/3600000;Eec_total /3600000;Wthermal_total/3600000];
• Write to data file Creates an Excel file with statistics and results. function [] = WriteData(XLSoutputfile, ResultData1,ResultText1,ResultData2,ResultText2,ntvs_text,Energy_text,SOC_text,n tvs,Energy,SOC,E_consumption,E_aircond, vprofile, vprofile_text,t,m)
% Create output file warning off MATLAB:xlswrite:AddSheet; %Turn off msg while creating new sheet %Delete old Excel Output File delete (XLSoutputfile);
75 %Create Excel Output File Excel = actxserver ('Excel.Application');
%Write Data xlswrite(XLSoutputfile, {'HYBCAR ST'}, 'Results', 'A1'); %Write titles xlswrite(XLSoutputfile, ResultData1, 'Results', 'B3'); xlswrite(XLSoutputfile, ResultText1, 'Results', 'C3'); xlswrite(XLSoutputfile, ResultData2, 'Results', 'F3'); xlswrite(XLSoutputfile, ResultText2, 'Results', 'G3'); xlswrite(XLSoutputfile, m, 'Results', 'B8'); xlswrite(XLSoutputfile, {'vehicle mass (kg)'}, 'Results', 'C8'); xlswrite(XLSoutputfile, max(E_aircond(:,1)/3600000), 'Results', 'F12'); xlswrite(XLSoutputfile, {'thermal energy consumed by air conditioning (kWh)'}, 'Results', 'G12');
xlswrite(XLSoutputfile, [ntvs_text, Energy_text, SOC_text], 'Matlab', 'A1'); %Write titles xlswrite(XLSoutputfile, [ntvs, Energy, SOC], 'Matlab', 'A2'); %Write data to file xlswrite(XLSoutputfile, m, 'Matlab', 'C8') xlswrite(XLSoutputfile, {'HYBCAR ST'}, 'Plot', 'A1'); %Write titles 2 xlswrite(XLSoutputfile, {'HYBCAR ST'}, 'Plot2', 'A1'); %Write titles 3
ExcelWorkbook=Excel.Workbooks.Open(XLSoutputfile); Worksheets = Excel.sheets;
Worksheets.Item('Sheet1').Delete; %Delete Empty Sheet 1,2,3 Worksheets.Item('Sheet2').Delete; %Delete Empty Sheet 1,2,3 Worksheets.Item('Sheet3').Delete; %Delete Empty Sheet 1,2,3
ActiveSheet=Excel.Sheets.Item('Results'); %Write Bar Plot barh(round(ResultData2));% % xlabel(ResultText2); Xlabel('Energy (kWh)'); set(gca,'YTickLabel', ResultText2) title('Overview: Energy consumption'); print -dmeta % Copy to clipboard ActiveSheet.Range('B14').PasteSpecial; %paste plot
% Plot: Consumption at constant speed plot(E_consumption(:,1),E_consumption(:,7)/3600000); ylabel('Energy (kWh)/100km'); xlabel('Velocity (km/h)'); title('Energy consumption for 100km at constant speed'); print -dmeta % Copy to clipboard ActiveSheet.Range('K14').PasteSpecial; %paste plot
ActiveSheet=Excel.Sheets.Item('Plot'); % Plot: Velocity plot(ntvs(:,2)/60,ntvs(:,3)*3.6); %Velocity vs time xlabel('time (min)'); ylabel('Velocity (km/h)'); title('Velocity vs. time'); print -dmeta % Copy to clipboard ActiveSheet.Range('B2').PasteSpecial; %paste plot % Plot: Altitude
76 plot(ntvs(:,2)/60,ntvs(:,5)); % altitude vs time xlabel('time (min)'); ylabel('Altitude (m)'); title('Altitude profile'); print -dmeta % Copy to clipboard ActiveSheet.Range('K2').PasteSpecial; %paste plot %Write Bar Plot barh(vprofile);% % xlabel(ResultText2); Xlabel('%'); set(gca,'YTickLabel',vprofile_text) title('Velocity contribution'); print -dmeta % Copy to clipboard ActiveSheet.Range('B28').PasteSpecial; %paste plot
ActiveSheet=Excel.Sheets.Item('Plot2'); % Plot: Power at wheel plot(ntvs(:,2)/60,Energy(:,7)*(1/(t*1000))); xlabel('time (min)'); ylabel('Power at wheel (kW)'); title('actual Power needed at wheel'); print -dmeta % Copy to clipboard ActiveSheet.Range('B2').PasteSpecial; %paste plot % Plot: SOC EES and TES in % plot(ntvs(:,2)/60,[SOC(:,3),SOC(:,5),SOC(:,8)]); %SOC Battery % xlabel('time (min)'); ylabel('State of Charge (%)'); title('State of Charge: Electric and thermal energy storage systems'); legend({'Electric Energy Storage';'Thermal Energy Storage'; 'Thermal Energy Storage 2'},'Location','best') print -dmeta % Copy to clipboard ActiveSheet.Range('K2').PasteSpecial; %paste plot % Plot: Power actual label_text = {'Potential Energy'; 'Kinetic Energy'; 'Air drag'; 'Rolling Resistance'; 'Electric devices'; 'Brake Energy recuperated'}; plot(ntvs(200:660,2)/60,[Energy(200:660,2)*(1/(t*1000)),Energy(200:660,3)*(1/(t* 1000)),Energy(200:660,4)*(1/(t*1000)),Energy(200:660,5)*(1/(t*1000)),Energy(200: 660,6)*(1/(t*1000)),Energy(200:660,8)*(1/(t*1000))]); xlabel('time (min)'); ylabel('Power (kW)'); title('(for selected data sets) Actual power consumed for:'); legend(label_text,'Location','best'); print -dmeta % Copy to clipboard ActiveSheet.Range('B28').PasteSpecial; %paste plot
Excel.Visible = 1;%Make it Visible ExcelWorkbook.SaveAs(XLSoutputfile); %Save % ExcelWorkbook.Close(false); %Close %Excel.Quit; close all;
77
Appendix II - Student Transport ITESM
Expreso Tec Route V1 V2 V3 V4 G1 G2 G3 G4 SN CL Average
Time for one segment (min) 50 42 40 50 35 36 50 55 45 50 45 Distance / segment (km) 17 15 18 16 16 15 22 17 20 22 18 Time calculated (min) 35 26 33 30 28 29 47 36 43 46 35 Stops (number) 17 12 18 12 11 10 8 12 13 13 13 Routes/day (both ways) 10 12 12 12 10 12 12 12 11 8 11 Routes max / bus / day (one way) 10 10 11 11 11 16 12 12 12 12 12
hours total / day 16 16 16 16 16 16 16 16 16 16 16
km / day 170 146 202 179 172 245 269 200 241 258 208 moving hours / day 8 7 7 9 6 10 10 11 9 10 9 idle hours / day 8 9 9 7 10 6 6 5 7 6 7 average velocity (km/h) 20 21 28 20 27 26 27 18 27 26 24
Circuito Tec Route A B average
Time for one segment (min) 30 30 30 Distance / segment (km) 7.8 9.2 8.5 Time calculated (min) 24 26 25 up to up to Routes/day (both ways) 26 26 Routes max / camion - day (one way) 26 26 26
hours total / day 14 14 14
km / day 202.8 239.2 221
moving hours / day 13 13 13
idle hours / day 1 1 1
average velocity (km/h) 15.6 18.4 17
78 Appendix III - Virtual Test Track This appendix describes how to obtain a virtual test track in MSC ADAMS/car based on a GPS track. In the present work, the vehicle runs a relative well defined route. For that reason, the virtual test circuit is rebuilding a loop starting end and ending in the main entrance of the university, running through some most representative streets in the neighborhood. Doing so, the simulations can be revised easier, simply comparing virtual results against real results. Procedure to create the virtual test track: • Define and measure roads A route, that represents well the main characteristics, is shown in the following figure:
Test track Circuito Tec Coordinates can be gathered by a handheld GPS. • Coordinate transformation Since the output of the Garmin handheld is in the geographic coordinate system and ADAMS/car expects Cartesian coordinates, it is necessary to perform a transformation. In the following passage, this process is explained for a single sample point. In general, the calculation is done automatically in an Excel sheet. Garmin provides the following data (selection) for each track point: N25 46.139 W100 24.043 and the corresponding altitude. This data has to be transformed into distances, in relation to a reference point. In the ADAMS/car model, the reference point is the start point of the simulation, thus in our example the entrance of Tecnológico de Monterrey. The transformation stated below implies some simplification, but is sufficient exact for our purposes. Details and further information for the transformation can be found in [48]. Expanding the Garmin data format we can obtain the following information: latitude longitude hemisfere N W degree 25 100 minute 46 24 decimalminute .139 .043 Garmin coordinate data First, we do transform the latitude: On a spherical surface at sea level, one latitudinal second measures 30.82 meters, one latitudinal minute 1849 meters, and one latitudinal degree is 110.9 kilometers. For our example, we have in north direction: 25 ⋅110900m + 46.139 ⋅1849m = 2857792.521m
79 The next step is to transform the longitude: The circles of longitude, meridians, meet at the geographical poles, with the west-east width of a second being dependent on the latitude. On the equator at sea level, one longitudinal second measures 30.92 meters, a longitudinal minute 1855 meters, and a longitudinal degree 111.3 kilometers. The decrease in distance can be calculated by the following aproximation: π ⋅ cos(φ) ⋅ M 180 r
With Φ representing the latitudinal angle and Mr is the earth's average meridional radius (6,367,449 m). In our example: 3.14 3.14 ( ⋅ cos(25) ⋅ 6367449m) ⋅100 + ( ⋅ cos(25) ⋅ 6367449m) ⋅ 24.043 = 10107.9m 180 60 ⋅180 These steps must be performed for all track points, to obtain x and y value. Then they will be related to the initial point (first track point is set to x = 0 and y = 0). For the last value z will be set to the altitude given by the GPS, with the first point defined as z = 0. The table created can be imported in ADAMS/car. The next step would be to implement obstacles.
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Appendix IV - Vehicle Comparison Electric TES Diesel- Electric (Deep TES(glass)- TES(BN)- TES(LiF)- (NaF/MgF2)- Diesel Electric (Li-Ion) Cycle) Electric Electric Electric electric first engine diesel diesel - - Stirling Stirling Stirling Stirling power (kW) 120 9 9 9 9 9 cost (MN) 80000 8000 330000 330000 330000 330000 cost future (MN) 80000 8000 225000 225000 225000 225000 weight (kg) 250 75 150 150 150 150 size (l) 200 72 96 96 96 96
first energy storage diesel diesel - - TES TES TES TES amount (kWh) 600 300 300 300 300 300 cost (MN) 3000 1000 100500 135000 333000 394800 cost future (MN) 3000 1000 100500 135000 333000 394800 weight (kg) 56 28 996 200 989 1509 size (l) 77 39 650 1056 433 433 second engine - electric electric electric electric electric electric electric weight (kg) 50 50 50 50 50 50 50 size (l) 18 18 18 18 18 18 18 cost (MN) 30000 30000 30000 30000 30000 30000 30000 second electric energy electric electric (li- (deep electric electric electric storage - (li-ion) ion) cycle) (deep cycle) (li-ion) (li-ion) electric (li-ion) amount (kWh) 10 75 75 15 10 10 10 cost (MN) 92000 690000 184000 36800 92000 92000 92000 cost future (MN) 55000 412500 97500 19500 55000 55000 55000 battery replacement 92000 690000 368000 73600 92000 92000 92000 battery repl. (future) 55000 412500 195000 39000 55000 55000 55000 weight (kg) 63 468.75 2640 528 63 63 63 size (l) 63 250 1116 223 33 63 63
Electric Diesel- Electric(Li- (Deep TES(glass)- TES(BN)- TES(LiF)- TES(NaF/MgF2)- total Diesel Electric Ion) Cycle) Electric Electric Electric electric cost (1000MXN) 83 223 1410 582 571 679 877 939 cost future (1000MXN) 83 149 855 323 414 500 698 760 weight (kg) 306 215 519 2690 1724 463 1251 1772 size (l) 277 191 268 1134 987 1203 610 610
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