DESIGN, IMPLEMENTATION AND EVALUATION OF AN AUXILIARY ENERGY SYSTEM FOR ELECTRIC VEHICLES, BASED ON ULTRACAPACITORS AND BUCK-BOOST CONVERTER

Transcripción

1 PONTIFICIA UNIVERSIDAD CATOLICA DE CHILE ESCUELA DE INGENIERIA DESIGN, IMPLEMENTATION AND EVALUATION OF AN AUXILIARY ENERGY SYSTEM FOR ELECTRIC VEHICLES, BASED ON ULTRACAPACITORS AND BUCK-BOOST CONVERTER MICAH E. ORTÚZAR Thesis submitted to the Office of Research and Graduate Studies in partial fulfillment of the requirements for the Degree of Doctor in Engineering Sciences Advisor: JUAN W. DIXON Santiago de Chile, July, 2005

2 PONTIFICIA UNIVERSIDAD CATOLICA DE CHILE ESCUELA DE INGENIERIA Departamento de Ingeniería Eléctrica DESIGN, IMPLEMENTATION AND EVALUATION OF AN AUXILIARY ENERGY SYSTEM FOR ELECTRIC VEHICLES, BASED ON ULTRACAPACITORS AND BUCK-BOOST CONVERTER MICAH E. ORTÚZAR Members of the Committee: JUAN W. DIXON LUIS MORÁN JOSÉ RODRIGUEZ MARCELO GUARINI BIMAL K. BOSE RAFAEL RIDDELL Thesis submitted to the Office of Research and Graduate Studies in partial fulfillment of the requirements for the Degree of Doctor in Engineering Sciences Santiago de Chile, July, 2005

3 A mi Familia y mis amigos, A todos los que me apoyaron.

4 AGRADECIMIENTOS Después de más de cuatro años de trabajo y estudio son muchos los cambios, muchas las experiencias vividas, profesional y personalmente. El apoyo de mi familia, desde distintos lugares del mundo, ha sido fundamental, me ha ayudado en momentos críticos y hoy me acompañan en este paso. A ellos, mi Papá, Mamá; a mis hermanos, a Margarita y Thomas, a todos ellos muchas gracias. Mis amigos han jugado un papel importante durante este período. Ellos han sido mi segunda familia y un apoyo emocional imprescindible. Un cariñoso saludo a todos ellos. También me siento profundamente agradecido de todos quienes me apoyaron al interior del departamento de Ingeniería Eléctrica. Un agradecimiento especial a Eduardo Cea por su ayuda y su amistad. Muchas gracias a Betty y Elena por su apoyo materno, gracias a Virginia y Carlos por cuidar de nosotros. Gracias también a Nelson Saleh por su ayuda y buena voluntad. Una mención especial merece el profesor Juan Dixon y su familia por quienes me sentí muy acogido. Juan se involucró a fondo en este proyecto y gracias a su empuje e iniciativa hemos llegado a puerto. Todos los resultados y logros de este proyecto se deben, en gran medida, a su iniciativa y su experiencia. También agradezco su amistad y comprensión a lo largo de varios años en que hemos trabajado juntos. Un saludo especial para Juan.

5 CONTENTS Pág. TABLE INDEX... 7 FIGURE INDEX... 8 RESUMEN ABSTRACT I INTRODUCTION I.1 Electric and hybrid vehicles I.2 Integrating different energy systems I.3 What are ultracapacitors and how do they work? I.4 State-of-the-art traction systems using ultracapacitors I.5 Objectives and hypothesis I.6 Methodology II STATIC CONVERTER DESIGN AND IMPLEMENTATION II.1 Introduction II.2 Power design II.2.1 Buck-boost topology II.2.2 Static converter components design and selection II.3 Safety features II.4 Thermal design II.5 Mechanical design III MONITORING AND CONTROL SYSTEM III.1 Introduction III.2 Control algorithms III.3 Communication layout III.4 Implementation via DSP III.5 Real-time monitoring software III.6 Failure detection IV URBAN CIRCUIT TESTS... 60

6 IV.1 Introduction IV.2 Test circuit IV.3 Tests results V RESULTS ANALYSIS V.1 Economic approach on results V.2 Related researches V.3 General discussion VI CONCLUSIONS REFERENCES A P P E N D I C E S Appendix A: Buck-Boost Converter Operation Analysis Appendix B: Disipated energy and heat generation in semiconductors Appendix C: Economic Evaluation Considerations Appendix D: TMS320F241 DSP Controller, Texas Instruments Appendix E: DSP code, Assembler Language... 94

7 7 TABLE INDEX Pág. TABLE 1-1: ENERGY DENSITY COMPARISON FOR GASOLINE AND DIFFERENT BATTERY TYPES...15 TABLE 4-1. URBAN TESTS RESULTS SUMMARY...64 TABLE 5-1. TOTAL MEAN COSTS COMPARISON WITH BATTERIES AS THE MAIN ENERGY SOURCE TABLE 5-2. TOTAL MEAN COSTS COMPARISON WITH HYDROGEN + FUEL CELL AS THE MAIN ENERGY SYSTEM TABLE B-1: INSTANTANEOUS POWER LOSSES WHILE CONDUCTING 150A IN SEMICONDUCTORS TABLE B-2: MEAN POWER LOSSES WHILE CONDUCTING 150A IN SEMICONDUCTORS TABLE B-3: MEAN POWER LOSSES THROUGHOUT TIME, WHILE CYCLING 150 A CHARGE AND DISCHARGE OPERATIONS FOR 19.09S WITHIN 1MIN PERIOD TABLE B-4: WATER-COOLED HEAT-SINK S REQUIRED THERMAL RESISTANCE TO ENSURE TEMPERATURE COMPLIANCE FOR EACH SEMICONDUCTOR AND CASE...90

8 8 FIGURE INDEX Pág. FIGURE 1-1: POWER CIRCUITS OF TWO DIFFERENT HYBRID CONFIGURATIONS: A) PARALLEL HYBRID; B) SERIAL HYBRID...18 FIGURE 1-2: PHYSIC PHENOMENA IN ELECTROSTATIC CAPACITORS, ELECTROLYTIC CAPACITORS AND ULTRACAPACITORS (ELECTRONIC DLC), (MILLER AND SMITH, 2004)...20 FIGURE 1-3: RAGONE PLOT OF SPECIFIC ENERGY (WH/KG) VERSUS SPECIFIC POWER (W/KG) FOR DIFFERENT ENERGY-STORAGE TECHNOLOGIES (EPCOS, 2001) FIGURE 1-4: ELECTRIC VEHICLE, CONVERTED FROM A CONVENTIONAL ICE-POWERED PICK- UP TRUCK CHEVROLET LUV FIGURE 2-1: STATIC-CONVERTER-INTERFACED CONNECTION...33 FIGURE 2-2: EV POWER DEMAND PROFILE...34 FIGURE 2-3: BUCK-BOOST TOPOLOGY EQUIVALENT CIRCUIT...36 FIGURE 2-4: BUCK OPERATION. ENERGY IS TRANSFERRED FROM THE BATTERY TO ULTRACAPACITORS FIGURE 2-5: BOOST OPERATION. ENERGY IS TRANSFERRED FROM ULTRACAPACITORS TO THE BATTERY FIGURE 2-6: 2700F ULTRACAPACITOR FROM EPCOS FIGURE 2-7: ULTRACAPACITOR BANK WITH CELL BALANCING UNITS INSTALLED...41 FIGURE 2-8: INDUCTANCE DESIGN CONFIGURATION AND FINAL APPEARANCE FIGURE 2-9: 3300 UF ELECTROLYTIC FILTER CAPACITOR FIGURE 2-10: PM400DSA060 FROM POWEREX...44 FIGURE 2-11: SNUBBERS CONNECTED TO SEMICONDUCTOR TERMINALS...45 FIGURE 2-12: SAFETY ELEMENTS IN POWER CIRCUIT...46 FIGURE 2-13: THERMAL MODEL OF ONE IGBT-DIODE PAIR FIGURE 2-14: WATER-COOLED HEAT-SINK...48 FIGURE 2-15: COMPONENTS LAYOUT WITHIN STATIC CONVERTER FIGURE 2-16: INSTALLED COMPONENTS IN CONVERTER CHASSIS...49

9 9 FIGURE 2-17: STATIC CONVERTER A) POWER CIRCUIT INSTALLATION, B) LOCATION IN FRONT COMPARTMENT...50 FIGURE 3-1: HEURISTICS-SOC-CONTROL ALGORITHM AND SIGNALS MANIPULATION...53 FIGURE 3-2: OPTIMAL CONTROL DATA GENERATION AND NEURAL NETWORK TRAINING PROCESSES FIGURE 3-3: OPTIMAL-CONTROL ALGORITHM, IMPLEMENTED USING NEURAL NETWORKS.56 FIGURE 3-4: COMMUNICATION AND COMMAND FLOW DIAGRAM FIGURE 3-5: DSP CONTROL BOARD, SIGNALS AND DATA PORTS...58 FIGURE 3-6: CONTROL AND DATA MONITORING/ACQUISITION SOFTWARE SCREENS FIGURE 4-1: URBAN CIRCUIT TEST COURSE...61 FIGURE 4-2: LOAD CURRENT AND BATTERY VOLTAGE WITHOUT AES S POWER SUPPORT...62 FIGURE 4-3: CURRENTS AND BATTERY VOLTAGE FOR A POWER-SUPPORTED SYSTEM....63

10 10 PONTIFICIA UNIVERSIDAD CATOLICA DE CHILE ESCUELA DE INGENIERIA DEPARTAMENTO DE INGENIERÍA ELÉCTRICA DISEÑO, IMPLEMENTACIÓN Y EVALUACIÓN DE UN SISTEMA AUXILIAR DE ENERGÍA PARA VEHÍCULOS ELÉCTRICOS, BASADO EN ULTRACAPACITORES Y CONVERTIDOR BUCK-BOOST Tesis enviada a la Dirección de Investigación y Postgrado en cumplimiento parcial de los requisitos para el grado de Doctor en Ciencias de la Ingeniería. MICAH E. ORTÚZAR RESUMEN El trabajo expuesto en esta tesis explora los factores clave que impiden a los vehículos eléctricos ser ampliamente aceptados en los mercados de transporte público y privado. En particular, se analizan las limitaciones que presentan los sistemas de almacenaje de energía para entregar altas potencias durante la aceleración o recibirlas durante el frenado. Para resolver estos problemas de potencia se propone la combinación de elementos de alta potencia específica con elementos de alta energía específica. En este contexto se ha diseñado, implementado y evaluado un Sistema Auxiliar de Potencia (SAP), basado en ultracapacitores y un convertidor Buck-Boost para ser usado en combinación con baterías de plomo-ácido. Los procesos de diseño e implementación se describen en detalle; también se presentan los resultados del proceso de evaluación. Finalmente se complementa la discusión de los resultados con un análisis desde el punto de vista económico de la implementación de este tipo de sistemas. La falta de elementos de almacenaje de energía eléctrica que presenten, simultáneamente, alta potencia específica y alta energía específica, y los altos costos de estos equipos, se consideran los mayores obstáculos para introducir vehículos de cero emisiones en los mercados de transporte público y privado. La combinación de elementos de alta energía específica, como celdas de combustible o baterías avanzadas, con elementos de alta

11 11 potencia específica, como los ultracapacitores, se presenta como la solución más viable para solucionar los problemas de baja autonomía y/o desempeño deficiente en el corto plazo. El SAP propuesto, basado en ultracapacitores y convertidor buck-boost, se ha diseñado e implementado en un vehículo eléctrico a escala real en combinación con baterías de plomo-ácido. Los procesos de diseño e implementación, así como el período de pruebas y sus resultados, se describen en detalle. Los resultados de la evaluación muestran que, con el SAE instalado y operando en el vehículo, la potencia disponible ha aumentado de 40 kw a 85 kw. Se han evaluado dos algoritmos de administración de energía, uno basado en heurística y el otro basado en técnicas de control óptimo aplicado a redes neuronales; el rendimiento del vehículo (km/kwh) aumentó en un 5.2% y un 8.9% con el primer y segundo algoritmo, respectivamente. Del análisis económico se concluyó que, si se consideran los costos solamente, con el sistema propuesto se requeriría que la vida útil de la batería se extendiese en un 50% o más para compensar los costos del SAE. Para mejorar la autonomía y para realizar, en el futuro, nuevos análisis al sistema desarrollado, se ha instalado una batería de Na/Ni-Cl 2 (conocida comercialmente como batería ZEBRA) en reemplazo de las baterías de plomo-ácido. Members of the Doctoral Thesis Committee: JUAN W. DIXON LUIS MORÁN JOSÉ RODRÍGUEZ MARCELO GUARINI BIMAL K. BOSE RAFAEL RIDDELL Santiago, July, 2005

12 12 PONTIFICIA UNIVERSIDAD CATOLICA DE CHILE ESCUELA DE INGENIERIA DEPARTAMENTO DE INGENIERÍA ELÉCTRICA DESIGN, IMPLEMENTATION AND EVALUATION OF AN AUXILIARY ENERGY SYSTEM FOR ELECTRIC VEHICLES, BASED ON ULTRACAPACITORS AND BUCK-BOOST CONVERTER Thesis submitted to the Office of Research and Graduate Studies in partial fulfillment of the requirements for the Degree of Doctor in Engineering Sciences by MICAH E. ORTÚZAR ABSTRACT This thesis addresses key issues that prevent electric vehicles from being broadly adopted by private and public transportation markets. In particular, the limitations of energy storage devices to deliver energy at high power rates during acceleration and accept high power regeneration during braking are analyzed. To solve available power problems, the combination of a high specific-power storage device with a high specific-energy storage device is proposed. Accordingly, an Auxiliary Energy System (AES), based on ultracapacitors and a buck-boost converter, has been designed, implemented and evaluated in an electric vehicle. The system s design and implementation are described; also an evaluation process and its outcomes are presented. Finally, an economic approach is applied to the general discussion on obtained results. The lack of a single energy storage element that presents, simultaneously, high specific power and high specific energy for electric vehicles, and the high cost of these devices, are identified as the main obstacles to successfully introduce zero-emission vehicles to public and private transport markets. The combination of high specific-energy storage devices or elements, such as advanced batteries or hydrogen (through Fuel Cells), with high specific power storage devices, such as ultracapacitors, is presented as the most viable solution to the problems of low autonomy and/or poor performance in the short term.

13 13 The proposed AES, based on ultracapacitors and a buck-boost static converter, has been designed and implemented in an electric vehicle in combination with lead acid batteries energy storage. The design and implementation processes of the proposed system are described in detail. Evaluation results demonstrated that, with the AES installed in the vehicle, available power was increased from 40 kw to 85 kw. Two energy management algorithms were evaluated, one based on heuristics and the other one based on optimal control techniques applied to neural networks; the vehicle s yield (km/kwh) was increased in 5.2% and 8.9% with the first and second algorithms, respectively. Economic analysis concluded that, in terms of costs only, with the proposed system a battery life extension of 50% would be needed to compensate for the AES s costs. For better autonomy and to perform developments in the future on the integrated system, a Na/Ni-Cl 2 battery (commercially know as ZEBRA battery) has been installed in exchange for the lead-acid battery. Members of the Doctoral Thesis Committee: JUAN W. DIXON LUIS MORÁN JOSÉ RODRÍGUEZ MARCELO GUARINI BIMAL K. BOSE RAFAEL RIDDELL Santiago, July, 2005

14 14 I INTRODUCTION I.1 Electric and hybrid vehicles In the early days of the automotive era, around the end of the 19 th century and beginning of the 20 th century, electric-powered and internal-combustion-enginepowered vehicles not only offered comparable performance characteristics, but also, electric vehicles where more reliable and safe. Accordingly, the first known automotive-hire service in the US (the Electric Carriage & Wagon Company, established on 1897) was served by electric vehicles (Kirsch, 1996). However, because of the low power and low autonomy (inability to run long distances without recharging) presented by electric vehicles (EV), this early advantage ended quickly when design improvements performed on internal combustion engines (ICE) made them reliable enough to be competitive. While a series of unfortunate events and final bankruptcy buried the EC&WC s future, the scarce autonomy EVs presented plus governing technical and economical conditions in the early 20 th century resulted in the general adoption of ICE-powered vehicles in the US and worldwide. During the fourth quarter of the 20 th century and due to several circumstances, such as the oil crisis, environmental issues and technological breakthroughs, the effort to develop viable EVs regained strength. Power electronics technology advances during the 70s, and subsequent improvements in the 80s, gave birth to new efficient and powerful inverters, which made viable the use of AC motors, which are simpler, more efficient and with more specific power than the classic DC motor. In addition, around the late 80s the brushless DC motor, invented in 1962 by T.G Wilson and P.H Trickey (Wilson and Trickey, 1962), was improved and made competitive. This machine, a version of the synchronous motor in which permanent magnets in the rotor establish the excitation field, behaves as a DC motor when adequate electronic control is applied. Hence, it incorporates the DC motor s advantage of high torque at low speeds, but also incorporates the advantage of AC-motor s simplicity and high specific power, while achieving an excellent dynamic behavior.

15 15 These advances dissipated doubts about efficiency and specific power in EVs. Nevertheless, EVs continued to be a poor competitor to ICE-powered vehicles, mainly due to the reduced capacity of energy storage devices (compared to specific energy in fossil fuels). This handicap, even though still present to date, has been constantly reduced due to technological advances (new battery technologies, fuel cells, etc.). Nowadays, electric propulsion systems efficiency rates are much higher, surpassing 80% (Eaves and Eaves, 2004), against a typical of around 18% for conventional gas engines (Fueleconomy, 2005) and 35-40% for modern Diesel engines in urban drive conditions. Still, energy density (Wh/l) and specific energy (Wh/kg) is considerably higher in fossil fuels than in most advanced batteries (API, 1988; Chan and Wong, 2004). This is shown in table 1-1. Table 1-1: Energy density comparison for gasoline and different battery types. 97 Octane Gasoline Batteries Lead-Acid NiMH Li-ion ZEBRA Zn/Air Energy Density [Wh/Lt] Specific Energy [Wh/Kg] 9, , These facts imply that, in vehicles of similar characteristics, the useful energy content in a gasoline tank is several times higher than that contained in an advanced battery pack of comparable weight and dimensions, which translates to lower autonomy for EVs. For example, a 30-liter gas tank can deliver 52 kwh of useful energy to the wheels (accounting for ICE loses); on the other hand, a 100-kg Li-ion battery can deliver 12 kwh to the wheels, almost 1/5 th of the energy delivered by gasoline weighting over 3 times more.

16 16 The aforementioned issue plus the problem of high initial cost and low specific power of most batteries conform the main obstacles to successfully introduce competitive EVs to the public and private transport industry. A comprehensive solution would require the development of more advanced batteries, achievable only through major research breakthroughs and investment in new technologies, or the integration of fossil-fuel energy-storage and electric power conversion and management (higher efficiency, lower noise, lower maintenance costs and lower environmental impact). The second scheme may achieve higher autonomy than pure EVs and higher efficiency than regular ICE-powered vehicles but, in order to reach maximum efficiency and minimum emissions, the main energy transformer (Diesel or gas engine, gas turbine, fuel cell, etc.) must work at its optimum power output which will contrast with the variable power requirements of the vehicle. Also, to reach its potential, the system should be able to recover energy from braking, which fossilbased energy-storage systems cannot do. Therefore, the integration of these dissimilar energy conversion mechanisms require a temporary auxiliary-energystorage device; in such a way the vehicle is able to recover, and reuse, energy from braking, and the main energy transformer (ICE, gas turbine, FC, etc.) is dimensioned to satisfy mean (not peak) power demand, reducing the acquisition cost. These auxiliary energy storage devices should have high power density, high specific power and high efficiency. Vehicles that use such a combination of different energy storage devices are commonly known as hybrid systems. I.2 Integrating different energy systems Recent commercial debut of hybrid vehicles is a proof of the improved efficiency and performance of electric powered vehicles, regardless of the original source of energy (electricity, gasoline, natural gas, hydrogen, etc.). Nevertheless, in terms of power-toweight ratio and cost, new hybrid models still do not strongly compete with traditional gasoline or diesel-powered vehicles; moreover, although hybrids yield (km/lt) is usually higher than most gas-powered vehicles, it is comparable to that of some diesel models. Also, the use of combustion engines and complicated gearboxes

17 17 surely influences cost and weight. On the other hand, power is an issue that must be resolved using appropriate auxiliary energy sources to support peak power requirements. Therefore, all new topologies that could offer improvements in terms of cost, weight and power should be explored and tested by researchers. In accordance, the project presented here studies the technical and economical viability of a seldom-explored hybrid configuration: the combination of batteries and ultracapacitors. At this stage, it is convenient to define different types of hybrids and the energy sources they use or could use. a) Parallel hybrid: is the combination in parallel of two different energy sources, both with independent mechanical outputs that are combined in a special gearbox to deliver or accept energy from the wheels (Fig. 1-A). This topology has been applied in most commercial models; it usually combines an internal combustion engine and an electric motor, which is connected to an electrochemical storage device, such as batteries. b) Serial hybrid: both energy sources deliver energy to the power train through the same electro-mechanical converter (an electric motor), to which they are electrically connected in parallel, as shown in Fig. 1-B. In this case, any two sources that deliver electrical energy could be used, as long as they are compatible in terms of electrical variables. This topology is also the only possible when the main energy source is a battery or hydrogen (through Fuel Cell), because (unlike ICE) they do not produce mechanical power.

18 18 AESU A) Gearbox Traction Motor Internal Combustion Engine Power Inverter + - Auxiliary Energy Storage Unit Gasoline or Diesel tank MESU B) Traction Motor Power Inverter MESU Main Energy Storage Unit (Batteries, Fuel Cell, Gas turbine, Diesel Engine, etc.) Auxiliary Energy Storage Unit AESU Figure 1-1: Power circuits of two different hybrid configurations: A) Parallel hybrid; B) Serial hybrid. The two (or more) energy sources involved in the hybrid configuration can be differentiated by their capability of storing energy and delivering power. A source of high energy density serves for an extended driving range and may be called the Main Energy System, because a vehicle could eventually run long distances powered by this source only, even though the power and/or efficiency characteristics could be poor. A powerful and efficient reversible source is the best compliment to the above mentioned and may be called the Auxiliary Energy System, because a vehicle running with this kind of source would perform well and efficiently but only for a few miles; thus, it depends on the previous one for medium and extended-range applications.

19 19 For the Main Energy System (MES), the most popular and proven choice has been the internal combustion engine (ICE), followed by the gas turbine and, lately, the Fuel Cell (FC). For the Auxiliary Energy System (AES), the most mentioned candidates are high power batteries, ultracapacitors and flywheels, of which only the first two are commercially available. All of these AES offer high efficiency, high power-density and reversibility (Rutquist, 2002). Because an ICE s natural output is mechanical power (at manageable speeds for gearboxes) and any unnecessary energy transformation is undesirable, it is usually applied in parallel configuration. For the gas turbine, and most other candidates for main energy sources, such as FCs, primary and secondary batteries, the series configuration is more suitable. Recent developments in the areas of FCs and batteries (Zinc-Air, ZEBRA, etc.) suggest a good chance of being a competitive alternative to the ICE for Main Energy System in the near future (DOE, 2003), using series hybrid configuration. The two commercially available storage-device alternatives to implement the AES are advanced batteries and ultracapacitors. Of these, the second present several advantages over batteries for this particular application. The higher specific power and higher efficiency are the most notable; but also, the longer cycle life and no maintenance characteristics translate conveniently into no-replacement-cost and lower-service-cost (Mazaika and Schulte, 2005). These facts motivated the research described in this thesis, regarding the development of an Auxiliary Energy System (AES) based on ultracapacitors and a static converter. This AES has been conceived to be used in series hybrid configurations (Jeong et al, 2002) in combination with different main energy systems. I.3 What are ultracapacitors and how do they work? Electronic double layer capacitors, DLC, or ultracapacitors, were first developed and patented in 1961 by SOHIO. The construction of an ultracapacitor consists of a pair of metal foil electrodes, each of which has an activated carbon (AC) fiber mat deposited on metal foil. The activated carbon side of each electrode is separated by an electronic barrier such as glass paper then sandwiched or rolled into a package. An

20 20 aqueous or organic electrolyte salt impregnates the activated carbon as shown in Figure 1-2. The electronic properties of an ultracapacitor are strongly dependent on the porosity of the activated carbon and on the molecular size of the electrolyte ions. Activated carbon electrodes used in ultracapacitors have specific surface areas of 1000 to 2300 m2/g and charge separation distances, d in Figure 1-2, on the order of 10 Angstrom or less (Miller and Smith, 2004). Figure 1-2: Physic phenomena in electrostatic capacitors, electrolytic capacitors and ultracapacitors (electronic DLC), (Miller and Smith, 2004). The electrostatic capacitor is generally constructed of metal films, ceramic, or glass, mica or other dielectric material. The electrostatic capacitor in Figure 1-2 consists of two metal electrodes separated by a dielectric of thickness d. A potential across the two metal electrodes creates a uniform electric field in the insulating medium the properties of which determine the voltage rating (Miller and Smith, 2004). An electrolytic capacitor is similar to the electrostatic unit in construction except for the presence of a conductive electrolytic salt that is in direct contact with the metal current collector, or cathode. The anode is made from an etched metal foil that has been anodized by application of an electric potential when the foil is immersed in an electrolyte during manufacture. An electrolyte different from the forming electrolyte

21 21 is used as the ionic conductor. When the formed anode foil with its alumina dielectric layer is rolled up along with the cathode foil, an insulating separator such as Kraft paper is placed on the outside of the anode foil to insulate it. The negative foil is typically the outside of the electrolytic can. When an external potential is applied across the electrolyte terminals, a uniform electric field is established across the anodized layer of alumina while a decaying electric field exists some distance, δx, into the electrolyte according to Poisson s equation. Because of the presence of an electric field that extends into the electrolyte, this capacitor will have a more limited breakdown voltage than an electrostatic capacitor. Electrolytic capacitor manufacturers design for breakdown voltages somewhat above the surge rating of the unit. Typically, higher surge rated electrolytic capacitors also have higher resistance, hence a higher equivalent series resistance, ESR, and therefore higher losses in power electronic circuits. Another consequence of an electric field in the electrolyte is the fact that capacitor current is now a function of both voltage change and capacitance change as a function of voltage (Miller and Smith, 2004). The voltage rating of ultracapacitors is constrained by the same phenomena of electric field presence within the electrolyte as in conventional metal foil electrolytic capacitors. Ultracapacitors with organic electrolytes have voltage ratings of <3.0 V per cell whereas with aqueous electrolytes the voltage rating drops to <1.23 V per cell, typically 0.9 V. In all ultracapacitors the terminal capacitance consists of the series combination of an anode DLC and the cathode DLC, so the net rated voltage is twice the value of the electrolyte decomposition voltage. Organic electrolyte ultracapacitors have higher decomposition voltages and higher specific energy but higher resistance than aqueous types. Low conductivity of organic-electrolyteultracapacitors results in higher ESR. ESR can be reduced in general by the addition of vapor grown carbon fiber to the AC. Because capacitance is proportional to the electrodes surface-area and inversely proportional to charge separation distance d, ultracapacitors advantage conventional capacitors, such as electrolytic and electrostatic capacitors, by their porous-acelectrodes enormous effective areas and the extremely small charge separation

22 22 distance (Miller and Smith, 2004). The drawback, despite their two capacitive layers instead of one, is the limited maximum voltage ultracapacitors withstand, usually between 2.3V and 2.8V. Ultracapacitors have greater specific power (more than 1.5kW/kg) than conventional or advanced batteries; and higher specific energy (of about 3 to 5 Wh/kg) than aluminum electrolytic capacitors (Dietrich, 2001; Burke and Miller, 2002). Their advantage over batteries, in terms of power, is due to their reduced equivalent series resistance (ESR) and that in these elements, unlike batteries, there are no chemical reactions involved in the process of charge and discharge. Therefore, the speed needed to deliver energy does not depend on the speed of such reactions or the ability of chemical components to recombine, but on electrostatic phenomena, which does not require molecular mutation to take place. Also, the longer cycle life and good behavior at low temperatures (Schneuwly and Smith, 2005) are important advantages. Figure 1-3 shows how ultracapacitors close the gap between batteries and electrolytic capacitors in terms of specific power and energy. Figure 1-3: Ragone plot of specific energy (Wh/kg) versus specific power (W/kg) for different energy-storage technologies (EPCOS, 2001).

23 23 Because they present low losses and high cycle life at high power demands, these elements of recent development cover a wide field of uses in power engineering applications, especially in schemes with high peak power demand and medium-low energy requirements (Cohen and Smith, 2002). Consequently, they present advantages for complementary use in electric vehicles (EVs) and hybrid electric vehicles (HEV), especially in their energy storage systems. Accordingly, there are an increasing number of studies in which ultracapacitors are used as a complement of the main energy system (ICE, FC, batteries, etc.) in hybrid vehicles. I.4 State-of-the-art traction systems using ultracapacitors Contents of this section show the bibliography analysis made during the thesisproject formulation period. Works reviewed in this process served as a knowledge base of the up-to-date developments in Auxiliary Energy System using ultracapacitors and their results. All publications discussed in this section are contemporary or previous to the project formulation. Works published thereafter, as well as publications presenting this project s results, are discussed in chapter V. Publications regarding ultracapacitors and auxiliary energy storage may be classified in three groups: i) the ultracapacitor industry reports and surveys, which deal with new material fabrication processes, specific characteristics measurements and testing under different operating conditions; ii) the speculative kind, whose approach is focused in forecasting long term industry trends and suggesting certain technology adoption for the particular application (analysis and conclusions presented in these essays are based on inference and experience acquired within the industry); iii) finally, a third group presents technical reports on practical applications, which expose useful data about behavior and performance obtained from experimental prototypes (or rigorous simulations using real data sets). Publications within this last group describe the state-of-the-art power topologies for EVs and HEVs, therefore an updated analysis on viability of these schemes can be extracted from them. The first group of publications presents studies that describe the state of the industry and its projections (Cohen and Smith, 2002). Production-models characteristics and

24 24 behavior under stress (and abuse) are measured to evaluate adequacy for different applications (Jehoulet et al, 2000; Varakin et al, 2001; Goesmann et al, 2002; Conte and Pirker, 2005). Projected future materials improvements and costs are also forecasted, based on research s preliminary results and suggested technology inversions (Burke and Miller, 2001). Most of these works are presented by ultracapacitor manufacturers, a fact that could suggest questionable objectiveness, but data contained in them should be reliable because they are subject to the industry scrutiny and benchmarking. The most useful information contained in these publications is related to behavior under abuse and safety considerations (Goesmann et al, 2002; Conte and Pirker, 2005). The second group of publications could be considered the vanguard analysis that may inspire future developments. Even though they lack real-life experimental results, these works are a sample of the engineering analysis process preliminary to any serious innovative attempt (Furubayashi et al, 2000; Mitsui et al, 2002; Dixon et al, 2000). The third classification group, consisting of practical applications reports, comprises most publications on ultracapacitors to be found. There is a wide variety of explored topologies and the analysis scope spread over an ample spectrum of profoundness. Miscellaneous applications from peripheral-load power-support (Folchert et al, 2002) to ultracapacitors-as-the-single-energy-source configurations (Barrade and Rufer, 2001) can be found. There are several reports on HEVs implementations using ultracapacitors in combination with non-reversible energy sources such as ICEs, FCs and primary batteries among others (Furubayashi et al, 2000; Burke and Miller 2001; Di Napoli et al, 2001; Lott and Späth, 2001; Varakin et al, 2001; Jeong et al, 2002; Okamura, 2002). A couple of publications describing idle stop systems were found (Furubayashi et al, 2001; Mitsui et al, 2002); this application consists of using ultracapacitors in urban buses to power the starter motor of ICEs, which are turned off at every stop to avoid inefficient and contaminating idling conditions. In these cases, ultracapacitors are used to avoid battery deterioration due to successive peak power episodes and to decouple this strongly-perturbing load from other on-board electronic loads. Finally, there is a constantly-growing number of reports that present

25 25 HEV implementations combining ultracapacitors as Auxiliary-Energy-Systems and different types of batteries as Main-Energy-Systems (Arnet and Haines, 2000; Härri and Egger, 2001; Heinemann et al, 2001; Wight et al, 2001; Wight et al, 2002). These works conformed the most valuable and updated information source when this project was elaborated, because they expose practical details and evaluation results of the early experimental implementations on the topology being evaluated in this research. A thorough review of the highlights of each publication found in this subgroup is presented in the following pages. Arnet (Arnet and Haines, 2000) presents the hardware and algorithm concepts used in the implementation of an AES based on ultracapacitors to be implemented in an electric vehicle using lead-acid batteries as MES. This design was part of Solectria Corporation s new energy storage devices development program. The static converter used in his design has a Buck-Boost topology, such as the one used in the project presented in this thesis. He also used algorithms that establish an inverse relation between the vehicle s kinetic energy and the ultracapacitor s state of charge (SOC). This work is contemporary with the first publication that emanated from this project (Dixon et al, 2000), in which general hardware configuration and algorithm concepts were presented; coincidently, Arnet s general approach is very similar to that exposed in (Dixon et al, 2000), therefore early conclusions on the feasibility of this implementation could have been extracted from his findings, but his results were preliminary and did not evaluate the general performance of the equipment in reallife operation. Also, some differences appeared in hardware design and algorithm implementation, hence Arnet s work and this project results could complement eachother. Härri (Härri and Egger, 2001) introduces an energy scheme concept he calls SAM (Super Accumulator Module), which consists on combining batteries and ultracapacitors using a topology he refers to as Virtual Parallel (VP). But the actual semiconductor configuration is not at all clear, nor is under what criterion the different operation modes are selected and the capacitor SOC controlled. Test results and performance data are not presented either. Therefore, this essay does not provide useful orienting insights.

26 26 Heinemann s work (Heinemann et al, 2001) is interesting because of exposed temperature management data and ultracapacitor behavior under different operation conditions. Also, his exploration of different energy management strategies presents interesting alternatives. Nevertheless, his findings are mostly simulations or small scale bench tests, and do not deliver conclusive data about the overall efficiency increase and/or available power. By 2002, Wight s publications (Wight et al, 2001; Wight et al, 2002) had the most illustrative results on ultracapacitors and lead-acid batteries combination for EVs. Data presented in both works summarize test procedures on real-life-scale vehicles using the static converter developed by Arnet and two different ultracapacitor brands, one in each publication. The first study shows a complete set of tests comprising urban, suburban, acceleration and dynamometer experiments on two identical vehicles, both equipped with lead-acid batteries as MES, the first one using the ultracapacitor-based AES and the second one (control subject), which did not have an AES. General conclusions showed that, when using that particular ultracapacitorbased AES, more available power was observed by the driver, acceleration was faster and more energy could be extracted from the batteries before reaching the discharged threshold minimum voltage (which allowed some increased autonomy). Nevertheless, even though more energy was extracted from the batteries, in most experiments the overall efficiency or yield (km/wh) was lower on the vehicle equipped with the AES. The second essay shows almost identical tests but using a different ultracapacitor brand. Results resemble very much those obtained in the first publication; the amount of energy extracted from batteries in the vehicle equipped with the AES was greater in all tests than that extracted from batteries in the control vehicle. This time the results on efficiency where non-conclusive. From available literature, at the time of the research project-formulation, it could be concluded that there was scarce experience of this kind of application and few reallife-scale tests had been performed. Results obtained by Wight suggested that the use of a high-specific-power AES, like the one proposed in this research, would lessen main battery deterioration due to the reduction of peak power demanded and therefore, would probably extend battery life. On the other hand, Wight s results

27 27 were not conclusive regarding overall efficiency impact that the use of this AES produced on EVs; there is a chance that efficiency results from his tests could have been influenced by the algorithm structure or its parameters and/or by the efficiency characteristics of his particular static converter. This reasoning suggested that, by incorporating an ultracapacitor-based AES to EVs, in addition to battery life extension and a better acceleration response, overall increase in vehicle efficiency could be achieved if improvements where performed on static-converter design and energy-management algorithm structure. I.5 Objectives and hypothesis The central objective of this research is to identify and improve some of the deficiencies that prevent clean and efficient transport technologies, such as EVs, from successfully competing against traditional pure-ice-based vehicles. This objective has been partially realized in previous analysis, identifying obstacles such as reduced autonomy, high cost and limited power of EVs and HEVs as the predominant barriers to achieve competitive clean vehicles. Reduced autonomy is a characteristic of pure EVs, which are powered with reduced energy-density electrochemical batteries. This problem has been partially overcome with the development of HEVs, equipped with Main Energy System (MES) of higher specific energy. Nevertheless, some of the energy converters used in these configurations are not as clean or efficient as it would be desirable, as in the case of Internal Combustion Engines (ICEs) and Fuel Cells (FCs) (Galliers, 2003). The problem of high cost, common to all clean mobility solutions, is particularly pronounced in the case of FCs, for which forecasts are not optimistic in the short and medium terms (Galliers, 2003; Chan and Wong, 2004). In the case of ICEs, although actual costs are not prohibitive, simple reasoning concludes that their complexity makes them expensive to maintain. Furthermore, if battery technologies are consistently improved and acceptable energy densities are achieved, it is not unreasonable to speculate that manufacturing costs of an electrochemical storage unit with fewer and simpler components could be lower than those of an intricate ICE. Of course complex manufacturing of special alloys and compounds have to be perfected

28 28 and made cheaper, but that is not impossible given the rate at which manufacturing processes are advancing. Hence, electrochemical batteries seem to be the alternative clean-energy-source that could first break the barrier of unacceptable costs and consistently bring them to competitive levels. The power issue is also prevalent in all technologies previously reviewed. In the case of HEVs efficiency considerations make it transcendental that their MES s work at constant or near-constant power levels, lower than the peak power demanded; also, cost constraints force the MES s power-rating reduction to levels near mean power demand. This leaves the burden of power on the AES, whose duty is to quickly deliver or accept bursts of energy as a response to the power-train s demands. The first elements used for AES implementations were high power batteries, but these require sophisticated charge equalization management (Schneuwly and Smith, 2005) and present a short cycle life, which had an impact in cost; that is why new energystorage elements such as ultracapacitors and flywheels are being tested and implemented in HEVs to address high power issues. In the case of battery-powered pure EVs, batteries present varied problems, such as low specific power, DODdependant cycle life (Mazaika and Schulte, 2005), inefficiency at high power demands, etc. This has motivated the experimentation of different technology combinations to satisfy separate energy and power needs. This is particularly important when primary batteries (such as Zn-air) or high-specific-energy but lowspecific-power batteries (such as Na/Ni-Cl 2 or ZEBRA) are used; in the case of these battery technologies, very promising in terms of specific energy, power support is fundamental. From this analysis it can be deduced that, in the medium term, the clean transport technology that will most certainly reduce its costs is battery electrochemical storage. Of which some chemistries have lately achieved important specific-energy improvements. Therefore, an efficient, cost-effective and reliable power support unit could close the circle and make battery-powered EVs a competitive choice in the near future. This reasoning has motivated the development of an ultracapacitor-based AES to be implemented in a lead-acid battery-powered vehicle. The lead-acid technology would

29 29 be subsequently changed to Na/Ni-Cl 2 (ZEBRA) of greater energy density and lower power density. The incorporation of this power support system would certainly increase available power, but it also raises obvious concerns about costs. On the other hand, a question regarding other EVs limitations arise: would this system have a negative effect on efficiency and therefore on autonomy? In this author s opinion, no, on the contrary: the reduction of maximum power demanded to the battery would increase battery operation efficiency and more energy would be recovered from regenerative braking even when the battery is fully charged; this would more than compensate for the expected losses produced in the static converter interfacing energy flow to and from ultracapacitors. If this reasoning proves correct, then costs could also be compensated, or even reduced, as a result of energy savings and battery life extension. This expectation, founded on intuitive reasoning, could be formulated as a hypothesis to be demonstrated. The adequate use of ultracapacitor-based Auxiliary-Energy-System in electric vehicles powered with lead-acid batteries, under congested city driving conditions, increases total energetic efficiency and extends its autonomy. That is, in driving conditions with a high number of stops and accelerations respective to the covered distance, the total energy spent (per kilometer) will be measurably lower in a leadacid battery equipped vehicle that uses a ultracapacitor-based AES than that spent in the same vehicle without the AES. The AES-equipped vehicle would also be able to cover a longer distance with one charge. The demonstration or refutation of this hypothesis will be the central aim of this thesis. A discussion on costs will also be included in the final analysis, but does not fall within the preset scope of this study. I.6 Methodology To demonstrate, or refute, the proposed hypothesis, a real-life scale AES prototype using ultracapacitors and a static converter was designed and constructed. The system was implemented in an EV and tested in an urban drive circuit. Finally, results were

30 30 analyzed and compared to those obtained without the use of the AES, leading to final conclusions, from which economical implications where drawn. The AES design and implementation were key processes to ensure the success of this project. The ease of power flow control, operational safety and overall system efficiency, depended directly on the adequacy of the power topology, as well as the thorough study and design of converter components and control system. A Buck- Boost power topology was used, where the ultracapacitor bank was connected at the low voltage side of the converter, allowing bi-directional power flow with variable voltage at the ultracapacitor terminals. The specially designed and constructed smoothing inductor (of 1.6 mh) and water-cooled heat-sink allowed high efficiency and high power rating by ensuring low amplitude ripple current and stable thermal management. A control and monitoring system was also designed and implemented using a DSP from Texas Instruments. This system allowed the implementation of different energy management strategies without hardware modification and provided real time information through its monitoring and data-logging features. The electric vehicle used in this work, was transformed from a conventional Chevrolet LUV truck, shown in Figure 1-4, which is similar in weight and shape to a Chevrolet S-10; its drive train is powered by a Brushless DC traction motor (32 KW nominal power and 53 KW maximum power) and a 54 kw inverter (Dixon et al, 2000). The vehicle s main energy system was formed by a pack of 26 lead-acid batteries connected in series (356 Vdc), which has been recently replaced with a ZEBRA (Zero Emission Battery Research Activity) battery (371 Vdc). The Auxiliary Energy System was implemented with a 20-Farad ultracapacitor bank and a Buck- Boost converter, with a nominal voltage of 300 Vdc, and a nominal current of 200 Adc.

31 31 Figure 1-4: Electric vehicle, converted from a conventional ICE-powered pick-up truck Chevrolet LUV. The test circuit was a slow and mid-speed driving urban route. Special care was taken to perform tests under similar environmental and technical conditions, such as ambient temperature, traffic conditions, tire pressure, etc. Tests were performed without regeneration, with battery-only-regeneration and with AES-assisted regeneration (two algorithms were tested). Available power (kw) and yield (km/kwh) were the measured performance indicators.

32 32 II STATIC CONVERTER DESIGN AND IMPLEMENTATION II.1 Introduction The battery pack, composed of 26 series-connected lead-acid batteries, has a no-load voltage that ranges from 358V, when recently charged, to 312V when discharged. This voltage is also load-dependant, reaching under 250V when heavily loaded and in low SOC condition. On the other hand, it can reach more than 400V when it has been recently charged and regenerative braking is applied. These are extreme conditions which not only deteriorate batteries and shorten their life, but may also damage power inverter (traction equipment has minimum and maximum voltage limits of 250 and 400 V respectively). This situation cannot be avoided unless the vehicle s drive train power is restricted according to battery voltage. This strategy would avoid inverter damage and could help preserve batteries longer, but would severely affect vehicle performance and efficiency (by restricting power and regeneration); it would limit available power when batteries are partially discharged and, over time, batteries aging would also noticeably affect performance. In other words, batteries would go through a long agony and this would be reflected in vehicle performance. This is where the Auxiliary Energy System fills the gap. As previously defined, the most adequate hybrid configuration to be implemented in a battery-powered EV is a series-hybrid topology, shown in Figure 1-1B. Even though it is called serial hybrid (because of the serial mechanical output), the Main and Auxiliary Energy Systems are connected in parallel. Therefore, if the Main Energy System (batteries) remains the same, the AES must be designed in such a way that it adapts to the pre-established power-circuit voltage-rating, and there is an adequate supporting-power-flow during peak power demand. Even though there have been some experiences connecting ultracapacitors directly to the Main Energy System (Jeon et al, 2005; Massé and Freeman 2005), a static converter should interface power connection between batteries and ultracapacitors for several reasons, but three are fundamental. First, batteries work at relatively constant voltage levels while capacitor s voltage is directly related to their SOC, therefore to