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Space for pictureReplace this square with a picture illustrating the content of the report. This pictureshould be “floating over the text”, in order not to change the position of the titlebelow (clic on Format: Object: Layout, and chose “In front of text”)Instructions for use of this templateReplace the yellow marked text with your own title, name etc on the first ninepages. Replace only the text and not the return-signs, comment marks [mp1] etc.Update all field in the document by choosing Edit: Select All (Ctrl A) and thenclicking the F9-button. Write your report using the formats and according to theinstructions in this template. When it is completed, update the table of contents.The report is intended to be printed double-sided.Battery Thermal Management Systems ofElectric VehiclesMaster’s Thesis in Automotive EngineeringJILING LIZHEN ZHUDepartment of Applied MechanicsDivision of Vehicle Engineering & Autonomous SystemsRoad Vehicle Aerodynamics and Thermal ManagementCHALMERS UNIVERSITY OF TECHNOLOGYGöteborg, Sweden 2014Master’s thesis 2014:42
MASTER’S THESIS IN AUTOMOTIVE ENGINEERINGBattery Thermal Management Systems ofElectric VehiclesJILING LIZHEN ZHUDepartment of Applied MechanicsDivision of Vehicle Engineering & Autonomous SystemsRoad Vehicle Aerodynamics and Thermal ManagementCHALMERS UNIVERSITY OF TECHNOLOGYGöteborg, Sweden 2014
Battery Thermal Management Systems of Electric VehiclesJILING LIZHEN ZHU Jiling Li;Zhen Zhu, 2014Master’s Thesis 2014:42ISSN 1652-8557Department of Applied MechanicsDivision of Vehicle Engineering & Autonomous SystemsRoad Vehicle Aerodynamics and Thermal ManagementChalmers University of TechnologySE-412 96 GöteborgSwedenTelephone: 46 (0)31-772 1000Cover:Saab 9-3 ePower electric vehicle made its debut at 2010 Paris Motor ShowChalmers / Department of Applied MechanicsGöteborg, Sweden 2014
Battery Thermal Management Systems of Electric VehiclesMaster’s Thesis in Automotive EngineeringJILING LIZHEN ZHUDepartment of Applied MechanicsDivision of Vehicle Engineering & Autonomous SystemsRoad Vehicle Aerodynamics and Thermal ManagementChalmers University of TechnologyABSTRACTElectric vehicles (EV) develop fast and have become popular due to their zero emissionand high tank-to-wheels efficiency. However, some factors limit the development ofthe electric vehicle, especially performance, cost, lifetime and safety of the battery.Therefore, the management of batteries is necessary in order to reach the maximumperformance when operating at various conditions.The battery thermal management system (BTMS) plays a vital role in the control of thebattery thermal behaviour. The BTMS technologies are: air cooling system, liquidcooling system, direct refrigerant cooling system, phase change material (PCM) coolingsystem, and thermo-electric cooling system as well as heating. These systems areanalysed through a trade-off between performance, weight, size, cost, reliability, safetyand energy consumption. According to the analysis two prime battery thermalmanagement systems are recommended: combined liquid system (CLS) and a variantsystem with PCM.The models of CLS and PCM system were built and simulated using softwareMATLAB/Simulink. The simulation results predict the battery temperature variationand the energy consumption of BTMS. Through simulating the PCM system model, theeffect of PCM on battery temperature variation was investigated and the proper PCMmass was estimated.Seen from the simulation results, BTMS is of great importance to control batterythermal behaviour. Further study could be more comprehensive and accurate throughcombining the simulation model with battery thermal electric and CFD models.Key words: EV, Battery thermal management, BTMS, Air cooling system, Liquidcooling system, Direct refrigerant system, PCM, MATLAB/SimulinkmodelI
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11.3Limitation11.4Thesis Outline2THEORY32.1Lithium-ion Battery2.1.1Mechanism of Secondary Lithium-ion battery2.1.2Thermal Issues of Li-ion battery2.1.3Operating Requirements33452.2Heat Transfer2.2.1Overall Heat Transfer Coefficient2.2.2Heat Balancing within Heat Exchanger7792.3On-Off Controller102.4Phase Change Material (PCM)12BATTERY THERMAL MANAGEMENT SYSTEMS (BTMS)3.141Systems Functions13133.2Technologies of BTMS3.2.1Air Cooling and Heating3.2.2Liquid Cooling and Heating3.2.3Direct Refrigerant Cooling and Heating3.2.4PCM3.2.5Thermoelectric Module3.2.6Heat Pipe3.2.7PTC Heater14141516171718193.3Evaluation of Different Technologies3.3.1Forced Air System3.3.2Liquid System3.3.3PCM d Cooling System3.4.1Combined Liquid System (CLS)3.4.2PCM Model (CLS PCM)222223MODELLING OF BTMS4.1Modelling Conditions4.2CLS Model4.2.1Battery Unit24252727III
4.2.24.2.34.2.44.2.54.2.6Air Conditioner (Active Cooling)Radiator (Passive Cooling)PTC Heater (Heating)PumpControl Unit28282929314.3PCM Model334.4Performance Parameters345RESULTS AND DISCUSSION365.1CLS Model5.1.1NEDC versus US065.1.2Cold and Hot Weather5.1.3Mild Weather363940445.2PCM Model5.2.1Comparison of Two Systems5.2.2Choose the PCM Mass4545466CONCLUSION497FUTURE PROSPECTS50REFERENCES52APPENDIX A: SYSTEM EVALUATION54APPENDIX B: MODELLING PROCESS57APPENDIX C: CLS RESULTS59APPENDIX D: CLS PCM RESULTS63IV
AcknowledgementsIt is our honour to be involved in this master thesis project. Experts at LeanNovaEngineering AB have been very welcoming, friendly and helpful throughout our thesis.They have continuously supported us with their comprehensive expertise andknowledge of automotive systems. We have gained a lot from this thesis not only theexperience but also the innovate spirit to solve complex problems.We would like to extend a warm thank you to our supervisor Peter Hayden who hasbeen involved in most of this thesis. With his expertise for HVAC system, we havemanaged to solve dozens of problems. He also inspired us with ideas, action andconfidence by talking with us patiently. The project would never have completedwithout his engineering skills and management.Thank you also to all our great team members. Zhongyuan Zhang has provided usstrong theoretical support in thermodynamic, MATLAB simulation and batterytechnology. Her guidance took us directly into the central issues without detour. ClaesZimmer and Anders Lindén have accompanied us from the beginning to the end withtheir knowledgeable comments and answers to all our questions.We would also like to thank our examiner Professor Lennart Löfdahl. In weeklyconversation during the project he has brought us his guidance and advice in everyaspect.Lastly, we would like to thank our friends and our family for all support andencouragement throughout the project and also our study in Chalmers.Jiling Li and Zhen ZhuGöteborg, 2014V
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NotationsSymbol List𝐴(Contact) areaLiLithium𝑞Heat transfer ratė𝑄𝑔𝑒𝑛Heat generation ratė𝑄𝑑𝑖𝑠Heat dissipation rateReReynolds number𝑇𝑎𝑚𝑏Ambient temperature𝑇𝑏𝑎Battery temperature𝑇𝑏𝑎,𝑖𝑛Battery initial temperature𝑇𝑑𝑒𝑠Desired temperature𝑇𝑓𝑙,𝑖𝑛Fluid inlet temperature𝑇𝑓𝑙,𝑜𝑢𝑡Fluid outlet temperature 𝑇𝑀Mean temperature difference𝑈Overall heat transfer coefficientAbbreviation ec.SEITab.TDCUS06Air Conditioner / Air ConditioningBattery Electric VehicleBattery Mangement SystemBypassBattery Thermal Management SystemEquationElectric VehicleFigureHeater / HeatingHeating, Ventilation and Air Conditioningkilo per hourNew European Driving CyclePagePhase Change MaterialPositive Temperature CoefficientRadiatorSectionSolid electrolyte interface/interphaseTableTesting Driving CycleA Supplemental Federal Test Procedure of USAVII
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1IntroductionIn this chapter, a brief synopsis of this report is presented. Section 1.1 Background,explains the original motivation of this thesis. Section 1.2 Objectives, points out whatthis thesis is aiming at. Section 1.3 Limitation, defines the circumscription of thesiswork and Section 1.4 Thesis Outline, introduces briefly the report structure.1.1BackgroundThere are nowadays different blending levels of hybrid electric vehicle and pure electricvehicle available on the current automobile market. According to the blending level,various size, type and number of battery cells are mounted in EVs. Unlike conventionalfuel, battery cells as an energy source have stricter requirement on workingenvironment. They are especially sensitive to temperature. To ensure a proper thermalworking environment, a Battery Thermal Management System (BTMS) will normallybe integrated with battery cells. Thus, knowledge about the proper workingrequirements of battery is vital, and what kind of management systems can sufficientlyand efficiently meet these requirements. With this cornerstone, the performance anddurability of battery pack can be maximized in an electric vehicle. Furthermore, theelectric range of vehicle is restricted due to limited capacity of the battery. It is veryuseful to investigate carefully the electric energy consumption of BTMS and to look forpotential savings. This investigation will help battery performance by reducing theenergy consumption of BTMS and extending the electric range of EVs.1.2ObjectivesThe main purpose of this master thesis is to develop a BTMS model for balancing thedifferent cooling and heating circuits within the battery pack to fulfil the performancerequirements. As prerequisites for the modelling, the requirements of the battery packwill be investigated at first through literature research. Then, several potential BTMSs,both in commercial stage and study phase, will be listed in a concept selection matrixwith their pros and cons. Two of them will be proposed as candidates for the followingsimulation. After the models are built up through simulation tool Simulink , they willbe tested in different initial conditions. Lastly, the corresponding performanceparameters of systems will be analysed and compared.1.3LimitationAlthough hybrid EVs are popular on the current market, this thesis concentrates onlyon battery electric vehicles and plug-in hybrid electric vehicles which are equipped withlarger battery packs compared with those on other EVs. The thesis focuses only onbuilding a simulation model for the thermal balancing within the battery pack. Thus,the exact heat generation model of battery cells is not discussed in detail; also this modelis taken from an exterior resource. Furthermore, the other cooling and heating circuits,such as chiller, radiator and heater, will be simplified. The delay, fluctuation or fadingof operation will be neglected. The pre- and post-conditions will be mentioned, but theoperation and system feature during test driving cycles is the main points of this thesisand thus will be intensively investigated and compared.CHALMERS, Applied Mechanics, Master’s Thesis 2014:421
1.4Thesis OutlineThis report is divided into seven main chapters. It starts with a general introduction inChapter 1 and a theoretical fundamental in Chapter 2 which offers basic knowledgeabout Li-ion battery, heat transfer theory, on-off control and PCM. Chapter 3 presentsthe basic required functions of BTMS and several commercial and potential solutionsin Section 3.2 with corresponding evaluation in Section 3.3. Depending on theevaluation two candidate systems are proposed in Section 3.4.Based on the system description in Chapter 4 the combined liquid system model is builtup at the first phase in Section 4.1. Then improvement is achieved with an additionalPCM unit which is described in Section 4.2. Chapter 5 presents the modelling resultsof both systems with defined performance parameters.In the Chapter 6 a final conclusion based on previous results analysis is discussed.Lastly, possible work for the future is recommended for completing and optimising themodels.To summarise the contributions: Jiling Li took main responsibility for Sections 2.1 2.43.3 3.4 4.2.4 4.2.5 4.3 5.2; and Zhen Zhu took main responsibility for Sections 2.2 2.33.2 4.1 4.2.1-4.2.3 4.2.6 5.1. The rest of the report was completed in co-operation witheach other.2CHALMERS, Applied Mechanics, Master’s Thesis 2014:42
2TheoryThis chapter highlights the most important fundamental points of thesis, presenting thetheoretical thoughts of automobile engineering, chemistry, thermodynamics andautomatic control behind numerical simulations.2.1Lithium-ion Battery2.1.1Mechanism of Secondary Lithium-ion batteryA lithium-ion battery consists of an anode, cathode and electrolyte as well as aseparator, see Figure 2.1. The anode, is the oxidised electrode which removes electronsto the external circuit during discharging. Correspondingly, the cathode, is the oxidisingelectrode which receives electrons from the external circuit. The electrolyte is themedium to transfer ions between electrodes inside the cell and the separator is used toisolate electrodes. Also, the solid electrolyte interface (SEI) is thin passivation layerwhich is formed on the surface of the carbon anode during the first charge. It slowsdown the reaction rate and decreases the current. (electropaedia, 2014)Figure 2.1The physical structure of lithum-ion battery (electropaedia, 2014)Secondly a Li-ion battery is rechargeable which means its electrochemical reactions arereversible. Lithium ions disperse from the negative to the positive when dischargingand go in reverse when charging. Lithium-ion battery use intercalated lithiumcompound as the electrode instead of metallic lithium. The electrochemical reactionsfor Li-cobalt in the positive electrode and negative electrode are expressed as following:The positive electrode reaction is:Li𝐶𝑂 𝑂2 𝐿𝑖1 𝑥 𝐶𝑂 𝑂2 𝑥𝐿𝑖 𝑥𝑒 (2.1)The negative electrode reaction:x𝐿𝑖 𝑥𝑒 𝑥𝐶6 𝑥𝐿𝑖𝐶6(2.2)The electrochemical reactions in the positive electrode and negative electrode for otherlithium batteries are similar.CHALMERS, Applied Mechanics, Master’s Thesis 2014:423
There are many types of lithium-ion battery which are named by cathode oxides andeach one has its own characteristics as followings:Table 2.1Reference names for Li-ion batteries (Battery University, 2014).Chemical nameMaterialShort formNoteLithium CobaltOxideLiCoO2Li- cobaltHigh capacity; forcell phone laptop,cameraLithium ManganeseOxideLiMn2O4Li- manganeseLithium IronPhosphateLiFePO4Li- phosphateMost safe; lowercapacity than Licobalt but highspecific powerand long life.Lithium NickelManganese CobaltOxideLiNiMnCoO2NMCLithium NickelCobalt AluminiumOxideLiNiCoAlO2NCALithium titanateLi4Ti5O12Li- titanatePower tools,e-bikes, EV,medical,hobbyist.Gainingimportancein electricpowertrain andgrid storageSeen from Table2.1, three types of lithium batteries are particularly suitable for an EVbattery, namely LiMn2O4, LiFePO4, and LiNiMnCoO2.2.1.2Thermal Issues of Li-ion batteryLithium-ion cells performance depends on both the temperature and the operatingvoltage. Lithium-Ion cells work well when cells operate within limited voltage andtemperature. Otherwise, damage will occur to the cells and will be irreversible.In over-voltage situations the charging voltage exceeds the bearable cell voltage,resulting in excessive current flows and at the same time, it causes two problems.At excessive currents the Lithium-ions are deposited more rapidly than intercalation tothe anode layers, Lithium ions are then deposited on the surface of the anode as metallicLithium. This is Lithium plating. It gives rise to the reduction in the free Lithium ionsand an irreversible capacity loss. (Electropaedia, 2014). There are two types of metallithium plating, namely homogeneous lithium plating and heterogeneous lithiumplating, but the lithium plating is dendritic in form. Eventually it can result in a shortcircuit between the electrodes.4CHALMERS, Applied Mechanics, Master’s Thesis 2014:42
As with over-voltage, under-voltage also brings about problems which give rise to thebreakdown of the electrode materials.For the anode, the copper current collector breaks down. It causes the increase of batterydischarge rate and battery voltage, however, the copper ions are precipitated as metalcopper which is irreversible. The situation is dangerous for it can result in short-circuitbetween anode and cathode. For the cathode, the cobalt oxide or manganese oxide willbe decomposed after many cycles under low voltage. Meanwhile, oxygen will bereleased and the battery suffers from capacity loss.The battery temperature should be controlled carefully. Both excess heat and lack ofheat will brings about problems.Chemical reaction rates have a linear relation to temperature. The decrease of theoperating temperature will reduce reaction rate and the capacity of carrying currentduring charging or discharging. In other words, the battery power capacity is decreased.Moreover, the reduction of reaction rate makes it harder to insert lithium ions intointercalation spaces. The result is the reduction of power and lithium plating causingthe capacity loss.High temperature increases the reaction rate with higher power output, however, it alsoincreases the heat dissipation and generates even higher temperatures. Unless heat isdissipated quicker than heat is generated, the temperature will be higher and finally athermal runaway will result.Thermal runaway consists of several stages and each stage will give rise to moreirreversible damage to cells. First, the SEI layer is dissolved to electrolyte at round80ºC. The primary overheating may result from excessive current or high ambienttemperature. After breakdown of the SEI layer, electrolyte begins to react with theanode. This reaction is exo-thermal which drives the temperature higher. Secondly, thehigher temperature causes the organic solvents to break down with the release ofhydrocarbon gases. Normally this starts at around 110 ºC. The pressure inside cells isbuilt up by the gas and the temperature is beyond the flashpoint. However, the gas doesnot burn due to the lack of oxygen. A vent is needed to release the gas in order to keepcells under proper pressure and avoid a possible rupture. Then, the separator is meltedand short-circuits occur between the anode and cathode at 135 ºC. Finally, the metaloxide cathode breaks down at 200 ºC and releases oxygen which allows the electrolyteand hydrogen gas to burn. This reaction is also exo-thermal and drives temperature andpressure still further. (Electropaedia, 2014)In addition, uneven temperature distribution is another problem of batteries. Typically,it is caused by the excessive local temperature, variable current in a cell and the thermalconductivity of the case, as well as the placement of positive and negative terminalsand so on. (Pesaran, 2001) It results in local deterioration and even thermal runawaywith reducing the battery lifetime. (Kizilel, et al., 2009)2.1.3Operating RequirementsThe battery temperature should be controlled within temperature limits to avoid thethermal issues and improve the performance. The temperature range affects the batterypower and battery cycle life, see Figure 2.2 and Figure 2.3. At the same time, thetemperature distribution should be even to guarantee the battery performance andCHALMERS, Applied Mechanics, Master’s Thesis 2014:425
lifetime. That is also the reason why the battery thermal management system isnecessary to the battery system.Figure 2.2Battery power and temperature (Matthe, et al., 2011).When temperature ranges from 20 C to 40 C, battery power reaches maximum, seeFigure 2.2.Figure 2.3Cycle life and temperature (Electropaedia, 2014).The cycle life goes down slowly below 10 C because of anode plating and drops offquickly above 60 C due to the breakdown of electrode materials, see Figure 2.3.Generally, the temperature must be controlled between 20 C and 40 C to ensure theperformance and cycle life. Moreover, the temperature distribution is controlled under5K to keep the safety and lifetime of battery (Pesaran, 2002). In addition
Master’s Thesis in Automotive Engineering JILING LI ZHEN ZHU Department of Applied Mechanics . The battery thermal management system (BTMS) plays a vital role in the control of the battery thermal behaviour. The BTMS technologies are: air cooling system, liquid cooling system, direct refrigerant cooling system, phase change material (PCM) cooling system, and thermo-electric cooling system .
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