Lecture_2_1 to 2_6c2eb005-0aa3-4678-b558-1528ec7692f6.pdf

SEE 627 - Electric Vehicles Dr. Amarendra Edpuganti Assistant Professor Department of Sustainable Energy Engineering IIT Kanpur 9/6/2024 1  Modul...

SEE 627 - Electric Vehicles Dr. Amarendra Edpuganti Assistant Professor Department of Sustainable Energy Engineering IIT Kanpur 9/6/2024 1  Module 2 – Energy Storage Systems Lecture 1 9/6/2024 2 CONTENTS Introduction Electrochemical Cell Cell Parameters 9/6/2024 3 INTRODUCTION Energy storage systems are the core technology which provides on board electrical energy for the electric vehicles (EVs). Currently four viable energy sources adopted in the EV market. 1. Electrochemical Batteries (Normally called as batteries) 2. Ultra Capacitors (UCs) 3. Ultra Flywheels (UFs) 4. Fuel Cells 9/6/2024 4 INTRODUCTION Battery Based EV Fuel Cell Based HEV 9/6/2024 5 INTRODUCTION Flywheel Based HEVs Ultra Capacitor Based HEVs 9/6/2024 6 INTRODUCTION Batteries are electrochemical devices which store energy during charging and produce electricity during discharging. The UCs are essentially capacitors with ultra high capacitances. UCs store and produce electrical energy by electrostatic/electrochemical means. 9/6/2024 7 INTRODUCTION The UFs are essentially electric machines spinning at ultrahigh speed. UFs stores and produce electric energy by electromechanical means. Fuel cells are electrochemical devices which directly convert chemical fuels into electricity. All the above sources can operate in bidirectional mode except fuel cell. 9/6/2024 8 INTRODUCTION Requirements of an energy storage system for the EV ✓Portable, compact, and light weight ✓Low initial cost ✓Continuous electric supply and high sustained power output ✓High energy and power stored content per unit mass and per unit volume ✓Highly efficient ✓Wide temperature range of operation ✓Long inactive shelf life and long operational life 9/6/2024 9 INTRODUCTION Requirements or specifications of an energy storage system for the EV ✓Sealed and leak proof ✓Rugged and resistant to abuse ✓Safe in use ✓High cycle life ✓Overcharge and over discharge capability ✓Maintenance free 9/6/2024 10 INTRODUCTION Fig. Life chain of a battery 9/6/2024 11  Electrochemical Cell 9/6/2024 12 Electrochemical Cell A cell is the most fundamental unit of a battery. During discharging – Chemical energy to electric energy During charging – Electric energy into chemical energy The performance of the cell depends on the material inside the cell. However, general principles of the working is independent of the material employed. Cell can be said to be either electrolytic cell or galvanic cell. Electrolytic cell: Electrical energy is converted to chemical energy (Charging of cell) Galvanic cell: Chemical energy is converted to electric energy (Discharging of cell) 9/6/2024 13 Electrochemical Cell Load Load e- e- M+ M+ Negative Electrode Negative Electrode Positive Electrode Positive Electrode 1st 2nd 1st 2nd Fig. A Galvanic Cell Fig. An Electrolytic Cell 9/6/2024 15 Electrochemical Cell Classifications of Electrochemical Cell Primary Cells Secondary Cells Galvanic cells Both Galvanic & electrolytic cells Non-rechargeable Rechargeable Not suitable for EVs Suitable for EVs 9/6/2024 16 Electrochemical Cell Components of an Electrochemical Cell Current Casing Collector General Rule:- Negative | Electrolyte | Positive 𝐸𝑐𝑒𝑙𝑙 = 𝐸 𝑟𝑖𝑔ℎ𝑡 − 𝐸 𝑙𝑒𝑓𝑡 𝐸𝑐𝑒𝑙𝑙 = 𝐸 + − 𝐸 − Electrode Electrolyte Seperator Fig. Design of an Electrochemical Cell 9/6/2024 17 Electrochemical Cell Components of an Electrochemical Cell The components of a cell is divided into two parts 1. Active Components- Active components are those directly involved in the reduction/oxidation (redox) reactions of the cell. Example- Electrodes 2. Non-active Components- Non active components are passive in the redox reaction. However, involved in the other side reactions, which occur during the charge and discharge of the cell. Example- electrolyte, the separator, current collectors and the casing 9/6/2024 18 Electrochemical Cell Components of an Electrochemical Cell 1. Electrodes An Electrode is an electrically and ionically conducting material. Two types of electrodes are used in the cells. ✓Metallic Electrodes or Blocking Electrodes Chemical reactions take place only at the outermost surface layer. ✓Insertion or Non-Blocking Electrodes Reactions take place at the surface as well as in the bulk of the electrode. 9/6/2024 19 Electrochemical Cell Components of an Electrochemical Cell 1. Electrodes Basically, electrodes are subjected to two different processes: ✓ Non-Faradaic Processes ✓ Faradaic Processes 9/6/2024 20 Electrochemical Cell Components of an Electrochemical Cell 1. Electrodes In the faradaic processes, chemical reactions, such as the oxidation- reduction reactions, are associated to transfer the charges. Example- Batteries and Fuel Cells In the non-faradaic processes, charges are distributed only by physical mean without any formation of the chemical bonds. Example- Capacitors (Except pseudo capacitor) 9/6/2024 21 Electrochemical Cell Components of an Electrochemical Cell 2. Electrolytes Electrolyte serves as catalyst to make a battery conductive. Electrolyte promotes the movement of ions from the cathode to the anode on charge and in reverse on discharge. The electrolyte of a cell consists of soluble salts, acids or other bases in liquid, gelled and dry formats. 9/6/2024 22 Electrochemical Cell Components of an Electrochemical Cell 2. Electrolytes The ion conductivity should be fast in order not to limit the redox reaction in any operational condition. The electrolyte must be compatible with the electrodes without losing its performance. It should be stable within the full voltage range of the cell. It should have low toxicity. 9/6/2024 23 Electrochemical Cell Components of an Electrochemical Cell 2. Electrolytes Generally, liquid electrolyte has a high conductivity, but low mechanical strength. Polymer and solid electrolytes are more flexible in format and can be made in several shapes. However, these electrolyte has low ion conductivity. Mechanical and electrical properties of an electrolyte should be good. 9/6/2024 24 Electrochemical Cell Components of an Electrochemical Cell 3. Separator Separator is most often a porous membrane soaked in electrolyte before the cell is assembled. It provides a barrier between the anode and the cathode while enabling the exchange of ions from one side to the other. In case of liquid electrolyte, separator is used to improve the mechanical property of the electrolyte. 9/6/2024 25 Electrochemical Cell Components of an Electrochemical Cell 3. Separator The small amount of current that may pass through the separator is self- discharge and this is present in all batteries to varying degrees. Self-discharge eventually depletes the charge of a battery during prolonged storage. 9/6/2024 26 Electrochemical Cell Components of an Electrochemical Cell 4. Current Collector Current collector is used to transfer the electron from one electrode to the other in the external circuit as effectively as possible. It ensures the best possible cell charge and discharge processes. 9/6/2024 27 Electrochemical Cell Components of an Electrochemical Cell 4. Current Collector A good current collector material should have following properties. ✓ Significant electrical conductivity ✓Must be stable w.r.t. electrochemical environment inside the cell. ✓ Should be able to remove the heat, which is unavoidably generated during the operation of the cell. ✓ Should have good mechanical strength. (Especially in case of non-metallic electrode materials) 9/6/2024 28 Electrochemical Cell Cell Design The performance of a cell is basically dependent on its design. Design of a cell most often totally independent of the cell chemistry used. The basic design parameters which is to be considered to control the performance of a cell are: ✓ Physical shape and size of cell ✓ Arrangement of electrodes and electrolytes ✓ Intended usage and performance demand 9/6/2024 29 Electrochemical Cell Cell Design On the basis of how electrodes and electrolytes are arranged, cells are classified as: M+ M+ Negative Electrode Reference Electrode Positive Electrode 1. Half cells Positive Electrode Charge Charge 2. Full cells M+ M+ Discharge Discharge (a) Full Cell Design (b) Half Cell Design 9/6/2024 30 Electrochemical Cell Cell Design 1. Half Cell Design In this design, full electrochemical operating redox cell is physically separated into a Half cell. Either the oxidation or reduction of the electrode of interest occurs and an electrode having well known and defined potential is used as counter (reference) electrode. 9/6/2024 31 Electrochemical Cell Cell Design 2. Full Cell Design In this design, two electrodes are used to construct a cell to fulfill the requirements for the specific application. This is the most common method to construct an electrochemical cell. Depending upon the arrangement of these two electrodes, full cell designs are divided into two parts : 1) Monopolar 2) Bipolar. 9/6/2024 32 Electrochemical Cell Cell Design (-) Negative Electrode (-) Negative Electrode Positive Electrode Positive Electrode Electron Conducting Negative Electrode Membrane Negative Electrode Positive Electrode (+) (+) Positive Electrode (a) Monopolar Full Cell Design (b) Bipolar Full Cell Design 9/6/2024 33 Electrochemical Cell Cell Design 2. Full Cell Design ❖ Comparison of Monopolar and Bipolar Cell Design The flow of electrons in bipolar cell design is short compared to Monopolar cell design. The overall cell resistance in bipolar design is low because of the reduced number of connections between the cells. 9/6/2024 34 Electrochemical Cell Cell Design Based on physical shape and size, cells are classified as ✓ Cylindrical Cells ✓ Prismatic Cells ✓ Pouch Cells (a) (b) (c) Fig. Various cell designs: (a) cylindrical; (b) prismatic; (c) pouch. 9/6/2024 35  Cell Parameters 9/6/2024 37 Cell Parameters 1. Open-Circuit Voltage The cell voltage is the potential difference between the two electrodes (volts (V)). Theoretically cell voltage is calculated by Nernst Equation. 𝑅𝑇 𝐶𝑜 𝐸𝑐𝑒𝑙𝑙 = 𝐸0 − ln 𝑛𝐹 𝐶𝑅 𝐸𝑐𝑒𝑙𝑙 = Cell Potential (in Volts) (at std. Temperature) E0 = Standard Cell Potential R = Universal Gas Constant (8.3145 JK-1mol-1) T = Temperature in Kelvin F = Faraday Constant (9.648533*104 C mol-1) 𝐶𝑜 and 𝐶𝑅 bulk concentrations of the oxidation and reduction reactants 9/6/2024 38 Cell Parameters 2. Terminal Voltage The cells are connected in series to achieve battery voltage required for the EV application. E is the open circuit voltage and R is the internal resistance. Terminal voltage is at its full charge voltage VFC when the battery is fully charged Vcut is the battery cut-off voltage, where discharge of the battery must be terminated V = E − IR 9/6/2024 39 Cell Parameters 3. Capacity (Theoretical QT) The amount of charge released by the energized material at an electrode associated with complete discharge of a battery is called the battery capacity. Faraday’s law of electrolysis states that the mass of the elemental material altered at an electrode is directly proportional to the element’s equivalent weight for a given quantity of electrical charge. mR is the mass of the reactant material in kg and Mm is the molar mass of that material in g/mol. n is the number of electrons per ion produced at an electrode F is the Faraday number or Faraday constant Coulomb (C) is the charge carried by 1A current for 1s Faraday number is given by the amount of electric charge carried by one mole of electrons = 96,485 C/mol 9/6/2024 40 Cell Parameters 3. Capacity (Theoretical QT) - Numerical Q) In the nickel-cadmium cell, nickel oxyhydroxide, NiOOH is the active material in the charged positive plate. During discharge it reduces to the lower valence state, nickel hydroxide Ni(OH)2 by accepting electrons from the external circuit: Discharge − 2𝑁𝑖𝑂𝑂𝐻 + 2𝐻2 𝑂 + 2𝑒 Charge 2𝑁𝑖(𝑂𝐻)2 +2𝑂𝐻 − (0.49𝑉) Cadmium metal is the active material in the charged negative plate. During discharge, it oxidizes to cadmium hydroxide, Cd(OH)2 and releases electrons to the external circuit: 𝐶𝑑 + 2𝑂𝐻− Discharge 𝐶𝑑(𝑂𝐻)2 +2𝑒 − + 2𝑂𝐻− (0.809𝑉) Charge The net reaction occurring in the potassium hydroxide (KOH) electrolyte is: Discharge Cd+2NiOOH-+2H2O Charge 2Ni(OH)2+Cd(OH)2 (1.299V) Estimate the mass of a 1.299 V, 100 Ah Ni-Cd battery. Neglect the mass KOH component of the electrolyte. 9/6/2024 41 Cell Parameters 3. Capacity (Theoretical QT) - Numerical 9/6/2024 42 Cell Parameters 3. Practical Capacity (Qp) The practical capacity QP of a battery is the actual charge released by the energized material at an electrode associated with complete discharge of the battery. The practical capacity is always much lower compared to the theoretical capacity QT due to practical limitations. Battery capacity measurement 9/6/2024 43 Cell Parameters 3. Practical Capacity (Qp) The practical capacity of a battery is defined in the industry by a convenient and approximate approach of Ah under constant discharge current characteristics, i.e., at a specified discharge rate (commonly referred to as C- rate) from 100% capacity to the cut-off voltage. The results show that the capacity depends on the magnitude of discharge current. The smaller the magnitude of the discharge current, the higher the capacity of the battery is. When the capacity of a battery is stated, the constant discharge current magnitude must also be specified. The nominal capacity of a battery is given at a 1C-rate, and it decreases with increasing C-rate. 1C-rate will charge/discharge the battery in 1 hour 9/6/2024 Constant current discharge curves 44 Cell Parameters 3. Practical Capacity (Qp) Capacity/SOD Fig. Capacity variations depending upon the discharge rate 9/6/2024 45 Cell Parameters 3. Practical Capacity (Qp) Battery capacity changes with discharge time. Qp = 42 Ah -10 hour discharge Qp = 46 Ah -20 hour discharge Fig. Capacity variations with discharge time 9/6/2024 46 Cell Parameters 4. Discharge rate The discharge rate is the current at which a battery is discharged under constant current characteristics. If QP is rated battery capacity and h is discharge time in hours, discharge rate = Qp /h Example: If the capacity of battery is 100 Ah ****Considering that battery capacity does not change with discharge rate 9/6/2024 47 Cell Parameters 5. Charge Rate The rate at which the charge take place is defined as the charge rate or C-rate. C-rate basically expresses the current capability of an electrochemical cell. 1C rate means that the charge current will charge entire battery in 1 hour Charge time decreases with charge rate. 9/6/2024 48 Cell Parameters 6. State of Charge (SOC) SoC represents the present capacity of the battery. Discharging: QT represents the rated capacity Charging: 9/6/2024 49 Cell Parameters 7. State of Discharge (SOD) SoD is a measure of the charge that has been drawn from a battery during discharge. 9/6/2024 50 Cell Parameters 8. Depth of Discharge (DOD) DoD is the percentage of battery rated capacity to which a battery is discharged 9/6/2024 51 Cell Parameters 9. Energy The energy of a cell refers the amount of useful work a cell can do until the cutoff voltage has been reached. The energy stored in a cell depends on its voltage and charge stored. The SI unit of energy is Joule (Watt-sec). This is an inconveniently small unit, so watt-hour used instead. Practical available energy : 𝑡𝑐𝑢𝑡 𝐸𝑝 = න 𝑣 𝑡 𝑖(𝑡)𝑑𝑡 0 9/6/2024 52 Cell Parameters 10. Specific Energy (Gravimetric energy density) It is the amount of electrical energy stored for every kilogram of battery mass (Wh/kg) If the mass of the battery MB is proportional to the mass of the limiting reactant of the battery mR, then SE is independent of mass. The specific energy of lead acid battery is 35–50 Wh/kg at C/3 rate. As practical energy EP varies with discharge rate, the value of SE is also variable. Energy density: It is the amount of electrical energy stored per cubic meter of battery volume. It has unit of Wh/L. This is also know as volumetric energy density. 9/6/2024 53 Cell Parameters 11. Power The power of the cell refers to the rate at which this work can be performed. The instantaneous battery terminal power is Using the maximum power transfer theorem in electric circuits, the battery can deliver maximum power to a load when the load impedance matches the battery internal impedance. Since Ev and Ri vary with the state of charge, Pmax also varies accordingly. 9/6/2024 54 Cell Parameters 11. Power Maximum power output is needed from the battery in fast discharge conditions in EVs, which occur when the electric motor is heavily loaded – e.g: Acceleration on a slope The performance of batteries to meet acceleration and hill climbing requirements can be evaluated with the help of rated power specifications, which are based on the ability of the battery to dissipate heat. The rated continuous power is the maximum power that the battery can deliver over prolonged discharge intervals without damage to the battery. These do not necessarily correspond to Pmax on p-i curve of battery characteristics. The rated instantaneous power is the maximum power that the battery can deliver over a very short discharge interval without damage to the battery. 9/6/2024 55 Cell Parameters 12. Specific Power It is the amount of electrical power obtained from every kg of battery mass. It is highly variable quantity because the power output of the battery depends far more upon the load connected to it rather the battery itself. Typically, lead acid battery’s maximum specific power is around 280 W/kg The term power density is used to refer to the power of the battery per unit volume (W/L) 9/6/2024 56 Cell Parameters Energy and Power The water in the bottle represents specific energy (capacity); the spout pouring the water govern specific power (loading). Fig. Relationship between specific energy and specific power. 9/6/2024 57 Cell Parameters Energy and Power Energy of an electrochemical cell can be improved ✓ By increasing the voltage ✓ By increasing the amount of active material 9/6/2024 59 Cell Parameters Energy and Power Power of an electrochemical cell can be improved ✓ By decreasing the voltage drop with increased current ✓ By decreasing the ohmic, kinetic, and mass transfer limitations of the cell (the second point can be achieved by using thinner electrode i.e. less active material ) 9/6/2024 60 Cell Parameters Energy and Power The conditions for the energy and power enhancement of a cell are completely opposite to each other. Therefore, a cell or battery can either be energy optimized or power optimized. Higher power output from an energy optimized cell will result in large polarization and consequently large losses. 9/6/2024 61 Cell Parameters Energy and Power This tradeoff between the energy and power for electrochemical cell is characterized by Ragone Plot. This plot illustrates the available energy density as a function of the available power density on a logarithmic scale. This plot can be used for any energy storage devices. However, this is firstly used for the performance comparison of a battery. 9/6/2024 62 Cell Parameters Energy and Power At point A in the plot, high specific power can be delivered but the delivered specific energy is very limited. At point B, high specific energy can be delivered during longer time periods with a low specific power output. Fig. Ragone Plot 9/6/2024 63 Cell Parameters Energy and Power Fig. Ragone plot of various battery technologies with specification at cell level for automotive applications 9/6/2024 64 Cell Parameters Energy and Power Fig. Ragone chart of various energy sources 9/6/2024 65 Cell Parameters 13. Efficiency The performance and the efficiency of a cell depend both on charge and energy transfer. Therefore, two efficiency metrics are used for electrochemical cells. ✓ Coulombic Efficiency ✓ Energy Efficiency 9/6/2024 66 Cell Parameters 13. Efficiency Coulombic efficiency is defined as charge transferred during the discharging process (Qdis) over the charge transferred during the previous charging process (Qcha). 0 𝑄𝑑𝑖𝑠 ‫𝑡𝑑𝐼 𝑠𝑖𝑑׬‬ η𝐶𝑜𝑢𝑙𝑜𝑚𝑏𝑖𝑐 = = 0 𝑄𝑐ℎ𝑎 ‫𝑐׬‬ℎ𝑎 𝐼𝑑𝑡 Coulombic efficiency is also known as Amphour (or Charge) efficiency. 9/6/2024 67 Cell Parameters 13. Efficiency Energy efficiency is defined based on the energy transferred according to: 0 ‫𝑡𝑑𝐼)𝑡(𝑉 𝑠𝑖𝑑׬‬ η𝑒𝑛𝑒𝑟𝑔𝑦 = 0 ‫𝑐׬‬ℎ𝑎 𝑉(𝑡)𝐼𝑑𝑡 High energy efficiency of battery is desirable for the EV applications. Energy efficiency is highly dependent on the how battery is used. 9/6/2024 68 Cell Parameters 13. Efficiency The reciprocal of Amphour (or charge) efficiency is known as Ampere-hour charging factor (αAh). 1 αAh = ηCoulombic The reciprocal of energy efficiency is known as Watt-hour charging factor (αwh). 1 αwh = ηenergy 9/6/2024 69 Cell Parameters 14. Cycle Life The number of discharge-charge cycles the battery can experience before its capacity drops to predefined value (Usually 80% is considered as end-of-life criteria, sometimes - 65%). Cycle life is estimated for specific charge and discharge conditions. Cycle life is affected by the rate and depth of charge-discharge cycles, end of charge voltage (EOCV)/SOC, temperature, and humidity. Cycle life decreases with increase in DOD, EOCV or SOC, temperature, and charge/discharge rate. A lowering of EOCV to 4.1 V (equivalent to 10% reduction of SOC) can be expected to yield a doubling the cycle life. An increase of temperature from 25°C to 45°C can be expected to yield a 20% reduction of cycle life. Cycle life of 3.7V Li-ion battery for different parameters 9/6/2024 70 Cell Parameters 14. State of Health It is a figure of merit of the condition of a battery compared to its ideal conditions. Caged SOH = Cn Caged is the capacity of aged battery. The units of SOH are percent points (100% = the battery's conditions match the battery's specifications) 9/6/2024 71 Cell Parameters 14. Other Parameters ESR (Equivalent Series Resistance) is the internal resistance present in any cell that limits the amount of peak current it can deliver. The MPV (mid-point voltage) is the nominal/normal voltage of the cell, and it is the voltage that is measured when the battery has discharged 50% of its total energy. 9/6/2024 72 REFERENCES 1. Iqbal Husain, “Electric and Hybrid Vehicles Design Fundamentals”, CRC Press, Taylor & Francis Group, 2021. Third Edition – Chapter 5 9/6/2024 73