International Journal of Electrical and Electronics Engineering Research (IJEEER) ISSN 2250-155X Vol. 3, Issue 4, Oct 2013, 35-42 TJPRC Pvt. Ltd. REGENERATIVE BRAKING FOR AN ELECTRIC VEHICLE USING HYBRID ENERGY STORAGE SYSTEM VIVEK KUMAR YADAV 1 & NAVJOT BHARDWAJ 2 1 K E C, Ghaziabad, Uttar Pradesh, India 2 I T S Engineering College, Greater Noida, Uttar Pradesh, India ABSTRACT This paper presents study and implementation strategy for deploying regenerative braking system in vehicles with hybrid energy source. Intelligent electronic control strategy for the whole process using Buck-Boost Converter is given, further it encapsulates elements of structure of regenerative braking system. Application of super capacitors as power buffer to minimize rapid power fluctuation under acceleration/deceleration and properties of super capacitors which makes it suitable for regenerative energy recapture are also stated. KEYWORDS: Super Capacitors, Bust-Boost, Regenerative Braking, Control Strategy, Electric Vehicle, Battery INTRODUCTION As the law of conservation of energy states that energy cannot be created nor destroyed, but only change from one form to another. During conventional braking kinetic energy of vehicle is converted into heat energy, this heat energy is wasted and dissipated into atmosphere. Regenerative braking makes restoration of kinetic energy possible. An electric car has an electric drive train, electric motor is used to power vehicle and motor is powered by batteries. When vehicle is accelerating or running at constant speed energy flows from battery to the motor, as soon as driver hits brake pedal energy flow is reversed, motor acts as generator and now wheels will power generator and experience braking force, power flow back to battery pack. So, some portion of energy lost is returned to batteries, generator shaft exhibits mechanical resistance which produce braking power. Drawback associated with this system is that it takes longer time to stop vehicle, so frictional brakes are also installed with regenerative braking system to provide braking during sudden stop. Electric vehicles are future of commuter transportation as they provide zero emission driving and highly efficient, motor's driving efficiency is greater than 80% which is about four times of an internal combustion engine. Regenerative energy saving ranges from 8% to 25% depending on driving conditions, it is more efficient for neighborhood purpose where brakes are applied frequently. The hybrid energy storage system consist of batteries and super capacitor. Super capacitors are significant as during braking they can capture large amount of energy in short interval of time which batteries cannot secondly during acceleration when there is significant rise in power demand super capacitor provide abrupt power which battery cannot provide, deep charging/discharging cycle of battery causes heating which ultimately reduces the life and capacity of battery. Super capacitor has high power density hence can accept/provide large power during braking and acceleration simultaneously. ELEMENTS OF REGENERATIVE BRAKING There are five elements of regenerative braking which are synchronized, but before understanding these it is important to discuss about deceleration modes.
36 Vivek Kumar Yadav & Navjot Bhardwaj There are two deceleration modes Foot off throttle but not on brake pedal, in this mode partial charging will occur and vehicle will decelerate gradually. Foot on brake pedal, in this mode higher amount of deceleration will be allowed and vehicle will stop more rapidly. During light braking pedal application generator will slow the car, if heavier brake pedal are applied the frictional brakes will also come into play. Angular sensors can be employed on brake and acceleration pedal to sense level of braking. The elements of regenerative braking are:- Brake Pedal Braking force is applied by driver on brake pedal. Hydraulic Booster Unit It consist of master cylinder and regulator it respond in two steps. First it sends electronic signal to Electronic Control Unit (ECU) that barking force has been demanded, next the master cylinder exerts hydraulic pressure on pedal stroke stimulator and the regulator feeds fluid to hydraulic pressure control unit. Brake Electronic Control Unit It senses braking demand and sends a friction demand to regenerative braking. It also calculate the force necessary to fulfill braking demand and instruct hydraulic pressure control unit to pass on a corresponding amount of hydraulic fluid. Pedal Stroke Stimulator It absorbs an amount of hydraulic pressure from master cylinder that corresponds to the amount of braking force applied by the regenerative braking. Hydraulic Pressure Control Unit It passes corresponding amount of hydraulic fluid to a four way cylinder. MOTOR AS GENERATOR In an electric vehicle motor is used to power vehicle, this motor may be three phase ac motor or dc motor. In this section we will study how these motor acts as generator during regeneration mode. In a three phase ac motor with squirrel cage rotor the copper bars are shorted by end rings, which allows current to flow with little resistance from one side of rotor to another. The rotor doesn't have a direct supply of electricity, when conductor is moved through magnetic field (created by stator) a current is induced. As rotating magnetic field is continuously rotating in stator with synchronous speed, the rotor is always trying to catch up. The interaction of magnetic field create torque, the amount of torque produced is related to relative position of rotor field to stator field. since the stator field is always ahead of rotor field when the accelerator is pressed, the rotor is always spinning to catch up and it is continuously producing torque. When driver releases the accelerator pedal, the power electronics module immediately changes the position of stator field to behind the rotor. Now rotor must slow down to align its field with the stator field. The direction of current in the stator switches the direction and energy starts to flow through Power electronics module, back to the battery. This is energy regeneration.
Regenerative Braking for an Electric Vehicle Using Hybrid Energy Storage System 37 In case of DC motor during regenerative braking, power cycle is reversed. Suppose we have very simple DC motor add power to its brushes motor will spin, remove the power and spin the motor shaft it will produce electricity nothing is reversed except the fact that the relay sending power to motor is now collecting power from motor. ENVIRONMENTAL CONCERN CO 2 emission is major concern in today scenario transparent sector alone is responsible for 25% to 30% emission of green house gases. Average CO 2 emission is 150 gm /Km for a passenger car. Estimate made by US Department of Energy shows that if 10% of total no of vehicles will have an electric drive train then CO 2 emissions would decrease 60 Million tons /Year. Regenerative Braking is small, yet very important step towards eventual independence from fossil fuels. This break allows extended driving range and time to plugged into external charger, it also conserve energy up to 20% rather than wasted as heat and reduce wear of vehicle brakes. ABOUT SUPERCAPACITORS Electric car can have different energy storage devices such as lead-acid, Ni-Cd, Ni-Metal Hydride. Li-ion and etc. Nowadays, research interest on connecting Li-ion batteries and super capacitors, as combination of two provide both high energy and power density. Super capacitors exhibits high power density which makes them suitable to provide/accept power at peak loads i.e. During transient state such as acceleration/ Braking simultaneously. While specific energy is provided by batteries to provide power in steady state, it is responsible for range of vehicle. Super capacitors have emerged as new technology in energy storage. They are also known as Ultracapacitors, Electrochemical Dual Layer Capacitors (EDLC). It has same fundamental equation as that of conventional capacitors but have very large surface area of electrodes and negligible distance between electrodes. Collective effect of these two factors allows large capacitance. Supercapacitors have low energy and high power density as compared to batteries, Table below shows comparison between different energy storage devices. Table 1: Table Shows Comparison between Various Energy Storing Devices Energy Storage Specific Energy Specific Power Efficiency Charging Life Cycle Device (WH/Kg) (W/Kg) (%) Time Lead-Acid 30-50 150-400 80 500-1000 6-12hr Ni-Hydride 60-80 200-300 70 1000-2000 6hr Li Polymer 150-200 350 90 5000 6hr Li ion (classic) 80-130 200-300 95 5000 1-6hr Super capacitor 1-10 6000-9000 85-98 >500,000 0.3-30sec Super capacitors store electrostatic energy as there is no transfer of electric or chemical charge the process is non faradic due to which super capacitor possess properties such as long life, large number of charging and discharging cycles, highly reversible stored charge, wide range of working temperature[65-40 ᵒC], ESR(Effective Series Resistance ) is very low too so negligible loss during high power demand. Area of electrode is increased by using activated carbon or carbon nano-tubes as electrode. Recent researchers suggest carbon nano-tubes as electrode as super capacitor electrode material, Nanotubes are grown a entangled mat of carbon nano-tubes with an accessible network of mesopores. These mesopores are interconnected allowing continuous charge distribution, that uses almost all available surface area [3000m 2 /gm]. Charge separation is created nearly at each solid-liquid interference. It is observed that distance between electrodes is in angstroms and area in thousands of square meter /gm of electrode.
38 Vivek Kumar Yadav & Navjot Bhardwaj CIRCUIT PROPOSED To study regenerative braking in electric vehicle, supercapacitor of 1000F, 2.5V nominal voltage, 2.7V maximum voltage and maximum current of 400 A is used. 144 units of such capacitors are connected in series to form a bank of capacitors. The capacity of bank is 7 farad, nominal voltage of 300V dc and maximum voltage of 360V dc The weight of bank is 45kg. Figure 1: Figure Shows Buck-Boost Converter Power circuit has two components buck-boost convertor using IGBT and supercapacitor bank. Power circuit is connected in parallel with main battery which consist of 26 batteries in series with a nominal voltage of 312V dc. Control System is designed so that super capacitor bank voltage is not allowed to fall below [120 V] one third of maximum voltage, allowing to store 112Wh of useful energy. It may seem very less energy but it can allow more than 40KW power for duration of 10 sec which is enough time for good acceleration and deceleration without affecting batteries. During acceleration [Boost Operation] converter introduces energy form super capacitor to battery pack T1 is switched ON and OFF at a controlled duty cycle. When T1 is switched ON energy is taken from supercapacitor bank and stored inductor L. When T1 is switched off energy stored in L is transferred into C, through D2 and then into battery pack. Inductor Ls is used to soft the current pulse going in battery. During braking(buck Operation) converter transfers required energy from battery pack to super capacitor. This operation is accomplished with a controlled PWM operation on T2. When T2 is switched ON energy goes from battery pack to super capacitor bank, L stores some part of this energy. When T2 is switched OFF remain energy stored in L is transferred into super capacitor bank. CONTROL STRATEGY To do above operations correctly. A promising control strategy is required, control strategy depends on size of super capacitor bank, larger the capacity of bank vehicle can accelerate/decelerate taking an almost average current, as super capacitor will provide all current above and below average current. Large number of super capacitor can't be employed as they are costly, so more complicated and intelligent control strategy need to be designed to use super capacitor efficiently. This control strategy will take in account all variable parameters such as instantaneous battery and super capacitor voltage, battery state of charge, instantaneous battery current, capacitor current, vehicle speed and etc. This system forms a closed loop control system which sense and reacts accordingly. This system avoids battery deep charging and discharging cycles using limited energy stored.
Regenerative Braking for an Electric Vehicle Using Hybrid Energy Storage System 39 Speed of vehicle needs to be taken in account because when vehicle starts or running at lower speed it needs power for acceleration hence super capacitor bank should be fully charged, opposite to this when vehicle is running at high speeds super capacitor bank should be empty in order to store energy from regenerative braking. At medium speeds, the super capacitor banks should have in-between charge. The state of charge should be monitored because fully changed battery will not accept any charge, so during this condition super capacitor must be empty contradictory to this when state of charge is low, super capacitors should be enough charged to power vehicle. The state of charge is determined by time integration of the battery current (Positive or Negative). As energy stored in super capacitor is proportional to V 2 cap this voltage indicates level of remaining charge. Super capacitor voltage is controlled by IGBT Pulse Width Modulation(PWM) strategy applied to buck boost converter and through interaction of variable such as vehicle speed and state of charge of battery. The instantaneous battery voltage and direction of Load current decide mode of operation for converter. During braking instantaneously voltage of battery bank rise rapidly, buck mode is activated and energy flow from motor to super capacitor bank whereas during acceleration instantaneously voltage of battery falls rapidly boost mode is activated and energy flow from super capacitors and batteries to motor. All the operations are controlled electronically by microcontrollers or microprocessors, which is pre-programmed to take appropriate action accordingly to different situations. The combinational control is used having 2 levels primary and secondary control to give each variable a right significance. Primary Control Primary control maintain adequate level of charge in super capacitor. This charge is known as reference charge and it is calculated through the speed of vehicle and state of charge of battery pack. Figure 2 shows how reference charge change accordingly to speed of vehicle. Higher the state of charge of battery and vehicle speed lower the reference charge and vice-versa. Simultaneously V CAP is measured and actual charge is calculated. An error signal is generated between actual and reference charge and passed through proportional and integral(pi) control, it calculate reference current(i REF ) necessary to maintain super capacitor bank adequate amount of charge. Load Current (I LOAD ) is also an important reference, when this current exceeds maximum absolute value set on battery pack(i BATT ), the reference current(i REF ) is also modified. Figure 2: Figure Shows Charge in Capacitor at Variable Speed and SOC of Battery
40 Vivek Kumar Yadav & Navjot Bhardwaj Secondary Control Figure 3: Figure Shows Block Diagram of Secondary Control The reference current from primary control finally goes to secondary control and decides the amount of compensating current (I COMP ) required to compensate charge of super capacitor. During acceleration I COMP became positive (I COMP taken from Super capacitor bank) while during regeneration I COMP became negative (I COMP goes into Super capacitors). Discriminator block will decide whether convertor work under Buck or Boost operation this is how adequate charge is maintained in super capacitor. But what happens when battery voltage(v BATT ) is below or above rated voltage, I REF is modified and a similar action will be performed as in case of super capacitor. To take in account all these situations some logic rules have been implemented and these logic rules are performed inside limiter block. Figure 4: Figure Shows Working of Limiter Block Limiter block calculate a band around vehicle's current(i LOAD ) and sets the compensating current (I COMP ) within limits of band. This band keeps battery voltage and current within limits. Figure 4 above shows action of limiter block with maximum bandwidth ±70A. EXPERIMENT RESULT An experiment was performed in Catholic University of Chile on streets with electric car for this experiment Chevrolet S-10 is used which is converted into electric vehicle. The experiment will provide information about V BATT, V CAP, I LOAD, I BATT, I COMP during acceleration and braking. The limiter block is preprogrammed to keep battery voltage and current into limits V min = 300V dc, V max = 360 V dc, I min =-70 A dc and I max =70 A dc.
Regenerative Braking for an Electric Vehicle Using Hybrid Energy Storage System 41 During Acceleration Figure 5: Figure Shows Acceleration from 40 to 60kmph Experiment show vehicle takes 4.1 sec to accelerate from 40 to 60 km/hr max current taken from battery is 200A during transient stage while this current is reduced during steady (Vehicle running at 60 km/hr) and reaches about 20A. During acceleration when load current begin to increase ICOMP becomes positive and most of current is taken from super capacitor bank. As the voltage of bank VCAP decrease, battery pack also starts to provide current to load in order to support super capacitor bank, battery pack can provide current within its limits (±70 A). If still vehicle is accelerating battery will provide its maximum (70 A) current and surplus current will be provided by super capacitor bank but as super capacitor bank is also set with minimum voltage (120 V) vehicle will stop accelerating. But in above case vehicle continues to accelerate as capacitor are large enough. As vehicle runs on constant speed (60km/hr) I comp become negative and super capacitor begin to charge and battery will provide the constant load current to run vehicle. During Braking Figure 6: Figure Shows Deceleration from 40kmph to Stop Experiment shows it takes 2.1 seconds to decelerate from 40 km/hr to 0 under regenerative braking. Both battery pack and super capacitor bank will receive this current. It can be observed from above simulations that battery pack receives current lower that its limit, it is because battery has achieved its maximum voltage [360 V dc] limit and maximum
42 Vivek Kumar Yadav & Navjot Bhardwaj amount of current is accepted by super capacitor bank. Even after stopping the vehicle battery keeps charging super capacitor bank. To make it ready for upcoming acceleration. FUTURE SCOPE As electric vehicles are more efficient than conventional vehicles. Research related to electric vehicles has increased significantly in last decade as energy crisis and carbon emission are increasing world-wide. Using regenerative braking efficiency and range of electric vehicle increases, introduction of super capacitors provide high acceleration and deceleration with minimum loss. It is interesting to know that if super capacitor achieves specific energy of 20Wh/kg which is presently 7 to 10Wh/kg it will be possible to implement electric vehicle using super capacitors only with a very less charging time. Super capacitors are also used to start internal combustion engine which promotes stop and start technology which save 5-8% of fuel and reduce vehicle emission. CONCLUSIONS The hybrid energy storage system containing battery and super capacitor contribute to rapid energy recovery and consumption during acceleration and braking, these provide high acceleration and deceleration with minimum loss of energy and stress on battery pack by reducing power demand from them. A large amount of energy can be recovered, Regenerative braking is highly efficient in city conditions. The validity of proposed model is proved by simulation result. The system uses IGBT Buck-Boost convertor and control strategy to control energy stored in super capacitor bank storage system structure and specification are investigated. REFERENCES 1. Maxwell Technologies Document number: 1015370.3. DATASHEET K2 SERIES ULTRACAPACITORS. Retrieved From: http://www.maxwell.com/products/ultracapacitors/docs/datasheet_k2_series_1015370.pdf 2. Dr. John M Miller, Maxwell Technologies, Inc. Energy Storage Technology, Markets and Applications, Ultracapacitors in combination with Lithium-ion. Retrieved From: http://www.maxwell.com/products/ultracapacitors/downloads 3. Marin S.Halper, James C. Ellenbogen. 2006. Supercapacitors: A Brief Overview MP 05W0000272 4. Juan W. Dixon, Micah Ortúzar and Eduardo Wiechmann. Regenerative Braking for an Electric Vehicle Using Ultracapacitors and a Buck-Boost Converter. Retrieved From: http://web.ing.puc.cl/~power/paperspdf/dixon/42a.pdf 5. Zdzisáaw Juda. 2011. Advanced Batteries and supercapacitors for electrical vehicle propulsion systems with kinetic energy recovery. Retrieved From: http://ilot.edu.pl/kones/2011/4_2011/2011_juda_advanced_batteries.pdf