Application of Mullti-Port Power Electronic Interface for Contactless Transferr of Energy in Automotivee Applications Mattheew McDonough, Pourya Shamsi, Babak Fahimi University of Texas at Dallas
[email protected]
Abstract- Plug-in Hybrid Electric Vehicles (P PHEV) and Electric Vehicles (EV) are gaining popularity due to political, environmental, and economical reasons. Ressearch is being done to charge these alternatively fueled veh hicles by means of Inductively Coupled Power Transfer (ICP PT). In this paper authors present a system to use a Multi Porrt Power Electronics Interface (MPEI) to control power flow from m/to several AC and DC sources/sinks within a PHEV or EV, especially including T). This paper shows Inductively Coupled Power Transfer (ICPT potential reduction in mass and improvemeent in driving range and fuel economy due to the use of a novel ch harging strategy.
I.
INTRODUCTION N
Plug-in Hybrid Electric Vehicles (P PHEVs) are widely accepted as effective solutions to provide a near term reduction to urban air quality probblems, fossil fuel dependence, and climate change cauused by Internal Combustion Engine Vehicles (ICEVs). Electric Vehicles (EV) offer a solution to the same issues. However, consumer’s range anxiety must be addressed before EVs are widely accepted. Range anxiety can be solved in two ways. w The method in the forefront of research is increasing the t energy storage capacity of the vehicle. This involves adding expensive, large, and heavy batteries to an already heeavy and expensive vehicle, relative to its ICE counterpart. An A alternative is to decrease the effective range between chaarges. This can be done with Inductively Coupled Power Transfer (ICPT). Vehicle charging using ICPT is proposed in [1]-[3]. This paper discusses the advantages of using a Multi-port Power Electronic Interface (MPEI) [4] to activelyy manage power to and from multiple sources. In this case thhe system contains (a)power grid interface via ICPT, (b)an ultracapacitor u pack, (c)a reduced size battery pack, and (d)mootor/generator unit. The MPEI, when combined with the approopriate batteries and ultracapacitor packs, is able to accept (and potentially broadcast) energy while in motion. Thiss will increase the overall range before a vehicle has to stopp to charge or even eliminate the need to stop and charge all toogether. MPEI coupled with ICPT can servee as a home based charger or bi-directional energy storage syystem for home use, as well. A typical, vehicular MPEI is show wn in Fig. 1. In this figure the MPEI serves as a smart controlller which transfers energy in a bidirectional manner to and from the grid via ICPT, the ultracapacitors, the batteryy pack, and the motor/generator. This configuration will accommodate the regenerative braking as well.
Fig. 1. Multi-Port Power Electronics Interface (MPEI) Within an Electric Autom mobile
ICPT is a way to transfer power wirelessly through the use of an air-core transformerr formed from a primary coil (typically buried in the drivingg surface) and a secondary coil (attached to the bottom of the vehicle). Power is taken from the grid and converted to DC C and then back into AC at judiciously chosen high frequuency. Fig. 2 shows a typical topology. The high frequency AC vooltage source supplies the ICPT primary coil with an AC curreent. This current generates an alternating magnetic flux, of which w part will be captured by the secondary coil. The MPEI will w use the induced voltage to charge the onboard batteries annd ultracapacitors, or will use the energy directly to propel thhe vehicle. Figs. 3 and 4 show two different types of ICPT connsidered in recent publications.
Fig. 2. Typical ICPT I Topology.
Fig. 3. Square Primaryy and Secondary Coil
Fig. 4. Track Type Primary Cooil and Square Secondary Coil
978-1-61284-246-9/11/$26.00 ©2011 IEEE
II. OUTLINE OF PROBLEM Battery characteristics from popular Extended Range EV (EREV), EV, and HEV models, respectively, are summarized in Table I. The added mass to convert a HEV into an EREV, PHEV, or EV partially negates the added benefit of being able to plug it in. Equations (1) – (4), from [5] show the direct dependence upon mass for three of the four factors that are summed for power (and hence energy) requirements of a vehicle. Here, V, , Ff, Fw, Fg, Mv, , g, fr, , pa, Cd, Af, and Vw are velocity of the vehicle, efficiency from power plant to wheels, force of friction from the road surface, force due to wind, force due to an inclined surface, mass of the vehicle plus mass of the load, mass factor (a constant to include the inertia of rotating components), gravitational acceleration constant, coefficient of rolling friction between the wheels and the road surface, angle of inclination of the road surface, density of the air, aerodynamic drag coefficient of the vehicle, frontal area of the vehicle, and the velocity of the wind, respectively.
III.
TECHNICAL OBJECTIVES
A. Optimal Ultracapacitor Capacity Selection The first objective of this study is to choose the optimal capacity for ultracapacitors in an ICPT based (P)HEV. The first step involves selection of a model vehicle and analyzing its power and energy requirements in various driving cycles. B. ICPT System Modeling & Optimal Battery Capacity Selection The second objective is to design an ICPT system to increase the range of the vehicle. A method for designing the system is discussed in [9] and was partially used in this study. The size of the battery should be chosen to store enough energy to propel the vehicle through a given segment/s of driving cycles to meet the required range.
C. MPEI’s Application in ICPT Powered Vehicles The fourth objective is to show the effectiveness of the MPEI system in automotive applications, specifically in applications with ICPT. MPEI is designed to control power flow in a bidirectional manner from multiple sources. MPEI is optimally designed to process the power to/from the ICPT (1) system, to/from the motor, to/from the battery and to/from the ultracapacitors. An MPEI is tested with ultracapacitors and (2) batteries to prove its ability to manage power from multiple sources with different control time constants. (3) D. Resulting Weight Reductions and Improved Range (4) The final objective is to demonstrate improved efficiency from weight reduction and improved range from the transfer These equations show that mass of the vehicle is a very of energy through the ICPT system. important factor for efficiency when efficiency corresponds IV. TECHNICAL APPROACH & RESULTS to the main objective in transportation of a load. With consumers expecting longer all electric range from EVs and A. Optimal Ultracapacitor Capacity Selection The software package, Cruise by AVL [10], was chosen PHEVs, battery packs are getting larger and cars are getting heavier. A new way to power vehicles is needed to reverse to do the vehicle simulations. The standard Cruise EV model this trend. ICPT has the ability to do this by continually with parameters modified to match a popular EV was used as powering the vehicle while it is performing its desired the baseline for comparison. The vehicle specifications in Table II were input to the function. A MPEI is needed in the vehicle to manage power Cruise software in combination with default specifications in distribution from the ICPT system to the drive shaft and the software. The FTP75 urban driving cycle was analyzed. energy storage devices, i.e. battery and ultracapacitors. While The profile is shown in Fig. 5. there has been some work published on system level control TABLE II of power in a (P)HEV [6]-[8], ICPT adds challenges that Vehicle Model Specifications have not been studied. ICPT will supply short bursts of power Weight 1587 Kg that will need to be stored efficiently. This will mean a larger Battery Type Li-ion ultracapacitor pack to accept more energy in a short period of Battery Capacity 24 kWh # of Battery Cells 192 time. The power handling capability of some parts will be Volts/Cell 4 V higher. MPEI will work well for ICPT applications. Motor Power
80
TABLE I Data for a Popular PHEV, EV, and HEV EREV EV HEV Battery Type Battery Capacity (kWh) Battery Mass (kg)
Li-ion
Li-ion
NiMH
16
24
1.31
170
300
29
Price Estimate ($)
~10000
~12000
~2500
Fig. 5. FTP75 driving Cycle
kW
The current requested from the battery (the only energy storage system in this simulation) was stored and is shown in Fig. 6. This current has many spikes and high magnitudes that reduces the life of the battery pack while increasing the size of the battery pack. The current data from Cruise was filtered to remove the high frequency contents and was clipped at 50 A to keep the battery current at a sufficiently low level. The filtered current is shown in Fig. 7. The remaining current is shown in Fig. 8 and will be provided by the ultracapacitors. The maximum power rating is determined by the largest current spike according to (5), where P, Imax, and Vmot are the power of the source, the maximum current requested from the source, and the voltage of the motor, respectively. For the vehicle chosen in this research, the ultracapacitor bank needs to be capable of delivering 25 kW. This value should be compared by the power absorption during regenerative braking and power broadcast from ICPT to select the final size. Notably the regenerative braking and power broadcast may occur simultaneously. (5) The ultracapacitor pack’s energy level should be managed by the MPEI. The ultracapacitor pack’s energy content should be divided into four sections as demonstrated by Fig. 9. The lowest energy section, one, defines the minimum SOC of the capacitor. Capacitors are safe to discharge completely; however, because the ultracapacitors voltage is a function of the energy content, at low energy states, the voltage is low. To draw reasonable amounts of power from the capacitors at low energy states requires unreasonably high currents. Therefore, the ultracapacitors should remain above section one
Fig. 6. Current Request By EV Motor
Fig. 7.Current to be Provided by Batteries
Fig. 8. Current to be Provided by Ultracapacitors
Section two would be the area during which the ultracapacitor would need to be recharged so that it can provide power for the next acceleration stage. Section two must be large enough to provide the energy that is consumed by the largest negative spike in Fig. 8. This occurs around 200 seconds. The energy in this spike is found by (6), where V and Iuc represent motor voltage and motor current drawn from the ultracapacitors, respectively. Here the range of integration, tmin to tmax is 188s to 205s. The energy calculated in this area is 62 Wh. (6) Section three should be chosen based on the choice of the ICPT track and could be reduced to zero if ultracapacitor size is of concern from a cost or weight point of view. This part would be able to store excess energy from the ICPT track to be either sent to the batteries or to be used in the vehicle. Section four should be sized according to the same logic as section two except for positive spikes in current. For simplicity and symmetry, the size of section four is chosen to be equivalent to section two. MPEI should be used to keep the ultracapacitors primarily in section 3. When the ultracapacitors are in section two, MPEI should charge them up to section three, assuming the batteries can provide the current to do so. This allows the ultracapacitors to provide energy for the next acceleration phase. When the capacitors are in section four, MPEI should use the energy in the ultracapacitors to charge the batteries until the ultracapacitors reach section three. This gives the capacitors room to store energy from the next ICPT charging phase or the next regenerative braking phase. The total ultracapacitor energy content found by this method for the vehicle under simulation should be no less than 124 Wh. B. ICPT System Modeling & Optimal Battery Capacity Selection As previously mentioned a method for designing the ICPT system is discussed in [9] and was partially used in this research. This method uses a distinct algorithm to determine a speed level where the vehicle is being charged by the ICPT system if it is under this speed. This would be analogous to installing the primary coils in areas where the vehicles are likely to be traveling slower (i.e. low speed limit areas, traffic lights, stop signs). Because the FTP75 driving cycle represents an urban environment, there are many times in which the vehicle is stopped. This produces unexpected results when the algorithm described was used with the simulation results. A speed level of 1 km/hr can be chosen and the vehicle can be fully charged at the end of the FTP 75 cycle if an ICPT system with a power level of 40 or 60 kW is chosen. When a 20 kW ICPT system is chosen, the vehicle recovers most of its energy used during the driving cycle. The flow diagram in Fig. 10 shows the algorithm used for the current research. The ICPT design used in the remainder of the paper assumes a 20 kW charger operating at times in which the vehicle is stopped or operating at below 1 km/hr.
Fig. 9. Regions of Charge of Ultracapacitor Pack
Fig. 11 MPEI for ICPT based Automobile Applications
Fig. 10. Flow Diagram for ICPT Charging Control. E=Current SOC of Energy Storage System Emax= Max SOC of Energy Storage System V=Velocity of the Vehicle
Table III summarizes the amount of energy used to propel the vehicle through the FTP75 driving cycle given the designed charging system. Row 1 represents one FTP75 cycle. The unmodified vehicle could propel itself through multiple FPT75 cycles, up to 157 km where its energy was depleted. Row 2 shows the energy storage required to match the original vehicle’s range. The battery should be chosen to match this range. Therefore, for the remainder of the research, the battery capacity will be 3.5 kWh. C. MPEI’s Application in ICPT Powered Vehicles MPEI, as described in [11]-[13], was originally designed as a multi-input, multi-output power electronic interface for stationary energy applications. However, because of the multi-input, multi-output similarities, it’s technology readily adapts to automotive applications. MPEI is made of multiple switch legs. These legs boost sources to the DC bus voltage sourcing power to the DC bus, and buck the DC bus voltage down to the levels tolerable by the output, sinking power from the DC bus. When the sources and sinks balance, stability is achieved in the MPEI. The mobile MPEI will be primarily made of one leg for each DC source (batteries and ultracapacitors), two legs for the ICPT system, and a three legs for the traction motor. In total, 14 power MosFets control power flow into and out of each sink/source, hereafter refered to as a source. The configuration is shown in Fig. 11. TABLE III FTP75 Cycle Energy Usage Energy Use at Maximum State Distance End of Cycle of Depletion 12 km 0.40 kWh 0.67 kWh 157 km 2.66 kWh 3.13 kWh
This configuration requires a common ground and creates a common DC bus. The control system in the MPEI actively manages power flow into and out of each source by varying the duty cycles on the respective leg(s) of the source(s). This has already been well proven in [11]. Fig. 12 shows MPEI monitioring battery conditions and maintaining a stiff DC bus while driving a three phase motor. For MPEI to meet the requirements of this paper, it must be able to determine source charging and discharging currents based on prewinding time constants. To test this, a current profile was generated to simulate motor current. The MPEI provides that current from the DC bus. When the DC buss voltage begins to fall, it creates a current request to the available sources. These sources respond according to their prewinding time constants. Fig. 13 shows the MPEI feeding the motor current request with currents from multiple sources at different response time constants. This current request profile (dark blue) is being quickly tracked by the magenta, representing the ultracapacitors. The battery current (aqua) adapts more slowly to protect the battery from current spikes, extending its lifetime.
Fig. 12. MPEI Current Waveforms Blue- Motor Current; Aqua-DC Bus Voltage; Green-Battery Voltage Magenta-Battery Current
The original vehicle consumed 1.87 kWh over one FTP75 cycle. A new simulation was performed with an identical vehicle, but at the reduce weight. The reduced weight vehicle consumed 1.63 kWh over the same cycle. The overall energy savings is .24 kWh per cycle, or 13% decrease in energy usage. The range of the vehicle being charged by a 40 or 60 kW system in the FTP 75 cycle is extended indefinitely. The range of a vehicle being charged with the 20 kW ICPT system will be more than 8 times greater. Table VI summarizes the results of this study.
Fig. 13. Multi-Input Current Waveforms and Desired Output Current Blue-Current Request Simulating Motor Current Aqua-Low Pass Filtered Current Provided by Batteries Magenta-Remaining Current Provided by Ultracapacitors
V. WEIGHT REDUCTION AND RANGE EXTENSION Table IV, partially derived from [14], shows some popular battery packs chosen for EVs and HEVs as well as the specifications from the EV that was used as the base model for this study. It also shows the required energy, power, and weight for the given battery pack. The battery pack from the chosen EV under study closely matches the best suited battery from Shin-Kobe. The weight of the battery pack required is 55 kg. The required energy from the ultracapacitor pack is 245 Wh at a maximum of 25 kW. Table V, shows the specifications of commercially available ultracapacitor [15]. Therefore, the mass of the ultracapacitor system would be 28 kg. The peak power of the entire system is 80 kW. The accepted weight of automotive power electronics is 5 kW/kg [16]. The total weight for the power electronics converter is approximately 16 kg. The weight of the battery pack, ultracapacitors and power electronics total to 99 kg. The original EV under consideration has a battery pack weighing 300 kg, while the vehicle, in total, weights 1587 kg. This is a weight reduction of 201 kg or 13%. TABLE IV Popular Batteries and Their Specifications ApplicW/kg 95% Manufacturer ation Ah V Wh/kg eff. Saft HEV 12 4 77 256 Saft EV 41 4 140 90 Shin-Kobe EV 90 4 105 255 Shin-Kobe HEV 4 4 56 745 Popular EV EV 80 266 Required Wh 3500 Required W 13300
VI. CONCLUSIONS ICPT is an effective way to increase the range and decrease the energy consumption (by reducing the weight of the vehicle) in EVs. With even relatively low power ICPT systems (20kW) vehicle range can be increased over 700% in urban driving cycles. This helps reduce range anxiety in consumers and provides greater incentive for green transportation. Aside from range increases, efficiency increase by 13% due to reduced weight. MPEI is an effective solution to managing power to and from multiple input-output ports in automotive application, especially applications involving ICPT. Together, these two technologies can bring significant improvements to electrified transportation. Future work should consider the challenges of highway driving and the challenges of integrating ICPT systems into Cruise. TABLE VI Summary of Results Original Modified EV EV Battery & Ultracapacitor Weight Total Vehicle Weight Energy Use per FTP75 Cycle Range with 20 kW ICPT Charging
VII. [1]
Maximum mass (kg) 227 148 52 347 55
[2]
[3]
[4] TABLE V Ultracapacitor Specifications Wh/kg W/kg Wh/kg needed W/kg Needed 4.5* 124 5900 25000 *Calculated from Vrated to ½ Vrated.
Maximum Mass (kg) 28
[5] [6]
[7]
300 kg
% Reduction
99 kg
67%
1587 kg
1386
13%
1.87 kWh
1.63 kWh
13%
157 km
> 1256 km
>700%
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