Sizing of Ultracapacitors and Batteries for a High Performance Electric Vehicle

2012 IEEE International Electric Vehicle Conference (IEVC) Sizing of Ultracapacitors and Batteries for a High Performance Electric Vehicle Wilmar Martinez, Member National University Bogota, Colombia whmartinezm@unal.edu.co Camilo Cortes, Member National University Bogota, Colombia caacortesgu@unal.edu.co Luis Munoz Los Andes University Bogota, Colombia lui-muno@uniandes.edu.co Abstract One of the main problems in autonomous electric vehicles is the energy storage, because of the use of low capacity batteries with low power delivery. This work shows a way to deal with the energy storage problem on a high performance electric vehicle capable to run a quarter of a mile in 10 seconds. The concept design of the system combines different storage technologies, ensuring the appropriate power delivery to the motors during the short time needed. In order to achieve this goal, it is necessary to determine the optimum size of each storage element to guarantee the vehicle high performance. This work proposes a methodology for evaluating different characteristics of ultracapacitors and batteries, such as mass, volume and cost. After determining the amount of the storage technologies, it is possible to make an optimal power management in the vehicle, that combined with the control of the motor improves the performance of the electric vehicle. Index Terms High Performance Electric Vehicle, Batteries, Ultracapacitors, Storage System. E I. INTRODUCTION LECTRIC vehicles have large problems of power and autonomy, i.e. they don t have enough torque and speed to supply the daily needs of the population, due to the reduced energy capacity of modern storage technologies devices [1]. Since 2010, National University of Colombia and Los Andes University have been working on a project whose aim is to design and construct a high performance electric vehicle that runs a quarter of a mile in 10 seconds or less. The vehicle under development will be used as a demonstrative platform of the capabilities of the electric vehicles (EV). In the long term, it is expected for the results of the project to be implemented in the development of commercial electric vehicles in Colombia. The project involves different work areas; one of these areas is the electric system, involving storage, control and power conversion methods. The storage system is a critical factor for the autonomy of a commercial vehicle [2], because the motor of the vehicle requires a great amount of power to generate large speed and torque [3]. For the particular case of this high performance vehicle, the storage system is different from the used for traditional applications because it requires a small amount of stored energy in comparison with a commercial vehicle, but it requires considerable power, associated with race conditions. However, the reduced time that the car will be working allows us to use conventional storage systems in this high power application. There are certain factors to consider for enhancing the autonomy of an electric vehicle mainly based on storage technologies. First, it is necessary to use batteries (BT) or a similar system (e.g. fuel cells) with high specific energy, which can provide a considerable amount of energy in a low weight package [4]. Second, the storage system must provide the sufficient amount of power according to the needed torque; as a result, it is necessary to combine batteries with other storage technologies [5]. Ultracapacitors (UC) are ideal for the second condition because they can provide a large amount of power in short periods of time [5] [6]. Consequently, it is necessary to determine the optimal amount of each one of these elements in order to optimize the energy management in the vehicle [7]. In this study, the energy required for a high performance electric vehicle was determined based on a model of the longitudinal dynamics of the vehicle, which also includes efficiency calculations. Using this energy, a study evaluating different combinations of several batteries and ultracapacitors was made, determining what the best option is in order to obtain the energy and power required as function of mass, volume and price. W. Martinez. Electrical and Electronic Engineering Department, National University of Colombia, Bogota. (e-mail: whmartinezm@unal.edu.co). C. Cortes. Electrical and Electronic Engineering Department, National University of Colombia, Bogota. (e-mail: caacortesgu@unal.edu.co). L. Munoz. Mechanical Engineering Department, Los Andes University, Bogota - Colombia. (e-mail: lui-muno@uniandes.edu.co). DOI: 10.1109/IEVC.2012.6183242

II. METHODOLOGY As the high performance electric vehicle should run a quarter of a mile in 10 seconds, the first step was to calculate the mechanic power required to reach the 10 seconds goal. A maximum speed of 212 km/h was obtained at the quarter of mile. The race time was 9.91 seconds. The next step was to set two batteries and two ultracapacitors in four different combinations. The maximum amount of each element in order to supply all the required energy was also calculated, with the purpose of having a base for the analysis. Finally, based on the calculus of the required energy and the properties of batteries and ultracapacitors, the amount of these elements was analyzed considering several features such as mass, volume and cost. A. Energy Calculus The vehicle parameters were defined based on a first geometrical layout. A model of the longitudinal dynamics of the vehicle was developed and used in order to estimate the mechanical energy consumption [8]. The longitudinal dynamics model is based on the characteristics of the selected motors. There are four 50 kw peak PM motor, with a peak torque T m,max=440 Nm and a maximum operative speed of 6500 RPM. The other parameters of the simulation are the vehicle mass m eq (with an initial guess of 750 kg, which includes the equivalent of the rotational inertias), a cross-sectional area A=2.5 m 2, with a drag coefficient C D=0.35 and a rolling resistance coefficient f 0=0.015. The vehicle was simulated with 4 wheels with radius r=30 cm. The gearbox reduction ratio was selected based on the results obtained from a set of simulations. The final reduction was N tf=3.5:1. There was an assumption about the tire characteristics and the traction control, by considering that the tires will have a traction coefficient high enough to provide sufficient traction to transmit the forces to the road with an average longitudinal slip under 5% to consider a whole transmission efficiency of tf=90%. The vehicle is design to run closer to Bogotá, where the air density is =0.9 kg/m 3. dv dt 1 m eq 4T m N r tf tf f F 0 z 1 ACDv 2 The model is a nonlinear ordinary differential equation. It was solved numerically, and the direct result is the speed profile of the vehicle during the race. Based on the postprocessing of that information, the angular speed and torque of each motor can be simulated, and based on those two quantities the instantaneous mechanical power can be computed. Finally, by integrating the power across the time, the mechanical energy is computed. The mechanical energy needed to run the quarter of a mile in 10 seconds is 527 Wh. Fig. 1 shows the results obtained for the speed during the race. 2 (1) Fig. 1. Results of the longitudinal dynamics simulations. Speed vs. time during the race. B. Ultracapacitors When sizing ultracapacitors, it is necessary to know the implications of several factors that affect the storage system. These factors include: capacitance, specific energy, peak current, mass, volume and price [7]. Some ultracapacitors were studied taking into account those factors. Two ultracapacitors were selected, one of them is a small unit and the other is a module. Table I shows the different specifications of ultracapacitors, UC 1 is a single small ultracapacitor and UC 2 is a module. These ultracapacitors have a high peak current, between 700A and 1900A, which is enough to supply the required energy in 10 seconds. TABLE I SPECIFICATIONS OF ULTRACAPACITORS C. Batteries Item UC 1 UC 2 Voltage [V] 15 128 Nominal Capacity [mah] 28.1 1120 Energy [Wh] 0.42 143.4 Peak Current [A] 730 1900 Capacitance [F] 20 63 Volume [L] 0.314 69.7 Mass [Kg] 0.23 60.5 Price [USD] 800 10400 There are several kinds of batteries: lead-acid, nickel cadmium, nickel metal hydride, lithium ion, lithium polymer, etc. Each kind of battery has different advantages and disadvantages [5]. Lithium ion technology is a good option according to the requirements of the project. This kind of battery has a higher specific energy as well as power density

and its price is appropriate for the overall budget. Table II shows a comparison of several battery types based on different power and energy features [9]. Based on this table it is possible to make the Ragone Chart shown in Fig. 2 [10]. It is possible to conclude that Zn Air Battery is one of the batteries with more specific energy, but its specific power is low. On the other hand, Li Ion Battery is a good option because it has high specific energy and power. In addition, Lithium is not a toxic element; meanwhile elements like Lead and Cadmium are very toxic for the environment. TABLE II SPECIFIC POWER AND ENERGY OF DIFFERENT BATTERY TYPES [9] Specific Power [W/kg] 2000 1600 1200 800 400 Item Specific Energy [Wh/kg] Specific Power [W/kg] Pb 30-25 80-300 NiZn 80-70 200 NiCd 60-50 200-500 NiMH 60-70 200-1500 NaNiCl 125 150 ZnBr 200-300 70 Li Ion 60-200 80-2000 Zn Air 200-300 70 Ultracapacitors 2-5 1000-2000 Pb NiCd NiZn NIMH NaNiCl ZnBr Li Ion Zn - Air Ultracapacitor Item BT 1 BT 2 Voltage [V] 4 20 Nominal Capacity [mah] 90 2000 Energy [Wh] 1.08 64 Peak Current [A] 5 20 Volume [L] 0.715 55.9 Mass [Kg] 3 37 Price [USD] 126 2800 III. ANALYSIS The first step was to calculate the energy that each of the elements (batteries and ultracapacitors) can deliver, based on their nominal capacity and operation voltage. Then, the amount of each element necessary to supply the required energy was estimated. This way, it was possible to calculate the maximum and the minimum amount of batteries and ultracapacitors needed for the application. Fig. 3 shows the amount of each element needed to supply the required energy. With these elements, different combinations of batteries and ultracapacitors were made, defining several scenarios. In the first scenario, the energy is supplied by batteries; in the second scenario, the energy is supplied by ultracapacitors; and in the third scenario, ultracapacitors and batteries deliver energy in different combinations. 0 0 50 100 150 200 250 300 Specific Energy [Wh/kg] Fig. 2. Specific power and energy of battery types [10] Different Li-Ion batteries available in the market were studied, taking into account the specific energy, peak current, mass, volume and price. Two batteries were selected, one of them is small unit (BT1) and the other is a module (BT2). Table III shows the specifications of batteries. These Li-Ion batteries have a nominal capacity between 90Ah and 2000Ah, these are good capacities compared to other battery technologies. This contributes to the storage system because there is a large amount of available energy. However, there is a critical factor with these batteries: they have a peak current between 5A and 20A. This fact produces a large constraint, since the energy stored in the batteries cannot be delivered in a short time. As a result, it is necessary to combine them with another storage technology. TABLE III SPECIFICATIONS OF BATTERIES A. Mass Fig. 3. Maximum amount of BT and UC Fig. 4 shows the simulations for the mass of each combination versus the number of the respective UC, i.e. Y- axis corresponds to mass in kg and X-axis corresponds to the amount of ultracapacitors. The red line represents the mass of batteries, the blue line represents the mass of ultracapacitors, and the green line corresponds to the total mass of the combined components. The intersection of blue and red lines corresponds to the case where each item provides half of the system mass. It is observed that the mass of ultracapacitors is lower than the mass of batteries, this is a great advantage for the weight of the vehicle, because this has a positive impact on the longitudinal dynamics ensuring a better time in the race. Taking this fact into account, it is possible to conclude that the best combination is UC2 - BT2 having a mass between 220

kg and 300 kg. The lower mass is due to the fact that these elements are modular. The UC1 - BT2 combination is also a good possibility, which means that single small batteries are not a good option for electric vehicles of high power. Moreover, the system mass decreases in every case as the number of ultracapacitors increase. Therefore, the best solution - in mass terms - for the storage unit of the high performance electric vehicle is to use 4 UC2 ultracapacitors to supply the 530 Wh energy, having a mass of 224 Kg. Fig. 4. Mass simulations. a) Combination UC1 with BT1, b) Combination UC1 with BT2, c) Combination UC2 with BT1, d) Combination UC2 with BT2 B. Volume Fig. 5 shows the simulations of volume versus quantity of ultracapacitors. It can be determined from the plots that the UC2 BT1 is the best combination with a volume between 250 ml and 350 ml. However, it can be noticed that all UC-BT combinations are viable, because their volume behavior is similar, i.e. the differences of volume from one combination to another have no significant influence on the vehicle performance. Nevertheless, the best solution in volume terms is to use 4 UC2 ultracapacitors, having a volume of 258 ml. C. Cost Cost is a critical criterion in this methodology of sizing batteries and ultracapacitors for a high performance vehicle, because ultracapacitors are expensive in comparison to lithium ion batteries. Fig. 6 shows the simulations of ultracapacitor - battery combinations for the cost analysis. These simulations can be divided into two groups. First, the combination that contains ultracapacitors type UC1 (the single small ultracapacitor) have a great cost increase when the number of UC increases. Moreover, this system cost is higher than the one from UC2 combinations, which means that single small ultracapacitors are not a good option for electric vehicles of high power. Second, combinations with UC2 present a lower cost. In the case of the UC2 BT1 combination it can be noticed that cost decreases when the number of UC increases, this is due to the fact that for lower quantities of UC1 it is necessary to have a higher quantity of batteries, producing a high cost for the storage system. Finally, UC2 BT2 is the best combination, having a cost between USD23000 and USD38500. However, in opposition to the mass and volume simulations, the best solution in cost terms is to use 8 BT2 batteries with a cost of USD23000.

Fig. 5. Volume simulations. a) Combination UC1 with BT1, b) Combination UC1 with BT2, c) Combination UC2 with BT1, d) Combination UC2 with BT2 Fig 6. Cost simulations. a) Combination UC1 with BT1, b) Combination UC1 with BT2, c) Combination UC2 with BT1, d) Combination UC2 with BT2

D. Review Table IV shows the review of mass, volume and cost analysis. It is possible to conclude that combinations containing single small units of ultracapacitors or batteries are not viable for the project, because it is necessary to have a great quantity of units to provide the required energy, which produces elevated mass and higher cost. As a result of this methodology, it was concluded that the best combination is the UC2 BT2, i.e. the best elements for storage systems in a high power vehicle are the modular ones. Two kinds of elements were evaluated: single small units and modular units. From mass and volume analysis it was established that the single small batteries are not a good option, and based on cost analysis, it was established that single small ultracapacitors are not viable. Modular units are the best choice for storage systems in high performance vehicles. The optimal solution, in mass and volume terms, is to use only ultracapacitors, due to their characteristics of low mass and volume, but in cost terms, ultracapacitors are viable but more expensive solution. TABLE IV ANALYSIS REVIEW MASS VOLUME COST UC1 BT1 X X UC1 BT2 UC2 BT1 UC2 BT2 X However, each type of analysis has its own best solution: in mass and volume terms the best option is to have 4 UC2 ultracapacitors. Meanwhile, in cost terms the best solution is to have 8 BT2 batteries. IV. CONCLUSION This paper shows a simple methodology for sizing ultracapacitors and batteries in a high performance electric vehicle. This methodology allows establishing a correct quantity of ultracapacitors and batteries to supply the necessary energy to run a quarter of a mile in 10 seconds or less, considering three criteria: mass, volume and cost of the storage system. X REFERENCES [1] J. Dixon, M. Ortúzar, and E. Wiechmann, Regenerative Braking for an Electric Vehicle Using Ultracapacitors and a Buck-Boost Converter Catholic University of Chile. pp. 1-4. 2008. [2] C. Bonfiglio and W. Roessler, A Cost Optimized Battery Management System with Active Cell Balancing for Lithium Ion Battery Stacks, Vehicle Power and Propulsion Conference, VPPC 09, pp. 304-309. 2009. [3] C. Shumei, L. Chen and S Liwei, Study on efficiency calculation model of induction motors for electric vehicle, IEEE Vehicle Power and Propulsion Conference, VPPC 08, pp. 1-5. 2008. [4] I. Aharon and A. Kuperman, Topological Overview of Powertrains for Battery-Powered Vehicles with Range Extenders ieee transactions on power electronics, Vol 26, no. 3, pp. 867-870. 2011. [5] A. Burke, Batteries and Ultracapacitors for Electric, Hybrid, and Fuel Cell Vehicles, Proceedings of the IEEE, Vol 95, pp. 808-811. 2007. [6] A. Burke, Ultracapacitors: why, how, and where is the technology, Journal of power sources, vol. 91, pp. 37-50, 2000. [7] J. Dixon, I. Nakashima, E. Arcos and M. Artuzar, Electric vehicle using a combination of ultracapacitors and zebra battery, IEEE transactions on industrial electronics, Vol 57, pp. 943-947. 2010. [8] T. Gillespie, Fundamentals of vehicle dynamics, 1 st Ed., SAE International, 1992. [9] P. Bossche, F. Vergels, J. Mierlo, J, Matheys and W. Autenboer, SUBAT: An assessment of sustainable battery technology, Journal of Power Sources (2005), Vol 162. Pp. 913-919. 2005. [10] International Energy Agency IEA, Technology Roadmap Electric and plug-in hybrid electric vehicles (EV/PHEV), pp. 10-14. 2011. It was established that a 530 Wh stored energy is needed in order to run a quarter of a mile in 10 seconds with an electric vehicle of 750 Kg with four 50 kw peak PM motor, a peak torque of 440 Nm and a maximum operative speed of 6500 RPM. Using this energy calculus it was possible to perform simulations with the purpose of determining the kind and quantity of ultracapacitors and batteries. However, there is a limitation with this approach: a constant mass was used for the energy calculus, but it is clear that the mass will vary depending on the case of storage system. Nevertheless, this simplification allows finding an initial solution for the problem of sizing a system of batteries and ultracapacitors.