Novel power management for high performance and cost reduction in an electric vehicle

Novel power management for high performance and cost reduction in an electric vehicle

Renewable Energy 22 (2001) 177±183 www.elsevier.com/locate/renene Novel power management for high performance and cost reduction in an electric vehic...

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Renewable Energy 22 (2001) 177±183 www.elsevier.com/locate/renene

Novel power management for high performance and cost reduction in an electric vehicle Xinxiang Yan*, Dean Patterson Northern Territory Centre for Energy Research, Faculty of Technology, Northern Territory University, Darwin, NT 0909, Australia

Abstract This paper presents a novel power management scheme to achieve high performance and cost reduction in an electric vehicle for short pro®le ¯eet application. The measured drive cycle of an internal combustion engine meter-reading vehicle has been analyzed for the optimization of the EV system design and power management. Zinc Bromine batteries will be employed to provide the continuous power for normal driving while ultracapacitors will be employed to provide peak power demand during acceleration and to take regenerative braking energy eciently during deceleration. The EV motor operates in constant torque mode at motor speeds below the base speed and in constant power mode at motor speeds over the base speed for high eciency and low cost. 7 2000 Elsevier Science Ltd. All rights reserved.

1. Introduction Electric vehicles are one of the promising solutions to urban pollution that is caused by conventional internal combustion engine (ICE) automobiles. Further, when electricity is generated from natural gas as it is in Darwin, Australia, substantial green house gas emission reduction is possible. Of all the technologies used in electric vehicles, the battery remains the main barrier to success. This is * Corresponding author. 0960-1481/01/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 1 4 8 1 ( 0 0 ) 0 0 1 0 3 - 8

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because the energy density, power density, lifetime, weight, volume, cost of the battery needs to be compromised, which has proved a very challenging task. The Northern Territory Centre for Energy Research is currently undertaking an electric vehicle research project to develop an urban EV for electricity and water meter-reading purposes, which is supported by the Northern Territory Power And Water Authority in Darwin, Australia, and an ARC SPIRT grant. This research addresses power management for performance optimization and cost reduction of the electric vehicle.

2. The drive cycle of an ICE meter-reading vehicle To optimize the design of the meter-reading EV, the drive cycle of an ICE meter-reading vehicle has been obtained. A data logging system was developed and installed in the conventional ICE meter-reading vehicle. It collected the data of the vehicle velocity, gradient of the roads, and the braking command during duty of the vehicle. The meter-reading vehicle does two typical jobs, `commercial' meter reading and `domestic' meter reading. When doing `domestic' meter readings, the driver drives to residential areas, stops the vehicle and walks to read meters from house to house. When doing `commercial' meter readings, the driver drives in urban business areas and reads meters from business to business, with the engine on most of the time. Figs. 1 and 2 show the typical daily drive cycles for `domestic' meter readings and `commercial' meter readings. The drive cycles obtained have indicated that this electric vehicle will be similar to a typical urban EV. That is, the meter-reading vehicle runs mostly in town at low speeds and runs at high speeds in highway for some time. Therefore, the research on this meter-reading EV will have great signi®cance to general application EVs.

Fig. 1. Typical daily drive cycle for the `domestic' meter readings.

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Fig. 2. Typical daily drive cycle for the `commercial' meter readings.

3. Analysis of the measured driving cycles Analyzing the real drive cycles of the meter-reading vehicle has resulted in many meaningful numbers, as shown in Table 1. It is shown that the average speeds are quite low and the engine idling time accounts for a considerable portion of the engine-on time, especially when doing `commercial' meter reading, up to 41%. When an ICE vehicle runs at very low speeds or idles, its eciency is extremely low while an electric vehicle can o€er much higher eciency than an ICE vehicle in such cases. It is also shown that braking was performed quite frequently. The ratio of the braking time over the vehicle moving time was very high, up to 19%. This implies a huge amount of regenerative braking energy available, which can contribute a lot to energy saving and consequently the extension of the drive range. These numbers con®rm that the meter-reading vehicle is a typical electric vehicle application. From the drive cycles shown previously we can see that the meter-reading vehicle needs to start and stop very often. This implies high peak power demand while the average power usage is much lower. A novel energy management has

Table 1 Analysis results of the typical daily drive cycles

Number of braking Total braking time (s) Total vehicle moving time (s) Total engine-on time (s) Idling time (s) Braking time over vehicle moving time (%) Idling time over engine-on time (%) Driving distance (km) Average speed (km/h)

`Domestic' meter reading

`Commercial' meter reading

189 788 8068 9279 1211 9.76 13.1 108.9 48.6

454 1616 8242 14059 5817 19.61 41.38 73.26 32

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been employed in the Meter-reading EV design, which can signi®cantly improve the vehicle's performance and reduce its cost, as described below.

4. Short term power management Currently the signi®cant problems with EVs compared to ICE vehicles are their relatively short drive range per charge and their high cost. These are determined by very large, expensive, heavy and short-lived battery packs. Lead acid batteries are cheaper than alternative technologies but have low energy density. Some advanced batteries such as NiMH and NiCD, do have higher energy density than the lead acid battery, however, they are extremely expensive. Automakers are paying exorbitant sums for the advanced batteries they use in the current generation of electric vehicles, over US $30,000 per unit. This situation makes it very dicult for EVs to be widely accepted and used. Battery performance is a question of tradeo€s, or a matter of optimization [1]. This is because the three leading performance criteria of a battery, energy, power and life are quite strongly related to each other. Generally speaking, improvements in any one come at the expense of one or both of the others. For example, the common way to improve the power performance of a battery is to use thinner electrodes, but this usually lowers both the energy density and the life expectancy. The Zinc Bromine battery, provided by ZBB in Western Australia, is one of a number of emerging battery technologies. It features: 1. 2. 3. 4. 5.

twice the energy density as lead acid battery; low cost, no more than a lead acid battery; long lifetime, up to 2500 cycles; recyclable; low power density and high impedance.

For a high-performance vehicle, the required peak power is many times the required average power. The Meter-reading EV will employ a short term power unit that consists of ultracapacitors and a bidirectional dc±dc converter in the driveline to load level the battery, and therefore the peak power requirements for the battery can be signi®cantly reduced [2±6]. This o€ers the opportunity to design and use EV batteries that are optimized for energy density, life and low cost with less attention being given to the peak power, such as Zinc Bromine battery which will be used in the meter-reading EV. Application of Zinc Bromine batteries in EVs has been reported in few publications. In fact, the ®rst four features of the Zinc Bromine battery listed above make it very suitable for EV application if the problem of low power density can be overcome. The battery energy density and peak power density requirements are decoupled in the meter-reading EV by employing the short term power unit to load level the battery. Fig. 3 shows this power management scheme. The peak power during acceleration is supplied by the ultracapacitors through a dc±dc converter while the continuous power for

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normal driving, which is relatively low, is supplied by the batteries. The batteries charge the ultracapacitors when the power demand of the electric vehicle is low. Energy recovery during regenerative braking is an important contributor to improvement in fuel/energy economy, especially for EVs in city driving. With their quick recharge capability, the ultracapacitors capture regenerative braking energy more eciently than the batteries, extending the range of the Meter-reading EV. This arrangement will make the Meter-reading EV a high-performance and lowcost electric vehicle. 5. Constant power operation of brushless dc machine for propulsion at high speed An axial ¯ux permanent magnet brushless dc machine (BDCM) is a natural choice for EV applications owing to its very high eciency, high reliability and easy control. The desired torque-speed characteristic for traction application is constant torque below the base speed of the motor, which is determined by the battery voltage, and constant power over the base speed that implies a reduction in the achievable torque as speed increases (Fig. 4). This torque-speed characteristic will make the motor highly ecient at high speed, minimizing the power rating of the motor and consequently, the cost, weight, and volume of the whole propulsion system. The single signi®cant problem with permanent magnet motors is their speed limitation, determined by battery voltage. The Meter-reading EV propulsion system employs an axial ¯ux permanent magnet brushless dc machine. For the constant power operation above the base speed, the battery voltage is boosted to a higher voltage through a boost converter. While the motor voltage becomes higher, the motor current becomes less. Therefore, constant power operation can be achieved.

Fig. 3. Short term power management scheme.

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Fig. 4. Optimized motor torque-speed characteristic.

It should be noted that the constant power operation of the brushless dc motor requires a bidirectional dc±dc converter with the battery connected to the lowervoltage side and the motor/inverter to the higher-voltage side. Bidirectional operation is necessary for the dc±dc converter due to the need for regenerative current control when the motor back EMF is higher than the battery voltage. On the other hand, the short-term power management requires another bidirectional dc±dc converter with the ultracapacitors connected to its lower-voltage side and the battery to its higher-voltage side. It is desirable to use a single bidirectional dc±dc converter for both the short-term power management and the constant power operation. A novel multi-functional dc±dc converter has been proposed for this application. The detailed operation principle of this multi-functional dc±dc converter is discussed in Reference 6.

6. Conclusions The real drive cycle of the ICE meter-reading vehicle has been obtained and analyzed. It veri®es the necessity and signi®cance of a good power management in an electric vehicle. Zinc bromine batteries and ultracapacitors will be employed in the meter-reading EV. While the batteries provide the continuous power for normal driving, the ultracapacitors provide the peak power demand for acceleration and take regenerative braking energy eciently. The EV motor operates in constant torque mode at motor speeds below the base speed and in constant power mode at motor speeds over the base speed. Such a novel power management can optimize the EV system performance and reduce its cost signi®cantly.

Acknowledgements The authors would like to thank Mr. John Swenson at the Northern Territory

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Centre for Energy Research for his advice and help in developing and installing the data logging system. References [1] Gary L Hunt. The great battery search. IEEE Spectrum, November 1998. [2] Burke AF, Hardin JE, Dowgiallo EJ. Application of ultracapacitors in electric vehicle propulsion systems. In: Proceedings of the 34th International Power Sources Symposium. 1990. p. 25±8. [3] Di Napoli A, Carioohi F, Crescimbini F. Ultracapacitor based bidirectional dc±dc converter prototype for recovery of the braking energy in EV motor drives. In: EPE '95. 6th European Conference on Power Electronics and Applications, Belgium. 1995. p. 19±21. [4] Nobuyuki Kasuga, et al. Ultra-capacitor and battery hybrid EV with high eciency battery load leveling system 1998;EVS-15. [5] King RD, Schwartz J, Cardinal M, Garrigan L Salasoo. Development and system test of a high eciency ultracapacitor/battery electronic interface 1998;EVS-15. [6] Xingiag Y, Patterson D, Kennedy B. A multi-functional dc±dc converter for an EV drive system. In: Proceedings (CD) of the 16th International Electric Vehicle Symposium and Exhibition (EVS 6), Beijing, China, 1999.