Experimental Results of Onboard Battery-Ultracapacitor System for Electric Vehicle Applications

Experimental Results of onboard BatteryUltracapacitor System for Electric Vehicle Applications F Gagliardi M Pagano DIPARTIMENTO DI INGEGNERIA ELETTRICA Universita degli Studi di Napoli Federico 11 Email marpananGLuninait G Maestri M Martone A Tarantino ANSALDOBREDA 80 100 Napoli Italy Email tarantinoantoniofansaldobredait Abstract Automotive drive systems which use more than one type of energy source are considered an important step towards low polluting emission and fuel economy All over the world the automotive industry is undertaking strategic initiatives to develop new technologies applied to these systems highefficiency highspecific power electric motors loadlevelling devices hydrogen and directmethanol fuel cells advanced batteries direct injection Diesel and Otto cycle engines are carrying out tests to evaluate their ability to reduce emission and increase vehicular economy The aim of these initiatives is to propose a framework for analysis design and control of vehicles within different energy generators The paper discusses on electrochemical capacitors called ultracapacitors and tests one of these to evaluate its ability for automotive applications The results of the tests effected on an ANSALDOBREDA prototype want to evidence device ability to store energy during braking operations and to boost on requirements to limit battery intervention Some considerations on ultracapacitors charge efficiency are also reported 1 Introduction Due to both the recent increase in gasoline prices and ongoing public and governmental pressures for environment aspects electric automotive drive systems which use more than one type of energy source are considered an important step towards low polluting emission and fuel economy The automotive industry over the world is undertaking strategic initiatives to develop new technologies applied to automotive systems High efficiency highspecific power electric motors load levelling devices hydrogen and directmethanol fuel cells advanced batteries direct injection Diesel and Otto cycle engines are carrying out tests to evaluate their ability to reduce emission and increase vehicular Q780373693l02l1700 0 2002 IEEE economy The aim of these initiatives is to define a framework for the analysis design and control of electric vehicles EV within different energy sources With reference to loadlevelling and accumulator devices three types of devices are now being particularly focused on flywheel batteries and ultracapacitor systems The flywheel energy storage system FESS has become a viable alternative to electrochemical batteries The advantages of flywheel systems adoption are the higher power density insensitivity to environmental conditions absence of hazardous chemicals and ease of checking the charge ie by checking the speed They can accumulate a lot of energy quantity during normal operations and feed loads when this is required to the detriment of stored kinetic energy The main limit of this type of device is rotating mechanisms which can determine problems in automotive applications Perhaps the most promising nearterm alternative for EV and Hybrid Electric Vehicle HEV is to combine the best characteristics of fueldriven engines primary power source and electric motor drives with static energy storage components ie battery andlor ultracapacitors which can be considered secondary power sources The presence of a secondary power source allows designers to size the combustion engine for cruising power requirements the secondary source handles peaks power demands during accelerations additionally they can be used for capturing regenerative braking energy and applying the saved energy to further accelerations Batteries are already very used as storage systems because their technology is familiar and well know however they present deficiencies for power storage also the later bom batteries ie prismatic batteries present maximum power density of about 1 kWKg Moreover they have difficulty to operate in cold environments it must be remembered that the proposed operating temperature range target for Electric Vehicles is 30 65 I and present limited life under extreme conditions These determine during 93 vehicle life repetitive replacements which include not only costs associated with the purchase and installation of new elements but also the costs of removal and disposal of old elements which notably are not environmental friendly Automotive industries 2 are now revolving their studies on a new technological family of capacitors called ultracapacitors 3 which have the main advantage of being static systems and also are suitable to satisfy the most important requirements of EV and HEVs secondary power source to provide boosts of power for short duration events In the paper a ultracapacitors system is investigated to evidence its suitability to EV and HEV applications researchers of ANSALDOBREDA and Electric Engineering Department are now carrying out tests to evaluate its ability to save energy during automotive system service and to boost in automotive accelerations 11 Ultracapacitors Due to the doublelayer cell voltage of 14 V to use ultracapacitors systems in electric power applications as for traditional accumulator cells it is necessary to assemble modules with series parallel andor series parallel cells In the following sections it is briefly discussed former on the functional principle of ultracapacitor cell and later on the tested ultracapacitors module 111 Electrochemical capacitor cell The ultracapacitor is a special electrochemical cell which presents an high value of capacity The chemical processes which characterise the cell take place on electrodesolution contact surfaces fig 1 0 0 0 0 0 On electrode surfaces separated by an electrolyte solution two dielectric doublelayers form Considering the plates plan the capacity value C can be expressed as S 6 c E where S is the section of pole surface 6 is the double layer thickness is the air dielectric constant E is the relative dielectric constant C can assume thousand of Faraday value for double layers thickness of about 15 A Born for electronic applications ultracapacitor cell has a voltage rate of 14 V with maximum charge currents of about 1000 A To take into account the chemical processes which happen into the cell it is normally possible considering not very high frequencies and absence of selfdischarge processes to analytically represent the cell by the equivalent circuit of fig2 34 i wl Fig2 Electrochemical capacitor equivaleiit circuit Two capacitors C and C2 which represent the two doublelayers realised on the positive and negative electrodes are considered in series with the Equivalent Series Resistance R which takes into account the effects of electrolyte solution resistance and the collectors resistance The quantity of stored energy depends on the value of resultant capacity whereas the value of leakage resistance determines the efficiency and the response time of the cell 112 Tested ultracapacitor module In electric power applications where voltage levels are upper than ultracapacitor cell ones it is necessary to assemble ultracapacitors modules which normally present cells in series configurations 5 in this manner the ultracapacitor device can be coupled with the EV engine systems The ultracapacitor module in exam fig3 consists of the series of 36 cells of 2700F23 V Siemens components 61x62164 mm3 the rated voltage is 79 V and the maximum chargedischarge current is 400 A for a specific power of 476 WKg Fig1 Electrochemical capacitor cell 94 Fig3 Tested ultracapacitor device The ultracapacitor module has been measured at the Electrical Engineering Department of University of Naples to verify the parameters and to confirm the possibility of adopting the macroequivalent circuit reported in fig2 From the analyses of tests it is evident that the discharge processes observed in laboratory permitted to neglect the device selfdischarge resistance Rf present in the Randles equivalent circuit 3 fig4 the measures results assessed a total capacity C series of C and Cz of 75 F and a equivalent series resistance Re of 36e3 R C 1 Rf Fig 4 Ratidles equivalent circuit Ill Tested EV prototype Since batteries have been most prohibitive to successful EV performances because they are power poor and also struggle to absorb regenerative braking power compromising potential EV fuel economy electric characteristics of ultracapacitors modules suggest to use these systems in EV applications to store energy during braking operations and to introduce in the system a specific impulsive power source To focus on these aspects the experimental tests are addressed to carry out considerations both on ultracapacitor charge modalities to save an amount of vehicle kinetic energy during braking operation and on ultracapacitors discharge modalities during impulsive energy requirements in vehicle accelerations A prototype equipped with the ultracapacitor module of 8 112 has been assembled at ANSALDOBREDA Naples fig5 The vehicle is a BOXEL prototype which weighs about 1200 Kg and is moved by a 10 kW 120 V motor of 34 Nm nominal torque the engine is powered by a Reghel RF096C300500 dctdc converter coupled with an interface device that controls battery and ultracapacitor sources fig6 a Fig5 AnsaldoBreda prototype vehicle Ultracapacttors Battery Chopper 7 Fig 6 EV drive system IV Experimental tests Many experimental tests have been performed to verify the characteristics of onboard ultracapacitors battery source In particular the tests performed on vehicle and on equipped test stand and characterised by steady conditions high power peaks and repeated braking operations focus on possibility to use ultracapacitors to store energy during short braking 95 instants and to help battery system during impulsive requirements 44 43 42 41 40 IV1 Energy storage ultracapacitors device The results reported in the following refer to tests effected during vehicle operations the prototype is accelerated using battery source and during repetitive braking operations the interface device is able to furnish the energy flow only to the ultracapacitor device The voltage on ultracapacitor during a series of experimental charge periods and the numerical ultracapacitor voltage with reference to the equivalent circuit of fig2 in a single period are respectively reported in fig7a 7b in particular in fig7b it is reported the simulated ultracapacitor state of charge for an initial voltage value of 40 V 75 153 23 308385463 54 618695773 85 928 101 108 Fig7a Experimental ultracapacitor voltage Fig 7b Numerical ultracapacitor voltage The results illustrate the possibility to adopt for this application the macroequivalent circuit of fig2 to represent the ultracapacitor features in particular they evidence the clear devices ability of storing energy during motor braking also during short times it is relevant hence to assess the efficiency of the operation Neglecting connections losses it is possible to express the charge efficiency CE defined by ratio among the ulriacayacitor saved energy Esaved and the eneisjfed to ultracapacitor Efed in the charge period T as where Vt is the instantaneous ultracapacitor voltage value and It is the instantaneous value of the current furnished to the ultracapacitor As the CE coefficient suggests the efficiency of charge process is function of ultracapacitor series resistance and instantaneous charge current in particular the low value of internal resistance favours high efficiency which coherently with ultracapacitor cell features can be reduced in environment characterised by low temperature 4 The charge action can assure high efficiency limitedly to maximum voltage values and losses due to high current peaks in fact differently to batteries ultracapacitors present high value of maximum charge current and in particular their life is mainly influenced by overvoltage value hence normally it is not necessary to control charge current In fig8 it is reported for different ultracapacitor initial voltages the CE of the tested system for r7s 0 98 0 97 0 93 40 45 50 55 60 65 70 75 vo VI Fig 8 Charge efficiency for 7 S Fig8 evidences how the efficiency value is however higher than 93 and the CE coefficient increases with the initial voltage This confirms that the efficiency increases when the saved energy decreases according to limit current peak during charge transitory It suggests for a prefixed power rate to prefer devices which present high capacity and voltage values also with reduced current rate favouring to assess low discharge values during transitory 96 IV2 Ultracapacitors impulsive source Some tests of batteryultracapacitor physic parallel have been effected in laboratory to control the ultracapacitor ability to furnish impulsive currents when electrically coupled with batteries As results of experimental tests the battery and ultracapacitor currents and the voltage on batteryultracapacitor device at a power requirement are reported respectively in fig 9a 9b Fig9a Battery Ib and ultracapacitor IC currents The experimental results point out battery and ultracapacitor intervention features to supplying a load of nominal 64 kW at 80 V The ultracapacitor intervention is characterised by high initial current peak which partially reduces the power requirement to the battery but the current rapidly decreases due to battery voltage regulation as the load is turn off the battery furnishes energy to the ultracapacitor which increases its voltage The tests evidence that to favour the battery during initial phase of load supply it seems suitable to decouple ultracapacitor module up battery and control the intervention of each one separately this effect can increase battery overall life which are depleted at a faster rate than if they are used for high power tasks V Conclusions Batteries are prohibitive to successful EV performances because they are power poor and also struggle to absorb regenerative braking power compromising potential EV fuel economy Until now some manufacturers have increased the number of batteries used in drive systems to combat the effects of battery powerpoor characteristics this increment of size creates a relatively sufficient amount of higher power but increases the need for EV maintenance and diminish the environmentally beneficial aspects of EV design The features of ultracapacitor modules strongly suggest to study the possibility of integrating the ultracapacitors into automotive systems to couple they with battery electric source limiting its rating and design overall electric source system with higher power densities characteristics The experimental tests reported in the paper confirm these aspects evidencing also the possibility to assess high efficiency during ultracapacitor operations and induce to planning a control system which regulating separately ultracapacitor and battery interventions optimises energy exchanges The actual high cost of ultracapacitor modules dont have however to worry because both cost projections assume reduction arising from incremental technological advances as well as cost reduction resulting from economies of scale of materials procurement and high volume manufacturing Moreover it is pointed out how the high overall life times of ultracapacitor can assure high EV reliability and reduce maintenance costs VI List of symbols E air dielectric constant E relative dielectric constant C ultracapacitor value CE charge efficiency 6 doublelayer thickness saved energy fed energy E V Electric Vehicle FESS flywheel energy storage system HEV Hybrid Electric Vehicle It instantaneous charge current R ultracapacitor equivalent series resistance R ultracapacitor equivalent parallel resistance S poles section Vt instantaneous ultracapacitor voltage VII References l Menahem A Kalhammer F R MacArthur D Advanced Batteries for Electric Vehicles An Assessment of Performance Cost and Availability State of California Air Resources Board Sacramento California 22 June 2000 97 2 Brusaglino G Rena PG Aspetti di integrazione di sistemi di accumulo di energia elettrica su veicoli stradali in Proceedings of Azionamenti Elettrici Evolirzione Tecnologica e Probletnaticlie Emergenti Vol I Bressanone 1820 March 2002 3 Conway BE Electrochemical Supercapacitors Scientific Fundamentals and Technological Application Kluwer AcademicPlenum Publiushers New York 1999 4 F Gagliardi M Pagano G Maestri M Martone A Tarantino 1 supercondensatori fondamenti modelli applicazioni in Proceedings of Azionamenti Elettrici Evohzione Tecnologica e Problematiche Emergenti Vol I Bressanone 1820 March 2002 5 Dietrich T Ultracap capacitors Charging characteristics and circuit design http Hwwsiemcnscninprinf Components 399 p 192 1 98