Electric Bicycle Using Batteries and Supercapacitors

D. M. Sousa, P. J. Costa Branco, J. A. Dente DEEC AC-Energia /CAUTL, Instituto Superior Tcnico, TU Lisbon

, DEEC AC-Energia, Instituto Superior Tcnico, TU Lisbon Av. Rovisco Pais, 1 1049-001 Lisboa

Lisboa, Portugal Tel.: +351 21 841 74 29, +351 21 841 74 32, +351 21 841 74 35.

Fax: +351 21 841 71 67. E-Mail: pcdsousa@mail.ist.utl.pt, pbranco@alfa.ist.utl.pt, edentepc@ist.utl.pt

URL: http://www.ist.utl.pt

Keywords Electric vehicle, Energy storage, Supercapacitor, Power converters for EV, Electrical drive.

Abstract In this paper, a traction system useful for an autonomous Electric Vehicle of individual use is described. The developed system is constituted in a first approach by two different power sources: one is constituted by batteries or by fuel cells, and the other by supercapacitors. This paper describes a technical solution joining and accomplishing the usage of two energy storage systems in the same traction system. In the developed system, the supercapacitors run as element that store energy temporarily and that can be used to retrieve energy. Starting from the functional characteristics of typical electrical vehicles and characterization of a typical routing profile, the energy consumption is obtained. In order to characterize and design the system, this is described in detail, namely the supercapacitors models, the battery, the power converters and the implemented strategy of control. According to the obtained results, a control strategy that allows an effective management of the stored energy in the system regarding the vehicles optimal functioning and increasing its autonomy is also presented and discussed. Based on experimental and simulation results, the advantages and disadvantages of the proposed solution are presented.

Introduction In the modern societies, the increasing needs of mobility means sometimes increasing the number of vehicles circulating. Ambient concerns, as for instance local pollutant emissions for the atmosphere, influence also, in nowadays, the technical decisions related with all kind of vehicles. In this context, new alternatives to the existing internal combustion engines are mandatory. So, vehicles with electric propulsion seem to be an interesting alternative [1, 2, 3]. Starting from this context, this research describes a solution that was developed and studied to be applied in electric vehicles of individual use as bicycles. The solution proposes the combination of two sources of energy, batteries and supercapacitors, and two DC-DC converters. On board, batteries and supercapacitors store the energy. Anyway, the proposed topology considers that fuel cells should be used in two ways: replacing the set of batteries or to charge the batteries and the supercapacitors.

As it is well known, in the typical electric traction systems the batteries drive the high currents and in the worst situation drive the current peaks demanded by the load. As it is well known, this type of operation decreases strongly the autonomy of the vehicles for individual use. The continuous and random operation of electrical vehicles requires and claims for systems improving the autonomy and the performance of the available ones. In this situation, a solution to improve the battery behaviour and its time life is to replace temporarily the battery by another power source or, as in the developed solution, to supply the system using other power source when undesired and transient situations occur [4]. In this case, the load is supplied by the complementary energy source avoiding, at least, deep discharges of the battery. The adopted solution uses supercapacitors, which drive the peaks of power required by the load.

Requirements of the system The first step in order to project the system is to establish the objectives of the work according to the energy consumption and the performance of the vehicle for individual use. To estimate the power required by this type of vehicles, we have considered that the forces applied to the vehicle are, as represented in figure 1, the following:

aMFa = (1)

sin= gMF g (2)

)(21 2tvACMF fDair = (3)

cos= CgMF Rr (4)

Where: Fa is the resulting force; Fg is the gravitational force; Fair is the air friction force; and Fr is the wheels friction force; parameter is the air density (1.29 kg/m3); Af is the frontal area of the vehicle; CD is the air friction coefficient (tipically 0.9 for a scooter and 0.8 for a bicycle); and CR is the wheels friction coefficient (usually between 0.008 and 0.014).

cos= CgMF Rr

sin= gMFg

)(21 2tvACMF fDair =

Fig. 1: Forces applied to the vehicle

Considering that the vehicle runs with a speed v, the power required by the system is:

cos)()(21sin)()()( 3 +++=

+++=

tvCgMtvACMtvgMtvtaMP

PPPPP

RfDVE

rairgaVE (5)

Assuming that the vehicle speed is equal to the angular speed of wheels with radius R, torque of the traction system can be estimated as:

cos)()(21sin)( 32 +++=

+++=

tvCRgMRtvACMRgMRtaMT

TTTTT

RfDVE

rairgaVE (6)

Anyway, to analyze and compare the performance of different vehicles for individual use, operating conditions in terms of speed and autonomy should be used. So, based on the Portuguese standards (NP EN 1986-1), a typical urban cycle (Figure 2), repeated 10 times, with the total duration of 1180 s should be fulfilled by the traction system in terms of torque and speed and by the power sources on board in terms of energy stored.

Fig. 2: Profile of the used urban cycle To fulfill the cycle above and taking into account the physical dimensions of a bicycle or a scooter, the nominal power required by this type of system stays in the range of 2 kW to 2.5 kW. So, based on the premises and conditions above, a traction system is proposed aiming the autonomy, efficiency and performance of this type of vehicles.

Global system The main elements constituting the global system are two power converters, two energy storage systems (in the basic implementation, batteries and supercapacitors) and the traction motor [5]. With the proposed solution, the most important objective is to increase the capacity of storing energy and vehicle autonomy, avoiding deep discharges of the batteries.

In order to achieve this goal, the global topology represented in figure 3 was investigated.

VSC

iSC' iSC

Vdc

iC

DC-DC 1 DC-DC 2

ia

Va

idc

L

Fig. 3: Global system

The power converters The proposed topology uses two power converters. Their main functions are:

The power converter DC-DC 1 (operating as buck or book converter (figure 4), in agreement with the level of charge of the supercapacitors) transfers energy from the supercapacitors to the battery.

The DC-DC 2 converter adjusts the supply voltage of the traction motor (in this case, a DC motor) to control its speed.

VSC

iSC'L

S1

S2

iSC

Vdc

iC

LOAD

idc

Fig. 4: Converter topology of the used DC-DC power converters

The traction motor The implemented traction system is based on a DC motor, which dynamic behaviour can be represented by:

=

++=

+=

TikdtdJ

kdtid

LirU

dtid

LirU

Loada

aaaaa

fffff

(7)

To project and analyse the system, knowledge of motor parameters is mandatory. In particular, the electrical time constant and the starting current allowed by the system, which according to experimental tests are, respectively:

msrLa

aa 30= (8)

ArU

ia

astart 20 (9)

The supercapacitors model The nominal voltage of each supercapacitor available is lower than the rated voltage of typical electric traction systems (12 V or 24 V, for instance) [6]. Therefore, in order to fulfill the rated voltage of these systems, it is mandatory to connect supercapacitors in series and in parallel modes. Anyway, to investigate the dynamic behaviour and the performance of the global system it is important to know the supercapacitor model, which electric equivalent model is represented in figure 5.

Fig. 5: Equivalent model of a supercapacitor The supercapacitor model is constituted by an inductance L, a resistance Ri and an impedance Zp connected in serie. The impedance Zp can be calculated using the expression:

jj

Cj

)coth()(Zp = (10)

To the available supercapacitors (2.5 V; 200F 30% (at 20C)), the parameters of the model were obtained by electrochemical impedance spectroscopy, which simulated and experimental spectra are shown in figure 6 [7, 8].

-10

0

10

20

30

40

50

24,5 25,2 24,7 24,2 24,1 24,4 25,1 26,3 28,3 31,7 35,9 37,5

Re(Z) [mOhm]

- Im

ag(Z

) [m

Ohm

]

ExpSimul

Fig. 6: Spectra of a supercapacitor impedance

Experimental and Simulation Results In a first approach to the problem, the supercapacitors were connected in parallel with the battery, as represented in figure 7 [9, 10]. To study and analyse the performance of such system, both experimental and simulation results were obtained. The implemented model (simulated using the @Matlab/Simulink) includes dynamic models to the used battery and supercapacitors [11, 12, 13].

ia

Va

VB

Rb

ib

VSC

Rsc

isc

Fig. 7: Circuit connecting in parallel the supercapacitors and battery The electrical equations representing the circuit are in a first approach the following:

=

==

+=

dtvdCi

RivRivviiia

SCSC

bbbSCSC

bSC

(8)

A prototype of this circuit was implemented in the laboratory, with a set of batteries (12V, 7Ah each one), a DC motor and a five supercapacitors in series (Ctotal = 200/5 = 40 F). To a random load diagram, the experimental and simulation results obtained are shown in figures 8 and 9. From these results, it is important to point out that current peaks are driven by the supercapacitors, thus avoiding deep discharges of the batteries. Furthermore, when the motor is braking the supercapacitors are charged.

-10.00

-5.00

0.00

5.00

10.00

15.00

20.00

25.00

30.00

0 60 120 180 240 300 360 420

Tempo [s]

Cor

rent

e [A

]

Carga Bateria SC

T im e ( s)

Cur

rent

(A

)

Load Battery Supercapac.

Fig. 8: Experimental results

Cur

rent

(A

)

T im e ( s)Load Battery Supercapac.

Fig. 9: Simulation results A reasonable agreement between the experimental results and the simulation ones is observed, leading that the assumption that the developed models constitute a good approach and the circuit behaves as foreseen analytically. Anyway, the above operating principle is only valid if an effective control of the energy transit between the supercapacitors and the battery is reached.

The control strategy The first approach to the problem of energy management is based on the calculation of an average value of the current iDC demanded by traction system. The average current should be supplied by the main power supply, which can be the set of batteries or a fuel cell [3, 14, 15]. The difference between the

instantaneous value of the load current ia and the average value of iDC will be the current supplied by the supercapacitors, iSC. When the current iSC is positive and the total voltage of the supercapacitors is higher than the energy availability, the break level V, a duty-cycle is applied to the semiconductor S1 (figure 4) running the DC-DC 1 converter as a boost converter. On other hand, if the supercapacitors does not have energy available, that is, the set of supercacitors is discharged or their voltage is under the break level, converter DC-DC 1 is switched off and the main power supply supplies the traction system. For negative values of iSC and if the supercapacitors do not have the maximum load V, converter DC-DC 1 runs as buck converter. When the supercapacitors voltage reaches the value V , converter DC-DC 1 remains in its stand-by mode. Anyway, if a fuel cell is used as the main power supply, condition iDC

Conclusion In this paper an electronic converter using two power sources connected through two DC-DC converters is described having potential application in electric bicycles or in other vehicles for individual use without internal combustion engines. The proposed system uses in its basic topology a set of batteries and a bank of supercapacitors to supply the traction system but is also designed to replace the batteries by fuel cells. The conception of the proposed system is also the first step to investigate the solutions and systems that allow to charge electrical vehicles in remote places or when the infinite power nets are not available. In this case, fuel cells can be used to store energy and to restore the energy of these types of autonomous vehicles. With the proposed solution, it is expected to increase the autonomy of electrical vehicles as electric bicycles or scooters and to avoid high current peak and fast discharges of the batteries. Therefore, a control algorithm managing the energy stored on board and the running of the proposed system is described. From the experimental and simulation results obtained it is important to point out that the proposed system has an appropriate performance in hard situations like high loads avoiding deep discharges of the batteries. Furthermore, it is also possible to adequate the algorithm of to the profile of the course and maximize the energy recovering. It is also important to refer that the running of the DC-DC converter either as buck or boost converters does not introduce perturbations in the system dynamics, in particular the vehicle speed remains constant. This work reflects also the real perspective of integration of multi energy storage systems in a unique traction system. The proposed solution reveals advantages from the point of the viewpoint of the traction system concerning overload situations and avoiding an unnecessary over dimensioning of all system. In the further work, the implemented power circuit (figure 3) will be analysed taking into account the amount of energy stored for unit of weight in the storage systems available and useful to these type of small vehicles.

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