8th International Conference on Power Electronics - ECCE Asia

May 30-June 3, 2011, The Shilla Jeju, Korea

978-1-61284-957-7/11/$26.00 ©2011 IEEE

[ThD2-3]

1

A management strategy for solar panel –battery –

super capacitor hybrid energy system in solar car Bin Wu1, Fang Zhuo1, Fei Long1, Weiwei Gu1, Yang Qing1, YanQin Liu2

1School of Electrical Engineering

Xi’an Jiaotong University, Xi’an, China

2Maintenance and Testing Company

Electric Power Bureau of Chendu, China

Abstract--This paper presents an application of solar energy

- battery – super-capacitor hybrid energy storage system in

solar electric vehicles. The key point is the proposed energy

management control algorithm. The entire system consists

of a solar panel, a boost converter, a battery, a super

capacitor, a bi-directional DC/DC converter, and a

brushless DC motor. This paper gives the details of the

control strategy to cooperate the three energy sources or

storage devices: solar panel, battery and super capacitor. A

hysteretic control algorithm for battery charging by solar

panel is given. A novel control method for bi-directional

DC/DC is proposed to keep the battery discharging current

within a certain limit and make full use of super capacitor.

The switching transient between super-capacitor charging

mode and discharging mode is also presented. Both

simulation and experimental results demonstrate the

effectiveness of the proposed strategy.

Keywords-- solar energy, battery, super-capacitor, hybrid,

hysteretic control, energy management

I. INTRODUCTION

In resent years, hybrid electric vehicle (HEV) has

aroused increasing attentions and many automobile

companies have put large investment in this area. The

reason is not only the coming energy crisis, but also the

global effort of cutting down the carbon dioxide emission

to prevent the global warming. Newly posed promotional

policies from worldwide governments also indicate this

trend. [1] introduces a system composed of a Engine

Generator, a Super-capacitor and a Battery as its energy

sources. But it is not zero emission and the efficiency is

limited for the complicated power electronic devices. [2]

presents a pure electric vehicle energy system with the

only super-capacitor as its unique energy source. Super

capacitor has the features of high specific power, low

internal resistance, capable of quick charge with high

current and perfect characteristic in wide temperature

range. Moreover, it has long cycle life (more than 100

thousand times) and capable of deep discharge. But the

problem is its low energy density, which constrains it

driving distance[3]~[6]. Some researchers are doing

research to improve the energy density of super capacitor

to make it work like a battery. But a more practical way is

to corporate the battery with a supper capacitor in a

vehicle, where each device performs their own duty based

on their own characteristics.

Thus, this paper develops an energy system for

pure electric car which includes the solar panel, battery,

super capacitor as its energy sources and the power

electronic devices as connecting and transforming part.

The solar panel would absorb the solar energy under

MPPT control strategy during the day to supplement the

energy cost of battery. The battery would feed the power

demand of the brushless motor directly which requires

48V DC source. As the energy demand is not large when

the car is cruising, the voltage drop of the battery is

negligible. When the car is climbing slope or accelerating,

the motor would cause a big current impulse. This current

impulse may lead to big voltage drop on battery which

would affect the power quality of the DC source. Besides,

large current discharge would also shorten the life-span

of battery[7]. Thus, we employ the supper capacitor and its

bi-directional DC-DC converter in parallel with the

battery. When the car is cruising, the super capacitor is

charging until to its rated voltage. When there is a current

pulse, the bi-directional DC-DC would be controlled as a

current source[8] to supply the extra load current in order

to prevent the battery current from exceeding certain

value, thus the voltage drop is avoided and longer

life-span of battery is acquired.

The II part of this paper would introduce the

configuration of the established system and its

fundamental function. The III part would introduce the

novel energy management control mode and their

exchange logics. The IV and V par t would give the

simulation and experimental results of the system based

on the control strategy. The VI par t would give the

conclusion.

II SYSTEM CONFIGURATION

The configuration of the solar car energy system is

present in Figure 1. The boost converter is responsible for

the MPPT control to charge the battery. The voltage of

battery is considered constant in the time scale of MPPT

control. The duty cycle of the boost converter is adjusted

only based on the output current and voltage of solar

panel. Meanwhile, the battery is supplying the brushless

DC motor directly. The supper capacitor and the

bi-directional Buck-Boost converter is used to store

certain amount of energy under cruising mode and

discharge to supply impulse current in accelerating and

climbing slope mode, shown as Figure 2(a), (b).

2

Figure 1 Solar car energy system

a) Cruising

b) Climbing slope/accelerating

Figure 2 Power flow in different working conditions

According to the configuration of this energy system,

many advantages can be achieved as following First, the

battery could power the motor directly without being

afraid of voltage drop, which would improve the

efficiency. Second, the large current discharging of

battery is avoided thus the life-span is extended. Third,

this system is also able to collect large feedback energy

through buck-boost converter to the supper capacitor if

there is a need. Fourth, the introduction of solar panel

could absorb energy from outside, which would enhance

the cruising capability of electric vehicle.

III ENERGY MANAGEMENT ALGORITHM

A. Battery charge control As the solar car is consuming energy as well as

collecting the solar energy when running during sunny

day, solar charger should be able to charge the battery

dynamically. First, it has to collect as much solar energy

as possible. Secondly, it has to prevent the overcharge of

the battery which may lead to unnecessary damage. Third,

charging mode switching must be stable and dynamically.

Figure 3 Battery charge control flow

Thus a simple charging hysteretic control method is

developed as the flow chart shown in Figure 3.

During MPPT mode, the popular hill-climbing

method is employed to track the maximum power output

of solar panel. As the voltage of battery is almost constant,

the input voltage of the boost converter is regulated by

the following equation based on boost continues

conduction mode:

1

outin

VV

D (1)

Where inV is also the output voltage of solar panel.

This voltage is adjusted dynamically to the right value by

D to realize MPPT. To guarantee a quick jump to the

continuous mode, some technique of starting the MPPT

mode should be employed. The electrical power from

solar panel is directly delivered to the load when there is

a demand and merely charging the battery when the car

stops.

The 54V constant charge mode is cooperated with

the MPPT mode. If the battery is charged by MPPT mode

and the voltage has reached 54V, the controller would

switch to this mode and try to limit the battery voltage to

54V. This mode would continue until the voltage drops

bellow 48V, then the MPPT mode starts. This control

strategy would effectively avoid the over charging of

battery, which may guarantee a safe working condition.

B Bi-directional DC-DC control

The main task of the super capacitor is to share

load current with battery during a short time when there

is a large current demand from the motor, such as

accelerating or climbing slope. The reason is that the

super capacitor is fit for large current discharging and

long cycle life. When the load demand decreases to

3

below the battery limited current, the battery would

provide extra current to charge the super capacitor to its

nominal value for the next discharging demand. The

super capacitor could be also used for motor feed back

energy collection in further research. The control strategy

is shown as Figure 4.

Figure 4 Bi-directional DC-DC control strategy

There are three modes for bi-directional DC-DC

control: Battery Current Discharging Mode (BCDM),

Super Capacitor Current Charging Mode (SCCCM) and

Super Capacitor Voltage Charging Mode (SCVCM).

They are switched according to the discharging current of

battery and the voltage of super capacitor. It he case

exploited in this paper, the switching logic is described as

Figure

In Figure 5, baI represents for the output current of

battery, scV represents for the voltage of supper capacitor.

According to this diagram, the output current of battery

would be controlled within the limit of 15A even if the

load demand exceeds this limitation. According to this

power management strategy, the super capacitor and

battery would work compatibly in the same system.

Details of each mode are discussed as followed:

Figure 5 mode switching of bi-directional DC-DC control

(1) Battery Current Discharging Mode(BCDM)

Iref

Iba

+

-

PI G1

G2

G1

G2

Iba

Isc

Vsc

Figure 6 BCDM control

In order to limit the battery output current in the

circuit topology as Figure 6, we employed the BCDM

mode. Basing on the theory of circuit, The bidirectional

buck-boost converter is flexible enough to adjust its

output current during a big fluctuation of load current to

keep the battery current constant. So the direct control of

the output current was being employed.

The control goal of this mode is when the load

current exceeds a certain value the reference of the

control loop would switch to the value that the battery

output current should be limited to. Meanwhile, the feed

back would be replaced by the actual battery current

sensed.

The control loop is magnificently different with

some other control goals in traditional DC-DC converters.

The exploitation of models and stability issues is not the

focus of this paper and would be discussed in others.

(2) Super Capacitor Current Charging Mode (SCCCM)

When the load current demand is smaller than the

battery rated current, the battery is charging the supper

capacitor. There a two ways to charging a super capacitor:

constant current charging or constant voltage charging.

Although super capacitor could endure big current,

constant voltage charging mode is also not advisable

when the voltage of super capacitor is fairly lower than

the reference voltage. Under this condition, constant

current charging should be considered first.

In this mode, the bi-directional DC-DC is

controlled as a Buck current source to charge the super

capacitor. The low voltage side is connected with a single

but large capacitor, which is 120F in the experiment.

Figure7 shows the simple control loop in this mode.

According to the buck circuit operation principles, the

small signal mode of buck converter is employed for the

PI parameters selection. The stability could be easily

achieved.

4

Figure 7 SCCCM control l

(3) Super Capacitor Voltage Charging Mode (SCVCM)

If the voltage of super capacitor has reached near the

target value, the control strategy of bi-directional DC-DC

has to switch to the SCVCM mode. The converter is

controlled as a buck converter whose control goal is to

keep the output voltage constant, shown as Figure 8.

This would control the super capacitor voltage at a

suitable value when the vehicle is under cruising

condition to prepare for a sudden discharging. Moreover,

it could make sure the voltage of super capacitor not

exceed its maximum bearable value.

Figure 8 SCVCM control

IV SYSTEM SIMULATION RESULT

The proposed control strategy for the hybrid energy

system of electric vehicles is simulated by PSCAD. The

system is built as Figure 1 shows. The circuit parameters

are selected as table 1.

TABLE I

PARAMETERS OF ENERGY SYSTEM

Components Values Unit

Super Capacitor 120 F

Battery(12V) 60*4(serial) AH

Pm 3.8 kW

Ps 150 W

L1 0.28 mH

L2 0.2 mH

C1 470 uF

C2 2000 uF

The simulation results are show for two cases. The

fist case is focused on the charging mode switching when

the super capacitor is charged. The second case is to

verify the control strategy of BCDM discussed in section

III. Details are as followed:

Case 1 SCCCM to SCVCM

The load current is small and the battery is charging

the super-capacitor. The constant current charging

controlled value is 10A. The results are shown as Figure

9.

As L2 is the inductance connected directly with the

super capacitor, from Figure 9 we can clearly find that the

Bi-directional DC-DC converter is charging the super

capacitor with constant current 10A for a period of time.

During that time, the voltage of super capacitor increases

linearly. When it reached near the target value 20V, the

converter starts to control the voltage stay at 20V.

Therefore, the charging current drops correspondingly

until to zero. This result demonstrates the smooth switch

between SCCCM and SCVCM.

Case 2 BCDM

The load current is stepping form 5A to 20 A.

According to the control algorism, assuming the battery

output current should be limited within 15A. Thus, the

super capacitor starts to discharge when the load current

is bigger than 15A through bi-directional DC-DC

converter.

The battery current is limited within 15A no matter

what the load changes. The simulation is shown in Figure

10 and Figure 11.

In Figure 10, when the current demand rises from

5A to 20A, the load starts to consume a constant power

exceeding the max power that battery is designed to

supply. Thus the bi-directional DC-DC begin to discharge

energy stored in super capacitor to supply the extra power

demand. As the voltage of super capacitor would drop

during the process of discharging, in order to keep a

constant discharging power, the capacitor current is f

increasing. When the load step back to 5 A, the current of

L2 change to become negative 10A, which means the

battery is starting to charge the super capacitor again.

Figure 11 shows that when the load becomes 20A,

the output current of battery is controlled around 15A as

expected. It clearly verified the effectiveness of BCDM

control strategy.

Figure 9 Super capacitor charging mode change

5

Figure 10 Bi direction DC-DC current change

Figure 11 Battery current change

V EXPERIMENT VERIFICATION

The control algorithm is also verified based on an

experimental prototype shown in Figure 15 and Figure 16.

Corresponding to waveforms of the simulation results in

Figure 9, Figure 10, Figure 11, the experimental results

are derived in Figure 11, Figure 12 and Figure 13. They

are much similar to the simulation results because the

control algorisms are the same.

The experimental parameters are exactly selected as

TABLE 1 shows. Compared with the simulation results,

the load is no longer an idea current source with a perfect

step change, but a brushless motor as Figure 16 shows. It

means the experiment would be more close to the actual

situation in electric vehicles.

According to the experimental results, the proposed

control strategy in hybrid energy system is solid verified.

The super capacitor is charged to 20 V which requires the

bi-directional DC-DC to work from SCCCM to SCVCM

as Figure 12 shows. The super capacitor is switching

from discharging mode to charging when the motor load

is turning low from a heavy load as the Figure 13 shows.

The battery output current is limited within 15A when the

motor load is heavy as Figure 14 shows.

As the experimental results shows, control modes of

BCDM, SCCCM, SCVCM are changed smoothly. The

transients demonstrate the effectiveness of the control

loop of each control modes.

Figure13 Bi-direction DC-DC current change

Figure 12 Super capacitor charging mode change

Figure14 Battery current change

In this paper, the experimental results are focused on

the bi-directional DC-DC converter control, which it the

key device in this hybrid energy system. The boost circuit

is not emphasized. However, as a whole system, the

energy form solar panel and its delivery to battery or load

is also important in this paper. The experimental results

of this part of circuit not shown, which is simple to

understand and realize.

6

Figure 15 Main circuit

Figure 16 The brushless motor load

VI CONCLUSION

In the paper, a hybrid energy system for pure electric

vehicle is presented. A topology for cooperating the

energy sources-solar panel battery super capacitor is

proposed and the control strategy is analyzed. The

topology employs the minimal power electronic

converters, which would reduce the loss, improve the

performance of electric vehicles and extend the battery

life-span. The simulation and experimental results verify

the proposed control strategy. Battery big current

discharging has been prevented. The super capacitor

charging and discharging conditions modes could be

switched smoothly based on the control algorithm.

REFERENCE

[1] Hyunjae Yoo; Seung-Ki Sul; Yongho Park; Jongchan

Jeong; , "System Integration and Power-Flow Management

for a Series Hybrid Electric Vehicle Using Supercapacitors

and Batteries," Industry Applications, IEEE Transactions

on , vol.44, no.1, pp.108-114, Jan.-feb. 2008

[2] Chunbo Zhu; Rengui Lu; Likun Tian; Qi Wang; , "The

Development of an Electric Bus with Super-Capacitors as

Unique Energy Storage," Vehicle Power and Propulsion

Conference, 2006. VPPC '06. IEEE , vol., no., pp.1-5, 6-8

Sept. 2006

[3] T. Smith, J. Mars, and G. Turner, “Using supercapacitors

to improvebattery performance,” in Proc. IEEE Conf.

PESC02, Jun., vol. 1, pp. 124–128.

[4] R. Schupbach, J. C. Balda, “The role of Ultracapcitors in

an Energy Storage Unit for Vehicle Power Management”,

IEEE Proceedings of the 58th Vehicular Technology

Conferrence, VTC 2003-Fall, Vol.3, 6-9October 2003,

Orlando,Florida.

[5] Farkas, R. Bonert, “Ultracapacitors as Sole Energy Storage

Device in Hybrid Electric Cars”, Power Electronics in

Transportation, October 20-21,1994, Dearbom, Michigan,

pp.97-101.

[6] Attaianese, C.; Nardi, V.; Parillo, F.; Tomasso, G., "High

performances supercapacitor recovery system including

PowerFactor Correction (PFC) for elevators", European

Conference on Power Electronics and Applications EPE’07,

pp. 1-10, 2-5 Sep2007

[7] Yamanaka, M.; Ikuta, K.; Matsui, T.; Nakashima, H.;

Tomokuni, Y.; , "A life indicator of stationary type sealed

lead-acid battery," Telecommunications Energy

Conference, 1991. INTELEC '91., 13th International , vol.,

no., pp.202-208, 5-8 Nov 1991

[8] A. Pfaelzer, M. Weiner and A. Parker, "Bi-Directional

Automotive 42/14 Volt Bus DC/DC Converter," SAE

Transitioning to 42-Volt Electrical Systems, 2000.