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Design and implementation of the low cost and fast solar chargerwith the rooftop PV array of the vehicle

Thanh-Tuan Nguyen, Hyung Won Kim, Geun Hong Lee, Woojin ChoiDepartment of Electrical Engineering, Soongsil University, 1-1 Sangdo 5-Dong, Dongjak-Gu, 156-743 Seoul, Republic of Korea

Received 8 April 2013; received in revised form 26 June 2013; accepted 8 July 2013Available online 2 August 2013

Communicated by: Associate Editor Elias Stefanakos

Abstract

In this paper, a comprehensive and detail design procedure for the low cost and fast solar charger with the rooftop PV array of thevehicle is presented. A simplied maximum power point tracking technique is adopted to take advantages of the simpler structure of thecircuit and the shorter charge time than those of the conventional solar charger. In the small-signal modeling of the converter, linearizedequivalent circuit models of the PV array and the leadacid battery are included for the accurate charge control all over the operatingrange. An experimental prototype is implemented by using a buck converter with a digital signal processor as a controller and tested byusing the Elgar TerraSAS solar array simulator to verify the feasibility and validity of the system and its control algorithm.

2013 Elsevier Ltd. All rights reserved.

Keywords: Solar charger; Simplied maximum power point tracking; Low cost; Shorter charge time

1. Introduction

As the concerns of conventional energy source exhaus-tion and environmental pollution increase, attempts arebeing made to replace fossil fuels with non-conventionalenergy sources in various sectors. In vehicular transporta-tion eld, automotive industry is undergoing a revolutionin the design of its electrical system. This is the result of increasingly sophisticated engine as well as the introductionof new electrically controlled functions ( Kassakian et al.,1996). Using rooftop photovoltaic (PV) arrays is an effec-tive way to aid in providing additional power which canpower the ventilation system or air conditioning systemin vehicles (Giannouli and Yianoulis, 2012 ). The advantageof using rooftop PV arrays is that even when the car isparked under sunlight, the arrays can produce electric

energy to charge the battery and then power the systemsto cool down the atmosphere inside the car. However, thereare several drawbacks in using PV arrays as an energysource for the charger such as high fabrication cost, lowenergy conversion efficiency and long charge time. Thus,when the PV array is used for charging the battery as anauxiliary energy source, the PV system need to be low incost and it is important to charge the battery within a shortperiod of time by optimizing the power generated by thePV arrays at certain operating conditions under the varyingirradiance and temperature level. An intelligent batterycharger for hybrid electric vehicle has been proposed toshow that the solar energy can be used as an additionalenergy source (Lu et al., 2007). However, only the designconcept was presented and thus the details of the systemremained unknown. In ( Pires et al., 2012; Traube et al.,2013), the electric vehicle charger which is able to chargethe battery of the vehicle from both PV array and the acgrid are presented, but it is an off-board type system thusis not applicable to the on-board charger. An analog max-

0038-092X/$ - see front matter 2013 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.solener.2013.07.006

Corresponding author. Tel.: +82 2 820 0652; fax: +82 2 817 7961.E-mail addresses: [email protected] (T.-T. Nguyen),

[email protected] (W. Choi).

www.elsevier.com/locate/solener

Available online at www.sciencedirect.com

Solar Energy 96 (2013) 8395
http://dx.doi.org/10.1016/j.solener.2013.07.006mailto:[email protected]:[email protected]://dx.doi.org/10.1016/j.solener.2013.07.006http://crossmark.crossref.org/dialog/?doi=10.1016/j.solener.2013.07.006&domain=pdfhttp://dx.doi.org/10.1016/j.solener.2013.07.006mailto:[email protected]:[email protected]://dx.doi.org/10.1016/j.solener.2013.07.006http://-/?-http://-/?-http://-/?-http://-/?-

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imum power tracker which is simple in hardware imple-mentation and fast in tracking time was implemented inJi et al. (2012). However, since the boost converter topol-ogy is used, the topology is not suitable for low voltageapplication such as the battery charger. Also it requiresthe voltage and current sensors to measure the peak power

of the PV array. In ( Ahmed and Shoyama, 2011 ), anattempt has been made to improve the performance of the maximum power point tracking (MPPT) algorithm interms of the number of required sensors and the trackingtime. The proposed MPPT algorithm requires only a cur-rent sensor associated with the PV array to measure theoutput current of it, however, additional sensors arerequired to implement the charge control algorithm forthe battery. The numerous kinds of maximum power pointtracking (MPPT) methods were summarized in Esram andChapman (2007) . Some of them suggest that it is adequateto maximize the load current to maximize the load powerwhen the load is of voltage source type. Thus the load cur-rent can be used as single control variable for the MPPT,thereby making it possible to reduce the number of sensors.

In this paper, a solar charger for vehicles which use roof-top PV arrays as an auxiliary energy source is presented.This charger employs a MPPT technique to force the PVarray generate maximum power and then deliver to chargea leadacid battery. The MPPT technique is implementedby controlling output current without sensing the voltageand current of the PV array thereby reducing the complex-ity and the cost of the system. Also the constant currentcontrol and the constant voltage control are used togetherwith the MPPT method to reduce the charge time and to

prolong the battery life.

2. Low cost and fast solar charger and its charge algorithm

Fig. 1 shows the block diagram of the solar chargerwith reduced number of sensors. A PV array is used asa power source which generates power to charge the bat-tery. A leadacid battery is used to store the energy fromthe PV array and a DC/DC buck converter is used tocharge it. The charger operates one of three chargemodes: MPPT charge, constant current (CC) chargeand constant voltage (CV) charge depending both on

the available energy from the PV array and the state of the charge in the leadacid battery. When the voltageof the battery is lower than the full charge voltage andthe charge current is lower than the rated charge currentof the battery, the MPPT mode is employed to charge thebattery with maximum power. In this mode, if the charge

current exceeds the rated charge current (0.15C in thiscase), the controller changes its control method to currentcontrol mode (CCM) to limit the charge current. Whenthe battery voltage reaches to its limit value, the constantvoltage mode (CVM) charge is performed until the bat-tery is fully charged.

The MPPT method can be implemented by maximizingthe charge current I out at the output of the buck converteras shown in Fig. 1. With this method, since two sensors fordetecting the voltage and current of the PV array are notrequired, the cost of the proposed solar charge systemmay be 510% lower than that of the system presented inKe and Makaran, 2009 . Further, it helps reduce the com-plexity of the system and also provides the exibility inselecting the PV array for the charger since no sensors asso-ciated with PV array are employed.

3. Principle of the maximum power point tracking techniquewith reduced number of sensors

The purpose of MPPT techniques is to automaticallynd the voltage or current value at maximum power pointat which the PV array should operate and generate its max-imum available power, even in varying operating condition(Esram and Chapman, 2007 ).

Among the various maximum power point trackingtechniques the P&O (Perturbation and Observation)method has been most widely used in practice due to itssimplicity of the implementation ( Enrique et al., 2010 ). InP&O algorithm, the output voltage of the PV array whichrepresents the operating point of the PV array is perturbedby changing the duty cycle of the power converter switch(Esram and Chapman, 2007 ). It can be seen from Fig. 2that when the operating point of the PV array is on the leftof the MPP and moving towards the MPP, then the output

Fig. 1. Block diagram of the proposed solar charger with reduced number

of sensors. Fig. 2. The characteristic curves of photovoltaic array.

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C i >1 Dmax I o max Dmax0:02 I pv max R pv max f sw

9

where f sw is the switching frequency (Hz), D max is the dutycycle at maximum output power, I o_max is the charge cur-rent when the PV array works at the MPP (A), I pv_max isthe PV array output current when the PV array works atthe MPP (A), and R pv_max is the equivalent resistance of the PV array at the MPP ( O ).

All the system parameters of the solar charger are listedin Table 1 .

5. Design of the charge controller including the linearizedequivalent circuit models of the PV array and the leadacidbattery

5.1. Linearized equivalent circuit model of the PV array

The characteristic equation of a PV array can be repre-

sented by (10).

i pv I ph I s exp v pv R si pv

V T g 1 v pv R si pv R p 10where i pv is the output current of the PV array (A), v pv is theoutput voltage of the PV array (V), Rs is the series resis-tance (X), R p is the shunt resistance ( X), I ph is the light in-duced current (A), g is the diode ideality factor, I s is thesaturation current of the PV array (A), and V T is the ther-mal voltage (V).

The equivalent circuit of a PV array can be illustrated asFig. 4(a).

As shown in Fig. 2, when the output voltage of the PVarray is lower than V mp the array behaves like a currentsource and when the output voltage is higher than V mp itacts like a voltage source. Practically, the series resistanceR s has dominant inuence on the operation of the PV mod-ule when it operates in voltage source region, while the par-allel resistance R p has stronger inuence than seriesresistance in the current source region ( Villalva et al.,2009). However, this non-linear equivalent circuit of thePV array is rarely used for the circuit analysis. In orderto reduce the complexity of the circuit analysis, lineariza-tion technique can be used to derive a further simpliedequivalent circuit of the PV array at a certain operatingpoint ( Villalva et al., 2010).

In order to obtain the slope of the linearized character-istic curve of the PV array at a certain operating point, apartial derivative of with respect to the PV voltage needsto be calculated.

@ i pv@ v pv V ; I

R s 1

I sgV T

eV R s I

gV T 1 R p

0@

1A

1

11

The linear model of the PV array can be described by the

tangent at an operating point ( V , I ) as (12).

Fig. 3. Proposed maximum power point tracking algorithm for the solar charger.

Table 1System parameters of the solar charger.

Switching frequency f s 60 (kHz)Inductor L 100 (l H)Inductor current I L 6.0 (A)Input capacitor C 220 (l F)Input voltage at MPP V 17.0 (V)Input voltage at point A V 12.0 (V)Input voltage at point B V 20.5 (V)Equivalent resistance of the PV array at MPP Req 1.25 (X)Equivalent resistance of the PV array at point A Req 31.18 (X)Equivalent resistance of the PV array at point B Req 0.39 (X)Equivalent capacitance of the battery C b 90,000 (F)Equivalent resistance of the battery Rb 0.02 (X)


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i pv g v pv V I 12

where g @ i pv@ v p v V ; I .

Thus the simplied equivalent circuit model of the PVarray can be represented by a voltage source and a seriesresistor as shown in Fig. 4(b). (where Req = 1/ g andV eq = V I / g ).

However, since the above linearized model is only validat a certain operating point, it is necessary to linearize thePV model at each operating point all over the operatingrange of the buck converter on the I V characteristicscurve.

5.2. Modeling of the buck converter including linearized PV array model and battery model

In this section, the control-to-inductor current and thecontrol-to-output voltage transfer functions are derivedby using small-signal modeling technique for the chargecontrol.

The linearized equivalent circuit model of the PV arrayin Fig. 4(b) is combined with a buck converter and anequivalent circuit model of the leadacid battery. The bat-tery is modeled with an RC series circuit, where Rb and C brepresent the equivalent series resistance and the equivalentcapacitance of the battery, respectively ( Jossen, 2006).These two parameters of the equivalent circuit of the bat-tery can be obtained by using current interruption testand coulomb counting method. In this case, the equivalentresistance Rb and the equivalent capacitance C b of the

leadacid battery are equal to 0.02 ( X) and 90,000 (F),respectively.The PWM converter can be modeled by the circuit aver-

aging technique. Thus, the averaged model of the solarcharger can be developed as shown in Fig. 5.

The control variable is the duty cycle d and the MOS-FET switches at the frequency of f s = 1/ T s. The MOSFETis on during the interval dT s and off during the interval(1 d ) T s. By applying the Kirchhoffs voltage law(KVL) for the secondary loop and the Kirchhoffs currentlaw (KCL) at node N, the voltage equation of the second-ary loop and the current equation at the node N can bewritten in (13) and (14).

vd L d dt

i L i LZ o 0 13

V eq v Req

C d dt

v i Ld 0 14

i L; v and d denote the average values of the inductor cur-rent, the input voltage of the buck converter and the dutycycle of the switching, respectively. In order to construct asmall-signal model, it is assumed that the averaged outputvoltage v, the averaged inductor current i L and the aver-aged duty cycle d waveforms will be equal to the corre-sponding dc value V , I L and D, plus some superimposedsmall ac variation ~v, ~i L and ~d (Erickson and Maksimovic,2001). Hence, we have

v V ~v; i L I L ~i L; d D ~d 15

where ~v

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5.3. Design of the current and voltage controller

According to the charge algorithm of the solar charger,the constant voltage mode and the constant current mode

can be performed separately. The current control mode isemployed if the charge current exceeds the rated chargedcurrent (0.15C) to limit the charge current at the ratedcharged value. In the other case, the voltage control modeis activated when the battery voltage reaches to the limitvoltage 14.0 (V) to charge the battery in the constant volt-age mode (Dunlop, 1997 ).

Fig. 6 shows the frequency responses of the control tooutput current and the control to output voltage of theconverter obtained by the PSIM simulation when the oper-ating point of the PV array is at MPP. In this gure, thefrequency responses of the output current and voltage aredrawn by using ac sweep method for the solar charger cir-

cuit and the transfer functions, G id (s) and G vd (s) obtainedby the small-signal modeling. Since both gain responseand phase response in each case are matched each other,the derived transfer functions can be considered valid.

Thus it can be used for investigating the dynamic charac-teristics of the converter and designing the controllers.

Fig. 7 shows that the Bode plots of the G id (s) at threedifferent operating points of the PV array. It is noticed thatwhen the operating point moves from point A in the cur-rent source region to point B in the voltage source region,the resonant frequency of the system varies slightly around350 (Hz) while the dc gain and the damping factor whichaffect to the dynamics of the system vary signicantly. Inthe design of the current controller a zero will be locatedat the resonant frequency of the converter to avoid theinteraction in phase. Since the small change in the resonantfrequency makes it easier to design the PI controller for the

Fig. 5. The averaged model of the solar charger.

Fig. 6. Validation of G id (s) and G vd (s) transfer function at the MPP using AC sweep.


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current control for all three cases, it can be considered asan advantage in designing the current controller.

Similarly, by investigating the G vd (s) transfer function atpoint A, MPP and point B, the Bode plots are drawn asshown in Fig. 8. The resonant frequencies also slightly varyin three cases at about 350 (Hz). However, the dc gains of

the Bode plots at MPP and point A are negative, thus it is

required to increase the low frequency gain of the system inthese cases by the controller.

It is clearly shown in Figs. 7 and 8 that the phase mar-gins in the three cases are more than 90 , which make theconverter operation stable enough. Since the gain at lowfrequency of the open-loop system (both of G id (s) and

G vd (s) transfer function) in the current source region is

Fig. 7. Bode plot of G id (s) at three certain operating points of the PV array.

Fig. 8. Bode plot of G vd (s) at three certain operating points of the PV array.

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low, a PI controller is employed to achieve a higher dc gainat the low frequency range in the closed loop control.

The transfer function for the closed-loop control sys-tems can be represented as (18).

T s G pi sG s

1 G pi sG s H s 18

where G pi (s) is the PI controller for the output current oroutput voltage, G (s) is the control to output current or con-trol to output voltage transfer function and H (s) is the gainof the sensing circuit.

As mentioned above each PI controller is designed tohave a zero frequency equal to one-tenth of the resonantfrequency of the converter to avoid the interaction in the

Fig. 9. Bode plots of the closed-loop system for output current control at three certain operating points.

Fig. 10. Bode plots of the closed-loop system for output voltage control at three certain operating points.


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phase response while still providing higher gains(Tymerski, 2009 ). Hence, the PI controller having zerofrequency at 35 (Hz) is designed as (19) and (20) for thecurrent control and voltage control, respectively.

G pi i s 0:43 s 95 s 19

G pi v s 17:85 s 3925

s 20

Fig. 9 shows the Bode plots of the transfer functions of the closed-loop current control with a PI controller at threedifferent operating points, point A, MPP and point B. Itcan be seen in Fig. 9 that with the PI compensator G pi _ i (s)shown in (19), the closed-loop system is stable in the rangeof PV operation with sufficient phase margins. The Bodeplot of the transfer function of the closed-loop voltage con-

trol is also shown in Fig. 10. It can be seen that with the PI

Fig. 11. PSIM simulation results of the low cost MPPT algorithm.

Fig. 12. System conguration.

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controller designed as (20) the stability of the closed-loopvoltage control system is ensured with high phase marginfor all three cases.

6. PSIM simulation result of the low cost MPPT algorithm

PSIM simulation was performed to verify the proposedMPP tracking algorithm. The circuit for the simulationconsists of a PV model which is available in the PSIM soft-ware as a library, a buck converter and an RC equivalentcircuit model of the leadacid battery. The MPPT algo-rithm was composed by the C language and put in a Cblock. The MPPT cycle was set at 30 ms. The duty cyclewas changed at each MPPT cycle by a constant step of 0.5% in this study. The test condition was set at 1200(W/m 2 ) of irradiance and 30 C of temperature. Under thiscondition, the maximum generated power from the PVarray consisted of 36 cells in series is 115.6 (W) at the PVarray voltage 17.0 (V) and its current 6.8 (A).

The Fig. 11 shows the simulation results. When the

MPPT starts to work, as the output current of the PV array

increases to 6.8 (A), the output voltage of the PV arraydecreases from the open-circuit voltage (21.2 (V)) to theMPP voltage (17.0 (V)). At the same time the batterycharge current increases until it reaches to its maximumvalue (7.6 (A)). The actual output power of the PV arrayincreases to its maximum available output power indicat-ing that the MPP has been tracked successfully under thetest condition. After the MPP is tracked, the output current

of the PV array oscillates around the maximum value. Ittakes about 1.8 s to track the MPP in this case. The MPPtracking performance will be veried by the experimentsin the following section.

7. Experimental results

The MPPT and the battery charge algorithm describedabove are experimentally tested with the Elgar TerraSASsolar array simulator and a 12 V 40Ah leadacid battery.The digital signal processor (DSP) TMS320F28335 fromTexas Instrument (TI) was used for full digital control of

the proposed MPPT algorithm and the charge method.

Fig. 13. The PV array outputs in MPPT mode.

Fig. 14. Battery voltage and current in MPPT mode.


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