A Stand-alone Photovoltaic Supercapacitor

Battery Hybrid Energy Storage System

M.E. Glavin, Paul K.W. Chan, S. Armstrong, and W.G Hurley, IEEE Fellow
Power Electronics Research Centre
National University of Ireland Galway, Galway, Ireland.
E-mail: margaret.glavin@nuigalway.ie

Abstract—Most of the stand-alone photovoltaic (PV) systems

require an energy storage buffer to supply continuous
energy to the load when there is inadequate solar
irradiation. Typically, Valve Regulated Lead Acid (VRLA)
batteries are utilized for this application. However,
supplying a large burst of current, such as motor startup,
from the battery degrades battery plates, resulting in
destruction of the battery. An alterative way of supplying
large bursts of current is to combine VRLA batteries and
supercapacitors to form a hybrid storage system, where the (a)
battery can supply continuous energy and the
supercapacitor can supply the instant power to the load. In
this paper, the role of the supercapacitor in a PV Energy
Control Unit (ECU) is investigated by using
Matlab/Simulink models. The ECU monitors and optimizes
the power flow from the PV to the battery-supercapacitor
hybrid and the load. Three different load conditions are
studied, including a peak current load, pulsating current
load and a constant current load. The simulation results
show that the hybrid storage system can achieve higher
specific power than the battery storage system.
(b)
Index Terms—Photovoltaic, Lead acid battery, Energy Figure 1. Block diagram of (a) conventional and (b) proposed
Control Unit (ECU), Supercapacitor. photovoltaic system

I. INTRODUCTION seconds at a particular time. Sizing the battery around this

can prove costly; in photovoltaic systems the batteries are
The world is approaching peak oil and the ability to replaced typically every 3-5 years depending on the
produce high quality, inexpensive, and economically application.
extractable oil on demand is diminishing. Peak oil and the By utilizing a battery supercapacitor hybrid energy
environmental impact of fossil fuel utilization, has storage system as shown in Fig. 1(b) the battery size can
encouraged a growth in the area of renewable energies be reduced and a higher SOC can be maintained. The
such as wind and solar power. supercapacitor has a greater power density than the
In remote areas stand-alone photovoltaic systems are battery, which allows the supercapacitor to provide more
most common. A typical stand-alone system Fig. 1(a) power over a short period of time. Conversely, the battery
incorporates a photovoltaic panel, regulator, energy has a much higher energy density when compared to a
storage system, and load [1]. Generally the most common supercapacitor allowing the battery to store more energy
storage technology employed is the VRLA battery and release it over a long period of time. In Table 1 the
because of its low cost and wide availability. Photovoltaic battery and the supercapacitor are compared under various
panels are not an ideal source for battery charging; the headings [4-6]. In the hybrid system the peak power
output is unreliable and heavily dependent on weather requirements of the load are supplied by the
conditions, therefore an optimum charge/ discharge cycle supercapacitor and the VRLA battery supplies the lower
cannot be guaranteed, resulting in a low battery state of continuous power requirements [7-10].
charge (SOC). Low battery SOC leads to sulphation and The proposed Energy Control Unit (ECU) aims to
stratification, both of which shorten battery life [2, 3]. optimize the battery supercapacitor hybrid storage system
Certain load applications require high current for a to reduce the size of the battery and extend the life of the
period of time e.g. motor starting applications; the starting battery by avoiding deep discharge through high currents.
current requirement can be 6-10 times the normal The ECU monitors the battery, supercapacitor and
operating current of the motor. Normally the peak current photovoltaic panel current, voltage and temperature in
requirements are satisfied by the VRLA battery. VRLA addition to the load power requirements. The ECU
batteries in this situation are large in order to deal with the estimates the battery and supercapacitor SOC, optimizes
high current being removed from the battery. The peak the energy from the photovoltaic panel and controls the
current demand might only need to be met for a few flow of energy throughout the system.

1688

978-1-4244-1742-1/08/$25.00 
c 2008 IEEE
 TABLE I. BATTERY VERSUS SUPERCAPACITOR PERFORMANCE [6]

Lead Acid Battery Supercapacitor

Specific Energy
10-100 1 – 10
Density (Wh/kg)
Specific Power
<1000 <10,000
Density (W/kg)
Cycle Life 1,000 > 500,000
Charge/Discharge
70 – 85% 85 - 98%
Efficiency
Figure 2. Photovoltaic cell model
Fast Charge Time 1 - 5h 0.3 – 30 sec

Discharge Time 0.3 – 3h 0.3 – 30s

Matlab/Simulink is used for the design and

optimization of the system. This paper outlines the
models of the various components. The proposed VRLA
battery supercapacitor hybrid storage model is described
and simulations are presented comparing the proposed
system with conventional battery storage under three
different load types; a peak current load, pulsating current
load, and a constant current load.

II. MATLAB/SIMULINK MODELS

A. Photovoltaic Model
Figure 3. I-V characteristics of BP 350 photovoltaic module
A simple photovoltaic cell equivalent circuit model
is shown in Fig. 2 [11]. The model consists of a current
source Iph (represents cell photocurrent), a series
resistance Rs (the internal resistance of each cell) and a
diode. The net output current of the photovoltaic cell is
the differences between the photocurrent Iph and the diode
current ID as described by the following equation,

§ § q (Vs  I s Rs ) · ·
Is I ph  I D I ph  I o ¨¨ e ¨ ¸  1¸¸ (1)
© © mkT ¹ ¹

where m is the ideality factor of the diode, k is

Boltsmann’s constant, T is the absolute temperature of the
cell, q is electron charge, Vs is the voltage applied across
the cell, and Io is the dark saturation current.
The cells are connected in series and parallel to form Figure 4. P-V characteristics of BP 350 photovoltaic module
a PV module. The model simulates a BP solar BP 350
50W photovoltaic panel in Simulink. There are 36 cells in
series and 2 parallel branches. In the model, the ideality A simple equivalent circuit battery model is shown in
factor, m, is equal to 2.0077 where it achieves the Fig. 5. The battery model takes into account the battery
maximum power point at Vs = 17.5V and Is = 2.9A at T = state of charge (SOC) and deep of charge (DOC). The
25oC. Fig. 3 and Fig. 4 illustrate the simulated I-V and P- battery' s usable capacity decrease s with increasing
V characteristics of the photovoltaic panel under various discharge current, the battery DOC measures the fraction
temperature conditions respectively. of the battery' s charge to usable capacity.The model
includes an open circuit battery voltage Eoc, internal
resistance R0 and two RC parallel branches [12-14]. The
B. Battery Model model equations are shown (2) - (6).
Batteries are the main storage technology used in PV
systems. The battery model is used to analyze the effects E oc E 0  K e (1  SOC ) (2)
of different charge rates, state of charge (SOC), and state
of health (SOH) of the battery. The optimum battery size R1 R10 e( K1 (1  SOC )) (3)
for a particular application can be obtained by performing
various test scenarios. Simulations are used to compare R 20
R2 (4)
different storage technologies without the need for DOC
expensive test beds.

2008 13th International Power Electronics and Motion Control Conference (EPE-PEMC 2008) 1689
 1
SOC 1 
Cn ³i batt dW (5)

1
DOC 1 
C (i avg ) ³
ibatt dW (6)

where: SOC is the state of charge of the battery

DOC is the deep of charge of the battery
Cn is the battery capacity
C(iavg) is the current-dependent battery capacity
(obtained in datasheet)
E0 is the open circuit voltage when the battery is
fully charge
Ke is a constant
R10 is the 1st RC branch constant in ȍ Figure 7. 0.2 pulse charge of Yuasa NP18-12 battery
IJ1 is the 1st RC branch time constant in sec
K1 is a constant
R20 is the 2nd RC branch constant in ȍ
IJ2 is the 2nd RC branch time constant in sec

Fig. 6 shows the simulated discharge characteristics

curves for a Yuasa Np18-12 lead acid battery for various
C-rates. From testing the Yuasa Np18-12 battery used has
E0 = 12.85, Ke = 1.7, R0 = 0.12ȍ for charging and 0.057ȍ
for discharging, R10 = 0.16ȍ for charging and 0.02ȍ for
discharging, K1 = 7, and R20 = 0.0055ȍ for both charging
and discharging. Fig. 7 and Fig. 8 show the 0.2C pulse
charge and 0.2C pulse discharge of the battery
respectively.

Figure 8. 0.2C pulse discharge of Yuasa NP18-12 battery

C. Supercapacitor Model
Fig. 9 shows the classical equivalent circuit model
for the supercapacitor [15]. The model consists of three
components, the capacitance, the equivalent series
resistance (ESR), and the equivalent parallel resistance
(EPR). The ESR is a loss term that models the internal
Figure 5. Battery model heating in the capacitor and is most important during
charging and discharging. The EPR models the current
leakage effect and will impact the long term energy
storage performance of the supercapacitor and C is the
capacitance. Equations (7)-(9) describe the ESR, EPR
and terminal voltage of the supercapacitor.

'V
ESR (7)
'i

(t 2  t1 )
EPR (8)
§V ·
ln¨¨ 2 ¸¸ C
© V1 ¹

³ (i
1 ec
vc ESR ˜ ic  c  ) dW  Vc _ init (9)
C EPR

Figure 6. Battery discharge characteristics

where: V1 is the initial self-discharge voltage at t1
V2 is the finial self-discharge voltage at t2
C is the rated capacitance

1690 2008 13th International Power Electronics and Motion Control Conference (EPE-PEMC 2008)
 ǻ9 change in voltage at turn on of load B. Battery Management System (BMS)
ǻ, change in current at turn on of load The Battery Management System (BMS) controls the
Vc_init is the initial capacitor voltage flow of energy from the photovoltaic panel to the battery
ic is the capacitor current and load. The BMS is responsible for calculating the
battery SOC, varying the DC-DC converter duty cycle,
The function of the voltage-dependent capacitor C and implementing the charging algorithm. The BMS is
can be obtained with curve fitting from the based on SOC estimation. The battery charging/
charging/discharging measurements. The model is discharging is dependent on both the battery SOC and the
verified with Nesscap 2.7V/600F supercapacitor. Fig. 10 load requirements as described by Table II.
shows the 10A charging, rest and 5A discharging of the The DC-DC converter implements Maximum Power
model with an ESR of 1mȍ and an EPR of 258ȍ. Point Tracking (MPPT), charges the battery, and delivers
energy to the load. Sensors and measurement circuits are
III. BATTERY STORAGE SYSTEM responsible for measuring the voltages and currents of the
solar panel, battery, and load along with the solar panel
A. Photovoltic Battery Storage Model and battery temperature. This information is used by the
The most common setup for standalone photovoltaic control algorithm to enhance the performance of the
systems, shown in Fig. 1(a), consists of a photovoltaic system, making the best use of the available energy to
panel, converter, load, and battery storage. The energy maintain the battery at a high SOC but also ensuring that
produced from the photovoltaic panel is stored in the the load demand is met at all times.
rechargeable battery to supply the load requirements
when discrepancies arise between available and required
IV. HYBRID STORAGE SYSTEM
energy. Deep discharge batteries are designed to be
discharged down to as much as 80% depth of discharge A. Photovoltic Hybrid Storage Model
(DOD) repeatedly and have thicker plates then car The proposed Hybrid storage model consists of a
batteries making them the preferable choice for PV VRLA battery bank and a supercapacitor battery bank as
storage. Generally the battery is sized to enable it to shown in Fig. 1(b). The hybrid system adopts the
supply power to the load for a period of 2-3 days, advantages of both technologies, high power density from
resulting in a large battery pack that will need to be the supercapacitor and high energy density from the
replaced every few years. battery. The supercapacitor supplies the high peak power
requirements and the battery bank supplies the low power
requirements, resulting in a reduction in the battery pack
size.

TABLE II. BATTERY MANAGEMENT CONDITIONS

Condition Action

PV Power = Load PV supplies load

Battery SOC High No battery charging
Figure 9. Supercapacitor equivalent circuit model
PV Power = Load PV supplies load
Battery SOC Low No battery charging

PV Power > Load PV supplies load

Battery SOC High No battery charging

PV Power > Load PV supplies load

Battery SOC Low PV charges battery

PV Power < Load PV supplies load

Battery SOC High Battery supplies load
PV supplies load
PV Power < Load Battery supplies load until
Battery SOC Low minimum SOC is reached then
shut down load
No PV Power
Battery supplies load
Battery SOC High

No PV Power
Shut down load
Figure 10. Supercapacitor charge/discharge characteristics Battery SOC Low

2008 13th International Power Electronics and Motion Control Conference (EPE-PEMC 2008) 1691
 B. Proposed Energy Control Unit (ECU) 800
S olar R adiatio n P rofile

The Energy Control Unit (ECU) controls the

700
complete photovoltaic system. The ECU is responsible
for charging the battery/supercapacitor hybrid and 600

Solar radiation (W /m )
2
supplying power to the load according to the conditions
500
outlined in Table III.
The power available from the photovoltaic panel is 400

used to supply load power, with excess energy being used 300
for battery and supercapacitor charging. The ECU
implements MPPT capturing the maximum power 200

available from the panel. Various sensors are utilized 100

throughout the system to measure the voltage and current
0
of the battery, supercapacitor and panel along with the 0 5 10 15 20 25
T ime (Hrs )
power requirement of the load. These observations enable
intelligent decisions to be made about how to best utilize Figure 11. Solar radiation profile
the available energy in order to avoid situations where the
load must be shut down due to low battery and
supercapacitor SOC under conditions of inadequate solar 9
P ea k C urrent L oad Pro file

irradiation.
8

7
V. SYSTEM LOAD COMPARISON
6
The battery management system (BMS) was

Current (A)
5
compared to the proposed hybrid energy control unit
4
(ECU) under different load profiles as outlined below.
The solar irradiation profile utilized for the simulations is 3
shown in Fig. 11. 2

1
A. Peak Power Load 0
0 5 10 15 20 25
Fig. 12 shows a peak current load application that T im e (H rs )
has been used to analyses the benefits of the
Figure 12. Peak current load profile
supercapacitor. Examples of peak load applications are
motor starting applications were the starting current
maybe 6-10 times the continuous operating current of the Battery Sup erc apac ito r S O C Peak C urrent Lo ad
motor. The profile of Fig. 12 has an initial current of 1.1

8.33A and a continuous current of 1.375A with the load

1
operating for 45mins every hour throughout the day. Fig.
13 shows the battery SOC with BMS, battery SOC with 0.9
ECU and supercapacitor SOC with ECU. In the Hybrid
system the battery supplies a continuous current of 0.8A, 0.8
SOC

a discharge rate of 0.05C, this current supplies power to

0.7
the load and also recharges the ultracapacitor. A 12V
1200F supercapacitor supplies the remaining load current. 0.6
The hybrid system results in the battery being maintained
at a higher SOC. 0.5 B MS b attery S OC
EC U b attery S OC
EC U s up erc ap acito r S OC
0.4
0 5 10 15 20 25
T im e (Hrs )
TABLE III. HYBRID SYSTEM CONDITIONS AND ACTIONS
Figure 13. Battery supercapacitor SOC for peak current load
Photovoltaic Battery Supercapacitor
Power SOC SOC
Supply Load >0 High High
B. Pulsating Load
Charge Battery >Load Low Low/High
The second load profile used in the analysis is a
Charge pulsating current load. A typical application is the
> Load High Low
Supercapacitor transmitting system. In the simulation, the supercapacitors
Shutdown None Low Low in the hybrid system deliver the pulse power while the
battery supplies the remaining constant current.

1692 2008 13th International Power Electronics and Motion Control Conference (EPE-PEMC 2008)
 Fig. 14 shows the profile of the pulsating current Battery Sup erc ap ac ito r S O C Co nstant C urrent Lo ad
1.1
load. The load operates for 200s out of 250s. The load has
a low continuous current of 0.42A and a high pulse 1
current of 2.08A with a duty cycle of 0.5 and a period of
20s, the load operating over 24 hrs. 0.9
Fig. 15 shows the battery SOC in BMS, battery SOC
0.8
in ECU and supercapacitor SOC with ECU. The Hybrid

SOC
system battery supplies a continuous current of 0.8A 0.7
(0.05C) with the remaining current being supplied by the
supercapacitor. The simulation results show that the 0.6
hybrid system allowed the battery to be maintained at a BMS b attery S O C
higher SOC. 0.5 ECU b attery S O C
ECU s uperc ap acito r S OC
0.4
0 5 10 15 20 25
C. Constant Power Load T ime (Hrs )

A constant current load of 1.04A (0.06C of the

battery) is simulated. The load was analyzed in both BMS Figure 16. Battery supercapacitor SOC for constant current load
and ECU. In the simulation, the battery current is limited
at 1.04A. Without pulse current in the load profile, all the
current is supplied from the battery in the hybrid system. VI. OPTIMIZATION
Fig. 16 shows the SOC in both systems. In the simulation, Photovoltaic are unreliable energy sources that are
the hybrid system has a lower SOC then battery system heavily dependent on weather conditions. The power
because the battery needs to charge the supercapacitors output of the PV panel increases with increasing
due to self discharge. irradiation but decreases with increasing temperature,
operating at its most efficient at high irradiation and low
2.5
P uls ating C urrent Load temperature. The power output from a BP 350 50W solar
panel for a average day in June (best conditions) and
December (worst conditions) is illustrated in Fig. 17, data
2 obtained from [19] for Newcastle, England; which could
be typical for a cloudy climate in northern Europe.
1.5 To ensure that the load requirements can be met
Current (A)

throughout the year, photovoltaic systems are sized for

worst case conditions, from Fig. 17 sizing is performed
1
according to December figures. Other considerations are

0.5
¾ The allowable dept of discharge for VRLA batteries
is 80%.
0
0 0.02 0.04 0.06
T ime (H rs )
0.08 0.1
¾ The days of Autonomy, which refers to the number of
days a battery system will provide a given load
Figure 14. Pulse current load profile without being recharged by the photovoltaic array or
other source is typically 3 to 5 days.

Battery Sup erc apac ito r S OC Puls e C urrent

1.1 BP350 50W P anel June/D e cemb er P o wer
25
June
1 D ec emb er
20
0.9

0.8 15
Power (W)
S OC

0.7
10
0.6

BMS b attery S O C 5
0.5 ECU b attery S O C
ECU s uperc ap acito r S O C
0.4 0
0 5 10 15 20 25 0 5 10 15 20 25
T ime (Hrs ) T im e (H rs )

Figure 15. Battery supercapacitor SOC for pulsating load Figure 17. BP350W June and December average power

2008 13th International Power Electronics and Motion Control Conference (EPE-PEMC 2008) 1693
 Energy audits are performed to obtain information Dom es tic L oad Pro file Ap ril
4000
about the load profile that needs to be supplied from the
PV system. Many appliances require higher starting power Mo nday
3500
T ues d ay
compared to operating power as outlined in Table IV [20]. W ed nes day
3000
The proposed ECU supplies this starting power from the T hursd ay
supercapacitor. Fig. 18 shows the domestic profile 2500
F rid ay
S aturday

Pow er (W)
obtained from a flat in Newcastle, England for a week in S und ay
April. The average power consumption was recorded over 2000

5 minute time intervals throughout 2005[21]. 1500

From Fig. 18 various spikes in power can be observed
1000
throughout the day. Spikes of approximately 9 times the
continuous power requirements are observed, this would 500
result in a large current being removed from the battery
reducing the battery SOC. To ensure adequate power is 0
0 5 10 15 20 25
available the battery pack size is increased to supply the T im e (Hrs )
large current. The supercapacitor can complement the PV Figure 18. Domestic load profile for week in April
panel and the battery to supply the high power
requirements, allowing for a smaller battery pack.
The domestic load profile on Monday shown in Fig. S olar R adiatio n P rofile
18 was scaled down and simulated with both the BMS and 500

the ECU models to obse rve the benefits of the 450

supercapacitor in a domestic application. Fig. 19 shows 400
the solar radiation for a typical April day in Newcastle.

Solar Radiation (W /m )
2
350
The output current from the MPPT is shown in Fig. 21.
300
Fig. 22 shows the SOC of the Np18-12 Yuasa lead
acid battery and a 12V 1200F supercapacitor. The battery 250

in the hybrid system supplied continuous current of 0.5A 200

(0.03C) with the supercapacitor supplying the remainder. 150
It is observed from Fig. 22 that the hybrid system
100
maintains the battery at a greater SOC with the final SOC
for the hybrid system being 72% while the BMS has a 50

battery SOC of 50%. 0

0 5 10 15 20 25
The hybrid system allows for an increase in battery T im e (H rs )
SOC as outlined in Table V with the addition of a 12V Figure 19. Solar radiation for April
1200F supercapacitor pack. Supercapacitors are currently
expensive components, but with the advances in
technology and the increasing growth in the market the MP P T O utp ut Cu rrent
cost is being reduced. 1.8
BMS M PP T c urren t
VRLA batteries are generally large in photovoltaic 1.6 ECU M P PT c u rrent
systems and although there is an increase in SOC with the 1.4
addition of the supercapacitor bank it will prove costly.
Additional optimization is required to find the optimum 1.2
Current (A)

balance between the VRLA battery and the supercapacitor 1

bank. Other battery and fuel cell technology can be
0.8
investigated to see if they have greater benefit.
0.6
TABLE IV. APPLIANCE STARTING AND CONTINUOUS POWER 0.4

Starting Continuous 0.2

Appliance Ratio
Power Power
0
0 5 10 15 20 25
Clothes Washer 5,042 225 22.4 T im e (H rs )

Figure 20. MPPT output current

Well Pump 1/2hp 1950 150 13

Clothes Dryer 4208 334 12.6 VII. CONCLUSIONS

An Energy Control Unit (ECU) for a photovoltaic
Fridge/Freezer 2700 600 4.5 battery supercapacitor hybrid system has been developed.
Simulations have been performed to compare the ECU to
Freezer the standard photovoltaic battery storage system under
2100 800 2.6
different load conditions; a peak current load, pulsating
current load, constant current load, and a domestic profile.
Vacuum cleaner 2012 818 2.5 Maximum Power Point Tracking (MPPT) allows the
maximum power to be gained from the photovoltaic panel

1694 2008 13th International Power Electronics and Motion Control Conference (EPE-PEMC 2008)
and the proposed ECU is responsible for calculating the ACKNOWLEDGMENT
battery and supercapacitor SOC. The ECU controls the
system based of the available power, battery/ This project was supported by Enterprise Ireland under
supercapacitor SOC and the required load power. From the Commercialisation Fund in Technology Development
the simulations performed the addition of a supercapacitor (CFTD).
bank will increase the battery SOC for peak and pulse
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