EE4504 – Design of Clean Energy

Report on: Design of 450W Solar Battery Charging System.

Prepared By:

Srinivasan Vivek (K1500002L)

Table of Content

Acknowledgement ..................................................................................................................... ii

List of Figures ............................................................................................................................ iii

Chapter 1 .................................................................................................................................... 3

1.1 Objective ........................................................................... Error! Bookmark not defined.

1.2 Overall deisgn circuit ....................................................................................................... .1

Chapter 2 .................................................................................................................................... 2

2.1 Choosing the Solar Panel .................................................................................................. 2

2.2 Calculation for the PWL model ........................................................................................ 2

2.3 Solar Panel Circuit Model ................................................................................................. 4

2.4 Simulation Result.............................................................................................................. 5

2.5 Commercial Part ............................................................................................................... 6

Chapter 3 .................................................................................................................................... 7

3.1 DC-DC converter ............................................................................................................... 7

3.2 DC-DC Sepic Converter ..................................................................................................... 9

3.3 Calculations ........................................................................................................................ 10

3.4 Simulation Result............................................................................................................ 11

3.5 Commercial Parts ................................................................................................................ 11

Chapter 4 .................................................................................................................................... 13

4.1 MPPT SYSTEM.................................................................................................................. 13

4.2 MPPT Controller ............................................................................................................. 13

4.3 Simulation Result ................................................................................................................. 15

4.4 Commercial Part. ............................................................................................................ 16

Chapter 5 .................................................................................................................................... 17

5.1 Battery Circuit Model ........................................................................................................... 17

5.2 Calculation .......................................................................................................................... 18

5.3 Simulation Result ................................................................................................................. 19

5.4 Commercial part.................................................................................................................. 19

Chapter 6 .................................................................................................................................. 20

6.1 Conclusion ...................................................................................................................... 20

References ............................................................................................................................... 22

i

Abstract

The report will cover the utilization of solar cells and the power distribution generated from

solar cells. This includes the modeling a solar cell from scratch, studying and understanding

the V-I characteristic curve of the model in comparison with the commercial product.

Other areas like the SEPIC converter circuit, individual components of the DC-DC converter

will also be studied through calculation and simulation. Maximum power point tracking

circuit is also designed to improve the efficiency of the designed circuit.

Lastly, a battery model is built to complete the system.

ii

Acknowledgement

Would like to express our sincere gratitude to A/P Ali Iftekhar Maswood and A/P Luo Fang

Lin for their dedicated teaching and guidance in this module. This module has enriched us in

the knowledge of Solar Energy and the type of Conversion Technique that we can used to

conserve energy. This also mark an important episode in our commitment in global warming

as well as making use of such renewable energy collected from the natural resources and by

replenishing them naturally without causing harm to the environment.

iii

List of Figures

Figure 1 Overall Design Circuit Diagram ......................................... Error! Bookmark not defined.

Figure 2 Chosen Solar Panel ........................................................ Error! Bookmark not defined.

Figure 3 PWL model and I-V curve .............................................. Error! Bookmark not defined.

Figure 4 Solar Panel Equivalent .................................................. Error! Bookmark not defined.

Figure 5 450W Solar Panel Equivalent Circuit ............................ Error! Bookmark not defined.

Figure 6 Simulated VI & Max Power Characteristic Curve ......................................................... 2

Figure 7 Buck Converter ............................................................................................................. 5

Figure 8 Boost Converter ........................................................................................................... 5

Figure 9 Buck Boost Converter .................................................................................................. 5

Figure 10 Sepic DC-DC Converter with Resistor ........................................................................ 7

Figure 11 Result for the SEPIC DC-DC converter........................................................................ 8

Figure 12 DC-DC SEPIC Converter with MPPT............................................................................ 8

Figure 13 Description of MPPT System ...................................................................................... 9

Figure 14 Truth table of XOR Gate .............................................. Error! Bookmark not defined.

Figure 15 Simulation of MPPT Controller ................................................................................ 13

Figure 16 Battery Equivalent Circuit ........................................................................................ 14

Figure 17 Simulation result for Battery charging and Output power ...................................... 15

1

Chapter 1

1.1 Objective

Design a solar panel that is able to charge up a 12-V Battery rated with 30 ampere-hour

(Ah). This will include the designing, developing and implementing a suitable DC-DC buck-

Boost converter to supply the battery load. Maximum PowerPoint Tracking (MPPT) will be

used to extract the maximum power from the solar panel. Apart from designing the circuits,

choice of material construction and sizing of the components will also be studied and

investigated in this project. PSIM software will be used as the simulator to study and

determine the response and behaviour of the circuit. Figure 1 in this chapter will

demonstrate the overall circuit for the design and in the later chapters will describe in

detail.

1.2 Overall Design Circuit

Figure 1: Overall Design Circuit Diagram

2

Chapter 2

2.1 Choosing the Solar Panel

To verify the proposed circuit module, we are choosing SSP150M solar panel and the

specification of the panel as below.

Figure 2: Chosen Solar Panel

Looking at the specification for 150W Sunshine Solar Panel, a total number of 3 solar

modules will be required to produce an output of 450W.

2.2 Calculation for the PWL model:

With the reference from the paper: “Improved Circuit Model of Photovoltaic Array” by

Mohamed Azab. We are going to calculate the four line segments from the I-V curve in

figure 2 with the following consideration.

- PV parameters at maximum power point (MPP): Vmax=18.10V and Imax=8.31A.

- Vertex points are located at: 0.9 MPP, MPP, and 1.1MPP.

- The model assumes that: Von1<Von2<Von3.

Figure 3: PWL model and I-V curve

The operation in each segment is explained as below:

Specification for 150W Sunshine Solar panel

Maximum Power (Pmax) 150W

Maximum Voltage (Vmax) 18.10V

Maximum Current (Imax) 8.31A

Open Circuit Voltage (Voc) 22.4V

Short Circuit Current (Isc) 8.66A

3

Segment 1: (VD < Von1)

Von1=0.9MPP=0.9*18.10=16.29V.

When the generated voltage is less than Von1 all the diodes are OFF, and no current flows

through the diode (ID1= ID2= ID3=0).

Thus, the current generated from PV flows through the load and only small portion can flow

through the shunt resistance Rsh, therefore the current is nearly constant in this segment.

Segment 2: (Von1 <= VD < Von2)

Diode D1 is ON.

Since, The PV current fails from 8.66A (Isc) to 8.31A (Imax), thus the current through diode D1

is:

ID1= 8.66-8.31=0.35A.

Ron1 is calculated from the following equation:

Ron1 = (Vmax – 0.9 Vmax) / ID1

= (18.10 – 16.29) / 0.35 = 5.171Ω.

Segment 3: (Von2 <= VD < Von3)

Diode D1 and D2 are ON.

The current through D1 is calculated as:

ID1 = (1.1 Vmax – 0.9 Vmax) / Ron1

= (19.91 – 16.29) / 5.171 = 0.7A.

Assume: Output Current (Io) = 70% of Imax.

Therefore, Io = 8.31 * 0.7

= 5.817A.

The current through diode D2 is:

ID2 = Isc – ID1 – Io

4

= 8.66 – 0.7 – 5.817 = 2.143 A

Ron2 is computed using the equation as shown:

Ron2 = (1.1Vmax – Vmax) / ID2

= (19.91 – 18.1) / 2.143 = 0.8446 Ω.

Segment 4: (Von3 <= VD <= VOC)

All Diodes are ON.

The PV load current is zero at open circuit point, the open circuit voltage is VOC=22.4V.

The current through D1 is calculated from the equation:

ID1 = (VOC – 1.1Vmax) / Ron1

= (22.4 – 19.91) / 5.171 = 0.4813 A

The current through diode D2 is

ID2 = (VOC – 1.1Vmax) / Ron2

= (22.4 – 19.91) / 0.8446 = 2.9484A

The current through diode D3 is:

ID3 = ISC – ID1 – ID2– Io

= 8.66 – 0.4813 – 2.9484 - 0 = 5.2303A

Therefore, R3 is calculated as:

Ron3 = (VOC – 1.1Vmax) / ID3

= (22.4 – 19.91) / 5.2303 = 0.476 Ω.

2.3 Solar Panel Circuit Model:

The voltage V1, V2 & V3 are selected as 16.29V, 18.10V and 19.91V respectively.

Assume, parallel resistor large as possible and shunt resistor small as possible.

Therefore, Rp=10KΩ and Rsh = 0.001 Ω.

5

Figure 4 Solar Panel Equivalent

After evaluating all the parameters, we can then group 3 x solar panel in series so, that we

could achieve the overall output power about 450W. which as shown in Figure 5 below.

Figure 5 450W Solar Panel Equivalent Circuit

2.4 Simulation Result

Figure 6 Simulated VI & Max Power Characteristic Curve

6

The maximum power from the 3 x 150W solar panel is 448W at the point, where the voltage

is 53.92V and the current is 8.32A. As, we know the output voltage and current of the solar

panel may vary according to the sun light. So, in order to charge the 12V30AH battery, we

need to maintain the voltage and current at the same time, we need to get the maximum

power from the solar panel. So, we will be using buck-boost converter with MPPT

technique.

2.5 Commercial Part

Sunshine Solar Panels 150W 12V Monocrystalline

Specifications

Product Code. SSP150M

Max Power. 150W ± 3%

Max Power Voltage. 18.10V

Max Power Current. 8.31A

Open Circuit Voltage. 22.40V

Short Circuit Current. 8.66A

Normal Operating Cell Temp. -45 to 80°C

Max System Voltage. DC1000V

Weight. 11.2Kgs

Dimensions. 669 x 1470 x 35mm

PRODUCT CODE: SSP150M

Price: $396

Part Number Description Manufacturer Price per

Part

Parts needed Total cost Remarks

SSP150M 150W solar

panel

Sunshine 396$ 03 1188$ 3 panel

will be

connect

in series

7

Chapter 3

3.1 DC-DC converter

DC–DC converter consists of passive store devices together with switches that simplify the

unregulated DC input voltages conversion to a regulated DC output voltage level. This

converter can be categorised by two topologies which are isolated and non-isolated where

these topologies will provide the characteristics of each converter and its output level. DC-

DC converters are designed to equip with LC filter to reduce the ripple content in output

voltage and current. From the ripple characteristics, the converters are classified to operate

in two distinct modes which are: continuous conduction mode (CCM) or Discontinuous

Conduction Mode (DCM) where the load current drops to zero between the switching

cycles. The common isolated topologies are fly back, forward, push-pull and bridge

configurations. While for non-isolated converter are buck, boost and buck-boost which are

widely used in industrial DC motor drives as the configuration could provide smooth

acceleration, high efficiency and fast dynamics.

Buck Converter

Figure 7 Buck Converter

This converter compromises of two energy storing elements and two switches. Buck

converter produces an output voltage that is lower than its input voltage. This converter is

also known as step down converter.

8

Boost Converter

Figure 8 Boost Converter

This converter produce output voltage that is greater than its input voltage. The inductor

and switch positions are interchanged compared to buck converters. This converter is also

known as step-up converter.

Buck-Boost Converter

Figure 9 Buck Boost Converter

This converter is a combination of the said two converters and it depends on the switching

duty cycle D, buck-boost converters are able to produce an output voltage magnitude that is

either greater or less than its input voltage. The output voltage produced is in opposite

polarity than its input voltage. In this design, DC-DC converters are used to transfer energy

from a solar cell to charge the battery where fixed DC output voltage and continuous load

current can be obtained.

9

3.2 DC-DC Sepic Converter

Figure 10 Sepic DC-DC Converter with Resistor

The Single-Ended Primary-Inductance Converter (SEPIC) is a DC/DC-converter topology that

provides a positive regulated output voltage from an input voltage that varies from above to

below the output voltage. Similar to buck-boost converter, SEPIC converter has an addition

pair of energy storage devices in inductor and capacitor and the main storage device in this

topology is also the capacitor.

The output voltage can be regulated by controlling the duty cycle of the switch and the duty

cycle presents the ratio of the ON and OFF period for a switch in one cycle. It also allows

regulation of the input voltage through controlling the duty cycle. SEPICs are useful in

applications in which a battery voltage can be above and below that of the regulator’s

intended output for example, a single lithium ion battery typically discharges from 4.2V to

3V and if other components require 3.3v then the SEPIC would be effective. SEPIC converter

can be operating in two modes which are continuous-conduction and discontinuous

conduction. In continuous conduction mode, the current through the inductor never falls to

zero while in discontinuous conduction mode, the current through the inductor allow to fall

to zero. The voltage drop and switching time of diode is critical to SEPIC’s reliability and

efficiency. The diode’s switching time needs to be extremely fast in order to not generate

high voltage spikes across the inductors, which could damage to the component. The

resistances in the inductors and the capacitors also play a major role to SEPIC’s efficiency

sosss

ssssss

10

and ripple. Inductors with lower series resistance allow less energy to be dissipated as heat,

resulting in greater efficiency. Capacitors with low equivalent series resistance should also

be used to minimize ripple and heat build-up.

3.3 Calculations

Consider,

Ripple is 5%, Frequency is 20KHz, L1 = L2, and C1 = C2.

- Input current, Ii = I for Pmax = 8.34A.

- Input voltage, Vi = V for Pmax = 53.95V.

Required,

- Output current, Io = 30A.

- Output voltage, V0 = 12V.

Calculations for computing Duty cycle:

𝑽𝟎=(𝑽𝒊*𝑫)/(𝟏−𝑫)

12/53.95 = D/(1-D)

D = 0.222/1.222

D = 0.182.

Calculations for computing inductor value:

𝐿1=(𝑉𝑖*𝐷*T)/Δ𝑖𝐿1

Δ𝑖𝐿1 = 5% of Input current, Vi=53.95, T=1/20000, D=0.182.

L1 = (53.95*0.182)/(20000*0.05*8.34)

= 1.177mH

Since, L1 = L2 = 1.177mH.

Calculations for computing capacitor value:

𝐶1= (𝐼𝑜*𝐷*T)/Δ𝑣𝐶1

= (30 x 0.182 x 2 x 10-5) / 0.6

11

= 455uF

Since, C1 = C2 = 455uF.

3.4 Simulation Result

Figure 11 Result for the SEPIC DC-DC converter

3.5 Commercial Parts

MOSFET

12

CAPACITOR (C1 & C2)

INDUCTOR 1 (L1)

INDUCTOR 2 (L2)

Part Number Description Manufacturer Price per

Part

Parts

needed

Total

cost

Remarks

IXTP28P065T-

ND

MOSFET P-channel

65V 28A to 220A

IXYS 2.79$ 01 2.79$ -

604D451F075

HP7

Capacitor, 450uF

with maximum

voltage is 75V.

Sprague 44.32$ 02 88.64$ For C1 and C2

Inductor with

maximum DC

current is 5A and

Inductance value

30mH.

Hammond 24.50$ 02 49$ For L1, we

required 15mH

so need to

connect the 2

pieces in parallel

RLB9012-

122KL

Inductor with

1.2mH

BOURNS 0.612$ 01 0.612$ For L2.

13

Chapter 4

4.1 MPPT SYSTEM

Solar cells must be operated at their maximum power point (MPP) in order to achieve

optimised performance. The maximum power point varies with illumination, radiation,

temperature and other effects that produce non-linear output efficiency. Hence, a

maximum power point tracker (MPPT) system is used to vary the operating point so that the

solar cells are able to deliver maximum power achievable. A single MPPT system usually

consists of a controller and a dc-dc converter connected between the solar panels and

battery. In this design, a Sepic converter is used as the dc-dc converter. Figure 12 shows the

overall MPPT system inside this design.

Figure 12 DC-DC SEPIC Converter with MPPT

4.2 MPPT Controller

In this simulation design, the controller consists of components that vary with that of actual

components found in the market. This is due to the simplicity that is given in the simulation

software. The components in the simulation include a multiplier, differentiators, an XOR

logic gate and a D flip-flop as shown in Figure 17.

14

Figure 13 Description of MPPT System

The multiplier is used to calculate the power of the solar cells by multiplying the voltage and

current signals from the solar cells itself. In this design, a voltage and current sensor is used

to sense the incoming signals and is then passed through the multiplier. The power signal,

derived from the multiplier, and the voltage signal will then go through separate

differentiators. If the power is increasing, the differentiator would output a negative value

and if the power is decreasing, the differentiator would out put a positive value. Similarly,

the voltage signal is passed through the same process.

From the circuit referred in Figure 14, the outputs of differentiators’ dV/dt and dP/dt were

fed into a comparator. The comparators operate in such a way that it compares both the

differentiated signals with respect to ground, and switches the output to indicate which is

larger. The comparator would output a positive ‘1’ value if the differentiated value is

positive and output a zero if the differentiated value is negative, smaller than ground. Once

both outputs of the comparators have been generated, the signals will then be fed to an

XOR Logic Gate. The truth table of an XOR gate referred to the circuit in figure below.

15

Figure 14 Truth table of XOR Gate

The output signal of the XOR Logic gate is often undesirable to be fed to the gate driver of

the transistor due to its high frequency switching which is unsuitable for the DC-DC

converter. Hence, a D Flip-Flop is used as a device to send the signal to the gate driver with

its input signal received from the XOR Logic gate to prevent the high frequency switching

from happening. A D Flip-Flop is also considered in this design due to its simplicity.

4.3 Simulation Result

Figure 15 Simulation of MPPT Controller

16

From the results we obtained, we can conclude the power across the load resistor is

approximately 425W. Hence, it can be deduce that the MPPT controller is working correctly.

4.4 Commercial Part.

Part Number Description Manufacturer Price per

Part

Parts needed Total cost Remarks

30amppt-OLD 30A 12/24V

MPPT charge

controller

TRACER 141$ 01 141$ -

17

Chapter 5

5.1 Battery Circuit Model

There are various existing designs to model a battery. In this system design, to design a 12V

30AH battery, we are incorporating Z.M. Salameh’s battery model design which takes into

account the dynamics during the charging and discharging phase. Figure 16, shows the

schematic of the model of the battery.

Figure 16 Battery Equivalent Circuit

Co = Overvoltage capacitance

Cbatt = Battery capacity

Roc = Charge overvoltage resistance

Rod = Discharge overvoltage resistance

Ric = Internal resistance for charge

Rid = Internal resistance for discharge

Rsd = Self-discharge resistance

All these parameters mentioned above are needed to be evaluated. However, only some of

these parameters which include the Cbatt and Rsd can be calculated while the rest can only be

estimated due to lack of some features in the datasheet. While the parameters of the Cbatt

and Rsd is calculated using the battery voltage and capacity.

18

5.2 Calculation

Assume, Co = 200F

Roc = 0.1Ω

Rod = 0.1Ω

Ric = 8.9mΩ

Rid = 8.9mΩ

Need to calculate, Rsd and Cbatt

Amount of total charge in 12V30AH battery is:

Q = 30A X 3600

= 108000 coulombs

Therfore, Cbatt = Q/V

= 108000/12

= 9000F

The state of charge of the battery is expected to be at 83% in 6months at a 20°C. Hence, the

charge expired in due 6 months is:

Q (discharge) = 108000 x 0.17

=18360 coulombs

The current flowing through the resistor Rsd, is then calculated using the following:

Current flowing through Irsd = 18360/ (6 x 30 x 24 x 60 x 60)

= 1.180 x 10-3 A

The voltage terminal at full charge is determined to be 14.5V as stated in the datasheet.

Therefore, the self-discharge resistance Rsd is then calculated to be:

Rsd = V/Isd

= 14.5/1.180 x 10-3

= 12.288KΩ.

19

5.3 Simulation Result

Figure 17 Simulation result for Battery charging and Output power

Figure 17, shows the voltage, current and output power at terminal. It can be seen that the

voltage is increasing while the current is decreasing gradually. This shows that the battery is

charging and the output power from the converter is 443.5W which is the maximum power

taking from the 450W solar panel.

5.4 Commercial part

Part Number Description Manufacturer Price per

Part

Parts needed Total cost Remarks

EVX12300 12V 30AH

AGM battery

CSB 74.99$ 01 74.99$ -

20

Chapter 6

6.1 Conclusion

In the chapter 2, we do model for the 150W solar panel and then three 150W panel

connected in series to get the output power of 450W and also the model V-I curve match

with the practical solar panel curve. All the results are verified by PSIM. The maximum

power we got, 448W.

In chapter 3, we design the DC-DC sepic converter by calculating the values for inductor,

capacitor and duty cycle to get the output voltage 12V, and the output current 30A with 5%

ripple. In order, to take the maximum power from the solar panel, in the chapter 4, we do

the maximum power point tracking circuit by sensing the output voltage and output current

from the solar panel. The MPPT circuit drives the MOSFET switch by ON and OFF to get the

maximum output power from the panel. From our design, we got the simulation result with

maximum power from the solar panel is 425W.

In chapter 5, we model the 12V 30AH lead acid battery and the output of the solar panel

with DC-DC converter and MPPT is given to the designed battery model. We got the

simulation result where the voltage is increasing and the current is decreasing which shows

the battery is charging. We able to get the final maximum output power is 443.5W

We able to find all the commercial components for the designed circuit, and it show the

circuit which we designed is physically feasible. Below, is the list of commercial components:

Part

Number

Description Manufacturer Price per

Part

Parts

needed

Total

cost

Remarks

SSP150M 150W solar panel Sunshine 396$ 03 1188$ 3 panel will be

connect in series

IXTP28P065

T-ND

MOSFET P-channel

65V 28A to 220A

IXYS 2.79$ 01 2.79$ -

604D451F07

5HP7

Capacitor, 450uF

with maximum

voltage is 75V.

Sprague 44.32$ 02 88.64$ For C1 and C2

Inductor with Hammond 24.50$ 02 49$ For L1, we required

21

maximum DC current

is 5A and Inductance

value 30mH.

15mH so need to

connect the 2 pieces

in parallel

RLB9012-

122KL

Inductor with 1.2mH BOURNS 0.612$ 01 0.612$ For L2.

30amppt-

OLD

30A 12/24V MPPT

charge controller

TRACER 141$ 01 141$ -

EVX12300 12V 30AH AGM

battery

CSB 74.99$ 01 74.99$ -

The total cost to build the designed model is $1545.

22

References

1. Azab, M. (2009). Improved circuit model of photovoltaic array. International journal

of electrical power and energy systems engineering, 2(3), 185-188.

2. Falin, J. (2008). Designing DC/DC converters based on SEPIC topology.Analog

Applications, 19-20.

3. http://pveducation.org/pvcdrom/solar-cell-operation/short-circuit-current.

Retrieved 14th April, 2016.

4. http://us.sunpowercorp.com/solar-resources/performance-reliability/solar-

efficiency/. Retrieved 14th April, 2016.

5. http://www.qrg.northwestern.edu/projects/vss/docs/power/1-what-are-solar-

panels.html. Retrieved 14th April, 2016.

6. NTU EEE, EE4504 Lecture notes

7. Salameh, Z. M., Casacca, M. A., & Lynch, W. A. (1992). A mathematical model for

lead-acid batteries. Energy Conversion, IEEE Transactions on,7(1), 93-98.