Characterisation Of Light Sources And Rechargeable Batteries

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02 Nov 2017

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Chapter 2

2.1 Introduction

Initially, the solar cell are analysed in indoor light condition at the lab with help of fig 1.5 from the chapter 1 and the output power of the cell can be varied based on the properties of cell, light condition and temperature. Generally the electrical energy is generated from the solar cell, which already discussed in the previous section. This sections discussing about properties analysis of photovoltaic cell and energy storage devices. Here, the battery is necessary to accumulate energy of the cell and the photo voltage source is used to charge the battery with element of power specification, as shown in fig 2.1. The batteries were charged at the time of light and the light resources should be normal. The battery device is helpful to give a constant source for load resistance, but solar cell source is always based on the falling of light and not at the load resistance. [4]

Fig 2.1 the structure of photovoltaic arranged for charge rule and storage [4]

The photo voltage is based on the falling of light at the solar cell in indoor condition and the generated source is connected to the load resistance circuit. However, the characteristics of solar cell is always depends on the light source and parameters. In standard test condition, the energy of cell is based on ISC and VOC and it is recorded from the curve of cell (ref fig 1.6). [20]

2.2 General properties of solar cell

The properties of cell is based on the nature of light source and temperature and it is necessary to consider the size of the cell for attain certain power. In this method, the solar cell values are noted based on the mounted ideal circuit and the characteristics of the solar cell are analysed.

2.2.1 Characteristic curve

In lab, the solar cell setup was constructed with help of ideal diode equivalent circuit, ref fig 1.5 (from the previous chapter). The crystal silicon solar cell are used with size of 1×1 cm2 cell and the productivity values are measured in indoor light at the time of voltage and current value are noted by change the load resistance value between zero and infinity in the mounted connection to form IV curve. The maximum output power can be calculated with help of V and I values and the Im & Vm are determined in the area of maximum power of the cell, as shown in fig 2.2. The value of shut and series resistance is calculated from the IV characteristic of the cell.

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Fig 2.2 Characteristic of solar cell for indoor light condition

For example, let us consider fig 1.6 (from the chapter 1) the shut and series can be written as follows,

RSH=∆VSC/∆ISC & RS=∆VOC/∆IOC (2.1)

From the fig 2.2, the values of RS, RSH, ISC, and VOC are calculated (refer fig 1.6 and 2.1 equation to find those values) and the values are tabulated in the table 2.1. Here, the ISC and VOC values are approximately equal i.e., the values are not clearly found at I=0 and V=0 because at the time of experiment, there is no low resistor such as below 10Ω in the lab. Therefore, the values are assumed approximately from the IV curve (fig 2.2).

2.2.2 Efficiency

The percentage of solar cell is determined from the equation 1.5 (refer chapter 1) and the power striking of the cell is calculated by using the irradiance energy of cell in the active area. Therefore, the irradiance is determined by light falling to the distance between cell and light, so the irradiance of cell is 25W/0.292 ≈ 300W/m2. Then, the irradiance is multiplied with active area of cell and here active area of cell is 1cm2. So, the power intensity of cell is 300×0.0001≈ 0.03W. From the IV curve, the maximum power is noted and the efficiency of the solar cell is calculated, the percentage is mentioned in the table 2.1.

2.2.3 Simulation of matlab and comparison

In this method, at last stage the cell is simulated in the matlab with the help of estimated value of RS, RSH, ISC, VOC, n and Isat. Here, the n is ideality factor and it is determined using different intensities of cell, the ISC and VOC are noted for different intensity and then the characteristic is shaped for noted VOC and ln(ISC). From the

Cell name

Size (cm2)

RSH

(Ω)

RS

(Ω)

ISC

(A)

VOC

(V)

n

Isat (µA)

c-Si

1

203.9

63.7

0.0037

0.505

6.07

0.015

Im

(A)

Vm

(V)

ÅŠ

(%)

0.272

0.000272

3.78

Table 2.1 Parameters values for c-Si cell

shaped curve for different intensity, the n is found by drawing slope in the curve and the saturation current of cell also determined by using equation 1.2(from chapter 1), the values of all the parameter of cell are tabulated in table 2.1. The matlab simulation are constructed based on one diode model (refer fig 1.5 from chapter 1) and the measured cell values are noted separately to find strength of the cell and those values are used to

Fig 2.3 Simulated IV characteristics and measured value of a c-Si cell

compare with matlab simulated result of the solar cell. The cell is simulated in matlab by giving estimated parameters of c-Si, which is mentioned in the table 2.1 and then noted value of c-Si cell is added in simulation and it is shown in the fig 2.3.

2.3 Power changes in the cell

The silicon solar cell is used to find the energy difference and the size of the cell is 6×3 cm2. In general, the power changes in the cell are depends on the nature of light condition. The Si cell is examined for whole day to measure strength of the cell in changing indoor light condition and the cell is connected with the system using logging software. The system is constructed by using the load resistance of the cell.

2.3.1 IV Characteristics

The silicon cell is connected as per equivalent circuit (refer fig 1.5) and the values of voltage are noted in indoor light by changing the load resistance. From the noted value, the IV curve is shaped and maximum power is estimated. Therefore, the maximum current and maximum voltage joining point in the IV curve is the load resistance (≈1500Ω) of the Si cell and that value can be used for functioning of the cell, as shown in the fig 2.4. As per 2.2 sections, the all parameters values of the Si cell are estimated, but there is no need to find those values for this section. [20]

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Fig 2.4 IV and PV characteristics of Si cell

2.3.2 Output power varies in day time

The Si cell is tested in the day time for indoor light and the cell is mounted with 1500Ω, which is load resistance for functioning the cell and that one is estimated in the previous section of 2.3.1 using IV curve. In this section, the solar cell is constructed with system for measuring the voltage and current in the day time using logging software, as shown in fig 2.5. The software is useful to record the values of current and voltage at the cell with different time interval in day time. Here, the current and voltage are changed continuously during day period because of change in light source and temperature. Therefore, the solar cell output power is changes over the day time and also parameters of cell are based on the changes in voltage and current. The solar cell is tested nearly more than six and half hours in a day. [22]

Fig 2.5 Block diagram for power affecting in Si cell measured using computer

Later, the recorded values are used to analysis the Si cell with help of different voltage, current and power of the cell for different time period in day time, as shown in the table 2.2. Generally, the power is easily estimated when the values of voltage and current are known for particular cell and the power of the cell is calculated at functioning cell with load resistance. From the recorded values (refer table 2.2), the different characteristics curve are expressed in Si cell for different time period with different values of current, voltage and power. In that characteristic, there is problem in current curve shape; unfortunately the current value is reached to zero but there is no zero data is present in measured value. There is a problem in creating graph using excel sheet and tried to create good curve, but unable to sort out problem in that shape of current. However, the curve is shaped for Si cell with various times, as shown in fig 2.6 and the maximum power is attained at the time of 16:00 hours. Therefore, the power of solar cell is changed according to the light condition and effects of different parameters.

Resistor=1500Ω (maximum power at the solar cell in 300k)

Time (hrs)

Voltage (V)

Current (mA)

Power (mW)

9:40

0.6342

0.4228

0.2682

10:00

0.6745

0.4497

0.3033

10:20

0.6783

0.4522

0.3067

10:40

0.6793

0.4529

0.3076

11:00

0.6836

0.4558

0.3116

11:20

0.6815

0.4543

0.3096

11:40

0.6764

0.4510

0.3050

12:00

0.67914

0.4528

0.3075

12:20

0.6765

0.4510

0.3051

14:00

0.6721

0.4481

0.3011

14:20

0.6715

0.4477

0.3006

14:40

0.7017

0.4678

0.3282

15:00

0.7094

0.4730

0.3355

15:20

0.7083

o.4722

0.3344

15:40

0.6943

0.4629

0.3213

16:00

0.7129

0.4753

0.3388

Fig 2.6 Characteristics varies at Si cell in

day time

Table 2.2 Noted data at Si cell

using logging software

2.4 Analysis of battery discharge

In this section, the detailed about the storage device which was used in the lab and the device is used to store the energy in the day time. Later, that energy might be needed as a source for the system. This section says about charging and discharging of the storage device.

2.4.1 Charging and discharging of Ni-MH battery

In the lab, the Ni-MH battery was used to analysis the charging and discharging the battery. At first, the battery out let is measured, it is 3.68V and then the battery was charged directly through voltage source with 500mAh because the current value is specified in the battery (≈600mAh). The battery was charged for thirty minutes through voltage source, after it was attained 3.90V and assumed as a fully charged. Initially, the battery is discharged to 3.87V by open circuit voltage with 6Ω resistor for an hour. Here, the 6Ω is used because of 600mAh current i.e., battery voltage to the current of the battery (3.6V/600mAh ≈ 6Ω). Again, the device is charged with voltage source and it became 3.90V, later tried to discharge the device with 300mAh i.e., discharge of device is measured for half C-rate (which is half value of specified current of battery). Therefore, the battery is discharge at 300mAh with 12Ω resistor (that means 3.6V/300mAh ≈ 12Ω). However, the battery is discharged by using 10Ω value because at the time of experiment they are unable to find 12Ω, so instead of that 10Ω is used and it is only closest resistor variable got in the lab. After two hours of discharging, the device is dropped to 3.09V and it was tabulated for each and every 10% of battery discharge in the table 2.3.

Charged percent of a battery (%)

Ni-MH 3.6 V battery (V)

Percent changes in placed battery (%)

100

3.9

8.333

90

3.81

5.833

80

3.72

3.333

70

3.63

0.833

60

3.54

-1.667

50

3.45

-4.167

40

3.36

-6.667

30

3.27

-9.167

20

3.18

-11.667

10

3.09

-14.167

0

<3.09

<-14.167

Fig 2.3 Valuation of 3.6V battery discharge at 300mAh

The characteristics curve is shaped for discharging 3.6V at 300mAh by using the table 2.3. Similarly, discharging was completed for same battery at 300mAh before charging the battery by voltage source. The discharge of battery is completed in a day and the characteristics are shaped. The discharging of 3.68V battery at 300mAh, it is dropped to 3.62V (≈ the battery is discharged at the rate of 1.6% per day) and it was expressed in the characteristics for both before and after charging, as shown in the below fig 2.7.

Fig 2.7 Self-discharge of 3.6V Ni-MH battery

2.5 Trickle charging the battery using Si cell

The 3.6V Ni-MH battery is used to charge the device in the lab by using Si solar cell in indoor light condition. In the previous section 2.4, the self-discharge of 3.6V Ni-MH battery is estimated; it is nearly 1.6% per day. The device is charged through certain circuit connection, which is based on the calculation and the rate of percentage of 3.6V battery is discharged.

2.5.1 Current estimation for charging 3.6V battery

The self-discharge of 3.6V battery is dropped to 3.62V from 3.68V in a day. Therefore, the calculation for 3.6V battery is based on the section 2.4. and estimation of current can be written as follows,

here, the time should be in sec since 60×60=3600sec in an hour,

for 24 hours of discharge can be 24×3600≈86400sec,

discharge the battery at 300mAh is 300×10-3×3600≈1080 Coulombs

therefore, the current needed for charging is estimated as follows,

1080/86400 ≈ 0.0125A ≈ 12.5mA

Fig 2.8 equivalent circuit for charging battery

So, the 3.6V battery is trickle charge with load current of 12.5mA using equivalent circuit for a Si cell, as shown in the below fig 2.8. Here, six Si solar cells are used for charging the 3.6V battery with a diode and 22kΩ and the output of the circuit is almost equal to the 4.2V. Therefore, this circuit is useful to charge the 3.6v battery and initially the battery voltage is almost 3.62V, later the device is connected with system setup and it is charged for a day, at the end of the day the battery out let is increased to 3.68V. So the trickle charger circuit is correctly designed by using previous section value.



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