Solar System V2

NATIONAL INSTITUTE OF TECHNOLOGY, DURGAPUR

DEPARTMENT OF ELECTRICAL ENGINEERING

ADVANCED ELECTRICAL ENGINEERING LABORATORY

Experiment No. 4A.

Objectives: To measure the current voltage characteristics of a crystalline silicon solar cell

a) Measurement by using 4 quadrant power supply and solar cell as load

b) Measurement by using solar cell as power source under illumination

Introduction

A crystalline silicon solar cell is basically a large area np junction diode as shown in Figure 1. In dark, the currentvoltage curve lies in quadrants I and III as for a diode and is described as:

I = I0 [exp (qV/kT)1] (1) Where, I is current flowing through the device, q is charge on electron, V is voltage across the device, T is temperature in deg Kelvin, k is Boltzmann constant, and I0 is reverse saturation current. Under illumination, the curve is shifted such that now it lies in quadrants I, III, and IV as shown

in Figure 1.The equation describing the IV under illumination is:

I= I0 [exp (qV/kT) 1] Iph (2) Where, Iph is the light induced current , and depends on the intensity of illumination. We see that when leads of the solar cell are shorted (V=0), a current Isc = Iph flows through the leads (short circuit current). Voltage across the leads is Voc if the leads are kept unconnected (open circuit voltage).

Figure 1: Crystalline Silicon Solar Cell (left) and its IV Characteristics Curve (right)


Figure 2a shows a schematic of a 4quadrant power supply with solar cell connected as load. A normal power supply in the laboratory can be used to measure IV in quadrants I and III (as required for the dark IV). For an illuminated solar cell, for the part of the IV curve in quadrant IV, current is negative, while the voltage is positive. It is not possible to measure negative

current at positive voltage by using a normal power supply. However, the 4quadrant supply is able to measure these characteristics since it can source as well as sink current for voltage of any polarity.

Figure 2a: 4Quadrant Power supply schematic with the solar cell connected as load

Figure 2b: Solar cell as source of power with a variable load resistor R

Figure 2b shows schematic of solar cell connected as source of power to variable R load. Under

illumination, the device behaves as a source of electrical power (solar cell) in the 4th

quadrant, with the nside terminal becoming ()ve with respect to pside terminal. It is customary to measure and present the

solar cell IV characteristics in only the 4th quadrant part and inverted as shown in Figure 3 [for a typical Bharat Electronics (BEL) solar cell which is used in the present apparatus]. The inverted IV (Figure 3) is described as:

I = Iph I0 [exp ( qV/kT) 1] (3)


BEL Solar Cell Area ~ 14 cm2, Test Conditions: Irradiance = 100 mW/cm2 Temperature = 25C VOC = 600 mV, ISC= 490 mA, FF = 0.75

Figure 3: IV Curve (inverted) of Solar Cell supplied by BEL

Apparatus Required:

Sr. No. Unit Description/Rating Qty

1 Solar Cell BEL Make cSi cell 1

2 Light Source: Halogen Lamps 50W ,230V

2

Black cloth

3 31/2 digit Voltmeter 0 to 1.999 V

1

4 Potentiometers 010 single turn,

2

0100 10 turn

5 31/2 digit Ammeter 0 to 1999 mA 1

6 4 Quadrant Supply 0 to +/10 V, 0 to +/1 A 1

Experiment:

In this experiment, we shall measure IV characteristics of a single solar cell by two different ways

to get characteristics as shown in Figure 1 (Part A) and as shown in Figure 3 (Part B).


The solar simulator housing has two solar cells (both cells will be used for the experiment on

two cells in series and parallel). In order to do the experiment with single cell, see the figure

below and use jumper between leads B and D as shown. This allows us to measure solar cell kept on the left side in the housing.

Figure 3: IV Curve (inverted) of Solar Cell supplied by BEL

Part A (Measurement using 4 quadrant power supply with solar cell as load (Figure 2a)):

i) To connect cell to 4quadrant power supply, keep DPDT switch S in the EXT position.

ii) Connect the power supply to the leads of the solar cell , brought out on right side of the

lamp/ solar cell housing.

Iii) Cover the solar cells with black cloth for measuring IV in the dark. Keep the lamps off

and shut the door in front of the housing.

iv) Measure IV by gradually turning the bias voltage knob located on the left side of the 4

quadrant supply (the right side knob should be kept in the extreme clockwise position

and should not be moved during the experiment).

The applied bias voltage (in volts) is read on the meter on the left. The meter on the right

measures the current in amperes. Measure IV over bias voltage from 0.7 V to +0.7 V.

v) REMOVE THE BLACK CLOTH AND TAKE IT OUT OF THE BOX. Turn on the fan.

Turn ON the two lamp switches L1 and L2.

vi) Adjust the bias voltage knob on the 4 quad supply till the current shows zero reading.

With this adjustment, the voltmeter reads open circuit voltage Voc. Wait until the reading becomes stable (Voc reading will decrease initially because of rise of cell temperature due to heat from the lamps).

When reading becomes stable, measure IV characteristics by gradually varying the bias voltage from 0.7 V to + 0.7 V.

vii) Turn off the lamps and turn the bias voltage knob till the voltage is zero.


Part B (Measurement using solar cell under illumination as power source (Figure 2b)):

For this measurement, meters on the simulator panel will be used. Change the position the

DPDT switch S to INT.

i) There are two potentiometers. Turn them both clockwise so that resistance is maximum.

ii) Turn on the fan. Turn on the two lamp switches L1 and L2.

iii) See the reading on temperature indicator. Wait (a few minutes) till the temperature

reading stabilizes.

iv) Note the readings on current meter and voltmeter. This condition is close to the open

circuit condition; voltage will be high and current low.

v) Reduce resistance by turning 0100 ohm potentiometer anticlockwise. This is a 10turn

potentiometer. Take readings of V and I as R is varied.

vi) After the 100 ohm pot reaches the minimum setting (fully anticlockwise), turn 0 10

ohm potentiometer gradually anticlockwise and record IV readings.

Vii) 10 ohm pot is single turn. Once it reaches the minimum setting, current will be the

highest and voltage is the minimum. This condition is close to short circuit.

After reaching the minimum of the two pots and recording the readings, set the 10 ohm pot to maximum (fully clockwise). Then set the 100 ohm pot also to the maximum (fully clockwise). Check the voltmeter reading. If it is the same as noted in the beginning of the

measurement, this checks that the temperature did not change during the above measurements.


Observation:

Part (A) Record IV measurements in the dark in a tabular form between 0.7 to 0.7 V.

Sr. No. Voltage (mV) Current(mA)

1

2

3

4

5

6

7

8

Repeat the experiment under lighted conditions and record the IV readings in another table. Plot

graph similar to Figure 1. Part (B) Record your IV measurements in a tabular form, as shown below:


1

2

3

4

5

6

7

8

Plot I versus V on a graph paper, and compare with Fig. 3. Extend the curve to touch the ordinate and abscissa. Get values of VOC and ISC.


CurrentVoltage (IV) graph:

PowerVoltage (PV) graph:


Results:

Part (A) Using plot of ln(I) versus V at forward bias values greater than 100mV in the dark find I0. Using

this plot, get ideality factor of the diode, n by using the relation:

ln(I) = ln(I0) + qV/nkT

Part (B) Multiply I and V and plot P(mW) versus V. Determine the maximum power point Pm. Determine corresponding Vm and Im . Knowing Voc, Isc, Vm and Im , determine FF. FF = ( Vm.Im)/ (Voc.Isc) In order to find the efficiency, , we need to know the intensity of light incident on the solar cell. We have estimated the light intensity of two lamps with clear glass filter by using a pyranometer. The estimated intensity Pin ~ 900 mW/cm

2. Taking the area of cell 14 cm2, estimate the total light power Pin incident on the solar cell. Find efficiency of the cell.

= (Vm. Im) / Pin


Experiment No. 4B.

Objectives: To measure the overall current voltage characteristics of two crystalline silicon solar cells connected i) in series and ii) in parallel.

Introduction:

Cells connected in series / parallel combinations

follow Kirchoffs laws. For cells in series, the voltages generated by different cells add, the

current flowing in all the cells is the same. In the

case of cells in parallel combination, the voltage

across the combination is same, but we can draw

more current than obtainable from a single cell. By

measurement on series and parallel combinations

of two solar cells, we get curves of total

currentvoltage behavior of such combinations as compared to that of single cell shown in the figure.

Figure 1: Current Voltage Curve of Solar Cell

Apparatus Required:


1 Solar Cells BEL Make cSi cell 2

2 Light Source: Halogen Lamps 50W ,230V 2

3 Voltmeter 0 1.999 V 1

4 Potentiometers 010 single turn,

2

0100 10 turn

5 Ammeter 01999 mA 1

6 Jumper wires 2


Experiment:

Measurements are done using solar cells under illumination as power source.

Methodology of measurement is the same as used for a single cell. One can do both types

of measurements (using 4 quadrant supply and combination of cells, keeping the switch S

in EXT position). The method described here is by using the combination of cells as

source. For this measurement, the switch S will be in position INT. Figures given below

show the cells connected in series or in parallel by using jumpers appropriately ( For

series combination , connect jumper between B and C; for parallel combination , connect

one jumper between A and C and second jumper between B and D ).

Figure 2: Solar Cells connected in series and in parallel

i) There are two potentiometers. Turn both clockwise so that resistance is maximum.

ii) Turn on two halogen lamps in the centre. Turn on the fan.

iii) See the reading on temperature indicator. Wait till the temperature stabilizes.

iv) Note the readings on current meter and voltmeter. This condition is close to the open

circuit condition, voltage will be high and current low.

v) Reduce resistance by turning 0100 ohm potentiometer anticlockwise. This is a 10

turn potentiometer. Take readings of I and V as R is varied.

vi) After the 100 ohm pot reaches the minimum setting (fully anticlockwise), turn 010

ohm pot anticlockwise and record IV readings. 10 ohm pot is single turn. Once it

reaches the minimum setting, current will be the highest and voltage is minimum.

vii) After reaching the minimum of the 10 turn pot and recording the readings, set The 10

ohm pot to maximum ( fully clockwise) . Then set the 100 ohm pot also to the

maximum (fully clockwise).Check the voltmeter and current meter readings are same

as in the beginning.


Observations:

Record your IV measurements in a tabular form, as shown below:


1

2

3

4

5

6

7

8

Plot I versus V on a graph paper, extend the curve to touch the ordinate and abscissa. Get values of VOC and ISC. Compare with the corresponding result from experiment 5.

Plot of current versus voltage for cells in series


Plot of current versus voltage for cells in parallel

Results: Verify the behavior of Isc and Voc of cells connected in series and in parallel.


Experiment No. 4B.

Objectives:To measure the current voltage characteristics of a crystalline silicon solar cell

a) at different light intensities b) at different temperatures

Introduction:

a) Dependence on light intensity:

The equation describing the IV characteristics of solar cell with illumination is:

I = Iph I0 [exp ( qV/kT) 1] (1) where Iph is the light induced current , and varies linearly with the light intensity. Since Isc = Iph , we expect the short circuit current Isc to vary linearly with the light intensity. From Eq (1) Voc = (kT/q) ln [ (Iph + I0)/I0] ~ (kT/q) ln [Iph/I0]. Thus, the open circuit voltage is expected to vary logarithmically with the light intensity. In the experiment, we shall vary light intensity by i) putting a frosted glass plate, and ii) wire mesh filter between the lamps and the

cell, which attenuate light uniformly at all wavelengths.

b) Dependence on cell temperature: The solar cell characteristics are temperature sensitive. In order to appreciate this, consider the solar cell equation I = Iph I0 [exp(qV/kT) 1]. In this equation, I0 increases strongly with increase of cell temperature, expressed as:

I0 = qA[ (Dp/Lp.ND) + (Dn/Ln.NA)]ni2= I00 exp { Eg/kT } = A B T3 Exp (Eg/kT)

where Eg is band gap of the semiconductor ( silicon in the present case) and A is cell area, B = q[ (Dp/Lp.ND) + (Dn/Ln.NA)]{ 2( 2me*k/h2)3/2} { 2(2mh*k/h2)3/2} contains factors which are relatively temperature independent. As a result, the temperature dependent solar cell equation is:

I(T)

= Iph (T) I00 [exp {( qV Eg)/kT} 1]

Thus we get, Isc (T) = Iph(T) , and

Voc

(T)

= Eg (T)/q + (kT/q) {ln Isc (T) ln (ABT3)} (2)

Voc of Si solar cells decreases by about 2.2 mV/C. Since the band gap decreases by about 0.5 meV /C with increase of temperature that causes part of decrease in Voc.


Rest of the decrease of Voc is caused by the second factor in equation (2). In comparison to Voc , the change in Isc is relatively small as shown in Fig 1. The slight increase in Isc is due to enhanced absorption of solar spectrum due to reduction in the band gap with increase of temperature. Fractional decrease of Voc is nearly 10 times larger than fractional increase of Isc. As a result solar cell efficiency decreases with increasing cell temperature.

Figure 1: IV characteristics of solar cell as a function of temperature

Apparatus Required:


1 Solar Cell BEL Make cSi cell 1

Light Source: Halogen Lamps 50W ,230V 2

2 and Stainless steel sieves

4

Light Attenuators 46%,29%,14%,7,5%

3 Voltmeter 01999 mA 1

4 Potentiometers 010 ohm , o100 ohm 2

5 Ammeter 01999mA 1

6 Temperature indicator and

Room Temp to 70C 1 controller


Experiment: This experiment will be done on a single cell with the solar cell as the source of power and

two potentiometers as variable load. In order to do the experiment, keep the Switch S in the

INT position as done in Part B of Expt. 4A.

Part A (Light Intensity Dependence of Solar Cell Characteristics)


ii) Turn on two halogen lamps. Turn on the fan.


iv) Note the readings on current meter and voltmeter. This condition is close to the

open circuit condition, voltage will be high and current low.

v) Reduce resistance by turning 0100 ohm potentiometer anticlockwise. This is a

10 turn potentiometer. Take readings of I and V as R is varied.

vi) After the 100 ohm pot reaches the minimum setting (fully anticlockwise), turn

010 ohm pot anticlockwise and record IV readings.

vii) 10 ohm pot is single turn. Once it reaches the minimum setting, current will be

the highest and voltage is minimum. After reaching the minimum of the 10 turn

pot and recording the readings, set the 10 ohm pot to maximum ( fully

clockwise) . Then set the 100 ohm pot also to the maximum (fully

clockwise).Check the voltmeter and current meter readings are same as in the

beginning to ensure temperature is constant.

viii) Replace the clear glass plate with frosted glass. This reduces light intensity to

about 70% of the intensity obtained by using clear glass. Repeat steps i) to vii).

ix) Put a stainless steel sieve attenuator on frosted glass and repeat steps i) to vii).

There are four wire mesh/stainless perforated sheet attenuators. Take readings i)

to vii) with all the four attenuators.


Light intensities measured with pyranometer using various attenuators are

shown in the table below.

Part B (Temperature Dependence of Solar Cell Characteristics) (Measurement using solar cell under illumination as power source (Fig 2b))


ii) Turn on two halogen lamps in the center. Turn on the fan.


iv) Note the readings on current meter and voltmeter. This condition is close to the

open circuit condition, voltage will be high and current low.

v) Reduce resistance by turning 0100 ohm potentiometer anticlockwise. This is a

10 turn potentiometer. Take readings of I and V as R is varied.

vi) After the 100 ohm pot reaches the minimum setting (fully anticlockwise), turn

010 ohm pot anticlockwise and record IV readings.

vii) 10 ohm pot is single turn. Once it reaches the minimum setting, current will be the highest and voltage is minimum.

After reaching the minimum of the 10 turn pot and recording the readings, set

the 10

ohm pot to maximum (fully clockwise). Then set the 100 ohm pot also to the

maximum

(fully clockwise). Check the voltmeter and current meter readings are same as in

the

beginning.


Viii) To set temperature press the SET button on temperature controller for about 45 secs.

The indicator reading will start fluctuating. Using up/down arrow keys, set the

desired

temperature. Push the SET button again. The reading now reads the actual

temperature of

the sample stage. Suppose the initial temperature was 35C and you have set the

desired

temperature 45C. The controller will cut of heating when the temperature

reaches 45C.

After this, the temperature will fluctuate between 44C and 46C. Repeat the

measurement

as in steps i) to vii).

ix) Set the controller to in steps to higher temperatures 50C, 60C and 70C and

after each

temperature stabilizes , repeat the measurements from steps i) to vii).

Observations:

Record your IV measurements in a tabular form, as shown below:

Sl. No.

Voltage (mV)

Current(mA)

Part (A): IV as function of light Intensity

Plot I versus V on a graph paper. Extend the curve to touch the ordinate and abscissa. Get values of VOC and ISC. Do this for all the attenuations. Plot Isc versus Light Intensity. Use intensity values as given above. Plot Voc versus Log natural light Intensity

Part (B): IV as function of Temperature

Make IV plots with measurements done at the lowest temperature without heating, and at 45 C, 50 C, 60 C and 70 C. Find Isc, Voc from the IV plots at different temperatures. Plot Isc versus temperature T. Plot Voc versus temperature T.


Plots of current versus voltage at various light intensities

Plot of Current versus voltage at various temperatures


Results:

Part (A) i) Find the dependence of Isc on light intensity ii) Find the dependence of Voc on light intensity

Part (B)

i) Find the coefficient of the rate of change of Isc from slope of Isc versus T ii) Find the coefficient of the rate of change of Voc from slope of Voc versus T

Solar System V2

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