2. A solar cell (also known as a photovoltaic cell or PV cell) is defined as an
electrical device that converts light energy into electrical energy through
the photovoltaic effect.
Input to solar cells is energy in the form of solar radiation and output from
the solar cells is energy in the form of electricity.
Types of materials:
mono-crystalline silicon, multi-crystalline silicon, amorphous silicon,
cadmium telluride
2
3. Individual solar cells can be combined to form modules
commonly known as solar panels/modules.
The common single junction silicon solar cell can produce a
maximum open-circuit voltage of approximately 0.5Volts
3
4. ➢ PV module generates electricity - sunlight falls on it.
➢ In night, the energy output from PV module is zero.
➢ PV module depends on the physical size of the module
➢ PV modules - characterized in terms of their power rating “Peak Power”
(Wp).
➢ E.g:Wp, 10 Wp, 18 Wp, 36 Wp, 50 Wp…,
➢ Electricity generated from a PV module is DC (direct current) in nature
➢ ‘Inverter’ – DC to AC
➢ DC electricity generated from the PV modules is stored in batteries.
PV modules are: mono-crystalline silicon solar PV module, multi-
crystalline silicon solar PV module, amorphous silicon solar PV module,
cadmium telluride solar PV module, CIGS solar PV modules, etc.
PV modules are connected together. This interconnection of PV modules is
called ‘PV module array’.
4
5. 5
The electricity generated by solar cell depends upon
▪the intensity (amount) of light
▪the area of a cell
▪the angle at which light falls on it.
➢Higher is the intensity of sunlight, the more is the electricity generated by solar
cell.
➢If area of a solar cell is increased, the current generated by it increases.
6. All solar cells, have only two terminals (positive and negative
terminals) as output.
Typically solar cells have front contact at the top, emitter-
base junction or p-n junction in the middle and the back
contact at the bottom.
At the emitter-base junction, the separation of negative and
positive charge takes place.
Electricity is supplied to a load by connecting its terminals to
the front and back contacts of a solar cell or solar module or
solar panel
6
7. Photons in the sunlight falling on the solar cell’s front face are absorbed by
semiconducting materials
Free electron-hole pairs are generated.
When solar cell is connected to a load, electron and holes near the junction
are separated from each other.
The holes are collected at positive terminal (anode) and electrons at
negative terminal (cathode).
Electric potential is built at the terminals due to the separation of negative
and positive charges.
Due to the difference between the electric potentials at the terminals we get
voltage across the terminals.
Voltage developed at the terminals of a solar cell is used to drive the
current in the circuit. The current in the circuit will be direct current or DC
current.
The amount of electricity generated is proportional to the
amount of light falling.
7
9. Short circuit current (Isc),
Open circuit voltage (Voc)
Maximum power point
Current at maximum power point (Im)
Voltage at maximum power point (Vm)
Fill factor (FF)
Efficiency (ᶯ)
Standard test conditions (STC)
which is corresponding to 1000 W/m2 of input solar radiation
and 25°C cell operating
temperature.
9
10. Short circuit current (Isc):
It is the maximum current a solar cell can produce. The higher the Isc, better is the
cell.
Depends on cell technology, cell area, amount of solar radiation falling on cell,
angle of cell, etc.
The current density is obtained by dividing Isc by the area of solar cell (A).
Open circuit voltage (Voc):
It is the maximum voltage that a solar cell produce. The higher the Voc, the better is
the cell. Depends on cell technology and operating temperature.
Maximum power point (Pm or Pmax):
It is the maximum power that a solar cell produces under STC. The higher the Pm,
the better is the cell.
maximum power point for a I-V curve of solar cells occurs at the ‘knee’ or ‘bend’
of the curve.
Pm or Pmax = Im * Vm
Fill factor (FF):
FF is the ratio of the areas covered by Im-Vm rectangle with the area covered by
Isc-Voc rectangle The higher the FF, the better is the cell.
10
12. A solar PV module is a device in which several solar cells are connected together to
generate more power than a single solar cell.
12
When solar cells are connected in series the voltage of each cell adds up while the current in
the series connected cells remain same as that of a single cell.
Suppose, terminal voltage of a solar cell is 0.5 V under operating conditions and two such
identical cells are connected in series, so, the terminal voltage of string of two solar cells
will be 0.5 + 0.5 = 1 V.
✓If 6 cells are connected in series, than terminal voltage of series of 6 cells will be 0.5*6 =
3 V .
✓If 36 solar cells are connected in series, then terminal voltage of series of 36 cells, or PV
string of 36 cells will be 0.5 *36 = 18 V.
14. 14
Current at maximum power point ( Im )
The Im will always be lower than Isc. Normally, Im is
equal to about 90% to 95% of the Isc of the module.
Voltage at maximum power point (Vm )
The Vm will always be lower than Voc. Normally, Vm
is equal to about 80% to 85% of the Voc of the PV
module.
PV modules and other electronic devices, used in PV
system, are compatible to work with voltage levels
multiple of 12 V, like 12 V, 24 V, 36 V (battery voltage
level) and 15 V, 30 V,45 V, etc. for PV module voltage
level.
16. For crystalline silicon solar cell technology, required voltage
for charging 12 V battery can be obtained by the series
connection of 36 solar cells.
Due to higher operational temperature (higher than
specified by STC, 25°C), the voltages (Vm and Voc) decrease.
The solar cell under encapsulation operates at higher
temperature resulting in loss of voltage by about 0.08 V.
Available cell terminal voltage
Vm = Vm (STC) – loss of voltage due to higher temperature=
0.50 – 0.08 = 0.42 V
Number of cell in series =15/0.42= 35.71 or approximately 36
cells.
16
17. Step 1: Find out the Vm (at STC) of a solar cell of given
technology
Step 2: Find out loss of voltage (loss of Vm) under
operating conditions.
Step 3: Available voltage = Vm (STC) – Loss of voltage.
Step 4: Note down the required PV module voltage.
Step 5: Divide the required module voltage by available
operating voltage to get the number of cells connected
in series.
17
18. A PV module of new solar cell technology is to be designed to charge a battery of 12 V. The Voc and Vm of the
cell of new technology under STC are 0.90 and 0.80 respectively. The cell’s voltage decreases by 1 mV every
degree centigrade rise in temperature. How many cells should be connected in series in this PV module, if cell
temperature under operation is 60°C. Make a drawing of PV module with this new technology.
Vm = 0.80 volt;Battery voltage = 12 V;Cell operating temperature = 60°C;Per degree centigrade decrease in
voltage = 1 mV or 0.001 V
Step 1: Find out the Vm (at STC) of a solar cell of given technology Vm (at STC) = 0.8 V.
Step 2: Find out loss of voltage (loss of Vm) under operating conditions.
The temperature corresponding to STC is 25 °C, the cell is operating at 60 °C, therefore, the cell is operating 60 –
25 = 35 °C above STC.
Loss in voltage per degree centigrade rise in temperature is 0.001 V,
Therefore, loss of voltage due to 35°C rise in temperature = 0.001*35 = 0.035 volt
Step 3: Available voltage at operating conditions = Vm (STC) – Loss of voltage
Available voltage at operating conditions = 0.8 – 0.035 = 0.765 V
Step 4: Note down the required PV module voltage.
The battery to be charged is 12 V, therefore, the PV module voltage in all operating conditions should be about 15
V.
Required PV module voltage = 15 V
Step 5: Divide the required module voltage by available operating voltage to get number of cells connected in
series.
Number of new type of cells to be connected in series =15/0.765 = 19.6 or about 20 cells.
18
19. Standard Test Condition (STC) - 1000 W/m2 of solar radiation at 25 °C of
cell temperature.
the output power of the module is known as ‘Peak power’ and written as
wattpeak or Wp.
Solar radiation intensity is lower than 1000 W/m2 and cell temperature
under operation is higher than 25 °C; both of these effects decreases the
power output of module.
The actual output power of PV module is less than the rated peak power or
Wp of PV module given by manufacturer.
Wattage of a PV module depends on the current and voltage of a PV
module at peak power condition.
Total current produced by a solar cell is obtained by multiplying the current
density of a cell with its area. Jsc * A = Isc
Current density of cell depends on a technology and manufacturer.
For the same technology cells, large area cells produce more current.
19
20. A manufacturer of crystalline silicon solar cell guarantees 33
mA/cm2 current density of his solar cells. If the cell area is
144 cm2, then estimate the Isc of solar cell.
Solution
Cell area is = 144 cm2
Current density Jsc = 33 mA/cm2 or 0.033 A/cm2
Isc = Jsc * A
=33 mA/cm *144 cm
=0.033 A/cm *144 cm = 4.75 A
20
21. The modules produce maximum power (for given light intensity) when
sunlight falls perpendicular to the surface of a module.
When the light does not fall perpendicular on the module, it always gives
less output
The effect of the angle of sunlight falling on a module on output power of
module
◦ (a) light is at an angle less than 90°
◦ (b) light is at an angle of 90°
◦ (c) light is falling parallel
21
22. For measuring the module parameters, following
equipments are required:
1. Ammeter or a multimeter
2. Voltmeter or a multimeter
3. Rheostat
4. Connecting wires
22
23. The open circuit voltage (Voc) and short circuit current (Isc) can be directly
measured with multimeter.
Voc- multimeter - voltmeter mode
Isc - multimeter - DC current mode or an ammeter
Voc-should not be any load connected to the module/should be in open circuit
condition
23
24. Ammeter connection to solar PV module for measuring short circuit current
(Isc).
The typical Isc values of modules could be in range starting from 0.1 A to 10 A.
24
25. 25
The measurement would require two multimeters (or
a voltmeter and an ammeter) and a rheostat
27. Count the number of cells connected in series in the module shown.calculate
the open circuit voltage (Voc) and voltage at maximum power point (Vm).
It is given that open circuit voltage of a single cell is 0.6 V.
Number of cells to be connected in series (n) = —36——————
Total series open circuit voltage (Voc) = Voc (single cell) number of cells
in series
Voc (module) = ——————— ———————
Voc (module) = ——19.2————— volts
Vm is normally 80% to 85% of Voc. Let us assume that Vm = ———
19.2*.80———— Voc
= ———15.36———— V
Therefore, Vm (module) = ———15.36———— volts.
27
28. The crystalline Si cell technology is the most commonly used technology.
Large area solar cells are available in either size 12.5 * 12.5 cm2 or 15 *15 cm2.
PV modules are typically designed to produce about15 volt (Vm) under operating conditions-
32 to 36 cells connected in series.
The current produced by the cells depends on the area and current density.
The current density of commercially available crystalline Si solar cell is about 30 mA/cm2 to
35 mA/cm2.
The maximum possible short circuit current (Isc) that we can get from modules with large
area solar cells is:
Isc = Jsc (mA/cm2) *Area (cm2)
= 35 * 12.5 *12.5= 5468 mA = 5.46 A
Isc = Jsc (mA/cm2) *Area (cm2)
= 35 *15 *15= 7875 mA = 7.87 A
Im = 0.09 *Isc
Im = 0.90 * 5.46 = 4.91 A (for 12.5 *12.5 cm2 cells)
Im = 0.90 * 7.87 = 7.08 A (for 15 *15 cm2 cells)
Wp = Im *Vm
Wp = 4.91 * 15 = 73.65 watt Wp = 7.08 * 15 = 106.2 watt
28
29. SPV high power module is having an area of 1.62 m2 gives a current at
maximum power point of 7.83 A and voltage at maximum power point of
29.4 V. The short circuit current of the module is 8.52 A and open circuit
voltage is 36.7 V. What is the fill factor, maximum power point and
efficiency of the solar cell? Consider STC.
29
30. When light falls on a PV module, same current is generated in all solar
cells which flow through PV module.
Due to some reason, if one of the solar cells gets shaded (no light falling on
one cell), then the current generated by that cell will be lower than the rest
of the solar cells.
Since the cells are connected in series, the shaded solar cell (generating
low or no current) will resist the current flow generated by non-shaded
solar cells generating full current.
Due to this, the shaded solar cell can become very hot, forming hot spots in
the PV module.
Therefore, local heating of solar cells in a PV module due to shading
should be avoided.
30
31. Bypass diode is used to avoid the destructive effect of hot spots or local
heating.
A diode, called bypass diode, is connected in parallel with solar cells with
opposite polarity to that of a solar cell.
Thus, in normal condition (no shading), the bypass diode is operated in
reverse bias condition, effectively in open circuit.
This reverse bias will act as a forward bias voltage for bypass diode since it
is connected with opposite polarity.
In this way, the bypass diode will carry the current, rather than shaded cell
(meaning bypassing the current from shaded cell). By bypassing the
current, the solar cell gets protected by heating and causing permanent
damage to the PV module.
31
32. 32
Ideally, there should be each diode for the each solar cell in the solar PV
module, but practically, due to cost reason, there are few bypass diodes
which are connected in PV module.
It is recommended that practically, there should be at least one diode for
each series combinations of 10 – 15 cells.
Bypass diodes are added in parallel to solar cells to avoid the formation of
hot spots and damage to PV modules
33. 33
Blocking diodes are added in PV system to avoid reverse flow of current
into the PV modules.
In day time, energy is generated by PV module and supplied to battery.
When there is no sunlight, like in the night, the SPV modules stop
producing the energy and become idle.
During night, charged batteries start supplying energy to the SPV modules.
This is loss of energy and should be avoided.
In order to avoid the flow of current from battery to solar PV modules, a
diode, called blocking diode is used to block the current flow.
34. 34
The diode by appearance is cylindrical in shape with a silver ring on it, and other
part is black in colour. The diode has two pins; one is positive and other one is
negative. The silver ring represents the negative terminal and other terminal is
positive.
Typical diode ratings
The rating of a bypass diode, which is used for 16 – 19 cells in series, is the total current
flowing through the series connected cells in a string.
One module of 36 cells contains 2 bypass diodes and a module with 72 cells contain 4
bypass diodes.
The rating of bypass diode is of order 5 – 10 ampere and 30 – 50 volts (depending on the
number of cells which are connected in series and their current and voltages).
The blocking diode ratings depend on the array current and the voltage because main
function of these diodes is to block the current coming from the battery during night when
modules do not produce any power.
35. 35
Schematic representation of a PV module
In a PV power plant, the PV module connected in series are referred a PV modules
string.
36. 36
A solar PV array is a combination of PV modules in series and parallel.
37. 37
If PV modules are connected in series, then their voltage gets added up.
Series combination of the PV modules is achieved by connecting the
opposite polarity terminals of modules together
The negative terminal of one module is connected with the positive
terminal of the other module.
When PV modules are connected in series, the voltage of the series
connected PV modules is the sum of the voltages of individual PV
modules.
39. 39
The number of PV modules connected in series depends on the amount of
voltage required from a PV module string.
Current of a PV module string is the same as the current of individual PV
module of a string, assuming all PV modules are identical.
40. 40
Calculate the number of modules to be connected in series to obtain the open
circuit voltage of the array as 40 V and/or maximum power point voltage of 32 V.
The modules available for connection are having the following parameters:Voc =
20 V, Vm = 16 V, Isc = 4 A and Im = 3 A
Step 1 Note down the voltage requirement of series connected PV modules:
Step 2 Note down the parameter of a PV module that is to be connected in series:
41. 41
Step 3 Estimating the number of PV modules to be connected in series:
42. 42
Step 4 Estimating the total power of the series connected PV modules:
43. 43
When PV modules are not connected with each other, the total peak power
produced by PV modules is the sum of the total peak power produced by
individual modules
Mismatch in voltages of series connected PV modules is
not an issue, mismatch in currents is.
44. 44
If the current producing capacity of the PV modules is not same, then the current
flowing in the series connected modules will be equal to the current of the
module with the lowest current producing capacity. Thus, the lowest current
producing module determines the current in series connected modules.
Due to this mismatch in current, the power generation from the series combination
of three modules will be less than the case when the three modules are working
independently.
Due to such power generation losses, it is advised that PV modules with difference
in Im should not be connected in series.
PV modules with difference in Im should not be connected in series. It results in
loss of power output
45. 45
The PV modules are connected in parallel in order to obtain higher currents
in PV systems. Module currents in parallel combination get added up.
In parallel combination of PV modules, the voltage of the combination
remain the same as that of individual module voltage (provided all modules
have identical voltage) where as the current of the parallel combination is
the sum of currents of all PV modules.
46. 46
The PV modules with different wattages should not be connected together
in series or parallel combination.
Find out the total current at maximum power point, Im, of parallel
combination of three PV modules. Current at maximum power point of
individual modules
47. 47
The Pm of a PV module (under STC) is the product of current at maximum
power point (Im) and voltage at maximum power point (Vm). When PV
modules are not connected with each other, the total peak power produced
by the PV modules is the sum of the peak power produced by individual
modules.
48. 48
The main purpose of parallel combination of PV modules is to increase the
current of combination. In a PV array, either individual PV modules can be
connected in parallel or PV module strings (series connected PV modules)
can be connected
The number of PV modules connected in parallel is obtained by dividing
the desired current from the parallel combination with current of individual
PV module.
49. Estimate the number of SPV modules to be connected in parallel to achieve
the current at peak power point of 42 A. The system voltage requirement is
16 volts.The modules to be connected are having parameters Vm = 16 V,
Im = 7 A,Voc = 20 V, Isc = 8.5 A.
Step 1 Note down the current requirement of parallel connected PV
modules or PV array:
49
Step 2 Note down the parameter of a PV module or PV module string that
is to be connected in parallel:
50. 50
Step 3 Estimating the number of PV modules or strings to be
connected in parallel:
Step 4 Estimating the total power of the series connected PV modules:
51. 51
Mismatch in Module Voltages Connected in Parallel
➢The voltage of parallel combination of PV modules is equal to the voltage of a single
module, if the module voltages are identical.
➢If there is difference in PV module voltages, then the voltage of the parallel
combination is determined by the PV module with lowest voltage.
PV modules of same current rating in series combination and PV modules of
same voltage ratings in parallel combination.
52. In a PV system, the number of PV modules is first connected in series
(string) as per the requirement of system voltage, and then many PV
module strings are connected together in parallel
52
53. 53
Estimate the number of PV modules to be connected together in order to design a solar
PV system for power generation with following requirements:Power = 10 kW, Voltage at
peak power = 200 V, Current at peak power = 50 A,The PV modules available for this
plant are having the following parameters:Vm = 35 V, Im = 8.5 A. Recalculate the
numbers. After calculation of number of PV modules, estimate the actual peak power of
the system
Step 1 Note down the voltage, current and power requirement of PV module array:
54. 54
Step 2 Note down the parameter of a PV module that is to be
connected in PV array:
Step 3 Estimating the number of PV modules to be connected in series and
parallel:
55. 55
Step 4 Estimating the total power of the series PV module array:
57. Five common factors that affect the power generated by solar cells. They are as
follows:
1. The conversion efficiency (h),
2. The amount of light (Pin),
3. The solar cell area (A),
4. The angle at which day light falls (q ), and
5. The operating temperature (T )
57
58. The ratio of electrical energy generated to the input light energy is referred
as conversion efficiency of solar cells.
The conversion efficiency of solar cell is fixed, based on material and the
manufacturing.
The maximum power output (Pmax or Pout) of solar cells depends on
voltage developed across cell terminal and current it can supply.
The cell area also affects the power output.
58
➢The solar cells with higher efficiency values will always give better performance.
➢The unit of solar cell efficiency is percentage (%), the unit of Pout is normally watt,
the unit of Pin is normally W/m2 or W/cm2 and unit of cell area is in m2 or cm2.
➢The solar cell efficiency is given for standard test condition (STC) and under the STC,
the value of input power density, Pin is taken as 1000 W/m2 or 0.1 W/cm2.
60. 60
Calculate the output power from a solar cell if its efficiency (in %) is 30,
24, 19,16 and 12, input power density is 1000 W/m2, and area of the solar
cell is 100 cm2.
61. 61
The output power of solar cell directly depends on the efficiency of solar
cell.
62. 62
Large amount of light falling means high generated power, less amount of
light falling means low generated power.
A solar cell produce 1 A current under 1000 W/m2 input solar radiation,
then under 500 W/m2 input solar radiation, the cell will only produce ½ A
current (because input radiation is half).
The solar cell current output is proportional to the amount of solar radiation
and voltage is relatively not affected by the variation in sunlight intensity.
The amount of power generated (Current *Voltage) by solar cell is
proportional to the amount of light falling on it.
The amount of power generated by the solar cells throughout the day keeps
changing (i.e., it is not constant).
64. Calculate the output power for solar cells of efficiencies 16%. When the input
power is say, 1000, 800, 600 and 400 W/m2 and area of solar cell is 100
cm2.
64
65. 65
When amount of light falling on a solar cell reduces, the output power
generated also reduces.
66. The current output is directly proportional to the cell area.
So, when solar cell area is large, the amount of electric current generated
by it will be large.
Similarly, less amount of electric current will be generated when the cell
area is small.
Current density (Jsc) of a solar cell is always fixed or constant.
Large solar cell area means high current, small solar cell area means low
current.
The output voltage is independent of cell area
Thus, at a given input sunlight intensity, if a 100 cm2 cell produces 0.5 V,
then cell of 100 cm2, or 200 cm2 or 50 cm2 or 10 cm2, etc. will produce
same 0.5 V.
66
67. Calculate new value of output current for solar cells of area 20, 30, 50, 80
and 100 cm2, when current density of cell is 35 mA/cm2.
67
68. 68
With increase in area of a solar cell, the amount of output current also increases.
69. 69
Solar cell produces maximum power (for given light intensity) when
sunlight falls perpendicular to the surface of solar cells.
When the light does not perpendicular to solar cells, it always gives less
output power than maximum possible output power.
70. 70
For a solar cell, higher temperature means lower power output.
For typical Silicon solar cell the output voltage decreases by 2.3 mV per
degree centigrade increase in temperature.
The decrease in voltage, power and efficiency with temperature is different
for different type of solar cells. For crystalline Si solar cells, for every 1°C
increase in temperature above 25°C, the decrease in value of voltage,
power and efficiency is given in Table
71. 71
➢The temperature coefficient of parameters are given as percentage of parameter value
at STC.
➢The values given in this table are typical values of the parameter, the actual value may
be different from one manufacturer to other.
➢Due to higher cell operating temperature than the STC temperature, the actual maximum
output power of modules is lower than STC value.
➢Using the temperature coefficient of the module parameter, the change in value of
parameter with increase in temperature can be estimated.
72. 72
➢For a solar PV module, higher temperature means lower power output.
➢For a typical crystalline silicon PV module, the output voltage decreases
by 0.35% per degree centigrade rise in temperature.
73. 73
If the actual operating temperature of the solar cell is 40 °C. The output
voltage of a solar cell at standard operating temperature is, say 0.7 V. The
output voltage decreases by 2.3 mV/°C. Calculate the new value of output
voltage?
The solar cells output voltage decreases if operating temperature is above 25 °C.
