solar cell

1. A solar cell is a p-n junction so it is basically a diode
but it is a special kind of diode.
If there is no sunlight striking the cell, then it
will behave exactly like a diode, allowing the
current to flow only in one direction
In sunlight, the cell would be modeled ideally by a
diode connected in parallel with a current source.
Solar Cell Model :

2. However, the real model consists of the following
components ~~
Ipv is a current
source that is
analogous to
the potential
current the
sunlight create.
RS is the internal
series resistance of the
cell.
Ideally RS=0
RSH is the shunt resistance. Some electron-
hole pair will recombine across the junction. It
is the resistance to the “current leak”.
Ideally RSH=∞
Cell output current
Cell output voltage

3. Applying KCL
Diode/cell
reverse
saturation
current
Bolzmann’
s constant
Charge
of an
electron
n is the model
parameter
1.5 for Si cell
Putting the value of ID to the
previous equation the cell
output current (I) becomes

4. The equation for the cell/diode reverse saturation current Irs is
Working
temperature
of the cell
Reference
temperature
And the equation for the light generated or photon current Ipv is
Cell saturation
current at Tr
Short circuit
current of the
cell
Short circuit
current temp.
coefficient
Solar
irradiation
(KW/sq.m)

5. But unfortunately the output power of a single PV cell is very small
Due to the low voltage generated in a PV cell (around 0.5V),
several PV cells are connected in series (for high voltage)
and in parallel (for high current) to form a PV module for
desired output.

6. Then the equation of the cell output current (I) becomes
Number of
cells in
parallel
Number of cells
in series

7. Characteristic curve of PV array:
Here I
assume
RS=0 &
RSH=∞
For simplicity
Output Voltage
Vs output
current curve
Output Voltage
Vs output
power curve
Those are for constant temperature & irradiation

8. Solar Irradiation Variation’s Effect
Variation of
Solar irradiation
100 mW/sq.cm
80 mW/sq.cm
60 mW/sq.cm
40 mW/sq.cm
20 mW/sq.cm

9. Temperature Variation’s Effect
Variation of
Temperature
VI curve
PV curve
28˚c
40˚c
50˚c
60˚c
70˚c

10. Influence of Partial Shadow :
Ideally PV modules should be built using identical cells so to have a
module with uniform characteristic. If irradiation cannot reach particular
solar cell – due to shadowing, dirt, snow and leaves – then the photon
current Iph of this particular solar cell is zero or very small. When
connecting PV cells/modules in series, if one of the PV cells/modules
has a much lower photo current Iph than others due to partial shading,
dust or degradation, it operates as a load for other cells and is reversed
biased. This cell/module will then dissipate energy rather than generate
hence leading to a cell temperature rise and if the temperature is too
high the cell/module can be damaged and affect the whole PV
module/array is compromised
The most common method to avoid this is
to put a bypass diode across a PV string
or modules

11. An array with 3 cells is considered
Low illuminated panels however make no contribution to the load power
as these are short circuited by the bypass diodes.

12. Why MPPT ??????
PV curve
If the Panel always
operate this voltage
then we always can
get the MAX. power
This voltage can be achieved by step up or step
down the panel output voltage.
This can be done by DC-DC converter by varying
Duty Cycle.

13. This is done by utilizing a DC-DC converter whose duty cycle is varied
by using a mppt algorithm.
Few of the many algorithms are:
• Perturb and Observe method
• Incremental Conductance method
• Parasitic Capacitance method
• Constant Voltage method
• Constant Current method
One of the most common is Perturb and Observe method. The
operating voltage is sampled and the algorithm changes the operating
voltage in the required direction and samples𝑑𝑑/𝑑𝑑. If 𝑑𝑑/𝑑𝑑 is
positive, then the algorithm increases the voltage value towards the
MPP until 𝑑𝑑/𝑑𝑑is negative. This iteration is continued until the
algorithm finally reaches the MPP. The voltage never actually reaches
an exact value but perturbs around the maximum power point (MPP).
The system oscillates
about the MPP if
perturbation step size
is large. The oscillation
can be minimized by
reducing the
perturbation step size.
But, a smaller
perturbation size slows
down the MPPT