BY PASS DIODE
Bypass Diodes are wired in parallel with individual solar cells or panels, to provide a current path around
them in the event that a cell or panel becomes faulty or open-circuited.
Solar photovoltaic panel are a great way to generate free electrical energy using the power of
the sun. You just place them wherever you want and away you go either as part of an off-grid
standalone system or as roof installed PV panels for a grid connected system. The power range
of a solar photovoltaic system is extremely wide, from a few milliwatts to hundreds of megawatts
due in part to the modularity of the solar panels.
Photovoltaic Shading
Photovoltaic cells are a type of semiconductor photodiode that directly converts the light hitting
their surface into electrical power. PV systems generate electricity by connecting solar
photovoltaic panels together in the form of an array and exposing them to the direct sunlight.
We would think then that during normal operation all the solar panels of a PV system would
experience the same solar conditions as they all form part of the same solar array.
However, the electrical generating performance and reliability of a PV system can be affected by
external factors, such as environment, temperature, humidity, positioning, and degree of solar
radiation which can all lead to power degradation.
But as well as these obvious environmental factors, one factor in particular that will lead to
mismatches between solar cells or whole panels, and power degradation within a solar array is
shading, that is the blocking of the sunlight onto the cell, or panel by leaves, trees, buildings or
antennas. This can be either full or partial shading, and depending on the degree of shading, will
cause a decrease of output power.
Series Connected Solar Cells
Photovoltaic (PV) panels are made from interconnected crystalline silicon cells and are therefore
sensitive to shading. In a standard PV panel, these solar cells are connected together in series,
result in high voltage but the same value of current flows through all the connected cells.
So as long as the sunlight hitting the surface of the PV panel is uniform, each photovoltaic cell
within the same panel will produce the same amount of electrical voltage, approximately 0.5
volts. Then for instance, at full sun a 2-watt PV cell will produce a constant current of about 4
amperes, (0.5 x 4 = 2 watts).
If, however, a cell becomes shaded by some external means, it will stop producing electrical
energy and behave more like a semiconductive resistance, strongly decreasing the total amount
of energy produced by the photovoltaic panel. For example, let’s assume we have three series
connected 0.5-volt photovoltaic cells with a solar irradiance of 1kW/m2 across all three
photovoltaic cells as shown.
Photovoltaic Cell Shading
Now let’s assume that Solar Cell No2 in the string has become either partially or fully shaded
while the remaining two cells in the series connected string have not, that is they remain in full
sun. When this occurs, the output of the series connected string will reduce dramatically as
shown.
Shaded PV Cell
What happens here is that the shaded cell stops producing electrical energy and behaves more
like a semiconductive resistance. The shaded cell generates less current than the other two cells
strongly decreasing the energy production of the series string. The result is that the power being
generated by the “sunny” cells is now being dissipated by the “shaded” cell which can, over time,
cause overheating (hot spots) and eventually destruction of the bad cell.
As the shaded cell causes a drop in its generated current. The unshaded good cells adjust to this
current drop by increasing the open-circuit voltage along their I-V characteristics curves resulting
in the shaded cell becoming reversed biased, that is a negative voltage now appears across its
terminals in the opposite direction.
This reverse voltage causes current to now flow in the opposite direction through the shaded cell
resulting in its consuming power at a rate depending on ISC and operating current, I. Thus, a fully
shaded cell will experience a reverse voltage drop under any current conditions and therefore
dissipate or consume electrical power rather than generate it.
Bypass Diode Protection
Bypass diodes have been connected in parallel across each of the three PV cells. These externally
(or internally) connected bypass diodes are connected in reverse bias mode across their
respective cell(s). They are electrically connected so that the diodes Cathode (K) terminal is
connected to the positive side of the pv cell, while the diodes Anode (A) terminal is connected to
the negative side of the cell. Thus, the diode is reverse biased.
When the three solar cells receive full sun, they each generate a voltage as normal, and as each
of the three bypass diodes are reverse biased across their respective cells any reverse current
(red arrows) trying to flow through them is blocked. Thus, being reverse biased, the diodes act
as if they are not there with the series string producing full output power (6 watts in the previous
example) as the three solar cells are working as expected.
However, if as before one of the PV cells becomes partially shaded due to leaves, trees or snow,
etc. the shaded cell does not produce and electrical energy as we have seen above and thus their
bypass diode takes over becoming activated as shown.
Shaded PV Cell with Bypass Diode Protection
Here under the condition of shading, cell two stops producing electrical energy and behaves like
a semiconductive resistance as we discussed before. Due to the shaded cell generating reverse
power, it forward biases the parallel connected bypass diode (i.e., it turns it “ON”) diverting
current flow of the two good cells through itself as shown by the green arrows above. Thus, the
bypass diode connected across the shaded cell maintains the operation of the other two PV cells
by creating an electrical path for the generated current to flow along.
Then although one cell is shaded (cell 2 in this example) the other two cells, 1 and 3 continue to
generate energy but at reduced power. Therefore, as seen in our previous example above, the
output would be using our 2-watt cell example from above and assuming no losses through the
bypass diode, 4 watts (1.0V x 4A).
One other advantage of parallel connected bypass diodes is that when forward biased, that is
when they are conducting, the forward voltage drop is about 0.6 volts thus limiting any high
reverse negative voltage generated by the shaded cell which in turn reduces hot spot
temperature conditions and therefore cell failure, allowing the cell to return to normal once the
shading has been removed.
Bypass Diode Integration
The integration of a bypass diode across each individual single cell as we have done above in our
simple example would be too expensive and not that easy to install. In practice, manufacturers
place bypass diodes across groups or sub-strings of PV cells (typically 16 to 24 cells) in the back
of panels or within the junction box of a solar module.
Thus, for example, two bypass diodes would be sufficient for a solar panel with a rated power of
about 50 watts containing between 36 to 40 individual cells. Many high-end solar panels have
them fabricated directly onto the semiconductor photovoltaic cell structure.
Blocking diode
A blocking diode allows the flow of current from a solar panel to the battery but prevents/blocks
the flow of current from battery to solar panel thereby preventing the battery from discharging.
The blocking diode is incorporated into the circuit to prevent the battery from discharging back
into the solar array at night when the load is being supplied from the battery store.
During the night, clouds or no load in the shades, the connected battery will provide the current
to the solar cells as they behave like a normal resistor. To overcome this issue, blocking diodes
are used to block the current flow back to the solar panels which prevents the draining of battery
as well as protect the solar cells from hot-spots due to dissipating power inside it which led to
damage the solar cell.
In short, the blocking diodes only provide a single path for current from the solar panel to the
battery and block the currents from the battery to the solar cells during night as solar cells are
acting as a load instead of generating energy.
Keep in mind that blocking diodes are installed in series with the solar panel. The following fig
shows a combination of blocking diodes connected in series and bypass diodes connected in
parallel with the solar panel.
As shown in fig below, a leaf is fallen on cell# 3. This way, the generated current will flow from
cell#1 and cell# 2 to the output as it is in normal operation. The current will flow through bypass
diode across cell# 3 which is affected and cell# 4 and to the loads then through blocking diodes
which is a reliable operation of solar power system as expected.
Junction Box
A junction box is a very crucial part of the solar module. It is the medium through which we interconnect
two or more solar modules. Similar to a battery it has 2 ends Positive and Negative, and these can be
tied attached to panels either serially or in parallel.
A solar panel’s junction box is attached to the back of the solar panel. It wires the connectors together
and is the output interface of the solar panel. A PV junction box is attached to the back of the solar
panel (TPT) with silicon adhesive. It wires the (usually) 4 connectors together and is the output interface
of the solar panel.
Specifications
When selecting solar junction boxes, it is important to consider the casings, which should have
strong aging and UV light resistance to protect them in severe outdoor environments. Like many
enclosures designed to withstand severe conditions, solar junction boxes are often rated with a
two-digit number based on the (Ingress Protection) IP Code, IEC 60529. Common IP ratings
include:
IP 54 is mostly protected from dust and cannot be damaged by light water originating from all
directions.
IP 55 is mostly protected from dust and cannot be damaged by low pressure water jets.
IP 65 is completely protected from dust and cannot be damaged by low-pressure water jets (i.e.,
from a 6.3 mm nozzle).
IP 67 is completely protected from dust and cannot be damaged by immersion in water up to 1
m deep.
IP 68 is completely protected from dust and cannot be damaged by immersion in water deeper
than 1 m.
Cooling mode and volume of their inner cavity are additional important specifications.
PV Box Selection
To choose a PV junction box, the current size of the component is the main information, one is
the working maximum current, one is the short circuit current. Of course, the maximum current
that the components can output when short-circuit current, according to the short-circuit
current calculation of the rated current of the junction box should be a relatively large safety
factor, according to the maximum working current calculation of the junction box is a smaller
safety factor.
Scientific way to selecte PV junction box should be according to the rule of cell voltage and cell
current change with light intensity.You have to know what the peak light is in your area, then
check the maximum current possible against the curve of the cell current with the intensity of
the light, and then choose the rated current of the PV junction box.