Welcome to the next teaching unit in the chapter on crystalline silicon solar modules.
In the last unit, we dealt with how solar cells are connected
to form cell strings in solar modules.
We have seen that shading has a great influence on the performance
of such a cell string and
therefore bypass diodes are integrated into the circuit.
Now, we look at the efficiency of a solar module.
Just as for a solar cell, we can determine the various
electrical parameters for a solar module,
such as
the short-circuit current, the open-circuit voltage, the fill factor
and the efficiency.
We have seen that the IV- curve of a
cell string can be derived from that of a single solar cell.
The parallel connection of solar cells increases the current
of the cell string, while the series connection increases the voltage.
What about the efficiency and the fill factor?
In the ideal case that no cells are shaded,
one would expect that the efficiency and the fill factor of
cells and the corresponding module are identical.
In practice, however, this is not the case,
since some additional losses occur in the module.
Firstly, there are ohmic losses in the cell connectors between the cells.
In addition,
there may be slight
deviations in the electrical properties of the individual cells.
These deviations are also called mismatch.
If one cell has a slightly lower short-circuit current than the others,
its current determines the string current of all cells
connected in series.
Correspondingly, for cells connected in parallel,
the string with the lowest voltage determines the module voltage.
Overall, module voltage and current are thus somewhat lower
than in the ideal case, in which all cells are identical.
As a result, the efficiency and fill factor are also lower.
In practice, the cells are therefore pre-sorted
according to similar short-circuit current and open-circuit voltage
before being connected to form cell strings,
in order to keep the cell mismatch as low as possible.
In addition, the efficiency also decreases due
to the packing density of the cells in the module.
We can define the packing density as the sum of the areas
of all cells in the module divided by the module area.
In the case of a mono crystalline silicon module, there are gaps
between the solar cells due to the pseudo-square shape of the wafers.
The area of multi crystalline silicon cells can be better utilized.
However, as we have seen in the first lesson,
these cells have a lower efficiency than mono crystalline solar cells.
All the reasons listed here lead to the fact
that the efficiency on module level is about 2-3% lower than on cell level.
At the present time - i.e.
in November 2021 - the efficiency record for crystalline
silicon modules is 24.4%.
The corresponding cell efficiency is 26.7%.
Let's have a look at the composition
of a typical crystalline silicon module.
Since the cells are very thin,
the cell strings are not very resistant to mechanical stress.
In addition, the cell connectors
and the contact grids on the solar cells are sensitive to moisture.
Therefore, the cell strings are embedded in two films of encapsulate.
For this purpose usually Ethylene vinyl acetate
- also called EVA - is used.
On the front side the module ends with a glass pane.
The glass pane protects the module from Moisture, dust,
rain, snow - but also from hail.
For PV modules,
special solar glass is used that contains less iron than normal window glass.
This makes it even more transparent to incident light.
Strong reflection occurs
at the glass pane, just like at the solar cell.
Therefore, the front glass of solar modules is also coated
with an anti-reflective layer.
Of course, this layer must be optimized for the refractive index of glass -
so different materials are used here
than on the cell surface.
Typical values for the solar transmission of unlaminated
solar glass coated
with an anti-reflective layer are 94.5 %.
However, this value increases to 98.5 %
when the glass is optically coupled with a solar cell,
as an optical interface with 4% losses is omitted.
Anti-reflective coatings
have the disadvantage that they cannot withstand the weather
indefinitely and can also degrade
depending on the climate in which the solar module is used.
Therefor there are also textured glasses.
Again, the function is quite similar to the texturing of solar cells.
When texturing the front glass,
it must be remembered that more dust also settles
on a textured glass and self-cleaning by rain is reduced.
On the rear side of the cell,
the module ends with a backsheet film .
This film provides additional protection against moisture
and insulates the back of the module.
In single-sided modules, a compound films
are usually used as backsheets.
These films are usually white
and are highly reflective.
Thus, it additionally serves as a diffuse backside reflector,
allowing photon recycling of light not absorbed by the solar cells -
with the help of total internal reflection
at the front cover glass.
Finally, the module is enclosed in a frame,
which is usually made of aluminum.
The frame increases the rigidity of the module,
serves to fix the module to the substructure and also protects the edges.
The space between the glass pane and the frame
is usually sealed with silicone or a rubber band.
The junction box is also attached to the rear side of the module.
The tin-plated copper conductors of the
cell strings are brought together in the junction box.
The junction box also includes the necessary connection cables
and plugs to connect the PV module
with other modules or an inverter.
In addition, the bypass diodes are installed here.
While there is a central junction box in the full-cell modules,
it is often divided into three parts in the half-cell modules.
Here, each bypass diode is located in its own Junction box.
Here you can see a
typical production chain of a PV module.
This production chain consists of several stages
in which special machines assemble the various parts of a module
in a mainly automatic process.
The processes of the different manufacturers
differ from each other, but most of them include the following steps.
In the first step, the cells are soldered together
in the "stringer" to form cell strings.
In the next step, the "lay-up", the laminate is prepared.
Here, the module is built backwards
- so the glass is at the bottom at this point.
An EVA film is placed on top of the glass
and then the prepared cell strings are positioned on top of it.
The whole stack is then finished
with the second EVA film and a backsheet film.
In the next step, this module package is placed in the laminator,
where it is heated to about 150 °C.
During this process, the EVA film
melts and flows into the cavities between the cells.
As it cures, the EVA film firmly bonds the glass, the cells
and the back sheet together and ensures that the components are optically coupled.
This bond is very durable and difficult to separate again later.
This is a particular problem for the recycling of PV modules,
which we will look at in more detail later.
After the lamination process, the laminate is framed
and the junction box is attached to the module.
Finally, the PV module is tested in a flasher
and optionally other tests such as an electroluminescence test are performed.
To summarize.
We have learned in this teaching unit that PV modules
have a somewhat lower efficiency than the cells themselves
due to a mismatch of the cells, additional ohmic losses in the cell connectors
and the packing density of the cells.
The core of the PV module
is a laminate consisting of the solar cells,
an encapsulation material, a front glass, and a back sheet.
In addition to the laminate, the module
also consists of a frame and a junction box.
You have now learned
about all the main steps on the way to the finished PV module
and know their importance for the functioning
of a PV module made of crystalline silicone solar cells.
With this unit, we thus conclude
the chapter on crystalline silicon solar modules.