The right solar module

Criteria for assessing the quality of

crystalline solar modules in practice

| Once installed, modules should last for well over 20 years and achieve a high output day after day
| Price and efficiency alone are not sufficient decision criterias
| Further characteristics of the module quality affect yield and durability – and thereby investment security

When assessing the total yield of a solar energy system, today the focus is often on the time it takes to break even or the length of
time that the state will fund the project. A basic assumption is that the system will operate without errors, although this is not always
true. Often, users base their solar module choice solely on the nominal degree of efficiency and peak price per watt. From a long-term
perspective, however, other criteria should be taken into consideration during the assessment. A good module should be able to
achieve a reliably high yield for well over two decades. This edition of the Practical knowledge series gives advice on what to look for
when choosing modules.

Module components

Cells

At the heart of the solar module is the solar cell. A careful combination
of efficient cells with uniform properties is the prerequisite for a high
degree of module efficiency, a long service life and high yields.

The most commonly used cells throughout the world are made
from polycrystalline silicon. These cells are always square and can
be easily identified by their uneven markings. Cells made from
monocrystalline silicon usually feature a somewhat higher degree
of efficiency. The typical "cropped" corners, however, reduce the
size of the active cell surface and the higher cell efficiency is not
fully converted into a higher overall module efficiency. In terms
of the output generated and durability, there is no difference
between the two types.

Practical tip No. 1: Note the number of busbars.

To be able to conduct the electricity generated by the cell to a
load, this must have an electrical contact. This is formed by the
metallic contact grid mounted on the cell surface that consists of
thin fingers and wide strips - the so-called busbar.
A number of module manufacturers use cells with two busbars,
as these cells can be obtained from various sources and often effect when subjected to occasional rainfall and is far less likely to result in the build-up of moss.
Modules with anti-reflective-coated glass are also available on the
market. Here, it is important to ensure that the durability of the
coating is guaranteed for the extended service life of the module
(at least 20 years). As with spectacle lenses, the anti-reflective
coating can age or even flake off, which in extreme cases can
lead to the deterioration of the module output.

Frame

The module frame protects the delicate outer edges of the glass film
laminate and, together with the special glass, guarantees stability and
torsional rigidity

Practical tip No. 3: Check the condition and quality of the frame.

The most robust and stable frames are those made from anodised
aluminium, which in contrast to coated frames are less prone
to scratches. Frames should not feature any closed cavities as
these can collect water, which will turn to ice in winter. This can
cause the frame to bend or to break completely. In winter, a flat
runoff edge along the cells can also prevent the formation of
ice and snow, the force of which can detach the frame from the
module. In the summer, contamination such as pollen or dust
cannot collect on flat frame edges, which means that moss is
unable to grow. Such contamination is particularly critical when
it covers parts of the cells, as this can considerably reduce the
output of the module.

A good frame is screwed together. Special screws connect all
parts of the frame in such a way that they are mechanically and
electrically conductive and thereby enable the module to be
earthed throughout. Frame parts that are not screwed together
but rather interlocked, and non-metallic corner connections are
not recommended. The frame should of course feature good,
clean workmanship without sharp edges or protruding screws.

Also ensure that the frame parts are properly attached to the
laminate. On the rear of the frame, on the inside edges, special
frame hinges ensure a clean and consistent seal. Ensure the
parallel and straight alignment of the hinge. Silicon can also
provide adhesion when applied evenly and flush and not sprayed
onto the frame. Silicon is, however, more susceptible to climatic
conditions and should therefore be avoided.

Junction box

The primary task of a junction box is to channel the direct current
generated by the module to the outside. If incorrectly designed or
manufactured, however, it can represent a risk of fire for the photovoltaic
system in certain circumstances.

possess a high degree of cell efficiency. To be able to conduct
the electricity generated particularly reliably, cells are being fitted
with three busbars, as this allows the load to be better distributed
and contact resistances between the cells to be reduced. This
enables cells with three busbars to achieve a higher degree of
module efficiency and at the same time improved reliability.

Glass

The glass is a stabilising, translucent support material on the module.
In contrast to window glass, highly transparent, thermally pre-stressed
toughened glass (solar glass) is usually used for modules.

Practical tip No. 2: Check the glass surface.

A wide range of dif ferent solar glasses are available on the
market: flat, unstructured, microstructured, heavily structured
glasses or even specially coated glasses. The task of these
glass types is to channel as much sunlight as possible into the
module. Heavily structured glasses, however, tend to be prone
to contamination, which can have a negative impact on the
module output over time. Better suited to European latitudes is
the use of microstructured glass, which features a self-cleaning

Practical tip No. 4: Check for soldered and encapsulated junction boxes.

Soldered or welded connections created in automated processes
are secure and stable. Proceed with caution with clamped,
plug-in or screw connections. If these are improperly manufactured,
they can become loose or rust and thus cause a short circuit.
The resulting electric arc can cause fires in the junction box.
Junction boxes cast with flame-retardant plastic are protected
against the ingress of moisture and air. This provides additional
safety.

Practical tip No. 5: Bypass diodes protect cells from damage.

Bypass diodes support optimised operation, even under
unfavourable operating conditions. If cells are covered temporarily,
e. g. by leaves, the bypass diodes channel the electricity from
the cells that are not covered across the covered cells and
thereby protect them against damage. When the cells are no
longer covered, the bypass diodes switch back to their original
state and the module is again able to achieve its full output. Good
modules have several bypass diodes, each of which should be
connected to a maximum of 16-20 cells. A special diode cooling
system dissipates the heat generated by the diodes.
It is also important to ensure that in the box, the connection area
of the cells is electrically and thermally isolated from the diode
area. Also avoid plugged-in diodes here. This guarantees
maintenance-free operation of the module throughout its entire
service life. It is not necessary to replace the diodes.

The hidden values

It is not only the materials and components used which testify to the quality of a module. Other features,
which guarantee the usability of the modules in standard applications and beyond, are also important.

Practical tip No. 6: Only positive performance tolerances will allow you to achieve the full output for your money.

Performance tolerances are specified by manufacturers in order
to indicate production-related deviations from the nominal output.
Minus tolerances indicate lower actual module outputs than the
nominal values specified (and thereby paid for). If a positive
performance tolerance is guaranteed, customers always receive
at least the specified output for their money. In practice, this can
be exceeded by up to a certain percentage and thus lead to a
higher yield. The specified positive tolerance should not be too
high, however, as the large variation in real measured values can
hinder string optimisation.

Look for sensible tolerance specifications: A positive tolerance
of 3% when the modules are sorted into 5 W classes does not
make mathematical sense (at this tolerance, a 220 W module
could achieve a maximum of 226.6 W – which corresponds to
the next level of performance).

Practical tip No. 7: Check for evidence of weak light performance.

The sun does not shine from a cloudless blue sky every day.
When the output of the modules is determined in a production
plant, exactly these conditions are simulated – these are the
so-called standard testing conditions (STC): 1000 W/m² irradiation
25° C module temperature, atmospheric depth AM 1.5. An
irradiation of 800 W/m² is known as weak light. This can be
converted into electrical energy extremely efficiently. Modules,
therefore, should possess evidence of good weak light performance
in order to ensure the highest possible average annual yields.

Guarantees

Product and performance guarantees may be offered by a manufacturer in addition to and independently of the legal seller's warranties which provide additional investment security.

Practical tip No. 12: Take note of warranty periods and transparent conditions.

The warranty periods for solar modules should be at least five
years. Some manufacturers also offer optional warranty extensions,
which provide additional investment security.

Performance guarantees apply to various periods and minimum
performance of the module (in percentages) (e. g. 20 years at
80 % of the nominal output).

The actual performance scope is defined by each manufacturer
on an individual basis. It is therefore important that you carefully
check any warranty conditions and ensure the clear, unambiguous
formulation of the requirements and precise performance
descriptions. As a general rule, warranty cases must be proven by
the customer. Transparent warranties prevent unpleasant surprises
in the event of damage and simplify the processing of claims.

Practical tip No. 13: Take note of the applicable laws, court of jurisdiction and accessibility of the warranty provider.

If claims have to be legally enforced in foreign countries, a range
of laws apply. In addition to high legal fees, valuable time can be
lost clarifying and eliminating damage, which can also result in a
loss of income. It has therefore been extremely beneficial for
European law to be made applicable and the court of jurisdiction
to be in Europe.

Practical tip No. 8: Permissible loads of 5400 Pascal provide static safety.

Modules are usually tested in accordance with the applicable
standard for pressure loads and tensile loads of up to 2400 Pa.
This often does not provide sufficient safety for special weather
conditions, as the modules are subjected to higher surface
loads in areas prone to snow, for example.Modules with a
bearing load of at least 5400 Pa guarantee planning reliability.
When mounted correctly, the modules can support snow drifts
of more than three metres.

Practical tip No. 9: Check for approval for upright and crossways installation as well as a low dead weight.

For installation, it should not only be possible to attach modules along their long sides – they should also have approved clamping points on their short sides. This allows them to be used securely both in upright and crossways format and to be installed flexibly in accordance with individual requirements.

It is also advantageous for modules to have a low dead weight,
which means that they can be installed on roofs with a low bearing
load. Today, light modules weigh less than 90 g/Wp.

Warning: The light weight should not be favoured over a low
pressure and tensile load. Top-quality modules meet both criteria.

Practical tip No. 10: Remember operational safety and
ease of installation..

FLight modules simplify transport and handling during installation.
Connection cables with a screw cap also make work quick and
easy. It is also important that the cable ends are fixed to the module
(e. g. clamped onto the junction box) for transport to the
installation site and thus do not represent a trip hazard.

Practical tip No. 11: Do not be confused by a large range of products.

Light modules simplify transport and handling during installation.
Connection cables with a screw cap also make work quick and
easy. It is also important that the cable ends are fixed to the module
(e. g. clamped onto the junction box) for transport to the
installation site and thus do not represent a trip hazard.

Certifications and directives

As with guarantees, certifications can offer additional security,
for example when using solar modules in coastal regions or in
agriculture.

The International Electrotechnical Commission (IEC) developed
safety standards for the international module trade.
The following IEC certificates are required for crystalline modules:
I IEC 61215: Crystalline silicon terrestrial photovoltaic (PV)
modules Design qualification and type approval
I IEC 61730: Photovoltaic module safety qualification
In the USA and Canada, the North American safety certificate
UL 1703 must be provided.
Examples of further national requirements in Europe are the CSTB
guideline for the French market and the MCS certificate for Great
Britain, which is strongly linked to the IEC safety standards.

Practical tip No. 14: Note the relevant certifications for the construction of solar energy systems in coastal regions or in agriculture.

When used in coastal regions, the IEC 61701 standard comes into
force, which certifies the corrosion resistance of the module in the
presence of salt fog.
For agricultural customers, the "Ammonia resistance" focus
test by the German Agricultural Society (DLG) will be of interest.
The DLG certificate certifies that modules can withstand chemical
loads from the air in barns.
The fact that the places of module manufacture are certified
in accordance with ISO 9001 and ISO 14001 is important from
quality and environmental perspectives. This guarantees
adherence to environment protection requirements as well as
a quality management system which consistently ensures the
high quality of the modules produced.