and clean
The Roboklin 25 tracked
vehicle achieves a maximum
cleaning rate of 1 MW in five
Photo: Messersi
To clean or not to clean? The decision to clean solar modules mounted
on roofs and on the ground ultimately depends on the location and the
cost. This, at least, is what experience shows, as there are hardly any
recent studies available. Service providers should be more transparent
about their costs and technology.
f many of the past and current manufacturers are
to be believed, cleaning solar modules is superfluous. After all, technologies like self-cleaning
surfaces such as the lotus effect or special coatings
are often touted enthusiastically by the manufacturers. All it takes is one strong rainfall and the modules
are as good as new. However, experts like Clas
Ziganner, CEO of the service provider Zenit-SIS, are
more sceptical: “Of course some of the dirt is washed
away by rain, but anyone who leaves his car standing for longer periods of time will notice a significant
build-up of grime.” Dirt tends to become encrusted
permanently, especially on the frames of the modules, warns Ziganner: “This can even result in moss
formation, which can significantly reduce the
system’s yield.”
Operators often have no clear idea of the effect
of dirt on the electricity yield. That is why Zenit-SIS
approaches cleaning in conjunction with monitoring
and maintenance. The monitoring of yields provides
important preliminary information, says CEO Clas
Ziganner: “Operators cannot easily spot drops in
yield due to dirt build-up, as the reduction occurs
gradually. Good monitoring can help the operator
recognise the lost yield early on and take countermeasures.” Because lower yields may also be the result of shading or technical faults, Zenit-SIS believes
in comprehensive customer care, which includes
monitoring, maintenance and repairs.
Cleaning can make sense
Christian Kerschl is the Manager of the Sonnenhaus
Institute and has his own planning company for photovoltaic and solar thermal systems. He believes
that drops in yield often go unnoticed. “Due to differences in solar irradiation, annual yields can vary by
up to 10 %. Only the operators of very large solar
farms will notice this; it is almost imperceptible in
smaller systems.” From his own experience, Kerschl
knows of cases where reduced yields correlated
closely with the soiling of solar modules. “On roofs
Sun & Wind Energy 1/2016
with a very low inclination, cleaning is absolutely
necessary after a few years because dirt is not
washed away by the rain or carried away by snow. In
Bavaria, when the snow slides off modules in the
spring the lower side remains quite brown,” he
The situation is even worse in agricultural settings and can also be quantified. This is especially
true for large installations mounted near livestock
sheds or directly on their roofs. The emissions and
dust are deposited on the modules and form a hard
crust. “This can result in yield drops of 15 %. On
farms, systems really should be cleaned once or
twice a year. For instance, if it doesn’t rain after the
rapeseed bloom, the yellow pollen can stick to the
modules for several weeks. The same is true for PV
systems mounted close to incinerators, chimneys or
large ventilation systems. These also lead to the formation of tough layers,” explains Emanuel Saß, certified appraisal specialist for photovoltaic systems.
The frame and surface of the modules also play a
role. “For example, between 2004 and 2006 lightly
textured glass surfaces were all the rage. These are
more prone to soiling. In addition, the dirt collects
on the lower frame of the module. From there, moss
and lichen begin to grow upwards,” he says.
Should cleaning become necessary, operators
should approach it systematically. This includes
making sure the process is approved by the manufacturers, liability insurance and instructing the
s­ ervice provider regarding the electrical systems, as
well as a certain level of knowledge. For instance,
­using cold water to clean piping hot modules has resulted in more than a few cracked panes. “Cleaning
is a tricky business. Some products are effective
while others are not. In addition, this is work on an
electrical system. That is why the system must be
switched off and why the service provider must
­possess a minimum of knowledge about electrical
installations,” says Saß.
Your vehicle and SunBrush mobile
– a winning team!
Mobile PV cleaning for modern PV operators.
The SunBrush mobile can be flexibly installed on any tractor, telescopic handler,
excavator, lifting platform or concrete pump
without having to modify the hydraulic
SunBrush mobil GmbH
Hauptstr. 24
87760 Lachen
0049 (0) 8332 9258080
Solar Edition
Encrusted dirt or moss formation can significantly reduce
the yield of modules.
Photo: Zenit-SIS
The Washtronic uses a cylinder unit to compensate for
irregularities in the ground,
ensuring that the brushes
always apply a constant pressure onto the modules.
Photo: Sunbrush
Robots are an effective
alternative that minimizes the
risk of accidents. The Gekko
from Serbot can grip onto any
Photo: Serbot
Specialised vehicles for PV farms
The cleaning companies argue that they help maintain value and recommend regular cleaning. “Fences,
cameras, inverters and transformers are regularly
maintained but not the supposedly self-cleaning
modules. Rain does not wash away all the dirt, moss
or lichen,” insists Markus Kort, Head of ProClean
Solar GmbH. He has been in the business of cleaning large solar farms for five years – mainly in France
and Italy because of the relatively low rainfall. He
uses a Unimog and specially developed equipment
to drive at 0.5 km/h through the long rows. A modified agricultural sprinkler with a wide reach uses
water from the tank on the vehicle and special
brushes for scratch-free machine-cleaning of the
modules. The water is distilled to prevent the buildup of limescale on the glass. “We follow the cleaning
guidelines of the manufacturers. This applies to the
water pressure when cleaning, the cleaning agents,
the brushes and the robot-controlled pressure applied to the modules. With this method we can clean
around 1 MW per day,” he says. He finds some of the
prices offered on the market completely exaggerated. For free-standing systems, Kort believes around
2,000 €/MW, or 2 €/kW, to be reasonable.
The automated cleaning of large PV farms with
long brush systems used on the module rows is
time-effective but requires some technical skill. On
the one hand, the vehicles need to be able to fit
through the narrow rows. On the other hand, the
contact pressure of the cleaning system on the solar
panels must always remain constant to avoid breaking the modules. This is no easy feat, as the ground
around free-standing modules is often uneven or
steeply sloping.
One of the companies that has got into this area
is Messersi, because there is clearly a need for effective cleaning equipment. The Italian company’s main
business is actually constructing special vehicles for
construction and agriculture but in 2011 it developed
a special vehicle for cleaning large PV farms. The result of this development is the Roboklin 25, which
has been sold worldwide. The vehicle is powered by
a turbo diesel engine from Kubota, the tracked undercarriage is able to drive up to 11 km/h and can
face all ground conditions and slops. This machine
has a maximum cleaning capacity of 1 MW in five
hours. The telescopic arms have maximum lengths
of 4 and 6.5 m. Two washing systems can be mounted on them, either a sprinkler system with a maximum pressure of 40 bar or a rotating brush up to
4.2 meters of length. Both systems are supplied by
a tank with a capacity of 2,400 litres. The cleaning is
controlled through ultrasound sensors. These ensure that the distance between the modules and the
cleaning system always remains constant, automatically compensating for irregularities on the
ground while the vehicle is in motion. The hydraulically controlled telescopic arms are also cleverly designed. They can clean from a distance of up to 9 m
between the vehicle and the module and an articulated arm lets the cleaning systems move freely to
the left or the right.
Long-term tests on brushes
A similar concept is used by SunBrush mobil
GmbH. “We have developed a patented Washtronic
Sun & Wind Energy 1/2016
control system that maintains the desired contact
pressure of the brush regardless of the ground’s
­irregularities,” explains the company’s Managing
Director, Franz Ehleuter. He also builds specialised
vehicles for the mobile cleaning of photovoltaic
systems and sells them to service providers. Originally, he built washing systems, and he also operates several PV installations in the German state of
Bavaria. “We noticed that if we didn’t clean them
or remove the snow in winter they did not function
properly,” he says. Before getting to work, he commissioned Ulm University of Applied Sciences to
test the brushes on new and flash-tested modules.
After some 4,000 cleaning cycles in the laboratory, the Ulm researchers could not detect any
­scratches or performance drops. In addition, the
yields of two systems on a roof were compared. In
comparison with a regularly cleaned 58 kW system,
the 1.8 kW control unit yielded only 40 % of the
­reference value after three years. This is only a
snapshot, “but of course customers always ask
­whether the cost of cleaning makes sense. We
keep it simple and start by cleaning just one string.
The operator can then read the difference on the
­inverter and transpose the results. The possible
advan­tages of cleaning should always be transpa­
rent,” he says. This includes the costs. Ehleuter
finds 2,000 to 2,300 €/MW for free-standing
­systems to be reasonable, and up to 12 €/kW for
systems on roofs, because the required effort in
­relation to the surface area is greater.
Robots for roof-mounted systems
Two options are available for cleaning roof-mounted
systems: the classical mop and robotic systems.
However, the homogenously sealed glass surfaces
can hardly be washed properly by hand. Added to
this are the dangers and the regulations regarding
accident prevention. “Robots also have the great advantage that they maintain the same pressure over
many hours of work,” says Ivo Lackner, Head of Sales
at the Swiss robot manufacturer Serbot. The company’s cleaning solution, named Gekko, is increasingly being used by service providers. The unit, including the supply lines for electricity and water, is transported to the roof on an elevating platform. To
achieve the necessary grip on the tilted modules,
Serbot uses a vacuum process.
According to Lackner this does not create too
much pressure because the robot only weighs
200 kg. “The modules are built to withstand snow
loads of 500 kg and wind loads of 280 kg,” he explains. Serbot also has positive examples at the
ready to promote the washing of modules. According
to the company, operators can calculate with an increase of 6 %. Cleaning installations with surfaces
greater than 100 m3 therefore makes sense. How­ever,
the maximum hourly rate of 400 m3 on free-standing
systems is a little low. “We are working on larger solutions but for free-standing installations machinecleaning with brush systems has a definite advantage,” he concedes.
Torsten Thomas, Ralf Ossenbrink
Solar Edition
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