We have been responsible at Solar Group Utilities for cleaning of large scale solar parks for many years and have used a variety of different cleaning methods, what follows is a balanced argument of mechanical vs manual cleaning methods, with our summary based on our own experiences.
The methods considered would be the following
- Cleaning robots which follow the framework of the modules/table
- Tractor/plant mounted rotating brushes
- Water fed soft brushes
Both systems are using purified/de-ionised water, no chemicals and no high pressure.
Solar photovoltaic modules installed on an angle will generally stay clean on their surface throughout the years due to rainfall, however the following can seriously cause the build up of detritus and subsequently effect production:
- Dirt washes down the face of the module and collects at the bottom where the frame meets the surface of the module, we call this the “bottom edge”
- Moss and lichen grows rapidly in this build up of dirt which can quickly shade the bottom of the module which depending on the layout can have a serious impact on production.
- Bird droppings can have a serious effect also, it is important these are removed as soon as possible as they are acidic and can have a permanent damaging effect on the surface of the module.
Both forms generally use rotating brushes, in our experience these struggle to clean the bottom edge adequately and leave significant dirt behind, meaning that the cleaning interval will have to be increased. It also relies on operator vigilance, if for instance the operator is in a tractor cab then he or she may not notice that a bird dropping has not been completely removed. In real life installations the frame work is often not perfectly straight and level this causes the modules to have high and low points, rotating brushes will struggle to clean these areas effectively and may put undue strain on the high points. Below is a video of typical results obtained by cleaning robots, in it you can see the dirt left behind on the bottom edge, also notice the running gear/belt is using the surface of the module for its purchase.
The results can be controlled much easier with manual cleaning, if a module is unusually dirty or has been soiled by bird droppings more time can be spent ensuring a consistent level of quality throughout the clean, particular attention can be paid to the bottom edge and any other locations on the module, below are some typical results obtained from manual cleaning, As you can see the bottom edge is spotless meaning that the interval between cleans can be increased. Unlike mechanical cleaning which can cause significant damage to the grounds and access the photovoltaic array, manual cleaning generally has no heavy plant involved so grounds are maintained in good condition.
While this is an extremely important consideration as the most expensive asset in the solar park is almost certainly the photovoltaic modules, it is however very simple consideration – almost all module manufacturers prohibit the use of mechanical cleaning due to the following unknown consequences:
- Unknown stress put on surface of modules – an example in the video above. This will potentially cause micro fractures.
- Damage to the modules caused by uneven ground and the subsequent stress that mechanical arms place on the module – this also has the possibility of causing micro fractures
- Micro scratches caused by rotating brushes.
Compliance for individual brushes/robots can be sought from the manufacturer however this can prove extremely expensive as independent testing and field tests may be required.
On the other hand all module manufacturers allow manual cleaning although it is still imperative to follow cleaning procedures if they are stipulated by the manufacturers, it is vital that any cleaning contractors are aware of any specific requirements for your solar park.
The production from any solar park is paramount and should be the most important consideration of any O&M activity, so in summary we will consider the facts from above:
Barriers to optimum output:
- Poor cleaning encouraging quick regrowth/re-accumulation of dirt/detritus, moss and lichens
- Poor cleaning dirt/guano left on the surface causing shading
- Micro fractures
- Micro scratches
- Damage to grounds/access
Mechanical cleaning in our experience can and often does cause all of the above which inevitably causes significant production loss throughout the years.
Manual cleaning is far less likely to cause all of the above, It cant always be guaranteed to have great results so careful management/inspection of any contractor is required to ensure quality, however production remains at optimum and asset managers are left satisfied.
At solar group utilities we can offer full scope O&M or individual contract services, we pride ourselves on quality.