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We have assembled some studies to provide an overview of the state of research in the field of the soiling of PV modules and its effects, either as a download or directly on these pages. Here a brief summary to give you a general idea: several studies by Prof. Dr. Dr. J. Hoffmann from thermovolt AG and by Prof. Dr. Häberlin and Ch. Renken from the Bern University of Applied Sciences discovered significant power losses of up to 13,8% in some cases for the tested PV systems. The phenomenon of dirt streaks could even be observed on solar cells in modules with a pitch of 65°.
Further studies will follow, particularly since the studies that are currently available were written at a time when modules still had to be cleaned expensively by hand (please refer to the section on solar farm costs/benefits). Some of these studies therefore erroneously assume that regular cleaning is not necessary despite the positive effects on account of the high costs and effort. The much cheaper and more efficient mechanical cleaning solution from ProCleanSolar was unknown at the time and could hence not be included in the assessment.

Ageing of photovoltaic modules
Prof. Dr.-lng. Johannes Hoffmann thermovolt AG

1. On the ageing process
Ageing effects can be observed in photovoltaic modules over the course of their working life. They occur in all types of cell, mono- or polycrystalline as well as amorphous cells.
Ageing reduces the efficiency of the cell and module which increases the longer the working life.
The reduction of the effective generated power which is caused by the ageing effect with a constant solar irradiation is relevant for practical power supplies.
When dealing with battery feeds from standalone systems, what is important here is the d.c. power generated by the solar cells and modules with a defined solar irradiation.
In the majority of cases there are a.c. and d.c. power feeds into the superior mains (parallel mains operation) through solar modules and grids or self-commutated converters. The a.c. or d.c. power fed into the superior grid is decisive for the efficiency of the photovoltaic system in this case. It is reduced by ageing effects despite a constant solar irradiation in both the solar cells and the converters.

There are two main reasons for this deterioration of the cell and module efficiency:
firstly, deposits and impurity layers accumulate on the exposed side of outdoor modules (e.g. dust, dirt particles, pollen, pollution, sometimes also algae growth on the edges, etc.) that attenuate and weaken the solar irradiation that generates the electric power directly in the cells. Nor are these impurity layers on the surface completely removed by the self-cleaning effect of rain. The longer the modules remain outdoors, the thicker the impurity layers become and thus the greater the attenuation of the useful solar irradiation.
This ageing effect can be combated by regular cleaning of the modules' irradiation surfaces. However, this type of cleaning is hardly ever carried out during the normal operation of photovoltaic systems on account of the high costs and effort.
A second reason for the reduction in efficiency is the irreversible inter-crystalline recombination processes in the cells that occur when the cell temperatures rise.
Neither of these two causes can be eliminated from an energy technology point of view, so that the reduction in the cell and module efficiency that accompanies the service life has to be taken into consideration in the design and profitability analysis of photovoltaic systems.

2. Processes to measure the ageing
The ageing can be practically determined under industrial conditions by measuring the d.c. and a.c. power with a given solar irradiation of both the module under investigation and simultaneously that of a new, unused comparative module from the same manufacturer with identical characteristics. The comparative module and the module under investigation should be geometrically aligned to ensure identical irradiation conditions for both modules.
Taking into account the former service life of the module under investigation the quotient q, where
P0 = effective electric power of the module under investigation
Pn = effective electric power of the new comparative module
q = p0/pn
is then a quantity for the ageing, defined here as the electrical per-unit output.
The process tolerances that are unavoidable during production of the modules are around ± 3%.
When calculating the expected electric power yields, these should therefore be reduced by the factor 0.97.
Prof. Dr. Dr. J. Hoffmann thermovolt AG



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