Page 45 - Energize October 2021
P. 45

TECHNICAL



        of the panel by 15 to 25%.  The level of soiling depends on the level of   Cracked glass
                         3
        airborne matter and will depend also on the slope of the panel array. The   Cracked glass may be due to stress on the panel due to bad installation
        lower the slope the worse the accumulation and the more difficult to clean.   or temperature stresses. Cover glass cracking can lead to moisture
        Fouling of panels is the deposit of solid matter such as bird droppings on the   ingress and damage to encapsulant.
        panel surface and can affect specific parts of the panel entirely. In addition
        to reducing the output of single cells to zero, this can result in hotspots   Delamination (snail tracks)
        which can damage the panel. Some bird droppings are corrosive and can   Delamination is the separation of the encapsulant film from the cover
        damage the panel glass. In urban areas the worst culprits are feral pigeons,   glass and from the module. This is normally seen as “snail tracks”
        which are wont to roost on rooftops and also on the array structure and are   or as an opaque area under the glass. (Figure 3). Snail tracks are an
        the only birds which foul their own nest and surroundings. Fouling of panels   indication of cracks in the module which can affect performance.
        may also be due to insect activity, such as ants, hornets or bees.   Delamination creates an opaque shield over the module, which results
                                                               in loss of performance.
        Wiring and connections
        Wiring and connection boxes are exposed to the elements and can
        develop faults over time. Insulation damage can occur due to heat and
        UV exposure as well as rodent or insect damage. Mechanical connections
        can corrode or become loose due to thermal expansion or contraction.
        Insect invasion of connection boxes can cause corrosion and overheating.
        Bird nesting can interfere with wiring. Bare insulation can result in arcing.

        Panel damage
        Although panel damage only accounts for a small percentage of problems,
        it could result in serious deterioration of the panel output. Panel damage   Figure 3: Delamination due to panel cracks (Author)
        can be determined by inspection. Most common visible damage is:
        •  Hotspots                                            Encapsulant failure
        •  Cracked glass                                       Encapsulant failure occurs mainly on the rear encapsulant sheet and can
        •  Delamination (snail tracks)                         be identified by bubbling or separation of the encapsulant. This can lead
        •  Encapsulant failure.                                to moisture and dirt ingress which can damage the modules Figure 4.
                                                                  Damage to panels cannot be rectified and the panel may have to
        Hotspots                                               be replaced. Damaged panels can still function for several years but will
        Severity of hotspots and remediation actions also vary widely. There   eventually lose significant capacity. The decision to replace will depend
        are three main causes of hotspots:                     on the extent of the damage.
        •  Shading or soiling: Caused by overhead objects (trees, poles, etc.),
          vegetation overgrowth, surface fouling, foreign objects on surface.
          Hotspots may also result from grime accumulation in corners or at
          edges due to inefficient cleaning.
        •  Mechanical damage: Broken glass, broken/bent frame, collisions of
          modules with each other or other objects, improper fixturing.
        •  Internal module failures: Cell material defects (shunts, high series
          resistance, etc.), cell cracks, local de-lamination, poor solder joints.

        Hotspot heating occurs in a PV module when its operating current
        exceeds the reduced short-circuit current (I sc) of a shadowed or faulty
        cell or group of cells. When such a condition occurs, the affected cell
        or group of cells is forced into reverse bias and dissipates power, which
        can cause local overheating. Damage caused by hotspots can vary from
        mild (80°C rise), moderate (80 to 200°C rise) causing cover glass cracking
        and encapsulant back sheet bubbles, leading to moisture intrusion and
        corrosion, to extreme (>200°C) which results in cell damage.















        Figure 2: Hotspots (PV Magazine)                       Figure 4: Encapsulant failure (DuPont)



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