Suitable Mounting System Materials

Aluminium alloys, galvanized steel, V2A and V4A stainless steel, and concrete (including precast concrete elements) are the materials that are most suitable for solar generator mounting systems, which
are also referred to as racking systems, racks and so on. As with standard tiled roofs, the support system for roof-integrated pitched-roof solar generators can be made of wood. Wood is also often used as a mounting system material for ground-based installations with framed modules in rural Southern European areas. For example, a large 5 MWp grid-connected system was recently realized in the Leipziger Land region (Figures 1.15 and 10.46-10.48) using a wooden mounting system. For ground-based installations, however, metal mounting systems are available with extremely large ground bolts that can be screwed right into the ground without a foundation. The main fastening elements for mounting systems are bolts and clips constructed of V2A or V4A stainless steel. It has been found that galvanized bolts often begin rusting after just a few years.

Inasmuch as aluminium alloys are normally corrosion resistant and are also easy to work on following installation without inducing corrosion in the areas where work was performed, they are the ideal material for module frames and mounting systems. Unfortunately, aluminium manufacturing is energy intensive, thus making it unpopular among potential PV system owners, who tend to steer clear of it. But since aluminium rack elements can be readily recycled with only negligible energy expenditure, the grey energy integrated into this material is recouped – which means that the purported ‘problem’ entailed by the use of aluminium racks is greatly exaggerated. In cases where robust corrosion protection is a key factor for an installation, the use of aluminium alloys is definitively justifiable.

In a temperate climate, galvanized steel with an intact zinc layer is well protected against corrosion for many years, but in most cases, after 10-20 years it begins to show signs of corrosion. However, this is normally only an aesthetic drawback that has no impact on sturdiness. Rainwater in conjunction with module-frame aluminium can accumulate locally and thus provoke contact corrosion. If the water is able to run quickly off such contact points and if these points are well ventilated, the corrosion that is observed at contact points between the aluminium and galvanized steel is of no real importance.

When combined with either aluminium alloys or galvanized steel, joining elements made of stainless steel exhibit no corrosion and are thus ideal for use with both materials. On the other hand, owing to the far thinner zinc layer and the mechanical stress engendered by bolt tightening, which often damages the zinc layer, galvanized nuts and bolts are less well suited for mounting systems.

A more critical problem, and one that should be avoided wherever possible, is direct transitions between aluminium/galvanized steel and copper lightning protection installations. In such cases, anti-corrosion measures such as the following should be realized in the contact area: use of suitable bimetal joining elements, or nickel/tin-plated transition elements that fall within the scope of the electrochemical series; and protecting joints from direct exposure to the elements, wherever possible.

More problematic than corrosion induced by local element formation is the perennial problem of DC installation electrolyte corrosion that occurs at the metal to electrolyte current transition point. Such corrosion can be avoided via proper insulation of all DC components and good protection of all bare module plug-in connectors and junction box elements against moisture. For systems with high DC voltages, in addition to these precautions insulation monitoring measures should also be implemented.

Updated: August 6, 2015 — 2:49 pm