Advice for Rooftop Solar PV Arrays

December 12, 2022
Rooftop Solar PV Array installation

Understanding how they work

PV panels are dark in colour, attract and absorb solar radiation, which in turn activates embedded silicon materials, creating electrical potential. Knowing some of the advantages and disadvantages to the building structure is a vital part of the relationship between the user, the installer, the PV installation itself and the rest of the building, especially the roof.

When it comes to the installation and maintenance of PV (solar) panels, the actual nature of the roof is often not taken into account by the property owner and can lead to a mismatch.

Whether the panels are fitted to new buildings or retrofitted onto existing commercial, industrial or residential buildings, it all begins with the roofing substrate that lies below the panels and the surrounding exposed areas.

From that point on, the user moves into an in-use/operating phase, with maintenance and other post-installation issues.

The overall design

The majority of PV installations are retrofit. However, with the strong movement towards energy efficient design and making the building impervious to loadshedding, a large number of new build warehouse and commercial are designed with fitted PV arrays.

Structural considerations

In the design stage the following issues are pertinent:

Load bearing structures

Are they designed to handle the weight of a retrofitted solar array?

  • Peak kg/m2 load points – the probability of roof damage being caused by uneven loading at specific points on the roof in the case of highly angled PV panels that do not disperse the weight evenly. This particularly applies to flat roofs, where the panel has to be elevated to a greater extent to achieve optimal efficiency.
  • Additional loading caused by force of wind
  • Angle to the sun is essential to achieve maximum efficiency of the array.
  • Downstream of manufacture, various South African rollformers offer formed roofing panels in various profiles and interlocking options. It is therefore important that the suspension system proposed for the PV array is tailor-made to fit the roofing profile.

Access points for installation and maintenance

Roofs are expensive to replace. Be aware of the damage caused by:

  • Installer and maintenance foot access.
  • Swarf and any other debris arising during installation and from other mechanical work on the roof, needs to be cleaned off immediately. Swarf particles contain metal, often posing a significant corrosion risk to the roofing surface, as well as causing discolouration of the roofing members.
  • Compatible walkways should be installed, limiting damage or abrasion to the roof surface. The PV array should permit easy access – for maintenance and inspection.
  • Electrical cables should not be placed directly on the roof sheeting and result in any accumulation of debris or other contaminants.

Metallic roofs: Corrosion and oxidation considerations

Solar panels should consist of silicon cells entrapped within, ideally, low iron, alkali free and lead free glass, with an anti reflective coating not containing titanium oxide and zinc oxide.

The panel is enclosed within an aluminum frame. By its very nature, aluminium, is slightly more cathodic in nature compared with zinc-based roofing substrates, so great care should be taken to ensure that these metals are insulated from each other. This not being the case, micro-current will flow from the roof material to the aluminium frame, causing early-stage pitting and breakdown of the surface of the roofing panel via the process called galvanic corrosion.

However, the advanced AZ coated Alloy used with the Zincal® product is less prone to galvanic corrosion when combined with any compatible aluminium frame. However, any other compatible component not enhancing corrosion on the roof sheeting can also be considered.

Thermal stability

A roof is a key component of the interior climate control of any building. By their nature, solar panel-mounted roofs are cool roofs. The panels shade the roof below from the direct rays of the sun for a considerable amount of time during the day.

Conversely, the roof also influences the structures loaded above it, noting that solar/PV panels exhibit temperature -sensitive efficiency levels.

Engineering studies show that a high roof temperature for PV panels is undesirable, impacting the roof sheeting lifespan and R Values of the roof. The following advice applies:

  • Install high solar reflectance roof sheeting to reflect away the inbound infrared radiation from exposed areas of the roof, before it gets absorbed and diffused.
  • Elevate the solar array at least 10 cm off the surface of the roof using an approved suspension system. This will permit the free movement of cooling air and also allow for evaporative drying of any water on the surface of the roof more quickly.
  • Consider insulating the back plate of each PV panel to prevent the panel cooling too quickly and also from warming up to a point where its optimum efficiency is impaired.

Roofing substrates

An ideal material to avoid a number of the above issues is Safal Steel’s Zincal ® product. It is lightweight in construction, allowing a higher degree of weight loading on the suspension structure of the roof.

Design the roof for PV and select the right roofing

It is a popular trend to specify a pre-painted steel roofing material from an inspirational design approach.

Good engineering practice suggests that a high solar reflective index (SRI) roof provides optimal performance. It needs to be noted that most of Safal Steel’s Colorplus® range offer exceptionally good solar reflectance and transmittance performance, even among the darker colours. So, too, does Zincal®

A Zincal® roof transmits 65 watts/m2 of heat compared with 120 watts/m2 for a galvanised roof and 150 watts/m2 for a clay tile roof (*).

One aspect of a solar array worth considering is the erosive effect that dripping water can have on painted surfaces. In many instances, an Al/Zn-coated metal roof provides better surface performance than a painted one.

Due to the fact that almost the entire roof or a visible section can be covered during the roofing process, a Zincal® roof is also well worth considering.

Local manufacture and high quality licensed technology

Our foundation product, Zincal® , is locally manufactured at our plant in KZN. We are the only African-licenced manufacturer of the patented Al/Zn technology based on the continent. As a licensee of the world-famous BIEC-technology, our systems are subject to monitoring and audit by our principal.

Designers and building professionals are encouraged to avoid cheap imported alternatives that originate from unknown sources, often in the Far East. There is only one original Zincal®. You will know us by the stamp mark of our brand on the back of the formed roof sheet.

Safal Steel offers a range of coated unpainted and pre-painted options in various TCT ranges and two mechanical strength standards – G550 and G275. There are many colours available in the Colorplus® range, as well as different textures. In-service warranties are available, subject to our terms and conditions.

Note (1): At a solar radiation level of 850W/m2 equivalent to 30°C ambient temperature.

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