Large flat roofs and green roofs offer ideal surfaces for photovoltaics, but the mounting systems have to meet demanding and sometimes conflicting requirements. They need to be quick and efficient to install, yet flexible enough to allow individual adjustments, whether on a conventional flat roof or a green roof.
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Juliane Hinz, product manager, and Ciaran Murphy, from international technical sales, both at K2 Systems, explained how this works in the joint webinar with PV Europe. They showed how the company reconciles speed and flexibility with new mounting systems that cover large roof areas quickly, whether planted or unplanted, and walked through how these arrays are planned.
More light for the plants
On green roofs, plants and modules have long competed for the same space. “Just a few years ago we regularly received calls from building owners telling us that our system fitted their roof well, but all their plants had died,” Murphy recalls. With Hinz, he presented the second generation of the company’s green roof system, Green Roof 2.0. They also introduced the new flat roof system D-Dome 6.7 Portrait, designed for the large modules previously associated mainly with ground-mounted arrays.
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Its predecessor, Green Roof Vento, already raised the modules high enough for light to reach the plants beneath. “The second generation is significantly more optimised and slimmer, both in terms of material and installation time,” says Murphy. East-west layouts can be built as a V-shape, with the modules meeting at their lower edges, or as an A-shape, with the modules meeting at the top. A south-facing layout is also possible, as are portrait and landscape formats and tilt angles of 10 or 15 degrees.
Greater flexibility built in
The result is a highly adaptable system. Hinz demonstrated the set-up live during the webinar. It starts with the Basic Rail 22, familiar to installers from other K2 systems. A front and a rear support sit on top of it and carry the Single Rail. “Installers always need the front support, regardless of whether they are installing in portrait or landscape format or want a 10- or 15-degree tilt,” she explains.
The height of the rear support alone sets the tilt angle. “A major advantage is that everything is pre-assembled and delivered under a single article number directly to your warehouse or to the construction site,” Hinz adds.
Substrate as ballast
Ballasting works differently. The Basic Rails are screwed onto trapezoidal polyethylene (PE) plates that sit beneath the growing substrate. “Our idea was to use the weight that is already on the roof rather than add further ballast,” Murphy explains. The approach works best on new buildings or roofs being refurbished.
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The system also works for retrofitting existing green roofs, using the same basic structure. Instead of PE plates, the Basic Rails are then screwed onto concrete blocks, which act as ballast. Murphy explains why: “Because once the substrate is in place, it means a lot of extra work to remove it, install the mounting system and then reapply it. It is possible, but then the installer risks damaging the roof membrane underneath.”
A reference project in Switzerland shows how the system performs at scale. K2 supplied the mounting structure for an array of almost 4 MW with around 6,500 modules, roughly 10 percent of which sit on a multi-storey car park. “Anyone planning such a large project also gets recommendations from us on how best to organise the teams and materials,” says Murphy.
Long rails for large modules
The second system presented in the webinar has its roots in Italy, where customers wanted to build low-tilt arrays using modules rated at 680 W to more than 700 W. “Our special projects team combined components from pitched and flat roof systems from our portfolio, which works almost like a metal construction kit,” Murphy says. The new D-Dome 6.7 Portrait system grew out of the first of these projects, an array of just over 1 MW.
Unlike most flat roof systems, D-Dome 6.7 Portrait mounts the modules in portrait format. “This way you need more rail per module, but overall fewer rails and therefore less material on the roof,” says Murphy. Installers can fix the modules directly to the domes along the long side, at the quarter points.
Portrait mounting saves material
Module manufacturers now require their very large panels to be clamped at the quarter points, which is not possible with landscape mounting. K2 Systems gets round this by mounting the modules in portrait format, while two neighbouring modules can still share a dome, just as with smaller modules in landscape format. No cross-bracing is needed. In addition, the V-shaped layout puts the larger gap at the top rather than at the bottom. “That gives you better hand access to the cabling, and warm air can flow out more easily from under the modules,” Murphy explains.
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The modules are always clamped on the long side, at the points where module manufacturers specify their load values. Because the large panels require greater stiffness, K2 uses the taller Basic Rail 36 for this system. “If you are on the construction site wondering whether you are holding a part for 7, 10 or 15 degrees, you will find the answer directly on the component,” says Hinz, pointing to the embossed degree marking.
Installers have flexibility when it comes to ballast. In Italy, for example, they lay long concrete sleepers across the base rails. “We define the minimum ballast based on our wind analysis, but how the array is then weighted down is largely left to the installer,” says Hinz.
Two degrees make the difference
Murphy demonstrated the planning process for both new systems directly in the K2 Base software. Green Roof 2.0 is already available there for all customers. Projects with the D-Dome 6.7 Portrait are currently planned internally by K2 experts, with installers sending in the project data via the website.
Murphy also shared several planning tips and tricks. Each PE plate of Green Roof 2.0 carries around 18 kg at a substrate density of 600 kg per cubic metre. “If the software indicates another 17 kg of ballast needed at a given point, in practice you simply add a second PE plate,” he says. For large projects, the installation plan is also presented more clearly so that no one loses their bearings on the roof amid a hundred pages of module blocks.
Software shows changes in tilt
Modules of different sizes can also be mounted with Green Roof 2.0. To do so, the installer simply shifts the supports within the base rail. This does slightly alter the tilt of the modules, and the software now shows this precisely. “Because when an investor puts millions of euros into a 3 MW project and calculates with 8 degrees of tilt, an actual tilt of 7 or 10 degrees can significantly change the payback period,” Murphy stresses.
Further details and practical tips for installing megawatt arrays on green and conventional flat roofs can be found in the webinar. Those who could not attend live can watch it on demand, free of charge, and see how both systems are assembled. (su)
