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Light touch … adapting C&I solar for rooftops that can’t take the strain

Solar installers who try to cut corners by avoiding stringent structural assessments risk the reputation of the entire industry – and can miss out on proving themselves in the tough C&I sector.

Because solar panels solve problems for the people who own them, not everyone likes to talk about the uncomfortable stuff. Are they attractive? Can you recycle them? How long do they really last? Another related thought that comes further down the list is: can my roof take the weight?

It’s hard to quantify the number of sites where a weak roof will hamper the installation of an optimal amount of solar, says Smart Commercial Solar managing director Huon Hoogesteger. His company hedges risk by calling in the experts. After bidding for a commercial job and designing a rooftop solution, Smart Commercial Solar will employ a structural engineer to verify the proposal.

“Once the client has accepted or indicated strongly they will accept [the proposal] we will then spend the money to structurally approve our design,” he says.

“The engineer will come back with recommendations that, yes, that design’s OK, or we’ll be told we cannot do that because the roof is not strong enough.”

Hoogesteger estimates that in about 10% of cases an engineer’s report will point out problems that will need to be overcome by either strengthening a roof or designing a smaller PV system.

An outcome of one in 10 is pretty good, “but that’s because we are not covering the entire rooftops of most buildings,” he says.

As an example, take the work Smart Commercial is doing for Australia’s largest hardware retailer. Of the 83 sites it has worked on so far, the rooftops are so vast that structural issues have been easily overcome by positioning arrays where the roof is “structurally adequate”. Solar systems at those sites have only covered about 20% of available roofspace anyway, he says.

Light touch

A checkerboard layout represents a compromise between what the roof could take and what the load required.

For a recent job for a well-known food brand the system designers came up with a checkerboard approach, where a reasonably fit bird could hop across the roof in any direction – including diagonally – without landing on a module (see picture).

“We didn’t have the luxury of a large roofspace in that job and there is a high-density load underneath. If we had a choice we would have filled that roof with solar panels,” he says.

Traditional panels weigh about 13kg per square metre, including racking and cabling. The structural engineer’s report for the site recommended about 60% of that. To make the most of what they had to work with, it was decided to spread a smaller system across the entire roof – hence the checkerboard.

At a different location owned by another client, the problem of designing a system to suit the customer’s energy needs and the building’s ability to bear weight saw a solution where arrays are concentrated above and around columns (see next photo).

Even the Sunman PV product, an encapsulated module with no frame or glass that weighs 4kg/sqm, was too heavy for the structure. The site needed 2.1MW, but the owner now makes do with 380kW.

House of cards

Although the site required 2.1MW, the owner is making do with 380kW of PV distributed astutely.

Placing too much weight on a weak roof is risky, but so is placing too much trust in a structural engineer. Smart Commercial Solar lost a 1MW-plus job after an appraisal by one structural engineer was reviewed and overturned by another, who found that the roof was not fit to take any solar at all. “We ended up losing the equivalent of $900,000,” Hoogesteger says. “We had equipment installed and we had lost revenue … we almost lost the entire contract.”

That’s bad news for one well-established, large solar installation company, but every person working in the sector should feel a chill about what might have happened.

“Thank God it was caught. If it hadn’t been, there was real risk the roof could have collapsed – in which case it would have killed people.”

There will always be consequences if a job is not done right, he says.

Aim higher or stay put?

Having described such a close call, what is Hoogesteger’s advice to installation companies that want to elbow into the commercial-and-industrial realm? “You can’t treat commercial installations as they do in the residential sector,” he tells EcoGeneration. “You need to do development applications, structural engineering, grid designs … they are all engineering sciences in themselves.”

Despite the scale, commercial work is still expensive. Apart from commissioning structural engineering reports, rooftops may have to be strengthened. “People think costs go down, but we’ve found that’s not really the case at all.” The risks also have a tendency of rising in lockstep with the capacity of a project.

“I know that some of our competitors have simply installed systems without structural engineering approval,” he says, “which is interesting … and typical of our industry.”

There is nothing wrong with an aspiring solar company taking a “glory shot”, he says, but the firm reality is that commercial solar requires detailed, quality engineering – both electrical and structural – and an understanding of all the parts of the installation.

“The costs are higher and the risks are magnitudes higher. If we don’t get it right, the roof may collapse, destroy a building or, worse, kill someone. Reputationally, the individual company may suffer – but broadly we all suffer. Do it right, and [C&I solar companies] get to do some very cool projects.”

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