The long read: Floating solar up to standard

From pv magazine 06/2021

On the surface (literally), floating PV (FPV) is a great way of generating clean energy without taking up land. But that’s only if the entire plant, from cells to cables, stays afloat.

Yet there are currently no specific standards for FPV, with existing projects constructed to terrestrial standards. The prevailing attitude, beyond the existing standards for terrestrial PV, seems to be “whatever floats your boat.” But “whatever” isn’t good enough when it comes to a nascent technology growing at the rate FPV is forecast to in the years ahead.

So, what should an FPV project be judged on? According to Michele Tagliapietra, a consultant at DNV and a project manager for the joint industry project behind the recent Recommended Practice (DNVGL-RP-0584) for FPV, “the top three components that should be handled with particular care … would be cables, mooring lines and the material of the floats themselves – with specific focus on the interconnection between the floats.”

Power and water

Cable standards in water start the concerns. FPV in Southeast Asia has two primary concerns – water (from both below and above) and humidity. According to Jan Mastny, head of global sales for Studer Cables, “what is very important to say is that today’s cable standards … do not consider any exposure to water or humidity.” This means that cables do not have to be tested for their water and humidity resistance prior to installation. Mastny, an International Electrotechnical Commission (IEC) member, says that part of the problem is that the range of standards currently applicable to solar PV were made as long as 10 years ago, when FPV was little more than a curiosity.

Lokesh Vinayagam, a research associate with the Solar Energy Research Institute of Singapore (SERIS) – where he is the O&M lead of the world’s largest FPV test bed – told pv magazine that FPV currently follows the same DC cable standards used for terrestrial applications.

“For systems on water, the DC cable should be highly water resistant, and for certain installations (e.g. near-shore/offshore floating PV), even marine grade,” he said. “Any unnoticed and continuous immersion in water for extended periods of time will lead to accelerated degradations and possible damages … this could cause the system to shut down.”

Of course, whether it be a lower energy yield from degradation, or downtimes and replacements, the opex of inadequate components only increases.

“At minimum, cables for FPV applications should be highly water resistant,” Lokesh said. This might seem pretty straightforward, but marine cables are often too expensive for FPV business models. “According to existing standards, PV cables need to be halogen-free,” Mastny explained. However, “in order to replace the halogen with other additives” for FPV, he said, “compounds are being used that are highly hydroscopic” – which is to say, absorbent.

“The cable is actually behaving like a sponge,” Mastny continued. “And it is only a question of time before the moisture gets to the conductor … and one of the first things the water does is increase the resistance in the cable, so if the conductor is exposed to moisture, it creates more resistance and the resistance is reducing the performance of the installation.”

Leaving humidity aside, the cable material shouldn’t matter if the cables are kept out of the water. Yet keeping cables out of the water isn’t easy. “The standards today are actually contradicting each other,” said Mastny. “For example, in the standard today, it is clearly written that the string cable must be flexible. But if you want to make a water-resistant cable you need a metallic layer as a protection, and if you add this layer, obviously the cable is no longer flexible.”

Of course, when it comes to near-shore and offshore saltwater FPV (which are not covered in the current version of the DNV Recommended Practice, because wider commercial viability is still thought to be five to 10 years away), the demands are only intensified.

“The important difference,” said Børge Bjørneklett, founder and CEO of Ocean Sun, a Norway-based offshore FPV outfit, “is the saltwater and the corrosive atmosphere, which put extra requirements on the power electronics.” Therefore, “the challenge is to create a protective environment for the panels and the power electronics,” and this requires the floating structure to be constructed with marine grade materials. “We use cable conduits in UV-resistant polyamide and have not had any problems,” said Bjørneklett.

Staying anchored

For anchoring and mooring, FPV projects are very dependent on underground and underwater conditions. Taking the SERIS testbed in Singapore as an example, the water is, according to Lokesh, “relatively calm and doesn’t induce much stress on the anchoring and mooring system.”

However, no FPV plant is motionless. Even in the relative calm of the SERIS Singapore testbed – as opposed to the much greater stresses of near-shore and offshore FPV – Lokesh said that all anchoring and mooring components receive regular checks for “corrosion, tension, and slackness.” He added that “certified divers or underwater drones might need to be employed to carry out periodic inspections over the lifetime of the system.”

Thankfully, when it comes to anchoring and mooring, there are other industries that have been doing it a lot longer than PV. And that knowledge can and is being transferred across.

“We need … a clarified and harmonious way of testing the float components and the connection between floats, because these are items that can cause problems in the long term,” said Tagliapietra. The lack of specific standards, and the tangle of various local and national standards, is one of the reasons DNV produced its Recommended Practice.

“If every manufacturer will push internally to have a structure that complies to all its [the Recommended Practice] requirements, being stable, having enough space for walkways, being resistant to cyclic load, being fireproof, being resistant to UV … if they all comply then we believe FPV would be safer and more reliable in the medium and long term,” said Tagliapietra.

Extraordinary environs

Putting a solar power plant on water is an extraordinary situation, and as such it deserves extraordinary standards. What needs to be done first is to acknowledge the problem.

The problems for FPV are twofold. Firstly, solar power plants are not naturally buoyant or at home in water. Only compromises can be made to get around this. And secondly, as mentioned, the standards which currently apply to FPV are terrestrially minded and therefore at least partly inadequate.

The ideal solution would be to redesign floating PV from the ground up, or rather, from the waterline up. But as Mastny noted, the best we can do is to compromise. “We are converting PV plants, not building submarines,” he said. As far as cables are concerned, Lokesh and Mastny agree that the best course of action is long-duration testing of water-resistance, cable management, and good O&M practices, so as to minimize faults and costs.

Of course, standards always move slower than the technologies to which they apply, and FPV is a young market in rapid development. But there is already movement on this front. While work on an international floating PV standard is underway in the IEC (driven in part by SERIS), Lokesh said there is no need to reinvent the wheel.

“The approach is to focus on topics that are different for floating PV as compared to ground-mounted installations. Also, we are looking at adjacent standards, such as marine electrical standards which we can apply to FPV,” he said.

Bjørneklett looks at FPV standards in a similar way to Lokesh and DNV, and also with relation to maritime standards. “Traditionally, the insurance companies and financiers will turn to the classification societies, certification bureaus, etc., and ask them for third-party advice. This is how it’s done in other maritime industries and works well for all parties. The classification societies have long experience in calculating the risks based on sound engineering analysis.”

Mastny, on the other hand, looks to the big EPCs as drivers of change. “The EPCs are those who can directly influence the specification of the product – nobody else has the power,” he said. “Sometimes I have the impression that EPCs are not utilizing their power for enough benefit.”

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