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9 December 2021

Can thin film make the case for offshore solar?

After last week’s article on the use of flexible thin-film modules in the [email protected] II floating solar pilot project in the Netherlands, we had a chat with Wim Soppe, a senior scientist at research firm TNO and the project’s manager. The project is a pilot for an offshore solar concept, but is located in a nearshore artificial lake with brackish water.

The first question to address is of course – what changes does thin film enable compared to other offshore floating approaches? Soppe explained that the thin-film modules only weigh 2.5 kilograms per square meter and have a very low wind profile because they are positioned horizontally. Further, the flexible floaters take up little wave energy since they mold to the shape of the waves. These three advantages allow for a much simpler, lighter and cheaper supporting infrastructure – inflatable floats instead of rigid platforms and lighter anchors because there’s far less drag from the wind.

The third element to stop this light, thin apparatus getting turned over by the wind is water-bags under the surface – again, not expensive. It may also be the case that this lighter system will see fewer breakages over the course of a 30-year project lifespan, due to less mechanical stress in general.

The floater’s weight is 2.5 kilograms, equivalent to module weight, so the “dry weight” of the system is 5 kilograms per square meter – only half a ton for a 100-square-meter installation, with tons of water in the bags beneath. Compare this to silicon PV which weighs 15 kilograms or more per square meter just for the module.

Asked what the difficulties were for using thin-film modules – why aren’t all offshore designs trying them out? – Soppe’s first answer is simply cost; silicon PV dominates the market and has mild advantages on efficiency and especially price, so that’s why so far most offshore solar concepts don’t use thin-film.

When asked about the issue of different amounts of light being received by the modules – since in a bendy setup modules end up at different angles – Soppe replied that it’s one of the things being examined in the [email protected] II pilot. Given the scale of a wave this of course doesn’t affect cells within a module, only one module to the next. It is not going to break anything, but it may have an impact on power output which will feed into LCOE calculations.

It’s also not a problem that’s specific to this approach – even a 10-meter rigid platform will be put at a different angle to its neighbors by waves, leading to mismatch losses on that string. TNO’s researchers call this phenomenon “Wave-Induced Mismatch Loss” or WIML, and have published a couple of papers on the topic, showing 5% losses with silicon PV. The equivalent figure for thin film is one of the things which TNO’s pilot project will seek to determine, and the bigger the efficiency loss is, the greater the motivation for a more dense approach to the electronic connections, such as miniaturized Maximum Power Point Tracking (MPPT) systems.

Asked about the previous [email protected] project, Soppe explained, it was different in that it used closed-cell foam for the floater and open-cell foam, which took on water, as the stabilizer. The verdict was that it functioned well but was too pricy; so that’s why [email protected] II uses water-bags as stabilizers and inflatable mattresses (similar to lifeboat dinghies) as floaters.

At this point it’s clear that cost, not power, is the foremost consideration for TNO’s floating solar design; Soppe estimates that module cost is 25% of project cost, a high proportion similar to that in ground-mounted utility-scale solar plants. “Land” costs are low or non-existent, and nor are there extraneous regulatory and installation costs such as often accompany rooftop solar installations. So the agenda is not to come up with the most efficient possible installation, but the most cost-efficient.

As for commercialization itself, TNO remains a research institute: it is BlueWater Energy Services, the main partner in the project’s consortium, which will take the project over once a design is settled on for scale in 2024 or so. Much of Bluewater’s past lies in the offshore natural gas industry and it’s only with floating wind and solar is it planning to take the leap into renewables. Another important consortium member is Genap, which specializes in plastic floaters.

Soppe does not see the North Sea as the main opportunity for offshore solar, whether paired with offshore wind, energy islands, or whatever else. Instead for obvious reasons the big opportunity will be sunny climes – most particularly the Persian Gulf and the Gulf of Mexico, the former of which also has calm weather and shallow depths.

Asked about whether it matters which type of thin-film photovoltaic is used – currently it’s CIGS made by Midsummer, a Swedish manufacturer – Soppe answered that several kinds are potentially viable – but thin crystalline silicon is only designed to endure bending a few times in an installation on a rooftop and amorphous silicon is lower efficiency, so mainly the alternatives are CIGS, CdTe, and perovskites – TNO is also looking into perovskites and Soppe brought them up without being asked specifically, saying that single-junction perovskites could be worth using in future.