The long read: Is the coast clear for solar to head offshore?

From pv magazine 02/2022

When it comes to land and reservoir-based PV systems we’ve more or less agreed on the basic design layout, but when it comes to offshore environments, FPV design is as mutable as the terrain. Thomas Reindl, deputy CEO of Solar Energy Research Institute Singapore (SERIS) at the National University of Singapore, lists a few of the offshore challenges, including “wind and wave forces, currents which take a strong toll on the mooring and anchoring system, high salinity which accelerates corrosion, and biofouling.”

There are also particular challenges facing the FPV strings. Depending on the floating platform design, Reindl says there is the potential for “mismatch in power generation in a PV string.” To navigate these hurdles, various approaches are floating to the top. And while many early offshore FPV designs stoically defied the elements, many more are now seeking solutions which harmonize with the ocean and its tributaries.

Act natural

Inspired by the Amazon’s giant water lily, Norwegian-headquartered Ocean Sun stands out from many other FPV mounting structure designs with its flexible circular surface membrane. The round design of the buoyancy ring, explains founder and CEO Børge Bjørneklett, “gives a favorable distribution of loads, similar to a wheel structure. For other floater geometries, high force can arise if hit by broadside waves.” Like the lily, the ring “undulates freely with the waves in the vertical plane,” spreading forces, reducing fatigue stress requiring fewer mooring lines and anchors, according to Bjørneklett.

Ocean Sun is a part of an EU-funded consortium with Fred. Olsen Renewables, Innosea, the Technological Institute of the Canary Islands (ITC), and the Ocean Platform of the Canary Islands (PLOCAN) to test its offshore FPV technology. The trial involves a 0.25MWp system in one of the sunniest parts of Europe, off the coast of Gran Canaria, where wave heights can reach up to 10 meters and winds are high to match. The test project began in January 2021 and will last for 30 months.

Three-point plan

Unlike the Bermuda Triangle, SolarDuck’s triangular design promotes safety and stability on the water. According to CTO and co-founder, Don Hoogendoorn, “triangular structures are used frequently to design light but very stiff structures. The Eiffel tower, cranes and oil rig’s legs all entail these triangular designed structures … Triangles are inherently stiff and also avoid torsional movements in the structure. Hence, lightweight solutions can be developed which will be lighter than, for instance, solutions based on rectangles.”

The triangular design of the SolarDuck system ensures scalability and flexibility, the company claims. “Triangular structures allow for three axes of rotation, while rectangular designs only allow for a movement over two axes,” Hoogendoorn notes. “Therefore, triangles will follow waves with bi-axial curvature much easier, resulting in lower loads on the structure and hence a more cost-effective design.” And since triangles tesselate together so well, there is much reduced risk during crew transfers from coupled systems.

So, while Ocean Sun prizes flexibility with its floating membrane, SolarDuck prizes stiffness, and overcomes any shortcomings on this front through elevation. By elevating the platform at least 3 meters above the surface, says Hoogendoorn, waves won’t hit the panels causing cracking and other failure modes. And since the panels are on the triangular platform, there is much reduced risk of fatigue between individually moving panels or cables.

Better with ballast

Seeing as solar panels can’t float by themselves, floaters are perhaps the primary ingredient of any FPV array and invite all manner of innovation. The most conventional FPV floaters are blow-moulded, such as those deployed by French supplier Ciel & Terre, meaning they gain buoyancy through inflation of a hollow form.

Another French company, HelioRec, is employing a different approach. Taking a cue from the maritime sector, HelioRec has introduced ballast to its floater system – in the form of water weight. The company’s CEO and founder, Polina Vasilenko, calls the floater design the “hydro-lock,” and says the additional mass provides more stability without extra cost.

The extra weight from the onboarded water also provides wind resistance, Vasilenko claims, “since the power plant has large inertia, and it is difficult to pull the system out of the water.” Nor is flexibility compromised for this extra stability, since the system’s flexibility is provided by UV and saltwater resistant “rubber connectors between floating modules, ensuring a long lifetime.”

HelioRec recently signed a partnership with Dutch maritime company Van Oord to evaluate whether FPV can be used to charge electric boats, such as unmanned survey vessels. “The idea is to build parking and charging stations for electric boats powered by floating solar,” says Vasilenko. The entrepreneur notes that she is passionate about accelerating the decarbonization of the maritime sector and believes that FPV can work “in combination with wind, H2 production and aquaculture to use the same sea area in the most efficient way.”

Limited land

Quiz question: What do Singapore, Malta, the Netherlands, and Japan have in common? It’s not a dearth of free space – the problem is that the space is not on land.

“In a place like Malta a 20MW, let alone a 100MW, solar park is unthinkable,” says Luciano Mule’Stagno, director of the Institute for Sustainable Energy and group leader of the Solar Research Lab at the University of Malta. “But we can easily find suitable places for several of these plants at sea. Coupled with the ever-falling cost of energy storage, I can see a place like Malta running on 100% renewables (FPV -plus-offshore wind) in 10 to 15 years.”

The same reasoning goes for large coastal cities, continues Mule’Stagno. “It’s mostly impossible to install large systems within the city – so the only current option is far-away farms with long transmission cables. A large floating system can be within 1 kilometer of where the consumption is concentrated.”

Reindl, agrees, noting the “great future potential” of offshore FPV “will likely start with near-shore installations for load centers such as megacities.” SERIS is currently involved in a 1.1MW near-shore FPV demonstration system with energy storage at one of Singapore’s southern islands. The data from the project will help researchers to “understand the performance of marine FPV systems and their potential challenges in tropical regions.”

Gerald Tan, founder and managing director of Singaporean offshore renewable energy developer G8 Subsea, also agrees with offshore FPV’s near-shore necessity. “I think one of the first principles of power generation is that you generate as close to where you consume as possible,” Tan tells pv magazine. Of course, that means the most suitable sites tend to be closer to shore. “Site selection is extremely important,” continues Tan. “Certainly one of the most critical aspects of project development … Near-shore sites are also far easier to install, manage and maintain. The next most important aspect is grid connection. We want to see adequate grid infrastructure on the land, so that we don’t have to spend too much on the cable landing station or the power management station ashore.”

One notable exception to these parameters of site selection is the potential of offshore FPV to operate in tandem with offshore wind. Tan wants to lead the way in the concept of the hybrid solution, which he says has clear benefits to cost and capacity. “You have the sea space between turbines, and that sea space could be used for FPV. Especially as there is so much expenditure on the cables already. And of course, we know that wind blows more at night and obviously solar is proficient during the day. So, by combining these two together I’m using the same infrastructure and the same efforts of construction to double up the capacity.”

Hoogendoorn is similarly optimistic about FPV’s potential in combination with offshore wind, noting that offshore FPV uses far less space per MW and GWh than offshore fixed wind.

“For example: offshore FPV in the North Sea has a factor of eight higher MW/km² and a factor two higher GWh/km² than offshore fixed wind.” Clearly then, hybrid systems could easily double the generation capacity of an offshore wind array. “This could be very beneficial in the space constrained North Sea. For areas around the equator these values are even higher.”

Last resort

Island nations and archipelagic regions are perhaps those which can most effectively utilize offshore FPV. “The atolls and islands of the Maldives are an immense opportunity for offshore FPV,” said Tan. “Anyone can see that the shores are eroding, and a lot of expense is spent on shipping diesel between islands which can be solar powered.”

Austria’s Swimsol is doing just that, having installed its SolarSea platform in various Maldivian atolls. Managing Director Martin Putschek tells pv magazine that when it comes to offshore FPV in these situations, and particularly at high energy- consuming locations like resorts, offshore FPV is not an alternative to rooftop solar, but complementary.

“In our projects with resorts, and also other customers, we utilize all suitable available rooftop space for solar panels and provide any additionally required solar capacity on our SolarSea platforms. The private sector has been expressing more interest in our FPV solution, especially when they realize that rooftop PV would simply not meet their power demand and save enough diesel.”

Putschek notes that these offshore installations have been gaining in popularity since they’ve been able to showcase their first commercial systems, which required a bit of convincing for the early adopting resort. “But now interest has grown immensely, and we are currently negotiating several FPV systems in the megawatt scale,” he says.

However, there are obstacles. For instance, “not all resorts have a suitable lagoon or sea space close by within their island boundaries, or resorts don’t want the system too close for the visual impact.” In such instances, installations have to be made in more distant public waters, requiring permissions and lease agreements with the government.

Offshore oil

Generating energy offshore through wind and solar may seem a rather novel idea, but the oil and gas industries have been doing it for decades. And while the oil and gas industries are slow to realize they’ve overstayed their welcome at the energy party, the expertise gained from decades working offshore is certainly welcome to renewable energy developers.

Indeed, many of those renewable energy developers have taken note of the changing times and jumped ship to green industry. G8 Subsea is just such an example. “It really helps that we are already a specialized offshore contractor,” G8’s Tan tells pv magazine. “A lot of our lessons learned come from our past experience in oil and gas, from projects in the North Sea. For instance, our teams are well trained in diving activities, safety protocols and the like.”

Considering there is a dearth of regulations when it comes to offshore FPV internationally, experience in health and safety is highly necessary. This is also true for the safety and performance of the projects themselves. “If you look at the present methods of installing megaprojects on water, all risks are amplified as soon as you go offshore,” continues Tan. “So, it makes a lot of sense to do as much assembly as possible onshore where people are safer, time is used more productively, and the electrical certification is conducted before we tow it out … Once everything is offshore, we are only spending a few days making sure everything is properly secured and inspected. This is much better than spending months or years with heavy-loading vessels offshore.”

Scaling up

Most of the world’s population lives within 100km of the ocean, meaning the potential market for offshore energy generation for coastal cities and populations is enormous. But key to any market is scale, and offshore FPV is no different.

One way to maximize stable growth is to spread the load of technological knowhow. This is what Ocean Sun is doing by licensing its technology, as it has done with Chinese developer Sunneng Technology. Sunneng is constructing a 1MW nearshore pilot array using the Norwegian company’s innovative membrane design. “By licensing the technology we can more quickly increase the volume of installations and, in turn, reduce cost of the components,” says Bjørneklett. “With enough volume of installations, the Ocean Sun system has potential to compete with ground mount systems in terms of capex.”

Bjørneklett adds that he expects “dramatic” growth for FPV. “Mainly because of maturity of the solutions and because it solves the problem of land access for large PV installations in the proximity of consumers,” he says.

Energy salvage

Like renewable energy generation generally, the pending issue is not whether we can generate substantial amounts of energy through renewable sources, but whether we can store that energy for stable use. When it comes to offshore projects, especially if the symbiotic relationship of floating wind and solar is capitalized upon, the ability to store that generated energy could rapidly accelerate the energy transition.

Enter Dutch startup Ocean Grazer, which has designed an offshore pumped hydro storage system that is buried in the seabed and can provide GWh-scale storage volume through stacking of 10MWh systems. The Ocean Battery does this by pumping water from low-pressure rigid reservoirs into high-pressure flexible bladders laid on the seabed.

CEO Frits Bliek describes the Ocean Battery as a “perfect match” for FPV by absorbing “the peak around noon to charge the ocean battery quickly and allow for reduced capacity to discharge the battery and inject the power spread out over the rest of the day.”

Environmental impact

Offshore PV developers and proponents of the application are keen to ensure that environmental considerations are a priority in their plans and projects. SolarDuck’s elevated design ensures light and oxygen flow to the sea surface beneath. And G8 Subsea doesn’t want to see the same heavy piling that previous offshore industries have implemented.

Stagno, from the University of Malta, explains that the systems his team are deploying are designed for environments of 50 meters depth or more, ensuring low impact on busier shallow environments. Of course, this is not always possible in the case of archipelagic regions, but unlike other energy generation, small-scale FPV is hardly a disturbance.

“We want to take care of aquatic life,” says Tan, who uses autonomous submarine drones to inspect FPV systems. “We took time to see if marine life was still active, and I’m very happy to say the aquatic life under floating solar farms is very healthy. Fish are starting to use it as a reef, as a shelter.”