Interest in floating solar is growing fast – but the challenges are also rising, especially for the moorings, writes Stevie Knight.
Not only do floating photovoltaic systems avoid taking up prime land, but they’re also more effective “as compared to ground-based installations the water’s cooling effect improves efficiency by somewhere between 10% and 20%”, explained Charles Gery of Seaflex.
While some are modest (2019 saw a 0.25MW Dutch plant) they can be large. In Asia a 5MW farm is set to cover an area roughly the size of six football pitches: it’ll need 70 mooring lines to hold it.
Further, a number are moving toward harsher environments. Solar farms on hydroelectric dams reduce evaporation and can utilise the existing power infrastructure - but they are particularly challenging. One installation at the mouth of the Rabagão River in Montalegre, Portugal has to handle depths of over 60m with a dynamic 30m range: further, its unsheltered position exposes it to 31m per second winds.
Moreover, some are in regions “where the wind speeds can reach 80m per second”, said Gery. Only last year Japan’s largest floating plant broke loose and caught fire after being hit by Typhoon Faxai. It “wasn’t a Seaflex project”, he underlined, and while a number of elements may have contributed to the disaster, there’s also an issue around traditional line technologies.
Gery explained that the common wire varieties “while fine in calm conditions, do nothing to dampen the loads that can come with higher winds or waves... we’ve seen these forces getting up to 25KN in typhoons” – that’s 2,550kg on each mooring point. Further, these conditions can also damage the floating plastic pontoon above as the stresses tend to be concentrated on a single spot.
The alternative, he explained, is to inset an elastic section between the two more traditional end pieces which redistributes the loads. This is the idea behind Seaflex’s technology: reinforced homogeneous elastomer hawsers attached by high-quality stainless steel plate. It is remarkably resilient: DNV GL monitored tests show a single Seaflex hawser can withstand a pull over a tonne, stretch out to twice its length and retract with no damage.
Therefore, each line can be equipped with between two and 10 individual elastic units depending on demand; the aforementioned Portugese project had rubber hawser sections of around 10m or 11m in each of the moorings.
While a straightforward principle, each location needs a tailored approach. For example, in the northern hemisphere, “you may need to raise the angle of the photovoltaic panels in order to make the most of the sunlight... and so even moderate winds can make the pontoon act like a sailboat”, said Gery.
As a result, owners are presented with mitigation choices. Would they be willing to lose a little from full-sun efficiency by turning the installation - if it significantly decreased the load on the moorings?
“The industry has now got to the point where we need to establish some best-practices,” Gery explained. Therefore Seaflex is leading the EU-funded FRESHER (floating solar energy mooring) programme with Swedish, Spanish and Portuguese partners to start the process of investigating underpinning guidelines for large-scale and even offshore deployment.
Interestingly, the move into marine locations could reshape other renewables. As Gery explained: “If you can co-locate floating solar with windfarm development, you may be able to share the transmission infrastructure.” Plus, it will likely balance some of the output, working when there’s sun but no wind.
However, “you can’t just take current technology and dump it offshore”, said Gery: there’s a wider range of environments and stressors to take into account. “For example, we will eventually need to look at the kind of forces that come from choppy seas and how that’s different to higher, longer wave periods,” he added.
Marine developments will also likely demand a much more robust floating pontoon, and there’s also the impact of seawater: although the salt might present issues for the panels, the moorings will probably be fine, “as one of Seaflex’s oldest installations has been working in salt water since 1982”, he concluded.