New study debunks several myths about floating PV

New research from the Delft University of Technology (TU Delft) has helped to debunk a number of myths about floating PV technology, while providing a more realistic picture of how this emerging market segment is struggling to compete with ground-mounted solar.

In “Innovative floating bifacial photovoltaic solutions for inland water areas” – recently published in Progress in Photovoltaics – the Dutch group identified knowledge gaps and challenges that need to be addressed to bring floating tech closer to commercial maturity.

“If we design floaters in such a way that they work as a reliable virtual ground, and tackle technical challenges such as wind and wave forces, hail storms, bird dropping, mismatches, and PID, among others, they will reach to a lower Levelized Cost of Energy than ground-mounted PV,” researcher Hesan Ziar told pv magazine. “Here at TU Delft, together with our partners, we are working on the different solutions for technical challenges associated with floating PV.”

In the Netherlands, the levelized cost of energy (LCoE) for standard floating PV systems could be less than €0.07/kWh.

“A lower LCoE may also be reached in projects combining floating PV and hydropower because it brings advantages such as more flexibility for the power production and preventing the water behind the dams from evaporating,” Ziar explained. “What we foresee, is not only the combination of floating PV and hydropower for inland water areas but also the combination of floating PV and wind turbines for offshore areas, which could also lead to lower LCoE for sustainable electricity.”

However, there is still much that needs to be done, especially in terms of technological improvements.

Albedo effect

In one example, the scientists looked at the use of bifacial solar panels in floating PV arrays. The scientists analyzed the albedo of water at various levels above the water surface with PV installations at a testing pond. They found that the albedo value of the soil on the shore was higher, at 15.64%, while the albedo of water (with depths of 0.5 and 1 meters) was 7.71% and 8.11%, respectively.

“Very low albedo of water predicts a low contribution of the reflected light energy for bifacial PV installation and, therefore, suggests the necessity for using reflectors,” the academics said, adding that the average irradiance-weighted albedo value for inland water is around 6.5%.

Their findings contradict the common belief that bifacial floating PV arrays may benefit from strong sunlight reflected from the water.

“As with any novel technology, bifacial and floating PV is still held back by a lack or inaccessibility of long-term field data to demonstrate its real-world performance under various conditions,” the researchers said.

Water soaking

Another widely held belief in floating PV research is that partial contact with water could help to reduce the operating temperature of solar panels.

“For water-soaking applications, it is always important to find the sweet spot that keeps the temperature low but does not drastically reduce the impinged irradiance,” the scientists said.

However, applying this technique to bifacial panels by putting their bottom frame in contact with the water did not result in significant gains in terms of temperature reductions or power yield increases.

“The temperature at the bottom part of the module is considerably lower for the case where the module is placed in contact with water, but this effect is hardly extended further to the PV cells placed at the bottom,” the researchers said, adding that the reported total yearly gain was limited, at around 0.17%. “Considering the low energy gain and the higher chance for the potential induced degradation (PID) effect, this option is left outside the perspective of a durable floating bifacial PV system.”

Other issues

The researcher recommend the use of active bird control techniques in floating PV systems, as the presence of birds and their droppings can partially reduce irradiance on the reflectors and the panels.

“A few passive actions can be done to reduce the birds' presence effects such as increasing the distance of the reflectors from the water level and placing them slightly tilted in the retractable system and keeping the floating island tumbled during the night time,” the scientists said. “However, this observation suggests more active bird control techniques, such as laser‐based bird control for inland FPV concept.”

They also found that floating PV systems may have stronger bio-fouling impacts. They noted the accumulation of micro-organisms, plants, algae, and small animals on the surface of the installations. They also said arrays suffer more from temperature spatial variance than land-based PV systems.

In addition, they determined that there are no significant differences in relevant water-quality parameters and water temperatures when PV systems are deployed on water. This contradicts the common belief that solar panels might have a positive impact on both values.

“However, already established aquatic plants accumulate three times less biomass under the FPV systems, and periods of hypoxia are much more frequent ” the researchers added.

Moreover, the research shows that only bifacial plants with reflectors and limited-angle horizontal tracking can currently outperform ground-mounted monofacial PV facilities, by providing an additional yearly yield of 17.3% (up to 29% in a clear-sky month). However, such installations have never been tested in real projects and their high costs could offset the gains achieved in power production.

According to Ziar, floating monofacial PV arrays experience the same issues as floating bifacial PV systems.

“All the challenges, except for the challenges related to reflectors such as their additional cost and high chance of being bio-fouled are extended to floating monofacial PV systems as well,” he concluded. “Therefore, considering that bifacial and monofacial PV have similar weight and size and does not have a big difference in price per watt-peak, then by implementing good reflectors and keeping them clean, bifacial floating PV will have a higher chance in the future.”