Pontoon-type structure for offshore floating photovoltaics

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A group of researchers at the Shanghai Jiao Tong University in China has developed a new floating structure for offshore floating PV that reportedly offers a series of advantages compared to conventional semi-submersible structures.

“We proposed a new concept for the offshore floating photovoltaic structure which is a pontoon-type floating structure designed with high freeboard to avoid wave slamming or overtopping onto the deck and the solar panels,” the research's lead author, Xianta Zhang, told pv magazine. “By choosing a suitable size for each unit, it has the feature of following the waves. Besides, we chose the rope-type multi-module connection which may be more reliable compared with the traditional hinged connection, which suffers from drawbacks such as poor stability and negative air gap.”

The research team described the floating structure as a pontoon-truss platform composed of four pontoons and a steel truss connected by soft ropes. It also features an angle range of 0 degrees to 14 degrees, which reportedly offers “exceptional” stability. “The four pontoons are floating on the free surface and a square zone is enclosed,” the scientists added, noting that this design is intended to increase installation capacity while reducing mooring costs.

The system also includes steel frames and deck beams. “The steel truss is fabricated by joining horizontal and vertical members with the pontoons, yielding a design that is both lightweight and effective in reducing wave run-up,” the group said. “The deck beams are then mounted on the topside, creating space for PV panel installation.”

The performance of the system was tested through a series of experiments and compared to that of a semi-submersible foundation containing four immersed horizontal pontons and four semi-submerged vertical columns.

This analysis showed that the semi-submersible system has a “marginal” advantage in the maximum restoring moment, which defines the rotational force that acts in the opposite direction of a rotating body, but it can achieve this only at heel angles over 20 degrees. By contrast, the pontoon-truss system was found to provide optimal stability within 10 degrees.

“In other words, this innovative platform exhibits remarkable stability at smaller inclination angles,” the academics explained. “Furthermore, the wind-heeling moment of the semi-submersible type is notably larger than that of this novel floater, despite their equal wind pressure areas, due to the semi-type’s relatively larger arm of wind force to the gravity center.”

The system was also found to have a “superior ability” to resist capsizing and to respond rapidly to wave run-up. “Both experimental and numerical results identify that this pontoon-truss design is feasible in sea environment and keeps away from green water and negative airgap, however, this conclusion is obtained from scaled basin test and ideal simulation,” the researchers stated.

The proposed floating structure was described in the study “Conceptual design and model test of a pontoon-truss type offshore floating photovoltaic system with soft connection,” which was recently published in Ocean Engineering. “It should be noted that the present study provides a new floating photovoltaic scheme but it is limited to the early design stage,” the academics concluded. “To promote this scheme, there are still some challenging and interesting points for future exploration.”

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