Dynamic responses of floating photovoltaic arrays under wave loads

Abstract

Floating photovoltaic (FPV) systems are a promising technology for harnessing the abundant solar energy in sheltered nearshore and large lake areas, however, few technologies have yet to make it to a commercial stage. In order to achieve commercialisation and ensure structural integrity, floating photovoltaic concepts need to undergo rigorous experimental testing and numerical analysis. This thesis undertakes an evaluation of floating solar applications in nearshore environments and lakes through a combination of experimental wave tank testing and numerical simulations. By studying the dynamic response of scaled and simplified FPV models when exposed to waves, the research offers valuable insights into the behaviour and challenges specific to the gable-slender concept, which contributes to the broader advancement of FPV technologies. The thesis first focuses on introducing FPV technology, the gable slender concept and existing literature aimed at modelling the responses of floating photovoltaic devices, identifying areas of opportunity for studying the responses of floating photovoltaic technologies. Theoretical formulations pertinent to investigating the responses of floating solar devices in wave conditions are then studied. In the third chapter, a novel experimental modelling technique is introduced through the design of two scaled and simplified experimental FPV models, which represent a small portion of an array-type device. Tank tests were then conducted in order to investigate the load and load effects, taking into account the unique wave-body interactions and multi-body hydrodynamic effects on the dynamic responses. Two numerical replications of the experimental models were then formulated in a fully non-linear time-domain modelling code in order to verify the reliability and accuracy and identify limitations of the modelling code to the application of floating solar technologies across a range of monochromatic sea states.

The primary contributions include:
The study sheds light on the diverse responses of cylindrical type FPV platforms to different monochromatic sea states. It revealed a range of behaviours from calm to turbulent motions, surge responses and fluid-structure interactions. The varying degrees of non linearity, in the responses and fluid-structure interactions, including effects such as overtopping and internal reflections, underscores the sensitivity and complexity of the dynamics exhibited by these cylindrical type platforms in large lakes or nearshore wave environments.

A notable negative heave offset was observed in the experimental analysis of both the single and twin cylinder models. A correlation between the magnitude of the offset and wave amplitude was identified, with the largest amplitude waves having produced the largest offset. This phenomenon is believed to be closely linked to overtopping, observed in most sea states where the offset occurred, and possibly due to insufficient reserve buoyancy to counteract the overtopping action. This could have implications for the reliability and safety of cylindrical type floating solar platforms, particularly in terms of panel vulnerability to water impact.

The thesis offers a detailed parametric analysis of the relationship between surge force extremes and wave parameters. Wave steepness was identified as one of the best linear correlations for both maximum and minimum surge forces across both of the single and twin cylinder models.

The research contributes novel findings on heave responses amplitude operators of the single and twin cylinder models, showing distinct patterns for both models across various sea states. It particularly highlights how shorter wavelengths affect the single cylinder model’s heave RAO, a novel contribution to the understanding of the dynamics of these types of structures.

The study is the first to compare the responses of single and twin cylinder models in irregular sea states. A spectral analysis of the heave, pitch and surge force responses revealed a complex dynamic coupling between heave and pitch motions. This insight into the interdependence of motion modes is a contribution to the field.

A contribution of this thesis is the evaluation of a numerical model using Airy and 5th Order Stokes waves against experimental data. The findings on the limitations of these models in predicting heave and pitch motions and surge force amplitudes under certain conditions showed that as the wave height increased, the accuracy in predicting the surge force amplitude of the single cylinder model decreased, with both the Airy and Stokes numerical model consistently underestimating this force. On average, the Airy model underpredicted the surge force amplitude by 22.76 % and the Stokes model by 17.25 % while, for the twin cylinder model, larger discrepancies were evident. On average the Airy numerical model underpredicted the surge force amplitude by 39.54 % and the Stokes model underpredicted the surge force amplitude by 36.19 % across all sea states.

Date of Award	Jul 2024
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	The Bryden Centre
Supervisor	Madjid Karimirad (Supervisor), Trevor Whittaker (Supervisor) & John Doran (Supervisor)