Support structure for offshore solar

Abstract

This thesis presents the potential of solar energy to answer the increasing demand for sustainable energy. Multiple floating solar parks are installed on inland water bodies. This trend is a result of the lack of surface for PV systems and a result of legislation on national and international governmental levels, for the reduction of greenhouse gasses. This inland technology is currently well established. Still, in densely populated areas such as the Netherlands supplementary surface could be beneficial to unlock the full potential of solar energy. Ocean and sea surfaces are a possible, but challenging solution to this problem. These offshore water surfaces could be used for electricity or synthetic fuel production. Offshore locations are attractive because of the abundant availability of surface. Still, an OFPV-system needs to overcome multiple technical and non-technical challenges and requirements. This challenge demands technical solution within the constraints of a suitable business case. Furthermore, the impact on people and ecology also needs to stay within ethical borders. An OFPV can be subdivided into three physical subsystems: an energy converting system, a position monitoring or mooring system and a floating support structure. Multiple concepts were proposed in literature or by the industry. All proposed concepts are variations of support systems that provide a surface for conventional PV-technology. It also was concluded that none of the proposed concepts is fully developed. Currently, only one system is in the testing phase. The current state of the technology, and the high potential of an OFPV are the drivers behind the search for a new concept design. In this thesis, the requirements for an OFPV support structure were formulated, and a brainstorm resulted in the buoy and beam concept for further research. The buoy and beam structure is a system in which submerged beams form a triangle grid are held together by floating buoys. The buoys support triangle platforms, by carrying the corners of the platforms. All connections between the buoys, beams and platforms are fully hinged. Because of the hinges, the structure can move with the waves and therefore mitigates the loads it experiences. In the second part of this thesis, the feasibility of the buoy and beam structure was assessed. It was concluded that the heave response is the most critical response to the feasibility of the concept. When the platform would heave too much, the waves will slam into the platforms and the solar panels, leading to damage and excessive loads on the structure. Consequently, the heave response and the relative wave height seen from the moving triangle platform are researches and modelled. The structure was identified as a hydrodynamic transparent structure. Consequently, the relevant hydrodynamic loads are obtained with the Morison equation and Airy wave theory. All damping terms are linearised. A differential equation incorporated the constrains for both, the construction and the hydromechanic loads. The mechanical equations can be linearised by assuming small angular displacements of the beams. This differential equation is implemented in a calculation tool in MATLAB. The code is verified over multiple steps. The tool made it possible to obtain the response based on the topology and the main dimensions of the buoys and the beams. With this calculation tool relations between the dimensions of the buoys and beams on one side and the frequency response, on the other hand, are identified by simulating different variations of a simplified starting design. The following conclusions were made on the eigenfrequencies. Firstly, it was noted that the eigenfrequencies of the system are in proximity to each other. Secondly, it was shown that the first and second eigenfrequency could be calculated in a simplified manner. Thirdly, it was seen that the buoy diameter to mass ratio has a positive linear correlation with respect to the eigenfrequency, i.e. the stiffness to mass ratio of the system severely influences the eigenfrequency. Furthermore, it was seen that an increasing beam length has a slight linear correlation with the eigenfrequency. Also, it was concluded that a large hexagon system (for example, nineteen buoys) could be beneficial because a hexagon structure creates a large surface with respect to the number of buoys. Moreover, in reality, the response should be lower than the model prediction, as a result of the high number of beams that cause damping. Additionally, a hexagon system should have a minimal change in response for different incoming wave angle. To obtain a feasible system, the response peaks of the structure should not overlap with the wave spectrum. Based on the additional simulation, it was concluded that it should be possible to design a system with a low eigenfrequency resulting in a system that will only slightly move with the waves. Additional relations between the dimensions of the buoys and beams and the response peaks were found. The requirements to obtain a low eigenfrequency and a modest response match, which positively influences the feasibility. Some further steps on the research of the heave response and the relative wave height are needed. The calculation tool can still be improved. A preciser estimation of the response peaks is of significant importance. Some further iterations must be executed to determine the full potential and feasibility of the concept design. Furthermore, in a scientific point of view, it will be highly interesting to validate the model on a model scale. In the development of the buoy and beam structure, additional critical responses need to be researched. The forces in the hinges and the response in horizontal direction need to be known for further assessment of the feasibility. Next to the technical aspects, the human, ecological and economic implications of the buoy and beam structure need to be researched.