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J. Mar. Sci. Eng. 2020, 8, 26 15 of 28 the used materials must preserve high levels of strength and stiffness and withstand the corrosive seawater environment, and require few or no maintenance, considering the inaccessibility of deep seawater area. For these reasons, fiber-reinforced polymeric composite materials, especially GFRP and CFRP, represent an attractive choice for the realization of tidal turbine blades [39]. GFRPs are generally preferred by designers due to the tradeoff between structural properties and low-cost. CFRP offers high performance and lower weight (by about 13% for the blade) if compared to GFRP [39] but at a 7/8 times higher cost. Due to this reason, the use of carbon fiber composite is limited to the main parts of the blades [39–41]. Differently from wind turbines, horizontal axis tidal turbines are affected by issues such as cavitation in blade tips and higher blade bending moments. In particular, GFRP blades during its lifetime (approximately 20 years) is subjected to an average speed of tides that varies between 2.4 and 4 m/s, resulting in early damage due to the strong bending moments. These extreme working conditions impose an upper limit on the dimension of the blades around 12 m. Conversely, CFRP provides a significant improvement in the maximum bearable stress, being the loads well below the traction (axial) and shear failure stresses. It is reasonable to conclude that GFRP is not a suitable material for the main structural components of large submerged turbine blades [42,43]. As above mentioned, submerged turbines are difficult to access for monitoring and maintenance, therefore the blades are designed in order to withstand the aggressive seawater environment for a long time. Besides the common issue related to fatigue loading, galvanic corrosion phenomena, as well as fouling and damages caused by the presence of marine organisms, need careful consideration. Previous studies discussed the effect of the exposure to the marine environment on different materials, commonly used in high-demanding applications (automotive, aeronautic, etc.), namely [40,44,45]: • glass fiber in an epoxy matrix, • carbon fiber in an epoxy matrix, • aeronautical aluminum alloy, • 314 and 316 stainless steel. • common steel, After one year of exposure, composite blades showed a very low level of fouling on the surface, while the metal components resulted affected by severe corrosion, except for ones manufactured in stainless steel [40]. Wind energy production does not generate any solid, liquid residues or gaseous emissions, nor involves the depletion of any form of fuel. A few concerns are related to the noise and visual disturbances, or eventual impact on flora and wildlife. These factors represent an obstacle to installation in proximity of inhabited centers, but they are not critical in several locations. Obviously, harvested power is hardly predictable and not constant, as it is dependent on wind speed. Offshore wind energy plants are exposed to higher and generally constant wind speeds, with the possibility of extending the plant over large areas without incurring in restrictions related to the environmental impact. However, the design of foundations and support structures, similar to the ones used in offshore oil and gas plants, and of turbines and blades exposed to the corrosive marine environment, has to meet additional constraints with respect to the in-shore wind farm. Therefore, offshore wind energy parks present relatively higher costs in terms of materials, installation, maintenance, and connection to the grid. Recent developments in new technologies, advanced material, and manufacturing processes are making the realization of offshore wind plants more affordable and convenient. Some studies pointed out that the cost of energy required for installation, the useful life and the disposal of an offshore wind plant is typically recovered in 6–12 operative months [41,42]. Considering a standard lifetime of approximately 20 years, the ratio between produced and consumed energy (efficiency factor) oscillates between 20 and 40. The study conducted by Vestas Wind Systems [42] revealed that the energy used in the production, installation, and disposal of a modern 3 MW (MegaWatt) plant should approachPDF Image | Marine Application of Fiber Reinforced Composites
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