The Advantages and Challenges of Carbon Fiber Reinforced Polymers for Tidal Current Turbine Systems - An Overview

A significant step in reducing the effects of greenhouse gases is obtaining electric energy from renewable sources. Electricity from tidal currents using underwater turbines is one of the most promising and well-liked technologies. The turbine systems are the key element in the tidal current energy. They are built using hydrodynamic principles to extract the most power possible from tidal ocean currents and are designed to last for extended periods in a maritime environment. The performance of tidal turbines is also significantly influenced by their materials, i.e., carbon fiber reinforced polymers (CFRP) used in them. This paper also reviews the CFRP materials used in tidal current turbine systems. Besides, an analysis of their advantages and challenges regarding CFRP materials that can impact tidal current turbine efficiency is further explored.


Introduction
Tidal current energy is one of the renewable energies that uses a turbine system to capture the tidal currents' kinetic energy and convert it into electricity [1][2][3][4][5][6].It is the most predictable [7][8][9], has reached the early stages of technological maturity [8,10], and is a promising source of energy [11][12][13][14][15].Even without abandoning research on conventional energy sources [16][17][18], research on ocean renewable energy is important.Tidal current energy is produced using a similar process to how wind energy is produced, except it is entirely immersed underwater.Tidal current energy systems widely used are horizontal and vertical axis turbines [19], as seen in Figure 1.[20], Kobold (right) [21] 1298 (2024) 012029 IOP Publishing doi:10.1088/1755-1315/1298/1/012029 2 Tidal current turbines must be installed in areas with challenging weather and sea environment (strong currents, turbulence, waves, and storms) [22], and it has complex stresses such as corrosion, mechanical, and biological stress [23][24][25].Moreover, this ocean current energy device is operated for a long time with minimal maintenance intensity and is submerged in turbulent seawater [7,26].On the other hand, the tidal turbine's blade is its most important component to generate tidal energy.Hence, tidal turbine blades must be made of a suitable material to lower the chances of failure.Due to their superior specific strength (strength/weight ratio) and specific strength/stiffness ratio as compared to glass fibers, carbon fiber reinforced polymers (CFRP) have become the favored material for making blades that are thinner, stiffer, and lighter [22,27,28].Therefore, this paper's goal is to study CFRP and its application to tidal current energy.The advantages and challenges of CFRP utilization are then emphasized to further the tidal current's development.

Carbon Fiber Reinforced Polymers (CFRP)
A composite material that is not metallic and is made of carbon fiber (the reinforcement material) and polymer resin (the fiber-holding matrix) is known as CFRP [29].In a CFRP composite, the carbon fiber contributes to the composite's strength and stiffness, while the matrix typically controls the chemicalheat resistance and load balancing [30].Figure 2 illustrates the standard CFRP structure.
As the first component of CFRP, carbon fiber (CF) is one of the most widely utilized reinforcement materials due to its outstanding physical qualities, like high stiffness and strength, high heat resistance, high conductivity, self-lubrication, and anti-corrosion [30].Carbon fiber is generated from substance precursors for polymers like cellulose, polyvinyl chloride, and polyacrylonitrile (PAN).These precursors are treated with heating and tensioning processes to create carbon fibers.The diameter of carbon fibers, which ranges from 5 to 10 μm, is extremely small [29], as shown in Figure 2.
Polymer resins have two primary types: thermoplastic resin and thermosetting resin [31].Thermoplastic resin is a polymer resin connected by intermolecular interactions.This creates a molecular structure that is either in the direction of a straight line or have branches, which is only capable of providing a very slight limitation on how far molecules can move.As a result, after curing, the thermoplastic resin is remeltable and tractable when heat and pressure are applied.Thermosetting resin, on the other hand, is a polymer resin linked with chemical connections to create a strongly cross-linked molecule.The mobility of molecular chains can be severely constrained by this cross-linked structure, which renders thermosets unmeltable and intractable when heated after curing [29].For typical thermosets, for instance, an epoxy matrix is preferred because it offers strong adhesion [31].The mechanical features of steel, CFRP, carbon fiber, and polymer resin are shown in Table 1.Between carbon fiber and S355 steel, it shows that despite having far lower densities and weights than steel, carbon fiber has greater tensile strengths [29].Also, polymer resins have slightly lower densities than carbon fibers, while carbon fibers have far higher strengths than polymer resins.Carbon fibers, however, cannot be used as stand-alone engineering materials because of their filamentary character.Therefore, when carbon fiber is combined with a polymer resin in a synergistic way, it produces CFRP, a typical orthotropic material with outstanding mechanical capabilities.There are three ways to make composite materials that can be distributed into four steps (layup, resin, joining, and finishing) [33]: • Vacuum-assisted resin transfer molding (VARTM) is a popular technique for making composite blades.The blade's top and bottom halves, as well as the two shear webs, are manufactured in separate pieces and bonded together to create the blade's final shape.Mold is used to create the parts, which are then filled with fiber materials and reinforcing root sections before being sealed with a vacuum bag.A vacuum is then employed to pull the resin into the fiber by pumping the resin between the bag and the fiber.Curing took place at 80°C for 8 hours.• Monocoque is a single-piece blade manufacturing method where fibers are placed out in a mold, and then the polymer resin is sucked into the fibers using vacuum.Curing took place at 80°C for 8 hours.Shear webs are not required since this single-piece process enables the production of blades that are stiffer than those produced using the VARTM process.• The heated mold method utilizes epoxy powders or thermally induced polyester, substituting the resin solution utilized in VARTM processes.The powder and fiber are heated in a 200°C ceramic mold.As a result, the powder melts, and composites are created.The blade's upper and lower parts, as well as a pair of shear webs is partly dried before being combined and cured in one piece within 70 minutes at 170°C.Some tidal current turbines comprise CFRP, i.e., Seaflow and Seagen (Figure 3).The two-blade rotor Seaflow has an 11-meter diameter.It is constructed of a carbon fiber-reinforced spar joined to fiberglass ribs and then covered in a pre-impregnated carbon fiber/epoxy resin matrix.The prepreg resin dried at a low temperature of 75°C in an ordinary industrial-grade oven [34,35].Figure 3. Seaflow (left), a 300-kW trial installation from Marine Current Turbines, has been running off the coast of Lynmouth, UK [35], and SeaGen (right), a 1200-kW tidal current turbine system from Simec Atlantis Energy, located in Northern Ireland [20] The SeaGen (Marine Current Turbines Ltd.) rotor blades have a 16-meter diameter.It was made up of a hollow intermediate-modulus unidirectional carbon fiber mixture box spar that serves as the primary load-balancing component, coupled with carbon ribs and a glass fiber composite envelope that is joined to the carbon fiber ribs.For both glass and carbon components, prepreg was employed and cured at 80°C [34,36,37].

Advantages
In comparison to traditional materials, CFRP is viewed as an appealing option due to its outstanding strength to weight ratio and high strength to stiffness ratio, flexibility, greater corrosion resistance, prolonged service life, and their anisotropy may be altered to allow 3-D blade deformation modification [28,[30][31][32]38,39].Structures with complex curves can also be produced using CFRP with ease and simplicity to maintain [22,36].

High strength and lightweight
CFRP material usage has significantly grown in recent years because of its strong specific strength and light weight.Table 2 displays the physical and mechanical characteristics of steel and composite.Comparing CFRP to steel and other composite materials, it is found to be lighter while yet having more strength.This causes a significant weight reduction and span extension for prestressed parts.However, composites usually have a lower Young's modulus than steel with the exception of CFRPs having a high elastic modulus [40,41].Also, compared to GFRP, CFRP provides superior performance while being 13% lighter (for the blade) [39].

Corrosion and fatigue resistances
CFRP has non-metallic and non-corrosive qualities that make it less susceptible to chloride-induced corrosion than steel, which can greatly increase the structure's corrosion resistance [41].Also, CFRP provides higher resistance to corrosion and fatigue, and it is an important design consideration for tidal turbines.Non-corrosive CFRPs are crucial in harsh corrosive environments since chloride ions and water molecules are corrosion-promoting elements [43].Also, carbon fibers have far better seawater corrosion resistance than glass fiber [27].Carbon fibers have the advantage over glass fibers, which lose tensile strength (by around 8% weekly) when subjected to 2% sodium chloride solutions.Glass fibers can also be corroded due to microbial attack when exposed for an extended period of time to natural polymers released by bacteria into their environment [43].Light weight, great strength, strong shock absorption capabilities, excellent durability, and favorable fatigue characteristics are the key benefits of a CFRP. Figure 5 compares the fatigue behavior of composites and other materials, and it shows CFRP has the highest fatigue and static strength.Additionally, the function of moisture content affects the fatigue strength of epoxy matrix CFRP.Moisture content can also influence how CFRP might fail in various of expected failure types, as shown in Figure 5.

Challenges
The challenges to tidal current turbine development are the hostile sea environment with its corrosive saline water, fouling growth, and other environmental concerns.These aspects must be addressed when developing tidal current turbine systems since they will impact the longevity and efficiency of tidal turbine blades.
In addition, CFRP can provide high performance, although it is 7/8 times more expensive than GFRP.However, the advantages of CFRP in terms of weight reduction, mechanical advantages, and durability would need to balance the higher cost [39].The inability to recycle CFRPs is another issue with their use.The composite CFRP material was created by combining carbon fibers with thermoset epoxy resin, resulting in non-recyclable materials.Only incineration can be used to eliminate composite materials containing these resins [33,39].

Biofouling
The development of marine biofouling on the tidal current turbine's structure might be a major challenge.Organic molecules, algae, anemones, bacteria, barnacles, bivalve molluscs, bryozoans, diatoms, fungi, hydroides, kelps, tubeworms-all examples of marine biofouling-can accumulate on maritime objects submerged in water [22,46,47].This marine biofouling can cause a considerable problem for tidal turbine systems [22,48] since it can change the hydrodynamic design, increase the structural weight, increase inertia and drag load, enlarge the surface roughness, reduce the overall efficiency and power production, and contribute to structural failures [20,22,49,50].Figure 6 depicts the marine biofouling on a tidal current turbine system.Farkas et al. [53] have researched how biofilm fouling affects the performance of marine current turbines using a computational fluid dynamics (CFD) method.Full-scale CFD simulations are run for eight tip speed ratios (TSRs) under six marine biofouling situations by modifying their surface coverage percentages and biofouling heights.The results show that biofilm has a considerable negative effect on the tidal current turbines efficiency.For the best TSR, the power coefficient drop is about -10.7% in the R1 marine biofouling scenario (with 500 μm biofilm height, 50% biofilm surface coverage, and a 195 μm roughness length scale).
Marine structures must have an antifouling coating to prevent the negative effects of marine biofouling growth.The antifouling paint on the market has a performance life of three to five years.On the other hand, tidal current turbines have a lifespan of 20 to 30 years.Tidal current turbine performance must be maintained by manual cleaning and paint reapplication once the coating lifespan has passed.Consequently, it is necessary to develop a more robust antifouling coating to reduce the frequency and expense of maintenance [22].

Seawater ageing and fatigue
The environment that tidal turbines must function in is quite hostile.Thus, it is critical to consider how seawater aging affects composites [36].Davies et al. [34] examined how unidirectional (UD) carbon/epoxy from prepreg was affected by natural saltwater ageing.The material gained weight ranging from 1% to 5% after 400 days of immersion at 20°C and 60°C (Figure 7).They exhibited interlaminar shear failures between the blades and the hub, which significantly decreased the fatigue life.The defects (such as voids and fractures) brought on by high-stress concentrations are also directly responsible for the fatigue strength of CFRPs [32].Additionally, Tual [28] used accelerated seawater aging studies to examine the behavior of three types of carbon epoxy composites in tidal turbines over a long period of time.The results showed that the kinetics of water diffusion were primarily unaffected by thickness and fiber orientation.Still, the water absorption kinetics and the water saturation condition were significantly influenced by the manufacturing method and the matrix resin type.The failure strength was reduced by 20% to 40%, although the elastic modulus and toughness were unaffected.The results of the interlaminar shear stress tests showed that seawater ageing impacted on the interfacial adhesion under shear loading in all three carbon/epoxy composites.
Li et al. [54] also investigated the impact of seawater ageing on the mechanical and physical properties of CFRP.The tensile strength of the CFRP samples decreased exponentially, and the moisture absorption content increased when the ageing process was prolonged.Additionally, the deterioration worsened with increasing temperature.As the ageing duration and ambient temperature increased, the interlaminar bindings were degraded.Enhancing the fiber/resin matrix interface bonds and interlaminar bonding may be a valuable strategy for increasing CFRP's resistance to deterioration in maritime environments.

Hydrodynamic loads
The hydrodynamic loads depend on inflow speed.As inflow speeds grow, the bending moments increase [55].In addition, high seawater density provides extra difficulties, such as excessive turbine strain caused by turbulence.The load on the blades can rise noticeably with even a little increase in flow velocity [22], which might cause failure.There are two primary reasons why rotor blades fail: 1) inadequate design strength owing to underestimating or unknown loads, and 2) manufacturing errors produce a structure with significantly lower strength than intended.The list of blade fractures or failures that were recorded for various prototypes is shown below [22,55,56]

Discussions
The benefits of using CFRP materials were covered in the preceding section.Comparing the CFRP to other composites, it performs better and is lighter.The advantages can be useful to researchers and related stakeholders who want to develop more robust tidal current turbine blades.However, it has a higher cost.Because of this, adopting a combination or hybrid composite, with carbon mainly employed to boost stiffness, constitutes a compromise between these competitive objectives [39].An efficient antifouling coating may stop marine biofouling problems.But issues like seawater aging and fatigue, and hydrodynamic loads that cause blade failure might be overcome with better materials.Composites can be expected to be joined using a range of methods, including adhesives and other materials.Reusable and natural-based composites can be options to unrecyclable composites, and they could be able to reduce waste through recycling and reusing [33].However, it is vital to comprehend how the environment effects on these material qualities relating to operation and maintenance, reliability, and safety issues [57].The impact of seawater and biofouling on these materials must therefore be thoroughly assessed in further research and work.

Conclusions
Tidal current turbine blades must be made of a suitable material to lower the failure chances of generating renewable energy.CFRP material has been preferred for making tidal current turbine blades that are thinner, stiffer, and lighter.The CFRP materials used in tidal current turbine systems are reviewed.CFRP offers advantages such as superior strength, weight reduction, mechanical advantages, durability, corrosion resistance, and fatigue resistance.The primary challenges that must be answered and solved in CFRP utilization are marine biofouling attached to the structures, seawater ageing and fatigue that will reduce the CFRP's physical and mechanical properties, and unpredictable hydrodynamic loads and turbulence.Consequently, further work is still needed to address the challenges to support the tidal current turbine developments.

Figure 2 .
Figure 2. The CFRP's structure (left) and the comparison human hair to carbon fiber (right)[29]

Table 2 .
Physical and mechanical characteristics of steel and composite[41] Material Density ρ (g/cm 3 ) Tensile strength   (MPa) Young's Modulus (has a lower mass compared to GFRP.Jaksic et al.[42] investigated the feasibility of using CFRP instead of GFRP in the design of tidal turbine blades to make the blades lighter and less expensive.The findings demonstrate that as blade length increases, blade mass also increases.When comparing the same lengths of CFRP and GFRP blades, Figure4also indicates a reduction in the blade mass distribution along the blade for CFRP blades.This difference reaches up to 24 kg/m at the blade's widest point, while the mass of the blade overall is reduced by 14.5%.

Figure 5 .
Figure 5.Comparison of the fatigue behavior of composites (left)[44] and failure diagram of carbon fiber-epoxy with regard to water content (right)[45]

Figure 7 .
Figure 7. Left: weight increase, and right: seawater ageing effects on wet fatigue behavior, 2 Hz, R=0.1, parallel specimens, at 60°C : • On December 11, 2006, Verdant Power's two tidal turbines that were deployed in the East River in New York City experienced blade failure (Figure 8).• On June 11, 2010, blade failure occurred in the Open Hydro turbine that was installed in Canada's Bay of Fundy when there was a very high inflow speed of around 5.15 m/s, or it exceeded the designed operating inlet speed by over 2.5 times.• In August 2010, a manufacturing flaw led to the Atlantis AR1000 turbine's blade fracture, which happened soon after it was installed and linked to the grid of the European Marine Energy Centre.

Figure 8 .
Figure 8. Blade of Verdant Power's tidal turbines that failed.