**1. Introduction**

Increasing environmental concern about the harmful consequences of global warming and carbon emissions has recently created new demands for renewable and sustainable energy sources such as wind, solar, biomass and geothermal energy [1]. Among these renewable energy sources, wind has become the focus of attention in recent years [2–7]. With the developing technology, wind power plants, which are a renewable energy alternative, are rapidly progressing to become one of the main energy sources in the world. For example, when the installations of wind power plants in Turkey are analyzed over periods of years, it can be seen that the interest in wind power plants in Turkey has increased over time. While the power of wind turbines in Turkey was 9253 MW according to the data of May 2021, it has increased to 10,976 MW according to the data for December 2022 [8,9]. WindEurope predicts that the installed power of the European Wind Power Plant, which is currently 220 GW in total, will reach 318 GW in 2025. It is also predicted that Turkey will reach a total installed power of 14 GW in the same period, ranking sixth in Europe [10]. In the report of the International Renewable Energy Agency (IRENA), titled "*Global Energy Transformation: A Roadmap to 2050*", it is stated that the electricity produced from wind energy is expected to constitute 36% of total electricity production in 2050. Accordingly, the share of wind energy in renewable energy sources is estimated to be 42% [10].

**Citation:** Çiftci, C.; Erdo ˘gan, A.; Genç, M.S. Investigation of the Mechanical Behavior of a New Generation Wind Turbine Blade Technology. *Energies* **2023**, *16*, 1961. https://doi.org/10.3390/en16041961

Academic Editor: Paweł Lig ˛eza

Received: 28 January 2023 Revised: 10 February 2023 Accepted: 13 February 2023 Published: 16 February 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Wind turbines, which are still being installed around the world, typically have a lifespan of about 20 to 30 years. Although some components of wind power plants, such as towers and generators, that have reached the end of their lives can be recycled with some processes, it is very difficult to recycle thermoset-based composite materials used in the turbine blades [11]. Because of these difficulties, these turbine blades are usually buried on land, while only a few can be used in the construction industry. The fact that these large parts of a large number of wind turbines installed throughout the world in the last thirty years cannot be recycled and will turn into scrap also creates a handicap in terms of environmental and climate change. For example, it is predicted that there will be 43 million tons of wind turbine blade waste worldwide by 2050 [12]. According to this prediction, 40%, 25% and 16% of the world's total turbine blade waste will belong to China, Europe, and the United States, respectively. Considering that each kilowatt of wind energy needs about 10 kg of wind turbine blade material, it is expected that shortly, humanity will waste about 200,000 tons of blades [12]. It is also estimated that the amount of blade material that will need to be recycled annually between 2029 and 2033 may reach 400,000 tons [13]. Finally, the amount of blade waste is expected to increase to 800,000 tons per year after 2050 [13].

The commonly used intervention for the recycling of wind turbine blades consists of transporting the blades to landfills as waste, as shown in Figure 1. In addition, the production of various products, such as fibers, pellets, construction materials and panels from composite fiber wastes, contributes to the study of recycling. These forms of applications, also referred to as cyclical and zero-waste solutions, demonstrate attempts at reducing the carbon footprint of turbine blades [14].

**Figure 1.** Knotted carbon-based bars used in the production of the new generation blade.

Energy production of wind turbines is largely dependent on the aerodynamics of the blades, and energy output can be increased by improving the blade's aerodynamic performance [15–21]. Another way to increase the amount of energy is to increase the blade dimensions, though this depends on the blade strength [22–25]. Considering the amount of energy produced by wind power plants and the dimensions of the turbine blades, the blades of medium- and large-scale turbines is produced with a different manufacturing concept compared with the blades of small-scale turbines, since they are subjected to higher bending moments and shear forces [26–28]. In the manufacturing of the blades of these medium- and large-scale blades, each layer of the blades consists of glass and carbon fiber reinforced epoxy materials. In these layered composites, the matrix material is epoxy, while the reinforcing materials are glass and carbon. Although the layers in these composite structures are heterogeneous within themselves, considering that turbine blades are formed by the repetition of these composite layers, it can be stated that the blades are produced in a homogeneous structure [29–33]. Small-scale turbine blades are produced by embedding chopped glass or carbon reinforcements into the matrix material homogeneously using plastic injection production technology in order to make production practical and fast.

It is vital that the composite structures used in the production of turbine blades can provide the desired strength values when exposed to wind force. According to the literature, the mechanical properties of the blade structures are commonly obtained by flapwise and

edgewise tests. Considering that the usage of these test setups is expensive, the digital image correlation (DIC) technique offers a convenient and economical alternative, especially for a classical flapwise test. The digital image correlation (DIC) technique was invented in the early 1980s. It is still a common non-contact and non-destructive experimental measurement technique for measuring the displacement of structures under various loads. It can be seen in the literature that the DIC technique has been extensively applied to tests of structures where the sample size is small, and the experimental setup is well established [34]. For example, Winstroth et al. [35] successfully measured the displacements of turbine blades using the DIC technique, even in a large-scale wind turbine of 3.2 MW. Wu et al. [36] used a new and economical optical technique based on three-dimensional digital image correlation (3D-DIC) for the health monitoring of wind turbine blades. In their study, the measurements of a 5-kW wind turbine with a diameter of 4 m were obtained using 3D-DIC. First, they painted rotor blades with random black and white dot patterns and placed two digital cameras in front of the wind turbine to measure the rotor blade deformations. With this test setup, the displacement and strain values of the blades were obtained dynamically. Mastrodicasa et al. [37] also used the DIC system to measure the deformation and three-dimensional stresses on a 50-m blade subjected to periodic loading. They also improved the calibration of the DIC system considering the size (~4 m × 3 m) of the focal areas for the cameras. The results obtained in their study show that the DIC technique can provide reliable information about the changes in strains and deformations of the structures that are subjected to even fatigue loads. Khadka et al. [38] tried to develop a new non-contact technique using a drone to take dynamic measurements from the DIC system. In their experimental study, the integration of DIC and drone was used to obtain strain data on wind turbine blades at remote locations.

Composites show themselves in structures produced by the combination of two or more different materials. In this context, when we look at the history of the use of composite materials, it is known that they are used in almost different areas and many different products. Reinforced concrete beam or column structures used in the construction industry are also examples of composite structures. In these reinforced concrete structures, this concrete material needs to be reinforced due to the low performance of the concrete material under tensile load, although it is sufficiently resistant against the compressive load. In order to meet this need, the concrete is reinforced by using steel bars in its material. With this concept, massive weights are easily carried by reinforced concrete beams and columns in buildings. Reinforced concrete design specifications place a limitation on the distances between steel bars used in the concrete material to prevent delamination or splitting-type failures within the reinforced concrete beams or columns. Considering that turbine blades exhibit a kind of cantilever beam feature under wind load and also that the strength of layered composite structures should be increased despite the delamination failures, it is predicted in this study that the turbine blades can be produced in an analogous way to reinforced concrete beams.

A heterogeneous turbine blade, which in this study we call a new generation turbine blade, is produced in way that is inspired by the reinforced concrete beams that are frequently used in the construction industry and is unlike traditional turbine blades with a homogeneous structure. In this production, the first to be reported in the relevant literature, continuous carbon-based bars were used instead of reinforcing steel bars in reinforced concrete beams, and recycled low-density polyethylene was also used instead of the concrete material. The most important reason for producing the new generation blades in a heterogeneous structure inspired by reinforced concrete beams is to prevent delamination-type failures, which can often be seen in traditional turbine blades. In other words, it is predicted that delamination failures will be minimized in the new generation blades, whose reinforcing material is heterogeneously embedded in the matrix material, so that the total contact surface area between the materials is increased. Another novelty of this study is based on the originality of the production technique of the new generation blades, as the manufacturing technique used in the production of the new generation blade differs from the production methodology of the traditional turbine blades. Unlike traditional blades, almost no resin or epoxy material was used in the production of this new generation blade. This new generation blade was produced by bonding the matrix material (recycled polyethylene) to its reinforcement by exposing it to heat and physical melting it. Another advantage of using this technique is to supply a recyclable turbine blade since the matrix material of the blade can be easily stripped from the reinforcing material through heat application. In addition, in this study, we aimed to produce a new generation blade that is more mechanically durable than a conventional one. The modelling of a classical heterogeneous system will require information regarding several characteristics, obtained from coupon tests of the different materials in composite structures and also from interfacial bonding tests between these different materials. Since the heterogeneity of the new generation blades makes their modelling difficult, all the obtained mechanical data must be defined during modelling. Instead of obtaining all these data, this study facilitates the modelling of the blade structural system as a whole. While the strength and displacement of the wind turbine blades are obtained by using the DIC system, static loads were applied to the blades. While applying these loads, the deformations were measured with the DIC system and moment–curvature curves were calculated. In ADINA finite element method software, and by using the moment–curvature data, the behavior of the new generation and commercial blades was modelled as a whole based on pressure distributions obtained from CRADLE Computational Fluid Dynamics (CFD) software at different wind speeds. Thereby, there was no need to define the mechanical properties of materials in the finite element model of the blades. This is another novelty of our study.

In this study, first, the new generation blade was produced in the same profile and size as the purchased commercial blade, with the manufacturing and measurement methods explained in Section 2. Then, experimental studies were conducted on the strength of both new generation and commercial blades using the DIC technique. An analysis of the mechanics and flow are described in Section 2. Then, commercial and new generation blades are modelled and their behaviors under different wind speeds and angles of attack are compared in Section 3.

#### **2. Materials and Methods**

The new generation blade mentioned in this study differs from traditional turbine blades both in terms of production technique and the difference of materials used in production. In order to measure and evaluate how these differences contribute to the new generation turbine blades, a commercially available traditional turbine blade was purchased first. Then, the new generation blade was produced with the same size and profile as the commercial one. The details on the production of the new generation blade are given as follows.

#### *2.1. Manufacturing of the New Generation Blade*

In order to minimize delamination type fractures, which are frequently encountered in traditional turbines, new generation blades are inspired by the structure of composite reinforced concrete beams, which are widely used in the construction industry. In this production, continuous carbon-based bars are used instead of reinforcing steel bars in reinforced concrete beams, and recycled low-density polyethylene is also used instead of the concrete material. In addition, as is known, deformations are formed on the steel bars of the reinforced concrete structures in order to increase the interfacial bonding strength between the steel bars and concrete material. In other words, due to the usage of the deformed steel bars, we can ensure that the steel reinforcements do not slide into the concrete material. This important because the reliability of the design calculations of the reinforced concrete beams depends on the fact that the concrete material in the beams works as a whole with the adjacent steel reinforcement without slipping. With this context, in order to produce the new generation blade of this study so that it operates under a similar principle, sliding of the carbon reinforcement within the blade must be minimized.

For this reason, knotted structures were arbitrarily formed on each carbon-based bar to be used for the production of the new generation blade. The visuals of these knotted structures are given in Figure 1. Additionally, only this much information will be given in terms of the creation of these knotted structures, which were developed within the company established by one of the authors, since the know-how on this subject is evaluated to be within the scope of trade secrets. As a matter of fact, it is vital to minimize any reduction in the strength capacity of carbon rods during the creation of knotted structures, and the know-how in this regard belongs to the company.

Figure 2 is plotted to represent the locations of the knotted bars within the airfoil of the new generation blade. According to Figure 2, the new generation blade was produced in two separate parts using two different molds. In order to realize the production of these separate parts, two molds with different shapes were first obtained (see Figure 2). Both blade pieces were produced by heating in a furnace after the placement of the knotted carbon bars and recycled polyethylene material in the molds. These pieces were then glued together using a trace amount of epoxy material as shown in Figure 2. It can be emphasized that the amount of epoxy in the new generation blade is negligible when compared with the amount of resin or epoxy used in traditional turbine blades. Additionally, it is thought that this epoxy, which is used a negligible amount, does not have a negative effect on the recycling of new generation blades. Because, as is known from the recycling industry, the materials are never recycled in pure form. Moreover, the new generation blade in this study has already been produced using an impure recycled form of low-density polyethylene (F2–12).

**Figure 2.** A representative display for the new generation blade. (**a**) Airfoil after combining two separate parts, (**b**) representative cross-sectional area of the part coming out of the first mold, (**c**) representative cross-sectional area of the part coming out of the second mold.

#### *2.2. Digital Image Correlation Method*

In this study, a digital image correlation (DIC) system was used to obtain the strength and displacements of both new generation blade and purchased commercial blades under static load. The DIC system captures repetitive images and then calculates the displacements of samples by tracking the deformation of the white dots applied to the surface of the samples using a cross-correlation method [39,40]. The DIC system used in this study includes 2 × 1 megapixel (MP) high speed cameras, 2 spot lamps, Zeiss precision F-Mount 35 mm lens, and ISTRA 4D software with a calibration target. The maximum frame rate of the camera is 1000 frames per second with a resolution of 1280 × 800 pixels. In order to activate the system, first the calibration process is performed. Calibration was performed before the experiment using a square work area of 500 mm × 500 mm. In the calibration process, first, the calibration board was displayed with 2 compact cameras and 2 spot lamps, and the speckled structure was defined in the system by transferring it to the software.

Both of the blades were made ready by being painted as a white dot (pattern) for the application of the DIC system as in Figure 3. The purpose of this process is to obtain deformation and displacement measurements by referring to the white dots of the cameras in the DIC system.

(**a**)

**Figure 3.** Making the necessary preparations for the experimental analysis of the blades in the DICsystem,(**a**)paintingworkrequiredfortheDIC system,(**b**)bladesurfaceafterpainting.

(**b**)

According to Figure 4a, the images of both the purchased commercial blade and the manufactured new generation blade—before being subjected to any load—were taken by using the commercial software of the DIC system. These first images will be used as the reference image for the blades, and according to this reference image, the deformation values of the blades under static load can be calculated. The color changes in the images in Figure 4b indicate the strain values formed on the surface of the blades under loading.

(**a**)

> (**b**)

**Figure 4.** Application of the DIC method on a selected blade (**a**) reference image (**b**) strain distributions on the blades.

While the strength and displacement of the wind turbine blades are obtained by using the DIC system, the static loads applied on the blades are given in Figure 5. In order to compare the 2-kW small-scale ISTRA BREZEE brand commercial wind turbine blade with the new generation blade which has the same size and profile, the loads shown in Figure 5 are separately applied at the root, middle and tip regions of both blades. Information about

the locations of the applied loads is given in Figure 5. While applying these loads, the deformations were measured with the DIC system.

**Figure 5.** Representative images of the loads of (**a**) 9.8 N, (**b**) 14.7 N, (**c**) 19.6 N applied on the blades when using the DIC system.

#### *2.3. Finite Element Modeling for the Blades*

It is important for the energy sector that the new generation blades can be modeled as in the traditional blades. In addition, the heterogeneity of the new generation blades makes their modeling difficult. The modeling of a classical heterogeneous system will require information about several characteristics, which can be obtained from the coupon tests of the different materials in composite structures and also from interfacial bonding tests between these different materials. Then all these obtained mechanical data must be defined during modeling. Instead of obtaining all these data, this study deals with facilitating the modeling of the blade structural system as a whole. In this modeling, ADINA R.D. Inc. commercial engineering simulation software program was used. As mentioned in the tutorial examples of this program, it is possible to combine the mechanical properties of the entire structural system into just one batch of moment–bending curve data by considering the beam element structure as a whole in the finite element method. In other words, the moment–curvature curve shows how the output data of a simple beam structure changes according to the input data when bending, and it briefly illustrates the relationship between input and output. In this respect, by using the moment–curvature data, the behavior of beam structures with a variable cross-sectional area when bending can be modeled as a whole. For this modeling, it is sufficient to model the blades as a cantilever beam element without even defining the cross-sectional areas. The blades are modeled in two dimensions and the displacements in the flapwise testing direction could be calculated by the finite element method. These calculated displacement values will then be compared with the experimental values obtained from the DIC flapwise tests. By the way, to maintain control over the mesh size in the finite element modelling, mesh sizes were increased until consistent displacement output data were obtained. In addition, there is no need to define the mechanical properties of different materials in the finite element model of the blades. The only data needed here is to know the moment–curvature data of both commercial and new generation blades. Experimental data using the DIC system were used to learn these moment–curvature data. In light of these experimental data, the moment–curvature data of the root, middle and tip regions were obtained by using the strain values at the blade surfaces. In other words, the moment–curvature calculation of each region was roughly calculated by using the strain data of the root, middle and tip regions of the turbine blades obtained from the DIC experiments in the formula in Equation (1) [41]. Finally, the bending behavior of composite blades, whether homogeneous or heterogeneous, can be easily modeled using the learned moment–curvature curve.

$$\text{Curvature} = \frac{\text{Strain}}{(\text{blade thickness}) \times (0.5)} \tag{1}$$
