Exploration of bioinspired small wind turbine blade manufacturing alternatives: Defining materials and processes

Definition of the case study

The procedure outlined above starts with creating a particular case study that concentrates on small HAWT blades. These turbines are classified according to their swept diameter, in accordance with the IEC 61400-1 standard Commission (2013). Small turbines are defined as having a swept diameter of less than 16 m, medium turbines range from 16 to 36 m, and large turbines have a swept diameter of 36 m or more. The objective is to analyze and explore alternative materials for turbine blades, covering all design variables to ensure the best performance, cost effectiveness, strength, durability, and system integration.

The location of HAWT implementation is directly related to the exploration of alternative material-process combinations. Environmental and meteorological conditions are crucial in determining the appropriate mechanical, aerodynamic, and dynamic blade design.

It is crucial to introduce the case study with a clear vision of the project’s current status and model characteristics, including turbine size, general dimensions, desired blade features, specific material and process requirements, and a well-defined objective. This ensures that the design conditions are met and efficient results are achieved, taking into account variables such as structural strength, durability, and performance under various environmental conditions.

General search for materials and processes

This section outlines essential factors for designing blades for small HAWTs. The focus is on blade length optimization to enhance energy capture, the utilization of effective airfoils to decrease wind resistance, guaranteeing structural integrity against different loads (e.g. mechanical, aerodynamic, seismic), utilizing high-strength composite materials, and achieving uniform stress distribution through an optimal design. Ensuring durability and longevity is crucial for safe operation in harsh environmental conditions, as is reducing structural loads through weight and density factors during installation. Furthermore, consistent energy production relies on maintaining efficiency under varying wind conditions.

This involves first defining the boundary conditions for blade design and then gathering reliable information on composite materials, manufacturing techniques, and molding processes from credible sources. Material evaluation involves evaluating suitability based on factors such as strength, stiffness, density, availability, and cost. Similarly, the evaluation of manufacturing processes takes into account affordability, expertise, and implementation.

In addition, this section examines distinct alternatives inspired by related applications, analyzes their feasibility and limitations, and suggests methods to improve performance. Comparative analysis, supported by software or material databases with validated properties, assists in the identification of optimal material-process pairings, providing data-driven insights for decision-making.

Material selection

To ensure optimal system performance, it is crucial to define specific design requirements when selecting materials. These requirements include load capacity, boundary conditions, external loads, regulations, and standards (Bošnjaković et al., 2022). The effectiveness of the system is dependent on a clear understanding of both physical and mechanical properties. During the evaluation of materials, it is crucial to consider key properties such as fatigue strength, stiffness, flexural strength, corrosion resistance, and lightweight characteristics. The design variables should be analyzed and the data should be assessed to evaluate the performance and suitability of the materials for the intended applications. The aim of this evaluation process is to estimate expected outcomes and identify the most appropriate materials for prototyping tests. After choosing the appropriate materials, the phase of design optimization should focus on minimizing weight and maximizing efficiency. Iterative improvements in product design strive to achieve an equilibrium that meets the performance criteria while permitting consistent reproduction. Thorough quality control measures are essential to ensure uniformity and sustained performance throughout the production of the final product.

Process selection

When selecting a manufacturing process, it is crucial to carefully consider the material to be used, as it directly affects the process. This step should define all manufacturing requirements, such as physical aspects for dimensioning and specified tolerances, to ensure proper functioning during assembly and operation. Avoiding errors and inconsistencies is important. Based on previous research, additive manufacturing, manual processes, and machining techniques can be evaluated to achieve high accuracy on intricate surfaces and to secure uniform reproducibility with minimal variations. After material selection, it is essential to review the compatibility between the chosen materials and manufacturing processes, since the molecular composition of the material can be altered by different processes, affecting the material properties. To analyze technical and economic factors, it is necessary to comparatively evaluate the viability and feasibility of implementing each manufacturing process. Proceeding with the manufacturing of components may be based on technological limitations, budget constraints, affordability, expertise, and other variables acting as boundary conditions.

Manufacturing and experimental testing of the chosen material-process pairs

After selecting material and process according to the defined selection criteria in the methodology, establish test objectives to demonstrate proper blade functionality based on a variable of interest. These objectives will serve as validation metrics, providing blade efficiency results on wind incidence in terms of aerodynamic performance, resistance, stiffness, vibration, and aerodynamic loads, among other factors. Procedures and protocols are established to carry out the tests and carefully select the required equipment and measurement tools. The number of prototypes is determined on the basis of the chosen manufacturing methods and materials. Testing is accomplished by utilizing either a tailored test bench or physical prototypes specifically designed for appropriate installation of manufactured prototypes. This facilitates the collection of relevant data for research. Analyzing the results enables validation of the initially proposed objectives and determines whether the product achieves the highest level of satisfaction with respect to the recorded variable.

Final analysis and conclusions

The ultimate analysis gathers the data acquired from the experimental tests and forms the basis for determining replicability, as well as the improvement of the optimal materials and procedures for the turbine blades. On interpretation of the results, the production process is evaluated to identify any patterns, variations, or trends that may affect the final product and could potentially lead to its failure. A product with consistently replicable results can be achieved through careful analysis, interpretation, and design improvement of the data obtained. The objective is to ensure that each iteration yields the desired variable readings. This iterative process will help develop a highly efficient and reliable product for future use.

Definition of the case study

A preliminary study on varying parameters showed that specific changes in blade geometry and composite laminate layout can result in further improvements in the performance of HAWT turbine blades. Grujicic et al. (2010) suggested that the geometry of the blade plays a significant role in determining the performance of a HAWT turbine. This concept is demonstrated experimentally by Bernal-del Río and Osorio-Gómez (2020), obtaining that the WLE blade outperforms conventional blades in terms of power generation. The WLE blade incorporates a bioinspired design featuring fins reminiscent of those of whales, which enhance aerodynamic efficiency and leverage the wind more effectively. The excellent performance of the WLE blade is due to its unique geometry, which presents various protrusions on its surface. This makes it challenging to select the right materials and techniques to adapt them to the complex surface, as shown in Figure 2. The figure indicates the radius and curvature values using colors that correspond to the minimum and maximum values. The minimum values are in dark areas, and the maximum is at the leading edge of warm tones.

OPEN IN VIEWER

Figure 2.

Wavy leading edge form—study surface.

It is acknowledged that the current manufacturing processes for the production of wind turbine blades are highly complex, requiring numerous variables and steps to achieve the desired final product that meets the aerodynamic standards without surface distortions Bucur et al. (2020). Therefore, the objective is to investigate feasible alternative options of material combinations and processes to enable local production of blades with more intricate designs, ensuring various factors such as affordability, durability, and aerodynamic smoothness. The results of this study have the potential to be replicated for other types of wind turbines that incorporate more conventional geometries. This could increase its relevance and applicability within the industry.

General search for materials and processes

Various materials and processes are currently used to fabricate airfoils. Steel and aluminum alloys are among these materials, but their high density limits their use to small-scale wind turbines. Composite materials are the most commonly used material in both small and large WTs. These materials consist of a matrix, such as polyurethane or epoxy resin, reinforced with materials such as fiberglass, carbon, or aramid fibers. These composites have the advantage of being lightweight and possessing high mechanical properties, such as tensile strength. However, their production may require specialized skills to ensure proper component formation, and their cost may be relatively steep. To make an informed decision, a thorough search is conducted in various databases to explore possible material and process combinations. Tools such as CES EduPack are utilized to gain a comprehensive understanding. Additionally, a systematic comparison is performed to determine the necessary characteristics for an appropriate WT blade design. These characteristics include:

Based on these factors mentioned above, comparative tables were developed among the various material families that are potentially suitable for implementation in the defined case study.

Metals

Metals with notable characteristics such as excellent electrical and thermal conductivity, high mechanical strength, versatility in production processes, minimal thermal deformation, and suitability within ambient temperatures are frequently used in commercial applications. These metals comprise machinable alloys, cast metals, stamped metals, forged metals, and dieless processed metals. Table 1 presents the metals that have been identified as having optimal qualities for use in HAWT blades.

OPEN IN VIEWER

Table 1.

Properties of Metals for blade manufacturing.

	Price [COP/kg]	Density [kg/m3]	Young’s Modulus [GPa]	Fatigue resist. [107cycles]	Fracture toughn. [MPa/m2]
Aluminum 6061	5840–6140	2.70e3	70	57–210	0–125 T4–235 T6–310
Aluminum 2024	5840–6140	2.70e3	73	57–210	0–185 T351–430
Aluminum 3003	5840–6140	2.70e3	69	42–160	0–110 HX2–140 HX9–245
Stainless steel	17,200–18,700	7.9e3	193	175–753	62–150
Brass	13,200–15,500	7.9e3–8.55e3	90–110	150–250	30–60
Cromolio	21,200–25,200	7.8e3–7.9e3	205–217	248–700	14–200
Magnesium alloy for casting	82,900–89,700	1.75e3–1.87e3	42–47	60–125	12–18
Titanium alloy	636,000–700,000	4.4e3–4.8e3	110–120	589–617	55–70

Based on the data analyzed, the combined strength and density of the aluminum and magnesium alloys make them strong candidates for manufacturing. While they are heavy, chromium-molybdenum and stainless steel offer excellent mechanical strength. However, metals like brass are not viable because of their low toughness relative to their density, which would require an increase in their thickness to meet application requirements. Therefore, an efficiently optimized manufacturing process could produce positive results with minimal material utilization.

However, the manufacturing processes available locally for these materials are limited to the following.

Machining two components on both sides and subsequently welding them together to create a smooth, high-quality surface finish.

Casting allows for the creation of intricate internal shapes within a component using a one-time mold. This process reduces material usage, strengthens areas prone to stress, and optimizes the weight-to-strength ratio of the component.

Dieless is an efficient and accessible technique to obtain two component sides and facilitate welding while remaining cost-effective.

In summary, the high initial cost of materials, including alloys, presents accessibility obstacles. The series 6000 aluminum alloys seem to provide ideal properties for customization through casting and/or machining in the manufacturing process, while the magnesium alloy can be effectively implemented using machining. However, despite their outstanding mechanical properties, metals are limited by their density, which results in increased weight. This weight issue complicates the resolution of rotor inertia, indicating that these materials may not be a high priority in manufacturing applications.

Polymers

This section discusses a broad variety of polymeric materials, including both amorphous and semicrystalline polymers. Examples of amorphous polymers are ABS, PVC, PC, PS, PMMA, etc., while PEEK, PET, PP, PTFE, etc. are examples of semicrystalline polymers. Both amorphous and semicrystalline polymers were selected as a result of their widespread use in current industries. Although they are uncommon in this specific application, they allow for a more comprehensive search to include commonly used polymers. An evaluation of material selection based on manufacturing processes was performed, taking into account thermoforming, CNC (milling-turning) and 3D printing, to determine feasible local manufacturing methods. Table 2 assesses the thermoforming process by analyzing the selected polymers to identify suitable materials for this process. Thermoforming involves temperature-dependent modifications to mold geometries and surfaces.

OPEN IN VIEWER

Table 2.

Polymers for thermoforming process of blades.

	Price [COP/kg]	Density [kg/m3]	Young’s modulus [GPa]	Fatigue resist. [107 cycles]	Fracture toughn. [MPa/m2]	Tensile strength [MPa]	Bending temp. [°C]
Poly-vinylchloride (PVC)	4300–4920	1300–1580	2.14–4.14	16.2–26.1	1.46–5.12	51	81
Poly-propylene (PP)	5220–5440	890–910	0.896–1.55	11–16.6	3–4.5	33	98
Polyethylene Terephthalate (PET)	5620–5740	1290–1400	2.76–4.14	19.3–29	4.5–5.5	86	79–115
Polymethyl Metacrylic (Acrylic)	8480–8820	1160–1220	2.24–3.8	15.2–32.7	0.7–1.6	55	87
Poly-carbonate (PC)	10,400–11,200	1140–1210	2–2.44	22.1–30.8	2.1–4.5	79	131–142

Based on Table 2, PVC is the most cost-effective material for thermoforming, PP has the lowest density, PET exhibits the best Young’s modulus, fracture toughness, and tensile strength, polycarbonate is the most resistant to flexural temperature, while acrylic is the most fatigue resistant material. The following polymer fabrication process is CNC machining, which uses computer-controlled turning and milling machines to remove layers of material from a raw piece. These procedures enable the production of parts with precision characteristics that cannot be achieved by manual labor or less efficient techniques. A comparison study of amorphous and semicrystalline polymers for the CNC machining process is presented in the case study in Table 3. It outlines design factors and criteria for informed decision-making on the potential for utilizing nontraditional geometries in manufacturing.

OPEN IN VIEWER

Table 3.

Polymers for CNC machining process of blades.

	Price [COP/kg]	Density [kg/m3]	Young’s modulus [GPa]	Fatigue resist. [107 cycles]	Fracture toughn. [MPa/m2]	Tensile strength [MPa]	Bending temp. [°C]
Acrylonitrile Butadiene Styrene (ABS)	1370–8720	1010–1210	1.1–2.9	11–22.1	1.19–4.29	41	93
Poly-propylene (PP)	5220–5440	890–910	0.896–1.55	11–16.6	3–4.5	33	98
Polyethylene Terephthalate (PET)	5620–5740	1290–1400	2.76–4.14	19.3–29	4.5–5.5	86	79–115
Nylon	7930–8600	1120–1140	2.62–3.2	36–66	2.22–5.62	90–165	-
Poly-carbonate (PC)	10,400–11,200	1140–1210	2–2.44	22.1–30.8	2.1–4.5	79	131–142
Poly-tetrafluoro ethylene, (PTFE)	45,500–51,900	2140–2200	0.4–0.552	5.75–7	1.32–1.8	20–34	260

Based on the data presented in Table 3, ABS is the most cost-effective polymer for CNC machining, while PP is the least dense, PET has better elasticity, and nylon has the highest resistance to fatigue, fracture, toughness, and tensile forces. PTFE stands out as having the best flexural temperature.

ABS, nylon, and polycarbonate are suitable for the 3D printing process that can achieve complex geometries through additive manufacturing. It is important to note the significant differences in mechanical properties between these polymers, which should be considered for various applications.

In summary, when polymers are considered, it is generally accepted that polycarbonate and nylon are viable choices. Polycarbonate is suitable for thermoforming, CNC machining, and 3D printing, whereas nylon is suitable only for CNC machining and 3D printing, but not for thermoforming. Moreover, both polycarbonate and ABS can be utilized for all three production methods. Of the materials examined, ABS proves to be the most cost-effective, while polypropylene presents with the lowest density. In contrast, nylon boasts the highest fracture toughness. With regard to processes, each material is deemed suitable and feasible given current technological capabilities.

Composites materials

Composites are materials that consist of two or more components intentionally designed to improve their properties for specific uses or applications.

They comprise a matrix that provides adhesion and transfers external loads to the materials contributing to the mechanical properties. Additionally, there is a reinforcement component that adds rigidity and strength. Appendix Table 1 shows the materials used as matrices, focused on resins and Appendix Table 2 shows the reinforcements, focused on fibers.

Table 4 illustrates the predominant combinations of matrices and reinforcements, showcasing their high demand characteristics and properties in the market.

OPEN IN VIEWER

Table 4.

Composite materials for manufacturing of blades.

	Price [COP/kg]	Density [kg/m3]	Young’s modulus [GPa]	Fatigue resist. [107 cycles]	Fracture toughn. [MPa/m2]
Epoxy Fiberglass	74,900–106,300	1.75E3–1.97E3	15–28	55–96	7–23
Epoxy/Carbon fiber (UD Layup)	115,000–160,000	1.55E3–1.56E3	110–131	150–300	6.12–20
Epoxy/Aramid Fibers (QI Layup)	147,000–242,000	1.38E3	23.5–30.9	2.5E3–3E3	2–4
Polyurethane/Fiber Glass Billets	180,000–210,000	1.8E3–2.1E3	12–14	12–27	3–6
Polyester Matrix Fiber Sheet	101,000–118,000	1.8E3–2.1E3	9–14	20–36	5–13
Phenolic/Fiberglass (Q1 Layup)	82,000–88,300	1.7E3–2E3	20.4–30	33–120	13–34

The epoxy resin reinforced with carbon fiber is notable for its outstanding physical and mechanical properties, making it a prevalent material in various industries. However, it is important to note that its high cost is one of the primary drawbacks compared to alternative options.

In terms of manufacturing methods, they can be categorized into open and closed molds.

For open molds, the following processes are used:

Hand Lay-up, which is a manual process of consecutively adding layers and bonding them together with resin.

Filament winding, which involves winding resin-soaked fibers onto a rigid, removable surface.

Vacuum bagging. This process involves manual placement of the component, consolidation of the composite layers through vacuuming, and application of temperature for curing.

For closed molds, the following processes are used:

Resin Transfer Molding (RTM): pressure and injects resin into dry fabrics that are already placed to optimize resin usage and improve mechanical properties.

Injection of pre-impregnated fibers with resin into closed molds.

Pultrusion: Stretches the material into a constant profile shape, followed by heat treatment to align and consolidate fibers.

Rotomolding: Resin and, in some instances, fiberglass are injected into a closed mold and the mold is rotated to ensure complete resin coverage and cure.

Now, the generalized search process for materials and processes can be implemented, followed by filtering the materials and processes found. This involves analyzing the various properties of each material and the respective processes to acquire an initial approach to the ideal materials for designing small WT blades. The CES Edupack software compares and analyzes materials and processes, utilizing a proposed filtering approach to analyze results.

Alternative materials

The CES Edupack software was utilized as a search tool to explore alternative materials. It presents results for materials mentioned above and also facilitates the discovery of new results by adapting search iterations based on designated criteria and search ranges for previously defined design variables.

It is clear that the use of composite materials, such as carbon fiber and different thermoplastic polymers outlined in Appendix Table 3, provides small wind turbine blades with distinct properties and adaptability to geometries that can withstand the loads encountered Alshammari et al. (2021).

It should be noted that higher values for Young’s modulus, strength, and fracture toughness indicate better material properties. Therefore, a direct comparison can be made between price and density. In this comparison, selection limits are established based on reference values for polymeric materials that meet the requirements, defining variables such as density (maximum limit: 1750 kg/m³), price (maximum limit: 120, 000COP/kg), Young’s modulus (minimum limit: 2:6 GPa), fatigue strength (minimum limit: 36 × 10⁷cycles) and fracture toughness (minimum limit: 2 . 22 M P a / m 2 ). These limits are used to filter out materials that do not meet the application requirements, resulting in Appendix Table 4, defined as the first filter.

It can be analyzed that materials such as wood have an acceptable density, in addition to its tentative price and its affordability in manufacturing by CNC machining. On the other hand, composites are the best in several design conditions with affordable manufacturing processes, but their price is somewhat high. To carry out a more specialized search, it was decided to move on to level 3 analysis in the CES Edupack software, where a wider range of materials is available. The implementation of improved selection limits aims to find the optimal material for the application, taking into account preliminary design variables such as:

Maximum density of 1560 k g / m 3 , which belongs to carbon fiber and is lower than the density used in the previous filter. Carbon fiber is widely used in blades.

Minimum Young’s modulus of 16.6 GPa , which belongs to PBT. This material offers higher stiffness compared to the nylon used in the previous filter, making it a better choice with superior mechanical properties.

Fatigue resistance of 52 × 10 7 cycles.

Fracture toughness of 4 . 62 M P a / m 2 , also belonging to PBT for the same reason as the change in stiffness from nylon to PBT.

Appendix Table 5 shows the main properties of the material most suitable for the application, based on the refined limits of the specified design criteria. It can be observed that there are more materials that meet even more demanding limits where Sisal and Juta materials stand, which are normally raw materials that are transformed into ropes, but stand out for their price, density, and mechanical properties, the worrying factor being the way they are processed. On the other hand, PET with 30% carbon fiber is an interesting option to evaluate, as well as fiberglass with polyester. Applying a final filter to all the searches compiled in the CES Edupack software, the most optimal alternative materials for manufacturing are determined and presented in Table 5, taking into account that in the search both nylon and fiberglass have already been previously mentioned previously, demonstrating the benefits in the application of the exposed case study.

OPEN IN VIEWER

Table 5.

Alternative materials for manufacturing of blades.

	Price [COP/kg]	Density [kg/m3]	Young’s modulus [GPa]	Fatigue resist. [107 cycles]	Fracture toughn. [MPa/m2]
Oak wood	2030–2240	850–1.030	20.6–25.2	42–52	9–10
Sisal	1840–2150	1450–1500	9.4–22	220–316	19.6–101
Magnesium alloy for casting	8290–8970	1.750–1.870	42–47	60–125	12–18
Hornbeam	6170–8230	700–890	14–18.1	43.1–52.1	7.2–8.7

Tensile structures

One proposed alternative is inspired by the way the wings of early airplanes were made, using tensegrity structures to demonstrate how it is possible to achieve complex, doubly curved surfaces by using an internal structure and a flexible material stretched around it. This approach focuses on the relationship and combination of physical properties at the structural level.

For the external surfaces, the possibility of manufacturing them locally could also be explored using thermoforming and rotational molding of polymers and then stiffening them with the internal structure. The proposal is conceived for WT blades with a length of 30 m, which means that at this size, the stress, tension, and fatigue resistance considerations are more demanding than for a blade of a small WT. Additionally, the elements that make the design possible can be divided into flexible and rigid elements. Within the flexible elements, there are cables that act as tensioners and help to reinforce the textile membrane, which is responsible for defining the shape of the surface, is resistant and has a low density. In addition, edge reinforcements are placed along the edges of the membrane to absorb the tension generated at the edges and anchor points.

However, there are rigid elements that form the edges to provide support and shape to the membrane, which is constructed in an arched form. Anchor points provide stability by dissipating vibrations to the maximum extent while maintaining the necessary tension in the membrane.

The topic of tensile structures consists of three main elements: the tensioned element (fabric), the internal structure, and the tensioning elements.

First, an evaluation of the types of fabric available is made, as shown in Figure 3.

OPEN IN VIEWER

Figure 3.

Different families of fabrics on the market.

Based on this, a search is made for commercial fabrics available in the industry that meet requirements and variables such as impermeability, tensile strength, fatigue resistance, durability, tear strength, wrinkle resistance, conformability, stiffness, sunlight resistance, and thermal stability. These fabrics are listed in Table 6.

OPEN IN VIEWER

Table 6.

Fabrics to consider for manufacturing of blades.

Fabric types			Composition
Elastic fabric	Clover 5JC62		15% elast. 85% polyester
	Sepentina 27200		4% elast. 96% polyester
No elastic fabric	Polyester	Ranger	67% PTF 33% polyester
	Nylon	Artillería	100% NTF
		Millenium
		Chaleco R
	Acrylic
	His PVC		Polyester PVC laminate
	Cotton	Siria 24620	100% Cotton
Thermo-formable fabrics	Polymer film
	His PVC		Polyester PVC laminate
	Mono Kote

From there, the question arises as to the different methods of tensioning and forming the different types of fabrics. The most common tensioning methods used in industry are analyzed and presented in Table 7.

OPEN IN VIEWER

Table 7.

Types of tensioning strategies.

Types of tensioning	Form processes	Additional process	Unions
Rope cordoning methods	Molding only	Polymer elbows	Sewing
Distribution of point stresses with rope (for falls)	Tensioned-elastic	Stretch and elbows polymers	Sewing
Leggins type (smaller than the shape)	Thermoforming	Heat and polymer elbows	Welding
Distribution with methods tension and eyelets	Embedded bars	Aluminum bars	Sewing
Distribution with horizontal tensors	Threads in trailing edge	Nylon/guaya strings and polymer elbows	Sewing
Flanged tension	Sleeve type	Polymer elbows	Sewing

Taking into account the information above, the idea is to propose a process for designing tensile structures and compare it with other manufacturing processes found, since tensile structures are an interesting alternative to evaluate in terms of cost, ease of fabrication, and overall benefits.

Material selection

Table 8 shows the materials with the best properties that are most suitable for the case study.

OPEN IN VIEWER

Table 8.

Materials selected for manufacturing of blades.

	Price [COP/kg]	Density [kg/m3]	Young’s modulus [GPa]	Fatigue resist. [107 cycles]	Fracture toughn. [MPa/m2]
Oak wood	2030–2240	850–1.030	20.6–25.2	42–52	9–10
Nylon (NTF)	7930–8600	1120–1140	2.62–3.2	36–66	2.22–5.62
PBT 50% Fiberglass	7530–7960	1710–1750	16.6–17.5	52.8–68.6	4.62–5.54
Epoxy carbon fiber (UD lay-up)	115,000–160,000	1550–1560	110–131	150–300	6.12–20
Epoxy/Fiberglass	74,700–106,000	1750–1970	15–28	55–96	7–23
Poly vinylchloride (PVC)	4300–4920	1300–1580	2.14–4.14	16.2–26.1	1.46–5.12
Polyester (67% PTF 33% POL)	70,000–100,000	3–10	5–10	20–55	2–6

Limitations related to affordability, availability, price, and implementation without experience are defined. From there, unconventional alternatives that meet the initial constraints are explored. These alternatives are classified on the basis of the implementation and exploration of new and different manufacturing processes. The proposed approach includes the use of polyester materials for tensile structures with the objective of analyzing and evaluating this alternative method for the defined case study. This option offers availability, affordability, and suitability for timely fabrication. In addition, composite materials will be implemented as they are widely used for microgenerator blades and offer a wide range of material availability. Wood is another natural resource with excellent properties, which makes it a tempting choice due to its low density, affordability, and ease of manipulation in manufacturing that requires less expertise.

Process selection

Based on the selected materials, a search for possible manufacturing processes is carried out, identifying a series of parameters that define them, such as manufacturing time, affordability, ease of access to machinery, repeatability, number of elements to be manufactured and adaptability to the complex surface presented in the case study.

Taking into account the above considerations, the materials selected are based on their mechanical and physical properties as well as factors such as affordability, availability, and expertise. The selected manufacturing processes include:

These selected manufacturing processes were chosen based on their compatibility with the selected materials and their ability to meet the requirements of the case study in terms of mechanical performance, cost-effectiveness, and ease of production.

Implementation of the selected materials and manufacturing processes

Serialized agglomerated plans and epoxy resin with filler

When working in wood, using the plane-serial method, a set of flat pieces is obtained and cut in such a way that, when properly assembled, they create solid three-dimensional figures. In this case, the combination of the serial plane process with the rotomolding or casting process in epoxy resin with a filler, in this case talc, although fiber can also be used as a filler, is used. This results in a blade with a good surface finish and the properties of a composite material. To allow the resin to penetrate and diffuse more easily, six guides were created in the 3D modeling. Two of these guides are in the front, two in the back, and in each corner, both in the predominantly curved corner and in the slightly flatter corner, a guide or space was left for the resin to flow more easily.

Methodology and result

Initially, it was considered to use corrugated cardboard for serial planar agglomerates and epoxy resin with the rotomolding manufacturing process. However, this proved to be impractical as the rotomolding process is not suitable for such small and thin parts. Additionally, when corrugated cardboard was tested for serial planar agglomerates, the laser cut pieces were too small, causing the corrugated cardboard to separate into its three components (two faces and an inner part). In addition, due to the low rigidity and malleability of the board, the laser cutting process was not precise, resulting in deflection and deformation of the board components.

Knowing that corrugated was not the ideal material, we turned to MDF because of its low cost and low density. However, when attempting to assemble the MDF pieces using glue, it was found that the process required one hundred and eighty-nine (189) pieces, and the smallest pieces could not be effectively glued due to their tiny size. However, it was still possible to develop about 75% of the blade parts using this method. The process involved using a CNC-machined oak mold as a guide for the female surface geometry of the blade. The oak mold and serial plans of the MDF agglomerate can be seen in Figure 4.

OPEN IN VIEWER

Figure 4.

Oak mold of the WLE blade and serial planes agglomerated in MDF.

For resin impregnation, a Smooth-On Star Mold 30 silicone mold is fabricated to replicate the geometry of the WLE blade. The MDF pieces are glued and placed inside the silicone mold. The epoxy is then poured into the mold, covering the material and taking the shape of the mold. Figure 5 shows how the piece looks after being removed from the mold with epoxy and talcum powder, and after applying putty and sanding to correct for imperfections.

OPEN IN VIEWER

Figure 5.

WLE blades manufactured with serial planes.

Rotomolding/casting of polyurethane resin

The implementation of epoxy and polyurethane resins is based on affordability, density, weight, price, and other relevant characteristics. However, due to the limited space for thickness in this case study, the rotational movement of the resin becomes challenging as it requires a uniform process. Therefore, manual pouring of the resin is also used, along with the addition of internal serial planes or ribs to provide rigidity and strength in the starting area of the blade.

Methodology and result

A silicone mold, Smooth-On Star Mold 30, is fabricated to replicate the geometry of the WLE blade. Polyurethane resin is then poured into the mold and the mold rotates for five (5) minutes. The drying time of the polyurethane resin is approximately 3 minutes, so rapid rotation (set at 20 rpm) is necessary to allow the resin to accurately copy the shape of the mold. The wooden support, as shown in Figure 6(a), is inserted at the exact location of the rotor axis. This is done to prepare the silicone mold for the resin casting process and to ensure that the resin does not leak between the mold surfaces. Subsequently, because the mold is not one-sided, there may be some excess material along the line where each side of the mold meets. Also, if a vibrating table is not available, the mold may have bubbles. This excess material and any imperfections are sanded and smoothed. Optionally, the piece can be painted with a light color, such as white, using polyurethane paint to ensure better preservation of the piece, as shown in Figure 6(b) and (c).

OPEN IN VIEWER

Figure 6.

Process of rotomolding/casting resin, final piece: (a) MDF blade with serialized plans, (b) freshly de-molded resin blades, and (c) surface corrections and final finishing.

Tensostructures

Tensile structures comprise a rigid support and a fabric or canvas. Their key feature is their capacity to span extensive areas using limited material. In this instance, an MDF framework with fortified edges is utilized, utilizing mild steel wire and resin. A slight steel rod is inserted inside to offer extra support. Lastly, a serpentine fabric sleeve envelopes the entire structure to adopt the desired shape.

This method was selected because it allows the construction of a blade with the majority of its volume consisting of air, leading to a reduction in weight. Moreover, the low manufacturing costs enable the production of blades at a reasonable price, offsetting any imperfections in the shape with the reduced production costs. The design is built around two shoulders at the two corners of the blade to ensure precision in the shape of the surface and edges, especially in the case of the WLE. Subsequently, there will be an internal base made of oak to endow it with sufficient mechanical properties to withstand fracture and fatigue, with the added advantage of having a low density. In addition, multiple oval-shaped structures will be used to cover the voluminous and concave areas of the front and back segments of the blade. Finally, the blade structure will be covered with 4% serpentine fabric to ensure uniform geometry, while internal hemp threads will be utilized to tension the fabric.

Methodology and result

Three millimeter (3 mm) thick MDF planks are cut and assembled into ribs that roughly follow the shape of the blade, as shown in Figure 7.

OPEN IN VIEWER

Figure 7.

Laser cutting and reinforcement of the ribs of the tensile structures.

The resin epoxy is then poured into the wavy region using a thermoformed mold to create a foundation for the WLE geometry. It is crucial to correctly position the blade inside the mold to ensure that the curves are in the right places during the pouring process. To solve problems with irregularities in the shape of the trailing edge, a mild steel wire is attached to the opposite profile of the poured edge to guide the shape more accurately. The interior is also strengthened with a 3 mm diameter and approximately 15 cm long iron rod placed in the middle of the piece. Once the resin has fully cured and the blade has been thoroughly strengthened, it is time to apply the fabric cover. The sleeve is sewn and the serpentine fabric is positioned at a right angle to the blade’s axis, so that the sleeve can take the intended shape. The edges of the sleeve are smoothed by ironing to ensure a surface that is free of knots or irregularities. Then, the fabric is tensioned and secured in its final position. This process produces a blade with distinctive geometry compared to others. Figure 8 demonstrates a comparison between a blade made with ABS and one created using the tense structure method.

OPEN IN VIEWER

Figure 8.

Comparison of ABS blade (left) with tensile structure blade (right).

However, despite its unique design, the blade could be produced quickly and at a substantially lower cost than the standard rate. Additionally, it performs its intended function of turning the rotor once it is installed in the wind turbine. After evaluating its effectiveness, the decision was made to administer a waterproofing treatment to one of the blades to increase the durability of the material in outdoor environments.

Experiment and results

Initially, the analysis begins with a comparison of variables such as Weight, Working Hours, and Price for each of the blade types: ABS 3D printing, polyurethane casting, serial planes + epoxy, and tensestructures. This analysis is shown in Figure 9.

OPEN IN VIEWER

Figure 9.

First comparison about weight, work hours and price values.

There is variability in the price range for each of the processes, ranging from 10,000 to 500,000 COP. Similarly, the weights can range from approximately 150 to 300 g, and there is a maximum time limit of 50 hours to work without delays due to mold handling or manual sanding and surface finishing. ABS stands out in terms of price and working hours, with the highest values around 500,000 COP and 24 hours, respectively. On the other hand, Tensestructures offered the least expensive option, costing approximately 10,000 Colombian pesos. Furthermore, polyurethane casting proved to be the most efficient method of completing the job, taking approximately 5 hours to complete.

Figure 10 quantifies the environmental impact using the CES Edupack software. This quantification is based on materials and commercial processes and calculates the weight according to the carbon footprint. The study found that serial planes made of epoxy resin had the highest environmental impact. The energy required for each raw material is also quantified in the software, providing a reference for comparison. It is apparent that serial planes require the most energy at around 25 J, while polyurethane cast requires the least at around 0.5 J.

OPEN IN VIEWER

Figure 10.

Second comparison carbon footprint and raw material energy respectively.

The performance validation of the blade is carried out by adapting to a prototype wind turbine at a fixed angle of attack of 39.5°. The prototype was validated for the wind direction and the tests were carried out in a controlled environment, producing average velocities of 2–3 m/s. The goal is to confirm its operability and ability to initiate without any intervention according to the set aerodynamic capacity. A series of experiments was carried out, capturing approximately 8000 data samples within the required time frame to obtain results (two trials of 20 and 40 minutes). The results are depicted in Figure 11.

OPEN IN VIEWER

Figure 11.

Third comparison in a controlled environment.

The blades manufactured in tensile structures attained the lowest RPM values, recording 143 and 152 during the 20- and 40-minute tests, respectively. On the contrary, the blades made of polyurethane resin casting and serial planes, which had a greater weight, as depicted in the above graphs, had the highest RPM values. Specifically, the former and latter recorded 170 and 168, respectively, in the 40-minute test. Based on the proportional relationship observed between RPM and generated energy, the polyurethane casting and serial plane blades have demonstrated high energy generation levels in the 40-minute test, registering at 40 m W / h and 39 m W / h , respectively.

It is important to note that estimating the data collection time is vital to ensure better approximations for the listed variables.