- Original Paper
- Open Access
Structural Analysis of Biologically Inspired Small Wind Turbine Blades
© The Author(s). 2017
- Received: 6 July 2017
- Accepted: 12 July 2017
- Published: 22 August 2017
Renewable energy resources are becoming more important to meet growing energy demands while reducing pollutants in the environment. In the current market, wind turbines are primarily restricted to rural use due to the large size, noise creation, and physical appearance. However, wind turbines possess the ability to run at any time of the day. Horizontal axis wind turbines remain the most widely used, but there is significant room for improvement in vertical axis wind turbines.
While vertical axis wind turbines are not reaching the same level of efficiency of horizontal axis wind turbines, there are significant benefits to researching improvements. One of the main benefits is to make use of vertical axis wind turbines in urban settings. In order to improve the efficiency of the vertical axis wind turbine, a biological approach was taken to design blades that mimic the shape of maple seeds and triplaris samara seeds. This approach was taken because due to its geometrical properties, typically extra lift is generated.
The results obtained through FEA simulations were consistent with the expected results for the application that was considered. The results obtained provide valuable insight for engineers to iterate and design optimum wind turbine blades taking advantage of biological phenomena applied to conventional airfoils.
The purpose of this paper is to provide structural analysis details into the design of a vertical axis wind turbine blades that mimic the geometry of maple and triplaris samaras seeds.
- Wind energy
- Wind turbine
- Maple seed
- Renewable energy
Increasing demands for energy and green engineering is leading to an increase in generation of renewable energy from resources such as solar, wind, and hydroelectric power. While solar energy is currently the most commercially used, wind energy has enormous potential for increase in efficiency and power output (Maehlum 2016). Further, vertical axis wind turbines have the potential to outperform horizontal axis wind turbines but have not achieved the maximum efficiency available (Dabiri 2011). This has led to more research into commercial applications of vertical axis wind turbines. While not all locations are ideal for wind turbines, they do hold an advantage over solar panels in that they can work constantly because they are not dependent on the sun being present (Boyle 2012).
The current market is primarily HAWTS, but VAWTS have shown the ability to provide more energy per land area, or a greater power density. The power density for a VAWT wind farm can achieve a magnitude of ten times greater than a HAWT wind farm (Dabiri 2011). The increase in power density is due to the ability of VAWTS to be placed closer together than HAWTS. If a wind turbine is too close to the next wind turbine, the airflow will be disrupted and impact the power output. The placement of wind turbines to improve power output is important but only has a major impact when turbines are in close proximity of one another. Single turbine efficiency is still the major selling point, especially in individual use and urban settings. The current major drawback in VAWTS is the overall lower efficiency compared to the efficiency of HAWTS. However, VAWTS have the advantages of being able to rotate at lower wind speeds, ability to function on wind from any direction, and are located lower to the ground, which reduces cost and can be more marketable for urban settings (Boyle 2012; Tummala et al. 2016).
Generation of FEA models
All the simulation work was completed using ABAQUS CAE software. Prior to conducting FEA simulations, the turbine blade size and geometries should be designed. While the turbine shaft was not as important as the blade designs, it was still desired to be designed efficiently to reduce the force required to get the turbine to rotate but strong enough to withstand high winds. Additional considerations were given to the size required for the desired application. As mentioned previously, the project is to design a VAWT that is small scale, designed to be used in an urban environment for things such as powering sensors on bridges and streetlights. That being said, the designed turbine measured about 30 cm in height and a blade length of about 16 cm. In addition, the initial prototypes were desired to be made of plastics through 3D printing (Object Eden260V from Stratasys). The 3D printing process allows for easy creation of prototypes; however, the thickness of the layers became an issue. Future physical testing was taken into consideration during FEA model creation to ensure values could be compared with a high degree of confidence. In particular, size of experimental rotors was limited by the size of wind tunnel test chamber. Parameters that were considered to be included in the sizing limitations were distance between the blade tip and wall and height of blades in the tunnel. The wind tunnel data can be scaled appropriately, depending on the Reynolds number conditions, for future FEA models of larger wind turbines.
The FEA simulations were broken down into nine groups to validate the results and impact of the blade geometries. The groups were normal blades, extended normal, maple seed shape, triplaris samara seed shape, normal with tubercles, extended normal with tubercles, maple seed with tubercles, triplaris samara with tubercles on the leading edge, and triplaris samara with tubercles on the trailing edge. The normal blades are straight blades with a constant cross-section width. The extended normal blades were designed similar to the normal blades with the only difference being the length of the blade. The blade length was extended to have the same surface area for the lower half of the blade as the maple seed blade to compare the structural integrity for the same pressure application over the same area. The maple seed blade was designed to follow the geometry of a maple seed as close as possible to generate leading-edge vortices and increase the lift of the wind turbine. The cross-section of the maple seed was difficult to generate and would not be reliable at high wind speeds. Given that the wind turbine is lift driven, the use of airfoils was determined to be the best option. The triplaris samara blades were designed in a similar fashion as the maple seed blades.
FEA simulation and results
Fixed rotationally through 3D solid elements
Blades and shaft are all one piece (no friction)
Geometry of blade was not simplified for symmetry or axi-symmetry
Geometry of blade attachment was simplified to remove issues with contact of mounting system and turbine shaft
Focus was on blade and not attachment mechanism
ABS Plastic material properties (Comparison of Typical 3D Printing Materials n.d.)
Young’s modulus E (GPa)
Ultimate tensile strength (MPa)
An initial basic mesh optimization was completed only on the maple seed and samara blades with at the zero angle of attack and zero pitch orientation with a wind speed of 15 m/s. For both blade types, the number of elements for each optimization level was as follows: coarse mesh of ~ 70,000 elements, moderate mesh of ~ 150,000 elements, fine mesh of ~ 200,000 elements, and very fine mesh of 250,000 elements. Ultimately, the data demonstrated small changes between the fine and very fine mesh patterns. An intermediate fine mesh (~ 220,000–230,000 elements) was used to ensure quality output while decreasing the simulation time. Tetrahedral elements were used throughout the entire model as they adapted best to the geometry of the blade.
There were no geometric simplifications applied to the blade itself or the applied loading. The mounting system was simplified to overcome the issues with contact of the mount mesh and shaft mesh. Without this simplification, the resolution of the mesh would have needed significant refinements, resulting in greater time requirement to complete simulations. The attachment method utilized was determined to provide a better analysis of the blade behaviors without bias in data from mounts.
Wind speeds and corresponding pressure loads
Wind speed (m/s)
Pressure load (Pa)
Where the pressure load is equal to the lift force, L, divided by the surface area of the bottom surface of the blade. Surface area was determined through a measurement tool in Creo Parametric. The lift coefficient was appropriately correlated through known NACA 0012 data.
The range of pressures was selected to cover a sufficient array of loadings that wind turbine blades would typically experience. Since the likelihood of higher wind speeds in urban setting is extremely low, it is deemed important to demonstrate ability or inability of the blades to withstand the wind loading conditions so as to meet performance benchmarks of commercial wind turbines. Analysis was completed for static loading to show a correlation between blade geometry and orientation to determine the loading patterns of each blade. Future work in this area will include cyclic loading conditions, which will give a better perspective to further narrow down orientations and geometries. In addition, cyclic loading will be utilized to analyze fatigue life and failure of blades. The premise of this work was to lay the foundation for different blade geometries and to understand the pressure and stress distribution as well as rigidity of geometries.
Preliminary study of stress distribution across the varying blade geometries
Not loaded cyclically, future considerations to fatigue and failure
Experimental validation has not yet been completed
Addition of tubercles to the blades was done with the intent of decreasing the drag and at the same time increasing the lift-to-drag ratio without sacrificing structure stability of the blades. When comparing the normal blades with and without tubercles, it was shown that the blades with tubercles had lower maximum stresses and displacements. This proved the tubercles did not lose any structural strength. The same was true for the extended normal blades with tubercles and without. Comparing the maple blades, there was a slight increase in maximum stress for the maple seed blades with tubercles. However, a majority of the maple blades with tubercles showed a decrease in displacement. These were also small changes. If CFD simulations were to show an increase in lift and decrease in drag, the changes would be enough to consider the tubercles successful.
This paper presented the initial design, blade analysis, and proposed physical creation of the micro wind turbine and blades. Due to the desire for a small wind turbine for urban applications, the proposed design was the actual size of the wind turbine. Removing the scaling provides a direct comparison, which is ideal for the number of variations applied to the modeling. The small size also allows for easy physical creation through 3D printing and injection molding. While physical testing has not yet occurred, the consideration of future testing taken during creation of the model will make it much easier to handle. The FEA analysis of the application of maple seed and triplaris samara seed wing geometry to wind turbine blades proved to be insightful. It was shown that structurally the wind turbine blades could withstand strong wind speeds of up to 55 m/s, while proving to be structurally comparable, and in many cases superior, to the strength of the normal blade. In addition, it was shown that the maple seed blades actually deflected less than the normal blades. Further modification gave a baseline justification that addition of tubercles would not hurt the structural integrity of the blades. While there were some increases in the stress felt by the blades, the deflection actually decreased in most blades. These results proved that more work should be done in the investigation on the impact of tubercles in the power generation of the blades.
This work will be applied to future FEA and CFD work on the micro wind turbine. Now that the structural improvements have been noted, several cross-sections will be tested for structural validation. It is important to validate this prior to CFD simulations. Physical testing will also occur to validate the structural strength of the design and compare the results. In addition, the amount of lift and power generated will be measured and compared to the CFD and FEA models.
CS made substantial contributions to the study specifically in the conception, design and running most of the simulation analysis. He also mentored the two undergraduate students who are co-authors. CS analyzed the simulation results and interpreted data. CS verified the simulation results obtained by the two undergraduate students. CS was involved in drafting of the manuscript as well as revising it critically for important intellectual content. SJ mentored the main author (CS) during his graduate studies and analyzed and verified the development of the concept, theory, and design as well as simulation results. SJ was involved in drafting of the manuscript as well as revising it critically for important intellectual content. LK contributed in the development of the concept and understood the theory behind the concept. LK was involved in creating some of the test case models, ran the finite element analysis, and acquired the simulation data. LK was involved in drafting of the manuscript. AM contributed in the development of the concept and understood the theory behind the concept. AM was involved in creating some of the test case models, ran the finite element analysis, and acquired the simulation data. AM was involved in drafting of the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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