Design and development of a vertical axis wind turbine for slip stream energy harvesting

 Corresponding author: Destine Mashava. Copyright © 2025 Author(s) retain the copyright of this article. This article is published under the terms of the Creative Commons Attribution License 4.0. Design and development of a vertical axis wind turbine for slip stream energy harvesting Rachel Deline Muyambo, Takudzwa Muhla, Givemore Kanyemba, Destine Mashava *, Gilbert Munhuwamambo and Innocent Mapindu Department of Industrial and Manufacturing Engineering, National University of Science and Technology, Bulawayo, Zimbabwe. Global Journal of Engineering and Technology Advances, 2025, 23(03), 010–020 Publication history: Received on 09 April 2025; revised on 29 May 2025; accepted on 01 June 2025 Article DOI: https://doi.org/10.30574/gjeta.2025.23.3.0166 Abstract Zimbabwe has over the last decade witnessed a steady surge with regards to the demand of electric vehicles (EVs), and on the global scale as well, this has highlighted the need for innovative charging solutions. These innovative solutions are required more in the developing nations, which historically have depicted insufficient infrastructure to support the widespread adoption of EVs. The major challenges which have been observed have been the high ratio of EVs as compared to the available charging pots, the erratic nature as well as inadequacy of electricity as well as the heavy reliance by developing nations on fossil-based fuels for the generation of electricity. The factor associated with fossilbased fuels thus diminish the environmental benefits that are synonymous with EVs which include the reduction of greenhouse gas emissions. The research sought to address these highlighted challenges by designing a Vertical Axis Wind Turbine (VAWT) and integrating it with a charging station which then harnesses electricity from the slipstream associated with passing vehicles. This action thus effectively converts the nation’s highways into renewable energy sources. The evaluation process of the design involved the analysis of aerodynamical forces, estimation of power generation potential, an assessment of operational loads, conducting Computational Fluid Dynamics (CFD) analysis and prototyping the system to demonstrate its practicability and potential scalability. The results of the research demonstrated the feasibility of implementing VAWTs along highways so as to provide a sustainable and decentralised power supply for EV charging stations, thus offering a much promising solution for the adoption of EVs in Zimbabwe. Keywords: Vertical axis wind turbine; Computational fluid dynamics; Electric vehicles; Renewable energy; Slipstream; Charging stations 1. Introduction Electric vehicles (EVs) have over the last decade, gained global traction due to the environmental benefits they offer, improved performance as well as reduced costs associated with maintenance. Global sales alone of EVs surpassed the 2 million marks in the first quarter of the year 2022 [1], which was an evident 75% increment from the first quarter of the previous year, 2021. However, despite this noticeable milestone, there has been a lag in the adoption of EVs in developing nations such as Zimbabwe, and this hinderance has largely been due to the insufficiency of charging infrastructure [2], electricity supply as well as an over-reliance on fossil fuels for purposes of power generation. This research proposed a Novel solution: the integration of Vertical Axis Wind Turbines (VAWTs) with EV charging stations so as to seamlessly harness slipstream energy from passing motor vehicles. By converting the highways into renewable energy sources, the proposed system seeks to provide a sustainable and decentralised power supply for the charging of EVs, thus reducing Global Journal of Engineering and Technology Advances, 2025, 23(03), 010–020 11 the strain on the national grid and also ultimately minimising greenhouse gas emissions. The proposed system is designed in such a way that it can be seamlessly integrated into existing road infrastructure, thus providing a sustainable and renewable energy source. Recent reports published by the International Energy Agency (IEA) have highlighted the rapid growth in terms of global EV adoption, with developing nations not being left out as they have embraced EVs as a sustainable transportation solution for their populations [3]. However, a significant barrier in the form of insufficient charging infrastructure, remains in the African landscape, and this has been exacerbated by the fact that electricity supply is inherently unreliable and the nations depend heavily on fossil fuels [4]. A slipstream is defined as a region behind a moving object in which a wake of fluid is moving at velocities comparable to that of the moving object, relative to the ambient fluid through which the object is moving [5]. Slipstreaming has found a popular use in formula 1 racing where the car in the front line pierces a hole through the wind and the cars behind it line up behind it to reduce the effect of air resistance [6]. The proposed principle of operation within this research is by virtue of harvesting high speed slipstream air to turn the turbine blades of vertical axis wind turbine which in turn produces electricity. This is achieved by generating electrical power through wind draft and lift forces produced by vehicles travelling on the road [7]. High speed vehicles push away air as they move, producing air kinetic energy ad heat. Placing wind turbines at the centre or the side allows this kinetic energy to be captured and converted in electrical energy. According to the Zimbabwean National Statistics Agency (ZIMSTAT), the number of registered vehicles in Zimbabwe reached approximately 1.5 million as of December 2022, with light motor vehicles accounting for 80.6% of the total [8]. Of these vehicles, the majority are concentrated in Harare, which is the Capital City. Over the years, the vehicle population has shown a consistent surge, with a recorded growth increment of 6.9% from 2021 to 2022 [8]. Each moving vehicle represents an opportunity to harvest the slipstream kinetic energy produced [9] [10] in order to generate electricity required to charge electric vehicles or to serve secondary functions such as lighting up street lamps, electrifying homes that are not connected to the grid or supplying lighting options that supplement electricity in the event of power cuts. Figure 1 shows the application of the slipstream principle. Figure 1 Slipstream Harvesting [11] 2. Design Parameters Determination The rotor, shaft, power train parameters were determined using mathematical calculations to achieve the vertical axis wind turbine for electric vehicle charging. 2.1. Rotor design The rotor of the turbine encompasses the rotor aerodynamics, air foil, the blades, swept area and tip speed ratio parameters. Global Journal of Engineering and Technology Advances, 2025, 23(03), 010–020 12 The rotor aerodynamics parameters are derived from the blade acting forces and angles diagram as shown in Figure 2. Figure 2 Acting forces and angles for rotor blade The formulas applied on the turbine determine the parameters are given as follows [12, 13]; The relative velocity is given by; 𝑊= 𝑉∞−𝑅𝜔 (1) Where; 𝑊 is the relative velocity, 𝑉∞ is the free stream velocity, is the 𝑅 is the radius of rotation, and 𝜔 is the turbine angular velocity The resulting angle of attack and airspeed flow are determined geometrically as: 𝑊𝑐 =𝑈+𝑉cos𝜃 (2) 𝑊𝑛=𝑉sin𝜃 (3) where: 𝑊𝑐 is the chord wise direction, 𝑊𝑛 the perpendicular direction to the chord. Thus, the angle of attack for an unpitched and pitched blade is defined as: 𝜃= tan−1(𝑉sin𝜃 𝑅𝜔 + 𝑉cos𝜃) − 𝛼 (4) The Rotor air foil, DU O6 -W - 200 air foil was selected due to its superior performance based on the performance curve chart given on Figure 3. This air foil model was used in the design of the rotor blades. Global Journal of Engineering and Technology Advances, 2025, 23(03), 010–020 13 Figure 3 Performance curves comparison [14] The selected number of blades was achieved using the performance curves obtained by varying turbine blade numbers as given on Figure 4 [14]. The 3-blade design was selected as it promotes stability, reduces torque ripple in the drive train and eliminates symmetrical loading. Figure 4 Varying blade number performance curves [14] The solidity values selected were based on the standard graphs of the relationship between the power coefficient and the tip speed ratio as solidity values are varied curves as given in Figure 5 [14]. Solidity values between 0.3 and 0.5 were chosen as they are ideal in maximizing both the power coefficient and the tip speed ratio. Global Journal of Engineering and Technology Advances, 2025, 23(03), 010–020 14 Figure 5 Effect of solidity on the power coefficient [14] The turbine swept area is the space that encloses the turbine as it rotates and it is dependent on the shape of the rotor configuration and is determined as: 𝐴= 𝐿 × 𝑑 (5) Where; 𝐿 is the blade length and 𝑑 is the blade diameter The tip speed ratio (TSR) is defined as the ratio of the tangential speed at the blade tip to the actual wind speed [15]. Values of TSR for VAWTs range between 2 and 2.5. The equation is below giving the TSR to be used for calculations; 𝑇𝑆𝑅=𝜔𝑅 𝑣𝑤 (6) Where; 𝜔 is angula velocity, 𝑅 is the blade radius and 𝑣𝑤 is the wind speed The turbine torque is a factor of the velocity, rotor swept area, air density and the efficiencies of the generator and the gearbox. The turbine torque is determined as: 𝑻𝒕= (𝟎.𝟓 × 𝑪𝒑 × 𝝆 ×𝑨 × 𝒗𝟑) 𝝎 (7) Where; 𝐶𝑝 is the turbine power coefficient, 𝜌 is the air density, 𝜔 is the angular velocity, 𝐴 is the rotor swept area, 𝒗 is the wind velocity. The turbine power is determined as: 𝑃𝑡 = 0.5𝑇𝑡 (8) Glass fibre - epoxy resin was selected for the blade material. The Fiberglass composites are good insulators, and do not react to an electric field. This material allows to achieve light weight blades and the razor-sharp edges allow the blades to cut through the wind, making it relatively silent. Although the material has low tensile strength, it has a low tensile modulus which gives the blades the ability to bend and strain without breaking. The chord length is determined as: 𝑐𝑙= 𝑆 × 𝜋 × 𝑑 𝑁𝑏 (9) Global Journal of Engineering and Technology Advances, 2025, 23(03), 010–020 15 Where; 𝑆 is solidity, 𝑑 is the blade diameter and 𝑁𝑏 is the number of blades The wind load is obtained as follows: 𝑊𝐿=𝑃𝑑 × 𝐴𝑏 (10) Where; 𝑃𝑑 is the dynamic pressure and 𝐴𝑏 is the blade area. 2.2. Shaft design The shaft is subjected to two main forces and torque as a result of the blades rotation due to wind orthogonal to the shaft. The stress which the shaft is subjected in the x and y direction is determined as follows [16]: 𝜎𝑥= 𝑃𝑤 𝐴 (11) 𝜎𝑦= 𝑊 𝐴 (12) Where; 𝐴 is the shaft cross section, 𝑃𝑤 is the wind load on the blade and 𝑊 is the turbine weight The torsion in the shaft is determined as follows: 𝜏= 𝑇 ×𝑟 𝑗 (13) Where; 𝑇 is the torque, 𝑟 is the shaft radius and 𝑗 is the polar moment of inertia The principal stresses 𝜎1 and 𝜎2 are determined as follows [16]: 𝜎1𝜎2=(𝜎𝑥 + 𝜎𝑦 2) ± √(𝜎𝑥− 𝜎𝑦 2)2+ 𝜏2 (14) The Von Mises stress theory is used to determine the factor of safety as follows [16]: 𝜎𝑜𝑣𝑚 = √(𝜎1− 𝜎2)2+(𝜎3− 𝜎2)2+(𝜎1− 𝜎3)2 2 (15) The factor of safety is then determined as [16]: 𝑆𝐹= 𝑆𝑦 𝜎𝑜𝑣𝑚 [16] Where; 𝜎𝑜𝑣𝑚 is the overall stress and 𝑆𝑦 is the yield strength 2.3 Spur Gears Design The formulas applied in the gear system design are given as follows [16]: The tangential tooth load 𝑊𝑇 on the gear is determines as: 𝑊𝑇 = 𝑃 𝑣 × 𝐷 (17) Where; P is the permissible tangential tooth load, 𝑣 is the pitch line velocity and 𝐷 is pitch circle diameter. The Lewis equation is used to determine the tangential tooth load on the pinion and is given as follows: 𝑊𝑇= 𝜎𝑤.𝑏.𝑝𝑐.𝑦=𝜎𝑤.𝑏.𝜋.𝑚.𝑦=(𝜎0.𝐶𝑣).𝑏.𝜋.𝑚 (18) Where; b is width of gear face 𝑝𝑐 is the pitch circle, 𝑚 is the module, 𝑦 is half of the thickness of the tooth; 𝜎0 is allowable static stress and 𝐶𝑣 is the velocity factor. Global Journal of Engineering and Technology Advances, 2025, 23(03), 010–020 16 The gear train dynamic load is determined using the Buckingham equation defines as: 𝑊𝐷= 𝑊𝑇+ 𝑊𝐼= 𝑊𝑇+ 21𝑣(𝑏.𝐶+ 𝑊𝑇) 21𝑣+ √𝑏.𝐶+ 𝑊𝑇 (19) Where, 𝑊𝑇 is the steady load dur to transmitted torque, 𝑊𝐼 is the incremental load dur to dynamic action, 𝑣 is pitch line velocity, 𝑏 is the face width of gears, 𝐶 is deformation dynamic factor The static toot load is determined as follows: 𝑊𝑆= 𝜎𝑒.𝑏.𝑝𝑐.𝑦=𝜎𝑒.𝑏.𝜋.𝑚.𝑦 (20) For safeguard against breakage, 𝑊𝑆 must be greater than 𝑊𝐷 and the maximum wear tooth load 𝑊𝑤 should not be less than the dynamic load. The wear tooth load is defined as: 𝑊𝑤= 𝐷𝑝.𝑏.𝑄.𝐾 (21) Where; 𝐷𝑝 is the pitch circle diameter of the pinion (mm), 𝑄 is ratio factor 𝐾 is the Load stress factor. 3. Results and Analysis Table 1 gives the determined parameters which were determined used in the development of the vertical axis wind turbine. The parameters include solidity, turbine swept area, number of blades, turbine solidity, swept area, tip speed rotation shaft diameter, power, torque shaft factor of safety, generated. Table 1 Design parameters of the vertical axis wind turbine. Table 1 Vertical axis wind turbine parameters Parameter Value Solidity 0.5 Blades 3 Shaft diameter 0.025 m Swept Area 0.78 m2 Tip speed ration 2.25 Code Length 0.5m Torque 44.3 Nm Power 6 919 Wh Factor of Safety 20 The shaft material used is the 50 C 4 grade of carbon steel and the factor of safety of 20 indicates a safe project design which is able to sustain load at higher wind speeds. Table 2 and Table 3 gives the bearing and gearbox specifications of the developed the vertical axis turbine. Global Journal of Engineering and Technology Advances, 2025, 23(03), 010–020 17 Table 2 Bearings specifications Parameter Value Bearing type Angular contact bearing Bore 25mm Outer diameter 47mm Width 12mm Minimum static load 18.6kN Maximum static load 31.9kN Angular contact bearings were selected because they provide greater speed and they are able handle combined radial and axial loads. Table 3 Gearbox Specification Parameter Value Gear type Spur Gear ratio 10: 1 Rotational Speed 175 rpm Shaft orientation Vertical Input horsepower 1.13 Efficiency 0.90 Spur gears application is due to their ability to transmit exact velocity ratios, transmit large power, are associated with high efficiency and are reliable. 3.1. Vertical axis wind turbine Simulation Figure 6. gives the simulation outcome carried out to establish the velocity contour around the blade according to its shape. Figure 6 Characterisation of a single blade velocity contour Figure 7 give the Multiple blade velocity characterisation of the three-blade vertical axis wind turbine. Global Journal of Engineering and Technology Advances, 2025, 23(03), 010–020 18 Figure 7 Multiple blades velocity contour characterisation Table 4 Gives the simulated power achieved under varying wind speed from a minimum speed of 5m/s to a maximum speed of 25m/s. Table 4 Simulated vertical axis wind turbine generated powered under variable wind speed Wind Speed (m/s) Wind speed (km/hr) Power generated (W) 5 18.0 58.5 10 36.0 465.9 15 54.0 1 572.5 20 72.0 3 727.5 25 90.0 7 280.2 Figure 8 gives the simulation carried out to characterise the turbine fracture as a function of force induced by excessive wind velocities. Figure 8 Vertical axis wind turbine fracture Simulation