Variable designs of vertical axis wind turbines—a review

Omnidirectionality and simple design make VAWTs more attractive compared to HAWTs in highly turbulent and harsh operational environments including low wind speed conditions where this technology shines more. However, the performance of VAWTs is lacking compared to HAWTs due to low turbine efficiency at downstream caused by large wake vortices generated by advancing blades in the upstream position. Introducing variable design methods on VAWT provides better adaptability to the various oncoming wind conditions. This paper presents state-of-the-art variable methods for performance enhancement of VAWTs to provide better direction for the wind industry. The variable VAWT design can increase the lift and torque, especially at the downstream regions by managing the blade-to-wake interaction and blade angle of attack (AoA) well, hence contributing to the performance enhancement of VAWTs. In addition, the self-starting capabilities have also been found to improve by employing variable methods with a better angle of attack on the turbine blades. Nevertheless, the complexity of varying mechanisms and structural rigidity are the main challenges in adopting this idea. Yet, it possesses great potential to develop higher-efficiency VAWT systems that can operate in a wide range of wind speeds.

1 Introduction

Vertical axis wind turbines (VAWTs) have gained renewed attention due to global efforts to reduce fossil fuel consumption and combat climate change. Wind turbines are broadly categorized into two types based on their rotational axis: horizontal-axis wind turbines (HAWTs) and vertical axis wind turbines (VAWTs) (Li et al., 2015; Rezaeiha et al., 2018a; Sagharichi et al., 2018; Guo et al., 2019; Hand et al., 2021; McKenna et al., 2022; Zhu et al., 2022). The HAWTs dominate the market (Manwell, 2009; Li et al., 2015; Santamaría et al., 2022) due to their higher efficiency compared to VAWTs (Hand et al., 2021; Peng et al., 2021). The performance limitations of VAWTs arise from turbulent flow characteristics resulting from blade rotation (Zhu et al., 2022), including issues like tip vortices and dynamic stall (Rezaeiha et al., 2018a). Despite these challenges, VAWTs possess several advantages:

(1) Omni-directional capabilities (Li et al., 2015; Rezaeiha et al., 2018a; Liu et al., 2019; Zhao et al., 2022).

(2) Low wind speed capabilities.

(3) Lower manufacturing costs (Zhu et al., 2018a; Zhao et al., 2022).

(4) Lower maintenance as yaw is absent in VAWTs (Li et al., 2015; Zhu et al., 2018a; Zhao et al., 2022).

(5) Closer to energy storage centres, which helps reduce transmission losses (Li et al., 2015).

(6) Lower noise emission (Li et al., 2015; Rezaeiha et al., 2018a; Liu et al., 2019; Battisti, 2021; Zhao et al., 2022).

(7) Requires small operational area (Li et al., 2015; Rezaeiha et al., 2018a; Zhao et al., 2022).

These advantages make VAWTs a viable option, particularly in confined and turbulent spaces such as urban environments (Hand et al., 2021) and areas with significant wake-to-wake turbine interactions (Whittlesey et al., 2010; Kinzel et al., 2012). The simplistic design of VAWT offers better adaptability to changing wind conditions, providing increased reliability (Belabes and Paraschivoiu, 2021; Hand et al., 2021). They are considered less intrusive in urban environments compared to HAWTs, contributing to their growing popularity (Hand et al., 2021). Recent research efforts, such as studies (Islam et al., 2008a; Bianchini et al., 2015; Rezaeiha et al., 2017; Guo et al., 2019; Su et al., 2021; Sun et al., 2021; Zhang and Qu, 2021; Azadani and Saleh, 2022), indicate a renewed focus on optimizing the aerodynamic efficiency of VAWTs for improved power harnessing, despite being overshadowed by HAWTs in the past decade.

VAWTs can be classified into two main types: lift-based turbines, such as Darrieus VAWTs, and drag-based turbines, including Savonius rotors, cup-type turbines, and crossflow turbines. Darrieus VAWTs feature aerofoil blades suspended at a distance from the shaft, generating lift through the interaction between the incoming wind and the aerofoils. Whereas the Savonius turbines, with concave vanes resembling the letter “S,” rely on drag forces to rotate and are often chosen for low-wind applications due to their self-starting capabilities (Manwell, 2009; Mathew and Philip, 2012; Bouzaher and Guerira, 2022; Zhao et al., 2022).

As shown in Figure 1, improvements in VAWT power output can be achieved through aerodynamic enhancements. Parametric and geometric studies, such as adjusting pitch angle (Fiedler and Tullis, 2009; Chen and Kuo, 2012; Guo et al., 2019), blade profile (Chen and Kuo, 2012; Danao et al., 2012; Roh and Kang, 2013; Mitchell et al., 2021; Ung et al., 2022; Abdalkarem et al., 2023), chord-to-radius ratio (Strickland et al., 1979; Rainbird et al., 2015; Bianchini et al., 2016), number of blades (Li et al., 2015; Qa et al., 2016; Rezaeiha et al., 2018b; Sarath Kumar et al., 2020), and rotor height (Brusca et al., 2014), contribute to optimizing turbine performance. Additionally, power augmentation methods involve incorporating additional components like deflectors, shrouds, guide vanes, diffusers, and concentrators to accelerate the incoming wind and enhance power generation (Wong et al., 2017; Wong et al., 2018a; Wong et al., 2018b; Wang et al., 2020; Wang et al., 2022).

Figure 1

Figure 1. VAWT performance studies.

The next approach involves varying design parameters based on the oncoming wind speed. Variable design in VAWTs refers to a method where a parameter or geometry of the turbine changes during operation to enhance performance. This adaptive approach allows the turbine to adjust its parameters for optimal efficiency, expanding the range of freestream velocities it can capture. Various variable design features have been implemented in VAWTs, including variable pitch (Kirke and Lazauskas, 2011; Sheng et al., 2013; Elkhoury et al., 2015; Zhang et al., 2015; Rezaeiha et al., 2017; Manfrida and Talluri, 2020; Aboelezz et al., 2022), variable gurney flaps at the trailing edge (Hao et al., 2017; Liu et al., 2022a), active flow control at the airfoil’s leading edge (Rezaeiha et al., 2019; Sun and Huang, 2021; Liu et al., 2022b), and variable oscillating flap blades (Xiao et al., 2013). These variable features can be categorized into passive and active controls (Li et al., 2015; Santamaría et al., 2022), where passive flow control involves mechanisms that vary parameters without a motor or actuator, while active control employs additional input power, such as a motor (Burton et al., 2001). The implementation of variable features in VAWTs has demonstrated an increase in the measured output coefficient of power ($C_P$) by mitigating stall effects on the blades at specific azimuthal angles.

Review works have been done for VAWT parametric (Islam et al., 2008a; El-Samanoudy et al., 2010; Aslam Bhutta et al., 2012; Hand et al., 2021; Zhu et al., 2022) and power augmentation study (Ouro et al., 2018; Bianchini et al., 2019; Bontempo et al., 2021; Villeneuve et al., 2021). However, there is a notable absence of review work specifically addressing the variable design of VAWTs. A variable operation involves chosen parameters or shapes constantly adapting to the oncoming wind while completing a single rotation. Therefore, this paper aims to provide a comprehensive review and evaluation of various variable designs for VAWTs from an aerodynamic perspective. To commence, Section 2 outlines the fundamental principles and governing equations of a VAWT. In Section 3, a detailed analysis of different variable systems employed in VAWTs is presented, highlighting their advantages, disadvantages, and other relevant findings. Section 4 discusses the future outlook of variable designs for VAWTs, and the paper concludes with Section 5.

2 Principles of VAWT

The governing principles for all lift-type VAWTs are consistent (Zhao et al., 2022). The upstream region consists of Quadrants 1 and 2, while the downstream region is referred to as quadrants 3 and 4 (Figure 2). Figure 2A illustrates a simple two-bladed H-Darrieus VAWT, and the forces form the basis of VAWT mechanics. As VAWTs are dependent on the freestream velocity ($U_{\infty }$), the resultant velocity ($W$) of the aerofoil can be expressed through the summation of vectors normal to the blade, $\overrightarrow{V_N}$ and along the chord line of the blade, $\overrightarrow{V_C}$ acting on the blade in Eq. 1 (Manwell JGMaALR, 2009; Mohamed, 2012; De Tavernier et al., 2019; Mohammed et al., 2019):

$\overrightarrow{W}=\overrightarrow{V_N}+\overrightarrow{V_C}(1)$

Figure 2

Figure 2. Principles of VAWT (A) Free-body diagram of a 2-bladed H-Darrieus turbine. (B) Aerodynamics of a single aerofoil.

$\overrightarrow{V_N}$ and $\overrightarrow{V_C}$ are expressed in Eqs 2, 3 (McKenna et al., 2022; Manwell JGMaALR, 2009; Mohammed et al., 2019). Here, $R$ represents the rotor radius, $\omega$ is the angular velocity of the turbine and $U_i$ is the axial flow velocity. Velocity components acting on the blade change at every azimuthal position throughout a full cycle. Therefore, to account for the varying magnitudes of velocity components, the azimuthal position of the blade, θ, is taken into consideration. 5 is the ratio between relative flow velocity of the turbine to the freestream velocity (Mohammed et al., 2019)

$\overrightarrow{V_C}=R\omega +U_i\mathit{Cos}\theta (2)$

$\overrightarrow{V_N}=U_i\mathit{Sin}\theta (3)$

As illustrated in Figure 2, $\overrightarrow{V_C}$ is the sum of two vectors, the blade tip speed and the velocity acting on the chord line of the blade profile. $U_i$ can be expressed in Eq. 4 as $U_{\infty }$ is the freestream wind velocity and $a$ is the induction factor giving consideration of the deceleration of $U_{\infty }$ passing through the turbine (Manwell, 2009).

$U_i=U_{\infty }\left(1-a\right)(4)$

$\frac{W}{U_{\infty }}=\sqrt{{\left(\left(1-a\right)\mathit{Sin}\theta \right)}^2+{\left(\frac{R\omega }{U_{\infty }}+\left(1-a\right)\mathit{Cos}\theta \right)}^2}(5)$

Tip speed ratio (TSR, $\lambda$) is expressed in Eq. 6 (Islam et al., 2008b; Manwell, 2009; Carrigan et al., 2012; Mohammed et al., 2019; Zhao et al., 2022). It is the ratio comparing the velocity tip speed of the blade in relative to the incoming wind velocity.

$\lambda =\frac{R\omega }{U_{\infty }}(6)$

Angle of attack, $\alpha$ and is expressed through Eq. 7 changes with every azimuthal angle. As shown in Figure 2B the changes of the angle of attack of an aerofoil in a cycle.

$\alpha ={\mathit{tan}}^{-1}\left[\frac{\left(1-a\right)\mathit{Sin}\theta }{\lambda +\left(1-a\right)\mathit{Cos}\theta }\right]-\beta (7)$

To measure the performance of any VAWT, the coefficient of moment (C_Q) and power (C_P) can be calculated and used as parameters to assess efficiency. Eq. 8 describes the moment coefficient where $Q$ represents moment, $\rho$ is the density of air or fluid, $A$ is the aspect ratio as expressed in Eq. 10. Area of rotor is the product of the turbine’s rotor diameter ($D$) and height ($H$) in Eq. 10.

$C_Q=\frac{Q}{0.5\rho A{U}_{\infty }^2}(8)$

Similarly, power coefficient is represented in Eq. 9. $P$ represents power.

$C_P=\frac{P}{0.5\rho A{U}_{\infty }^3}(9)$

$A=DH(10)$

Solidity, σ, is define through Eq. 11, where N is the number of blades, c represents the blade chord length, and D is the turbine rotor diameter (Manwell, 2009; Carrigan et al., 2012; Li et al., 2015; Hassan et al., 2016; Sagharichi et al., 2018; Wang et al., 2021; Zhao et al., 2022). This equation characterizes how tightly dense the rotors are when interacting with the oncoming wind (Rezaeiha et al., 2018b; Zhao et al., 2022). In cases where the turbine has high solidity, less freestream wind can pass through the rotor, making the turbine more solid, and vice versa.

$\sigma =\frac{Nc}{D}(11)$

Solidity plays a crucial role in VAWT performance, influencing wake-to-wake interaction, self-starting capability, and optimal operational TSR (Sagharichi et al., 2018). In contrast, low-solidity turbines are preferable in high-wind conditions for increased C_P.

3 Variable design

This section comprehensively reviews variable designs for VAWT applications, encompassing H-Darrieus and Savonius turbines. Each subchapter provides a detailed review, including variable control mechanisms. Figure 3 illustrates the anatomy of variable designs in VAWTs.

Figure 3

Figure 3. Anatomy of variable VAWT designs.

3.1 Variable pitch (VP)

Studies on fixed-pitch vertical axis wind turbines (FP-VAWTs) have indicated an increase in power yield (Bianchini et al., 2015; Rezaeiha et al., 2017). Rezaeiha et al. (Rezaeiha et al., 2017), investigated a NACA 0018 3-bladed H-turbine and observed a 6% increase in C_P and up to 2% increase in C_Q with a pitch angle (β) of −2°. Zhang et al. (Zhang L. et al., 2021) reported a 2.88% increase in power compared to a turbine with $\beta$ = 0°. The introduction of pitch angles led to spikes in lift and tangential forces in quadrant 2, contributing to an overall increase in generated power. Studies by (Rezaeiha et al., 2017; Elsakka, 2020) concluded similarly on the impact of pitch angle on a turbine with a single blade across a cycle. Pitched aerofoils can also serve as a preventive measure against structural failure under high mechanical fluctuation loadings at strong wind speeds (Islam et al., 2008a). Additionally, simulated results showed a 26.3% reduction in resonance on the shaft compared to experimental findings (Zhang L. et al., 2021).

variable pitch (VP) motions consist of sinusoidal (Figure 4A) and cycloidal (Figure 4B) methods (Zhao et al., 2018; Zhao et al., 2022). In the sinusoidal VP method, the blades’ pitch angle varies in the shape of a sinusoidal function across azimuthal angles (Pawsey, 2002; Paraschivoiu et al., 2009). This motion help keep the blades below stall conditions at critical azimuthal regions, particularly towards the downwind direction. Although this variation improves lift forces by maintaining local angle of attack (AoA) below stall angles, the sudden change in pitch angle has the potential to induce mechanical fatigue loading, reducing the operational life of the turbine (Zhao et al., 2018). Despite these potential disadvantages, research on VP-VAWT has demonstrated significant improvements, with a 30% increase in power generation compare to an FP-VAWT (Paraschivoiu et al., 2009). Esfahani et al. (2015) also show increase propulsion force using an elliptical motion trajectory base on a sinusoidal motion on a single aerofoil.

Figure 4

Figure 4. Variable pitch systems: (A) Sinusoidal, (B) Cycloidal variable motion (Zhao et al., 2018), (C) newly proposed variable pitch motion (Zhao et al., 2018) and (D) the comparison of C_P between the variable Trochoidal motion turbine to fixed pitch. (Zouzou et al., 2019).

The cycloidal method depicted in Figure 4B was based on the Voith-Schneider system (Camporeale and Magi, 2000), commonly applied in hydro-turbines or vertical axis tidal turbines (VATT). Performance studies for cycloidal actuation have shown a higher power yield than a fixed turbine. Hwang et al. (2009) reported a 25% increase in turbine performance with an active control cycloidal design. However, the sinusoidal motion generally yields higher performance than the cycloidal motion. Zhao et al., 2018 proposed a new variable pitch method that allowed the turbine to remain at maximum power output for an extended duration, this resulted an 18.9% increase in power at a TSR of 4.5. This method outperformed an FP-VAWT, with a 50% surged in C_P (Figure 4C). A similar approach by Manfrida and Talluri (Manfrida and Talluri, 2020) introduced a dynamic pitching law, did not yield significant improvement compared to the sinusoidal VP motion.

Innovative control methods, such as the variable pitch trochoidal motion proposed by (Xu et al., 2019a; Zouzou et al., 2019), aimed to enhance variable pitch designs. The trochoidal motion positions the blades at 0° pitch angles at 90° and 270° azimuthal positions, lead to significant improvements in C_P. Experimental results indicated an operational range for this turbine at lower TSRs between 0.4 and 0.7, similar to the operating range of a Savonius turbine in low wind conditions (Figure 4D). Despite a 30% reduction in C_P, attributed to mechanical inefficiencies during experiments, the VP H-VAWT in this study outperformed the Savonius turbine in low wind conditions. Elkhoury et al. (2015) also reported mechanical losses contributing to lower power generation at higher TSRs.

Eq. 12 proposed by Xu et al. (2019b) led to a 78.6% increase in maximum power compared to a fixed-pitch turbine. In wind tunnel tests, the optimised control outperformed the sinusoidal variable pitch method by 45.5% in C_Pmax during the wind tunnel test. This equation, linked pitch angle to azimuthal blade angle, allows researchers to determine the optimal blade pitch at various azimuthal positions.

$\beta ={\displaystyle \sum _j^3}\left({\displaystyle \sum _k^3}{a}_{jk}{\lambda }^{k-1}\mathit{Sin}\left(j\theta \right)\right)(12)$

Where $\beta$ is blade pitch; $a_{jk}$ ($j=1,2,3andk=1,2,3$) are fitted coefficients; $\lambda$ is tip speed ratio; $\theta$ is azimuthal angle measured in radians.

Several researchers (Paraschivoiu et al., 2009; Sheng et al., 2013; Jakubowski et al., 2017; Zhao et al., 2018; Manfrida and Talluri, 2020; Zhang L. et al., 2021; Guevara et al., 2021) have propose various mathematical approaches to optimize pitch angles. Numerical tools such as artificial neural networks (ANN) (Abdalrahman et al., 2017) and genetic algorithm (GA) (Li et al., 2018; De Tavernier et al., 2019) are used for optimising variable pitch settings. However, these approaches are only simulated results, and experimental validation is yet to be conducted (Li et al., 2018). Furthermore, the wind energy industry has not yet standardize to a singular equation that relates blade pitch angle, AoA, and azimuthal angle.

The benefits of VP control include delaying dynamic stall development by enlarging the leading-edge vortex (LEV) with optimal AoA, resulting in increased lift. This approach mitigates vortex shedding, reducing blade wake-to-wake interactions and drag forces (Zouzou et al., 2019; Rainone et al., 2023). Variable pitch designs also exhibit better self-starting capabilities (Erickson et al., 2012; Elkhoury et al., 2015; Zhang et al., 2015; Sagharichi et al., 2016; Sagharichi et al., 2018; Sagharichi et al., 2019; Guevara et al., 2021), widening the operational range of the turbine (Erickson et al., 2012). Additionally, VP control proves useful for preventing damage from operation beyond the recommended wind working range through aerodynamic braking (Paraschivoiu et al., 2009; Kirke and Lazauskas, 2011; Lazauskas and Kirke, 2012; Jain and Abhishek, 2016; Sagharichi et al., 2016). However, the developmental cost may outweigh the advantages (Paraschivoiu et al., 2009). Simulation studies indicate that VP-Vertical Axis Wind Turbines (VAWTs) may lose omnidirectional abilities, especially when the wind comes from the advancing side. The addition of a yawing mechanism may suggests its limitation (Jain and Abhishek, 2016). Performance declines noticeably if the wind comes from the advancing side, and a yawing mechanism was suggested to be added. Despite promising performance yields, mechanical control systems remain a significant challenge for market readiness. Recommendations to install sensors on each blade to enhance turbine efficiency (Staelens et al., 2003).

Figure 5 illustrates various control mechanisms for variable pitch, employing rotational disks or cams mounted on the shaft to guide linkage bars. The design by (Kiwata et al., 2010) using a 4-bar linkage shows insignificant improvement in self-starting capabilities for the FP-VAWT but achieve higher rotational velocity. Figure 5G depicts a collective pitch control system with a rubber belt linking the blades, controlled by a servo motor. Experimental data evidence a 20%–30% lower output than simulation results, attributes to mechanical and aerodynamic losses such as friction and vortex shedding (Zhang et al., 2015). Long-term fatigue stresses from abrupt dynamic loading during pitch changes could be a potential issue over time (Zhao et al., 2018). Both active and passive control methods contribute to the total weight of the turbine, and an increase in moving parts introduces points of failure and friction. Findings to skin friction to be influential in limiting the turbine’s predicted performance (Xu et al., 2019b).

Figure 5

Figure 5. Design of variable pitch control mechanisms by (A) Sagharichi, et al. (Sagharichi et al., 2018) (B) Kiwata et al. (Kiwata et al., 2010) (C) Jain and Abhishek (Jain and Abhishek, 2016) (D) Benedict, et al. (Benedict et al., 2013) (E) Erickson, et all (Erickson et al., 2012). (F) Xu, et al. (Xu et al., 2019a; Xu et al., 2019b). (G) Zhang et al. (Zhang et al., 2015).

The implementation of VP-VAWT, requires careful consideration of factors such as wind speed, control methods, and mechanisms. For high TSRs, sinusoidal, cycloidal, or collective pitch motions have been applied effectively in high-wind areas. In contrast, the trochoidal system has shown effectiveness at low TSRs (Zouzou et al., 2019). The 4-barred linkage mechanism, illustrated in Figures 5A–G, is a commonly employed method. Less popular approaches, like the collective pitch system shown in Figure 5G, may be less suitable for practical applications. For active control, a servo-controlled system is an option, but it comes with the drawback of consuming power, reducing the net power generated, especially in low-wind conditions. In contrast, a passive mechanical control might be more suitable for low-wind scenarios, as it involves fewer moving parts, leading to reduce maintenance issues. Active control methods are generally better suited for high-wind conditions. While most studies have relied on simulations and control experiments, there is a need for real-world testing to gain a deeper understanding of this system. Testing in open environments with actual wind scenarios could provide valuable insights. Exploring the application of VP motion beyond H-VAWT to wind farming applications is a potential avenue for further development. Establishing a standard equation or control for determining optimal pitch variations could be beneficial for VAWT designers (MacPhee D. and Beyene A., 2016). However, the commercial development of a reliable and affordable variable pitch system remains a challenge due to the complexity of the control mechanism (Zhu, 2007). Table 1 provides a summary of VP-VAWT methods.

Table 1

Table 1. Summary of VP-VAWT methods.

3.2 Adaptive flap

Inducing lift on an aerofoil can be achieved by adding flaps, a method commonly used in fixed-wing aircraft. Trailing edge (TE) flaps on the suction side had been experimentally and numerically confirmed to improve lift by 10% on a glider (Meyer et al., 2007), preventing recirculation of flow towards the leading edge and delaying early flow separation (Meyer et al., 2007; Wang and Schlüter, 2012; Bechert et al., 1997; D and Singh, 2016; Schluter, 2010; Kernstine et al., 2008; Bramesfeld and Maughmer, 2002). This section explores the different approaches to implementing variable adaptive flaps for VAWT.

In a numerical parametric study, Liu et al. (2019) utilized a static TE flap positioned on the upper surface of an aerofoil to alleviate flow separation (Figure 6A). At a flap angle of 23°, the flow separation region decreased by 12%, and the separation point was postponed to 0.407c from 0.425c. This resulted in the elimination of recirculation flow at the TE. Free flap pitching showed that moments on the flap initially increased and then decreased with increasing AoA.

Figure 6

Figure 6. Adaptive flap studies on the (A) trailing edge adaptive flap motion (adapted from Liu et al. (2019), (B) variations of flap CM at different flap position angles, (C) flap moment operational control and (D) streamline velocity contour of flap angles of 0$°$, 14$°$, 23$°$, 32$°$ at AoA = 14$°$ (Hao and Li, 2020).

Shown in Figure 6B, Hao et al. (Hao et al., 2019; Hao and Li, 2020) introduced an adaptive flap method with a self-adjustable flap angle based on the blade’s (AoA). In Figure 6B, the moment of the flap increased as flap angle reach its maximum. The blade’s AoA rose to 20°, indicating significant boundary layer separation. Figure 6C illustrates the operational conditions of the adaptive flap method, and Figure 6D shows velocity streamline plots at different flap angles. Optimal flap angles delayed separation and reduced wak e interactions between blades. The spring control system was explored, but potential mechanical failures and long-term fatigue loadings were identified as concerns. Eq. 13 outlines the linear torque control approach, establishing a direct relationship between the coefficient of moment of the flap ($C_{Mf}$) and pitch angle (${\beta }_{Flap}$) (Hao et al., 2019; Hao and Li, 2020). Additionally, Eq. 14 is defined for a leading edge flap (Hao et al., 2019).

$C_{Mf}=0.00006*{\beta }_{Flap}-0.0001(13)$

$C_{Mf}=0.00005*{\beta }_{Flap}-0.0003(14)$

As pressure at the bottom surface of the flap remains constant, pressure on the upper side decreases slowly, preventing flow separation from moving toward the LE. When ${\beta }_{Flap}$ is less than optimal, a higher-pressure difference is found at the end of the flap. Conversely, when ${\beta }_{Flap}$ is higher than optimal, the pressure difference acts inversely in the negative direction for the second half of the flap length. When ${\beta }_{Flap}$ is at the optimal angle, pressure difference at the end of the flap is equal to zero. Therefore, the authors propose the composite torque method in Eq. 15 (Hao and Li, 2020).

$C_{Mf\_control}=C_{Mf\_back}-C_{Mf\_front}(15)$

Where $C_{Mf\_control}$ is the difference between the coefficient of moment of back half of the flap $C_{Mf\_back}$ and front half of the flap. When the flap is below the optimal angle, $C_{Mf\_control}$ is positive; when $C_{Mf\_control}$ is negative, it means the flap is beyond optimal. Eq. 15 adjusts the flap angle, with Eq. 14 offering consistency but limiting the lift-to-drag ratio. Simulations show reduction in vortex shedding with flaps, lowering aerodynamic turbulence. Contrasting aerofoils with and without flaps for VAWT, C_P surged by 54% and 12.8% at low (λ = 0.6) and high (λ = 1.2) TSRs respectively. The flap is most active at 90° ≤ θ ≤ 180°. At low TSR, $\theta$ = 120$°$, the flap is found to be ineffective; at high TSR, the flap angle increase to mitigate the recirculation. Beyond 150$°$-180$°$, the flap fails to delay flow separation due to changing freestream velocity, reducing lift forces beyond optimal (Hao et al., 2019; Hao and Li, 2020). Liu et al. (Liu et al., 2019) simulated improvements in power yield by 24.2% (λ = 1.3) and 23.7% (λ = 1.4).

In a study investigating flap length, chordwise position, and the number of flaps, Hao et al., 2019 differentiated between medium BL separation (14°≤ AoA ≤16°) and large BL separation (AoA = 20°). For medium BL separation, the flap’s chordwise position was optimal towards the TE. Conversely, at large BL separation, the flap position should be closer towards the leading edge (LE) for effectiveness, resulted in a 41% increase in the C_L (Hao et al., 2019). The flap length needed to be carefully selected to prevent flow recirculation spillage, as excessive backflow from the TE to the LE can lead to early flow separation. Additionally, an overly long flap can be considered wasteful in manufacturing and may require more force and power to move.

The double flap configuration significantly outperformed the single flap, showing a 44% and 9.3% improvement at an AoA of 20° and 18° respectively. This configuration addressed issues seen in the single flap setup, reducing the formation of turbulent eddies at high AoAs. An added advantage was the lower noise emission of the adaptive flap system (Liu et al., 2019).

Figure 7A illustrates the second category of adaptive flap where the trailing edge moves independently to the main aerofoil body. This concept is introduced by Xiao et al. (Xiao et al., 2013) in VATT application and holds potential for performance enhancement in hydrofoil turbines (Zhou et al., 2021). Simulation work discovers about 28% improvement in turbine’s performance. In Figure 7A, w represents the slot width and $\gamma$ means the slot angle. This idea aims to mitigate BL separation and TE vortex (Xiao et al., 2013). The oscillating motion is describe in Eq. 16 (Xiao et al., 2013):

${\omega }_f={\beta }_0\sin \left(n\omega t+\frac{\pi }{2}\right)(16)$

Figure 7

Figure 7. Other innovative methods of employing an adaptive flap design like (A) oscillating variable flap (Xiao et al., 2013), (B) L-shaped moveable GF-flap (Motta et al., 2016) and (C) movable GF with C-dimple (Liu et al., 2022a).

Where ${\omega }_f$ is the angular velocity of oscillating flap (rad/s); ${\beta }_0$ is flap amplitude ($°$); $\omega$ is the angular velocity of the turbine, $t$ is the time, $n$ is a non-dimensional ratio between the turbine revolution ($T_{turb}$). The revolution of the oscillating flap ($T_{flap}$) defined in Eq. 17 (Xiao et al., 2013).

$n=\frac{T_{turb}}{T_{flap}}(17)$

The variable flap configuration with γ = 60°, w = 0.03c, and chordwise position (L) 0.7c demonstrated a 28% increase in maximum power yield, reducing dynamic stall and vortex shedding at the wake by 43%. A parametric study by (Xiao et al., 2013) revealed that the slot angle (γ) significantly influenced flow separation, delaying dynamic stall at high TSR and widening the operational range. Torque performance increased notably between 100° < θ < 240° at γ = 60°, with a fixed flap angle improving torque at 180°. Oscillation amplitude (${\beta }_0$) of 15° enhanced torque and power outputs by 12.98% and 23.72%, disrupting TE vortices. Oscillation frequency had the least impact on torque performance (Xiao et al., 2013). This variable flap configuration showed benefits in improving lift forces, delaying dynamic stall, and providing self-starting abilities to the turbine. Similar conclusions were reached by Zhou et al., 2021.

The discussed variable flap designs show promise for VAWT applications, but their implementation was relatively new and lacked real-world testing. Recommendations included exploring double-flap configurations with individual control and addressing challenges in control systems and sensor integration. Passive-control designs needed attention to spring stiffness and damping. Designs by (Xiao et al., 2013; Liu et al., 2019; Zhou et al., 2021) can mitigate flow separation but fell short of typical VAWT performance. Advancements in control systems and physics, building upon Eqs 13–15, were needed for industry application readiness. Long-term benefits were yet to be confirmed.

3.3 Gurney flap control

A gurney flap (GF) is a short flap added at the end of the TE of an aerofoil, to be position on either the pressure side (Giguere et al., 1995; Myose et al., 1996; Li et al., 2002; Lee, 2009; Zhang et al., 2009; Motta et al., 2016; Bianchini et al., 2019) or the suction side (Bianchini et al., 2019; Ni et al., 2019; Li et al., 2021). There are also configurations with double GFs on both sides (Zhu et al., 2018b; Bianchini et al., 2019; Zhu et al., 2019). Early studies focus on GFs apply directly to the aerofoil (Giguere et al., 1995; Myose et al., 1996; Li et al., 2002; Lee, 2009; Zhang et al., 2009) with applications in racing vehicles demonstrating performance improvement (Liebeck, 1978). The GF is typically oriented perpendicular to the chordline, although some instances feature angled GFs (Shukla and Kaviti, 2017; Masdari et al., 2020). Liebeck (Liebeck, 1978) conducted the first study on GFs, revealing significant lift improvement with a slight increase in drag when the flap length. Fixed GFs have been applied to VAWTs for performance enhancement (Chen et al., 2020; Masdari et al., 2020; Chakroun and Bangga, 2021; Li et al., 2021; Ni et al., 2021; Syawitri et al., 2022). On the pressure side, GFs slow down flows, increasing pressure and creating a separation eddy before the flap leading edge, leading to higher pressure difference at the TE and increased lift force (Liebeck, 1978; Ismail and Vijayaraghavan, 2015). In some instances, a combination of a variable gurney flap and a TE adaptive flap design was used to enhance the aerodynamic performance of a rotary aircraft and serve as a load control (Figure 7B) (Motta et al., 2016).

Hybrid designs combining GFs and dimples have shown the best performance improvement for VAWTs. Studies preferred NACA 0012 and 0015 profiles, finding that NACA 0018 and 0021 fell short in achieving maximum lift. In a study (Zhu et al., 2019), GF-dimples on the suction side (outboard) of the blade at λ = 3.1 increased maximum $C_P$ by 17.92% outperforming inboard GF-dimples (5.8% rise) and configurations with GF-dimples on both sides (2.7% rise). Average torque output improve by 24.2% at the same TSR using a 3-bladed, NACA 0021, H-VAWT with a solidity ratio (σ) of 0.25 (Zhu et al., 2018a). The success of the GF-dimple combination lies in increasing pressure from the GF and reducing drag from the dimple, suitable for various TSR conditions and effective in protecting the turbine from gusts (Shukla and Kaviti, 2017). Zhu et al. (2019) found that outboard Gurney flap-dimple configurations improve turbine torque at low $\lambda$ and high solidity conditions. However, its suitability for H-Darrieus turbines is limited due to a drastic torque decline at high λ in the upwind section (Liu et al., 2022a).

In Figure 7C, a variable c-dimple-GF significantly enhanced power production by up to 37.5% (Liu et al., 2022a). The actuated c-GF demonstrated higher performance and effective flow separation mitigation, particularly at θ = 60° during high TSRs. Early activation of the flap can hinder self-starting capabilities and C_P production, though the starting azimuthal angle has minor influence. The trade-off with this variable system was the persistence of vortex shedding like with a fixed GF, as boundary layer separation is delayed. Liu et al. (Liu et al., 2022a), proposed an actuation motor to move the GF, requiring sensors for activation, but the associated costs and weight impact on the total turbine have not been investigated. Comparatively, a fixed GF with a dimple cavity cut-out was simpler and more cost-effective (Zhu et al., 2019; Li et al., 2021), and a direct comparison between the two methods was yet to be conducted.

Liu et al. (2022a) proposed implementing a variable gurney flap using a retractable flap with an actuator, similar to those on aircraft wings. The actuator needed to be small enough to fit within the aerofoil, and for low wind speed conditions, a passive control with a contracting spring can be considered. However, the impact on overall power generation and the need for rapid movement in low TSR turbines should be carefully evaluated. It's essential to note that only simulated results had been presented, and further experiments to be recommended to validate these findings.

3.4 Deforming aerofoil

Morphing blades made from flexible materials, such as rubber (Tavallaeinejad et al., 2022), silicon (Lachenal et al., 2013; Pohl and Riemenschneider, 2022), offers a passive alternative to complex variable pitch controls (Zhu, 2007; MacPhee D. and Beyene A., 2016). These blades respond to changing aerodynamic forces by flexing or deforming, improving lift and delaying boundary layer separation (Lachenal et al., 2013; Wolff et al., 2014a). The technology, demonstrated in simplified models and studies on single aerofoils, has shown promising applications in HAWTs (Hoogedoorn et al., 2010; Wang et al., 2011; Wang et al., 2012; MacPhee and Beyene, 2013; Capuzzi et al., 2014; Mo et al., 2015; Dai et al., 2017; Pavese et al., 2017; MacPhee and Beyene, 2019; Cavens et al., 2020). It can be controlled through active (Daynes and Weaver, 2012a; Daynes and Weaver, 2012b; Wang et al., 2012; Wang et al., 2019; Leonczuk Minetto and Paraschivoiu, 2020; Pohl and Riemenschneider, 2022; Tong et al., 2023) or passive (Castelli et al., 2013; MacPhee and Beyene, 2013; Liu and Xiao, 2015; MacPhee DW. and Beyene A., 2016; Wang et al., 2016; MacPhee and Beyene, 2019; Cavens et al., 2020; Roy et al., 2021) methods by actuators and flexible materials.

A simplified model of a deformable TE using a foil was studied experimentally and numerically, showing enhanced thrust generation (Zhu, 2007). The flat plate formed a curvature, resembling cambering effects. Excessive flexibility negatively impacted turbine performance, but with the right amount of feathering, flow separation was effectively managed (Zhu, 2007; Liu and Xiao, 2015). However, the turbine’s performance was limited due to chaotic flow characteristics downstream caused by flexing and twisting along the blade span (Liu and Xiao, 2015; Bouzaher et al., 2016). At optimal settings, C_P increased by 8% (Liu and Xiao, 2015).

Macphee and Beyene (MacPhee D. and Beyene A., 2016; MacPhee DW. and Beyene A., 2016) improved H-Darrieus turbine performance by 10% and reduced blade oscillation loads by 8.3% in a simulated study with a passively morphing VAWT. Notable enhancements occurred at azimuthal positions of 10°, 130°, and 250°, attributed to decreased vortex shedding and dynamic stalling. The morphing design also exhibited self-starting capabilities. The maximum C_P increase of 9.6% was achieved with a Young’s Modulus of 0.5 MPa, while raising the modulus led to decreased performance. However, when Young’s Modulus (E) was raised turbine’s performance started decreasing. This aligns with the observation in (Wolff et al., 2014a) suggested that the flexibility of the blade was correlated to the blade’s dimensions as mentioned in (MacPhee DW. and Beyene A., 2016) emphasizing the correlation between blade flexibility and dimensions [146]. The passive pitch control system, governed by Eq. 18, showed the blade pitch angle ($\left.{\beta }_P\right)$ correlated to (E) (MacPhee DW. and Beyene A., 2016).

${\beta }_P=-\mathit{arctan}\left(\frac{d_{2,tail}}{c+d_{1,tail}}\right)(18)$

Where, $d_{i,tail}$ represents the TE displacement in the $i$ th direction positive (downward) or negative (upward) flexure of the TE (MacPhee DW. and Beyene A., 2016). By (Roy et al., 2021), performed simulations and experimental investigations revealed a 90% increase in turbine performance and a 26.67% expansion of the operational range with flexible blades. The flexible blades lead to an enhanced lift-to-drag ratio by efficiently mitigating stalls at critical azimuthal positions. Moreover, there is a 6.88% decrease in normal forces acting on the aerofoil, contributing to an extended operational lifespan. The use of silicon rubbers for aerofoil tests, with specified material properties ($\rho$ = 1,050 kg/m³, Poisson ratio ν = 0.45, and E = 0.4 MPa), is illustrated in Figure 8A.

Figure 8

Figure 8. Describes the experimental results of a single morphing blade for VAWT application using a (A) silicon rubber morphing blade (Roy et al., 2021). (B) Flexible blade motion at different azimuthal angles (Bouzaher et al., 2016). (C) Cambering effects of flexible blade compared to a rigid one (Bouzaher and Hadid, 2017). (D) Coefficient of moment and (E) power plotted against the blade’s azimuthal angle of a cycle (Bouzaher and Hadid, 2017).

Bouzaher et al. (Bouzaher et al., 2016; Bouzaher and Hadid, 2017) investigated the effects of flexions at the LE and TE of an aerofoil for VAWT and VATT applications, employing active control of flexible pitch. The study considered coefficients of moment, power, lift, and drag with varying oscillating frequency ($zi$) and amplitude ($a_0$). The group found that with a flexible blade torque and lift coefficient increased from 0.25 to 0.4 and 1.83 to 3, respectively compared to a non-flexible one. The absence of vortex shedding at the trailing and leading edges, attributed to flexible TE, contributed to elevated lift forces. This widened the pressure difference between upper and lower surfaces, reducing far TE vortices and lowering wake disturbances. Similar findings were reported by (Huang and Wu, 2013; Wang et al., 2019; Leonczuk Minetto and Paraschivoiu, 2020). Natural camber adjusted to changing local AoA during operation, resulting in improved aerodynamic performance and torque outputs (Figure 8B). Figure 8C illustrated a substantial increase in C_Q with external blade deformation, enhancing flow reattachment and reducing stall effects (Bouzaher et al., 2016).

From the parametric studies (Bouzaher et al., 2016; Bouzaher and Hadid, 2017), proposed an implementation of $zi$ = 2 and $a_0/c$ = 0.3 or 0.5 as value were very close. The optimal setting was found to be $zi$ = 2. At lower TSRs, $zi$ = 2 and 3 yielded higher efficiency, while at higher TSRs, zi = 4 performed better with a 38.2% increase in performance compared to the static bladed turbine (Figures 8D,E). At this flapping frequency, it performed within a wider range of TSRs, but power input to the system also increased with rising TSR. Beyond that, when $zi$ = 8, there was a 50% drop in C_P compared to the rigid turbine. In dealing with oscillating amplitude effects, C_P was at 25% in the upward trend when $a_0/c$ is at 0.3 and 0.5. With this parameter, an unforeseen delay in LE vortex expansion in the second quarter was observed after the blade had pass stall conditions which was not ideal. This delay was caused by the lower flexing curvature of the blades; a higher flexing curvature might have initiated enlargement earlier. Another, huge downside was that at $a_0/c$ = 0.5, it consumed 5%–10% more input power than $a_0/c$ was 0.3 and 0.1.

Wang et al. (2019) enhanced blade $C_L$, $L/D$ and delayed dynamic stall effects through active blade morphing and increased cambering (Figure 9A). Deformation ratio to blade chord length determined the amount of morphing, with significant lift enhancement observed as the deformation amplitude increased (Tong and Wang, 2021). Polydimethyl-siloxane membrane (PDMS), a rigid material, was used to avoid passive bending in the experimental and numerical study of a single NACA 0012 blade (Wang et al., 2019; Tong and Wang, 2021) (Figure 9B). A synergistic smart morphing aerofoil proposed by (Pankonien et al., 2013; Pankonien et al., 2014a; Pankonien et al., 2014b; Pankonien et al., 2016) integrated with a flexible trailing edge, was investigated for H-VAWT application (Tan and Paraschivoiu, 2017; Leonczuk Minetto and Paraschivoiu, 2020) (Figure 9C). Actively controlled by a flexure box, only quadrant 2 was morphed inwards while the trailing edge remained neutral. This inward cambering at quadrant 2 enhanced C_P by 46.2%, reduced dynamic stall effects, and limited wake vortex shedding (Leonczuk Minetto and Paraschivoiu, 2020). Study by (Tan and Paraschivoiu, 2017) concluded that this morphing method outperformed a VP design. In contrast, a study of a fixed VAWT by Elkhoury et al. (Elkhoury et al., 2015) found that a cambered aerofoil had little impact on turbine performance, with a symmetric aerofoil producing better output. To address this, implementing variable cambering, and leading edge camber, as used to mitigate dynamic stall effects, could be a potential solution (Kerho, 2007).

Figure 9

Figure 9. Mechanical design of the synergistic smart morphing aerofoil (A) Schematic diagram of blade variation (Wang et al., 2019; Tong and Wang, 2021). (B) mechanism of active control morphing blade (Wang et al., 2019). (C) Mechanism of synergistic smart morphing aerofoil (Pankonien et al., 2014a).

Active morphing VAWTs, exemplified in (Bouzaher and Hadid, 2017), achieved a 35% power gain through LEV amplification (Bouzaher et al., 2016; Bouzaher and Hadid, 2017). Application examples of adaptable aerofoil morphing methods can be found in HAWT (Daynes and Weaver, 2012a; Daynes and Weaver, 2012b; Tripp et al., 2018; Cavens et al., 2020; Pohl and Riemenschneider, 2022), aerospace (Bornengo et al., 2005; Fichera et al., 2019) and motorsports (Bornengo et al., 2005). Material properties significantly influenced flapping motion, with the starting positions on the aerofoil having minimal impact (Bouzaher et al., 2016; Bouzaher and Hadid, 2017). Active morphing VAWT designs discussed in (Pankonien et al., 2014a; Wang et al., 2019) may not be suitable for small to medium-sized or low-wind VAWTs, potentially negating morphing aerofoil advantages (Wolff et al., 2014a; Wolff and Seume, 2016). Incorporated morphing systems at specific span sections, seen in (Wang et al., 2012) or adopting configurations suitable for large morphing VAWTs, as suggested by Baghdadi et al., 2020 could offer alternative solutions. Hoke et al., 2015 achieved a 37.3% thrust improvement by combining sinusoidal VP motion with a deforming aerofoil, broke down and delayed LEV formation to enhance suction forces on the blade.

In conclusion, actively morphing aerofoils is more suitable for large VAWTs and HAWTs. Integrated morphing flat plates in VAWT designs also show potential applications (Tian et al., 2014; Tavallaeinejad et al., 2022). However, potential downsides include wear and tear on joints and the complexity of actuation mechanisms. Both actively and passively morphing blades require flexible materials with periodic loading. Joining flexible and solid materials can be complex, requiring special adhesives and potentially impacting VAWT sustainability. The recyclability of flexible materials is crucial for environmental friendliness. Ultraviolet (UV) rays and heat from the sun can reduce the life expectancy of flexible materials by exciting the surface molecules, leading to the breaking of chemical bonds and the deterioration of the material’s properties (Awaja et al., 2011; Nasri et al., 2022). Despite these challenges, there are opportunities for this design to enhance VAWT performance. Exploring the internal structure for passive morphing VAWT and investigating other flow energy harvesters with morphing capabilities are potential avenues for improvement. Table 2 provides a summary of deforming aerofoils.

Table 2

Table 2. Summary of deforming aerofoils.

3.5 Movable mass blocks

A passively variable movable mass block for VAWT was suggested by Zhang et al. (Zhang LJ. et al., 2021) to reduce shaft radial loadings and enhance the shaft’s lifespan. The design relies on centrifugal forces of mass blocks to counteract radial loads on the rotating shaft, with a mass block placed on each strut. Independent movement of the mass blocks along the arm is facilitated by a support rod (Figure 10A). Eq. 19 allows the prediction of the required mass and traveling displacement of the mass blocks at specific azimuthal angles.

$F_{ri}=M_i\times {\omega }^2\times x_i(19)$

where $F_{ri}$ is centrifugal force produced by the $i$ th mass block, the mass of the mass block ($M_i$) multiplies with turbine angular velocity ($\omega$) and linear displacement of the block from the shaft ($x_i$) (Figure 10B). Figure 10C illustrates the motion of each mass block and its centrifugal forces in a complete cycle. During operation, the mass blocks move linearly along the strut, compensating for forces pulling in opposing directions in a predictable pattern. This study determined a control method by investigating the mass and displacement of the blocks.

Figure 10

Figure 10. Shows the movable mass block (A) design mechanism, (B) kinematic diagram of the movable mass blocks system and (C) centrifugal forces of MassBlocks across different azimuthal angles (Zhang LJ. et al., 2021).

CFD simulations indicated a 52.4% reduction in radial loading, leading to a 51% decrease in total shaft deformation. Increasing mass block weight to 0.075 kg resulted in only 1% shaft damage, with radial loading dropping by 97.1%. The optimal mass for each block was determined to be 0.065 kg (Zhang LJ. et al., 2021).

Since introducing mass blocks, the turbine’s performance remains uncompromised. However, an unforeseen issue arises with the turbine’s self-starting capabilities and start-up wind velocity significantly impacted due to added weight, necessitating a larger moment of inertia for rotation. Consequently, the turbine experiences an extended start-up duration. Zhang et al. (Zhang LJ. et al., 2021) propose a solution by calibrating mass block displacement along the arms base on a displacement increment equation to increase the moment of inertia from 0.018 kgm² to 0.036 kgm², this shorten the start-up duration by 40.6%.

While this variable method shows promise in reducing radial loadings and extending the turbine shaft’s lifespan, concerns arise about the durability of the struts. Higher loads may lead to bending over time, particularly affecting the struts along the track where the mass blocks oscillate (Zhang LJ. et al., 2021). Wear and tear on the struts could become a future issue as materials degrade, impacting bending deformation at the arms.

Notably, medium to large-scale H-Darrieus turbines typically have two supporting arms to the blade, unlike the single arm used in this study. Further studies can explore the robustness of the equations. Additionally, the control method for the wind turbine is yet to be explored, and its impacts on the shaft parameters in a final design should be carefully considered.

3.6 Synthetic jet

Rezaeiha et al. (Rezaeiha et al., 2019) proposed an active control suction slot to be placed at the LE of the aerofoil in chordwise to enhance aerodynamic performance by delaying the flow separation with smaller laminar separation bubble (LSB). It prevented turbulent flow and boundary layer detachment. An increased in C_L and a minimization of the C_D were observed on the blade. Thus, delaying dynamic stalls forming at higher AoAs. At 50$°\le \theta \le$ 160$°$ showed clear aerodynamic advantages. Flow separation was pushed down the blade chordwise towards the TE. Significantly smaller vortex eddies at $\theta$ beyond 100$°$ where usually large drag, flow separation and dynamic stalls were often present. LSB notably reduced and suppressed dynamic stalls.

Optimal settings increased C_P by 247% at a TSR of 2.5 with suction slots at 8.5% of the chord length. TSR, turbulence intensity, and Reynold’s number were influential in governing slot positions. Moving slots further down chordwise to 13.5%, 18.5%, and 28.5% resulted in decreasing power yield by 208%, 119%, and 81%, respectively. Slots moving nearer to the trailing edge (TE) shifted the suction effect away from the leading edge, leading to a greater development of dynamic stall (Rezaeiha et al., 2019).

Sun and Huang (Sun and Huang, 2021) confirmed similar findings and extended their study to multi-suction slots on NACA-021 series aerofoils for an H-Darrieus turbine. Tested different maximum thickness positions relative to a single suction slot and observed that as the maximum thickness of the aerofoil moved towards the TE, the optimum positions for the suction slots also moved down away from the LE. Similar findings in (Yen and Ahmed, 2013), indicating that synthetic jet actuation had little influence on pressure distribution, particularly in the early stages of suction development along the LE when flow separation had not occurred. Using lift and drag ratios as measured outputs (Sun and Huang, 2021), proposed a double suction slot along the chord at positions of 10% and 30% for low TSR turbines and, for high TSR turbines, positions at 30% and 50% of the aerofoil chordwise.

An earlier research report published by the National Advisory Committee for Aeronautics (NACA) concluded positively on the addition of a backward-opening slot to blow air from a single static aerofoil, showing improved maximum lift, reduced drag forces, and better control on stall effects (Knight and Bamber, 1929). Liu et al. (2022b) applied similar boundary layer control techniques and introduced a blow-suction synergy (BSS) method to address dynamic stall issues at lower TSRs. The study focused on optimizing the LE suction slots and jet coefficient (Cμ), determining optimal settings through an orthogonal experimental design method ($C_{\mu }$ = 0.01 and suction slot at 0.2c). Compared to fixed VAWT, three control strategies were explored, leading to a 166% increase in maximum lift and a postponed stall angle at θ = 14°–24°. The BSS method effectively balanced sucking and blowing effects, addressing vortex development risks and reducing blade wake interactions. Energy production improved from low to medium TSR (Liu et al., 2022b).

The BSS also enhanced turbine stability, decreased negative tangential forces and provided smaller vortex shedding with higher wake dissipation at the TE in upstream regions (Figure 11B). The proposed control strategy involved intermittent blowing and suction, with suction at the inner side and blowing at the outer side between 0° and 180° azimuthal angles, and the opposite configuration (suction at the outer side and blowing at the inner side) from 180° to 360° azimuthal range. Although no working models or experiments had been conducted, the suggested includes placing air pumps in the blades with valves at the slots to control fluid flow, emphasizing intermittent control for reduced power consumption (Liu et al., 2022b).

Figure 11

Figure 11. Comparison between (A) conventional to (B) upward parabola control functions (Zhu et al., 2018a).

Zhu et al. (Zhu et al., 2018a), explored suction slot numbers, jet momentum coefficients, and control strategies (Figure 12A). Optimal power yield improvement was found with two suction orifices at the TE. Contrary to the previous studies (Rezaeiha et al., 2019; Liu et al., 2022b). This study demonstrated that double trailing-edge slots, upward-parabola control strategy, and a jet coefficient of 0.035 achieved a 15.2% increase in power coefficient (Figure 11). The relationship between orifice slots and C_P was suggested to be inversely proportional. Conventional methods (Figure 11A) faced issues with uneven loading fluctuations, but alternative controls, particularly the upward parabola (Figure 11B), proved efficient with an 80.10% reduction in actuator energy consumption. Synthetic jet flow control delayed flow separation and dynamic stalls, improving pressure distribution, lowering blade-wake interactions, and reducing trailing edge vortices. The paper proposed a piezoelectric synthetic jet actuator (Figure 12B) (Liu et al., 2022b).

Figure 12

Figure 12. Two different types of synthetic jet mechanism. (A) Design parameters of aerofoil for multi-slot study and sketch of synthetic jet actuator (Zhu et al., 2018a). (B) Blow-suction synergy method (BSS) (Liu et al., 2022b).

Concerns also arise about the total weight of the turbine, especially the blades. For instance, in (Liu et al., 2022b), enclosing pumps within the blade could substantially increase the overall blade mass. As discussed earlier in variable methods, mass is critical, impacting self-starting capabilities and reducing initial advantages. Furthermore, more blade energy is necessary for the turbine’s self-starting. The cost implications from design to fabrication of such a turbine must be thoroughly evaluated.

3.7 Variable swept area

The expandable Savonius, akin to morphing aerofoil VAWTs, uses rotating concave plates to alter its sweep area (Bouzaher, 2022; Bouzaher and Guerira, 2022). In 2D CFD studies by Bouzaher et al. (Bouzaher, 2022; Bouzaher and Guerira, 2022), torque increase by 90.6% at maximum expansion. Figures 13A,B illustrates the design with three segments per concaved blade, connected by hinges, moving elliptically. As the advancing blade expands, the concave curvature increases, accelerating fluid flow and improving self-starting capabilities. With a constant 1 m diameter, improvements range from 6% to 112% across deformation amplitudes (Bouzaher, 2022).

Figure 13

Figure 13. The different variable swept area that are currently being studied on. (A) Elliptical motion (Bouzaher and Guerira, 2022) (B) Mechanical design of expandable Savonius design (Bouzaher and Guerira, 2022). and (C) the principles of a deformable Savonius turbine (Sobczak et al., 2020) and (D) Variable swept area VAWT (Pietrykowski et al., 2023).

Marinić-Kragić et al. (Marinić-Kragić et al., 2019) achieved a 26% improvement (C_P = 0.28) with a flexible system at TSR = 0.9. In contrast to prior systems (Bouzaher, 2022; Bouzaher and Guerira, 2022), this study used an elastic material, allowing vanes to deform and create expansion and contraction. Plastic reinforced with glass fibers was employed for flexibility, possessing mechanical properties of E = 30 GPa, ν = 0.28, and ρ = 1,600 kg/m³. Vortex magnitudes remained constant, but C_P increased due to a widened pressure difference at θ = 90°, where maximum deformation and power generation occurred. The material’s mechanical properties were the main limiting factor, and a rotational speed modulator was proposed to maintain safe operational conditions beyond the material’s stress capacity.

Another similar concept of deformable variable Savonius investigated by (Sobczak et al., 2020; Marchewka et al., 2021), enhancing turbine power performance by 90% compared to a fixed Savonius (Figure 13C). CFD results indicated maximum deformation between θ of 45° and 180°, with the highest C_P recorded at θ = 105° and an eccentricity of 15%. A maximum $C_P$ was recorded at $\theta$ = 105 $°$ with an eccentricity of 15%. Like the previous study, increased pressure distribution resulted from the expansion of the advancing vane and contraction of the retreating vane. Flow characteristics remained relatively stagnant (Marinić-Kragić et al., 2019; Sobczak et al., 2020; Marchewka et al., 2021). Although, patented (Obidowski et al., 2019), experiments are pending.

For Darrieus VAWT, improvements were observed at low TSR conditions (Fadil et al., 2020; Abdolahifar et al., 2023; Pietrykowski et al., 2023; Abdolahifar et al., 2023) found a drop in performance at low and high TSRs (0.69≤ λ ≤ 1.5) with a V-shaped blade turbine, attributing it to violent non-uniform vortex separation due to the aerofoil shape. Helical and H-VAWTs exhibited 42% and 54.4% higher torque, respectively, than the V-shaped blade with twist. However, Pietrykowski et al. (Pietrykowski et al., 2023) reported improved power generation with a variable swept area design (Figure 13D), showed an optimum C_P = 0.08 at low TSR (0.3–0.4). The swept area increased at lower wind speeds (0.785 m²) and decreased at higher speeds (0.285 m²) with θ at 120° and 30°, respectively. Operational TSR ranged from 0.1 to 0.6.

Manufacturing complexities are the main downside. If costs are reasonable and materials durable, there is a potential for commercialisation. Further studies on control can expedite implementations. In comparison, the reviewed expandable Savonius design is currently more suitable for low wind conditions.

4 Analysis of variable designs

The review summarizes various designs in Table 3, comparing them based on C_P increase. The complexity is defined by the number of moving parts provided at the readers’ convenience. However, to gauge the true productivity potential of these variable methods, a cost-benefit analysis will be conducted from production to end-of-life. Realistic manufacturing processes are excluded due to the absence of attainable data on market preferences. Nevertheless, this analysis provides engineers with a sufficient holistic overview and understanding of the developmental stages of these variable designs.

Table 3

Table 3. Summary analysis of variable methods.

5 Future perspective

The review underscores performance improvements in VAWTs through variable designs, but commercialization awaits additional evidence. Future research directions in variable methods for VAWTs are outlined in the conclusion.

• Numerical studies lack practical implementation details, omitting factors like mechanical losses and real-world challenges. Future research, involving wind tunnel experiments and real-world implementation, is necessary to address the limitations of variable VAWTs comprehensively.

• Variable mechanism complexity will be highly influential for a variable method to realise commercialisation characterising the ease of implementation, manufacturing, maintenance and cost effectiveness. It will be crucial for low wind turbines.

• Deforming aerofoil is a potential viable method as it requires no added mechanical control system for passive control designs. However, material exploration should be conducted to determine the right material strengths and properties to be use for VAWT. UV damage and material recyclability should be included in designing process.

• As for other variable methods too, should investigate alternative materials that can be lighter, higher stress tolerances and sustainable to be replaced.

• VP-VAWTs can be an effective method for turbine storm protection. More studies can be done to investigate the efficiency passive aerodynamic braking with this method. Integration of this variable method can be investigated for large VAWTs.

• Although, variable solidity by (Huang et al., 2023; Tong et al., 2023) found to have good potential due to its simplicity and had proven, but more evidence is required to deeper understand this variable method.

• Combination of variable methods can be investigated. A passively deformable aerofoil coupled with a variable solidity can be seen as a good combination to optimise flow of the turbine whilst extracting the most energy from low to high wind speeds.

• Investigations to merry flow augmentation methods like (Wong et al., 2017; Wong et al., 2018a; Wong et al., 2018b; Wang et al., 2022) with variable designs to enhance power generation especially at low wind speed regions.

• Field testing and validation will give engineers a deeper understanding to the actual performance values when turbines are subjected to varying wind speeds and weather conditions. Many unforeseen weaknesses can be rooted out during this process that may influence final costings and sustainability.

• The use of artificial intelligence can be implemented to shorten the duration of studies in improving and optimising the variable design factors like pitch control (Abdalrahman et al., 2017), design of a Savonius VAWT (Teksin et al., 2022) and pattern trends of blade designs on VAWT (Noman et al., 2022).

6 Conclusion

The increasing demand for VAWTs in the wind energy market suggests significant potential for variable designs to enhance turbine performance and reduce aerodynamic losses by minimising stall effects and size of vortex eddies. The following outlines conclusions drawn from a review of methodologies in prior state-of-the-art studies.

(1) VP-turbines hold strong commercialization potential due to progressing from numerical studies to mechanical designs. It demonstrates self-starting capabilities, significant aerodynamic improvements, and increased power generation. There’s potential for use as a storm protection system, preventing the turbine from operating beyond its capabilities. However, a downside is the potential for drastic and intrusive pitch changes during intervals, which may damage the turbine. Performance improved between 20% and 50%. Active and passive control shown great advantages with minimal differences. As for passive controls, sinusoidal control method had shown to outperform other systems.

(2) Adaptive flaps break down trailing edge vortices, reducing wake disturbances and improving power yield in the downwind section. While common in aviation, this approach is relatively new in VAWT technology. Active control may be more effective for this variable method, with multiple flaps towards the blade’s trailing edge proving more effective than a single flap. Flap angle operational range should be between 20$°$ and 50$°$ depending on the AoA. The maximum recorded improvement was mentioned to be 54%.

(3) GF control enhances pressure distribution at the trailing edge by slowing incoming flow velocity, creating separation bubbles that reattach the flow. Additional modifications, such as inward dimples, can be combined with GFs. A small sensor initiates the motor and varies the GF, making a smaller flap a cost-effective method due to the reduced force needed to change the GF’s angle. GF-dimple places at the outboard improved more by 5.8% than inboard dimples. Early initiation of GF-dimples will inhibit self-starting capabilities.

(4) A deformable aerofoil offers promising passive control for small, low wind speed turbines. The use of elastic material reduces moving parts, increasing aerodynamic stall mitigation in investigations. The properties of the elastic material play a crucial role in the turbine’s performance, flexibility, weight, durability, and potential costs. Performance improvement was seen to be ranged between 9.6% and 90% with E = 0.5 MPa and 0.4 MPa. Active control deformation also seen an increase by 35% however, the significance from complexity of mechanism to power the control system and maintenance have not been explored.

(5) Moveable mass blocks, controlled by centrifugal forces, reduce shaft fatigue to only 1% damage, extending the turbine’s operational life, without aerodynamic improvements. The mass of the blocks used were 0.065 g on each of the turbine strut.

(6) The synthetic jet, with suction and blowing slots, prevents flow separation, delays dynamic stall, and enhances turbine performance. LE edge slots positioned around 8.5% of the chord had proven to have positive impact. TE slots had an opposite effect reducing performance up to 208% recorded. Control system development is ongoing.

(7) Expandable Savonius turbines, using elastic materials and segmented vanes for adjustable operation, hold promise for boosting power generation in low wind speed conditions. The working mechanics resemble those of a deformable aerofoil.

Author contributions

K-YL: Formal Analysis, Investigation, Writing–original draft. AC: Conceptualization, Methodology, Supervision, Writing–review and editing. J-HN: Supervision, Writing–review and editing. K-HW: Conceptualization, Funding acquisition, Project administration, Supervision, Writing–review and editing.

Funding

The author(s) declare that financial support was received for the research, authorship, and/or publication of this article. This research was funded by the Ministry of Higher Education Malaysia through the Fundamental Research Grant Scheme (FRGS), grant number FRGS/1/2020/TK0/USMC/02/3.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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Nomenclature