Addressing VAWT Aerodynamic Challenges as the Key to Unlocking Their Potential in the Wind Energy Sector

Abstract

While the wind turbine industry has been primarily dominated by horizontal-axis wind turbines, the forefront of knowledge of these turbines has revealed significant challenges in various aspects, including manufacturing, structural design, cost, and maintenance. On the other hand, the advantages associated with Darrieus vertical-axis wind turbines (VAWTs) demonstrate significant potential that can address the existing challenges of the wind turbine industry. Current work aims to investigate the practicality of this potential for the wind energy sector. To this end, the benefits of employing Darrieus turbines for domestic and industrial applications, isolated operation, and on/offshore windfarm applications have been explored. It is apparent that Darrieus VAWTs are better suited to a wide range of environments, whether they are deployed in isolation or integrated systems, and whether they are utilized on a small or large scale. Darrieus VAWTs are adaptable to urban unsteady variable wind, are less expensive on large scales, provide higher power density at the windfarm level, and provide stability for offshore platforms. Nevertheless, challenges remain in fully harnessing VAWT potential rooted in their complex aerodynamics. This serves as a primary challenge for VAWTs to address the challenges of the wind turbine industry in line with the 2050 roadmap.

1. Introduction

As an infinitely available, environmentally friendly energy source, renewable energy harvesting has been the world’s target for electricity generation for years [1]. Wind and solar energies are promising power sources that are widely available around the world and govern an important portion of large-scale power generation through renewable energy sources. Yet, power harvesting is limited by efficiency, cost, maintenance, and the available space that wind turbines and solar panels need to be installed. The importance of wind energy harvesting is gaining prominence following the 2050 roadmap. According to the special report of the International Energy Agency in 2021 (net zero by 2050) [2], it is anticipated that two-thirds of the total energy supply should be sourced from wind, solar, bioenergy, geothermal, and hydro energy by 2050. Around 90% of this energy goal should be met through solar and wind power, necessitating a 20-fold increase in solar capacity and an 11-fold increase in wind capacity from 2021 to 2050. Therefore, it is evident that the wind energy industry needs to expedite research and invest in advancing wind turbine designs while being cost-effective.

Based on their applications, wind turbines can be categorized into: isolated small-scale and large-scale, and onshore and offshore for windfarm applications. Each category involves a unique design, depending on turbine size and operating conditions.

Small wind turbines can be considered micro (diameters of 1.5 m (or 0.5–1.25 m) and less with annual energy generation of about 1000 kWh, 0.004–0.25 kW), mini (diameters of 1.5–2.6 m (or 1.25–3 m) and annual energy of 1000–2000 kWh, 0.25–1.4 kW), and domestic scale (diameters of 2.7–9 m (or 3–10 m) and annual energy of 2000–20,000 kWh, 1.4–16 kW) [3,4,5]. The application of the micro type is for low-power purposes, such as battery charging, simple lighting, and remote devices, while the domestic type can be used for residential households, farms, and telecommunications [3]. In urban environments, the turbine will be small and work under low Reynolds numbers in instantaneous winds with a noticeable atmospheric boundary layer.

Higher capacity wind turbines can be considered commercial (diameters of 10–20 m, 16–100 kW), medium (diameters of 20–50 m, 100–1000 kW), and large (diameters of 50–100 m, 1000–3000 kW) [4,5]. As the turbine size and height increase, it is possible to utilize a higher quality wind and work under higher Reynolds numbers, while concerns regarding impacting the environment, structural stress, noise, manufacturing, and maintenance arise. At the windfarm level, the power density and the aerodynamic interaction of wind turbines are found to be important factors determining the overall efficiency of the site. Although offshore sites are dominated by accessible, high-quality wind, wind turbine installation, maintenance, and platform design [6] are found to be challenging, especially with large-scale turbine sizes.

In general, wind turbines are classified into horizontal-axis and vertical-axis wind turbines (HAWTs and VAWTs, respectively); each type has unique aerodynamics, design, and, accordingly, distinctive advantages while suffering from some drawbacks. Among the challenges that have existed in developing wind energy harvesting, HAWTs have found the opportunity to thrive in the wind energy industry, while VAWT commercialization has not found a comparable chance, and their blossoming has been limited to the 1980s in North America and the 2010s in Europe [7]. The hesitant behavior that has arisen during the development of VAWTs occurred despite them having demonstrated better suitability over HAWTs in some environments and from some points of view. Thus, a question arises about whether VAWTs can be revived in the wind turbine industry to accompany HAWTs in addressing the energy challenge at both small and large scales.

Current literature has been trying to acknowledge VAWTs’ suitability for different applications in the wind turbine industry, where further development of HAWTs is presenting significant challenges. However, the potential of VAWTs needs to be discussed in more detail. The objective of the present work is to depict a picture for researchers in the field to help them understand the potential of VAWTs that makes them worthwhile to study and develop in the wind energy industry. The potential of VAWTs in the wind energy sector is explored, while the factors that have limited their wider adoption are discussed. These challenges primarily arise from the unsteady aerodynamics of VAWTs, an issue that, despite being recognized, remains inadequately understood. Thus, as aerodynamics is found to be the most important factor that affects both the efficiency and structural strength of wind turbines, this study will be presented from the perspective of aerodynamics.

2. VAWTs and HAWTs in General

Over time, different types of wind machines have been introduced, and the design of the most successful ones has been under development since then. To provide a general overview of different wind machines’ performance, their efficiency vs. turbine TSR (tip speed ratio) is shown in Figure 1. As seen, HAWTs and Darrieus VAWTs (probably the curved-blade type) are of the highest efficiency, while Savonius VAWTs have the best performance at low TSRs.

To identify the challenges that VAWTs could potentially address, it is essential to begin by comparing the merits and demerits of VAWT and HAWT configurations. Then, it will be possible to determine the specific areas where VAWTs have the potential to be advantageous. The advantages and disadvantages of each type of wind turbine are pointed out in

Table 1. General advantages and disadvantages associated with VAWTs and HAWTs.

Turbine	Advantages	Disadvantages
HAWTs	Higher power generation [10]. Relatively smooth torque [11]. Simpler aerodynamics. Self-starting ability if they are faced with the wind [10,12]. The structural stress caused by bending moments is less than VAWTs due to the correlation between high bending moments and larger cross-sectional areas of the turbine blades [7]. Shorter shaft [13]. Lateral stresses do not affect bearings [11].	Dependency to wind direction. Require yaw mechanism. Decrease in efficiency in urban areas and skewed winds [14,15]. Lower power density in farm design. Higher installation and maintenance costs. Higher rotational speed. Higher noise [10]. Large tower inference [10]. Can not generate power at wind speeds below 6 m/s and above 25 m/s, which is the general cut-out speed [16]. Upscaling issues [12]. Significantly higher number of bearings [17].
VAWTs	Independent of wind direction. Low installation and maintenance costs [18]. Low noise. Simpler blade design. Lower rotational speed. 6% more maximum efficiency than the Betz limit associated with HAWTs [19]. More swept area in a similar occupied area compared to a HAWT rotor [20]. More friendly with avian wildlife [21]. Suitable for urban areas, low-quality winds, and severe wind climates [4,22]. Less susceptible to flow turbulence, which provide more flexibility in integrated layout [23]. Easier access to the generator [13]. Higher power density in farm design [24,25,26,27]. Faster wake recovery [28,29] and lower wake generation [26]. More feasible wake manipulation [23]. The possibility of enhancing the windfarm efficiency through synergistic clusters [23]. The possibility to change the aspect ratio of the turbine according to the condition the turbine needed to be installed [18]. Perform better in skewed flow conditions [30]. Do not have significant issues in upscaling [31]. Can generate power at very low and extreme wind speeds, i.e., between 2 and 65 m/s [16]. Can survive in extreme weather conditions of frost, ice, sand, salt, and humidity [16]. Better stability in floating offshore platform [32].	Lower power generation in general. Higher fluctuation amplitude in torque. Too much unsteady cyclic aerodynamic loads lead to structural failure [7]. Self-starting issues, except for the Savonius types. Complex aerodynamics. Usually, spokes are needed to connect the blades to the shaft. Longer shaft [13]. Heavy load on the bearing. VAWTs have received less research and development attention compared to HAWTs.

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As seen in the table, the advantages of VAWTs are undeniable from practical and environmental viewpoints. Following the wind speed range study of 2–11 m/s in Ref. [33], when HAWTs are facing the wind, VAWTs tend to show lower efficiency at both low and high wind speeds. This difference is particularly significant at higher wind speeds [33]. However, in real-world applications, it was observed that VAWT power generation can surpass HAWT performance by nearly three times within a 24 h period across a wind speed range of 0–24 m/s, with an average wind speed of 2.2 m/s. This advantage is attributed to the intermittency of changing wind directions, which directly impacts the total power generation over the course of a day [33]. Killing birds is one environmental disadvantage of large wind turbines. According to Ref. [34], within a period of 4 years (2014–2018) in South Africa, over 800 endemic birds were killed by colliding with wind turbines. This is more serious for HAWTs as the speed of their blade tip is much larger than VAWTs and they rotate at higher altitudes [21].

Discussion on VAWT specifications will be made in Section 4, where they are categorized based on the application of the turbines.

Due to the distinctive benefits offered by VAWTs, there is an anticipation for their increased prevalence, particularly in the fields of local power generation, urban settings, offshore, and windfarm applications, which include conditions with variable wind in terms of direction and intensity and conditions in which the power density, manufacturing, and maintenance costs are important.

However, attempts to develop large-scale VAWTs failed in the past, with structural failures identified as one of the contributing factors [13]. Therefore, despite the growing interest in VAWTs in recent times, their progress has been limited by their failure to meet the initial expectations. On the other hand, the advantages of HAWTs have led to their widespread development and commercialization, solidifying their position as the predominant leader in the market. It is evident that the distinctive advantages of VAWTs have prevented them from being abandoned during the peak of HAWTs’ popularity. Nonetheless, the persistent challenges related to VAWTs’ unresolved drawbacks have accounted for their underdevelopment. Therefore, it is crucial to learn from past mistakes and address current deficiencies in design, installation, maintenance, and performance. This is essential if VAWTs are to secure an active role in the competitive future energy market [7].

3. Types of VAWTs

To better comprehend the limitations that hinder VAWT development, it is necessary to glance at the various types of VAWTs and their respective specifications. They are categorized into drag-based and lift-based types, named Savonius and Darrieus, respectively.

3.1. Savonius VAWTs

The drag-based VAWTs are recognized as the oldest wind-driven machines and were used in ancient Persia (currently known as Iran) for grain grinding [7]. The Savonius VAWTs were initially adapted for the generation of electricity in 1925 by a Finnish engineer [35]. These types of wind turbines are notably simple to manufacture and do not have self-starting problems [35]. However, they are of low efficiency as the advancing and returning blades generate opposite torques, and the positive/negative torque of the advancing/returning blade reduces/strengthens as the TSR increases. At static conditions, both advancing and returning blades experience the same velocity, while the higher drag coefficient of the advancing blades leads to overall high positive torque generation, which results in the self-start ability of these turbines [36]. As the TSR increases, the output torque continuously declines due to the unfavorable reduction in relative velocity on the advancing blades and its increase on the returning blades. The former causes a reduction in positive torque, and the latter strengthens the negative torque [37]. Thus, provided the TSR is about one or below, the turbines can generate power as the speed of their blades has not exceeded that of the wind. This limits the operational range of the turbines and is the reason why gaining high power is not physically possible for them. Although there are different methods to improve their efficiency by adjusting the direction of incoming flow or changing the blade assembly and shapes, the operational range of Savonius turbines is fairly proven to be from start to TSRs of about one, where they produce their maximum power [38]. Nevertheless, due to the ease of start and manufacturing, Savonius turbines have secured a place in the market where operating at low wind speeds and low rotational speeds are desirable.

3.2. Darrieus VAWTs

The Darrieus turbines were first introduced in the 1920s in France [7]. As these turbines convert lift force into power, it is physically possible for them to rotate at high speeds and thus generate much higher power compared to the Savonius type [12]. However, the power of Darrieus turbines is still lower than that of HAWTs, and they suffer from difficulties in starting and a high fluctuation amplitude of torque in each revolution [39,40]. The conventional blade shapes of Darrieus turbines are straight (H-type, giro-mill, or cyclo-turbine), curved (eggbeater or Troposkein shape or $\varphi$ shape), and helical.

During the 1970s and 1990s, the curved-blade design emerged mostly in North America as the primary model for large-scale Darrieus turbines. The largest one, with a 64 m diameter, could reach the rated power of 3.8 MW at 22 m/s wind speed [7]. However, these turbines faced challenges such as fatigue at the blade joints and lower-than-expected power generation [13]. Additionally, damage to the base bearing caused further operational issues [7]. Consequently, a considerable number of turbines had to be dismantled in 2004 [13]. This failure adversely affected the commercialization of Darrieus VAWTs by resulting in the financial support being directed into the HAWT industry, thus introducing structural issues as the main obstacle to VAWT development rather than their aerodynamic efficiency [13]. Nevertheless, it is clear that a significant portion of the structural failures in VAWTs can be attributed to their prominent variable cyclic aerodynamic loads [7]. Readers interested in the literature on relevant projects involving large-scale VAWTs are referred to Ref. [7], which highlights the following reasons that may account for holding back the commercialization of Darrieus VAWTs: Early structural failure, lack of long-term financial support from government and private companies, and the development of HAWTs in the market.

During a similar timeframe as the introduction of curved-blade turbines, straight-blade turbines were also developing in Europe, and until now, their utilization has continued, unlike the curved-blade type. There appear to be two phases of Darrieus VAWT development larger than 100 kW, one around the 1980s and 1990s and the other during the last decade [7,28]. While in the former phase, the curved-blade turbines were dominant, the latter phase shows a revival for H-rotor turbines [28]. In between, research on VAWTs partially continued, mostly in urban areas and on a small scale with a capacity of less than 10 kW [41].

In regard to efficiency comparison, Figure 2 presents an overview of the performance of two well-known Darrieus VAWTs—a 100 kW two-bladed H-rotor and a 500 kW curved-blade turbine—alongside a typical HAWT [22]. As observed, the performance of VAWTs tends to be lower compared to that of HAWTs. However, VAWTs, particularly H-rotor designs, are capable of reaching peak efficiency at lower TSRs. This characteristic makes them more attractive from several perspectives—environmental, practical, and structural. Operating at lower TSRs reduces noise and mechanical stress, potentially leading to a longer lifespan and less maintenance. Practically, the lower operational speed can simplify the design. Structurally, the reduced stress on the turbine blades and support structure at lower TSR can result in more durable and cost-effective designs.

4. Discussion on Darrieus VAWT Potential

Given the distinguishable advantages of Darrieus VAWTs in different environments, the potential of small and large-scale Darrieus turbines for isolated operation and on/offshore windfarm applications is discussed here.

4.1. At Small Scale

In urban settings, the interaction of the atmospheric boundary layer with buildings, topography, and vegetation makes the inflow conditions for wind turbines highly turbulent, such that the inflow’s inclination and direction experience instantaneous changes [14]. These inflow conditions fairly align with the operating conditions of Darrieus VAWTs. Tall buildings in urban areas can serve as effective turbine towers, allowing for the cost-efficient utilization of high-quality wind resources [42]. The better suitability to skewed flow conditions, which are dominant on building rooves, and the lower nominal rotational speed are also other factors that make VAWTs more appropriate for urban environments. These turbines have also proved to be valuable in capturing untapped energy for lighting purposes from the wake of moving vehicles on highways [43,44]. Integrated VAWTs are also viable for installation in the ridges and corners of buildings to benefit from the augmented airflow around buildings [45]. In this regard, different designs for building integrated and augmented wind turbines have been developed [42]. Darrieus VAWTs are also much appreciated in remote areas without dominant sunlight and steady wind [46]. They are also suitable for powering telecom towers in off-grid areas [47].

According to Ref. [48], for a small-scale VAWT, the levelized cost of energy (LCOE [USD/kWh]) continuously goes down with an increase in annual average wind speed. In addition, the annual operating and maintenance cost (AO) of small VAWTs is predicted to be slightly lower than that of large HAWTs. According to the study conducted based on the wind data of four locations in Oklahoma City, the levelized cost of energy for VAWTs is equal to or less than that of the average national electricity price where annual average wind speeds exceed 4.3 m/s [49].

The integration of small VAWTs and solar systems can be a promising solution for sustainable power generation through combining renewable energy sources. One such example can be found in Ref. [50], in which an efficient hybrid solar–wind–rain eco-roof system was introduced for urban and rural applications. Refs. [51,52,53] also discuss the viability of a hybrid system, which includes the integration of photovoltaic panels with a Darrieus wind turbine. However, in the case of hybrid energy systems in terms of cost of energy, Ref. [54] indicates that using HAWT-based hybrid systems is more cost-effective than that of VAWT-based systems. Ref. [55] suggests a novel, battery-free VAWT-compressed air energy storage reverse osmosis system as a sustainable desalination solution for off-grid environments with a scarcity of water. In this study, the most cost-effective configuration of eleven VAWTs and a pressure exchanger obtained a levelized cost of water of 1.63 USD/m³ and an annual water production of 9400 m³. At an average wind speed of 5 m/s, the normalized daily water production per square meter of turbine-swept area at the study site was 0.19 m³/m²/day.

In water, Darrieus hydrokinetic turbines have proven themselves to be a promising option for local power extraction from rivers, especially in rivers that are shallow (mostly a few meters) and with low flow velocity (about 1 m/s) [56,57]. In these conditions, a large swept area is needed to generate enough power [57], while axial flow turbines (HAWTs operate in water) are not efficient unless they operate in deep water with high flow velocity [58]. In this regard, the aspect ratio of the Darrieus turbine can be changed so the turbine can be fully immersed and will be able to deliver the desired power. In tidal applications in which intermittent forward and backward movements of water current exist, Darrieus turbines stand out due to their ability to maintain the same rotational direction during both forward and backward currents. This allows them to generate power throughout the entire tidal cycle.

In such environments, HAWTs fall behind, emphasizing the advantage of Darrieus VAWTs in such applications. However, their poor ability to start and generate sufficient power is worsened at a small scale due to the low Reynolds number, and this is a disincentive to their promotion. Nevertheless, the advantages of Darrieus VAWTs in local power generation are unique, and by addressing their drawbacks, they can still be a viable candidate in these environments. A recent comparative study evaluated the potential of different micro wind turbines for operating in poor wind quality environments, considering the criteria of self-starting ability, the necessity of a yawing mechanism and gearbox, the design simplicity, power, performance in turbulent conditions, wide operational range, and amount of material required [10]. It was concluded that hybrid Darrieus–Savonius VAWT is the best choice compared to others, including conventional types of Darrieus, Savonius, and HAWT turbines [10]. However, it is evident that a hybrid configuration is not a flawless solution, as the Savonius rotor adversely interferes with turbine power generation after passing the starting phase.

Regardless of the type of wind turbine, it is worth noting that research on small-scale wind turbines is considerably limited compared to their larger, commercial counterparts, in spite of the more complex wind conditions they often face [3]. To name a few in the literature that has investigated VAWT installation in urban areas [4], the effect of building height on the performance of roof-mounted VAWT [59], and shrouded vertical axis wind turbines applicable for urban structures [60]. Some successful installations can be mentioned as the Pearl River Tower in Guangzhou, China [61], Lincoln Financial Field in Philadelphia, PA, USA [62], and Hess Tower in Houston, TX, USA [63]. However, the underperformance of wind turbines can be attributed to insufficient wind resource assessments and the inappropriate selection of wind turbine types [64]. To successfully harness the potential of generating power through the wind in urban areas, it is crucial to have a better understanding of input wind conditions, which requires accurate local wind data measurements for each specific site. These measurements are crucial for making informed decisions regarding turbine type, size, and installation location [3]. In addition, the government should be encouraging the installation of small-scale wind turbines in urban areas, but currently, the existing regulations are strict and, therefore, disincentive in most countries [3].

To provide an overview of the already installed turbines, Ref. [65] investigated more than 200 commercially available small-scale wind turbines from different aspects.

• Almost 50% of the total turbines are of vertical-axis type.
• Taking the average of the minimum, maximum, and mean cut-in speeds across three small-scale turbine sizes, vertical-axis turbines have minimum, maximum, and mean values of 1.1 m/s, 4.6 m/s, and 2.5 m/s, respectively. For horizontal-axis turbines, the corresponding values are 1.8 m/s, 4.6 m/s, and 3.0 m/s, respectively.
• Taking the average of the minimum, maximum, and mean specific capital costs (capital cost divided by maximum electric output) across three small-scale turbine sizes, vertical-axis turbines have minimum, maximum, and mean values of 0.48 EUR/W, 21.88 EUR/W, and 3.93 EUR/W, respectively. For horizontal-axis turbines, the corresponding values are 0.58 EUR/W, 9.48 EUR/W, and 2.49 EUR/W, respectively.

These results suggest that while VAWTs are better suited for small-scale applications due to their unique characteristics, their lower power output leads to inferior performance when considering specific capital costs.

4.2. At Large Scale

Darrieus VAWTs provide several significant advantages on a large scale, and they have the potential to generate cheaper electricity than HAWTs at very large capacities [7]. When it comes to the multi-megawatt scale, concerns about the manufacturing of HAWT blades significantly arise due to variations in bending stress modes and the cubic growth of blade weight accompanying its length increment [18,66]. On top of that, the challenges regarding the transportation of the turbine components cannot be neglected [18]. Furthermore, as the structure’s weight increases, the lateral and longitudinal bending moments exerted on the tower of HAWTs also escalate, particularly intensifying during yawing motion [18].

On the other hand, due to simpler blade design and manufacturing, the associated cost of Darrieus turbines on a multi-megawatt scale can be lower than that of HAWT blades. The blades of VAWT can be produced longer since the gravity-induced bending stress is considerably less than that of HAWT blades [18]. In this regard, Darrieus VAWTs can be the solution to challenges regarding different bending stresses, manufacturing costs, transportation, installation, and maintenance [18]. In addition, as the Reynolds number increases at larger scales, the performance of VAWT improves. However, the inertia and friction of VAWTs due to the heavy load from the shaft are noticeable at megawatt scales [13]. In the case of H-rotor VAWT, the range of 30 MW is found to be the structural upper scale, as at this scale, the effects of gravity on key joints become significant [67]. Due to the constraints associated with upsizing HAWTs, VAWTs could serve as a viable alternative for power capacities exceeding 10 MW, as they encounter fewer technical barriers in scaling up [13]. HAWTs face limitations due to gravitational fatigue, where the blades experience tension-compression cycles during rotor rotation [31].

However, it should be noted that cyclic load fluctuations of VAWTs, as well as the high low-root bending moment, should be considered as critical aspects. According to Ref. [68], a 5 MW 2-bladed VAWT blade can undergo 3.5 times a higher bending moment compared to that of a HAWT. This moment difference becomes more significant as the wind speed increases.

In terms of power generation, VAWTs can exhibit a 41% higher electrical power coefficient on average at 15% lower rated wind speeds with respect to HAWTs with a similar rating or capacity. The average Annual Energy Production (AEP) of VAWTs can be at least 13% higher than that of HAWTs with 2–6 MW capacities [69].

To harness the potential of high wind speeds that are beyond the working range of HAWTs, Ref. [70] suggests a strategy that integrates a VAWT into a HAWT with a common tower at those wind speed conditions. The extra torque of HAWT blades was transferred to the vertical-axis rotor drivetrain at high wind speeds. The mechanical power obtained by the integrated system excelled at both isolated HAWT and VAWT.

4.3. For On/Offshore Windfarm Applications

In windfarm applications, Darrieus VAWTs have proven to have higher power density and higher efficiency if they are placed within a certain desired layout. Due to lower wake generation and faster wake recovery, Darrieus VAWTs can be placed closer to each other compared to HAWTs [26]. The efficiency of each VAWT in a windfarm can surpass that of an isolated turbine due to the positive impact of the turbines on each other when in clusters, e.g., between 1.8% and 24% increment of power coefficient [71,72,73,74,75,76]. In addition, wake manipulation control methods are more feasible to implement in Darrieus VAWTs to avoid the negative effects of wake interaction [23]. According to Ref. [26], the annual energy production of a VAWT windfarm can surpass that of a HAWT farm in case the number of installed turbines exceeds a certain threshold. In this regard, the same area of land with 510 VAWTs could lead to 8.92 TWh/y of total annual energy production, which is 11.2% higher than that of 460 HAWTs with 8.02 TWh/y. The power density of windfarms can also be further increased via the inclusion of VAWTs amongst HAWTs to harness the unused available power and enhance the HAWT’s efficiency through a local increment of wind velocity created by VAWTs [77]. Compared to a cluster with merely HAWTs, co-locating HAWTs and VAWTs resulted in about an 18% (averaged over all wind directions) increase in windfarm power production [78].

The employment of twin rotors in farm design has been shown to be effective in the enhancement of overall efficiency [79,80,81,82,83,84]. The best layout for VAWT windfarm is suggested to be staggered triangular clusters with two counter-down rotating wind turbines upstream and one wind turbine downstream [71]. Readers interested in the farm design of VAWTs are referred to Refs. [85,86,87], for more information. Despite the potential of VAWTs in windfarm applications, obtaining the optimal configuration of VAWT, as well as the optimal layout, is found to be the ongoing goal for utilizing the potential of VAWTs in windfarm settings.

The trend towards large-scale offshore development is on the rise as the system’s cost-effectiveness increases with scale [41]. In offshore applications, the viability of a wind turbine is determined by its size. There are two critical perspectives to consider in this regard: the economic aspect and safety. Wind turbines need to be of substantial size to function effectively from both economic and safety standpoints. The platform of wind turbines should meet the costly stability requirements that need to be considered for all-sized turbines. In addition, small wind turbines are more vulnerable to severe sea conditions and may even be completely submerged in some situations. Therefore, small wind turbines are found to be uneconomical and unsafe for offshore applications [6]. In these conditions, the challenges posed by excessive weight, large dimensions (particularly height), and turbine maintenance become increasingly pronounced for HAWTs [18]. However, it should be noted that the utilization of small wind turbines near offshore sites is a different topic, as size and safety criteria are different in these situations. In this regard, distance to shore is defined as a factor for turbine installation [88]. More information on the properties of the offshore sites can be found in Ref. [88].

Taking into account the advantages that VAWTs offer in floating offshore applications, especially in light of the substantial challenges confronting HAWTs, research has identified VAWTs as a viable alternative that addresses HAWT drawbacks. The low center of gravity and low center of thrust of VAWTs have made them a feasible candidate for floating offshore applications [26,89,90]. Having a more stable structure and negligible gyroscope effects, VAWTs are better suited for offshore applications and large scales [18]. The heavy turbine rotor can be supported through buoyancy, reducing the supporting structure costs [91,92].

Although it is probable the aerodynamic performance of large-scale VAWTs may not exceed HAWTs, the nominal blade cost of a VAWT array is only 2.16% of that of a HAWT when multiple wind turbines are installed on a single platform [6]. However, for a single wind turbine installed on a single platform, the nominal blade cost of a HAWT is lower than that of a VAWT [6]. Furthermore, the VAWT array can reduce the load fluctuation amplitude and increase its frequency, which improves the overall stability of a floating platform [6]. According to Sandia National Laboratories, in 2012, VAWTs have prominent advantages compared to HAWTs when it comes to offshore applications, while the disadvantages of the longer VAWT blades and a lack of reliable braking systems remain challenging [93]. H- or Y-rotor configurations can be the most cost-effective designs [93]. It was believed that a viable economic VAWT for offshore applications should possess the following specifications: (I) molded composite blades that incorporate aerodynamic fairings around all joint structures; (II) using thick NLF (Natural Laminar Flow) airfoil profiles; (III) variable speed with regenerative braking; and (IV) direct-drive power train with vertically mounted, multiple generators [93].

The emergence of large-scale VAWT in offshore floating applications has opened new possibilities for the revival of these wind turbines. Currently, the analysis of their aerodynamics is in the developmental phase [41]. The main floating VAWT projects are (a) NOvel Vertical Axis wind turbine (NOVA) (5–10 MW) [94], (b) DeepWind (5–20 MW) [95], (c) INFLOW (26 MW) [96], (d) H2OCEAN [97], (e) SeaTwirl [98], and (f) X-rotor [32,99,100]. The NOVA project was about V-rotor turbines, and the X-rotor project is undergoing its continuation [32]. This is a hybrid vertical and horizontal axis wind turbine design. The design aims to maximize the swept area and minimize the overturning moment, as well as address the issues of the turbine at low rotational speed using a secondary rotor [32]. The DeepWind project was about Troposkien-shaped turbines, and the main challenge was the blades [32]. The INFLOW and H2OCEAN projects have been concluded and are not moving forward [32]. The SeaTwirl project is about an H-rotor design, which is concluded, and the S2 technology is under construction [32]. Detailed information on the potential and challenges of floating VAWTs for offshore applications can be found in Ref. [32].

Table 2. Main floating VAWT projects, adopted from [32].

Name	Period	Budget	Current Status
NOVA [32,94]	2009–2010	GBP 2.8 M	V-rotor led to an ongoing project on X-rotor.
DeepWind [32,95]	2010–2014	EUR 4.18 M	Concluded with the main challenge being the blades.
INFLOW [32,96]	2011–2015	EUR 21.5 M	Concluded.
H2OCEAN [32,97]	2012–2014	EUR 6.5 M	Concluded.
SeaTwirl [32,98]	2019–2022	EUR 3.5 M	Concluded, and S2 technology is ongoing.
X-rotor [32,99,100]	2021–2023	EUR 3.9 M	Ongoing, in continuation of NOVA.

presents a summary of the mentioned projects, adopted from Ref. [32].

The pronounced advantages of VAWTs at deep-water offshore can be summarized as the: (I) insensitivity of VAWT to wind direction allows for larger rotors, (II) simplicity and accessible drivetrain of VAWT reduce the Operation and Maintenance (O&M) costs (thanks to the Rotor Nacelle Assembly being located at the base of the turbine), (III) lower center of gravity of VAWT decreases the substructure costs (reduces the topside moment of inertia and enhances system stability), (IV) less number of components, and (V) improved wake dynamics, which dissipate closer to the water surface, enabling closer turbine installations and reducing the required maritime area [32,90]. Having utilized VAWTs, the life-cycle cost for an offshore wind project can be mainly reduced in the areas of turbine O&M support structure, electrical infrastructure, and logistics and installation [90]. Furthermore, floating VAWTs have better scalability, while HAWTs face limitations due to gravitational fatigue, where the blades experience tension-compression cycles during rotor rotation [31]. In addition, the progress of HAWTs benefits the development of VAWT since VAWTs can leverage the already established technology for blade manufacturing, generator design, and foundation design for offshore sites [18].

4.4. Closure Discussion

In general, urban environments with high turbulence demand the use of small-scale turbines, while offshore applications exceeding 20 MW in power capacity pose challenges for HAWTs [28]. As the wind energy market expands into various settings, including urban, onshore, and floating offshore windfarms, VAWTs can emerge as compelling and viable alternatives, thanks to their suitability for these environments [28]. However, despite the advantages attributed to Darrieus VAWTs and their potential to gain traction in the wind energy market, their critical aerodynamic and structural shortcomings appear to be the primary hindrance to their successful development [101].

The limited development of Darrieus turbines is primarily due to critical disadvantages such as poor self-starting capability, high torque fluctuations, structural failures, and relatively lower power output. These efficiency and structural challenges are largely attributed to the complex, unsteady aerodynamic behavior of these turbines.

The aerodynamics of Darrieus VAWTs are unique and different from that of non-rotating blades. Various unsteady phenomena occur during the rotation of the turbine blade while it undergoes continuous changes in angle of attack (AOA) and relative velocity. Within each rotation, two cycles of dynamic stall, one on each side of the blade, occur. The circular path of the blade alters the flow structure and, thus, the aerodynamic loads. Additionally, the occurrence of various post-stall phenomena and their complex interactions (including blade wake) further complicates the analysis of aerodynamics. On top of these, the quality of variation in AOA and relative velocity varies at different geometries and operating conditions, which means each geometry and operating condition represents unique flow characteristics. All factors attest to the level of complexity in the aerodynamics of these turbines.

According to Ref. [102], the following reasons are attributed to the drawbacks of Darrieus VAWTs: (A) Start challenges; (a1) deep stall conditions at most azimuth angles; (a2) small/large torque arm length of lift/drag when the lift/drag is high at low/high azimuth angles; (a3) blade low Reynolds number; and (a4) friction caused by the load from the shaft. (B) High fluctuation amplitude of torque; (b1) the nature of non-uniform aerodynamic load of the blade due to instantaneous changes in the relative velocity and AOA; (b2) variable torque arm length of lift and drag due to changes in AOA; (b3) distinctive difference between the torque generated in the upwind and downwind regions; and (b4) sudden changes in aerodynamic loads due to dynamic stall. These factors contribute to structural failure, noise, vibration, and generator complexity. (C) Relatively poor power; (c1) sudden drop in aerodynamic torque as a result of dynamic stall; (c2) reduction/increasing of torque arm length of lift/drag when TSR increases with the aim of weakening dynamic stall; and (c3) the inconspicuous role of downwind region in power generation.

Upon examining the complexity of the VAWT aerodynamics and the root causes of its drawbacks, one can see that fully addressing the drawbacks requires certain insight that considers the sources of each drawback. Such a high-level insight into the flow complexity has not yet been fully grasped [28], making it challenging to accurately predict and analyze VAWT behavior. Furthermore, the issue of different aerodynamics between a real-sized model and its scaled-down counterpart remains unresolved [103], introducing uncertainties in behavior prediction.

Therefore, developing innovative designs and potential solutions for Darrieus VAWTs remains challenging without a thorough understanding of their aerodynamics and key influencing parameters. That being said, it appears that the starting point for revitalizing VAWTs in both local and integrated power generation markets and enhancing the efficiency of wind energy harvesting, ultimately contributing to a healthier environment, should begin with a deeper understanding of their intricate aerodynamics and then devise possible solutions based on the established insight. This insight should also extend to the windfarm level, which plays a pivotal role in the revival of VAWTs. Consequently, addressing the aerodynamic challenges faced by VAWTs and focusing on studying their performance at the farm level will be of paramount importance, thereby facilitating the integration of VAWTs into the future energy market.

Readers who are interested in more details on the complexity of the aerodynamics of Darrieus VAWTs, the root causes of their aerodynamic drawbacks, the effective operational and geometrical parameters, and the aerodynamics of conventional Darrieus VAWTs are referred to Ref. [102].

5. Conclusions

Although VAWTs have been held back from developing a place in the wind energy industry, they have unique potential that has protected them from being renounced as an alternative. These wind turbines have proven themselves to be reasonable choices for many applications in both small and large-scale urban and on/offshore environments in both isolated and integrated systems.

The following conclusions can be drawn from the current work:

• On a small scale, for urban applications and local power generation, VAWTs are found to be a promising option as they have the ability to adjust themselves to changing flow conditions. In water, their ability to generate power within shallow rivers and throughout the whole tidal cycle is outstanding.
• On a large scale, the associated costs of maintenance and manufacturing for VAWT can be significantly lower than those of HAWT. The increase in Reynolds number can also lead to aerodynamic performance improvement compared to small-scale sizes.
• At windfarms, the power density of Darrieus VAWTs is higher. Their aerodynamic performance can benefit from being placed in an array, resulting in performance improvements overall as well as compared to isolated turbines. Integration of VAWTs and HAWTs in a windfarm can also be a solution to further enhance power density.
• At offshore sites, the maintenance convenience associated with VAWTs is more pronounced. Also, VAWTs have much more stability than HAWTs, which means their platform design can be simpler. In addition, integrated VAWTs on one platform can significantly reduce their oscillation and thus improve the stability of the platform.

The potential of Darrieus VAWTs for the future wind energy industry can be seen as promising solutions to some existing challenges in promoting wind energy utilization through efficiency improvement and financial considerations. Yet, the aerodynamic challenges of VAWTs need to be addressed. Thus, the addition of deeper insights into the aerodynamics of VAWTs will help unravel the performance challenges and lead to the development of more widely adopted designs that optimize their current performance potential while ensuring structural safety and economy. This, in turn, paves the way for VAWTs to play a more substantial role in wind energy and contribute to a more environmentally sustainable future.