Review on the technical perspectives and commercial viability of vertical axis wind turbines

Ocean Engineering 182 (2019) 608–626

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Review

Review on the technical perspectives and commercial viability of vertical axis wind turbines Jing Liu a, *, Htet Lin a, Jun Zhang b a b

Energy Research Institute @ NTU, Nanyang Technological University, Singapore College of Aerospace Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, 20016, China

A B S T R A C T

A review of existing technical challenges and their potential solutions in the development of Vertical Axis Wind Turbines (VAWTs) is presented in the context. Over the last decade, VAWTs have received increasing attention from researchers and scientists due to their inherent features such as omni-directional property, simplification of the blade geometry, lower noise generated, and easy accessibility attributed to the ground-based generators. However, their major drawbacks such as lower power coefficient, poor self-starting capability, and fatigue issues due to cyclic aerodynamics loads, make them less competitive than Horizontal Axis Wind Turbines (HAWTs). This paper will share a new perspective on the state of the art VAWTs to explore the new opportunity and possibility for their commercial viability. A comprehensive comparison was performed between VAWTs and HAWTs focused in different sections on their aerodynamic performance, efficiency, power density in a wind farm, and self-starting ability. The objective of the review paper is to identify technical barriers, design challenges and the future of commercial VAWTs in the wind turbine market, currently dominated by conventional HAWTs. The paper will address what the major technical barriers for VAWTs to compete against HAWTs in future. Moreover, the review will discuss if the innovative technologies proposed in the last decade are commercially feasible to overcome drawbacks of VAWTs. The paper will also explore if VAWTs are more competitive in offshore application through the lessons learned from onshore turbines installed.

1. Introduction It is well known that the wind energy market has been consolidated in the technologies of turbine design and operation. Wind energy has received growing interest and has been in the upward trend of wind power getting more penetration to the grids. Global Wind Energy Council (GWEC) has released in the report that global installed total wind power capacity is 539,123 MW by 2017, and including 3.5% offshore wind power (GWEC, 2017). China, U.S. and Germany are the top 3 countries contributed 62% of the global installed wind power. The tariff price of wind energy has reached below US$ 0.02/KWh in some countries. About 6% of the country’s electricity is delivered from the wind turbine fleet in U.S. and Canada. Uruguay’s electricity is all 100% from renewable energy while 35–40% is from wind energy. Wind turbines are the commonly used devices to harness wind en­ ergy. They convert kinetic energy from the wind into mechanical energy which rotates generators to deliver electricity. There are primarily two types of turbine according to the orientation of the rotating axis: Hori­ zontal Axis Wind Turbines (HAWTs) and Vertical Axis Wind Turbines (VAWTs). The rotary axis of HAWTs is parallel to the wind direction while that of VAWTs is perpendicular to the wind direction. Although both commercialized HAWTs and VAWTs are available in the market,

HAWTs are more prevailing in the market of both onshore and offshore applications over the past 30 years due to their mature technology and promising performance and they have a long record of operational success in the history. Small sized kilowatts VAWT manufacturers take €nger, only one-quarter market portion of HAWT ones (Pitteloud and Gsa 2017). They seem to stagnate in the development and market share in generating business revenue. Megawatt-scale VAWTs are rarely notice­ able in the current market. On the contrary, the size of HAWTs has grown larger recently, for example, Vestas turbines (2 MW–4.2 MW), Siemens (2.3 MW–4.3 MW), and GE turbine (4.8 MW). Lengths of tur­ bine blades have been manufactured longer and larger as shown in Fig. 1. The converged design and technologies of the HAWT rotors have been established to be three-bladed, upwind type with yaw mechanism, and pitch control system, thanks to decades of their development and extensive research. The growing turbine size plays a critical role in the reduction of levelized cost of electricity (LCOE), but meanwhile brings more chal­ lenges in construction and operation and maintenance (O&M). As the blades are designed longer and larger, more challenges arise due to significant weight of the top rotor and heavy mechanical and electrical system housed inside the nacelle of HAWT. Their aerodynamic perfor­ mance is scalable as derived from aerodynamic theory. However, dramatically increased mass of the topside rotor deteriorates scaling-up

* Corresponding author. E-mail address: [email protected] (J. Liu). https://doi.org/10.1016/j.oceaneng.2019.04.086 Received 24 October 2018; Received in revised form 11 March 2019; Accepted 26 April 2019 Available online 10 May 2019 0029-8018/© 2019 Elsevier Ltd. All rights reserved.

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Ocean Engineering 182 (2019) 608–626

Abbreviations AOAs BEM CFD CIR COG COR CR DMS DOE DOF DTU FAWT FLOWE FVM GWEC HAWT LCOE LLFVW MS NACA O&M RANS SNL

TLP TSR TU Delft VAWT VIM

angle of attacks Blade Element Momentum theory Computational fluid dynamics counter-inner-rotating center of gravity counter-outer-rotating co-rotating double-multiple streamtube model department of energy degree of freedom University of Denmark Floating Axis Wind Turbine Field Laboratory for Optimized Wind Energy Finite Volume Method Global Wind Energy Council Horizontal Axis Wind Turbine levelized cost of electricity Lifting Line Free Vortex Wake multiple streamtube model National Advisory Committee for Aeronautics operation and maintenance Reynolds-averaged Navior-Stoke Sandia National Laboratory

tension leg platform tip speed ratio Delft University of Technology Vertical Axis Wind Turbine vortex induced motion

Nomenclature d distance of the downstream turbine to the upstream turbines [m] s rotational axis distance between two rotors [m] Ur rated wind speed [m/s] power coefficient Cp A swept area of the rotor [m2] ρ wind density [kg/m3] U incoming wind speed [m/s] ω rotational speed [rad/s] L blade length [m] R rotor radius [m] c blade chord length [m] N number of blades AR aspect ratio σ solidity D turbine rotor diameter [m] Re Reynolds number wr Local resultant flow velocity [m/s]

behavior of the turbine system because the increased weight owing to bigger rotor size is greater than aerodynamic load increased (Malcolm, 2003). Self-weight induced cyclic gravitational loading causes fatigue issues on the blades and their associated structures of HAWT (Shires, 2013). Although the bigger size of turbine can benefit cost reduction, the cost of O&M is also significantly increased especially in harsher offshore environment and operational need of the specific vessels and installation equipment. On the contrary, increasing the scale of VAWT doesn’t bring

such negative impact relatively because of the base-mounted generator and gearbox systems. Therefore, the scaling-up limit of the top rotor can €jd et al., 2016). In addition, the blade profile reach up to 30 MW (Apelfro of HAWT is relatively more complicated with cylindrical shape at the rotor base connected to complex aerodynamic profile until the blade tip. A large mould for producing such blade is necessary as shown in Fig. 2 of an offshore turbine blade manufactured by Siemens. MHI Vestas has even launched a 9.5 MW offshore HAWT in 2017 which is 164 m in

9.5 MW 6 MW 4 MW 2 MW 90m

2000 (V902.0 MW™ at a glance)

164m

154m

117m

2010 (V1174.2 MW™ at a glance)

2014 (SWT6.0-154 Offshore Wind Turbine)

2017( MHI Vestas Offshore V164-9.5 MW)

Fig. 1. Size evolution of largest installed wind turbine (MHI Vestas Offshore V164-9.5 MW, 2017; V90-2.0 MW at a glance, 2000; V117-4.2 MW at a glance, 2010; SWT-6.0-154 Offshore Wind Turbine, 2014). 609

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Fig. 2. Large mould of an offshore wind turbine blade (IRENA’s Renewable Energy Innovation Outlook, 2016).

diameter (The world’s most powerful available wind turbine gets major power boost, 2017). Its huge components are very costly for maritime transportation and O&M which are much exposed to offshore environ­ ment. In this point of view, VAWT has an advantage of having extruded blade profile and thus, large mould is not necessary. Compared with offshore wind turbines, aerodynamic performance of onshore ones is normally susceptible to the turbulence flow caused by complex topography of land terrain. Their gigantic structures also bring issues of obstruction view of the scene, noise pollution and damage to the flying birds, and shadow flicker during sunny days. Thus, available wind farms have to be confined to non-forested hills and non-urban and ice-free areas. Moreover, onshore wind turbines have a relatively longer downtime window due to non-predictable and unstable wind resources. Offshore devices are more able to tap the richer and better quality wind resources and the negative impact such as shadow flicker, noise pollu­ tion on human population is also minimized. Annual installed offshore wind capacity has been expanding fast recently. Many large operating offshore wind farms have been installed in high geographic latitudes,

European countries, and those areas with higher and consistent wind speed owing to the heating of the earth’s surface by the sun radiation. Captured offshore wind capacity is approximately 1.5–2 times of onshore wind capacity in Denmark, Germany and Netherlands (Offshore Energy Outlook, 2018). Another trend of offshore wind energy is tap­ ping into further deeper waters of more than 40 m depth. To accom­ modate such water depth, evolution of developed offshore platforms of HAWTs is shown in Fig. 3, from bottom fixed monopile or jacket to floating tension leg platform (TLP), semi-submersible, or spar. However, the installation of offshore wind farms need to avoid vessel traffic routes and fishing areas. The impact of offshore wind turbines on marine spices and seabirds is assessed in the reference (Bailey et al., 2014). To reduce cost, engineers are nowadays diverting their attention to alternative technologies instead of immoderately enlarging HAWTs. Offshore VAWTs pose an alternative solution because their mechanical and electrical system can be mounted on the platform deck so that workers can easily access the system and complete O&M in a much safer and less costly way. On the other hand, center of gravity (COG) is in

Fig. 3. Different platforms for offshore wind turbines (Arapogianni et al., 2013). 610

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consequence much lower than that of HAWT with an equivalent power capacity. This can reduce the size of the designed platform and anchored foundation. Additionally, VAWT can rotate in any wind direction without yaw system, and hence less restraints to the hydrodynamic motion and the design of the platform. This will increase uptime of operating window compared to that of using HAWT platform in the harsh sea environment. At present, megawatt-scale VAWTs are still in their infancy and concept design stage. Thus, it is a big question to be answered if they could move forward and become an alternative in the offshore wind energy market. Innovative and advanced technologies are urgently needed to alleviate some inherent drawbacks of VAWTs, for example, lower power coefficient, cyclic aerodynamics loads, and poor selfstarting capability. In this article, a comprehensive literature review will be provided to fully understand the existing bottlenecks in the VAWT technologies and to explore technical perspectives. The paper will elaborate complicated aerodynamic load and performance of VAWTs involved and their associated mechanical and electrical tech­ nology on their commercial viability in future. Therefore, the content of the review paper is organized as follows: Section 2 provides a comprehensive description on the advantages and disadvantages of HAWTs and VAWTs in order to fully understand drawbacks of VAWTs. Section 3 summarises the development history of onshore VAWTs. The lessons learned in the past VAWT projects will be emphasized to highlight their technical bottlenecks. Section 4 introduces the aerodynamic design tools of VAWTs and their corresponding technologies to theoretically explore working fea­ tures and shortcomings of different rotors. Design tools for turbine rotors are also discussed in this section on their reliability, accuracy, and computational efficiency. They play crucial role in optimizing rotor performance. Section 5 to 7 discuss the advanced technologies to overcome the drawbacks of VAWTs such as cyclic aerodynamics loads, lower power coefficient, and poor self-starting capability. A holistic review on VAWTs is provided for both new researchers and experts to continue to grow harnessing wind energy.

advantage of self-starting ability. Therefore, they are commonly found as small-sized turbines in the urban and remote areas with relatively low wind speed. For the lift-based straight-type VAWTs, the advantage is their simple and extruded blades, hence lower manufacture cost. A patent of troposkien-type turbine was filed in 1926 by G. J. M. Darrieus, a French engineer (Darrieus, 1931). This type of turbine has the lowest minimized bending stress which was offset by the increased centrifugal stresses at higher rpm (Ashwill, 1992; Sutherland et al., 2012). The concepts of helical-type rotor are developed to minimise their cyclic aerodynamics loads (Scheurich et al., 2011). They have relatively lower vibration load due to the helix structure of each blade and vibration is thus balanced off. Besides the abovementioned Savorius rotor and straight-type & troposkien-type Darrieus rotors, Bhutta et al. (2012) also reviewed various innovative configurations in the past decades on their advan­ tages and disadvantages. Troposkien-type rotor has potential of scaling up to megawatt-sized turbines in future. Innovative rotors, such as two-tier Darrieus-Masgrowe rotor, combined Savonius and Darrieus rotor, and two-leaf semi-rotary rotor have both high starting torque and power efficiency. Cross-flex-type rotor can be integrated into the building and produce the acceptable power output. Currently, they are only popular in urban application and are still not competitive over HAWTs in the market. It is well known that Megawatt-scale HAWTs have their long suc­ cessful commercial history in both onshore and offshore applications as mentioned above. Their technologies are more mature and converged in aerodynamic shapes, design procedure, operation, installation, etc. Turbine blades have been established to be non-uniform and twisted in the spanwise distribution. Pitch control is used to regulate angle of at­ tacks (AOAs) in high wind speed in order to alleviate the occurrence of stall. However, VAWTs have fallen far behind in their development and commercialization because of their several unsatisfactory performances. Firstly, their power efficiency is lower than that of HAWTs. For HAWTs, the blades are designed purposely to be non-uniform and twisted to maximize aerodynamic performance accounted for different tangential speeds and AOAs from the blade root to the tip. Thus, dynamic stall can be reduced. However, the complicated profile of the blades results in the high cost of manufacturing because the whole piece of blade mould must be prepared to maximize structural strength (Siemens 6.0 MW Offshore Wind Turbine Brochure, 2012). One example of the mould is illustrated in Fig. 2. The blade roots are typically thick in the cylindrical shape for mechanical strength purpose, but some are in NACA profiled shape. At the blade tip, its tangential speed is highest, where the tip is mostly developed to be thinner to reduce both the aerodynamic loss and induced noise. For VAWTs, the shape of the blades is normally uniform without twist. Hence, turbine blades are less costly in the manufacture. As a turbine rotates, AOA of each blade keeps changing together with varying rotational azimuth angles. This phenomenon causes cyclic aerodynamic performance and hence, fatigue issues of structures at the

2. Advantages and disadvantages of VAWTs and HAWTs VAWTs are generally categorised as drag- and lift-based devices, the former one utilises wind drag on the blades driving to rotate, for example, S-type Savonius, the latter one utilises the lift on the blades, for example, straight-type, troposkien-type, and helical-type Darrieus (Fig. 4) (Wiel, 2015). Lift-based devices are the most popular for tur­ bines sized up to hundreds of kilowatts. Drag-based turbines are not preferred due to high solidity, heavier weight, and fairly low efficiency, etc. For example, the power coefficient of Savonius turbine is not more than 25% (Zamani et al., 2016). However, the drag-based ones have the

Drag-based VAWT

Lift-based VAWT

Fig. 4. Schematic view of different types of VAWTs from left to right: S-type Savonius wind turbine, straight-type, troposkien-type, and helical-type Darrieus wind turbine. 611

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risk of failure. Only the blades in the upstream can produce rotational torque whereas others may be in the downstream turbulent region creating resistance to the rotation. As a result, the possibility of self-starting mostly depends on the rotor’s initial azimuth angles of the blades, where the overall torque is relatively high. Consequently, power efficiency (Cp) is greatly reduced because of the occurrence of the stall over a wide range of rotational azimuth angles (Arab, 2017). This inherent property is determined by the orientation of the rotational axis. Furthermore, the rated tip speed ratio (TSR, λ) of VAWT is designed to be lower than that of HAWT and is generally less than 4. But modern three-bladed HAWTs are typically designed with a rated TSR from 6 to 9 (Schubel and Crossley, 2012). Therefore, VAWTs create lower aero­ dynamic noise emission and less disturbance to human population €jd, 2016). However, higher gear ratio is required to increase (Apelfro rotational speed from a low-spinning rotor to a high-spinning drivetrain. It should be noted that gearbox becomes much heavier and more expensive in such cases. Secondly, only VAWTs’ upstream blades can produce rotational torque and thus, the self-starting capability is poor. The fluctuation of the aerodynamic performance of VAWTs due to the changes of azimuth angle also deteriorates such situation. The blades fail to provide positive torque when the stall happens. On the contrary, all HAWTs’ blades contribute rotating torque simultaneously regardless of rotational azi­ muth angles. Thus, a breeze can easily blow the rotors to rotate if inertia and friction of the system can be overcome. However, HAWTs require yaw mechanism to deal with varying wind direction to secure the rotors always facing the incoming wind to maximize power output. In contrast to HAWTs, VAWTs are omni-directional and yaw mechanism is not thus required. Therefore, they have fewer components than HAWTs and hence, less maintenance frequency. In offshore application, the omnidirectional characteristic leads to less restraints to the hydrodynamic motion and the design of the floating platform resulting in better floating stability (Fowler et al., 2014; Cheng, 2016; Lei et al., 2017). Lastly, the vertical orientation of the long shaft brings both pros and cons of VAWTs. The major advantage is that the drivetrain system of VAWT can be installed at the ground level. Thus, it is relatively safe and easy to be accessed for maintenance. However, this may sacrifice selfstarting ability and inherent efficiency. The long steel causes an in­ crease in the overall weight of the rotor which will be borne by bearing mechanism below. Moreover, increased inertia requires more forces to move the top rotor, especially for large-scale devices. Higher torsional stress and inertia from the top rotor are unfavorable for self-starting and turbine efficiency. On the contrary, HAWT’s weight is borne by their supporting tower. Consequently, it is much easier to drive the generator to rotate.

Additionally, VAWT’s supporting tower is shorter than HAWT’s one. The rotor of VAWT has lower rotational speed owing to lower TSR. Thus, tangential speed of its blades is much lower than that of the tips of HAWTs. For HAWTs, the clearance between the lowest blade tip and the ground must be relatively large for safety purpose. From aerodynamic perspectives, VAWTs can benefit from skewed wind direction and can produce power well (Bianchini, 2012). For example, mostly installed troposkien-type Darrieus wind turbine rotors are relatively close to the ground while the towers of HAWTs are usually as high as possible where the wind is much uniform and strong. This will be a tradeoff between the increase in cost and the increase in wind energy extracted. Moreover, VAWTs are less susceptible to shadow flicker, compared to HAWTs, which often disturbs residents in the installed area. Such shadow emission periodically casts shadow of each blade of HAWT onto their surrounding buildings and land area during sunny days (Shadow flicker protection system for wind turbines, 2018). Helical-type VAWTs produce minimal shadow flicker due to one end of each blade being curved in a helix structure. In offshore application, shadow flicker factor for rotor design can be ignored since the turbines are sighted far from residential areas. In summary, a comprehensive comparison was provided in this section between VAWTs and HAWTs and is tabulated in Table 1. The primary unsatisfactory factors of VAWTs mainly lie in lower power ef­ ficiency, poor self-starting capability, and cyclic aerodynamics loads and associated fatigue issues. Turbine blades alternatively contribute rota­ tional torques by varying rotational azimuth angles. Only the upstream blades can provide rotational torque while stall occurs for the down­ stream blades. This characteristic prohibits for further commercializa­ tion. Thus, VAWTs fall far behind HAWTs in generating business revenues. However, small sized VAWTs still have broad applications in the urban or rural area due to their omni-directionality, insensitivity to turbulence, less costly in materials and maintenance, low TSR, and less noise, for example, architecture roof turbines and off-grid hybrid wind and solar street light system with maglev VAWT (Aluminum Alloy Vertical Axis Wind Turbine Generator 300 W TYPMAR CXF-30012V/24V; Ragheb, 2014; Pendharkar, 2012). Advanced solu­ tions are still required to be further research and developed for their competitive potential in future wind turbine market. 3. Research and development history of onshore VAWTs The most successful commercialized onshore VAWTs are developed by Sandia National Laboratory (SNL) early 1970’s to 1990’s. They are two-bladed troposkien-type VAWTs, sized 17 m in diameter and rated power at 95 kW (Fig. 5 (a)). More than 500 units were in commercial

Table 1 Comparison between VAWTs and HAWTs on their pros and cons. VAWTs

HAWTs

Pros

Cons

Pros

Cons

Orientation of rotational axis

Vertical Omin-directional No yaw system Simple shape Less costly Low noise Not required Less components Less maintenance Less costly Less restraint to the hydrodynamic motion. Ground-based drivetrain system Easy access to drivetrain system

Horizontal High Cp Constant aerodynamic loads Self-start ability High efficiency High TSR and Cp

Require yaw system

Blade

Low Cp Cyclic aerodynamic performance Poor self-starting capability Fatigue Low efficiency Dynamic stall Low TSR and Cp -

Long Resist self-starting Sacrifice efficiency High weight High torsional stress and inertia

Short Can self-start High efficiency Low weight

Yaw system

Shaft

612

Constant aerodynamic loads Self-starting ability

Complicated profile Costly Noise emission Required Restrain to hydrodynamic motion Costly Frequent maintenance Drivetrain system at high level Difficult access drivetrain system

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Ocean Engineering 182 (2019) 608–626

(a)

(b)

(c)

(d)

Fig. 5. Onshore VAWTs developed/constructed early 1990s: (a) FloWind (Sutherland et al., 2012); (b) VAWT-850 (Shires, 2013; D’Ambrosio and Medaglia, 2010); � (c) T1-turbine (M€ ollerstr€ om et al., 2015; M€ ollerstr€ om, 2015); (d) Eole 4 MW turbine (Benmeddour et al., 2010).

was operating between 1987 and 1993 and produced 12 GWh of electric energy (Shires, 2013). However, it went out of service in 1993 due to the damage of its bottom large and expensive bearing (Benmeddour et al., 2010). In a nutshell, failure of the onshore VAWTs is primarily because of fatigue issues regardless of LCOE and poor self-starting capability. Although turbine performance is not disappointing in terms of power efficiency, research and development activities seem stagnated because of less funding support from government and the success of their competitor HAWTs. It is less frequent to hear the released news on newly operational onshore VAWTs sized in hundreds of kilowatts after their prosperous period from 1970’s to 1990’s. Nowadays, smallscaled wind energy converting devices have been prevailing in the City Street or on the building rooftop in the urban area full of strong turbulence due to the disturbance of buildings. Small VAWTs are increasingly popular for installations in urban settings due to their aesthetically-pleasing shape, lower noise produced, and easy building integration (Kumar et al., 2018). For instance, it was reported that over 200 Quiet Revolution VAWTs were installed between 2005 and 2016 (Quiet Revolution VAWT at RWE Stadium Essen, 2015; Quiet revolution wind turbines, 2016), one case is shown in Fig. 7 (a). A 14 kW helical-type VAWT was installed for electric vehicle charging in Spain (see Fig. 7 (b)) (UGE installs its first wind-powered EV charging station, 2012). Sometimes, small VAWTs are attractively designed into kinetic sculptures and become a dual-purpose public art display as shown in Fig. 7 (c) and (d), and they are all drag-based VAWTs. Some VAWTs installed in urban area were also reviewed by Kumar et al. (2018) including aforementioned combined Savonius and Darrieus rotor and modified Darrieus rotors. As pointed by authors, their viability depends on their self-starting ability, reliability, and cost. To date, researches and laboratory-based tests on VAWTs are still going on in many universities and companies, such as Stanford Uni­ versity (Brownstein et al., 2016), Technical University of Denmark (DTU) (Vita, 2011; Paulsen et al., 2012, 2014), Cranfield University (Collu et al., 2014; Blusseau and Patel, 2012), Delft University of Technology (TU Delft) (He, 2013; Claessens, 2006; Kemp, 2015), etc. A number of researchers are also keeping eyes on the conceptual design of future offshore VAWTs due to higher wind energy available and lower turbulence near the sea surface without disturbance of complicated land terrain. However, they are still in the theoretical stage and lack of practical operation data, for examples, 5 MW DeepWind by a consortium led by DTU, 5 MW TWINFLOAT by N�enuphar, 10 MW Aeogenera X by a consortium led by Cranfield University, and 6 MW Spinfloat by a con­ sortium led by French wind power specialist ASAH LM. These concepts are funded by government or private companies, their aim is to explore a more favorable offshore device to reduce LCOE. But there is a long way to go to prove their feasibility due to technical bottlenecks of existing VAWTs.

Fig. 6. Joint between different sections with different airfoil (Sutherland et al., 2012).

operation on land. However, nearly all the turbines were removed by the end of 2004 because the wind farm can only generate one-tenth elec­ tricity of its original designed in 1990’s (Gipe, 2009). This failure is due to the fatigue occurred in the joints alone the blades (see Fig. 6) (Sutherland et al., 2012). There are totally two joints along each blade to connect different airfoils, NACA 0021 at both ends and SAND 0018/50 in the middle. This weak design turns VAWTs into a negative influence on their commercial perspective. U.S. department of energy (DOE) terminated almost all VAWT-based research funding after the closure of FloWind. This commercialization has proved that VAWTs can produce competitive electricity compared with the HAWTs. Beside 17 m commercialized VAWT, SNL also developed a 34 m VAWT (500 kW) test bed to optimize turbine performance (Ashwill, 1992). Cp reached as high as 40% presented in the field test report (Sutherland et al., 2012). A 500 kW VAWT-850 straight-type rotor was also constructed in 1990’s by VAWT Ltd as shown in Fig. 5 (b). It was Europe’s biggest VAWT at that time (Price, 2006). It also suffered blade failure just within a few months of operation due to a manufacturing fault and one fiber­ glass blade broke (Shires, 2013; D’Ambrosio and Medaglia, 2010). A unique direct drive T1-turbine was designed and built by the Vertical Wind AB and Uppsala University (Sweden) in 2010 (Fig. 5 (c)) (M€ ollerstr€ om et al., 2015). The generator was mounted on the ground and connected to the rotor by a long steel shaft. T1-turbine is currently in operation and owned by Uppsala University for tower dynamics and €llerstro €m, 2015). noise study (Mo � Eole – the world’s largest 110 m high VAWT, installed at Qu� ebec, Canada in the 1980’s (Fig. 5 (d)). Its rated output power is 3.8 MW. It 613

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(a)

(b)

(c)

(d)

Fig. 7. (a) Quiet Revolution VAWT installed at RWE Stadium Essen (Quiet Revolution vawt at RWE Stadium Essen, 2015). (b) 14 kW wind turbine integrated in an electric vehicle charging station, Spain (UGE installs its first wind-powered EV charging station, 2012). (c) Wind Tree display at Place de la Concorde, Paris (The ’Wind Tree’ that could heat your home, 2015). (d) The Breathe Kinetic Sculpture with capability of powering tiny lights at night at Marina Bay, Singapore (BreathePublic Art Installations).

4. Aerodynamic design of VAWTs

Cambered airfoil is an asymmetry between two sides of the chord line. The symmetrical 4 digit NACA series are indicated as NACA 00xx, xx represents the thickness of the airfoil. NACA 0012 and NACA 0015 are popular profiles and applied in VAWTs’ design (Horst, 2015). The research on airfoil selection criteria, effect of airfoil thickness, camber on turbine performance, and blade stall is discussed in the reference (Kemp, 2015) and will not be repeated in this paper. Besides NACA profiles, there are FX-series, DU-series, S-series, etc (Hashem and Mohamed, 2018) which can also be used for wind turbine design. However, it has been numerically proved that symmetrical airfoil S 1046 has maximum Cp of 40% (Mohamed, 2012; Tjiu et al., 2015). For this type of airfoil, the maximum thickness 17% occurs at 30.8% of chord. In conventional NACA series, the maximum thickness is

Airfoils are widely chosen as blade profile of the wind turbine. They were developed with wind tunnel tests by the United States National Advisory Committee for Aeronautics (NACA) during World War II (The Aerodynamics of the Wind Turbine, 2012). In contrast to blades of HAWTs which rotate at relatively steady AOAs in a certain wind speed, AOAs of VAWTs’ blades are varying due to rapid changes of the rota­ tional azimuth angles. The pressure and suction surfaces of the blade always alternate in every rotation circle. As summarized by Kemp (2015), the symmetrical NACA series without camber is normally employed for VAWTs design in order to produce equivalently torque between upstream half and downstream half of the rotation circle.

(a)

(b)

Fig. 8. An example of Cl and Cd from XFOIL at Re ¼ 1e5. Deep stall happens at α ¼ 15.50 (a) and the ratio of Cl/Cd drops when Cl is nearly 1.2 (Claessens, 2006). 614

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at 30% of chord. The blades of VAWTs experience a much wider range of AoAs than that of the blades of HAWTs. They are susceptible to dynamic stall due to the rapid changes of rational azimuth angles. The dynamic stall is characterized by the flow separation on the blade surface (Claessens, 2006). At a lower AoA, the boundary fluid moves regularly attached to the surface of the airfoil. But at the higher AoA, separation happens at the trailing edge of the airfoil. The stall occurs when AoA is larger than stall angle of airfoil. At stall angle, there is a sharp increase in drag (Liu et al., 2017). Vortices which generated from both leading and trailing edges of the blades due to flow separation will shed into the wake (Horst, 2015). Large wake field full of fierce turbulence can cause the embedded blades to produce decreased non-uniform force. The separation moves to the leading edge as further increase in AoA, and this phenomenon is called deep stall (Claessens, 2006). As shown in Fig. 8 (a), deep stall happens at AoA ¼ 15.50 where the value of Cl suddenly decreases from approximately 1.2 to 0.6. The ratio of Cl and Cd also starts to drop which significantly affect the efficiency of the airfoil (Fig. 8 (b)). After stall occurrence of airfoil, the blades continue to rotate into the upstream laminar flow and then lift is recovered when AoA > 160. For the case in Fig. 8, AoA has to be lower than 15.50 to get the comparable lift value 1.1 due to hysteresis effect (Claessens, 2006). It takes a long time for blades to recover from the effect of the vortices in the wake. Addition­ ally, the downstream blades have to pass through the turbulent flow produced by the upstream blades. The oscillatory aerodynamic loads on VAWT occur in all 6 degree of freedom (DOF) when deep and dynamic stall happens. Thus, the aerodynamic forces on blades are oscillating over the whole rotational circle. Blades are prone to fatigue, aero-elastic vibration and produce noise. Cyclic aerodynamic over a rotational rev­ olution is the inherent characteristic of VAWTs and different from that of HAWTs. It may shorten structural life span. As discussed in Section 3, turbines in Fig. 5 proves such findings. Nowadays, HAWTs are the wind industry’s preferred products due to aforementioned success stories. Rotor design tools have been developed and verified to be more reliable and accurate such as QBlade, BLADED, AeroDyn, Harp_Opt, etc. All these methods employ Blade Element Mo­ mentum theory (BEM), namely an aerodynamic theory. BEM method has been widely integrated into the design tool of HAWTs because of its simplicity and high computational efficiency. Coupled with genetic al­ gorithm, it is able to generate optimized turbine blade profile with in­ puts from look-up table of lift and drag polar data (Liu et al., 2017). Aerodynamic design tools for predicting VAWT performance include local circulation models, momentum models, and vortex models (Cas­ tillo Tudela, 2011). Circulation models are based on potential theory, Eulerian equation. Momentum models are derived from Newton’s sec­ ond law of the motion and are an extension of BEM method. By employing actuator disc theory, VAWT rotor is represented as a disc (top view) and there is pressure jump as the fluid flows through the actuator disc. A deceleration of the wind speed after the disc is responsible for the pressure jump and therefore an induced velocity is determined. Mo­ mentum models consist of multiple streamtube model (MS) and double-multiple streamtube model (DMS) (Marten et al., 2013). The rotation disc is discretized into multiple streamtubes in the cross-flow direction. The difference between them is that DMS employs twice actuator disc theory in both upstream and downstream half circle of the rotation in each streamtube while MS is only used in the upstream half circle (Fig. 9). As a result, DMS is more advanced than MS model. But this momentum model is limited to solidity less than 0.2 and is not valid for the larger range of solidity (Castillo Tudela, 2011). Moreover, their accuracy is doubtful in quantitatively predicting aerodynamic loads when the dynamic stall occurs (Arab et al., 2017). Some physical phe­ nomenon will be lost since the rotor is run as a steady-state situation. Therefore, momentum analytical models are suitable for optimizing rotor design due to their high computational efficiency (Hamedi et al., 2015). Vortex models are for modelling wake field of a rotating turbine.

900 Downstream circle

Upstream circle

Streamtube

U

00

1800

U Wr 2700 Fig. 9. DMS model.

They are less computationally costly and suitable for the qualitative optimization of rotor design in industrial activity (Marten et al., 2017). Lifting Line Free Vortex Wake (LLFVW) is one of the vortex models and it employs non-linear lifting line theory to capture dynamic loading on the blades and Free Vortex Wake algorithm to simulate vortices shed from the trailing edges into the downstream wake. Transient rotating behavior is accounted for in this model. Calculation of load on the blades is identical with BEM method and requires lift and drag polar data of airfoil. Torque curve from LLFVW is consistent with BEM results, which was proved by running the NREL Phase VI HAWT rotor (Marten et al., 2016a). A nonlinear LLFVW has been implemented in the open source QBlade v0.9, a wind turbine aerodynamics tool. This method is appli­ cable to simulate both HAWTs and VAWTs (Marten et al., 2018). Its accuracy and broad application even outperform unsteady BEM method with fewer assumptions. Vortex model is applied in another turbine design tool CACTUS which was originally developed based on VDART3 code by Strickland at Texas Tech. Later, SNL participated in the code development specified for the troposkien-type VAWTs (Murray and Barone, 2011). In contrast to the traditional BEM method, CACTUS employs dynamic stall models in the load calculation. All the wind turbine design tools stated above are summarized in Table 2, together with the programming language of source code. In contrast to the BEM method, LLFVW is more representative on the physics of wake modelling. The transient vortices shed from the blade trailing edges are simulated by calculating circulation at the quarter chord position of the blade in the lifting line formulation (Marten et al., Table 2 Particulars of aerodynamic design tools of the wind turbine blades.

615

Turbine design tool

Type of turbine

Theory

Developer

Source code

HARP_Opt

HAWT

BEM and Genetic Algorithms

Matlab

QBlade

HAWT & VAWT

CACTUS

HAWT&VAWT

CFD

HAWT&VAWT

BEM (HAWT) and DMSM (VAWT) LLFVW (HAWT&VAWT) BEM and using a free vortex line description of the turbine wake flow Navior-Stoke Equation

NREL and the University of Tennessee Hermann F€ ottinger Institute of TU Berlin Strickland and SNL

ANSYS Fluent & CFD, StarCCMþ, OpenFOAM, etc.

Depend on different software

Fortran

Fortran 9x

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2016b). Blade tip loss is also considered in this model. Thus, the tran­ sient flow behavior around the rotor and trails of vortices shedding into the wake are all account for. However, in the momentum method, the turbine rotor is assumed to be a virtual actuator disc (Liu et al., 2016). The wake is modeled with a momentum balance across the actuator disc. The coefficient of the thrust is implemented to calculate the wake-induced flow. It is assumed that inflow is uniform and steady in the disc region. This one-dimensional solution is not able to consider the vortices shed from the blade tips. Therefore, the tip loss correction has to be taken into account to minimise aerodynamic prediction errors (Shen et al., 2005). In addition, this method can only obtain a steady-state flow result. The transient flow behavior in the rotor region is therefore simplified and cannot be captured (Liu et al., 2016). But it is still acceptable to predict the turbine performance and the wake flow in the initial design stage. Another advantage of LLFVW method is its employed yaw correction model which is used to deal with floating wind turbine. BEM method also includes yaw correction but it gives larger error than LLFVW method in modelling HAWTs (Marten et al., 2016a). LLFVW has a large improvement on the accuracy and convergence problems to simulate VAWTs over the DMS model which has been implemented in QBlade started from v0.6. BEM method also has been implemented in FAST in v0.8 and coupled to a structural model. But LLFVW has not been coupled to any structural model and only applied for aerodynamic simulation of the rigid rotors (Marten, 2016). Computational fluid dynamics (CFD) analysis is the most accurate method in predicting turbine performance and their numerical results are comparable with the wind tunnel testing results. The CFD applica­ tion in both scientific research and commercial industries is growing rapidly in recent decades. It includes but not limited to investigate the underlying physics, optimize turbine performance, and reduce design cost. However, its powerful simulation solvers sacrifice computational time and efficiency compared to aerodynamic design tools, and a few weeks may be needed to obtain the approximated solution depending on computational resources used. Nowadays, computational efficiency has been greatly increased with the development of super parallel computing technology. ANSYS FLUENT, Star-CCMþ, and OpenFOAM are popular CFD software. All of them use the same discretization technique, Finite Volume Method (FVM), to approximate the solution of the unsteady Reynolds-averaged Navior-Stoke’s (RANS) equations (Versteeg and Malalasekera, 2007). The flow characteristics such as vortex shedding from the blades, flow separation and pressure distri­ bution on the blade surfaces and the wake-induced flow, can be fully resolved. CFD analysis can be used to verify the results from aero­ dynamic design tools. This is helpful for improving the accuracy and reliability of the rotor design and eliminating the uncertainty of

(a)

aerodynamic performance such as oscillation frequency of the rotor, flow interference, output power, self-starting performance, etc. There are a number of scientific investigation on the effect of turbine particulars and the environmental condition on the power efficiency of VAWTs. A research observed that the maximum power coefficient is shifted to lower TSR region with an increase in solidity with CFD anal­ ysis (Mohamed, 2012). A broad range of high power coefficient can be obtained with lower solidity ratios of 0.1 and 0.16. The narrow blade chord causes less blockage to the incoming flow and this forms a wide gap between blades for the flow to pass through. The wake effect is minimized and the chance of earlier stall is reduced. Solidity effect on the power coefficient was studied in the reference by Hezaveh et al. (2017) and the same conclusion was obtained. Besides, CFD analysis could be performed to study the effect of aspect ratio on the power co­ efficient, which is negligible in the condition of fixed solidity and the rotor swept area A. Different shapes of the diffuser outside of the rotor was investigated by Hashem and Mohamed (2018), and a factor of power augmentation is about 4 as compared to the bare VAWT (Hashem and Mohamed, 2018). In the raining environment, the turbine will suffer a power loss (Wu and Cao, 2018). CFD has been applied to experiment with the innovative designs of VAWTs. As shown in Fig. 10(a), the nature-inspired shape of the turbine was studied numerically in TU Delft (Wiel, 2015), the Tulip project is mainly intended for suburban and industrial areas. Numerical simula­ tion was performed to study cycloidal wind turbine by active pitch control of blade motion (Hwang et al., 2005; Erickson et al., 2011) (Fig. 10(b)). The control mechanism is employed to change the pitch angle of the blade relative to the wind direction. Fig. 10 (c) is the J-shaped VAWT, the specific shape is used to improve self-starting capability of the turbine. In CFD simulation result, there is a long wake in the downstream of the turbine because a portion of the kinetic energy in the coming flow is converted into the mechanic energy (Liu et al., 2016). However, it is unrealistic to measure the velocity of the wake in the laboratory when the wake is more than 30D long (Stallard et al., 2013). Thus, CFD analysis plays an essential role in studying the downstream flow. Fig. 11 shows the numerical prediction results of the wake at the different TSR. The higher the TSR is, the more kinetic energy is extracted by the tur­ bine. Vortices shedding frequency is also much higher. It was learnt that high turbulence in the flow can re-energize the wake to recover effi­ ciently. On the contrary, if the rotor is working at a lower TSR, the wake is extended relatively long. As a result, less kinetic energy is harvested by the rotor. The wake requires long downstream distance to dissipate viscous vortices until is recovered to the incoming flow. As a summary, in this section, aerodynamic design techniques used in various design tools for wind turbine blades are discussed. Different

(c)

(b)

Fig. 10. Innovative VAWTs: (a) nature inspired flower shaped turbine (Wiel, 2015); (b) concept of Cycloturbine (Hwang et al., 2005); (c) 11 J-shaped wind turbine (Zamani et al., 2016). 616

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1 Pt ¼ Cp ⋅ ρAU 3 2

Turbines are typically designed to deliver retained high rated output power (maximum power) after U is higher than the rated wind speed Ur. Cp reduces when U � Ur through stall regulation or passive stall for €jd et al., 2016). Such turbine ideally VAWTs (Liu et al., 2017; Apelfro works at a constant maximum Cp when U � Ur. To produce a determined output power, the size of the turbine rotor is generally determined by the rated Cp and the rated wind speed Ur. For a commercialized Megawatt-scale wind turbine, Ur is typically at a range of 11 m/s to 15 m/s, which is usually 1.5 times annual averaged wind speed measured at hub height (Malcolm and Hansen, 2006). This value is critical in the rotor design and in determining the optimal cost of the system. The lower Ur requires the higher swept area to reach prescribed output power. This leads to the longer blades and larger swept area for the same power output. The highest thrust on the rotor, which is the main fundamental load, usually happens in the stall regime instead of at the rated wind speed for VAWT, where this happens at the rated wind speed for HAWT. Consequently, high-cost material is required to meet the required structural strength. Therefore, the higher Ur is recom­ mended to reduce rotor size and minimise the dominant fundamental load to an acceptable level for both VAWT and HAWT. Generally, wind speed is rated at 14–15 m/s for VAWT and 11–13 m/s for HAWT (Blonk, 2010). Within this higher range, the highest thrust occurs at Ur for VAWT. Especially in the offshore application, higher wind speed results in smaller rotor size and lower foundation load - leading to less material used and hence a light topside structure. The dimension of the designed platform is thus reduced and higher stability requirement is easily met (Fowler et al., 2014). For example, according to Eq. (5), the required swept area of the rotor at Ur ¼ 11 m/s is less than half of that at Ur ¼ 15 m/s. This reduction is remarkable and significantly affect the support tower size, material used, system cost, and even the size of the offshore platform. In Fig. 12, the solid lines present the relationship between A and Pt at different Ur, which is from 11 m/s to 15 m/s. This calculation is based on Eq. (5) and Cp is assumed to be 30%. The output power is linearly pro­ portional to the swept area A of the rotor (solid lines). Various wind turbines either from published papers or commercialized products are indicated in Fig. 12. The square symbols represent VAWTs and the circle ones represent HAWTs. Different colors of the symbols represent the different Ur of the turbine which is consistent with the colors of the lines. The rated Cp of the turbine rotor is less than 30% if the symbol is above the same colored solid line and vice versa. The rated Cp is the value when the turbine works at rated power. That means the lower rated Cp requires large swept area A and the higher rated Cp requires small A in order to reach the prescribed power capacity. For example, Darwind XE128 5 MW, its rated Cp is 37%, and A is below the red solid line and smaller A is qualified to reach rated 5 MW. It utilises the direct drivetrain to minimise the friction and inertia loss of the gear in practice. But for Gamesa G132, the same 5 MW turbine, its rated Cp is 27% at the rated wind speed 13 m/s (Gamesa G132-5.0 MW). Its Cp can be up to 48% at the wind speed of 8 m/s. A is above the blue solid line and it has to be large enough to fit its prescribed power capacity. For VAWTs, it has to be pointed out that all megawatts ones marked in Fig. 12 (a) are concept designs and their efficiency is apparently less than 30% at the rated power (Akimoto et al., 2011; Paulsen et al., 2012). There are very limited products with published performance data in practice for VAWTs less than 1 MW. Thus, only 2 rotors in 5 MW are given in Fig. 12 (b). Power coefficient Cp of all the rotors shown in Fig. 12 is summarized in Fig. 13. The rated wind speed Ur of each rotor is differentiated with the color which is consistent with Fig. 12. All rated Cp values are lower than Betz limit 59.3% (Hau, 2013; Abdulrazek, 2012; Ragheb and Ragheb, 2011; Fraenkel, 1986). Betz limit is the theoretical maximum percentage of kinetic energy which can be extracted by an ideal turbine

Fig. 11. Wake in the downstream at different TSR (Rezaeiha et al., 2018).

BEM techniques and models have been developed by researchers to be utilised in the optimization of HAWT performance and obtaining outline of the blade’s profile with low computational costs. It has also been integrated into the structural analysis model. Vortex model is found to be more powerful and accurate for performance prediction of VAWTs and has been used in open source software QBlade. It has been also briefed by Bhutta et al. (2012) in which broad design techniques are introduced such as, to estimate blade vibratory stresses, structural and aero-elastic, system damping, etc. The literature review found that CFD can most accurately predict turbine performance and flow visualization. 5. Power coefficient of VAWTs compared to that of HAWTs The efficiency of either VAWT or HAWT is a function of solidity (σ ), aspect ratio (AR), number of blades (N), TSR and Reynolds number (Re) (Saber et al., 2015), � � 1 Cp ¼ f σ; AR ; N; TSR; (1) Re where, σ ¼ Nc/2πR, c is blade chord length and R is rotor radius. AR ¼ L/ D, L is blade length. λ ¼ ωR/U, ω is the rotational speed, U is the incoming wind speed to the rotor. Reynolds number Re is a dimen­ sionless number related to the resultant speed of the rotating blades, Re ¼

ρcjwr jR μ

(2)

where wr is the local resultant flow velocity and is defined as ! �! wr ¼ U

�! ωr

(3)

The vector arithmetic is illustrated in Fig. 9. The energy available in the incoming wind is the cube of the wind speed: 1 P ¼ ρAU 3 2

(5)

(4)

where ρ is the wind density, and A is the swept area of the rotor. A ¼ π R2 for HAWTs and A ¼ 2LR for VAWTs. The power produced by the turbine is determined by the power co­ efficient Cp and available wind energy contained in the incoming wind over a swept area A,

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(a)

(b) Fig. 12. (a) The relationship between output power and turbine radius at different rated wind speed and (b) the zoomed-in results of (a) for turbines less than 1 MW.

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6. Advanced technologies to improve power efficiency and cyclic aerodynamics The Cp values of VAWTs are lower than that of HAWTs for several reasons. HAWTs employ yaw system which controls the projected area of the rotor to be in line with the wind direction. Ideally, all the blades harness identical kinetic energy from wind continuously and uniformly, which results in a constant aerodynamic force (Borg, and Collu, 2015). But for VAWTs, only the blade in the upstream half circle can produce the positive tangential force while the other blades produce zero or negative tangential force as indicated in Fig. 14(a). Thus, the output torque on the straight-type VAWT rotor displays cyclic behavior in accordance with the varying rotational azimuth angles. The averaged efficiency of VAWTs is normally lower than that of HAWTs where every blade produces constant torque continuously (Abdulrazek, 2012). The aerodynamic forces on working VAWTs highly oscillate whereas they are fairly constant on HAWTs in operation (Borg, and Collu, 2015). As mentioned before, VAWT has the inherent property of cyclic output power, and thrust on the VAWT rotor is due to the effect of the dynamic stall as the turbine rotates (Fig. 14(b)) (Carrigan, 2012). These will excite vibration on the turbine which is prone to fatigue of blades, bearing, shaft, etc. It leads to risk of failure of VAWTs to produce electric power, for example, the turbines shown in Fig. 5. The cyclic property of aerodynamic loading on blades can be smoothened by using the helical-type design (Scheurich et al., 2011). This kind of turbines requires lower start-up wind speeds. However, they exhibit a lower power output than straight-type ones (Marsh et al., 2015; Alaimo et al., 2015). It has been found that an evenly distributed vortices were generated along the trailing edges of the helical blades whereas the vortices were mainly accumulated at the tips of the straight blades. Thus, helical blades could create more drag in comparison to straight blades. In some rotor designs, the blades of cycloturbine are proposed to be mechanically pitched to the optimized AOA relative to the wind direc­ tion and hence to produce fairly constant aerodynamic loads (D’Am­ brosio and Medaglia, 2010). The concept of Cycloturbine is shown in Fig. 10 (b). Cycloturbine with free-pitching blades has been proved to be worse with a dynamic mechanism, in terms of self-starting and effi­ ciency compared to the conventional pitch fixed turbine (Rathi, 2012). But if the blade can be actively pitched to an AOA at maximum tangential force, cycloturbine could have the better self-starting ability

Fig. 13. Power coefficient at rated wind speed for various dimension rotors.

without losses such as viscous loss, pressure drag on blades, and tip vortex. Its value is derived based on the continuation of flow regime when the downstream velocity is 1/3 of the upstream velocity (Ragheb and Ragheb, 2011). However, in practice, the typical power coefficients of turbines are lower than Betz limit because of rotating wakes, finite number of blades, and no-zero Cd value on the blades (Lysen, 1982). As shown in Fig. 13, Cp of VAWTs in megawatts is much lower than HAWTs, especially the ones are designed at higher Ur from 13 to 15 m/s. The size of installed VAWTs in practice is only limited to kilowatt-scale. Their efficiency is not much inferior to that of HAWTs, especially for 0.5 MW VAWT rotor designed by SNL which can reach approximately 41% (Sutherland et al., 2012). In this design, both thicker end sections with NACA 0021 profile provides structural strength support while the thinner SAND 0018/50 is used in the middle to minimise stresses and maximize captured energy (Ashwill, 1992). SAND 0018/50 is developed by SNL, and the last two digits 5 and 0 indicate the maximum thickness is at 50% of the blade chord from the leading edge, which is positioned backward compared to the conventional NACA 0018. For normal NACA series, the drag is higher at the lower AOAs. It is thus difficult to self-start at the lower wind speed. The benefit of SAND 0018/50 is the lower drag values at lower AOAs and thereby improving their self-starting property. However, the troposkien-type VAWT with SAND 0018/50 profile is experimentally proved to be less efficient (Claessens, 2006).

1 revolution

Power

Time (b)

(a)

Fig. 14. Streamline and produced power for each turbine blade (a) (Horst, 2015) and cyclic power (b). 619

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and display a great improvement in lower TSR based on the DMS model results. CFD method also proved the possibility of performance enhancement of VAWT using variable pitch angle (Rezaeiha et al., 2017). One approach of active pitch control mechanism was introduced in the research by Hwang et al. (2005). Four blades of the straight-type VAWT were connected with four-pitch link bars. The optimal pitch an­ gles synchronize with the changes in wind speed and direction to diminish stall occurrence. 2D numerical results show not only 30% higher power of cycloturbine than that of classical VAWT but also improved cyclic tangential force on the rotor (Hwang et al., 2007). Besides, the application of moulded composite blades can reduce the risk of fatigue due to the cyclic aerodynamic performance of rotor and smoothen the joints on the rotor to reduce aerodynamic loss (Sutherland et al., 2012). Fiberglass composites had been used for VAWT-850 (see Fig. 5 (b), Price, 2006). But one of blades broke within one year’s operation due to a manufacturing fault. The expensive tower had to be built with concrete to bear large cycling fatigue loads. The composite blades of HAWT have been tested and can have a lifespan of between 18.66 years and 24 years (Shokrieh and Rafiee, 2006). The study results may not be applicable to composite blades of VAWT because of the different sources of cyclic loads. A potential way to improve the efficiency of VAWTs and reduce aerodynamic fatigue load is to apply a shield to diminish the negative tangential force on the downstream blades. The shield idea was first presented by Shepherd (1990). Schematic view of the shield concept is illustrated in Fig. 15 (a) and (b). This idea was numerically investigated recently and applied to the counter-inner-rotating (CIR) VAWTs (Jin et al., 2017). A deflector was applied in the upstream of two turbines to act as a shield. The numerical results show that the deflector can improve the performance of VAWTs. However, the shield concept is

applicable to the fixed wind direction. This will sacrifice the advantage of omni-directional characteristic of VAWTs. In addition, the cyclic behavior in power was also improved instead according to the numerical results of the rotor torque coefficient in the reference (Jin et al., 2017). Recently, the shield idea is employed in a venturi or a cycloidal diffuser design (Hashem and Mohamed, 2018) as shown in Fig. 15 (c). CFD study has proved its power improvement by a factor of approximately 4. Wong et al. (2018) proposed the concept of a flat plate to accelerate the flow by properly positioning it in the upstream (see Fig. 15 (d)). The higher wind velocity from the top edge of the flat plate can benefit the power extraction and self-starting capability. Importantly, this concept doesn’t influence the omni-directional characteristic of VAWTs. Some different configurations were also presented and discussed by Bhutta et al. (2012). However, all above rotors are only found to be applicable in small scale size. Various researchers have also paid attention to improve the effi­ ciency of VAWTs by optimizing the geometry of the rotor. More recently, Windspire VAWT was designed purposely with high aspect ratio As ¼ 5 (As ¼ L/D) (Fig. 16). It was found that the turbine with low solidity (σ ¼ 0.04) produces a broad flat power curve and exhibits higher

Fig. 16. (a) Windspire VAWT rotor and (b) two-dimensional numerical results (Giorgetti et al., 2015)0.

Shield

(a)

(b) VAWT position Flat plate

(c)

(d)

Fig. 15. A windmill operation in 1966 (Shepherd, 1990): (a) side view; (b) top view. (c) Cycloidal diffuser. (d) A flat plate deflector in the upstream of the turbine to accelerate the flow (Wong et al., 2018). 620

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Fig. 19. (a) Vortex shedding in the wake of schooling fish; and (b) configura­ tion of wind farm based on the concept of schooling fish (Whittlesey et al., 2010).

00 Fig. 17. Relation of output power and aspect ratio of HAWTs and VAWTs in the condition of the same rotor diameter (Paraschivoiu, 2002).

U

maximum Cp (Giorgetti et al., 2015). This is consistent with the study by Hezaveh et al. (2017). It is the solidity which has the significant effect on Cp instead of aspect ratio. As of VAWTs can be higher than 1 whereas As ¼ 1 for HAWTs as discussed in (Paraschivoiu, 2002). Higher As, more power can be captured by the turbine in the condition of the same so­ lidity and turbine diameter as shown in Fig. 17 (Paraschivoiu, 2002). An increase in swept area A results in the increase in output power. The lower power coefficient for VAWT can also be compensated by working in a pair (Dabiri, 2011). Recently, the published field test data of wind farm with the large foot-print area is published on the counter-rotating 1.2 kW VAWTs (manufactured by Windspire Energy Inc.). This study was done by Prof. Dabiri from California Institute of Technology (Dabiri, 2011). The test site is called the Antelope Valley the Field Laboratory for Optimized Wind Energy (FLOWE). Each turbine is 1.2 m in diameter and 4.1 m in length as shown in Fig. 18 (a). His wind farm has 9 pairs of counter-rotating VAWTs and is configured as in Fig. 18 (b). The wind farm with these small VAWTs can outperform 6–9 times conventional wind farms of HAWTs on power density with 4D (D is the turbine rotor diameter) turbine spacing (Dabiri, 2011). The counter-rotating concept and wind farm arrangement take advantage of schooling fish concept as shown in Fig. 19 (a). The vortex (see Fig. 19 (b)) created by front fish can provide propulsion to the neighboring fish (Whittlesey et al., 2010). Likewise, the turbine array arranged the way the schooling fish can improve the performance of individual ones. According to another study by Giorgetti et al. (2015), two close arranged counter-rotating VAWTs suppress the downstream vortex of the flow

900

s D 1800 (a) CR

(b) COR

(c) CIR

Fig. 20. Configurations of the paired rotating rotors: (a) co-rotating (CR); (b) counter-outer-rotating (COR); and (c) CIR.

and prevent energy dissipation, thus output power is enhanced by more than 10% higher than the isolated turbine. The vortex suppression can also cause an increase in power for the downstream turbines because of the lack of wake interaction (Zanforlin and Nishino, 2016). Prof. Dabiri tested counter-rotating VAWTs of a wind farm in his testing site and concluded that each turbine can deliver 10% more power. As shown in Fig. 20, the possible arrangements of VAWTs can be corotating (CR), counter-outer-rotating (COR), and counter-inner-rotating (CIR) where s is rotational axis distance between two rotors. The nu­ merical study presents that all three arrangements can benefit efficiency increase with s at a range of 1.5D to 3D compared with the isolated one (Shaheen and Abdallah, 2017). The averaged power coefficient in­ creases as s reduces. This is consistent with the numerical results pre­ sented by Giorgetti et al. (2015). The configuration of CIR shows the best results in the power enhancement. However, these conclusions above are limited to a fixed incoming wind direction of 900. It has been proved that the aerodynamic performance of each turbine is dependent on the incoming wind speed which has investigated in a wind tunnel (Ahma­ di-Baloutaki et al., 2016). For fixed s ¼ 2D, configurations CR and CIR exhibit a power decreasing trend with the increase of wind speed. Among the three configurations, COR exhibits strongest flow interaction in the wake, which can achieve fast wake recovery about 87% of the incoming flow at downstream10D away. CIR VAWTs can reduce energy dissipation in the regime between two turbines due to the oriented same flow direction. Thus, more kinetic energy is available to be extracted by each turbine (Giorgetti et al., 2015). The concept of CIR VAWTs was studied in a wind tunnel by Ahmadi-Baloutaki et al. (2016). The output power of both turbines is improved when s ¼ 2D except at the higher wind speed of 14 m/s as stated in the reference. The smaller spacing s in the range of (1.5D, 3D),

Fig. 18. (a) Two counter-rotating VAWTs at FLOWE. (b) Wind farm with 18 units (Brownstein et al., 2016). 621

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the more improvement in power. Shaheen and Abdallah (2017) numerically proved that there is 16% increase in power compared to the isolated turbine when s ¼ 1.5D. Giorgetti et al. (2015) also numerically showed around 10% increase when s is lower than 2D. 11% increase in power efficiency was obtained in another wind tunnel testing when s is between 1.5D and 2D (Shyu, 2014). Moreover, the vortex shedding and turbulence induced by the rota­ tional blades are suppressed in the arrangement of CIR rotors. This will enhance power output and reduce the aerodynamic load on the turbine structure for the downstream turbines (Bremseth and Duraisamy, 2016). Ahmadi-Baloutaki et al. (2016) tested the performance of turbine working in the shadow of CIR VAWTs in the wind tunnel. The output power was enhanced and the increase was mainly dependent on s and d, where d is the distance of the downstream turbine to the upstream turbines as illustrated in Fig. 21. The highest power efficiency happens when s ¼ 1.5D and d ¼ 3D. As s ¼ 2D, little flow interaction was observed between two CIR turbines in the experimental results by Lam and Peng (2017). The flow along the centerline between two turbines appears to be undisturbed, and hence the downstream turbine can likely operate as in the free stream flow when 1D < d < 2D. This is, however, not appli­ cable to the turbine operating in the downstream of COR. It is hard to solve it fundamentally since this inherent property is determined by the orientation of the rotational axis. In summary, although Cp of VAWTs in megawatts is much lower than HAWTs, the kilowatts ones are not much inferior, for example, troposkien-type VAWTs developed by SNL. Megawatt-scale VAWTs are still in the conceptual stage and have a lack of practical operation data. It is well known that although development of VAWTs started decades ago, rarely there are successfully commercialized products. Thus, it is hard to convince people about their feasibility and reliability due to their negative publicity. Nevertheless, scientists and engineers are continuing to put their efforts on overcoming the technical challenges. Many innovative ideas were proposed to enhance the output power, including but not limited to, shielding and deflector application, cycloturbine, and turbines working in a pair. CIR VAWTs working in a group could potentially be a promising solution. Although power coefficient of iso­ lated HAWT is higher than that of isolated VAWT, turbines must be spaced 3D to 5D in the lateral direction and 6D to10D in the longitudinal direction in modern wind farms to achieve 90% kinetic energy recovery (Giorgetti et al., 2015; Kanner, 2015). According to the study conducted by Ahmadi-Baloutaki et al. (2016), the disadvantage of lower Cp of VAWTs can be compensated by employing CIR turbine group and putting them closer to increase power density. Cyclic behavior in power and thrust is really a critical factor in VAWT design and it directly determines turbine lifespan. Nearly all past failure of onshore VAWT cases are related to this characteristic, for example, the turbines shown in Fig. 5. To smoothen cyclic aerodynamics, the advanced state-of-the-art technologies include employing helical blades, cycloturbine, applying deflector, and so on. However, no technology demonstration has ever shown their reliability in megawatts power output. Composite material utilization in blades manufacturing is also

recommended for future turbines. 7. Self-starting property Self-starting capability of VAWTs is another one of the most con­ cerned factors in the design for areas where wind speed is lower. It is well known that the rotating blades can only produce positive tangential force within a certain range of the azimuth angles due to the occurrence of the stall as rapid changes of AOAs (Arab et al., 2017). This inherent characteristic is determined by the specific orientation of the rotating axis relative to the wind direction. The rotors produce cyclic torque as shown in Fig. 14 (b). As a result, the possibility of self-starting somehow depends on the rotor’s initial azimuth angles of the blade. The chance of self-starting is high at the blade position where the overall produced torque can overcome inertia and friction of the system. External assis­ tance to self-start could be a practical solution, for example, a bi-directional gearbox is normally used to run the generator as a motor for a brief period of time to spin the rotor (Sutherland et al., 2012; Apelfr€ ojd et al., 2016). Another obstacle of self-starting could be the longer rotational shaft of VAWT. The natural frequency caused by the torsional vibration rep­ resents the fundamental frequency due to its lowest value for both HAWT and VAWT design. It is even more concerned for VAWTs if the generator is at the ground level resulting in a long drivetrain, as shown in Fig. 22 of a 200 kW VAWT (Apelfr€ ojd et al., 2016). Three blades are attached to the hub with struts. The hub is connected to the direct driven generator bridged by a long rotating steel shaft. This long drivetrain suffers generator-damped torsional vibrations in the rotating shaft of VAWT (Eriksson and Bernhoff, 2005). Its smallest eigen-frequency has to be designed to be larger than the greatest rotor frequency in order to avoid vibrational resonance. The eigen-frequency of the torsional vi­ bration is dependent on the generator damping. It has been theoretically proved that the direct driven synchronous generator has higher eigen-frequency than the induction generator with a gearbox in the condition of same shaft dimension and material. The eigen-frequency can be improved by the increase of the mass of the shaft if the induc­ tion generator has to be used. Although the ground-based generator makes it easy for maintenance personnel to access and hence to reduce maintenance cost, this advan­ tage seems insufficient to justify the increased friction and inertia of the

d U

s D

Centreline

Fig. 21. Configuration of counter-rotating VAWT and a triangular tur­ bine array.

Fig. 22. Converter system of a 200 kW VAWT (Apelfr€ ojd et al., 2016). 622

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the end of the spar (Paulsen et al., 2011). The technology of rotating spar float is also applied in the Floating Axis Wind Turbine (FAWT) concept (Fig. 24 (b)). The rollers and generators are arranged around the cy­ lindrical spar surface above the sea surface and are driven by contacting the cylindrical surface of the rotating floater (Akimoto et al., 2011). The torque from the rotating spar float will drive the rollers and generators to produce power. The drivetrain system is designed to be synchronized with the pitch and heave motion of the spar float. The rotor support directly comes from buoyancy forces. Power take-off mechanism system can be designed light. Only the thrust forces are loaded on the bearing mechanism which is much lower than the weight of the turbine rotor. The tilt of the spar is caused by the Magnus effect of the current. It is hereby highlighted that the strong current can also cause vortex induced motion (VIM) especially for deeper drafter float and hence the regular lateral oscillation. This is a significant challenge in terms of turbine design, fatigue of structure, lifespan, etc. Researchers have carried out a number of investigation using labo­ ratory testing or numerical computing on the aerodynamic character­ istics of self-starting and have tried to find feasible solutions. It has been found that the turbines with thicker blade profiles perform better in selfstarting than the ones with thinner blades (Batista et al., 2011). The thicker blades are able to contribute relatively high torsion to spin the rotor. However, the thinner blade is normally utilised in the larger scale over tens- or hundreds of kilowatt scale turbines in order to reduce the mass of the turbine (Bansil, 2014). Additionally, a lighter rotor reduces the inertia of the rotor and less starting torque is required. NACA 0012 to NACA 0018 are the classical and appropriate blade profiles applied in VAWTs. The lower mass of the turbine is less costly not only in material but also in installation, and importantly, results in lower moment of inertia. The self-starting capability is also dependent on the number of VAWT blades. It has been concluded that three-bladed rotors are normally better than two-bladed rotors in lightly loaded condition irrespective of the initial azimuth position of the blades (Dominy et al., 2007). In the reference (Zamani et al., 2016), the J-shaped blade profile was proved numerically to perform better in self-starting. It produces higher torque at lower TSR than the conventional blade profile. Decades ago, hybrid VAWTs had been developed which used Savonius rotor to assist self-starting at low wind speed while the Darrieus rotor was to augment efficiency at higher wind speed (Shankar, 1979). This arrangement combines their respective advantage of the self-starting ability in Savonius rotor and high efficiency in Darrieus rotor. There are two possible ways to arrange these hybrid rotors, Savonius rotor is either stacked below the Darrieus rotor (configuration 1) or tucked inside the Darrieus rotor (configuration 2). Configuration 1 is more sensitive to the variation of wind speed than configuration 2 because it has shorter rotational axis (Wakui et al., 2005). However, it harnesses less wind energy due to the flow interference between the inner blades and outer blades. The radius ratio of these configurations is a critical parameter to maximize the power output (Wakui et al., 2005). Jacob and Chatterjee (2019) investigated configuration 2 using experiment and numerical modelling. It was found that the overall produced power is the sum of power from the individual rotor. The rotational speed of the whole system is dependent on the radius ratio of the two rotors. However, its longer rotational shaft determines its favorability in small scale power application. The hybrid VAWTs in a few thousand watts have been commercialized and are widely popular in the urban area. In this section, possible solutions are explored on the self-starting capability of the VAWTs. Several researchers and engineers have attempted to improve the blade profile to increase the produced torque from the blade. However, this improvement is found to be minimal because of the stall of the blade with varying rotational azimuth angles. The self-starting capability is attributed to the cyclic torque output of the turbine. The heavy rotors due to the long shaft on the top of the drive­ train system are detrimental to the situation. The high percentage of the power is consumed to drive the generator due to the high friction and

Fig. 23. (a) Troposkien-type VAWT, and (b) electrical and mechanical system assembly (Tjiu et al., 2015).

system due to the heavy load from the long shaft, especially for the megawatt scale turbines. As a result, the torque from the rotor has to bear the long shaft to start the connected generator to rotate. That means, strong wind is necessary in order to generate high rotational torque. Even worse, the long shaft will also increase the risk of fatigue on the gears and eventually, increase the frequency of turbine maintenance. Troposkien-type VAWT has a short shaft connection between the rotor and the generator as shown in Fig. 23 because the rotor is closer to the ground. However, the blades have to be much longer and larger to capture prescribed power output because of the lower wind speed at the lower altitude of the wind profile. This will increase the cost of manufacturing and material used. However, for this kind of VAWTs, extra upper bearing is needed at the top of the rotor in order to allow the rotor to rotate freely. To stabilize the top side of the rotor, guy wires are used with one end anchored on the ground. Thus, such turbine requires a large footprint area. The tension in wires also causes additional down­ ward force burdened on the base structure (Cheng, 2016; Sutherland et al., 2012). The downward force will be transferred to the lower bearing mechanism, which is further detrimental for self-starting. On the other hand, one extra upper bearing will also increase the total cost of the turbine. To reduce the load from the long shaft and large rotors on the bearing mechanism, floating offshore VAWT could be a solution. The spar float can be designed to rotate together with the rotor as one body. As shown in Fig. 24 (a), DeepWind concept, the buoyancy of the float can balance the system weight and hence the downward force on the bearing mechanism is reduced (Vita, 2011). Additionally, the rotating spar al­ lows relatively large motion for the operating turbine. However, in this concept, it is technically challenging to put the submerged generator at

Blades

Blades

Wind

Rollers and generators Arm

Rotating spar Rotating spar

Generator Mooring lines

(a)

(b)

Fig. 24. Offshore floating VAWTs: (a) DeepWind concept and (b) curved FAWT concept (Akimoto et al., 2011). 623

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inertia of the rotor. Offshore VAWTs could be the solution by balancing rotor weight with the buoyancy of the spar float. However, Magnus ef­ fect and VIM are another significant concerns for the spar type VAWTs designs and could be eliminated by deliberated design of the float.

popular, partly because of their aesthetic shapes. In future, it is expected that kinetic sculpture combined VAWTs could be increasingly popular as a dual-purpose public art display. References

8. Conclusions

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This paper gives a new perspective on the state of the art of VAWTs. A comprehensive comparison was performed between VAWTs and HAWTs on the aerodynamic working principle and design tools, power effi­ ciency, self-starting capability, yaw mechanism, fatigue issues, noise, etc. The aim of the paper is to identify the technical barriers and design challenges and to explore the feasibility of deviation from traditional HAWTs to VAWTs for the future wind turbine market. Like a coin has two sides, although the orientation of vertical axis determinates their omnidirectional characteristic and easy accessible ground-based drive­ train system, it also brings a series of technical barriers such as lower Cp, self-starting issue, and cyclic aerodynamics and associated structure fatigue. It is well known that lower-level-installed drivetrain system can reduce the height of COG of VAWTs and the size of the support structure, which significantly benefit cost reduction for floating platforms in the offshore application. It certainly alleviates the challenges of the struc­ tural design. However, this advantage seems insufficient to justify the increased friction and inertia of the system due to the heavy load from the long shaft or struts and hub, especially for megawatt-scale turbines. In this review article, holistic information of VAWT designs is pro­ vided for both new researchers and experts to understand their technical perspectives and commercial viability. An overview of lessons learned from the installed VAWTs in the field is presented to support the development of optimal VAWTs. Advanced technologies proposed in the latest publications are respectively reviewed in this paper in order to mitigate technical barriers. In terms of power efficiency, the troposkientype rotor has proved its inspiring power output. Techniques such as shielding and deflector ideas, cycloturbine, and paired CIR VAWTs are also proposed to enhance the output power. Although the power coef­ ficient of isolated HAWT is higher than that of isolated VAWT, this disadvantage can be compensated by employing turbines in arrays and putting VAWTs closer to each other to increase power density. The major hindrance for VAWTs is from cyclic aerodynamics of the rotors. Nearly all past failure onshore cases as shown in Fig. 5 are inherent results of this characteristic. Proposed helical-type design and Cycloturbine can reduce fluctuation of such periodic performance but the recently developed technologies have not been proved to feasible for megawatt-scale turbines yet. Moreover, the reinforced composite ma­ terial has to be used to maximize structural strength to resist the fatigue loads. To improve self-starting ability, combined Savornius and Darreus rotor could be a feasible solution but only applicable in kilowatts sized device. Another way is to employ an offshore spar buoy with a generator connected at the bottom of the structure. The buoyancy of the spar float can balance off the heavy mass of the top rotor and support tower and downward forces to reduce system inertia for self-starting. In addition, richer offshore wind resources can drive the VAWTs to self-start at higher wind speeds. In the present, VAWTs have fallen far behind HAWTs in publicity and generating business revenues in both onshore and offshore applications. In the 4th quarter of 2017, 30 MW Hywind Scotland offshore floating HAWT farm started to deliver electricity 25 km off the coast shore in Scotland. Each turbine is rated 6 MW. Advanced innovative solutions are required to develop the competitive VAWTs in the global trend of larger turbine rotor size to reduce LCOE. Due to the discussed constraints of upsizing HAWT, VAWT could be an alternative to be developed for over 10 MW in power capacity thanks to its lesser technical barriers of scaling-up. The offshore application poses a great potential for VAWTs but is still in its infancy and concept design stage. Nowadays, in the rural or urban area, the commercialized kilowatts VAWTs are increasingly 624

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