lable at ScienceDirect

Renewable Energy 75 (2015) 50e67

Contents lists avai

Renewable Energy

journal homepage: www.elsevier .com/locate/renene

Review

Darrieus vertical axis wind turbine for power generation I:Assessment of Darrieus VAWT configurations

Willy Tjiu a, *, Tjukup Marnoto b, Sohif Mat a, Mohd Hafidz Ruslan a,Kamaruzzaman Sopian a

a Solar Energy Research Insititute (SERI), National University of Malaysia, UKM Bangi, Selangor 43600, Malaysiab Faculty of Industrial Technology, “Veteran” National Development University, UPN Jogjakarta 55283, Indonesia

a r t i c l e i n f o

Article history:Received 15 December 2013Accepted 18 September 2014Available online

Keywords:DarrieusVAWTH rotorMusgroveGiromillArticulating

* Corresponding author. SERI, Level 3, PerpustakaaKebangsaan Malaysia, 43600 Bangi, Selangor, Malafax: þ603 8921 4593.

E-mail address: willy_tjiu@yahoo.com (W. Tjiu).

http://dx.doi.org/10.1016/j.renene.2014.09.0380960-1481/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

This paper aims to assess the Darrieus vertical axis wind turbine (VAWT) configurations, including thedrawbacks of each variation that hindered the development into large scale rotor. A comprehensivetimeline is given as a lineage chart. The variations are assessed on the performance, components andoperational reliability. In addition, current development and future prospects of Darrieus VAWT arepresented. The Darrieus VAWT patented in France in 1925 and in the US in 1931 had two configurations:(i) curved blades and (ii) straight blades configurations. Curved blades configuration (egg-beater or phi-rotor) has evolved from the conventional guy-wires support into fixed-on-tower and cantilevered ver-sions. Straight blades configuration used to have variable-geometry (Musgrove-rotor), variable-pitch(Giromill), Diamond, Delta and V/Y rotor variations. They were stopped due to low economical value,i.e. high specific cost of energy (COE). Musgrove-rotor has evolved into fixed-pitch straight-bladed H-rotor (referred as H-rotor in this paper for simplicity). H-rotor, in turn, has evolved into several varia-tions: Articulating, Tilted and Helical H-rotors.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

During the Cold War and energy crisis in 1970s, wind turbineswere recognized and developed for its potential in power genera-tion since wind energy resource was unaffected by political andeconomic insecurity. Interest in developing wind energy technol-ogy had sprouted Darrieus VAWT out of the vacuum. An alreadyknown wind turbine technology for electricity generation at thetime was HAWT pioneered by Poul la Cour in Denmark in 1891 [1].Until currently, only variable-pitch Darrieus VAWT configurationknown as giromill that is deemed as efficient as HAWT with coef-ficient of performance (CP) of about 0.5 [2,3]. For a 500 kW variable-pitch giromill at mean wind site of 5.4 m/s, Darrieus VAWT powergeneration cost was found out to be about 18e39% less than theHAWT counterpart [2]. However, the practical implementation hasbeen challenging for Darrieus VAWT researchers. Unlike HAWTblades which see relatively steady angle of attack (AOA) of theincoming wind, VAWT blades undergo inconsistent AOA which

n Tun Sri Lanang, Universitiysia. Tel.: þ603 8921 4596;

changes rapidly between the positive and negative angles. Inaddition, VAWT blades encounter turbulent wind in the leewardside due to the vortices created by the blades passing through thewindward side. These phenomena present Darrieus VAWT de-signers a complicated aerodynamic problems not experienced byHAWT blades.

Darrieus VAWT was intensely investigated for about two de-cades, mainly at National Research Council (NRC) in Canada, SandiaNational Laboratories (SNL) in the US, and The Carmarthen BayWind Energy Demonstration Centre in the UK. Attempts in buildinglarge scale Darrieus VAWT were carried out by DominionAluminium Fabricators in Canada [4], Alcoa in the US [5], and JamesHowden and Co., Wind Energy Group, Ltd. and VAWT, Ltd. in the UK[6]. Recent innovations on Darrieus VAWT have contributed tosimpler and predictable characteristics, which improve the reli-ability and performance of the turbine. The innovations differdistinctively from the previous developments in the 1970se1990s,especially in terms of design complexity and the components used.

2. Evolution of Darrieus VAWT

After the WWI, G.J.M. Darrieus, a French aeronautical engineer,invented a VAWT by adopting airfoil profile for the blades. He

W. Tjiu et al. / Renewable Energy 75 (2015) 50e67 51

patented the design in France in 1925 and in the US in 1931 and putthe working principle as a biomimicry of birds' wings by stating, “Itis thus possible to give these blades a stream line section analogous tothat of the wings of birds, that is to say, offering the minimum resis-tance to forwardmovement and capable of converting into mechanicalenergy the maximum available amount of energy of the fluid by meansof the useful component of the traverse thrust which this section un-dergoes” [7]. The patent covered two major configurations: curvedand straight blades as shown in Fig. 1.

The curved and straight-blades configurations have evolved intoseveral variations, as shown in Fig. 2. Curved-blades configurationhas been known as egg-beater or phi-rotor due to the similar look.There are several variations of phi-rotor, such as guy-wired, fixed-on-tower and cantilevered versions (details on these types areavailable in the following sections). Similarly, straight-bladesconfiguration has several variations. Diamond, V/Y and delta (D)variations have been documented [8,9]. Another variation, avariable-geometry VAWT or often called Musgrove-rotor had beenreplaced by fixed-pitch H-rotor (referred only as “H-rotor” in thispaper for simplicity). Currently, H-rotor has been actively investi-gated, including multi-megawatt rotor for offshore application(details on the topic are available in the Part II of this article).Furthermore, improvements on H-rotor sprout another three var-iations: Articulating, Helical and Tilted H-rotor. Details on Articu-lating and Helical H-rotor are given in the following sections, whileTilted H-rotor is given in the Part II of this article.

3. Support structures for Darrieus VAWT

G.J.M. Darrieus did notmention specific support structure for hisinvention in the patent. However, curved-blades configurationwithcable or guy wires support has been very popular due to the intenseresearch in the US and Canada. Nevertheless, several supportstructures have been implemented for both curved- and straight-blades Darrieus VAWTs, as shown in Fig. 3. Although the illustra-tions in Fig. 3 are depicted using curved-blades, it is applicable forstraight-blades configuration as well.

Guy wires support (A) has been widely used for phi-rotor. Guywires cannot be readily mounted on top of the rotor shaft instraight-blades configuration without extending the rotor shaft orthe use of support arms for the wires. Alternatively, a combinationof cantilever support and guy wires (B) has been used for straight-blades configuration. Guy wires support has been less preferable inrecent years [10e12] due to several drawbacks, including increasedaxial load on the bearings due to wire tension in (A), vibrationsinduced by the rotor and the wind, and large land area required tomount the wires [2]. Fixed-on-tower (C) requires a customized

Fig. 1. Original illustrations by G.J.M. Darrieus in 1931 patent: curved blades (left) and straigh(f2) ¼ hubs, (f) and (g) ¼ rotor shaft [7].

generator for a particular tower since the generator stator coils aremounted on the tower's stationary shaft, while generator rotor areattached to the lower hub of the rotor shaft. In addition, the sta-tionary shaft diameter to height ratio is preferably about 0.01e0.02[13]. Cantilevered-rotor (D) has been used with great success. It hasseveral advantages compared to other types due to its simplicity inmanufacturing and maintenance. The components manufacturingis flexible since the drivetrain is not embedded into the rotor andstator assemblies as in (C). In addition, the drivetrain is detachablefor simpler onsite maintenance [14]. Among these four types,cantilevered-rotor will most likely be dominant in future DarrieusVAWT development.

4. Tailored airfoils for Darrieus VAWT

Airfoils used for commercial Darrieus VAWTs are usually basedon the airfoils used in aviation industry. The most common profilesused are the symmetrical NACA airfoils [2,12,15], with thicknessusually ranges from 12% (NACA 0012) to 21% (NACA 0021). Somemanufacturers camber the airfoils in order to capture more energyat either side of the rotor [16,17]. However, no significant differencein the performance has been reported as compared to the DarrieusVAWT with symmetrical airfoil, since cambering the airfoil causesan increase of tangential force in one half, but decreases the force inthe other half of the swept region [18].

SNL found that a way to improve the performance was bydesigning airfoils specifically tailored for Darrieus VAWT [19]. Theyargued that standard aviation airfoils are not intended for DarrieusVAWT since the operating regime of a VAWT blade is very differentfrom an airplane blade, which can be summarized in Table 1 [20].

Summary of the intended tailored airfoil characteristics by SNLcompared to the experimental results obtained are shown inTable 2 [20]. The tailored airfoil exhibits more reliable turbineoperation via better tip speed ratio (TSR) range cut-off near thepeak CP condition over the standard NACA 4-series, and is imple-mented on variable-speed turbine [21]. The tailored airfoil isemployed at the transition and equatorial sections to provide over-speeding regulation. While for the root sections at which the TSR islower than the equatorial section, standard NACA 4-series is used.This is due to the customized natural laminar flow (NLF) airfoils bySNL have sharp leading edge, which make them more suitable forhigh TSR. The root sections in a phi-rotor experience higher AOA, sothat the rounded leading edge of standard NACA 4-series airfoilsperforms better. The combination reduces the COE and increasesthe turbine reliability and lifetime [21].

Based on the results by SNL, future Darrieus VAWT bladesshould use the combination of standard NACA 4-series and NLF

t blades (right). Annotations in the figure: (a) ¼ blades, (e) ¼ supporting plates, (f1) and

Fig. 2. Timeline of Darrieus VAWT development.

W. Tjiu et al. / Renewable Energy 75 (2015) 50e6752

airfoils for phi-rotor and whole NLF airfoils for H-rotor. Fortunately,standard NLF airfoils like NACA 65-series look similar to the SANDairfoils. Therefore, with the help of modern analytical software,various NLF airfoils can be investigated for use as Darrieus VAWTblades. Fig. 4 shows the comparison of SAND airfoils to NACA 65-series. Standard symmetrical NACA 4-series are represented asdotted lines to serve as the comparison baselines.

A recent investigation using computational fluid dynamic (CFD)has been performed on 20 shape of airfoils listed in Table 3 alongwith the simulation results [22]. Unfortunately, the author neither

included symmetrical NACA 65-series nor SAND airfoils in thesimulation. Fig. 5 shows Selig S 1046, the best performing airfoil inthe simulation with CP of 0.4051. The simulation, however, wasneither optimized for certain TSR nor Reynolds number. The authorsimulated several rotor solidities between 0.1 and 0.25 with TSRranges from 2 to 10. The S 1046 has a similar trailing edge shape tothe NLF airfoils shown in Fig. 4. The leading edge is also slightlysharper than the standard NACA 4-series, but it is not as sharp asthe NLF airfoils. However, the main difference between S 1046 andSAND/NACA65-series is the location of the thickest point. S 1046

Fig. 3. Types of support structures for curved- and straight-blades Darrieus VAWTs.

Table 1Operating conditions of a Darrieus VAWT blade and an airplane blade.

Parameter Blades of a Darrieus VAWT Blades of an airplane

AOA Operate in unsteady conditions;oscillate between positive andnegative AOA twice per revolution,which are often exceeding ±90� .

Operate in nearlysteady conditionsat near zero AOA.

Stall Encounter stall frequently,especially in strong wind.

Encounter stall onlyin unusual operatingconditions.

Reynoldsnumber(Re)

Between a few hundred thousandand a few million.

Usually betweenthree andthirty million.

Table 3List of airfoils simulated with the corresponding CP [22].

Airfoil CP max Airfoil CP max Airfoil CP max Airfoil CP max

NACA 0010 0.2345 NACA 63415 0.1711 AG18 0.0123 FX66S196 0.2074NACA 0015 0.2947 NACA 63418 0.2772 S 809 0.3428 FX77W256 0.1639NACA 0018 0.2964 AH93W174 0.2469 S 9000 0.1696 FX71L150 0.2961NACA 0021 0.2679 AH93W215 0.2541 S 1046 0.4051 FXL142 0.3311NACA 6312 0.1290 AH94W301 0.2130 S 1014 0.2769 FXLV152 0.3576

W. Tjiu et al. / Renewable Energy 75 (2015) 50e67 53

has the thickest point similar to NACA 4-series at about 30% ofchord from the leading edge, while the NLF airfoils' thickest point isat 50% (except for SAND 0015/47, which is at 47% from the leadingedge).

Table 2Comparison between actual and intended characteristics of SAND airfoils.

Requirements of tailored-airfoil Actual characteristics oftailored-airfoil

Increase the maximum CP(higher power generation).

Modest value of maximum CP.

Force blade stall at a windvelocity closer to the maximumCP (Over-speed and powerregulation in strong winds).

- Low drag at low AOA, andhigh drag at high AOA.

- Sharp stall.

Allow the turbine to operate athigher rotational speed(higher power generation andlowering the cost ofdirect-drive generator).

Higher operational speed isachieved by using lowthickness/chord ratio.

Fig. 4. Tailored airfoil by SNL (left) [20] and standard NACA 65-series (right).

5. Egg-beater or phi (4) rotor

5.1. History of phi-rotor

Darrieus VAWT had experienced a vacuum period for about fourdecades when South and Rangi of the NRC of Canada reinvented thephi-rotor design in 1968 [23]. The local community called it “Rangi-South Wind Turbine” [24], being unaware it was previouslyinvented by Darrieus. Thereafter, Darrieus VAWT caught the in-terests of many researchers [25e38], and various dynamic analysison the performance were formulated, including blade momentum,vortex, and finite-difference models.

Unfortunately, not long after the investigations into phi-rotorgained momentum, several machines experienced problems inthe drivetrain, control system and brakes. The failure started withthe first large scale phi-rotor of 224 kWmanufactured by DominionAluminium Fabricators under NRC supervision inMagdalen Islands,Canada [4]. The machine crashed to the ground in 1978, a year afterits operation. In the US, Alcoa built several phi-rotors under SNLsupervision, including the 12.8 m, 17 m and 25 m diameter withgenerating capacity of 30e60 kW, 60e100 kW and 300e500 kW,respectively. However, similar fate with the turbine in MagdalenIslands, turbines built by Alcoa had various problems. The 12.8 mturbine collapsed in 1980 when the rotor column vibrated andbuckled due to over-speed. The 25 m turbine crashed in 1981 whensoftware error in the controller failed to actuate the brake in strongwinds [5].

The last and biggest phi-rotor built by SNL was called “Test Bed”with rotor diameter of 34 m, which was operational in 1988. Therotor had a swept area of 955 m2 and height-to-diameter ratio of1.25. It achieved 500 kW rated power at 37.5 rpm in mean velocity

Fig. 5. Selig S 1046 airfoil in comparison with standard NACA 0017 airfoil [22].

W. Tjiu et al. / Renewable Energy 75 (2015) 50e6754

of 12.5 m/s. The peak CP of 0.409 was obtained at TSR of 6.34 [39].Abundant data on large scale phi-rotor as well as the “Test Bed” areavailable at SNL website. However, the largest Darrieus VAWT inthe world was a phi-rotor built in Canada with rated power of4 MW, rotor height of 96 m and diameter of 64 m. Construction ofthe phi-rotor called Eole began in 1982, andwas completed in 1988.However, the Eole was mostly operated in reduced speed, and thepower output was limited to 2.5 MW to ensure longevity [40]. Theturbine was successfully operated for 5 years until 1993 when itwas damaged during a storm [41]. Repairing the damage wasdeemed too costly since the whole rotor needed to be dismantled.Instead, the Eole was utilized as a tourism icon to show theachievement of Canadian wind energy sector in attempting largescale Darrieus VAWT.

The “Test Bed” inspired FloWind Corp. to commercially marketDarrieus VAWT under auspices of SNL and NREL. Extended height-to-diameter ratio was developed with the blades made of com-posite materials. Until the 1995, FloWind had installed more than800 Darrieus VAWTs in the Altamont and Tehachapi passes inCalifornia [41]. However, despite the successful operation of theturbines, the company went bankrupt in 1997 due to productionfleet financing could not be obtained. Thereafter, VAWTwas out-of-favor and virtually all government sponsors on VAWT researchwere terminated [8].

Until recently, fixed-on-tower and cantilevered phi-rotors havegained popularity. The new designs utilize tubular tower, and doesnot use guy wires. The designs offer simpler and more reliablesystem than the conventional guy-wired phi-rotor. 50 kW fixed-on-tower rotors are developed by ArborWind in collaboration withJohnson System, Inc. (JSI) [10]. The target markets include rural use,large industrial, farm and green houses. Large scale fixed-on-towerphi-rotor with power rating of 200 kW has been attempted byMcKenzie Bay International, Ltd. (MKBY) in collaboration withClean Green Energy, LLC. (CGE) [11]. However, high cost prohibitsthe commercialization of the rotor. Instead, smaller cantileveredversions of 20e65 kW are currently developed [11]. A 60 kW can-tilevered phi-rotor is also developed by VAWTPower Management,Inc. (VMI) in cooperation with the US Department of AgricultureConservation and Production Research Laboratory and SNL of theUS Department of Energy (DOE) [12]. VMI stated that the design isan innovation of the earlier concepts developed by SNL, NRC of

Fig. 6. Three-bladed DOE/Sandia 17-m guy-wired phi-rotor. (a) P

Canada, Alcoa, Agway, the National Rural Electric CooperativesAssociation, FloWind and Vawtpower, Inc [12]. In addition, SNLprovides technical assistance and instruments to measure the rotorperformance. Furthermore, new interest in Darrieus VAWT formulti-megawatt offshore wind power generation has granted SNL$4.1 million from the US DOE. The project was started in 2012, andwill be completed in 2017.

5.2. Assessment on phi-rotor

Rotor illustration and the components of guy-wired phi-rotor inthe early 1970s development are shown in Fig. 6a and b, respec-tively [42]. Two and three-bladed rotors were manufactured, andwere structurally enhanced with struts forming “X” sign. The strutswere detrimental because they added costs, and lowered the per-formance due to parasitic drag and turbulent flow formed by them.The struts were eliminated in the following designs since the tro-poskein blades were able to withstand stresses in high rotationalspeed.

The phi-rotor was supported by guy cables mounted at the topof rotor shaft to the ground at equally-spaced angles. Thrust bear-ings were used at the top and bottom of the rotor, enabling it torotate freely. Mechanical brake was mounted at the bottom of rotorshaft to ensure safe operation in strong winds. Torque sensors viaflexible couplings were utilized to monitor anomalies in the systemand to regulate the generators power. The early design of guy-wiredphi-rotor exhibited many disadvantages due to complex arrange-ments as well as mechanical losses in the components.

In the development, researchers at SNL examined the guy-wiredphi-rotor design more thoroughly and made several conclusions,such as: (i) two-blade design is more cost-effective than the orig-inal three-blade design, (ii) struts should be kept short or possiblyeliminated since they add parasitic drag and cost, and (iii) the bladeairfoil shape should be tailored for VAWT application [40]. Inaddition, the brake system had been positioned directly below therotor lower hub in order not to obstructmaintenancework on othercomponents while keeping the rotor stationary, and to prolong thegearbox lifetime since braking force was not transmitted throughthe gearbox.

The guy-wired phi-rotor blades were manufactured via stan-dard extrusion method using aluminum alloy, which were then

hotograph [79] and (b) major components illustration [42].

W. Tjiu et al. / Renewable Energy 75 (2015) 50e67 55

bent conforming troposkein shape [5]. As implemented in the “TestBed”, a blade was divided into three sections: root, transition andequatorial section. The sections were equipped with extrudedNACA 0021 (1.22 m chord), SNL 0018/50 (1.07 m chord) and SNL0018/50 (0.91 m chord) airfoil profiles, respectively [39]. SNL codedenoted natural laminar flow airfoils developed by Sandia for useon Darrieus VAWT [21]. There seemed to be no standardization inthe naming of the tailored airfoils, for example, SNL 0018/50 mightbe referred to as SAND 0018/50 or SANDIA 0018/50. Nevertheless,the codes for airfoil thickness and thickest location remain thesame. Fig. 7a and b shows the “Test Bed” with illustrations on theblade sections and geometry. Upper root section was longer thanthe lower one in order to maintain the shape under bending stressdue to gravitational loading.

The airfoil profile tailored to a particular section of a bladeserves two purposes: structural strength and aerodynamic perfor-mance. Different propelling forces between the root and equatorsection would cause localized edgewise bending on a blade ofuniform dimension from the root to the equator section. Thisedgewise bending is insignificant for small scale rotor, but for largescale rotor like the “Test Bed” it would be detrimental. Thus, inorder to minimize the fatigue, airfoil chord dimension was altered,so that the propelling forcewould bemore uniform from the root tothe equator. In addition, the chord of the root section was madethicker for the same purpose of reducing fatigue due to bending,and also to compensate for lower TSR and higher AOA. Fig. 8a and bshows typical components of a guy-wired phi-rotor based on the“Test Bed”. Generally, the major components consist of thefollowing:

� rotor assembly (rotor column, upper and lower hubs, andblades),

� shaft assembly (interconnection shaft, brake disk and caliper,rubber isolator, and torque sensor),

� base structures (gearbox, generator, foundations, and groundequipment station), and rotor support structures (supportstands, upper and lower rotor bearings, guy wires andtensioners).

Fig. 7. The “Test Bed”. (a) Photograph

VMI has been testing VP100 (shown in Fig. 9a) since 2006, whichis a three-bladed cantilevered phi-rotor with 60 kW rating. The totalstructure is about 23.7 m tall, while rotor height and diameter are13.5 m and 15 m, respectively. The blades use NACA 0015 profilewith 0.35 m chord length, 0.053 m chord thickness and 0.0053 mwall thickness [12]. The blades were made of extruded aluminumalloy, which were bent into troposkien shape. The blades were thenfittedwith twohinges at the ends,whichwere epoxied into position.The hinges allow vibration in the rotor assembly without stressingthe aluminum blades, which is an innovation of the rotor. VP100 isconnected to vertical gearbox and generator considering the1200 rpm generator used in the system [14], while the rotor speed isonly 62.4 rpm [12]. The high rotational speed of generator suggeststhat a speed increaser is used in the system. The main reason ofusing a combination of a speed increaser and a generator is to get thereliability improvement over a multi-stage gearbox, while keepingthe cost reasonably below a direct-drive generator [43]. Fig. 9bshows an artist's impression of themajor components of the VP100.

Maintenance work demonstration on the VP100 showed that ittakes only four hours to replace the 800 pounds (363 kg) generatorwithout using crane as in typical HAWT maintenance. In addition,only hand tools and light jacks are used in the process since thegenerator is placed on the ground. VMI claims that the VP100maintenance cost is much lesser than the HAWT counterpart, sincethe use of crane adds thousands to tens of thousands of dollars inthe servicing cost of the wind turbine [14].

MKBY and CGE successfully installed a 200 kW fixed-on-towerphi-rotor in Ishpeming, Michigan in 2010, after having installa-tion problems in the previous year. A troposkien blade wasdeformed when lifted by a crane, which prompted for redesigningthe core structure of the blade. In the next attempt, a frame wasconstructed to hold a blade while being lifted by a crane to beassembled to the rotor shaft. However, the turbine has not been asatisfaction, primarily due to the high cost in manufacturing. Inaddition, the installation was too expensive and complex [11].Fig. 10a and b shows the 200 kW and its major componentsdescription, respectively. The turbine has an outer (rotor) shaftwhich rotates around an inner (stationary) shaft. The stationary

[40] and (b) Blade geometry [39].

Fig. 8. (a) General view and (b) drivetrain view of the “Test Bed” [40].

Fig. 9. A 60 kW cantilevered phi-rotor by VMI. (a) The VP100 photographed during operation [80] and (b) an artist's impression of major components of the VP100.

W. Tjiu et al. / Renewable Energy 75 (2015) 50e6756

Fig. 10. The 200 kW cantilevered phi-rotor by MKBY and CGE. (a) Photograph of the rotor in operation [11] and (b) major components illustration of the rotor [13].

W. Tjiu et al. / Renewable Energy 75 (2015) 50e67 57

shaft functions as the holding post for the rotor assembly as well asfoundation post. Truss structure is used for additional foundationsupport for the whole system.

MKBY and CGE are currently developing cantilevered phi-rotorcalled “Wind-e20”, which is scheduled for completion in 2013.Wind-e20 is the 20e65 kW version based on the improvements onthe 200 kW model [11]. Wind-e20 has several unique features,including remote-controlled foldable blades for safety in strongwinds, typically above 38 m/s. The blades are made of straightsections with joints that are powered by hydraulic pumps, so thatduring strong winds the hydraulic actuator pulls the blades close tothe shaft, similar to the closing of an umbrella. In addition, theblades are equipped with airbrakes, particularly at the equatorialsection. The airbrake movement is electronically controlleddepending on the wind velocity [44]. Fig. 11 shows an artist'simpression of the major components and the blades position dur-ing normal operation (right) and folded in strong winds (left).

In another development, ArborWind in collaboration with JSIhave been manufacturing 50 kW fixed-on-tower phi-rotor similarto the 200 kWmodel used by MKBYand CGE. However, the turbinebuilt by ArborWind and JSI does not include a ground-mountedstationary shaft, and the blade is not manufactured in multiplesmall sections. The lack of fully extended stationary shaft reducescost in trade off with higher bending stress on the shaft mountedon the truss structure. In addition, a fixed blade further reduces coston the hydraulic pumps and complexity in the manufacturing.Considering the troposkien shape of the blade, it is able to with-stand centrifugal force in strong winds. The goal is to produce aspecific COE between 9 and 12 cents per kWh [10]. Fig. 12a and bshows the commercial prototype of the 50 kW and its majorcomponents, respectively.

5.3. Shape of the phi-rotor blade

In the early development, phi-rotor was hailed for its advantageof using troposkein blades, which took the shape of a jumping ropeenduring high centrifugal force. Therefore, the blades could bemade slender, light and low cost via relatively simple extrusionmanufacturing method [5]. However, phi-rotor performance variesdepending on the blade curvature as shown in Fig. 13 [23]. Effi-ciency is influenced by the length of the relatively straight sectionat the equator to the rotor height [45], which is denoted by ze/H.

Therefore, SNL neither used ideal troposkien, catenary norparabola shapes for the phi-rotor due to the curvature effect.Instead, SNL used straight-line for the top and bottom parts andcircular arc-shape for the middle of the rotor [19]. The reasonbehind such configuration is to increase the ze/H ratio while stillhaving the ability to endure centrifugal force. In addition, bladecurvature is affected by the H/D ratio. In term of H/D ratio, puretroposkien shape has the H/D ratio of about 0.9, while the “TestBed” had the H/D ratio of 1.25. Furthermore, Paraschivoiu [23]suggested that future phi-rotor will use extended height-to-diameter (EHD) with H/D ratio between 1.3 and 1.5, which makesthe ze/H ratio closer to unity. However, the trade-off in increasingthe H/D and ze/H ratios is the increment in operational bendingstresses since the shape has become nontroposkien [8].

5.4. Disadvantages of phi-rotor

Recent innovations by MKBY and CGE, ArborWind and JSI, andVMI have demonstrated significant advantages of the fixed-on-tower and cantilevered phi-rotor over the conventional guy-wired phi-rotor, while still using the acclaimed fatigue-free

Fig. 11. An artist's impression of the Wind-e20 and its major components.

W. Tjiu et al. / Renewable Energy 75 (2015) 50e6758

troposkien blades. Nevertheless, the phi-rotors are the product oflessons learned in the guy-wired phi-rotor, which was the mostextensively investigated design among the Darrieus VAWT varia-tions. Based on the failures in the design, several disadvantages ofphi-rotor, especially the guy-wired type, are such as:

� High axial load on support bearings due to rotor assembly andguy-wires.

Tare and zero-wind losses are relatively small and can beneglected compared to the total power produced [39]. Tare loss isthe power loss due to bearing friction of a rotor without the bladesattached, while zero-wind loss is the friction loss with the bladesattached and spun at nowind. However, a recent report released bySNL in 2012 stated that the bearings, especially the bottom supportbearing must be designed to support both the rotor weight anddownward force due to the wires tension. Therefore, the requiredhigh capacity of the support bearings can contribute significantly tothe capital cost of the turbine [8].

� Uneven wind velocity across rotor height

The swept-area of phi-rotor is bound by the troposkien shapeand is determined by rotor height-to-diameter (H/D) ratio. Withthe tendency to use higher H/D ratio in order to get higherequatorial section [23], rotor height increases more than theincrement in rotor diameter. For large-scale on-land phi-rotorwhich is located on the ground, the effect of uneven wind ve-locity is more severe due to terrain roughness. The rotor's upper

section may experience much higher wind than the lower sectionnear the ground surface, which causes uneven lift force producedacross the blades length that contributes to instability, bendingand torsion stress on the blades [46e48]. On the other hand,straight-bladed configuration type has the flexibility in adjustingthe swept-area. Rotor height and diameter can be independentlyadjusted to suit particular design. In addition, H-rotor type ismostly mounted on a tower, which further reduces uneven windvelocity variation.

� Gravity-induced bending stress on the blades

In phi-rotor, gravity-induced bending stress is the force todeform the troposkien shape due to the blades own weight. For asmall-scale phi-rotor less than 100 kW, gravitational loading on thebladesmay be neglectedwith respect to centrifugal force. However,weight of the blades becomes significant in large rotor since thelength of a typical phi-rotor blade is three times a HAWT bladewiththe same swept area and solidity [40]. When the rotor is stationary,the bending stress on the blade is static. However, when the rotorstarts to rotate, the static bending stress becomes dynamic and isovercome by centrifugal force depending on the rotational speed ofthe rotor. The bending stress oscillates in accordance with thecentrifugal force, which is affected bywind velocity, turbulence andwake effect at the downwind side.

Paraschivoiu [23] and Sutherland et al. [8] mentioned thatgravity-induced stress is related to rotor height-to-diameter (H/D)ratio. Lower H/D ratio leads to greater gravitational stresses, but thetype of airfoil can be tuned to minimize gravity and radial aero-dynamic influences. This is the reason why the “Test Bed” wasequipped with thicker root section than the equatorial section, andupper root was longer than the lower one, i.e. to maintain bladeshape when the rotor is stationary as well as sustaining stressesendured by the blades in motion.

Gravity-induced bending stress is less vulnerable for straight-bladed configuration since the blades are shorter and have lowerbending moment, i.e. the blades are more rigid at the same chordlength and thickness as a phi-rotor blades. In addition, they arepositioned vertically and are suspended by support arm(s), so thatthey are not subjected to constant bending stress due to gravity.Support arm is the component which endures gravity-inducedbending stress, and it can be made stronger and tapered from theshaft to the blade.

� Wake due to large rotor column

The rotor column of a phi-rotor needs to sustain high tensionproduced by guy wires as well as cyclic torque produced by theblades, so that buckling strength is the most important aspect of arotor column requirement [23]. However, large rotor columnextending across the height causes blades in leeward position tosuffer from turbulent flow region known as wake, especially inlarge-scale rotor. A wake not only reduces performance, but alsocauses vibration on the blades and support structures. Fig.14 showsthe vortices andwakes generated by the blades and rotor column ofa typical Darrieus VAWT [23].

� Rotor height limitation

Despite the low cost and simplicity in supporting a phi-rotor,guy wires have a drawback of instability over a long distance,including the catenary effect. In addition, guy wires also endureintermittent rotor and wind forces which make them vibrate andoscillate. The oscillation frequency and operating mode of guywires were studied extensively in order to avoid resonances with

Fig. 12. A 50 kW cantilevered phi-rotor manufactured by Arborwind and JSI: (a) Photograph [10] and (b) artists impression of the major components.

W. Tjiu et al. / Renewable Energy 75 (2015) 50e67 59

rotor vibration [47]. Therefore, it is difficult to build a very tall rotorequipped with guy wires in order to take advantage of higheraltitude winds.

� Large footprint to mount guy-wires.

Fig. 13. CP of phi-rotor in respect to curvature ratio [23].

Since guy-wires are fixed above the rotor assembly, large landarea is required for anchoring them. This restricts the imple-mentation of phi-rotor at limited and utilized area, such as infarming land. In addition, the use of guy-wires is not practical foroffshore application. Nevertheless, higher H/D ratio phi-rotor re-quires smaller footprint.

6. Variable geometry VAWT (Musgrove-rotor)

6.1. History of Musgrove-rotor

Variable geometry Darrieus VAWT or also known as Musgrove-rotor was invented by Peter Musgrove, a British aeronautical en-gineer in the mid-1970s [49]. The rotor was a modification of thestraight-blades Darrieus VAWT by employing blades reefing

Fig. 14. Vortices and wakes of a typical Darrieus VAWT [23].

Fig. 15. An experimental model of Musgrove-rotor [50].

Fig. 17. The VAWT-450 Musgrove-rotor [6].

W. Tjiu et al. / Renewable Energy 75 (2015) 50e6760

mechanism to prevent the rotor from over-speeding in strongwinds. The turbine consisted of two sets of straight blades sup-ported on a horizontal beam similar to the shape of an “H” letter.The horizontal beam, also taking the shape of an airfoil, was in turnsupported by a tower at the middle of the beam. The drivetrain andgenerator were located at the base of the tower. Each set of theblades consisted of two equal portions feather-able about thehorizontal beam, for which in reefed position they took the shape ofa double-arrow “4”, thus reducing the swept area as well as thelifting force of the blades tangential to the radial line of the rotor.The rotor was operational in the wind velocity of up to 30 m/s.Fig. 15 shows the installation of an early experimental model ofMusgrove-rotor [50], while Fig. 16 shows the major componentsdiagram [49].

Musgrove-rotor had similar components to the guy-wired phi-rotor. However, Musgrove-rotor was equipped with two stages ofspeed increaser (upper and lower gearbox) as shown in Fig. 16. Theconsideration of using multi-stage gearbox is to reduce the numberof poles needed for the generator, therefore, reducing the cost ofgenerator. Transformer was used to step-up the AC voltage beforeinjecting it into transmission lines on electricity grid system.Promising results in the early development made the UK govern-ment financially supported the scaling-up of Musgrove rotor in thelate 1970s [51]. The first large scale Musgrove rotor was completedin 1986 by VAWT Ltd., and was named VAWT-450 (based on theswept area of 450 m2). It had rotor diameter of 25 m and ratedpower of 130 kW at 11 m/s wind velocity. Fig. 17 shows the VAWT-450 in reefed position. Several Musgrove rotors with 100 kW ca-pacity were also built by VAWT Ltd. on Isles of Scilly and Sardinia[6].

6.2. Assessment on Musgrove-rotor

Manufacturing process of straight blades is simpler than curvedblades. However, the main disadvantages of variable geometry

Fig. 16. Major components of Musgrove-rotor [49].

VAWT were the unnecessarily complex design of reefing mecha-nism, large concrete structure and high cost in building the turbine.In addition, the Musgrove-rotor consisted of many componentswhich hindered its cost-effectiveness. Despite the disadvantages,after learning that there was a rotational speed limit of the fully-extended blades, Musgrove-rotor development was terminatedand shifted to H-rotor.

7. Giromill or cycloturbine

7.1. History of giromill

Another variant of straight-blades Darrieus VAWT is giromill oralso known as cycloturbine. The term “giromill” was constructedfrom two words: cyclogiro and windmill coined by MCAIR, whichdeveloped cyclogiro airborne vehicle and adapted it to the versionof the windmill [52]. It was developed in the US around 1976, atabout the same time of Musgrove-rotor in the UK. Giromill is a H-rotor with variable-pitch, so that wind's AOA to the blade ismaintained relatively constant at certain negative angle for one halfand certain positive angle for the other half of revolution at certainwind velocity. The pitching method include mechanical and elec-trical actuators, such as using a cam and push-rod mechanism [53],hydraulic mechanism, and DCmotor connected to a blade pivot axisvia a timing belt [52].

After the successful feasibility study, a three-bladed pre-commercialization prototype giromill was built in 1980 underfunding from US DOE. Fig. 18a and b shows the MCAIR giromill [40]and its components description [54], respectively. The giromill hada diameter of 58 ft (17.7 m) and rotor height of 42 ft (12.8 m), whichproduced constant power of 40 kW at 8.9e17.9 m/s wind velocity.The drivetrain concept was similar to the Musgrove-rotor, exceptfor the placement of the brake disc and the single stage gearboxutilized on the giromill. However, despite the successful develop-ment of MCAIR giromill, the US government chose a two-bladeddownwind HAWT with similar power rating. The decision wasbased on higher annual energy generation and lower COE.

7.2. Assessment on giromill

A giromill is able to achieve maximum CP of 0.5 [3,52], which ismore efficient than other Darrieus VAWT variations presented inthis paper. Although variable-pitch mechanism in giromill showshigher performance than fixed-pitch Darrieus VAWT, the

Fig. 18. The MCAIR 40 kW prototype giromill. (a) The 40 kW giromill during testing [40] and (b) components of the giromill [54].

Fig. 19. The VAWT-850 fixed-pitch H-rotor with the VAWT-450 Musgrove-rotor in thebackground [6].

W. Tjiu et al. / Renewable Energy 75 (2015) 50e67 61

mechanism is costly. Complexities of the pitch-change system andsupport structures for changing the pitching angle reliably over theservice time make the giromill not cost effective and have pre-vented it from being manufactured in large-scale basis.

8. H-rotor

8.1. History of H-rotor

Despite its simplicity, H-rotor was developed later than Mus-grove and giromill rotors although it was mentioned in the originalDarrieus patent. Experience gained from the VAWT-450 showedthat the reefing mechanism in Musgrove design was unnecessarybecause passive stall of the airfoils in vertical position during strongwind naturally prevented the blades from over-speeding. Thus,another turbine was built as a H-rotor by VAWT Ltd. in 1988 andwas named VAWT-850 which had rated power of 500 kWand rotordiameter of 38 m [55]. Fig. 19 shows the VAWT-850, whoseconnection of support bar and blades was simpler than theMusgrove-rotor at the background. The turbine was completed inAugust 1990 and was tested until February 1991 when one of theblades broke due to an error in the fiberglass blades manufacturingprocess [56].

Current large scale H-rotor is developed by Vertical Wind AB, awind energy research company based in Sweden in collaborationwith Uppsala University. After successful initial investigations on2 kW and 12 kW prototypes [57,58], the company produced a largescale turbine of 200 kW [59,60]. The production of a 200 kW H-rotor was started in October 2009, and has been operational sinceApril 2010. Fig. 20a and b shows the rotor and an artist's impressionon the major drivetrain components, respectively. The structuralconcept of the H-rotor is similar to the giromill built by MCAIR.However, the H-rotor developed by Vertical Wind is much simpler

since the rotor does not have wind detection and blade pitchingmechanism as well as a gearbox.

Vertical Wind AB also reported that fewer moving partscompared to conventional wind turbines gives higher availabilityand reliability as well as lower maintenance cost. The companyclaims that direct-drive generator provides excellent cost efficiencysince it is placed on the ground, and hence, does not need to beoptimized for the weight and size. In addition, costs related togearbox failure are eliminated. Furthermore, the H-rotor is quieterthan a HAWT of similar size. The success story was receivedenthusiastically by the Swedish Energy Authority, E.ON and Fal-kenberg Energy, for which four turbines will be installed there [61].

Fig. 20. A 200 kW H-rotor by Vertical Wind. (a) Photograph of the H-rotor [60] and (b) artist's impression on the main components of the rotor.

W. Tjiu et al. / Renewable Energy 75 (2015) 50e6762

8.2. Assessment on H-rotor

In the 1970se1980s, glass-fiber reinforced plastic (GFRP) wasnot common for being used as Darrieus VAWT blades. Until Mus-grove and MCAIR started developing straight-bladed configuration,it was found out using aluminum via extrusion method was notsuitable for H-rotor blades due to cyclic flapwise bending stress.Therefore, the recent straight-bladed Darrieus VAWT configura-tions use GFRP and carbon fiber composites similar to the HAWTblades, which is able to sustain continuous cycles of edgewise andflapwise bending stress during the blades service life. By the use ofGFRP and carbon fiber composite, the benefit of stress-enduringtroposkien-shaped aluminum blades for phi-rotor is compensatedby the stress-enduring composite materials for H-rotor blades. Inaddition, the aerodynamic drag caused by struts or support arms inH-rotor is also compensated by the increased performance of therotor, since blade equatorial portion-to-rotor height (ze/H) ratiobecomes unity, as described earlier.

The H-rotor program in UK was terminated after the failure ofVAWT-850 due to the prohibitively high cost in building the con-crete tower and support structure [6]. Similarly in the US, H-rotorwas not attempted by the government despite successful towerand drivetrain components installation in the giromill program.Current development by Vertical Wind in the Sweden hasimproved the designs of H-rotor by Musgrove in the UK andgiromill by MCAIR in the US. However, cyclic torque in large scale,especially in multi-megawatt range, requires investigations intostrong and light-weight rotor shaft, since an extended rotor shaft isprone to vibration and fatigue, primarily due to torsional stress onthe shaft.

A retrospective analysis by SNL in 2012 stated that H-rotor has ahigh potential for cost-effective offshore wind power generation

[8]. In particular, support bar of a H-rotor can be used as an aero-dynamic braking system in strong winds, which has been a majorconcern in Darrieus VAWT design. Airbrake system has been astandard aerodynamic brake for commercial airplanes, whichdeploy extended flaps during landing. In sport cars, aerodynamicbraking system has been used in conjunction with mechanicalbrake to provide higher deceleration rate by deploying the rearspoiler upward. Therefore, H-rotor has a potential to embed similaraerodynamic braking system on the support bar cost-effectively,without modifying the blades.

9. Helical H-rotor

9.1. History of helical H-rotor

H-rotor was modified into another variant in which the bladeswere twisted along the perimeter to form helical shape. Surpris-ingly, the modification was intended as a water turbine since theinventor, Professor A.M. Gorlov of Northeastern University, is anexpert in hydro power. The invention was granted US Patents no.5,451,137 & 5,451,138 on 19th September 1995. Although the tur-bine was originally designed as a water turbine, the disclosedpatents stated that it could be used for hydro-pneumatic, hydro,wind and wave power systems [62,63].

Fig. 21aec show the comparison of Helical H-rotor for water andwind turbines. The main difference between them is that the waterturbine has a much higher solidity, which is the ratio of bladescoverage area to turbine swept area. The hydrofoil's chord of theGorlov water turbine blades is made longer and thicker in order toincrease the structural strength. In addition, the rotating speed isreduced, so that the chance of cavitation is minimized. Further-more, the Gorlov water turbine rotates much slower than the

W. Tjiu et al. / Renewable Energy 75 (2015) 50e67 63

QuietRevolution and Turby wind turbines, which is beneficial to themarine lives.

The QR5 turbine shown in Fig. 21b is manufactured by QuietRevolution in the U.K [64]. The rotor size is 5.5 m (H) by 3.1 m (D),and has a rated power of 8.5 kWat 16 m/s wind velocity. The cut-inand cut-out wind velocities for the turbine are 5.5 m/s and 26 m/s,respectively. The turbine employs state-of-the-art components,which include carbon fiber composites for the rotor assembly anddirect-drive permanent magnet generator. Another helical H-rotorshown in Fig. 21c is developed by Turby BV, a Dutch manufacturerwhich produces 2.5 and 10 kW turbines [15]. The company hasbeen cooperating with Delft Technical University to produce the

Fig. 21. Helical H-rotor for: (a) water turbine (Gorlov Helical Turbine) [81], (b) an

turbine. Similar to the QR5, the Turby blades are manufacturedusing carbon fiber aramide composite. In addition, direct-drivepermanent magnet generator is also used. Turby has an overallCP of 0.3 from the wind power to electricity. It utilizes NACA 0018profile for the blades, and is operated at TSR of about 3. The cut-inand cut-out wind velocities of the turbine is 4 m/s and 19 m/s,respectively, while the rated power is reached at 13 m/s [65].

9.2. Assessment on helical H-rotor

Helical H-rotor improves the performance of H-rotor bydistributing a blade profile along the perimeter of the rotor

d (c) are wind turbines by QuietRevolution [64] and Turby [15], respectively.

Fig. 22. Geometry of the modeled Darrieus VAWTs: (a) H-rotor, (b) Phi-rotor and (c) Helical H-rotor [67].

Fig. 23. Power coefficient variations of a typical phi (4) rotor, H-rotor and helical H-rotor [67].

W. Tjiu et al. / Renewable Energy 75 (2015) 50e6764

uniformly, and thus, making the swept area as well as blade sec-tions constant to the wind at all instances of turbine rotation.Therefore, rotor torque fluctuation is significantly reduced whenthe helical shape covers a full 360� rotation. Benefits of havingregular torque include better power output regulation and reducedcyclic stress on the drivetrain. In addition, noise is reduced andslightly higher effective chord is obtained [66]. Currently, helixdesign is getting popularity not only because of better performance,but also for the esthetic value, in which modern elegant designharmonizes the elements in the space.

Comparison of Helical H-rotor to H-rotor and phi-rotor has beendone [67,68]. Fig. 22 shows modeled geometry of H-rotor, phi-rotorand helical H-rotor. The rotors were modeled with 3 blades spacedequally at 120� using symmetrical NACA 0015 with chord-to-radiusat mid-span of 0.15, aspect ratio of 20 and TSR of 5. The modeledrotors performance is shown in Fig. 23 where torque fluctuationvaries three times every rotation. The graphs show that a phi-rotorhas the most fluctuation with variation of about 0.3 CP, followed byH-rotor with 0.2 CP, and the least fluctuation is achieved by thehelical H-rotor with variation of about 0.03 CP. However, despite thebenefits gained, true helical blades are more expensive tomanufacture.

10. Articulating H-rotor

Another recent variation of Darrieus VAWT is the articulating H-rotor developed by Blackhawk Project, LLC. The wind turbineconcept is based on a helicopter rotor that adjusts automatically tothe wind pressure, so that vibration and mechanical stresses arereduced. Bruce Boatner, who invented the articulating H-rotor in2006, is an engineer and helicopter pilot. The articulating H-rotorreceived US Patent no. 7,677,862 on 16th March 2010 [69].Currently, Blackhawk, LLC is testing TR-10, a prototype model of1.5 kW at the Center For Advanced Energy Studies (CAES), Idaho

National Laboratory since 2009. The rotor has a diameter of 10 ft(3 m) and height of 7 ft (2.1 m).

The working principle of the wind turbine is based on gimbal orswashplate-like mechanism, in which the blades are free to oscil-late or tilt around the rotor hub, i.e. the articulation point. Elasto-meric dampeners are used to prevent the blades from over-tilting.Linkages are connected from the hub to the blades, so that pitchangles are altered depending onwhich blade is being pushed by thewind. Fig. 24 shows an illustration of the 1.5 kW articulating H-rotor with annotation on its major components. Pitch-control via

Fig. 24. An illustration of the 1.5 kW prototype by Blackhawk, LLC [82].

W. Tjiu et al. / Renewable Energy 75 (2015) 50e67 65

articulating motion allows the turbine to self-start at light windsdespite of having low solidity, higher torque during operation, aswell as for aerodynamic braking. Another advantage of articulatingmotion is that the blades swiftly adapt to the wind force, thusreducing vibration as often occurs in stiff and fixed blades. Thefeature is highly advantageous for urban application, where thewind is more turbulent.

11. Fish-schooling formation

The effort to study Darrieus VAWT in array configuration hasbeen bio-inspired by the nature. Migrating birds and fishes showthat they have more stamina in traveling farther as a group. Bypositioning themselves precisely at certain coordinates, the ani-mals are able to gain from the vortices shed by the animals ahead.

Fig. 25. Biomimicry of a Darrieus VAWT wind farm to vortices pattern formed by a school ofVAWT wind farm configuration [75].

This phenomenon has been investigated for wind turbine appli-cation, and has been shown to be beneficial for vertical axisconfiguration. A recent investigation [70] using stereoscopic par-ticle image velocimetry (PIV) shows the wake and vortices formedby a two-bladed H-rotor clearly. The H-rotor dimensions are 1 mrotor diameter, 1 m rotor height and 0.06 m NACA 0018 chordlength, which rotates at TSR of 4.5 in a wind stream velocity of9.3 m/s. The PIV images show the fast wake recovery of the H-rotor,in which after only 1.5 rotor diameter distance downwind, thecycloidal wake is no longer detectable and is replaced by largevortical structures due to the roll-up of co-rotating small vortices[70]. The utilization of these vortices is the basis of VAWT fish-schooling formation.

Darrieus VAWT has an advantage in turbulence compared to theHAWT, so that they can be formed into arrays to harness morepower in a given area. In limited area of urban population, thisarrangement would be advantageous. Unlike HAWTs that experi-ence higher fatigue and performance loss when positioned close toeach other [71e74], Darrieus VAWTs wind farm study suggestedslight reduce (or even increase in some cases) in performancedepending on the array configurations [75]. For a clustered tur-bines, Darrieus VAWT pairs at downwind position recover the ef-ficiency towithin 5% of an isolated turbine at four diameter spacing,while HAWTs require 15-20 diameter spacing [76]. Similar phe-nomenon has been observed for Savonius VAWT [77,78]. However,research on the topic is still very scarce, and large Darrieus VAWTcluster such as in a typical HAWT wind farm has not been per-formed to observe the large-scale wake effects on the pairs for-mation. Nevertheless, the studies showed the potential of smallinter-turbine spacing in Darrieus VAWT to reduce the size andimpacts of wind farm.

Fig. 25a and b shows a biomimicry configuration of DarrieusVAWT wind farm based on wake vortices of fish schooling studiedby Weihs in 1975 [75]. Both acw vortex (anticlockwise) and cwvortex (clockwise) represent dipoles of wake vortices formed by theschool. The dipoles position are used as the placement of DarrieusVAWTs. The distances between the dipoles are indicated by 2a, 2band 2c. 2a is the downstream distance of two vortices in the sameline; 2b is the lateral distance between acw and cw vortex of aparticular fish; and 2c is the distance between two adjacent fishes

swimming fishes, where: (a) Wake vortices of schooling fish and (b) proposed Darrieus

Fig. 26. Performance of two-closely spaced H-rotors. Normalized CP is showed in respect to: (a) incoming angle of the wind and (b) TSR [76].

W. Tjiu et al. / Renewable Energy 75 (2015) 50e6766

[75]. However, it was found that adjacent turbines with the samerotational direction tend to reduce the performance of the pair. Onthe other hand, the performance is generally unaffected by counter-rotating turbines.

Investigation into a single pair of H-rotors is shown in Fig. 26a.The H-rotors (indicated by two counter-rotating circles) are spacedat 1.65 rotor diameter. The red/bold line is the normalized CP at allangle, except for some angle range which have been omitted due toinconsistent wind flow below 15min. The three circles surroundingthe H-rotors are normalized power indicators at 0.5, 1.0 and 1.5,respectively from the smallest circle. The investigation showed thatat certain angle, the average power generated by both turbines isless than that of an isolated turbine. However, at other angles theaverage power of the pair is higher than that of an isolated. Overall,the average power generated by the pair at all angles is slightlybetter than an isolated turbine as shown in Fig. 26b. The figureindicates that slower turbine rotation benefits the pair in a trade-offwith more critical speed regulation. The vertical dashed-line is thedesigned operating TSR of the H-rotors [76].

12. Conclusion

Darrieus VAWT had experienced ups and downs since the in-vention in 1920s. Several variations on both curved- and straight-blades configurations have been investigated. Current develop-ment shows that guy-wired rotor is getting less popular due tomany disadvantages, while cantilevered-rotor using tubular ortruss structure is becoming more dominant for both curved- andstraight-blades configurations. The reliability of cantilevered-rotorhas ignited new interest in Darrieus VAWT both in small and largescale. Novel variations have emerged to provide better performanceand lower COE. Darrieus VAWT has produced several variations,most notably Helical H-rotor. In addition, investigations into clus-tered Darrieus VAWT have been currently taking place, which showpromising results over an HAWT wind farm.

Acknowledgment

The authors would like to acknowledge the Ministry of Science,Technology and Innovation (MOSTI) eMalaysia, for sponsoring thisproject under the PRGS/1/11/TK/UKM/03/2 grant.

References

[1] Hau E. Wind turbines: fundamental, technologies, application, economics. 2nd

ed. Berlin: Springer-Verlag Berlin Heidelberg; 2006.

[2] Brulle RV. Feasibility investigation of the giromill for generation of electricalpower. Technical discussion, vol. II. Energy Research and DevelopmentAdministration; 1977. COO/2617e76/1/2.

[3] Brulle R. McDonnell 40 kW Giromill wind system e phase II: fabrication andtest. McDonnell Aircraft Company; 1980. RFP-3304.

[4] Templin RJ. Design characteristics of the 224 kW Magdalen Islands VAWT. SEEN80e16453 07-44. NASA Lewis Research Center; 1979.

[5] Johnson GL. Wind energy systems. Prentice-Hall; 1985.[6] Price TJ. UK large-scale wind power programme from 1970 to 1990: the

carmarthen Bay experiments and the Musgrove vertical-axis turbines. WindEng 2006;30:225e42.

[7] Darrieus GJM. Turbine having its rotating shaft traverse to the flow of thecurrent, US Patent No. 1,835,018; 1931.

[8] Sutherland HJ, Berg DE, Ashwill TD. A retrospective of VAWT technology.SAND2012e0304. Sandia National Laboratories; 2012.

[9] Manwell J, McGowan J, Rogers A. Wind energy explained: theory, design andapplication. 2nd ed. Great Britain, U.K: John Wiley & Sons Ltd.; 2009.

[10] Vertical Axis Wind Turbine, Johnson System, Inc., 2013. Available from: http://www.johnsonsysteminc.com/green-energy/verticle-axis-wind-turbine/[accessed 01.03.13].

[11] ISHPEMING, MICHIGAN PROTOTYPE, McKenzie Bay International, Ltd., Avail-able from: http://www.mckenziebay.com/#!Ishpeming%20Turbine/c1oat[accessed 01.03.13].

[12] Mackenzie J. Clines corners wind energy demonstration project. VAWTPowerManagement, Inc.; 2005. Available from: http://www.vawtpower.blogspot.com/2005_05_01_archive.html [accessed 24.02.13].

[13] Nigam DK, El-Sayed MEM. Vertical axis wind system, US Patent No. 7,948,111;2011.

[14] Mackenzie J. Operational testing nears completion. VAWTPower Manage-ment, Inc.; 2007. Available from: http://www.vawtpower.blogspot.com/2007_07_01_archive.html [accessed 03.12.13].

[15] Holdsworth B. Options for micro-wind generation: part two. Renew EnergyFocus 2009;10:42e5.

[16] 6 kW vertical axis wind-power turbine (VAWT), Shanghai Muce wind powerequipment Co., Ltd., Available from: http://www.vawtmuce.com/picture.asp?ClassName¼Wind-Power%20Type [accessed 30.06.14].

[17] UGE-9M, Urban Green Energy, Inc., Available from: http://www.urbangreenenergy.com/products/UGE-9M [accessed 30.06.14].

[18] Deglaire P, Engblom S, Agren O, Bernhoff H. Analytical solutions for a singleblade in vertical axis turbine motion in two-dimensions. Eur J Mech B Fluids2009;28:506e20.

[19] Blackwell BF, Sheldahl RE, Feltz LV. Geometrical aspects of the troposkien asapplied to the darrieus vertical-axis wind turbine. In: ASME des. eng. tech.conf., Washington, D.C.; 1975.

[20] Berg DE. Customized airfoils and their impact on VAWT cost of energy. In:Windpower '90, Washington, D.C; September 1990. p. 24e8.

[21] Klimas PC. Tailored airfoils for vertical axis wind turbines. SAND84e1062.Sandia National Laboratories; 1984.

[22] Mohamed MH. Performance investigation of H-rotor Darrieus turbine withnew airfoil shapes. Energy 2012;47:522e30.

[23] Paraschivoiu I. Wind turbine design e with emphasis on darrieus concept.Quebec: Presses Internationales Polytechnique; 2002.

[24] Steward H. Eggs that stay firm, fences that bounce and a new look in wind-mills. The Gazette Saturday Edition 07 October 1972. Montreal.

[25] Biswas S, Sreedhard BN, Singh YP. Dynamic analysis of a vertical axis windturbine using a new windload estimation technique. Comput Struct 1997;65:903e16.

[26] Wilson RE, Lissaman PBS. Applied aerodynamics of wind power machines.Oregon State University; 1974.

W. Tjiu et al. / Renewable Energy 75 (2015) 50e67 67

[27] Strickland JH. The darrieus turbine: a performance prediction model usingmultiple streamtubes. SAND75e0431. Sandia Laboratories; 1975.

[28] Templin RJ. Aerodynamic performance theory for the NRC vertical-axis windturbine. LTR-LA-160. National Research Council; 1974.

[29] Loth JL, McCoy H. Optimization of darrieus turbines with an upwind anddownwind momentum model. J Energy 1983;7:313e8.

[30] Wilson RE, Walker SN. Fixed wake analysis of a Darrieus rotor. SAND81e7026.Sandia Laboratories; 1981.

[31] Shankar PN. Development of vertical axis wind turbines. Proc Indian Acad Sci1979;C2:49e66.

[32] Rajagopalan RG, Fanucci JB. Finite difference model for vertical axis windturbines. J Propuls 1985;1:432e6.

[33] Paraschivoiu I. Double-multiple streamtube model for studying vertical-axiswind turbines. J Propuls 1988;4:370e7.

[34] Holme O. A contribution to the aerodynamic theory of the vertical-axis windturbine. In: Int. Symp. Wind energy syst; 1976. C4.

[35] Strickland JH, Webster BT, Nguyen T. A vortex model of the darrieus turbine:an analytical and experimental study. J Fluids Eng 1979;101:500e5.

[36] Swamy NVC, Fritzsche AA. Aerodynamic studies on vertical-axis wind turbine.In: Int. symp. wind energy syst., Cambridge, England; 1976. p. 73e80.

[37] Worstell MH. Aerodynamic performance of the DOE/Sandia 17-m-diametervertical-axis wind turbine. J Energy 1981;5:39e42.

[38] Castelli MR, Englaro A, Benini E. The Darrieus wind turbine: proposal for anew performance prediction model based on CFD. Energy 2011;36:4919e34.

[39] Ashwill TD. Measured data for the Sandia 34-meter vertical axis wind turbine.SAND91e2228. Sandia National Laboratories; 1992.

[40] Spera DA. Wind turbine technology: fundamental concepts of wind turbineengineering. 2nd ed. New York: ASME Press; 2009.

[41] Cooper P. Development and analysis of vertical-axis wind turbines. In:Tong W, editor. Wind power generation and wind turbine design. South-ampton: WIT Press; 2010.

[42] Reuter RC, Worstell MH. Torque ripple in a vertical axis wind turbine.SAND78e0577. Sandia National Laboratories; 1978.

[43] Polinder H, Pijl FFAvd, Vilder GJd, Tavner P. Comparison of direct-drive andgeared generator concepts for wind turbines. IEEE Trans Energy Convers2006;21:725e33.

[44] WIND-e20: Overview. 2013. Available from: http://winde20turbine.com/component/allvideoshare/video/latest/wind-e20-overview.html [accessed03.12.13].

[45] Blackwell BF, Reis GE. Blade shape for a troposkien type of vertical-axis windturbine. SLA-74e154. Sandia National Laboratories; 1974.

[46] Owens BC, Hurtado JE, Paquette JA, Griffith DT, Barone M. Aeroelasticmodeling of large offshore Vertical-axis wind turbines: development of theoffshore wind Energy simulation toolkit. SAND2013-2203. Texas A&M Uni-versity in collaboration with Sandia National Laboratories; 2013.

[47] Ashwill TD. Initial structural response measurements and model validation forthe Sandia 34-meter VAWT test bed. SAND88e0633. Sandia National Labo-ratories; 1990.

[48] Popelka D. Aeroelastic stability analysis of a darrieus wind turbine.SAND82e0672. Sandia National Laboratories; 1982.

[49] Musgrove PJ. The variable geometry vertical axis windmill. In: Int. symp. windenergy syst., Cambridge, England; 1976. p. 87e100.

[50] Vaahedi E, Barnes R. Dynamic behaviour of a 25m variable-geometry vertical-axis wind-turbine generator, generation, transmission and distribution. IEEProc C 1982;129:249e59.

[51] Anderson MB, Groechel KM, Powles SJ. Analysis of data from the 25 m variablegeometry vertical axis wind turbine. In: Proc. ninth British wind energy assoc.wind energy conf., Edinburgh; 1987. p. 333e9.

[52] McConnell RD. Giromill overview. In: The wind energy innovative syst. conf.,Colorado Springs, 23e25 May; 1979.

[53] Moran WA. Giromill wind tunnel test and analysis. COO/2617e4/2. Technicaldiscussion, vol. 2. McDonnell Aircraft Company; 1977.

[54] Anderson JW, Brulle RV, Birchfield EB, Duwe WD. McDonnell 40 kW Giromillwind system e phase I: design and analysis. RFP-3032/1. McDonnell AircraftCompany; 1979.

[55] Meir R, Page LD, Bedford LA. The UK department of energy wind energyprogramme. In: Proc. tenth British wind energy assoc. wind energy conf.,London; 1988. p. 5e8.

[56] Prestage M. Dream of power has gone with the wind. London: The Inde-pendent; 1991. p. 3.

[57] Eriksson S, Solum A, Leijon M, Bernhoff H. Simulations and experiments on a12 kW direct driven PM synchronous generator for wind power. Renew En-ergy 2008;33:674e81.

[58] Sjokvist S, Eriksson S. Study of demagnetization risk for a 12 kW direct drivenpermanent magnet synchronous generator for wind power. Energy Sci Eng2013;1:128e34.

[59] Eriksson S, Semberg T, Bernhoff H, Leijon M. A 225 kW Direct driven PMgenerator for a vertical axis wind turbine. In: European wind energy conf. &exhib. 2010, Warsaw, Poland, 20e23 april; 2010.

[60] Ottermo F, Bernhoff H. An upper size of vertical axis wind turbines. WindEnergy 2013. http://dx.doi.org/10.1002/we.1655.

[61] Ny vindkraftteknik f€or framtiden, E.ON Sweden AB, 2012. Available from:http://www.eon.se/om-eon/Om-foretaget-old/old-aktuellt/Nyheter-Privatkund/Unikt-samarbete-skapar-framtidsutsikter-for-ny-vindkraftteknik/[accessed 09.12.13].

[62] Gorlov AM. Unidirectional helical reaction turbine operable under reversiblefluid flow for power systems, US Patent No. 5,451,137; 1995.

[63] Gorlov AM. Unidirectional reaction turbine operable under reversible fluidfrom flow, US Patent No. 5,451,138; 1995.

[64] QuietRevolution e QR5, Quiet Revolution Ltd., Available from: http://www.quietrevolution.com/qr5/qr5-turbine.htm [accessed 16 January 2013].

[65] Bussel GJWv, Mertens S, Polinder H, Sidler HFA. The development of Turby, asmall VAWT for the built environment. In: Global wind energy conf. 2004,Chicago, USA, 30 march; 2004.

[66] Bussel GJWv, Mertens S, Polinder H, Sidler HFA. TURBY: concept and real-isation of a small VAWT for the built environment. In: EAWE/EWEA Specialtopic Conf. “The science of making torque from wind”, Delft, The Netherlands;2004. p. 509e16.

[67] Scheurich F, Fletcher TM, Brown RE. The influence of blade curvature andhelical blade twist on the performance of a vertical-axis wind turbine. In: 29thASME wind energy symp., Orlando, Florida, 4e7 January; 2010.

[68] Scheurich F, Brown RE. Modelling the aerodynamics of vertical-axis windturbines in unsteady wind conditions. Wind Energy 2013;16:91e107.

[69] Boatner B. Vertical Axis wind turbine with articulating rotor, US Patent No.7,667,862; 2010.

[70] Tescione G, Ragni D, He C, Ferreira CJS, Bussel GJWv. Near wake flow analysisof a vertical axis wind turbine by stereoscopic particle image velocimetry.Renew Energy 2014;70:47e61.

[71] Thomsen K, Sorensen P. Fatigue loads for wind turbines operating in wakes.J Wind Eng Ind Aerodyn 1999;80:121e36.

[72] Grady SA, Hussaini MY, Abdullah MM. Placement of wind turbines using ge-netic algorithms. Renew Energy 2005;30:259e70.

[73] Gonzalez JS, Rodriguez AGG, Mora JC, Santos JR, Payan MB. Optimization ofwind farm turbines layout using an evolutive algorithm. Renew Energy2010;35:1671e81.

[74] Ammara I, Leclerc C, Masson C. A viscous three-dimensional differential/actuator-disk method for the aerodynamic analysis of wind farms. J Sol En-ergy Eng 2002;124:345e56.

[75] Whittlesey RW, Liska S, Dabiri JO. Fish schooling as a basis for vertical axiswind turbine farm design. Bioinspir Biometics 2010;5:1e6.

[76] Dabiri JO. Potential order-of-magnitude enhancement of wind farm powerdensity via counter-rotating vertical-axis wind turbine arrays. J Renew SustEnergy 2011;3:1e12.

[77] Golecha K, Eldho TI, Prabhu SV. Study on the interaction between two hy-drokinetic savonius turbines. Int J Rotating Mach 2011. http://dx.doi.org/10.1155/2012/581658.

[78] Shigetomi A, Murai Y, Tasaka Y, Takeda Y. Interactive flow field around twoSavonius turbines. Renew Energy 2011;36:536e45.

[79] Worstell MH. Measured aerodynamic and system performance of the 17-mresearch machine. In: Proc. vertical axis wind turbine (VAWT) des. tech.semin. ind., Albuquerque, New Mexico; 1980. p. 233e58.

[80] Mackenzie J. Turbine before the squall. VAWTPower Management, Inc.; 2010.Available from: http://www.vawtpower.blogspot.com/2010_07_01_archive.html [accessed 24.02.13].

[81] Rourke FO, Boyle F, Reynolds A. Tidal energy update 2009. Appl Energy2010;87:398e409.

[82] Boatner B. A summary overview of the blackhawk wind turbine. 2010.Available from: http://coen.boisestate.edu/windenergy/files/2011/10/BlackhawkOverview-BruceBoatner1.pdf [accessed 23.02.13].