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

Download 5,66 Mb.

Pdf ko'rish

bet	1/3
Sana	23.01.2022
Hajmi	5,66 Mb.
	#406031

1 2 3

Bog'liq
Darius1

5 authors

See discussions, stats, and author profiles for this publication at:

https://www.researchgate.net/publication/266799978

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

Darrieus VAWT conﬁgurations

Article

Renewable Energy · March 2015

DOI: 10.1016/j.renene.2014.09.038

CITATIONS

141

READS

17,606

5 authors

, including:

Some of the authors of this publication are also working on these related projects:

Green Desiccant materials

View project

Darrieus VAWT

View project

Willy Tjiu

Institut Teknologi Indonesia

PUBLICATIONS

267

CITATIONS

SEE PROFILE

Mat Sohif

Universiti Kebangsaan Malaysia

218

PUBLICATIONS

5,352

CITATIONS

SEE PROFILE

M.H. Ruslan

Universiti Kebangsaan Malaysia

217

PUBLICATIONS

5,673

CITATIONS

SEE PROFILE

Kamaruzzaman Bin Sopian

Universiti Kebangsaan Malaysia

1,187

PUBLICATIONS

27,267

CITATIONS

SEE PROFILE

All content following this page was uploaded by

Willy Tjiu

on 14 February 2019.

The user has requested enhancement of the downloaded file.

Review

Darrieus vertical axis wind turbine for power generation I:

Assessment of Darrieus VAWT con

ﬁ

gurations

Willy Tjiu

, Tjukup Marnoto

, Sohif Mat

, Mohd Ha

ﬁ

dz Ruslan

Kamaruzzaman Sopian

Solar Energy Research Insititute (SERI), National University of Malaysia, UKM Bangi, Selangor 43600, Malaysia

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 2013

Accepted 18 September 2014

Available online 9 October 2014

Keywords:

Darrieus

VAWT

H rotor

Musgrove

Giromill

Articulating

a b s t r a c t

This paper aims to assess the Darrieus vertical axis wind turbine (VAWT) con

ﬁ

gurations, including the

drawbacks of each variation that hindered the development into large scale rotor. A comprehensive

timeline is given as a lineage chart. The variations are assessed on the performance, components and

operational reliability. In addition, current development and future prospects of Darrieus VAWT are

presented. The Darrieus VAWT patented in France in 1925 and in the US in 1931 had two con

ﬁ

gurations:

(i) curved blades and (ii) straight blades con

ﬁ

gurations. Curved blades con

ﬁ

guration (egg-beater or phi-

rotor) has evolved from the conventional guy-wires support into

ﬁ

xed-on-tower and cantilevered ver-

sions. Straight blades con

ﬁ

guration 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 speci

ﬁ

c cost of energy (COE). Musgrove-rotor has evolved into

ﬁ

xed-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.

1. Introduction

During the Cold War and energy crisis in 1970s, wind turbines

were recognized and developed for its potential in power genera-

tion since wind energy resource was unaffected by political and

economic insecurity. Interest in developing wind energy technol-

ogy had sprouted Darrieus VAWT out of the vacuum. An already

known wind turbine technology for electricity generation at the

time was HAWT pioneered by Poul la Cour in Denmark in 1891

[1]

.

Until currently, only variable-pitch Darrieus VAWT con

ﬁ

guration

known as giromill that is deemed as ef

ﬁ

cient as HAWT with coef-

ﬁ

cient of performance (

P

) of about 0.5

[2,3]

. For a 500 kW variable-

pitch giromill at mean wind site of 5.4 m/s, Darrieus VAWT power

generation cost was found out to be about 18

39% less than the

HAWT counterpart

[2]

. However, the practical implementation has

been challenging for Darrieus VAWT researchers. Unlike HAWT

blades which see relatively steady angle of attack (AOA) of the

incoming wind, VAWT blades undergo inconsistent AOA which

changes rapidly between the positive and negative angles. In

addition, VAWT blades encounter turbulent wind in the leeward

side due to the vortices created by the blades passing through the

windward side. These phenomena present Darrieus VAWT de-

signers a complicated aerodynamic problems not experienced by

HAWT blades.

Darrieus VAWT was intensely investigated for about two de-

cades, mainly at National Research Council (NRC) in Canada, Sandia

National Laboratories (SNL) in the US, and The Carmarthen Bay

Wind Energy Demonstration Centre in the UK. Attempts in building

large scale Darrieus VAWT were carried out by Dominion

Aluminium Fabricators in Canada

[4]

, Alcoa in the US

[5]

, and James

Howden and Co., Wind Energy Group, Ltd. and VAWT, Ltd. in the UK

[6]

. Recent innovations on Darrieus VAWT have contributed to

simpler and predictable characteristics, which improve the reli-

ability and performance of the turbine. The innovations differ

distinctively from the previous developments in the 1970s

1990s,

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 pro

ﬁ

le for the blades. He

Corresponding author. SERI, Level 3, Perpustakaan Tun Sri Lanang, Universiti

Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia. Tel.:

603 8921 4596;

fax:

603 8921 4593.

E-mail address:

willy_tjiu@yahoo.com

(W. Tjiu).

Contents lists available at

ScienceDirect

Renewable Energy

j o u r n a l h o me p a g e :

w w w . e l s e v i e r . c o m/ l o ca t e / r e n e n e

http://dx.doi.org/10.1016/j.renene.2014.09.038

0960-1481/

Renewable Energy 75 (2015) 50

patented the design in France in 1925 and in the US in 1931 and put

the working principle as a biomimicry of birds' wings by stating,

“

It

is thus possible to give these blades a stream line section analogous to

that of the wings of birds, that is to say, offering the minimum resis-

tance to forward movement and capable of converting into mechanical

energy the maximum available amount of energy of the

ﬂ

uid by means

of the useful component of the traverse thrust which this section un-

dergoes

”

[7]

. The patent covered two major con

ﬁ

gurations: curved

and straight blades as shown in

Fig. 1

The curved and straight-blades con

ﬁ

gurations have evolved into

several variations, as shown in

Fig. 2

. Curved-blades con

ﬁ

guration

has been known as egg-beater or phi-rotor due to the similar look.

There are several variations of phi-rotor, such as guy-wired,

ﬁ

xed-

on-tower and cantilevered versions (details on these types are

available in the following sections). Similarly, straight-blades

con

ﬁ

guration has several variations. Diamond, V/Y and delta (

)

variations have been documented

[8,9]

. Another variation, a

variable-geometry VAWT or often called Musgrove-rotor had been

replaced by

ﬁ

xed-pitch H-rotor (referred only as

“

H-rotor

”

in this

paper 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, while

Tilted H-rotor is given in the Part II of this article.

3. Support structures for Darrieus VAWT

G.J.M. Darrieus did not mention speci

ﬁ

c support structure for his

invention in the patent. However, curved-blades con

ﬁ

guration with

cable or guy wires support has been very popular due to the intense

research in the US and Canada. Nevertheless, several support

structures 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 for

straight-blades con

ﬁ

guration as well.

Guy wires support (A) has been widely used for phi-rotor. Guy

wires cannot be readily mounted on top of the rotor shaft in

straight-blades con

ﬁ

guration without extending the rotor shaft or

the use of support arms for the wires. Alternatively, a combination

of cantilever support and guy wires (B) has been used for straight-

blades con

ﬁ

guration. Guy wires support has been less preferable in

recent years

[10

12]

due to several drawbacks, including increased

axial load on the bearings due to wire tension in (A), vibrations

induced by the rotor and the wind, and large land area required to

mount the wires

[2]

. Fixed-on-tower (C) requires a customized

generator for a particular tower since the generator stator coils are

mounted on the tower's stationary shaft, while generator rotor are

attached to the lower hub of the rotor shaft. In addition, the sta-

tionary shaft diameter to height ratio is preferably about 0.01

0.02

[13]

. Cantilevered-rotor (D) has been used with great success. It has

several advantages compared to other types due to its simplicity in

manufacturing and maintenance. The components manufacturing

ﬂ

exible since the drivetrain is not embedded into the rotor and

stator assemblies as in (C). In addition, the drivetrain is detachable

for simpler onsite maintenance

[14]

. Among these four types,

cantilevered-rotor will most likely be dominant in future Darrieus

VAWT development.

4. Tailored airfoils for Darrieus VAWT

Airfoils used for commercial Darrieus VAWTs are usually based

on the airfoils used in aviation industry. The most common pro

ﬁ

les

used are the symmetrical NACA airfoils

[2,12,15]

, with thickness

usually ranges from 12% (NACA 0012) to 21% (NACA 0021). Some

manufacturers camber the airfoils in order to capture more energy

at either side of the rotor

[16,17]

. However, no signi

ﬁ

cant difference

in the performance has been reported as compared to the Darrieus

VAWT with symmetrical airfoil, since cambering the airfoil causes

an increase of tangential force in one half, but decreases the force in

the other half of the swept region

[18]

.

SNL found that a way to improve the performance was by

designing airfoils speci

ﬁ

cally tailored for Darrieus VAWT

[19]

. They

argued that standard aviation airfoils are not intended for Darrieus

VAWT since the operating regime of a VAWT blade is very different

from an airplane blade, which can be summarized in

Table 1 [20]

Summary of the intended tailored airfoil characteristics by SNL

compared to the experimental results obtained are shown in

Table 2 [20]

. The tailored airfoil exhibits more reliable turbine

operation via better tip speed ratio (TSR) range cut-off near the

peak

C

P

condition over the standard NACA 4-series, and is imple-

mented on variable-speed turbine

[21]

. The tailored airfoil is

employed at the transition and equatorial sections to provide over-

speeding regulation. While for the root sections at which the TSR is

lower than the equatorial section, standard NACA 4-series is used.

This is due to the customized natural laminar

ﬂ

ow (NLF) airfoils by

SNL have sharp leading edge, which make them more suitable for

high TSR. The root sections in a phi-rotor experience higher AOA, so

that the rounded leading edge of standard NACA 4-series airfoils

performs better. The combination reduces the COE and increases

the turbine reliability and lifetime

[21]

Based on the results by SNL, future Darrieus VAWT blades

should use the combination of standard NACA 4-series and NLF

Fig. 1.

Original illustrations by G.J.M. Darrieus in 1931 patent: curved blades (left) and straight blades (right). Annotations in the

ﬁ

gure: (a)

blades, (e)

supporting plates, (f1) and

(f2)

hubs, (f) and (g)

rotor shaft

[7]

.

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

airfoils for phi-rotor and whole NLF airfoils for H-rotor. Fortunately,

standard NLF airfoils like NACA 65-series look similar to the SAND

airfoils. Therefore, with the help of modern analytical software,

various NLF airfoils can be investigated for use as Darrieus VAWT

blades.

Fig. 4

shows the comparison of SAND airfoils to NACA 65-

series. Standard symmetrical NACA 4-series are represented as

dotted lines to serve as the comparison baselines.

A recent investigation using computational

ﬂ

uid dynamic (CFD)

has been performed on 20 shape of airfoils listed in

Table 3

along

with the simulation results

[22]

. Unfortunately, the author neither

included symmetrical NACA 65-series nor SAND airfoils in the

simulation.

Fig. 5

shows Selig S 1046, the best performing airfoil in

the simulation with

of 0.4051. The simulation, however, was

neither optimized for certain TSR nor Reynolds number. The author

simulated several rotor solidities between 0.1 and 0.25 with TSR

ranges from 2 to 10. The S 1046 has a similar trailing edge shape to

the NLF airfoils shown in

Fig. 4

. The leading edge is also slightly

sharper than the standard NACA 4-series, but it is not as sharp as

the NLF airfoils. However, the main difference between S 1046 and

SAND/NACA65-series is the location of the thickest point. S 1046

Fig. 2.

Timeline of Darrieus VAWT development.

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

has the thickest point similar to NACA 4-series at about 30% of

chord from the leading edge, while the NLF airfoils' thickest point is

at 50% (except for SAND 0015/47, which is at 47% from the leading

edge).

5. Egg-beater or phi (

) rotor

5.1. History of phi-rotor

Darrieus VAWT had experienced a vacuum period for about four

decades when South and Rangi of the NRC of Canada reinvented the

phi-rotor design in 1968

[23]

. The local community called it

“

Rangi-

South Wind Turbine

”

[24]

, being unaware it was previously

invented by Darrieus. Thereafter, Darrieus VAWT caught the in-

terests of many researchers

[25

38]

, and various dynamic analysis

on the performance were formulated, including blade momentum,

vortex, and

ﬁ

nite-difference models.

Unfortunately, not long after the investigations into phi-rotor

gained momentum, several machines experienced problems in

the drivetrain, control system and brakes. The failure started with

the

ﬁ

rst large scale phi-rotor of 224 kW manufactured by Dominion

Aluminium Fabricators under NRC supervision in Magdalen Islands,

Canada

[4]

. The machine crashed to the ground in 1978, a year after

its operation. In the US, Alcoa built several phi-rotors under SNL

supervision, including the 12.8 m, 17 m and 25 m diameter with

generating capacity of 30

60 kW, 60

100 kW and 300

500 kW,

respectively. However, similar fate with the turbine in Magdalen

Islands, turbines built by Alcoa had various problems. The 12.8 m

turbine collapsed in 1980 when the rotor column vibrated and

buckled due to over-speed. The 25 m turbine crashed in 1981 when

software error in the controller failed to actuate the brake in strong

winds

[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. The

rotor had a swept area of 955 m

and height-to-diameter ratio of

1.25. It achieved 500 kW rated power at 37.5 rpm in mean velocity

Fig. 3.

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

Table 1

Operating 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 and

negative AOA twice per revolution,

which are often exceeding

Operate in nearly

steady conditions

at near zero AOA.

Stall

Encounter stall frequently,

especially in strong wind.

Encounter stall only

in unusual operating

conditions.

Reynolds

number

(

)

Between a few hundred thousand

and a few million.

Usually between

three and

thirty million.

Table 2

Comparison between actual and intended characteristics of SAND airfoils.

Requirements of tailored-airfoil

Actual characteristics of

tailored-airfoil

Increase the maximum

(higher power generation).

Modest value of maximum

P

.

Force blade stall at a wind

velocity closer to the maximum

P

(Over-speed and power

regulation in strong winds).

- Low drag at low AOA, and

high drag at high AOA.

- Sharp stall.

Allow the turbine to operate at

higher rotational speed

(higher power generation and

lowering the cost of

direct-drive generator).

Higher operational speed is

achieved by using low

thickness/chord ratio.

Fig. 4.

Tailored airfoil by SNL (left)

[20]

and standard NACA 65-series (right).

Table 3

List of airfoils simulated with the corresponding

P

[22]

Airfoil

max Airfoil

max

NACA 0010 0.2345 NACA 63415 0.1711 AG18

0.0123 FX66S196

0.2074

NACA 0015 0.2947 NACA 63418 0.2772 S 809

0.3428 FX77W256 0.1639

NACA 0018 0.2964 AH93W174

0.2469 S 9000 0.1696 FX71L150

0.2961

NACA 0021 0.2679 AH93W215

0.2541 S 1046 0.4051 FXL142

0.3311

NACA 6312 0.1290 AH94W301

0.2130 S 1014 0.2769 FXLV152

0.3576

Fig. 5.

Selig S 1046 airfoil in comparison with standard NACA 0017 airfoil

[22]

.

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

of 12.5 m/s. The peak

of 0.409 was obtained at TSR of 6.34

[39]

Abundant data on large scale phi-rotor as well as the

“

Test Bed

”

are

available at SNL website. However, the largest Darrieus VAWT in

the world was a phi-rotor built in Canada with rated power of

4 MW, rotor height of 96 m and diameter of 64 m. Construction of

the phi-rotor called Eole began in 1982, and was completed in 1988.

However, the Eole was mostly operated in reduced speed, and the

power output was limited to 2.5 MW to ensure longevity

[40]

. The

turbine was successfully operated for 5 years until 1993 when it

was damaged during a storm

[41]

. Repairing the damage was

deemed too costly since the whole rotor needed to be dismantled.

Instead, the Eole was utilized as a tourism icon to show the

achievement of Canadian wind energy sector in attempting large

scale Darrieus VAWT.

The

“

Test Bed

”

inspired FloWind Corp. to commercially market

Darrieus 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 than

800 Darrieus VAWTs in the Altamont and Tehachapi passes in

California

[41]

. However, despite the successful operation of the

turbines, the company went bankrupt in 1997 due to production

ﬂ

eet

ﬁ

nancing could not be obtained. Thereafter, VAWT was out-of-

favor and virtually all government sponsors on VAWT research

were terminated

[8]

.

Until recently,

ﬁ

xed-on-tower and cantilevered phi-rotors have

gained popularity. The new designs utilize tubular tower, and does

not use guy wires. The designs offer simpler and more reliable

system than the conventional guy-wired phi-rotor. 50 kW

ﬁ

xed-on-

tower rotors are developed by ArborWind in collaboration with

Johnson System, Inc. (JSI)

[10]

. The target markets include rural use,

large industrial, farm and green houses. Large scale

ﬁ

xed-on-tower

phi-rotor with power rating of 200 kW has been attempted by

McKenzie Bay International, Ltd. (MKBY) in collaboration with

Clean Green Energy, LLC. (CGE)

[11]

. However, high cost prohibits

the commercialization of the rotor. Instead, smaller cantilevered

versions of 20

65 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 Agriculture

Conservation and Production Research Laboratory and SNL of the

US Department of Energy (DOE)

[12]

. VMI stated that the design is

an innovation of the earlier concepts developed by SNL, NRC of

Canada, Alcoa, Agway, the National Rural Electric Cooperatives

Association, FloWind and Vawtpower, Inc

[12]

. In addition, SNL

provides technical assistance and instruments to measure the rotor

performance. Furthermore, new interest in Darrieus VAWT for

multi-megawatt offshore wind power generation has granted SNL

$4.1 million from the US DOE. The project was started in 2012, and

will be completed in 2017.

5.2. Assessment on phi-rotor

Rotor illustration and the components of guy-wired phi-rotor in

the early 1970s development are shown in

Fig. 6

a and b, respec-

tively

[42]

. Two and three-bladed rotors were manufactured, and

were structurally enhanced with struts forming

“

”

sign. The struts

were detrimental because they added costs, and lowered the per-

formance due to parasitic drag and turbulent

ﬂ

ow formed by them.

The struts were eliminated in the following designs since the tro-

poskein blades were able to withstand stresses in high rotational

speed.

The phi-rotor was supported by guy cables mounted at the top

of rotor shaft to the ground at equally-spaced angles. Thrust bear-

ings were used at the top and bottom of the rotor, enabling it to

rotate freely. Mechanical brake was mounted at the bottom of rotor

shaft to ensure safe operation in strong winds. Torque sensors via

ﬂ

exible couplings were utilized to monitor anomalies in the system

and to regulate the generators power. The early design of guy-wired

phi-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-wired

phi-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 possibly

eliminated since they add parasitic drag and cost, and (iii) the blade

airfoil shape should be tailored for VAWT application

[40]

. In

addition, the brake system had been positioned directly below the

rotor lower hub in order not to obstruct maintenance work on other

components while keeping the rotor stationary, and to prolong the

gearbox lifetime since braking force was not transmitted through

the gearbox.

The guy-wired phi-rotor blades were manufactured via stan-

dard extrusion method using aluminum alloy, which were then

Fig. 6.

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

[79]

and (b) major components illustration

[42]

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

bent conforming troposkein shape

[5]

. As implemented in the

“

Test

Bed

”

, a blade was divided into three sections: root, transition and

equatorial section. The sections were equipped with extruded

NACA 0021 (1.22 m chord), SNL 0018/50 (1.07 m chord) and SNL

0018/50 (0.91 m chord) airfoil pro

ﬁ

les, respectively

[39]

. SNL code

denoted natural laminar

ﬂ

ow airfoils developed by Sandia for use

on Darrieus VAWT

[21]

. There seemed to be no standardization in

the naming of the tailored airfoils, for example, SNL 0018/50 might

be referred to as SAND 0018/50 or SANDIA 0018/50. Nevertheless,

the codes for airfoil thickness and thickest location remain the

same.

Fig. 7

a and b shows the

“

Test Bed

”

with illustrations on the

blade sections and geometry. Upper root section was longer than

the lower one in order to maintain the shape under bending stress

due to gravitational loading.

The airfoil pro

ﬁ

le tailored to a particular section of a blade

serves two purposes: structural strength and aerodynamic perfor-

mance. Different propelling forces between the root and equator

section would cause localized edgewise bending on a blade of

uniform dimension from the root to the equator section. This

edgewise bending is insigni

ﬁ

cant for small scale rotor, but for large

scale rotor like the

“

Test Bed

”

it would be detrimental. Thus, in

order to minimize the fatigue, airfoil chord dimension was altered,

so that the propelling force would be more uniform from the root to

the equator. In addition, the chord of the root section was made

thicker for the same purpose of reducing fatigue due to bending,

and also to compensate for lower TSR and higher AOA.

Fig. 8

a and b

shows typical components of a guy-wired phi-rotor based on the

“

Test Bed

”

. Generally, the major components consist of the

following:

rotor assembly (rotor column, upper and lower hubs, and

blades),

shaft assembly (interconnection shaft, brake disk and caliper,

rubber isolator, and torque sensor),

base structures (gearbox, generator, foundations, and ground

equipment station), and rotor support structures (support

stands, upper and lower rotor bearings, guy wires and

tensioners).

VMI has been testing VP100 (shown in

Fig. 9

a) since 2006, which

is a three-bladed cantilevered phi-rotor with 60 kW rating. The total

structure is about 23.7 m tall, while rotor height and diameter are

13.5 m and 15 m, respectively. The blades use NACA 0015 pro

ﬁ

with 0.35 m chord length, 0.053 m chord thickness and 0.0053 m

wall thickness

[12]

. The blades were made of extruded aluminum

alloy, which were bent into troposkien shape. The blades were then

ﬁ

tted with two hinges at the ends, which were epoxied into position.

The hinges allow vibration in the rotor assembly without stressing

the aluminum blades, which is an innovation of the rotor. VP100 is

connected to vertical gearbox and generator considering the

1200 rpm generator used in the system

[14]

, while the rotor speed is

only 62.4 rpm

[12]

. The high rotational speed of generator suggests

that a speed increaser is used in the system. The main reason of

using a combination of a speed increaser and a generator is to get the

reliability improvement over a multi-stage gearbox, while keeping

the cost reasonably below a direct-drive generator

[43]

Fig. 9

shows an artist's impression of the major components of the VP100.

Maintenance work demonstration on the VP100 showed that it

takes only four hours to replace the 800 pounds (363 kg) generator

without using crane as in typical HAWT maintenance. In addition,

only hand tools and light jacks are used in the process since the

generator is placed on the ground. VMI claims that the VP100

maintenance cost is much lesser than the HAWT counterpart, since

the use of crane adds thousands to tens of thousands of dollars in

the servicing cost of the wind turbine

[14]

.

MKBY and CGE successfully installed a 200 kW

ﬁ

xed-on-tower

phi-rotor in Ishpeming, Michigan in 2010, after having installa-

tion problems in the previous year. A troposkien blade was

deformed when lifted by a crane, which prompted for redesigning

the core structure of the blade. In the next attempt, a frame was

constructed to hold a blade while being lifted by a crane to be

assembled to the rotor shaft. However, the turbine has not been a

satisfaction, primarily due to the high cost in manufacturing. In

addition, the installation was too expensive and complex

[11]

.

Fig. 10

a and b shows the 200 kW and its major components

description, respectively. The turbine has an outer (rotor) shaft

which rotates around an inner (stationary) shaft. The stationary

Fig. 7.

The

“

Test Bed

”

. (a) Photograph

[40]

and (b) Blade geometry

[39]

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

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) 50

67

56

shaft functions as the holding post for the rotor assembly as well as

foundation post. Truss structure is used for additional foundation

support for the whole system.

MKBY and CGE are currently developing cantilevered phi-rotor

called

“

Wind-e20

”

, which is scheduled for completion in 2013.

Wind-e20 is the 20

65 kW version based on the improvements on

the 200 kW model

[11]

. Wind-e20 has several unique features,

including remote-controlled foldable blades for safety in strong

winds, typically above 38 m/s. The blades are made of straight

sections with joints that are powered by hydraulic pumps, so that

during strong winds the hydraulic actuator pulls the blades close to

the shaft, similar to the closing of an umbrella. In addition, the

blades are equipped with airbrakes, particularly at the equatorial

section. The airbrake movement is electronically controlled

depending on the wind velocity

[44]

Fig. 11

shows an artist's

impression 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 JSI

have been manufacturing 50 kW

ﬁ

xed-on-tower phi-rotor similar

to the 200 kW model used by MKBY and CGE. However, the turbine

built by ArborWind and JSI does not include a ground-mounted

stationary shaft, and the blade is not manufactured in multiple

small sections. The lack of fully extended stationary shaft reduces

cost in trade off with higher bending stress on the shaft mounted

on the truss structure. In addition, a

ﬁ

xed blade further reduces cost

on 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 a

speci

ﬁ

c COE between 9 and 12 cents per kWh

[10]

.

Fig. 12

a and b

shows the commercial prototype of the 50 kW and its major

components, respectively.

5.3. Shape of the phi-rotor blade

In the early development, phi-rotor was hailed for its advantage

of using troposkein blades, which took the shape of a jumping rope

enduring high centrifugal force. Therefore, the blades could be

made slender, light and low cost via relatively simple extrusion

manufacturing method

[5]

. However, phi-rotor performance varies

depending on the blade curvature as shown in

Fig. 13 [23]

. Ef

ﬁ

ciency is in

ﬂ

uenced by the length of the relatively straight section

at the equator to the rotor height

[45]

, which is denoted by

Therefore, SNL neither used ideal troposkien, catenary nor

parabola shapes for the phi-rotor due to the curvature effect.

Instead, SNL used straight-line for the top and bottom parts and

circular arc-shape for the middle of the rotor

[19]

. The reason

behind such con

ﬁ

guration is to increase the

ratio while still

having the ability to endure centrifugal force. In addition, blade

curvature is affected by the

/

D

ratio. In term of

ratio, pure

troposkien shape has the

ratio of about 0.9, while the

“

Test

Bed

”

had the

ratio of 1.25. Furthermore, Paraschivoiu

[23]

suggested that future phi-rotor will use extended height-to-

diameter (EHD) with

/

D

ratio between 1.3 and 1.5, which makes

the

ratio closer to unity. However, the trade-off in increasing

the

H

/

and

ratios is the increment in operational bending

stresses since the shape has become nontroposkien

[8]

5.4. Disadvantages of phi-rotor

Recent innovations by MKBY and CGE, ArborWind and JSI, and

VMI have demonstrated signi

ﬁ

cant advantages of the

ﬁ

xed-on-

tower and cantilevered phi-rotor over the conventional guy-

wired phi-rotor, while still using the acclaimed fatigue-free

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) 50

troposkien blades. Nevertheless, the phi-rotors are the product of

lessons learned in the guy-wired phi-rotor, which was the most

extensively investigated design among the Darrieus VAWT varia-

tions. Based on the failures in the design, several disadvantages of

phi-rotor, especially the guy-wired type, are such as:

High axial load on support bearings due to rotor assembly and

guy-wires.

Tare and zero-wind losses are relatively small and can be

neglected compared to the total power produced

[39]

. Tare loss is

the power loss due to bearing friction of a rotor without the blades

attached, while zero-wind loss is the friction loss with the blades

attached and spun at no wind. However, a recent report released by

SNL in 2012 stated that the bearings, especially the bottom support

bearing must be designed to support both the rotor weight and

downward force due to the wires tension. Therefore, the required

high capacity of the support bearings can contribute signi

ﬁ

cantly to

the capital cost of the turbine

[8]

Uneven wind velocity across rotor height

The swept-area of phi-rotor is bound by the troposkien shape

and is determined by rotor height-to-diameter (

/

D

) ratio. With

the tendency to use higher

/

D

ratio in order to get higher

equatorial section

[23]

, rotor height increases more than the

increment in rotor diameter. For large-scale on-land phi-rotor

which 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 section

near the ground surface, which causes uneven lift force produced

across the blades length that contributes to instability, bending

and torsion stress on the blades

[46

48]

. On the other hand,

straight-bladed con

ﬁ

guration type has the

ﬂ

exibility in adjusting

the swept-area. Rotor height and diameter can be independently

adjusted to suit particular design. In addition, H-rotor type is

mostly mounted on a tower, which further reduces uneven wind

velocity variation.

Gravity-induced bending stress on the blades

In phi-rotor, gravity-induced bending stress is the force to

deform the troposkien shape due to the blades own weight. For a

small-scale phi-rotor less than 100 kW, gravitational loading on the

blades may be neglected with respect to centrifugal force. However,

weight of the blades becomes signi

ﬁ

cant in large rotor since the

length of a typical phi-rotor blade is three times a HAWT blade with

the same swept area and solidity

[40]

. When the rotor is stationary,

the bending stress on the blade is static. However, when the rotor

starts to rotate, the static bending stress becomes dynamic and is

overcome by centrifugal force depending on the rotational speed of

the rotor. The bending stress oscillates in accordance with the

centrifugal force, which is affected by wind velocity, turbulence and

wake effect at the downwind side.

Paraschivoiu

[23]

and Sutherland et al.

[8]

mentioned that

gravity-induced stress is related to rotor height-to-diameter (

/

D

)

ratio. Lower

/

D

ratio leads to greater gravitational stresses, but the

type of airfoil can be tuned to minimize gravity and radial aero-

dynamic in

ﬂ

uences. This is the reason why the

“

Test Bed

”

was

equipped with thicker root section than the equatorial section, and

upper root was longer than the lower one, i.e. to maintain blade

shape when the rotor is stationary as well as sustaining stresses

endured by the blades in motion.

Gravity-induced bending stress is less vulnerable for straight-

bladed con

ﬁ

guration since the blades are shorter and have lower

bending moment, i.e. the blades are more rigid at the same chord

length and thickness as a phi-rotor blades. In addition, they are

positioned vertically and are suspended by support arm(s), so that

they are not subjected to constant bending stress due to gravity.

Support arm is the component which endures gravity-induced

bending stress, and it can be made stronger and tapered from the

shaft to the blade.

Wake due to large rotor column

The rotor column of a phi-rotor needs to sustain high tension

produced by guy wires as well as cyclic torque produced by the

blades, so that buckling strength is the most important aspect of a

rotor column requirement

[23]

. However, large rotor column

extending across the height causes blades in leeward position to

suffer from turbulent

ﬂ

ow region known as wake, especially in

large-scale rotor. A wake not only reduces performance, but also

causes vibration on the blades and support structures.

Fig. 14

shows

the vortices and wakes generated by the blades and rotor column of

a 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 endure

intermittent rotor and wind forces which make them vibrate and

oscillate. The oscillation frequency and operating mode of guy

wires were studied extensively in order to avoid resonances with

Fig. 11.

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

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

rotor vibration

[47]

. Therefore, it is dif

ﬁ

cult to build a very tall rotor

equipped with guy wires in order to take advantage of higher

altitude winds.

Large footprint to mount guy-wires.

Since guy-wires are

ﬁ

xed above the rotor assembly, large land

area is required for anchoring them. This restricts the imple-

mentation of phi-rotor at limited and utilized area, such as in

farming land. In addition, the use of guy-wires is not practical for

offshore application. Nevertheless, higher

/

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 modi

ﬁ

cation of the

straight-blades Darrieus VAWT by employing blades ree

ﬁ

Fig. 12.

A 50 kW cantilevered phi-rotor manufactured by Arborwind and JSI: (a) Photograph

[10]

and (b) artists impression of the major components.

Fig. 13.

of phi-rotor in respect to curvature ratio

[23]

Fig. 14.

Vortices and wakes of a typical Darrieus VAWT

[23]

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

67

59

mechanism to prevent the rotor from over-speeding in strong

winds. The turbine consisted of two sets of straight blades sup-

ported on a horizontal beam similar to the shape of an

“

”

letter.

The horizontal beam, also taking the shape of an airfoil, was in turn

supported by a tower at the middle of the beam. The drivetrain and

generator were located at the base of the tower. Each set of the

blades consisted of two equal portions feather-able about the

horizontal beam, for which in reefed position they took the shape of

a double-arrow

“

4

”

, thus reducing the swept area as well as the

lifting 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 of

Musgrove-rotor

[50]

, while

Fig. 16

shows the major components

diagram

[49]

Musgrove-rotor had similar components to the guy-wired phi-

rotor. However, Musgrove-rotor was equipped with two stages of

speed increaser (upper and lower gearbox) as shown in

Fig. 16

. The

consideration of using multi-stage gearbox is to reduce the number

of poles needed for the generator, therefore, reducing the cost of

generator. Transformer was used to step-up the AC voltage before

injecting it into transmission lines on electricity grid system.

Promising results in the early development made the UK govern-

ment

ﬁ

nancially supported the scaling-up of Musgrove rotor in the

late 1970s

[51]

. The

ﬁ

rst large scale Musgrove rotor was completed

in 1986 by VAWT Ltd., and was named VAWT-450 (based on the

swept area of 450 m

). It had rotor diameter of 25 m and rated

power 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 curved

blades. However, the main disadvantages of variable geometry

VAWT were the unnecessarily complex design of ree

ﬁ

ng mecha-

nism, large concrete structure and high cost in building the turbine.

In addition, the Musgrove-rotor consisted of many components

which 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 terminated

and shifted to H-rotor.

7. Giromill or cycloturbine

7.1. History of giromill

Another variant of straight-blades Darrieus VAWT is giromill or

also known as cycloturbine. The term

“

giromill

”

was constructed

from two words: cyclogiro and windmill coined by MCAIR, which

developed cyclogiro airborne vehicle and adapted it to the version

of the windmill

[52]

. It was developed in the US around 1976, at

about 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 is

maintained relatively constant at certain negative angle for one half

and certain positive angle for the other half of revolution at certain

wind velocity. The pitching method include mechanical and elec-

trical actuators, such as using a cam and push-rod mechanism

[53]

hydraulic mechanism, and DC motor connected to a blade pivot axis

via a timing belt

[52]

After the successful feasibility study, a three-bladed pre-

commercialization prototype giromill was built in 1980 under

funding from US DOE.

Fig. 18

a and b shows the MCAIR giromill

[40]

and its components description

[54]

, respectively. The giromill had

a diameter of 58 ft (17.7 m) and rotor height of 42 ft (12.8 m), which

produced constant power of 40 kW at 8.9

17.9 m/s wind velocity.

The drivetrain concept was similar to the Musgrove-rotor, except

for the placement of the brake disc and the single stage gearbox

utilized on the giromill. However, despite the successful develop-

ment of MCAIR giromill, the US government chose a two-bladed

downwind HAWT with similar power rating. The decision was

based on higher annual energy generation and lower COE.

7.2. Assessment on giromill

A giromill is able to achieve maximum

P

of 0.5

[3,52]

, which is

more ef

ﬁ

cient than other Darrieus VAWT variations presented in

this paper. Although variable-pitch mechanism in giromill shows

higher

performance

than

ﬁ

xed-pitch

Darrieus

VAWT,

the

Fig. 15.

An experimental model of Musgrove-rotor

[50]

Fig. 16.

Major components of Musgrove-rotor

[49]

Fig. 17.

The VAWT-450 Musgrove-rotor

[6]

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

67

60

mechanism is costly. Complexities of the pitch-change system and

support structures for changing the pitching angle reliably over the

service 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 original

Darrieus patent. Experience gained from the VAWT-450 showed

that the ree

ﬁ

ng mechanism in Musgrove design was unnecessary

because passive stall of the airfoils in vertical position during strong

wind naturally prevented the blades from over-speeding. Thus,

another turbine was built as a H-rotor by VAWT Ltd. in 1988 and

was named VAWT-850 which had rated power of 500 kW and rotor

diameter of 38 m

[55]

Fig. 19

shows the VAWT-850, whose

connection of support bar and blades was simpler than the

Musgrove-rotor at the background. The turbine was completed in

August 1990 and was tested until February 1991 when one of the

blades broke due to an error in the

ﬁ

berglass blades manufacturing

process

[56]

Current large scale H-rotor is developed by Vertical Wind AB, a

wind energy research company based in Sweden in collaboration

with Uppsala University. After successful initial investigations on

2 kW and 12 kW prototypes

[57,58]

, the company produced a large

scale turbine of 200 kW

[59,60]

. The production of a 200 kW H-

rotor was started in October 2009, and has been operational since

April 2010.

Fig. 20

a and b shows the rotor and an artist's impression

on the major drivetrain components, respectively. The structural

concept 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 pitching

mechanism as well as a gearbox.

Vertical Wind AB also reported that fewer moving parts

compared to conventional wind turbines gives higher availability

and reliability as well as lower maintenance cost. The company

claims that direct-drive generator provides excellent cost ef

ﬁ

ciency

since it is placed on the ground, and hence, does not need to be

optimized for the weight and size. In addition, costs related to

gearbox failure are eliminated. Furthermore, the H-rotor is quieter

than a HAWT of similar size. The success story was received

enthusiastically by the Swedish Energy Authority, E.ON and Fal-

kenberg Energy, for which four turbines will be installed there

[61]

.

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

ﬁ

xed-pitch H-rotor with the VAWT-450 Musgrove-rotor in the

background

[6]

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

67

61

8.2. Assessment on H-rotor

In the 1970s

1980s, glass-

ﬁ

ber reinforced plastic (GFRP) was

not common for being used as Darrieus VAWT blades. Until Mus-

grove and MCAIR started developing straight-bladed con

ﬁ

guration,

it was found out using aluminum via extrusion method was not

suitable for H-rotor blades due to cyclic

ﬂ

apwise bending stress.

Therefore, the recent straight-bladed Darrieus VAWT con

ﬁ

gura-

tions use GFRP and carbon

ﬁ

ber composites similar to the HAWT

blades, which is able to sustain continuous cycles of edgewise and

ﬂ

apwise bending stress during the blades service life. By the use of

GFRP and carbon

ﬁ

ber composite, the bene

ﬁ

t of stress-enduring

troposkien-shaped aluminum blades for phi-rotor is compensated

by the stress-enduring composite materials for H-rotor blades. In

addition, the aerodynamic drag caused by struts or support arms in

H-rotor is also compensated by the increased performance of the

rotor, since blade equatorial portion-to-rotor height (

) ratio

becomes unity, as described earlier.

The H-rotor program in UK was terminated after the failure of

VAWT-850 due to the prohibitively high cost in building the con-

crete tower and support structure

[6]

. Similarly in the US, H-rotor

was not attempted by the government despite successful tower

and drivetrain components installation in the giromill program.

Current development by Vertical Wind in the Sweden has

improved the designs of H-rotor by Musgrove in the UK and

giromill by MCAIR in the US. However, cyclic torque in large scale,

especially in multi-megawatt range, requires investigations into

strong and light-weight rotor shaft, since an extended rotor shaft is

prone to vibration and fatigue, primarily due to torsional stress on

the shaft.

A retrospective analysis by SNL in 2012 stated that H-rotor has a

high 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 major

concern in Darrieus VAWT design. Airbrake system has been a

standard aerodynamic brake for commercial airplanes, which

deploy extended

ﬂ

aps during landing. In sport cars, aerodynamic

braking system has been used in conjunction with mechanical

brake to provide higher deceleration rate by deploying the rear

spoiler upward. Therefore, H-rotor has a potential to embed similar

aerodynamic 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 modi

ﬁ

ed into another variant in which the blades

were twisted along the perimeter to form helical shape. Surpris-

ingly, the modi

ﬁ

cation was intended as a water turbine since the

inventor, Professor A.M. Gorlov of Northeastern University, is an

expert 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 disclosed

patents stated that it could be used for hydro-pneumatic, hydro,

wind and wave power systems

[62,63]

Fig. 21

c show the comparison of Helical H-rotor for water and

wind turbines. The main difference between them is that the water

turbine has a much higher solidity, which is the ratio of blades

coverage area to turbine swept area. The hydrofoil's chord of the

Gorlov water turbine blades is made longer and thicker in order to

increase the structural strength. In addition, the rotating speed is

reduced, so that the chance of cavitation is minimized. Further-

more, the Gorlov water turbine rotates much slower than the

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) 50

67

62

QuietRevolution and Turby wind turbines, which is bene

ﬁ

cial to the

marine lives.

The QR5 turbine shown in

Fig. 21

b is manufactured by Quiet

Revolution 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 kW at 16 m/s wind velocity. The cut-in

and 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

ﬁ

ber composites for the rotor assembly and

direct-drive permanent magnet generator. Another helical H-rotor

shown in

Fig. 21

c is developed by Turby BV, a Dutch manufacturer

which produces 2.5 and 10 kW turbines

[15]

. The company has

been cooperating with Delft Technical University to produce the

turbine. Similar to the QR5, the Turby blades are manufactured

using carbon

ﬁ

ber aramide composite. In addition, direct-drive

permanent magnet generator is also used. Turby has an overall

CP of 0.3 from the wind power to electricity. It utilizes NACA 0018

pro

ﬁ

le for the blades, and is operated at TSR of about 3. The cut-in

and 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 by

distributing a blade pro

ﬁ

le along the perimeter of the rotor

Fig. 21.

Helical H-rotor for: (a) water turbine (Gorlov Helical Turbine)

[81]

, (b) and (c) are wind turbines by QuietRevolution

[64]

and Turby

[15]

, respectively.

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

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

ﬂ

uctuation is signi

ﬁ

cantly reduced when

the helical shape covers a full 360

rotation. Bene

ﬁ

ts of having

regular torque include better power output regulation and reduced

cyclic stress on the drivetrain. In addition, noise is reduced and

slightly higher effective chord is obtained

[66]

. Currently, helix

design is getting popularity not only because of better performance,

but also for the esthetic value, in which modern elegant design

harmonizes the elements in the space.

Comparison of Helical H-rotor to H-rotor and phi-rotor has been

done

[67,68]

Fig. 22

shows modeled geometry of H-rotor, phi-rotor

and helical H-rotor. The rotors were modeled with 3 blades spaced

equally at 120

using symmetrical NACA 0015 with chord-to-radius

at mid-span of 0.15, aspect ratio of 20 and TSR of 5. The modeled

rotors performance is shown in

Fig. 23

where torque

ﬂ

uctuation

varies three times every rotation. The graphs show that a phi-rotor

has the most

ﬂ

uctuation with variation of about 0.3

, followed by

H-rotor with 0.2

, and the least

ﬂ

uctuation is achieved by the

helical H-rotor with variation of about 0.03

. However, despite the

bene

ﬁ

ts gained, true helical blades are more expensive to

manufacture.

10. Articulating H-rotor

Another recent variation of Darrieus VAWT is the articulating H-

rotor developed by Blackhawk Project, LLC. The wind turbine

concept is based on a helicopter rotor that adjusts automatically to

the wind pressure, so that vibration and mechanical stresses are

reduced. Bruce Boatner, who invented the articulating H-rotor in

2006, is an engineer and helicopter pilot. The articulating H-rotor

received US Patent no. 7,677,862 on 16th March 2010

[69]

Currently, Blackhawk, LLC is testing TR-10, a prototype model of

1.5 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 or

swashplate-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 pitch

angles are altered depending on which blade is being pushed by the

wind.

Fig. 24

shows an illustration of the 1.5 kW articulating H-

rotor with annotation on its major components. Pitch-control via

Fig. 22.

Geometry of the modeled Darrieus VAWTs: (a) H-rotor, (b) Phi-rotor and (c) Helical H-rotor

[67]

Fig. 23.

Power coef

ﬁ

cient variations of a typical phi (

) rotor, H-rotor and helical H-

rotor

[67]

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

67

64

articulating motion allows the turbine to self-start at light winds

despite of having low solidity, higher torque during operation, as

well as for aerodynamic braking. Another advantage of articulating

motion is that the blades swiftly adapt to the wind force, thus

reducing vibration as often occurs in stiff and

ﬁ

xed blades. The

feature is highly advantageous for urban application, where the

wind is more turbulent.

11. Fish-schooling formation

The effort to study Darrieus VAWT in array con

ﬁ

guration has

been bio-inspired by the nature. Migrating birds and

ﬁ

shes show

that they have more stamina in traveling farther as a group. By

positioning themselves precisely at certain coordinates, the ani-

mals are able to gain from the vortices shed by the animals ahead.

This phenomenon has been investigated for wind turbine appli-

cation, and has been shown to be bene

ﬁ

cial for vertical axis

con

ﬁ

guration. A recent investigation

[70]

using stereoscopic par-

ticle image velocimetry (PIV) shows the wake and vortices formed

by a two-bladed H-rotor clearly. The H-rotor dimensions are 1 m

rotor diameter, 1 m rotor height and 0.06 m NACA 0018 chord

length, which rotates at TSR of 4.5 in a wind stream velocity of

9.3 m/s. The PIV images show the fast wake recovery of the H-rotor,

in which after only 1.5 rotor diameter distance downwind, the

cycloidal wake is no longer detectable and is replaced by large

vortical structures due to the roll-up of co-rotating small vortices

[70]

. The utilization of these vortices is the basis of VAWT

ﬁ

sh-

schooling formation.

Darrieus VAWT has an advantage in turbulence compared to the

HAWT, so that they can be formed into arrays to harness more

power in a given area. In limited area of urban population, this

arrangement would be advantageous. Unlike HAWTs that experi-

ence higher fatigue and performance loss when positioned close to

each other

[71

74]

, Darrieus VAWTs wind farm study suggested

slight reduce (or even increase in some cases) in performance

depending on the array con

ﬁ

gurations

[75]

. For a clustered tur-

bines, Darrieus VAWT pairs at downwind position recover the ef-

ﬁ

ciency to within 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 VAWT

cluster 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 small

inter-turbine spacing in Darrieus VAWT to reduce the size and

impacts of wind farm.

Fig. 25

a and b shows a biomimicry con

ﬁ

guration of Darrieus

VAWT wind farm based on wake vortices of

ﬁ

sh schooling studied

by Weihs in 1975

[75]

. Both

acw vortex

(anticlockwise) and

vortex

(clockwise) represent dipoles of wake vortices formed by the

school. The dipoles position are used as the placement of Darrieus

VAWTs. The distances between the dipoles are indicated by

and

is the downstream distance of two vortices in the same

line;

is the lateral distance between

acw

and

cw vortex

of a

Download 5,66 Mb.

Do'stlaringiz bilan baham:

1 2 3