Savonious Blade Design-comparison

8/17/2019 Savonious Blade Design-comparison

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International Journal of Modern Engineering Research (IJMER)

www.ijmer.com Vol. 3, Issue. 5, e! " #ct. $%&3 !!"$'"$'* I+ $$-'"**-5

Experimental Comparison Study for Savonius Wind

Turbine of Two & Three Blades At Low Wind Speed

Mohammed adi /li Lecturer University of Mustansiriya

ABSTRACT: In this project, experimental comparison and investigations were carried out to study the performance and to makea comparison between two and three blades Savonius wind turbine

!or this purpose, two models of two and three blades were designed and fabricated from "luminum sheet, each of them hasan aspect ratio of # " s $ %&' $(), the dimension is # % $ *++ mm height and diameter ' $ *++ mm) and the blades weremade of semi cylindrical half of diameter #d $ (++ mm) -he two models were assembled to have #overlap e $ + and a

separation gap e. $ +)

-hese two models were tested and investigated by using a subsonic wind tunnel that was fabricated for this purposeunder a low wind speed due to many reason mostly that the savonius wind turbine has its maximum performance at #/ $ -S0$ () and a high starting tor1ue at low wind speed

It was observed from the measured and calculated results that the two blades savonius wind turbine is moreefficient, it has higher power coefficient under the same test condition than that of three blades savonius wind turbine

-he reason is that increasing the number of blades will increase the drag surfaces against the wind air flow and causes toincrease the reverse tor1ue and leads to decrease the net tor1ue working on the blades of savonius wind turbine

9. IT!"#$CTI"%

0he renewa1le energ2 is considered as a new technolog2 and an alternating energ2 source to 1e used instead of fossil fuel, its continuous rising cost of it and due to growing concern to reduce the effects of climate change, such as glo1alwarming, generated 12 etensi4e and deli1erate use of fossil fuels, mainl2 in the electric !ower generating !lants andtrans!ort.

lo1al warming will continue unless de!endence on fossil is reduced, thus the 6ind !ower has a 7e2 role inreducing greenhouse gas emissions

8ref. &9.

0oda2, the most commonl2 used wind tur1ine is the ori:ontal /is 6ind 0ur1ine (/60), where the ais of rotation is !arallel to the ground. owe4er, there eist other t2!es of wind tur1ines, one of which will 1e the !rimar2 focus

of this !a!er, the Vertical /is 6ind 0ur1ine (V/60). 0hese de4ices can o!erate in flows coming from an2 direction, andta7e u! much less s!ace than a traditional /60

8ref. $9, and V/60 are definitel2 a credi1le source of energ2 for the future

8ref. &9.

V/60s ha4e a num1er of ad4antages o4er /60s, such as8ref. 39

1)im!le construction, the2 can 1e made from oil 1arrels cut in two hal4es.2)Etremel2 (low cost), sim!licit2 reduces cost of construction, and aids installation.

3)0he2 can acce!t wind from an2 direction, thus eliminating the need for re"orienting towards the wind.

V/60s wor7 well in !laces with relati4el2 low wind strength, and constant winds, V/60s include 1oth a drag"t2!econfiguration, such as the a4onius rotor, and a lift"t2!e configuration, such as the ;arrieus rotor

8ref. -9.

35. !ICILES "' SA("I$S !"T"! WI# T$!BIE%

a4onius tur1ines are one of the sim!lest tur1ines. /erod2namicall2, the2 are drag"t2!e de4ices, consisting of twoor three 1lades (4ertical < half c2linders). / two 1lades sa4onius wind tur1ine would loo7 li7e an == letter sha!e in crosssection (figure &).

0he sa4onius wind tur1ine wor7s due to the difference in forces eert on each 1lade. 0he lower 1lade (the conca4ehalf to the wind direction) caught the air wind and forces the 1lade to rotate around its central 4ertical shaft. 6hereas, theu!!er 1lade (the con4e half to wind direction) hits the 1lade and causes the air wind to 1e deflected sidewa2 around it.

'i)ure *+,% S-hemati- drawin) showin) the dra) for-es exert on two blade Savonius.


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@ecause of the 1lades cur4ature, the 1lades e!erience less drag force ( ! convex) when mo4ing against the wind than the 1lades when

mo4ing with the wind ( ! concave). ence, the half c2linder with conca4e side facing the wind will e!erience more drag force thanthe other c2linder, thus forcing the rotor to rotate. 0he differential drag causes the a4onius tur1ine to s!in. Aor

this reason, a4onius tur1ines etract much less of the windBs !ower than other similarl2 si:ed lift t2!e tur1ines 1ecausemuch of the !ower that might 1e ca!tured has used u! !ushing the con4e half, so sa4onius wind tur1ine has a lower efficienc2.

imilarl2, the three 1lade sa4onius wind tur1ine is constructed from three half c2linders, the2 are arranged at (&$%o) relati4e

to each other as shown in figure ($).

'i)ure */,% S-hemati- drawin) showin) the dra) for-es exert on three blade Savonius.

61. T0E !ESEA!C0 1"AL%

0he goal of this research is to carr2 out a stud2 and ma7e a com!arison in !erformance 1etween two and three 1ladessa4onius wind tur1ine at low wind s!eed, the reasons to stud2 them at low s!eed are

1. In man2 areas in the world a!art from coastal region, the a4erage wind s!eed is relati4el2 low and 4ariesa!!recia1l2 with seasons. It is around $% 7mCh.

2. / a4onius rotor reDuires (3% times) more surface for the same !ower as a con4entional rotor 1lade wind tur1ine.0herefore it is onl2 useful and economical for small !ower reDuirements

8ref. 59.

3. It has a high starting torDue a a4onius rotor can theoreticall2 !roduce energ2 at low wind 4elocities 8ref. *9.4. It is difficult to !rotect them from etreme winds 8ref. 9.

5. 0he !ea7 !ower coefficient for an2 a4onius rotor occurs at a ti! s!eed ratio (less than &)8ref. 9

.6. Fower wind s!eeds found at lower heights, thus V/60 li7e sa4onius can 1e installed close to the ground without an

etended !ost with the generator and the dri4en train mounting at the 1ase near the ground le4el which ma7es thesecom!onents easier to ser4ice and re!air.

I(. E2E!I3ETAL !I1 #ESI1%The Wind Tunnel:

/ su1sonic wind tunnel was designed and fa1ricated for the e!erimental !art of this research as shown in theschematic drawing and the e!erimental rig construction 8figure 39. 0he wind tunnel was designed using !l2wood modelingand fa1ricated at la1orator2 wor7sho!. 0he wind tunnel is ($5% cm) long which consists of fan section (with circular mouthentr2), rectangle section, con4erging section and sDuare eit section with straightener section.

'i)ure *4,% S-hemati- drawin) & the Experimental ri) with Savonius blade model.


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1)0he Aan section < 0he aial flow fan is mounted and encased in a circular mouth casing. 0he ca!acit2 of the fan is -5%%m

3Chour at &5%% r!m. 0he fan is dri4en 12 a single !hase %.5 76 motor.

2) 0he Rectangular section < it is made of !l2wood to ma7e the entr2 of the air 1lowing from the fan into the di4ergingsection and to deca2 tur1ulence.

3)0he ;i4erging section < it is a contraction :one made of !l2wood to !roduce a uniform 4elocit2 distri1ution with wea7 tur1ulence in traightener section.

4) 0he traightener section < it is at the entrance of the test section to 1rea7 the large scale distur1ances and eddies. 0hetraightener com!rises a num1er of !lastic sheet meshed together to form a sDuare cells sha!e to straighten the air flow andtheir ais !arallel with the direction ais of the tunnel.

5)0est ection " it has four sides made of !l2wood. #ne of the 4ertical sides has a window with glass !late for seeing thetesting models. ?art of the u!!er side can 1e o!ened to change the testing models. 0here is a hole in the u!!er !late for inserting a wind s!eed instrument. 0he four sides ma7e sDuare section of dimension (3% cm G 3% cm) with a length of (&$%cm).

6) 0he /ir !eed Regulator < it used to control the air s!eed flowing through the wind tunnel to re!resent a change inwind s!eed.

The Savonius rotor blades:

0he sa4onius rotor 1lades (the model) was designed and fa1ricated. 0he model was tested in the wind tunnel for 4arious air flow s!eed conditions.0he material choice to fa1ricate such models for two and three sa4onius wind tur1ine de!ends mainl2 on man2 criterions.

/luminum is the 1est choice due to its light weight, corrosion resistance, rigidit2, rec2cla1le materials, eas2 to construct andlow cost

8ref. 9.

0he dimensions of these rotors were selected to ha4e an as!ect ratio (/ a H C ; H &), thus the 1lades (scoo!s) were made of semi"c2lindrical scoo!s of diameter (d H &%% mm), height of ( H $%% mm) and thic7ness of (%.%' mm).

0hese 1lades were assem1led and mounted on two /luminium disc sheets (End !lates) of diameter (;o H $&% mm) andthic7ness of (&.5' mm) to o1tain a 1etter !erformance (figure -). @oth the semi" c2lindrical 1lades and the two end !lateswere assem1led to a central shaft.

'i)ure *5,% The fabri-ated models of two & three blades Savonius wind turbine.

The measuring instruments:

0he e!eriment was carried out at different wind s!eeds 82 H $ mCs, to 2 H * mCs9 and data was recorded at room

tem!erature.;ifferent digital instruments were used to measure and record the reDuired data. 0he rotational s!eed (+) of the rotor 1lades!eed was measured using a !hoto " contact tachometer (model ;0"$$*). 0he rotational s!eed was measure 12 using a

digital tachmeter to calculate the angular 4elocit2 ( , and then the rotor ti! 4elocit2 (2 rotor ). 0he wind s!eed ( 2 ) wasdetermined using the digital 0hermo < /nemometer (model /M"-$&%E). 0he anemometer 4ane was mounted and fied inthe wind tunnel " test section 1etween the straightener and the testing sa4onius wind tur1ine.

0he static torDue was measured 12 measuring the tangential force eerted on the sa4onius rotor shaft 12 digital electronicforce meter (3ei%eng model).

22. T0E BASIC C"CETS%

0he !erformance of sa4onius wind tur1ine can 1e e!lained according to the following three 1asic rules that are stilla!!lica1le

8ref. 9

1. 0he s!eed of the 1lade ti!s is ideall2 !ro!ortional to the s!eed of wind.

2. 0he maimum torDue is !ro!ortional to the s!eed of wind sDuared.3. 0he maimum !ower is !ro!ortional to the s!eed of wind cu1ed.

0he !erformance of an2 7ind of wind tur1ine can 1e e!ressed in the form of torDue coefficient (4 t ) and the coefficient of !ower (4 p) 4ersus the ti! s!eed ratio ( ).

The swept area * As ,:


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/s the rotor turns, its 1lades generate an imaginar2 surface whose !rojection on a 4ertical !lane to wind direction is calledthe swept area

8ref. '9.

0he amount of energ2 !roduced 12 a wind tur1ine !rimaril2 de!ends on the rotor area, also referred to as cross"sectionalarea, swe!t area, or interce!t area.0he swe!t area for a4onius wind tur1ine can 1e calculated from the dimensions of the rotor (Aigure 5).

Savonius area = The swept area = As = H * D

6here % H the rotor height (m). ' H the

rotor diameter (m).

'i)ure *6,% S-hemati- dia)ram of Savonius rotor wind turbine.The Tip speed ratio * ,:

0he ti! s!eed ratio is the ratio of the !roduct of 1lade radius and angular s!eed of the rotor to the wind 4elocit28ref. &%9

. 0heti! !eri!heral 4elocit2 of the rotor (Vrotor ) is defined as (Aigure *)

'i)ure *7,% S-heme of a Savonius rotor showin) the tip velo-ity of the rotor.

= ∗

6here 2 rotor H the ti! s!eed (the !eri!heral 4elocit2 of a4onius rotor) (mCsec)5 H the angular 4elocit2 of a4onius rotor (radCsec).d H the diameter of the semi"c2lindrical a4onius rotor (m).

+ow the 0i! !eed Ratio (0R) of a tur1ine is e!ressed as

= = =∗

6here 2 $ the wind s!eed (mCsec)

The Torue Coe!!i"ient # C t $:

It is defined as the ratio 1etween the actual torDue de4elo!ed 12 the rotor (- ) and the theoretical torDue a4aila1le in the wind(- w)

8ref. &&9, thus the torDue coefficient (4 t ) is gi4en 12

= =

=

∗ ∗ ∗

6here 4 t H the torDue coefficient

- H the rotor torDue ( + . m )

- w H the wind a4aila1le torDue ( + . m ) 6 H the air densit2 (7gCm

3)

/nother conce!t that can used to measure the wind tur1ine !erformance is the static torDue (- s), which measures the self"starting ca!a1ilit2 of the tur1ine. tatic torDue is defined as a maimum 4alue of the torDue when rotor is 1loc7ed i.e.without a1ilit2 to rotate

8ref. &$9. o, the static torDue coefficient is gi4en 12

= = =

∗ ∗ ∗

6here 4 ts H the static torDue coefficient- s H the rotor static torDue ( + . m )

0he static torDue of different angle of attac7 ( 7 ) relati4e to the wind direction was measured at e4er2 (3%o) to (3*%

o). Aigure

() shows the angle of attac7 ( 7 ) for 1oth two and three 1lades sa4onius wind tur1ine.


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'i)ure *8,% S-heme of a Savonius rotor showin) the an)le of atta-9 * ,.

0he torDue is defined as the force acting tangentiall2 o4er the rotor 1lade, o!erating at a distance of rotor radius (d ) from thecentre, it is gi4en as

= .

6here I H the rotor moment of inertia 8 (7g . m$) or (+ . m . s

$) 9

H the rotor angular acceleration ( & C s$ )

0he moment of inertia tells us =how much energ2 is stored in a rotating shaft or a1out how much energ2 it will ta7e toaccelerate the shaft to a !articular 4elocit2. 0his is called the second moment or moment of inertia= and it is eDual to 8ref. &39= ∗

Referring to (figure ), the moment of inertia for a semi"circular 1lade sha!e can 1e calculated according to the followingeDuation

= .

6here r H the radius (the distance of the infinitesimal element of mass from the origin) (m) H ∗ ∅

dm H the infinitesimal element of mass (7g) H . . . . ∅ ∅

t H the 1lade thic7ness (m)

0herefore, the moment of inertia for one 1lade ( I (b) 1ecomes eDual to

'i)ure *:,% S-hemati- drawin) for a semi;-ir-ular shape for moment of inertia -al-ulation./ /

=

∅ . ∅ = . . .

∅ . ∅. . . .

= . ∅. ∅ = .

6here= . . . (7g)2

0hus, the moment of inertia for two and three 1lades of sa4onius 1ecomes

= . = .

Referring to figure ('), the total moment of inertia for the sa4onius wind tur1ine is eDual to

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'i)ure *


11/16

= + .

→ = =

&ower Coe!!i"ient * C p , Anal'sis:

?ower coefficient, (4 p) of awind tur1ine is the ratio of maimum !ower o1tained from the wind to thetotal !ower a4aila1le in the wind

8ref.

&-9.

0his h2!othesis shows therelationshi! 1etween the !ower coefficient (4 p) and the wind s!eed(2 ), which e!resses the 1asic theor2of the a4onius wind tur1ine.?rinci!all2 the !ower that thesa4onius rotor can etract from thewind ( 8 w) is less than the actuala4aila1le from the wind !ower ( 8 a).0he a4aila1le !ower ( 8 a), which isalso the 7inetic energ2 (E) of thewind, can 1e defined as

6here ma H wind mass flow rate

stri7ing the swe!t area of the windtur1ine (7gCsec).

H 6 . " s 2

@ut, the swe!t area( " s H % 9 ' ),therefore the actual !ower 1ecomes

=

0he !ower that therotor etracts fromthe wind is

6here 8 w H the !ower that the rotor etracts from the wind (6att).0he !ower coefficient (K

0hee!erimentBs !rocedure was

carried out andtested in the wind

tunnel andthereDuiredmeasurement wereo1tainedto stud2

the !erformance of thetwo 1ladesand three 1ladessa4oniuswindtur1ineand ma7esthecom!arison 1etweenthem tosee whichone is 1etter in

!erformance than the other.0he !erformance 8the dimensionless

!arameters torDue coefficient (4 t ) and

!ower coefficient (4 p)9 was e4aluated asfunction of the dimensionless !arameter the ti! s!eed ratio ( / ) at low winds!eeds in terms of starting acceleration

and maimum no"load s!eed.Aigure (&%) shows the !lot

1etween the wind (air) s!eed and therotor re4olution (r!m) for 1oth two andthree 1lades sa4onius wind tur1ine, ita!!ears that as the wind s!eed increasesfrom 8(% mCs) u!to (3 mCs)9 where thesa4onius wind tur1ine is initiated andstarts to mo4e. /t this wind 4elocit2where the wind tur1ine starts to mo4e,the wind 4elocit2 is called the cut ins!eed, the low cut"in s!eed for this t2!eof wind tur1ine which is a1out ($.5 mCs)and two 1lades sa4onius is a little 1it

lesser than three 1lades.

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R o t o r

600

500

400

300

200

100 100

0 0

1 2 3 4 5 6 1 2

Wind speed(m/s)

'i)ure *+=,% The )raph showin) the rotorrpm versus wind speed for two & three

blade.

Aigure (&%) showshigh rotational s!eed, thecause is that slim rotor withsmall diameter can get higher rotational s!eed 1ut lower

tor Due

> and 4ice 4ersa rotor with 1igger rotational diameter !roduces a 1igger torDue 1uta lower rotational s!eed.

It is o1ser4ed from figure(&&) that static torDue for two 1lade sa4onius wind

tur1ine 4aries with the angle of attac7 8angle of rotation ( L )9 for a winds!eed of (2 H 5.3 mCs) , the statictorDue was measured at e4er2 (3%

o to

3*%o), it a!!ears the torDue that can

1e !roduced during each re4olution isan oscillator2 torDue.

Two

Bla

de -Static

Torque0.06

0

.

0

4

0.02

0.00

0 30 60 90 120 150 180 210 240270 300330 360angleofattack

'i)ure

*++,%

The

stati-tor?ue

variatio

n with

an)le of

rotationfor two

blade

savoniu

s windturbine.

Aigure (&$) showsthe static torDuecoefficient for two 1lade sa4oniuswind tur1ine for

dif f er ent wi

nd s !eed (5.3

-.* mCs), thestatic torDuecoefficient 4arieswith increasingthe angle of rotation, it startsto increase from(%

o to 3%

%) to

reach itsmaimum 4alueof (%.3 %.*5)res!ecti4el2 andthen goes downto decrease from(3%

o to &5%

o) to

reach it lowest4alue of (%.&& %). It isnoticea1le thattorDue 4alues are2ielding thes2mmetr2 for flow angleshigher than (&%Nto 3*%

o). /t angle

of (&5%o and

33%o), the static torDue

coefficient has itlowest 4alue and for lower wind s!eed itma2 has a negati4etorDue.

'i)ure *+/,% The stati

- tor?ue-oeffi-ientvariationwith an)leof rotationfor twobladesavoniuswindturbine atdifferentwind speed.

Aigure (&3)shows thestatic torDuecoefficientfor three 1ladesa4oniuswindtur1ine for (5 mCs)wind s!eed,the static

torDue coefficient4aries with increasingthe angle of rotation, itstarts to increase from(%

o to *%

%) and then

goes down to decreasefrom (*%

o to &$%

o), It

is noticea1le thattorDue 4alues are2ielding the s2mmetr2for flow angles higher than (&$%N) from (&$%

o

to $&%o) and ($-%o to33%

o).

0he static torDue for 1oth two and three 1lade is found to 1e !ositi4e at an2 angle,high enough to o1tainself"starting conditions.

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0.2 0.4 0.6 0.8 1.0


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Tip Speed Ratio

'i)ure *+5,% The tor?ue

-oeffi-ient variation with the

tip speed ratio for two &

three blades savonius wind

t

u

r

b

i

ne.

0he reason isthat increasingthe num1er of

1lades will increase the dragsurfaces against the wind air flow and causes to increasethe re4erse torDue that leadsto decrease the net torDuewor7ing on the 1lades of

sa

Ai

1lades sa4onius wind tur1inehas it highest 4alue of (%.$&) atthe ti! s!eed ratio of (%.), thethree 1lades has a 4alue of (%.&) at the ti! s!eed ratio of (%.).

Two

Blade

s0.30

0.20

0

.

0.

0.0 0.2 0.4 0.6 0.8 1.0

Tip SpeedRatio

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Tree Blades

P o w e r

c i e n t

0

.

2

0

0.

1

5

0.10

0.05

0.00

0.0 0.2 0 .4 0.6 0.8 1.0

TipSpeedRatio

'i)ure *+6,% The power

-oeffi-ient variation with the tip

speed ratio for two & three

blades savonius wind turbine.

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