wind energy explained - 2

8/14/2019 Wind Energy Explained - 2

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Wind Energy Explained - 2

Manjunath Terwad


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Agenda

Airfoil Terminology

Airfoil Behavior

Graph of lift and drag coefficient for

different Re Stall

Flow Regimes

Drag Machines

Blade Element theory.


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Airfoil Terminology

:M e a n

C h a m b e r Lin e lo cu s of th e p o in ts b e tw ee n h a lfw ay o f

u p p e r a n d lo w e r surfa ce:C h o rd Lin e co n n e ctin g th e e d g e s

: C h a m b e r It s th e d ista n ce b e tw ee n th e m e a n ch a m b e r lin e.a n d ch o rd m e a su re d p e rp e n d icu lar to ch o rd

: .A n g le o f A tta ck A n g le b e tw ee n re la tiv e w in d a n d ch o rd lin e


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Lift and drag Coefficients


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Airfoil Behavior

T h e th e o re tica l liftco e fficie n t o f a fla t p la te

:is

T h is w o u ld m e a n th a t lift co e fficie n tC l as but this is not the

actual case which is due to the.assumption like air being inviscid

In reality air has some viscosity.and this results in stalling


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Lift and drag coefficients fordifferent Reynolds Number


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Stall:?

S ta ll is re d u ctio n in th e liftco e fficie n t fa cto r g e n e ra te d b y th e

a irfo il a s a n g le o f a tta ck in cre a se s.a b o ve a ce rta in critica l v a lu e

D u e to sta ll w a ke is fo rm e d a b o v e.th e a irfo il su rfa ce


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Flow regimes for given Re

Attached Flow Regime : At 7the flow is attached to the uppersurface . Here Cl as and Cd is

low. High lift/stall development

region: when the angle of attack

exceeds critical value 10-16 degreethe Cl and Cd . This is useful in

speed regulation during high wind.

Stalled regime: Cl , Cd are almost


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http://en.wikipedia.org/wiki/File:StallFormation.svg


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Drag Machines

=A surfacearea


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The power coefficient Cp is

C p . .is zero at the speed ratio of zero and at 1 i e no.relative velocity between the surface and wind Also

/ .the peak is at 1 3 of speed ratio at 0 08 which is lower.than Betz limit 0 59The biggest disadvantage is the rotor surface cannot

.move faster than the wind speed


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The Blade Element Theory: This is about the analysis of the forces at the section of the

blade, as a function of blade geometry.

This relates the blade shape with the rotors ability toextract power from the wind.

Assumptions in Blade element Theory:

The Blade is divided into N sections (or elements).

There is no aerodynamic interaction Between Elements.

Forces are determined solely by lift and drag characteristicsof airfoil shape of blades


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=Relative wind velocity vector sum of velocity at rotor and wind.velocity due to rotation of blade

( - )U 1 a= ,Rotational Component vector sum of blade section velocity induced

angular velocity at the blades due conservation of angular momentum


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:ConclusionsFrom the 2 equations obtained from the blade

element theory we can observe that as the dragincreases the torque decreases and the thrust force.increases

These equation can be used to find out the ideal.blade shapes for optimum performance


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Thank You

wind energy explained - 2

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