Turbines and Wind Turbines

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Turbines and Wind Turbines

Classification


A turbine extracts energy from a fluid that

possesses high head H;

2 types of turbines

Impulse Turbine

High head is first converted

to high-velocity jets through

a nozzle;

The jets hit turbine blades

as they pass by.

Usually bigger than reaction

turbines.

Reaction Turbine

Fluid fills the blade

passages, and pressure drop

occurs within the impeller.

Dynamic devices of either

radial-, mixed- or axial-flow

type.


Low head, high flow devices

First efficient inward-flow turbine

was built by James B. Francis in

1849.

Radial- and mixed-flow design

are called Francis Turbines.

At lower head, a more compact

design will use an axial flow.

Propeller turbine

Kaplan type (adjustable blade)


High head, relatively low power.

Reaction turbines would require too

high a speed n and the high pressure

would require a thicker casing;

In the impulse turbine, the high

pressure is converted to high

velocity jets (atmospheric pressure)

through a nozzle. The jets hit a

bucket (elliptical split-cup shape)

The first efficient design was by

Lester A. Pelton (1880), so they are

called Pelton wheels.


The turbine parameters are similar to pumps, but defined as

a function of the output brake horsepower.

gH

Q

C

H

= = f ( C )

2 2

P

C

Q

= = f ( C )

3 P

n D

nD

bhp η

= ρgQH =

f

(C

CP )

with:

C P

=

bhp

ρn

3 D 5

The maximum efficiency point is called the normal power.


Francis turbine,

n = 600 rpm, D = 2.25 ft, N sp = 29


A power specific speed can also be defined:

N

'

=

* 1/ 2

P

1/ 2

Rigorous form:

sp 5/ 4 1/ 2 5/ 4

C

C

*

H

=

n(

bhp)

ρ ( gH )

Lazy form:

N sp

=

( rpm)(

bhp)

[ H ( ft)]

5/ 4

1/ 2

Turbine Type N sp range C H range

Impulse 1 – 10 15 - 50

Francis 10 -110 5 – 25

Prop: water 100 – 250 1 – 4

Prop: gas, steam 25 - 300 10 – 80


Rated Power: largest power

guaranteed by the

manufacturer

As opposed to normal power

which is delivered at

maximum efficiency.


Four-blade Dutch-type windmill

American multiblade farm

HAWT:

Horizontal Axis

Wind Turbine

Historically,

wind energy to

mechanical

power


The most popular design is

the 3-blade HAWT.

Efficient but awkward since

they require extensive

bracing and gears combined

with an electric generator.


VAWT: Vertical Axis Wind Turbine

Offers a simplified design, since the

gears are on the ground and the blades

strength requirements are lower.

But, less efficient and not self-starting.

Sold by Di Natura (Malta)

Darrieus (1925) wind turbine

Cap-Chat (Qc), biggest in the world


Manufacturers


The maximum power available to the propeller of a wind

turbine is:

P avail

1

= mV &

2

The actual efficiency, and hence actual power produce, of a

wind turbine is expressed in terms of the power coefficient:

C P

=

2

=

P

1 ρ

2

AV

1

2

ρAV

The maximum ideal limit for C P is given by the Betz number:

C P, max

=

3

0.593

3

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