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Renewable Generation & Control

Lecture 6:Lecture 6:

HYDROPOWERHYDROPOWER

Supplies nearly 1/6 of the world’s electricity and 85% of

the world’s electricity from renewable sources

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6.1 Size of hydroelectric plant6.1 Size of hydroelectric plant

18 GW scheme at the Three Gorges

An example of Large Hydro ( > 10 MW)

• Small Hydro (1 – 10 MW)

• Mini Hydro (100 kW – 1 MW)

• Micro Hydro (< 100 kW)

And finally Pico Hydro (< 5 kW)…

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6.2 Large Hydro with Reservoir

Hoover dam – 221.4 m high

379.2 m wide - 2 GW

A reservoir enables seasonal variations in river flow to be controlled• Greater electricity generation• Flood control• Use as “peaking plant”

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6.3 Types of hydropower6.3 Types of hydropower(according to storage)(according to storage)

(Las Juntas, Peru: 25 kW)

Hydropower with a reservoir(Llyn Brianne, Wales: 4 MW)Dual purpose: Water supply + electricity generation

Run-of-river hydropower

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Pumped storage hydropowerPumped storage hydropower

Run-of-river hydropower (Las Juntas, Peru)

© First Hydro

(Dinorwic, Wales: 1.8 GW)(Dinorwic, Wales: 1.8 GW)

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Variation of electrical demand

http://www.nationalgrid.com/uk/Electricity/Data/Realtime/Demand/demand24.htm

Base load Excess capacity

Pump at Night

Pump storage

provides this energy

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Water flow variationWater flow variation

Flow available will vary with seasons and from year to year.

Available flow is shown by a flow duration curve.

Qav is (rainfall - evaporation)

× catchment area.

(converted to m3/s)

Without storage, a

hydropower plant will be

sized to use a flow rate < Qav.

Flow duration curve

0

1

2

3

4

5

6

0 0.2 0.4 0.6 0.8 1

Probability

Q/Qav

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Calculating the Power

Hydropower uses the potentialenergy of the falling water:

mass flow × head × gravity

Power is determined from:

volume flow rate, Q,

and head, H.

If H is in m and Q in m3/s,

then the power is in W.

efficiencyoverall

smg

mkg

waterofdensity

QgHP

=

=

=

=

η

ρ

ηρ

2

3

/81.9

)/1000(

H is 7 m, Q is 0.5 m3/s

if η = 0.73 then P = 25 kW.

Las Juntas, Peru

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www.hydropower.com.cn

There is a pressure loss along the Penstock pipe that brings the water to the turbine.

Pipe losses are related to pipe diameter, length roughness and flow velocity.

The frictional resistance is proportional to kinetic energy in the pipe:

2

2

2kQ

g

vh f

== ζ

hHH grosst f−=

Turbine head:grossH

Head loss in PipesHead loss in Pipes

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l Turbines are classified as to whether they are

l IMPULSE (uses the kinetic energy of the water flow to cause rotation) Jet velocity is

l Examples include Pelton Wheels and Turgo turbines

l REACTION (where the pressure of the water turns the turbine)

l Examples include Francis turbines, Kaplan turbines, and propeller turbines

Types of turbinesTypes of turbines

gHv j 2=

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Different runner types/shapesDifferent runner types/shapes

Kaplan runner

Francis runners

High specific speed

Medium specific speed

Pelton runner

High head,

Low flowLow head,

High flow

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Types of turbinesTypes of turbines

A dimensionless parameter used for selecting a particular turbine over another is its SPECIFIC SPEED

ω = speed of rotation (rad/s)

P = power of turbine (W)

ρ = density of water (1000 kg/m3)

H = head of water (m)

( ) 45

gH

P

s

ρω

ω =

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Ranges of specific speedsRanges of specific speeds

l Francis turbine ωS = 0.3 – 2.5

l Kaplan (propeller) ωS = 1.5 – 5

l Pelton wheel ωS = 0.05 - 0.5

The graph shows the maximum efficiency available for a given specific speed and type of turbine.

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PeltonPelton Wheel 1Wheel 1

Potential energy of water is converted to kinetic energy in a nozzle

A jet of water strikes blades or “buckets” on a rotating wheel

When the velocity of the jet is 2x velocity of the bucket, the water flows off with ̴ 0 resultant velocity

NozzleBucket

Adjustable Spear Valve

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PeltonPelton Wheel 2Wheel 2

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PeltonPelton wheel 3wheel 3

To increase flow rate,

multiple jets are used

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TurgoTurgo turbineturbine

l Looks like a Pelton Wheel cut in half

l Can handle higher flow rates than Pelton Wheel

l Jets don’t interfere as much

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6.17 Reaction turbine6.17 Reaction turbine

l Potential energy is converted to pressure energy

l The turbine impeller is driven by the pressure difference across it;

l The water flows through enclosed passages.

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Francis turbine Francis turbine

Francis turbine runner from 3 gorges dam

Radial inflow – axial outflow

Medium head/flow ratio

Adjustable Guide Vanes

Fixed Stay Vanes

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Typical Power station layoutTypical Power station layout

Snowy Mountains Hydro

Control Valve

Multi-pole Synchronous

Generator (Salient-pole)

Francis (reaction) turbine

Draft tube

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Power station with Kaplan TurbinePower station with Kaplan Turbine

Direct-drive multi-pole generator

Tilting-pad Thrust bearing

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CrossCross--section of a plantsection of a plant

Draft Tube

Kaplan turbineSump

Height in m above sea level

Trash Rack

Services

Trash Rack CleanerMachine Hall

Machine Crane

Weir Crane

Downstream

Weir level

Upstream

What is the gross Head?

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Large Kaplan turbineLarge Kaplan turbine

Adjustable Guide Vanes

Adjustable Runner Blades

Draft Tube (recovers kinetic energy)

Scroll or Volute

(increases kinetic energy)

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Which one to chose?Which one to chose?

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6.25 River Trent Catchment6.25 River Trent Catchment

Beeston Weir

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6.26 Calculating the resource 6.26 Calculating the resource -- exampleexample

The Trent at Beeston has a weir of height 2.9m

The catchment area is approx 5,900 km2

The average rainfall is approx 900 mm/year across the catchment

The evaporation is 475 mm/year

Power is determined from: volume flow rate, Q,

and head, H. – calculate Q from above values.

If H is in m and Q in m3/s, then the power is in W.

If H is 2.9 m,Q is 80 m3/s

and η = 0.87 then

P = 2.9 x 80 x 1000 x 9.81 x 0.87 = 2.0 MW

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6.28 Economies of Scale6.28 Economies of Scale

Cost/kW v Plant Capacity

0

2000

4000

6000

8000

10000

12000

0.001 0.01 0.1 1 10 100 1000 10000

MW Capacity

US $/kW

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6.28 Hydropower and the Environment6.28 Hydropower and the EnvironmentPhoto: R. Antonio/Gamma

Photo: H

ydroplan

Large Hydro – problems with

• river ecology

• displaced population

• sedimentation

• methane emissions

Small Hydro (run-of-river)

• little storage area

• less effects on ecology

• very low CO2 emissions

• but lower power capacity

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ConclusionsConclusions

� Mature and relatively clean technology

� Low greenhouse gas emissions

� Cost-effective

� Large potential in developing countries

BUT

Most large hydropower schemes are gradually silting up,

changing river and estuary environments.

Use of smaller schemes and better design may be able to

reduce negative environmental impacts

Largest growth is likely to be in Small hydro in future.

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Hydro ExampleHydro Example

1. (a) Specific speed for a water turbine is defined as:

( ) 45

gH

P

s

ρω

ω =

a. Explain how the specific speed is related to the type of turbine.

b.A hydro site has a gross head of 100 m and the pipe line head losses, assumed to be proportional to the square of the flow rate (Q2), are estimated to be 12 m when the flow rate is 0.05 m3/s. Determine the net head (H), when the turbine flow rate is 0.025 m3/s.

c.The small water turbine for this site has a maximum efficiency of 75%. Determine the power output (P) and the specific speed if the turbine runs at 1000 rpm at a flow rate of 0.025 m3/s.

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Pumped Storage ExamplePumped Storage Example

A reversible pump turbine has a specific speed ωs = 0.888 and an efficiency of 88%. The

turbine is to be installed at a site where the gross head is 70 m and the pipe line head loss

is 2 m when operating at the rated flow of 2.5 m3/s.

a) Determine the power output of the turbine at rated flow.

b) From the specific speed, calculate the turbine speed.

c) How many poles would a direct drive synchronous generator have for a frequency of 50

Hz?

d) The system is to be operated as a pumped storage scheme. If the motor/generator

efficiency is 97%, and the pump-turbine has the same efficiency when pumping, show that

the overall (electrical power in to electrical power out) efficiency of the scheme is 71%.