5 HYDRO

Question 1

What proportion of world-wide energy is produced by hydro?

Answer 1

16% of worldwide elec generation

~900GW of capacity

Question 2

???? Why does output of hydro vary?

Answer 2

Capacity factor is around 40%

Question 3

Advantages of hydro generation

Answer 3

• operating costs are very low. Many stations are operated remotely with few staff permanently on site.
• the power output of turbine generators can be adjusted rapidly. (ie can be controlled to support S and D)
• If a reservoir is used, then water can be stored and used when required
• <=100 year lifetime. As long as elec/mech works replaced every ~40years

Question 4

Disadvantages of hydro generation

Answer 4

• typically high capital costs of hydro schemes (US$2000-4000/kW)
• schemes with large reservoirs can result in a significant loss of land with major environmental impacts - can affect local populations
• power output of run-of-river schemes varies and depends on precipitation. Run-of-river schemes without any storage can have cap. factors as low as 30%
• decomposing submerged vegetation in large reservoirs can lead to significant emissions of methane (powerful greenhouse gas)
• the failure of a large dam can lead to fatalities

Question 5

What is a tailrace and why is it important in REACTION turbines?

Answer 5

It is where the water exits. It forms part of the head in reaction turbines.

Question 6

What is a penstock?

Answer 6

The pressurised pipe in which water travels from the dam to the turbine generator.

Question 7

Main components of a hydro-scheme

Answer 7

• TRASH RACK to filter out water-borne debris
• PENSTOCK - to carry water to the turbine
• SPILLWAY - to divert excess water - ensures pressure on dam structure isn’t too high to collapse penstock (cheaper than scheme not having capacity for high water levels)
• GOVERNOR SYSTEM and VALVE - to control input power
• TURBINE
• GENERATOR
• DRAUGHT TUBE (on reaction turbines)

Question 8

Hydrological cycle and where hydro scheme placed.

Answer 8

Sea/lake water is evaporated by solar radiation into clouds.
Clouds rise towards precipitation where 𝜑=100% (mountainous areas)
Rainfall then travels downwards towards hydrostation placed on the river. (some doesn’t make its way to the hydrostation as transpiration and evaporation occur along route and cause losses)

Question 9

What does hydro potential depend on?

Answer 9

• the head available
• hydrology of the river catchment (between water source and hydrostation)
• civil works constraints
• end energy use/demand

Question 10

How to calculate amount of run-off

Answer 10

= (rainfall - evaporation - surface absorption) * catchment area

Question 11

Why is hydro resource complex? (for small hydro??)
+ how to mitigate for this.

Answer 11

It isn’t constant (like the wind). Can’t predict the weather.

Need to take measurements of flow over as many years as possible to evaluate if site is suitable for hydroscheme.

Question 12

How does type of soil affect amount of discharge?

Answer 12

Sandier soil absorbs water making it harder for water to flow so less discharge.
Rockier soil is less absorbent and so amount of discharge is higher than for sandy soil. The flow is a lot more variable as the rainfall runs straight off the rocks instead of being absorbed.

Question 13

What are the general definitions of hydro schemes by rating?

Answer 13

LARGE > 50MW
SMALL = 5-50MW
MICRO < 25MW

Question 14

What are the general definitions of hydro schemes by head?

Answer 14

HIGH > 200-300m
MEDIUM = 30-200m
LOW < 30m

Question 15

what does run-of-river mean?

Answer 15

the scheme doesn’t have an impoundment

Question 16

What turbines are typically within a high-head hydro scheme?

Answer 16

Impulse / pelton

Question 17

Which hydro schemes are the most common?

Answer 17

medium head

Question 18

Example of a medium head hydro scheme

Answer 18

Hoover dam in US

Question 19

2 main classes of turbine

Answer 19

IMPULSE

REACTION

Question 20

Operation of an impulse turbine.

significance of pressure

Answer 20

Operates by extracting Ek from an open jet of water.

No pressure drop across the runner - the jet is at Patm

Question 21

3 examples of impulse turbines

Answer 21

Pelton
Turgo
Cross Flow

Question 22

What type of head schemes are impulse turbines typically used for?

Answer 22

high heads (>250m) and for small units

Question 23

Operation of reaction turbine

significance of pressure

Answer 23

operate by changing the direction of the flow.

The water arriving at the runner is still under pressure and the pressure drop across the turbine runner accounts for a significant part of the energy.

Question 24

Examples of reaction turbines

Answer 24

Francis
Kaplan (propeller type)

Question 25

What is the function of the splitter within Pelton (impulse) turbine buckets?

Answer 25

As the jet strikes the bucket, the water splits and is turned back on itself to extract the energy. As water leaves the jet, the energy is changed from pressure to Ek.

Question 26

How much power can a Pelton turbine produce?

Answer 26

up to 1MW (depending on the head)

Question 27

How to control torque of pelton wheel?

Answer 27

JET DEFLECTOR - to deflect jet away from buckets to reduce torque SPEAR VALVE - to alter jet diameter to alter pressure and ∴ force on the buckets; altering torque.

Question 28

pelton turbines: What angle is the relative velocity of jet turned as it hits the bucket and turns back on itself?

Answer 28

𝛽 = 165º

Question 29

PELTON: why do we want the absolute velocity leaving the bucket to be as low as possible?

Answer 29

As the lower the vale of V2 = the higher the amount of Ek that was extracted

Question 30

PELTON: What is bucket friction coefficient?

Answer 30

CB Represents loss due to friction between water and bucket.

Question 31

What is Euler's turbine equation?

Answer 31

Torque is equal to rate of change of angular momentum.

Question 32

Why is practical runner efficiency lower than theoretical runner efficiency?

Answer 32

due to frictional losses

Question 33

When does max runner efficiency occur?

Answer 33

at U = 0.5(V1) ie when linear velocity of jet equals half the absolute velocity of jet

Question 34

Positives and negatives of cross flow turbines

Answer 34

-ve: - low head - low flow - low power +ve: - doesn't need to be cast -> can be fabricated out of sheet metal ∴ useful in developing countries without sophisticated casting capabilities

Question 35

How does water enter and leave different reaction turbines?

Answer 35

FRANCIS: enters radially. leaves axially. PROPELLER: enters axially. leaves axially.

Question 36

benefits of propeller turbines over Francis turbines

Answer 36

Flexibility. Can control input flow, vanes and speed of rotation by controlling torque applied on the generator. The propeller controls the shaft.

Question 37

Characteristics of Archimedes screw generators

Answer 37

Usually < 500kW rating Low head ∴ low power Low speed High torque Large devices Modern ~20years old Easy to add to side of weir and divert flow through it. For small schemes

Question 38

Why is the Archimedes screw so large?

Answer 38

Has to be big for the buckets to be large enough to compensate for the small head. (only few m head)

Question 39

Aspects to consider when developing small hydro schemes

Answer 39

Hydraulics Hydrology Civil Eng (large number of components) Mech Eng Elec Eng

Question 40

What are the most important factors determining viability of hydro projects?

Answer 40

Hydro Resource Environmental impact

Question 41

How to save money in high head hydro schemes?

Answer 41

Put a canal on hillside, and place penstock vertically above powerhouse, to reduce the required size of the penstock as intake is lower down the hill. Ensure penstock isn't travelling the whole way from the source to the powerhouse.

Question 42

What is the significance of a penstock on cost of project?

Answer 42

~50% of project cost. (High pressure component)

Question 43

What is the use of a leat in low head schemes?

Answer 43

They divert watercourse to powerhouse. Lower head but a lot cheaper as is a simple civil eng construction. Building the scheme around an existing weir can increase flow (ie through Archimedes screw)

Question 44

What is an environmental statement?

Answer 44

Explains the need for the scheme and provides details to allow the planning authority and general public to make decision on application. Costs ~5-10% project. Document that describes the physical characteristics of the scheme and its land use requirements.

Question 45

What does a small hydro Environmental statement include?

Answer 45

Licenses to abstract/impound water Risk of flooding/change in height of water Changes in flow of rive & impact on fish Impact of construction

Question 46

Which schemes are most likely to be accepted?

Answer 46

Short construction time low capital costs high operating costs

Question 47

Operation of an impulse turbine

Answer 47

DRIVEN BY WATER IMPACT Ek Extract kinetic energy from the water. ie they capture the speed of the water. eg Pelton captures the speed of water and converts it into rotational energy Turgo spins and generates electricity as water is accelerated and hits the turbine. (impulse turbines can only be operated in one direction whereas Kaplan and Francis can be reversed)

Question 48

Operation of a reaction turbine

Answer 48

DRIVEN BY PRESSURE The runner is immersed in the water flow and operates by hydrodynamic lift forces or pressure Francis - pressure of the water at the bottom of the penstock presses against the blades causing the turbine to rotate Kaplan - can change the angle of attack of the water flow.

Question 49

How to calculate Annual Capacity Factor

Answer 49

= (annual energy generated) / (generator rating)