5 HYDRO Flashcards
What proportion of world-wide energy is produced by hydro?
16% of worldwide elec generation
~900GW of capacity
???? Why does output of hydro vary?
Capacity factor is around 40%
Advantages of hydro generation
- 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
Disadvantages of hydro generation
- 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
What is a tailrace and why is it important in REACTION turbines?
It is where the water exits. It forms part of the head in reaction turbines.
What is a penstock?
The pressurised pipe in which water travels from the dam to the turbine generator.
Main components of a hydro-scheme
- 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)
Hydrological cycle and where hydro scheme placed.
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)
What does hydro potential depend on?
- the head available
- hydrology of the river catchment (between water source and hydrostation)
- civil works constraints
- end energy use/demand
How to calculate amount of run-off
= (rainfall - evaporation - surface absorption) * catchment area
Why is hydro resource complex? (for small hydro??)
+ how to mitigate for this.
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.
How does type of soil affect amount of discharge?
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.
What are the general definitions of hydro schemes by rating?
LARGE > 50MW
SMALL = 5-50MW
MICRO < 25MW
What are the general definitions of hydro schemes by head?
HIGH > 200-300m
MEDIUM = 30-200m
LOW < 30m
what does run-of-river mean?
the scheme doesn’t have an impoundment
What turbines are typically within a high-head hydro scheme?
Impulse / pelton
Which hydro schemes are the most common?
medium head
Example of a medium head hydro scheme
Hoover dam in US
2 main classes of turbine
IMPULSE
REACTION
Operation of an impulse turbine.
significance of pressure
Operates by extracting Ek from an open jet of water.
No pressure drop across the runner - the jet is at Patm
3 examples of impulse turbines
Pelton
Turgo
Cross Flow
What type of head schemes are impulse turbines typically used for?
high heads (>250m) and for small units
Operation of reaction turbine
significance of pressure
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.
Examples of reaction turbines
Francis
Kaplan (propeller type)
What is the function of the splitter within Pelton (impulse) turbine buckets?
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.
How much power can a Pelton turbine produce?
up to 1MW (depending on the head)
How to control torque of pelton wheel?
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.
pelton turbines: What angle is the relative velocity of jet turned as it hits the bucket and turns back on itself?
𝛽 = 165º
PELTON: why do we want the absolute velocity leaving the bucket to be as low as possible?
As the lower the vale of V2 = the higher the amount of Ek that was extracted
PELTON: What is bucket friction coefficient?
CB
Represents loss due to friction between water and bucket.
What is Euler’s turbine equation?
Torque is equal to rate of change of angular momentum.
Why is practical runner efficiency lower than theoretical runner efficiency?
due to frictional losses
When does max runner efficiency occur?
at U = 0.5(V1)
ie when linear velocity of jet equals half the absolute velocity of jet
Positives and negatives of cross flow turbines
-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
How does water enter and leave different reaction turbines?
FRANCIS: enters radially. leaves axially.
PROPELLER: enters axially. leaves axially.
benefits of propeller turbines over Francis turbines
Flexibility. Can control input flow, vanes and speed of rotation by controlling torque applied on the generator. The propeller controls the shaft.
Characteristics of Archimedes screw generators
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
Why is the Archimedes screw so large?
Has to be big for the buckets to be large enough to compensate for the small head. (only few m head)
Aspects to consider when developing small hydro schemes
Hydraulics
Hydrology
Civil Eng (large number of components)
Mech Eng
Elec Eng
What are the most important factors determining viability of hydro projects?
Hydro Resource
Environmental impact
How to save money in high head hydro schemes?
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.
What is the significance of a penstock on cost of project?
~50% of project cost. (High pressure component)
What is the use of a leat in low head schemes?
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)
What is an environmental statement?
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.
What does a small hydro Environmental statement include?
Licenses to abstract/impound water
Risk of flooding/change in height of water
Changes in flow of rive & impact on fish
Impact of construction
Which schemes are most likely to be accepted?
Short construction time
low capital costs
high operating costs
Operation of an impulse turbine
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)
Operation of a reaction turbine
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.
How to calculate Annual Capacity Factor
= (annual energy generated) / (generator rating)
