Hydro

Question 1

True or false: Hydroelectric is the leading renewable power source globally

Answer 1

True

Question 2

What are the 2 processes that hydro depends on?

Answer 2

1. The water cycle to provide a continuous flow of water from high altitudes (mountains) to sea level
2. Conversion of potential energy from the water to electricity (at a hydro power station)

Question 3

Describe the water cycle (4 stages)

Answer 3

1. Vapour formation - the sun extracts water vapour from the sea (evaporation), from plants & soil (evapotranspiration), from ice/snow (sublimation)
2. Transportation - water vapour rises, gaining potential energy, cools & condenses to form clouds, air currents transports clouds across the world and cloud particles can collide
3. Precipitation - clouds give up their potential energy & fall as rain/sleet/snow/hail
4. Return to oceans - fallen precipitation makes it way back to the ocean depending where it fell (surface run-off from mountains into lakes, infiltration into aquifers, ground water discharge & freshwater springs, ice will accumulate as ice caps and melt over many years)

Question 4

What is evapotranspiration?

Answer 4

Refers to the evaporation of water from the soil and transpiration of water from plant’s stomata

Question 5

Why is gravity so important to hydro power?

Answer 5

The water cycle causes water vapour to gain potential energy as it evaporates and hydro power relies on water vapour falling & capturing the loss of this potential energy as it gains kinetic energy

Question 6

Draw and label a typical hydro power station

Answer 6

1. Reservoir
2. Dam
3. Intake
4. Control gate
5. Penstock
6. Turbine
7. Generator
8. Transformer
9. Outflow
10. Power house
11. Power lines

Question 7

Describe how a hydro power station works

Answer 7

1. Water is stored in a reservoir to store the potential energy
2. A controlled amount of water is permitted through the intake and control gate (PE > PE + KE)
3. The kinetic energy & pressure drop of the water turns the turbine (PE + KE > ME)
4. The turbine rotations drive a generator which creates electricity (ME > EE)

Question 8

What is hydro power?

Answer 8

The production of electrical power from the gravitational potential energy of water

Question 9

What is the purpose of the dam?

Answer 9

To store potential energy

Question 10

Why can’t the hydro generation process be 100% efficient?

Answer 10

Water leaves with a small velocity (kinetic energy) to allow the water to continue travelling away from the damn.

Question 11

True or false: Efficiency is higher for smaller turbines

Answer 11

False - efficiency is higher for larger turbines

Question 12

How are hydroelectric dams characterised by head height?

Answer 12

1. Low head (< 30 m)
2. Medium head (30 - 300 m)
3. High head (> 300 m)

Question 13

Describe a low head hydro dam

Answer 13

• Often do not store water
• Examples are a weir on a river or a tidal barrage (store water at high tide)
• Some do not have a dam at all and the flow of water generates electricity directly (run-of-the-river system)
• They require lots of water flowing

Question 14

Describe a medium head hydro dam

Answer 14

• Located in more mountainous regions where the rivers flows down steep terrain
• Dams are constructed behind which a reservoir of water is stored
• Water from the reservoir can be taken to the power generation system where electricity is generated - this is difficult as it is not located where the power is needed.
• High power for small volumetric flow because the head height is very large

Question 15

Describe a high head hydro dam

Answer 15

• Typically consist of a small dam and reservoir and a long section of pipework down a steep mountainside
• Very important in the national grid because they can be adjusted easily and quickly to produce the power to rapidly respond to changes in demand

Question 16

Is the three gorges damn low, medium or high head?

Answer 16

Medium ~ 182 m

Question 17

Is a tidal barrage a low, medium or high head hydro dam?

Answer 17

Low

Question 18

Give some early examples of hydro-mechanical power?

Answer 18

Textile mills, water mills, saw mills

Question 19

True or false: The first known use of hydro-mechanical power occurred around the 3rd century BC.

Answer 19

True

Question 20

Name the 4 different water wheel types

Answer 20

1. Undershot water wheel
2. Breastshot water wheel
3. Overshot water wheel
4. Pitchback water wheel

Question 21

List the features of an undershot water wheel

Answer 21

1. Cheap and simple to build
2. Little environmental impact as they do not require major change of the river
3. Low efficiency because they gain no advantage from head – only use KE
4. Can only be used where the flow rate is sufficient to provide torque
5. Best suited to shallow streams in flat country

Question 22

List the features of an breastshot water wheel

Answer 22

1. The blades form buckets and the water fills these about halfway up the wheel.
2. Popular in the United States
3. More efficient than undershot wheels, but less efficient than overshot wheels
4. Require a good trash rake and typically have a masonry “apron” closely conforming to the wheel face.
5. Work best with steady, high volume flows.

Question 23

List the features of an overshot water wheel

Answer 23

1. Water fills buckets near the top of the wheel slightly beyond the axel with the water passing over the top of the wheel.
2. The weight turns the wheel, and the water flows out into the tail race when the wheel rotates enough to invert the buckets.
3. Uses the PE stored in the water so does not require rapid flowing streams.
4. The overshot design can utilise both the kinetic and potential energy of the flowing water.
5. No need for complex sluice and tail race configurations.

Question 24

List the features of an pitchback water wheel

Answer 24

1. A variety of the overshot wheel
2. Water is introduced just behind the top of the wheel.
3. Will continue even if the water in the wheel pit rises well above the height of the axle, when other overshot wheels would be stopped or damaged.
4. Suitable for streams that experience extreme variations in flow
5. Gains some power from the kinetic energy of water flowing current past the bottom of the wheel as well as the potential energy.

Question 25

Draw and label the 4 different water wheel types

Answer 25

1. Undershot - water is only underneath the wheel. Wheel turns with water flow 2. Breastshot - water comes in elevated like an apron shape and wheel sits in the crevice 3. Overshot - water comes in an elevated pipe from directly above the wheel and the buckets carry the water down 4. Pitchback - water comes from an elevated pipe onto the RHS of the wheel so it rotates in the other direction to the overshot. Buckets carry water down.

Question 26

How do you characterise turbines by power output?

Answer 26

1. Large (> 10 MW) 2. Small (< 10 MW) 3. Mini (< 1 MW) 4. Micro (< 100 kW)

Question 27

What is the difference between a reaction & impulse turbine?

Answer 27

1. A reaction turbine is one that operates submerged in water and uses the pressure drop to extract energy 2. An impulse turbine is one that spins freely due to the momentum of the water striking the turbine blades

Question 28

Name 3 reaction turbines

Answer 28

1. Francis 2. Fixed pitch propeller 3. Kaplan

Question 29

Name 3 impulse turbines

Answer 29

1. Turgo 2. Pelton 3. Crossflow

Question 30

List the features of a Francis turbine

Answer 30

1. By far the most common 2. Radial flow reaction turbine 3. Horizontal or vertical orientation 4. Guide vanes used to direct water 5. A scroll case or volute maybe used 6. Water changes direction passing through the turbine 7. Optimum speed is just less than the incident water speed 8. Not suitable for very high (high speeds) or low head heights (twist) 9. 10 < H < 350 m

Question 31

List the features of a Kaplan (Propeller) turbine

Answer 31

1. Very large volume flows 2. Small head heights (2 < H < 40) 3. Kaplan turbines can vary the pitch of the blades to optimise efficiency 4. Optimum blade speed is faster than the water flow 5. The outer blade edge moves faster than the inner so blade is twisted

Question 32

List the features of a Pelton turbine

Answer 32

1. High head heights >250m (50 < H < 1300) 2. Wheel with a series of double cups around rim 3. High speed jet focused onto the splitting edge divides jet into the two cups & transfers momentum 4. Power adjusted by jet size 5. Most efficient when cups moving at just under half jet speed 6. Several jets can be used to increase output with small wheel

Question 33

List the features of a Turgo turbine

Answer 33

1. It is a variation on the Pelton design with a single cups instead of doubles. 2. Handles larger volumes of water so has an advantage at medium head heights over Pelton designs 3. 50 < H < 2500 m

Question 34

List the features of a Crossflow turbine

Answer 34

1. Water enters as a flat sheet rather than a jet 2. Used instead of Francis turbines in small scale plants with outputs below 100kW due to its simplicity and therefore lower cost.

Question 35

Describe a pumped storage hydroelectric plant (large hydro)

Answer 35

1. Water is stored at a high altitude reservoir and is discharged into the low altitude reservoir through the turbines during times of peak electricity demand 2. It is pumped back from the lower reservoir to the higher reservoir during off-peak times using reversible turbines 3. Electricity costs to pump water back up is high

Question 36

What applications are small or mini hydro plant suitable for?

Answer 36

1. ‘Run-of-river’ systems with little or no storage and therefore they have limited negative environmental effects. 2. Suited to mountainous fast flowing streams or lowland areas with wide rivers 3. The exit flow from water treatment or sewage plants are becoming increasingly popular sites for mini hydro systems 4. Suitable for a small community or industrial plant, often in isolated areas 5. Small hydro plants can be connected to the grid where possible as a source of low-cost renewable energy. 6. Due to the limited construction work and deviation of the river system they have relatively low environmental impact. 7. Need to find a balance between stream flow and power production.

Question 37

What's a key advantage for the pitchback water wheel over the overshot water wheel (hint: flooding)

Answer 37

In the event of a flood, the high water level, for an overshot wheel, will cause drag which will reduce its ability to rotate. Whereas a pitchback wheel, the water will still exit the same way, even if the water level is high, so it remains operational.

Question 38

What area of the UK is best suited to hydro power? Why?

Answer 38

Scottish highlands - lots of rainfall & very mountainous (high head height)

Question 39

What time of year does hydro power peak? Why?

Answer 39

Winter (Q1 and Q4) because it is colder (demand increases) and there is more rainfall

Question 40

Why is the build of new hydro sites in the UK limited?

Answer 40

Many of the economically attractive sites have already been used. AND Most unutilised, good sites for hydro are protected due to environmental concerns.

Question 41

What continents have the best new hydro development prospects?

Answer 41

Asia, South America & Africa

Question 42

List the social disadvantages of hydro power

Answer 42

- Dam Failure / Seismic Risks (terror, war) - Geologically inappropriate Location (e.g. sedimentary rock causing landslides) - Potential Blackouts - Finite Lifetime - Land Loss (Population Relocation and Environmental Damage) - Affected by variable water supply - Altered Water Flow (Reduced/increased Sediment Build Up and Reduced aquatic life) - Greenhouse Gas Emission

Question 43

What are the risks associated with dam failure? How can it be managed?

Answer 43

Lots of reasons dam failure can occur: - earthquakes - terror/act of war/sabotage - heavy rainfall (wave over dam wall) Managed by having smaller dams but they still pose a risk after decommissioning due to lack of maintenance

Question 44

What is the risk associated with blackouts?

Answer 44

Bad weather can cause power plant turbines, connections or substations (e.g. generators) to short circuit resulting in a sudden loss of power.

Question 45

What limits the lifetime of a hydroelectric power plant/dam?

Answer 45

Silt that would usually accumulate in a river accumulates at the dam wall, reducing the storage capacity. Once it reaches the top of the spillway the dam will fail so maintenance is required. This is also dangerous to enter the turbine blades as it causes wear so operations should stop. Typical lifetimes can be 20 to 70 years

Question 46

What are the social issues associated with land loss due to hydro power development?

Answer 46

People are displaced/left homeless (40-80 million people worldwide have displaced as a direct result of dam construction). Ancestral and cultural sites can be lost and attachment to places can cause social issues.

Question 47

What environmental issues are associated with hydro power development?

Answer 47

- Destroys productive land - Aquatic ecosystems both upstream and downstream suffer - The build up on silt upstream of the dam means there is no silt downstream, which can lead to scouring of river beds and loss of riverbanks downstream - Dissolved O2 content of the water changes from pre-construction conditions due to warmer temperature of water exiting plant from friction, more O2 is dissolved effecting aquatic faunal populations.

Question 48

What can be done to mitigate the effect of aquatic life?

Answer 48

Use canals to divert a river at a shallower gradient to increase the head of the scheme

Question 49

What effect does variable water supply have on a hydro plant?

Answer 49

Lower river flows because of drought, climate change, upstream dams or diversions will reduce the electricity production capacity resulting in power shortages in areas that depend heavily on hydroelectric power

Question 50

How does hydro power development produce greenhouse gas emissions?

Answer 50

In tropical regions, reservoirs may produce substantial amounts of methane and carbon dioxide from plant material decaying anaerobically. Large shallow reservoirs (i.e. produce <100 W/m2), located in warm climates, where no clearing of the vegetation was undertaken prior to flooding the valley can have greenhouse gas emissions higher than those of a conventional oil-fired thermal generation plant. (underwater logging) Reservoirs in Canada and northern Europe have greenhouse gas emissions typically ~ 2% to 8% those emitted by fossil-fuel power generation.

Question 51

Summarise the 7 disadvantages associated with hydro power

Answer 51

1. High initial installation costs 2. Long lead time in construction of large projects 3. Variable output dependent upon weather 4. Possible dam or supply failure 5. Flooding of large areas of land with associated loss of agriculture, environmental damage and relocation of communities 6. The altered water content and flow impacts upon the river flow and aquatic life 7. Anaerobic decay of un-cleared vegetation can produce CH4

Question 52

What new opportunities are available with hydro power?

Answer 52

They can also provide: - Irrigation - Flood control - Aquaculture - Water sports/tourist attractions

Question 53

How can the amount of silt in a reservoir be managed?

Answer 53

Production of silt can be minimised by: - special weirs - upstream forestation projects Silt build-up can be minimised by: - bypassing - dredging

Question 54

Summarise the 8 advantages of hydro power?

Answer 54

1. Low cost source of low carbon electricity 2. Renewable resource with high conversion to electricity efficiency 80%+ 3. Low greenhouse gas emission 4. Scalable technology from micro to large scale production 5. Can be used for base load, peaking and pumped storage applications 6. Inexpensive to run due to low maintenance costs once installed 7. New opportunities such as irrigation, aquaculture, flood prevention and water sports 8. Long lifetimes relative to other power sources