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CPE451 > Hydro > Flashcards

Hydro Flashcards

1
Q

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

A

True

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2
Q

What are the 2 processes that hydro depends on?

A
  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)
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3
Q

Describe the water cycle (4 stages)

A
  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)
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4
Q

What is evapotranspiration?

A

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

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5
Q

Why is gravity so important to hydro power?

A

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

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6
Q

Draw and label a typical hydro power station

A
  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
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7
Q

Describe how a hydro power station works

A
  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)
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8
Q

What is hydro power?

A

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

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9
Q

What is the purpose of the dam?

A

To store potential energy

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10
Q

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

A

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

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11
Q

True or false: Efficiency is higher for smaller turbines

A

False - efficiency is higher for larger turbines

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12
Q

How are hydroelectric dams characterised by head height?

A
  1. Low head (< 30 m)
  2. Medium head (30 - 300 m)
  3. High head (> 300 m)
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13
Q

Describe a low head hydro dam

A
  • 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
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14
Q

Describe a medium head hydro dam

A
  • 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
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15
Q

Describe a high head hydro dam

A
  • 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
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16
Q

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

A

Medium ~ 182 m

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17
Q

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

A

Low

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18
Q

Give some early examples of hydro-mechanical power?

A

Textile mills, water mills, saw mills

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19
Q

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

A

True

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20
Q

Name the 4 different water wheel types

A
  1. Undershot water wheel
  2. Breastshot water wheel
  3. Overshot water wheel
  4. Pitchback water wheel
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21
Q

List the features of an undershot water wheel

A
  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
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22
Q

List the features of an breastshot water wheel

A
  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.
23
Q

List the features of an overshot water wheel

A
  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.
24
Q

List the features of an pitchback water wheel

A
  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.
25
Q

Draw and label the 4 different water wheel types

A
  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.
26
Q

How do you characterise turbines by power output?

A
  1. Large (> 10 MW)
  2. Small (< 10 MW)
  3. Mini (< 1 MW)
  4. Micro (< 100 kW)
27
Q

What is the difference between a reaction & impulse turbine?

A
  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
28
Q

Name 3 reaction turbines

A
  1. Francis
  2. Fixed pitch propeller
  3. Kaplan
29
Q

Name 3 impulse turbines

A
  1. Turgo
  2. Pelton
  3. Crossflow
30
Q

List the features of a Francis turbine

A
  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
31
Q

List the features of a Kaplan (Propeller) turbine

A
  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
32
Q

List the features of a Pelton turbine

A
  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
33
Q

List the features of a Turgo turbine

A
  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
34
Q

List the features of a Crossflow turbine

A
  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.
35
Q

Describe a pumped storage hydroelectric plant (large hydro)

A
  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
36
Q

What applications are small or mini hydro plant suitable for?

A
  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.
37
Q

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

A

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.

38
Q

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

A

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

39
Q

What time of year does hydro power peak? Why?

A

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

40
Q

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

A

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

41
Q

What continents have the best new hydro development prospects?

A

Asia, South America & Africa

42
Q

List the social disadvantages of hydro power

A
  • 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
43
Q

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

A

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

44
Q

What is the risk associated with blackouts?

A

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

45
Q

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

A

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

46
Q

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

A

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.

47
Q

What environmental issues are associated with hydro power development?

A
  • 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.
48
Q

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

A

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

49
Q

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

A

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

50
Q

How does hydro power development produce greenhouse gas emissions?

A

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.

51
Q

Summarise the 7 disadvantages associated with hydro power

A
  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
52
Q

What new opportunities are available with hydro power?

A

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

53
Q

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

A

Production of silt can be minimised by:
- special weirs
- upstream forestation projects

Silt build-up can be minimised by:
- bypassing
- dredging

54
Q

Summarise the 8 advantages of hydro power?

A
  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

CPE451 (7 decks)

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