Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Power Generation 22nd C > Hydro, Tidal & Wave Energy > Flashcards

Hydro, Tidal & Wave Energy Flashcards

1
Q

Provide an expression which gives the maximum theoretical power available at a vertical location H metres below a water reservoir, when the water is
allowed to flow through and active area of A ?

(Pelton Wheel)

A 
p = Qgh
p = Avgh
v = (2hg)^0.5
p = A (2gh)^0.5 gH
p = A (2g^3H^3)^0.5
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What is the maximum potential energy available in a lagoon of cross sectional area A in a location where the maximum tidal height variation is H?

A

Energy = ρHAgH/2 =[ρH2Ag]/2

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What’s the equation for power generation from waves?

A

P = 1/32pi *rho * g^2 H^2T (where T is period and H is wave height)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Explain how the energy available in a given tidal power generation scheme is fixed by“nature”, whilst actual power generated (over finite time periods) can be varied

A

By allowing the flow rate to vary at the operators command (the total volume flow is fixed)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Which way direction do the moon and earth rotate?

A

anticlockwise

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What is sidereal month?

A

27.3 days - time for the moon to orbit the earth relative to a fixed point

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What is a synodic month?

A

earth and moon rotating about each other and re-align with respect to the sun after 29.5 days

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What would happen to the period of the tides if (a)it the moon was a ‘stationary satellite’ and (b) stationary

A

(a) infinite high tide period (high tide in the same location)
(b) two tides each day exactly 12 hours apart (half a day)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

why is the tidal period 12 hours and 25 minutes?

A

because the moon is rotating anticlockwise too but at a much slower speed

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

How would you estimate the theoretical maximum

mean power generation potential over a complete tidal cycle?

A

P = E/T = mgh0.5/T=rhoAh^2*g/2T

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What is tidal resonance? (3)

A
  • when the period for tidal currents are such that they are in tune with the natural frequency of the local topology.
  • This can lead to constructive interference between the tidal ebb and flow and the natural timescales for flows.
  • As viscous dissipation is relatively low, the amplitude of the tidal range can increase considerably.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

How is the earth-moon system in equilibrium?

A

net centrifugal forces = net gravitational forces (but varies over the surface of the earth)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What is the process and final result of deriving the tidal impact of the moon (or sun)?

A

Process:

1) force balance of gravitational each force = gravitational moon force (deltamg = deltamg)
2) g = GM/r^2 and sub into force balance
3) (GM/(r+deltar)^2 - (GM/r^2)) = moon same
4) divide by G and factor m &M

Final result:
tide height = (m_m/m_e)(r/d)^3*r

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Why is the earth’s rotation velocity reducing?

A

the gravitational pull on the bulge which occurs before the moon is directly overhead which as a azimuthal component retarding the earth (2x10-5 seconds year

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

how do we calculate the synodic month?

A

synodic month = sidereal month * (1 + (sidereal month/year))

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

The distance between the earth and the moon can be reduced by up to 8% (elliptical orbit, what impact does this have on tidal force?

A

tidal generating force is proportional to the cubed of distance, therefore 8% translates into 25% difference in tidal force

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

at what angle from earth moon plane do we get zero tides?

A

54.& degrees

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

what’s the angle between the sun-earth plane and the earth-moon plane?

A

5 degrees. both not perpendicular to the rotational axis of the earth

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

why don’t the tidal bulges occur directly under the moon? Calculate the impact on time (3)

A
  • earth is spinning and tends to drag water
  • due to shear drag on the ocean’s floor
  • tide 3 degrees or so ahead

3/360 x 24 x 60 = 12 minutes

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

Other than earth/moon/sun gravitational forces and centrifugal forces due to the earth’s rotation, what else impacts the earth’s tides? (4)

A
  • moon’s orbit is not circular
  • inclination of the moon’s orbit in relation to earth/sun orbit
  • earth density varies
  • earth spinning,, going around the sun and not perfectly spherical
21
Q

Why do tides occur?

A

complex interaction of gravitional forces (vaires on earth surface) between the moon-earth and sun-earth, as well as a centrifugal effects (same everywhere) resulting from the earth’s rotation

22
Q

what is the equation for shallow wave speed?

A

v = gy^0.5

23
Q

what are the three typical methods for generating power with tidal?

A
  1. Flood generation
  2. Ebb generation
  3. two way generation

could also do offshore lagoons

24
Q

What is the equation of an idealized ocean wave?

A

P = rho g^2 H^2 T /32 pi

approx P = H^2T

25
Q

Define significant wave height

A

average height of the highest one third of waves and a zero up crossingperiod

26
Q

What is the equation for the average power of irregular waves?

A

Ps = alpha . H^2 . T

Where alpha = 0.49 kw(sm^3)

27
Q

What are the two equations for velocity and wave length in deep water?

A

V = gT / 2pi (velocity)

L = (gT^2)/2pi

28
Q

What is the condition for deep water?

A

D> L/2

29
Q

What is the condition for shallow water?

A

D

30
Q

What is the velocity of a wave in shallow water

A

v = (gD)^0.5

31
Q

Where is the energy in a wave?

A

95% is in the layer contained h = L/4 from surface

32
Q

Why are some tides during the year bigger than others?

A

The sun also has a gravitational pull on the surface waters; this can be additive (spring tide) or opposed (neap tide) to that of the moon. This produces tides which can be greater or smaller than the average.

33
Q

What are the 2 effects that can impact the size of local tides?

A

Topology of the land masses, which funnel tides into an ever decreasing area, and (by continuity) results in increasing tidal range

Resonance - where the period associated with surface waves in a particular stretch of ocean/sea/estuary can synchronise with the tidal period of 24.8/2 = 12.4 hours (or a suitable multiple.

34
Q

what are the physical mechanisms which contain the tidal variation (specifically what prevents it getting bigger or smaller)?

A
  • At resonance, the limits on the tidal variation will be set by dissipative effects of shear on the estuary bottom and sides and general turbulence dissipation.
  • The minimum will be set by the forcing function of the tidal range in the adjacent sea/ocean.
35
Q

Where along the eustry would you expect the largest tidal variation?

A

The exact distance at which the variation will be greatest will depend on the exact topology of the local geography; it will be determined by the balance of forcing funnelling and resonance forces and dampening frictional and dissipative forces

36
Q

State the positives of hydropower (3)

A

low (almost zero) C footprint
long life expectancy
- can provide multi-benefits (irrigation, flood control, water supply)

37
Q

What are some of the negatives associated with hydropower?

A
  • may require lots of space,
  • displacement of activities and people
  • may lead to unforeseen consequences (silting, arguments about water rights,..),
  • difficult(very expensive ) to reverse.
38
Q

what is the equation used to determine the speed of a water jet on a Pelton wheel?

A

u = (2gH)^0.5

39
Q

State the equation used to determine the power generated from a Pelton wheel

A

p = 0.5 rho Q u^2

40
Q

why do waves exist?

A
  • The major driver for surface waves is the wind shearing phenomena at the water-air interface.
  • Atmospheric pressure variations also affects surface waves.
  • Complex coupling between the surface wave topology, the air speed immediately above it results in enhanced lift at wave crests, which tends to increase the wave heights.
41
Q

how does depth impact waves?

A

The deeper the sea the greater the wave amplitude and speed can be.

42
Q

why is all energy in wave at the top?

A

There are no energy source terms to drive or feed wave motion below the surface, indeed, at the sea bed there is zero vertical motion. Any motion below the surface of the waves to ensure continuity and constant hydrostatic pressure at a given height (ρgy effect)tends to be dominated by horizontal rather than vertical motion.

43
Q

why is KE approx = PE?

A
  • The mass of water contained in the crest of the wave and so above the mean sea level has a potential energy (it can drop back into the tough).
  • However fluid in the crest of the wave has zero vertical velocity, the vertical velocity increases to its peak value as the water drops to the mean sea level and then decelerates again to reach zero at the trough.
  • As dissipation is small, energy is simply being exchanged between potential and kinetic energies as individual water packets move through their orbital paths.
  • Hence the amount of kinetic is equal to the amount of potential energy.
44
Q

Why is it important for a wave device to be roughly equal to the length of the wave?

A

If, however the structure has a similar size to the waves, it is able to experience the relative movement between successive wave and is able to draw energy from this relative movement

45
Q

what is the equation for the height of saw shaped wave z ? (limit of integration for z)

A

z = 2hx/L

46
Q

Pelton wheel key information - 2 equations and one condition

A

v = (2gh)^0. 5 from ke=mgh

p = 0.5 rho Q u^2

for maximum power, rim speed in 0.5 of jet speed

47
Q

why do shallow and deep waves have different speeds?

A

shallow v = (gd)^0.5 - damping get bigger as d gets smaller

deep v = ((g/2pi)* L )^0.5

speed is more related to wave length

48
Q

What happens to tidal energy if it is not used?

A

If the tidal energy is not ‘used’ and most of it is not, it gets dissipated as frictional losses on the sea beds, estuary and coastal erosion etc. It also means that the total kinetic energy of the system of planets is slowly decreasing. Spins will slow and distance between the celestial bodies will increase

49
Q

what is the gravitional constant?

A

6.67 x 10-11

Power Generation 22nd C (9 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions