L3: Wave Energy Flashcards
What are the three factors that will affect a wind wave formation?
Wind speed, fetch, duration
What does the size of a wave correlate to?
The amount of wind energy transferred
What are the key advantages of wave over wind and solar energy?
It has a higher energy density than both and has a high availability (90% of the time vs 20-30)
What are three key environmental advantages to wave power?
- Most wave energy converters are off land
- No major visual pollution
- Little noise (the waves are noisier than the converters)
What does linear wave theory assume about wave motion?
It has simple harmonic motion
How is wave number defined?
k = 2*pi/wavelength (lambda)
How can the wave frequency be calculated in rads/sec?
omega = 2pif (f is in Hz)
Give the equation for wave dispersion
omega^2 = gktanh(kh)
Where omega = wave freq (rad/s), g = gravity, k = wave number, h = water depth
Under what circumstance can the wave dispersion equation be simplified, and how?
Where the depth = over half of the wavelength, tanh -> 1 so omega^2 = gk
Give the equation which describes the wave profile at position x, time t
eta(x, t) = Acos(kx - omega*t)
Where A =amplitude (half of height), k = wave number, x = position, omega = freq (rad/s), t = time
What is V(p)?
Phase velocity, the speed at which the wave front is propagating
What is V(g)?
Group velocity, the speed of energy transfer
What are V(p) and V(g) analogous to?
V(p): Speed
V(g): Acceleration
Give an overview of wave energy density
Energy density = mean energy per unit horizontal area
Total energy density = PE + KE = rhogh + 0.5mv^2
Give the equation for power per meter. How can watts be found from this?
P = 0.5rhogA^2V(g) (W/m)
Multiply by wave width to get watts
How is energy transfer (group) velocity unique for single frequency waves?
As there is no change in wave length, V(g) = 0. Use V(g) = V(p)
Which spectrum moment represent the area underneath the wave spectrum?
0th moment
How are zero-crossing points defined?
When a wave moves from a crest to a trough OR vice versa - but should not use both.
How is the zero-crossing period defined (words and equation)?
Time between any two successive zero-crossing points.
T(z) = m(0)/m(1) where m(n) is the nth spectral moment of the sea state
Why is the zero-crossing period not an ideal representation of the sea state?
If there’s a large portion of short period waves, mean T(z) will be significantly reduced
What does the peak period T(p) correspond to? Why is it not a pertinent model for multi-modal sea-states?
The period at which S(f) is the highest. Multi-modal sea states will have more than S(f) one peak
What is the energy period T(E) in the case of a unidirectional sea-state? Give the equation relating it to spectral moments
The period which the power transported per metre of crest length is proportional to
T(E) = m(-1)/m(0)
How is significant wave height calculated?
H(s) = H(m(0)) = 4sqrt(m(0))
What are the alternate names of uni- and omni- directional waves?
Uni: Long crested
Omni: Short crested
How can the spectrum S be expressed when taking into account directionality?
S(f, theta) = S(f) * G(theta)
with the integral of G(theta)*dtheta between 0 to 2 pi equalling 1
Describe the operation of surface-following buoys
- Measures elevation directly
- Moves with waves
- Built-in motion sensors detect elevation
- Technique does not work well with high frequency waves (but these do not tend to contain much energy)
Describe the operation of remote measurement
- Can measure wide range of area
- Examples: radar, satellite w/ altimeter
- Measures elevation directly
Describe the operation of acoustic doppler velocity profilers
- Measures water particle velocity
- Fires pulses of acoustic energy along four beams
- Acoustic energy is scattered by water particles
- Reflections received by instrument
- Signal is doppler shifted w/ respect to transmitted signal
- Allows velocity to be calculated
Describe the operation of a pressure sensor
- Detects pressure change of waves
- Laid on seabed
- Has depth limitation as the dynamic pressure change becomes increasingly small compared to static pressure from depth
What does each cell represent on a sea-state scatter diagram?
Shows the number of waves occurring within the corresponding ranges of H(s) and T(e)
How can a WAC and sea-state diagram be used to calculate the mean power output and total energy output for a year?
Multiply corresponding cells together then sum all components. Divide result with total number of counts in sea-state diagram
To get annual energy, multiply result by total seconds in 1 year
Explain shoaling
As a wave propagates into shallower water, the wave group velocity changes but the energy flux remains constant
Conservation of energy means the wave height must increase - when it exceeds breaking limit, wave breaking occurs
What are the three kinds of wave breaking and where do they occur?
- Spilling breakers, nearly horizontal beach
- Plunging breakers, steep beach
- Surging breakers, very steep beach
Why is breaking not desirable?
It is a source of energy dissipation so reduces available wave power
Describe refraction
When a wave propagates at an angle to the coast, dispersion equation shows that the wave in shallower water will travel slower, resulting in the turning of the direction of wave propagation
Define omni-directional wave energy resource
Includes all wave energy
Define directionally-resolved wave energy resource
Includes all wave energy crossing a line orthogonal to the mean direction of wave propagation
Define exploitable wave energy resource
Includes all wave energy crossing a line orthogonal to the mean direction of wave propagation limited to 4 times the mean wave energy density
