Week 5 - Wave Energy Flashcards
How is water waves created?
Water waves are generated by friction between wind and water surface which transfers energy from the wind to the water.
The sea surface shape becomes unstable.
Wind creates differential pressure distribution and waves grows.
Waves do not grow indefinitely and eventually energy lost as a result of mechanisms such as white-capping.
What are the features of waves?
- Energy is continually transformed from kinetic and potential and back again.
- Water particles move in circular paths, known as orbitals (in shallow water, orbitals are elliptical).
- Wave progresses but particles do not.
- The orbital diameter decreases rapidly with depth below surface (negligible beyond half a wavelength down).
- Implies wave energy devices should lie at or close to the sea surface.
- Surface motion defined by top orbital (wave height H is twice amplitude a).
- In deep water the velocity of wave progression or ‘phase velocity’ c depends only on wave period T:
c = gT/(2pi)
and wavelength is:
lamba = gT^2/(2pi) - In deep water long wavelength waves travel faster than short wavelength waves (“dispersion).
- In shallow water, friction means depth d governs velocity not wavelength:
c = sqrt(g*d)
At intermidiate depths:
c = f(d,lamba)
How is deep, intermediate and shallow water defined?
Deep: d > wavelength / 2
Intermediate: Wavelength / 4 < d < Wavelength / 2
Shallow: d < wavelength / 4
How is energy in a wave calculated?
P = roh * g^2 * H^2 * T / (32*pi)
[Power per metre of wave crest (W/m)]
How is the ‘sea state’ defined?
For real or irregular waves it is impossible to measure each component wave so the ‘sea state’ in a time interval is defined by characteristic height and period.
What governs the velocity of waves in deep, intermediate and shallow water?
Wave period; both wave period and water depth; water depth – detail in slides
What is the difference between a regular and an irregular wave?
Regular - single frequency and height; Irregular - mix of waves of different
heights, periods and phases (real sea)
What are the three main wave period measures and what are their differences?
T_z is the zero-up crossing period (average interval for elevation to rise across zero).
T_p is the peak crossing period (Modal period)
T_e (energy period) used for wave energy as it accounts for energy in waves and ignores small, low energy, waves, giving a longer period: T_e ~=1.12 T_z
What measurements sources have been used for estimating wave energy?
Weather ships, short term buoy deployments, marine automatic weather stations (large permanent buoys), offshore platforms, satellite: synthetic aperture radar.
Wave energy converters exploit energy by moving in heave, pitch and surge. What do these mean and name a device/concept which exploits them?
Heave is vertical motion (simple buoy),
pitch is rotating motion in same direction as waves (Duck, mostly), surge is backwards/forwards motion in direction of waves (Oyster). Sway (side-to-side)
motion is also possible but limited energy there (partly exploited by Pelamis but
heave dominant).
Wave energy devices can be typically classified in several ways. What are these
and what are the key criteria for classification?
Water depth: shoreline, near-shore, deep water; Fixing: Bottom standing, floating, submerged; Number: solo, spine based or array; Absorber type: attenuator, terminator or point absorber; Reaction type: slack-moored or reaction from seabed; Power take-off: pneumatic, direct drive electrical, hydro; hydraulic.
Describe 4 wave energy conversion concepts, including what you believe to be the
advantages and disadvantages of each.
Oscillating Water Column: (+) simple mechanical system; largely proven
technology; possible to integrate within coastal defense. (-) significant civil
engineering required, largely confined to shallow water.
Floating buoy: (+) typically can handle waves from any direction; fairly simple to
‘tune’ to waves in theory; relatively simple to install; deployable in deep or shallow
water. (-) power takeoff depends on the mooring system which can be problematic;
lots of converters required to create significant amounts of power as individual
converters are small.
Flexible membrane systems: (+) suitable for deep or shallow water; shares features
with OWC. (-) unproven at large scale; membranes relatively fragile, prone to
fatigue/creep.
Articulated cylinders: (+) suitable for deep water; power take off can be enclosed
within the converter itself; proven at scale. (-) complex mechanical systems;
physically large machines; require space to allow them to orientate in waves.
. There are a series of factors that make wave energy exploitation particularly
challenging. Briefly list these and outline why they feature.
Wave patterns are irregular in amplitude, phase and direction; wide range of wave
intensities and probability of hurricanes producing waves of freak intensity;
constructing power devices for these types of wave regimes; of maintaining and
fixing or mooring them in position; transmitting power to land; extremely difficult
to couple irregular slow motions to electrical generators; many types of device.
What are the main categories of cost used in investment appraisal of generation?
Capital cost, operating costs (including fuel), decommissioning.
What determines the revenue of a generator?
Volume of sales and the price applying to each unit sale. Expanding on this would include consideration of whether generator is selling direct into the market or via a
power purchase agreement.
