Marine Renewable Energy Flashcards
What are the main sub-categories of marine renewable energies?
wave energy
tidal energy
offshore wind
ocean thermal energy
What is the formula for levelized cost of energy?
(OPEX + CAPEX) / E
exes over E in £/kWh
Ocean waves a re a concentrated form of what energy?
solar energy
the characteristics of waves depends on what two factors?
fetch
duration of wind
what is wind fetch?
length that wind acts on ocean surface
What is a wave?
a disturbance travelling through a medium
The total power of a regular wave in deep water is measured by what unit?
per unit length of wave front
What is the formula for total wave power?
P=(1/8pi )rhog^2a^2*T
a=amplitude
T=wave period
Closer to shore, the magnitude of harvested wave power is usually . .
less than in the sea
Seasonal variations in the potential energy gained from wave energy are much larger in which hemisphere?
north - stormy winters can generate more wave energy
What is a WEC?
wave energy converter - a floaty boi
a.k.a PTO (Power take off), transforms mechanical energy into electricity.
how does as WEC work?
piston compress fluid that is stored in a high-pressure accumulator.
pressure diff between high and low pressure accumulators is used to drive electrical generator
wave energy absorption can be defined as a [] process
wave interference
what is the saying which describes wave interference?
“to destroy a wave means to create a wave” - placing a WEC system in a wave causes destructive interaction and therefore waves of its own
energy gained from WEC is maximised when it experiences what phenomenon?
resonance?
WEC technologies are classified using:
WEC working principle
dimension/configuration
location - offshore, nearshore, shoreline
Which three main types of WECs can be defined based on their working principle?
oscillating water columns
oscillating bodies
overtopping devices
oscillating water columns can exist in which two types?
floating or fixed
WECs can be either
heaving or pitching
what is the difference between heaving or pitching WECs?
heaving - bob up and down
pitching - pivot back and forth
How does an overtopping device operate?
Waves approach the device and climb up its slope to be stored.
This water can then be released and harnessed for its kinetic energy
In what area are overtopping devices best suited?
Areas with large amounts of land to build on with tall oncoming waves
What is a limitation of overtopping devices?
They are costly and large and must be optimised to their environment
What are some types of overtopping devices?
fixed or floating
What is the main difference between the two types of overtopping devices?
Fixed ones are either anchored or built into the land
Floating ones float whilst being connected to the sea bed via mooring lines or cables. - more expensive but better for deep sea and less prone to damage as it floats with the water
The periodic rise and fall of water in the oceans is know as
tide
The difference in sea level between high and low tide is known as
tidal range
Incoming flow of water is called
flood
outgoing flow of water is called
ebb - bye bye
The flow of water is known as
tidal current/tidal stream
why do we observe two tides per day?
cause by rotation of the Earth along its own rotation axis and not the rotation of the moon around the Earth.
The existence of two low-tides and two high-tides per day is called what?
semi-diurnal cycle - 2 full periods every day
What happens when the sun and moon are aligned(and therefore their gravitational forces)?
spring tide - heightened tide paired with orthogonally placed lowest tide
What happens when the sun and moon are perpendicular (and pulling 90 degrees apart)?
neep tide - lowest high tide, tallest low tide
The cycle of neap/spring tide has a period of how many days?
14
What is tidal range defined as?
difference in sea level between high and low tide
what does electricity generation from the tidal range utilise?
the gravitational potential energy an artificial change in height produced by an built obstacle has
in nearshore locations tidal currents are [] around headlands or through constrictions
accelerated
Electricity generation from tidal currents capitalizes on what form of energy which is found in free flowing water?
kinetic energy
Which two forms do tidal range power plants normally take?
tidal barrage or tidal lagoon
What are the main components of a tidal range power plant?
sluice gates
bulb turbines
impoundment
What kind of structure might a tidal barrage have?
hundreds of metres long, spanning entire river estuaries
What kind of structure might a tidal lagoon have?
encloses an area of coastline with high tidal range behind a breakwater
What are the types of tidal stream devices?
horizontal axis turbine
vertical axis turbine
oscillating hydrofoil
Archimedes screw
tidal kite
what is the main advantage of the tidal kite?
relative velocity felt by the turbine attached to the kite is increase, good for low energy sites
what are some environmental consequences of tidal energy shcemes?
disturbance to marine life
must coexist with fishing industry
large installations can affect: sediment transfer paths, tidal currents
water quality might be affected
How is the flow of water at the sea bed affected by the sea bed itself?
friction provided by sea bed slows the flow down
what is the main difference between first and second generation tidal stream devices?
first generation sit on the sea bed
second generation are floaty bois
how were 1st gen support structures for horizontal-axis turbines designed?
with extremely heavy weights to resist sliding along the sea bed and bending on the vertical lever arm
1st gen structures of tidal stream devices were . .
expensive
limited to use on shallow sites
2nd gen support structures include:
buoyancy modules (large floaty pills)
mooring systems under tension
why can’t a turbine extract all the kinetic energy from a flow?
the flow would stop
