Chapter 8 Ocean Technology Flashcards
(41 cards)
Ocean Energy manifest itself in form of?
- Tides 300 TWh+/a
- Currents 800 TWh+/a
- Thermal gradients 10 000 TWh+/a
- Salinity gradients 2000 TWh+/a
- Waves 80000 TWh+/a
What are the main possibilities of utilising tidal energy ?
Tidal stream generators
Tidal stream systems make use of the kinetic energy from the moving water currents to power turbines, in a similar way to underwater wind turbines. This method is gaining in popularity because of the lower ecological impact compared to the second type of system, the barrage.
Tidal barrages
Barrages make use of the potential energy from the difference in height (or head) between high and low tides, and their use is better established. These suffer from the dual problems of very high civil infrastructure costs and environmental issues.
Explain how Tidal barrages work!
- utilise the potential energy in the difference in height (= head) between high and low tides
- are essentially dams across the full width of a tidal estuary
- relatively high civil infrastructure costs
- very few viable sites worldwide
- a number of environmental issues
and how do Tidal Stream Generators (TSGs) work?
- They utilise the kinetic energy of moving water with stream turbines an are very similar to wind turbines
- They have lower cost compared to tidal barrages and also lower ecologincl impact
How are the tides formed?
Due to lunar gravity and centrifugal forces
The gravitational force between moon and earth + centrifugal force induced by the rotation of earth cause the formation of water bulge.
effect of lunar gravity: ~ ± 27 cm
Due to solar gravity as well
effect of solar gravity: ~ ± 12 cm
theoretical amplitude of tides: neap tide: ± 15 cm
spring tide: ± 39 cm
real tides differ greatly, due to:
- resonance effects
- local amplification due to flow constraints (continents, islands etc.)
Is tidal energy a renewable energy source?
Tidal power, sometimes called tidal energy, is a form of hydropower that exploits the rise and fall in sea levels due to the tides, or the movement of water caused by the tidal flow. Because the tidal forces are caused by interaction between the gravity of the Earth, Moon, and Sun, tidal power is essentially inexhaustible and classified as a renewable energy source. In fact though, the ultimate energy source is the rotational energy of the Earth, which will not run out in the next four billion years, although the Earth’s oceans may boil away in two billion years.
Tidal energy potential?
Thus we need to distinguish between the potentials for tidal barrages and the one for hydrokinetic (stream) turbines.
The tidal energy potential that can be utilised with tidal barrages is difficult to determine, as the power potential is in proportion to the tidal range multiplied with the storage capacity behind the dam. The latter is not a fixed quantity at all. Thus resource maps are only giving the tidal range. High values of the average tidal range indicate an opportunity to build an economically feasible tidal barrage.
For energy conversion with tidal stream generators, resource maps are either giving the yearly average of the flow velocity or of the energy density.
Cave: as the power density is proportional to the 3rd power of the velocity,
the yearly averages cannot be simply converted into each other!
(Add formula)
For Dynamic Tidal Power projects, considerations similar to those as for barrages apply: The potential depends greatly on the length of dam that is feasible to build. Velocity maps are giving an indication of potentially good locations.
Where are the highest tidal ranges found?
The highest tidal ranges are found in places where the “tidal wave” meets obstructions and in bays where resonance phenomena occur.
Waht is the M2 tidal wave?
In most locations, the largest constituent is the “principal lunar semidiurnal”, also known as the M2 (or M2) tidal constituent. Its period is about 12 hours and 24.2 minutes, exactly half a tidal lunar day.
What are the three possible layouts of a tidal barrage?
- Singel bassin plant
- Twin bassin plant
- Tidal Lagoon
What are the operating modes of a single bassin plant?
- single acting:
- double acting:
- Example is la Rance power plant
- built in 1966
- double acting with pumping phase
- dam length 750m
- reservoir area 22km2
- 24 reversible Kaplan bulb turbines D=5.35
- average tidal range: 8.5 m
- total installed turbine power: 260 MW
- yearly generation: approx. 550 GWh
- Example is la Rance power plant
- double acting with pumping phase:
When is building a tidal barrage economically feasible? With which type of Turbine are they equipped with?
tidal barrages are economically feasible where high tidal amplitudes are found (e.g. French channel coast) and where existing bays or estuaries can be closed with a short dam.
Because of the low static heads (max. 10m) they are normally equipped with Kaplan bulb turbines.
How is a tidal barrage built?
The barrage method of extracting tidal energy involves building a barrage and creating a tidal lagoon. The barrage traps a water level inside a basin. Head (a height of water pressure) is created when the water level outside of the basin or lagoon changes relative to the water level inside. The head is used to drive turbines. The largest such installation has been working on the Rance river, France, since 1967, with an installed (peak) power of 240 MW, and an annual production of 600 GWh (about 68 MW average power)
What are the main components of a tidal barrage?
The basic elements of a barrage are caissons, embankments, sluices, turbines, and ship locks. Sluices, turbines, and ship locks are housed in caisson (very large concrete blocks). Embankments seal a basin where it is not sealed by caissons.
The sluice gates applicable to tidal power are the flap gate, vertical rising gate, radial gate and rising sector.
How is the twin basin plant formed ? What are its pros and cons?
Two basins each having a sluice gate to the sea:
- basin 1 is filled at high tide,
- basin 2 is emptied at low tide
- turbine operation from basin 1 to basin 2
+ continuous electricity generation, decoupled from tidal periods
- limited economical feasibility because very big reservoirs are required (small level variations compared to tidal amplitude).
How does a tidal lagoon work?
tidal lagoon:
- avoids closing off estuaries with a barrage
- circular dam structure in areas of high tidal range and low water depth
- proposed for Severn estuary – claimed annual production: 33 TWh
- 3 basin scheme permits continuous production
- not realised as yet but first project is planned in swansea
What are the basic principles of a tidal stream generator?
- deceleration of the flow,
- conversion of the kinetic energy into mechanical power,
- electricity production via generator
What are the stream turbine types, all feasible at average flow velocities > 2 m/s ?
- horizontal axis turbines (marinised wind turbines, mostly 2- or 3- bladed)
- vertical axis turbines (Darrieus, Voith-Schneider, Gorlov)
- oscillating blades
- some exotic other devices, like resistance runners, kites etc…
Compare horizontal axis stream turbines with HAWTs (wind)?
functionally identical with wind turbines:
- same fluid dynamical laws
- same components
but:
- much higher energy density
- hostile marine environment: sand, salt, marine growth
- all electromechanical equipment permanently submerged
- poor accessibility for maintenance
What is the enegry density formula for HASTs? P/A= ?
P/A= =0.5* rho* cinf [W/m<span>2´</span>]
You wanna give some examples of HASTs?
Voith Siemens Hydro:
110 kW prototype,
rotor dia 5.3m, c=2.9m/s deployed 2011 in Korea
Seaflow MCT:
150 kW prototype, successfully sea tested from 2003 to 2005
SeaGen MCT:
2*16m rotor diameter, P=1.2MW deployed 2008 in Strangford Loch
What do u know abt vertical axis stream turbines?
- very similar to their wind turbine counterparts, i.e. Darrieus rotors
- –> same considerations apply, i.e. energy density, marine environment
- due to the higher fluid density and lower speed, centrifugal forces are not dominant
- –> straight blades (H-rotor) instead of parabolic shape
-
special rotor type: Gorlov turbine
- patented in 1995 by Prof. Alexander M. Gorlov
- helically twisted blades –> low torque fluctuations
- low specific speed –> fish friendly
Now we come to the Oscillating hydrofoils type, what kind of forces are used in this type?
How is the mechanical energy transformed to electrical and what challenges rely in the conversion process?
Give an Example of this type of wave technology!
are utilising the lift forces occurring on a wing which is met by the flow under an angle of attack.
in contrary to rotating machinery, the blade angle must be changed at the end of each stroke
Conversion of the reciprocating motion into electric energy:
electric linear generator –> (expensive, no energy storage)
hydraulic cylinders, pressure accumulator, hydrostatic motor, electric generator
multitude of load cycles –> wear and fatigue problems
Example> The Pulse Device: in the mouth of the River Humber in the UK
What is wave energy?
- Wave energy, also known as ocean energy or sea wave energy, is energy harnessed from ocean or sea waves. The rigorous vertical motion of surface ocean waves contains a lot of kinetic (motion) energy that is captured by wave energy technologies to do useful tasks, for example, generation of electricity, desalinization of water and pumping of water into reservoirs.
- When wind blows across the sea surface, it transfers the energy to the waves. They are powerful source of energy. The energy output is measured by wave speed, wave height, wavelength and water density.
- Wave energy or wave power is essentially power drawn from waves. When wind blows across the sea surface, it transfers the energy to the waves. They are powerful source of energy. The energy output is measured by wave speed, wave height, wavelength and water density.
- waves can travel 1000 km without significant power loss
- -> the energy of remote storm events can be utilised locally
