1.1 Introduction
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It is another sustainable and endless energy source, which could significantly contribute to the renewable energy mix. In general, increasing the amount and diversity of the renewable energy mix is very beneficial as it increases the availability and reduces the need for fossil fuels.
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Electricity from wave energy will make countries more self-sufficient in energy and thereby less dependent on energy import from other countries (note: oil is often imported from politically unstable countries).
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It will contribute to the creation of a new sector containing, innovation and employment.
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Electricity from ocean wave can be produced offshore, which thereby does not require land nor has a significant visual impact.
1.2 The Successful Product Innovation
WEC type | Capture width ratio (%) |
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Floating overtopping device | 17 |
Oscillating water column | 29 |
Point absorber | 16 |
Pitching flap (bottom fixed) | 37 |
1.3 Sketching WECs and Their Environment
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The hydrodynamic subsystem is the primary wave absorption system that exploits the wave power (see Chap. 6). It can be of different types depending on the technology, e.g. oscillating body, oscillating water column and overtopping principle, and it is connected to both the reaction and PTO subsystems against which it will actively transfer forces and motions.
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The power take-off subsystem converts the captured wave energy (by the hydrodynamic subsystem) into electricity (see Chap. 8). The PTO systems can be based on different principles, of which some of the most common are hydraulic PTO, direct drive mechanical PTO, linear generators, air turbine and low head water turbine.
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The reaction subsystem maintains the WEC into position relative to the seabed (e.g. mooring system) and provides a reaction point for the PTO and/or support for the hydrodynamic subsystem(s) (e.g. fixed reference or support structure) (see Chap. 7).
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The control (and instrumentation) subsystem is the intelligent part of the system as it takes care of the control of the WEC and its measurements. It mainly consists of the processors for the automation and electromechanical processes, the sensors and their data acquisition, the communication and data transfer, and the human interface.
1.4 Rules of Thumb for Wave Energy
1.4.1 The Essential Features of a WEC
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Survivability: The WEC requires a reliable mooring system and preferably a passive safety system that can effectively reduce extreme loads. With passive meaning that the safety mechanism can be activated (automatically) without requiring external interaction, such as electricity or other.
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Reliability and maintainability: Easy access and inspection of the most essential parts of the WEC. In addition, it would be very beneficial if most (or all) maintenance could be done on the WEC itself at location, without having to bring it back to a harbour.
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Overall power performance: The WEC must consist of an efficient wave energy absorbing technology and PTO. It has to produce a sufficiently smooth electrical power and have a high capacity factor. Otherwise, too much energy will be lost over the whole wave-to-wire power conversion chain.
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Scalability: At full scale, a WEC needs to be a multi-MW device in order to be economically viable. In order to be able to continue significantly improving its LCoE, it needs to be scalable, meaning that it should be capable of further enlarging its dimensions (like offshore wind turbines do). Many WECs unfortunately reach their optimal dimensions at too low dimensions, making it not possible for them to become multi-MW WECs (>5 MW). This does not include the multiplication of WECs as this will not have a significant influence on the average infrastructural and technology costs and thereby will not significantly improve the LCoE of the WEC or project.
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Environmental benefit: WECs are expected to be sustainable energy systems and are thereby expected to have a great environmental benefit and a minimal environmental footprint.
1.4.2 Economic Rules of Thumb
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The CapEx per installed MW is approx. 4 million euros.
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The OpEx/MWh is approximately 30 Euro.
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The LCoE is approximately 120 Euro/MWh.
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The general development, infrastructure and commissioning costs, referred to as the base CapEx, of a 3.6 MW offshore wind turbine in a project are in the range of 7.2 million Euros. This includes the development and consent, the installation and commissioning and a part of the balance of plant category, but excludes the tower, the foundations and the technology itself. This cost corresponds to about 45 % of the CapEx [10].
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The resulting “base” CapEx cost for a 3.6 MW WEC is expected to be slightly less, approx. 6 million Euro, as especially the installation cost should be significantly lower. For smaller WECs, it is expected to be approximately 2 million Euro for a 750 kW WEC.
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Sharing basic equipment over different wave absorbing bodies, such as mooring systems, weather stations, communication systems, electricity cables and others.
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Sharing parts of the power take-off (PTO) system, which (usually) results into higher capacity factors and smoother electrical power output.
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The whole system can be commissioned at once, thereby sharing installation and servicing works and equipment, e.g. it only requires one vessel for handling one system.
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Larger structures are more easily accessed as they are more stable, which enables easier inspection of the system and some maintenance could be done on board, without the need of retracting the system to a safe/controlled area.
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The wave-to-wire efficiency (η w2w ) is the overall efficiency of the system delivering the absorbed energy from the waves to the grid. This value is also based on many underlying specifications, such as the wave conditions, the availability of the system and the maximum power rating, and so needs to be taken very carefully.
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The capture width ratio (CWR) describes the effectiveness of the converter to absorb the energy in the waves. This value is based on many underlying specifications, such as the wave conditions and the size of the wave activated body, and needs thereby to be handled very carefully.
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The WEC weight/installed kW ratio is also often used to indicate how much material is used relative to the power rating of the WEC. This can be a bit misleading as it does not particularly show the type of material (e.g. steel or concrete). It should at least be divided between active structural (load carrying) material and ballast material, as their difference in cost can be as great as a factor 100.
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The capacity factor (also called capacity factor) is the ratio between the average produced power and the installed power on the WEC. It describes the utility rate of the PTO system and is very interesting as it gives an idea of what the WEC delivers (average produced power) and what it costs (driven by installed power). However, this value is also wave condition dependant (location).
WEC type | Relevant dimension (m) | |
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Point absorber | Diameter | 12–20 |
OWC | Lengtha
| 12–20 |
OWSC | Thicknessa
| The thicker the better |
Floating structures e.g. overtopping WEC | Length | Longer than a wavelength |
1.4.3 WEC Design Rules of Thumb
“A good wave absorber must be a good wave-maker.”
WEC type | Max-to-mean ratio |
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Single wave-activated body with one-way PTO | 15–30 |
Single wave activated body with two-way PTO | 10–12 |
OWC with two way PTO | 10–15 |
10 side-by-side located wave-activated bodies (in the wave direction) with two-way PTO | 3–7 |
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Although WECs are typically more efficient in steep waves, they result in larger surge offsets, relative to their rest position.
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Surge motions of a moored floating structure are especially large under the event of breaking waves, which also result in significantly higher wave loads on the structure.
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The durability of the mooring system is even further challenged under short-term repetitive wave events such as wave groups (which is very common).
1.4.4 Power Take-Off Rules of Thumb
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Reduces the complexity of the PTO system and the possible amount of load cases.
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Optimises its efficiency and facilitates its control as the exact motion of the wave activated body is known.
PTO system | Efficiency (%) |
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Hydraulic | 65 |
Water | 85 |
Air | 55 |
Mechanical | 90 |
Direct drive | 95 |
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Temporarily store/smooth energy.
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Handle short-term power overload.
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Handle sudden system faults and possible control losses.
1.4.5 Environmental Rules of Thumb
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Good average wave energy content, e.g. >15 kW/m, as this is the source of energy.
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Good average wave steepness, e.g. >1.5 %, as the performance of WECs is significantly higher in steep waves.
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Low max-to-mean ratio in terms of significant wave heights, as you build (≈pay for) the WEC design to endure a 100-year wave while it produces energy (≈earnings) relative to the average wave condition.
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Low monthly wave energy content variation, as it facilitates stable power production and improves the capacity factor when the wave climate is consistent over the whole year. However, this makes installation and maintenance more difficult as weather windows are less frequent and shorter.
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Proximity to the coast, infrastructure and end-user as it significantly reduces CapEx and OpEx costs related to the project.
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Reasonable water depth (e.g. 30–60 m), which can seriously affect the mooring and cabling cost.
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South and West coasts below the tropic of Capricorn (e.g. Australia, New Zealand, South Africa and Chile): high average wave power and low seasonal variability and low 100-year wave to mean wave ratio.
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East coasts below the tropic of Capricorn (e.g. Australia, New Zealand, South Africa, Argentina, Uruguay and South Brazil): medium average wave power, with low seasonal variability and low 100-year wave to mean wave ratio.
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West coast of United States: medium average wave power, with low seasonal variability and low 100-year wave to mean wave ratio.
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North Atlantic (Europe and East coast US): high average wave power and steep waves, but high seasonal variability and high 100-year wave to mean wave ratio.