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Tidal turbines
CHAPTER 10
Tidal turbines Sanaz Roshanmanesha, Farzad Hayatib, Mayorkinos Papaeliasa a
School of Metallurgy and Materials, The University of Birmingham, Birmingham, United Kingdom School of Engineering, The University of Birmingham, Birmingham, United Kingdom
b
1 Overview Tidal energy is still a developing renewable energy source with only a few commercial projects having been commissioned to date. Although there are several different device designs that have been proposed and investigated for harvesting tidal energy, most of them have only been tested either at model or small scale. A handful of designs have managed to gather sufficient financial support to be tested at sea at full scale. There are currently plans for significant utility-sized tidal energy plants in the short-to-medium term. Nonetheless, these still face uncertainty regarding their eventual implementation due to financing issues that have not been entirely resolved yet. Tidal energy production offers several advantages over conventional renewable energy sources such as photovoltaics and wind energy. For example, it does not require access to valuable land resources and is unobtrusive. Furthermore, it can produce electricity continuously at a predictable rate, thanks to our accurate knowledge of the tidal cycle. It is undeniable that the available tidal energy which can be exploited in a financially viable way is significant. Appropriate sites for tidal energy exploitation exist throughout the world’s seas. These sites are often within acceptable distance from the shore helping reduce the overall connection costs of future tidal farms to the grid. As the construction of new wind farms and photovoltaic parks is getting closer to saturation point due to less and less land being available for commissioning such projects, alternative renewable energy sources that are not land-demanding such as tidal energy will become more attractive to investors. Most tidal turbine designs have significant similarities in comparison with wind turbines. Tidal turbines harvest the kinetic energy of water currents caused by the tides due to the movement of the moon in the same way that wind turbines harvest the kinetic energy of the wind. However, tides are completely predictable and therefore tidal energy harvesting is far Non-Destructive Testing and Condition Monitoring Techniques for Renewable Energy Industrial Assets https://doi.org/10.1016/B978-0-08-101094-5.00010-1
© 2020 Elsevier Ltd. All rights reserved.
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more predictable than wind. Due to the variability of wind speed over time, the amount of energy harvested from wind farms can fluctuate significantly. Hence, integration of more than 30% of wind energy in the energy mix is challenging due to the risk of blackout caused by grid instability. The operation of tidal turbines, on the other hand, is predictable and therefore, the amount of energy produced is more stable. This allows tidal energy to be more easily integrated to the energy mix in large scale. Tidal turbines, like wind turbines can be of the direct-drive or geared type. The majority of commercially available tidal turbines is based on the use of a gearbox. The use of the gearbox is often necessary due to the very low speed of rotation that tidal turbines operate. Direct-drive tidal turbines require a much larger number of magnets for electricity production and as a result, their cost is much higher whilst the weight of the generator increases substantially. The use of the gearbox allows the low-speed rotation of the rotor to be multiplied through the various stages of the gearbox, reaching speeds above 1100 Revolutions per Minute (RPM) at the output stage (generator). Unfortunately, tidal turbines, like wind turbines, operate in a turbulent environment which means loading conditions vary continuously. Tidal turbine gearboxes apart from the fact of being submerged underwater several metres below the surface of the sea need to be able to operate under a profoundly adverse variable loading environment. Long-term experience with large-scale multi-MW wind turbines has shown that variable loading causes a high level of wear and tear in gearbox components. This is primarily due to the high stresses and variability in lubrication quality. Thus, it is not surprising that gearbox-related faults are among the most important causes of downtime. Moreover, very few wind turbines reach their end of the design lifetime without having their gearbox replaced or refurbished. Tidal turbines face even harsher operational conditions, meaning gearbox-related faults are likely to develop over prolonged in-service operation. Water is 832 times denser than air. As a result tidal turbine rotors can be made much smaller in size than wind turbine rotors for harvesting the same amount of energy. As tidal turbine rotors increase in size, they need to sustain much higher loads under harsh environmental conditions. The rotor is subject to accidental impacts and fouling build-up causing imbalance and deterioration of the overall hydrodynamic efficiency of the blades with time. Types of damage that can affect the drive-train of a tidal turbine include misalignment, imbalance, looseness, broken gear teeth, bearing defects,
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lubrication variability resulting in dry contact of the rotating surfaces, and excessive wear. It is very important to be able to detect accidental impacts on the rotor and damage evolution in the drive-train. Certain commercial tidal turbines make use of vibration condition monitoring systems in the same way as most multi-MW wind turbines.
1.1 Early tidal turbines Early tidal electricity generators have been based on the use of tidal barrages at river estuaries. A tidal barrage consists of several tidal turbines placed along a dam-like structure. The dam walls prevent any water flow along the structure, so all the energy of the water currents is forced to pass at a higher pressure through the turbine locations only. As the water passes through the turbines, the blades of the rotors are forced to rotate. The kinetic energy of the rotors is converted to electricity by the built-in generator. A fairly well-known example of a tidal energy barrage system is the La Rance Tidal Power Station in France, which was built in 1967. The La Rance Power Station shown in the photograph of Fig. 1 has a power rating of 240 MW [1]. It consists of 24 bulb tidal turbines with a power rating of 10 MW each. Their rotor spins at approximately 94 RPM. The positions of some of the tidal turbines along the dam structure are labelled in the photograph in Fig. 2. The annual capacity factor of the La Rance Power
Fig. 1 Aerial photograph of the La Rance Tidal Power Station in France. (Source: Public Domain, https://commons.wikimedia.org/w/index.php?curid=3182853).
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Fig. 2 Photograph of the La Rance Tidal Power Station indicating the location of some of the tidal turbines along the dam. (Photograph adapted from Dani 7C3 - Own work, CC BY 2.5, https://commons.wikimedia.org/w/index.php?curid=915904).
Table 1 List of the largest barrage type tidal power stations in the world currently in operation Location
Site
Basin area (km2)
Mean tide level (m)
Power rating (MW)
Installation year
France Russia Canada China
La Rance Kislaya Guba Annapolis Jiangxia
22 1.1 15 1.4
8.55 2.3 6.4 5.08
240 0.4 18 3.9
1967 1968 1984 1985
Station, which cost €95 million in 1967 to construct, is 28%. This is similar to the annual capacity factor of several onshore wind farms.The power generated by the La Rance Power Station reaches 540 GW/h per annum. Since its construction the La Rance Tidal Power Station has produced in excess of 27 TWh. The cost per MWh produced is less than €18, which makes it highly competitive in comparison with other renewable energy sources. Table 1 lists the largest tidal power stations currently in operation. A problem with tidal barrages is their impact to the environment [2]. The operation of tidal barrages can pose challenges to aquatic animals. In 2004,
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it was reported that a whale had been trapped in the basin of the Annapolis barrage turbine which is located in Canada [3]. However, as technology advances, the threats to wildlife are reduced albeit not irradicated completely [4, 5].
1.2 Modern tidal power generators Tidal turbines are large-scale rotating systems consisting of several subcomponents. They are designed to efficiently convert the kinetic energy of tidal currents to electrical energy through the transmission of torque to the generator. The typical design of commercial industrial tidal turbines is very similar to that of offshore wind turbines. Tidal turbines can be installed underwater in rows like offshore wind turbines. The advantage of tidal turbines in comparison with wind turbines is the fact that operation is independent of weather and climate change. Tides are predictable allowing integration of tidal power to the energy mix more easily. As mentioned earlier, water density is 835 times that of air. Hence, energy harvesting is more efficient in terms of the rotor size required to harvest the equivalent amount of energy. However, due to the much higher stresses sustained by the blades in the water their length is much smaller, whilst rotor speed is only a few RPM. Due to the limited industrial experience with tidal energy systems, their construction, operation and maintenance costs remain higher than those of offshore wind turbines, the exception being the better technologically understood tidal barrages. Nonetheless, as the learning curve is gradually overcome it is expected that costs associated with tidal turbines will be reduced to similar levels to those of offshore wind turbines. Currently, there exist more than 30 different types of tidal turbine designs which are under different stages of development. One of the most advanced designs is the MCT SEAGEN geared tidal turbine. Geared tidal turbines are very similar to geared wind turbines with the main difference being related to the blade design and the requirement for water-tightness of the nacelle. Tidal turbines, like wind turbines, are also subject of variable loads. Therefore, similar issues with the drive-train, particularly the gearbox, are anticipated. As mentioned earlier, variable loading conditions do affect the lubrication quality increasing considerably fatigue damage accumulation. Instant misalignments due to variable loads or permanent misalignment can further exaggerate fatigue damage resulting in final failure much earlier than anticipated in the design.
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Tidal power generators can be classified into four different categories [6] depending on the key characteristics as described below. • Vertical axis tidal turbines: This type of tidal turbines has the main rotor shaft installed transverse to the direction of the water motion. Within this particular design the gearbox and generator can be installed either at the bottom near the seabed or top of the device near the surface for better accessibility during maintenance. • Polo tidal turbines: This type of turbines is usually based on vertical axis. Their structure is similar to hamster wheels with the blades on the circumference of the wheel. This type of design can have variable pitch blades to increase versatility. • Venturi effect turbines. This tidal turbine design is based on the Venturi effect.The Venturi effect can be produced using a duct narrowing in the centre to accelerate the tidal flow. The generator in this type of turbine is located in the centre of the duct for maximum efficiency. • Horizontal axis tidal turbines: Unlike vertical axis turbines, in this type of design the main rotor shaft is parallel to the direction of the water flow. This is very similar to horizontal axis wind turbines (HAWT) but in the case of tidal turbine the device is fully submerged in the water. An example of each of these design types is shown in the schematics of Fig. 3. Recently, a new type of tidal turbine was prototyped by BioPower Systems which is based on the oscillation of a hydrofoil, generating a motion similar to that of a fish tail fin.This design is illustrated in the schematic of Fig. 4. A technology status review carried out recently by Khan et al. [7] showed that the horizontal axis turbines are the most popular type of tidal stream turbines. About 43% of all designs are of the horizontal type followed by vertical axis turbine designs with 33%.The plots in Fig. 5 illustrate the number of tidal turbine designs that have reached commercial maturity in comparison with conceptual designs proposed, tested at different scales and precommercial systems. Tidal power generators are complex systems which can be described in simpler terms by dividing them into the several subsystems and subassemblies that they consist of. Fig. 6 shows a generalised chart summarising the basic configuration of tidal turbines. Some of these main subassemblies will be discussed next.
1.3 Rotor blades Rotor blades are a critical structural component of the tidal turbine since they are responsible for harvesting the kinetic energy of the water and
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Fig. 3 Different types of tidal turbines; (A) Vertical axis; (B) Polo turbine; (C) Venturi (duct) effect; (D) Horizontal axis. (Source: (A) Fan, Z. Tidal Power Energy, Renewable Energy in Future. 2011; pp. 30–11, 2011. (B) Boyle, G. Renewable Energy: Power for a Sustainable Future. 3rd ed.; Oxford University Press: Oxford, 2004. (C) Mestres, M., et al. Analysis of the Optimal Deployment Location for Tidal Energy Converters in the Mesotidal Ria de Vigo (NW Spain). Energy 2016, 115 (Part 1), 1179–1187. (D) Borthwick, A.G.L. Marine Renewable Energy Seascape. Engineering, 2016, 2 (1), 69–78).
transmitting it through the main drive-train to the generator. The water currents will directly act on the surface of the rotor blades forcing them to rotate. The blades may have a fixed pitch to simplify the design. Most modern tidal turbines use blades with variable pitch which can adapt to the water current speed. The blades are commonly bi-directional to enable them to harvest tidal energy in both directions. The variable pitch enables the blades to turn their hydrodynamically active surface in both directions depending on the direction of the tide maximising the energy production. Unlike wind turbines, the forces applied on the tidal rotor blades are much greater.Table 2 compares some of the turbine parameters between horizontal axis wind and tidal turbines with a power rating of 1 MW. As it is shown, the thrust in the tidal turbine can be twice the level of that applicable for
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Fig. 4 Oscillatory-hydrofoil turbine concept, developed by BioPower Systems company. (From Marsh, P., et al. Three-Dimensional Numerical Simulations of Straight-Bladed Vertical Axis Tidal Turbines Investigating Power Output, Torque Ripple and Mounting Forces. Renew. Energy 2015, 83, 67–77). 50 45 40 35 30 25 20 15 10 5 0
17%
7%
43%
33%
Other
Cross-flow
Vertical
Horizontal
Other
Cross-flow
Vertical
Horizontal
Fig. 5 Current technological landscape of tidal turbine designs.
a wind turbine of equivalent power rating. Furthermore, the tidal turbine blades can suffer from erosion by solid particles, fouling by marine growth, corrosion and tribo-corrosion [9]. Failure in the rotor blades in a tidal turbine has much more serious consequences compared with wind turbines. Due to the hostile underwater conditions and limited available time windows caused by poor weather conditions, recovery and repair operations of tidal turbines take considerable amount of time and effort.
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Tidal turbine configuration
Rotor blade assembly
Main drive shaft
Generator drive train
Supporting structure
Nacelle enclosure
Fixed yaw
Hub assembly
Couplings
Foundation / pile
Sensors
Variable yaw
Seals
Direct-drive generator
Stability
PLC / SCADA controls
Fixed pitch
Bearings
High-speed generator
Gravity
Bilge pump
Variable pitch
Tachometer
Gearbox
Position
Electricity out cabling
Controls
Brakes
Electrical control
Floating
Atmosphere
Fig. 6 Generalised tidal turbine configuration. (Modified from Grosvenor, R.I.; Prickett, P.W. A Discussion of the Prognostics and Health Management Aspects of Embedded Condition Monitoring Systems. In Annual Conference of Prognostics and Health Management Society; 2011). Table 2 Comparison between similar wind and tidal turbine parameters (1 MW power rating) [8] Parameter 3
Medium density (kg/m ) Rotor diameter (m) Number of blades Rotational speed (RPM) Velocity for maximum power (m/s) Mass flow rate (tonnes/s) Thrust (tonnes)
Wind turbine
Tidal turbine
1.2 52 3 7–2000 12–13 30–40 50
1000 20 2–5 7–20 2.5 900 100
Similar to wind turbines, the material of choice for the blades in the tidal turbine is usually glass or carbon fibre reinforced composite materials [8]. However, in certain cases aluminium or steel alloys have been employed instead. Several tidal turbine failures have been caused by blade failure [10]. Therefore, it is important to assess the condition of the blades to ensure repairs or replacement can be carried out in good time, before a failure leads to loss of production. Various blade monitoring techniques such as fibre-optic sensors or strain gauge measurement sensors can be used to evaluate the structural health of tidal turbine blades. Alternative accelerometers or acoustic e mission sensors
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can be employed. There are also several techniques available which can be applied for the prevention of fouling and corrosion around seals and surfaces [10]. However, there is a need for advances in detection of impact damage. Acoustic emission sensors are ideal for the detection of impact events. Manual inspection can be carried out either visually or using ultrasonic testing. In the case where carbon fibre-based reinforced composites are used, eddy current testing can also be employed for the detection of delamination damage.
1.4 Drive train The drive train is one of the main subsystems of the tidal turbine. In geared designs, it consists of the main bearing, rotor shaft, gearbox, and generator, whilst in the direct-drive designs, it consists of the main bearing, rotor shaft, and generator only.The drive train is responsible for transferring the torque from the rotor blades to the rotating generator in order to convert the kinetic energy into electrical power. Each of these components will be briefly described below.
1.5 Gearbox The gearbox in tidal turbines is responsible for transmitting the hightorque, low-speed drive shaft energy generated by the blades motion into high-speed, low-torque energy driving the generator for producing electricity. The speed required by the generator varies depending on the required grid electricity and frequency and the number of magnets used. The overall concept is identical to that used in wind turbines. However, the gear ratio is usually higher than in wind turbines, since the rotor speed is typically much slower underwater, albeit it has higher torque. Common MWscale wind turbine gearbox designs comprise three stages, one planetary and two h elical-parallel stages with a ratio between 60 and 80 [11], while an equivalent tidal turbine designs typically employs four-stage gearboxes with three planetary stages and one helical-parallel stage with a ratio of about 200 [12]. The gearbox is equipped with a lubrication system to ensure appropriate level of lubrication of rotating components and reduce the gearbox deterioration rate. It may also be equipped with a breaking system at the high-speed output to reduce the speed and stop the turbine in case of emergency shutdown or maintenance. The tidal turbine gearbox can be monitored using various types of sensors including oil temperature for detection of overheating bearings, vibration systems, or acoustic emission. In addition, online oil sensors can
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be employed for assessing the oil viscocity and the presence of debris. Oil samples can also be retrieved manually when the turbine is raised out of the water for planned maintenance activities. During this time, visual inspection can be carried out after the gearbox case has been opened. However, visual inspection usually has limited value unless damage in components viewed is already visible.
1.6 Generator Tidal turbines can make use of either synchronous or asynchronous induction generators. Asynchronous generators run above the grid synchronous frequency. They are particularly popular in wind turbines due to their ability to produce power at variable speed. They are simpler compared to other types of turbines, do not require a commutator, are light weight and can generally withstand harsh operational conditions. However, they do require a cooling system. In contrast, synchronous generators are bulkier and heavier than asynchronous generators and run at a lower speed. This removes the need for the presence of a gearbox and the generator can run in direct-drive mode. Generators consist of the generator bearings and windings. Both of these components can develop faults. Generator bearings can be monitored with accelerometers or acoustic emission sensors.Windings can be monitored by trending the power output with varying water speed. Trending the power output can help identify possible faults affecting the windings prior to the occurrence of final failure.
1.7 Power converter To enable the tidal turbine to be connected to the grid, a power converter is required. The main role of the power converter is to convert the variable frequency from the tidal turbine generator into the grid frequency. This is usually done in three steps. First, a rectifier unit converts the generator alternating current (AC) into direct current (DC). Then it feeds it to a DC link capacitor. The DC link capacitor is subsequently connected to a switching inverter producing an AC current at fixed frequency. Fig. 7 shows a typical AC-DC-AC converter. After conversion, the generated voltage will be fed into a transformer to step up the voltage to several kilovolts (kV) in order to reduce the transmission loss and connect the turbine into the main grid. The power converter can be monitored using a variety of sensors to assess different parameters that can influence its operation including current, voltage, temperature, humidity and vibration sensors.
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Rectifier unit
DC link capacitor
3-Phase inverter unit
Fig. 7 A typical AC-DC-AC converter with DC Link Capacitor to translate high frequency generated power into the grid frequency.
1.8 Low-voltage consumed power Some tidal turbines are required to be powered from the grid before starting to produce power. In addition, some systems such as the controller, safety systems and brakes are required to be powered uninterruptedly even when the turbine is idle. These subsystems are usually powered from the grid. However, a second source of power is normally installed on-board, such as low voltage batteries that are kept charged from the grid. These batteries provide uninterrupted power to the low-voltage electrical systems of the tidal turbine ensuring their operation at all times.
1.9 Control and management systems The control system of the tidal turbine is very similar to that of a wind turbine control system. The controller is responsible for monitoring and controlling the overall operation of the tidal turbine. This includes but not limited to, the amount of tidal flow, the position and pitch of blades, blade and rotor speed, braking, power output, etc. It also monitors other critical parameters such as the status of the cooling and lubrication systems. The controller can manage the operation of tidal turbine independently as well as being managed remotely by the supervising engineers in order to shut down the turbine safely in cases such as emergency, planned maintenance or inspections.
1.10 Supporting structure The forces acting on the tidal turbines submerged beneath the surface are very high and complex in nature. The blades give rise to high axial forces rendering essential the securing of the turbine structure to the seabed. The supporting structure is responsible for holding the turbine in place against
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the acting axial forces. There are several different types of supporting structures available to secure the tidal turbine. However, they can be typically grouped into two main categories; fixed structures using gravity-base or monopile-support structures and floating structures using mooring-support. The schematics in Fig. 8 show a few different types of supporting structures used in different tidal turbine designs. Depending on the sea environment, the amount of force acting on the tidal turbine and type of tidal turbine any of these supporting structures may be used to secure the turbine in place during operation.
(A)
(B)
(E)
(C)
(F)
(D)
(G)
Fig. 8 Different type of tidal stream turbine supporting structure after [13]. (A) surface piercing seabed monopile; (B) seabed mounted monopile; (C) floating pontoon; (D) seabed tethered; (E) telescopic seabed monopile; (F) floating SPAR buoy, tethered to seabed; (G) ducted seabed mounted.
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Horizontal axis
Vertical axis
3%
12% 8%
27%
37%
52%
33%
BSM
FSM
28%
NSM
N/A
BSM
FSM
NSM
N/A
Fig. 9 Variation of the turbine supporting structure in vertical axis and horizontal axis tidal turbines (after [7]). These techniques are known as Bottom Structure Mounting (BSM), Floating Structure Mounting (FSM) and Near-surface Structure Mounting (NSM).
The review of tidal energy technology carried out by Khan et al. [7] has shown that bottom structure mounting (BSM) is very popular within horizontal axis turbines, whilst vertical axis turbines are mostly supported using near surface structure mounting (NSM). The plots in Fig. 9 summarise the result for the two different types of tidal turbines.
1.11 Power transmission One big challenge in the implementation of tidal turbines is the electricity connection link required for the transfer of produced electricity to the shore. One solution is to use fixed subsea cables on the seabed, similar to offshore wind farms. However, in the case of floating and semifloating tidal turbines this will be very difficult and usually an umbilical cord is required to be used from the turbine to the seabed. Apart from the power cables the data cables are also installed alongside the power cables, for transmission of critical data to the shore. These data are used for both remote monitoring and controlling purposes. Fig. 10 illustrates the cross section of a typical subsea cable. Modern tidal turbines are designed to operate unsupervised. Nonetheless, a supervisory overriding control is available to enable to control the turbine in the case of emergency or maintenance. The communication between the control unit onboard the tidal turbine and the offshore Supervisory Control and Data Acquisition (SCADA) unit is achieved by
Tidal turbines
Armour
157
Conductor
Insulation Optical fibre Outer sheath
Fig. 10 Cross-section of a typical subsea cable. (Source: Alcorn, R.; O'Sullivan, D. Electrical Design for Ocean Wave and Tidal Energy Systems. IET, 2013. ISBN: 978-1-84919-561-4).
using data cables. Depending on the generator power rating and distance of the tidal turbine to the shore, there is normally an offshore substation installed on a floating or fixed platform in addition to the subsea transmission line and onshore substation [13].
References 1. Wolf, J.; et al. Environmental Impacts of Tidal Power Schemes. Proc. Inst. Civil Eng. Mar. Eng. 2009, 162(4), 165–177. 2. Canadian Press. Whale Still Drawing Crowds at N.S. River; 8 April 2009; Available from: http://www.theglobeandmail.com/news/national/whale-still-drawing-crowds-at-nsriver/article1140088/, 2004. (Accessed April 5, 2017). 3. Brown, R. S.; et al. Understanding Barotrauma in Fish Passing Hydro Structures: A Global Strategy for Sustainable Development of Water Resources. Fisheries 2014, 39(3), 108–122. 4. Roach, J. Fish-Friendly Dams? Scientists Race to Reduce Turbine Trauma; Available from: http://www.nbcnews.com/science/environment/fish-friendly-dams-scientists-racereduce-turbine-trauma-n79936; 2014. (Accessed April 5, 2017). 5. Fan, Z. Tidal Power Energy, Renewable Energy in Future. pp. 30–11 2011. 6. Borthwick, A. G. L. Marine Renewable Energy Seascape. Engineering 2016, 2(1), 69–78. 7. Khan, M. J.; et al. Hydrokinetic Energy Conversion Systems and Assessment of Horizontal and Vertical Axis Turbines for River and Tidal Applications: A Technology Status Review. Appl. Energy 2009, 86(10), 1823–1835. 8. Grogan, D. M.; et al. Design of Composite Tidal Turbine Blades. Renew. Energy 2013, 57, 151–162. 9. Wood, R. J. K.; et al. Tribological Design Constraints of Marine Renewable Energy Systems. Philos. Trans. Roy. Soc. A: Math. Phys. Eng. Sci. 2010, 368(1929), 4807–4827. 10. Liu, P.;Veitch, B. Design and Optimization for Strength and Integrity of Tidal Turbine Rotor Blades. Energy 2012, 46(1), 393–404.
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11. ACCIONA Wind Power. Technical Specification of AW1500 PLATFORM; Available from: http://www.acciona-windpower.com/products-and-services/aw1500/; 2014. (Accessed March 15, 2017). 12. Aqua Dynamo Unprecedented Power of Aqua; Available from: http://www.aquadynamo. com/; 2016. (Accessed March 15, 2017). 13. Alcorn, R., O'Sullivan, D., Eds.; Electrical Design for Ocean Wave and Tidal Energy Systems; IET, 2013; ISBN: 978-1-84919-561-4.
Further reading 14. Boyle, G. Renewable Energy: Power for a Sustainable Future; 3rd ed.; Oxford University Press: Oxford, 2004. 15. Mestres, M.; et al. Analysis of the Optimal Deployment Location for Tidal Energy Converters in the Mesotidal Ria de Vigo (NW Spain). Energy 2016, 115(Part 1), 1179–1187. 16. Marsh, P.; et al. Three-Dimensional Numerical Simulations of Straight-Bladed Vertical Axis Tidal Turbines Investigating Power Output, Torque Ripple and Mounting Forces. Renew. Energy 2015, 83, 67–77. 17. Grosvenor, R. I.; Prickett, P. W. A Discussion of the Prognostics and Health Management Aspects of Embedded Condition Monitoring Systems. In: Annual Conference of Prognostics and Health Management Society; 2011. 18. Kotzalas, M. N.; Doll, G. L. Tribological Advancements for Reliable Wind Turbine Performance. Philos.Trans. Ser. A: Math. Phys. Eng. Sci. 2010, 368, 4829–4850.
Tidal turbines Sanaz Roshanmanesha, Farzad Hayatib, Mayorkinos Papaeliasa a
School of Metallurgy and Materials, The University of Birmingham, Birmingham, United Kingdom School of Engineering, The University of Birmingham, Birmingham, United Kingdom
b
1 Overview Tidal energy is still a developing renewable energy source with only a few commercial projects having been commissioned to date. Although there are several different device designs that have been proposed and investigated for harvesting tidal energy, most of them have only been tested either at model or small scale. A handful of designs have managed to gather sufficient financial support to be tested at sea at full scale. There are currently plans for significant utility-sized tidal energy plants in the short-to-medium term. Nonetheless, these still face uncertainty regarding their eventual implementation due to financing issues that have not been entirely resolved yet. Tidal energy production offers several advantages over conventional renewable energy sources such as photovoltaics and wind energy. For example, it does not require access to valuable land resources and is unobtrusive. Furthermore, it can produce electricity continuously at a predictable rate, thanks to our accurate knowledge of the tidal cycle. It is undeniable that the available tidal energy which can be exploited in a financially viable way is significant. Appropriate sites for tidal energy exploitation exist throughout the world’s seas. These sites are often within acceptable distance from the shore helping reduce the overall connection costs of future tidal farms to the grid. As the construction of new wind farms and photovoltaic parks is getting closer to saturation point due to less and less land being available for commissioning such projects, alternative renewable energy sources that are not land-demanding such as tidal energy will become more attractive to investors. Most tidal turbine designs have significant similarities in comparison with wind turbines. Tidal turbines harvest the kinetic energy of water currents caused by the tides due to the movement of the moon in the same way that wind turbines harvest the kinetic energy of the wind. However, tides are completely predictable and therefore tidal energy harvesting is far Non-Destructive Testing and Condition Monitoring Techniques for Renewable Energy Industrial Assets https://doi.org/10.1016/B978-0-08-101094-5.00010-1
© 2020 Elsevier Ltd. All rights reserved.
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more predictable than wind. Due to the variability of wind speed over time, the amount of energy harvested from wind farms can fluctuate significantly. Hence, integration of more than 30% of wind energy in the energy mix is challenging due to the risk of blackout caused by grid instability. The operation of tidal turbines, on the other hand, is predictable and therefore, the amount of energy produced is more stable. This allows tidal energy to be more easily integrated to the energy mix in large scale. Tidal turbines, like wind turbines can be of the direct-drive or geared type. The majority of commercially available tidal turbines is based on the use of a gearbox. The use of the gearbox is often necessary due to the very low speed of rotation that tidal turbines operate. Direct-drive tidal turbines require a much larger number of magnets for electricity production and as a result, their cost is much higher whilst the weight of the generator increases substantially. The use of the gearbox allows the low-speed rotation of the rotor to be multiplied through the various stages of the gearbox, reaching speeds above 1100 Revolutions per Minute (RPM) at the output stage (generator). Unfortunately, tidal turbines, like wind turbines, operate in a turbulent environment which means loading conditions vary continuously. Tidal turbine gearboxes apart from the fact of being submerged underwater several metres below the surface of the sea need to be able to operate under a profoundly adverse variable loading environment. Long-term experience with large-scale multi-MW wind turbines has shown that variable loading causes a high level of wear and tear in gearbox components. This is primarily due to the high stresses and variability in lubrication quality. Thus, it is not surprising that gearbox-related faults are among the most important causes of downtime. Moreover, very few wind turbines reach their end of the design lifetime without having their gearbox replaced or refurbished. Tidal turbines face even harsher operational conditions, meaning gearbox-related faults are likely to develop over prolonged in-service operation. Water is 832 times denser than air. As a result tidal turbine rotors can be made much smaller in size than wind turbine rotors for harvesting the same amount of energy. As tidal turbine rotors increase in size, they need to sustain much higher loads under harsh environmental conditions. The rotor is subject to accidental impacts and fouling build-up causing imbalance and deterioration of the overall hydrodynamic efficiency of the blades with time. Types of damage that can affect the drive-train of a tidal turbine include misalignment, imbalance, looseness, broken gear teeth, bearing defects,
Tidal turbines
145
lubrication variability resulting in dry contact of the rotating surfaces, and excessive wear. It is very important to be able to detect accidental impacts on the rotor and damage evolution in the drive-train. Certain commercial tidal turbines make use of vibration condition monitoring systems in the same way as most multi-MW wind turbines.
1.1 Early tidal turbines Early tidal electricity generators have been based on the use of tidal barrages at river estuaries. A tidal barrage consists of several tidal turbines placed along a dam-like structure. The dam walls prevent any water flow along the structure, so all the energy of the water currents is forced to pass at a higher pressure through the turbine locations only. As the water passes through the turbines, the blades of the rotors are forced to rotate. The kinetic energy of the rotors is converted to electricity by the built-in generator. A fairly well-known example of a tidal energy barrage system is the La Rance Tidal Power Station in France, which was built in 1967. The La Rance Power Station shown in the photograph of Fig. 1 has a power rating of 240 MW [1]. It consists of 24 bulb tidal turbines with a power rating of 10 MW each. Their rotor spins at approximately 94 RPM. The positions of some of the tidal turbines along the dam structure are labelled in the photograph in Fig. 2. The annual capacity factor of the La Rance Power
Fig. 1 Aerial photograph of the La Rance Tidal Power Station in France. (Source: Public Domain, https://commons.wikimedia.org/w/index.php?curid=3182853).
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Fig. 2 Photograph of the La Rance Tidal Power Station indicating the location of some of the tidal turbines along the dam. (Photograph adapted from Dani 7C3 - Own work, CC BY 2.5, https://commons.wikimedia.org/w/index.php?curid=915904).
Table 1 List of the largest barrage type tidal power stations in the world currently in operation Location
Site
Basin area (km2)
Mean tide level (m)
Power rating (MW)
Installation year
France Russia Canada China
La Rance Kislaya Guba Annapolis Jiangxia
22 1.1 15 1.4
8.55 2.3 6.4 5.08
240 0.4 18 3.9
1967 1968 1984 1985
Station, which cost €95 million in 1967 to construct, is 28%. This is similar to the annual capacity factor of several onshore wind farms.The power generated by the La Rance Power Station reaches 540 GW/h per annum. Since its construction the La Rance Tidal Power Station has produced in excess of 27 TWh. The cost per MWh produced is less than €18, which makes it highly competitive in comparison with other renewable energy sources. Table 1 lists the largest tidal power stations currently in operation. A problem with tidal barrages is their impact to the environment [2]. The operation of tidal barrages can pose challenges to aquatic animals. In 2004,
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147
it was reported that a whale had been trapped in the basin of the Annapolis barrage turbine which is located in Canada [3]. However, as technology advances, the threats to wildlife are reduced albeit not irradicated completely [4, 5].
1.2 Modern tidal power generators Tidal turbines are large-scale rotating systems consisting of several subcomponents. They are designed to efficiently convert the kinetic energy of tidal currents to electrical energy through the transmission of torque to the generator. The typical design of commercial industrial tidal turbines is very similar to that of offshore wind turbines. Tidal turbines can be installed underwater in rows like offshore wind turbines. The advantage of tidal turbines in comparison with wind turbines is the fact that operation is independent of weather and climate change. Tides are predictable allowing integration of tidal power to the energy mix more easily. As mentioned earlier, water density is 835 times that of air. Hence, energy harvesting is more efficient in terms of the rotor size required to harvest the equivalent amount of energy. However, due to the much higher stresses sustained by the blades in the water their length is much smaller, whilst rotor speed is only a few RPM. Due to the limited industrial experience with tidal energy systems, their construction, operation and maintenance costs remain higher than those of offshore wind turbines, the exception being the better technologically understood tidal barrages. Nonetheless, as the learning curve is gradually overcome it is expected that costs associated with tidal turbines will be reduced to similar levels to those of offshore wind turbines. Currently, there exist more than 30 different types of tidal turbine designs which are under different stages of development. One of the most advanced designs is the MCT SEAGEN geared tidal turbine. Geared tidal turbines are very similar to geared wind turbines with the main difference being related to the blade design and the requirement for water-tightness of the nacelle. Tidal turbines, like wind turbines, are also subject of variable loads. Therefore, similar issues with the drive-train, particularly the gearbox, are anticipated. As mentioned earlier, variable loading conditions do affect the lubrication quality increasing considerably fatigue damage accumulation. Instant misalignments due to variable loads or permanent misalignment can further exaggerate fatigue damage resulting in final failure much earlier than anticipated in the design.
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Tidal power generators can be classified into four different categories [6] depending on the key characteristics as described below. • Vertical axis tidal turbines: This type of tidal turbines has the main rotor shaft installed transverse to the direction of the water motion. Within this particular design the gearbox and generator can be installed either at the bottom near the seabed or top of the device near the surface for better accessibility during maintenance. • Polo tidal turbines: This type of turbines is usually based on vertical axis. Their structure is similar to hamster wheels with the blades on the circumference of the wheel. This type of design can have variable pitch blades to increase versatility. • Venturi effect turbines. This tidal turbine design is based on the Venturi effect.The Venturi effect can be produced using a duct narrowing in the centre to accelerate the tidal flow. The generator in this type of turbine is located in the centre of the duct for maximum efficiency. • Horizontal axis tidal turbines: Unlike vertical axis turbines, in this type of design the main rotor shaft is parallel to the direction of the water flow. This is very similar to horizontal axis wind turbines (HAWT) but in the case of tidal turbine the device is fully submerged in the water. An example of each of these design types is shown in the schematics of Fig. 3. Recently, a new type of tidal turbine was prototyped by BioPower Systems which is based on the oscillation of a hydrofoil, generating a motion similar to that of a fish tail fin.This design is illustrated in the schematic of Fig. 4. A technology status review carried out recently by Khan et al. [7] showed that the horizontal axis turbines are the most popular type of tidal stream turbines. About 43% of all designs are of the horizontal type followed by vertical axis turbine designs with 33%.The plots in Fig. 5 illustrate the number of tidal turbine designs that have reached commercial maturity in comparison with conceptual designs proposed, tested at different scales and precommercial systems. Tidal power generators are complex systems which can be described in simpler terms by dividing them into the several subsystems and subassemblies that they consist of. Fig. 6 shows a generalised chart summarising the basic configuration of tidal turbines. Some of these main subassemblies will be discussed next.
1.3 Rotor blades Rotor blades are a critical structural component of the tidal turbine since they are responsible for harvesting the kinetic energy of the water and
Tidal turbines
149
Fig. 3 Different types of tidal turbines; (A) Vertical axis; (B) Polo turbine; (C) Venturi (duct) effect; (D) Horizontal axis. (Source: (A) Fan, Z. Tidal Power Energy, Renewable Energy in Future. 2011; pp. 30–11, 2011. (B) Boyle, G. Renewable Energy: Power for a Sustainable Future. 3rd ed.; Oxford University Press: Oxford, 2004. (C) Mestres, M., et al. Analysis of the Optimal Deployment Location for Tidal Energy Converters in the Mesotidal Ria de Vigo (NW Spain). Energy 2016, 115 (Part 1), 1179–1187. (D) Borthwick, A.G.L. Marine Renewable Energy Seascape. Engineering, 2016, 2 (1), 69–78).
transmitting it through the main drive-train to the generator. The water currents will directly act on the surface of the rotor blades forcing them to rotate. The blades may have a fixed pitch to simplify the design. Most modern tidal turbines use blades with variable pitch which can adapt to the water current speed. The blades are commonly bi-directional to enable them to harvest tidal energy in both directions. The variable pitch enables the blades to turn their hydrodynamically active surface in both directions depending on the direction of the tide maximising the energy production. Unlike wind turbines, the forces applied on the tidal rotor blades are much greater.Table 2 compares some of the turbine parameters between horizontal axis wind and tidal turbines with a power rating of 1 MW. As it is shown, the thrust in the tidal turbine can be twice the level of that applicable for
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Fig. 4 Oscillatory-hydrofoil turbine concept, developed by BioPower Systems company. (From Marsh, P., et al. Three-Dimensional Numerical Simulations of Straight-Bladed Vertical Axis Tidal Turbines Investigating Power Output, Torque Ripple and Mounting Forces. Renew. Energy 2015, 83, 67–77). 50 45 40 35 30 25 20 15 10 5 0
17%
7%
43%
33%
Other
Cross-flow
Vertical
Horizontal
Other
Cross-flow
Vertical
Horizontal
Fig. 5 Current technological landscape of tidal turbine designs.
a wind turbine of equivalent power rating. Furthermore, the tidal turbine blades can suffer from erosion by solid particles, fouling by marine growth, corrosion and tribo-corrosion [9]. Failure in the rotor blades in a tidal turbine has much more serious consequences compared with wind turbines. Due to the hostile underwater conditions and limited available time windows caused by poor weather conditions, recovery and repair operations of tidal turbines take considerable amount of time and effort.
Tidal turbines
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Tidal turbine configuration
Rotor blade assembly
Main drive shaft
Generator drive train
Supporting structure
Nacelle enclosure
Fixed yaw
Hub assembly
Couplings
Foundation / pile
Sensors
Variable yaw
Seals
Direct-drive generator
Stability
PLC / SCADA controls
Fixed pitch
Bearings
High-speed generator
Gravity
Bilge pump
Variable pitch
Tachometer
Gearbox
Position
Electricity out cabling
Controls
Brakes
Electrical control
Floating
Atmosphere
Fig. 6 Generalised tidal turbine configuration. (Modified from Grosvenor, R.I.; Prickett, P.W. A Discussion of the Prognostics and Health Management Aspects of Embedded Condition Monitoring Systems. In Annual Conference of Prognostics and Health Management Society; 2011). Table 2 Comparison between similar wind and tidal turbine parameters (1 MW power rating) [8] Parameter 3
Medium density (kg/m ) Rotor diameter (m) Number of blades Rotational speed (RPM) Velocity for maximum power (m/s) Mass flow rate (tonnes/s) Thrust (tonnes)
Wind turbine
Tidal turbine
1.2 52 3 7–2000 12–13 30–40 50
1000 20 2–5 7–20 2.5 900 100
Similar to wind turbines, the material of choice for the blades in the tidal turbine is usually glass or carbon fibre reinforced composite materials [8]. However, in certain cases aluminium or steel alloys have been employed instead. Several tidal turbine failures have been caused by blade failure [10]. Therefore, it is important to assess the condition of the blades to ensure repairs or replacement can be carried out in good time, before a failure leads to loss of production. Various blade monitoring techniques such as fibre-optic sensors or strain gauge measurement sensors can be used to evaluate the structural health of tidal turbine blades. Alternative accelerometers or acoustic e mission sensors
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can be employed. There are also several techniques available which can be applied for the prevention of fouling and corrosion around seals and surfaces [10]. However, there is a need for advances in detection of impact damage. Acoustic emission sensors are ideal for the detection of impact events. Manual inspection can be carried out either visually or using ultrasonic testing. In the case where carbon fibre-based reinforced composites are used, eddy current testing can also be employed for the detection of delamination damage.
1.4 Drive train The drive train is one of the main subsystems of the tidal turbine. In geared designs, it consists of the main bearing, rotor shaft, gearbox, and generator, whilst in the direct-drive designs, it consists of the main bearing, rotor shaft, and generator only.The drive train is responsible for transferring the torque from the rotor blades to the rotating generator in order to convert the kinetic energy into electrical power. Each of these components will be briefly described below.
1.5 Gearbox The gearbox in tidal turbines is responsible for transmitting the hightorque, low-speed drive shaft energy generated by the blades motion into high-speed, low-torque energy driving the generator for producing electricity. The speed required by the generator varies depending on the required grid electricity and frequency and the number of magnets used. The overall concept is identical to that used in wind turbines. However, the gear ratio is usually higher than in wind turbines, since the rotor speed is typically much slower underwater, albeit it has higher torque. Common MWscale wind turbine gearbox designs comprise three stages, one planetary and two h elical-parallel stages with a ratio between 60 and 80 [11], while an equivalent tidal turbine designs typically employs four-stage gearboxes with three planetary stages and one helical-parallel stage with a ratio of about 200 [12]. The gearbox is equipped with a lubrication system to ensure appropriate level of lubrication of rotating components and reduce the gearbox deterioration rate. It may also be equipped with a breaking system at the high-speed output to reduce the speed and stop the turbine in case of emergency shutdown or maintenance. The tidal turbine gearbox can be monitored using various types of sensors including oil temperature for detection of overheating bearings, vibration systems, or acoustic emission. In addition, online oil sensors can
Tidal turbines
153
be employed for assessing the oil viscocity and the presence of debris. Oil samples can also be retrieved manually when the turbine is raised out of the water for planned maintenance activities. During this time, visual inspection can be carried out after the gearbox case has been opened. However, visual inspection usually has limited value unless damage in components viewed is already visible.
1.6 Generator Tidal turbines can make use of either synchronous or asynchronous induction generators. Asynchronous generators run above the grid synchronous frequency. They are particularly popular in wind turbines due to their ability to produce power at variable speed. They are simpler compared to other types of turbines, do not require a commutator, are light weight and can generally withstand harsh operational conditions. However, they do require a cooling system. In contrast, synchronous generators are bulkier and heavier than asynchronous generators and run at a lower speed. This removes the need for the presence of a gearbox and the generator can run in direct-drive mode. Generators consist of the generator bearings and windings. Both of these components can develop faults. Generator bearings can be monitored with accelerometers or acoustic emission sensors.Windings can be monitored by trending the power output with varying water speed. Trending the power output can help identify possible faults affecting the windings prior to the occurrence of final failure.
1.7 Power converter To enable the tidal turbine to be connected to the grid, a power converter is required. The main role of the power converter is to convert the variable frequency from the tidal turbine generator into the grid frequency. This is usually done in three steps. First, a rectifier unit converts the generator alternating current (AC) into direct current (DC). Then it feeds it to a DC link capacitor. The DC link capacitor is subsequently connected to a switching inverter producing an AC current at fixed frequency. Fig. 7 shows a typical AC-DC-AC converter. After conversion, the generated voltage will be fed into a transformer to step up the voltage to several kilovolts (kV) in order to reduce the transmission loss and connect the turbine into the main grid. The power converter can be monitored using a variety of sensors to assess different parameters that can influence its operation including current, voltage, temperature, humidity and vibration sensors.
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Non-destructive testing and condition monitoring techniques
Rectifier unit
DC link capacitor
3-Phase inverter unit
Fig. 7 A typical AC-DC-AC converter with DC Link Capacitor to translate high frequency generated power into the grid frequency.
1.8 Low-voltage consumed power Some tidal turbines are required to be powered from the grid before starting to produce power. In addition, some systems such as the controller, safety systems and brakes are required to be powered uninterruptedly even when the turbine is idle. These subsystems are usually powered from the grid. However, a second source of power is normally installed on-board, such as low voltage batteries that are kept charged from the grid. These batteries provide uninterrupted power to the low-voltage electrical systems of the tidal turbine ensuring their operation at all times.
1.9 Control and management systems The control system of the tidal turbine is very similar to that of a wind turbine control system. The controller is responsible for monitoring and controlling the overall operation of the tidal turbine. This includes but not limited to, the amount of tidal flow, the position and pitch of blades, blade and rotor speed, braking, power output, etc. It also monitors other critical parameters such as the status of the cooling and lubrication systems. The controller can manage the operation of tidal turbine independently as well as being managed remotely by the supervising engineers in order to shut down the turbine safely in cases such as emergency, planned maintenance or inspections.
1.10 Supporting structure The forces acting on the tidal turbines submerged beneath the surface are very high and complex in nature. The blades give rise to high axial forces rendering essential the securing of the turbine structure to the seabed. The supporting structure is responsible for holding the turbine in place against
Tidal turbines
155
the acting axial forces. There are several different types of supporting structures available to secure the tidal turbine. However, they can be typically grouped into two main categories; fixed structures using gravity-base or monopile-support structures and floating structures using mooring-support. The schematics in Fig. 8 show a few different types of supporting structures used in different tidal turbine designs. Depending on the sea environment, the amount of force acting on the tidal turbine and type of tidal turbine any of these supporting structures may be used to secure the turbine in place during operation.
(A)
(B)
(E)
(C)
(F)
(D)
(G)
Fig. 8 Different type of tidal stream turbine supporting structure after [13]. (A) surface piercing seabed monopile; (B) seabed mounted monopile; (C) floating pontoon; (D) seabed tethered; (E) telescopic seabed monopile; (F) floating SPAR buoy, tethered to seabed; (G) ducted seabed mounted.
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Horizontal axis
Vertical axis
3%
12% 8%
27%
37%
52%
33%
BSM
FSM
28%
NSM
N/A
BSM
FSM
NSM
N/A
Fig. 9 Variation of the turbine supporting structure in vertical axis and horizontal axis tidal turbines (after [7]). These techniques are known as Bottom Structure Mounting (BSM), Floating Structure Mounting (FSM) and Near-surface Structure Mounting (NSM).
The review of tidal energy technology carried out by Khan et al. [7] has shown that bottom structure mounting (BSM) is very popular within horizontal axis turbines, whilst vertical axis turbines are mostly supported using near surface structure mounting (NSM). The plots in Fig. 9 summarise the result for the two different types of tidal turbines.
1.11 Power transmission One big challenge in the implementation of tidal turbines is the electricity connection link required for the transfer of produced electricity to the shore. One solution is to use fixed subsea cables on the seabed, similar to offshore wind farms. However, in the case of floating and semifloating tidal turbines this will be very difficult and usually an umbilical cord is required to be used from the turbine to the seabed. Apart from the power cables the data cables are also installed alongside the power cables, for transmission of critical data to the shore. These data are used for both remote monitoring and controlling purposes. Fig. 10 illustrates the cross section of a typical subsea cable. Modern tidal turbines are designed to operate unsupervised. Nonetheless, a supervisory overriding control is available to enable to control the turbine in the case of emergency or maintenance. The communication between the control unit onboard the tidal turbine and the offshore Supervisory Control and Data Acquisition (SCADA) unit is achieved by
Tidal turbines
Armour
157
Conductor
Insulation Optical fibre Outer sheath
Fig. 10 Cross-section of a typical subsea cable. (Source: Alcorn, R.; O'Sullivan, D. Electrical Design for Ocean Wave and Tidal Energy Systems. IET, 2013. ISBN: 978-1-84919-561-4).
using data cables. Depending on the generator power rating and distance of the tidal turbine to the shore, there is normally an offshore substation installed on a floating or fixed platform in addition to the subsea transmission line and onshore substation [13].
References 1. Wolf, J.; et al. Environmental Impacts of Tidal Power Schemes. Proc. Inst. Civil Eng. Mar. Eng. 2009, 162(4), 165–177. 2. Canadian Press. Whale Still Drawing Crowds at N.S. River; 8 April 2009; Available from: http://www.theglobeandmail.com/news/national/whale-still-drawing-crowds-at-nsriver/article1140088/, 2004. (Accessed April 5, 2017). 3. Brown, R. S.; et al. Understanding Barotrauma in Fish Passing Hydro Structures: A Global Strategy for Sustainable Development of Water Resources. Fisheries 2014, 39(3), 108–122. 4. Roach, J. Fish-Friendly Dams? Scientists Race to Reduce Turbine Trauma; Available from: http://www.nbcnews.com/science/environment/fish-friendly-dams-scientists-racereduce-turbine-trauma-n79936; 2014. (Accessed April 5, 2017). 5. Fan, Z. Tidal Power Energy, Renewable Energy in Future. pp. 30–11 2011. 6. Borthwick, A. G. L. Marine Renewable Energy Seascape. Engineering 2016, 2(1), 69–78. 7. Khan, M. J.; et al. Hydrokinetic Energy Conversion Systems and Assessment of Horizontal and Vertical Axis Turbines for River and Tidal Applications: A Technology Status Review. Appl. Energy 2009, 86(10), 1823–1835. 8. Grogan, D. M.; et al. Design of Composite Tidal Turbine Blades. Renew. Energy 2013, 57, 151–162. 9. Wood, R. J. K.; et al. Tribological Design Constraints of Marine Renewable Energy Systems. Philos. Trans. Roy. Soc. A: Math. Phys. Eng. Sci. 2010, 368(1929), 4807–4827. 10. Liu, P.;Veitch, B. Design and Optimization for Strength and Integrity of Tidal Turbine Rotor Blades. Energy 2012, 46(1), 393–404.
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11. ACCIONA Wind Power. Technical Specification of AW1500 PLATFORM; Available from: http://www.acciona-windpower.com/products-and-services/aw1500/; 2014. (Accessed March 15, 2017). 12. Aqua Dynamo Unprecedented Power of Aqua; Available from: http://www.aquadynamo. com/; 2016. (Accessed March 15, 2017). 13. Alcorn, R., O'Sullivan, D., Eds.; Electrical Design for Ocean Wave and Tidal Energy Systems; IET, 2013; ISBN: 978-1-84919-561-4.
Further reading 14. Boyle, G. Renewable Energy: Power for a Sustainable Future; 3rd ed.; Oxford University Press: Oxford, 2004. 15. Mestres, M.; et al. Analysis of the Optimal Deployment Location for Tidal Energy Converters in the Mesotidal Ria de Vigo (NW Spain). Energy 2016, 115(Part 1), 1179–1187. 16. Marsh, P.; et al. Three-Dimensional Numerical Simulations of Straight-Bladed Vertical Axis Tidal Turbines Investigating Power Output, Torque Ripple and Mounting Forces. Renew. Energy 2015, 83, 67–77. 17. Grosvenor, R. I.; Prickett, P. W. A Discussion of the Prognostics and Health Management Aspects of Embedded Condition Monitoring Systems. In: Annual Conference of Prognostics and Health Management Society; 2011. 18. Kotzalas, M. N.; Doll, G. L. Tribological Advancements for Reliable Wind Turbine Performance. Philos.Trans. Ser. A: Math. Phys. Eng. Sci. 2010, 368, 4829–4850.