The physical constraints of most sites with sufficient energy density to allow cost- effective tidal power generation result in the need to install spatially distributed arrays of devices. This is not the case for wind turbines where there is more energy available at greater heights and it is structural constraints that limit the power of the machine [126]. As tidal current sites have predominantly bi-directional flow, the obvious approach is to place devices close together in a line perpendicular to the flow. Physical constraints, such as the width of channels, shipping lanes or insufficient water depth will often make it impossible to site enough turbines in a single line. Consequently multiple lines across the flow are likely to be required, in the form of an array similar to those used for multi-directional wind farms. By placing multiple turbines behind one another the inflow conditions into the downstream turbines will be dependent on the wake and blockage effects of the upstream row of turbines. The ability to propose a viable tidal energy scheme relies on an accurate assessment of the tidal resource at a specific location [127]. Detailed knowledge of how the local flow regime varies with depth and over short time scales comparable to the blade rate of rotation is
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required for the specification of the blade strength and fatigue characteristics as well as the loading imposed on the support structure [128]. The interaction of an upstream turbine wake with a downstream turbine will make an important contribution to the unsteady fluctuations in velocity as well as to the overall kinetic energy available to a downstream tidal turbine.
Turnock et al [129] developed a method of replicating the wake downstream of a marine current turbine using a coupled RANS-BEMT approach. This method allows calculation of the power prediction from an array of turbines placed in close longitudinal and lateral proximity to each other. Extracting energy from the flow creates a turbulent region with reduced velocity behind the turbine. Turbulent mixing over the velocity gradient between the free stream and wake region starts to mix the two flows transferring momentum back into the wake. The turbulent mixing propagates both outwards and towards the wake’s centreline, broadening the wake whilst reducing the velocity deficit. This continues far downstream where the velocity deficit becomes zero.
Figure 5-13 shows the broadening wake of a turbine, with locations of average radial velocity extraction taken at intervals of two diameters downstream.
Figure 5-13: Locations of average radial velocity extraction
Blockage due to the presence of the turbine leads to acceleration of the flow either side of the turbine to maintain a constant mass flow rate, the smaller the turbine separation the more pronounced this effect becomes. This increased velocity results in larger velocity gradients which leads to more rapid mixing of the wake. The wake mixes more quickly with a reduced
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turbine separation downstream. Figure 5-14 illustrates the wake structure with a two diameter lateral separation.
Figure 5-14: The turbine wake structure with a lateral separation of two diameters
Due to the blockage effect it was found to be beneficial to place the turbines in a staggered grid, Figure 5-15b), as opposed to a rectilinear grid, Figure 5-15a), in order to minimise detrimental wake influence on turbine performance and increase energy extraction from a tidal stream site. The subsequent analysis considers four HATTs in a staggered grid with a lateral spacing of 2D and a downstream spacing of 2D.
Figure 5-15: Turbines in a) a rectilinear array and b) a staggered array with two diameter lateral and longitudinal spacing -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 u/U y/ D x=1.0D x=2.0D x=3.0D x=4.0D x=5.0D x=6.0D x=7.0D x=8.0D x=9.0D x=10.0D a) b)
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The turbines in this work are all 20m diameter, three bladed, HATTs. The only change being made to the devices is the type of blade in use. It was found using the RANS- BEMT analysis that for a 2D lateral and longitudinal spacing in a staggered grid the power absorption (P/Po) of the downstream turbines was increased to 1.13 due to the accelerated
flow resulting from blockage. For a single turbine situated in a tidal stream with a maximum spring peak current velocity of 2.0m/s, the Po is 677.1kW and the annual energy capture of the
device is 2.12MWh. For four individual turbines in a single row the annual energy capture is 8.48MWh, whereas for the four turbines situated in a staggered grid in the same flow regime, the total annual energy extracted is 8.99MWh. These turbines do not have any vaning or variable pitch mechanisms and therefore only operate throughout half the tidal cycle.
The second situation is one of four turbines in a staggered grid, but in this instance with bi-directional blades. The bi-directional turbine has the benefit of operating on both directions of the tidal flow and, as such, whilst the peak energy capture is less than that of a fixed blade rotor the overall annual energy capture will be higher. The annual energy capture of a single HATT with bi-directional blades in the same tidal stream as the fixed blade device is 3.59MWh; for four devices in a row this is 14.36MWh; and for four turbines in a staggered grid it is 15.29MWh. This is 70% greater than a standard, unidirectional, fixed blade turbine.
The third situation is one where the four turbines in the staggered grid have passively adaptive composite blades. The annual energy capture of a single turbine with passively adaptive, bend-twist coupled blades is 2.32MWh; that of four turbines in a row 9.28MWh; and that of four turbines in a staggered grid 9.88MWh. This a 10% improvement on the fixed blade turbines, however not as good as the bi-directional blade turbines due to the fact that it only operates in one direction of the tidal flow. It is apparent that turbines utilizing passively adaptive blades are more efficient than those with standard fixed blades (Chapter 4), and much more efficient than those with a bidirectional blade. The bi-directional devices are considerably more efficient when considering the whole tidal cycle. As mentioned previously variable pitch blades that can rotate 180o would be one method of achieve bi-directionality, alternatively a mechanism allowing the turbine to yaw into the flow could also be employed. Both of these methods increase design complexity, however, and are difficult to maintain in the extreme subsea environment in which HATTs operate. The cost of getting boats and
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divers out to the devices in the narrow operational windows of slack tide is large and hence maintenance is a key variable in the through life assessment of tidal turbine design.
Combining the concepts of both the passively adaptive, bend-twist coupled central spar and asymmetric snap-though laminates it may be possible to create a tidal turbine blade which is a sealed unit and that operates very efficiently in both directions of the tidal flow. The blade presents an asymmetric foil section to the flow in one direction, which adapts to local inflow conditions as a result of the couple spar; when the tide reverses, the asymmetric laminate skins are activated and the blade “snapped through” to create the same asymmetric foil section, but in the reverse direction. This enables the turbine to have optimal energy capture throughout the tidal cycle, whilst removing the need for variable pitch or vaning mechanism and therefore facilitating maintenance. This concept is covered in detail in Appendix E. The actively adaptive bladed turbine will have the same power curve as the passively adaptive bladed turbine but as the device can operate effectively in both directions of tidal flow it will have twice the annual energy capture. Hence the annual energy capture of a single device using actively adaptive blades would be 4.64MWh; that of a single row of four devices would be 18.56MWh; and that of a staggered grip of four turbines would be 19.77MWh. This is an improvement of almost 120% when compared to the base fixed pitch bladed turbine and around 25% more efficient than the bi-directional bladed turbine.