This work describes a patented system for tidal stream energy exploitation which is made by a submerged floating body supporting two hydrokinetic turbines, tethered to the seabed through a flexible cable and a single anchoring point, and able to align with tidal current direction changes. In this paper, the main results of the project development are reported and, starting from the first generation system, some advancements and configuration changes are discussed. In particular, the effectiveness of a shrouded turbine configuration (used in the previous arrangement) has been reconsidered after a cost/benefit analysis. Moreover, a new blade design approach has been introduced in order to comply with the requirements of a fixed pitch control strategy.
Among other marine renewable energy sources, marine current energy has received particular attention in research activities due the high theoretically available resource and to some attractive features, such as a reduced impact (particularly in the case of submerged devices) and a higher level of predictability in comparison to other sources (like wind energy), in the case of tidal current energy, due to the regular nature of astronomical tides. Reference (Black and Veatch Consulting Ltd, 2004) estimates a global available energy resource of about 600 TWh/y; of such global theoretical value, due to technical limitations or to constraints posed by other environmental and economic considerations (for example, impact on local environment), only a 20% fraction could effectively be extracted, resulting in an effective resource of about 120 TWh/y.
Several possible concepts have been proposed in harnessing tidal current energy and a large technical literature has been produced on this subject. Vikas, Rao and Seelam (2016) reported a brief review of general considerations and possible device configurations. Another review of several types of tidal systems can also be found in (Roberts, et al., 2016). In the case of current hydrokinetic devices, the kinetic energy contained in water stream flows is directly used for energy exploitation. Current streams can be generated by different types of water mass motion, such as tide induced motions in localized areas, oceanic current flows or river flows. Hydrokinetic turbines have been proposed in several configurations:
bottom mounted hydrokinetic turbines, similar to on-shore wind turbines, with rigid seabed foundations, such as Atlantis Lockheed AR1500 (Simec Atlantis, 2018), the SeaGen twin turbine system, originally developed by MCT (Douglas, Harrison, & Chick, 2008), or the Verdant Power hydrokinetic turbine (Sale, Jonkman, & Musial, 2009);
hydrokinetic turbines mounted on floating structures, both emerged (such as the Marine Power Orbital (Marine Power, 2018) or (nEDO IHI, 2017)) and submerged (such as system described in (Coiro, et al., 2017));
kite-like devices, which use a floating support structure, with a hydrokinetic turbine, tethered to seabed and following a controlled trajectory in order to increase the effective relative speed, such as Minesto device (Munjulury, et al., 2017), (Minesto UK Ltd, 2018).
