The paper presents an innovative system for the collection of energy from river and tidal currents, designed with the objective of combining high performance, cost-efficiency and simplicity. The proposed system consists of a kinetic turbine able to be immersed inside water currents and kept in equilibrium by the action of a central deflector and a steel cable anchored to the shore. The size and the orientation of the deflector are defined according to the working conditions and desired equilibrium position. The paper also describes the design parameters of a demonstrative installation at Punta Pezzo (Villa San Giovanni, Italy), located in the Strait of Messina. In the selected site, nearby the coast, the peak current speed reaches 3 m/s (6 kn). The turbine and its components have been designed assuming that the machine will always work under maximum power coefficient conditions. This implies a variable rotational speed, so the use of an inverter becomes mandatory. Preliminary performance estimations show that the system can provide electrical power of about 470 kW, with 43% efficiency when the system works under optimal conditions.
Tidal currents [1], [2], [3] and [4] are considered a renewable resource of particular interest taking into account the increasing demand of renewable energy production worldwide due to the strict environmental legislations and the need of gradually replace fossil fuel-based energy sources. Many sites around the world are characterized by the presence of water currents with velocities greater than 3 m/s [5]. Unlike marine currents, tidal currents, are bidirectional and located near coastal areas. These high energy currents are generated by gravitational forces and located in river or marine canals, estuaries, or narrow straits, always located nearby the coast or beside the margin of specific aquatic environments. These tidal currents (as opposed to river and ocean currents) are not affected by climate changes and occur with a constant cyclical period, being perfectly predictable. The installation of traditional off-shore (DeltaStream, Evopod Tidal, Free Flow, Gorlov Helical, Lunar Energy Tidal, Open Center, Seagen, Kobold Turbines [1], [2], [6], [7], [8], [9] and [10]) tidal devices intended for the collection of these resources are still at an early stage of development and none of their has demonstrated superiority over the others. They are considerably expensive, time consuming and require high skilled personnel. The installation of on-shore systems is significantly cheaper, simpler and faster. However, their profitability is still conditioned by the presence of sufficient resource, as well as an optimal bathymetric profile, where the depths fall quickly near the coast. The environmental impact of state of the art technologies is also a relevant feature that has been considered for the design of the present turbine, avoiding any interference with the seabed flora and fauna.
The development of a tidal energy farm in a site with sufficient tidal potential (i.e current speeds over 3 m/s) leads to more attractive revenue opportunities, although installation costs may rise the faster the current is, due to the more intense stresses that the system has to cope with. Higher powers can be generated by using one or more turbines fitted to the frame and directly linked to the coast. The main challenge of such a configuration is to keep the turbine(s) in equilibrium inside the currents and harness the maximum amount of energy (i.e oriented frontally with respect to the flow). It is therefore necessary to devise a structure anchored to the coast, able to operate with strong tidal currents and a turbine oriented frontally with respect to the flow, even when it changes direction.
In this context, a basic research has been carried out in collaboration with two companies, Sintenergy Ltd. and Develpack srl of Reggio Calabria, in order to achieve scale models for laboratory testing and measuring. After designing and validating an optimized prototype, it is possible to build real-scale systems for deployment in suitable areas for large diameter turbines. Rotors between 10 and 12 m. are capable of delivering hundreds of kW and be competitive in the renewable energy market.
In the present paper, an innovative system that harnesses the energy of tidal currents has been designed. The described system is disruptive in terms of installation and operational costs, given the fact that the turbine is kept in equilibrium using an extensible cable connected to the shore. As a result, all operations can be carried out from the coast and the civil works needed to support the turbine can be executed in a cost-effective and simple way. In addition, the system overcomes the frequent environmental problems connected to the presence of protected species as states a European Community Law, known as Natura 2000, which forbids any interference with the seabed (such as electric cables lay out and foundations).
Furthermore, the turbines are adaptable to multiple site conditions and can be optimized considering the site’s bathymetric profile and flow speed patterns.
With respect to the classification described in [2], the proposed system belongs to the horizontal axis hydrokinetic turbines group, with the exception that it stands in equilibrium with the aid of a buoyancy system and a steel cable anchored to the coast. The system is suitable for collecting energy from tidal (and river) currents, but not for ocean currents [3], which are normally far away from coastal areas.
Due to its special hydrodynamic features, the proposed system stands in equilibrium using a structure subjected to exclusively tensile stresses, unlike other tidal technologies that have to cope with flexural loads due to the action of the current’s drag forces. This fact results in a significant cost reduction (both in terms of materials, installation works and manpower needs) and an enhanced environmental compatibility compared to existing technologies. In addition, the use of traditional and onshore-based equipment, both for installation and maintenance operations, makes the proposed system significantly competitive in the Renewable Energy market (in terms of capex and opex) and highly compatible with the surrounding environment.
To date, the collaboration between the Mechanical Engineering Department of Unical and Sintenergy Ltd. is focused in developing further solutions for the optimization of the system in order to reach an important goal: the construction and installation of the first real scale prototype, located at the pier of Punta Pezzo, in the province of Reggio Calabria.
From a first evaluation, the energy output of the system is expected to be around 436,000 kWh/year, with an annual income of about 148,000 €. The proposed solution outlines the base of an intensive research aimed to the manufacturing and commercialization of different products related to this new self-regulating turbines.
