Dr.Alberto Moroso Marine Engineer, Ponte di Archimede S.P.A. Messina, Italia
1.THE ENERMAR SYSTEM
The purpose of the ENERMAR project is to demonstrate that the exploitation of the renewable energy contained in the marine currents, by means of an innovative patented turbine called KOBOLD, is a convenient method if compared with the exploitation of other renewable energy sources.
A pilot plant is moored in the Strait of Messina, close to the Sicilian shore, in front of a village called Ganzirri, close to the lake of the same name
This plant will be useful to demonstrate on the field the characteristics of limited environmental impact, and the performances of both the system and its components.
In this site the expected currents speed is 2 m/s (4 knots), the sea depth is 20 metres and the plant is moored at 150 metres offshore. (see Appendix Figure 3). The current is never still, its period of inversion is equal to 6 hrs and 12 minutes, the period of amplitude is equal to 14 days.
2. OBJECTIVES OF THE ENERMAR SYSTEM
The objectives of this project are the following:
To test the first pilot plant in the world, consisting of a support floating structure and a patented Kobold turbine, with the necessary devices to produce and manage the electric energy. The whole system has been designed to have characteristics of solidity and high efficiency.
To identify the optimisation regarding the whole system and its single components;
To promote the industrial development of the ENERMAR project and its commercialisation, once demonstrated the convenience of the exploitation of marine currents compared with other sources of alternative energy.
3. EXPERIMENTAL AND NUMERICAL ACTIVITIES FROM AN IDEA TO THE PROTOTYPE
The first models of the turbine were built and tested in the water tank belonging to Department of Naval Architecture of University of Naples. The blades were freely to oscillate up to 90 degrees (with respect to the radial direction).
In a second phase a numerical code ad hoc developed at Dept. of Aeronautical Engineering of University of Naples was used to predict the turbine behaviour and output power.
The numerical activity was coupled with extensive experimental activities consisting in wind-tunnel tests of a larger model of Kobold turbine.
In fact a second model was built and tested in the wind tunnel of Dept. of Aeronautical Engineering, University of Naples.. This model was built to be tested with different number of blades and to optimize the blade’s articulation angles. The model had a diameter of 2.2 meters, blades height was of .8 meters and blades chord was of .17 meters. It was tested with 2, 3, 4 and 6 blades. The blade airfoil was NACA 0018 and due to the high number of possible parameters variation, hundreds of tests were performed.The first Kobold turbine model tested had the blade oscillating between angles of 0 and 90 with respect to radial direction. To optimize the angles and the position of blade counterweight, a special arrangement was made. This blade articulation angle arrangement led to enough power produced close to the starting point (the turbine was self-starting due to blade drag at 90 to the wind current) but it was unable to speed-up due to negative power in the successive rpm range. In the case, power was added to speed-up the turbine. The 3 blade configuration was chosen for the real prototype (in fact the maximum rotor gross power is the same of 4 blade arrangement, but with obvious less losses due to blade sustaining arms and minor construction costs).
Optimization of blade articulation angle was then performed to solve the problem of negative power in the low rpm range (in fact, as already said, in this condition the turbine was unable to accelerate up to the maximum power point).
The turbine has been tested several times, modifying its characteristics according to the numerical and experimental test results. All the investigations led to the definition of the turbine well defined kinematic characteristics.
The theoretical evaluations has taken into account various mathematical models, suitable to describe and foresee the Kobold turbine behaviour both form dynamic and kinematic point of view.
4. PROTOTYPE DESIGN AND CONSTRUCTION
After wind-tunnel tests and numerical calculation performed on the wind-tunnel model, a real prototype study has been started. Some analysis and practical considerations on prototype dimensions, led to a choice of a 3 blades turbine with a diameter of 6 meters. The blade height was chosen to be 5 meters and the blade chord was equal to .4 meters. This time, the numerical code used to predict turbine performances, was fed by the aerodynamic data of an high lift airfoil, used for the blade sections, called HL-18, purposely designed at Dept.
Of Aeronautical Engineering to be cavitation free and to optimize the turbine performance.
Each blade had to be sustained by two arms and these had to be streamlined using another ad hoc designed airfoil.
5. THE ENERMAR PLANT TURBINE PROTOTYPE
The core of the ENERMAR plant is the patented Kobold turbine. It can be defined as a Vertical Axis Hydro Turbine able to convert the kinetic energy contained in the marine currents into mechanical energy. The Kobold turbine has been designed to satisfy, at the highest possible level, either the environment safeguard or efficiency needs, as well as the necessities of low construction and maintenance costs.
The characteristics of the Kobold turbine are the following:
direction of rotation independent of marine current direction.
a very high starting torque, that makes the turbine able to start spontaneously, also in loaded conditions, without the necessity of any starting devices.
The ENERMAR plant is composed by the turbine prototype, the design of which has been previously explained, and an electrical generator. The whole system is mounted on a floating platform in Messina Strait. The turbine consists of a transmission shaft, built with special steel, and three couples of radial arms, each of them holding a blade.
From the mechanical point of view, the Kobold turbine has been designed following simple and effective principles, so as to need for its whole useful life very limited maintenance interventions.
The generator is brushless, three phases, synchronous, 4 poles, 128 Kw, is connected to a control unit delivering energy to the net through a gearbox (ratio 90:1) to increase the rotational speed from the 18 rpm of the turbine to the 1500 rpm of the generator shaft (necessary to achieve the frequency of 50 hz)
6. ENVIRONMENTAL IMPACT
The environmental benefits, subsequent to the diffusion of this kind of plant can be easily understood: it is sufficient to consider that the world-wide sea current exploitable energy is about equal to the present world energy demand. For this purpose this technology, if sufficiently diffused, could contribute to satisfy a relevant percentage of the world energy electric demand, with reasonable costs and no effects on the environment.
The ENERMAR plant environmental impact has been evaluated particularly from the point of view of the compatibility with the sea, flora and fauna.
The environmental impact and compatibility study, carried out by the University of Messina, has reached the following conclusions:
The environmental impact is negligible;
The ENERMAR units are compatible with the Italian rules for the installation and removal of sea structures.
7. STATE OF THE ART
The Enermar pilot plant has been installed in a suitable place in the Strait of Messina.
The average sea tidal current speed at the installation site is around 2 m/s.
The first set of tests consisted in systematic data collection of the mechanical behaviour of the turbine. Even with a low speed current (1.2 m/s is the cut-in speed), the rotor started rotating very fast, without any external power supplied. Global efficiency of the system has been measured as ratio between the produced electrical power and the theoretical power available in the current relative to the intercepted area: , where S = Diameter* Blade Height (S=30 m2 in case of Kobold turbine), is water density and V is the current speed. The measured global efficiency has been measured to be around 23% (see figure 20) which is comparable to long time well developed wind turbine and then this first results can be considered excellent even because on-going improvements in the mechanical transmission system will certainly rise the global efficiency very soon.
Furthermore the system is equipped with an automatic data acquisition system that not only acquire all data (current speed, torque, rpm, voltage, current) but also automatically control the loads of the turbine.
