ISWEC: مکانیسم ژیروسکوپی برای بهره برداری از قدرت موج

In the past four decades, hundreds of Wave Energy Converters (WECs) have been proposed and studied, but so far a final architecture to harvest wave power has not been identified. Many engineering problems are still to be solved, like survivability, durability and effective power capture in a variable wave climate. ISWEC (Inertial Sea Wave Energy Converter) is a system using the gyroscopic reactions provided from a spinning flywheel to extract power. The flywheel works inside a sealed floating body in order to be protected from the outer environment and grant a reliable and durable operation. The article summarizes the design procedure of a 1:45 scaled ISWEC device with rated power 2.2 W and the tank tests performed with a simplified plain float to verify the actual prototype power capabilities. The article then focuses on the implementation of a non-linear coupled model (mechanics + hydrodynamics) to improve the float shape in order to maximize the power absorption. The final result is a float shape capable to absorb a power almost three times bigger (5.96 W) than the initial float shape.

Oceans are a huge reservoir of different types of renewable energy. Tidal energy is perhaps the most known source of energy from the ocean because of its effective use since the sixties. However the oceans provide different kinds of renewable energy, such as OTEC power (Ocean Thermal Energy Conversion), marine current power, salinity gradient power and finally wave power [1], [2] and [3]. Wave power is the science studying the extraction of energy from sea waves. The field is explored since the seventies when the Duck, one of the first Wave Energy Converter (WEC), was proposed from Salter at the University of Edinburgh in 1974 [4] and [5]. The Duck is a slack-moored cam-shaped floating body, constrained to rock around a spine moored to the seabed and extracting energy thanks to the relative motion between Duck and spine. Since then hundreds of devices have been proposed and studied and some of them have reached a pre-commercial stage [6], [7], [8], [9] and [10]. However, unlikely from what happened for other types of renewables, so far a final architecture to extract wave power has not been identified. One of the problems to be solved in a WEC is the so called “reaction problem”: in order to extract power from the sea surface with a force, a reaction to that force must be provided. In his analysis [11], French highlighted that the reaction force can be given in four different ways: reacting on a large structure bigger than the wavelength and therefore hydrodynamically stable, reacting to the seabed, reacting to a mass that is part of the WEC and reacting against a part of the sea. Except from the third choice, the others possibilities have to use mechanical parts in relative motion working immersed into sea water or spray. Those parts can be protected against corrosion, but they could remain a problem in a WEC durability. In this article the system ISWEC (Inertial Sea Wave Energy Converter) is proposed [12] and [14]. ISWEC uses a gyroscope to create an internal inertial reaction able to harvest wave power without exposing mechanical parts to the harsh oceanic environment. In fact ISWEC externally presents as a monolithic float. The float rocks in reaction to the incoming wave and the gyroscopic system is sealed inside. The gyroscope drives a PTO system (Power Take Off) converting mechanical power into electrical power. Furthermore, by switching off the gyroscopic system, ISWEC behaves as a bulky dead body and thus increasing its chances of survival in case of storm. In this article ISWEC is analyzed from the mechanic and hydrodynamic points of view in order to determine its power conversion capabilities and to maximize the power output for a given design wave.

The parametric analysis carried out in the previous paragraph meant to assess the power generation capacity of the ISWEC system. The analysis has been carried out on the 1:45 prototype present and the Department of Mechanics and formerly tested at the wave tank of the University of Edinburgh. The analysis is presented giving the relative variations with respect to the rated conditions and therefore it can be used to obtain a first idea of the influence of the main parameters on a general ISWEC system. One of the first results coming out from the analysis is that ISWEC works at its best when tuned with the incoming wave. Thus a variable PTO stiffness is desirable to optimize the system for different wave frequencies. It is also showed that the amount of power required from the PTO to create a spring effect is almost negligible with respect to the power exchanged by the damping component. Therefore it is highly convenient to avoid mechanical or pneumatic springs and to use the PTO itself to supply the required stiffness. Regarding to the PTO damping action, it must be regulated in order to get the best power absorption without overloading the PTO. The main system parameter however is the flywheel velocity, that can be used heavily to regulate the power absorption. In fact for each wave frequency there is a gyro optimal velocity (combined with a resonant stiffness and a proper damping) granting the maximum power absorption. The ISWEC control logic must take into account even the power lost to maintain the gyro in rotation and deciding therefore a kind of economic gyro speed granting the maximum profitability of the device.