Calculation of the performance of resonant wave energy converters in real seas

Abstract

It is well known that the performance of point-absorber wave energy converters (WECs) depends upon resonance with the wave frequency. Indeed, the ideal performance of a resonating point-absorber WEC in a regular sea that can be represented by a simple sinusoid is well known, provided all motions are small and remain in the linear region. However, the performance of such a device in a more realistic, irregular sea that is not represented by a simple sinusoid cannot be so readily calculated. The first difficulty lies in modelling the hydrodynamic behaviour of the device. Recent developments in representing the hydrodynamic diffraction and radiation forces have enabled relatively simple simulation models to be developed, such as those presented and used in this paper. The second difficulty lies in the design of the device itself. In a regular sea with a known wave frequency, the settings of the power take-off system can be defined at well-known optimum values. It is shown in the present paper that, even when the wave frequency is not constant, the local wave frequency can be estimated, and this estimate can be used to adjust the power take-off system settings to maintain quasiresonance and, hence, approach the level of performance in a comparable regular sea. In this manner, for irregular seas it is possible to identify a dominant wave frequency over a relatively short time period and to use this frequency continuously to adjust the power take-off system settings, so as to adapt to the current sea conditions. This is likely, in some sea conditions, to involve the power take-off supplying power over part of the cycle, rather than absorbing it. This will increase the demands placed on the power take-off -particularly on its efficiency when the direction of power flow has to be reversible. The relative performance of such a tuneable point-absorber WEC is assessed in the paper. It is shown that the power converted in irregular seas could be as much as 50 per cent of the rated power, where the latter estimate is equivalent to the power converted in a corresponding regular sea.

Point Absorber Wave Energy Harvesters: A Review of Recent Developments

“…The sea states are mostly random, but studies have proved that they can be quantified over regions and months of the year with ocean climate studies based on either wind observation, or buoy implementation to record the wave data over a long time, such as the WERATLAS program [10] (European Wave Energy Atlas) designated to indicate the available wave energy in the European shores. Researchers were able to derive a mean of the significant wave height and period, hence the power contained within waves, for specific regions around the world during every month of the year [3,[11][12][13]. This variance in the sea state is one of the first challenges for ocean wave energy harvesting.…”

“…This necessitates big devices with very large masses in order to coincide the device's natural frequency with the ocean waves frequency and achieve resonance with the incoming waves, which results in design, manufacturing, transport, implementation, mooring, and maintenance difficulties due to the massive volumes and masses.  Theoretical difficulties: Wave energy harvesting is very multidisciplinary containing boundary element methods of hydrodynamics [8,9,13,[16][17][18][19][20][21][22][23][24][25][26][27][28], finite element methods of fluid mechanics [29][30][31][32][33], mechanical to electrical energy transfer [34], power electronics [35,36], and control theories [37][38][39][40][41]. Hydrodynamics for example are theoretically intensive, containing complicated diffraction and radiation wave theories [2] and sometimes non-linear high order wave theories [32,42,43].…”

Large-scale ocean wave energy plant modeling

“…Shek et. al used the software to model a single heaving buoy point absorber with direct drive PTO and reactive force control [13]. Yavuz et.…”

A Remotely Controlled Sea Level Compensation System for Wave Energy Converters

“…Research on the production of electrical energy from ocean waves has evolved significantly over the years, with the development and testing of various types of wave energy converters (WECs). Different techniques and strategies have been implemented in order to improve the performance of WECs both at the simulation and theoretical level [1][2][3][4] and at the sea testing level [5][6][7]. The analysis and the evaluation of the performance of WEC technologies have also been studied and can be referred to previously-published works; see [8][9][10].…”