ALI ABDOLALI
Ph.D. in Coastal Engineering
From Venice Gate to wave power extraction
The principle of wave energy converter design is based on the mechanical principle of maximizing the fractional ratio of power that can be extracted from the incoming waves. One category of these devices includes flap gates hinged on a bottom axis or on a foundation. They are free to oscillate in unison with the waves (one degree of freedom). The mechanical principle of these devices is the mass-spring damped forced oscillator [1,2]. The idea dates back to 80s, when a series of gates proposed for the well-known European project of protection of Venice lagoon against flooding (Figure 2). The difference of WEC and Venice gates is in the purpose, which is to maximize the oscillation amplitude of the gates to be transformed into electricity by mechanical coupling.
Figure(1) Three rows of five neighbouring flaps
The best performance of device is when device oscillation frequency coincide incoming wave frequency leading to resonance behavior. So, during the design, the natural frequency of body should be near the frequency of incoming wave. A device with variable natural frequency or a set of devices with more than one natural frequency which cover a wide range of sea state can be economic. Since the sea state is random and waves do not have a single frequency, the device should adapt itself according to the incoming wave and more importantly, in phase with incoming wave to do not make chaos.
In order to control such a system, varying PTO can force the device to stay in-phase with waves. However, it implies reverse flow direction during part of wave cycle and make the situation complicated and impractical. Another solution is latching the device in a fixed position during certain intervals of wave cycle. When the wave farm is considered, this latching method makes difficulties. The innovation of Venice Gate is treating the problem differently.
Figure (2) Venice Lagoon Mobile Defense System
The pioneer idea of Venice gates for wave energy extraction is not only covering a wider range of natural frequency, but also, taking the advantage of trapped modes. During physical modeling of Venice Gates on one array of neighboring gates aligned on a common axis, it is revealed that at certain frequencies ω of the incoming waves the gates can be excited to oscillate in opposite phase at frequencies ω/ 2 with very large oscillation amplitude. This resonance mechanism showing the existence of trapped modes was identified by Mei et al [3]. As in the case of edge waves on a beach, if the gates are in a channel, resonance is possible through a nonlinear sub-harmonic mechanism as shown by Sammarco et al. [4]. The perfect trapping is not seen in the case of gate barriers in open sea due to radiation. Another motivation for the innovative idea of Venice gate is that a single unit is not economic to implement, the idea of planning wave farm should be considered in order to satisfy financial aspects [5].
Figure (3) Snapshot of the free-surface elevation for two flaps in (a) an in-line configuration, (b) the staggered configuration for wave amplitude A=1 m and period T=6 s
References
[1] P. Sammarco, S. Michele, M. d’Errico, Flap gate farm: From Venice lagoon defense to resonating wave energy production. Part 1: Natural modes, Applied Ocean Research, Volume 43, October 2013, Pages 206-213, ISSN 0141-1187.
[2] Michele, S., Sammarco, P., d’Errico, M., Renzi, E., Abdolali, A., Bellotti, G., Dias, F., 2015, Flap gate farm: From Venice lagoon defense to resonating wave energy production. Part 2: Synchronous response to incident waves in open sea,Applied Ocean Research, Volume 52, August 2015, Pages 43-61, ISSN 0141-1187, http://dx.doi.org/10.1016/j.apor.2015.05.002.
[3] Mei CC, Sammarco P, Chan ES, Procaccini C. Subharmonic resonance of proposed storm gates for Venice lagoon. Proceedings of the Royal Society of London A 1994;444:257–65 .
[4] Sammarco P, Tran H, Mei CC. Subharmonic resonance of Venice storm gates in waves. I. Evolution equation and uniform incident waves. Journal of Fluid Mechanics 1997;349:295–325
[5] Renzi, E., Abdolali, A., Bellotti, G. and Dias, F., 2014, Wave-power absorption from a finite array of Oscillating Wave Surge Converter. Journal of Renewable Energy, Volume 63, Pages 55–68. Doi: http://dx.doi.org/10.1016/j.renene.2013.08.046​