Marine Renewable Energy, 2nd Volume

A series of experimental tests on flow-induced motion (FIM) and energy conversion of two rigidly coupled triangular prisms (TRCTP) in tandem arrangement were conducted in a recirculating water channel with the constant oscillation mass m_osc. The incoming flow velocity covered the range of 0.395 m/s ≤ U ≤ 1.438 m/s, corresponding to the Reynolds number range of 3.45 × 10⁴ ≤ Re ≤ 1.25 × 10⁵. The upstream and downstream triangular prisms with a width of 0.1 m and length of 0.9 m were connected by two rectangular endplates. Seven stiffness (1000 N/m ≤ K ≤ 2400 N/m), five load resistances (8 Ω ≤ R_L ≤ 23 Ω), and five gap ratios (1 ≤ L/D ≤ 4) were selected as the parameters, and the FIM responses and energy conversion of TRCTP in tandem were analyzed and discussed to illustrate the effects. The experimental results indicate that the “sharp jump” phenomenon may appear at L/D = 2 and L/D = 3 significantly, with substantially increasing amplitude and decreasing oscillation frequency. The maximum amplitude ratio in the experiments is A*_Max = 2.24, which appears after the “sharp jump” phenomenon at L/D = 3. In the present tests, the optimal active power P_harn = 21.04 W appears at L/D = 4 (U_r = 12.25, K = 2000 N/m, R_L = 8 Ω), corresponding to the energy conversion efficiency η_harn = 4.67%. Full article

This paper presents the results of laboratory tests on a hybrid wave energy converter concept, the O²WC (Oscillating-Overtopping Water Column) device. The proposed device aims at providing an alternative to the classical OWC concept, storing part of the wave energy of the highly energetic sea states in a second chamber at atmospheric pressure, through overtopping phenomena. In this way, the maximum airflow rate and air pressure in the OWC chamber are reduced, possibly aiding the safe functioning of the air turbine, and allowing to exploit the excess of energy instead of dissipating it through by-pass valves. The performance of the device is investigated under different incident wave conditions, for different design parameters. The height of the overtopping threshold from the second chamber of the device which allows to maximize the performance has been selected. Results show that the decrease of the primary conversion efficiency of the OWC component of the device caused by the decreased air pressure in the OWC chamber can be partially compensated by the additional energy stored in the overtopping chamber of the O²WC device. Overall, the studied O²WC device has capture width ratio values ranging between 0.3 and 0.7. Full article

The tidal turbines represent a new frontier for extracting energy from tides source. Despite the technology being mature, new solutions aimed at improving performance, reliability with reduced environmental impact, manufacturing and installation costs are currently under investigation. The Rim-driven turbine (abbreviated as RDT) was recently proposed. A RDT resembles a ducted turbine (abbreviated as DT), as both contain blades and a duct. The present study aims at investigating the detail performance and flow field of a RDT in a real flow based on the China Zhaitang Island’s tidal current data. To show the difference between the RDT and DT, simulations are also performed on the corresponding DT. It is found that the power and thrust for the two configurations exhibit time-periodic behavior that is consistent with the wave frequency. At axial flow, the fluctuation amplitude on the power and thrust increase with the increase of tip speed ratio. The RDT has higher power output when operating at lower tip speed ratio and has a potential reduction in flow resistance and disturbance with respect to the DT. At yawed flow, the fluctuation amplitude on the power and thrust decrease with the increase of yaw angle. The RDT has less capable of compensating the effect of yawed inflow in reducing the power than the DT at larger yaw angle. In addition, the power and thrust generates micro-amplitude fluctuation integrated into the main waveform, which the frequency is consistent with the turbine rotation frequency. The wake characteristics analysis reveals that the yawed flow field is more turbulent, and the two configurations suffer strong unsteady flow separation along the whole span. Strong interactions are observed between the rotor’s main wake and the duct’s upper wake. The yaw angle primarily determines the downstream wake deflection direction and significantly changes the wake shape and vortex structures. Meanwhile, the wake flow is found to recover more quickly at larger yaw angle. Besides, due to the open-center of RDT, a part free-stream flow is allowed to travel through and forms an obvious high velocity zone. The presence of open-center of RDT has avoided the low velocity zone, improved the wake structure and accelerated wakes recover, which seems to give an advantageous effect in operating a RDT. Full article