In order to estimate the power performance of the Floating dual chamber Oscillating Water Column (FOWC) device and optimise geometrical configuration of the system as well as the PTO damping values, an in-house code is developed based on the Boundary Element Method (BEM) approach. Detailed descriptions regarding the numerical model and obtained results for various design configurations are presented by Rezanejad and Guedes Soares (2021a).
Several attempts (more than 50 different case studies with different design values) have been carried out using the developed numerical tool to reach an optimized design of the device to harvest maximum power in Portuguese nearshore zones (by assuming that the PLEXIGLAS® material should be used for the model due to the budget limitations to build the model). The 1/50 scale model (that is tested in the wave tank) and its geometrical specification has been shown in Figures 1 and 2, respectively (see the detailed information regarding the experimental set-up and corresponding results in the paper published by Rezanejad et al. (2021b)). The water depth is chosen to be equal to 80 cm in the experiments (40 m in the prototype case). The overall weight of the 1/50 scale model, including the ballast weights as well as the sensors installed inside the chambers is around 80.0 kg.
Reproduction of the PTO mechanisms in physical models of OWC devices is not a straightforward task since the technologies suitable for full-scale devices, usually, do not lend themselves to down-scaling. Hence, the influence of the PTO unit is applied by employing a hole on the cap of the two chambers. In this case, a pressure drop proportional to the square value of the air volume flux passing through the hole is induced. This influence is relatively equivalent to the effect of the impulse turbines (the PTO system intended to implement in the prototype of the FOWC device) on the air flow passing through them. Three holes with various diameters were applied to induce High (corresponding to the smallest hole), Low (corresponding to the biggest hole) and Medium (corresponding to the hole with the intermediate size) damping conditions on each chamber. Only one hole has remained open in each test for each of the chambers while the other holes are sealed in order to apply the proper value of the PTO damping to the system. Hence, nine different PTO damping conditions were combined and applied to the model in the test campaign.
Both long crested regular and irregular wave conditions were investigated in the experimental study. Two wave heights (equal to H=4 and 8 cm) were applied in the regular wave tests. The following twelve wave periods were studied when the regular waves with the height equal to 4 cm was generated: T=0.8,1.0,1.1,1.2,1.3,1.4,1.5,1.6,1.7,1.8,2.0 and 2.2 s. In the case of producing the regular waves with the height equal to 8 cm, only six wave periods were considered: T =1.2,1.4,1.6,1.8,2.0 and 2.2 s. Tests were carried out with irregular long crested waves characterized by a JONSWAP spectrum (peak enhancement factor of 3.3). The following peak periods were selected T_p =1.2,1.4,1.6 and 1.8 s with the significant wave height equal to H_s =4 cm. Limited survivability tests were also carried out in the test campaign. Air pressure and surface elevations were monitored inside the chambers of the FOWC system to measure the captured power. The 6DOF motions of the device were recorded using Qualisys Motion Capture System. The water surface elevations were recorded in various locations in the tank to estimate the incident and reflected wave patterns (based on Mansard and Funke (1991) incident / reflected wave decomposition methodology).
In this regard, Figure 3 represents the corresponding experimental results associated with the constant PTO damping condition (smallest holes, between the three available holes, are implemented in the model to simulate the influence of the PTO system which is called hereafter as High damping condition). As can be seen in Figure 3, the primary hydrodynamic performance η (which shows the ratio of the absorbed pneumatic power to the incident wave power) has a value between 40 to 100% in a wide range of wave periods (varying between 1.2 to 1.9 s). In other words, the primary efficiency of the device is higher than 40% (and goes up to 100%) in a wave period range that varies between 8.5 and 13.5 s in prototype scale (geometric scale of the device is 1/50). Moreover, the primary efficiency of the FOWC device in random waves (T_p =1.2,1.4,1.6 and 1.8 s) varies in the range between 28 to 50% (with the average value of 41%) as represented in Figure 3. The hydrodynamic performance of the device reached approximately the unit value in the regular waves with T=1.5 s and H=4 cm. In other words, the device would be able to harness all the energy of the incident wave propagated toward it, which is another beneficial aspect of the device that is rarely detected in other floating WEC devices. In this context, Babarit (2015) performed a comprehensive research study to provide a database of capture width ratio (hydrodynamic efficiency) of various well known technologies of WECs. The hydrodynamic performance of fixed and floating OWC systems (including conventional BBDB devices) is reported to have the average efficiency of 29%, which reveals the superiority of the introduced WEC device in this proposal (as its average efficiency in all the examined random wave condition is around 41%) compared to those technologies.
A part of the numerical results with the experimental data is shown in Figure 4. In this figure, the numerical and experimental results (the pink and green colour curves in the figure) generally follow a similar trend of variation with respect to the wave period (for the case of constant damping condition), which proves the relative accuracy of the implemented numerical model (the in-house code that used to design the FOWC model). Moreover, the black curve in Figure 4 represents the variation of maximum primary efficiency with respect to the regular wave period obtained from the numerical analysis. The maximum efficiency is higher than 70% in the periods varying in the range between 1.17 to 1.64 s. The above mentioned 70% efficiency bandwidth would be in the range between 9.0 to 12.8 s in the prototype case which is compatible with the wave characteristics of Atlantic Ocean. This maximum efficiency can be reached by using optimal control in the PTO system to impose proper damping to the device.
As was explained in the Innovation & Applications section, each of the OWC and BBDB chambers of the device (the fore and rear chambers) has a dedicated role to capture the energy of waves in a certain frequency bandwidth. The first chamber (OWC part) has the main role to harvest energy of waves in the wave periods less than 1.3 s (which is relatively close to its natural resonance frequency). In this condition, the pitch motion of the device is relatively small which allows the fore chamber to capture most part of the energy of waves (see Media 1 in below). On the other hand, the role of the rear chamber (BBDB part) becomes dominant in capturing the energy of waves in periods greater than 1.3 s. This range of frequency is relatively close to the natural resonance frequency of the BBDB part which enhances the tendency of this chamber to absorb more energy. In this condition, the pitch motion of the device allows the BBDB part to harvest a major portion of energy (see Media 1 in below). The consecutive occurrence of the indicated resonance frequencies of BBDB and OWC chambers as well as their mutual interactions causes a significant boost in the efficiency of the device as was discussed earlier.
- Babarit, A., 2015. A database of capture width ratio of wave energy converters. Renewable Energy 80, 610-628.
- Rezanejad, K. and Guedes Soares, C., 2021a. Hydrodynamic Investigation of a Novel Concept of Oscillating Water Column Type Wave Energy Converter Device. Journal of Offshore Mechanics and Arctic Engineering, 143(4), p.042003.
- Rezanejad, K., Gadelho, J.F.M., Xu, S. and Guedes Soares, C., 2021b. Experimental investigation on the hydrodynamic performance of a new type floating Oscillating Water Column device with dual-chambers. Ocean Engineering, 234, p.109307.