4.3.1. Case 1: Position of Hydraulic Cylinder

The influence of HA position on the power of the generator was firstly investigated in this study. For this case, the position of HA was manipulated by adjusting the *L*<sup>2</sup> and *L*3, as indicated in Figure 1. In this case, *L*<sup>2</sup> was incrementally varied by 0.1 m within the range of 0.1 to 0.7 m. To be fair, during adjustment of the HA position, the initial rod length of HA from point A to B (*L*4) was maintained as default for every sequence. Figure 8 depicts that the averaged power generated from the generator varies with the vertical mounting distance of HA for different sea states. The figure clearly illustrates that the averaged power generated from the generator increases along with the increase of the horizontal mounting of HA for all sea states. Then, it decreases after reaching the optimal mounting position. From the figure, the optimal values for *L*<sup>2</sup> are different for each sea state. For the small wave height and period sea states (sea states B and D), the optimal value for *L*<sup>2</sup> is smaller than for the case of large wave height and period sea states. At the optimal mounting position, averaged generated power for the sea states A to E were around 72 W, 42 W, 84 W, 47 W and 79 W, respectively. At the lower *L*2, averaged generated power for all sea states were significantly reduced compared to the bigger value of *L*2, as depicted in Figure 8. For example, at *L*<sup>2</sup> equal to 0.1 m, the averaged generated power from the generator for the sea states A to E were reduced by 88%, 91%, 82%, 90% and 84% of their optimal values. While at *L*<sup>2</sup> equal to 0.7 m, the averaged power generated from the generator for sea states A to E were reduced by 43%, 53%, 32%, 51% and 38% of their optimal values, respectively. The percentage of averaged power reduction also indicates that the mounting position of HA relies on the wave height and wave period. The percentage of averaged power reduction shows that the mounting position of HA was more affected by low height and a small period for the sea states. Technically, the huge reduction of the averaged power generated at the lower *L*<sup>2</sup> is due to the larger HPTO force applied to the WEC device. So, from these investigation results, it can be suggested that the distance of *L*<sup>2</sup> should be equal or larger than *L*<sup>3</sup> to prevent huge losses due to the mounting position of HA.

**Figure 8.** Averaged power of generator versus horizontal mounting distance corresponding to different sea states.

#### 4.3.2. Case 2: Piston Size of the Hydraulic Actuator

The influence of the effective area of the piston in both hydraulic actuator chambers is also a concern in this study. As previously shown in Equation (6), the effective piston area relatively affects the amount of feedback HPTO force *FHPTO* applied to the WEC device. From Equations (7) and (8), the effective area of the piston in both hydraulic chambers can

be calculated via the diameter size of piston and rod (*dp* and *dr*). Since a double-acting with single rod hydraulic actuator was considered in this study, the effective piston area in chamber B is affected by the rod size. This means that the effective piston area in chamber A is larger than that in chamber B. To investigate, the wide range of *dp* was used to examine the performance of the HPTO unit. In this case, the values of *dp* was incrementally varied by 0.005 m within the range of 0.025 to 0.060 m. In this case, the minimum range was selected based on the smallest piston size that is currently available in the hydraulic equipment market. While the value of *dr* remains as the initial parameter setting. Figure 9 presents the effect of the piston and rod diameter on the averaged power generated from the generator in different sea states. From the figure, it can be observed that the averaged generated power is influenced by the piston size of the hydraulic actuator. The figure depicts that the averaged generated power first increases with the increase of the piston diameter size and then starts to decrease after obtaining the optimal value of *dp*. From the figure, it is clearly shown that the smaller significant wave height and peak period sea states were more affected by the piston size. The result shows that the average generated power for sea states B and D started to decrease after 0.045 m size of the piston, while for the bigger wave height and period sea states, such as sea state A, C and E, the average generated power started to decrease after 0.050 m and 0.055 m, respectively. This may be due to the lower wave forces during sea states B and D compared to wave forces for sea states A, C and E. Technically, more high-pressured fluid can be supplied to the hydraulic motor by a hydraulic actuator with a larger piston size. As a result, the average generated power at the optimal point for sea states A, C and E were found to be 94 W, 118 W and 110 W. Meanwhile, the average generated power at the optimal point for sea states B and D were found to be 64 W and 70 W, respectively.

**Figure 9.** Averaged power of generator versus piston diameter corresponding to different sea states.

#### 4.3.3. Case 3: Volume Capacity of HPA

The accumulator plays an important role in mitigating the power fluctuation during a dynamic process of wave power conversion. By using HPA and a robust control strategy, the HPTO unit enables conversion of the high fluctuating wave power into a smooth and continuous electrical power. Since the HPA is more important to the HPTO unit, a further investigation into the main parameters of HPA, such as volume capacity (*VHPA*,*cap*) should be conducted. Hence, the effect of the volume capacity of HPA on the HPTO performance was explored in the present study. A wide range of *VHPA*,*cap* was used to evaluate its effect on the averaged generator power corresponding to the different sea states. In this

case, the value of *VHPA*,*cap* was incrementally varied by 2 L within the range of 0.5 to 10.5 L, as previously mentioned in Table 4. Figure 10 presents the effect of *VHPA*,*cap* on the averaged generated power from the generator for the different sea states. The figure showed that the averaged power generated from the generator is influenced by the increase of *VHPA*,*cap*, particularly for large significant wave height and peak period sea states, while there was a less significant effect for small significant wave height and peak period sea states. From the result, the average generated power, particularly for sea states C and E significantly reduced by the increase of *VHPA*,*cap*. By changing the value of *VHPA*,*cap* from 0.5 L to 10.5 L, the average generated power for sea states C and E were reduced by 28% and 20%, respectively. This power reduction can be attributed to more energy accumulated in the HPA, rather than directly flowing to the hydraulic motor when the large capacity of the HPA is implemented.

**Figure 10.** Averaged power of generator versus volume capacity of HPA corresponding to different sea states.

#### 4.3.4. Case 4: Pre-Charge Pressure of HPA

The pre-charge pressure of HPA (*PHPA*,0) is another important parameter of the HPTO. Technically, *PHPA*,0 determines how much hydraulic fluid will remain accumulated in the HPA. The charging process of HPA begins when hydraulic fluid flows into the fluid chamber when the HPTO unit pressure is greater than the *PHPA*,0. During charging, the gas is compressed to store energy. Once the HPTO unit pressure is below *PHPA*,0 level, the high-pressure nitrogen gas in the ballast forces hydraulic fluid from the fluid chamber into the hydraulic motor. For such a dynamic process, the investigation of the effect of the *PHPA*,0 on the power of the generator was considered. In this case, the value of *PHPA*,0 was incrementally increase by 10 bar within the range of 20 to 80 bar. Figure 11 presents the variation of *PHPA*,0 on the averaged generated power from the generator for different sea states. As can be seen in the figure, for all sea states the average power of the generator slightly increased, and then tended to be steady after the *PHPA*,0 reached an optimal of *PHPA*,0. The figure showed that a higher level of *PHPA*,0 can be used for large significant wave height and peak period sea states. For example, the level of *PHPA*,0 can be set up to 70 bar for sea states C and E, while, the highest *PHPA*,0 for sea states, B and D was only

up to 40 bar. This difference is due to the different operating pressure of HPTO in each sea state, in which the operating pressure of HPTO is higher in sea states C and E rather than sea state B and D. The averaged generated power at the optimal point for sea state A, C and E were reached up to 80 W, 124 W and 108 W, while for sea states B and D the averaged generated power at the optimal point only reached 43 W and 49 W, respectively.

**Figure 11.** Averaged power of generator versus pre-charge pressure of HPA corresponding to different sea states.

4.3.5. Case 5: Displacement of Hydraulic Motor

The fluid displacement of the hydraulic motor (*DHM*) is another important influencing parameter in the HPTO unit. Referring to Equation (14), *DHM* directly affects the power and torque of the hydraulic motor. Therefore, it is necessary to explore the effect of the *DHM* on the power of the hydraulic motor and generator. From the preliminary survey, the smallest size hydraulic motor currently available from the available hydraulic equipment for HPTO application 6 cc/rev. Thus, the variation range of *DHM* was set within 6 to 20 cc/rev and the value of *DHM* was incrementally varied by 2 cc/rev. Figure 12 illustrates the effect of *DHM* on the averaged generated power from the generator corresponding to different sea states. From the figure, it is clearly shown that the averaged generated power increases with the increase in *DHM* and then starts to decrease after reaching an optimal value of *DHM* for all sea states. The figure shows that a higher value of *DHM* can be implemented with a large significant wave height and peak period sea states. The result indicates that the value of *DHM* can be considered up to 10 cc/rev for sea states C and E, but only up to 8 cc/rev for sea states B and D. This may be due to the high operating pressure of HPTO during the large significant wave height and peak period sea states. At the optimal value of *DHM*, the average generated power for sea states A to E can reach up to 72 W, 44 W, 102 W, 50 W and 90 W, respectively. Apart from that, the result also shows that the overestimated value of *DHM* was more affected in HPTO performance at the small significant wave height and peak period sea states. For example, the average generated power in sea states B and D was reduced by up to 59% and 60% once the value of *DHM* was increased from the optimal (8 cc/rev) to 18 cc/rev; while for sea states C and E, the average generated power in sea states C and E were reduced by up to 46% and 47% once the value of *DHM* was increased from the optimal (10 cc/rev) to 18 cc/rev. However, this was vice-versa

during the underestimated value of *DHM*, in which the HPTO performance in the large wave height and period sea states was more affected by the *DHM*. The figure clearly shows that the averaged generated power for sea states C and E were significantly reduced by 53% and 50%. Therefore, the result reveals that the underestimation and overestimation of the *DHM* can significantly reduce the power generated from the generator.

**Figure 12.** Averaged power of generator versus fluid displacement of HM corresponding to different sea states.
