4.3.6. Case 6: Damping Coefficient of Electrical Generator

The damping coefficient (*dG*) is different for each generator. As mentioned in [11], the *dG* of each generator is subjected to its type, capacity, etc. Technically, generated output power is affected by the *dG*. Since the generator is one of the major components of the HPTO unit, the effect of the *dG* on the performance of the HPTO needs to be investigated. Thus, the effect of *dG* on the generated power from the HPTO unit was investigated in this study. In this case, the *dG* was varied from 0.1 to 1.2 Nm/(rad/s) with increments of 0.1 Nm/(rad/s) in each sequence. Figure 13 depicts the effect of *dG* on the averaged generated power corresponding to five different sea states. From the figure, it can be seen that the optimal *dG* that achieves the highest averaged power was sensitive to changes in significant wave height and peak wave period. During the nominal sea state, the optimal *dG* was found to around 0.3 Nm/(rad/s), while, for the short and long peak wave period sea states (sea states B and C), the optimal *dG* was found to be around 0.35 and 0.26 Nm/(rad/s). Meanwhile, for the small and large significant wave height sea states (sea states D and E), the optimal *dG* was found at around 0.34 and 0.24 Nm/(rad/s), respectively. The result also indicates that the overestimation of *dG* badly reduced the averaged generated power for all sea states, particularly sea states A, C and E. As can be seen in the figure, the average generated power for sea states A to E was reduced by 73%, 62%, 72%, 63% and 74% of its optimal value, respectively, due to overestimation of *dG* by up to 1.3 Nm/(rad/s). Hence, from the results, it can be said that the generator damping coefficient needs to be optimally controlled to maximize power absorption from the ocean. Thus, the use of several damping control strategies, as suggested in [17], can be considered in the HPTO unit.

**Figure 13.** Averaged power of generator versus damping of generator corresponding to different sea states.

#### **5. Conclusions**

A comprehensive analysis of the effects of the important HPTO parameters on performance in generating usable electricity was conducted in the present study. Six critical parameters of the HPTO unit, including vertical mounting and the piston size of the HA, volume capacity and pre-charge pressure of HPA, displacement of HM and damping coefficient of the generator were considered. A simulation study was conducted using MATLAB/Simulink software, in which a complete model of WEC with the HPTO unit was developed using the Simscape fluids toolbox in MATLAB/Simulink. Five different irregular sea state inputs were used to evaluate the effect of each HPTO parameter against the different significant wave heights and peak periods. From the investigation, the following conclusion can be drawn:


the hydraulic motor. Thus, an appropriate *VHPA*,*cap* should be selected based on the HPTO capacity to avoid this power reduction.


The present investigation studies may help researchers and engineers of WECs to improve the efficiency of their systems. The optimization of the critical parameters above is another attractive issue in terms of maximizing the generated power from the HPTO unit. Thus, it is suggested that further research regarding the HPTO parameter optimization using heuristic optimization algorithms should be conducted.

**Author Contributions:** M.A.J., conceptualization, methodology, software, data curation, analysis, writing—original draft; M.Z.I., writing—review and editing and supervision, project administration, funding acquisition; M.Z.D., conceptualization, methodology, writing—review and editing and supervision; Z.M.Y., software, data curation, analysis and writing—original draft; A.A. data curation, analysis and writing—review and editing. All authors have read and agreed to the published version of the manuscript.

**Funding:** This project was funded by the Ministry of Higher Education (MOHE) under the Fundamental Research Grant Scheme (FRGS/1/2019/TK07/UMT/01/1).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data available on request due to restrictions of privacy.

**Acknowledgments:** The authors would like to thank the Ministry of Higher Education (MOHE) and Universiti Malaysia Terengganu (UMT) for financial support for this research.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


**Etzaguery Marin-Coria 1,\*, Rodolfo Silva 1,\*, Cecilia Enriquez 2, M. Luisa Martínez <sup>3</sup> and Edgar Mendoza <sup>1</sup>**


**Abstract:** Although the technologies involved in converting saline gradient energy (SGE) are rapidly developing, few studies have focused on evaluating possible environmental impacts. In this work, the environmental impacts of a hypothetical 50 kW RED plant installed in La Carbonera Lagoon, Yucatan, Mexico, are addressed. The theoretical support was taken from a literature review and analysis of the components involved in the pressure retarded osmosis (PRO) and reverse electrodialysis (RED) technologies. The study was performed under a three-stage scheme (construction, operation, and dismantling) for which the stress-inducing factors that can drive changes in environmental elements (receptors) were determined. In turn, the possible modifications to the dynamics of the ecosystem (responses) were assessed. Since it is a small-scale energy plant, only local impacts are expected. This study shows that a well-designed SGE plant can have a low environmental impact and also be of benefit to local ecotourism and ecosystem conservation while contributing to a clean, renewable energy supply. Moreover, the same plant in another location in the same system could lead to huge modifications to the flows and resident times of the coastal lagoon water, causing great damage to the biotic and abiotic environment.

**Keywords:** salinity gradient energy; RED; PRO; coastal systems; stress factors; receptors; environmental impact
