Experimental Study of a Wave Energy Converter Using a Unidirectional Cascaded Gear System in a Short-Wave Period

A wave energy converter (WEC) based on a direct mechanical drive system (DMDS) exhibits limited performance when the sea state stands for a short period. This study aims to increase the efficiency of a WEC-DMDS mechanical system applied under short-wave conditions. A novel WEC is designed by applying cascaded gear and reducing the flywheel inertia to achieve better efficiency in a short-wave period. By applying a short-wave period of less than 2.84 s for the actual scale, the UCG-WEC can produce a CWR of 18.5%, mechanical efficiency of 87%, and a maximum power of 200 W. These values are much better than those obtained previously, where zero efficiencies were achieved for the same wave period range. In addition, this model performs well under both high-and low-wave steepness conditions. This study also evaluates variations in lever length and effective height. The C-type configuration, with a relative length ratio of 0.74, is found to be the optimal choice.

Direct mechanical drive system; Efficiency; Short-wave period; UCG-WEC; Wave energy

Among the ocean energy sources, wave energy deserves consideration because of its ability to produce more than 1–10 TW of electrical energy, which can fulfill the daily energy needs of humans (Farrok et al., 2020). The considerable potential and benefits of wave energy have motivated researchers to design various wave energy converter (WEC) models (Chen et al., 2019). Of these, an oscillating buoy WEC is the most well-known model, which can harness the wave and gravitational energies simultaneously (Li et al., 2013). This WEC has attracted considerable attention because of its several merits, such as a relatively simple design (Rahmati & Aggidis, 2017), higher efficiency, and more feasibility along coastline areas with low energy density (Shi et al., 2019).
    However, an oscillating buoy WEC has a smaller geometry than the wavelength, which makes the absorption efficiency unfavorable (Falcão, 2010). To harness the benefits while addressing the weakness of this WEC, it has been integrated with a power take-off (PTO) mechanism. Several PTO methods have been proposed to extract wave energy, with the most familiar types being a hydraulic converter and an electrical direct drive system. However, a hydraulic converter often experiences an oil leakage problem, which causes pollution and damage to the marine environment (López et al., 2013). Meanwhile, electrical component protection and air gap arrangement are the main drawbacks of electrical direct drive systems (Mueller & Bakker, 2005). Consequently, fabricating a WEC from such systems is not favorable due to the design complexity and production cost. Hence, the employ of mechanical gear or a direct mechanical drives system (DMDS) has been proposed to convert wave energy to the maximum possible extent, with system simplicity, affordable fabrication costs, and ease of repair (Têtu, 2017; Yang et al., 2019).
        A WEC-DMDS has been extensively studied. Lok (2010) conducted experiments on a 1:66.7 scaled WEC based on a gear-flywheel system at a wave height of 2.24–4.48 cm, wave period of 0.75–1.45 s, and maximum captured width ratio (CWR) of 60%. Chandrasekaran and Harender (2012) conducted experiments on a rack-chain-gear WEC model using regular waves, considering a device scale of 1:8.8, wave height of 5–30 cm, and wave period of 1–3 s. According to the results, the highest power was achieved at 30 cm wave height and 2.5 s wave period. Chandrasekaran and Raghavi (2015) designed a lever-gear-flywheel WEC scaled at 1:6, which was tested at 24–30 cm wave height and 3 s wave period in a regular wave. The highest efficiency of 23% was achieved using a lever length of 1.7 m. A similar WEC model using a rack-gear-flywheel system was equally carried out by Peng et al. (2015) and Binh et al. (2016), obtaining final efficiencies of 14% and 28.47%, respectively. Another model using a counterweight-multiplying gear system was examined by Han et al. (2015), which yielded an efficiency of up to 47%.
    However, all the abovementioned WEC models were mostly tested at wave periods between 7 and 12 s at the prototype scale, which is not affected by local wind seas (Ahn et al., 2019). In contrast, Têtu (2017) found that the main problem of WECs based on the DMDS concept is their performance limitation when the sea state stands for a short period. This result was also supported by Yang et al. (2019), who examined the prototype scale of a WEC-DMDS. According to their result, for an energy wave period, Te, of less than 3 s (classified in local wind seas), the efficiency was below 5%, which is even lower until 0%. This happens for the following reason: because of a short-wave period, the lever movement is not in an optimal position; thus, the buoy produces a shorter amplitude in the heave motion. If this amplitude is converted into rotational motion, it yields a short rotation, which is not sufficient to rotate the generator. In addition, this phenomenon can occur under sea-state conditions that have high wave steepness; thus, this problem needs to be further investigated. To address this problem, designing a mechanical system as effectively as possible is an optimal solution. Therefore, this study focuses on an oscillating buoy WEC based on a DMDS concept called the unidirectional cascaded gear wave energy converter (UCG-WEC). This design aims to address the drawbacks of a DMDS-WEC when applied in a short-wave period. This design is realized using a cascaded gear system and flywheel that can work when the wave goes up and down to produce a suitable rotation from a short heave motion.   

According to the experimental results, the UCG-WEC can work appropriately in a short-wave period, especially for T < 2.84 s. The maximum efficiency of the UCG-WEC is approximately 18.5% for CWR and 87% for mechanical efficiency. These efficiencies lead to a maximum power of 200 W for actual conditions. This result is achieved in the C configuration, which has a relative length ratio of 0.74. This study shows that modifying the DMDS configuration can increase the efficiency of a WEC for a sea state that has certain limitations. Compared to the previous experiment, the UCG-WEC can produce considerable energy under short-wave conditions, and its efficiency can be increased. In addition, the UCG-WEC performs well under both high-and low-wave steepness conditions.

        The authors express their gratitude to the Ministry of Education and Culture of Indonesia and Lembaga Pengelola Dana Pendidikan (LPDP) for providing the research grant and college opportunity. This research was also supported by the Laboratory of Energy and Coastal Environment, Department of Ocean Engineering, Institut Teknologi Sepuluh Nopember (ITS), Indonesia.

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