This study investigates the effects of external electrical resistance on a point absorber wave energy converter to achieve the best efficiency through model tests. The point absorber wave energy converter is composed of three systems, which extract, transmit and convert wave energy into electricity. Two types of efficiency, extraction-transmission and power efficiencies, are discussed. Under the conditions of the study, the results show the maximum extraction-transmission efficiency can reach 73% and the power efficiency can reach 67% for the wave height of 20 cm, the wave period of 1.6 s, and the speed ratio of 1:9.46.
Starting from the 1973 oil crisis, people have been looking for alternative energy sources and have actively investigated a lot in relevant research and development for renewable energy sources. Among different sources of renewable energy, marine energy has been regarded as the cleanest, the safest, the most predictable, and infinite source (Vicinanza et al., 2013).
Over the last few years, thousands of concepts and ideas of wave energy converters (WECs) have been proposed in the world to extract energy out of oceans (Falcão, 2010; Pelc and Fujita, 2002), but only about 200 out of them have reached the status of model tests (Hayward et al., 2012) due to different economic or engineering reasons, e.g. structural strength, energy conversion efficiency, etc. In terms of WECs, they can be categorized as types of attenuator, point absorber, and terminator according to its size and the nearby wave characteristics (Clément et al., 2002; Cruz, 2008; Falcão, 2010; Khan et al., 2017; López et al., 2013). Also, they can be categorized as types of floating structure, oscillating water column, oscillating wave surge/impact, overtopping, and pressure differential according to their mechanism of energy extraction (Drew et al., 2009; López et al., 2013; Mustapa et al., 2017; Vicinanza et al., 2013).
According to the statistics, the average wave heights and wave periods in Taiwan are approximately 0.5~1.0 m and 5.0~7.0 s in summer; 3.0~5.0 m and 8.0~10.0 s in winter. Therefore, the capacity of wave energy near Taiwan in winter is higher than that in summer. However, the electric power consumption in Taiwan in summer is generally much higher than that in winter due to the usage of air conditioning. As a result, the efficiency of a WEC for small wave heights in summer in order to support the power consumption is an important issue in Taiwan.