• Energy Research

Since intermittent and highly variable power supply is undesirable, quantifying power yield fluctuations of wave energy converters (WECs) aids with assessment of potential deployment sites. This paper presents analysis of 3-hourly, monthly, seasonal, and inter-annual variability of power output of the M4 WEC. We compare expected performance from deployment at two wave energy hotspots: off Albany on the south-western coast of Australia and off the European Marine Energy Centre (EMEC) at Orkney, UK. We use multi-decadal wave hindcast data to predict the power that would have been generated by M4 WEC machines. The M4 machine, as a floating articulated device which extracts energy from flexing motion about a hinge, is sized according to a characteristic wavelength of the local wave climate. Using probability distributions, production duration curves, and coefficients of variation we demonstrate larger variability of the 3-hourly power yield at Orkney compared to Albany. At longer timescales, seasonal trends are highlighted through average monthly power values. From a continuity of supply perspective, we investigate occurrences of low production at three different threshold levels and calculate duration and likelihood of such events. Orkney is found to suffer from more persistent lows, causing a more intermittent power output. We also consider the effect of machine size on its power performance. Smaller machines are found to more effectively smooth out the stochastic nature of the underlying wave resource.

AbstractWe look at the variability of the power produced by the three-float M4 wave energy converter for locations in the North-East Atlantic and North Sea using the NORA10 hindcast data from 1958−2011. The aim is to investigate whether the produced power is also strongly affected by the climate variability (such as the North Atlantic Oscillations) in the winter, just as the ocean wave power resource as observed in previous studies. In this study, we demonstrate the use of proxy indices in combination with the climate indices to reconstruct a historic practical wave power climate from 1665−2005. We also conduct sensitivity studies to assess the changes in the practical wave power variability in response to perturbing the machine size, the power take-off coefficient, the response bandwidth and the power limit of the power take off. We find that the resultant temporal variation is still dominated by the climate variability. However, the overall variability important for power availability and energy supply economics is smaller than that of the ocean wave power resource because of the finite capture bandwidth of the M4 machine. The statistical methodology presented here is also potentially relevant to other wave energy converters in similar locations.

Abstract In this study we compare wave climates and their potential for wave energy conversion for the two energetic but quite different sites of Albany and Orkney. Energy capture is based on the M4 machine with well defined characteristics. The M4 machine is a self reacting system with 3 floats, each float with a circular cross-section when viewed from above. The smaller two floats are rigidly connected by a beam, and the largest float is connected to the mid float by a beam with a hinge. The machine generates power through the relative angular motion of this hinge above the middle float. The machine performance was previously assessed for various locations in the eastern North Atlantic including the European Marine Energy Centre (EMEC) site west of the Orkney Islands, Scotland, for wave power output (Santo et al., 2016a) and extreme response (Santo et al., 2017). In this study, we apply the analysis to a location off Albany on the south coast of western Australia, an area well-known for almost continuous exposure to long period swells. We use Australian Department of Transport (DOT) wave buoy data measured in 60 m of water over the period 2009 − 2017 . The hourly data is close to continuous but contains some gaps corresponding to ∼ 13% of the total duration, these are patched to form a continuous wave record. Having sized the machine based on mean wave period, extreme wave height statistical analysis is performed using storm-based identification and a peaks-over-threshold technique, following Santo et al. (2016b), providing information relevant for any wave energy converter at the location. From operability and power scheme economics, we then compare the optimal size of machine, practical power output and the associated variability in power produced by an M4 machine at Albany to the open North Atlantic location off the Orkneys. This is performed with the methodology outlined in Santo et al. (2016a). For survivability, it is important to identify extremes of machine motion. Hence, extreme responses are also compared for the central hinge angle of the machine in survival mode with the power take-off turned off. We find that a much larger machine is required at Albany, because of the longer waves compared to Orkney. However, at the two very different locations the power/cost ratios are similar.