Generator selection for offshore oscillating water column wave energy converters

B. Electrical Machines in Renewable Energy Generation Synchronous generators (SG) form the backbone of traditional fossil fuel power generation. They are cost effective at multi-MW power levels, rotate at constant speed, and can be easily controlled to provide both real and reactive power as required. However the characteristics of renewable energy sources are very different from fossil fuel supplies in that they are by and large, highly variable energy sources. For instance, in wind or wave power generation, a fixed speed generator will extract a much smaller fraction of the available power than a variable speed generator [7]. Likewise, under heavy gust or swell conditions, a fixed speed generator will experience severe shock loads on the generator shaft, whereas if the speed is allowed to increase, the inertia of the system will absorb some of the extra power input. This led to the adoption of asynchronous generators in wind turbines where the slip range was utilised to provide a small measure of speed variation. Extensions to the speed range were provided for by pole changing or rotor resistance variation [8] In recent years, the improvement in cost and performance levels of high power switching transistors has led to the adoption of fully variable speed controlled generators. These have typically taken the form of either gear-coupled Doubly Fed Induction Generators (DFIG) with power electronics control of the rotor voltage and frequency, or direct-coupled Synchronous Generators (SG) with power electronics control of the stator voltage and frequency, and either brushless field excitation or permanent magnet excitation (PMG). On examination of the technology employed by the worlds leading wind turbine manufacturers, it can be concluded that the geared DFIG represents the state of the art in the world’s leading wind energy manufacturers, with the adoption of the direct-coupled brushless SG/PMG representing a more recent trend, and in actual numbers, the DFIG still representing around 65% of the market share in wind turbines[8],[9]. There are clear reasons for the choice of these two generator technologies. In the case of the DFIG, system cost, size, and power efficiency is optimum for a variable speed system, since the power electronics must only be rated at 30-50% of the system rated power. In the case of the PMG, initial cost is significantly higher due to the high cost of the large permanent magnets, as well as the 100% rating of the power electronics. However, the dominant factor in this case is reliability and maintenance reduction. This is due to the lack of brushes in the system, as well as the direct coupling, which eliminates the gearbox with its associated maintenance. Furthermore, direct coupling enhances bearing lifetime due to the significantly reduced number of revolutions per year of the generator, as a result of the lower rotational speed. What is clear from the evolution of wind energy to a stage of relative maturity is that there appears to be a convergence towards one or two electrical machine technologies, and that there is a clear technical and economic rationale for such. The purpose of this paper is to seek to identify whether a similar rationale can be identified in the case of wave energy conversion. While it is likely that practical experience will be a strong deciding factor in the coming years, it is also possible to learn from the evolution of wind technology. II. OWC WEC GENERATOR COMPARISON Gearing is not typically an issue in OWC WECs as the turbine can be designed to rotate at speeds consistent with
Oscillating water column
Figure 2: Floating OWC device (Bent Backward Duct Device)
Figure 1: Prototype wave energy converters: (t) Ocean Energy, (b, l-tor) Wavebob, Pelamis.
I. INTRODUCTION Renewable energy technology is steadily gaining importance in the world energy market, due to the limited nature of fossil fuel supplies. Wind power has been rapidly increasing its penetration levels in the past decade, and its technology is maturing to the point where the electrical system configuration is converging to a small number of options. Wave energy has historically struggled to break through to a commercial implementation, but recent years have begun to see a maturing of some technologies, with several commercial companies now conducting sea trials with quarter-scale prototypes and actively developing full-scale prototypes. Three such prototypes are depicted in Figure 1.