Several types of self-rectifying turbines have been proposed in the last 40 years, and new ideas are still being pursued to find an efficient reliable PTO system for the OWC systems, the main ones being: Another solution is to use a self-rectifying air turbine that converts an alternating air flow into a unidirectional rotation.
However, this configuration is complicated, has high maintenance cost and for prototype size the valves become too large to be a viable option. Non-returning valves to rectify the air flow combined with a conventional turbine is one solution. The main challenge comes from the bidirectional nature of the flow.
The idea is to drive a turbine with the oscillating air pressure in an enclosed chamber as a consequence of the oscillating water level, induced by the ocean waves (see Fig. 8.2.2 Air TurbinesĪir turbines as a mean for converting wave power into mechanical power are mostly used in oscillating water column (see Chap. The types of PTO systems can be categorized into five main categories and are described in the following sections. The concept of control and its importance for PTO systems will then be introduced.Ī systematic comparison of the different types of PTO is a difficult task to accomplish as limited data is available and one particular device can be bound to only two types of systems. This chapter aims first at giving an overview of the different types of PTO systems. As previously mentioned, it is also difficult to design due to the variability of the energy source, the environment in which it is placed and scaling issues. The PTO system is a crucial component of a wave energy converter. They can first be tested at a larger scale where costs are significantly increased. To add to the difficulties, PTO systems are difficult to test at small scale as friction becomes an issue. This variety means that PTO systems are still at the development stage with little experience gained for large scale devices. For example, oscillating water column type of device utilised an air turbine coupled to the electrical generator, while point absorber type of converter can use different PTO systems depending on their configuration and may require cascaded conversion mechanisms. Many different types of PTO systems have been investigated, and the type of PTO system used in a wave energy converter is often correlated with its type. As for the rest of the device, the PTO system should be robust, reliable and should require as little maintenance as possible.Īs opposed to the wind energy sector, there is no industrial standard device for wave energy conversion and this diversity is transferred to the PTO system.
WECs are placed in very harsh environment, leading to a high wear-rate and are difficult to access due to their location and/or unfavourably weather conditions. The impact of the PTO efficiency and the reduction in cost of the PTO system on the LCoE were the PTO variables studied Fig. A study made by the Partnership for Wave Energy in Denmark investigated the influence of the PTO system for four different wave energy converters. Footnote 1 The influence of the PTO on the LCoE is schematized in Fig. The reliability of the PTO system affects the availability (the energy production) and the operation and maintenance cost. The PTO system affects directly the capital cost of a device by accounting for typically between 20–30 % of the total capital cost. The PTO system has a direct effect on the efficiency of power conversion hence, it has a direct impact on the annual energy production. The PTO system is of great importance as it affects not only directly how efficiently the absorbed wave power is converted into electricity, but also contributes to the mass, the size and the structural dynamics of the wave energy converter.īy having this direct influence on the wave energy converter, the PTO system has a direct impact on the levelised cost of energy (LCoE). The primary converter can for example be an enclosed chamber for an oscillating water column or a point absorber buoy. The power take-off (PTO) of a wave energy converter is defined as the mechanism with which the absorbed energy by the primary converter is transformed into useable electricity.
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