A direct drive power take off system has the electrical generator and moving part of the device joined together with no intermediate mechanical systems. As such the resulting system is mechanically simple, with less moving parts then either a gearbox or hydraulic system.

The electrical machine is required to convert slow reciprocating motion directly into electrical power without the luxury of having the motion rectified and increased. Slow speed high power conversion inherently requires high forces to be reacted. For an electrical machine this implies the provision of a large airgap surface area. A 100 kW conventional machine with an average airgap shear stress of 20 kN/m2 whose motion was just 1 m/s, as might be the case in a wave energy converter, would have to react a force of 100 kN, requiring an airgap area of around 5 m2. These large areas have dissuaded most designers from the direct drive route and imply that it may not be suitable for all MECs. Modern electrical machines, with higher shear stresses, provide the opportunity for more reasonable size machines.

Figure 2.3: Previous direct drive proposal

The direct drive concept was first proposed in 1978 by Baz and Morcos [52], the linear version of which is shown in Figure 2.3 and its rotary counterpart in Figure 2.4. They appropriately based their design on the simple principle that electric current will be induced in a coil if it is moved relative to a magnetic field. As a wave passes through the device, the float rises, which in turn alters the position of the coils with respect to a permanent magnet. The coils experience a change in flux linkage and by Faraday's law an emf is induced. Limitations in available permanent magnet material and the utilisation of such simple electrical machine topology led the authors to conclude that the low conversion efficiency of the linear and the low speed oscillation of the rotary versions prohibited their use. The research programme hence diverted towards devices with inherent speed amplification and rectification, involving the use of a fly wheel coupled to the float via a sprocket and chain configuration.

motfirï magn

Figure 2.4: Direct drive proposal with rolling magnetic field

The use of linear permanent magnet machines directly linked to the device has been suggested more recently [53]. This system, shown in Figure 2.5, was proposed as a method of controlling the oscillation of the sea surface. It was not suggested that this device be used as the power take off mechanism however, and the linear machine was used as a control mechanism. It allowed the controller to eliminate the difference between the actual and desired chamber pressure by acting as additional damping for parts of the cycle.

Two groups are presently engaged in the research of direct drive as the primary source of power take off for wave energy converters, the AWS concept mentioned previously and further in Chapter 8, and a UK based company: Direct Thrust Designs plc. The latter of these have patented a tubular machine, whereby the armature winding moves relative to a permanent magnet translator, Figure 2.6 [54].

linear motor damper linear motor

Figure 2.5: Linear motor as part of control strategy

Figure 2.6: Direct Thrust Designs plc 500 W prototype WEC

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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