Electronic conversion for direct drive

A major concern, common to all power take off strategies, is the lull in output power inherent with reciprocating motion and also the variation of net power due to atmospheric conditions. The predictable nature of tidal currents gives protection for the latter of these. Although it may be possible to operate several devices out of phase in an array and minimise the short term lull in power, some smoothing will still be necessary. Furthermore, it is advantageous to have a stand alone device with its own power smoothing, both to allow single deployment and also give protection against unit failure, i.e. preventing a fault in one device disabling an entire array. This is an area where both hydraulic power take off and, to a lesser extent, pneumatic systems, offer a reasonably simple solution, in terms of gas pressure and kinetic energy storage respectively.

In a direct drive system it is preferable if the energy can be stored in an electrical form, by means of a battery or capacitor. For a generator subject to sinusoidal motion, the RMS voltage and current are both V2 smaller than would be obtained if the velocity were constant. An energy buffer is required if there is a desire to feed the grid continuously at the average rate.

A sister project at the University of Durham intends to investigate the electronic support equipment required to facilitate direct drive power take off. For completeness a summary is contained here but the reader is directed towards [55] for more details. The system consists of an active rectifier which converts the AC to DC, a capacitor assisted DC link and then a DC-AC converter.

linear electrical 3-phase mains generator grid voltage

Figure 2.7: Three phase AC/DC/AC converter linear electrical 3-phase mains generator grid voltage

Figure 2.7: Three phase AC/DC/AC converter

Figure 2.7 shows a two stage inverter which may provide this role. On the left hand side of the diagram there are three energy sources, representing the three phases of a typical electrical machine. Each of these phases are fed into an active rectifier which will optimise the power conversion within the electrical machine. The D.C. link and capacitor have two roles. Firstly they may be used to provide reactive power to the generator, and secondly they act as an energy store to smooth power flow. The DC to AC inverter, shown on the right hand side of Figure 2.7, must provide a constant frequency constant voltage signal to the grid.

The power storage function could, in theory, be fulfilled by a battery but because of the short duration of discharge in combination with the requirement for a lack of maintenance, a capacitor is proposed here.

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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