The Denniss-Auld Turbine

The turbine used in an OWC is a key element in the device’s economic performance, and is considered by wave energy experts as a significant barrier to commercialising OWCs.

Most turbines are designed to function for gas or liquid flowing in one direction and at constant velocity with the blades designed to take advantage of the optimal “angle of attack”. However, when the flow is not always from the same direction or a constant velocity, traditional turbines become ineffective.

Previous attempts to address this difficulty have mostly resulted in turbines with varying degrees of efficiency. The Oceanlinx turbine, however, uses a different method - variable pitch blades which, with the slower rotational speed and higher torque of the turbine, improves efficiency and reliability and reduces the need for maintenance.

The turbine uses a sensor system with a pressure transducer which measures the pressure exerted on the ocean floor by each wave as it approaches the capture chamber, or as it enters the chamber. The transducer sends a voltage signal proportional to the pressure which identifies the height, duration and shape of each wave. The system is calibrated to prevent small-scale “noise” from activating it.

The signal from the transducer is sent to a Programmable Logic Controller (PLC) which adjusts various parameters in real time, such as the blade angle and turbine speed. These are calibrated in the algorithm based upon the particular conditions and energy content of the site at any particular point in time.

The Generator

The generator, which is coupled to the Oceanlinx turbine, is designed so that the electrical control will vary the speed and torque characteristic of the generator load real-time to maximise the power transfer.

An induction machine will be used for the generator, with coupling to the electricity grid provided by a fully regenerative electronic control system. The grid interconnection point and the control system are located in a weatherproof building external to the air duct. The voltage of the three phase connection at this point is 415 V L-L at 50 Hz.

The electrical interface between the generator and the mains supply comprises two bi-directional DC/AC 3-phase inverters, which each operate under independent microprocessor control. The inverters are coupled to each other on the DC sides with sufficient stabilising capacitance to prevent undesirable interaction. The generator side inverter senses the generator speed and provides the appropriate voltage and frequency control so that the generator operates according to the optimised algorithm.

The generator can be soft started electrically by ramping up applied power at a predetermined rate. The control has been developed so that it incorporates more advanced algorithms, such as ramping up the generator speed electrically in anticipation of an approaching wave, so that the turbine operates at optimum speed as soon as the air movement begins. A flexible microprocessor enables the trying of several algorithms in sequence to monitor the results while still permitting easy return to the original algorithm.

The DC link/mains inverter senses the mains voltage and waveform zero crossings, with appropriate filtering. With the appropriate phase and pulse width modulation, power is transferred in either direction with harmonies and power factor variation contained within the electricity authority’s requirements. The system is normally configured to operate at a power factor of 0.95 or better.

It is, however, possible to operate at a leading power factor with suitable control, to give the electricity authority a measure of dynamic power factor compensation.

The diagram below illustrates the key aspects of the Oceanlinx technology.