Getting straight to the hub of the matter

Figure 1. How the Oceanlinx operating water column device operates

The core technology of Oceanlinx, its latest recruit, is an improved and patented version of the standard oscillating water column technology. It claims to have improved the system design, the turbine, and construction techniques generally.

The company was founded ten years ago in Australia as Energetech, but has recently changed its name and moved the centre of its operations to the UK where there are considerable opportunities in wavepower development. It has five other projects in the pipeline worldwide, in Hawaii, Namibia, the USA (Rhode Island) and two in Australia (Port Kembla and Portland). They vary in size from 450 kW to 27 MW and are at various stages of development. The most advanced is at Port Kembla, New South Wales, a project grant-aided by the Australian government to the tune of A$750 000. A full-scale operational unit has been constructed and successfully tested over a two year period, and a power purchase agreement signed.

Wave hub

So far, three other companies have been chosen to use Wave Hub, namely Ocean Power Technologies, Fred.Olsen and Westwave. Their WEC devices will all connect to a termination and distribution unit, which will relay the generated electricity, via a sub-sea cable, to a substation on the shore, which will connect to the national grid. Each will be granted a lease of sea area covering two square kilometres. A sub-sea transformer will be provided with the capacity to deliver a total of 20 MW into the local distribution network.

Earlier this year SWRDA announced that it had approved £21.5 million of funding to construct the Hub, subject to UK government approval. The Department of Trade and Industry has also committed £4.5 million to the project. It is estimated that the Hub could generate £76 million a year for the regional economy. It would create at least 170 jobs and possibly hundreds more by creating a new wave power industry in South West England.

SWRDA estimates that the wave energy farm could generate enough electrical energy to save 24300 tonnes of carbon dioxide every year when displacing fossil fuels. This would support south west England’s target for generating 15% of the region’s power from renewable sources by 2010.

Oceanlinx chief executive Tom Engelsman said: ‘South west England has one of the best wave climates in Europe. We are delighted that SWRDA has given us an opportunity to prove the efficiency and reliability of our power generation technology in this commercial scale project in the UK. It is a very significant development for Oceanlinx.’ Wave Hub project manager Nick Harrington from SWRDA said: “Oceanlinx has a proven technology that is sufficiently advanced to take advantage of the unique facilities that Wave Hub will provide, and the company’s involvement reinforces the Hub’s status as a project of international significance. We’re delighted to have them on board.”

So far, other WEC device testing facilities have been on a significantly smaller scale. There is a small test site off the coast of Galway, Ireland, but it is not grid-connected. The Portuguese authorities have a 400 kW pilot plant in the Azores. The most advanced test site currently is the European Marine Energy Centre site, off Billia Croo, off the western coast of Orkney. It was here that the first WEC device, Ocean Power Delivery’s Pelamis, delivered power to the national grid. But the Wave Hub installation has considerably more capacity, 20 MW compared to 8 MW at the Billia Croo site and will be able to test up to 30 WEC devices simultaneously, six times as many as at the EMEC wave centre.

Although there are more than 1000 WEC techniques patented worldwide, in reality there are difficulties with wave energy technologies. Variations in wave amplitude, phase and direction make it very difficult to achieve maximum efficiency in energy generation. Harsh weather conditions can cause structural damage to equipment floating on the surface. Larger WEC installations may have an impact on marine ecology, fisheries, recreational users and navigation. Development has been slow and wave energy is not yet competitive with conventional generation methods, except in isolated communities which are not connected to the national grid. The average price is just below €0.10/kWh, more than double the average electricity price in the European Union.

Oceanlinx is claimed to have the lowest production cost per unit of any device in its wave power peer group, according to EPRI, and it can be used either as a generator or coupled with a reverse osmosis ‘filter’ separator as a desalination unit. Its turbine can be fully replaced in only 5 hours of downtime. Its key unique selling point is that its variable vane system allows it to continue rotating in the same direction on both the pumping and the ‘in’ strokes, that is, when air is being drawn into the chamber by the receding wave.

Technology overview

As a wave passes the Oceanlinx device, the water inside the OWC (a chamber which is open underneath the waterline) rises and falls, compressing and displacing the air inside, driving it past a turbine which is housed at the narrowest point in the chamber.

The air is accelerated to its highest velocity as it passes through the turbine, allowing for maximal extraction of the energy. The oscillatory wave motion causes a similar oscillatory airflow through the chamber, and the turbine converts energy on both the up and down strokes.

The innovative apects of the device lie in the turbine itself, which converts the energy in the airflow into mechanical energy to drive an electrical generator. The chamber and turbine are the essence of the Oceanlinx wave energy system

The Denniss-Auld turbine

The turbine used in an OWC is a key element in the device’s economic performance, and has been cited by wave energy experts as the single most important 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 at constant velocity, traditional turbines become less effective.

Previous attempts to address this difficulty have mostly resulted in turbines with varying degrees of efficiency. The Denniss-Auld turbine, however, uses a different method – variable pitch blades which, with the slower rotational speed and higher torque of the turbine, improve efficiency ( roughly twice as effcient, says Oceanlinx) and reliability, and reduce 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 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.

Figure 2. Overall schematic of the Oceanlinx platform. This is a floating device moored by chains.

The generator

The generator 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.

In this application an induction generator will be employed, 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, each of which operates under independent microprocessor control. The inverters are coupled to each other on the DC side 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 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 harmonics and power factor variation kept within the electricity authority’s requirements. The system is normally configured to operate at unity power factor at all times.

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. Dynamic simulated inertia using ultracapacitors, recently developed by the Australian Commonwealth Scientific and Research Organisation, is used to smooth out mains current fluctuations while allowing the turbine to be loaded to its best operating point.

Current Oceanlinx projects

A Portland, Australia site is in the advanced permitting stage for the deployment of eighteen 1.5 MW units, giving a total capacity of 27 MW. When approved this will be the largest wave energy project in development worldwide.

A t Port Kembla (New South Wales, Australia) a power purchase agreement has been signed with Australian utility Integral Energy for the supply of electricity from the prototype 450 kW unit. This will be the first offshore wave energy company to be connected to the commercial grid, an event expected to take place before the end of 2007.