Wave of the future

16 April 2001

Australia’s first oscillating water column is due to be completed in six months time and is expected to provide the region of Wollongong with renewable, reliable and competitively priced power. Grant McConnell describes how previous design problems have been resolved

The potential for wave energy is phenomenal. More than twice the world’s current annual usage of energy is washed up on our shores each year. The generation of wave power is not only environmentally friendly but is essentially free once the initial infrastructure costs are met. These facts have led to the design of a number of devices to harness the power of waves.

One of the most advanced of these is the oscillating water column (OWC) device designed by Tom Denniss, managing director of Energetech Australia. This device enables the energy in ocean waves to be converted into electrical energy. It does so by focusing a wide wave front, generally 40m across, into a chamber by means of a parabolic bay. Described as a unique feature, the Energetech wave energy system allows the height of the incoming wave to be amplified by a factor of around three at the focus point of the parabola. The rising and falling motion of the water at this focal point, where the vertical chamber is situated, causes a rush of air back and forth which drives a turbine located at the narrowest point of the chamber.

Generation potential

The amount of power generated by the Energetech wave energy system is dependent on the local wave climate. Typically, the device should generate between 250kW and 1MW. To put this in perspective, the average household uses between 0.5kW and 1kW. The system is, therefore, capable of supplying the power needs of about 200 to 2000 households, depending on the local wave climate and domestic energy usage. It can supply the needs of remote communities or industries, or simply be fed into the local power grid.

The system can be situated on any rocky shoreline, appended to an existing breakwater, or incorporated into a new break- water. The chamber itself is attached to the rock face or breakwater and is partially placed over land. Obviously the electrical output is greater where the wave climate is more favourable. Australia has one of the best wave climates in the world, especially on the southern shores. The west coasts of Europe, Canada and the US also have an abundance of wave energy.

The OWC itself is 40m across at the opening of the parabolic wall and 20m back to the turbine. The wave chamber is 10m across at its widest and the wall sits on the ocean bed with around 9m showing above the water. In the case where the system is to be incorporated into a breakwater, it is possible to build an opening under the water line leading into the air-filled chamber where the turbine is situated. While there would not be the amplifying advantages of the parabolic wall it would be possible to place a greater number of devices along the break-water than the width of the parabolic wall would otherwise allow.

Construction of the wave energy device is believed to be less costly and less damaging to the environment than existing methods. The parabolic wall is initially built as a concrete outer shell which can be floated or otherwise transported to the construction site where it is then sunk and filled with sand and rubble. This avoids the high costs involved with underwater and in situ concrete construction.

Design problems

In the past, the oscillatory motion of the air has been the major source of design problems for this style of wave energy device. Commonly used turbines which have been used can be relatively inefficient in converting the oscillatory motion of the airflow into electricity, and can often stall completely. Such efficiency and stalling problems have been the bane of a number of wave power projects around the world.

The Energetech turbine can solve these problems. This turbine is able to spin at high speed in the same direction regardless of the direction of the air, thus utilising the oscillating airflow efficiently and without stalling.

The design of the turbine blades has helped the device to overcome previous difficulties associated with earlier wave power turbines. The blades are constantly adjusted by mechanical actuators which are linked to pressure sensors positioned in the airflow. This ensures the blades are at the optimal angle to the airflow to produce the greatest drive at any given point in time. The blades form a symmetrical aerofoil in cross-section, unlike previous symmetrical bladed turbines. A net force is exerted on the flat side of each blade as air flows by, due to the Bernouilli effect and the subsequent difference between the flat and rounded sides of the blades. This effect is the same phenomenon that gives aeroplanes their lift, and is due to the lower pressure on the rounded side as the air has to travel further to get around the blade.

The combination of these technologies has led to a wave energy system which is efficient in converting the potential energy of the incoming wave front into commercially marketable electricity.

The system will produce power at a cost which is inversely proportional to the power in the waves. The actual wave power itself is free, but when the amount of power produced is measured up against the cost of implementing the system, the cost per kilowatt-hour is very competitive.

Moderately good wave climates should produce power at a cost of under 5 cents per kWh, while ideal wave climates will allow power to be generated for under 4 cents per kWh. This compares very favourably with other alternative power sources. For example, wind power costs between 4 and 14 cents per kW, while solar power is usually more than double at over 29 cents per kWh.

The supply of energy through ocean waves is the most consistent compared with other renewable energy sources such as wind and solar. The sun does not shine for at least half the day and the wind can drop off in a matter of minutes, but waves persist over a much longer time scale. This is an important consideration in the delivery of electricity on a commercial basis.

Given the relatively inexpensive nature of the power supplied by such wave energy systems and the minimal maintenance costs, it is a possible solution to power problems in isolated communities with available coastline; particularly those located on remote islands which are traditionally powered by diesel generators. The cost of the electricity supplied by these generators is usually more than 19 cents per kWh.

While the wave power device would incur a large initial cost, the low maintenance and long life of the system – estimated at more than 25 years – means the device can be seen as an economically viable solution. If the electricity usage of a local population is greater than what one wave energy system can provide, it is possible to construct a grid of multiple systems with no loss of efficiency. These grids can also be used in regions where there is an existing power source supplying a large number of homes. The power supplied by a grid of wave energy systems can be fed into the existing power grid. This is certainly a relevant application of the Energetech technology with many countries now requiring a percentage of the power supplied by their utilities to come from renewable energy sources.

Full scale construction

The Energetech wave energy system is pollution-free. It is a significant advance towards the lowering of greenhouse gas emissions, and should be a vital part of international efforts to limit global warming.

Supplementing the power generated by burning fossil fuels with the power generated from one Energetech wave energy system can prevent as much as 10,000t of CO2 and 30t of SO2 being emitted into the atmosphere each year.

Energetech Australia is currently undertaking its first full scale construction of its wave energy system at Port Kembla in Wollongong, just south of Sydney, Australia. The project will cost around US$1M due to the prototype nature of the project and the analysis and refinement that will occur. The expected cost of the system in the future however, should be half this figure.

The Port Kembla project is expected to take six months and will be completed in October 2001. Integral Energy, the region’s main power provider, has entered into an agreement with Energetech Australia to buy the electricity supplied by the wave energy system. This project would not have been possible without the support and enthusiasm of the Port Kembla Port Corporation and the local council, along with the US$392,550 grant from the Australian Greenhouse Office.

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