Archimedes Wave Swing

by W van Zanten, CADDET Dutch National Team

Introduction

The Archimedes Wave Swing is a simple concept for converting wave energy into electricity. It was invented by Fred Gardner and has been investigated at the Energy Centre of the Netherlands (ECN) as the first phase of a development project. In this system, energy is drawn from ever-present sea swells and converted into electricity.

The Principle

The swelling sea is a large mass-spring system which oscillates with a fixed frequency. Waves move over large distances without much loss of energy. Average wave length is about 120 m and the maximum power a wave carries is about
 100 kW/m of wave front in high seas.
 The Archimedes Wave Swing (AWS) is an inventive way to convert some of that energy into electricity. It consists of a number of inter-connected, air-filled chambers situated below the sea's surface. These are topped with movable floats, like hoods on the air chambers, and waves cause them to oscillate vertically. As a wave crest moves over a hood, the pressure on it rises, the trapped air is pushed into another chamber, and the hood starts to sink. The process is reversed in a wave trough. Each wave repeats this process.

The Device

At full scale the floats would be large: each one would have a diameter of about 20 m and weigh about 1,000 tonnes. They are open at the bottom allowing water to enter from below. The float is designed to remain in balance with the amount of water above it. When, during a wave crest, the water level over the device rises, the trapped air is transported into another chamber and the float tends to sink. In a wave trough, the float rises and the air pressure diminishes. By connecting three floats together, this oscillating movement is exaggerated. Buoyancy forces compel the floats to sink and rise with the sea swell. As a wave progresses, it will cause the hoods of adjacent chambers to swing, hence the name "Archimedes Wave Swing".
 The floats act as flywheels. Their movement is converted from vertical oscillation to rotation and is used to generate electricity. The top of each float is shaped like a funnel, which affords a point-absorbing effect. This element of the design is crucial to its efficiency, as it enables the device to extract energy from an area larger than its own diameter.
 The entire system would be positioned about 20 m below sea level to protect it against the extreme forces of wave peaks. Although local storms generate peaks in wave energy, they do not penetrate very far below sea level. Besides providing protection to the system, this also allows it to be constructed more cost-effectively.

Development

The development project was started in 1995. The first phase, to build and test a 1/20 scale model, has been successfully completed. This phase includes safeguarding the design patents, building a test site and verifying the principle of the AWS.
 In the second phase, in mid 1996, the unit will be scaled up to an 8 MW prototype. Preliminary design work indicates that a power of 2.7 MW might be acheived by a three-chamber unit. A location for the prototype system also has to be found. The final phase is to build and operate the system and a location somewhere on the Portuguese coast is currently being considered, where 35 kW/m of wave front is expected.

Conclusion

This daring concept could be the start of new research on wave energy. Preliminary studies indicate that the Archimedes Wave Swing could become a viable renewable energy technology. But first the AWS has to prove itself; with this project, the first steps are being taken.
 For more information contact the CADDET Dutch National Team in Sittard.

The CADDET Renewable Energy Newsletter is a quarterly magazine published by the CADDET Centre for Renewable Energy at ETSU, UK.

The articles published in the Newsletter reflect the opinions of the authors. They do not necessarily reflect the official view of CADDET.

Enquiries concerning the Newsletter should be addressed to
Pauline Toole, Editor, CADDET Centre for Renewable Energy, ETSU, Harwell, Oxfordshire OX11 0RA, United Kingdom. Tel: +44 1235 432968, Fax: +44 1235 433595.