Electric sea-snake may provide low-cost energy from waves

Thursday, 29 January, 2009

A gigantic rubber tube — called Anaconda because of its snake-like appearance — may provide a practical solution to the problem of producing affordable electricity from the energy in sea waves.

Thanks to its UK inventors’ ultra-simple design, it would be cheap to manufacture and maintain, enabling it to produce clean power at lower cost than other types of wave energy converter — a crucial factor in the potential growth of wave power applications because cost has been a major barrier to the deployment of such converters up to now.

In other ways their attractions are considerable — wave-generated electricity is carbon free, helping the fight against global warming. Together with tidal energy it is estimated that wave power could supply up to 20% of today’s UK electricity demand.

The Anaconda device is closed at both ends and filled with water. It is designed to be anchored just below the sea’s surface, with one end facing the oncoming waves. Each wave hitting the end squeezes it and causes a bulge wave — a wave of internal pressure — to form inside the tube.

As the bulge wave runs through the tube, the initial sea wave that caused it runs along the outside of the tube at the same speed, squeezing the tube more and more and causing the bulge wave to get bigger and bigger. The bulge wave then turns a turbine fitted at the far end of the device and the power produced is fed to shore via a cable.

Because it is made of rubber, the Anaconda is much lighter than other wave energy devices, which are primarily made of metal, and dispenses with the need for hydraulic rams, hinges and articulated joints. This reduces capital and maintenance costs as well as the likelihood of breakdowns.

The 'electric sea-serpent' is still at an early stage of development. The concept has only been proved at very small scale in laboratory studies; therefore, important questions about its potential performance still need to be answered.

Funded by the UK’s Engineering & Physical Sciences Research Council (EPSRC) — and in collaboration with Anaconda’s inventors and with its developer, Checkmate SeaEnergy — engineers at the University of Southampton, southern England, have begun a program of larger-scale laboratory experiments and novel mathematical studies designed to do just that.

Using tubes with diameters of 0.25 and 0.5 m, their experiments will assess Anaconda’s behaviour in regular, irregular and extreme waves. Parameters measured will include internal pressures, changes in tube shape and the forces to which mooring cables would be subjected.

As well as providing insights into the device’s hydrodynamic behaviour, the data will form the basis of a mathematical model that can estimate exactly how much power a full-scale Anaconda would produce.

The maths the team has to undertake are unusual and particularly demanding because Anaconda’s response to pressures induced by surface waves is much more complex than that of a ship or an offshore structure.

It has many more degrees of freedom, and motions of each kind — vertical and horizontal bending, bulging, stretching, ovalling, twisting — all interact because of the compliant nature of the rubber.

When built as envisaged, each full-scale sea-snake would be 200 m long, 7 m in diameter and deployed in water depths of between 40 and 100 m. Initial assessments indicate that Anaconda would be rated at a power output of one megawatt (roughly the electricity consumption of 2000 houses) and might be able to generate power at a cost of six pence per kW/hr or less.

Although about twice as much as the cost of electricity generated from traditional coal-fired power stations, this compares very favourably with generation costs for other leading wave-energy concepts.

“The Anaconda could make a valuable contribution to environmental protection by encouraging the use of wave power,” said Southampton’s Professor John Chaplin, who is leading the project. “A one-third scale model could be completed in 2009 for sea testing and we could see the first full-size device deployed off the UK coast in around five years’ time.”

One of the new ideas recently identified by the Carbon Trust’s Marine Energy Accelerator initiative as having potential to deliver breakthrough reductions in the cost of energy, the Anaconda is the brainchild of Francis Farley, an experimental physicist, and Rod Rainey of Atkins Oil & Gas.

Manufacturing rights for the device belong to Checkmate SeaEnergy, part of the Checkmate Group.


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