rocking for more power

Tom Shelley reports on what looks to be a robust and relatively low cost means of extracting energy from wave power

Tom Shelley reports on what looks to be a robust and relatively low cost means of extracting energy from wave power A new device being developed to extract energy from wave power looks to be both simple, elegant and relatively low cost. It is derived from what happens naturally, if a boat is moored in a rough sea with something loose on deck. Sailors will be only too aware of the forces involved and the damage that can ensue. Add an elastic mooring rope, and the effect can be further enhanced. A team of semi retired engineers who specialise in developing green energy ideas are currently progressing an experimental generating system based on the idea from small scale sea trials to larger scale. Living on an island, surrounded by the seas that can sometimes be very rough indeed, the United Kingdom is in a unique position to exploit wave power. According to the DTI renewables web site at, wind-generated waves on the ocean surface of the world have a total (estimated) power of 90 million GW. Due to the direction of prevailing winds and the size of the Atlantic Ocean, the UK has wave power levels that are among the highest in the world. The Hydraulics and Maritime Research Centre in Cork estimates that the wave power resource around the UK is approximately 120 GW or 21/2 times the total electricity demand. Ken Upton, William Groombridge and their friends in their 4P charity in Alicante in Spain, URL, have previously come up with circles of kites (Eureka November 1996), and waving underwater hydrofoils undergoing repeated stalls (Eureka October 2000). Such a technology forms the basis of the 150kW 'Stingray' tidal stream generator installed in 2002, and its proposed 500kW successor. The Spanish based team call their latest moored rocking boat with loose cargo technology, the "Wave Rocker". Their latest idea is to have a floating catamaran or similar shape, optimised to be stable laterally but rock longitudinally. A mass is free to move up and down the length of the deck, acting on a piston as it reaches each end of its travel. Fluid displaced by the piston generates the power. Alternatively, the piston could be replaced by a magnet or armature running through coils. In addition, the mooring is deliberately made to be elastomeric, with an elastomeric section bridging the neck of a loop in an otherwise conventional mooring rope. This is so that the inertia of the moving hull, tensioning and being pulled by the rope, adds to that of the moving mass on deck. As an oncoming wave lifts the front of the craft, the centre of gravity moves back and the mass on the deck moves towards the stern, delivering the first power stroke. The mooring rope loop opens out because of increased drag on the hull. The craft is then lifted to the top of the wave and levels out. As the wave passes, the craft moves down the wave slope, tilting forwards. Its motion is increased by the tension on the mooring rope drawing its downwards. The moving mass moves towards the bows, delivering the second power stroke as the craft runs into the next wave and digs into it. The passage of the subsequent wave repeats the cycle. The team believes that the effect could probably be further enhanced by adding vortex drag foils to the hull or adopting a special hull shape. The foils could be feathered when the elastomeric rope was contracting. It might be possible to extract additional energy by replacing the elastomeric rope with a fixed rope attached to a winch drum equipped with a "Tensator" type spring. A scaled up, 4m long car roof rack portable version is currently being built from polycarbonate roofing sheets filled with construction foam and PET bottles, reinforced with an old car bumper, alloy door frames, and other disused items, but held together with modern materials. A patent has been applied for. Ken Upton comments that while most of the materials used are recycled in order to keep costs down, "The resin and glass and carbon fibre cost hard cash." The team is looking for investors, partnerships, or other input that can be used to help progress the idea further. Upton believes the construction techniques employed could also be used to build unsinkable work boats, of floating platforms for construction. A full sized yacht with Chinese junk type sails largely built out of recycled materials by team members has been sailed extensively for some years. The team is working with several institutions in the UK equipped with wave tanks and would like to acknowledge assistance provided by the Amateur Yacht Research Society, an organisation for which Ken Upton is the Iberia co-ordinator. 4P is a registered charity. Other wave power ideas previously discussed in Eureka include using waves to compress air in a concrete chamber driving a Wells turbine (December 1990). This, in the form of station called 'Limpet' is installed on the Isle of Islay and is operated by Wavegen ( It generates 500kW. In April 2003, we described the OWEL scheme for directing waves into a narrowing channel and extracting energy from the air trapped and compressed between their crests. In July 2001, we described a scheme using reciprocating floats and linear motors that got as far as a pilot plant. Both approaches are still being researched and further developed as is the Wave Dragon in Denmark and a multitude of other schemes. These include moving buoys, nodding ducks and twisting water snakes. They all work but suffer from the need to engineer sufficiently strongly to withstand worst case winter weather. Moored failures have in the past sunk in winter storms while some shore mounted installations have simply disappeared without trace. DTI wave power site Cyberlifeboat projects in Spain Amateur Yacht Research Society 4P team in Spain Wavegen Pointers * New wave power generating system is based on what happens if loose cargo is left on deck, free to move under the action of a rough sea * The effect is enhanced by the additional use of an elastomeric mooring cable. * The development is still presently at the small scale experimental stage but is being trialed in the sea * Wave energy contains about 1,000 times the kinetic energy of wind * Unlike wind and solar, power from ocean waves is produced at all times of day * Wave energy needs only 1/200 the land area of wind * Wave energy devices are quieter and much less visually obtrusive than wind devices The oceans cover a little more than 70 percent of the Earth's surface. This makes them the world's largest solar energy collector and energy storage system. According to Dr Tom Thorpe of ETSU, the highest energy waves are concentrated off the western coasts in the 40 deg-60 deg latitude range north and south. The power in the wave fronts varies in these areas between 30 and 70 kW/m with peaks to 100kW/m in the Atlantic SW of Ireland, the Southern Ocean and off Cape Horn. The capability to supply electricity from this resource is such that, if harnessed appropriately, 10% of the current level of world supply could be provided.