Waves provide a clean blow to energy

The problem of obtaining 'green' energy from wave power looks as if it may have finally been cracked. Tom Shelley reports

By channelling waves into a narrowing channel, it looks possible to obtain economically substantial amounts of energy from the air trapped and compressed between the crests. Using computational fluid dynamics to extrapolate results obtained from a small working model, 3MW could be generated from a 180m long floating machine. One of many ideas currently being developed around the world, it comes at a time when Denmark has just commissioned the worlds largest offshore wind farm and launched its 237 tonne pilot scale Wave Dragon machine. The new OWEL (Offshore Wave Energy Ltd) design is the brainchild of Professor John Kemp, working in conjunction with Sycamore Innovation Management, IT Power, Business Link Wessex and QinetiQ. Professor Kemp describes himself as a former teacher of marine navigation, and has spent a lifetime devoted to matters maritime. He recently explained to Eureka that he had begun by studying all possible sources of 'green' energy appropriate to the UK. Since the country is surrounded by turbulent seas, he selected wave power in deep water. Shallow water waves dissipate 80% of their energy by the time they strike. This leads to a requirement to moor a wave power generator in water 40m deep or more. Fixed systems have to be massively constructed so he chose, "ship design as opposed to harbour wall design". Other requirements he envisioned included a design that would be "simple, robust, cheap and with no moving parts in contact with sea water". He further decided that he would make use of the horizontal movement of waves, as opposed to vertical, because it is ten times greater. This, he explained, is because it is easier to extract energy from fast movements than slow ones, and from a force moving in one direction rather than reciprocating. His other inspiration came because he spent a lot of time on oil tankers, where storm waves are not a problem, because they just flow over the deck. The concept he came up with bears some slight resemblance to an oil tanker, but consists of a horizontal duct floating in water, as opposed to a tank. As the waves enter, they entrain air, which is compressed by inwardly tapering side plates. The compressed air passes to a manifold and to reservoir from which its energy may be extracted by a turbine. Suitable turbines already exist commercially, or the air could be used to provide input to a gas turbine. (Gas turbine compressors normally represent two thirds of their power consumption). Residual wave energy is dispersed by baffles so the waves do not reflect. Storm waves pass over the top of the duct. The first working model was 2m long, 60cm wide and 40cm deep. It was tested in a wave tank at Southampton Institute under the direction of naval architect Omar Khattab. According to Professor Kemp, it demonstrated the principle after which he had a "more professional" model built. For the next stage, the professor obtained a DTI SMART Award for a feasibility study, working with David Nicholas of Wessex Business Link, who introduced him to Sycamore Innovation Management and IT Power. This led to the creation of a 'virtual company', converted to a limited company on receipt of the SMART Award. The feasibility study was undertaken in conjunction with QinetiQ Haslar, using its computational fluid dynamics capabilities. The approach was to use CFD to undertake the feasibility study, but use tank testing to validate and refine the hydrodynamic and aerodynamic model. It was soon found that the initial computer modelling was underestimating likely performance by an order of magnitude. Furthermore it is evident that power efficiency improves with scale. Waves approaching the North West coast of Scotland have power levels of around 70kW/m width, of which 50kW/m might realistically be expected to be captured. Since 85% of the waves are 125m long or less, a full sized machine should be about 180m long and could be expected to produce 3MW from 3m high waves. The next stage is to be the design, construction and testing of a 20m model in the New and Renewable Energy Centre at Blyth. OWEL is currently looking for suitable partnerships and investment. The competition Nearest to large scale commercialisation is the Danish Wave Dragon. This is also deep water moored, but consists of two curved, converging arms, which channel and concentrate waves to overflow a lip. The upper platform onto which the waves overflow has inlets to turbines. Its main drawbacks are the possible vulnerability of the construction with its outstretched arms, and the low head at which the turbines have to operate. Nonetheless a 237 tonne prototype has been launched and towed to the Danish Wave Energy Test Station at Nissum Bredning. The prototype is designed to be full-sized relative to the wave climate there, corresponding to 1:4.5 for a North Sea Wave Dragon and a 1:5.2 scale for a Wave Dragon in a 36 kW/m wave environment. Due to scale effects the rated power will be 20kW representing 4MW when deployed in a relatively low-energy (24kW/m) wave environment and 7MW in a 36kW/m environment. The second generation 0.5MW Wavegen Limpet on the Isle of Islay, revealed in Eureka in December 1990, continues to work well, but the number of sites on which such machines can be built is limited. Interestingly it too converts wave oscillations to airflow. Reciprocating airflow turns the Wells turbine in the same direction, regardless of airflow direction. Professor Stephen Salter's 'Nodding Ducks' have been widely acclaimed but seem to have been a bit too complicated to be economical. Lack of space prevents our describing the Pelamis Sea Snake or the innumerable schemes involving oscillating buoys or floats. The technologies involved in many of them are challenging and ingenious. A disproportionate number of them are British, and it is to be hoped that the eventual commercial rewards will come as a result of efforts in this country, and not one of the others. Pointers * 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 Dr Anthony T Jones, Practical Ocean Energy Management Systems says: "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 the World Energy Council, the global energy available from wave energy conversion is 2TWh/yr. Tapping just 0.2 percent of this energy would satisfy the current global demand for electricity." Offshore Wave Energy Sycamore Innovation Management IT Power QinetiQ Haslar Wave Dragon Wavegen