It is easy to make a device that will respond vigorously to the action of sea waves. Indeed, it is quite hard to make one that will not. However, the conversion of the slow, random, reversing energy flows with very high extreme values into phase-locked synchronous electricity with power quality acceptable to a utility network is very much harder. This paper describes a range of different control strategies of varying degrees of sophistication and then describes possible conversion equipment for high-pressure oil and water and low-pressure air. Like many renewable energy sources, waves would benefit from some form of energy storage, particularly if they are to be used in weak island networks. Flywheels, gas accumulators, submerged oil/vacuum accumulators, thermal stores and reversible fuel cells are discussed, with emphasis on the coupling hardware. This leads on to a description of a new type of hydraulic machine with digital control which has been specially designed for high efficiency and flexible control of multiple erratic sources. An explanation of the rather low performance of air turbines in the random air flows caused by sea waves suggests the need for a variable-pitch version of the Wells turbine and the design of a reciprocating-flow wind-tunnel with recirculating energy recovery in which it could be tested.
This paper extends ideas presented to the World Renewable Energy Conference [1, 2]. One idea involves the impedance of flow channels and its relevance to the maximum tidal-stream resource. Estimates of the inertial and damping terms of the impedance of the Pentland Firth suggest a much higher resource size than studies based purely on the kinetic flux, because adding extra turbines will have less effect on flow velocities than in a low impedance channel. This very large resource has pushed the design of the turbine towards the stream velocities, depth, and seabed geology of this site. A second idea is an algorithm to control the pitch of close-packed vertical-axis generators to give an evenly distributed head. Finally, there are suggestions for a seabed attachment aimed specifically for conditions in the Pentland Firth and intended to allow rapid installation of a self-propelled tidal-stream generator.
The waves off the coast of western Europe are among the most powerful in the world and could generate enough electricity to satisfy the demands of several countries. Even allowing for shipping lanes, the deep Atlantic waters between Iceland and the southern tip of Portugal could accommodate up to 300 GW of wave-energy "plant" and provide 600 TW h of clean, renewable energy every year. But the challenge of making wave-generated electricity at an acceptable price is formidable, and throws up many fascinating design problems.
A modified version of the Edinburgh Duck wave energy converter has been studied recently at the University of Edinburgh. From the design point of view the key innovation was a modification of the wetted profile. Wave energy is converted into useful work by the same pitching motion as in the original Duck, but by means of a circular cylinder with an off centred axis of rotation. This recent study was focused on a Duck version designed for vapour compression desalination rather than electricity production. An hydrodynamic numerical model (WAMIT) was used to predict first-order hydrodynamics quantities and to select and optimize configurations. The results obtained showed that it was possible, following the appropriate control strategies, to obtain similar energy absorption capabilities as the in the cam shaped original Duck. A 1:33 scale model was built to validate the numerical predictions. This paper extends the already published numerical predictions and experimental results obtained with this model. Experimental tests in random waves and measurements of the mooring forces for different submerged volumes will be reported for the first time.
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