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As fixed pitch self rectifying turbines can only be efficient for only a narrow range of airflows research was carried out on the design of variable pitch blades.
The following advantages of variable pitch were found:
higher air to shaft conversion efficiency was achieved at higher air flows (this could double the value of all preceding investment);
it is possible to choose whether to use smaller turbines which get into the efficient operating region at lower airflows;
overspeed protection without brakes can be provided;
a moderate amount of reactive loading can be applied to widen the frequency band of efficient operation giving perhaps 1.5 times the hydrodynamic productivity or to allow the use of smaller civil structures;
a minimum loading profile to ingested water can be presented.
If variable pitch is used with a low inertia variable speed link it is possible to:
maintain the proper damping coefficient over a wide range of flows;
use different damping coefficients for different wave spectra;
reduce idling drag losses during lulls;
apply larger amounts of reactive loading.
If variable pitch is used with hydraulics and energy storage it is possible to:
combine the outputs of adjacent turbines to further widen the usable sea state range;
run true synchronous generators at a steady mean rating with lower capital costs and less disturbance to slender island networks;
avoid conversion losses valued at the capital cost of a generator every year.
A fast hydraulic machine has been developed that uses radial rather than axial pistons. Variable displacement is achieved using electronic control without swash plate or bent axis geometry. Oil can be pumped to several different destinations and, at the same time, it can work as a motor taking oil from others. It has an enormous low pressure manifold and adjusts its own bearing clearances to suit different conditions of pressure. There is no churning loss and almost no compressibility loss. Theoretical calculations of its shear and leakage losses show high efficiency particularly at part load. It is intended for use in the power trans of wave energy plants but smaller versions may open new applications on land.
Air turbines were studied to meet theneeds of designers of onshore and off-shorewave power converters. The useof the Wells turbine as a prime moverin wave power plants was assessed indetail. Results were collected on theperformance of the Wells turbine andguidelines developed for selecting themost appropriate air turbine for a givenwave power application. By decreasingcapital and operating costs this tech-nologyshould help make wave energya more attractive option for developers, industry and consumers.
An alternative system for a Wells turbine has been proposed which provides the advantages of using guide vanes with the ideal matching of the biplane type of design. In place of a fixed biplane rotor twin rotors are utilized which rotate in opposite directions. The result is that the second rotor extracts the majority of the swirl energy from the air flow and, in addition, because of the change in the angle of attack of the air between the 2 rotors the efficiency is increased and the onset of stall can be postponed on the downstream plane.
The preliminary work carried out on this form of the Wells Turbine has proved promising, the second rotor clearly extending the operating range of the turbine for different air flows. This has bee accompanied by a small increase in efficiency which was less than expected in the practical tests. It would be desirable to repeat the tests with a larger diameter rig where more accurate results could be obtained and where speed control of the rotors was available in order to obtain the characteristics over the complete range of behaviour.
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