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Making a variable-pitch turbine and high-speed valve for the Azores oscillating water-column



Oscillating water columns are the simplest kind of wave
energy device and are likely to be the first into
service. Air in a box which is open to the sea below the
surface is alternately compressed and decompressed by
wave action and is used to drive a turbine. The most
popular form of turbine produces torque in one direction
regardless of the direction of air flow. The range of
efficient operation in the irregular air flows of real
seas could be increased if blade pitch could be varied on
a wave-by-wave basis to avoid stall and increased drag at
higher air flows.

It is also desirable to stop the air flow column at short
notice to protect the turbine from sea water or to allow
a control method known as latching. This widens the
efficient band of operation by delaying the movement of
the water column to bring it into phase with the applied
wave force. It is necessary to open or close an annular
passage with an outside diameter of 1.7 metres in less
than 0.1 second, every few seconds, with minimum use of

Technical Approach

The chief design problem of the turbine is that the
centrifugal load on the pitch bearings is high and the
blades must move much more frequently than those of
aircraft propellers. Conventional rolling bearings would
have a very short life. A plano-spherical hydrostatic
bearing which could take a load of 40 tonnes and yet be
rotated by a finger was built for part of the Joule II
programme. In this project it will compared with a
torsion strap made from a carbon-fibre composite and then
build a turbine hub with fifteen bearings and a pitch
actuator. The blades will be built by ART Inverness and
the installation in the Azores by IST Lisbon.

The stop valve will consist of an annular membrane made
from polyurethane. It can be inflated so as to block the
turbine passage or deflated down onto the core of the
annular passage to allow air flow. The inflation will be
done with a double-acting piston which can be locked at
either end of its travel by a very fast brake. When the
brake is released to open the valve the potential energy
in the membrane compartment will accelerate the piston
and so be converted into kinetic energy. This will be
used to compress air in a second compartment. The brake
will be reapplied at exactly the moment of zero velocity.
A small amount of air will then be pumped from the lower
to the higher pressure compartment to make up for the
system losses. This means that instantaneous power flows
of about a megawatt are recovered time after time and the
mean power consumption will be only a few hundred watts.

Expected Achievements and Exploitation

It is expected to make wave energy turbines safer and
more efficient widening the range of air flows and
extending the frequency response.

Funding Scheme

CSC - Cost-sharing contracts


University of Edinburgh
Mayfield Road
EH9 3JL Edinburgh
United Kingdom

Participants (2)

Applied Research and Technology Ltd
United Kingdom
50 Seafield Road
IV1 1LZ Inverness
1,Avenida Rovisco Pais 1
1049-001 Lisboa