Apparatus and method for extracting energy from a fluid

A method of controlling the frequency of movement of water in an oscillating water column to substantially correspond to the frequency of an incoming and outgoing wave from a body of water which is in fluid communication with the column, the method comprising the steps of:

• a. arranging a duct for receiving the oscillating water column, the duct comprising:
• i. a first portion arranged in use to be substantially submerged below the mean surface level (MSL) of the body of water in which it is located, the first portion having an opening arranged for receiving the incoming wave from the body of water, and
• ii. a second portion which depends from the first portion and which is arranged in use to extend above the MSL, the second portion for receiving water from the incoming wave after it flows through the first portion,
• such that the oscillating water column is established in use within the duct as a result of repeated movement of water into and out of the duct, the flow of water out of the duct also being via the opening, but in a direction opposite to the direction of the incoming wave;
• and
• b. changing the configuration of at least one flow control device which is in direct fluid communication with a flow passage at an interior of the second portion of the duct which extends above the MSL, said device(s) arranged in use to move between a first configuration in which the device is open, allowing a flow of displaced air to exit the flow passage in the second portion when the oscillating water column is being received into the duct, and a second configuration in which the device restricts air flowing therethrough into the flow passage in the second portion;
• such that the frequency of the oscillating water column which flows into and out of the duct substantially corresponds to the frequency of the incoming and outgoing wave from the body of water;
• and
• wherein the frequency of the oscillating water column in use can be varied by the step of changing the cross-sectional area of the flow control device(s) as a proportion of the surface area of the second portion which extends above the MSL, by the selective movement of one or more of the flow control device(s) between the first and second configurations.