Oscillating Water Column

Oscillating Water Columns (OWCs) are a type of Wave Energy Converter (WEC) that harness energy from the oscillation of the seawater inside a chamber or hollow caused by the action of waves. OWCs have shown promise as a renewable energy source with low environmental impact. Because of this, multiple companies have been working to design increasingly efficient OWC models. OWC are devices with a semi-submerged chamber or hollow open to the sea below, keeping a trapped air pocket above a water column. Waves force the column to act like a piston, moving up and down, forcing the air out of the chamber and back into it. This continuous movement force a bidirectional stream of high-velocity air, which is channelled through a Power-Take-Off (PTO). The PTO system converts the airflow into energy. In models that convert airflow to electricity, the PTO system consists of a bidirectional turbine. This means that the turbine always spins the same direction regardless of the direction of airflow, allowing for energy to be continuously generated. Both the collecting chamber and PTO systems will be explained further under "Basic OWC Components."

The PTO system is the second main component of an OWC device. It converts the pneumatic power into a desired energy source (i.e. sound or electricity). The PTO system design is very important to the efficiency of the Oscillating Water Column. It must be able to convert airflow going both out of and into the collecting chamber into energy. Turbines that accomplish this are called bidirectional turbines.

The Wells turbine, designed in the late 1970s by professor Alan Arthur Wells at Queen's University Belfast, is a bidirectional turbine that uses symmetrical airfoils (see Fig. 1). The airfoils will spin the same direction regardless of the direction of airflow. The Wells turbine has both benefits and drawbacks. It has no moving parts other than the main turbine rotor, making it easier to maintain and more cost effective. However, it sacrifices some efficiency at high airflow rates because the airfoil's high angle of attack creates more drag. The angle of attack is the number of degrees the airfoil is from being parallel with the airflow. Wells turbines are most efficient at low speed airflows.

The Hanna Turbine [2] U.S. patent 8,358,026, was invented by environmental activist John Clark Hanna in 2009. The Hanna Turbine was developed to improve upon the pioneering Wells Turbine. As with the Wells, the Hanna device has no moving parts that come in direct contact with the ocean. The turbine has two rotors with back-to-back asymmetrical airfoils. Both rotors are mirror images with low angles of attack. The airfoils have higher lift coefficients and less drag than the Wells Turbine. This makes the Hanna design less prone to stalling and offers more torque with a larger operating window. The Hanna design also drives two generators that operate outside of the enclosed air duct in a relatively dry environment. This allows for easy maintenance of the generators.

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