8.3 Air Turbine Power Take-Off for OWC Devices

Air turbines are crucial for converting wave energy in Oscillating Water Column (OWC) devices. They harness the bidirectional airflow created by wave motion to generate power. Different turbine types, like Wells and impulse turbines, offer varying efficiencies and operational characteristics.

OWC power conversion relies on the oscillating motion of waves to compress and expand air in a chamber. This pneumatic power is then converted to mechanical energy through the turbine. Efficient airflow rectification and turbine design are key to maximizing power output in OWC systems.

Air Turbine Types for OWC

Wells Turbine

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• Uses symmetric airfoil blades that rotate in the same direction regardless of airflow direction
• Operates with bidirectional airflow without requiring a rectifying valve system
• Provides a simple and compact design for OWC applications
• Suffers from lower efficiency compared to conventional unidirectional turbines (around 40-70%)
• Experiences stalling at high flow rates, leading to a sudden drop in efficiency
  • Occurs when the angle of attack exceeds a critical value
  • Results in flow separation and a decrease in lift force on the blades

Impulse Turbine

• Uses asymmetric blades that are designed to extract kinetic energy from the airflow
• Requires a rectifying valve system to ensure unidirectional rotation
  • Guide vanes or flaps are used to direct the airflow onto the blades
• Offers higher efficiency compared to Wells turbines (up to 50-80%)
• Provides better starting characteristics and operates over a wider range of flow rates
• Examples include the Denniss-Auld turbine and the Mutriku OWC plant in Spain

Self-Rectifying Turbines

• Designed to operate with bidirectional airflow without the need for a rectifying valve system
• Utilizes special blade geometries or arrangements to achieve self-rectification
• Examples include the Savonius turbine and the McCormick counter-rotating turbine
  • Savonius turbine uses a simple rotor with S-shaped blades
  • McCormick turbine consists of two counter-rotating rotors with curved blades
• Offers lower efficiency compared to impulse turbines but provides a simpler and more robust design

OWC Power Conversion

Oscillating Water Column (OWC) Principle

• Utilizes the oscillating motion of ocean waves to compress and expand air in a chamber
• As waves enter the chamber, the water level rises, compressing the air and forcing it through a turbine
• As waves recede, the water level falls, creating a vacuum and drawing air back through the turbine
• The continuous wave motion creates an oscillating airflow that drives the turbine

Pneumatic Power Conversion

• The oscillating airflow in the OWC chamber represents pneumatic power
• The power available depends on the pressure differential across the turbine and the volumetric flow rate
• The pneumatic power ($P_p$) can be expressed as: $P_p = \Delta p \cdot Q$
  • $\Delta p$ is the pressure differential across the turbine
  • $Q$ is the volumetric flow rate
• Matching the turbine characteristics to the pneumatic power available is crucial for optimal power conversion

Airflow Rectification Strategies

• Bidirectional airflow in OWC systems can be handled using different strategies
• Self-rectifying turbines (Savonius, McCormick) inherently operate with bidirectional flow
• Non-self-rectifying turbines (impulse turbines) require a rectifying valve system
  • Guide vanes or flaps are used to direct the airflow onto the blades in a consistent direction
  • Ensures unidirectional rotation of the turbine despite the oscillating airflow
• Wells turbines utilize symmetric airfoil blades that rotate in the same direction regardless of flow direction

Turbine Efficiency Considerations

• The efficiency of the air turbine significantly impacts the overall power output of the OWC system
• Turbine efficiency ($\eta_t$) is the ratio of the mechanical power output to the pneumatic power input: $\eta_t = \frac{P_m}{P_p}$
• Wells turbines typically have efficiencies ranging from 40-70%, with peak efficiencies around 50-60%
• Impulse turbines can achieve higher efficiencies, up to 50-80%, with peak efficiencies around 70-75%
• Improving turbine efficiency through optimized blade design, materials, and control strategies is an active area of research