Wave turbines – Wave power

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Content – Energy sources

 


 
Capture of energy from ocean surface waves to generate electricity or mechanical work.

There is an energy transfer from the wind passing over the ocean surface and to the sea.

The determining factors for wave height are; the wind speed, the duration of time the wind has been blowing, the distance over which the wind excites the waves (fetch) and the depth and topography of the seafloor.

The wave power is determined by wave speed, wavelength and the density of the water.

Normally and for pratical use the wave motion may be considered strongest at the surface.

Wave power or wave energy flux

Formula
If the water depth is larger than half the wavelength, the wave energy flux is:
 
P = \dfrac{\rho g^2}{64\pi}H^2_{mo}\approx (0,5\dfrac{kw}{m^3s})H^2_{mo}T_e
 
P = Wave energy flux per unit of wave-crest length (kW)
Hm0 = Significant wave height (m)
Te = Wave energy period (s)
ρ = Water density
G = Acceleration by gravity.
 
Wave energy and wave-energy flux
 
The mean energy density waves on the water surface is:
 
E = \dfrac{1}{2}\rho gH^2_{mo}
 
E = The sum of kinetic and potential energy density per unit horizontal area. The potential energy density is equal to the kinetic energy, both contributing half to the wave energy density E.

Devices
Wave power devices are generally categorized by the method used to capture the energy of the waves, by location and by the power take-off system. Locations are shoreline, nearshore and offshore. Types of power take-off include:

  • Hydraulic ram
  • Wlastomeric hose pump
  • Pump-to-shore
  • Hydroelectric turbine
  • Air turbine
  • Linear electrical generator

Point absorber buoy
This device floats on the surface of the water, held in place by cables connected to the seabed. Buoys use the rise and fall of swells to drive hydraulic pumps and generate electricity.

Surface attenuator
These devices act similarly to point absorber buoys, with multiple floating segments connected to one another and are oriented perpendicular to incoming waves. A flexing motion is created by swells that drive hydraulic pumps to generate electricity.

Oscillating water column
Oscillating water column devices can be located on shore or in deeper waters offshore. With an air chamber integrated into the device, swells compress air in the chambers forcing air through an air turbine to create electricity

Overtopping device
Overtopping devices are long structures that use wave velocity to fill a reservoir to a greater water level than the surrounding ocean. The potential energy in the reservoir height is then captured with low-head turbines. Devices can be either on shore or floating offshore.

Oscillating wave surge converter
These devices typically have one end fixed to a structure or the seabed while the other end is free to move. Energy is collected from the relative motion of the body compared to the fixed point. Oscillating wave surge converters often come in the form of floats, flaps, or membranes. Some of these designs incorporate parabolic reflectors as a means of increasing the wave energy at the point of capture. These capture systems use the rise and fall motion of waves to capture energy. Once the wave energy is captured at a wave source, power must be carried to the point of use or to a connection to the electrical grid by transmission power cables.