Wave Energy

• Cause by surface winds
• Period of about 6 seconds

Transfer of energy from wind to sea

• V_c - absolute velocity
• V_u - peripheral velocity
• V_w - relative velocity

• Forward component of F_L
• Produces the thrust which drives the blade forward

The rate at which the phase of the wave propagates in space

• A water particle whose mean position below the surface z moves in a circle of radius:
• 

• Wave motion induces pressure changes beneath the column
• Pressure changes cause the water level in the chamber to oscillate
• Oscillating water level cause airflow changes in the chamber

Consists of aerofoils whose symmetry directs the air in the same direction -> allows turbine to power generator regardless of airflow direction

Water is 800 times denser than air

• The waves on the open sea have great energy potential
• Negligible environmental impacts

• Shore
• Near shore
• Offshore

Nearer the coastline the average energy intensity of a wave decreases due to interaction with the seabed.

Greater power available in the waves

• Engineering challenges
• Construction costs
• Maintenance and/or installation costs
• Transmission costs and losses
• Environmental impacts
• The scale of electricity production

• Fixed (TAPCHAN and OWC)
• Floating (floating devices and wave profile devices)

Fixed WEC devices are either built into the shoreline or fixed to the seabed 

The number of suitable sites is limited.

• tapered channel systems
• consist of a tapered channel feeding into a reservoir that is constructed on a cliff
• The narrowing of the channel causes the waves to increase their amplitude (wave height) as they move towards the cliff face
• The kinetic energy of the moving wave is converted into potential energy as the water is stored in the reservoir
• The stored water is then fed through a water turbine to generate electricity.

• Low maintenance costs
• Reliable
• Power on demand

Not suitable for all costal regions

• COnsistent waves
• Good average wave energy
• Tidal range less than 1m
• Deep water near shore
• Location for a reservoir

Air velocity can be increased by reducing channel cross sectional area

systems that are floating in the ocean, either close to shore or offshore

• Powerbuoy
• Wave Profile WEC devices (The Whale device, Pelamis, Salter duck)

• Mechanical power is obtained by floats making use of the motion of water
• Float moves up and down with the water
• An air turbine is run by the compressed air which in turn drives an electrical generator producing electricity
• The generated electricity is then transmitted to the shore through an underwater cable.

They float on or near the sea surface and move in response to the shape water rather than just the vertical displacement of water

• OWC based device for offshore operation
• A 120 kW prototype with 3 OWCs in a row has been operating since 1998 1.5 km off Nansei Town, Japan at 40 m depth

• The Pelamis is a semi-submerged, articulated structure composed of sections linked by hinged joints
• The wave induced motion of these joints is resisted by hydraulic rams which pump high pressure oil through hydraulic motors via smoothing accumulators
• The hydraulic motors drive electrical generators to produce electricity

• Generates electricity through the harmonic motion of the floating part of the device
• The devices rise and fall according to the motion of the wave and electricity is generated through the motion.

The Duck rotates with a nodding motion as the wave passes. This motion pumps a hydraulic fluid that drives a hydraulic motor, which in turn, drives an electrical generator

• Sea waves have high energy densities
• Clean source of renewable energy
• Natural seasonal variability of wave energy follows the electricity demand
• Negligible land use demand
• Secure energy in remote regions
• Stimulate declining industries

• The peak-to-average load ratio in the sea is very high and difficult to predict
• The structural loading in extreme weather conditions may be as high as 100 times the average loading
• High construction costs
• Cost of electricity transmission from deep ocean sites

• Navigation hazard to ships
• Visual impact
• Effect recreation
• Marine environment

dV=dx dz

u = c/2