Author: Graham Brennan, transport and ocean R&D programme manager, Sustainable Energy Authority of Ireland
Ireland is not blessed with many mineral or fossil fuel resources, but perhaps our greatest untapped natural resource lies in the vast ocean area to our west and its associated renewable energies. The Atlantic Ocean off west of Ireland is one of the most energetic seas in the world, with annual average wave power reaching 75,000 Watts per metre wave width. This compares with 45kW/m for Portugal and 30kW/m for the North Sea.
Any wave energy converter (WEC), therefore, has the potential to generate more electricity west of Ireland than anywhere else in Europe, giving Ireland an obvious advantage. To give an example of the energy contained off our coast: if it was possible to capture 1m width of this sea and convert it with 10 per cent efficiency, in one year we could produce enough electricity to power 27 electric vehicles, each travelling 16,000km.
This all sounds great, so what are we waiting for? We are waiting for someone to build a device that is strong enough, efficient enough, reliable enough, survives long enough and is cheap enough to persuade an energy utility company to buy one, a bank to finance it and someone else to insure it. In this two-part article, part one will present the engineering steps involved in designing a WEC and part two will explore the challenges faced by WEC developers and governments who are trying to make wave energy a reality.
Before starting, it is worth mentioning tidal energy briefly. Most of the tidal current energy is centred off the coast of Antrim. However, its potential contribution to our energy requirements is dwarfed when compared to the scale of the accessible wave energy available to us in the Atlantic. While many of the same engineering challenges apply to tidal, wave energy will be the focus of our discussion.
MEASURING WAVE ENERGY
[caption id="attachment_8794" align="alignright" width="2136"] A wave-monitoring buoy measures wave profile and direction and transmits data to shore which is used to calculate annual wave power and energy at any given offshore site[/caption]
The first question a curious person may ask is, ‘How do you know how much energy is in the sea?’ Waves are driven by the wind, starting as little ripples and growing to towering heights. The harder and longer the unobstructed distance the wind can blow, the higher the resulting waves will be.
Waves transmit energy over enormous distances, but yet the water particles involved in this process only move in local circular patterns with very little friction. Throw a cork on a wave at sea and watch its circular motion as the wave overtakes it. The cork follows the nearly same path as the water surrounding it. By summing the kinetic and potential energy of the circular moving particles in the water column, the energy contained in an ideal regular sinusoidal wave can easily by determined from its wave height or amplitude alone.
But real ocean waves are random-looking things, so how is it possible to measure the energy in such a scenario? Early researchers recognised that wave heights (and therefore energy) contained in apparently random wave trains followed a statistical distribution. When the boundary conditions were maintained for long enough (for example, wind speed and direction), the same distribution of wave heights occurred again and again.
Measuring the energy contained in this full distribution (or energy spectrum) with wave period allowed the energy for a given sea state to be determined per metre squared of sea surface area. Researchers developed spectra empirically to suit different regions of sea around the world. The spectrum is ‘fitted’ to the sea by entering the key parameters measured from the sea which are: