In the first of a two-part article, Graham Brennan examines the potential benefits that harnessing wave power could bring to Ireland and the engineering steps involved in designing a wave energy converter
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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.

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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

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

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:

  • the significant wave height (Hs or the average height of the highest 33 per cent of waves measured in the sample); and
  •  the peak period (Tp or the period at which most wave energy typically occurs).

Of course, the ‘boundary conditions’ for the sea are changing all the time, so investigators record wave height and period data every 30 minutes at the designated site. For each of these samples, Hs and Tp are calculated. The site is monitored for at least one year, and the number of times that each sea state (i.e. Hs, Tp combination) has occurred is recorded on a table.

Using this table (or matrix), together with the energy spectrum for that sea, it is then possible to determine the total annual energy and the average annual power per metre width of wave crest available at a given offshore location. So now we know how the wave energy and power are calculated for a target location out at sea.

WAVE TANK TESTING

Wave tanks can model real seas found at most target sites. Here a 1:15 scale model is undergoing real sea long crested wave tests at Ecole Centrale de Nantes

The cost of building a full-scale device from scratch and placing it into the sea to test is prohibitively expensive and potentially dangerous, so designers start small and work their way upwards building confidence at each stage.

In order to successfully simulate the fluid/solid loads on a model device at small scale, engineers make use of the Froude number. Holding the Froude number constant from tank to sea ensures that inertial and gravitational forces remain in proportion to each other at all geometric scales in the development process.

The power take-off mechanism is sometimes included or more commonly the effect of its resistance is included through use of a fluid flow device. Pressures and fluid flows are measured on board the device to calculate the available pneumatic or hydraulic power produced by the device which would then be available to generate electricity. The model is used to measure mooring and structural loads, which is important in reducing the final mass and cost of the design.

Typically designers will optimise their designs at 1:50th scale, working up to 1:15th scale confirming power and energy capture at each scale. Sophisticated wave tanks such as the one at the Hydraulics Maritime Research Centre (HMRC) in University College Cork are used to create waves both regular and ‘irregular’ (i.e. real sea waves). These tanks are so advanced they can recreate almost any desired wave energy spectrum including directional variability. In this way, the waves measured at a site in the Atlantic for instance can be simulated at scale in a test tank.

The power performance of the device is determined for a select number of sea states (Hs and Tp). The performance for the remaining sea states are extrapolated or modelled by computer until finally a power matrix is produced indicating the likely output for the device for all likely sea states. Combining this matrix with the likely occurrence of each sea state – provided earlier for the planned target site for one year of data – results in the first estimate of the annual energy yield for the device.

By accurately modelling a geometrically correct scale model in its scaled sea environment, it is possible to make an early estimate of the cost of electricity which can be expected for a full-scale device before further investments are made. HMRC has been at forefront of the development of prototype test procedures for many years. These protocols allow potential investors to gauge the status and prospects for any new WEC technology.

OPEN-SEA TEST SITES

Renewable powered surface buoy which connects to seabed acoustic sensors – IBM acoustic monitoring project Galway Bay (courtesy of Marine Institute)

Tank tests cannot confirm the reliability of electrical power output so promising devices must next be moved to an open sea test site. The Sustainable Energy Authority of Ireland (SEAI) and the Marine Institute have developed a 1:4 scale test site in Galway Bay, where maximum wave heights may reach 8m. A one-year test at this scale can cost of the order of €1-2million, which is much less than a full-scale test. SEAI provides funding for tests at this scale.

This open sea test facility is approximately 0.75km2 and lies 1.5km off shore from Spiddal. The Ocean Energy Buoy and the Wavebob were the first devices to be tested there and several companies (both national and international) are currently looking to use the site. IBM is constructing an underwater acoustic monitoring system, which will be used to monitor the environmental noise impact of WECs at the site.

Other plans for the test site include the development of a floating power-buoy system designed to measure and dissipate electrical output from 1:4 scale devices. The system will also supply ancillary power to test devices and will provide data connections to control and monitor the device remotely. Normally, each developer must include these services on their own device. Therefore, the power buoy will further reduce the cost and accelerate the pace of technology development. Independent performance reports can then be produced and supplied to technology developers greatly improving the credibility of the results gathered.

Following successful 1:4 scale trials, a developer is now in a position to consider a grid connected 1:1 full scale trial. Ideally, one year of 1:4 scale test data would be used by the developer to convince investors to provide the funds needed for the larger trial. A full-scale trial may cost in the region of €10 million or more to complete. Given the risks involved, developers can expect to source the majority of this funding from private investors who will expect some share of the business in return.

The European Marine Energy Centre (EMEC) offers a facility to grid connect wave and tidal energy converter devices. EMEC is based on the Orkney Islands and has an average annual wave power of 24kW/m in 50m water depth at a distance of 2km from shore.

Ireland’s planned experimental test site, the Atlantic Marine Energy Test Site (AMETS) off Belmullet in Co Mayo has average annual power of 58kW/m in 50m water depth at a distance of 5km from the shore. This figure rises quickly to 70kW/m at a distance of 11km, offering one of the most extreme waves regimes of any location in the world.

GRID-CONNECTED SYSTEM

So, the very high costs involved require engineers to start with cheaper scale models and work upwards to an eventual, full scale, grid-connected system. The wave environment for a chosen target sea location can be modelled successfully in modern tank testing facilities. A number of open-sea test sites have emerged over recent years to allow large-scale systems to be validated and tested for performance, reliability and safety. We now have a rough summary of how a prospective WEC technology is moved from the drawing board to the sea.

In part two, we will examine what it will take to make wave energy a credible alternative source of electricity and the steps needed to create a viable market for this important energy resource for Ireland.

http://www.engineersjournal.ie/wp-content/uploads/2013/10/Wavescan-belmullet-002-1024x768.jpeghttp://www.engineersjournal.ie/wp-content/uploads/2013/10/Wavescan-belmullet-002-300x300.jpegDavid O'RiordanElecenergy,tidal,UCC,wave
  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...