Ian Furlong

Title – Tidal energy: "Why we should reap the benefits of being an island nation."

Abstract

In this Wiki page we will discuss what exactly tidal energy has to offer Ireland in terms of being a viable renewable energy. To get there we will need to understand how the tides are created and is there consistency in their patterns. Consistency means predictability, which would be a significant plus for tidal energy over other forms of renewable technologies.

We will review some of the devices and techniques that are emerging as leaders in this field and as with any technology discuss the potential drawbacks associated. Tidal in-stream energy conversion (TISEC) will be the primary form of tidal energy that will be reviewed for reasons that will be explained in the following chapters.

We will also need to determine the amount of tidal energy that exists around the coastline of Ireland and what amount we can expect to feasibly extract from this source. With this amount of feasible energy known we will then compare the costs involved in getting this energy into the grid.

Finally we will review the outlook for the technology and for Ireland and will conclude with how TISEC compares to other renewable energy technologies.

Background

Since the industrial revolution the world energy consumption or demand for energy has increased year on year and there doesn’t appear to be any deviation from this any time soon. The pie charts below highlight the increasing world energy consumption (measured in Mtoe – million tonnes of oil equivalent) over the last 38 years and also gives a breakdown of the percentage of each fuel source that produced the energy. It is interesting to note that worldwide energy demands have almost trebled and the largest fuel source contributors (the finite fossil fuels of oil, coal and gas) have maintained their majority status in this period. Meaning we are now, quicker than ever before depleting these non-renewable fuel sources and still not increasing the renewable energy supplies to any significant degree.

Figure 1 : Energy consumption comparison between 1973, 2006 and 2010.

But how much energy do we need? Well in February 2011 at a European Council meeting[1], energy consumption data prepared by the IEA (International Energy Agency) was reviewed by the commission. According to the data the IEA predict a levelling of the consumption of energy in the European Union and also OECD countries in future years, but overall world energy consumption will grow due to global population increase and economic catching up in other regions. Overall, the IEA predict world energy demand could grow by 45% between 2006 and 2030 and expect countries such as India and China to double their energy demand. The graph below visually shows the IEA’s prediction. In chapter 5 we will look a little closer to home and assess what is Ireland’s energy demand.

Figure 2 : World energy consumption prediction. Source: IEA, World energy outlook 2010

So do we have the capacity to fulfil this predicted demand? Well a number of forecasters have already predicted that oil production will peak in the next decade and exhaust within the next 50 years.

You see oil was formed by geological processes millions of years ago and is typically found in underground reservoirs of dramatically different sizes, at varying depths, and with widely varying characteristics[2]. The largest oil reservoirs are called “Super Giants,” many of which were discovered in the Middle East. Because of their size and other characteristics, Super Giant reservoirs are generally the easiest to find, the most economical to develop, and the longest lived. The last Super Giant oil reservoirs discovered worldwide were found in 1967 and 1968. Since then, smaller reservoirs of varying sizes have been discovered in what are called “oil prone” locations worldwide -- oil is not found everywhere.

Geologists understand that oil is a finite resource in the earth’s crust, and at some future date, world oil production will reach a maximum -- a peak -- after which production will decline. This logic follows from the well-established fact that the output of individual oil reservoirs rises after discovery, reaches a peak and declines thereafter. Oil reservoirs have lifetimes typically measured in decades, and peak production often occurs roughly a decade or so after discovery. It is important to recognize that oil production peaking is not “running out.” Peaking is a reservoir’s maximum oil production rate, which typically occurs after roughly half of the recoverable oil in a reservoir has been produced. In many ways, what is likely to happen on a world scale is similar to what happens to individual reservoirs, because world production is the sum total of production from many different reservoirs.

So just to recap on these points, fossil fuels are a finite resource, meaning there will come a time when we have discovered all the oil and gas wells and mined all the coal. We are also in a critical time of our planets’ history when we are consuming fossil fuelled energy at an increasing rate year on year. Something will have to give because before long this planets’ dependency on fossil fuels will lead to a significant economic meltdown on a global scale. So what is the alternative or solution to this? By developing renewable energy sources to significant levels of contribution, must surely be the answer. Imagine if the major contributor of our energy sources was from renewable energies, all our fears about depleting energy sources would abate. Renewable energy sources such as Solar, Wind, Tidal and Wave are all sources that exist around us. The complexity is in harvesting this energy source and transforming it efficiently and economically into useable energies to fuel our planet. The obvious advantage these sources have over fossil fuels is that they infinite – (baring any astronomical catastrophe such as a meteor hitting the Earth) – meaning security of supply.

Another reason for moving towards renewable energy sources as an alternative to fossil fuels is that it is very probable that using fossil fuels changes the climate[3]. Climate change is blamed on several human activities, but the biggest contributor to climate change is the increase in greenhouse effect produced by carbon dioxide (CO2). Most of the carbon dioxide emissions come from fossil-fuel burning. And the main reason we burn fossil fuels is for energy. So to fix climate change, we need to implement new ways of sourcing energy.

In terms of selecting a renewable energy that may benefit Ireland, I selected Tidal energy; due to fact that we are an island nation and not land locked it has to be a form of renewable energy we can benefit from. It is one of the oldest forms of energy as tide mills, where put to use on the Spanish, French and British coasts, dating back to 787 A.D. Tide mills consisted of a storage pond filled by the incoming tide through a sluice and emptied during the outgoing tide through a water wheel. The tidal energy technology has moved on a lot since then with the construction of large tidal barrages, the first being the La Rance Barrage in France. It took six years to build, between 1960-1966 and has a 250MW capacity.

Figure 3 : La Rance Barrage, France. Source: DCU EE535 Tidal energy lecture notes

Currently the tidal energy technology is moving towards tidal in-stream energy conversion (TISEC). There are numerous device designers around the world working on building the most efficient energy harnessing devices, but the principal design is similar to a wind turbine. One such designer is a Norwegian energy company, Hammerfest Strøm established in 1997 by the utility company Hammerfest Energi.[4]

Figure 4 : Hammerfest Strøm tidal turbines on seabed. Source: www.hammerfeststrom.com

The Hammerfest Strøm tidal turbine is best described as an underwater wind turbine, but with shorter blades that rotate slower. Each turbine will have an installed effect in excess of 1MW, and arrays may consist of hundreds of turbines. The tidal power devices are installed at 40-100 meters depth in tidal streams with velocity in excess of 2.5m/s. The turbines are installed on the seabed, and are not creating any visible or audible pollution above the surface and allow vessels to operate without restrictions.

In this Wiki page I hope to find out what exactly Tidal energy (with particular emphasis on tidal in-stream energy conversion) has to offer Ireland and what the potential draw backs of this technology are.

Tidal Energy

What is tidal energy?

Tidal energy, also called tidal power, can be defined as a form of hydropower that converts the energy of the tides into useful forms of power - mainly electricity. It can be described as energy that can be obtained from the changing sea levels. In other words, tidal energy is a direct result of tide shifting from low to high.

In basic terms it is actually very similar to hydropower and with the recent developments in tidal in-stream energy rather than using a dam to create the energy in the water to turn the blades of a turbine, the oceans tides, which happen to exist without any human creation provide the necessary energy to turn the turbine blades. So large scale construction projects of massive concrete structures to dam rivers or construct tidal barrages, which can take considerable time to complete and cost significant capital investments to construct, are not required in tidal stream energy. Harnessing this energy is all done out in the oceans and undersea level, so issues such as noise pollution or the planning objections of the “not in my backyard” brigade which you get with wind farms are also not an issue for tidal energy. So, on the surface it would appear that tidal energy is the least invasive technology on the eye in the renewable energies toolbox, but on the other hand we must also appreciate that this technology is operated in possibly the harshest environment of all, undersea level, where the most durable materials are extremely put to the test. Also tidal energy like any technology has to have some drawbacks, which will be reviewed in chapter 3. Also in chapter 3 we will look at the some of the various device designs in harnessing tidal in-stream energy. But before we get into that we need to understand how consistent or variable the energy supply is that we can extract from ocean tides? Are the ocean tides significant enough to make it cost efficient to harness the energy? To answer these questions we will need to understand how tides are formed and how tidal energy is measured.

Factors that affect the tides and their patterns

Tides are created because the Earth and the moon are attracted to each other, just like magnets are attracted to each other. The moon tries to pull at anything on the Earth to bring it closer. But, the Earth is able to hold onto everything except the water.

Gravity is one major force that creates tides. In 1687, Sir Isaac Newton explained that ocean tides result from the gravitational attraction of the sun and moon on the oceans of the earth. Newton’s law of universal gravitation states that the gravitational attraction between two bodies is directly proportional to their masses, and inversely proportional to the square of the distance between the bodies. Therefore, the greater the mass of the objects and the closer they are to each other, the greater the gravitational attraction between them.

Figure 5 : Highlighting the fact that the moon influences our tides more than the sun. Source: NOAA Tides and Currents

The relationship between the masses of the Earth, moon and sun and their distances to each other play a critical role in affecting the Earth's tides. Although the sun is 27 million times more massive than the moon, it is 390 times further away from the Earth than the moon. Tidal generating forces vary inversely as the cube of the distance from the tide-generating object. This means that the sun’s tidal generating force is reduced by 3903 (about 59 million times) compared to the tide-generating force of the moon. Therefore, the sun’s tide-generating force is about half that of the moon, and the moon is the dominant force affecting the Earth’s tides.[6]

Figure 6 : Tidal bulge due to Moon's gravitational pull. Source: NOAA – National Oceanic and Atmospheric Administration

The gravitational attraction of the moon causes the oceans to bulge out in the direction of the moon. Another bulge occurs on the opposite side, since the Earth is also being pulled toward the moon (and away from the water on the far side). Ocean levels fluctuate daily as the sun, moon and Earth interact. As the moon travels around the Earth and as they, together, travel around the sun, the combined gravitational forces cause the world's oceans to rise and fall. Since the water is always moving, the Earth cannot hold onto it, and the moon is able to pull at it. Because the Earth rotates through two tidal “bulges” every lunar day, coastal areas experience two high and two low tides every 24 hours and 50 minutes. High tides occur 12 hours and 25 minutes apart and it takes six hours and 12.5 minutes for the water at the shore to go from high to low, or from low to high.

Solar tides are about half as large as lunar tides and are expressed as a variation of lunar tidal patterns, not as a separate set of tides. When the sun, moon, and Earth are in alignment (at the time of the new or full moon), the solar tide has an additive effect on the lunar tide, creating extra-high high tides, and very low, low tides, which are called spring tides. One week later when the sun and moon are not aligned, the gravitational forces cancel each other out, and the tides are not as dramatically high and low. These are called neap tides. During each lunar month, two sets of spring tides and two sets of neap tides occur.

Figure 7 : Spring and neap tides. Source: DCU EE535 Tidal energy lecture notes

Spring tides and neap tide levels are about 20% higher or lower than average tides that occur throughout the month. Offshore, in the deep ocean, the difference in tides is usually less than 1.6 feet, but as the tide approaches a coastline the difference between high and low tides increases. In bays and estuaries, this effect is even more amplified. For example in the Bay of Fundy, in Nova Scotia, Canada, and the range between high and low tides is 44.6ft, which is the highest tidal difference in the world.

Since the moon moves around the Earth, it is not always in the same place at the same time each day. So, each day, the times for high and low tides change by 50 minutes. The reason for this is that over time, the positions of these celestial bodies change relative to the Earth’s equator. The changes in their relative positions have a direct effect on daily tidal heights and tidal current intensity.

As the moon revolves around the Earth, its angle increases and decreases in relation to the equator. This is known as its declination. The two tidal bulges track the changes in lunar declination, also increasing or decreasing their angles to the equator. Similarly, the sun’s relative position to the equator changes over the course of a year as the Earth rotates around it. The sun’s declination affects the seasons as well as the tides. During the vernal and autumnal equinoxes—March 21 and September 23, respectively—the sun is at its minimum declination because it is positioned directly above the equator. On June 21 and December 22—the summer and winter solstices, respectively—the sun is at its maximum declination, i.e., its largest angle to the equator.

Just as the angles of the sun, moon and Earth affect tidal heights over the course of a lunar month, so do their distances to one another. Because the moon follows an elliptical path around the Earth, the distance between them varies by about 31,000 miles over the course of a month. Once a month, when the moon is closest to the Earth (at perigee), tide-generating forces are higher than usual, producing above-average ranges in the tides. About two weeks later, when the moon is farthest from the Earth (at apogee), the lunar tide-raising force is smaller, and the tidal ranges are less than average. A similar situation occurs between the Earth and the sun. When the Earth is closest to the sun (perihelion), which occurs about January 2 of each calendar year, the tidal ranges are enhanced. When the Earth is furthest from the sun (aphelion), around July 2, the tidal ranges are reduced (Sumich, J.L., 1996; Thurman, H.V., 1994).

Figure 8 : Aphelion and Perihelon tidal affects. Source: NOAA Tides and Currents

Three basic tidal patterns occur along the Earth’s major shorelines. In general, most areas have two high tides and two low tides each day. When the two highs and the two lows are about the same height, the pattern is called a semi-daily or semidiurnal tide. If the high and low tides differ in height, the pattern is called a mixed semidiurnal tide. Some areas, such as the Gulf of Mexico, have only one high and one low tide each day. This is called a diurnal tide. The coasts around Ireland and UK experience semidiurnal tides.

Figure 9 : Types of tides around the globe. Source: NOAA Tides and Currents

Tides are most commonly semi-diurnal (two high waters and two low waters each day), or diurnal (one tidal cycle per day). The two high waters on a given day are typically not the same height (the daily inequality); these are the higher high water and the lower high water in tide tables. Similarly, the two low waters each day are the higher low water and the lower low water. The daily inequality is not consistent and is generally small when the moon is over the equator.

Figure 10 : Distribution of tidal phases. Source: NOAA – National Oceanic and Atmospheric Administration[7]

The magnitude of tides can be strongly influenced by the shape of the shoreline. When oceanic tidal bulges hit wide continental margins, the height of the tides can be magnified. Conversely, mid-oceanic islands not near continental margins typically experience very small tides of 1 meter or less.[8]

The shape of bays and estuaries also can magnify the intensity of tides. Funnel-shaped bays in particular can dramatically alter tidal magnitude. As mentioned earlier the Bay of Fundy in Nova Scotia is the classic example of this effect, and has the highest tides in the world—over 15 meters. The image below of a tidal monitoring station in Alaska taken at high and low tide illustrates the dramatic effect that geographic location can have on tidal range. At increasing latitudes (as one moves further from the equator and closer to the poles) there often is a dramatic increase in tidal range—in this case, approximately 45 feet.

Figure 11 : Alaskan tidal monitoring station. Source: NOAA Tides and Currents

Local wind and weather patterns also can affect tides. Strong offshore winds can move water away from coastlines, exaggerating low tide exposures, whereas onshore winds may act to pile up water onto the shoreline, virtually eliminating low tide exposures. High-pressure systems can depress sea levels, leading to clear sunny days with exceptionally low tides. Conversely, low-pressure systems that contribute to cloudy, rainy conditions typically are associated with tides than are much higher than predicted.

How the tides are measured?

Before computers were used, water level data was recorded on a continuously running pen and ink strip chart. These records were collected by observers once a month and mailed to central agencies for manual processing. In the 1960s, data were recorded onto mechanically punched paper tapes that were read into a computer for processing. Water levels were recorded at 6-minute intervals. Observers maintained and adjusted the clocks, and calibrated the gauges with the tide readings. Tide stations were visited annually to maintain the tide houses and clean biological fouling from the underwater surfaces. During these annual visits, the components and support structures also were checked for stability. Although these systems worked well, they had their limitations. Stations were subject to recording errors and marine fouling, and were constantly in need of maintenance. In addition, the measurement and data processing equipment could not provide users with information until weeks after the data was collected.

Advances in technology have helped solve many of the problems associated with the old tidal recording systems. Microprocessor-based technologies allow for customized data collection and have improved measurement accuracy. While older tidal measuring stations used mechanical floats and recorders, a new generation of monitoring stations uses advanced acoustics and electronics. Today's recorders send an audio signal down a half-inch-wide sounding tube and measure the time it takes for the reflected signal to travel back from the water's surface. The sounding tube is mounted inside a 6-inch diameter protective well, which is similar to the old stilling well. In addition to measuring tidal heights more accurately, the new system also records 11 different oceanographic and meteorological parameters. These include wind speed and direction, water current speed and direction, air and water temperature, and barometric pressure. Like the old recorders, the new measuring stations collect data every six minutes. However, the timing is now controlled on the new stations by a Geostationary Operational Environmental Satellite (GOES). In addition, all of the raw and processed data are available over the Internet.

World locations that can benefit from tidal energy

The tidal fluctuations at any point on earth can be thought of as the cumulative effect of over a hundred harmonic constituents or periodic components working together, each with their own amplitude, phase and period (or frequency)[10]. The tide at any location can be completely described and predicted by summing the contributions from all sinusoidal constituents, assuming their amplitudes, phases and frequencies are known. Symbolically, the tide level z(t) is given by:

Where z0 is the mean value, and Ai, ωi and φi represent the amplitude, frequency and phase lag for each of the M tidal constituents.

There are well over 100 active constituents affecting the tide at any site, having periods ranging from about 8 hours to 18.6 years. However, the contributions from most constituents are very small, so that in practice, reasonably accurate predictions can be obtained by considering the five to eight leading constituents while disregarding the others. The principal semidiurnal (twice daily) constituents are known as M2 (moon, twice daily) and S2 (sun, twice daily).

Figure 12 : Global distribution of the amplitude of the M2 tidal constituent; Source www.aviso.com

From the map above you can clearly see that the red regions indicate where exactly in the seas and oceans around the globe that have the highest tidal amplitudes. The region from North West Africa up the West coast of mainland Europe and around the British Isles is in the “red” zone, which indicates a good location to exploit tidal energy. This is positive new for Ireland as it allows us to pursue tidal energy further as a possible renewable energy source. Taking a closer look at the tidal streams around Ireland will ascertain the optimum locations to focus the efforts in harnessing tidal energy. The tidal currents are generally low along the west and south coasts, are relatively strong in the St. George’s and North Channels and are moderate along much of the east coast. The current strengths are considerably influenced by the local bathymetry. One part of the flood travels up the west coast and around the north of Ireland, whilst the other part floods north up the Irish Sea. The tidal streams meet in the area of St John’s Point. A consequence of the way the tides flow around Ireland is that there is a phase difference in the times of high and low water along different parts of the coast[11].

Figure 13 : Depth averaged peak spring tidal currents. Source: 2010 Tidal Current Energy Resources in Ireland Report

From the map above the primary areas for Ireland to harness tidal energy should be off the East and North coasts. At this point we now know that the tides are a regular and predictable phenomenon due to the gravitational interaction with the Moon and Sun and the Earth's rotation, making tidal energy practically inexhaustible and can be classified as a secure supply of renewable energy. We also have identified that harnessing tidal energy from the seas around Ireland and specifically off the North and East coast is a feasible possibility. Now we need to review how tidal energy is measured and explore in more detail the various types of in-stream tidal energy devices.

How tidal energy is measured?

So now we know all the factors that influence the tides and have accurate measuring devices measuring tidal heights, ocean depths and water current speed, so calculating the energy or power output from the tides is a matter of performing calculations from the data we have.

The tide can be used to produce electricity in two ways:

• Potential energy: The energy from the head of water is called potential energy. By using low pressure turbines this energy can be converted into electrical energy. Tidal barrages utilise this method to harness the energy.

• Kinetic energy: The kinetic energy can be converted into electrical energy by using tidal turbines. Tidal streams utilise this method to harness the energy.

We will explore the calculations based on the two tidal energy techniques, tidal pools and tidal streams.

Power density calculation from tidal pool

To estimate the power of an artificial tide-pool, imagine that it’s filled rapidly at high tide, and emptied rapidly at low tide[9]. Power is generated in both directions, on the ebb and on the flood. (This is called two-way generation or double-effect generation.) The change in potential energy of the water, each six hours, is mgh, where h is the change in height of the centre of mass of the water, which is half the range. (The range is the difference in height between low and high tide; see the figure below.) The mass per unit area covered by tide-pool is ρ × (2h), where ρ is the density of water (1000 kg/m3). So the power per unit area generated by a tide-pool is:

2ρhgh

6 hours

Assuming perfectly efficient generators, plugging in h = 2 m (i.e., range 4 m), we find the power per unit area of tide-pool is 3.6 W/m2. Allowing for an efficiency of 90% for conversion of this power to electricity, we get power per unit area of tide-pool approx. equal to 3W/m2.

Figure 14 : Cross section of a tide pool. Source: Sustainable energy – Without the hot air; Mackay 2007

So to generate 1 GW of power (on average), we need a tide-pool with an area of about 300 km2, therefore a circular pool with diameter 20km would do the trick. (For comparison purposes, the area of the Severn estuary behind the proposed barrage is about 550km2.

Power density calculation from tidal stream

The tidal current at a site is generally expressed in terms of two orthogonal components[10], Ux(t) and Uy(t). By convention, Ux is the east-west component (with eastward flow positive) and Uy is the north-south component (with northerly flow positive). The flow speed U(t) is given by the vector sum of these two components:

And the flow direction is given by:

When the amplitudes, phases and frequencies of the dominant velocity constituents are known (from analysis of measurements or from numerical simulations), the time histories of Ux and Uy can be estimated from:

Where ai, bi and ωi are the amplitude, phase and frequency of the ith constituent for Ux(t), and ci, di and ωi are the amplitude, phase and frequency of the ith constituent for Uy(t). The kinetic power (P) in a tidal stream is proportional to the cube of the flow velocity. Hence, the power increases rapidly with increasing speed. When the flow speed doubles, the kinetic power increases eight-fold. The instantaneous kinetic power density (p) of a tidal stream can be written as:

Where ρ ≈ 1,027 kg/m3 is the density of seawater, and U is the instantaneous flow speed [m/s]. The units of power density are Watts/m2. The annual mean power density for a site is equal to the average value of p(t) over the year. The kinetic power (P) flowing across an area A oriented normal to the flow can be written:

If we assume that the flow speed is uniform over the area A, then this simplifies to:

It may be helpful at this point to briefly consider the simple model whereby the speed of the tidal current at a specific location can be described by a simple sinusoidal function as follows:

Where Umax is the maximum speed and T is the period of the tidal cycle. Then the average speed over a half cycle is:

Or 64% of Umax. The average power density over a half cycle is:

Or 42% of the maximum power density. Expressed in terms of mean velocity, the average power density over a half cycle is:

One important result from this simple sinusoidal model is that the average power density over a half cycle is 1.64 times greater than the power density of the average speed over the same half cycle. We will now consider the instantaneous power generated by a turbine-style energy converter located in the tidal stream. The instantaneous generated power (Pg) is given by the product of the kinetic power density of the tidal stream without the device (p), the area swept by the turbine rotor (At), and the overall efficiency (η):

The instantaneous power generated per unit cross-sectional area can be written as:

The efficiency (η) incorporates losses associated with hydrodynamic, mechanical and electrical aspects, as well as any disturbance to the tidal stream resulting from the presence of the device. For example, according to the well-known Betz law, which applies to both wind turbines and hydraulic turbines, a theoretical maximum of 59% of the kinetic energy in a flow can be converted to mechanical energy using a turbine. Forty-one percent of the incident energy is required to ensure that the fluid has enough kinetic energy to depart from the turbine.

Another factor that can impact efficiency is the angle of incidence between the flow and the plane of the rotor – the efficiency may decrease whenever the flow approaches the rotor obliquely. Other factors affecting the hydrodynamic efficiency include the presence of velocity shear (variation) across the rotor, velocity fluctuations induced by surface waves (and other sources), and the intensity and character of any turbulent fluctuations that may be present. For example, gravity waves propagating across the ocean surface (generated by winds or ship traffic) may generate cyclical velocity fluctuations throughout the water column which can impact the efficiency of an energy converter.

The efficiency of the device will generally vary significantly with flow velocity, and for tidal flows, the flow velocity varies over time in a predictable manner. The time-varying power generated by a device with area At is given by:

And the time-varying power generated per unit area is:

Tidal in-stream energy conversion case studies

There are many similarities between wind and tidal current generating systems both in terms of devices and the nature of the driving force. Compared to wind technology, tidal systems are in their infancy but of late some of the TISEC projects are now moving towards the deployment and production phase.[11]

The current first generation devices are limited to water depths between 10m and 50m. Second generation devices should appear after a period of approximately ten years and should be capable of operating in depths exceeding 50m.

Tidal current generators are classified in terms of the form of motion of the primary interface with the water and are either rotational or oscillatory and can be classified as follows:

· primary motion – rotational or oscillatory,

· orientation of the prime mover – horizontal or vertical,

· sea bed connection – moored or fixed structure,

· type of secondary converter – mechanical, hydraulic, electrical.

There are numerous TISEC companies each with their own device designs. In this assignment we will review only 3 case studies from different device designers which are linked to projects around Ireland, UK and France.

Hammerfest Strøm AS – Sound of Islay 10 x 1MW Turbine Array

Hammerfest Strøm AS has installed a 300kW horizontal axis turbine at Kvalsund in northern Norway. The general arrangement of the machine is shown below. The 20m diameter 3 blade rotor is connected to the generator via a gearbox housed in a 10m long 2m diameter nacelle weighing 54 tons. This is located on top of a tubular steel tripod structure weighing 20 tons and held to the sea bed by three 200 ton anchor weights. The water depth is 50m and the nacelle height is 20m below the surface. The structure is completely submerged and maintenance is carried out by divers. The test site has a mean current of 1.8m/s. The blades pitch to optimise performance in the varying tidal flows and pitch is reversed for flow in the opposite direction. The estimated energy output is 0.7GWh equivalent to an average power production of 80kW. Hammerfest Strom report that the turbine has been through a complete deployment, operation, retrieval, maintenance and redeployment cycle, and has proven to be both efficient and reliable, but unfortunately no information is available regarding the costs and performance. This system can and has been deployed in deeper water than the Marine Current machine, which in its present configuration is limited to 30m depth. The Hammerfest Strom unit has certain disadvantages such as difficult access and deep working conditions for divers and in addition there will be more blockages from the tripod structure compared to the monopole with a probable reduction in turbine efficiency due to the increased turbulence when the rotor is downstream from the support structure. When the rotors are upstream from the support structure both machines should have similar hydrodynamic efficiencies.[4] ScottishPower Renewables’ (SPR) tidal energy project in the Sound of Islay will utilise the 1MW Hammerfest Strom device, with a machine due to be installed at the European Marine Energy Centre in Orkney before the end of the year, in preparation for the Islay project. The Islay project will consist of 10 x 1MW Hammerfest turbines in an array and is due to be completed in 2013.

Here is brief guide to the operation of the Hammerfest tidal turbine: The water current is driving the turbine. The power is optimised by regulating the pitch of the blades. The power is converted to electricity in the nacelle (production module) The nacelle contains control system, gear box and generator. The electrical power is transmitted onshore by means of a subsea cable.

Figure 15 : The Hammerfest HS300 turbine. Source: www.hammerfeststrom.com

Marine Current Turbines Ltd. – SeaGen Monopile Twin Turbine Strangford Lough

Marine Current Turbines Ltd. Initially tested and demonstrated a 12m diameter two bladed horizontal axis machine off the north Devon coast known as the Seaflow project.

Figure 16 : The Seaflow project off the coast of Devon, showing the rotor in the operational position and in the raised servicing position. Source: 2010 Tidal Current Energy Resources in Ireland Report

It has a rated power of 300kW in a current of 2.7m/s and the system is mounted on a 2m diameter monopole driven into the sea bed. The entire installation process was carried out from a jack-up platform. The nacelle houses the bearing assembly, water seals, and the blade pitching mechanism, a two stage epicyclical gearbox and the generator. The nacelle can be jacked clear of the water for maintenance and inspection and so avoids the use of divers. The mechanism used is similar to that on the legs of a jack-up barge. A twin blade rotor is used so that the entire turbine is clear of the water when the nacelle is raised. The generator is not grid connected and the electrical output is dumped into a resistor bank housed in the control room at the pile head clear of the water. The plant has been running frequently for research purposes during the past year and a considerable amount of data has been collected. However, the information has yet to be released. Following discussions with the development team it is reported that performance has exceeded expectations. Early indications suggest that the hydrodynamic conversion efficiency is greater than 40% and is comparable to a modern wind turbine. The team has also developed a mathematical model for optimising the system in terms of size, capacity and cost. It takes into account loading on the various components to predict their size and weight and applies parametric costing information to assess the capital outlay and running costs of the plant. Independent reviews are in close agreement with the overall projected costs and productivity claimed by the development team. The next stage in their development process was the SeaGen research project in 2008. SeaGen is the world's first large scale commercial tidal stream generator and it is four times more powerful than any other tidal stream generator in the world. The first SeaGen generator was installed in Strangford Narrows between Strangford and Portaferry in Northern Ireland in April 2008 and was connected to the grid in July 2008. It generates 1.2 MW for between 18 and 20 hours a day while the tides are forced in and out of Strangford Lough through the Narrows, without causing any pollution, which approximately matches the energy requirements of Portaferry or Strangford villages.

Figure 17 : Location of SeaGen twin turbine in Strangford Narrows. Source: http://en.wikipedia.org/wiki/File:Strangfordloughmap.jpg

The SeaGen generator weighs 300 tonnes and consists of twin axial-flow rotors of 16 metres (52 ft) in diameter, each driving a generator through a gearbox like a hydro-electric or wind turbine. These turbines have a patented feature by which the rotor blades can be pitched through 180 degrees allowing them to operate in both flow directions – on ebb and flood tides. The power units of each system are mounted on arm-like extensions either side of a tubular steel monopile some 3 metres (9.8 ft) in diameter and the arms with the power units can be raised above the surface for safe and easy maintenance access. The power generated by the system is being purchased by Irish energy company, ESB Independent, for its customers in Northern Ireland and the Republic. The turbines 1.2MW capacity generates enough power to meet the average electricity needs of around 1000 homes. The SeaGen turbine cost £12 million.

Figure 18 : The SeaGen twin turbine in the raised position in Strangford Lough. Source: http://en.wikipedia.org/wiki/File:Seagenraised.jpg

OpenHydro - Paimpol-Bréhat 2012 Tidal Farm

OpenHydro is an Irish tidal energy technology company whose business is the design and manufacture of marine turbines for generating renewable energy from tidal streams. The company’s vision is to deploy arrays of tidal turbines under the world's oceans, silently and invisibly generating electricity at no cost to the environment. OpenHydro has achieved a number of industry firsts including being the first to deploy a tidal turbine at the European Marine Energy Centre (EMEC), the first to connect to and generate electricity from tidal streams onto the UK National Grid and the first to successfully demonstrate a method of safely and economically deploying and recovering turbines directly on the seabed. The deployment and recovery method delivers a step change in the economics of tidal energy.[12]

Figure 19 : OpenHydro’s open-centre seabed turbines. Source: http://www.openhydro.com/images.html

The functionality and survivability of equipment in an underwater environment demands simplicity and robustness, which OpenHydro claim their Open-Centre Turbine meets, with its slow-moving rotor and lubricant-free operation minimising risk to marine life. In September 2011 OpenHydro announced that the project to deploy the first of four 16m tidal turbines off the coast of Paimpol-Bréhat is now in the final stages of preparation. This is the first stage of a project which in 2012 will create the world’s largest tidal array generating power onto the French grid. Each turbine has the capacity to generate over 2MW of energy, so this initial array will provide 8MW of energy to the grid, enough for 4000 homes. The first turbine will be towed to the deployment site off the island of Bréhat, near Paimpol in Côtes-d’Armor, where it will be deployed on the seabed at a depth of 35 meters. For two months the turbine and subsea base, which have a combined weight of 850 tonnes and a height of 22 metres, will be tested. The objective of the test is to prepare for the implementation of the world’s largest tidal array which is scheduled for installation in 2012. The Paimpol-Bréhat tidal farm project was initiated by EDF in 2004 and work began in 2008 with the support of local stakeholders. The project is unique in the world and carries a budget in the order of €40 million.

Drawbacks to TISEC

In this section we review some of the drawbacks to tidal in-stream energy conversion, including the restrictions to exploit the resource from the waters around Ireland. Due to the fact that tidal turbines are only now moving into a production phase, there is little data collected and released on the environmental impacts they might be having on marine life. What follows are items raised by the SEAI report on tidal energy, which was published in 2010.[11]

Turbine Spacing – Optimum Array Arrangement Still Being Researched

    • At present research is still required on the spacing of tidal energy devices. Although it is considered best to place devices in a close spaced linear array across the flow as a ‘tidal fence’. It is not always possible to extract the tidal stream resource in a single close spaced ‘tidal fence’ and successive lines of units may have to be considered. However, there is no published data from model experiments to verify predictions and prototype arrays have yet to be built. Although there are significant differences, wind energy is the closest existing technology to that of tidal energy and here tower spacing of five rotor diameters is often used. With tidal streams it is appropriate to consider blockage factor in a line perpendicular to the main flow direction and device spacing in the direction of the flow. Prior to detailed scientific work being undertaken it is proposed that a blockage coefficient of between 10% and 15% be used with an upstream/downstream spacing of 10-20 diameters. Closer longitudinal spacing might be possible if the location of units is staggered on successive lines. It is noted that horizontal axis machines with a diameter of 70% of the mean water depth and a lateral spacing of 5 diameters would give a blockage coefficient of 11%.

Typical Site Installation – Foundations

    • Support structures for tidal energy devices are a vital component in the development of tidal energy – which leads to very costly installation costs. Similar to other offshore structures, the devices will be subject to loadings mainly due to selfweight, wave and current loading. In removing energy from tidal currents, the devices have to withstand horizontal forces which depending on the local maximum current velocity and rotor size may be in the order of 1,000 kN to 3,000 kN. The review of existing technologies in the preceding sections of this chapter has highlighted the main foundation types as being the mono-pile as used with the MCT device and a combination of tripod and gravity structure which has been used by Hammerfest Strøm and the OpenHydro turbines. There are a variety of installation systems including gravity, piled and moored. The most suitable system will be dependent on sea bed conditions and degree of exposure at the site.

Electrical Issues

    • Electrical losses within a tidal current farm are of concern as they will affect its profitability. Onland losses in wind farms are generally low (2% at rated output) as wind farms are normally located close to the grid. These losses can be expected to be greater (around 5% at rated output) for tidal current farms because of the distance from the shore. Losses can be reduced by increasing the voltage at which power is transmitted or by increasing the number of cables. Electrical equipment has effectively two loss components, one which is proportional to load (resistance losses in the copper) and one that is related to voltage (magnetic field losses). The latter component is effectively constant since the voltage does not vary significantly with load.

Wave Climate of Offshore Site

    • In cases where the structure extends above mean sea level, it will be exposed to breaking waves in shallow water and must be capable of withstanding shock pressures. Wave and current forces will also act on the submerged structure from the existing seabed level to the maximum wave crest level. On the west coast of Ireland, waves of over 12 metres occur in the offshore area, with wave directions mainly in the south west and north west sector. There is a high proportion of swell wave activity in this area with swell waves greater than 1 metre for more than 50% of the time. Swell wave periods can exceed 21 seconds with 12-15 second periods occurring quite frequently.

Defined Area Restrictions In Ireland the Department of Communications, Marine and Natural Resources document provides details of area restrictions for the following reasons:

    • Sites outside major shipping lanes

    • Sites outside military zones and restricted areas

    • Sites which do not interfere with existing pipelines and cable

    • Other regions include areas which are used for aggregate extraction and licensed dump sites

Environmental Issues - The preliminary considerations included the following:

    • Benthic ecology - The benthic ecology in the vicinity of the marine current turbine may be affected by the foundations and seabed disturbance associated with installation of devices.

    • Fisheries and spawning grounds - The effect of the structures and related cabling on existing stocks or their food sources. The physical disruption or reduction in available fishing grounds. The impact of structures and cabling on habitats such as spawning gravel for herring.

    • Marine Mammals - There are significant numbers of cetaceans and seals in Irish coastal waters. For example in the Shannon Estuary there is a colony of Bottlenose Dolphins. Vertical and horizontal axis turbine devices could cause problems for marine mammals in terms of collisions and impact on their echo sounding ability.

    • Recreational users - The value of the recreational resource is based strongly on their location in an attractive environment. They may be affected in some way by any visual and audible impact of the turbine. The water based activities may also be adversely affected if access to areas of open water previously available to them is restricted for safety reasons.

    • Migrating and other sea birds - A possible concern is the effect on migratory seabirds and/or coastal colonies. Whilst the above water structures may not interfere greatly with seabirds in flight, the below water structures could prove to be a hazard to diving birds. There is also concern that lighting of the structures at night could affect migrating birds. This may be mitigated against by using suitable lighting.

Economics of tidal energy for Ireland

Despite the potential drawbacks and the current restrictions, we need to review if tidal energy is a feasible source of supplying renewable green energy for Ireland. To do this we need to first analyse what is Ireland’s current energy demand and what is this predicted future requirements. Then we need to determine what amount of energy tidal in-stream could provide now with current technologies and what the expected output from tidal energy will be in the future.

What are Ireland’s energy requirements?

According to the SEAI report 2010, the predicted electricity demand for the Republic of Ireland and Northern Ireland in the year 2010 was estimated to be 42TWh/year. The graph below shows the electricity demand for the Republic of Ireland only, and plots the continuous rise from 1995 levels of ~16TWh to current 2011 demands of 32TWh and predicted future requirements in 2020 ~43TWh.

Figure 20 : Historic and projected electricity demand for ROI. Source: 2010 Tidal Current Energy Resources in Ireland Report

So now let us review what amount of energy TISEC can currently contribute and what amount we can expect in the future.

What amount of energy can TISEC provide Ireland?

The Sustainable Energy Association of Ireland (SEAI) has calculated the theoretical power from the formula:

P(mean) = 1/2 ρ Ks Kn V3 (peak)

Where:

· V3(peak) is the maximum spring tide velocity

· Ks is the velocity shape factor = 0.424,

• Kn is the neap/spring factor (neap 60% of spring) = 0.57,

• ρ is the density of water (t/m3)

They calculated the theoretical resource to be 230TWh/year which represents over 5 times the predicted electricity consumption in Ireland for the year 2010. However, technological limitations, physical, environmental and commercial constraints make it impracticable to extract all of this energy.

When the technological limitations are applied to the theoretical resource, the resultant resource is referred to as the technical resource. See table below for the different resource categories and the estimated energy that can be expected. The table also shows the energy expected in percentage terms of 2010 electrical demand.

Figure 21 : SEAI Energy resource categories. Source: 2010 Tidal Current Energy Resources in Ireland Report

The theoretical resource is the gross energy content of tidal and marine currents within the 12 mile zone around Ireland, and has been calculated by using computational models and model verification.

The technical resource has been calculated for areas where the peak tidal velocity is greater than 2.0m/s and is equal to 10.46TWh/yr which represents approximately 25% of the predicted electrical consumption for the year 2010.

The practical resource was calculated for sites with water depth between 20m and 40m and with a peak tidal velocity greater than 1.5m/s. The calculated resource was 2.633 TWh/year and represents approximately 6.27% of the predicted electrical consumption for the year 2010. However, depending on the rate of technological development this level of energy extraction is not expected to be available before 2010 and possibly not before 2015.

Due to the fact that environmental impact statements would be required for each site, the accessible resource has not been adjusted for the environmental constraints. The accessible resource is therefore the same as the practical resource, i.e. 2.633 TWh/y representing 6.27% of Irelands’ predicted electricity consumption in 2010.

In order to calculate the viable resource, detailed computational hydraulic models were produced and suitable turbine arrays were selected for the 7 sites for which detailed modelling was carried out in determining the practical resource. The viable resource for all sites identified was calculated as being the practicable resource at 2.0m/s, since energy extraction at lower velocities is not considered to be economical with first generation devices. The viable tidal energy resource for Ireland has been estimated as 0.915 TWh/yr and represents 2.18% of whole of Ireland’s electricity consumption for 2010.

Figure 22 : SEAI viable energy resource. Source 2010 Tidal Current Energy Resources in Ireland Report

So at present with current technological constraints the amount of viable energy that can be extracted around Ireland’s coastlines is 0.92TWh/yr, and based on the 2010 electrical consumption of 42TWh, the viable energy makes just 2.18% of a contribution. We need to focus on the technical resource category, which could yield as much as 25% of the 2010 electrical consumption.

The fact that TISEC technology is only starting to come on stream, (The Strangford Lough SeaGen project was officially plugged into the gird on July 17 2008, making it the first commercial-scale underwater device to feed into an electricity network) it is expected that advances in the technology will make it possible to deploy devices at depths greater than 50 metres, and design new, more efficient devices capable of generating cost effective energy from currents less than 2.0m/s. This is predicted to be the case by 2020. Consequently another issue that exists for emerging technologies is that the costs are front loaded. Costs will naturally decline with advancements in installation techniques, device prototyping and testing will be replaced with steady manufacture and installation. In the next section we will review the costs associated in more detail.

What are the costs associated?

Again owing to the fact that the technology is only emerging for Ireland, the SEAI 2010 report estimate the costs associated by extensive modelling of the various sites. In this section we will review these projected costs.

Foundation costs will vary significantly depending on seabed conditions, water depth and site location. As water depth increases there is a potential increase in hydrodynamic forces on the structure and hence an increase in the structural bracing requirements. Steel foundation costs are more difficult to predict for water depths greater than 30m, with environmental loads becoming more variable with depth and the whole concept of the foundation changes in respect of bracing elements, configuration and installation methods. Typical foundation and structure costs are expected to be in the order of €300,000 – €500,000 per system.

Grid costs as with foundation costs depend on the location of the site and distance from nearest grid. Indicative costs for a site located 10km from the shore are shown in the next figure.

Figure 23 : SEAI estimated grid connection costs for various sites. Source 2010 Tidal Current Energy Resources in Ireland Report

It has been assumed that the land connections can be made by a 20 kV to 38 kV line and that no underground cabling is required apart from 1 km from the sea to a connection point on land. The turbine farm to land undersea cable will be rated at 11 kV to 38 kV and will be buried. The interconnection between turbines and associated node will be at 1.1kV and will be buried. Generation will generally be at 1.1 kV and will be grouped around nodes where transformation to a suitable higher voltage will take place. No consideration has been given to the costs of obtaining permission for new power lines. Whilst costing has been indicative only it does illustrate the variation between sites. It is evident that a detailed study will be required for each site before more accurate costs for the electrical connection can be established.

The costs associated with operation & maintenance (O&M) of tidal energy devices are expected to be in the order of 2%-5% of the total energy costs and will depend on the number of devices in the tidal farm. Reasons for such high O&M costs include:

Ø Access to the turbines will be restricted to calm periods only. It is unlikely that transfers from access craft to structures will be carried out in conditions exceeding 1.5m significant wave height.

Ø Replacement of damaged or failed components will necessitate the use of a floating or integral crane or jack-up platform. This will be an operation which may also be prone to weather delays.

Ø O&M costs will be associated primarily with the turbines but some of these costs will be associated with the foundations and cabling which require inspection at regular intervals.

The model was then optimised for each site by selecting a rated velocity for the turbines which minimises the cost of generating electricity. A compromise is always needed between a system that is oversized and hence too costly and a system that is undersized and incapable of delivering sufficient energy hence the model can find the rated velocity and power which results in the most cost-effective solution.

The energy captured was then calculated and reduced to allow for 90% machine availability to arrive at the actual energy capture per unit, and hence per site.

Finally the capital cost of each project is given together with the average cost of generation. Capital investment varies from €1700/kW installed in the Shannon estuary to €3700/kW at the Codling Bank and the unit generating costs corresponding with these figures are in the 10 cent/kWh to 20 cent/kWh range.

Although these costs are high and as mentioned earlier they should be considered in the context of the following:

Ø This is a new technology at an early stage of development. Wind turbine generated electricity today costs approximately 25% in real terms of its cost when first generated some 20 years ago. It is expected that costs will reduce to somewhere in the region of 50% to 75% of today’s costs as technology improves and the benefits from increased scale of manufacture are realised.

Ø Most of the proposed projects are quite small, less than 10MW, and all energy technologies cost more when deployed on a small scale. This is particularly true for marine technologies where relatively large overhead costs apply.

Ø The projected costs are all inclusive. They cover the anticipated infrastructure costs and overheads as well as the pure technology costs. Many energy technologies are promoted on the basis of the system costs but without overheads such as connection costs.

Ø The technology considered here is “first generation” and is constrained by as yet undeveloped installation systems. It is expected that if this technology proves to be effective, then more cost-effective “second generation” technology will evolve which may be suitable for use in deeper water and on a larger scale. This could provide access to resources which are not available at this time.

Ø The unit energy costs apply for the period of 20 years over which the capital cost is amortised. An installation, especially in a sheltered location such as the Shannon Estuary, has the capability of being operated for much longer (albeit with replacement of major drive train components every ten years). The ESB are currently charging domestic consumers 14.76 cents per KW, and from the SEAI estimates the first 20 years of the tidal site investment, the generating costs range between 10-20 cents per KW.

Once the capital cost is written off, the generating costs will fall to a much lower level. In this respect the technology is analogous to hydro power where initial generating costs tend to be relatively high, but the long life of these systems permits attractively low cost operation in later years.

Benchmark Ireland against other nations in TISEC

This year REN21 published its latest Renewables Global Status Report.[13] It states that ocean energy is the least mature of the technologies, but that interest is growing in a wide range of possible technologies, including wave, tidal (barrages and turbines), osmotic power, and ocean thermal energy conversion (OTEC) systems. The 240MW La Rance tidal barrage began generating power off the French coast in 1966 and continues to produce about 600 GWh annually. Additional tidal projects came on line since then in Canada, Russia, and China, with an estimated 262 MW of capacity in operation by 2001. Otherwise, ocean energy saw little further development until recently. By the end of 2010, only tidal barrage systems had achieved commercial scale, and they accounted for most of the world’s installed ocean energy capacity. However, in 2010 there were a handful of pre-commercial projects generating power with a range of technologies. Although existing capacity remained low relative to other renewable technologies, numerous projects were in development or under contract, and at least 25 countries were involved in ocean energy development activities. At year’s end, an estimated total of 6MW of wave (2MW) and tidal stream (4MW) capacity had been installed by the 18 member countries of the International Energy Agency (IEA) Implementing Agreement on Ocean Energy Systems. However it was in Ireland that the world’s first commercial-scale tidal turbine (SeaGen 1.2 MW) passed the milestone of providing 2GWh of electricity to Northern Ireland and Republic of Ireland from the waters off Northern Ireland in Strangford Lough. The report also states that ocean energy continues to be at an immature stage of development, managing just $40 million of asset financing. However, ambitious plans emerged during 2010 for multi-MW projects off the coasts of countries such as the U.K. and Portugal. From an article in the tidal today website[14], that in the race to deploy tidal stream technology, there are three clear front-runners: UK/Ireland, Canada, and the US. The reason being they have key demonstration projects, the first array developments planned, and relatively good policy support. In the UK, utilities like SSE Renewables and Scottish Power Renewables have signed lease agreements to develop sites in the Pentland Firth, which “represent potential for up to 600 MW of capacity and a step change in the industry”, according to Frank Wright, Renewables Manager at analyst Douglas Westwood (DW). In 2010, The Crown Estate in the UK awarded licence rights to OpenHydro, in conjunction with SSE Renewables, to develop a major 200MW tidal farm in the Pentland Firth.[15] Scottish Power’s 10MW Sound of Islay tidal array project has been given the go ahead by the Scottish Government. And in other UK industry developments, £6.4m in funding from the European Regional Development Fund was confirmed for Tidal Energy Limited (TEL) to manufacture its 1.2MW DeltaStream device for deployment in Wales next year. Most of these projects were in Europe, with the majority operating off the coasts of Portugal and the United Kingdom for short-term testing and demonstration, and a few prototypes were initiating first steps toward commercialization. The next two nations leading the charge is the US and Canada. Meanwhile developments further afield, Canada and the US have stepped up their efforts to exploit their significant tidal regions in areas such as the Bay of Fundy and Vancouver Inlets, with companies like Ocean Renewable Power progressing numerous projects. However the most significant tidal stream activity is in South Korea’s Jeollanam-do region, where Voith Hydro and local partner Renetec are working on the Seaturtle Tidal Park, which will be 150MW initially, but could scale up to 600MW. Meantime, in June, Hyundai Heavy Industries’ also confirmed it had completed site trials for a 500kW tidal current system in the same region. So thanks to OpenHydro, Ireland is being mentioned amongst the other countries who are leading the way with making TISEC commercially viable. Hopefully it will not be too long before OpenHydro or another TISEC company set about installing in-stream tidal farms that will supply the Republic of Ireland electricity grid directly.

Conclusions

Despite the wave of activity, just 4MW of tidal stream capacity was installed globally by end 2010, so there is a hefty mountain to climb if commercial reality is to be achieved.[14] In the World Wave & Tidal Market Report, 2011-2015, Douglas Westwood forecasts that 129 tidal devices will be installed worldwide over the next five years, with 64 of them in the UK, 25 in Canada and 18 in the US. In total though they come to just 90MW – this compares to a forecast 11,000MW for offshore wind in the same period.

At best, says Wright (Renewables Manager at Douglas Westwood), “it will be a decade before any significant commercial success for tidal. Finance is the major stumbling block. Most technologies in development are the work of small independent companies that lack the means to finance testing and technology scale-up. Financing device development from concept to full-scale prototype can cost £25m-£45m. So R&D funding and other grants continue to be essential,” he says.

“With public finances constrained at present, it will be extremely challenging to maintain, let alone increase funding for tidal energy in the short to mid-term,” Wright warns. That’s been seen clearly in the UK and is one of the reasons the country’s Energy and Climate Change Committee has just set up a new inquiry to assess the success of existing Government support on the sector and investigate the potential impact of Government spending decisions.

While there has been some new funding for tidal, such as ERDF’s support for DeltaStream, typically these can only be secured with some element of matching private investment. This “chicken and egg” situation is common in the sector globally and needs to be resolved urgently. The problem is compounded by the fact that in general “private investors consider the risk levels to be excessively high, a trend exacerbated by the financial crisis”.

So “more thought needs to be given to joining up funding streams to move technology from prototype to full commercialization,” stresses Wright. Stronger incentive mechanisms, such as Nova Scotia’s feed-in tariff, are the answer, he says, “FITs have proved extremely effective in stimulating renewable energy in Europe. Where appropriate resource exists, tidal energy projects can be installed commercially given adequate financial incentives to make them viable”.

OpenHydro believe the ultimate renewable solution will depend on a blend of renewable technologies such as tidal, wind, wave and solar working together, and that tidal will play a significant part in this mix given the predictable nature of the energy and the size of the resource.

Recapping the benefits of tidal energy include:

Ø Energy is produced from a resource that is entirely predictable.

Ø Tidal turbines are located beneath the ocean surface and cannot be seen or heard.

Ø Water is 830 times denser than air meaning that, for a given electricity output, tidal turbines can be much smaller than equivalent wind turbines.

Figure 24 : OpenHydro’s comparative view of tidal energy versus other renewable technologies. Source: http://www.openhydro.com/images.html

Commercial application of ocean energy technologies remains limited, and the relative immaturity of tidal energy technology has prompted a wide range of exploratory R&D activities, with various devices racing to reach commercial readiness. In the past four years, horizontal axis turbines have been launched by several companies in Europe and North America, and an “oscillating hydrofoil” prototype was tested in 2009. Vertical axis (or cross-axis) turbines have reached the operational testing phase. These developments have been strongly backed by public R&D funding, particularly in Europe, North America, and South Korea. Governments in these regions are contributing grants and funding, typically in the range of $10–100 million per project or program, for testing facilities, demonstration projects, and basic research.

On Irelands’s shores we have the first TISEC device to be connected to the national grid, which has the ability to supply approximately 7GWh of electricity each year. Taking the average domestic electricity consumption as 5400KWh per year, would mean that from this one, 1.2MW TISEC twin-rotor turbine, 1300 homes would be supplied with its electricity requirements. This demonstrates the potential energy that exists in our seas/oceans that have consistency of supply, zero emissions and no visible or audible effect on the public. The figure below shows how that in a 24hr period or each day the SeaGen turbine in Strangford Lough is capable of exporting 20.21MWh of electricity to the grid. Multiplied by 365 days in a year gives us the 7GWh of electricity available from the tides ebb and flow in Strangford Lough.

Figure 25 : SeaGen exported power in 24hr ~20.21MWh. Source: MCT’s SeaGen Tidal Presentation 2011.

It is difficult to determine the exact costs per KWh that is to be expected with TISEC. The SEAI report on tidal energy 2010 estimated that the cost to generate 1KWh was in the region of €0.20-€0.10. For comparative purposes to other renewable technologies we will convert this to USD. So the estimate becomes $0.26-$0.13 per KWh.

MCT have published a small amount of data on their expected evolution of generating costs.[16] From the following figure we deduce that the range is £0.23-£0.05, which in USD becomes $0.36-$0.08.

Figure 26 : MCT’s planned projects with projected evolution of generating costs per KWh. Source: MCT’s SeaGen Tidal Presentation 2011

So taking the figures from the only TISEC device connected to the grid, the cost per KWh will initially be approx. $0.36, and as the technology advances the prediction is to get this cost down to $0.08. When TISEC generated costs get to this level it will place it as one of the cheaper sources of renewable technologies. The figure below is from the REN21 2011 report, which details the typical energy costs associated with the different renewable technologies.

Figure 27 : Status of Renewable energy Technologies: Characteristics and Costs. Source: Renewables 2011 Global Status Report

In general, TISEC technologies remain in an emerging phase of development. So while the sector maybe 15–25 years behind wind energy, it is poised to follow a similar path. It is expected however, that in the next 5 years the technology will take another significant leap in terms of cost efficiency in extracting the energy. This is when Ireland should reap the benefits of being an island nation and let our coastal waters power our homes.