Tidal energy generation in the waters of Alderney

June 1st, 2007 by Robin Le Page

Robin Le Page is a Guernsey Civil Engineer who wrote this paper in collaboration with Alderney Renewable Energy Ltd.

The paper was published by Loughborough University in May 2007.



In the UK the Renewable Energy Obligation will oblige utilities to source at least 10 % of their electricity from renewable sources by 2010. However land based renewable energy technologies are already facing issues owing to land use conflicts. Tidal stream energy generation is a technology which has been developing rapidly over the last 10 years and could provide an efficient source of renewable energy in the future, if potential sites are identified and developed.

The aim of this study was to determine whether the tidal streams in the waters of Alderney in the Channel Islands have the potential to produce energy viably. The method used was based upon the analysis of in-situ Acoustic Doppler Current Profiler (ADCP) data collected during 2006 from two sites in the waters of Alderney, the Swinge (North of Alderney) and the Race (between Alderney and Normandy). An ADCP was deployed at the Swinge site for 9 days, 31st August – 9th September and simultaneously at the Race for 43 days, 31st August – 12th October. The results showed that the mean spring tidal velocity at both the sites exceeded 3 m/s, while the maximum turbine diameter theoretically deployable at the Swinge site was 14 m and that of the Race site 22 m, thus a single turbine would be capable of producing 1.8 GWh and 4.4 GWh at rated capacities of 0.9 MW and 2.8 MW respectively.

1.  Introduction

Interest in the development of clean renewable energy sources is now higher than ever before. This is being driven by a variety of factors. In the last 150 years, since the industrial revolution, atmospheric CO2 concentrations have risen as much as in the previous 20,000 years [1]. Increasing awareness of issues related to the burning of fossil fuels is a driving factor in the use of renewable energies [2].

Both the UK government and the EU have committed themselves to meeting internationally negotiated targets to combat global warming. In the UK the Renewable Energy Obligation will oblige utilities to source at least 10% of their electricity from renewable sources by 2010. In order to achieve these, large scale increases in electricity generation from renewables, resources will be required and must be developed more ‘aggressively’ [3,4].

There are various solutions to the over use of fossil fuels. The alternatives include the deployment on a large scale of land based renewables such as solar and wind power or the ‘revival’ of nuclear energy [5]. However land based renewable energy technologies are ‘already’ facing constraints owing to continual conflicts over land use, including constrictive planning regulations and policies [4,6]. Land based renewables, are difficult to impose upon a reluctant public due to the numerous perceived negative aspects, and in any case ‘such solutions seem likely to be inadequate’, with the low efficiency of photovoltaic cells and the large land areas that wind turbine farms necessitate [5].

It is, therefore, particularly timely to seek truly sustainable methods of power generation which neither take up significant space on shore nor cause significant pollution or other environmental impacts. The need is obvious and “failure in this field could drive our society towards much more risky and dangerous solutions” [5]. An important factor is that the seas offer large open spaces where future new energy technologies can be deployed on a large scale, ‘perhaps’ with considerably less impact on the environment or other human activities [4].

Therefore with the necessity both in political and environmental terms to develop new technologies to produce renewable energy, the purpose of this study was to investigate the potential of tidal stream energy technologies and promising sites; specifically the island of Alderney, which has been recognised by the energy industry as one of the best potential tidal power locations, better than most of the UK and all of the other Channel Islands [7]. The aim of this study was to determine whether the tidal streams in the waters of Alderney have the potential to produce viable energy. This was achieved through the analysis of current velocity data obtained using ADCPs at two sites, the Swinge and the Race of Alderney (Figure 2).

2.  The Tidal Stream Energy Resource

The most powerful tidal currents exist in certain locations where the sea flows are ‘concentrated’. As the tidal wave approaches land, it is amplified both in tidal range and current speed. This occurs where waters are channelled through constraining topography, such as straits between islands, shallows between open seas and around the tips of headlands [3,4,8,9].

There has been considerable interest in recent years in the exploitation of tides and tidal currents in coastal areas to generate electricity; [10,11].

2.1  Extracting energy from tidal streams

The harnessing of the energy in a tidal flow requires the conversion of kinetic energy in a moving fluid, into the motion of a mechanical system, which can then drive a generator. This energy conversion is analogous with that of wind turbines [12,13]. Energy generation from tidal currents requires the placement of underwater turbines in locations with high tidal current velocities. The flow of water across the turbine blades causes the blades to rotate, generating electricity in proportion to the velocity of the tidal current and the characteristics of turbine [8]. The root of most feasible technology is the hydrofoil.

Most devices can be characterised into four fundamental types; horizontal axis systems, vertical axis systems, variable foil systems and venturi based systems. In horizontal axis turbines the root of the hydrofoil is mounted such that the blade is at right angles to the axis of rotation. In vertical axis turbines, the blades are mounted parallel to the axis of rotation, such as a Darrieus turbine. Variable hydrofoils involve the hydrofoils moving up and down similar to the motion of a dolphin’s tail. Venturi based systems use pressure changes in a flow contraction to drive secondary hydraulics or pneumatics [9,13].

Numerous projects currently exist, with the aim of developing a cost effective technology which can deliver useful energy efficiently. Many companies [14-17] have produced and tested prototypes of varying power ratings, from 10 kW – 300 kW, though this is continually changing with new projects emerging frequently due to the resource being freely available to all. A number of commercially available solutions are expected to be available within the next 3 years, capable of producing up to 1 MW.

Figure 1 - An artist's impression of an array of Marine Current Turbine Ltd's SeaGen turbines. A prototype has been installed in Strangford Narrows, Northern Island (click image to expand - image courtesy of MCT)

2.2  Site Selection Criteria

The key criteria for selecting a site for tidal stream energy generation are numerous: (1) 2 – 3 m/s minimum spring peak current (4-6 knots); in order to achieve an economic energy production; (2) a relatively uniform seabed, to minimise both turbulence and the loss of velocity near the seabed; (3) uniform flow with strong currents for long periods to maximise power available; (4) minimum depth 15 m, to provide adequate space for a rotor; (5) maximum depth 40 m, to remain within capability of existing jack-up barges, for installation purposes; and preferably such conditions extending over a wide enough area to permit the installation of a large enough array of turbines to make the project cost effective; (6) close to the coast, to keep cabling costs down; (7) no major conflicts with other sea users; (8) avoiding overly sensitive environmental sites [4].

Economic analysis of the technical concepts being developed by Marine Current Turbines Limited [20] has suggested that a minimum of approximately 2 m/s mean spring peak is needed and ideally 2.5 m/s or faster (i.e. 4-5 knots) [4].

2.3  Potential sites and recent assessments

A number of official studies have been carried out over the last 15 years, [8,10,18,19] some encompassing the whole of Europe and the UK, others specific sites. Additionally many academic papers [20,21] have been published on the subject, all of which include examination of promising areas for development.

The English Channel is a frequently discussed location, [3,9] some particularly energetic sites include the Alderney Race and the Big Russell [4,22]. The Pentland Firth is one of the most promising locations in the world for future development. Equally the Orkney and Shetland Islands contain a number of potential sites [2,4,9,13,22].

Many of the more specific and ‘accurate’ resource assessments have been conducted within the last couple of years. Advances in technology and development of prototype tidal stream generators, have improved understanding of the way in which the resource will be exploited and the efficiencies which can be achieved. Additionally reliable velocity data has been quite limited, in the past. Until recently it was very difficult to measure very strong currents at sea using mechanical flow meters, but newly developed technology such as Acoustic Doppler Current Profilers (ADCP) will enable rapid improvement of our knowledge and understanding of this resource. A boat-mounted ADCP can gather more accurate and detailed velocity profiles in a few hours compared with what would have taken months using traditional equipment. Computer-based flow modelling also offers a valuable relatively new tool for identifying promising sites and rapidly assessing their energy potential [12].

One of the more recent studies was conducted by Black and Veatch in 2004-05 [18], and included an assessment of the UK tidal resource, as well as identification of specific sites, of the top fifteen most ‘energy rich’ sites, three were in Alderney’s waters. Thus Alderney is undoubtedly worth investigating.

(click map to expand)”] (click map to expand)”]

3.  Methodology

The primary aim of this study was to determine the potential of the areas of water around Alderney to provide an economically viable source of energy, using precise ADCP velocity measurements.

3.1  Basic principles of an ADCP

An Acoustic Doppler Current Profiler (ADCP) as its name indicates measures the velocity of water using the Doppler shift principle, which states that if a source of sound is moving relative to the receiver, the frequency of the sound at the receiver is shifted from transmit frequency [23].
The transducers of an ADCP generate pulses of sound at a given frequency along a narrow beam of sound. As the sound travels through the water, it is reflected in all directions by particulate matter (e.g., sediment, biological matter) but some portion of the reflected energy travels back along the transducer axis toward the transducer where the processing electronics measure the backscattered frequency, and hence the Doppler shift. The Doppler shift measured by a single transducer thus quantifies the velocity of the water along the axis of the acoustic beam. By measuring the return signal at different times following the transmit pulse, the ADCP measures the profile of water velocity at many distances from the transducer. The profile of water velocity is thus divided into range cells, or bins, where each cell represents the average of the return signal for a given period of time [24].

3.2  Instrumentation used and site setup

The two ADCPs used in this study were both the RDCP (Recording Doppler Current Profiler) 600 manufactured by Aanderaa Data Instruments (AADI), Norway. At each site the ADCP was deployed in its self-contained mode looking upwards from the seabed. Deployment was similar to that of laying lobster pots with the RDCP being secured to two 30 m back lines anchored at each end with bundle chain weights. The buoy lines from the chain weights were 40 m in length and were marked on each end with a float.

3.3  The Race of Alderney

The Race of Alderney is the strait between the Channel Island of Alderney and Cap de la Hague (Figure 2) on the West coast of France (49°44’N, 1°56’N). The Race is approximately 4 miles wide and lies between Race Rock (49°42’N, 2°08’W) and a rocky bank with a least depth of 17 m over it, which lies approximately 3.5 miles WSW of Cap de la Hague. The Race derives its name from the “great rates attained by the tidal streams through it” [25]. The maximum and average depths of the Race are 46 m and 40.1 m respectively [18,26]. The instrument was located at position A in Figure 2, with the recording interval set at 30 minutes, with 25 cells, each of 2 m length, with a cell overlap of 50 %. .

The Race flows in a north-easterly direction for a period of 6 hours commencing at 6 hours before Dover High Water (DHW). At 1 hour before DHW it changes direction to flow south-west for approximately 6 hours. The times at which tidal streams begin to run in different parts of the Race do not vary appreciably, however the current velocities are subject to considerable variation. The strongest streams are found on the east side of the Race [25]. Peak flow velocities across the Race are between 3.5 m/s and 5 m/s depending on whether the flood or ebb is in question, and where in the Race is being considered [18,25-28].

3.4  The Swinge

The Swinge is a channel, ranging in width from 1.4 miles (2.2 km) to 1.1 miles (1.7 km), which leads between Burhou (49°44’N, 2°15’W) and the North-West coast of Alderney, about 1.25 miles South-East [25]. The Swinge floods and ebbs at similar times and in similar directions to the Race. The Swinge has an average depth of 15 m [27]. The instrument was located at position B in Figure 2, with a setup similar to that of the Race, though 15 cells due to the shallower depth.
The mean spring rate of the North going stream is 1.1 m/s – 2.8m/s, and the mean neap rate is between 0.4 m/s – 1.1 m/s. While the South going stream has a maximum mean spring rate of 3.5 m/s, and a maximum mean neap rate of 1.4 m/s. The greatest flow rates within the Swinge occur near the central channel [28].

3.5  Power output and energy calculations

With knowledge of the current velocities that occur at these two sites, and their variation with time it will be possible to calculate power outputs that are achievable, as well as energy generation over a set time period. Moving water carries kinetic energy. The energy per second intercepted by a device of frontal area A0 (m²) in a current of speed U (m/s) is given by:
P(t) = (1/2)ρA₀U³(t),      (1)

where ρ is the water density (kg/m³). However the power that can be converted to a useable mechanical form is limited for a device in open water flow, due to losses to:

PT(t) = (1/2)CΡρA₀U³     (2)

where Cρ is the power coefficient. Cρ is essentially the percentage of power that can be extracted from the fluid stream (efficiency) and takes into account losses due to Betz’ law and those assigned to the internal mechanisms within the converter/turbine. The value of Cρ for a turbine in a flow of incompressible fluid is limited to a maximum theoretical value of 59 %. Cρ for a real device is generally a function of the ratio between the speed of the turbine tip and the flow speed, which is commonly known as the tip speed ratio. Cρ will be in the range 0.35 – 0.5 for marine current turbines [21,29].
A design Cρ of 37.5 % was used throughout the power calculations contained within this study on the basis that this will provide a conservative value.
The total available annual energy yields of the two sites were determined by investigating the variation of power output with time. Energy production was assessed on a fortnightly, monthly and annual basis by considering the power outputs computed using equation (2). By assuming a tidal cycle of 6 hours duration and with knowledge of the peak power output for each cycle, the energy produced over each 6 hour period was calculated and totalled to give the energy generated during a fortnight period.

(click graph to expand)

4.  Data Analysis

4.1  Quality check parameters

There are a number of factors that can contribute to poor quality data, or data that does not meet certain ‘quality requirements’, such as an excessive volume of suspended sediment, side lobe interference and waves at the water’s surface. The quality and reliability of the data obtained using the two ADCPs was investigated before any data analysis took place.

There are some parameters that will especially reveal good or adequate measurements. These parameters are: signal strength, single ping standard deviation, heading, pitch and roll, and battery voltage. Aanderaa data instruments who manufacture the RDCP 600 state a number of recommendations in the product literature [30] which are summarised as follows:

(1) Signal Strength: > -45dB;

(2) Single Ping Std: < 20 cm/s;

(3) Heading/Pitch/Roll: rapid changes implies that the measurement volume has changed during the recording interval, and this is not ideal;

(4) Battery voltage: the battery voltage is OK if Ping Count reading equals the configured ping count.

Other than these parameters the data was also checked using a thorough comparison with secondary data. The most widely available is that produced by the Admiralty (UK Hydrographic Office) and illustrated in their Tidal Stream Atlas [28].

This is published in the form of charts, which illustrate mean spring and neap tidal stream velocities, at various specific locations. With knowledge of the mean tidal range at Dover for each day the current velocity for each hour can be calculated due the linear relationship between them. This however illustrates surface current rates (0 – 5 m), as it was collected using pole log-ship observations, however it does provide a useful basis with which to illustrate times of high/low current velocities. This data was also used to predict current velocities where the ADCP did not meet the necessary quality check parameters. A straight comparison of ‘quality’ ADCP data against tidal atlas data for the same time period allowed mathematical relationships between the two data sets to be determined and thus these trends were then used to forecast data for the ADCP that had to be discarded.

4.2  Data quality

The weights used to anchor the instruments were inadequate to hold the ADCPs in position during the increased flow rates that occurred during the first spring tide of the testing period (9th August). This resulted in the instruments shifting position and therefore problems with the measurements resulted. Thus it was necessary to discard the data that did not meet the quality check parameters, and was unusable and try to ‘salvage’ as much data as possible from the remaining measurements.

4.3  Results

It was necessary to discard over 50 % of both sites ADCP data, however on comparison with the Admiralty data [28], the trends were clear (Figure 4), which allowed a relationship to be derived and thus prediction of the discarded ADCP data was logical, accurate and reliable.

Figure 4. Comparison of ADCP data and Admiralty Tidal Atlas data sets for the Swinge site (click graph to expand)

The ADCP data and predicted data showed that the mean spring tidal velocity in both the Swinge and the Race exceeded 3 m/s, while the mean neap velocity at both sites was between 1 – 1.2 m/s. There was also considerable variation between the flood and ebb of the tide as illustrated by Figure 4. These velocities compare well with the UK Atlas for Offshore Renewable Energy, [19] which illustrates a peak flow in the region of 3.5 m/s for both sites, the Environmental Change Institute’s UK marine resource assessment study [8], which rated the peak velocity in the Race as 4 m/s, and Black and Veatch’s UK Tidal Stream Resource Assessment [18] which stated a velocity of 4.4 m/s at spring tide for the Race. Some of these exceed the ADCP data however this is due to the locations which these values consider being in the east Race, which contains faster flow rates than those of the west.

5.  Conclusions

5.1  Current velocities

Therefore both the Swinge and the Race which have a mean spring peak tidal current of greater than 2.5 m/s as shown by both the primary (ADCP) and secondary (Admiralty) data sets and are entirely suitable sites for tidal stream energy generation, not only due to their high current velocities, but also due to a number of other key site criteria as detailed in 2.2 being met. At both sites the minimum depth of 15 m is exceeded, while the maximum depth of 40 m is not. Proximity to land is another benefit, the north coast of Alderney in the case of the Swinge, and the coasts of Alderney and Normandy in the case of the Race. Additionally no major conflicts will exist with other sea users, due to the dangerous nature of both passages [25].

5.2  Power output

The peak power output of a 14 m diameter turbine in the Swinge would be 0.9 MW, while a 22 m diameter turbine installed in the Race would produce 2.8 MW. It is important from a realistic viewpoint to consider the theoretical power ratings of some of the more well developed technologies, such as; Stingray [16] (150 kW), OpenHydro [17] (250kW), Seasnail [15] (750 kW), and Seagen [14] (1 MW). An array of smaller turbines, of approximately 6 – 14 m diameter (250 kW – 1 MW power rated) is far more realistic in the short term than 20+ m diameter turbines rated at 3 – 4 MW. Table 1 below includes the maximum power outputs for a variety of turbine sizes, as well as the energy that would be generated by them over the duration of a year. Conversely it is important from a realistic viewpoint to note that the power output during a neap tide is in the region of 30 % of that at a spring tide, i.e. the rate of energy generation is not constant.

(click table to expand)

5.3  Energy generation

The power produced by a 14 m diameter turbine located in the Swinge would generate 75 MWh of energy per fortnight, which equates to approximately 1800 MWh per year. The average domestic household uses between 4.1 and 6.3 MWh of electricity per year. Thus the turbine (14 m) would be capable of meeting the electricity needs of between 280 and 440 domestic homes. The energy generation of a 22 m diameter turbine located at the Race site for a month period, is approximately 370 MWh, which equates to 4440 MWh/year. Thus the turbine would generate enough electricity to power almost 1000 homes, equivalent to the energy needs of the entire population of Alderney.


The author would like to thank the following people and organisations for their help throughout the course of this project: Alderney Renewable Energy for providing the ADCP data and their continual support, the UK Hydrographic Office for their swift correspondence and permission to use numerous charts and other Admiralty publications, Aanderaa Data Instruments for supplying the instrumentation literature and a copy of RDCP studio, Guernsey Harbour and Harbour Master for their help, Prof. Peter Fraenkel of Marine Current Turbines for his assistance, Guernsey Electricity for supplying information, and Prof. Koji Shiono for his guidance.


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