Strategic Tidal Stream Assessment for Alderney


Title: Strategic Tidal Stream Assessment for Alderney
Authors: Craig, J.
Publication Date:
January 01, 2008
Document Number: ED 43120001
Pages: 69
Technology Type:

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Attachment: Access File
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Craig, J. (2008). Strategic Tidal Stream Assessment for Alderney. Report by Alderney Commission for Renewable Energy. pp 69.

The Alderney Commission for Renewable Energy (ACRE) is a body set up by the States of Alderney to license, control and regulate all forms of renewable energy within the island of Alderney and its Territorial Waters. ACRE commissioned AEA to prepare a strategic assessment of the impact on the island and its community of tidal and/or wave energy development within the Territorial Waters. This report examines the technical implications of using these technologies and the environmental and socio-economic impacts of such a development.


This strategic assessment is based on an open centred, bottom mounted tidal stream generator developed by OpenHydro. Although the focus of this assessment is based on OpenHydro's technology other possible tidal stream and wave power technologies have been reviewed. It was not possible to cover all of these technologies in the same level of detail as OpenHydro's technology, but the main differences and similarities have been highlighted.


The report is intended to inform ACRE of the key issues and implications of tidal energy development, particularly at a large scale. It considers the impact of the technology based on two very large hypothetical developments within the Territorial Waters, one of 1GW and a larger one of 3GW, both located in the Alderney Race. The report will enable ACRE to evaluate the overall cost/benefit of the full scale development and to ensure that necessary controls and conditions are in place before any long term licence is granted.


Strategic Techno-economic impacts


Most tidal stream device concepts are based on horizontal axis turbines although some vertical axis devices and even oscillating hydrofoil concepts have been proposed. Devices would be secured to the sea floor by piles, bolted or rely on gravity. Some devices are bottom mounted and operate close to the sea floor. Deployment and retrieval would require cable connection from a surface vessel to either lower or raise the device to the surface before transfer to or from a shore base. OpenHydro‟s device and at least one other rival concept use this principal. Other devices rely on a surface piercing pile foundation to raise or lower them from or into the sea before detachment. A further permutation is a floating device anchored to the sea floor via cables.


The projected capital and operating costs and energy capture are highly uncertain as these technologies are still in an early stage of development. Estimates should therefore be treated with some caution. Few devices have advanced to a prototype stage and only limited trials have been achieved in the open sea. OpenHydro has been testing a small test device for 12 months, but performance data have not been released.


The most logical export market for large scale tidal energy development is France. Exporting power to the UK would require a 100 km DC link, four times the distance to France.


Wave Energy


The economics of wave energy devices depend strongly on the annual average power density at the chosen deployment site. The capital and operating costs of a facility are fixed but the revenue depends on the energy input from the waves, i.e. on the power density.


Wave power density is around 10-15 kW/m which is too low for the technology to operate profitably. A 10 km array might be able to generate between 438 and 657 GWh/year.


In conclusion, Alderney's wave energy resource is much smaller than its tidal resource, although it would be large enough to supply around half of the Channel Islands needs, provided that the devices could be operated profitably. However, the annual average power levels are too low to enable the current generation of wave energy converters, and possibly the next too, to operate profitably.


Strategic Environmental impacts


A 1GW array could potentially reduce CO2 emissions by between 360,000 te(CO2) and 700,000 te(CO2) per year), through displaced fossil fuel generation.


Reviews of environmental issues have identified a number of potential impacts that could arise during the installation and operation of tidal stream devices and cable laying. These impacts include disturbance to sediment, noise and vibration linked to drilling, adverse effects to fish caused by Electro Magnetic Forces (EMF) and collision risk with marine mammals. Potential changes in tidal currents in the vicinity of devices may change sediment dynamics and the local benthic ecology. There may also be indirect effects on sea birds, if for example, their prey was disturbed. However, there is a lack of evidence on the scale of these possible impacts. Preliminary research has been commissioned to evaluate some of these issues. Environmental monitoring of prototype devices is also planned to establish a better picture of potential impacts.


It is possible that some environmental impacts may be limited in the case of Alderney. The impact of sediment disturbance during cable laying could be minimised by the natural rapid dispersal of the sediment. The use of gravity based foundations, such as structures that would be used by OpenHydro‟s open centred turbine, would not require drilling to secure them to the sea floor. This would avoid one source of noise and vibration. In contrast, other tidal energy technologies rely on drilling to secure foundations to the sea floor. The resultant noise could cause temporary disturbance to fish and marine mammals.


There is evidence from the offshore wind industry that structures implanted into a marine environment act as artificial reefs, attracting some species of fish, crustaceans and other invertebrates. However, the extent of the colonisation associated with tidal stream devices is not known.


It is recommended that tidal energy development around Alderney would benefit from generic environmental research in the following four areas:


  • Assessment of the potential impacts related to energy depletion from a large tidal stream device scale array.
  • Assessment of the potential impact of tidal stream energy on sea birds and their prey.
  • Review and assessment of the impact of EMF from submerged cables and tidal stream generators.
  • Review and assessment of the potential for tidal stream devices to induce colonisation and increase biological productivity.


Strategic Socio-Economic impacts Financial Benefits


Revenue to the States of Alderney from seabed rental could be between £0.93M and £1.87M per annum from a 1GW array depending on the amount of electricity generated. A 3GW array could yield between an annual income of between £2.8M and £5.6M.


An estimated 6,149 tonnes of CO2 emissions from the use of fuel oil generated electricity could be saved if tidal energy replaced the island's existing power plant.


Complete electrification of the island would displace the equivalent of an estimated 3,176 tonnes of fuel oil at a cost of £1.9M. The cost-benefit to the island would depend on the cost of tidal energy as well as the cost of fuel oil. Assuming a cost of €150/MWh or £0.144/kWh for tidal energy the cost of supplying the bulk of the island‟s energy demand (excluding transport) would be £6.4m/year or more than three times the cost of saved fuel (at 2006 prices). Therefore, with a tariff of £1.07/MWh and tidal cost of £0.114/KWh complete electrification would only be economic if the oil price were to reach three times the 2006 level, and would become increasingly so with further rises in the price of oil beyond that.




Fishing is an important tourist industry for Alderney contributing up to 5% of the island's tourist income. There is also a small but important commercial fishing industry that includes lobsters and crabs as well as fin fish.


Fishing grounds near Alderney and the Casquets are often located on large sand banks. Although these areas are unsuitable for tidal energy developments, the management of fishing vessels and those involved in tidal energy development would need to be carefully controlled to avoid collision. There is also the possible potential for temporary disturbance to fishing grounds.




Large bottom mounted devices possibly up to 2MW in size could be deployed in the Race in less than 40m depth. Devices of this size occupy up to 20m of water leaving less than 20m clearance above the device. This is potentially too shallow for large commercial vessels therefore a permanent exclusion zone would need to be applied. The use of pile mounted, surface piercing structures, or floating devices, would present obvious navigation hazards and would require a permanent exclusion area. They would also create some visual impact. Smaller bottom mounted devices (<2MW) would occupy less depth, allowing the safe passage of large commercial vessels. They would, however, generate less energy.


During construction, and to a lesser extent during operation, there would be an increased presence of vessels operating from both Cherbourg and Braye Harbour.


Tourism and Recreation


Alderney is heavily reliant on tourism and any large scale infrastructure development would need careful consideration to avoid changing the island‟s character.


Tidal energy development could have some impact if a large substation was built on the island. However, if this facility was located in Mannez Quarry it would be largely concealed from the rest of the island. Electricity generated from a tidal energy array in the Race could be transmitted directly to France avoiding the development of large substations on Alderney. Surface piercing technologies would create some visual impact depending on their proximity to the island creating some modification to the seascape. There is also the potential for conflicts of interest between developers and those involved in recreational pursuits.


The active promotion of the technology would create added interest for tourists, particularly if there was a dedicated visitor centre.




During construction of a large scale array an estimated 130 personnel could be directly involved in tidal energy development. This does not include pre-construction activities such as resource assessment, environmental monitoring or survey work. Most of the work force would need to be based in Cherbourg but up to ~24 could be based on Alderney. These estimates do not include additional service related jobs generated by the presence of migrant workers.


Buildings and Infrastructure.


Alderney is a small island of only 11 km2. Much of the land area has either protected conservation status or has specific land use designations which permit only limited development. Any new infrastructure on the island would, therefore, be subject to stringent planning requirements which may limit the extent of any land based facilities.


Installing subsea cables will also need careful consideration. Longis Bay on the east coast might be a possible route for cables linked to an array in the Race. However, this location has a sensitive sublittoral habitat. One possible solution would be to run the cables along the side of the existing causeway to Raz Island.


New supplementary accommodation might also be necessary for construction personnel.


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