Gero Di Piazza

Increasing the level of renewable energy in power generation is a high priority in many countries, yet tidal power potential remains highly undeveloped. A project in the UK aims to kick-start interest in this technology, and prove that it is a viable option for the future.

Although the UK government has a long-term renewable initiative plan for ‘green’ energy to account for 10 per cent of power generation by 2010 and a further ten per cent by 2020, renewable energy projects continue to experience the effects of the ‘NiMBY’ (not-in-my-back-yard) syndrome. This acts as a frustrating barrier to developers of renewable energy projects, and wind power projects in particular. Local residents often oppose wind power projects on the basis of their visual impact and as well as the noise generated by the turbines.

At present, renewable energy contributes less than one per cent to total power generation in the UK. Planning rules to allow faster approval of renewable projects are desperately needed to meet the targets. Although wind power is said to be the fastest growing source of electric power in Europe, environmental and planning constraints continue to be a barrier to project developemnt in the UK. A shift to increased development of offshore wind is therefore expected.

Developments of wind farms in the UK is therefore quite limited with only 80 projects consisting of 1007 turbines with a total capacity of 557.5 MW.

The problems experienced by wind power developers, however, is creating an opportunity for the development of other forms of renewable energy. And while solar power may not have a great future in northern Europe, the region has yet to extensively exploit the power of its shorelines and river estuaries.

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Figure 1. The UK’s Bristol Channel will be the next port of call for the latest tidal wave pilot project in Europe

The development of renewable energy is also being encouraged by the Kyoto Protocol. Although the UK’s Energy Minister, Brian Wilson, has emphasised the importance of nuclear power in meeting Kyoto’s commitments, the focus on environment-friendly energy goes on. The UK government seems to have developed a new sense of urgency and wants project developers to make bold moves to enable renewable energy targets to be met.

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Apart from the visual impact factor, wind power has disadvantages due to the intermittence and variability of the wind – this leads to problems with the grid and with the need to provide surplus capacity in the energy system to allow for periods when wind power is not available.

Tidal power

Tidal power is considered to be attractive because of the guaranteed nature of the tidal system which leads to a predictable and regular production cycle. This results in reliable energy yield forecasts. This factor differentiates tidal from wave power, which is highly dependent on the wind environment and as a result, like wind power, suffers from problems of intermittence. The phasing of the tidal cycle with peak energy output occurring at different times depending on location also offers the opportunity for a more steady state energy regime from a network of tidal devices around the coastline of the UK.

Like wind power, the energy available from tidal flows depends on the density of the medium and the cube of the velocity. Since seawater is more than 800 times denser than air and tidal stream currents can be between 4.8 km/s and 12.8 km/s, it is clear that there is a considerable energy resource available if the appropriate technology is found and utilized.

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Figure 2.The turbines will be positioned on the Severn Estuary sea bed, 61 m below the surface of the water

Tidal power operates by building a barrier across a river estuary. The tidal flow drives turbines to produce electricity. Europe’s only tidal power station is at Rance in northern France. A number of sites in the UK could be developed to provide tidal power. The drawback is that these schemes affect the habitat of sea birds and fish because they alter the tidal currents.

According to the World Energy Council, If every reasonable project in the UK were to be exploited for tidal power the yield could be over 50 TWh a year representing 20 per cent of the electricity demand in Britain. About 90 per cent of this potential is at eight large estuaries including the Severn, Dee, Morecambe Bay, Solway, Humber and the Wash.

The technology used to extract power from tidal cycles is based on either of two types of technology:

  • Tidal barrages where a barrier is constructed to obstruct the tidal variation and energy is produced when the variation in water levels is used to drive a turbine. These barrages are typically placed across the entrance of an estuary and France’s model has been operating for many years.
  • Tidal stream generators which capture the energy in the tidal current created by the rise and fall of the tide.

Funding

Tidal generation is one of the most reliable and predictable ways of generating electricity. Some would argue that it is illogical not to harness such a useful way of generating power. Tidal behaviour can be predicted one year in advance. This makes life a lot easier for financiers. For they will get an accurate report on generation and likely profits – essential in giving the go ahead for funding. Industry insiders believe that tidal power has the potential to become something which will last indefinitely and produce huge amounts of sustainable energy.

The UK government has made public its desire to pursue tidal energy. Available figures claim there are thousands of MW of tidal power available surrounding the UK. This is why tidal projects are likely to receive further funding to carry out demonstration and development projects.

The UK’s Department of Trade and Industry (DTI) has awarded £1.6 million ($2.5 million) for a demonstration project which is seeking to confirm the viability of a tidal stream generator developed by Tidal Hydraulic Generators Ltd., (THGL) a company established by project manager Richard Ayre.

Following a period of research and investigation work, funding was obtained by THGL from the Pembrokeshire Coast National Park to allow trials of a basic turbine system consisting of a 6 m blade driving a local generator. This early prototype was used to examine factors such as the optimum blade configuration, taking account of different current velocities, and to assess the impacts on the marine environment.

The next stage of the project will involve the manufacture and installation of a five-turbine device in the Severn Estuary/Bristol Channel – off the west coast of England – in 2004. This device will weigh in the region of 150 t and is expected to produce a maximum of 1 MWh at peak operating times. A novel aspect of the device is the use of a low pressure hydraulic system driven by the turbines and converted to high pressure using an accumulator. The high pressure is then used to drive a single generator � this compares with other systems where each turbine has its own generator.

THGL believes that this innovative design will result in lower operating costs, higher reliability and less down-time. The fluid used in the hydraulic system is vegetable oil, selected to avoid environmental damage in the event of a failure in the system.

Ayre noted that at the initial planning stages, the design team had to consider that the turbines had to be at least 61 m below the surface of the water to avoid any wave effect. In fact a variety of different configurations were looked at so as not to disturb the sea bed. Ayre said: “They [turbines] have got to be ballasted to the seabed, and have to be dynamically stable on their own, which led to the design that we have. The National Parks paid for trials to be carried out to verify if commercial quantities of power could be generated and that’s where we are at the moment. “we had a diamometer on the rig, we now have a generator on the rig. But although the rig has a 6 m diameter blade, it is not being run at its full capacity because obviously we might just break something. But we have some very interesting figures and that’s why the DTI is keen to go forward.

“The next stage is a seabed rig in the Bristol Channel where there are much higher current flows.”

The pilot scheme will not be large enough to be linked to the national grid. There will be one central generating station, which will generate 1 MW of electricity. The system will undergo reliability trials to make sure the water seals, hydraulic pumps and so on all work well.

The turbines will have a ten year life span. For servicing to take place, they need to be retrieved from their subsea positions. Initially therefore, the pilot project will be located in a fairly shallow area to enable engineers to reach it at low tide, and there are plans to put it in a progressively deeper area. The unit will be run for a period of one year, at which point technicians will take it apart to examine wear and tear, and identify any’weak spots’.

Project aims

International technical and management consultancy Babtie Group is supporting THGL by providing a project management service, undertaking bathymetric surveys of the proposed site, designing of the foundations and assisting with the interface with the stake holders such as the Crown Estate, the Environment Agency and the navigation bodies. The Environmental Impact Statement will consider the effect of the device on local sea life and the fishing industry and will consider the implications of construction work and the impact of any onshore facilities which may be required.

The programme for the project aims to install the device early in 2004 with design work and environmental work being undertaken in 2003. The trials will consider the output from the device taking account of losses of efficiency due to the interaction of the current flow past adjacent turbines and different orientation of turbines will be used to allow the design of the production device to be optimized. The trial will also allow marine growth to be considered and the disturbance to marine life to be investigated. If the demonstration device is shown to be viable, it is hoped that a production version will be developed which will assist in meeting the UK’s renewables targets in 2010.


Figure 3. A computer generated image of what the tidal project will look like
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The sites for production of tidal stream power have been investigated and a number of areas are considered to be worthy of further investigation. Sites with high tidal current velocities and high flow volumes are attractive and these are often in areas where the tidal flow is channelled and concentrated. Areas such as the Pentland Firth between the North of Scotland and Orkney, Barry Island in the Bristol Channel, Rathlin Island in Northern Ireland and the Mull of Galloway offer tidal current velocities between 4.8 km/s to 9.6 km/s and have the potential as tidal stream power locations.

Those connected with the project admit the whole process is one big learning curve. Even though the technology is available to build a powerful generator, the Babtie team and those surrounding them believe more will be gained with separate pilot projects.

Ayre noted: “In reality we have enough information to design one straight away but when you have four or five choices on what type of rubber seal to refit, it is no good fitting one that will break down on the seabed. You need to know more about what type of electrical connectors we use rather than the shape of the turbine blades or the thickness of the shaft. So the side issue is equipment – which ones do we identify as having greater reliability.”