The South West has a significant renewable energy resource in the waves and tides of the sea. This was recognised in 2012 by the creation of the South West Marine Energy Park, the first in the UK, stretching from Bristol to the Isles of Scilly.
The tides are due to the influence of the gravitational pull of the moon (and to a lesser extent the sun) on the seas. Schemes that use tidal energy rely on the twice daily tides and the resultant upstream flows and downstream ebbs in estuaries or in some cases tidal movements out at sea.
Electricity can be generated by tidal barrages which extract energy from the rise and fall of the tides or tidal stream devices which rely on the flow of water past them
Tidal barrages are built across a suitable estuary and have some similarities with low head hydroelectric dams. Typically the barrage allows the incoming tide to pass through its sluice gates and at high tide the water is trapped behind the barrage by closing these gates. A height difference is allowed to develop between water on either side of the barrage and water is then allowed to flow through the turbines of the barrage, generating power. The power output of a tidal barrage is proportional to the area of the basin of water behind it and the square of the tidal range .
The 740m long tidal barrage at La Rance in Brittany, France was until recently the world’s largest tidal power plant. It is a 240MW scheme with 24 turbines operating in a tidal range of up to 12m, with a typical operating head of 5m. It generates nearly 500 million kWh of electricity a year (equivalent to the electricity consumption of about 100,000 typical UK homes).
The Severn Estuary has a large tidal range and proposals for a tidal barrage date back to the 1920s. The most favoured site is between Weston-Super-Mare and Lavernock Point near Cardiff in an area where the tidal range is about 7m. The scheme could be rated at 8.7GW, generating 17,000 million kWh per year, enough for about 3.6 million households. Some estimates for the cost of the project have been as high as £34 billion and the barrage would lead to a lower tidal range upstream, affecting exposure of mud flats for birds.
Tidal Stream Devices
Instead of exploiting the vertical rise and fall of the tides, these devices harness the flow of water due to the tidal current, causing movement of the device. Some designs are similar to underwater wind turbines while others use a hydroplane. The power output is strongly sensitive to the velocity of the tidal flow (proportional to the cube of the velocity). It is also proportional to the swept area, but because the density of sea water is much greater than air, the length of turbine blades for tidal turbines can be much shorter than for wind turbines. Areas like the Pentland Firth in the North of Scotland and the Channel Islands have high tidal currents and are ideal for these devices. Although the velocities are typically lower in the Bristol there are still many suitable sites
Marine Current Turbines
Marine Current Turbines in Bristol tested the 300kW SeaFlow device off the coast of Lynmouth between 2003 and 2009. The turbine had a single 11m diameter rotor and was attached to a tubular monopole. The turbine could be raised above the sea surface for maintenance.
The world’s first commercial tidal stream turbine was installed in the fast flowing narrows of Strangford Lough in Northern Ireland in 2008. This device, known as the SeaGen has two rotors and is capable of producing 6 million kWh per year (equivalent to the consumption of about 1275 homes)
Further projects are planned off Anglesey and in the Pentland Firth. An alternative to the Severn Barrage has been proposed consisting of a tidal fence of SeaGen devices.
The OpenHydro tidal turbine is an open centre turbine and also avoids the need for a gearbox, potentially reducing maintenance. A 250kW model was tested off Orkney, at the European Marine Energy Centre (EMEC) in 2008 and became the first tidal turbine to generate power to the UK electricity grid. The world’s first large scale grid connected tidal energy farm is currently under construction off the coast of Brittany at Paimpol-Brehat. This €40 million project for the French power company EdF consists of four OpenHydro 2MW tidal turbines. The turbines have a diameter of 16 m and when installed on the gravity base will have a height of 22 m, operating in water of 35 m depth. The first of the turbines was tested on site for three months during 2011. The project is likely to be completed during 2013 and is expected to generate sufficient electricity to supply 4000 homes.
While many tidal stream devices consist of horizontal axis turbines, the design from Pulse Tidal involves oscillating hydrofoils which sweep up and down as a result of water flowing past them. A 100kW Pulse-Stream 100 has been tested in the River Humber since 2009. A 1.2MW machine will be tested off the coast of Lynmouth from 2014 at the site previously used by SeaFlow. This design has the advantage of being able to generate commercially attractive amounts of power in relatively shallow water sites like Lynmouth. Electricity will be exported to the grid in these studies unlike the previous SeaFlow tests. During operation, the system sits on the sea-bed and is fully submerged. However a buoyancy system allows the device to be brought to the surface for maintenance without the need for cranes.
The power of the wind has been used for hundreds of years to mill grain, pump water and for other mechanical applications like sawing wood. Small wind turbines for charging batteries have been manufactured since the 1930s. Since the 1980s the technology of wind turbines for electricity generation has been sufficiently mature for a large manufacturing industry to develop. Wind turbines are now the most cost effective way to generate electricity from renewable sources. The UK has the best wind resource in the whole of Europe. Germany and Spain lead Europe in overall wind turbine installations but the UK leads the way with offshore wind.
Fullabrook Wind Farm was the largest onshore wind farm in England when it was completed in 2011. It comprises 22 Vestas V90 wind turbines each with a rated capacity of 3MW. The hub height is 65m with 45m turbine blades, making a total height of 110m to the blade tip. Over one year the wind farm will produce in excess of 150 million kWh, equivalent to the annual electricity consumption of about 32,000 households or more than 75% of North Devon homes.
An Enercon E-48 wind turbine with a rated capacity of 800kW was installed at the head office of Philip Dennis Foodservice Ltd at Mullacott Industrial Estate in 2012. This has a 56m hub height and blade tip height of nearly 80m. While the Vestas V90 has a gear-box in the nacelle at the top, the Enercon E-48 does not and is a direct drive turbine.
Electricity is the third highest cost for Philip Dennis after diesel and wages. The electricity consumption of the Ilfracombe site is about 1.55 million kWh per year. The turbine is likely to generate between 1.5 and 2.0 million kWh per year (equivalent to the annual consumption of about 375 homes).