This article aims to give the reader an understanding of tidal and wave power. Tidal power schemes are mainly built around coastal areas and estuaries, thus working with the tidal phenomenon. Wave power is used in the open sea, but in close proximity to the land for transmission reasons. Whereas the use of tidal power itself is not a new idea, within the last century wave power is relatively new, and more and more research is being done into a variety of wave power devices which will be briefly looked at further within the article.

Current Worldwide Tidal Power

Tidal Barrages

Although tidal barrage power is not a completely new idea, only a few barrage plants are in operation. The first began operating in 1966 at the Rance river, St Malo in France. The plant produces 240MW. There are only four other tidal power generation plants in the world:

• Annapolis in Canada – built in 1984 producing 20MW.
• Jiangxia in China – built in 1980 producing 3.2MW.
• Kispaya Guba in Russia – built in 1968 producing 1.7MW.
• Uldolmok in South Korea – built in 2009 producing 1MW.

A 254MW plant being built on the Sihwa Lake, South Korea and due to be completed during 2010.¹

There have been numerous proposals for a barrage scheme across the Bristol Channel in the UK, however none have been built to date. The latest proposal is known as the Severn Barrage and would house 216 turbines, generating 8,640MW combined, and producing 5% of the UK’s energy demand (based on 2002).

How it works

In terms of a Tidal barrage, the tidal power harnesses the kinetic energy of a flow of water in three ways:

• Ebb generation involves allowing an incoming tide to flow through opened sluice gates of a barrage. When at high tide the gates are closed. After a period of time, when the tide lowers, a head is created. This acts the same way as a dam. The head of water is released through the turbines and therefore generates electricity.
• Flood generation involves generating electrical energy from the incoming tide.
• Two-way operation maximises the use of both the ebb and flood generation techniques. More expensive turbines will be needed that can generate power in both directions.

Although generation can occur for a relatively long period throughout the day, it is not possible for continuous generation (24/7) to occur using a tidal barrage. For example, there will need to be a period of time for a head to be created for ebb generation to work. Two-way generation will result in four short bursts of electrical generation, accounting for each change in tide during the day. Also, as the tides change (due to the moons cycle of 24.8hr periods), tidal generation will continuously change day by day. This causes a problem, when generation is at its maximum during a tide of a particular day, it may not match the demand.

Advantages of Tidal Barrages

• Aside from construction and maintenance costs, tidal power generates electricity for free (as it does not need any fuel).
• It will be able to produce electricity reliably due to predictable tides.
• It may provide transport connections (saving the motorist time and fuel consumption).
• The tidal range limits may be reduced and therefore water activities such as water-skiing and fishing could benefit from this.

Disadvantages of Tidal Barrages

• Environmental consequences of affecting the natural tidal flow will have devastating consequences on natural habitats upstream of the barrage. Mudflats are one such area of particular conservational importance. Tidal flows may not retract from mudflats as they once did. A mitigating circumstance to overcome these environmental consequences may be to ‘re-home’ the wildlife by artificially creating mudflats elsewhere.
• Maintenance issues arise, particularly if a turbine or sluice gate is in need of repairs. The barrage would have to be temporarily switched off and therefore would not be producing power for a turbine to be repaired.
• There are only a few sites suitable for the construction of barrages.
• Migrating fish would have to be considered. This could possibly be mitigated by building ‘fish ladders’ through the barrage.
• The ecosystem of saline/freshwater may be affected.
• Tidal resonance will be affected, the tidal range will reduce, and such phenomenon’s as the Severn Bore (If the Severn Barrage were to be built) would disappear.

‘Underwater Tidal Farms’, Tidal Fences and Tidal Reef’s

Tidal barrages are not the only way of harnessing energy from incoming and outgoing tides. Three other ideas are currently being researched presently; Underwater Tidal Farms, Tidal Fences and Tidal Reefs.

Underwater Tidal Farms are in essence an underwater ‘wind’ farm. Water is around 800 times denser than air. It is therefore possible to extract the same amount of energy from a tidal machine that is much smaller in size than a wind turbine, and therefore much cheaper to build (in terms of construction to energy generation).

Recently, in the summer of 2010, the world’s largest and most powerful tidal turbine was unveiled in Scotland. It had been developed by ‘Atlantis Resources’ and is known as the AK1000. It has two 18m diameter rotors and its column stands at 22.5m.²

Tidal Fences are an underwater partial barrier that would be much friendlier to the environment. It would still harness the ingoing and outgoing tides but would not be able to generate as much electricity as a tidal barrage would.

A tidal reef would use reversible turbines that are fixed to an estuary’s floor and would harness the incoming and outgoing tides. This would have minimal impact on fish that swim up the estuary as it would be completely submerged allowing fish to swim over it.

Wave Power Devices

Wave power is the transportation of energy through ocean surface waves Waves are generated by friction created by wind. The further a wind is allowed to impact on the surface of water, the more energy the waves will have. The distance of open ocean that is allowed to be affected is known as the ‘fetch’. The UK (particularly on the South West, West and North) has the advantage of having a large fetch and therefore more energy is available that may be harnessed by a wave power device. Proximity to the coastline is important if a wave power device is to be installed. Waves will increase in amplitude (height) as it travels over shallower water and wave power devices harness this movement. More important though are the grid connection costs, as the further a device is away from the land, the higher the cost involved with transmitting electricity.

A group of wave power devices is known as an ‘array’.

The limitations of wave devices are not just for offshore generation but also for coastal generation. The ‘Limpet³’ is one such wave device installed in 2000 on the coast of western Scotland. It is rated at 500kW and generates energy through harnessing a wave to fill and empty a contained structure. This contained structure houses air and a ‘Wells’ contra-rotating turbine. The drawing in and out of air within this container through the turbine generates electricity.

There is a great potential for harnessing energy from waves. There are many innovative and different wave power devices being researched and tested although some are better than others. One of the first promising wave devices was the ‘Salter’s Duck⁴’ and was developed as a solution to the 1970’s oil crisis. The Duck was hugely efficient, generating 81% of a wave’s energy. However, famously it was never developed any further due to some miscalculations by a factor of 10 of an estimation cost of its energy production. The miscalculation was only discovered a few years ago and so is expected to be redeveloped. The Duck works by bobbing up and down every time a wave travels underneath it.  A pendulum connected to a generator inside the Duck’s shell moves backwards and forwards and this generates electricity.

Simple Diagram of Salter’s Duck

The ‘Pelamis⁵’ or ‘Wave Dragon’ is a wave device that sits semi-submerged on the ocean surface. It has tubes hinged together (like a column) which move freely with the amplitude of a wave. The movement of these joints causes a hydraulic ram to push fluid into a hydraulic motor generating electricity. Each device is currently rated at 750kW and is 180m long 4m diameter in size.

The ‘Wave Hub⁶’ is a research project that was due to be installed in the summer of 2010 16km off of Hayle, Cornwall. It will be a fixed to the sea bed. The best way to describe the wave hub is as a ‘socket’ accepting energy generated from wave power arrays. As it stands, four different technologies will be allowed to connect to the hub in order to do tests. The hub will measure the strength of the waves and also enter into a PPA (Power Purchase Agreement) on behalf of the developers. The wave hub also provides planning consent for wave arrays, reducing the lengthy task of getting planning permission for each technologies research. The wave hub is connected to the grid through an onshore substation at Hayle by an underwater cable that then is built within the sand dunes running up to the substation.

Environmental Impacts

• Could create safe havens for sea life as shipping and fishing would not be permitted in an area where a wave power array is sited.
• Tests have been carried out and are ongoing into whether an array of wave power devices affects the sonar of cetaceans.
• Some devices are noisy (i.e. the Limpet). Noise is created by air rushing through the turbine, however the sound created by the waves in the first place will most likely drown this noise out.
• There has been speculation about the affect that an array has on surfing. Computer modelling helps to determine a suitable site so that an array can be far enough from the coast not to affect the surfing industry.

Other Problems

Shipping: As wave power devices are based out at sea it is obvious that an array of them will create a hazard to shipping and fishing areas. Just like rocky coastal areas are defined on a map, wave power arrays will also have to be included.

Storm Surges: A wave power device would have to undergo occasional great stress as a result of storm surge. A storm surge would almost indefinitely create a fault with the device. To militate against this, it may be possible to temporarily sink a device until the storm surge has passed or tow it back into a harbour where it could be sheltered.

Renewable Obligation Certificate

Wave power is classed as an ‘emerging renewable technology’, and therefore under the Renewable Obligation Certificate will receive 2 ROC’s per MWH that it generates. In comparison, an ‘established’ technology such as energy from Landfill Gas receives 0.25 ROC’s per MWH that is generates. Incidentally, wave power is deemed to be a ‘large generator’ and so does not fall under the Feed-In-Tariff within the UK.

If you would like to find out more about renewable technologies then take a look at our informative pages.

References

1. Korean Tidal Barrage; http://www.newsworld.co.kr/cont/article2009/0909-52.htm

2. Atlantis Resources Homepage; http://www.atlantisresourcescorporation.com/about-atlantis/history.html

3. Limpet, WaveGen Homepage; http://www.wavegen.co.uk/what_we_offer_limpet_islay_wavecam.htm

4. How Salter’s Duck Works; http://science.howstuffworks.com/environmental/green-science/salters-duck1.htm

5. Pelamis, Ocean Power Delivery; http://www.pelamiswave.com/index.php

6.Wave Hub South West Research and Development; http://www.southwestrda.org.uk/working_for_the_region/key_sw_projects/cornwall__the_isles_of_scilly/wave_hub.aspx