Tidal Power

If there is one thing we can safely predict is the coming and going of the tide, it is a reliable and predictable source of energy. This gives this form of renewable energy a distinct advantage over other sources that are not as predictable and reliable, such as wind or solar. The Department of Trade and Industry has stated that almost 10% of the United Kingdom’s electricity needs could be met by tidal power. Geographically the UK is in a prime location not only does the sea move due to tide but prevailing winds also help reinforce the movement of water.

A question you may be asking is why do the tides come and go? It is all to do with the gravitational force of the Moon and Sun, and also the rotation of the Earth.

The gravitational attraction of the moon and sun affect the tides and the magnitude of this attraction depends on the mass of the object and its distance away. The moon has the greater effect on earth despite having less mass than the sun because it is so much closer. The gravitational force of the moon causes the oceans to bulge along an axis pointing directly at the moon. The rotation of the earth causes the rise and fall of the tides. When the sun and moon are in line their gravitational attraction on the earth combine and cause a “spring” tide. When they are as positioned in the first diagram above, 90° from each other, their gravitational attraction each pulls water in different directions, causing a “neap” tide.

The rotational period of the moon is around four weeks, while one rotation of the earth takes twenty hours; this results in a tidal cycle of around twelve and a half hours. This tidal behaviour is easily predictable and this means that if harnessed, tidal energy could generate power for defined periods of time. These periods of generation could be used to offset generation from other forms such as fossil or nuclear which have environmental consequences. Although this means that supply will never match demand, offsetting harmful forms of generation it is an important starting point for renewable energy.

Current Technologies
There are two options for getting energy from the tide, a tidal barrage or utilising tidal streams.

The Tidal Barrage System
This is where a dam or barrage is built across an estuary or bay that experiences an adequate tidal range. This tidal range has to be in excess of five metres for the barrage to be feasible. The purpose of this dam or barrage is to let water flow through it into the basin as the tide comes in. The barrage has gates in it that allow the water to pass through. The gates are closed when the tide has stopped coming in, trapping the water within the basin or estuary and creating a hydrostatic head, the larger the tidal range the larger the hydrostatic head and so the larger the potential energy that can be used. As the tide recedes outwith the barrage, gates in the barrage that contain turbines are opened, the hydrostatic head causes the water to come through these gates, driving the turbines and generating power. Power can be generated in both directions through the barrage but this can affect efficiency and the economics of the project.

This technology is similar to Hydro-power or wave-power (which will be a topic for discussion at another time), something that there is a lot of experience with in Scotland. There is potential for a project of this kind in Scotland, one place in particular which has been looked at is the Solway Firth in south west Scotland, where there is a tidal range of five and a half metres.
The construction of a barrage requires a very long civil engineering project. The barrage will have environmental and ecological impacts not only during construction but will change the area affected forever. Just what these impacts will be is very hard to measure as they are site specific, and each barrage is different. Damage to local habitats will have to be reviewed and considered.

The following diagram is a simplified version of a tidal barrage.

There are different types of turbines that are available for use in a tidal barrage. A bulb turbine is one in which water flows around the turbine. If maintenance is required then the water must be stopped which causes a problem and is time consuming with possible loss of generation. When rim turbines are used, the generator is mounted at right angles to the to the turbine blades, making access easier. But this type of turbine is not suitable for pumping and it is difficult to regulate its performance. Tubular turbines have been proposed for the UK’s most promising site, The Severn Estuary, the blades of this turbine are connected to a long shaft and are orientated at an angle so that the generator is sitting on top of the barrage. The environmental and ecological effects of tidal barrages have halted any progress with this technology and there are only a few commercially operating plants in the world, one of these is the La Rance barrage in France.

Pumping
The turbines in the barrage can be used to pump extra water into the basin at periods of low demand. This usually coincides with cheap electricity prices, generally at night when demand is low. The company therefore buys the electricity to pump the extra water in, and then generates power at times of high demand when prices are high so as to make a profit. This has been used in Hydro Power, and in that context is known as pumped storage.

The capital required to start construction of a barrage has been the main stumbling block to its deployment. It is not an attractive proposition to an investor due to long payback periods. This problem could be solved by government funding or large organisations getting involved with tidal power. In terms of long term costs, once the construction of the barrage is complete, there are very small maintenance and running costs and the turbines only need replacing once around every thirty years. The life of the plant is indefinite and for its entire life it will receive free fuel from the tide. The increase in the costs of fossil fuels is making tidal power a more attractive proposition. The economics of a tidal barrage are very complicated. The optimum design would be the one that produced the most power but also had the smallest barrage possible.

Social Implications
The building of a tidal barrage can have many social consequences on the surrounding area. During the construction of the barrage, the amount of traffic and people in the area will increase dramatically and will last for a number of years. The La Rance tidal barrage in France took over five years to build. This will also bring revenue to the area from the tourism and hospitality industry that will accommodate all the different types of visitors that the barrage will bring. This will give a boost to the local economy.

The barrage can be used as a road or rail link, providing a time saving method of crossing the bay or estuary. There is also the possibility of incorporating wind turbines into the barrage to generate extra power. The barrage would affect shipping and navigation and provision would have to be made to allow ships to pass through.
The bay would become available for recreation as the the waters would be calmer not immediately after the barrage but further in towards the land. This would be another tourist attraction and become a feature of the area.

Environmental Aspects
Perhaps the largest disadvantages of tidal barrages are the environmental and ecological affects on the local area. This is very difficult to predict, each site is different and there are not many projects that are available for comparison. The change in water level and possible flooding would affect the vegetation around the coast, having an impact on the aquatic and shoreline cost systems. The quality of the water in the basin or estuary would also be affected, the sediment levels would change, affecting the turbidity of the water and therefore affecting the animals that live in it and depend upon it such as fish and birds. Fish would undoubtedly be affected unless provision was made for them to pass through the barrage without being killed by turbines. All these changes would affect the types of birds that are in the area, as they will migrate to other areas with more favourable conditions for them.
These effects are not all bad, and may allow different species of plant and creature to flourish in an area where they are not normally found. But these issues are very delicate, and need to be independently assessed for the area in question.

Tidal power is perhaps one of the most reliable forms of green energy available but with the impacts on local economy for the short term and impacts to the local ecology there are far more impacts to be considered when compared to a wind farm or solar farm. Wave power has less generating ability but the impacts are reduced compared to tidal but the level of reliability of power is high, This form of energy generation shall be considered in the next entry.