Tidal resource

The motion of the tides is caused by the gravitational pull of the moon and sun. This motion varies according to a number of cycles. The main cycle is the twice daily rise and fall of the tide as the Earth rotates within the gravitational field of the moon. A second, 14-day cycle is caused by the moon and sun being alternately in conjunction or opposition. This results in spring and neap tides. There are other cycles that add 6-month, 19-year, and 1600-year components but these are much smaller.

Therefore, tidal energy is energy generated through the motion of the planets. As such, it is one of the few renewable energy sources that do not depend either directly or indirectly on radiant solar energy.

The actual size of the tidal movement depends on geographical location. Tidal amplitude in the open ocean is around 1 m but this increases nearer to land. Amplitude can be substantially enhanced by the coastal land mass and by the shape of river estuaries. Under particularly propitious conditions, such as those found in the Severn estuary in southwest England or the Bay of Fundy in Canada, the tidal amplitude will increase substantially. The Bay of Fundy, for example, has a recorded maximum tidal reach of 16.2 m, while that of the Severn estuary is 14.5 m. Note, however, that mean tidal amplitudes in these regions are likely to be much smaller than this.

The energy that can be extracted from tidal motion waxes and wanes with the tide itself. Under most conditions power output is not possible continuously. Tidal movement is, however, extremely predictable and the timing of the tides can be calculated with great accuracy. This makes tidal power a valuable form of renewable generation because, although intermittent, it is entirely reliable in its behavior.

The World Energy Council has estimated the global annual energy dissipation as a result of tidal motion to be 22,000 TWh. Of this, 200 TWh are considered economically recoverable based on the use of tidal barrages. Aggregated national estimates (see below) suggest that the total recoverable energy is much higher than this but not all of it will be economic. Current production from tidal energy is probably around 1 TWh.

There has been considerable interest in tidal power since the 1960s and a number of countries have identified sites where tidal power production would be possible. However, although a number of pilot projects have been launched, large-scale schemes have generally been judged too expensive to build.

One of the most thorough research projects examining national tidal potential was carried out in the United Kingdom between 1983 and 1994. This project looked at a range of possible schemes in England and Wales. It concluded that if every practicable tidal estuary with a spring tidal range of more than 3.5 m was exploited, around 50 TWh of power could be generated each year. This represented around 20% of the electricity consumption in England and Wales in the mid-1990s. The United Kingdom’s best site is the Severn estuary. The country probably has the best tidal regime in Europe but the European Atlantic coast offers a variety of other potential sites.

In Canada, the Bay of Fundy has the highest tides in the world. This region, on Canada’s east coast, has been the subject of intense examination. A comprehensive study of the region, carried out in the mid-1960s, focused on sites with a total generating capacity of nearly 5000 MW. However, tentative schemes to build projects were abandoned during the changing economic climate at the end of the 1970s.

Russia has significant potential for tidal generation, particularly in the White Sea on the Arctic coast and in the Sea of Okhotsk. A site at Mezenski Bay on the White Sea could provide 15 GW of generating capacity and an annual output of 40 TWh, while a second at Tugurki Bay has a potential generating capacity of 7800 MW and 20 TWh/year. The Russian state utility has estimated that total Russian tidal potential is 250 TWh/year.

Korea has a variety of tidal sites and is home to the world’s largest tidal power plant at Sihwa that began operating in 2011. The country has plans for further large tidal schemes. India also has substantial tidal potential. The Gulf of Kutch on the northwest coast has been studied and a 600 MW project proposed. Meanwhile, the Gulf of Khambhat has an estimated generating capacity of 7000 MW. The Indian government has put the country’s tidal potential at 10,000 MW but it could be much larger than this.

China has studied various potential sites. Its southeast coastline is thought to offer particularly good opportunities. Mexico has looked at a site on the Colorado estuary, Brazil and Argentina have studied projects, and the United States has examined a site in Alaska.

Australia’s northwestern coast has some of the highest tidal ranges in the world and there are a number of inlets that could be harnessed to generate electricity. A novel two-basin project was proposed near the town of Derby but the scheme was rejected by the Western Australian government in 2000 in favor of a fossil fuel plant.

Tidal power does not need to be tied to estuaries. In the 1960s, France developed plans for an offshore project in Mont St. Michel Bay. The scheme was shelved when the country decided to invest heavily in nuclear power. The Mont St. Michel project involved a tidal plant that did not make use of an estuary. Instead, a circular barrage, or bund, was to be constructed that would completely enclose an area of open sea. This type of plant would operate in exactly the same way as an estuary plant, with water flowing into the enclosed reservoir when the tide rises, and flowing out through turbines during the ebb tide. While this approach would involve enormous construction costs, it does have the merit of allowing a large tidal plant to be built where no suitable estuary exists.

Operating tidal barrage power plants

Harnessing tidal motion to generate mechanical power has a long history. Tidal basins were being used in Europe to drive mills to grind grain before 1100 AD. These plants were widely replaced when the Industrial Revolution introduced steam engines and fossil fuel, but a few survived though there are none now operating commercially. The exploitation of tidal ebb and flow to generate electricity has been less well tried. Table 9.1 shows the most important tidal power plants that have been built. As the table indicates, the largest is at Sihwa in South Korea, followed closely by La Rance on the northwest coast of France close to St. Malo.

The 240 MW La Rance plant was built using specially devised bulb turbines. A small turbine of similar design was bought by the Russian government during the 1960s, but it is not known whether it was ever deployed, although there has been speculation that the 400 kW Kislaya Guba project represents the final resting place for that turbine.

After La Rance, the third largest tidal barrage project is at Annapolis Royal on the Bay of Fundy in Canada. China has also developed some small-scale projects, of which the largest is at Jiangxia. Work on tidal power generation began in China in 1958 and there are thought to be seven projects in operation today with an aggregate capacity of 11 MW.

Tidal power plant design

The main component of any tidal barrage power plant is the dam or barrage that is built across the mouth of a tidal estuary or inlet. This barrage is fitted with special sluice gates that are opened and closed during different stages of the tide. It is also fitted with hydropower turbines, and these too are equipped with gates so that seawater can either be allowed to pass through them or prevented from doing so as the tide changes (Figure 9.2).

The simplest and most common form of power generation with a tidal barrage is ebb-generation. Under this scheme water is allowed to pass from the sea across the barrage and into the lagoon or basin behind it as the tide rises. Once high tide has been reached, the sluice gates within the barrage are shut, trapping the seawater within the lagoon.

The tide is now allowed to fall on the seaward side of the barrage to create a head of water across the barrage that will drive the trapped water through the hydro-turbines. The length of time during which the water is held will depend on the specific project but will normally be until the tide has fallen to around half its tidal range. At this point the gates closing the turbines are opened, allowing the water from the lagoon to flow through them and back to the sea. Generation will normally continue until close to or after low tide.

When generation stops, the gates protecting the turbines are closed again and the sluice gates opened so that as the tide turns, water will once again pass the barrage into the lagoon. The cycle is then repeated through the next tide.

It is possible to reverse the mode of operation and generate power on the flood tide instead of the ebb tide. In this case the sluice gates are kept closed at low tide so that no water can pass. When the tide has risen by about half its range the turbine gates are opened, allowing water to flow through them and into the lagoon, generating power in the process. Generation continues until levels on either side of the barrage are similar when the turbine gages are closed and the sluice gates opened, permitting the lagoon to empty again. While this is operationally simply ebb-generation in reverse, it is not commonly employed because it leaves the tidal basin behind the barrage exposed to low-tide conditions for extended periods, a situation that can have damaging environmental effects. (However, the particular conditions at the Sihwa tidal barrage plant in South Korea have made this mode of operation preferable.)

It is also possible to generate power during both the ebb and flow tide. The French plant at La Rance was designed to operate in this way but the plant in fact only operates in ebb-generation mode. The main problem with two-way generation is that calculations suggest that economic gains are small and unlikely to be cost effective because it necessitates the additional expense of either two-way turbines or two sets of turbines, one for each direction. On the other hand, it allows generation to take place for much more of the tidal cycle and leads to an overall lower peak power since the head of water that develops is never as high as with single-direction operation. This would in principle allow smaller turbines to be used.

A further operational mode, one that has been employed at La Rance in France, is to use the turbines as pumps to pump additional water across the barrage. Pumping takes place at close to high tide, creating a larger head of water than would be available from the tidal range alone. It is possible to generate up to 10% more power using pumping than without it, and the economics are attractive since the pumping takes place when the head height across the barrage is very small, therefore requiring little additional energy while energy is returned from a much higher head.

Two-basin projects

A conventional one-basin tidal barrage project can only generate power during a part of each tidal cycle. To get around this a variety of two-basin projects have been proposed. This adds complexity but allows either continuous generation or generation for a longer period than a single-basin design.

One type of two-basin design comprises two single basins, each with its own barrage controlling the flow of seawater in and out (Figure 9.3). These two basins are then connected by a channel into which turbines are fitted. In operation, one of the basins opens its sluice gates only close to low tide, keeping the water level within its lagoon as low as possible. Meanwhile, the second opens its sluice gates toward high tide so that the water level within its lagoon is always high.

Water is then allowed to flow from one lagoon to the other through the channel linking the two. The flow rate and the capacity of the turbine within the channel is sized so that there is always more water in the high-water lagoon than in the low-water lagoon so that there will always be a head of water to drive the turbine.

The best developed project of this type was one proposed for construction near Derby in Western Australia. The project involved building barrages across two adjacent inlets and creating an artificial channel connecting the two basins formed by these barrages. A power station with turbines capable of generating 48 MW was to be stationed on this artificial channel. However, the project was never built.

An alternative two-basin scheme design has a primary reservoir that acts like a normal ebb-flow tidal plant, generating power on the ebb tide. However, on the seaward side of the primary basin is a second smaller basin. During the generation phase of the main basin, some power is used to pump water into the second basin, creating a storage basin from which power will always be available for generation whatever the state of the tide. The economics of such a scheme are relatively low at around 30% cycle energy efficiency.

Bunded reservoir

Instead of building a barrage across an estuary, it is theoretically possible to enclose an area of a tidal estuary or tidal region off the shoreline with an embankment or bund such as the St. Malo project discussed earlier so that it does not affect any part of the coast. The principle involved is the same, creating a reservoir that can be filled at high tide and then allowed to empty when the tide has fallen.

Although the St. Malo project never progressed beyond the design and planning stage, tidal lagoons of this type are being proposed again, together with what are proposed to be cost-effective ways of building them. Such designs have environmental advantages because they do not affect a tidal estuary or coastal land region itself. In addition, they can be built so that they do not obstruct waterways or shipping routes. Shallow tidal flats in areas of high tidal reach are judged to be the most economical sites for constructing such plants. There is some interest in such projects at the beginning of the second decade of the 21st century but no scheme of this type has so far been constructed.

Tidal barrage construction techniques

The construction of a tidal barrage represents the major cost of developing tidal power. As a result, much of the research work carried out into tidal power has focused on the most efficient way of building the barrage.

Construction of the French tidal power plant at La Rance was carried out behind temporary coffer dams, enabling the concrete structure to be built under dry conditions. While La Rance was completed successfully using this approach, the method is generally considered too expensive as a means of constructing a tidal barrage today. There is also an environmental problem attached to completely sealing an estuary for the period of construction, which might easily stretch into years. For that reason, such an approach is unlikely to be adopted for the future.

A novel approach suggested for the construction of a barrage across the River Mersey in England borrows something from the construction of La Rance. The idea proposed here was to procure a pair of redundant bulk carriers (e.g., oil tankers) and sink them on the riverbed parallel to one another, sealing the ends and filling the enclosed space with sand to create an island. Concrete construction would be carried out on the island as if it were dry land. To create a watertight structure, diaphragm walls would be fabricated of reinforced concrete; the turbines and sluice gates required for the operation of the power station would subsequently be fitted to this concrete shell.

Once the first section of the barrage had been completed the bulk carriers would be refloated, moved along to the next section, and sunk again. This process would be repeated, until the barrage had been completed. The River Mersey barrage has not been built, so the efficacy of the method has yet to be tested.

Where an estuary is shallow, an embankment dam could be constructed instead of a concrete dam using sand and rock as its main components. Sand alone would not make a stable embankment; wave erosion would soon destroy it. Therefore, some form of rock reinforcement would be required on the seaward side. Concrete faces on both sides of the embankment could provide further protection. The sand needed for construction of such an embankment might be recovered from the estuary by dredging. Rock could also be removed from the riverbed by blasting or brought to the site from elsewhere. Rock is a more expensive construction material than sand so its use would have to be minimized to keep costs as low as possible.

While all these methods have their attractions, the construction method most likely to be used to build a large barrage today would involve prefabricated units called caissons. Made from steel or concrete, the caissons would be built in a shipyard and then towed to the barrage site where they would be sunk and fixed into position with rock anchors and ballast. Some caissons would be designed to hold turbines, others would be designed as sluice gates, and a third type would be blank. These would be placed between the other two types to complete the barrage.

Caisson construction was the favored approach in a study for construction of the Severn barrage in England completed in 1989 under the auspices of the Severn Barrage Development Project. A turbine caisson for this project would have weighed over 90,000 tons and would have had a draft of 22 m. The minimum height of the vertical faces would be 60 m. As a result of their size, special facilities would have been needed to construct them. Prefabrication of the caissons was expected to reduce construction time to a minimum. Even so, the Severn project was scheduled to take 10 years to complete.

For offshore lagoon construction, one company that is promoting such schemes proposes using a conventional rubble-mound breakwater. Barrage failure would have minimal safety consequences in an offshore lagoon because the project is self-contained, so this relatively cheap means of construction should be possible.

Turbines

The turbines in a tidal power station must operate under a variable, low head of water. The highest global tidal reach, in the Bay of Fundy in Canada, is 15.8 m and the mean tidal reach probably half of this range; most plants would have to operate with much lower heads than this. Such low heads necessitate the use of a propeller turbine, the turbine type best suited for low-head operation. The fact that the head varies appreciably during the tidal cycle means that a fixed-blade turbine will not be operating under its most efficient conditions during the majority of the tidal flow; consequently, a variable-blade Kaplan turbine is usually employed. As is the case with most low-head hydropower plants, tidal power plants usually employ a series of small turbines running along the barrage since these can exploit the available energy more effectively than a small number of large turbines.

The most compact and efficient design of propeller turbine for low-head applications is the bulb turbine in which the generator attached to the turbine shaft is housed in a watertight pod, or bulb, directly behind the turbine runner (Figure 9.4). The whole turbine–generator assembly is then hung inside a chamber that channels the water flow through the turbine blades to extract the maximum energy possible. The La Rance tidal plant employs 24 bulb turbines, each fitted with a Kaplan runner and a 10 MW generator. Bulb turbines were new when La Rance was built and construction of the plant involved some experimental work; of the 24 turbines, 12 had steel runners and 12 had aluminum bronze runners. Experience has led the operators to prefer the steel variety.

The turbines at La Rance were designed to pump water from the sea into the reservoir behind the barrage at high tide to increase efficiency. This was found to cause severe strain on parts of the generator and the design had to be modified. Work was carried out between 1975 and 1982. Since then the plant has operated smoothly and with high availability.

The more recent Sihwa tidal plant in South Korea also uses bulb turbines. In this case the plant is equipped with ten 26 MW bulb turbines with variable-blade propeller units. This power plant is built into a sea wall erected in 1994 to create an inland lagoon where water was collected for irrigation. Since then industrial pollution of the lagoon has made the water unusable. The tidal plant forms part of a scheme intended to flush the lagoon to reduce pollution levels. Unlike most other tidal barrage plants, it generates on the flow tide.

An alternative to the bulb turbine is a design called the Straflo turbine (Figure 9.5). This is unique in that the generator is built into the rim of the turbine runner, allowing the unit to operate in low-head conditions while keeping most of the generator components out of the water. A single large Straflo turbine generator was installed at the Annapolis tidal power plant at Annapolis Royal in the Bay of Fundy, Canada. This 18 MW unit is the only one of similar size that has been built, so experience with the design in limited.

Turbine speed regulation

The speed of a conventional turbine generator has to be closely regulated so that it is synchronized with the electrical transmission system to which it is attached. To aid frequency regulation under the variable conditions of a tidal power plant, a set of fixed blades called a regulator are often placed in front of the turbine blades to impart a rotary motion to the water. The use of these blades in conjunction with a variable-blade Kaplan turbine provides a considerable measure of control over the runner speed.

In small applications where such tight speed control may not be essential and where costs are critical it may be possible to use one method of control—either a variable-blade turbine or a regulator—rather than both. An isolated unit that does not connect into the grid could operate without regulation.

An alternative option is to use a variable-speed generator. This electronic solution will permit the turbine to run at its optimum speed under all conditions while delivering power at the correct frequency to the grid. This allows some efficiency gains. However, the solution is more costly than a conventional generator with mechanical speed control of the turbine. Variable-speed generators are being used on some hydropower schemes today (see Chapter 8). Capacity is limited but that is unlikely to be a problem with a tidal power plant where unit size is generally small.

Sluices and shiplocks

The sluices in a tidal barrage must be large and efficient enough to allow the tidal basin behind the barrage to fill with water quickly. Unless the water level behind the barrage effectively follows that on the seaward side, the efficiency of the plant is reduced. Where the water is sufficiently deep, efficient sluices can be built using the concept of the Venturi tube. Such a design will transfer water through the barrage extremely efficiently but it must be completely submerged. More conventional sluice gates usually need to be larger than Venturi tubes to provide the same rate of transfer.

Many of the rivers suitable for tidal development carry significant ship-borne trade and water traffic. To enable ships and boats to continue to use a river, ship locks must be included in the barrage. There must also be facilities to allow fish and other forms of marine life to pass the barrage. This is particularly important if the river is one used by migratory fish such as salmon.

Environmental considerations

Construction of a barrage across a tidal river is bound to affect the conditions on both sides of the structure. Water movement patterns will be changed, sedimentation movement will be affected, and the conditions at the margins of the estuary on both the landward and seaward side of the barrage will be altered. For a barrage across an estuary, the movement of marine animals is likely to be restricted too. This could have a dramatic effect on both marine and avian life.

The major effect of the barrage will be on water levels and water movement. Water levels will be altered on both sides of the barrage and the tidal reach may change behind the barrage, although the effect will be reduced as the distance from the barrage increases. Some areas that were regularly exposed at low tide will be continuously under water after the barrage is constructed. Though the volume of water flowing down the river should remain the same, patterns of movement will be changed.

Sedimentation will be affected in complex ways. The tidal waters of an estuary frequently bear a great deal of sediment. Some is brought in from the sea, some carried downstream by the river. Changes in current speeds and patterns caused by the interpolation of a barrage will affect the amount of sediment carried by the water and the pattern of its deposition. This will, in turn, affect the ecosystems that depend on the sediment.

Other areas of concern involve animal species. The effect on fish, particularly migratory species, is significant. Fish gates can be built to permit species to cross the barrage. Many can also pass through the sluice gates. However, there is a danger that fish will pass through the turbines too, being injured in the process. Various methods have been explored to discourage fish from the vicinity of the turbines, with patchy success.

Many species of birds live on mud flats in estuaries. There is a possibility that such mud flats would disappear after a barrage had been built, and with them the birds whose habitat they formed. Salt marshes adjacent to estuaries are also likely to be affected. Studies have been conducted at potential U.K. barrage sites to try and estimate the scale of such effects but much work remains to be done in this area.

The effects of a tidal lagoon or bunded reservoir are likely to be less dramatic than those associated with a coastal barrage. Since none has yet been built, the range of effects is currently unknown.

Elsewhere, global experience with tidal power plants is limited. What evidence there is suggests that such projects have no major detrimental effect on the environment. The evidence from La Rance, in particular, has provided no serious cause for alarm. Even so, it would be dangerous to make any assumptions. An extremely careful environmental impact assessment would form a vital part of any future tidal project.

It is important to remember when considering tidal barrage and similar projects that while these projects change the local environment, they do not destroy it. As with hydropower projects discussed in Chapter 8 there will be ways of mitigating the effects, and the new environment created after barrage construction may be as rich as the one it replaces. However, any tidal project is likely to provoke fierce debate.

Cost of electricity generation from tidal barrage power plants

A tidal barrage power plant will generally be a highly capital-intensive project and the cost of electricity during the period associated with repayment of any loan associated with the project will be higher than for a similarly sized hydropower plant, because the tidal power plant cannot operate continuously but only during certain periods of the tidal cycle.

While the high initial capital outlay is likely to provide a disincentive for many project developers, the longevity of a tidal barrage power plant means that once the loan has been repaid the plant will enjoy a long life providing low-cost electricity. The tidal power plant at La Rance, France, has been operating for over 40 years and is likely to continue for another 40 years while the electricity produced by the plant is cheaper than French nuclear power.

The most recently constructed tidal power plant is the Sihwa plant in South Korea. This 260 MW project is estimated to have cost $250 m, or just under $1000/kW. However, the power plant was installed into an existing barrage so the costs for this scheme cannot be considered typical.

One of the most intensely studied barrage projects is that on the Severn River in the United Kingdom. The latest study was carried out by the U.K. government and published in 2010.¹ This study examined a number of possible schemes across the river, concluding that the largest, with an installed capacity of 8640 MW, would have a cost between £23.2 bn and £34.3 bn or £2690–3970/kW. This figure was disputed by a consortium that claimed the barrage could be built for £17–£18 bn, a unit cost of around £2000/kW. The lifetime of the plant was estimated in the U.K. government report to be 120 years. While the initial cost of a barrage will be large, it can be partially mitigated if the structure can also provide a road or rail crossing. In the case of the Severn barrage, where no transport crossing was proposed, the U.K. government suggested that the cost of electricity from the plant would be more expensive than offshore wind or nuclear power.

Another proposed large tidal barrage project is at Incheon, South Korea. This 1320 MW scheme has an estimated cost of $3.4 bn, or $2580/kW. Work on the project is currently scheduled to start in 2014 with completion in 2019.

Tidal lagoons have been claimed to be cheaper to build than barrages but opinions differ. An independent review of a proposed 60 MW tidal lagoon project in the same region as the proposed Severn barrage in the United Kingdom put the cost of this project in 2006 at £234 m, or £3900/kW. This was significantly higher than the cost proposed by the company that had developed the scheme of £82 m or £1370/kW.

The preceding costs are higher than typical costs for hydropower plants of a similar size. Even so they could be economically viable. In most cases, however, such projects will only be built if they can serve additional uses and so attract public sector investment to help support the financing.