Tidal energy

Tidal energy is predictable to the minute for at least the rest of the century. Tide levels can be affected by storm surges as experienced dramatically in the UK in 1953. The British Isles benefit from some of the greatest tidal ranges in Europe. In summary, there are at least four technologies that can exploit the action of the tides, offering reliable electricity in the multi-gigawatt range. They are:

• The tidal barrage

• The tidal fence or bridge

• Tidal mills or rotors

The tidal barrage

Trapping water at high tide and releasing it when there is an adequate head is an ancient technology. A medieval tide mill is still in working order in Woodbridge, Suffolk. In the first quarter of the twentieth century this principle was applied to electricity generation in the feasibility studies for a barrage across the River Severn.

Tidal power works on the principle that water is held back on the ebb tide to provide a sufficient head of water to rotate a turbine. Dual generation is possible if the flow tide is also exploited.

A Royal Commission was formed in 1925 to report on the potential of the River Severn to produce energy at a competitive price. It reported in 1933 that the scheme was viable. Since then the technology has improved including a doubling of the size of generators. This increases the volume of water passing through the barrage by the square. A further study was completed in 1945 and the latest in-depth investigation was concluded in 1981. In all cases the verdict was positive, though the last report was cautious about the cost/benefit profile of the scheme in the context of nuclear energy. Despite this supporting evidence the UK still shows reluctance to exploit this source of power. Recently a discussion document produced by the Institution of Civil Engineers stated in respect of tidal energy:

it appears illogical that so potentially abundant an option will be deferred perpetually when the unit power costings involved are estimated to be reasonably competitive with all alternatives except combined cycle gas turbines.

Power generation is obviously intermittent but the spread of tide times around the coasts helps to even out the contribution to the grid.

The only operational barrage in Europe is at La Rance, Normandy. It is a bidirectional scheme, that is, it generates on both the flow and ebb tides. Two-way operation is only beneficial where there is a considerable tidal range and even then only during spring tides. Annual production at La Rance is about 610 gigawatt hours (GWh). Despite its success as a demonstration project, the French government elected to concentrate its generation policy on nuclear power which accounts for about 75 per cent of its capacity.

Tidal Barrage

Figure 3.2 Up to now, schemes proposed in the UK have been one direc-

Basic tidal barrage tional, generating only on the ebb tide. The principle is that water is held upstream at high tide until the downstream level has fallen by at least 2.0 metres. The upstream volume of water is supplemented by pumping additional water from downstream on the flood tide. This is reckoned to be more cost effective than bidirectional generation in most situations (Figure 3.2).

The technology of barrages was transformed by the caisson techniques employed in the construction of the Mulberry Harbour floated into place after D-Day in the Second World War. It is a modular technique with turbine caissons constructed on slipways or temporary sand islands. According to the Department of Trade and Industry's Energy Paper Number 60, November 1992: 'The UK has probably the most favourable conditions in Europe for generating electricity from the tides.' In fact, it has about half of all the European Union's tidal generating potential of approximately 105 terawatt hours per year (TWh/y) (ETSU). The DTI report concludes:

There are several advantages arising from the construction of tidal barrages in addition to providing a clean, non-polluting source of energy. Tidal barrages can assist with the local infrastructure of the region, create regional development opportunities and provide protection against local flooding within the basin during storm surge.

Around the world numerous opportunities exist to exploit tidal energy, notably in the Bay of Fundy in Canada where there is a proposal to generate 6400 MW. China has 500 possible sites with a total capacity of 110 000 MW.

Professor Eric Wilson, a leading tidal expert in the UK, sums up the situation by saying that a tidal power scheme may be expensive to build, but it is cheap to run. 'After a time, it is a gold mine.'

In 1994 the government decided to abandon further research into tidal barrages for a variety of reasons ranging from the ecological to the economic. In market terms a normal market discount rate heavily penalises a high capital cost, long life, low running cost technology. The economic argument could be countered if the market corrections stated earlier were to be implemented. However, another concern has grown in stature, namely, the threat from rising sea level amplified by an accelerating rate of storm surges.

Following the 1953 floods, it was decided that London should be protected by a barrage. It was designed in the 1970s to last until 2030. However, the threat from rising sea level was hardly a factor in the 1970s; now it is a major cause of concern that the barrage will be overwhelmed by a combination of rising sea level, storm surges and increased rainfall and river rundown well before that date. In the year 1986/87 the barrage was not closed once against tidal and river flooding; in 2001 it closed 24 times. A further complication is the Thames Gateway project which includes 120 000 new homes below sea level. If one flood breaks through the Thames Barrier it will cost about £30 billion or roughly 2 per cent of GDP (Sir David King, Government Chief Scientist, The Guardian, 9 January 2004). All this combines to make a strong case for an estuary barrage that will protect both the Thames and the Medway and, at the same time, generate multi-gigawatt power for the capital (Figure 3.3).

One of the arguments against tidal barrages is that they would trap pollution upstream. Since rivers are now appreciably cleaner than in the 1970s, thanks largely to EU Directives, this should not now be a factor. The Thames is claimed to be the cleanest river in Europe, playing host to salmon and other desirable fish species. A group of engineering companies has renewed the argument in favour of the River Severn barrage, indicating that it would meet 6 per cent of Britain's electricity needs whilst protecting the estuary's coastline from flooding (New Scientist, 25 January 2003).

The tidal fence

There is, however, an alternative to a barrage which can also deliver massive amounts of energy at less cost/kWh, namely, the tidal fence or

Thames Flood Barrier

Figure 3.3

River Thames flood risk zones below 5m contour and suggested barrage

Figure 3.3

River Thames flood risk zones below 5m contour and suggested barrage bridge which has only recently come into prominence. The tidal fence system, for example as designed by Blue Energy Canada Inc., consists of modular shell concrete marine caissons linked to form a bridge. Vertical axis Davis Hydro Turbines are housed between the concrete fins. Multiple Darrieus rotors capture energy at different levels of the tide. The rotors are 10.5 m in diameter and rotate at 25 rpm, each turbine having a peak output of up to14 MW. They can function within a tidal regime of at least 1.75 m. The generators are housed in the box structure bridge element which can also serve as a highway or platform for wind turbines (Figure 3.4).

From the ecological point of view the system has the advantage over the barrage option of preserving the integrity of the intertidal zones. Wading birds have nothing to fear. The slow rotation of the turbines poses minimum risk to marine life, with large marine mammals protected by a fence with a backup of an automatic braking system operated by sonar sensors. At the same time the system allows for the free passage of silt.

In terms of energy density, the tidal fence outstrips other renewable technologies:

Wind Solar (PV) Wave Tidal fence

1000 kWh/m2 1051 kWh/m2 35-70 000 kWh/m2 192 720 kWh/m2

(Source: Blue Energy Canada Inc.)

Blue Energy has designed a major installation at Dalupiri in the Philippines. It is a four-phase project with the first phase comprising a 4 kilometre tidal fence between the islands of Dalupiri and Samar in the

Figure 3.4

Blue Energy tidal fence concept

San Bernardino Strait. The estimated maximum capacity of the 274 turbines housed in the tidal fence is 2.2 GW guaranteeing a base daily average of 1.1 GW. The structure is designed to withstand typhoons of 150 mph and tsunami waves of 7 m.

The potential for the UK

Many speculations have been offered regarding the ultimate generating potential of various renewable technologies. The data which are used here have been extracted from a paper from the Tyndall Centre in the University of Sussex, UK 'Electricity Scenarios for 2050' Working paper 41, 2004, by Jim Watson which, in turn, cites data from the DTI 1999 and the RCEP 2000. The Tyndall paper suggests that the optimum output from renewables is 136.5 GW as defined in the first of four scenarios Many of these are intermittent and unpredictable. An exception is tidal energy which is predictable and this is where the tidal fence comes into its own.

The British Isles offer considerable opportunities for the application of this technology. Blue Energy has already identified the Severn estuary as a suitable site. The Open University Renewable Energy Team has selected 17 estuary sites suitable for medium to large-scale barrage systems (Boyle, G. (ed.) (1996) Renewable Energy - Power for a Sustainable Future, Oxford University Press). On the assumption that

Scale Barrage System Canada

these sites would be equally suitable for tidal fences, they add up to a linear capacity of 208 km. If only half of the full estuary width were available to house turbines in each case, this would produce a peak output of about 60 GW and a daily average of 30 GW. This is based on an extrapolation from the Dalupiri scheme and is therefore only a rough estimation. However, it should be enough to cause a reappraisal of the tidal potential of the UK, especially as the cost is highly competitive. The installed cost at present is estimated to be US$1400 per kW.

Since the output from the tidal fence is predictable and peak output may not coincide with peak demand from the grid, it is an appropriate system to combine with pumped storage to even out the sinusoidal curves.

Tidal currents

The European Union has identified 42 sites around the coasts of the UK which have sufficient tidal velocity to accommodate tidal turbines. It is estimated that tidal stream energy has the potential to meet one quarter of the electricity needs of the UK which amounts to about 18 GW. With a load factor of 0.50, this technology would deliver 9 GW. A 1993 DTI report claimed that the Pentland Firth alone could provide 10 per cent, or about 7 GW, of the UK's electricity demand. However, the greatest potential source of tidal currents is located off the islands of Guernsey. According to Blue Energy they have the potential to generate 26 GW or more than one third of the UK's generating capacity.

There are several technologies being researched, including the Stingray project which exploits the tidal currents to operate hydroplanes which oscillate with the tide to drive hydraulic motors that generate electricity. The hydroplanes are profiled like an aircraft wing to create 'lift'. It is still at the development stage and its final manifestation will operate in streams in both directions.

However, the most likely technology to succeed in the gigawatt range are the vertical or horizontal turbines. The tidal fence vertical turbine is claimed to be ideal for tidal streams since it has multiple rotors which can capture tidal energy at different depths. The minimum velocity of tidal flow to operate a tidal fence is 1.75 m/s or 3.5 knots. The strength of the current tends to be strongest near the surface so a vertical series of rotors could accommodate the different speeds at various depths. An ideal site could be the Pentland Firth.

The tidal mill

Horizontal axis turbines are similar to wind turbines but water has an energy density four times greater than air, which means that a rotor 15 m in diameter will generate as much power as a wind turbine of 60 m diameter. They operate at a minimum velocity of about 2 m/s. Since the tidal flow is constant, underwater turbines are subject to much less buffeting than their wind counterparts.

Figure 3.5

Tidal stream turbines or tidal mills, serviced above water

According to Peter Fraenkel, Director of Marine Current Turbines, the best tidal stream sites could generate 10 MW per square kilometre. His company has built a 300 kW demonstration turbine off the Devon coast and has a project for a turbine farm in the megawatt range (Figure 3.5). This company is presently investigating the opportunities around Guernsey and Alderney.

Offshore impoundment

An alternative to estuary tidal generation is the concept of the tidal pound. The idea is not new as mentioned earlier. The system is ideal for situations in which there is a significant tidal range and shallow tidal flats encountered in many coasts of the UK. The system consists of a circular barrage built from locally sourced loose rock, sand and gravel similar in appearance to standard coastal defences. It is divided into three or more segments to allow for the phasing of supply to match demand. According to Tidal Electricity Ltd, computer simulations show that a load factor of 62 per cent can be achieved with generation possible 81 per cent of the time. Tidal pounds would be fitted with low-head tidal generating equipment which is a reliable and mature technology.

In its Memorandum submitted to the House of Commons Select Committee on Science and Technology this company claimed that 'The UK has very large tidal ranges and many suitable for sites . . . that could conceivably generate thousands of megawatts.' It has been estimated that impoundment electricity could meet up to 20 per cent of UK demand at around 15 GW. With a load factor of 0.62 this amounts to 9.3 GW.

Besides having the potential to generate substantial amounts of electricity, tidal pounds can also provide coastal flood protection which was an important factor in determining the viability of the first large-scale project in the UK off the North Wales coast. In 1990 Towyn near Rhyl experienced devastating floods. The pound will be about 9 miles wide and 2 miles deep and located a mile offshore. It should generate 432 MW. The life expectancy of the structure is 100 years.

This is a popular holiday coast and it is expected that the project will become an important visitor attraction. There is talk of added attractions like a sea-life musuem and an education centre. The tidal barrage at La Rance in Normandy attracts 600 000 visitors a year.

This is perceived as a cost-effective technology thanks in part to the extra revenue from the Renewables Obligation Certification. Because it is located in shallow water construction costs are much less than for barrage systems. It is relatively unobtrusive and much kinder to marine life than a tidal barrage. It offers predictable power with a load factor which is significantly better than, for example, wind power.

In total the potential capacity of the various technologies that exploit the tides around Britain is in the region of 65 GW. The variation in high water times around the coasts coupled with pumped storage help to even out the peaks and troughs of generation before any account is taken of the range of other technologies.

Wave power

Wave power is regarded as a reliable power source and has been estimated as being capable of meeting 25 per cent or 18 GW of total UK demand with a load factor of 0.50, giving a reliable output of about

9 GW, and is already contributing 500 kW to the grid.

The World Energy Council estimates that wave power could meet

10 per cent of world electricity demand.

The most favoured system uses the motion of the waves to create an oscillating column of water in a closed chamber which compresses air which, in turn, drives a turbine. There are both inshore and offshore versions either in operation or projected. The first inshore version in the UK was positioned on an inlet in the Scottish Isle of Islay (Figure 3.6).

It was designed by Queen's University, Belfast, and has an output of 75 kW which is fed directly to the grid. The success of this pilot project justified the construction of a full-scale version which is now in operation.

A 25 metre slit has been cut into the cliffs facing the north Atlantic at Portnahaven to accommodate a wave chamber inclined at 45 degrees to the water. Two turbines are driven by positive pressure as air is compressed by incoming waves, and negative pressure as the receding waves pull air into the chamber. The rather clever Wells turbine rotates in one direction in either situation. It is rated at 500 kW which is enough to power 200 island homes.

Currently under test in the Orkneys is a snake-like device called Pelamis which consists of five flexibly linked floating cylinders, each of 3.5 m diameter. The joints between the cylinders contain pumps which force oil through hydraulic electricity generators in response to the rise and fall of the waves. It is estimated to produce 750 kW of electricity. The manufacturer, Ocean Power Devices (OPD), claims that a 30 MW wave farm covering a square kilometre of sea would provide power for 20 000 homes. Twenty such farms would provide enough electricity for a city the size of Edinburgh.

Like Scotland, Norway enjoys an enormous potential for extracting energy from waves. As far back as 1986 a demonstration ocean wave power plant was built based on the 'Tapchan' concept (Figure 3.7). This consists of a 60 m long tapering channel built within an inlet to the sea. The narrowing channel has the effect of amplifying the wave height. This lifts the sea water about 4 m depositing it into a 7500 m2 reservoir. The head of water is sufficient to operate a conventional hydroelectric power plant with a capacity of 370 kW.

A large-scale version of this concept is under construction on the south coast of Java in association with the Norwegians. The plant

Working Principle Tidal Fences

Figure 3.6

Principle of the Isle of Islay OWC wave generator

Figure 3.6

Principle of the Isle of Islay OWC wave generator

Figure 3.7

Wave elevator system, the 'Tapchan'

Figure 3.7

Wave elevator system, the 'Tapchan'

Tide Mill Principle

will have a capacity of 1.1 MW. As a system this has numerous advantages:

• The conversion device is passive with no moving parts in the open sea.

• The Tapchan plant is able to cope with extremes of weather.

• The main mechanical components are standard products of proven reliability.

• Maintenance costs are very low.

• The plant is totally pollution free.

• It will produce cheap electricity for remote islands.

The total for the three tide and wave technologies, taking account of load factors, could come to about 74 GW.

If we substitute these figures for the quantities indicated in Jim Watson's paper (op. cit.) for wave, tidal stream and tidal barrage of around 16 GW, and add the remaining renewable technologies from this source amounting to 119 GW taking account of load factors, the total comes to about 193 GW. This amounts to more than twice the present electricity generating capacity of the UK.

The other part of the equation is the demand side and Watson's scenarios include a reduction in electricity demand of up to one third. Assuming significant gains in energy efficiency, even if half the natural assets of the UK are exploited to produce carbon-free electricity, this leaves an appreciable margin of supply over demand. The logical use for this surplus capacity is to maximise pumped storage and to create hydrogen from electrolysis. This could provide further backup capacity from megawatt grid connected fuel cells in addition to fuelling the growing population of hydrogen powered road vehicles expected over the next decade.

It has been estimated that converting transport to hydrogen would require 143 GW of electrical power to extract hydrogen from water via electrolysis.

There is no doubt that the UK has the natural assets to enable it to be fossil fuel free in meeting its electricity needs by 2030. However, this would require an immediate policy decision by the government to make a quantum leap in its investment in renewable technologies, especially the range of opportunities offered by the tides. Tidal energy could more than fill the void in supply left by the demise of nuclear. What is needed is cross-party political support so that the subject of renewable energy is removed from the cut and thrust of politics.

In his 'green speech' in March 2003 Prime Minister Blair stated that he wanted Britain 'to be a leading player in this green industrial revolution . . . We have many strengths to draw on. Some of the best marine renewable resources in the world - offshore wind, wave energy and tidal power.' This chapter suggests a 'road map' that would enable actions to be matched to words.

Chapter Four

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  • Ambrosia
    How is climate effected by tidal energy ?
    5 years ago

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