Barriers against tidal power3 September 2003
In the first of a series of articles, E Van Walsum argues that the most promising tidal power projects around the world will not be developed unless environmental concerns are faced head-on
ENORMOUS amounts of tidal energy are being dissipated twice a day, every day on the world's ocean shelves. Coastal zones around the world where the average tidal range is in excess of 4.6m would be the most likely targets for early tidal power development. The table right shows a summary of potential tidal power plants (TPPs) which have been identified through numerous studies during the last half century. The table indicates that 1,288,133GWh of tidal energy can be readily developed around the world as soon as the hurdles to environmentally sustainable tidal power have been cleared.
How to go about realising the world's tidal power potential in an ecologically sustainable fashion is the main contribution this paper intends to provide to the current debate. This first part of this three-part series concentrates on single-basin TPPs. Because of the suggestions made in the past to consider multi-basin schemes for the development of tidal power in France, the UK, the US and more recently for the Derby TPP in Western Australia's Kimberley district, the second part of this series will deal with multi-basin TPPs.
Traditionally, engineering feasibility studies concentrated on finding the most economically attractive scheme. That scheme was then subjected to environmental scrutiny. This approach was considered acceptable in the past. On the basis of a prominent example, it will be shown that such a conventional study sequence in analysing a project's feasibility is wasteful when it comes to tidal power. Along our world's shorelines, the environmental concerns are so numerous and serious that, if they are not faced head on from the start, a great deal of engineering time and effort will be wasted on a project that was doomed to being discarded on environmental grounds.
Tidal power around the world
Two of the TPPs in Table 1 are not potential plants but are energy producers already in successful operation. The 240MW experimental La Rance tidal power project in Brittany, France was commissioned in 1966. This plant (operated by Electricite de France) is equipped with 24 bulb-type turbine generators. The turbines measure 5.35m diameter with generators rated at 10MW. These machines were designed to generate energy on either the incoming or outgoing tide, to pump at periods of slack tide either into or out of the basin and to serve as orifices, passing water either into or out of the basin. The plant therefore could, and quite often did, operate as a single high-basin plant, generating energy on the outgoing tide. With the given versatility of its turbine generator equipment, the plant also operated equally well as a single low-basin plant, generating energy during the incoming tide. In addition, it operated at times as a single-basin double-effect plant, generating energy on both the incoming and outgoing tides. With the plant's pumping capabilities it pumped at certain slack tides water either into or out of the basin to create extra head and volume at a minimal cost in energy. All this shifting from one operating mode to another was aimed at producing energy on the demands of a sun-oriented economy. While the pumping feature is a clever idea, it can only be used when excess generating capacity for pumping is available in the power system and when there is a demand for the extra energy.
With this large degree of flexibility, edf could experiment with various modes of single-basin TPP operation aimed at finding the most profitable mode. Initially, environmental concerns did not play a large role in EDF's deliberations. As environmental considerations came into play, the La Rance plant has generally been operated in one mode.
The Annapolis pilot TPP in Canada's Bay of Fundy on the Atlantic coast in the province of Nova Scotia features one Straflo‚ rim-type turbine generator with a 7.6m diameter turbine and a generator with a 20MW capacity. It is a modern version of the axial flow turbine with rim-type generator, patented by Leroy Harza in 1919. This single high-basin plant was inaugurated in 1984 and has been in successful operation since.
In Table 1, only projects considered by the author to be technically feasible within today's industry and economically viable in the foreseeable future have been listed.
The table shows that appreciable amounts of tidal energy can be generated in extremely isolated locations such as Ungava Bay in northern Québec, Canada, and Siberia, Russia. If and when the hydrogen economy materialises, these remote resources would become ready to be harnessed to provide the energy for hydrogen manufacture by hydrolysis. For Ireland and West Africa, the author is unaware of any tidal power assessments.
Excluded from Table 1 are four operating TPPs in China. These plants were omitted because, in terms of energy produced per year, they are insignificant. Yet what is not generally realised is that China, in a very modest way, has a more varied experience in designing, building and operating tidal power plants than any other nation in the world. The Chinese have experimented with a variety of plants. By the end of 1984, there were eight TPPs in operation in the country. Since 1984, four of those plants have been closed down. China's tidal power experience may be described by three of their plants:
The Jiangxia experimental TPP is located in Zhejiang province, about 200km to the south of the scenic city of Hangzhou.The plant was built in the dry within the left bank, behind cofferdams, and operates in double effect. The first 500kW bulb unit was commissioned in May 1980, with the second, a 600kW unit, in June 1984. By the end of 1985, five units were in operation. The third to fifth units each had a rated capacity of 700kW. The installed capacity with five units amounts to 3200kW. The sluice structure, originally built as part of a land reclamation project, has five openings 4.2m high by 3.3m wide, controlled by reinforced concrete gates. The highest basin level is restricted to 1.2m. Approximately 3.8 km2 of land was reclaimed in the basin above El. 1.2m, which was utilised to plant orange trees, sugar cane, cotton and rice. The inter-tidal zone of the basin with an area of 1.2km2 is used for oyster culture and clam fishery. The basin area at lowest low water is 0.8km2. The plant is still in operation, producing 6GWh of energy per year.
The Shashan TPP began as a single, high-basin plant. Starting out with a wooden turbine, the plant provided mechanical energy for the grinding of grain. In 1964, the wooden turbine was replaced by a steel runner with a matching 40kW generator. The plant produced 0.1GWh in 1984, which was used for irrigation and by 860 rural households. (Cheng 1985). This plant has since been closed down.
The Haishan TPP is noteworthy as it is the only linked-basins plant in existence in the world a plant featuring a high and a low-basin with the power plant in between these two basins, generating energy from water flowing from the high into the low-basin. The plant is located on Maoyan Island in Zhejiang province where it serves an isolated community of 760 families. The plant was designed for two 75kW units of which only one was installed and commissioned in 1975. This unit operated continuously. The energy was used partly to pump fresh water for domestic and irrigation use into the community reservoir. The plant has since been upgraded to an installed capacity of 0.25MW, producing 0.34GWh per year.
From an environmental perspective, it would seem that China is not concerned about changing the existing environment, as long as the newly created one is clean and productive and in that sense sustainable. That approach might well be acceptable in view of the small scale at which tidal power has been developed in China to the present. One would expect however that, like most other seas, the Yellow and East China Seas are host to migratory fish species which need access to coastal bays, estuaries and rivers for their feeding and procreation To what extent this has been taken into account by China's TPP engineering community is not clear.
Not listed in Table 1 are coastal zones with average tidal ranges of less than 4.6m. Such zones might be considered for tidal power developments at a later time.
In order to come to grips with the challenge of realising the world's tidal power potential in an environmentally sustainable fashion, the author has chosen by way of example the upper Bay of Fundy on Canada's east coast in the provinces of New Brunswick and Nova Scotia.
Bay of Fundy
Extensive engineering and economic studies into the feasibility of TPPs in Canada's Bay of Fundy from the 1960s until the late 1980s defined three preferred sites in the upper Bay of Fundy where economical, large-scale TPPs might be constructed. These sites were Shepody Bay, Cumberland Basin and Cobequid Bay.
It will be shown herein that the conventional engineering approaches used in these studies are wasteful as they lead to conclusions which, although economically optimum, proved to be environmentally unacceptable. The decisive environmental constraints could have been identified at the start of the studies to serve as a guide to the engineers in defining sustainable TPP alternatives.
Being well aware that hydro plants tend to be more economical the higher the hydraulic head, the studies emphasised single, high-basin TPPs the operation of which results in the largest possible heads obtainable (without resorting to pumping). Such plants feature a single basin, enclosed by a dam, equipped with sluices and turbine generators through which that basin would be filled to the highest possible level during the incoming tide. During this basin filling operation, the turbine generators would not generate energy but act as orifices. By closing all sluices and turbine generators, the water in the basin is kept at its highest level until the ocean waters have sufficiently receded for the turbine generators to start generating energy on the outgoing tide. Generation stops as soon as the available head has become inadequate. The turbine generators are then closed down and the water level in the basin is kept constant until the ocean waters rise up again with the incoming tide and the water in the basin can again be replenished to its highest possible level, ready to repeat the cycle. This type of TPP will herein be referred to as a Type X plant. Numerous analyses have confirmed that such plants produce, under almost all conditions, the maximum possible amount of energy out of any given tidal power basin.
At Fundy, with its extremely large tides, it appeared that under spring tide conditions, a double effect plant will yield more energy. Nevertheless, the emphasis of all Fundy studies remained centred on the Type X plant concept. Such a plant produces two blocks of tidal energy per 'moon day' of 24h 50min. With such a plant at each of the three preferred sites, the amounts of energy produced annually and the unit costs of such energy in C$ of 1976 could be predicted.
In the year 2000 the total amount of energy generated by New Brunswick Power was of the order of 18,000GWh and by Nova Scotia Power 12,000GWh. Tidal power from Fundy could therefore be a major contributor to the local energy scene. A study update (TPC, 1982) put the spotlight on a single-basin, single-effect plant at Cumberland Basin. The question as to what kind of single-basin TPP should be built was not decided (Clark, 1993).
The author presented his engineering approach to the development of the Bay of Fundy's tidal power resources in an article in 1999. Placing the emphasis from the start on the environment led in the year 2003 to different conclusions.
The fraternity of marine biologists and oceanographers in Canada's Maritime provinces and New England, aware of a multitude of pressures affecting the environment of the region, joined forces under the name of BoFEP the Bay of Fundy Ecosystem Partnership. Participating are scientific enterprises, community groups, resource users, government agencies and private sector groups from both Canada and the US. Various environmental effects which should be addressed when contemplating the construction of a TPP in the upper reaches of the Bay were identified.
At several sites around the world, substantial tides coincide with a high silt content in the tidal waters. This is not surprising since the turbulent tidal flows tend to erode the shorelines and stir up silt from the estuarine floor. Some exceptions occur in tidal areas in a hard, non-erodable rocky environment. In the past, dams across tidal estuaries were usually built to protect agricultural land from flooding or to provide an economical crossing for terrestrial traffic. The designers of such dams set out to meet the specified requirements at the lowest possible cost. The resulting dams often interrupted tidal movement. This resulted in massive accumulations of silt. With TPPs however, a vigorous tidal movement is essential for the plant's profitable performance a totally different proposition.
With single-basin TPPs such as the La Rance and Annapolis plants, silt-laden water flows in and out of the basin at the same end. Some sedimentation at the head of such basins is unavoidable. At La Rance, after 33 years of operation, it was concluded that the dredging of 1Mm3 of marine sediment and 1Mm3 of fluvial sediment will be required, (C.O.E.U.R. 1999). At Annapolis, after 19 years of operation, the need for dredging has not as yet become evident.
In order to avoid siltation which would impede the free flow of water in and out of tidal power basins, engineers designing, building and operating such plants should aim to: avoid stagnant water; create simple flow patterns, straight in and straight out of the basins, preferably along the same route; and encourage strong currents by having the largest possible volumes of water moving in and out of tidal power basins. At certain locations it is feasible to create within a tidal basin uni-directional flow, ie water flows into the basin at one end, through the basin and out at its other end. This results in a very effective flushing mechanism and is the most favourable configuration for the purpose of siltation control. This configuration, which will be referred to as the flow-through layout, could be applied to either a single-basin Type x or Y plant or linked-basins plant.
B. Shorebird survival
Every year, hundreds of thousands of Sandpipers return from their spring breeding grounds in the far north on Hudson's Bay via the Bay of Fundy to their over-wintering grounds along the coasts of South America. As they invade Fundy's mud-flats, they feed mainly on a tiny, shrimp-like creature, the Corophium, which thrives in the Fundy mud (BoFEP 1996-1). For these and other shorebirds to survive, the preservation of existing and the creation of new tidal flats will be essential. In designing a TPP, engineers should therefore aim to: maximise the tidal range within the basins, resulting in extensive tidal flats; provide opportunities for new tidal flats to develop within and outside the tidal power basins; operate the plant in a consistent pattern; and keep the new mean water levels within the tidal power basins as close as possible to the natural ones, thus preserving the most productive portion of the inter-tidal zone.
C. Fish-stock survival
Numerous species of migratory fish utilise the estuaries of the Bay of Fundy. Tagging experiments indicate the fishes originate from stocks derived over the entire North American Atlantic coast from Florida to Labrador. These migrations are an integral part of the life history of the respective species and appear to be controlled in part by the near-shore movements of ocean currents, (Dadswell, 1994). Tests with a high-frequency-sound fish diversion system at the Annapolis plant proved to be close to 50% effective for some species, ineffective for others. (Gibson et al, 2002). Similar results have been reported for strobe-light fish diversion systems. Screens or nets, preventing fish from approaching the turbines, would have to be designed for a large range of fish sizes, and would likely get choked with debris and reduce the head across the turbines. Fish by-pass sluices were proven to be ineffective (Dadswell, 1994).
To secure the survival of migrating fish stocks, the designers and operators of any new TPP will have to remove or minimise the various causes of fish mortality at TPPs (Dadswell et al, 1986). This includes minimising the head across the turbines; the rate of pressure drop and velocities through the turbines; and minimising water velocities through the turbines, their rotating speed, number of blades and the number of obstructions within a water passage maximising the turbines, size and efficiency, avoiding turbulence all along the water passages, operating the plant in a consistent pattern.
D. Sea mammal survival
Seals and porpoises frequent Annapolis Basin. Dolphins prefer the clear waters of the open ocean although some might stray at times close into shore. Whales do frequent Nova Scotia's coastal waters but generally do not migrate beyond the outer reaches of the Bay of Fundy, (BoFEP, 1996-2), (Kraus et al, 1982). There is however no evidence of collisions between any of these sea-mammals and the Annapolis TP turbine. The Whale Center of New England reported to the writer that there is research going on aimed at finding ways to avoid collisions between whales and large ships. Such research might also explain what kept sea-mammals away from the Annapolis turbine during the last 19 years. Co-operation between biologists, studying sea-mammal behaviour, and the tidal power engineering community is recommended.
E. Drainage of agricultural lands
Around the Bay of Fundy, the farmers' main concern is the effective drainage of the low-lying farm lands protected by dikes. Such drainage is achieved by means of flap gates set in culverts, traditionally referred to as aboideaus. These flap gates open automatically as soon as the level of the tidal water drops below that of the fresh water behind the dikes. When creating a tidal power basin, the agricultural community will want to see a mean-water level in the basin at the same level as or below the natural mean water level and, if possible, an increased area of arable land.
F. Changes in the tidal regime
The computer modelling of the Bay of Fundy Gulf of Maine Basin has shown that major TPPs can have a noticeable effect on its tidal regime. Such a side effect might be good or bad for certain creatures or interests, but the number of claims for compensation for perceived damages would be numerous and would likely multiply with time. To avoid such a legal hassle, the effect of a TPP's operation on the tidal regime beyond the plant's borders should not be noticeable. To achieve this objective it may be necessary to: maximise the amount of water flowing in and out of the tidal basin per tidal cycle, maximise the tidal range; operate the plant as close as possible to the rhythm of natural tides; operate the plant in a consistent pattern.
Single basin TPPs
As recommended at the conclusion of the Fundy engineering studies of the 1960s, 1970s and 1980s, a single-basin TPP at Cumberland basin should be considered. The range of possibilities implied by this recommendation is illustrated by the versatile La Rance single basin TPP. By accepting for the future development of tidal power the ecological dictum that a TPP shall be operated in a consistent manner, one type of plant shall be chosen. Thence our choice for a single-basin TPP is to be made from: a single, high basin single-effect TPP, hereinafter referred to as Type X; a single, low-basin single effect TPP, hereinafter referred to as Type Y; and a single-basin, double effect TPP, hereinafter referred to as Type Z.
The same ecological dictum eliminates pumping. When comparing for a specific tidal power basin the energy output of a Type X plant with that of a Type Y, it is clear that the high-basin Type X will yield more energy since its reservoir has a larger capacity than that of the low-basin Type Y, due to the slope of the shorelines. When energy production is the objective, then the choice to be made will be between Types X and Z. (A low-basin Type Y plant could be considered if land reclamation were also an objective).
As mentioned earlier, it is feasible at certain locations to create within a tidal basin uni-directional flow. Such a flow-through layout is particularly attractive when siltation is expected to create problems. For all other environmental factors this special layout has no further advantages or disadvantages. The general discussion on choosing the optimum single basin plant will therefore be limited to choosing between single effect, Type X and double effect, Type Z. In places where a flow through layout is feasible, it can be readily applied to Type X, giving it a competitive edge over Type Z when siltation is an important consideration. The flow-through layout, if applied to Type Z, would require a doubling of the generating capacity, making it economically unattractive.
In order to compare a Type X plant with a Type Z, one will have to take note of the various aspects of a TPPs newly created tidal regime within and outside its basin, the maximum head across and water velocities through its turbines, the duration of periods of stagnant flow per tidal cycle, the flow pattern characteristics and the number of times a fish would pass through the turbines on its way in and out of a TPPs basin. These aspects are jointly referred to as a TPP's parameters.
The numerical values of some of the parameters of the Type X and the Type Z TPP as determined for Fundy's Cumberland Basin, are presented in the second and third columns of Table 3 respectively. Each of the listed parameters influences one or more of the five environmental effects A, B, C, E and F.
By considering how each of these effects is influenced by the various parameters, score can be kept as to how the environmental performance of the Type X plant compares with that of the Type Z plant as shown in the 4th to 8th columns. Thus it becomes clear that the Type Z plant is environmentally superior to the Type X plant for each of the environmental effects listed. All in all, the single-basin double-effect plant sets the toughest environmental standard, forming the benchmark on the basis of which other competing schemes will have to be judged.
Preferred study route
For a single-basin TPP at Fundy's Cumberland Basin, a Type Z plant is qualitatively the preferred environmental choice. In predicting its
environmental consequences it will be possible to model mathematically what the new tidal regime will be within and outside the TPP's basin which will take care of effects E and F, in the larger Bay of Fundy - Gulf of Maine Basin.
Some engineers claim that similar quantitative judgments can be made on effect A, Siltation (Whitehouse 2000). Within the near future, this may well be confirmed by other sediment specialists. The ecological effects B and C defy mathematical modelling since the behaviour of the several migrating species is difficult to predict and then to express in numerical terms. The only feasible way of gauging such ecological responses will be by means of a small-scale, pilot-plant-type experiment. The preferred study route should therefore start off with the design, construction and operation of an environmentally oriented Type Z pilot TPP. After a few years of operation of such a plant can the consequences of a large scale project on effects B and C be judged and developments on a larger scale can be undertaken. The outcome of the proposed pilot plant operations can at present not be foreseen. It will be assumed for the time being that those pilot-scale tests will prove to be successful so that the double effect, single basin TPP at Cumberland Basin may be assumed as the standard against which all other TPP's are to be judged.
In order to arrive at realistic estimates of cost, the author used reference BFTPRB 1977, Reassessment of Fundy Tidal Power, as his source of information. That report proposed for Cumberland Basin a Type X TPP equipped with 37 synchronous turbine generators, the turbines having a runner diameter of 7.5m and the generators a 31MW limiting output. The proposed sluice capacity was 24 sluices, each measuring 12.2m x 12.2m. The annual energy output of this Type X plant was by author calculated to be 3154GWh with an energy cost, based on the use of fish-friendly turbine generators, of 2.7¢/kWh ('76 Can$). Similarly, the same report proposed for Shepody Bay a Type X TPP with 53 similar turbine generators and 30 sluices of identical size. The annual energy output of this plant was by author calculated to be 4461GWh with an energy cost of 3.4¢/kWh ('76 Can. $), again based on the use of fish-friendly turbine generators. (Note, all costs are maintained in 1976 C$ as presented in the major source of costs, the BFTPRB1977 report. To obtain costs in 2003C$, multiply costs as given by 2.7).
To assess the cost of a TPP designed and operated to meet all sustainability requirements, the author assumed for Cumberland Basin a Type Z plant with 61 variable speed turbine generators with runners of 7.5m diameter and a generator limiting capacity of 19MW without any sluices. The elimination of all sluices is justified by the fact that such sluices would have to be built to resist pressure from both sides and should open and close when under pressure. Simple flap gates would not do the job. Vertical lift gates would be required for a total of only two operating hours per tidal cycle of 12hr and 25min. Such gates would not return their cost of installation, operation and maintenance.
The annual energy output of a Type Z plant at Fundy's Cumberland Basin as described above was calculated to be 3771GWh. The cost data were by extrapolation derived from BFTPRB 1977, but with turbine generator costs for fish-friendly machines based on information supplied by two manufacturers in February 2003. This resulted in an energy cost of 3.4¢/kWh.
Similarly for Shepody Bay, a Type Z plant was assumed, featuring 83 similar turbine generators and no sluices. The annual energy output of this plant was by author calculated to be 5191GWh at an energy cost of 4.1¢/kWh.
The La Rance and Annapolis TPPs were built to advance the art of low-head hydraulic turbine design. Technically speaking, the state-of-the-art bulb turbine generators at La Rance, as well as the Straflo turbine generator at Annapolis, were successful prototype machines in that they eventually met all technical expectations. Through these plants, the state of the art of low-head turbine generator design was advanced, setting the stage for profitable future applications. In the design and construction of these machines, as well as in the way these plants were initially operated, very little attention was paid to environmental considerations. To advance quantitatively on the environmental front, a Pilot TPP of Type Z will be required, ie a single- basin, double-effect plant, operated for optimum environmental performance. Five of the most prominent environmental effects in the Fundy setting can be partly optimised by the design, construction and method of operation of the plant. In low-head hydro power plants, in addition to choosing the right kind of plant and operating mode, the ideal solution to fish survival would be to make the turbines fish-friendly so that fish of all species and sizes can pass through the turbines.
Since 1996, several manufacturers of hydraulic turbines have been working on ways and means to make their machines more fish-friendly. To tap into this, it is suggested to start a pilot TPP with a call for tenders for the mechanical-electrical machinery for such a plant, based on a specification that would reflect all the various ways in which the most prominent environmental effects can be environmentally optimised in a qualitative sense. Once the machinery has been chosen the plant would be designed around it in close cooperation with the machinery manufacturers.
Within the realm of low-head hydro power, TPPs are particularly attractive in that they do not flood land, displace people or cause water-borne diseases. TPPs can be created in a number of configurations and operating modes. From among those numerous possibilities, single-basin TPPs were assessed for environmental sustainability. It was found that in the upper reaches of the Bay of Fundy, with its specific environmental concerns, single-basin TPPs operating in double-effect, i.e. generating energy on both the incoming and outgoing tides, are qualitatively superior to other types of single-basin TPPs.
In order to decide on the most effective course of study to follow in determining the optimum, sustainable TPP for a given location, the environmental concerns for that site should be identified at an early stage.In areas where the mean tidal range is in the order of 9m and up, large-scale, single-basin, double-effect TPPs can offer an economical and environmentally benign source of electrical energy provided environmental issues are studied quantitatively through the operation of a pilot TPP and the results of such studies prove that a TPP can exist within a thriving tidal environment.