In part two of a series of articles on the development of tidal power projects, E Van Walsum tales a look at the environmental issues associated with double basin plants
PART ONE of this series was restricted to single-basin tidal power plants (TPPs), however, this second part will deal with double-basin TPPs. Based on what until now has been accomplished in TPP design, construction and operation, Part one concluded that in the upper reaches of Canada?s Bay of Fundy with its specific environmental concerns, single-basin TPPs operating in double effect (DE) – generating energy on both the incoming and outgoing tides (Type Z plants) – are environmentally superior to other types of single-basin TPPs. According to Part one, such plants could be considered environmentally benign provided environmental issues are studied quantitatively through the construction and operation of an environmentally designed pilot TPP and the results of such studies prove that a TPP can exist within a thriving tidal environment.
Anticipating favourable results from such an environmentally designed pilot TPP, a Type Z plant at Cumberland Basin, as described in Part one, will be used as the standard against which the environmental performance of other TPPs will be assessed.
Part one further concluded that, 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 and accepted as a guide.
As a first example of a double-basin scheme, Shepody Bay, Site A6, and Cumberland Basin, Site A8, in the upper reaches of the Bay of Fundy are chosen. This means that the environmental concerns as outlined in Part 1 will apply equally to this double-basin scheme.
In order to arrive at realistic estimates of cost, the author used as his main source of costs reference BFTPRB 1977, Reassessment of Fundy Tidal Power. Since the costs in that report are stated in Canadian dollars of 1976, all costs quoted herein are stated in the same ?76Can. To obtain costs in ’03Can$s, multiply the costs as shown by 2.7.
Paired Basins scheme with a Type Z plant
A paired-basins scheme is defined as a scheme consisting of two basins, each with its own TPP, operating individually in its own consistent manner while feeding the energy, generated by both plants, into the same market. Based on the findings of Part one, the only type of single-basin TPPs environmentally acceptable for these two sites would be Type Z plants. Hence a Paired-Basins scheme at these two sites would, of necessity, consist of two such Type Z plants.
As was seen in Part one, the Type Z plant at Cumberland Basin with 61 variable speed, fish-friendly turbines of 7.5m diameter would produce 3771GWh in four blocks per moon day at a unit price of 3.4¢/kWh. A similar plant at Shepody Bay with 83 similar turbines would produce 5191GWh in four blocks per moon day at a unit price of 4.1¢/kWh. For the two plants combined, the yield would be 8962GWh/year at an average price of 3.8¢/kWh.
Thus the two Type Z plants could produce together 8960GWh with a total of 144 machines. Such a scheme would become attractive if there were a phase difference between the tides of these two sites of approximately two to three hours. In this way the two sites, apart from each producing the maximum possible amount of energy, would together produce continuous, base-load energy plus fuel-displacement energy. However, with these sites being in close proximity to each other, the difference in the phase of the tides is minimal. Although this energy comes in four blocks per moon-day, absorption of all the energy in the surrounding utility systems would still be difficult. Therefore, other types of two-basin plants at these two sites might be more advantageous.
Paired Basins scheme with a Type Y plant at Shepody Bay and a Type X plant at Cumberland Basin
A more attractive form of paired basins scheme might consist of one Type X plant, generating energy at the outgoing tide and one Type Y plant, generating energy on the incoming tide. In this way, the two plants together would produce four instead of two blocks of energy per moon day of 24h50min. It was established in Part 1 that Type X and Y plants would be environmentally unacceptable at the given locations with their extreme tides and specific environmental effects.
However, in order to get an appreciation of the relative economies of different double-basin schemes, the cost aspects of a paired-basins scheme with Shepody Bay as a low-basin, Type Y plant and Cumberland Basin as a high-basin, Type X plant were analysed. It was assumed that fish-friendly turbine generators and sluices, fitted with automatic flap gates, would be used. This paired-basins scheme with a total of 90 turbine-generators of 7.5m diameter, rated at 31MW, would produce 6921GWh of energy per year at an estimated cost of 3.5¢/kWh.
A Linked-basins scheme is defined as a scheme consisting of two basins, one high and one low, with one TPP between the two basins, generating energy in single effect by flow from the high- into the low-basin.
The layout for such a linked-basins plant at Fundy’s two adjoining basins is shown in the upper part of Figure 2. The high Cumberland Basin is linked by the power plant to the low -Shepody Bay.
Figure 2 was originally published in ATPPB (1969) and again in BFTPRB (1977). It shows in the lower part the type of operations proposed. On the left is shown a pure energy production on demand type of operation which produces energy during times of peak demand only. Obviously, such energy comes at a high price since the amount of energy produced by the TPP would be greatly restricted. Moreover, the operation of such a plant would change from day to day and would thus be unacceptable on environmental grounds.
Shown in the right bottom part of Figure 4 is a continuous energy production type of operation, producing a certain amount of continuous or base-load energy supplemented by fuel-displacement energy. Environmentally, this type of operation could be acceptable since it would be consistent from day to day.
The conclusion drawn in the 1969 and 1977 Fundy reports with respect to such a linked-basins plant was the same in both reports: ‘the at-site cost of energy would be considerably higher than from either site operated as a single-basin development.’
With that, the issue of multiple-basin schemes was laid to rest as far as the Government-sponsored series of Fundy TP studies was concerned.
In literature concerning tidal power, linked-basin schemes were always aimed at producing either energy on sun-time-based demand or at producing continuous, base-load energy. The author could find no evidence that the operation of a linked-basins plant was ever aimed at producing the maximum possible amount of energy at the lowest possible cost per kWh. Such an operation would be consistent from one day to the next, never concerned with sun time but only with producing the maximum amount of energy at any time.
One way of operating a linked-basins plant for maximum energy production is shown in Figure 3. As suggested by George C. Baker – member of the Tidal Power Management Committee and driving force behind the construction of the Annapolis pilot TPP and its subsequent use for ecological studies – such a plant would be supplied with a large number of sluices for filling the high and emptying the low basin. All these sluices would be fitted with automatic flap gates, filling the high-basin automatically as soon as the level of the sea rises above that of the high-basin and emptying the low basin automatically as soon as the level of the sea falls.
At t=0, the plant would be generating energy while the high-basin?s level would be kept high through the large number of filling gates. As soon as the sea level falls below that of the high-basin at t=a, generation would stop and the levels of both basins would be kept constant. Generation would start up again at t=b, i.e. as soon as sea level drops below that of the low basin. While water flows from the high-basin through the turbines into the low-basin, the low-basin level is kept low by the large number of emptying gates. Generation would stop as soon as sea level rises above that of the low-basin at t=c. The level of both basins would again be kept constant until sea level rises above that of the high-basin at t=d. Now generation starts up again until the end of the cycle.
Such a linked-basins plant at Shepody Bay and Cumberland Basin, with 53 turbine generators of 7.5m diameter, would produce annually 5625GWh in four blocks per moon day at 3.6¢/kWh. By comparison, the paired-basins scheme with Shepody Bay as a low-basin and the smaller Cumberland Basin as a high-basin would produce 6921GWh at 3.5¢/kWh, also in four blocks of energy per moon-day, with a total of 90 machines, also with turbines of 7.5m diameter.
Economically speaking, a linked-basins plant as described above would have the upper hand if the closure dam for the enlarged Cumberland Basin could have been shorter. For example, with a closure dam of 1625m instead of 7200m, energy cost for the linked-basins would be 2.9¢/kWh. This confirms the conclusion of the 1969 and 1977 Fundy reports that a linked-basins plant at the head of the Bay of Fundy would produce energy at a higher cost than single-basin plants at the two sites. At the same time however it is clear that for geographically more favourable locations, linked- basin plants designed and operated to produce the maximum amount of energy at the lowest possible unit cost do merit investigation.
Being at the upper reaches of the Bay of Fundy, the environmental concerns as identified in Part one for this environment still apply. A Type Z plant at Cumberland Basin is used as the standard against which the environmental performance of this linked-basins plant is to be judged.
Figure 3A shows the water flow and energy production patterns during an incoming mean tide. Consider what happens in the high-basin. During this incoming tide, part of the flow coming in from the sea is moving directly through the turbines into the low-basin. This is very favourable for the maintenance of a clear channel between the intake sluices and the TPP. This local, vigorous flow will however have little effect on what will happen at the far end of the high basin. To avoid excessive siltation at the head of a basin, aim to avoid stagnant water, create simple flow patterns, and encourage strong currents by having the maximum possible amounts of water moving in and out of the basin.
The amount of water which flows from the sea into the high-basin and stays in that basin during the following period of stagnant flow can be measured approximately by the tidal range within the basin During this incoming tide, the level of the low basin goes up from its near minimum to its maximum.
Figure 3B shows the water flow and energy production patterns during an outgoing mean tide. The high-basin releases all its stored water through the turbines. That the curved flow from the high-basin into the turbines may result in some heavier particles being deposited on the outside of the curved flow will be of little consequence since that local area will be swept clean again during the next incoming tide.
The low basin will also release its stored water during this outgoing tide. Siltation within the low-basin would similarly be kept in check if the measures mentioned above are completed. The amount of water moving in-and-out of the low-basin proper, i.e. not counting the water which moves from the high-basin straight through the turbines and out into the sea, can again be approximately measured by the tidal range within the low-basin. The part of the low-basin between the TPP and the dewatering sluices will be subjected to vigorous flow patterns assuring the maintenance of a deep channel in this area.
Column A of Table 4 indicates that the linked-basins plant does not measure up to the Type Z plant as far as siltation is concerned. Type Z?s tidal range is superior to that of both the high and the low-basin. The larger the tidal range, the more vigorous the flow. The same applies to the stagnant flows, in this case, the shorter the stagnant period, the better.
With respect to birds, fish, farming and changes in the tidal regime, keeping in mind what criteria govern environmental sustainability for these remaining four concerns, it seems that the linked-basins plant does not measure up to the environmental standards, set by a Type Z plant at Cumberland Basin as shown on Table 4.
The upper parts of Figure 4 show a situation which is hypothetical but realistic. It shows an island at some distance from the mainland in tidal waters. The mean tidal range in this instance is assumed to be 5.64m instead of the 10m at the head of the Bay of Fundy. With the mean water level at El. 7, basin areas at different water levels were assumed to be: 1,650,000m2 for the high-basin at HHW, prior to dam construction and 1,500,000m2 for the low-basin at El. 8m (by comparison, the area of the smaller Cumberland Basin of the Type Z plant at HHW is 96,960,000m2. This linked-basins plant is therefore of pilot-plant proportions).
The layout shown is idealised to the extent that the intake sluice-caissons, power house caissons and dewatering sluice-caissons all stand shoulder-to-shoulder next to each other, filling the space between shores for 100%. Such an ideal layout would require a perfect balance between the size of the two basins and the distance between the mainland and the island at the locations of the sluice and power house structures. During the incoming tide, this would result in a truly linear, one-directional flow from the sea into the high-basin and through the turbines into the low-basin. During the following outgoing tide, that linear flow pattern would resume in the same direction, from the high-basin through the turbines into the low basin and out through the dewatering sluices. With this flow pattern, turbulent flow will occur exactly in the areas where most of the sediment would have settled down during the preceding stagnant-water period. This flow-through layout is therefore ideal for sites with a high silt content. In most practical applications where a perfect balance cannot be achieved, the flow-through layout would still maintain most of its siltation prevention capabilities and keep the essential channels clear of sediment.
The lower portions of Figure 4 show the operating pattern of this linked-basins TPP for maximum energy production. This mini linked-basins plant was assumed to be equipped with one SE, double regulated turbine generator with a turbine runner diameter of 5.35m (like the La Rance machines) and a generator limiting capacity of 6.4MW. The total throat area of the sluices for both intake and dewatering sluices was assumed to be 375m2 and the annual energy output of this plant was calculated to be 25GWh. This being a hypothetical plant, its environmental concerns are not defined.
So far, all environmental concerns and remedies were treated as if they were all of equal weight. With the flow-through type layout an exception was made since its flow through-properties are superior in solving sedimentation problems. The linked basins flow-through plant was therefore given a weight of 10 instead of a simple 1 for simplicity and effectiveness of its flow patterns. This makes the hypothetical linked-basins plant superior to the Type Z plant for the sedimentation concern.
In the shorebird survival category, the Type Z plant is clearly superior. The shorebirds however are prominent in the upper Bay of Fundy but far less so elsewhere. This strike against the linked-basins scheme might therefore not be serious.
Due to the more moderate tidal regime as chosen for this linked-basins plant, its fish-friendliness, is more favourable than that for the standard of comparison. As for farming, the low basin scores favourably, the high-basin unfavourably. Sea-mammal survival, may well turn out to be a non-issue, based on the experience with the Annapolis plant from 1984 up to 2003.
In order to generate revenue at the earliest possible time, the high-basin of the linked-basins plant would be constructed first as construction Phase I. This single-basin plant would then be operated upon completion as a Type X TPP as defined in Part one of this series.
The upper part of Figure 5A shows the completed Phase I during the incoming tide, the upper part of Figure 5B during the outgoing tide. The energy production patterns for this Type X plant are shown in the lower parts of Figure 5. The energy output of this Type X plant was calculated to be 16GWh per year.
While Phase I would thus be producing revenue, the closure dike of the low basin with its dewatering sluices would be constructed as Phase II upon completion of which the linked-basins TPP would start operation. Upon completion of the low-basin, the annual energy production will go up to 25GWh, an increase in the plant’s energy output of 56%. As long as the additional cost of construction of Phase II is less than 56% of the Phase I cost, this linked-basins flow-through type TPP would make economic sense.
Part 1 dealt primarily with the environmental barriers, obstructing the development of single-basin TPPs in the upper reaches of the Bay of Fundy. While environmental effects are different for each site, the methodology of coming to grips with such effects, as outlined in Part 1 for single-basin plants is equally applicable to double-basin TPPs in any location.
For the general case, the double effect, single-basin TPP remains the first choice from an environmental perspective. However, where geological conditions permit, and where the preservation of fish stocks is most important, a linked-basins plant with a flow-through configuration is recommended. Such a plant would require a single-effect, fish-friendly turbine-generator. In either case, the construction of a pilot plant, designed from the beginning to address the environmental concerns listed in Part 1, is recommended.
lsum, consulting civil engineer, who sadly died on Monday 18 August 2003.
Memorial donations can be made to: The Saku Koivu Foundation, c/o Montreal General Hospital Foundation, 1650 Cedar Ave. Room E6129, Montreal, Quebec, H3G 1A4
For the engineering and environmental aspects of this paper, the author wished to express his thanks to engineers and environmental scientists alike who, each in their own way, communicated their views to others. Some of those are listed, directly or indirectly, under References.