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New promising solutions for Tidal Energy (26th ICOLD Congress – Q.96)

Published on June 20, 2015 in Tidal Energy

By F. Lemperiere (HydroCoop), N. Nerincx (ISL), C. Bessière (Ingerop)

1. PAST SOLUTIONS FOR TIDAL ENERGY

Hydropower and Tidal Energy have about the same theoretical potential, above 20 000 TWh/year. The possible energy supply per km2 of tidal basin (20 GWh/km2 with a tidal range of 5 m) is higher than the average energy supply per km2 of dam reservoirs (3 500 TWh for 350 000 km2, i.e. 10GWh/km2). Other conditions are also better for tidal energy: there is no resettlement of population, monthly and yearly energy are always about the same and the risks from accidents are low. However Worldwide Hydropower generation is 3 500 TWh/year and Tidal generation is 1 TWh/year.

Environmental impact has been involved for explaining this surprising gap but the tidal energy impacts seem actually much more acceptable than dams impacts. In fact the past designs for Tidal Energy have been directly based upon Hydropower solutions (Tidal Plants) or Wind Farms solutions (In Stream Turbines). They were poorly adapted to the very specific data of Tidal Energy and their relevant cost is thus usually too high even for the best sites. This is the reason of the poor utilization of Tidal Energy.

1.1. Traditional Tidal Plants

The usual past solution, as for Hydropower, stores water by dykes in a reservoir (basin) and uses the corresponding energy through Tidal Plants, i.e. turbines (usually Bulb Units) placed in a concrete structure. The cost per MWh for creating tidal basins may be very acceptable but the key problem is the very low head associated with a good utilization of tidal energy.

The best way for operating a tidal Basin is both ways as per fig.2. Power is obtained 8 hours from 12, the conditions within the basin are similar to the natural ones but the average head between sea and basin is only about 40% of the average tidal range Hm, i.e. 3 m for exceptional sites and 1,5 or 2 m for most tidal potential; the flow may be well over 100 000 m3/s for a capacity of some GW. Heads are thus much lower than for traditional Hydropower and flows much higher.

The efficiency of hydropower turbines (including bulb units) is very poor for such heads and the past tidal studies did thus focus on sites of very high natural tidal range to be operated one way as per fig.1. The water volume to be used is one third of the volume of the Two Ways Solutions (fig.2) but the head is about two thirds of the tidal range Hm, i.e. 4 or 5 m for a tidal range of 6 or 7 m. Energy is supplied only 4 hours from a 12 hours tide and is yearly 2 000 hours only of the rated power. Tidal conditions in the basin are much modified and this may be unacceptable. The plant structure in open sea is 30 to 40 m high, its civil engineering expensive. Even in best sites as in the Severn (U.K.) the cost per MWh remains hardly acceptable with this solution.

Fig. 1: One Way Operation

Fig.2: Two Ways Operation

 

Various studies tried to associate several basins in favourable sites, bulb units supplying energy full time but this did not increase much the head or reduce the costs per MWh.

A better solution specifically designed for tidal energy, the orthogonal turbine studied in Russia, is well adapted to both ways operation under low head and turbines are quite simple. However the civil engineering is expensive (fig. 3) and the power per m of structure is under 500 KW even for high tides. The cost may be acceptable for some very good sites but too high for most tidal world potential which is for a tidal range between 3 and 6 m.

Fig.3 : Orthogonal turbine

1.2. Instream turbines

The other basic solution (In-Stream Turbines), tested since a decade, is a copy of the very successful Wind Farms Solution. The wind speed in many world places favours wind plants units of 3 to 5 MW onshore and 10 MW offshore. It is thus likely that within twenty years Wind Energy will equal Hydropower or Nuclear Energy. Using the same principle and basic design in tidal streams turbines seems thus attractive and has justified many studies, tests and first years of operation on some sites. The theoretical potential is significant but there are few world sites where it is possible to get cost effective energy. A first reason is the rather low water speed and most designs are limited to units of 1 MW in best sites. A second reason is that in most best sites the marine conditions of waves, foundation, maintenance, long electric links increase the cost to a very high level. Anyway this solution uses only a very small part of the natural energy. The World Tidal Stream Potential in natural sites seems thus few hundred TWh/year and less than 100 TWh/year at an acceptable cost.

Similarly the wind energy should have little future if the world wind speed would be half of the present one and if the only places for wind farms would be mountains over 3 000 m.

1.3.

With the various solutions studied till now, the cost effective world potential of tidal energy is very low.

2. A NEW SOLUTION: THE TIDAL GARDENS (T.G.)

The principle is to create sites where in-stream turbines may operate in best conditions of cost and efficiency and use a large part of the available energy.

2.1.

There are worldwide many places where large basins of hundreds km2 may be created along shore because the sea depth is less than 25 m and soil conditions favourable for building dykes within 10 or 20 km from shore. Instead of using costly traditional tidal plants in the dykes for generating power, the basins are linked to sea by wide channels in which are placed many in-stream turbines (fig. 4 and 5).

Fig.4: large basin

Fig.5: channel for in-stream turbines

The channels sides are limited by dykes and the bottom lined by concrete. The channels may be closed by gates similar to gates used with traditional tidal plants designs. This solution deserves a specific name such as Tidal Gardens or Tidal Channels (T.G. or T.C.)

Along a six hours half tide the channels, gates are closed when the basin and sea are at same level and remain then closed one or few hours only; then the channels are opened and operated at about the same water speed such as 3 or 4 m/s corresponding to the optimal utilisation of in stream turbines. It is possible to keep this speed full time through adapting the number of operating turbines to the prevailing water head between sea and basin.

As example, for a mean tide (fig.2), the channels are closed along 2 hours then are all open along 4 hours with the same flow (and water speed). Quite all turbines are operating along two hours and a part is stopping along the next two hours according to the reducing water head. The flow is kept the same up to few minutes before an equal level between sea and basin. During spring tides, the channels are opened 5 hours from six, during neap tides 2 or 3 hours. The flow (and speed of water) is thus quite the same during all operation. During very low neap tides, some channels may remain fully closed.

As analysed in 2.4 the cost per MWh of tidal Gardens is much lower than the cost of traditional Tidal Plants or In Stream Turbines in natural sites.

Beyond cost saving, the impact on environment is also better because the tidal conditions in the basin and along shore are close to the natural ones (shifted by 2 hours); high waves and exceptionally high water level are avoided.

Tidal Gardens are a new solution but it is based upon well known technologies: In-stream Turbines which may be even simplified and large dykes and caissons at sea. So there is no need of inventing the technologies, which should be simply optimized and possibly standardized worldwide for huge quantities.

Contacts have been established with industrial actors, as Electricité de France, to examine in further details:

(1) the likely hydraulic operation of a schematic “tidal garden site”

(2) the optimisation method of “tidal garden” design criteria such as number of channels, channel size, dyke track, etc …

The hydraulic behavior and the associated design criteria of a “tidal garden” site depend on:

–   the given site configuration : coastline shape, tidal range, sea bottom geological conditions, possible existing infrastructures

–   power generation objectives : power outputs performance expectations, ancillary services, intermittency characteristics, storage needs, …

–   socio-environmental needs and conditions : impact on aquatic ecosystems, impacts on sediment transport and morphology, opportunities or constraints linked to other uses (navigation, fishing, …).

2D hydrodynamics numerical modelling of a schematic “tidal garden” site is under progress to better understand its hydraulic behavior, and meet the above objectives. Results should be available in the next months and will be soon published.

2.2. Potential per km2

A rough evaluation of the Production may be based upon an operation with a mean tidal range according to fig.2.

For a basin area S in km2 and a tidal range Hm, the likely volume of water used in an half tide is about, in m3, 0,9 x 106 S Hm with an average head close to 0,35 Hm. The losses of energy in turbines and channel cannot be known precisely before detailed studies and tests. An overall loss of one third appears a reasonable figure. A different value would modify the production but not much the cost per MWh because the number of turbines is proportional to the expected production.

The power supply per half tide is in MWh:

And should be multiplied by 2 x 705 for evaluating the yearly supply. The direct result in GWh/km2 is about 0,8 Hm2. Some margin should be taken and evaluations limited to 0,7 Hm2.

The necessary generating capacity could be in theory based upon a power supply over 4 000 hours of the rated capacity. It is reasonable for more operating flexibility to use a figure of 3 500 hours and thus a necessary capacity, in MW/km2 of 0,2 Hm2 for a yearly production in GWh/km2 of 0,7 Hm2.

2.3. Design bases

A typical site for Tidal Gardens (Fig. 4) would be a large basin open to the sea by channels where 10 or 20 lines of in-stream turbines would be placed. The area of the basin could be several hundred km2 or possibly thousands of km2, with about one channel per 100 km2. Smaller basins could be used with one channel. Most future sites would be along the shore. A typical basin could then form a semi-circle along the shore but more favourable sites could be available such as narrow small or large gulfs.

The concept of a channel (TG) linking the basin to the sea is shown in Fig.5.

The length would be based on the mean tidal range as well as on turbines data. The width could be around 500 m for very large basins, or 100 to 200 m for small ones. The depth could be 15 to 20 m below the low sea level; this may require some dredging or filling. To allow for a significant water speed, the bottom should be lined, for instance by 0,50 m of concrete placed in calm water. In stream turbines may have an horizontal or vertical axis. Various lay out of turbines in the channels may be used.

The channel sides would be formed by dykes 25 m high, supporting a low head and greatly reduced wave impact. They could be as shown in Fig. 6(a).

The channel would be separated from the sea by gates, to be opened for about 4 hours within a six-hour half tide. The differential head on the gates would be quite low, but the wave impact might be high. Solutions similar to those for spillway gates could be used, but the specific conditions may favour solutions specific also for the construction method. Innovative designs would also be possible.

For the main closure dyke, recent progress in breakwater design and dredging efficiency favours a solution as shown in Fig. 6(b), which would be suitable for an optimal construction program of largest schemes limited to 6 or 7 years all included. For smaller sites and length of dykes under 5 or 10 km, using rockfill dykes may be less expensive according to quarries availability.

Fig.6(a): channel dike

Fig.6(b): dam breakwater

2.4. Cost

A first comparizon is made with traditional tidal plants designs, i.e. bulb units operating one way under a 4 or 5 m head.

The cost per MWh of such tidal plants includes two parts:

– The main dyke and sluices (gates) allowing the basin filling. For a same basin using Tidal Gardens, the main dyke and the Sluices are about the same but the yearly energy is about one third higher and the cost per MWh thus 25% lower.

– The largest part for the plants and especially for their civil engineering. The cost per MW of electromechanical part may be similar for Tidal Gardens but the civil engineering cost per MW of channels is much lower than for tidal plants; the overall cost per MW is thus significantly lower and the yearly supply is 3 500 or 4 000 hours of the rated power instead of 2 000 hours. The cost per MWh is thus reduced by about 60%.

– And the total cost per MWh is close to half of tidal plants cost for best tidal sites and less than half for a tidal range of 4 or 5 m. It may be in the range of 50 €/MWh for turbines and channels and 10 to 50 € / MWh for the main dyke.

– The cost per MWh of Tidal Gardens is also lower than In Stream Turbines in natural conditions. The water speed is higher, always the same, in the same direction. Turbines may be placed and maintained in calm water easily, anchored and linked electrically at low cost and used more hours per year. Conditions of operation and maintenance are much different. There are few places where the cost in natural conditions may be under 150 €/MWh when the cost of Tidal Gardens will be usually under 100 €/MWh.

– The cost per MWh of the main dyke and ancillary works varies significantly with each site. It may be as low as about 20 € / MWh for favourable shore shapes such as narrows gulfs but also for very large schemes where the length of dykes per yearly TWh is few Km or where the local conditions (sea depth and available quarries) favour low cost dykes.

Ancillary works may include shipping facilities; their cost per MWh will be usually much less than the cost of main dykes.

A part of the dykes cost may also be paid by the relevant facilities such as shore protection and possibilities of low cost energy storage or wind farms and industrial or touristic developments. For many world sites the cost per MWh of the main dykes and ancillary works may thus be well under 50 € / MWh and the total cost of power under 100 € / MWh.

Such costs will increase slightly for tidal ranges as low as 3 or 4 m and significantly under 3 m.

3. Evaluation of yearly energy

To illustrate the potential yearly energy estimation as indicated in point 2.2 we develop in this paragraph an application to the French Channel coast.

3.1. TIDES CHARACTERISTICS

The reference site is Saint-Malo where we had the opportunity to treat the 1981 – 2007 period for tidal energy estimation purpose (amplitude and timing of the corresponding 19,053 tides).

The average tide amplitude at Saint-Malo on this period is 7.86 m. A correlation between tides amplitude and timing is obvious for this given site.

Other correlations are underlined in the following figure where comparisons are made with Saint-Malo, for the other 4 harbours, considering tides amplitudes, and time of maximum sea levels.

These comparisons are based on 5 neap tides (P1 to P5), 12 mean tides (M1 to M12) and 5 spring tides (G1 to G5).

Fig.7 – Tides amplitudes of the other Channel 4 harbours compared to those at Saint-Malo

In summary:

  • At Roscoff the tide amplitude is roughly ever about 70 % of Saint-Malo’s one, leading to an estimated average tide amplitude of 5.50 m.
  • At Cherbourg the tide amplitude is roughly ever only about 50 % of Saint-Malo’s one, leading to an estimated average tide amplitude of 3.93 m.
  • At Dieppe as at Boulogne-Sur-Mer, the ratio varies with the amplitude, and represents respectively +12% and +7% for neap tides, 85% and 80% for mean tides, 75 and 70% for spring tides, that means roughly in average an amplitude of 6.70 m at Dieppe and 6.33 m at Boulogne-Sur-Mer.

The differences in time table are in average:

  • Advance of 62 minutes at Roscoff (about one hour)
  • Delay of 113 minutes at Cherbourg, 284 minutes at Dieppe and 300 minutes (5 hours) at Boulogne-Sur-Mer.

Combining operation of various basins located close to these various harbours could allow a quite continuous production.

3.2. POSSIBLE YEARLY ENERGY PRODUCTION

For a basin area S in km², a two ways operation of this basin, a ratio of installed capacity in MW/km² of 0.2 Hm², the possible yearly energy production in GWh/km² is summarized in table 8.

Site locationInstalled capacity
(MW/km²)
Yearly energy
(GWh/km²)
Saint Malo12.543
Roscoff621
Cherbourg310.5
Dieppe931
Boulogne-Sur-Mer828

Table 8: Possible yearly energy production

4. Environmental and economical impacts

Environmental and economical impacts of large scale projects such as Tidal Garden should be thoroughly studied, without any bias, and with modern methods.

Experience from previous off and near-shore projects (offshore wind farms, in-stream turbines, tidal plants) can be a valuable base for these studies, but attention must be paid that Tidal Garden is a specific concept with its own features and must be so considered. Main topics identified today are described hereunder.

4.1. Tides

Tidal range is a main parameter to assess environmental impacts as it affects marine environment through temperature, salinity, currents, light,… Tidal range in the basin should then remain as close as possible from the natural tidal range.

Preliminary calculation (analytic and model) showed about 2 hours time shifting, 20-30 % range attenuation at spring tide and 0-10 % attenuation at neap tides.

4.2. Marine environment

Main marine environment component are salinity, sedimentation, tide range, currents and turbulence. Those should be the less possible affected by the project.

Impact analysis is first carried out at global basin scale, rather than at local scale at the entry/exit points or close to the dykes.

  • Longshore drift will be modified as shores will be protected against swell,
  • Currents are modified as well but impact should be moderated except at entry and exit points.
  • Preliminary calculations show that water remains in the basin approximatively as long as without tidal garden. Temperature should then barely be impacted.

These preliminary considerations should be taken with care and confirmed with hydro-sedimentary model. It should be noted that hydro-sedimentary working of marine environment is a complex phenomenon, affected by several human and non human parameters, that does not always lead to equilibrium or acceptable situation for the populations.

4.3. Biodiversity

Main threats on biodiversity are listed hereunder with impacts during construction at first and during exploitation after, with some preliminary comments. Opportunities created by tidal gardens are detailed later.

  • Impacts during operation

Dyke physical presence• Marine habitat destruction – 3 to 5 km²/TWh/year to be compared to few for offshore wind farms, 10 km²/TWh/year for photovoltaic energy or 100 km²/TWh/year for onshore HPP
• Recolonisation possible
• Impassable for marine fauna
• Special attention to dyke localisation with respect to sensitive areas
Channels and turbines• Relatively high velocities – risk depending on the specie
• Collision risk depending on turbine structure, depth and location, specie’s dimension, displacement ability; collision risk to be mitigated by appropriate design.
Channels and turbines• No cable laying in the seabed (cables along the dykes)
Vibration and noise• Similar to usual in-stream turbines (those being however located in naturally high and noisy currents area);
• Limited knowledge on this topic available, to be built up for both T.G and in-stream turbines
Pollutions (painting, chemical products)• Similar risk as for other marine works
• Regulated by law, risk mitigated by know-how
Electromagnetic fields• Lower than for other marine energies (see cables, above)
• Limited knowledge on this topic available

Note: At the basin’s scale, tidal gardens are quite permeable for biodiversity. This should be confirmed by in-depth studies and experimentation, but one should remember that French tidal plant in “La Rance” estuary, which has no direct connection between the sea and the estuary (unlike tidal gardens) is reputed permeable to life, for planktonic organisms and bigger animals.[1]. Study of migration schemes, rest, feeding and reproduction areas is mandatory to correctly assess project’s impacts and take them into account in the layout.

  • Impacts during construction

Impacts during construction should be similar to those caused by large-scale marine works. Noise and vibration, sediment suspension, pollution, (permanent or not) habitat destruction, may be generated by dredging and reclamation operations, concreting, equipment placing.

Impact assessment should follow well established methods, typically used for large scale dredging operation, port or offshore wind farms construction. Those works are regularly carried out and techniques exist in order to mitigate impacts.

Pile driving or drilling• Pile driving should be avoided or very reduced.
Sea bed changes (Dredging/reclamation)• Risk of resuspension of sediments and induced turbidity increase;
• Commonly monitored and mitigated in marine works
Ships and heavy equipment presence• Similair to other marine works
• Methods to be adjusted to local environment

4.4. Other impacts

Some other impacts are listed below.

Landscape• Dykes and gates only are visible, 10 to 15 m above low tide
• Near shore dyke stretches to be compared to large bridges that are usually higher
• Offshore stretches to be compared to visual impact of 150 m high windmills
Navigation and communication routes• To be avoided as much as possible during site selection
• Technically possible to maintain communication cables
• Options to be further studied for navigation (open and turbine free channel, sluice,…)
Safety• No risk of dam break induced flood (compared to HPP)
• Safety measures to be taken, restricted access area to be delimited, as for any energy production site

4.5. Socio-economic Environment

Main socio-economic impacts of such a project are similar to those of an offshore wind farm project: no population displacement, increased economic activity thanks to large scale works (civil, mechanical, electrical works; economic activity for maintenance and operation).

Numerous jobs can also be created thanks to additional purposes that can be conferred to the tidal garden project, as listed below.

Important also is the current uses of the tidal garden area, especially the tidal garden basin. Use can be fishing, shellfish farming, aggregate pit operations,… The analysis should be carried out carefully, case by case depending on each site.

4.6. Future prospects

This preliminary impact review does not claim to be exhaustive, but lists main expected impacts of tidal gardens.

From this review following conclusions can be drawn:

  • Many topics have been studied, more or less extensively and many uncertainties are not specifically linked to tidal gardens, but more generally to marine renewable energies, that should include in the future the tidal gardens;
  • Impacts should be compared to other large scale marine works on one hand, and to other renewable energies (marine or not) on the other hand.
  • Hydrodynamic and hydrosedimentary modelling is mandatory to assess impacts on the environment;

Environmental impact assessment should then be carried out carefully, respecting national and international state of the art methodologies, without bias. As such, French experience in the La Rance estuary, widely studied and documented should not be transposed to tidal gardens. The estuary environment is particularly sensitive, and stands for about 1 % only of the French power potential.

Finally, environmental compensatory measures should be sought out even if those are barely implemented in renewable marine energy project today, but this topic is being currently studied[2].

5. Utilization beyond energy supply

A tidal garden project should be set up as main component of town and country planning. Use of resources should be efficient and utilization beyond energy supply by turbines can play a major role in the decision process.

Some opportunities for utilization beyond energy supply of a tidal garden project are described below.

5.1. Marine environment promotion

Environment modification due to dyke and channel construction is a threat for biodiversity but could be also an opportunity if designed in sensible way, amongst others thanks to the reef effect.

New marine structures are new areas that will be colonized by several organisms, depending on depth, and particularly in structures haven been designed taking this aim into account.

Areas near the dykes will be refuge areas, as navigation and fishing will be prohibited.

Those opportunities should be carefully studied in order to analyze the biodiversity change in terms of biomass and variety of species. Special attention should be paid to non native and invasive species.

These opportunities could be considered as a compensatory measure to the project in order to mitigate the impacts, keeping in mind that it is not possible to compensate a habitat loss by another habitat.

Those considerations are applicable to any marine renewable energy project. Knowledge today is limited, and common efforts should be made to enhance the know-how.

In a more prospective way, adequate gate operation could contribute to limit saltwater to rise back in estuaries subject to salinization.

5.2. Shore protection

  • Marine submersion

Several shorelines are affected by swells from open sea and from wind. With a representative wave period from Vendée and Charentes-Martimes (France) shoreline (4 to 6 seconds), dykes from tidal gardens have a very significant effect on the transmitted wave height and therefore contribute to protection against marine submersions.

  • Shoreline erosion

There is a direct link between transmitted swell and shoreline erosion. Shoreline drift is responsible for sandy material loss at several beaches along the shore. Decrease in swell intensity will decrease this drift. It can then protect the shoreline against erosion but can lead to threats to biodiversity. A hydro-sedimentary model coupled with a thermic model is mandatory to assess the impact on this complex phenomenon that may not be naturally acceptable to populations. Local effect at entry and exit points makes no doubt; local bathymetry will be affected.

5.3. Additional Energy production and storage

A tidal garden development project can create opportunities to set up wind farms within the basin at controlled costs.

Cable laying costs and offshore construction costs increase the budget of offshore wind farms. Those extra costs are mostly saved in a tidal garden project: cable costs can be shared with the existing tidal garden projects, and swell in the basin is significantly reduced, which makes design, work and operation condition much better.

As a calculation basis, wind farms could be set up on half the basin. With about 10 MW/km² installed operating about 2.500 hours per year, that gives about 12.5 GWh/km². This increase of the project power production could be done at competitive cost, and could be coupled to an energy storage device, as described below. It can be particularly interesting at site with moderated tide range.

However, wind farms could seriously increase environmental impact of the project.

Energy produced by a tidal garden is very predictable at long term, but irregular over half a tide and over 14 days. Demand is also irregular over 24 hours and over longer time period, with a more or less predictive global evolution.

It is relevant to plan a storage device for the energy produced by the tidal garden, by means of pumped-storage hydroelectricity. Different designs should be studied, but all of them take advantage of reduced swell in the basin: PSH with one reservoir, the other being the sea; PSH with two reservoirs in the basin; conventional marine PSH, upper reservoir being located onshore.

5.4. Industrial and non industrial activity

Different types of industrial and non industrial activities could take advantage of a tidal garden project. Some of them are listed below, along the dykes or within the basin. This list is obviously non exhaustive and should be adjusted and completed site by site.

In the basin• Favorable conditions to large scale aquaculture or shellfish farms, thanks to calm and constantly renewed water
• Construction materials extraction at main sedimentation areas created by change in hydrosedimentary working
Along the dykes (basin side)• Fishing ports and marinas in calm water; sandy beaches or artificial islands for nautical and tourist activities
• Deep see ports sheltered by the dykes, land reclamation for chemical plants, refineries, LNG terminals,…

5.5. Future prospects

Utilization beyond turbines energy production can be critical in the decision process for a tidal garden as it allows significant increase of the benefits and sharing of the costs. This can be compared to large multi-purpose hydropower dams.

Wind farms and PSH make also possible to optimize global energy production for the project.

Finally, additional usages of a tidal garden project shows low environmental impacts increase compared to benefits.

This shows that a tidal garden project should be considered as a global land planning project, able to economically stimulate a large area, beyond pure energy production.

6. The world potential

– The technically feasible potential is linked with the natural tidal range Hm and with the water area where depth is acceptable for dykes construction. A very rough evaluation could be made where Hm is over 3 m and a water depth below low tides level less than about 20 m. Such world area of 400 000 Km2 (20 000 km x 20 km) with an average tidal range of 4 m would allow technically an energy supply of (0,7 x (4)2 x 400 000/ 1 000), i.e. about 5 000 TWh/year.

– The economically feasible potential is based upon the cost comparison with other acceptable energy sources available mid century. The range of acceptable cost including transport may be close to 100 €/MWh but varies with countries. Some very rough evaluations of potential are given below for 20 countries. For most of them, the possibilities are totally linked with the advantages of the Tidal Gardens new solution. Many choices may also be favoured by the extra facilities from calm water basins: shore protection against waves or abnormal water levels, energy storage, low cost large wind farms, industrial or touristic development which may pay some part of the dykes.

The costs of energy transport may be a key element for comparison because a significant part of low cost tidal energy is one or few thousands Km far from customers (a transport cost of 10 €/MWh for 1 000 Km may be possible for very large capacities).

Storage is also a key problem for most renewable energies. Relevant facilities from tidal basins may be very useful for all energy sources. The need of storage for tidal energy may be reduced if the tides timetable is not the same for various tidal sites of a country.

Ten Countries have a very large cost effective potential: Russia, Canada, Australia and China may have each about 200 TWh/year, France, U.K., India, Brazil, South Korea, Argentina about 100 TWh/year.

Russia has the largest world potential, essentially in 3 places.

– The Western site which may include the Mezen Site already studied in detail and probably the very large site of Chechskaya (8 000 km2) of lower tidal range and perhaps the White Sea (fig.9). The total supply may be well over 100 TWh/year, 1 000 km from Moscow and St Petersbourg.

– The Tugurskaya site in Southern Okhotsk Sea may probably be extended as per fig.10 to a much wider area of lower tidal range; 100 TWh/year could be used in Siberia, China (Harbin) or Japan.

– The Penzhinskaya site in Northern Okhotzh Sea has a potential of 200 TWh/year but the extremely cold conditions and the distance from Customers may prevent or at least delay its utilization.

Fig.9: Chechskaya site

Fig.10: Tugurskaya site

         Canada has two sites with high tides:

– The well known Fundy Site where favourable sea depth favours short dykes and very low cost (fig.11) for about 40 TWh/year rather close to Montreal and New York.

– The Ungava Bay may supply 100 TWh/year but the cost delivered to Montreal or New York may be over 100 €/MWh.

Possibilities on Pacific Ocean seem much lower.

Australia has an excellent potential on Northern Coast West of Darwin with a tidal range of 7 m and a possible supply up to 200 TWh/year. It is very far from most Australia needs and closer to Java. The tidal Gardens Solution favours the Eastern Site North of Brisbane (fig.12) where tidal range is under 5 m but the sea depth conditions favourable: 50 TWh/year may be supplied at low cost 1 500 km from Sydney. Some TWH/year may also be supplied very close to Melbourne.

Fig.11: Fundy site

Fig.12: Eastern site North of Brisbane

China has a huge potential along over 3 000 km and the sea depth favours the construction of dykes 20 km from shore. But the tidal range is as average about 3 m and could hardly be used with past solutions. The Tidal Gardens are cost effective in Chinese conditions and could supply 100 or 200 TWh/year very close to energy needs. The additional facilities of energy storage, low cost wind farms, shore protection, industrial or touristic development could be extremely important, at least from Guangdong to Qingdao.

Brazil has about same conditions as China: long coasts, reduced sea depth and rather low tidal range. The possibilities seem essentially along 1 000 Km in the Northern Coast west of San Luis where tidal range is about 3 m. Supplying 50 to 100 TWh/year seems a reasonable target. Management of the Amazon Delta may not be an utopia.

France has much potential close to needs and the experience of La Rance since 50 years. The potential is close to 100 TWh/year where tidal range is over 6 m; it is close to 150 TWh/year with the Tidal Gardens Solution which may apply not only in the Channel but also in the West Coast. 3 of 8 possible large sites are presented in fig.13 and may supply 80 TWh/year with 200 kms dykes at an attractive cost.

U.K. The Tidal Gardens solution may increase dramatically the cost effective potential; the Severn site may be possibly much enlarged (fig.14) accepting a slightly reduced tidal range and large sites North of Liverpool may also supply over 35 Twh/year. There is also a significant potential in the Eastern Coast. The total tidal potential may be close to 80 TWh/year. A small part of the basins may be devoted to Energy Storage used also for wind energy.

Fig.13: 3 possible large sites in France

Fig.14: Severn example

South Korea has few sites with a tidal range of 6 m but very large areas with 4 or 5 m. The cost effective potential if using Tidal Gardens may reach 100 TWh/year; it may be an excellent opportunity for the overall economy of the country.

United States have high tidal range in large areas in Alaska but the sea depth is too important in quite all sites. However an excellent site is close to Anchorage and may supply up to 50 TWh/year at low cost. It is much more than local needs but the cost delivered to Seattle by sea electric line may be acceptable mid century.

Argentina has 3 sites:

The San Antonio Gulf is limited to few TWh/year by the sea depth.

The two Gulfs of Puerto Nuevo with 4 000 Km2 and about 4 m tidal range may supply over 30 TWh/year at low cost (fig.15).

The Patagonia may supply some 50 TWh/year associated with the huge offshore or onshore wind potential there. The cost delivered to Buenos Ayres or Sao Paulo may be acceptable.

Fig.15: Golfo Nuevo site

India has essentially three sites.

The two Western sites of Kutch and Bhavnagar Gulfs which total 4 000 Km2 with a tidal range of about 5 m and may supply at a rather low cost 50 TWh/year and be associated to solar energy with a common energy storage.

– There is also a possibility in the Bengal Gulf with low tidal range but very large areas.

There are also ten countries which may supply each between 10 and 50 TWh/year if using the Tidal Gardens solution.

In America: Panama and Chili.

In Africa: Mozambic (Beira)

In Europe: Netherlands and Germany

In Asia: Pakistan, Bangladesh, Vietnam, Myanmar, North Korea.

The realistic world tidal potential seems thus in the range of 1 500 TWh/year with 100 000 Km2 of basins. Adding low cost wind Mills on one third could add 1 000 TWh/year (30 GWh/year per km2). Present Hydropower supplies 3 500 TWh/year with 350 000 Km2 of Reservoirs. Nuclear Power supplies 3 000 TWh/year.

The impact on shore protection may be very important for countries with large deltas such as Vietnam or Bangladesh.

Some countries which have favourable or acceptable tides have little potential because the sea is too deep close to shore: it is the case of most Africa, Portugal, Ireland, Colombia, most Alaska,…

7. Possible schedule of tidal energy utilization

Most of the potential is for large schemes of some GW and up to 10 GW and relevant investments for one site are similar to investments for very large hydropower schemes or for nuclear plants. It is thus likely that these sites will not be developed before 2025, i.e. before the experience and optimization from smaller schemes. But there are worldwide hundreds sites for some hundreds MW where the cost per MW may be slightly higher than for the best huge sites but however acceptable. It will be the opportunity for optimizing designs and equipments for the main development.

Ten countries have the technical capacity and the potential justifying an early implementation of such preliminary schemes. It will be also the best way of checking the impacts of the solution and of improving them.

The tidal world yearly investments will probably not be very high before 2025. But it could be 50 Billions / year after 2030 because it will include the investment for power supply and also for facilities such as energy storage, wind farms, industrial developments.

8. Energy storage at sea beyond tidal areas

A small part of tidal basins may be used for energy storage by basins of which the dykes are built in calm water. But where there is no tidal basin, there are many other possibilities of storing energy along shore or offshore i.e. of using the sea as one basin of a Pumping Storage Plant (PSP).

– An upper basin may be placed on a cliff and the sea used as low basin.

– A basin may be created along shore and used as high basin: there are many alternatives for choosing the operating head and relevant dykes height. This head may well be as low as 10 or 20 m or over 50 m.

– A basin may be fully offshore and used as low basin or high basin.

– The cost of Energy storage fully at sea is generally acceptable only for rather large schemes over 500 MW except in Islands where much smaller schemes may be cost effective.

– A great advantage as compared with traditional PSP in mountains is the much better possibility of modifying the operation in very short time because the two basins are very close and are not linked by tunnels. Some hundreds GW of PSP at sea may be built before 2050.

9. Shore protection beyond tidal schemes

Beyond areas of possible tidal schemes the need of shore protection will also increase along the century for three reasons:

– The human risk from tsunamis or typhoons.

– The much increasing cost of buildings and infrastructures.

– The increase of oceans level and relevant disastrous impacts on some places such as deltas.

Where protections are not possible onshore the cost of offshore dykes may be acceptable, between 10 et 50 Millions €/Km. They could withstand some hours the impact of tsunamis or typhoons with a 10 m head, and/or withstand full time heads of few m:

– Adding to dykes some sluices and/or pumping stations will favour the choice full time of the optimum water level along shore.

CONCLUSION

Technical solutions studied up to now are poorly adapted to the very specific data of tidal energy and thus too costly. A new solution, the “Tidal Gardens” may be cost effective for 1 500 TWh/year in 20 countries even with natural tidal ranges as low as 3 or 4 m.

The environmental impacts seem better than for other renewable energies. The large relevant basins may be used also for very large low cost wind farms, industrial and touristic developments. Energy storage by PSP at sea and shore protection are favoured by such tidal plants but may also be obtained without Energy supply where tidal range is very low.

After 2030 the yearly investment for various dams at sea could be higher than the past or future yearly investment for traditional dams.

SUMMARY

The Energy potential of Rivers and of Tides is about the same. Hydropower supplies 3 500 TWh/year and Tidal Energy 1 TWh/year.

The reason of this surprising gap is not the environmental impact which may be actually more favourable for tidal energy (Shiwah Plant in Korea was made for improving impacts).

The true reason is that the traditional plants design successful in Hydropower and studied since 60 years for tidal energy is poorly adapted to the specific requirements of cost effective tidal energy, i.e. a very low operating head of about 2 m and flows of dozens or hundreds thousands m3/s.

A more recent solution is based on In Stream Turbines (similar to Wind Mills) which may be cost effective with prevailing water speed over 3m/s: a row of such turbines may use a water head to 0,20 m but there are few natural world sites with favourable data of water speed and local conditions (waves, access, links to grid, ….) and the cost effective potential is very low.

A new solution (Tidal Gardens) uses Large basins along shore linked to sea by wide channels in which are placed 10 or 20 rows of In Stream Turbines; they are built and operated in optimal conditions of water speed, construction, maintenance and link to grid. They may use a large part of available energy and operate both ways, i.e. most time. The optimal water speed i.e. full capacity may be kept through adjusting time of channels opening and of number of operating turbines. This solution has three key advantages as compared with traditional plants:

–  The cost par MW is lower and the yearly energy supply is 4 000 hours of the capacity instead of 2 000, thus halfing the cost per MWh.

– This attractive cost applies also for natural tides of 3 to 5 m, the number and investment of turbines being proportional to the tidal range. The tidal energy is thus not limited to exceptional sites and may be used in twenty countries instead of 5 or 10.

– The possibility of using a two ways operation keeps in the basin and along shore the tidal conditions close to the natural ones (shifted by 2 hours) and huge waves or detrimental very high water levels are avoided.

Negative and positive Environmental impacts deserve careful studies and comparisons, for a same energy, with other energy sources. They seem better for these tidal schemes than impacts from traditional Hydropower. Large basins of calm water along shore favour many opportunities: basins for energy storage built in calm water and using few per cent of the main basin area, low cost wind farms producing as much as tidal energy per Km2, fish farming, touristic development along shore, industrial and harbour development along the main dyke of the basin 20 km from shore. Shore protection against waves and possible control of the highest water levels may be extremely useful in many countries and mitigate the impact of oceans level increase.

The world cost effective tidal energy supply may be 1 500 TWh/year half of the present Hydropower or nuclear Energy. It is linked with natural tidal range over 3 m and moderate sea depth (dykes less than 30 m high). Huge wind energy in tidal basins shall be added possibly for 500 or 1 000 TWh/year. About Ten Countries could supply each 50 to 200 TWh/year: Russia, China, Canada, Australia, France, U.K, India, Brazil, Argentina. Other countries could each supply 10 to 50 TWh/year: U.S. (Alaska), Netherlands, Germany, Panama, Vietnam, Pakistan, Mozambic, Myanmar, North Korea,…

Most potential is by 100 very large sites between 1 and 10 GW to be implemented after 2025.

But there are hundreds of sites of hundreds of MW: 10 or 20 could be implemented before 2025 for optimizing the technical solutions and precising the impacts. Larger sites, using similar solutions, will thus be developed very safely.

[1] Le Mao P., 1985, Peuplements piscicole et teuthologique du bassin maritime de la Rance, impact de l’aménagement marémoteur, EN-SAR, 125 p

[2] UICN France (2014) : Développement des énergies marines renouvelables et préservation de la biodiversité. Synthèse à l’usage des décideurs. Paris, France


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