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Reintroduction tidal flow Grevelingenmeer

Ecological recovery Grevelingenmeer and renewable energy production go hand in hand

TIDAL POWERSTATION BROUWERSDAM

After a construction period of more than 6 years, the building of the Brouwersdam was finished in 1971, closing off the Brouwershavense Gat and connecting Goeree-Overflakkee with Schouwen- Duiveland. The Brouwersdam is the seventh construction built within the frame of the . With the Brouwersdam being finished, the creation of the Grevelingenmeer became a fact, offering better protection of the southwest delta area against storm floods from the North Sea. Unfortunately the construction of this dam also had a less positive side: the design of the dam appeared not to be sustainable.

With the full closing off of the Brouwershavense Gat, the tidal movement was gone. In 1978 a small water passage, the Brouwerssluis, was realized in order to enable a certain amount of water to flow in and out. The capacity of this water passage is sufficient to prevent the water from changing from salt into freshwater and to enable a natural exchange of fish. It is insufficient, however, to refresh the entire water resources of the lake. And fresh seawater is what nature needs most in this salt water lake. What 'environmental activists' already were afraid of, happened: by lack of tidal movement and lack of fresh seawater flushing through the lake, the natural value of the Grevelingenmeer seriously deteriorated. There is hardly any oxygen present in the deeper water layers. From the bottom up to six meters below the surface, the percentage of oxygen is as good as nil (especially in the summer). Without sufficient measures being taken soon, also the upper six meters will run out of oxygen, which will badly affect nature. The most obvious measure to be taken in order to revitalize this natural area is to reintroduce the tidal movement by constructing a large water passage. Exploratory calculations show that a water passage with a maximum capacity of 4.000 m3/s is sufficient to realize a tidal effect of approximately 0,5 meter. A larger tidal effect is not advisable since water levels of the Grevelingenmeer would become too high and/or too low. Translation of article from Civiele Techniek 1/2 2015 by Jarda van Spengen

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Tidal powerstation

The construction of such a large water passage demands a big investment. At this moment various possibilities are being explored to optimize the output of such a big investment; it might be economically interesting, for instance, to build an entire tidal power station instead of a only a water passage. That way renewable energy can be generated from the water flowing in and out. If the extra costs of the power station (compared to the costs of only a water passage) are less than the expected profits realized by the energy production, the cost-benefit analysis is positive. Since 2008 various studies have been carried out on the possibilities of a water passage with or without a tidal power station. Up to now all calculated expenses proved to be too high. Therefore also the cost-benefit analyses of a tidal power station turned out negative. Not only the (construction) design itself is vital when it comes to sustainability; also the existing knowledge, know-how and expertise concerning hydraulic concrete constructions can make a big difference with regard to building sustainable power stations.

Within the frame of the European Pro-Tide project, the Province of Zeeland has asked Iv-Infra to put in their expertise within the field of hydraulic concrete constructions. Pro-Tide is a European partnership between parties from France, Belgium, the United Kingdom and the , promoting the research and development of tidal energy production. During the project 'Tidal Diver Waterdunen' Iv-Infra's expertise in this field had clearly been demonstrated. With this project, also the feasibility of generating tidal energy had been explored.

The task assigned to Iv-Infra was to design a hydraulic construction and clever execution method for a tidal power station, in such a way that the building costs would be considerably lower than is the case in the MIRT and supplementary studies. Besides that, the surplus costs of a tidal power station should not be higher than the profits gained by the production of renewable energy. Also the construction should fulfil four tasks: it should function as a dam, it should regulate the water, it should make traffic possible by means of a road on top of the construction and it should generate energy from the tidal movement. Three different types of constructions had to be designed: using a diffuser, a pipe and a venturi. The costs for civil engineering comprise at least 50% of the total costs of a tidal power station. The other 50% are related to the realisation of bulb turbines that are often used in rivers, or the bi-directional turbines which have been derived from the bulb turbines. In order to make the production of tidal energy profitable, a well thought-out civil engineering design is of great importance.

Work method

Designs and cost calculations of earlier studies appeared not to be comparable, since secondary conditions, points of departure and definitions varied per study. During work sessions sketches and work methods for each type of device (diffuser, pipe and venturi) were conceived, discussed and analysed. Favourable variants were compared and rated on the basis of a multi-criteria analyses, in which building costs, risks, energy production, maintenance and adaptability of the construction were the most important criteria. Pro- Tide NL made a selection of three preferred variants, which were elaborated by Iv-Infra.

Traditional diffuser

Figure 1 Cross section: diffuse shaped device

Translation of article from Civiele Techniek 1/2 2015 by Jarda van Spengen

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A diffuser shaped device is built in order to generate a maximum of energy from the water flow. This power station contains a traditional bulb turbine, such as is used in rivers, or it makes use of bi-directional turbines, which were initially developed for the production of tidal energy. Both turbines are relatively efficient but expensive. Because of the presence of the diffusers making use of these types of turbines, there is a considerable amount of resistance, therefore the flow capacity is limited. In order to realize an output of 4.000m3/s in spite of the limited flow capacity, the construction has to be very long: about 660 m parallel to the dam. The design of the concrete construction is mainly determined by the essential shape of the diffuser, conducting the water through the turbines. The width of the construction, 35 m, is also determined by the shape of the diffuser, on both sides of the turbine, and by the stability requirements (tilting, sliding away, coming down, pushing up). The turbine is located in the middle of the construction; the technical room is located directly above it. The regulation and high tide slides are located near to the turbines, which makes it possible to use the technical room of the turbines for maintenance of the slides. The construction is filled up with sand, giving it sufficient extra weight, ensuring vertical balance. The construction is built in parts on an external building location and is installed on location later. It comprises 86 turbines, divided over six elements of about 100m each. An external lifting device is needed to put the elements into position.

Narrow pipe

One of the features of a narrow tube shaped device is that it involves a minimum of civil costs. In principle, various types of turbines can be installed within the construction, for instance free-flow hydraulic turbines. These turbines are relatively inexpensive, so the total investment is limited. The flow capacity is high because there is a relatively large flow surface, therefore, logically, less energy is produced by the flowing water than is the case with the diffuser that has bulb turbines. These last few years a lot of research has been done with regard to the (further) development of free-flow turbines. The output of the turbines (= the energy that can be produced by the flowing water), will increase in the future.

Figure 2 Cross section: narrow tube shaped device

Because of the larger flow surface of 8 x 8 m2, the necessary length of the construction is smaller than is the case with the diffuser shaped device, namely 136 m. The slides and the turbines are placed off-centre, at the seaside, therefore the provincial road N57 only has to be diverted temporarily during the building process. After realization of the project, the N57 will get back its original position. Should one succeed in finding a suitable building site, the entire construction can be moved and installed at its final destination as a whole. This has a big advantage: it is not necessary to put into place and connect all kinds of different elements.

Translation of article from Civiele Techniek 1/2 2015 by Jarda van Spengen

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Linear VETT turbine

A device with a venturi and 'linear VETT turbines' is the most innovative compared to the other types of constructions. The turbines are relatively cheap, but their efficiency is still limited in comparison with the bulb turbines or the bi-directional turbines.

Figure 3 Cross section: Venturi and linear VETT turbine

The 'Venturi Enhanced Turbine Technology (VETT)' was designed by VerdErg. In the linear VETT the water stream is narrowed within the venturi and runs faster, which has the effect that the rising level of the water (and the water pressure) at the exit of the venturi is lower than at the inlet.

This difference in pressure is used for the benefit of a secondary water flow, about 20 percent of the flow capacity, which flows through a separate inlet and is drawn in and accelerated by lower pressure. The (small) turbines are located in the secondary water flow, see figure 3.

The floor of the construction has a width of over 45 m., sufficient to meet the stability requirements. In order to ensure sufficient flow capacity as well, a construction length of 200 m is needed. The construction is divided into 8 elements which each have ten venturi holes and two turbines, one at the seaside and one at the Grevelingen side. It is possible to let down a large number of small slides into the narrowest hole, so that the water level can be regulated and high tide water can be kept out.

Result There are three designs that are technically realizable, two of which compete with the inlet construction of MIRT Grevelingen with regard to civil building costs: the narrow pipe-shaped construction and Linear VETT. All designs meet the requirement to let through 4.000 m3/s. However, there is a big difference in construction length and building costs on the one hand, and energy output on the other. This has to do with the output of the turbine and the resistance the water encounters when flowing through the hydraulic device. Figure 4: working principal of Linear VETT The building costs of the narrow pipe construction are even about 40 percent lower than is the case with the inlet construction, which makes the investment in a tidal power station economically interesting too. Various factors play a part with regard to the reduction of building costs: the chosen construction method, the fact that the civil construction has been studied in greater detail, and above all the use of the present caissons as a soil barrier during the building process. By using the caissons a smaller area is needed for building, less roads have to be diverted and a temporary replacement of the water dam is not necessary. In short: designing pays off. Translation of article from Civiele Techniek 1/2 2015 by Jarda van Spengen