The consideration of constrained flow solutions for the purpose of implementing a tidal power plant in the Brouwersdam reconnecting Lake Grevelingen and the North Sea

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After the construction of the Grevelingendam (1965) and Brouwersdam (1971), the tidal movement in Lake Grevelingen disappeared. This caused a deterioration of ecological parameters influencing the surrounding flora and fauna and consequently a decline of the water quality set in. Amongst others, Rijkswaterstaat and the provinces of Zeeland and South-Holland aim to reconnect Lake Grevelingen with the North Sea to infuse oxygen enriched water into the lake and reintroduce an attenuated tide in the lake, thereby improving the quality of the local ecology. To reach the desired water quality, culverts will be installed in the Brouwersdam, thereby restoring the connection between the two water bodies. As part of an integrated approach, Rijkswaterstaat intends to implement turbines into the culverts to not only generate power, but also perform water management. The conclusion from the literature study is that implementation of constrained flow devices, wherein the entire mass flow is guided through the turbine, is preferred as these turbines are able to modulate discharge through the tidal barrage. Moreover, constrained flow devices have a higher theoretical power output than the other considered hydropower methods applicable in a tidal barrage. In order to broaden the insight into the behaviour of the water level in Lake Grevelingen, due to the construction of the tidal barrage, three hydraulic configurations are evaluated wherein input water level data from the North Sea is used to model the tidal variation in the lake while simultaneously estimating the energy output of the system. Considering a culvert array of 18 culverts, measuring 8 m by 8 m, the first considered case takes into account 11 of the culverts equipped with unidirectional turbines, while the remaining culverts are unequipped. The second case involves all 18 culverts with unidirectional turbines, while the third case is set up with 18 bidirectional devices. Additionally, to provide a larger view on the possibilities, a multivariable analysis of the three cases is carried out wherein power generation and water management requirements are considered. Herein varying the cross-sectional area of the culverts and their numbers, though, for the configuration consisting of equipped and unequipped culverts, also the number of empty culverts is varied. From these analyses, one can conclude that installing unequipped culverts and unidirectional turbines in the array will diminish the controllability, whereas bidirectional turbines increase the controllability of the flow which is desirable. Furthermore, full time turbine modulation, wherein the turbines are not generating the optimal amount of power over the full scope of the tidal range for the purpose of water management is preferred over intermittent turbine modulation, wherein turbine modulation only occurs when the water level surpass certain water levels, because the turbines can be altered continuously, which increases the controllability of the discharge. Nevertheless, for the culvert arrays with bidirectional turbines applies that the required values of the monthly averaged and maximum tidal ranges are insufficient. Even though, the water level in Lake Grevelingen does not exceed the overshoot/undershoot boundaries, established by Rijkswaterstaat, the entire vision of water management is only partially fulfilled due to the lack of amplitude in the tidal range.

The study concludes with the selection of the following culvert array existing of 24 culverts measuring 9 m by 9 m executed with bidirectional turbines. From an economic feasibility study, wherein, amongst other involved parameters, an installed capacity of 36 MW, a market price of electricity of € 0.13 per kilowatt hour for the first 15 years and € 0.049 per kilowatt hour for the second 15 years and a loan period of 15 years, follows a payback period of 15 years and a levelised cost of energy value of € 0.058 per kilowatt hour.