The conceptual design of a Tidal Power Plant in the Brouwersdam

Abstract

Zeeland, located in the south west of The Netherlands, has suffered major casualties and damage due to the flood in 1953. After the disaster, the Dutch Government appointed the so-called Deltacomittee, provided with the task of advising on the execution of the Deltaplan. As part of the Deltaplan, North Sea estuaries has been closed off from the North Sea in order to provide sufficient safety against flooding in the future.
The 11000 hectare Lake Grevelingen was one of the closed off North Sea estuaries. Two dams and two islands enclose the lake and prevent fresh water flowing in. Characteristic of former estuaries are deep gullies that remain intact as any kind of sediment transport is prevented. Contact with the North Sea is prevented due to the construction of the Brouwersdam, the western border of Lake Grevelingen. Direct after finishing the construction works of the Brouwersdam, Lake Grevelingen showed reducing oxygen gradients and thus a decreasing water quality, especially at the deep gullies. The construction of a stop lock, the Brouwerssluice, was conducted to prevent the further decrease of the oxygen gradient.
Today, more than 40 years after finalizing the Brouwerssluice, the low oxygen gradient tends to rise to shallower areas and reaches critical values. Measures, such as a water inlet system are due to be constructed.
Introducing a tidal range of 50 cm has shown to be a valid solution to overcome the decreasing water quality. The tidal range allows thinking of power generation when water flowing in or out Lake Grevelingen. Generating tidal energy produces revenues and provides renewable energy. Therefore, the design of a Tidal Power Plant in the Brouwersdam has become the goal of this thesis.
Since caissons were used to close Lake Grevelingen off from the North Sea, as a starting point, it has been determined whether these caissons allow reusing with a new function; forming the powerhouse of a Tidal Power Plant. Mainly due to the concrete cover of the caisson, has shown to be inappropriate. Adapting the caissons becomes expensive and very time-consuming.
The starting point of the power plant construction was found in the turbine dimensions. In total six turbine types have been analyzed. Based on their efficiency, fish friendliness and ability to generate power in two directions, two optimal turbine types for low head power generation have been found: free-stream (Tocardo) and a modified bulb turbine (Pentair Fairbanks Nijhuis).
Due to the low head differences between lake Grevelingen and the North Sea, maximum energy generation will be required even for very low head differences. Hence, turbines able to generate energy at such low head differences will be required. In the Preliminary turbine design, two turbine types provided the most promising features: The Pentair Fairbanks modified Bulb turbine and the Tocardo free-stream turbine.
A total cross-sectional discharge area of 960 m2 would lead to a tidal range of 50 cm at Lake Grevelingen. Applying in total 18 turbines in a squared sluiceway with an inner space of 8.24 meter allows introducing a tidal range of 50 cm at the lake.
The free-stream turbine has already proven to generate power in low head environments in previous projects. The PFN modified bulb turbine has not yet been tested on such scale, but due to the expected higher efficiency rate, the energy generation of the PFN lies supposedly much higher than the Tocardo free-stream turbine. Therefore, it has been decided to temporarily apply the free-stream turbine until the PFN turbine has shown to meet the fish friendly requirements while ensuring high efficiencies. With current knowledge, the annual production of the Tocardo and PFN turbine has been estimated at 20 GWh and 80 GWh respectively. The energy production of the freestream turbine is based on the tidal data obtained from 2016. The same tidal data has been applied to the PFN turbine while accounting for the sea level rise as well. Since it is expected the PFN turbine will be in commission at 2030, an estimated sea level rise of 25 cm has been added to the 2016 tidal data.
The thickness of the concrete members in the preliminary design has been set at 500 mm. The conceptual design of structural elements has been dedicated to checking the assumption. In total four base cases have been studied, providing local and global forces emerged from uneven bedding of the rubble foundation layer. The rubble foundation layer functions as a bed protection foundation on which the Tidal Power Plant is located.
The influence of uneven bedding of the rubble foundation bed has been computed by considering the Tidal Power Plant as a beam on an elastic foundation. Due to an uneven bedding, one-dimensional base cases Hogging and Sagging are introduced. Sagging represents a beam on elastic foundation subject to a reduced foundation stiffness at the midsection of the beam. Hogging has been described as the opposite situation of sagging, a reduced bedding stiffness at the beam edges.
The structure’s strength has been determined while applying the hogging and sagging bases cases together with external and local forces. The external forces emerged from the hydraulic pressures in transverse direction and lateral earth pressures in longitudinal direction. The local forces concern the self-weight of an element or locally applied loads, such as the ballast load or road construction. The bending and shear resistance of the concrete members has been checked accordingly.
In addition to the forces obtained from hogging and sagging, a coupled 2-dimensional approach has been applied. Two base cases have been considered in the 2-dimensional plane. The uneven bedding induces the rise of torsional moments in the beam on an elastic foundation. The torsional resistance of the concrete members has been checked. Together with local and global one-dimensional forces, the governing reinforcement ratio has been determined for relevant concrete members. Combining the findings of the one-dimensional approach with the two-dimensional approach has resulted in reinforcement ratios exceeding 3%. Comparing this with the 1 % economical reinforcement ratio value, it has been concluded the 500 mm thickness of the concrete member is insufficient.
In further analyses, it is recommended to provide a cost estimation including revenues made from the energy production. The feasibility of the Tidal Power Plant will be determined by this cost estimation.