The fact that the head varies appreciably during the tidal cycle means that a fixed-blade turbine will not be operating under its most efficient conditions during the majority of the tidal flow consequently a variable-blade Kaplan turbine is usually employed. Such low heads necessitate the use of a propeller turbine, the turbine type best suited for low-head operation. The highest global tidal reach, in the Bay of Fundy in Canada, is 15.8 m and the mean tidal reach is probably half of this range most plants would have to operate with much lower heads than this. The turbines in a tidal power station must operate under a variable, low head of water. Paul Breeze, in Hydropower, 2018 Turbines The primary objective of this chapter is to equip those working within or researching marine renewable energy with an understanding of the fundamentals of tidal energy from both oceanographic and engineering perspectives. It covers methods of analysing and predicting the tides, and how tides can be used to generate electricity through arrays of tidal stream devices, and tidal range schemes (lagoons). In this chapter, we explain the origin of the tides, and how tides evolve as they propagate over shelf sea regions. However, regardless of its predictability, tidal energy shares a key feature with the majority of renewable energy sources-it is intermittent, from diurnal (once per day) and semidiurnal (twice daily), to fortnightly (spring-neap) timescales. This is in contrast to other more stochastic renewable energy sources such as wind and wave and, in part, solar.
Tidal energy is presently one of the more favoured forms of marine renewable energy because, due to its origins in (astronomical) tide generating forces, it is predictable. Reza Hashemi, in Fundamentals of Ocean Renewable Energy, 2018 Abstract
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