The strength and phase of the tidal stream

Abstract

The strength and timing of tidal stream energy in a region is affected both by external hydraulic factors (the driving potential for currents) and by the internal configuration of channels and reservoirs. If we consider a single tidal frequency and take a linear approximation, the dynamics and kinematics can be reduced to an analogous electrical circuit. The external factors can then always be represented by either a voltage source and series impedance (Thévenin circuit) or by a current source and a parallel impedance (Norton circuit). A simple, short channel between two major basins depends on inertia and friction and can be represented simply by a resistor and inductor. Longer channels or channels connected to harbours can be represented using additional circuitry and impedances, including capacitors. The analogy to “capacitance” is provided by wetted area and it is shown that an area of >100km2 is sufficient to alter the strength and timing of the tidal stream for Gigawatt-scale resources. In particular, features in the currents of Pentland Firth can be explained by the capacitive properties of Scapa Flow. Options for extracting tidal energy can be explored by adding load resistors to the circuit.