3. Tidal Stream

3.2. Energy Source and Location

As with tidal range impoundment plants, tidal stream technologies rely on the tides created by the gravitational pull of the moon and sun on the seas. Impoundment uses the rise and fall of sea level, and the potential energy of heads of water trapped in a basin, but tidal stream uses the kinetic energy of the currents flowing in and out of tidal areas.

 The tidal current resource follows a sinusoidal curve with the largest currents generated during the mid-tide. The ebb-tide often has slightly larger currents than the flood-tide.

In most places the movements of seawater are too slow and the energy availability is too diffuse to permit practical energy exploitation. The strength of the marine currents generated by the tide varies, depending on the position of a site on the Earth, the shape of the coastline and the bathymetry. Along straight coastlines and in the middle of deep oceans, the tidal range and marine currents are typically low. Generally, the strength of the currents is directly related to the tidal height of the location.

There are also some locations where the water flows continuously in one direction only, and the strength is largely independent of the moon’s phase. These currents are dependent on large thermal movements and run generally from the equator to cooler areas. The most obvious examples are the Gulf Stream and the Strait of Gibraltar, where in the upper layer, a constant flow of water passes into the Mediterranean basin from the Atlantic.