A variant of tidal energy is tidal stream (or marine current) technology, which aims to exploit the strong tidal currents which are found in shallow seas, particularly where natural constrictions exist, such as around headlands or between islands. Devices similar to submerged wind turbines would be used to exploit the kinetic energy in tidal currents. As with wind energy, a cube law relates instantaneous power to fluid velocity. So a marine current of 2.5 metres per second (5 knots), not an unusual occurrence at such locations, represents a power flux of about 8 kW/m². The minimum velocity for practical purposes is around 1 metre per second (2 knots), i.e. about 0.5 kW/m². The main siting requirement is thus a location having flows exceeding about 1.5 metres per second for a reasonable period.

The various turbine rotor options generally coincide with those used for wind turbines. The two main types are the horizontal axis, axial-flow turbine (with a propeller type of rotor) and the cross-flow or Darrieus turbine, in which blades rotate about an axis perpendicular to the flow. The more promising rotor configuration seems to be the conventional axial flow rotor.

The maximum flow velocity tends to be near the sea’s surface, so marine current turbine rotors ideally need to intercept as much of the depth of flow as possible, but especially the near surface flow. Options for securing a rotor include mounting it beneath a floating pontoon or buoy, suspending it from a tension leg arrangement between an anchor on the seabed and a flotation unit on the surface, and seabed mounting (feasible in shallow water, but more difficult in deeper water). Floating devices have the problem of providing secure anchors and moorings. Seabed-mounted devices seem more straightforward to engineer. One option is a mono-pile set into a socket drilled into the seabed, which seems the most cost-effective solution, just as it is for offshore wind turbines.

Ocean thermal energy conversion

Two main processes are used for power production from this source, both based on the Rankine (steam/vapour) cycle:

• The open cycle system flash evaporates warm seawater into vapour (at reduced pressure) and then draws it through a turbine by condensing it in a condenser cooled by cold seawater.
• The closed cycle system uses warm seawater to boil a low temperature fluid, such as ammonia, which is then drawn through a turbine by being condensed in a heat exchanger with cold seawater and then recycled back to the boiler by a feed pump.

Offshore OTEC is technically difficult because of the need to pipe large volumes of water from the seabed to a floating system, the huge areas of heat exchanger needed, and the difficulty of transmitting power from a device floating in deep water to the shore. The latest thinking is that OTEC needs to be applied as a multipurpose technology: for example, the nutrient-rich cold water drawn from the deep ocean has been found to be valuable for fish farming. In addition, the cold water can be used directly for cooling applications in the tropics such as air conditioning. If OTEC takes off, it is likely to be with energy as a by-product.