Tidal Currents are the ocean water mass response to tidal range. They are generated by horizontal movements of water. These movements are modified by seabed bathymetry. Tidal current flow result from the rise and fall of the tide; their timing and magnitude are highly predictable.The main difference between the turbines used in tidal and river (or ocean) is that currents flows are unidirectional, whilst tidal currents reverse ﬂow direction. For this reason, tidal current turbines have been designed to generate in both directions. The advantage of using tidal energy is that tides are more predictable than other renewable energies. A greatest use in tidal power in a future is promising since enhanced designs and turbine technologies are being developed.
There are variations of the height of a tide over a day.
Tidal stream generators are devices that use the kinetic energy of moving water to power turbines. These generators are the least ecologically damaging among the forms of tidal power generation. The suitable places to locate these systems are areas with fast currents where natural flows are concentrated between obstructions (at the entrances to bays and rivers, around rocky points, headlands, or between islands).
There are three kinds of tidal stream generators:
Axial turbines have rotary blades with axis parallel to the flow direction.
These turbines can be deployed either vertically or horizontally. Most water turbines have axial or radial flows; in a cross-flow turbine the water passes through the turbine transversely, or across the turbine blades. Cross-flow turbines are mostly used in mini and micro hydropower units of less than two thousand kW and with heads less than 200 m.
The shrouded turbine is used to increase the flow rate. The shrouded tidal turbine is an emerging tidal stream technology that has a turbine enclosed in a venturi shaped shroud or duct, producing a sub atmosphere of low pressure behind the turbine. They can operate in shallower slower moving water with a smaller turbine at sites where large turbines are restricted.
Artificial photosynthesis has the intention of constructing systems doing the same type of processes that the natural photosynthesis. One of the simplest designs is where the photosensitizer is linked in tandem between a water oxidation catalyst and a hydrogen catalyst:
Photoelectrochemical cells are heterogeneous systems that use light to produce electricity or hydrogen. Dye-sensitized solar cell is a hopeful, recent type of solar cell. This cell depends on a semiconductor for current conduction on one electrode and it is covered by a coating of an organic or inorganic dye that acts as a photosensitizer; the counter electrode is a platinum catalyst for H2 production. These cells have a self-repair mechanism and solar-to-electricity conversion efficiencies rivaling those of solid-state semiconductor ones.
Direct water oxidation by photocatalysts is a more efficient usage of solar energy than photoelectrochimical water splitting because it avoids an intermediate thermal or electrical energy conversion step. Some ruthenium complexes are able to oxidize water under solar light irradiation. They have photostability. Improvement of catalyst stability has been tried resorting to polyoxometalates, in particular ruthenium-based ones.
There are another emerging methodologies as the hydrogen-producing artificial systems and NADP+/NADPH coenzyme-inspired catalyst and photobiological production of fuels.