Ocean Energy (OE) involves the generation of electricity from the tides, the waves, the currents, the salinity gradient, and the thermal gradient of the sea or the ocean. The ocean is an enormous and predictable source of renewable energy with the potential to satisfy an important percentage of the worldwide electricity supply.
Globally, the theoretical potential of OE has been estimated over 100.000 TWh/year (as a reference, the world’s electricity consumption is around 16.000 TWh/year). The global technical resource exploitable with today technology is estimated to be in the order of 45.000 TWh/year for wave energy; tidal current energy is in the order of 2.200 TWh/year, salinity gradient energy in the order of 20.000 TWh/year, and of OTEC in the order of 33.000 TWh/year. Conversion of the wave resource alone could supply a substantial part of electricity demand of several countries in Europe, such as Ireland, UK, Denmark, Portugal and Spain and others. The electricity demand on islands in remote areas could entirely be met by converting a small fraction of the available OE resource.
Unlike other RES, ocean energy is not captured from a single source, but, instead, is stored in a variety of forms: the energy of waves, the kinetic energy of marine and tidal currents, the potential energy of tides, and salinity or thermal gradients. As a consequence of this variety the number of concepts for ocean energy conversion is very large. A first, basic division is grounded on the specific source of energy that the technology is tapping into.
Tidal energy conversion techniques exploit the natural rise and fall of the level of the oceans caused principally by the interaction of the gravitational fields in the Earth-Sun-Moon system. Some coastlines, particularly estuaries, accentuate this effect creating tidal ranges of up to ~17 m.
The vertical water movements associated with the rise and fall of the body of water, and horizontal water motions termed currents, accompany the tides. These resources therefore have to be distinguished between tidal range energy (the potential energy from the difference in height—or head—between high and low tides), and tidal current energy (the horizontal movement, i.e. the kinetic energy of the water in a tidal current).
Potential energy associated with tides can be harnessed by building barrages or other forms of engineering constructions across an estuary. Tidal barrages consist of a large, dam-like structure built across the mouth of a bay or an estuary in an area with a large tidal range. As the level of the water changes with the tides, a difference in height develops across the barrage. Water is allowed to flow through the barrage via turbines. This generation cycle means that, depending on the site, power can be delivered twice or four times per day on a highly predictable basis.
The principle of conversion is very similar to the technology used in traditional hydroelectric power plants. In France, the La Rance Barrage has a capacity of 240 MW, and has been producing 600GWh/year since 1966. Other barrages for hundreds of MW of installed power are currently under discussion in the U.K.
Rather than using a dam structure, tidal current devices are placed directly “in-stream” and generate energy from the ?ow of the tidal current. There are a number of di?erent technologies for extracting energy from tidal currents. Many are similar to those used for wind energy conversion, i.e. turbines of horizontal or vertical axis (“cross flow” turbine, as well as others such as, venturis and oscillating foils). Additionally, there are a variety of methods for ?xing tidal current devices in place, including seabed anchoring, via a gravity base or driven piles, as well as ?oating or semi-?oating platforms ?xed to the sea-bottom via mooring lines.
Currents are not generated by tides only, but also by winds, and temperature and salinity differences. The concept for harvesting the kinetic energy from marine, also known as ocean currents, is essentially the same as with tidal currents.
OTEC uses the heat stored in the oceans to generate electricity. Due to solar heating, the top layer of the water is much warmer than deep ocean water. Where the temperature difference between the warmer, top layer of the ocean and the colder, deep ocean water is about 20°C (36°F), the conditions for OTEC are most favourable. These conditions exist mainly in coastal areas located close to the Equator. The amount of energy available in the temperature gradient between hot and cold seawater can be substantially larger than the energy required to pump the cold seawater up from the lower layers of the ocean. The warm water from the surface is used to boil a working ?uid (or, in open cycle systems, the seawater itself under low pressure), which is then run through a turbine and condensed using cold seawater pumped up from the depths. Some energy experts claim that once it reaches cost-competitiveness with conventional power technologies, OTEC could produce billions of watts of electrical power.
At the mouth of rivers where fresh water mixes with salt water, energy associated with the salinity gradient can be harnessed using pressure-retarded reverse osmosis process and associated conversion technologies. Another system is based on using freshwater upwelling through a turbine immersed in seawater, and one involving electrochemical reactions is also in development.
The possibility of generating electrical power from the sea has been recognized for many years (the first patent on wave energy conversion was issued as early as 1799, and, already in 1909, a harbour lighting system in California was powered with a wave energy system). However, significant research and development of wave energy conversion began only rather recently: in fact, although there was a renewed interest on wave energy after the oil crisis of 1973, it subsided again a few years later.
Five years ago, especially in Europe, the sector experienced a resurgent interest. Today, wave energy conversion is being investigated in a number of EU countries, major activity is also ongoing outside Europe, mainly in Canada, China, India, Japan, Russia, and the USA.
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