Ocean Thermal Energy (ETM)

Unfinished technology and not yet profitable

However, the relatively small temperature difference between surface and deep water limits the energy efficiency of an ocean thermal power plant. This low efficiency must be compensated by a considerable water flow.

However, pumping cold water requires energy consumption, which can be considerable. According to Ifremer, the cold water pumping power can be reduced to 20% of the gross power produced by the ETM plant if you opt for piped water. large diameters. Thus, in the future, coastal thermal power plants should use significant infrastructure with pipelines of 8 meters or more in diameter.

On the other hand, the establishment of thermal power plants is limited by topographic conditions : it must be located in intertropical zones, in which a temperature gradient of at least 20 °C is available between deep and surface waters and where depths reach almost 1,000 m. These criteria are a prerequisite for producing an amount of electricity 3 to 5 times greater than what it consumes itself race.

The areas suitable for the use of ocean thermal energy are approximately between 30°N and 30°S. The more we move away from this zone, the more the rays reach the surface of the ocean obliquely and the more its thermal capacity decreases.

The surface of the oceans, where the temperature gradient between deep and surface waters is greater than 20 °C, is estimated to be almost 60 million km2approximately 3x larger than North and Central America.

India, Japan, Taiwan and the United States are the main countries, with France currently investing in ocean thermal energy. Almost 35 countries located wholly or partially in intertropical zonecan aspire to use this source of energy.

A return that does not yet cover costs

The efficiency of the ETM device indicates the ratio of the amount of electrical energy produced to the energy of evaporation of the working fluid. The maximum efficiency that can be achieved is the Carnot efficiency, i.e. r (Carnot) = 1 – (cold temperature in K / hot temperature in K). This efficiency is 6.8% for a closed-cycle plant with a cold water temperature of 4°C (ie 277.15 K) and a hot water temperature of 24°C (ie 297.15 K). compared to nearly 40% for a gas turbine. Note that the ETM device also consumes energy to pump seawater and vacuum the evaporator in the case of open cycle.

According to the study, the global potential for the production of thermal energy from the seas in the world could reach 10,000 TWh/year.OUCH-OES dedicated to marine energy, which accounts for more than half of global electricity consumption.

In France, production potential could technically reach 1.4 TWh or the equivalent of 0.3% of gross domestic consumption, assuming an installed capacity of 0.2 GW is reached in this horizon.

The current investment cost is 20 million EUR/MW installation, which leads to production costs €500/MWh, a high number, but already lower than the marginal operating costs of thermal power plants on isolated Pacific islands. Cost reductions through learning and scale effects to reach the threshold of 10 million EUR/MW of installed energy would make this energy competitive in the very large intertropical sector.

Environmental benefits

If the water at depth is rich in CO2THE carbon factor of this energy source is very advantageous. For example, an ETM power plant would emit almost 100 times less carbon dioxide than a thermal power plant.

Another advantage is that the power plant refuses to pump water at a depth that can create a artificial levitation (emergence of water to the surface), which, according to observers, could contribute to an increase in biological production. Upwellings bring rich nutrients to the surface and improve photosynthesis, the basis of food for marine fauna (which also reduces the risk of eutrophication).

Please note that the ETM can enhance the energy independence of countries with coastal areas meeting the above conditions. These countries are often still strong dependent on fossil fuels to produce electricity. This ambition motivates investments from Australia, Taiwan and, above all, Japan.

Finally, if the ETM plant does not release pollutants, but some risks sometimes reported: fish kills caused by water pumping, use of chlorine or low doses of biocide to prevent biofouling and marine sediments, etc. These risks are a priori greatly reduced.

The study must also make it possible to specify the impacts of the possible anthropogenic disturbance on the thermal structure of the seas (laboratories in Hawaii, Ditmars, Paddock, Vega, France Énergies Marines, etc.).

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