Energy from the Oceans

By Gianmaria Sannino – Senior Researcher at ENEA

Gianmaria Sannino opened his speech during ReThink – Circular Ocean Forum in Genoa, with a brief and current introduction concerning the correlation between climate change and sea-level rise, explaining how the oceans absorb heat and they expand by increasing their volume. In addition to melting glaciers, in fact, the oceans’ temperature increase, is the second reason that causes the raising of the level, which is among the causes of the disaster that took place in November in Venice.

The sea can also be exploited as an intelligent energy source, Sannino showed a quote by Joseph Conrad from the book Typhoon, which says “… he had never seen the immeasurable force and excessive anger, the anger that passes and runs out without ever subsiding – the anger and fury of the irritated sea”, which turns out to be a fictionalized definition of what marine renewable energy is.

Even Victor Hugo already in 1874, with a quote taken from the novel “Novantatrè”, emphasizes how the sea is a source of energy that the earth should make use of: “Think of the movement of the waves, the ebb and flow, the coming and going of the tides. What is the ocean? a huge lost force. How stupid the earth is, not to use the ocean! “

The global marine energy potential can be a very powerful resource, it is estimated that the amount of marine energy we can extract is equal to 1,200 TWh / year, while the global wave is estimated to be 29,500 TWh / year, these data are surprising if we consider that the current global electricity demand is 25,600 TWh / year.

It would be easy to ask why it is not exploited, the first reason is that this energy is not distributed evenly; tides, waves and currents have different intensities depending on the regions of the planet.

But what are the energy sources in the oceans? We need to make distinctions:

  • Wave movement: kinetic energy transferred from the wind to the surface of the oceans
  • Tides: potential energy derived from the lunisolar gravitational attraction of the oceans
  • Tidal currents: kinetic energy of marine currents deriving from the rise and fall of the water column due to the tides
  • Sea currents: kinetic energy of the currents due to the difference in density between water masses and/or surface wind
  • Temperature gradients: thermal energy derived from the temperature difference between the surface layers of the oceans heated directly by the sun and the deeper colder layers
  • Salinity gradient: osmotic energy deriving from the difference in salinity between the oceans and the freshwater present at the mouth of the rivers
Global map of the M2 tidal component of the surface elevation (NASA, 2006)

Enea, with its forecast models, has set up a map to highlight the amount and the localization of marine energy in the Mediterranean, this one was never considered as a source of energy from previous studies, that have always focused on the potential of northern Europe.

Enea’s studies show up a very valid wave energy potential, especially in the Sicily channel and in the west of Sardinia. Italy would be able to exploit this type of energy because it has about 13 kW per meter, 1/6 of the energy present in Northern Europe, but, 

Distribution of average power for unit crest in the Mediterranean between 2001 and 2010

What types of waves can, therefore, be exploited? From a study, it turns out that the most energetic waves are those about 4 meters high and that they hit the coast every 8 seconds, having this information is fundamental to synchronize the instruments.

Enea has also created a model to analyze the potential of currents and tides in the Mediterranean, in particular in the Strait of Messina, and it imagined installing 40 instruments capable of extracting 1 GW/hour of power, sufficient energy to feed all of Messina or Reggio Calabria. Unlike the waves, which depend on the wind, the tides are easier to control because they depend on the moon, so you can know with more precision and regularity how much energy can be extracted.

Why bet on energy from the sea? The reasons are many, for example: to cut CO2 emissions; because the energy from the sea gives more stability than the energy of the wind, the energy from the sea could create many jobs (in this regard, Fincantieri is already working on a marine energy project) and it would be very useful on the isles.

However, it is not easy to exploit it, several devices have been created but nobody understood which is the most efficient.

The first patent for a device able to extract marine energy was created in 1799, someone had already thought of it, but technological convergence has not yet been realized. It is difficult to capture the energy and maintain it, but it is impossible to deny how many benefits the Earth could obtain from this type of energy.

“The huge mass of a ship … responds to the slightest swell. If you imagine this ship suspended at the end of a lever, you will conceive the idea of ​​the most powerful machine that has ever existed “

Pierre-Simon Girard, 1799

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