The energy of wave motion
In this article we will discuss the energy of wave motion using a sustainable approach. We will try to learn a simple, yet global vision of all that is necessary for its exploitation to produce useful electrical energy.
The renewable source
How many times it must have happened that we sensed the amount of power inherent in the sea! For example, on a day of “rough sea” where great waves crash on the cliffs, or during a scene from a movie or even while browsing our social apps.
Therefore, man has been using his wits to try and exploit this great energy. There is as a series of technologies that are related to many opportunities given by the sea, all included in the category of Oceanic Energies. Among these we identify: the energy of sea currents, the salinity gradient power (osmotic power), tidal energy, thalassothermal energy (OTEC) and the energy of wave motion, the one that we will further discuss.
The nature of the resource
The energy of wave motion indirectly descends from solar energy. In fact, solar energy heats up the air in the atmosphere causing temperature differences between different portions of air masses, which tend to mix creating wind as an effect. Then the wind interacts with the sea surface transferring its kinetic energy and producing the waves.
The typical values of energy flow associated with wave motion in deep waters (> 100-200 m) varies between 6-70 kW/m, depending on the portion of sea surface interacting with the wind and the duration of the phenomenon. The value of energy in “shallow” waters is lower, this depends on many factors causing losses of kinetic energy such as local depth or some characteristics of the seabed (steepness, roughness and composition).
The comparison with wind energy
Since waves descend directly from the wind, this thought might arise spontaneously: ” why am I struggling to exploit an energy resource that descends from another? Shouldn’t I rather focus on offshore wind energy? Well, the answer is negative and the explanation is shown in the picture below:
The comparison between speed distributions shows that the converters associataed with wind energy have installation height limits (structural resistance) that enables a far limited placement compared to the actual speed distribution. On the contrary, the converters associated with wave motion are placed exactly in the peak area: the density of the energy flow per unit of vertical area is as much as 5 times higher for wave motion than wind energy.
WAVES ARE A FAR MORE CONCENTRATED FORM OF ENERGY THAN THE WIND!
Besides, the variability of waves is lower than that of the wind thanks to higher forces of inertia, therefore they can be studied with a more accurate predictability.
In any case, the main issue is the cost of the converters: it’s really high because of the complexity of the technologies.
Capacitance and distribution of the power of the resource in the world
The maximum theoretical average capacitance that can be developed by wave motion is estimable in 1-10 TW, so, assuming that there is only 1 TW available (ideally because we are assuming to use all available oceanic surface), this would correspond to 8760 TWh in a year, which, compared to the actual annual demand of electrical energy in the world (in 2018 it was about 25000 TWh, source:IEA), would mean the satisfaction of 1/3 of the demand!
The distribution of density of the average annual power of the wave motion energy is well explained in the chart below:
Since when are we using this resource?
The first patent certifyinmg the exploitment of this energetic resource dates back to the father and the son of the Girard family from Paris in 1799. Anyway, the actual founder of modern technologies is Yoshio Masuda (1925-2009), an officer of the Japanese Navy and inventor of the OWC converter (Oscillating Water Column). He promoted the first large converter of wave motion energy to be installed in the open ocean in 1976: the Kamei (working with the japanese company Jamstec).
Another pioneer is Michael E. McCormick who introduced improvements to the OWC technology, and is also the author of many writings on the conversion of wave motion energy.
Lastly, we need to mention J.Falnes and K.Budal who developed most of the theory of the control of converters.
Current state of the art
We will henceforth explain in detail the functioning principles of the currently used technologies. But first let’s visualize the general classification as shown in the summary document of the “19th INTERNATIONAL SHIP AND OFFSHORE STRUCTURES CONGRESS“.
oSCILLATING WATER COLUMN
These converters have a really simple principle of functioning. They have a body in which three main areas are distinguishable: a closed volume, with one of its “walls” that define it being the surface of moving water, the external environment and a turbine that enables the exchange between the first two area. Therefore, they produce energy using the functioning of air turbines (Turbine Wells).
The turbine is activated by the passage of air, whose motion is regulated by the water column whose height depends on the wave motion conditions.
The converters functioning by this principle are classifiable as stationary or moving main body, and in case it is stationary, they can be installed in secluded places or close to a breakwater.
Stationary body OWC in secluded installation. The converter is from 2015 and it is situated in Australia (1 MW) Stationary body OWC in breakwater implemented installation. The converter is from 2008-2012, it is situated in Spain, Mutriku (6x18.5 kW) Moving body OWC. The converter is a prototype from 2018-2019, it is situated in Spain.
These converters produce energy in a different way than those above, because they use linear electrical generators that are either directly attached to the oscillating body or through hydraulic circuits.
They practically exploit the relative motion created between the oscillating body and the waves.
The converters that function by this principle are classifiable as floating on the surface or fully underwater.
Floating OB, the converter is from 2012 and is situated in Brasil. Fully underwater OB, the converter is from 2011 and is situated in Australia (80kW)
This type uses hydraulic turbines to produce electrical energy.
It practically uses the varying height of the waves to carry the water near a hydraulic turbine.
The converters that function by this principle are classifiable as movable or fixed.
Movable OT, situated in Denmark Fixed OT, situated in Norway
Sustainability of the conversion process
As we can learn by reading the previous articles “Lo Sviluppo Sostenibile” and “L’Ecodesign“, a sustainable approach to any engineering technology requires that we do not dwell only on the main aspects! We must keep an eye on anything that contributes to the achievement of the goal of that technology.
Therefore, a sustainable judgment regarding the conversion systems of wave motion energy requires the achievement of level of competence also on:
They are of fundamental importance to maintain the motion of movable converters below a limit, besides the relative interaction with the converter affects the laws of energy absorption. That’s why we must study the technologies considering the oscillating body and the anchoring as a single system. Alongside this aspect we must consider the design of these objects that need to be capable of enduring variable environmental conditions, thus requiring a significant study on materials and resistances.
The connection to the coast for the transportation of electrical energy is realized using cables, not the usual underground lines. This is a rather significant aspect because the cables have high capacitance, which greatly increases the reactive generated power at the expense of active power. This implies the need to design an adequate system of transmission with the implementation of reactive compensator, thus with inductances. conditions of transportation are complicated because of yet another reason : reactive power increases with cable voltage and length, so a direct current transmission is chosen over the alternatig current, in order to reduce losses, but this raises the prices!
Marine environment conditions
Every component has to be adequately designed to be protected from corrosion conditions and maximize their operational lifespan.
It is necessary to find the best solution to electrically connect a park of converters and arrange in a way that prevents them from negatively interacting with each other, which would have negative consequences on the laws of energy absorption.
A problem that is common to all energies with alternating production. For a deeper analysis we recommend the article: “Conservare energia: energy storage systems“.
Maritime safety issues
Accurate information needs to be gathered about risk management and maritime safety regulations, by referring to the IMO (International Maritime Organization) regulations.
Ultimately, the current biggest limit hindering the sustainability of the process is the economic aspect. As you can read in the IRENA Technological Brief of 2014, the LCOE (levelaize cost of energy) of wave motion energy is about 330-630 €/MWh, that is a value way above market price. Then again, the foreseeable technological growth gives reason to believe that in 2030 a LCOE of 113-226 €/MWh will be reached, thus much more competitive with the current technologies for the conversion of energy from other renewable sources.
We can infer that the exploitation of this renewable source is not currently sustainable but it has high potential, it is then fundamental not to stop researching and investing.
Situation in Italy
Italy is in a rather favorable position compared to the other countries bordering the Mediterranean Sea. To understand this statement we attach some explicatory charts of the potential of energetic flow, divided by seasons and calculated on the average of the 2001/2010 decade (Source: ENEA-“Valutazione del potenziale energetico del moto ondoso lungo le coste italiane“):
To get into the specifics of the Italian coastline we attach the following charts, still calculated on the average of the 2001/2010 decade and divided by seasons (Source: ENEA-“Valutazione del potenziale energetico del moto ondoso lungo le coste italiane“):
Converters already installed in our peninsula
Until now, the installation of electrical generation devices that use wave motion and tide currents has been prototypical in Italy, in particular ISWEC, REWEC3 and R115/H24, not having yet achieved the status of consistent generation put on the market.
Below are shown some converters placed in Italy.
This conversion system is of the movable OWC type.
This conversion system is of the fixed OWC type, installed on the breakwater of the city harbor. In particular, it is made of 124 OWC conversion devices with the particular “U” technology, whose functioning has some improvements compared to the traditional one. (Interesting presentation by ENEA and WaveEnergy.it)
- Pictures of the waves by Russel Ord. To see all his photographic creations you can browse www.russellordphoto.com
- Electricity 2018 – www.iea.org
- Offshore Renewable Energy – www.researchgate.net
- Wells Turbine – www.researchgate.net
- Wave Simulation – www.researchgate.net
- 1 Document Wave Energy – www.irena.org
- 2 Document Wave Energy – www.irena.org
- 3 Document Wave Energy – www.enea.it
- Appunti Professor Falcao (Instituto Superior Técnico Lisboa)
- Lo sviluppo sostenibile by Andrea Carrabba
- L’Ecodesign by Andrea Carrabba
- Eolico – come sfruttare il vento by Marco Ciancarini
- Eolico galleggiante by Davide Burdo
- Il solare – i miti da sfatare by Davide Burdo
- Conservare energia: energy storage systems by Francesco Prodi