EDS: the Energy Double System for harnessing wave energy in the nearshore zone

Our proposal is to install a particular wave energy converter (WEC) designed for the nearshore zone, the Energy Double System (EDS), to produce clean electric energy. Its name is because it can harness both vertical and horizontal wave forces, through two different bodies placed in the same structure: a heaving float and a surging paddle. This allows to maximize the energy absorbed from each wave, since EDS can proficiently exploit both the vertical force of all the waves (through the float) and the huge horizontal force of the waves that are about to break (through the paddle). EDS can be attached to existing structures, and it is modular. Please visit our websites http://www.fluidlab.polimi.it/main-topics/wave-energy-converter and https://www.tecnomac.it/it/eds-energy-double-system.html.

Waves in the nearshore are directionally focused and have a huge horizontal force, because of the effect of the sea bottom. The EDS is designed for dealing with such wave characteristics. The EDS combines a heaving float, supported by an arm hinged on a fixed structure, and a surging paddle, top-hinged on the same arm, between the float and the fixed hinge. The supporting arm is aligned along the wave propagation direction.

The EDS is scalable according to the available wave power, with typical float diameter ranging from 3 to 8 m.

The energy is extracted by a PTO (Power take-off) connected to the arms of the bodies. This takes place through the rotation of a single main shaft to which is connected a permanent magnet generator controlled by an A.F.E. system.

Tests on the scale model of the EDS, carried out at the Hydraulics Laboratory of the Politecnico di Milano, have revealed an efficiency of more than 40%, measured as the rate between the power absorbed by the EDS and the oncoming wave power. The main publications on the EDS are:

-        Marchesi  E.,  Negri  M.,  Malavasi  S.,  2020.  Development and analysis of a numerical model for a two-oscillating-body wave energy converter in shallow water. Ocean  Engineering  214, 107765. https://doi.org/10.1016/j.oceaneng.2020.107765

-  Negri  M., Malavasi  S.,  2018. Wave energy harnessing through oscillating bodies.  Energies 11,  2730. https://doi.org/10.3390/en11102730.

-        Malavasi S., Negri M., 2015. Double system wave energy converter for the breaker zone. Special Issue Ocean energy: Ongoing research in Italy. Rivista EAI, ENEA. https://doi.org/10.12910/EAI2015-047

One or more module of EDS can be installed outside the external breakwaters of the port or along the coast at a sufficient water depth (about one diameter D of the float). The number of EDS to install will depend on the need of electricity.

A preliminary estimate of the mean annual power Pout that can be produced with the EDS, utilizing a section of breakwater or coast of length L, is the following:

Pout= Pw ∙ L/2 ∙CWR

Where:

-         Pw [kW/m] is the mean annual available wave power for unit crest width [kW/m]

-        L [m] is the length of the coast occupied (the factor ½ is because there must be some space between the EDS modules). Therefore, the total available wave power is Pw ∙ L/2

CWR is the mean annual capture width ratio of the EDS, which can be interpreted as the mean annual absorption efficiency of the EDS.

The mean annual available wave power Pw in the island is included in the range 1—2 kW/m (see the references). The table below reports two examples of calculation, assuming different values of the mean annual available wave power. It is assumed an annual mean efficiency CWR=30%. The optimal diameter D of the EDS increases as available wave power increases. The calculations are made considering a section L=100 m of the breakwaters or of the coast.

Although this is a first estimation and must be refined, it gives an idea of the power that can be produced. This calculation considers the annual mean power. Actually, the power extracted will present oscillation during the year, with a large part of the energy produced in winter.

		Case 1	Case 2
Mean annual wave power available	PW [kW/m]	1	2
Mean annual power produced	Pout [kW]	15	30
Energy produced in a year	Eout [MWh/year]	131	263
Diameter of the float	D [m]	3	4
Number of modules	n	17	13

References:

Besio G., Mentaschi L., Mazzino A. 2016. Wave energy resource assessment in the Mediterranean Sea on the basis of a 35-year hindcast. Energy 94 50-63. http://dx.doi.org/10.1016/j.energy.2015.10.044

Pisacane G., Sannino G., Carillo A., Struglia M.V., Bastianoni S. 2018. Marine Energy Exploitation in the Mediterranean Region: Steps Forward and Challenges. Frontiers in Energy Research 6,109. http://dx.doi.org/10.3389/fenrg.2018.00109

Impact

https://www.tecnomac.it/it/eds-energy-double-system.html