**Luca Martinelli \*, Matteo Volpato, Chiara Favaretto and Piero Ruol**

Department of Civil, Architectural and Environmental Engineering, Università di Padova, 35122 Padova, Italy; matteo.volpato@unipd.it (M.V.); chiara.favaretto@dicea.unipd.it (C.F.); piero.ruol@unipd.it (P.R.) **\*** Correspondence: luca.martinelli@unipd.it

Received: 28 December 2018; Accepted: 25 March 2019; Published: 29 March 2019

**Abstract:** This paper investigates on a Wave Energy Converter (WEC) named Energy & Protection, 4th generation (EP4). The WEC couples the energy harvesting function with the purpose of protecting the coast from erosion. It is formed by a flap rolling with a single degree of freedom around a lower hinge. Small-scale tests were carried out in the wave flume of the maritime group of Padua University, aiming at the evaluation of the device efficiency. The test peculiarity is represented by the system used to simulate the Power Take Off (PTO). Such dummy PTO permits a free rotation of two degrees before engaging the shaft, allowing the flap to gain some inertia, and then applying a constant resistive moment. The EP4 was observed to reach a 35% efficiency, under short regular waves. The effects, in terms of coastal protection, are small but not negligible, at least for the shortest waves.

**Keywords:** wave energy converters; Power Take Off; EP4; latching; wave flume; floating; moorings; renewable electricity generation systems; SDEWES 2018

### **1. Introduction**

The availability of renewable energy in the oceans [1] has long inspired many inventors of wave energy converters. In Europe, since the 1990s, several devices were developed by north-west European companies [2]. Conversely, due to scarce wave energy resources in the Italian seas [3,4], for many years the Italian contribution to the development of wave energy converters has been limited to the pioneering work of Boccotti, through the REWEC 3 J-type WEC, patented in 1998 [5].

The first WECs developed in the North European Countries reached a rather high technological readiness level [6,7] but suffered from the harsh oceanic environment at demonstration phase. Many developers eventually closed their companies or suspended their activities, losing part of their competitive advantage in favor of more recent concepts, so that now it is not too late to start with new patents.

In view of such problematic R&D processes of WECs, some authors even suggested that a mild wave climate, like the Mediterranean one [8,9], is suited to the development and, for some cleverly conceived concepts, to the commercial phase. In recent years, ENEA (the Italian National Agency for New Technologies, Energy and Sustainable Economic Development) started to gain interest in the marine energy sector, fostering new initiatives.

As a result, Italian inventors started proposing many new concepts. In the scientific literature we can find, beside the REWEC 3, the ISWEC [10], OBREC [11], SeaBreath [4], ShoWED [12], and DEIM [13], Tecnomac (EDS, [14]). Other devices have not published the tests in the scientific literature, but were presented to business events, e.g., 40South Energy (through several devices, the most recent being the interesting H24 module, http://www.40southenergy.com/2018/09/2294/), Swaths, Generma, Onda, WaveAbsorber, WEM/WOM, EP4.

The last of these devices is the one studied in this paper. The full name is Energy and Protection, 4th generation (EP4), patented by Dario Bernardi. In order to overcome the disadvantages of the

low energetic content of the Italian sea, the EP4 is designed to achieve a secondary objective, i.e., the protection of the coast from the wave action, thus providing a defense against erosion (similarly to other concepts, see [15]).

Following a well-established R&D roadmap (see e.g., [16]), the device was tested in a physical model hydraulic laboratory under regular waves, on a small scale. The inventor built a 1:10 model, with no generator, and tested it briefly at sea, to achieve a proof of concept, i.e., the ability to rotate a shaft. The same model (integrated to fit the flume frames) has been tested in the wave flume of Padova University.

The aim of the paper is to assess the device effectiveness in terms of power production and ability to protect the rear beach.

For a real WEC, the hydraulic, electrical or mechanical Power Take Off (PTO) is the system that allows the generation of electricity. On a small scale model, the real physical PTO cannot be modeled in detail, and no electricity is produced. To maintain the same overall dynamic behavior, a system is used that just mimics the damping and resistance effects, restraining the WEC movements in similitude to the physical PTO. Such system is termed dummy PTO.

It is well known [17,18] that an appropriate control technology has the capability to significantly affect the amount of energy taken from WECs, and the choice of the PTO significantly affects the final result. Therefore, the dummy PTO was built in agreement with the inventor's requirements.

Finally, the restraining force and movements were measured, in order to obtain the amount of energy dissipated by the system, to be interpreted as the "converted energy".

The paper layout is as follows. After this introduction, a classification of the different types of dummy PTOs used in the hydraulic tests is presented. Then, the experiments are described, including the facility, the scale model of the EP4, the selected dummy PTO, the test programme, the instrumentation and the analysis methods. Results are then described in terms of free oscillations, energy harvesting capacity and wave attenuation. Finally, the conclusions are drawn, with some comments on future developments.

#### **2. Classification of Dummy PTOs**

All WEC devices, under the wave action, are able to produce some movements or to spin a wheel. The ability to produce a movement (or rotation, e.g., of a turbine wheel) is considered a "proof of concept", i.e., a proof that wave energy has been converted into kinetic energy in some instant.

In order to evaluate the amount of harvested energy it is possible to restrain the movement with a force (or torque, in case of a turbine), that mimics the effect of a real PTO. Such restraining force (torque) *R* is sometimes called the PTO "load". The device that allows to apply such load is sometimes called "dummy" PTO.

The product of *R* by the velocity (or angular velocity) *v* is the power dissipated by the dummy PTO. The average of the dissipated power is assumed to be the "converted energy" *E*c:

$$E\_c = \prec \mathcal{R}(t) \; \upsilon(t) > \tag{1}$$

Obviously, the velocity *v* is directly affected by the load. If the load is too large the device is totally restrained, with *v* = 0, and from Equation 1 we see that *E*c = 0. If *R* = 0, *v* achieves the maximum value but, again, *E*c = 0. The optimal value of the load is the one that gives the largest value of *E*c.

It should be pointed out that the optimal *R*(*t*) is not constant, and may depend on the present and future state (position and velocity) of the WEC.

At prototype scale, the load is likely to be controlled (through the inverter) in real time, with substantial advantages in the achieved efficiency. During physical model tests, a similar system can be achieved with motor controlled by a PLC (available at the University of Padova) enabling the "active" PTO to exert any reaction force, based on the measured information of the system state (see e.g., by [19]). This investigation, however, increases significantly the costs of the experiments.

This level of accuracy is not suited for devices at an initial stage of development, such as the case of the EP4. More frequently, passive PTOs are designed. On the one hand, the type of PTO significantly changes the overall performance at the lab scale and it must be selected with care. On the other hand, the inventor is usually not financially supported and the cost of the experiment must be kept extremely low.

A simple classification of the existing passive PTOs is given below, and it is based on the achieved value of *R*:


The passive PTO is usually realized with adjustable (and repeatable) "settings". The optimal load is found by selecting the "setting" that gives the larger value of *E*c, usually under regular waves.

The settings are assumed to depend on the wave period but not on wave height. There is no proof that the ideal load found under regular wave conditions (of given period) is optimal also under irregular waves (of same peak period), although this has been verified for some experiments (e.g., [23]).

A final remark should address the PTO design load. The cost of the real PTO depends on the maximum value of power that may be converted. When the input power exceeds a threshold, the device may be either limited or disconnected, entering a "safe" mode. In [24], a quick way to select the design value on the basis of the hydraulic model tests is suggested. Whenever the power exceeds the design value, and this is a frequent case for irregular extreme waves, it is not realistic to assume that energy is harvested. Therefore Equation 1 should be modified on the basis of the PTO expected behavior.

#### **3. Experimental Investigation on the WEC**

#### *3.1. The Facility*

Physical model tests on the EP4 were carried out in the 36 m × 1.0 m × 1.4 m wave flume of Padova University (Figure 1). The wavemaker is an oleodynamic roto-translational paddle equipped with a hardware wave absorption system. To perform the tests, a fixed bottom was used. For average tide conditions, water depth was 0.5 m at the paddle, and 0.4 m at the structure. An array composed of four wave gauges (WG) was located 9.2 m in front of the model to measure the incident and reflected waves. Another gauge was placed 2.0 m behind the device. The instrumentation used in the tests also comprised of a load cell and a video-camera used to monitor the displacements. A MATLAB-based code extracts the flap rotation in time *θ*(*t*). After a lowpass filter, the signal is derived in time to obtain the rotational velocity *v*(*t*).

**Figure 1.** Wave flume and test setup with wave gauge and structure positions 3.2. The EP4 device.

The EP4 is a floater hinged at the base, free to oscillate, and connected by a chain to an upper shaft (Figure 2). The floater is 84 cm long, 37 cm wide, with total volume 0.013 m3. In the initial position it bent toward the incident waves (the wave generator is at the left, in the figure). A stabilizing bar (black in the figure), placed at the side of the center of floatation, assures that only one position is stable (unless the water level was extremely low, in which case there are two stable positions). The model was built in scale 1:10.

**Figure 2.** EP4 in the flume, with hanging load that applies a friction restraining the bar rotations.

The chain in Figure 2 was attached to an upper disk. The most simple PTO is the application of a constant resistive friction force to the disk (i.e., case no. 1 in the previous Subsection), and the actual force time history *R*(*t*) is represented in Figure 3, upper panel. Obviously the resistive force changed sign whenever the oscillation (the sinusoidal continuous line) reversed its direction. The dashed line represents the device velocity. A sinusoidal behavior is plotted for simplicity, being very similar to the actual measured signal *v*(*t*).

**Figure 3.** Scheme of the time history of the applied PTO load.

When the oscillation changed direction, i.e., when the paddle was at the extreme right or extreme left, the velocity was null. In these conditions, the friction force restrained the device movements, with no benefit, since the velocity was low and therefore the total contribution to the produced energy was minimal (Equation (1)).

By a simple procedure, i.e., enlarging the hole that connects the crown gear to the shaft (Figure 4), it was possible to delay the moment when the upper shaft was engaged.

**Figure 4.** Details of the dummy PTO.

Therefore, the force was applied with some delay, after the inversion of the motion. Figure 3 lower panel shows this case. The rotating flap can therefore accelerated, arriving to a larger value of the velocity (with no useless restraining force).

A similar delay would occur in a possible improved design where the shaft is coupled to a "double gear and clutch system" used to spin a flywheel in the same direction. In order to engage with the flywheel, the velocity must exceed a certain threshold, or else the clutch does not engage. Therefore, the selected dummy PTO is coherent with the future developments.

A few settings of the PTO loads were preliminarily analyzed, and two were found more significant. In both cases, the restraining forces engaged after a rotation of approximately 1 degree. The two settings of the PTO were measured in static conditions to be 5.5 and 7.7 Nm. The friction force under dynamic conditions, used for the evaluation of *E*c (Equation (1)), was assessed to be much lower and approximately half of this value, and equal to *L*<sup>1</sup> = 2.9 Nm, *L*<sup>2</sup> =4.1 Nm when the shaft was engaged, 0 otherwise (see Figure 3, lower panel).

### *3.2. Test Programme*

The test programme includes an initial analysis of the free oscillation, with evaluation of the natural period of oscillation *T*N, that was found to be 6 s. This value appears to be too large (19 s at prototype scale) to favor resonance, and this is a first noteworthy conclusion.

Then, 3 water levels were considered, i.e., target depth, low tide and high tide. The scaled values are 0.40 m, 0.35 m and 0.45 m respectively. Free oscillations were measured for all water depth, whereas the power production was only evaluated for d = 0.40 m (see Table 1). Wave height and period vary in the range 2–8 cm and 1–4 s, according to Table 2 (free oscillations) and Table 3 (with the applied load). A number of additional tests were also carried out with different loads, following in a confused pattern, that need not be presented here as they do not contribute to the conclusions.


**Table 1.** Summary test programme.


**Table 2.** Free oscillation tests.


H = 4 cm X X X

H = 8 cm X X X

H = 6 cm

#### **4. Results**
