**Laura Castro-Santos 1, Dina Silva 2, A. Rute Bento 2, Nadia Salvação <sup>2</sup> and C. Guedes Soares 2,\***


Received: 22 September 2018; Accepted: 6 November 2018; Published: 14 November 2018 -

**Abstract:** This paper develops a methodology to determine the economic feasibility of implementing offshore wave energy farms on the Portuguese continental coast. This methodology follows several phases: the geographic phase, the energy phase, the economic phase, and the restrictions phase. First, in the geographic phase, the height and the period of the waves, the bathymetry, the distance from the farm to the shore, from farm to shipyard, and from farm to port, are calculated. In the energy phase the energy produced by each wave energy converter is determined, and in the economic phase, the parameters calculated in the previous phases are used as input to find the economic parameters. Finally, in the restrictions phase, a limitation by the bathymetry will be added to the economic maps, whose value will be different depending on the floating offshore wave energy converter (WEC). In this study, three wave energy converters have been considered, Pelamis, AquaBuOY, and Wave Dragon, and several scenarios for electric tariffs have been taken into account. The results obtained indicate what the best WEC is for this study in terms of its levelized cost of energy (LCOE), internal rate of return (IRR), and net present value (NPV), and where the best area is to install wave energy farms.

**Keywords:** feasibility study; floating offshore wave farm; WEC; IRR; LCOE; ocean energy; marine energy

## **1. Introduction**

The first wave power patent was from the 18th century and during the centuries, lots of types of devices have been developed [1]. Wave energy converters (WEC) are the devices that can extract the energy from ocean waves. There are many ways the WECs can be classified. However, depending on their working principle, they can be classified as oscillating water columns, oscillating bodies, and overtopping devices [2,3]. The oscillating water column device works using an air turbine (Pico [4], LIMPET, Sakata, Mutriku [5], Mighty Whale [6], Ocean Energy, SPERBOY [7], Oceanlinx [8], and REWEC3 [9]); the oscillating bodies work with a hydraulic motor, a hydraulic turbine, and a linear electrical generator (AquaBuOY [10], IPS Buoy [11], FO3 [12], PowerBuoy [12], Wavebob [13], Pelamis [14,15], PS Frog [16], SEAREV [17], AWS (Archimedes Waveswing Submerged) [18], WaveRoller [19], and Oyster [20]); and the overtopping concept works with a low-head hydraulic turbine (TAPCHAN [21], SSG (Sea Slot-cone Generator) [22], and Wave Dragon [23]). They can also be classified by considering the water depth that they were designed to operate in; fixed (less than 50 m of depth or onshore) or floating (more than 50 m of depth) or the distance to shore [24]. Other WECs are: the Wavestar, which produces electricity due to the motion of the floats that are attached by arms to the platform [25]; the SeaBeavl, which is a "taut-moored

dual-body" WEC designed to improve the maintenance process [26]; and Falcão et al. analyze the hydrodynamics of IPS Buoy [27]. This paper will be focused on floating WECs, which have the advantage of operating in deep water where more wave energy can be found and in a larger range of water depths, increasing the number of locations where they can be deployed. Veigas et al. [28] studied the wave and offshore wind energy potential of a Spanish island located in the Atlantic Ocean. This study is important because it presents maps of the areas selected. Liu et al. [29] studied the energy conversion of a prototype WEC buoy in China and they considered a farm of buoys. Martinelli et al. [30] developed a method to select the design power of WECs in the first steps of the development of the product. In addition, Martinelli et al. develop an analysis based on statistic aspects of the power from OWC (Oscillating Water Column) [31]. Zanuttigh et al. [32] analyze the feasibility of the use of WECs for coastal protection and they consider the Adriatic coast. Arena et al. [33] analyze the resonant WECs. In 2011, Portugal had an experimental project for wave energy: the WaveRoller prototype, with a budget of five million euros [13]. This is the country selected to develop the case study because it is a country very involved with ocean energies, having a pilot area in its north-west coast.

The present paper will be focused on three wave energy converters: Pelamis, AquaBuOY, and Wave Dragon. These devices are not in commercial exploration and the main reason for their selection is that they represent different types of devices, different sizes, and different energy outputs. The Pelamis is an articulated structure similar to a snake and has "cylindrical sections linked by hinged joints" [2]. It has been developed in the UK and it should be installed aligned with waves [2]. It has been tested in several sizes from 1998 to 2011 [34,35]. The AquaBuOY combines the hose-pump and the IPS Buoy, being a small and modular WEC. Wave Dragon is based on the principle of wave overtopping and has "two wave reflectors focusing the incoming waves towards a doubly curved ramp, a reservoir and a set of low-head hydraulic turbines" [2,36].

Regarding the assessment, performance, and feasibility analysis: Rusu et al. [37] developed the wave energy resource for Portugal; Bozzi et al. [38] analyzed the wave energy feasibility in Italy considering three WECs: AquaBuOY, Pelamis, and Wave Dragon; Iuppa et al. [39] analyzed the case of Sicily; Guedes Soares et al. [40] considered several coastal locations to determine their efficiency; Vannucchi et al. [41] considered several Italian coasts: "Tuscany, Sardinia, Liguria et Sicily"; Dalton [42] analyzed the "non-technical barriers of wave energy in Europe"; Dalton et al. [43] considered the Pelamis in three different scenarios in Europe and the USA; and O'Connor et al. [44] analyzed the analysis of Pelamis and Wavestar in Europe.

Bozzi et al. [38] decided on the offshore location of a wave farm in Italy only by considering a particular set of points of the geography, while in the present paper all points of a particular geography are considered (in this case Portugal). Iglesias et al. [45] and Veigas et al. [46] considered several points of the geography (in this case, Galicia) but only the available energy was considered, while in the present paper the economic aspects of each point of the geography is considered (Portugal). Beels et al. [47] did not take into account the maps of the geography, and the economic calculations were very elementary, while in the present paper detailed economic aspects are calculated for all locations producing economic maps. O'Connor et al. [48] considered some economic aspects of wave energy but they did not consider the map of all the locations of the selected region, however, the present paper shows the map of all the locations. In the present method, all points of the geography are calculated, and this is very important because, for example, in a point where the wave resource is very good it may not be possible to install wave energy farms because there are restrictions. In this sense, the present method allows the addition of restrictions (in this paper only bathymetry is considered, but other restrictions can also be added).

The aim of the present paper is to develop a methodology to calculate the economic feasibility of floating offshore wave energy farms following several phases: geographic phase, energy phase, economic phase, and restrictions phase. In the geographic phase, some parameters (the height and the period of the waves [49,50], the bathymetry and the distance farm to shore, farm to shipyard, and farm to port) are calculated to be used as input values in the economic phase. The energy phase determines the energy produced by each wave energy converter (WEC). It can be calculated considering several

methods, depending on the data available and the precision wanted. Afterwards, in the economic phase, the economic parameters are calculated considering the inputs provided in the previous phases. Finally, the restriction phase, imposed by the bathymetry, will be added to the economic maps, whose value will be different depending on the floating offshore WEC. The WECs considered for this study are Pelamis, AquaBuOY, and Wave Dragon, whose energy yields are calculated using several scenarios for electric tariffs. They have been selected because they represent several types of devices, although some of them are not in operation nowadays. Results indicate what the best WEC is in terms of its LCOE (levelized cost of energy), IRR (internal rate of return), and NPV (net present value) and where the best area to implement a floating offshore wave energy farm is located. The method proposed was applied to Portugal, where a high wave energy resource can be found.
