The Mutriku Breakwater Wave Plant: Improvements and Their Influence on the Levelized Cost of Electricity (LCoE)

Alberto Peña; Iñigo Bidaguren; Urko Izquierdo; Gustavo Adolfo Esteban; Jesús María Blanco; Iñigo Albaina

doi:10.26748/KSOE.2024.099

Abstract

The Basque Energy Agency commissioned the Mutriku Wave Power Plant, using an Oscillating Water Column technology in 2011, and although the cumulative total power has been over 3 GW/h until 2023, the initial expectations of efficiency and LCoE have not been accomplished. This paper proposes a series of modifications to the breakwater and chamber geometry of the plant to increase energy production and efficiency. These proposed improvements were tested in experimental campaigns in the modular wave flume in the Energy Engineering Department at the University of the Basque Country, using a mock-up of the original installation. Some results also came from validated numerical simulations, showing that the response amplitude operator and the capture width ratio values were better than those in the original L-shaped power plant. The best configurations appear to be U-shaped and when harbor walls are used. In conclusion, this paper reports the proper configurations and localizations for new wave power plants that could be commercially competitive.

1. Introduction

Before 2011, in Mutriku, a village in the Biscay Bay (Basque Country), entering the harbor and the inner docks, where many accidents occur at the entrance, was very dangerous because of the extreme sea conditions. The Basque Government approved a project to solve this problem by extending the existing breakwater (see Fig. 1). The Basque Government contacted the Basque Energy Agency (EVE) to use the site to generate wave power. The technology chosen for this purpose was the oscillating water column (OWC) because of its proven efficiency (Torre-Enciso et al., 2009). The technology comprised 16 chambers, where Wells turbines were installed, with a predesigned total capacity of 296 kW. On the other hand, only 14 Wells turbines were used because the chambers at both ends could not provide the turbines with sufficient energy because of their inappropriate location angle in relation to the incident waves. Therefore, the total capacity was 259 kW (Ibarra et al., 2021). Fig. 2 shows the final structure of the Breakwater Wave Plant in Mutriku.

Ocean energy technology is still in the pre-commercial stage, and to be competitive, the European Union has an economic objective to reduce the LCoE to a minimum of 200 €/MWh and 150 €/MWh by 2025 and 2030, respectively (European Commission, 2016). In Mutriku, this value is currently double that aim.

Using new operational and maintenance (O & M) strategies, the operational expenditure (OpEx) may be decreased to 17% (M’zoughi et al., 2024), resulting in a 5% decrease in the LCoE. On the other hand, at the University of the Basque Country, research is focusing on increasing the annual energy production (AEP) by improving the design of the breakwater wave plant based on experimental and numerical campaigns. Therefore, action must be taken regarding energy production operation, maintenance, and the initial cost before this technology can be commercialized. This study will show the best geometry modifications that experimental and numerical studies have detected thus far and how these changes influence the improvement of the LCoE value in OWC devices such as the one in Mutriku. The results revealed an expected 25% increase in value if the proposed modifications were considered in new designs.

2. The LCoE Calculation Applied to the Mutriku Breakwater Wave Power Plant

The LCoE can be expressed simply by the relationship between costs and energy production as Eq. (1):

(1)

LCoE=∑0nC∑0nE

where C is the total costs, and E is the total electrical energy produced in kWh or MWh. Both were calculated from the beginning (0) to the end of life (n). Therefore, it must be estimated more than calculated (Johnston et al., 2020).

The calculation is complex because of the multiple inputs required: water depth, wave period, shore distance, technology development (turbines), site cost, incentives, cost of money, operation and maintenance, and infrastructure. Eq. (1) must take all these inputs into account. As mentioned before, the project was developed by the EVE, as explained in the Energy Agency of the Basque Government (2024). The total initial investment was 6.7 million euros: 2.3 million euros for plant construction and 4.4 million for dock enlargement. The Scottish company Wavegen, which was sold to the Voith group in 2005, constructed the facility. This company had a plant in Tolosa (Basque Country), and the turbines were made there. The plant finally closed in 2013. The installed capacity of 259 kW led to a cumulative energy production of 1 GWh in 2016, 2 GWh in 2020, and 3 GWh in 2023.

The O&M cost was estimated to be 16–25% of the total lifetime cost of offshore wind technology (IRENA, 2021). This cost is uncertain on the ocean, and the data availability is limited. This estimated cost can be reduced to 11–18% (Zhu et al., 2019) if maintenance is properly scheduled and the best strategies are implemented. Artificial intelligence methods were applied to search for the best strategies. Nevertheless, there is still the problem of the real environment situation when wave plants start to be commercially developed. Until then, only expected costs can be estimated. The harsh environment of Mutriku (elevated humidity, saline corrosion, and strong waves) leads to failures and breakage situations. If all these damages can be predicted, better strategies can be used to avoid or delay them, and the O&M can be reduced (Lekube, 2018).

A more developed formula, Eq. (2), (de Castro (2024), has been used to calculate the LCoE in Mutriku under the actual conditions.

(2)

LCOE=∑0n(Capitalt+O&Mt+Fuelt)·(1+r)-t∑0nMWh·(1+r)-t

where Capital_t is the total cost in year t. That is, investment in year t; O & M_t is operation and maintenance in year t; Fuel_t is the fuel costs in year t (not applied in wave energy); (1+r)⁻^t is the real discount rate; MWh is the electricity produced in year t.

The Mutriku Wave Plant was inaugurated in 2011 and the estimated lifetime was 25 years. With a supposed discount rate of 6%, an O & M cost at 25%, the initial inversion, and the accumulated energy production of 3 GWh, nowadays, the LCoE of Mutriku was elevated, approximately 500 €/MWh; it is expected to be 140 €/MWh in 2035 if nothing changes, i.e., far from the expected values.

3. Improvements in the Mutriku Breakwater Wave Power Plant Proposed by the University of the Basque Country

3.1 Computational Fluid Dynamics (CFD) Model Validation

A 1:36 scaled model was manufactured (see Fig. 3) according to the dimensions of one of the actual chambers in the Mutriku Wave Plant (Bidaguren et al., 2021). The experimental campaigns with this model were driven in the wave flume located in the Energy Engineering Department laboratory at the Bilbao School of Engineering (Fig. 4).

The top of the experimental chamber has interchangeable orifice plates, a water level sensor, and a pressure gauge. This mock-up was developed to obtain the capture width ratio (CWR) from experiments. The CWR measures the amount of pneumatic power harnessed from the mechanical power of the incident wave, and it is the parameter that will be used to observe the effect on the energy production and onto the LCoE. A CFD model was validated against the experimental results; Peña et al. (2024) reported the conclusions of this numerical and experimental campaign.

Fig. 5 shows the meshing of the original mock-up modeling, with a total of 1.9 million hexahedral cells. Fig. 6 shows a graphic representing the evolution of the CWR versus the wave period, using the geometry shown in Fig. 5. The characteristics of this model are orifice diameter, D = 12 mm; wave height, H = 30 mm; tide, h = 163 mm. The diameter of the orifice in the experiments is meant to be related to the demands of the PTO (Power take-off) turbine.

3.2 Experimental Improvements

The experiments changed the original geometry (Izquierdo et al., 2023). The original design was an L-shape configuration, and several campaigns were made in the wave flume in the Energy Engineering Department by adding different elements: U-shape geometry (with transversal walls), lateral walls (LW), a combination between the lateral walls and transversal walls (LW+U), and harbor walls (HW) at different angles.

The mock-up without the upper orifices was used in all the campaigns, and the conclusions were taken using the response amplitude operator (RAO) parameter. In these experimental campaigns, lateral walls (Fig. 7) showed improvement in the water surface displacement inside the chamber, increasing the RAO. In addition, the U-shape configuration shows an increasing RAO.

Izquierdo et al. (2023) presented the results of these campaigns. They concluded that the option of LW showed promising results, and including a front wall (W) in this configuration did not provide the expected results. The lateral walls were an excellent option among all the different geometries, including using lateral walls with different degrees of inclination to redirect the flow toward the central part of the chamber (Fig. 8).

Considering the experiments conducted thus far, the following graphics showed excellent agreement between LW and HW (Fig. 9).

The scaled-down periods of the maximum frequency of Mutriku waves are marked in red with a dashed line (Fig. 9), and its implementation would offer substantial improvements, as shown in Fig. 10.

4. CWR Computational Calculation: Comparison Between the Original and the LW Geometry

Focusing on the results obtained with LW, the best results were achieved for h = 163 mm at T = 1.6 s and 430 mm away from the breakwater (RAO value of 4.66), and h = 232 mm at T = 1.8 s and 430 mm away from the breakwater (RAO value of 5.52).

No experimental campaigns have calculated the CWR needed to calculate the LCoE. On the other hand, to observe the effects of the proposed modifications in the LCoE calculation, the validated 3D numerical model, described previously (Figs. 5 and 6), was used with the addition of 430 mm long lateral walls (Fig. 11), measured from the breakwater.

The main characteristics were the same as the original model (Fig. 5), plus the addition of the mentioned walls: orifice diameter, D = 12 mm; wave height, H = 30 mm; tide, h = 163 mm, and in the present case, only the period of 1.5 s because it is one of the most frequent periods in Mutriku. On the other hand, it was chosen because it is still in the Lemouté Stokes part, which is the wave model used.

The CWR calculated in the simulation of the original geometry was 0.65, as shown in Fig. 7. This value increased to 0.85 when lateral walls were considered. The calculated value considered the pressure inside the chamber (the pneumatic power of the turbine) and the height of the free surface inside the chamber (the wave energy). Fig. 12 shows the evolution of these two parameters.

Finally, this increase can be translated into a lower LCoE. The calculations made by CFD modeling predicted an increase in energy production of approximately 25%, leading to an LCoE of 410 €/MWh and 109 €/MWh in 2025 and 2035, respectively.

The LCoE could be reduced to approximately 80 €/MWh in 2035 by adding the effect of the optimization of O&M, decreasing the percentage to 18%. If the improvements were to be made today in the original site, the cost of the capital needed for the modification would show a high LCoE of 470 €/MWh in 2025. Nevertheless, the main objective of the research was to study different ways of decreasing the LCoE value in OWC-type wave power plants, taking the example of Mutriku as a starting point. The cases tested experimentally and numerically foresee a promising future for these installations, as shown in this document.

5. Conclusions

This paper presents the calculation of the LCoE in the Mutriku Breakwater Wave Plant, which had a value of approximately 500 €/MWh, far from the economic objective of the European Union. Several proposed concepts for the geometry of the plant are being studied in the Energy Engineering Department at the University of the Basque Country through experimental campaigns and numerical simulations. A CFD model validated against experimental results was used to model one of the geometric modifications that appear to be promising: the addition of harbor walls. The improvement in the CWR value was approximately 23%. Hence, the current LCoE value would be around 400 €/MWh if the geometry has been this way. For the calculation of the LCoE, an O&M cost of 25% was considered. Nevertheless, as mentioned in the document, this cost can be decreased by modern artificial intelligence techniques, which can be used to predict the most efficient way to perform operation and maintenance tasks. The cost could be lowered to 18%, and the LCoE could reach a value as low as 294 €/MWh. More experimental studies are needed to corroborate the numerical results and to design new geometries for future plants. From the economic point of view, this work indicates a promising future for this type of energy, but more work on improving the designs is needed. The calculation of the LCoE has considerable uncertainty, taking into account the different legislations in each country, as well as the calculation of some parameters, such as operation and maintenance or the very changeable wave energy. The efforts of the Energy Engineering Department in Bilbao focused on improving energy production, taking the Mutriko plant as an example. Future research plans to study other sites.

Conflict of Interest

The authors declare that they have no conflict of interest.

Funding

This study was carried out by the research group ITSAS-REM (IT1514-22) funded by the Basque Government, and through project PIBA_2021_1_0024, also funded by the Basque Government.

Acknowledgements

These results are part of a project enclosed into the long-term politics until 2050 in the European Union called Blue Growth and A Clean Planet for All.

Fig. 1

Original Breakwater in the port of Mutriku

Fig. 2

The final structure in Mutriku Breakwater Wave Power Plant

Fig. 3

Manufactured model (scale 1:36). General view

Fig. 4

Experimental wave flume. General view

Fig. 5

CFD 3D numerical model

Fig. 6

Capture width ratio vs. wave period

Fig. 7

Lateral wall (LW) installed in the mock-up inside the flume

Fig. 8

Typology of short (left) and long (right) harbor walls

Fig. 9

Comparison of the best alternative corresponding to LW and H W (alternative 4). RAO for a mock-up tide height of h = 163 mm (left) and h = 232 mm (right)

Fig. 10

Wave climate measured at Mutriku (Faÿ et al., 2020)

Fig. 11

Computational model of the mock-up geometry with lateral walls

Fig. 12

Evolution of the pressure inside the chamber (top) and the height of the free surface inside the chamber (down)

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