**1. Introduction**

The increased environmental awareness together with the fossil fuel availability reduction, have fostered the development and integration of renewable energy sources (RES) in the last decades [1]. In this sense, hydro, solar, and wind energy sources have been extensively researched and consolidated, whereas other RES as wave energy remain a few steps behind in terms of technology readiness level (TRL). Regarding wave energy, and given the significant potential enclosed in the waves [2], several wave energy farms have been deployed and tested, particularly in areas with high oceanic resource [3]. Thus, the numerous projects developed during the past years in Europe, United States, and Asia have pushed the wave energy technology to a precommercial state (TRL 7), and the first commercial projects are expected to be implemented in niche markets in the near future [2,4–6].

This paper focuses on wave energy converters (WECs) as the device selected for extracting the wave energy and converts it into electricity. For a thorough review of WECs and other wave energy devices, see [7].

The aforementioned integration and connection of wave energy farms, each one composed by a set of WECs, is not exempt of drawbacks. As with other non-dispatchable

**Citation:** Navarro, G.; Blanco, M.; Torres, J.; Nájera, J.; Santiago, Á.; Santos-Herran, M.; Ramírez, D.; Lafoz, M. Dimensioning Methodology of an Energy Storage System Based on Supercapacitors for Grid Code Compliance of a Wave Power Plant. *Energies* **2021**, *14*, 985. https://doi.org/10.3390/en14040985

Academic Editor: Alon Kuperman Received: 5 January 2021 Accepted: 6 February 2021 Published: 13 February 2021

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RES (wind and solar energy), wave energy farms can have a considerable impact on the stability, quality, and reliability of the power grid [8,9]. Given the intermittent nature of the source, i.e., the waves, the power generated by a WEC also follows an intermittent profile with severe power fluctuations [10]. As a result, the power grid can suffer from sudden frequency deviations, and voltage distortion such as harmonics or flicker [9–11]. These frequency and voltage issues may cause the wave energy farm to not be compliant with the grid codes of the transmission system operators (TSOs) and, hence, make wave energy farms not suitable for being included in the electricity generation mix of a country [8]. In [12,13] an introduction about the study on grid compliance and standardization of renewable generation and other energy systems connected to the power grid is provided. In [13–17] the grid codes of different countries (European and non-European) and the corresponding projects about renewable power are compared and summarized. The impact of grid code regulations on stability is also analyzed. Additionally, these papers provide the information about the future trend of grid code requirements. The main requirements in most of grid codes include reactive power, frequency regulation, fault ride through, power quality, and communication.

Several solutions for solving this issue have been proposed in the literature, including advanced control strategies [18–20] and proper WECs location in the wave energy farm [21]. In [22] an active control for a tidal turbine is developed to grid code requirements in terms of active and reactive powers (P, Q) and, conversely, the voltage at the point of common coupling and reactive power (U, Q). However, the most promising solution for solving power quality issues, which encompass the vast majority of articles of this topic, is the utilization of an energy storage system (ESS) for a combined operation together with the wave energy farm. In this sense, the ESS will store energy when the power generated by the WECs overcomes a certain value, and it will deliver power when required by the control strategy [21].

Different energy storage technologies have been analyzed in the literature for being integrated with WECs, aiming for grid code compliance: battery energy storage system (BESS), flywheel energy storage system (FESS), and supercapacitor energy storage system (SCESS) or a hybrid energy storage system (HESS). All three technologies are able to provide a fast response in the range of 10–50 ms when the control strategy demands it [23,24].

BESSs have been studied in [25] for a combined operation of a wind turbine and a WEC, including a predictive control in order to not overcome the maximum power fluctuations allowed by the Irish grid code and the UK national grid code. The work published in [26] uses a BESS for smoothing the output power of a WEC, in order to have an acceptable power quality for a general grid code defined by the authors.

FESS have been studied together with a wave farm in [27] for controlling the power output of the complete installation, aiming for being compliant with the Nordic grid code. The authors in [27] develop a control based on three stages, achieving a reduction up to 85% in the power output peak. A control strategy is proposed for power smoothing in a hybrid wave energy plant. The control strategy is based on a power filtering process.

Grid code compliance with SCESS has been studied in [28,29], validating control strategies for compensating WEC power oscillations. A finite predictive control is applied to the study proposed in [28], where the SCESS is located in the DC link of the back-toback power converter. The analysis performed in [30] include an experimental validation in a laboratory test bench of a control that includes a state of charge compensator for the SCESS. In [30] a SCESS is studied to smooth the power extracted from waves by a grid connected linear electric generator. A complete model of the system is developed in Simulink-MATLAB to test the system under varying system conditions (faults with and without the SCESS).

Another possibility is to use a hybrid energy system (HESS) consisting of two different storage technologies. For example, in [29] a power management system for a grid connected OWC wave energy converter with a HESS (battery/SC) is proposed. Performance of the control system is evaluated in two case studies, fixed and variable operating speed. In [31] an optimization algorithm is aimed at splitting the power required between the battery and SCs of a HESS. This ESS is used to smooth the power oscillations for a wave energy conversion connected to the public grid. The proposed algorithm, among other things, considers minimizing the losses of the storage system and maximizing the battery lifetime.

Apart from those three ESSs, other authors decided not to select a specific ESS, but studied the application of a generic ESS together with a WEC. In this sense, authors in [32] combined a wave farm model with a small ESS for being compliant with the Irish, UK, and Nordic grid code. The study includes a modeling strategy for the wave-to-wire system of WEC arrays. Moreover, the work published in [33] studied the integration of a generic ESS for achieving the grid code requirements at the point of common coupling (PCC), using real location data. The authors also propose a real-time technique for the centralized control of the wave farm, which is validated in critical scenarios such as weak grids. In [34] an adaptive energy filter is proposed to smooth renewable power fluctuations in function of the input power level. The novelty of this paper resides in the robustness of the proposed filter against changes in the level of the generated power (fluctuating in the case of renewable sources). This method is generic and it can be applied to any ESS (BESS, FESS, SCESS, and SMES).

As it has been described above, the combined effects of using a WEC with different ESS has been analyzed, and the benefits for meeting the grid code requirements are clearly stated in the literature. However, from an industrial point of view, knowing that including ESSs is beneficial for integrating wave energy in the grid is as important as knowing the proper size of the ESS, in terms of power and energy. ESSs are expensive equipment, and under or oversizing can result in premature degradation and incompliancy with the grid codes, or unnecessary economic investment, respectively. Hence, ESS dimensioning studies for wave energy become of the utmost importance.

In this regard, the works published in [3,24] propose a dimensioning methodology for a generic ESS integrated with a WEC, without focusing on a specific technology, and based on efficiency and grid code compliancy in terms of frequency. Authors in [24] include a methodology validation for non-regular waves. A further validation with real data from Tenerife is performed in [3]. Moreover, authors in [35] proposed a SCESS dimensioning methodology, which includes an ageing model without temperature dependence, and with no particular focus on grid code compliancy.

For wave energy applications, SCESSs and FESSs perform better that other ESSs in terms of efficiency [36]. Although SCESSs and FESSs present similar characteristics in terms of TRL, power and energy range and number of cycles [23], SCESS is the ESS technology selected by the authors to perform the analysis of this work, since power and energy levels are appropriate, a better economic alternative has been found in terms of €/kW and the system fits better within the available space at the power plant.

A case study is proposed to illustrate the methodology, with the real profile of a wave power generation plant and a real SCESS. To reduce fluctuations in the power delivered to the power grid a specific active power ramp rate (%/min) is set. It is a characteristic parameter of the grid code standards. The proposed methodology allows the establishment of other types of limitations that conform to the existing grid codes in different countries.

The paper is structured as follows: in Section 2 the wave power generation plant on which this study is based is described, in Section 3 the SCESS cabinet on which the complete SCESS (technology, power loss calculation, and efficiency map) is based is described, in Section 4 the results obtained from a simulation model that integrates both the SCESS and the wave generation profile are presented and commented and, finally, in Section 5 some final conclusions are detailed.

#### **2. Wave Power Generation Plant**

The study of using supercapacitors (SCs) as a storage solution for systems that harvest energy from waves to comply with the grid code is based on the wave power generation plant located in the breakwater at the harbor in Mutriku (Spain). The plant was inaugurated in 2011 and has been delivering power into the electric grid ever since. The technology used to extract energy from waves is called the oscillating water column (OWC) with a total installed power of 296 kW, comprising 16 equal turbines of 18.5 kW each [37]. The principle of these oscillating water column converters (OWC-WECs) is based on the pressure variation of the air contained in a chamber when the wave motion enters it. When the wave arrives, the contained air is expelled at high pressure through an orifice located at the upper part of the chamber. The air drives a turbine whose shaft is coupled to an electric generator. When the wave retreats, the water level in the chamber decreases. This causes the pressure inside the chamber to drop and the air to be sucked in through the upper hole. The turbine always rotates in the same direction regardless of the direction of air circulation, so that the rotation in the shaft is more or less continuous. The connection to the local distribution grid of the generation plant is made through a 460 V/13.2 kV power transformer. Figure 1a shows the electrical diagram of the wave power generation plant connected to the grid. Figure 1b shows an aerial image of the power plant.

**Figure 1.** (**a**) Electrical diagram of the wave power generation plant and (**b**) aerial photo of the wave power generation plant integrated in the breakwater of Mutriku port.
