*3.3. Wave Energy*

Another source of renewable energy is the energy extracted from waves. Currently there are different systems capable of extracting energy from waves, commonly known as wave nergy converter (WEC). As in the case of other renewable sources, the energy is converted and delivered into the grid by means of a power take-off (PTO). The intermittency associated to the wave energy causes a continuous imbalance between the power generated and the power demanded leading to potential power quality problems, such as voltage and frequency deviations, specially in weak electric grids with high penetration of renewables. The period of the wave energy generated is in the range of 1–10 s, causing frequency variations and voltage distortion, such as flicker and harmonics at the grid connection point [65–68]. Figure 16 shows a general scheme of WEC solution.

**Figure 16.** Simplified electrical diagramam of a WEC+PTO with linear generator connected to the grid.

Nunez Forestieri, J. [69] proposes an integrative sizing and EMS based on reinforcement learning (RL) for a HESS (SC/undersea energy storage) applied to grid connected operation of an offsore wave energy source. Different power profiles are used to verify the adaptability of the reinforcement learning-based energy management system (RLEMS). Real-time simulations confirm that the power and energy of the HESS is reduced when EMS is con-sidered in the sizing stage. The number of SC cells and the rate power of the undersea energy storage calculated with the proposed RL-based sizing allows to reduce the required capacity (power and energy) of the HESS to regulate power oscillations. Real-time simulation results are also presented that validate the viability of the proposed method (sizing and EMS) for applications in grid-connected renewable generation systems.

Rajapakse, G. [70] applies a predictive control model to smooth the power delivered to the network of an oscillating water column (OWC) wave energy conversion (WEC). Due to the nature of the resource, as well as the duration of the high-power pulses generated by the air turbine plus permanent magnet synchronous generator (PMSG) on which the study is based, SCs technology is considered the most suitable for this purpose. SCESS is connected to the intermediate DC stage of a back-to-back converter through a bidirectional DC/DC converter. Simulation results are shown in which the model predictive control (MPC) strategy is used, taking as one of the criteria that the SoC of the ESS remains within the limits established to extend the useful life. The THD of the output current obtained in simulations is lower than 5%, below the grid code requirement.

#### **4. Power Grid Connection Applications**

In this Section, the applications related to electrical systems, especially in electric grids and microgrids, are collected. Within them, the most published topics have been listed, describing in detail the use of SCs as well as the most relevant bibliography. Those studies are related to the limitations of the renewable energies sources, especially with their oscillatory nature, and the requirement of introducing flexibility in the electrical systems. This entails the integration of an ESS in order to increase the stability of the grid, absorbing or delivering energy, improving the voltage and frequency regulation of the electrical systems.

#### *4.1. Grid Regulation: Voltage and Frequency Compensation*

The increasing trend of integrating RES into the electric grids induces in the uncertainty in their operation and control. Their massive penetration into the power systems forces to increase the flexibility of the electric grid, due to the vulnerability of RES towards the unforeseeable variation of meteorological conditions. Related to this issue, ESS are a potential solution to support RES penetration, especially the hybridization of multiple ESS forming a hybrid energy storage system (HESS). This system has ability to fulfil all the requirements of a certain application. However, the limitation of the solution is its complex control strategy, since it plays a key role in optimizing the capabilities of each technology. Related to this scenario, the uses of SCs in the literature are focused on improving the performance of the RES and the electric grids, collecting those studies in the following topics:


Babu, T.S. [71] presents a review of the control strategies proposed in the literature for HESS. The paper classifies the control techniques into interconnection topologies, classical control strategies, advanced control techniques and real cases studies, being briefly discussed with their limitations, see Figure 17. The study collects the challenges faced when an implementation of HESS for standalone microgrid or a grid connected is made. This paper shows a guide to several control techniques implemented for HESS on grid connection applications.

**Figure 17.** Main interconnection topologies for a HESS, formed by a high power storage (HPS) and a high energy storage (HES): (**a**) passive, (**b**) semi-active and (**c**) active adapted from [71]; (**d**) Classification of HESS control techniques adapted from [71].

The study [72] proposes a strategy to manage a HESS in renewable generation systems which currently require controlling bidirectional power flow. The device is composed by a direct connection of a battery and a SCs unit linked through a dc-dc converter. The proposed strategy includes a power control loop which distributes the power flow through each device, achieving an optimized performance, providing grid-frequency regulation and maximizing the lifespan of the batteries, reducing their number of cycles. As in other researches, the SCs perform the fast response, absorbing the high-frequency term and the batteries provide the long-term power fluctuations. The HESS are controlled using a droop control strategy that considers the converter characteristics, SC voltage levels, and power demand.

Manandhar, U. [73] presents a new energy management scheme proposed for a grid connected HESS, composed of the battery and the SC, under different operating scenarios. The objective of the proposed energy management scheme is to reduce the stress in the battery system, controlling the dynamic power sharing between the battery and grid. The study presents a faster DC link voltage regulation to a generation and load disturbances, a reduced rate cycle on the BESS based on its state of charge. Finally, the SCs are in charge of absorbing the high-frequency power fluctuations, reducing the stress on the BESS, and maintaining the SOC limits of energy storage within the safe operating region.

Akram, U. [74] presents an innovative design and operation framework for a BESS and a HESS used for frequency regulation in the electric grid. The proposed design considers the total system cost, the investment, replacement and maintenance cost, as well as the penalty imposed due to not supplying the required regulation service. Moreover, this study shows a comparison based on cost per unit between two scenarios: a HESS and a BESS used both for frequency regulation. The results show that the HESS is more economical.

Nguyen-Huu, T. [75] proposes a coordinated operating control of a HESS (SCs unit and a battery bank) that provides frequency regulation service. The control, based on a droop control with the state-of-charge (SoC) feedback, includes the power flow scheme between the ESSs considering the coverage of the frequency band for each device, as well as the SoC of the SCs and batteries. Moreover, this study provides a guideline for dimensioning the HESS based on based on the smoothing time constant, droop rate, and renewable energy source power rating. The benefits of this method are improving the lifespan of the battery, estimated using a real-time state-of-health (SOH) method based on the temperature, SOH, and throughput degradation.

Pham, V.L. [76] proposes a triple active bridge converter for what will be DC grid in the future. This system is an isolated bidirectional DC-DC converter, used in DC grids and integrated energy systems, composed by different types of renewable energies and storages, such as the photovoltaic and battery systems in grid connection applications or fuel cells and battery/SC in EVs. The advantages of the triple active bridge converter include multiple interfacing ports with isolation, achievable implementation of centralized controls, and improved flexibility of electric systems.

Georgious, R. [77] presents a control strategy for a buck-boost bidirectional converter used in a HESS for DC microgrids. The HESS connected to the DC bus is formed by a Li-ion battery bank and a SCs unit, combined to achieve the energy and power requirements. The control strategy shows a DC bus short-circuit fault-tolerant scheme which provides a protection to HESS and the converter during a short-circuit fault.

Arkhangelski, J. [78] presents a study of a hybrid renewable energy system (HRES), which includes a HESS formed by SCs and batteries, as a reliable source to connect to the grid. This grid connection imposes restrictions relating to the power delivered and its harmonic content. The aim of SCs is to absorb the high-frequency fluctuations of the power along with smoothing the power of the batteries. This study proposes the use of a low-pass second order filter, which splits the high-frequency component for the SCs and the low-frequency component for the battery system. This solution greatly increases the reliability and durability of the HRES. The results show that the proposed strategy improves the lifetime of the batteries (see Figure 18).

**Figure 18.** Scheme of model studied adapted from [78].

Malkawi, A.M.A. [79] shows the benefits of using a SCs-based ESS along with batteries in a HESS to mitigate the impact of high and fast current variations on the losses and lifespan of the batteries. The system is used in DC nanogrids and microgrids with distributed renewable sources, see Figure 19. This paper presents a HESS controlled as a single unit or each ESS module independently, since if the SC interface is controlled independently from the battery interface, the SCs are able to produce both high and short current pulses, reducing the voltage variations, improving the voltage regulation.

**Figure 19.** Scheme of the SC nanogrid adapted from [79].

Fang, J. [80] proposed a HESS comprising a battery system and SCs to manage the coordinated control of the ESSs as virtual synchronous generators (VSGs). The study uses a control where the SCs attendant the fast-changes power modeled as an inertia and the batteries provide the remaining parts of the VSGs, compensating with slow dynamics and a droop control, the long-term power fluctuations.

#### *4.2. Microgrids*

The use of SCs in a microgrid is linked to a HESS, i.e., the use together with batteries. Within this approach, researchers are focused on improving the performance of a microgrid, analyzing the following topics:


Khalid, M. [81] presents a comprehensive review of the research development of the hybrid storage topic over the last two decades. In this paper, each application-focused is thoroughly and independently investigated. The HESS-focused application comprises battery and SC modules, which have complementary characteristics improving their scope in various fields. The review collects research works about regulation of renewable energy sources; grid regulation, especially voltage and frequency compensation; energy storage enhancements, including lifespan improvement, and capacity reduction; and regenerative braking in electric vehicles.

Torkashvand, M. [82] compared a battery ESS and a hybrid energy storage system combining SCs with Li-ion batteries and lead-acid batteries for islanded microgrid applications. This study presents the economical effective of the hybridization, as well as the dimensioning calculation of the ESS to use in the energy management and frequency control of microgrids operating in islanded mode. The results show that the HESS with SC has considerable cost reduction.

Zhu, Y. [83] proposed a strategy based on droop control method for a HESS, comprised of a battery and a SC module, under unbalanced load and nonlinear load conditions. The battery system works in droop mode, providing energy and fundamental active power, i.e., the static performance. Meanwhile, the SC module works in compensation mode, providing the reactive power required, as well as transient changes in the power conditions. This strategy provides better system performance, especially in unbalanced and nonlinear load conditions. Moreover, microgrid stability and the battery lifespan are increased, as well as the power quality.

Oriti, G. [84] presents a novel power flow control system for a remote military microgrid with a HESS, combining batteries and SCs to increase the battery life redirecting the higher frequency current over the SCs. Moreover, this analysis considers several configurations for SCs and filter parameters to achieve the highest cash flow for the overall system, reducing the fuel consumption for the diesel generator. Finally, these results are linked with the sensitivity analysis of the economics of the military microgrid.

Oriti, G. [85] describes an economic analysis combined with a novel power flow control strategy for an energy management system (EMS) involving a HESS. This device is formed by a battery and a SCs module. The aim of the study is increasing the lifetime of the batteries, introducing a SCs module on the EMS to absorb the higher frequency currents, leaving the slow current changes for the batteries. Moreover, the lifetime effect over the economics of the system is analysed.

Akram, U. [86] describes a methodology for the joint capacity optimization of two renewable energy generation system (wind and solar PV) and a HESS, comprised of batteries and SCs. The optimization problem of the sizing the HESS, solar and wind systems of the microgrid comprises the objective of minimising the cost, improving the reliability and reducing the greenhouse gases emissions. The results show that a microgrid with a HESS is more reliable and has lower greenhouse gases emissions and an economical benefit.

Ghosh, S.K. [87] proposes an energy management system (EMS)-based control scheme for DC microgrids with solar photovoltaic systems as the primary generation and energy storage systems, comprised by a battery ESS and SCs. The main feature of the study is to improve the dynamic performance of the microgrid during severe transients, especially in changes of the load demands and power oscillations of the PV units. Moreover, the EMS aims to increase the lifespan of the battery ESS and improve the voltage stability. The control strategy uses proportional-integral (PI) controllers to regulate the switching control actions based on the decision of the EMS achieving the desired objectives.

Kamel, A. [88] presented in the above studies, a control strategy based on a classic PI controller for an EMS and an isolated microgrid is described. It combines PV panels, FC as power sources and batteries and SCs as ESS. The system includes a maximum power point tracking (MPPT) to maximize the harvested energy from PV units. The aim is to optimize the energy management in the microgrid and the cost savings, using different control strategies, and reduce the hydrogen consumption. The PV array supplies the main power and the FC compensates the transient fluctuation of the solar source. Meanwhile, the battery and SC are used to solve the problems of slow response of the FC during the fast change of the load power and to remove the peak power from the system.

Wu, T. [89] introduces an improved hierarchical control strategy which considers the energy storage status of a distributed hybrid energy storage system, leading to the inconsistency of the remaining capacity of the energy storage system in the process of system operation, improving the stability of the microgrid.

Yu, M. [90] proposes a new control method for a HESS to improve the power quality and the fault ride-through capability of islanded forest microgrids. The system is composed by a wind turbine as source and batteries and SCs as energy storage. The method includes a basic control scheme represented by a mode-based sectional coordinated control, and an improved strategy for the HESS, using the batteries to smooth the low-frequency power fluctuations in the long-term, meanwhile the SCs suppress the high-frequency oscillations. A predictive control of the converters is adopted to reduce control delay and ensure the effectiveness of the energy storage power converters. Moreover, as an additional energy storage unit, a wind turbine is used, analysing its capacity of suppressing the huge power disturbance thanks to its large rotating kinetic energy, improving the fault-ride through capacity of the microgrid.

#### **5. Conclusions**

The present manuscript describes the most relevant papers that propose the integration of SCs in electric traction drives, renewable energy sources, and grid connection applications.

Regarding the publications related to electric traction drives, the largest number of them are related to the use of SCs in EVs. Regarding heavy-rail catenary supplied vehicles, most publications focus on the analysis of a DC catenary voltage (1.5–3 kV) against AC (25 kV), because DC voltage levels facilitate the integration of SCs without additional power electronics. SCs in heavy-rail vehicles are used to regenerative braking energy recovery and to stabilize the supply voltage. Energy savings with an ESS is around 12–20% and economic viability will depend on the incentives of each country for the energy returned to the grid. In relation to heavy-rail diesel-electric vehicles, there are hardly any publications because there are few vehicles of this type, due to thatsupply energy is based on fossil fuel sources. In light-rail rapid transit vehicles, SCs are proposed as one of the most appropriate technologies to function as a supplementary power source to the main one, absorbing high power peaks and recovering part of the braking energy. In this application there are also papers that highlight, in terms of cost, Li-ion capacitors (higher energy density) technology compared to EDLC technology.

Regarding electric drive for road vehicles, most of the papers suggest the use of SCs to work in a coordinated manner with batteries as an on-board hybrid storage system (HESS). In the case of public transportation catenary-supplied vehicles, the ultimate goal is to replace the catenary with a HESS with charging points in different sections of the road. Different battery technologies are compared, and strategies are proposed in order to split the required power between SCs and batteries. SCs/lithium-ion batteries combination is the one that offers the best results from a technical point of view. It is also contemplated to replace EDLC technology with Li-ion capacitors due to the latter having a higher energy density (reduction in weight and volume of the ESS), which is an important aspect in onboard systems. The papers related to HEVs study the feasibility of alternative powertrain architectures to the parallel configuration (generally considered the one that offers the best overall efficiency) to reduce fuel consumption when SCs are used as the only ESS. On the other hand, the papers related to EVs study the inclusion of SCs as part of the power system to extend the useful life of the batteries. In both cases (HEVs and EVs), an EMS is necessary to maximize the efficiency of the entire system. Controllers based on fuzzy logic and adaptive algorithm are considered essential tools to optimize the power distribution between SCs and batteries in the case of EVs.

Regarding the papers related to the inclusion of SCs in renewable energy systems (wind and PV solar), most of them consider a HESS (SCs/batteries). In the particular case of wave energy systems, SCs are considered as single and sufficient ESS due to the nature of the resource (high power and low energy peaks) and due to that main requirement is reduction of power oscillations. Solar PV and wind power systems need higher energy density ESS (e.g., batteries) in addition to SCs. The papers related to the inclusion of SCs in solar and wind applications are based on studying the optimal configuration for the connection of the HESS. Multi-objective optimization algorithms are also proposed for dimensioning of energy storage system and control strategies (e.g., low pass filter) to split the required power between both ESS. On the other hand, in wave energy applications the use of reinforcement learning-based energy management system is proposed in the SCESS sizing methodology to reduce and optimize the power and energy required. On the other hand, it is necessary to take into account the SoC of the ESS in the control strategy for the operation of the system.

Regarding the power grids applications, SCs are focused on improve their performance. The results of the studies show that the use of SCs together with batteries as a HESS improves the voltage and frequency stability of the electric grids, as well as the flexibility of the system allowing to introduce a higher number of renewable energy plants. Moreover, the SCs allow to use an advanced control strategy for the HESS, improving their efficiency and their capabilities against frequency and voltage fluctuations. Finally, the use of a HESS, composed by a high energy system and a SC based ESS, allows to dimension the system with high accuracy in order to fulfill the grid codes requirements and minimize its cost and maintenance.

In a nutshell, some generic conclusions of the use of SCs in the mentioned applications are:


• Remuneration policies for energy returned to the grid and grid code compliance will play a key role in integrating ESS into industrial applications.

**Author Contributions:** Conceptualization G.N. and J.T.; investigation, G.N., M.B., M.S.-H. and J.T.; writing—original draft preparation, G.N., J.T. and J.N.; writing—review and editing, M.S.-H., M.L. and M.B.; visualization, G.N., J.N. and M.S.-H.; complete review and adding contents, M.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**

