**1. Introduction**

Energy storage systems have begun to play a fundamental role in recent years, being one of the most used solutions to improve industrial processes. These devices increase both the production systems performance, improving the energy efficiency, reliability, and flexibility of electrical systems. This topic has nowadays a high relevance due to the expectations of a high integration of renewable energies in electrical grids.

There are different types of energy storage systems (ESSs), divided according to their nature or their operating cycle duration, listed in Tables 1 and 2 respectively. From those tables, the market niches of each ESS could be extracted, considering those technologies with short cycle to applications with fast response, e.g., frequency stability or regenerative braking. Meanwhile, those devices with long cycle could be used in long term applications, as for example massive energy storage or backup system for critical loads.

This paper focuses on a promising energy storage system, supercapacitors (SCs), also known as electrolytic double layer capacitors (EDLC), which have better trend than other competitors. This ESS has a high technology readiness level (TRL), TRL8, and a very promising track record, since it closes the gap between batteries and conventional capacitors, competing with other technologies with similar characteristics such as fly-

**Citation:** Navarro, G.; Torres, J.; Blanco, M.; Nájera, J.; Santos-Herran, M.; Lafoz, M. Present and Future of Supercapacitor Technology Applied to Powertrains, Renewable Generation and Grid Connection Applications. *Energies* **2021**, *14*, 3060. https://doi.org/10.3390/en14113060

Academic Editor: Alon Kuperman

Received: 20 April 2021 Accepted: 20 May 2021 Published: 25 May 2021

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wheels, see Figure 1, Tables 1 and 2. SCs are characterized by high power density and low specific energy.

As described before, SCs focus on applications that require charge-discharge cycle times ranging from a few seconds to several minutes. Based on this and considering the objective of improving the performance of the industrial processes and the electrical systems, countless research articles have been published, as well as research & development projects have been developed in recent years. The aim of these studies is to create new industrial products of ESS based on SCs. This great amount of studies related to SCs evidence the current need for this technology, having inter-annual market growth and encouraging impressive expectations for the future [1].

**Figure 1.** Storage time against the power delivered for electrical energy storage technologies, adapted from [5].



**Table 2.** Classification of the ESSs based on their cycle duration (based on [3,4]).


After showing the operating ranges of some of the most widespread storage technologies in the industry, a comparison between the SCs and their competitors (batteries and

flywheels) is collected in Table 3, where the advantages and disadvantages are described in order to establish criteria to select the most suitable techonology for an application.


**Table 3.** Comparison based on benefits and drawbacks of the Batteries, flywheels and Supercapacitors.

Regarding the comparison in the above table, the supercapacitors show better performance in some areas, especially regarding cost (specially when cost in terms of power terms, \$/W), compared with flywheels, and in the dynamic response and their aging, in applications with short cycle and high power delivered. Supercapacitors will be more preferred than batteries in general for applications where high power, low energy, and large cycling requirements are demanded.

One important aspect when designing and dimensioning SC-based ESS is to define a model of the system which represents its performance under a particular application profile or conditions with high accuracy. Countless SC models for industrial applications have been published in the literature, classified in three main categories: equivalent circuit models, electrochemical models and intelligent models [6]. From these categories, equivalent circuit models are suitable for industry applications, since they describe the SCs behavior using basic common components: capacitors, inductances, and resistors (RLCs) [7]. Within equivalent circuit models, three subcategories can be established, according to their complexity and accuracy: RC models, transmission line models, and frequency domain models [8–11].

Following the SCs model analysis, the next step is to define the main parameters that define the SCs performance:

• The first and main feature is the capacitance. This parameter represents the energy storage behavior of the SCs, being a function of the voltage and the frequency [12]. This variation, shown in Figure 2a, is between 15% and 20% of the rated capacity [13]. This effect is important in ESS applications since the ESS operating voltage impose a capacitance value different to its rated value, which provokes less stored energy. Moreover, the operation frequency of the SCs modifies the capacitance value. Figure 2b shows that there is a cut-off frequency, usually around 1 Hz depending on the materials and manufacturing processes, where the rated capacitance drastically decreases. Therefore, SCs are usually considered for fast charge–discharge cycles, from tens of seconds to minutes.

**Figure 2.** (**a**) Capacitance variation respect to the voltage; (**b**) Capacitance variation as a function of the frequency adapted from [7].


as a voltage-controlled temperature-dependent current source. The self-discharge current value is given by the manufacturer in the datasheet as a constant value [16].

**Figure 3.** (**a**) ESR variation respect to the voltage, adapted from [7]; (**b**) ESR variation as a function of the frequency.

Navarro G. [18] presents a study in which all these parameters are analyzed and aging and voltage imbalance in a large series connection of SCs are calibrated. Table 4 collects the main parameters of commercial cells for several manufacturers.

With respect to the lifespan of the SCs, several limits are established that imply the SCs replacement. A SC cell is considered to have achieved the end of its useful life when any of the following point are fulfilled:


Finally, the last step in the designing and dimensioning process is to evaluate the system losses. A SC-based ESS comprises the SC cells and an interface DC/DC power converter. Power losses take place in both devices, imposing the following issues [15]:


Considering a literature review, there are many articles focusing on organizing and summarizing the new published research advances of this type of technology, as collected in [26]. This paper collects both small- and large-scale applications. For this purpose, the applications have been classified in four main groups: electric traction applications; renewable generation systems; microgrid and power grid connection applications; and autonomous power systems for energy distribution and ships and aircraft applications.


**Table 4.** Main parameters of SCs cell for different manufacturers.

Figure 4 shows the percentage of scientific publications in the last five years related to SCs for the four application groups:


**Figure 4.** Classification of the research publications related to SCs in the last decade respect to their application.

This paper describes the most important studies done for the three main groups which represent most of the SCs present applications. Those groups collect almost 90% of the whole researches related to SCs.

The information is structured into the following sections. Following this introduction, in Section 2 the applications related to the electric traction are described. This section collects both the railway and the road vehicles. This is followed in Section 3, describing in detail the applications related to the renewable generation systems, which include solar, wind, wave energy and hybrid generation systems. In Section 4, those studies that analyze the electrical systems connected to the grid are detailed; within them, there are topics as frequency regulation, voltage drop problems, etc. Furthermore, this group includes those researches related to microgrids. Finally, the paper concludes with Section 5, which contains a summary of the most important aspects covered, as well as the future prospects of the technology based on the information provided for different applications.
