## 1 Introduction  
A report by the Intergovernmental Panel on Climate Change (IPCC) noted that so far the global temperature has risen by 1 \({}^{\circ}\)C due to warming by 0.2 \({}^{\circ}\)C per decade and may reach a critical value of 2 \({}^{\circ}\)C by the 2060s if the proposed environmental policies will not be implemented urgently worldwide [1]. This global warming has caused the warmest 18 years to date as well as several meteorological events that fall into the extreme intensity class, and for some time now it has been increasingly recognized and accepted that greenhouse gas (GHG) emissions are responsible for most of these climate changes [1,2]. As it is well known, the largest source of GHG emissions from human activities (formed mostly by carbon dioxide, accounting for 80% of the emissions) is represented by the energy-producing industry and by transportation, both of which are responsible together for more than a half of the total emission [3]. Given this situation, it becomes obvious there is the need to increase research and technology development efforts to make the transition to a low-carbon, secure, and competitive economy [4]. Part of this problem solution is perceived to be the increase in using renewable energy sources (RESs) [5]. Unfortunately, the rapid deployment of RESs involves addressing issues related to their disadvantages because there are some [6]. Their vast majority is characterized by intermittency and fairly wide diurnal and/or seasonal variability [7]. Due to their inherent nature, RESs are vulnerable to climate change and are also geographically unevenly distributed, and so the redistribution of energy, with all the losses involved, becomes inevitable [8]. During the last few years, technology advances have progressed in establishing a carbon-free energy chain via a partnership between electricity and hydrogen, mediated by fuel cells, which act as efficient and pollution-free energy converters, allowing direct conversion of hydrogen to electricity and thus favoring RES growing scale applicability [9]. To this effect, the proton exchange membrane fuel cell (PEMFC) emerged as a candidate solution [10], due to the advantages such as the high efficiency, zero emission, low noise, and flexibility in controllable power output. In addition, if we refer to PEMFC performance, then it should be mentioned that it strongly depends on operating parameters (such as load profile, temperature, pressure, relative humidity, reactance stoichiometry, etc.) that can cause significant changes in PEMFC performance [11]. Nevertheless, the slowness in some of the balance of plant auxiliary subsystems for gas (fuel and oxidizer), heat, and water management operation, and also in electrochemical processes occurring in fuel cells themselves, makes an FC-based energy storage or power supply system unable to provide a quick startup and to follow fast load changes [12]. It is a fact that in real life, electricity consumption characteristics are hilly and, moreover, quite often fast transients occur, so it appears obvious there is the need to use in power systems, apart from fuel cells, some wide dynamics energy storage devices to help the power source match the load demand [13]. Together, they form a so-called hybrid power system (HPS). Usually, hybridization is made using batteries and/or ultracapacitors (UCs) [14]. As it is known, a lithium-ion battery has a high energy ratio, a relatively long life compared to other technologies for making batteries, and can generate high power [15], and an ultracapacitor has high power density and very short response time [16]; together or separately, they can counterbalance the disadvantages of pure FC power systems [17]. Regardless of the application that a HPS works in, be it mobile or stationary, the power distribution among FCs, batteries, and/or ultracapacitors is a basic problem to be solved through an energy management strategy (EMS) [18].