**Preface to "Fuel Cell Renewable Hybrid Power Systems"**

The incredibly rapid increase in the world's energy demand over the last decade, along with the request for sustainable development, can be addressed using microgrids based on hybrid power systems combining renewable energy sources and fuel cell systems. This book includes innovative solutions and experimental research as well as state-of-the-art studies in the following challenging fields: fuel cell (FC) systems—modeling, control, optimization, and innovative technologies to improve the fuel economy, lifetime, reliability, and safety in operation; hybrid power systems (HPSs) based on renewable energy sources (RESs) (RES HPS)—optimized RES HPS architectures; global maximum power point tracking (GMPPT) control algorithms to improve energy harvesting from RESs; advanced energy management strategies (EMSs) to optimally ensure the power flow balance on DC (and/or AC bus) for standalone RES HPSs or grid-connected RES HPSs (microgrids); RES HPS with an FC system as a backup energy source (FC RES HPS)—innovative solutions to mitigate RES power variability and load dynamics to energy storage systems (ESSs) by controlling the generated FC power, DC voltage regulation, and/or load pulse mitigation by active control of the power converters from hybrid ESS; FC vehicles (FCVs)—FCV powertrain, ESSs topologies and hybridization technologies, and EMSs to improve the fuel economy; optimal sizing of FC RES HPSs and FCVs; the changes in climate that are visible today and are a challenge for the global research community.

The stationary applications sector is one of the most important energy consumers. Harnessing the potential of renewable energy worldwide is currently being considered to find alternatives for obtaining energy by using technologies that offer maximum efficiency and minimum pollution. In this context, new energy generation technologies are needed to both generate low carbon emissions as well as identifying, planning, and implementing the directions in which the potential of renewable energy sources can be harnessed. Hydrogen fuel cell technology represents one of the alternative solutions for future clean energy systems. Hence, the first chapter presents the potential applications of hydrogen energy in hybrid power system using SWOT (strengths, weaknesses, opportunities, threats) analysis. The main strategies to be used for integrating the hydrogen-based and classical energy sources in the hybrid power system were identified and detailed. In addition to research, technical, and implementation factors, hydrogen integration also depends on legislative and energy decision-makers, potential investors, and final beneficiaries.

In Chapter 2 is an analysis of fuel cell (FC) system integration in a hybrid power system using three control variants of the FC power based on the required-power-following (RPF) control mode, which ensures the load demand under variable renewable energy. One variant is to control FC power via the FC boost converter, and the other two variants are via air regulator and the fuel regulator. The FC system will compensate the power flow balance on the DC bus and operate the battery stack in charge-sustained mode. Thus, the FC power will be mainly given by the positive difference between the load demand and renewable power. If renewable power is higher than the load demand, then this excess will power an electrolyzer to maintain the charge-sustained mode of the battery. Seven control architectures have been investigated using a fuel economy optimization function, resulting in 15% fuel savings for the best RPF-based strategy compared to the commercial strategy based on feed-forward control.

Chapter 3 compares power losses in the case of current operating conditions of electricity distribution networks (EDNs) and modern microgrids based on renewable energy sources. Optimal allocation of capacitor banks is performed using five metaheuristic algorithms to minimize the power losses on the IEEE 33-bus system and a real 215-bus EDN from Romania.

Sensitivity analysis to evaluate the critical parameters in the design of a new fuel economy strategy for a proton-exchange membrane fuel cell (PEMFC)-based hybrid power system is presented in Chapter 4. The fuel economy strategy uses load-following control and the global extremum seeking (GES) algorithm to minimize fuel consumption. The multimodal behavior in dither frequency and parameter keff is highlighted for the optimization function defined as a mix of the FC net power and fuel efficiency, with the latter being weighted with parameter keff. The results show that the best fuel economy can be obtained for 100 Hz dither frequency and keff = 20.

Chapter 5 addresses the optimal sizing of a PEMFC-powered electric truck to minimize vehicle production and use costs. Property costs were estimated for various design parameters such as the cost of hydrogen and powertrain components, and mileage.

In Chapter 6, a low-power station based on direct methanol fuel cell (DMFC) stacks is designed and tested in a real environment. The generated power can be increased by using multiple DMFC stacks in parallel and an appropriate energy management strategy for the power flows.

Chapter 7 presents the advantages of using a high-temperature (HT) proton-exchange membrane fuel cell (PEMFC) and a carbon capture/liquefaction system in a hydrogen-fueled ship application. The steam methane reforming and steam methanol reforming technologies are evaluated from the point of view of the energetic and exergetic performances, respectively of the occupied space. Compared to fuel cell vehicles (FCVs), the use of electric vehicles (EVs) in the transport of goods and passengers has rapidly increased in number and manufacturer diversity. Hence, it is necessary to optimally plan the location of charging stations near work offices in mall parking areas and in certain locations on frequently used routes.

Chapter 9 presents and tests a charging station according to the standard of the International Electrotechnical Commission (IEC) 61851-1. In this chapter, it is demonstrated that a Simulink dynamic model based on an open source for the proton-exchange membrane fuel cell (PEMFC) system can be easily used in real-time design solutions by exporting the generated code as C/C++. The calculation time is reduced taking into account only important PEMFC parameters in the modeling without significantly depreciating the modeling performance.

As reported in recent studies on microgrids based on renewable energy, wind and photovoltaic energy are the most widely used renewable energy sources used in hybrid energy systems grid-connected. Chapter 10 presents an advanced fuzzy logic control for wind turbines to improve their behavior during transient regimes after grid failures. To improve the efficiency of solar energy conversion, the last chapter studies the design and safe use of a Stirling engine combined with a solar concentrator. As this book presents the latest solutions in the implementation of fuel cells and renewable energy in mobile and stationary applications such as hybrid and microgrid power systems, we hope the chapters within will be of interest to readers working in related fields.

> **Nicu Bizon** *Editor*
