Hydrogen refueling stations: a review of the technology involved from key energy consumption processes to related energy management strategies

Citation: Pereira, R.; Monteiro, V.; Afonso, J.L.; Teixeira, J. Hydrogen Refueling Stations: A Review of the Technology Involved from Key Energy Consumption Processes to Related Energy Management Strategies. Energies 2024,17, 4906. https://doi.org/10.3390/en17194906 Academic Editor: Ahmed F. Zobaa Received: 15 July 2024 Revised: 12 September 2024 Accepted: 28 September 2024 Published: 30 September 2024 Copyright: © 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). energies Review Hydrogen Refueling Stations: A Review of the Technology Involved from Key Energy Consumption Processes to Related Energy Management Strategies Rafael Pereira 1,2,* , Vitor Monteiro 1, Joao L. Afonso 1and Joni Teixeira 2 1Centro ALGORITMI/LASI, University of Minho, 4800-058 Guimarães, Portugal; [email protected] (V.M.); [email protected] (J.L.A.) 2Petrotec, 4805-661 Guimarães, Portugal; [email protected] *Correspondence: [email protected] Abstract: Over the last few years, hydrogen has emerged as a promising solution for problems related to energy sources and pollution concerns. The integration of hydrogen in the transport sector is one of the possible various applications and involves the implementation of hydrogen refueling stations (HRSs). A key obstacle for HRS deployment, in addition to the need for well-developed technologies, is the economic factor since these infrastructures require high capital investments costs and are largely dependent on annual operating costs. In this study, we review hydrogen’s application as a fuel, summarizing the principal systems involved in HRS, from production to the final refueling stage. In addition, we also analyze the main equipment involved in the production, compression and storage processes of hydrogen. The current work also highlights the main refueling processes that impact energy consumption and the methodologies presented in the literature for energy management strategies in HRSs. With the aim of reducing energy costs due to processes that require high energy consumption, most energy management strategies are based on the use of renewable energy sources, in addition to the use of the power grid. Keywords: hydrogen refueling stations (HRSs); hydrogen production; hydrogen compression; hydrogen storage; energy consumption; energy management strategies 1. Introduction The transport sector is highly regarded as a considerable and growing source of air pollution across the world [ 1 ]. This sector consumes the largest amount of fossil fuels worldwide, with a direct impact on carbon dioxide (CO 2 ) emissions [ 2 ]. In 2022, this sector accounted for more than 20% of global CO 2 emissions [ 3 ]. CO 2 emissions have significantly increased each year, reaching over 37 gigatons of CO 2 in 2023, compared to less than 5 gigatons in 1940 [ 4 ]. To reverse this negative trend, the European Union signed the Paris Agreement and agreed on the aim of carbon neutrality by 2050 [ 5 ]. Due to growing concern about the high values of pollution as well as the negative repercussions of the excessive use of fossil fuels, new alternative solutions focused on sustainable mobility were developed, with an increase in the development of environmentally friendly vehicle technologies, such as electric and hydrogen vehicles [ 6 ], namely battery electric vehicles (BEVs) and fuel cell electric vehicles (FCEVs). FCEVs can use hydrogen as fuel and are considered one of the most prominent options, with a major role in the future of transport [ 2 ]. Compared to BEVs, the main advantages of FCEVs are the higher travel range and the quick refueling time, although they face challenges due to the cost and durability of the technology, security issues and the lack of hydrogen infrastructure [ 7 ]. By overcoming these barriers, the successful deployment of fuel cell technology could be a key element to reach the European Union’s CO2emissions goal by 2050. Energies 2024,17, 4906. https://doi.org/10.3390/en17194906 https://www.mdpi.com/journal/energies Energies 2024,17, 4906 2 of 16 The transition to hydrogen-based mobility demands the further infrastructure development that must be able to meet the demand for hydrogen [ 8 ]. Hydrogen refueling stations (HRSs) are an example of this infrastructure, being intended for the refueling of hydrogen vehicles, such as FCEVs. As far as the presence of HRSs in the world is concerned, several countries have already implemented them, some of which are open to public supply, while others are dedicated to the private supply of a certain number of vehicles. Between 2021 and 2023, there was a significant increase of 60% in the number of HRS installations, with over 1100 operational HRSs being reached globally by 2030 [ 9 ]. Within this number, the Asia-Pacific region accounts for more than 780 HRSs, followed by Europe, with a number close to 250 installations, and the American continent with 80 infrastructures established. Aligned with the relevance of HRSs, this paper presents a review on the technology involved, from the main energy consumption processes to the related energy management strategies, where the main contributions can be highlighted as follows: (i) a comprehensive literature review is conducted about the main processes concerning HRS, with a special focus on the technologies utilized, the processes that impact the energy consumption required for HRS operation and the related operation strategies; (ii) it fills the gap in the current literature focusing on an ample review, as presented in this paper, since the presentation of some of the covered topics is dispersed in various articles and without the necessary contextualization of combined topics; (iii) a review is undertaken on the approaches of the operation methodologies for the energy management proposed for HRSs, since there are no studies that summarize them and highlights the methods on which they are based in the literature; (iv) description of the types of gaseous hydrogen storage tanks and presentation of advantages and disadvantages of each type of electrolyzer and compressors type; (v) the research carried out and the covered topics are aligned with an industrial perspective framed in a cooperative project related with an HRS. Following the Introduction, this review presents the following: hydrogen as an alternative fuel in Section 2; hydrogen production in an HRS in Section 3; the distinct types of hydrogen storage contextualized in an HRS in Section 4; the technologies of compression in an HRS in Section 5; hydrogen refueling process specifications in Section 6; the energy consumption in an HRS in Section 7; and, finally, the main conclusions are in Section 8. 2. Hydrogen as an Alternative Fuel One of the essential elements in the operation of HRSs is the fuel to be used to refuel vehicles, which is hydrogen. In recent years, hydrogen has gained importance on the world stage, with increased interest in its use as a potential candidate to replace conventional fossil fuels [ 10 ]. However, despite its recognition as an ideal and emerging energy carrier [ 11 , 12 ], hydrogen has some particularities of its own that need to be considered in the applications in which it is involved. Hydrogen is the most abundant chemical element in the universe, forming numerous compounds [ 13 ]. In its pure state, hydrogen is a flammable, invisible, colorless, and odorless gas in the form of a diatomic molecule, made up of two hydrogen atoms. However, despite being simple and abundant, hydrogen is not naturally present in the form of a gas on Earth [ 14 ]. Most of the hydrogen present on the planet is, in fact, a compound of oxygen, under the form of water [ 15 ], while some hydrogen can also be found in various organic compounds, namely hydrocarbons [14]. Under normal conditions of pressure and temperature (0 ◦ C and 1 atm), this chemical element is in the gaseous state, and it has a density of 0.0899 g/L, which is 14.4-times lighter than air [ 16 ]. Of all substances, hydrogen has the particularity of presenting the second lowest boiling point and melting point [ 17 ]. At ambient pressure, the boiling point and melting point are − 253 ◦ C and − 259 ◦ C, respectively [ 17 ]. In its liquid state, hydrogen has a density of 70.8 g/L [ 15 ]. In terms of its mass, hydrogen has a higher energy density than most of the fuels used [ 18 ]; however, in terms of volume, hydrogen in its gaseous state has a very low volumetric energy density in comparison to fossil fuels, meaning that a high volume of hydrogen is needed to store the equivalent quantity of energy [ 19 ]. Hydrogen Energies 2024,17, 4906 3 of 16 also has flammability limits, which are between 4% and 75%, considering the volume percentage of air, and if the hydrogen concentration rises above the lower flammability limit, a minimal quantity of energy could ignite hydrogen because of hydrogen’s low ignition energy (0.02 mJ) [20]. 3. Hydrogen Production in a Hydrogen Refueling Station Hydrogen can be produced from various sources, using diverse methods, and transported to HRSs or, as an alternative, it can be produced at an HRS [ 21 ]. Depending on where the hydrogen is produced, an HRS can be classified into two types [ 22 ]. When an HRS has the equipment to produce hydrogen, the production of hydrogen takes place on site and the stations are called on-site HRSs, whereas if the hydrogen is produced elsewhere and then transported to the HRS, these stations are defined as off-site HRSs [23]. There are two categories of possible production methods, in which hydrogen can be produced using non-renewable energy sources, such as fossil fuels, or renewable energy sources [ 24 ]. For fossil fuels, production using hydrocarbons stands out, such as natural gas, coal and oil. Production by steam methane reforming, where the natural gas is converted into hydrogen [ 25 ], is the most widely used for the world’s hydrogen supply [ 26 ]. Regarding the production of hydrogen from renewable energy sources, it can use biomass, with decomposition and oxidation actions, or it can use solar or wind energy, through the decomposition of water. Figure 1summarizes the categories mentioned for the various hydrogen production methods. Energies 2024, 17, x FOR PEER REVIEW 3 of 16 a high volume of hydrogen is needed to store the equivalent quantity of energy [19]. Hydrogen also has flammability limits, which are between 4% and 75%, considering the volume percentage of air, and if the hydrogen concentration rises above the lower flammability limit, a minimal quantity of energy could ignite hydrogen because of hydrogen’s low ignition energy (0.02 mJ) [20]. 3. Hydrogen Production in a Hydrogen Refueling Station Hydrogen can be produced from various sources, using diverse methods, and transported to HRSs or, as an alternative, it can be produced at an HRS [21]. Depending on where the hydrogen is produced, an HRS can be classified into two types [22]. When an HRS has the equipment to produce hydrogen, the production of hydrogen takes place on site and the stations are called on-site HRSs, whereas if the hydrogen is produced elsewhere and then transported to the HRS, these stations are defined as off-site HRSs [23]. There are two categories of possible production methods, in which hydrogen can be produced using non-renewable energy sources, such as fossil fuels, or renewable energy sources [24]. For fossil fuels, production using hydrocarbons stands out, such as natural gas, coal and oil. Production by steam methane reforming, where the natural gas is converted into hydrogen [25], is the most widely used for the world’s hydrogen supply [26]. Regarding the production of hydrogen from renewable energy sources, it can use biomass, with decomposition and oxidation actions, or it can use solar or wind energy, through the decomposition of water. Figure 1 summarizes the categories mentioned for the various hydrogen production methods. Figure 1. Hydrogen production methods [24]. The concept of green hydrogen is associated with the hydrogen production process that involves the use of energy from renewable energy sources, with water electrolysis being a common method in this type of production. Electrolysis refers to a process in which a non-spontaneous chemical reaction is caused by the transmission of an electric current through an electrolyte [27]. The process of water electrolysis is carried out by adding an electrolyte to the water, which makes it conductive and allows for a direct current to pass through it [28], promoting the occurrence of the chemical reactions inherent to the process. In this process, the chemical reactions result in the breakdown of the water molecule (H2O), which splits into oxygen and hydrogen, both in the gaseous state, through the production of an electrical potential difference between two electrodes in an electrolyte [29]. The electrolyzer consists of two electrodes: the cathode, representing the Figure 1. Hydrogen production methods [24]. The concept of green hydrogen is associated with the hydrogen production process that involves the use of energy from renewable energy sources, with water electrolysis being a common method in this type of production. Electrolysis refers to a process in which a non-spontaneous chemical reaction is caused by the transmission of an electric current through an electrolyte [ 27 ]. The process of water electrolysis is carried out by adding an electrolyte to the water, which makes it conductive and allows for a direct current to pass through it [ 28 ], promoting the occurrence of the chemical reactions inherent to the process. In this process, the chemical reactions result in the breakdown of the water molecule (H 2 O), which splits into oxygen and hydrogen, both in the gaseous state, through the production of an electrical potential difference between two electrodes in an electrolyte [ 29 ]. The electrolyzer consists of two electrodes: the cathode, representing the negative pole where the reduction reaction takes place, and the anode, representing the positive pole where the oxidation reaction takes place. There is a separator between these electrodes to ensure Energies 2024,17, 4906 4 of 16 the separation of the resulting products, and when electricity is applied, with a potential difference that is sufficient to break down the water molecule, oxygen is produced at the anode and hydrogen at the cathode [ 30 ]. If renewable energy sources are used as a source of electricity, the water electrolysis process allows for hydrogen production with zero CO 2 emissions [31]. The electrolyzer is the component responsible for the local hydrogen production in an HRS, being the equipment that carries out the process of water electrolysis, as illustrated in Figure 2. Although electrolyzers are a possible sustainable solution, they are a technology that still presents various economic obstacles, with the water electrolysis process accounting for only 4% of the world’s hydrogen gas volume [32]. Energies 2024, 17, x FOR PEER REVIEW 4 of 16 negative pole where the reduction reaction takes place, and the anode, representing the positive pole where the oxidation reaction takes place. There is a separator between these electrodes to ensure the separation of the resulting products, and when electricity is applied, with a potential difference that is sufficient to break down the water molecule, oxygen is produced at the anode and hydrogen at the cathode [30]. If renewable energy sources are used as a source of electricity, the water electrolysis process allows for hydrogen production with zero CO2 emissions [31]. The electrolyzer is the component responsible for the local hydrogen production in an HRS, being the equipment that carries out the process of water electrolysis, as illustrated in Figure 2. Although electrolyzers are a possible sustainable solution, they are a technology that still presents various economic obstacles, with the water electrolysis process accounting for only 4% of the world’s hydrogen gas volume [32]. Figure 2. Electrolyzer system for hydrogen production with power electronics representation. There are several types of electrolyzers, and their distinction is related to the method of water electrolysis, namely the type of electrolyte used, the operating conditions and the respective ionic agents [33]. Electrolyzers can be classified into four types: alkaline water Figure 2. Electrolyzer system for hydrogen production with power electronics representation. There are several types of electrolyzers, and their distinction is related to the method of water electrolysis, namely the type of electrolyte used, the operating conditions and the respective ionic agents [ 33 ]. Electrolyzers can be classified into four types: alkaline water electrolyzer (AWE); proton membrane electrolyzer (PEM); alkaline anion exchange membrane electrolyzer (AAEM); and solid oxide electrolyzer (SOE). Since these types of electrolyzers present distinct characteristics, which can be useful according to the final appli- Energies 2024,17, 4906 5 of 16 cation, Table 1presents an intuitive summary regarding the advantages and disadvantages of each type of electrolyzer. Table 1. Main advantages and disadvantages of each type of electrolyzer [29,33–35]. Technology Advantages Disadvantages AWE Mature technology Low capital cost High stability Longer lifetime Low hydrogen purity Low operational pressure Lower current density Corrosivity of electrolyte Slow startup Gas permeation PEM High hydrogen purity High current density Compact and simple design Quick response time High dynamic operation High membrane cost Acidic environment Lower durability AAEM High hydrogen purity Simple design Low cost Low concentrated liquid electrolyte Low lifetime Low ionic conductivity Low membrane stability SOE High efficiency Low capital cost Requires low energy Safety and sealing problems Uses brittle material Unstable electrodes The AWE’s electrolyte is an aqueous alkaline solution containing 20% to 40% of potassium hydroxide (KOH) or sodium hydroxide (NaOH) [ 34 ]. The electrodes are separated by a diaphragm that allows negative hydroxide ions (OH − ) to pass through to the anode and separates the hydrogen and oxygen, resulting in a hydrogen purity of between 99.5% and 99.9% [ 32 ]. PEM uses a solid polymer electrolyte and has an electrolytic membrane to separate the reactions at the cathode and anode, allowing only positive hydrogen ions (H + ) to pass through to the cathode, resulting in a hydrogen purity of 99.9% [ 33 ]. AAEM is a combination of AWE and PEM, presenting the advantages of both types [ 34 ] and a hydrogen purity of 99.9% [ 33 ]. This type has a low-concentration aqueous alkaline solution as its electrolyte and, like PEM, uses an electrolytic membrane, which, however, only allows the passage of negative hydroxide ions (OH − ) to be transported through the membrane to the cathode to produce hydrogen [ 32 ]. The operating temperatures for AWE, PEM and AAEM are lower than 100 ◦ C, whereas SOE stands out for operating at very high temperatures, namely above 650 ◦ C [ 36 ]. This type of electrolyzer uses a solid electrolyte made up of ceramic materials that enable the conduction of oxide ions (O 2 − ) [ 33 ], with a hydrogen purity of 99.9% [ 29 ], and although the electrolysis process takes place at high temperatures, the energy required is lower when compared to other methods [32]. For low-temperature water electrolyzers, including AWE, PEM and AAEM, water quality is also a pertinent parameter because water impurities can impact the performance and the lifetime of these types of electrolyzers [ 37 ]. Regardless of the amount of water needed to operate the electrolysis process, the electrolyzer requires high-purity water. The accumulation of ions can lead to corrosion of the electrolyzer components, with cationic impurities being the most problematic impurity for the PEM, while for the AAEM and the AWE, anionic impurities represent a direct threat [ 37 ]. The further development of electrolyzers with impurity tolerance could contribute to a reduction in capital and maintenance costs, as well as increasing their durability [38]. 4. Types of Hydrogen Storage in Hydrogen Refueling Stations Hydrogen storage is an essential factor in HRSs, and the storage of this fuel is considered to be at least as important as its production [ 35 , 39 ]. The cost of hydrogen storage is a Energies 2024,17, 4906 6 of 16 fundamental challenge that needs to be faced so that hydrogen can become a competitive and viable energy vector [ 19 ]. For HRSs, this is a considerable cost, representing more than 20% of the capital expenditure (CAPEX) [ 40 ]. Hydrogen storage can be divided according to its physical states, and, in HRSs, hydrogen can be supplied as a gas or as a liquid [ 41 ]. Based on these differences, HRSs can be categorized into gaseous hydrogen stations or liquid hydrogen stations [ 42 ], and, therefore, gaseous and liquid storage is the main type of storage used. Liquid hydrogen storage involves cryogenic tanks that maintain hydrogen in its liquid state at cryogenic temperatures [ 43 ]. Since hydrogen has a much higher density in the liquid state than the density in the gaseous state, it results in a greater mass and, consequently, a greater amount of stored hydrogen. For this reason, hydrogen storage in the liquid state can contain a greater amount of stored hydrogen compared to the gaseous state, thus providing the advantage of a high volumetric storage density [ 44 ]. However, the fact that hydrogen is liquid at a very negative temperature, with a boiling temperature of − 253 ◦ C, implies that the liquefaction process must involve high investment costs, as well as the consumption of large amounts of energy to keep the hydrogen in its liquid state [ 35 , 45 ]. The high energy required and the high hydrogen boil-off losses are some of the crucial challenges for the successful deployment of this type of storage [ 44 ]. As this type of storage still presents complex processes and challenges to overcome, after the hydrogen is supplied to HRSs in a liquid state, it undergoes a vaporization process and is converted into its gaseous form through a heat exchanger so it can then be compressed to a specified pressure [41]. The storage of hydrogen in the gaseous state is the most mature technology, and this type of storage is the most widely used method [ 46 , 47 ]. This storage method uses highpressure compressed gas tanks, which are also called cylinders or vessels. The need to use high pressures is because hydrogen needs to be stored in significant quantities to occupy a smaller volume, since hydrogen has a low density in its gaseous state. Higher storage pressure increases the energy content per unit volume, resulting in a higher volumetric energy density [ 48 ]. Therefore, the compatibility of materials is a factor to be taken into consideration to guarantee safe and efficient storage, and storage equipment needs to be robust and made of materials resistant to high pressures [ 49 ]. These storage tanks can also be categorized into five types [ 50 ], according to the types of materials used in their manufacture. These types are described in Table 2and are illustrated in Figure 3. Table 2. Description of the types of gaseous hydrogen storage tanks. Type Material Characteristic I All-metal gas tank II Metal-lined gas tank hoop-wound with fiber III Metal-lined gas tank fully wound with fiber IV Polymer-lined gas tank fully wound with fiber V All-composite liner-less gas tank Energies 2024, 17, x FOR PEER REVIEW 7 of 16 weight and higher working pressure compared to the types described previously. Type III uses a metal liner, while Type IV uses a polymer liner, allowing for a reduction in weight and an improvement in storage density [52]. Type IV is the most used type due to its high capacity and lightweight design but is the most expensive [19]. Type V is fully made of composite materials, such as carbon fiber or fiberglass [50] and has a great potential for raising the density of hydrogen storage; however, it is still in the development stage, with some constraints that need to be addressed to ensure that they can be developed and offer a viable alternative to Type IV [53]. Figure 3. Five types of gaseous hydrogen storage tanks. 5. Compression in Hydrogen Refueling Stations As mentioned in Section 4, despite the distinct existing forms of hydrogen storage, the refueling process that takes place in HRSs is with the use of hydrogen in its gaseous state. The supply of hydrogen in the gaseous state can take place through local production at an HRS, using electrolyzers, or it can be undertaken through different transportation methods, usually via tube trailers with pressures of over 180 bar [41]. If the hydrogen is produced by electrolyzers, the pressure of the hydrogen produced is considerably lower, ranging from 30 bar to 40 bar [40], depending on the type of electrolyzer. To be able to fuel the different types of vehicles, the pressure of the hydrogen needs to be increased, since the maximum tank pressure of a light vehicle is 700 bar, while the maximum tank pressure of a heavy vehicle is 350 bar. The compressor is the equipment responsible for being able to raise the hydrogen pressure to the required storage pressure, providing a higher output pressure compared to the input pressure, namely at values above 200 bar [40]. 5.1. Types of Compressors Compressors can be classified into two categories: mechanical compressors and nonmechanical compressors. The mechanical compressors use moving parts to compress hydrogen, in which gaseous hydrogen is squeezed into smaller compartments using mechanical energy, thus obtaining higher pressures [54]. On the other hand, the non-mechanical compressors are characterized by the absence of moving mechanical parts [55], which is an advantage over mechanical compressors since it is possible to reduce installation and maintenance costs [56]. In an HRS, the most used type of compressor is mechanical compressors, namely piston compressors and diaphragm compressors [40,48]. Diaphragm compressors are distinguished by the fact that there is no direct contact between the piston and the compressed gas, with the piston’s reciprocating movement being transmitted via a hydraulic fluid to a diaphragm [57]. As the diaphragm is responsible for isolating the compressed gas and separating the two fluids, these compressors are ideal for applications that require high gas purity, and the fact that these compressors have several pressure stages and can achieve high pressures makes them commonly used for compressing hydrogen in HRSs Figure 3. Five types of gaseous hydrogen storage tanks. Energies 2024,17, 4906 7 of 16 Type I is an all-metal gas tank and, of all the existent types, is the cheapest segment, the easiest to fabricate and, due to the high density of metal materials, the heaviest [ 51 ]. Type II differs from Type I by being wrapped in carbon fiber and allows for a higher working pressure of around 300 bar, while Type I has a working pressure of around 200 bar [ 51 ]. Type III and Type IV are used as solutions for vehicle applications, with a lower weight and higher working pressure compared to the types described previously. Type III uses a metal liner, while Type IV uses a polymer liner, allowing for a reduction in weight and an improvement in storage density [ 52 ]. Type IV is the most used type due to its high capacity and lightweight design but is the most expensive [ 19 ]. Type V is fully made of composite materials, such as carbon fiber or fiberglass [ 50 ] and has a great potential for raising the density of hydrogen storage; however, it is still in the development stage, with some constraints that need to be addressed to ensure that they can be developed and offer a viable alternative to Type IV [53]. 5. Compression in Hydrogen Refueling Stations As mentioned in Section 4, despite the distinct existing forms of hydrogen storage, the refueling process that takes place in HRSs is with the use of hydrogen in its gaseous state. The supply of hydrogen in the gaseous state can take place through local production at an HRS, using electrolyzers, or it can be undertaken through different transportation methods, usually via tube trailers with pressures of over 180 bar [ 41 ]. If the hydrogen is produced by electrolyzers, the pressure of the hydrogen produced is considerably lower, ranging from 30 bar to 40 bar [ 40 ], depending on the type of electrolyzer. To be able to fuel the different types of vehicles, the pressure of the hydrogen needs to be increased, since the maximum tank pressure of a light vehicle is 700 bar, while the maximum tank pressure of a heavy vehicle is 350 bar. The compressor is the equipment responsible for being able to raise the hydrogen pressure to the required storage pressure, providing a higher output pressure compared to the input pressure, namely at values above 200 bar [40]. 5.1. Types of Compressors Compressors can be classified into two categories: mechanical compressors and nonmechanical compressors. The mechanical compressors use moving parts to compress hydrogen, in which gaseous hydrogen is squeezed into smaller compartments using mechanical energy, thus obtaining higher pressures [ 54 ]. On the other hand, the non-mechanical compressors are characterized by the absence of moving mechanical parts [ 55 ], which is an advantage over mechanical compressors since it is possible to reduce installation and maintenance costs [56]. In an HRS, the most used type of compressor is mechanical compressors, namely piston compressors and diaphragm compressors [ 40 , 48 ]. Diaphragm compressors are distinguished by the fact that there is no direct contact between the piston and the compressed gas, with the piston’s reciprocating movement being transmitted via a hydraulic fluid to a diaphragm [ 57 ]. As the diaphragm is responsible for isolating the compressed gas and separating the two fluids, these compressors are ideal for applications that require high gas purity, and the fact that these compressors have several pressure stages and can achieve high pressures makes them commonly used for compressing hydrogen in HRSs [ 48 ]. Another solution for mechanical compressors is ionic liquid compressors, which are designed to improve compression efficiency [ 56 ] and, therefore, have the most potential in HRSs [ 58 ]. Non-mechanical compressors are subdivided into electrochemical compressors, in which compression is carried out through electrochemical reactions, and metal hydride compressors, in which compression is performed using thermal processes that exploit the properties of certain metals, alloys and intermetallic compounds to absorb hydrogen [ 56 ]. Table 3 presents the advantages and disadvantages of each type of compressor described above for HRSs. Energies 2024,17, 4906 8 of 16 Table 3. Advantages and disadvantages of compressor types [56,57]. Types of Compressors Advantages Disadvantages Piston compressor Mature technology Adaptability to a large range of flow rates High discharge pressures Embrittlement phenomena Several moving parts Manufacturing and maintenance complexity Difficulty in managing heat transfer Presence of vibrations and noise Not suitable for high compression ratios Diaphragm compressor Mature technology Low cooling requirement Ideal for handling pure gases and explosives Diaphragm failure Complex design Limited throughput Ionic liquid compressor High efficiency High compression ratio Low energy consumption Low noise emissions No gas contamination Reduced wear and long service Quite isothermal compression Small number of moving parts Liquid leaks Cavitation phenomena Corrosion Metal hydride compressor Thermally driven compressor Absence of moving parts Compact design Safety Absence of noise High-purity hydrogen production High desorption temperature High heat of absorption Limited heat transfer Necessity of using appropriate alloys Low efficiency High weight Low compression rates Electrochemical compressor Low cost of operation Absence of moving parts High-purity hydrogen production Very high compression efficiency Use as a purifier Difficulty in manufacturing the cell assembly Difficulty in realizing a perfect sealing High cell resistance Not suitable for very high discharge pressures Low compression rates 5.2. Compression and Storage Systems In HRSs, different approaches and systems for storing and compressing hydrogen can be presented, with two types of schemes standing out: cascade refueling and direct refueling [41]. The purpose of the cascade-based refueling system is to store hydrogen at certain pressure levels, decreasing the average pressure variation during refueling [ 59 ]. In this method, there are usually three high-pressure tanks, placed between the compressor area and the dispenser, and the compression is carried out through three pressure levels [ 60 ]. This number of compression levels corresponds to the three pressure levels possible, with one tank representing the lowest pressure (low-pressure tank), intermediate pressure (mediumpressure tank) and highest pressure (high-pressure tank) [ 61 ]. During the refueling process, hydrogen begins to flow from a low-pressure-level tank before the pressure reaches a specific limit through a reduction valve, located downstream of the storage tank [ 59 ]. After that, the refueling system switches to the next pressure-level storage tank, and when the pressure in the medium-pressure tank is depleted to a set level, the high-pressure tank is used until the vehicle tank is completely refueled [ 62 ]. The compressor turns on immediately when the storage tanks are emptying to fill the high-pressure tank and, subsequently, the medium-pressure tank and the low-pressure tank [63]. The direct refueling method is a more simplistic approach with only a single-tank storage unit, in which refueling is regulated by the pressure difference between the high- Energies 2024,17, 4906 9 of 16 pressure storage tank and the vehicle storage tank [ 59 ]. In this method, a storage tank is presented at a pressure of between 350 bar and 500 bar, using just one compressor to increase the pressure to the desired refueling pressure [41]. As part of the two aforementioned systems described, the cascade system is the most adopted configuration for HRSs, with the relevant advantages of allowing for reduced energy consumption and costs [ 59 , 62 ]. However, in these three high-pressure tanks, the volume ratio of each stage does not reach the universally recognized ideal configuration [ 42 ], and so the pressures are usually chosen according to the price and physical limitations of the storage tanks [64]. 6. Refueling Process Specifications Refueling vehicles in an HRS requires compliance with safety conditions and the fulfillment of certain requirements established in the existing refueling protocols. When it comes to refueling hydrogen in light and heavy vehicles, there is a technical standard that defines the refueling specifications, the “SAE J2601”. The aim of this standard, developed by the Society of Automotive Engineers (SAE), is to guarantee the transfer of hydrogen fuel safely and efficiently from the dispenser to the vehicle and to provide compatibility among vehicles and HRSs [ 65 ]. For this purpose, performance requirements and safety limits are defined, especially limits related to the hydrogen pre-cooling temperature, the pressure at the dispenser and the maximum permitted flow rate. Within these parameters, the hydrogen minimum temperature is − 40 ◦ C, the maximum temperature of the vehicle hydrogen is restricted to 85 ◦ C, the mass filling rate of hydrogen cannot exceed 60 g/s and the pressure of the high-pressure hydrogen storage tank needs to be lower than 125% of the nominal working pressure [ 66 ]. The dispenser unit is the equipment responsible for refueling the vehicle in accordance with the hydrogen pre-cooling temperatures defined in the “SAE J2601” refueling protocol [60]. According to the “SAE J2601” standard, HRSs can adopt two methods for the refueling process: “Table-based” and “MC Formula” [ 67 ]. The “MC Formula” method is a formulabased approach that uses a dynamic pressure ramp rate (PRR) calculated during refueling, with the target end-of-fill pressure being obtained continuously [ 68 ]. The calculation is carried out using a set of empirical formulas and coefficients, depending on parameters, such as the vehicle initial tank pressure, tank capacity, ambient temperature and the temperature of the pre-cooled fuel leaving the dispenser. Adaptive control is used to carry out this calculation over a range of pre-cooling temperatures from − 40 ◦ C to − 17.5 ◦ C [ 69 ]. The “Table-based” method refers to a look-up table approach that uses a fixed PRR, providing a fixed end-of-fill pressure target [ 68 ]. This approach is based on tabulated values, presented in a varied set of individual tables that are determined depending on the vehicle tank storage capacity (2–4 kg, 4–7 kg and 7–10 kg), the type of dispenser interface (with or without communication of information on the temperature and pressure parameters of the vehicle tank), the delivered pressure classes (350 bar and 700 bar) and the different pre-cooling temperature categories ( − 40 ◦ C, − 30 ◦ C or − 20 ◦ C) [ 69 ]. In this method, the ambient pressure defines the speed of refueling, particularly the average pressure ramp rate, and the combination of ambient temperature and initial pressure specifies the desired pressure to be reached when the refueling is completed [69,70]. The temperature is a fundamental factor to be taken into account since excessive temperatures can negatively influence the tank material’s mechanical properties and the filling efficiency [ 66 ]. To shorten refueling times, high pressure differences are required between the vehicle tank and the tanks installed in the HRS, so heat is generated when refueling the vehicle as the gas expands due to the hydrogen-negative Joule–Thomson coefficient [ 71 ]. During the refueling process, the Joule–Thomson coefficient effect happens when gaseous hydrogen at a high pressure flows through the valve assembly, resulting in an increase in the hydrogen temperature [ 72 ]. Therefore, when the vehicle needs to be refueled, the high-pressure storage system in the HRS is cooled to a set pre-cooling temperature, Energies 2024,17, 4906 16 of 16 88. Barhoumi, E.M.; Okonkwo, P.C.; Ben Belgacem, I.; Zghaibeh, M.; Tlili, I. Optimal Sizing of Photovoltaic Systems Based Green Hydrogen Refueling Stations Case Study Oman. Int. J. Hydrogen Energy 2022,47, 31964–31973. [CrossRef] 89. Song, L.; Gao, W.; Zhang, L.; Li, Q.; Ren, H. Economic Analysis of a Photovoltaic Hydrogen Refueling Station Based on Hydrogen Load. Energy 2023,16, 6406. [CrossRef] 90. Minutillo, M.; Perna, A.; Forcina, A.; Di Micco, S.; Jannelli, E. Analyzing the Levelized Cost of Hydrogen in Refueling Stations with On-Site Hydrogen Production via Water Electrolysis in the Italian Scenario. Int. J. Hydrogen Energy 2021,46, 13667–13677. [CrossRef] 91. Tang, O.; Rehme, J.; Cerin, P. Levelized Cost of Hydrogen for Refueling Stations with Solar PV and Wind in Sweden: On-Grid or off-Grid? Energy 2022,241, 122906. [CrossRef] 92. Di Micco, S.; Minutillo, M.; Perna, A.; Jannelli, E. On-Site Solar Powered Refueling Stations for Green Hydrogen Production and Distribution: Performances and Costs. E3S Web Conf. 2022,334, 01005. [CrossRef] 93. Jing, S.; Yonggang, P.; Jia, X. A Photovoltaic-Assisted in-Situ Hydrogen Refueling Station System and Its Capacity Optimization Method. In Proceedings of the 2023 5th Asia Energy and Electrical Engineering Symposium (AEEES), Chengdu, China, 23–26 March 2023. 94. Maurer, W.; Rechberger, P.; Justl, M.; Keuschnigg, R. Parameter Study for Dimensioning of a PV Optimized Hydrogen Supply Plant. Int. J. Hydrogen Energy 2022,47, 40815–40825. [CrossRef] 95. Shams, M.H.; Niaz, H.; Liu, J.J. Energy Management of Hydrogen Refueling Stations in a Distribution System: A Bilevel Chance-Constrained Approach. J. Power Sources 2022,533, 231400. [CrossRef] 96. Lu, D.; Sun, J.; Peng, Y.; Chen, X. Optimized Operation Plan for Hydrogen Refueling Station with On-Site Electrolytic Production. Sustainability 2022,15, 347. [CrossRef] Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.