Hydrogen-Enabled Power Systems: Technologies’ Options Overview and Effect on the Balance of Plant

Abstract

Hydrogen-based Power Systems (H2PSs) are gaining accelerating momentum globally to reduce energy costs and dependency on fossil fuels. A H2PS typically comprises three main parts: hydrogen production, storage, and power generation, called packages. A review of the literature and Original Equipment Manufacturers (OEM) datasheets reveals that no single manufacturer supplies all H2PS components, posing significant challenges in system design, parts integration, and safety assurance. Additionally, both the literature and H2PS projects’ database highlight a gap in a systematic hydrogen equipment and auxiliary sub-systems technology selection process, and how this selection affects the overall H2PS Balance of Plant (BoP). This study addresses that gap by providing a guideline for available technology options and their impact on the H2PS-BoP. The analysis compares packages and auxiliary sub-system technologies to support informed engineering decisions regarding technology and equipment selection. The study finds that each package’s technology influences the selection criteria of the other packages and the associated BoP requirements. Furthermore, the choice of technologies across packages significantly affects overall system integrity and BoP. These interdependencies are illustrated using a cause-and-effect matrix. The study’s significance lies in establishing a structured guideline for engineering design and operations, enhancing the accuracy of feasibility studies, and accelerating the global implementation of H2PS.

1. Introduction

Excluding rare pure natural (geologic or white) hydrogen (H₂), other forms of hydrogen are an energy carrier, not an energy source; thus, it must be produced using energy to split it from hydrogen-containing materials such as water or hydrocarbons [1]. The inputs of energy, the production technology and the feedstock determine the carbon footprint or the colour code of the produced hydrogen, e.g., Green, Blue, Grey, Pink, etc. Although hydrogen technologies are over 100 years old in principle, recent efficiency improvements, capital costs decline [2], and awareness of global warming and climate change have accelerated the role of hydrogen in the economy, called the “Hydrogen Economy” [3]. Hence, the principle of utilising hydrogen to decarbonise the energy sectors is gaining momentum with tremendous support from Governments, academia, research centres, and industries [4,5]. Many countries have set a target to become zero-emission by 2050, where hydrogen plays a crucial role in achieving this target [2,5,6].

Using hydrogen to store Renewable Energy (RE), called Power to Gas [6], is a promising approach to decarbonise the electricity sector. Hydrogen is a superior energy carrier due to its high gravimetric energy density (141.9 MJ/kg), significantly surpassing liquefied petroleum (46–50 MJ/kg) and natural gas (50–55 MJ/kg) [7]. Therefore, Hydrogen-enabled Power Systems (H2PS) are gaining accelerating momentum globally [2,8] to reduce the cost of energy and dependency on fossil fuels [9,10]. Many remote, isolated, stand-alone microgrids worldwide with abundant RE resources have started evaluating the feasibility of 100% RE-H2PS, which has led to the installation of many trials and demonstrations of H2PS, and many more are underway [11,12,13,14,15]. Green hydrogen is the most proposed and used in H2PS development [15] for grid-support and off-grid power systems using various hydrogen equipment technologies [11,16]. A typical 100% RE-H2PS comprises an electrolyser, storage, and FC packages with all the necessary equipment for integration into the hosting or served power systems [9,11]. The operation’s philosophy is to utilise excess RE to split water into hydrogen and oxygen, where hydrogen is stored, and oxygen is vented [9]. The stored hydrogen can be used to regenerate electricity via FCs when RE resources are insufficient [6,11,17]. Hydrogen Compressed Gas (H2CG) storage technology is the most used in the installed H2PS worldwide; however, other promising technologies like liquid hydrogen or material-based technologies are gaining momentum [11,18].

A scan of the worldwide H2PS projects’ database and Original Equipment Manufacturers (OEM) products’ datasheets revealed that no manufacturer produces all parts of the H2PS. A comprehensive literature review reveals no detailed Balance-of-Plant (BoP) guidelines, system integration requirements, and systems engineering design procedures from the BoP perspective. This presents challenges for feasibility studies, engineering design, system integration, and safe operations, as the system parts and technologies’ selection determine the cost and complexity of the system [19].

This study focuses on the H2PS equipment’s packages and technologies variation and their effect on the holistic system BoP to establish a guideline to support informed decision-making regarding the preliminary technologies’ selection process and, therefore, the cost estimation. In addition, an H2PS-BoP guideline is essential to facilitate a systematic hydrogen-enabled systems’ Engineering Design and Operations (EDO).

2. Method

The study was constructed at three levels of BoP: the hydrogen components at the OEM level, delivered packages, and the holistic H2PS from an EDO perspective, as illustrated in Figure 1.

The H2PS-BoP at level one discusses briefly (at high-level of detail) the hydrogen components, namely, Electrolysers and FC stacks, typical construction parts, and stacking methods at the manufacturer’s facility, i.e., cell and stack fabrication. This level of detail is beyond the scope of the EDO processes, as hydrogen components (equipment) are typically designed and packaged by the OEM. However, these details are crucial for low-level maintenance, conducted by the OEM or the personnel they trained to do so. On the other hand, the storage equipment’s high-level detailed BoP is discussed according to the storage technology used, as the components vary and differentiate according to the storage technology and the specific parameters, such as operation temperature, pressure, application, and required discharge duration.

The H2PS-BoP at level two discusses the hydrogen-delivered packages, including the equipment, essential auxiliary, and sub-systems. This level of BoP is crucial for the EDO and maintenance of H2PS. Therefore, from an EDO perspective, hydrogen equipment packages’ typical components, such as functionality, interconnectivity, and interoperability, are discussed and listed for each part of the holistic H2PS. This study discusses the packages as an individual part of the holistic H2PS regardless of the assembly process and orientation. We conducted a comprehensive review of multiple OEM product specifications and datasheets [20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44], alongside selected knowledge-sharing reports from existing H2PS projects [3,11,13,14,19,45,46,47,48,49,50,51,52,53,54,55,56,57], to identify and categorise the distinct technology packages and solutions in use.

The holistic H2PS auxiliary and sub-systems are discussed at level three BoP. In addition, the external support systems for feedstock supply, cooling, control, safety, and waste management are discussed for both case-specific designed and pre-engineered-packaged H2PS.

3. Water Electrolysis

Water electrolysis is the process of splitting water (H₂O) into hydrogen (H₂) and oxygen (O₂) by passing Direct Current (DC) through the electrolysis stack. The total reaction is explained in Equation (1) [58].

2 H₂O (liquid) + energy → 2 H₂ (gas) + O₂ (gas)

(1)

where Energy = 237.2 kJ/mol (electricity) + 48.6 kJ/mol (heat) [59].

3.1. Electrolysis Technologies

Despite the many different types (technologies) of electrolysers, there are three well-known types of electrolysis technologies, namely, Alkaline Electrolyser (ALKE), Proton Exchange Membrane Electrolyser (PEME), and Solid Oxide Electrolyser (SOE) [60], as shown in Figure 2.

These electrolysers work under the same principle but in different ways [61]. The ALKE and PEME are the most used technologies in today’s H2PS applications, while the SOE is gaining potential for Combined Heat and Power (CHP) applications, as the SOE operation temperature is very high compared to the other electrolysis technologies [62].

Electrolysers Emerging Technologies

A range of rapidly developing but not commercially ready water electrolysis technologies may require different BoP and engineering controls, such as the following:

• Anion Exchange Membrane Electrolyser (AEME) possesses the advantages of both ALKE and PEM electrolysers technologies without their weak points [63].
• Proton-Conducting Ceramic Electrolyser (PCCE) is another emerging high-temperature and comparably high-efficiency technology.
• Unitized Regenerative Fuel Cell (URFC) is an electrolyser and FC in one stack arrangement, which can carry out the electrolysis of water in electrolysis mode and function in regenerative mode as a FC. The combination of both electrolysers and FCs in a single cell/stack requires special arrangements and engineering controls [64].
• Capillary-fed Electrolysers (CFE) is a new category of electrolysis, introduced in 2022 by Hysata company in Wollongong, NSW, Australia. The concept was published in an article by Hodges A. et al. (2022) [65]. The efficiency mentioned in the literature is over 80% [65,66].

This study focuses on the most well-known electrolyser technologies and the most mentioned emerging technologies in the literature regarding H2PS. Some selected cell and stack specifications and properties of electrolysers are compared in

Table 1. Types of electrolysers: cell and stack selected specifications and properties comparison.

Specifications	ALKE	PEME	SOE	AEME	PCCE
Electrode/Catalyst (O2 side) (Anode)	Nickel coated. perforated stainless- steel [61]	Iridium oxide [61]	Perovskite-type (e.g., LSCF, LSM) 1 Ni0.85Co0.15-YSZ [61,67]	High surface area Nickel or NiFeCo alloys [61]	Perovskite-type (e.g., LSCF, LSM [63]
Electrode/Catalyst (H2 side) (Cathode)	Nickel coated. perforated stainless- steel [61]	Platinum nanoparticles on carbon black [61]	Ni/YSZ 1 [61]	High surface area nickel [63]	Ni/YSZ, Ni-BZY/LSC, BCFYZ [63]
Electrolyte	Potassium Hydroxide (KOH) or potash lye 57/mol/L [61] Or NaOH 30 wt% [59]	PFSA membranes Nafion membrane [61]	oxide-ion conducting ceramics [63] Yttria-stabilized- Zirconia (YSZ)Y2O3 + ZrO2 1 [61]	DVB polymer support with KOH or NaHCO3 1 mol/L [63]	(Y,Yb)-Doped Ba(Ce,Zr)O3−δ [63]
Separator/Diaphragms	ZrO2 stabilised with PPS mesh [61]	Solid electrolyte (above) [61]	oxide-ion conducting ceramics electrolyte, as above [61]	Solid electrolyte (above) [63]	Solid electrolyte (above) [63]
Porous Transport Layer Anode	Nickel mesh (not always present) [63]	Platinum-coated sintered porous Titanium [61]	Coarse Nickel mesh or foam [63]	Nickel foam [63]	Coarse nickel mesh or foam [63]
Porous Transport Layer Cathode	Nickel mesh [61]	Sintered porous Titanium or Carbon cloth [61]	None	Nickel foam or carbon cloth [63]	None
Bipolar Plate Anode	Nickel-coated stainless steel [61]	Platinum-coated Titanium [61]	None	Nickel-coated or stainless steel [63]	None
Bipolar Plate Cathode	Nickel-coated stainless steel [61]	Gold-coated Titanium [61]	Cobalt-coated or stainless steel [61]	Nickel-coated or stainless steel [63]	Cobalt-coated [63]
Frames and sealing 1	PSU, PTFE, EPDM [61]	PTFE, PSU, ETFE [61]	Ceramic glass [61]	PTFE, silicon [63]	Ceramic glass [63]
Ion Movement	hydroxyl (OH−) [61]	Hydrogen Ion (H+) [61]	Oxygen Ion (O2−) [61]	hydroxyl (OH−) [63]	Hydrogen Ion (H+) [63]
Operating Temperature (°C)	70–90 [61]	50–80 [61] 30–80 [68]	700–850 [61] 800–1000 [63]	40–60 [63]	300–600 [63] Typically, 500
Cell Operating Pressure (bar)	1–30 [61] Up to 448 bar [60]	1–70, typically 30 [61] Min 11 bars cathode pressures to keep the H2/O2 mixture below 2% [68], Up to 700 bars [69,70]	<10 [61] Typically 1 bar [63]	<35 [61]	1 [63]
Electrical Efficiency (Stack) (kWh/Kg H2)	50–78 [61] 47–66 [63]	50–83 [61] 47–66 [63]	45–55 [61] 35–50 [63]	51.5–66 [63]	None
Electrical efficiency (system) (kWh/Kg H2)	50–78 [63]	50–83 [63]	40–50 [63]	57–69 [63]	None
Hydrogen Purity (%)	99.9–99.9998 [61,63]	99.9–99.9999 [61,63]	99.9 [63]	99.9–99.999 [63]	None
Nominal Cell Current Density (A/cm2)	0.2–0.8 [61,63]	1.6–2.3 [61] 3.7 @ 80 °C [68] 1–3 [63]	0.3–1 [63] Up to 3.6 @ 950 °C [68]	0.2–2 [63]	None
Voltage range (Volts)	1.4–3 [63]	1.4–2.3 [63]	1–1.5 [63]	1.4–2 [63]	None
Load range (%)	15–100 [63]	5–130 [63]	30–125 [63]	5–100 [63]	None
Stack size	1 MW [63]	1–2 MW [63]	5 kW	2.5 kW [63]	None
Water feed	Cathode side [59]	Anode side [59]	Cathode side [59]	Cathode side [63]	Anode side [63]
Cold start to nom. Load (min)	<50 [63]	<20 [63]	>600 [63]	<20 [63]	None
Coupling with Variable RE (Solar, wind, and wave)	Awkward, i.e., long time to reach steady-state [63], performance deteriorates under part-load [59]	High dynamic range [63]	Awkward and require excess heat energy [63]	Awkward [63]	None
Lifetime (K hours)	60 [61]	50–60 [61] up to 80 [63]	20 and improving [61]	>5 [63]	None
Capital costs (small-scale stack)	800 to 1500 EUR/kW [60]	1400 to 2100 EUR/kW [60]	Unknown [60]	None	None
Capital costs (package) (USD/kW)	270 [63] 750 [71] 1000 EUR/kW [72] 571–1268 [73]	400 [63] 800 [71] 1800 EUR/kW [72] 385–268 [73]	>2000 [63] 677–2285 [73]	None	None
Capital costs (system) Minimum 10 MW (USD/kW)	500–1000 [63] 50 USD/kW additional for the BoP [73]	700–1400 [63] 50 USD/kW additional for the BoP [73]	Unknown [63] 50 USD/kW additional for the BoP [73]	None	None
State of Development	Mature/Marketed [61,63]	Mature/Marketed [61,63]	Developing [61]	None	None

¹ PFSA = Perfluorosulfonic acid; PTFE = Polytetrafluoroethylene; ETFE = Ethylene Tetrafluorethylene; PSF = poly (bisphenol-A sulfone); PSU = Polysulfone; YSZ = yttrium and scandium oxides; DVB = divinylbenzene; PPS = Polyphenylene sulphide; LSCF = La_0.58Sr_0.4CO_0.2Fe_0.8O_3−δ; LSM = (La_1−xSr_x)1−yMnO_3.

.

3.2. Electrolyser’s Cell and Stack Arrangements

This section discusses typical electrolysis cells and stack arrangements (level 1) in high-level detail, as the cell and stack are not directly related to the EDO of H2PS. The designers typically use pre-assembled electrolyser stacks or packages, while the maintenance team may replace parts according to the OEM instructions and specifications. On the other hand, high-temperature electrolysers such as SOEs are not commercially available at this time and are not commonly used in H2PS, as they require high-quality excess heat energy [74]. Therefore, this study focuses on the AKLE and PEME cells and stacks arrangements.

The typical electrolysis cell consists of two electrodes, an anode and a cathode, plated with a catalyst and enclosed by an electrolyte as an ionic conductor (charge carrier) [61]. The two electrodes are separated by a membrane (separator) to pass a selected ion from one electrode side to the other side [62], as shown in Figure 3a. Typically, each electrode with the catalyst, electrolyte/membrane, and gas diffusion layer is assembled in what is known as the Membrane Electrode Assembly (MEA) [63], as illustrated in Figure 3b.

The design of the electrolysis cell complexity can vary significantly depending upon the technology type and application [63], but, in general, several essential components found in most electrolysis cells, as illustrated in Figure 3a,b, are as follows [63]:

• The Membrane Electrode Assembly (MEA) consist of the following:

  a.

  Electrodes (anode and cathode);

  b.

  Catalyst (technology-specific);

  c.

  Membrane/Electrolyte (technology specific);

  d.

  Gas diffusion layer.

• Flow plates.
• Gaskets, clamping mechanism and seals.

The electrolyser’s hydrogen production capacity (called size) increased by cells cascading, i.e., multiple cells connected in series to form an electrolyser stack, as illustrated in Figure 4 [63]. Typically, these cells are separated with spacers (insulating material between two opposite electrodes or bipolar plates), seals, frames (mechanical support), and end plates (to avoid leaks and collect fluids). However, each technology requires some specific stack arrangements, which also vary between manufacturers as compared in Table 1 above. The manufacturing cost factor is the cost of the materials used as catalysts and the mechanical construction, which depends on the electrolyser’s technology and varies between manufacturers [61,63].

In general, several essential components can be found in most electrolysers’ stacks [63,75]:

• Feed stock (water) flow interconnection and recirculate water.
• Bipolar plates interconnecting cells back to back (anode to cathode).
• Anode and Cathode end plates at both ends of the stack.
• Cooling plates (coolant flow).
• Power connection arrangements.
• Stack mechanical arrangements, such as the following:

  a.

  Bolts;

  b.

  Layers’ stiffness;

  c.

  Machining gaskets;

  d.

  Seals;

  e.

  Clamping mechanism.

Figure 5 shows some examples of actual electrolyser stacks.

3.3. Electrolyser Package

The electrolyser stack requires support, auxiliary, and control systems to function, i.e., produce hydrogen [64,70]. These systems vary depending on the technology type and the scale. The electrolyser stack with all required sub-systems is referred to as an electrolyser package in this study. In general, the electrolyser packages’ BoP (level 2), as illustrated in Figure 6 (a and b), comprises (depending on the stack technology and size) many supporting systems, such as (but not limited to) the following [60,63,76]:

• Power supply rack (transformer and/or rectifier), Power Management System (PMS) and production control.
• Water supply, including storage, treatment such as desalination and/or purification and de-ionisation, and feeding systems, such as water pumping.
• Liquid electrolyte storage and circulating system (lye system) for the AKLE [76] or ion exchanger circulating system for the PEME.
• Cooling system and/or heat exchangers.
• Pressure control system.
• Monitor, control, and Human interface, including remote control, communication, software, and PLC.
• Hydrogen buffer tank.
• Hydrogen purification system, i.e., impurities removal and deoxygenation to remove remnant oxygen.
• Hydrogen dryers.
• Ventilation system.
• Venting valves and pipework.
• Sensors and alarm system.
• Interconnecting wires and pipelines.

Most electrolysers’ packages supporting sub-systems are generally the same, while technology-specific systems vary [76]. The feedstock (water) system complexity and integration depend on the water source, i.e., whether the water source is desalinated, collects rainwater, or has potable water mains [77]. Typically, the water supply system is not considered a part of the electrolyser package, but purification (demineralisation and deionisation) and the feed pump are part of the package, even if some or all parts are located outside the main package enclosure [78]. The water purity depends on the electrolyser’s technology but is typically required to be ultrapure and deionised feedstock [79]. The feedwater circulating system at the anode side of the stack in the PEME packages is essential to avoid the release of metal ions over time [76]. While liquid electrolyte storage tank and circulating system (typically pumps) can be found in the AKLE packages [80]. Also, the power supply rack may be located in the same enclosure or outside, depending on the electrolyser package scale. The power supply system varies depending on the power source, type, scale, and coupling with the hosting power system or grid connection requirements.

Typically, for small-to-medium-scale electrolysers, the stack and required support sub-systems, such as control and auxiliary systems, are delivered in one container (enclosure), as shown in Figure 7. However, some OEMs use more than one container or locate some sub-systems in other parts of the H2PS enclosures for safety or ease of access during operations and maintenance [26].

4. Fuel Cells

4.1. Fuel Cell Technologies

The Fuel Cell (FC) uses hydrogen as a fuel to generate electricity in the form of direct current (DC) by an electrochemical reaction; i.e., in a reverse way compared to electrolysers, where the anode of the electrolyser cell becomes the cathode of the FC, and the cathode becomes the anode. However, the directions of the migration of anions and cations with respect to current flow are unchanged [81].

The overall reaction is explained in Equation (2) [82] and as compared in Figure 8. Whilst hydrogen gas is the feedstock (fuel), oxygen is typically obtained from the air (~21% is oxygen).

2 H₂ (gas) + O₂ (gas) → 2 H₂O (liquid) + Electrical and Heat Energy

(2)

The FCs were used for stationary and mobile electricity generation at a range of sizes. It is the heart of the H2PS to utilise the stored hydrogen for power regeneration, though hydrogen can be used as a fuel for internal combustion engines or hydrogen gas turbines to generate power [6]. The trend is to use FCs in the H2PS, as they are more efficient compared to hydrogen-burning generators [59]. However, if the generated heat is utilised in a combined heat and power (CHP) system, low-temperature FCs are considered less efficient. This study focuses on FCs for power regeneration as part of the holistic H2PS BoP.

There are five well-known types of hydrogen FCs, which are distinguished by their electrolytes and the reactions that take place on the electrodes, namely [81,83,84]:

• Alkaline Electrolyte (AFC);
• Proton-Exchange Membrane (PEMFC);
• Phosphoric Acid PAFC;
• Molten Carbonate (MCFC);
• Solid Oxide (SOFC).

These technologies under the lens of this section are shown in Figure 8, and selected relevant specifications are compared in

Table 2. Types of Fuel Cells: selected cell and stack specifications and properties comparison.

Specifications	AFC	PEMFC	PAFC	MCFC	SOFC
Electrode/Catalyst/Reaction (Anode)	Carbon electrodes Nickel coated. perforated stainless steel [81] H2 + 2(OH)− → 2H2O + 2e− [75,85]	H2 → 2H+ + 2e− [85]	Platinum (Pt) alloys as the catalyst (0.10 mg Pt cm − 2) [81] H2 → 2H+ + 2e− [85]	Ni–Cr or Ni–Al [81] H2 + CO3 = → H2O + CO2 + 2e− CO + CO3= → 2CO2 + 2e− [85]	cermet of yttria-stabilised zirconia (YSZ) and Nickel [81] H2 + O= → H2O + 2e− CO + O= → CO2 + 2e− CH4 + 4O= → 2H2O + CO2 + 8e− [85]
Electrode/Catalyst/Reaction (Cathode)	Carbon electrodes, Silver-plated nickel screen [81] ½ O2 + H2O + 2e− → 2(OH)- [85]	Platinum nanoparticles on carbon black [61] Pt and ruthenium (Ru) 0.2 mg Pt/cm2 [81] ½ O2 + 2H+ + 2e− → H2O [85]	Pt supported on carbon black Pt alloys as the catalyst (0.50 mg Pt cm−2) [81] ½ O2 + 2H+ + 2e− → H2O [85]	nickel oxide Lithiated NiO [81] ½ O2 + CO2 + 2e− → CO3= [85]	strontium-doped lanthanum manganite (LSM), La1−XSrXMnO3 [81] ½ O2 + 2e− → O= [85]
Electrolyte/Membrane	Liquid [86] Potassium Hydroxide (KOH), typically 33 wt.% [81]	Solid [86] sheet of electrolyte (Perfluorosulfonic acid) [87]. Solid or quasi-solid membrane. Nafion membrane [81]	Liquid [86] Inorganic acid, concentrated phosphoric acid (H3PO4) (100 wt.%) [81]	Solid [86] Eutectic mixtures of Li–Na carbonate (Li2CO3–Na2CO3) Recently: α-LiAlO2 β-LiAlO2 [81]	Solid [86] Zirconia doped with 8–10 mol.% yttria (YSZ) with a small amount of alumina [81]
Bipolar Plate	titanium, stainless steel, or nickel [88]	Graphite or stainless steel, titanium, aluminium, and several alloys [61,81]	multilayer porous graphitic carbon bonded on either side of a thin, non-porous carbon layer Ribbed bipolar plate [81]	thin sheets of stainless steel [81]	Tubular Design. Doped lanthanum chromite for the interconnect in HT-SOFCs [81]
Water or Steam Produced	Anode [81,89]	Cathode [81]	Cathode [81]	Anode [81]	Anode [81]
Ion Movement	hydroxyl (OH−) [81]	Hydrogen Ion (H+) [81]	Hydrogen Ion (H+) [81]	CO32− [81]	O2− [81]
Operating Temperature (°C)	50–230 [81] 60–70 [86]	30 to 100+ [81] Up to 180 [90] Up to 200 [86]	~220 [81]	600–700 [81]	600–1000 [81]
Heat Quality	Poor but usable for hot water supply [81]	Poor but usable for hot water supply [81]	Usable typically, 150–180 °C [81]	Very good Suitable for CHP [87]	Very good Suitable for CHP [87]
H2 Fuel Purity Requirements	Pure H2, CO2 poison the FC CO >10 ppm [81]	Pure H2 and susceptible to poisoning by sulphur and CO [81], i.e., <10 ppm CO	H2, (low S (H2S and COS) < 50 ppm, poisoned by a few CO ppm, tolerant to CO2) Or other fuel that will need refining or processing [81]	H2 (No S) or Various hydrocarbon fuels O2 and CO2 to be supplied to the cathode to form CO3 pure CO can be used as a fuel [81,83]	Impure H2, (No S), or fuel that will need refining or processing. CO can serve as a fuels’ S tolerance [81,83]
Current Density (A/cm2)	0.2–0.5 [81]	High densities [81] 0.6–2.0 [91]	0.2–0.4 [81]	Typically, 0.16, Pressurised stacks 0.5 [81]	0.2–1.0 [91] 1 at 1000 °C
Operating Pressure (bar)	Ambient–4 bar [91]	Ambient–5 bar [91]	Ambient–8 bar (typically ~1 to 3 bar) [91]	N/A	Ambient (atmospheric pressure) [91]
Stack Efficiency (%)	Up to 70 [81] Around 60 [92]	50–60 [92] 40 reformed fuel [87] 60, and (87 CHP) [86]	>50 (LHV H2) [81] Over 80 [92] (80% CHP) [86]	60–80 [92] 50 [87] (80% CHP) [86]	>50% (LHV) [81] 60–80 [92] 60 [87]
Cell Voltage “open-circuit (no-load) voltage (OCV)” (Volts)	0.7–0.9 [88]	0.6 to 0.8 [93]	0.6–0.75 [81]	0.75–0.8 [81]	0.7–0.9 [91]
Typical Stack Size (kW)	1–100 [87]	1–100 [87] 1 MW [81]	5–400 [87]	300 modular, up to 3 MW [87]	1–2000 [87]
Cold start to Full Load	Quick start-up [87] Reservoir holding the electrolyte solution needs to be heated [81]	Quick start-up and load following [87]	Long start-up time [87]	Long start-up time [87]	Long start-up time and Limited number of shutdowns [87]
Lifetime (K hours)	>50 [88]	>40 [81]	40 [81]	40 [81]	40 [87]
Estimated Cost per kW (USD) [94]	500–1500	1200–3000	3000–5000	3000–6000	2000–5000
State of Development	Commercial [81]	Commercial [81]	Most Commercially Developed [81,85]	Mature/Marketed [81]	Commercial [81]

.

Fuel Cells Emerging Technologies

There are less well-known types of FCs, as they are still under development, such as Direct Borohydride (DBFC) [81], Direct Carbon (DCFC) [81], Biological (Microbial) MFC [81], and Reversible Fuel Cell [83]. These types of FCs are not used in the H2PS yet, while other technologies possess promising potential, such as Anion-Exchange Membrane Fuel Cells (AMFC) [81] and Unitized Regenerative Fuel Cells (URFC) [64].

4.2. Fuel Cell Package

This study focuses on the typical types of FCs found in existing and developing H2PS; therefore, it does not discuss Direct Methanol Fuel Cells (DMFCs) and other liquid-fed FCs because they do not use pure hydrogen directly to regenerate electricity. FCs fundamentally consist of similar construction parts (in general) of the electrolyser’s stack construction and arrangement, i.e., level 1, as shown in Figure 3, Figure 4 and Figure 5 [81,83,95,96]. However, there are some key differences in their BoP at levels 1 and 2 due to the fact that FCs work in opposite directions compared to electrolysers. Individual fuel cells are typically combined in series into a fuel cell stack to increase the single cell voltage from well below 1 V to a sufficient voltage suitable for applications [97]. The FC stacks’ desired produced power and heat depends upon several factors, such as fuel cell type (technology), cell size, the operating temperature, and the pressure of the hydrogen gas supplied to the cell [83]. The FC’s feedstock (fuel) is pure hydrogen gas, but each type of FC operates at a specific pressure and minimum accepted hydrogen purity [81].

Key Differences Between Electrolysers and Fuel Cell Stack Designs

There are some key differences between electrolysers and FCs stack designs and construction due to their different functions, such as the following [81,98]:

(1)

Electrode materials.

(2)

Electrolyte type.

(3)

Operating conditions, such as operating heat and pressure.

(4)

Gas management, i.e., modified, or different geometry.

(5)

Auxiliary and sub-systems, e.g., air instead of water pumping system.

(6)

Cell connections.

These differences are more related to the manufacturers than the H2PS designers, as shown in Table 1 and Table 2. However, the designers should consider the equipment and supporting systems BoP for the FC package to function, i.e., regenerate electricity.

The FC stacks’ BoP requires different auxiliary sub-systems compared to the electrolysers’ packages [81]. In addition, the requirements for the FC package depend on the application; i.e., there are different auxiliary sub-systems for stationary power regeneration compared to the FC in mobility applications [81]. This study focuses on stationary power generation.

The essential auxiliary sub-systems within the FC package depend on the size (capacity) of the FC stack. Figure 9 illustrates an example of PEMFC system (package) integration and interconnections in a single line diagram. In general, the FC package comprises the following (but not limited to) auxiliary and sub-systems:

• Hydrogen clean-up (purification) processors, e.g., carbon dioxide separators and sulphur removal, depend on the produced and/or stored hydrogen quality [17,81].
• Air humidifier unit to prevent membrane dehydration [8,81,83,92].
• Hydrogen pressure regulation (control) skid depends on the system’s storage technology and operating pressure [99,100]. Typically, FCs operate in a low-pressure hydrogen inlet [101]; practically, it is between 8 and 16 bars [102].
• Hydrogen recycling system for re-use of unconsumed hydrogen [81].
• Air delivery units, i.e., air compressor or blower with a control unit [83,92,100].
• Water management, i.e., water removal (prevent flooding), water purge (prevents freeze-out damage if the ambient temperature falls below zero °C), drainage and/or reclaim unit, typically a condenser and water pump [81].
• Electrolyte circulating system for FCs using liquid electrolyte, i.e., AFC, typically a tank and pump with pipes [103].
• Power-conditioning equipment, e.g., DC/DC converter and DC/AC inverter, to comply with the hosting grid connection requirements [81,83,92,100].
• Intercooling system, which depends on the FC capacity, e.g., fans for air cooled or liquid coolant with heat exchange mechanism [90,92]. This required a third channel in the bipolar plate [90].
• Overall control is called Fuel Cell Control Unit (FCCU) [90].
• Human–Machine Interface (HMI) and PLC interface for remote control [104].
• Power supply, typically battery for small-medium scale FCs, to provide power for controllers, valves, DC/DC converter, and pumps on start-up [81,100,104].
• Electrical and Thermal insulation and protection [81]. This is essential for high-temperature FCs [105,106].
• Enclosure ventilation system to prevent hydrogen leakage (if any) from accumulating within the closure [81].
• Venting valves and pipework [100].
• Sensors, safety systems [17], and alarm system [100].
• Electrical interconnecting wires [81].
• Pipework (hydrogen, air and water) [100].
• Operating noise reduction mechanism [107].

The FC integrated system or package can be enclosed in one or more containers. Figure 10 shows some examples of FC packages.

5. Hydrogen Storage

5.1. Technologies

Hydrogen is gaseous at Standard (Room) Temperature and Pressure (STP) with a low volumetric energy density, i.e., a large footprint for low energy density compared to hydrocarbon fuels [17,81]. It is difficult to store in large quantities safely and cost-effectively at the STP. Therefore, different hydrogen storage technologies have been developed, and many other technologies are still being developed [15]. Nevertheless, hydrogen compression and cooling/heating are the main processes required for hydrogen storage technologies, regardless of the type of technology and storage containers [108]. Typically, the storage package comprises two parts: hydrogen process and storage vessels with all required auxiliary systems.

The manufacturing process, parts of the compressors, and thermal reactions within the storage vessel internally (at level 1) are out of the scope of the H2PS engineering design, though they can significantly change the operations and maintenance requirements. The system designers have no role in the level (1) BoP. Therefore, this study focuses on the storage package BoP at levels (2) and (3).

The choice of storage technology depends on land availability (footprint), safety, and cost-effectiveness. These different hydrogen storage technologies can be categorised into three main categories, namely, physical-based, material-based, and chemical-based (hydrogen carrier), as illustrated with some selected examples in Figure 11.

5.2. Physical-Based Storage Technologies: BoP

Compression and cooling are the two main process requirements for all physical-based hydrogen storage, regardless of the type of technology and storage container shape, size, and make [17]. The physical-based technologies mean pure hydrogen is stored in its pure form at any matter phase without chemically reacting or binding with any other substance [108].

There are three well-known technologies to store pure hydrogen physically, which are defined by the pressure–temperature operating regimes, as follows [109]:

• Hydrogen Compressed Gas (H2CG) operates at high pressures, as high as 70 MPa, and near ambient temperature.
• Hydrogen Cryo-compressed Gas (H2CcG) typically operates at around and above 350 bars and temperatures less than −120 °C (150 K).
• Liquid hydrogen (also called Cryogenic Liquid) operates at low pressures, i.e., typically less than 6 bars (<0.6 MPa) and low temperatures near the normal boiling point of the hydrogen, i.e., −253 °C (20 K).

Figure 12 shows the hydrogen pressure–temperature operating regimes, phase of matter and density at different temperatures and pressures.

The BoP for the physical-based hydrogen storage technologies at level 2 (storage packages) depends on the compression type, cryogenic cooling system, storage container type, and operating parameters. This section discusses the common parts, equipment, and sub-systems for the well-known physical-based hydrogen storage technologies at level 2, i.e., packages.

The underground hydrogen storage in salt caverns, exhausted oil and gas fields or aquifers is acquiring attention as a physical-based pathway using H2-CG technologies [6]. However, it is geographically challenged; therefore, it is out of the scope of this study in the context of H2PS-BoP.

5.2.1. Compressed Gas Package

The Hydrogen Compressed Gas (H2CG) package comprises two main parts: the compressor unit and the storage vessel (typically low to 1000 bar pressure cylinders). Depending on the size of the H2PS (i.e., stored hydrogen mass), the compressor unit may come in one or two enclosures. In contrast, the multi-cylinder orientation arrangement determines the footprint of the vessel. The H2CG package BoP typically comprises (but is not limited to) the following:

• The compressor unit (package) type depends on the required pressure, mass flow rate, and technology’s techno-economic choice [110,111]. The compressor is the main part of H2CG technology, which operates as an intermediate system between the production and the storage vessel. Typically, the compressor unit is delivered from the OEM as a fully functioning unit packaged in one or more enclosures, which includes the following [25]:

  (a)

  Power supply switchboard (depends on the compressor size).

  (b)

  Bypass and pressure relief valves (manual or auto-actuated).

  (c)

  Hydrogen and purge gas venting system.

  (d)

  Control system and/or PLC unit (depending on the size of the system and its integration into the holistic H2PS requirements) for operations and remote control, if required.

  (e)

  Safety systems, including hydrogen detectors, shut-off (stop) functions, and alarm systems, are typically automatic.

  (f)

  Compressed air system and pipework if valves are air-actuated.

  (g)

  Compressor cooling system for internal and compressed hydrogen outlet cooling. Cooling the outlet compressed hydrogen gas depends on the mass flow rate and the operating temperature of the storage vessel.

  (h)

  Hydrogen pipelines.

Note: Using high-pressure electrolysers can eliminate the need for the compressor unit unless more compression is required.

2.

Storage vessels are typically cylinders where the compressed hydrogen is stored within. The storage vessels system, in general, includes the following:

(a)

Storage vessel, where its type depends on the operation pressure, footprint, application, and cost-effectiveness.

(b)

Check and shut off (isolation) valves, safety raptures, inlet/outlet valves, and pipelines.

(c)

Ventilation system in the case of enclosed storage vessels.

(d)

Control system for operations and remote control (if required).

(e)

Safety systems, including leak detectors, isolation valves, and alarm systems.

(f)

Compressed air system and pipelines (if valves are air-actuated).

(g)

Venting system for hydrogen and purging gas.

5.2.2. Cryogenic Compressed Gas and Liquifying Package

Hydrogen gas can be compressed with cryogenic cooling to be less energy-intensive than liquified hydrogen and have more volumetric density than H2CG [17]. Hydrogen Cryogenic Compressed Gas (H2CcG) at high pressure, i.e., above 150 bars (typically 350 bars [109]) and temperature between the boiling temperature (−253 °C) and typically less than −124 °C [109], can have a higher density than liquid hydrogen, as shown in Figure 12 [17]. In addition, cryogenic operations at a low temperature, as low as −196.15 °C (77 K), maximise hydrogen absorption when an electrochemical compressor is used, resulting from the possibility to compress hydrogen to a high pressure of up to 700 bar in a single step [69]. Nonetheless, a notable advantage of H2CcG over Liquid Hydrogen (LH2) is that it requires less energy to produce, i.e., the theoretical compression to 200 bars and cooling work to −193 °C (80 K) is approximately 10 MJ/kg, which is similar to the work required to compress hydrogen to roughly 500 bars compared to 22–50 MJ/kg of the liquefaction work [109].

On the other hand, cryogenic (refrigeration) below −252.9 °C (usually rounded to −253 °C) [112], even with low pressure (as low as 6 bars), hydrogen becomes Liquid Hydrogen (LH2) [109], as shown in Figure 12.

In contrast, typical H2CcG or LH2 system BoP comprises the same main parts of the H2CG system, with the following differences:

(a)

Cryogenic refrigerator is the heart of these technologies, typically one of the technologies, depending on the scale (hydrogen mass) and the system’s cost-effectiveness [109,113].

(b)

Cryogenic and/or liquid hydrogen containers (vessels), typically made of stainless steel and aluminium [114], are specifically designed to reduce heat leakage by thermal insulation. The H2CcG and LH2 vessels must be specifically designed to handle extremely low temperatures [115]. Specifically, LH2 tanks are expected to be built at larger (capacity) units compared to H2-CG tanks, as they do not handle high pressure [116]. Additionally, in LH2 plants, liquid hydrogen boiloff management system is crucial to reduce the heat transfer (losses) from the surroundings when hydrogen is stored for a long time [17,116].

(c)

Vaporisers are used to vaporise LH2 when hydrogen is needed [17]. Vaporisers use heat to convert liquid hydrogen back into its gaseous state. Several types of vaporisers exist, including ambient air vaporisers, steam-heated vaporisers, electric vaporisers, and microwave-induced plasma [117].

(d)

A hydrogen boiloff venting system is crucial, and safety concerns must be addressed as vented cold hydrogen is denser than ambient air and may fall back to the plant grounds [118].

(e)

Cryogenic pumps transfer LH2 from the vessel to the vaporiser and/or the buffer tanks [103].

Cryogenic-suitable materials of any type in contact with LH2 or H2CcG, such as package’s sub-systems, auxiliary, valves, and pipelines, shall be embrittlement-resistant and capable of withstanding extremely low temperatures, i.e., sufficiently flexible to provide for the effect of expansion and contraction due to temperature changes [110,114,119].

5.3. Material-Based Storage Technologies: BoP

Material-based hydrogen storage technologies have the potential for high volumetric density at lower pressure (close to ambient conditions), significantly improving safety [120]. Materials chemistry, structure, and properties have attracted the interest of many leading researchers as a promising, safe, and high-density technology to store hydrogen [121]. The choice of the material to store hydrogen depends on the required storage capacity, operating conditions, safety, and cost-effectiveness. Thermal management is at the heart of material-based technologies that control the hydrogen loading and release rates at desired levels [122]; some examples of hydrogen storage materials are shown in Figure 13.

The BoP in the context of a material-based hydrogen storage package in use within the H2PS is significantly different compared to physical-based storage pathways [122]. These technologies work on low pressure, which may eliminate the need for a compressor unit. Also, the hydrogen uptake and control of release are managed via a thermal management system [123]. A typical material-based storage package consists of the following:

• A thermal management system controls the hydrogen uptake (hydrogenation) and the release at the required flow rate [92]. The uptake reaction is exothermic; i.e., heat has to be removed while heat is required to enable the endothermic decomposition of the hydride (dehydrogenation) [123]. Therefore, the released hydrogen must be cooled and compressed to the FC inlet-specific operation pressure [124]. The required release heat, typically, can be obtained from an external source or utilise the heat releases from the regenerating package, i.e., the FC [123]. The external heat energy can be obtained from burning a portion of the released hydrogen to control the release flow rate [124], which adds complexity to the package. The H2PS-associated battery may be used for the startup of the release process. Also, the package includes a cooling and/or cryogenic refrigerator for the intake process and control [125].
• The material-specific storage vessel type and specific design depend on the type of material in use, storage size, operating pressure, and temperature range.
• Typically, the package requires moisture sensors in addition to the H₂ and O₂ sensors [124].
• A hazardous waste-materials disposal management system depends on the type of storage material and the system operation conditions [85].
• A low-pressure compressor might be required (depending on the storage system H₂ release pressure) to pressure the released H₂ to the FC inlet operating pressure [124].
• Power supply switchboard (depends on the compressor size and the thermal management technology).
• Pressure relief valves (typically auto-actuated).
• Hydrogen and purge gas (typically nitrogen) venting system.
• Control system and/or PLC unit (depending on the system’s size and integration requirements into the H2PS) for operations and remote control (if required).
• Safety systems include a shut-off (stop) function and an alarm system, typically automatic.
• Compressed air system and pipelines (if valves are air-actuated).
• Compressor cooling system for internal cooling and the released H₂ cooling after compression.

5.4. Chemical-Based (Hydrogen Carriers): BoP

Hydrogen storage by chemically binding with other materials is a promising pathway to efficiently and cost-effectively ease the H₂ storage difficulties [124]. However, this pathway is impractical for H2PS when hydrogen is produced on-site, as converting hydrogen to synthetic hydrocarbon fuels, LOHC, or non-organic hydrogen carriers adds significant complexity to the system [104]. Therefore, this H₂ storage pathway is outside the scope of this study.

6. Holistic H2PS Systems’ BoP

The literature review reveals many research studies, industry knowledge-sharing reports, and OEM datasheets related to the H2PS parts and equipment. However, it is rare to find publications related to the BoP of a holistic H2PS. Therefore, this study depends on the authors’ experience, personal communications, and publicly shared information from the implemented H2PS worldwide [11,13,14,45,46,47,48,49,50,51,52,53,54,55,56,57,126,127].

Typically, H2PSs comprise three main components or packages: hydrogen production, storage, and power regeneration. Each package relies on additional auxiliary systems and sub-systems to operate effectively. These packages are integrated and interconnected to make a holistic H2PS. The holistic H2PS must meet the integration requirements of the hosting power system or the system it serves, both at the input and output. Figure 14 visually represents how the typical packages in an H2PS at level 3 (the holistic system) are integrated with the supporting auxiliary and sub-systems.

The hydrogen packages are typically fully functional devices, incorporating various supporting auxiliary, sub-systems, and peripherals from original equipment manufacturers (OEMs). These peripherals can include control, sensors, safety devices, and essential equipment for package functionality. The peripherals included may differ based on the intended use of the package, the type of technology employed, and the processes followed by the OEM company. The previous sections have covered the balance of plant (BoP) for each package at levels 1 and 2, considering the equipment type and technology used. The subsequent sub-sections will discuss the BoP of the holistic H2PS at level 3 from an EDO perspective.

6.1. Non-Hydrogen Auxiliary and Sub-Systems

Typical H2PS, as shown in Figure 14, requires many non-hydrogen systems to function effectively and operate safely. The minimum (i.e., but not limited to) requirements are as follows:

• Renewable Energy (RE) power generation plants, such as solar PV, wind turbines, oceanic energy, etc., depend on the area’s RE resources availability, whilst the plant size is related to the scale of the H2PS.
• Water supply, storage, and purification systems and related pipework, whereby complexity depends on the H2PS scale and water sources, i.e., seawater, underground, or potable water.
• The cooling system and its related pipework depend on the H2PS scale and packages’ technologies, i.e., the cooling requirements.
• The compressed air system depends on the H2PS scale and the pneumatic devices in use.
• Non-product stream treatment and disposal include brine water, coolant, lubricants, and any other waste related to specific technologies in use. The treatment and disposal process shall follow environmental codes and regulations at all levels.
• Point of connection switchboard and electrical connections to and from the H2PS I/O terminals, i.e., cables, trays, converters, inverters, and protection systems. The electrical equipment shall be compatible with an explosive atmosphere requirement, depending on the safe distance from the hydrogen environment.
• Electrical buffer system to manage the load rejection and shedding, as well as the start-ups and shutdowns of the H2PS equipment. These buffers are typically batteries and/or supercapacitor banks. The buffer system’s size depends on the scale of the H2PS and the hosting or served power system size and orientation.
• Fire detection and suppression systems associated with the holistic H2PS.
• Civil work and safeguards include concrete foundations, footpaths, heavy machinery access, barriers, and fences. Note: Civil works are out of the scope of this study.

The non-hydrogen systems are typically designed to follow the control philosophy and align with the holistic H2PS control system integration requirements and operational communications protocols, such as connections to plant PLC.

6.2. Interconnections and Interoperability

The holistic H2PS typically acts as an energy storage system consisting of three main packages and many other sub-systems amalgamated to function as one device, like a battery bank functionality. The holistic system packages and subsystems’ interconnections and interoperability are the main challenges for system engineering designers. These challenges depend on the package’s technology type, which determines the type of equipment connecting any two packages and the integration into the holistic H2PS. The packages’ mutual interdependence is discussed in the following sections.

6.2.1. Production-Storage Packages’ Interconnection

The interconnection between the production (electrolyser(s)) and the storage packages depends on the electrolyser outlet pressure and flow rate (production rate) and the specific inlet operation pressure and control philosophy of the storage package. The required interconnections and interoperability equipment can be summarised (but are not limited to) as follows:

(1)

Hydrogen transfer pipework with proper mechanical support and safety valves.

(2)

The hydrogen buffer (bladder) tank depends on the storage package inlet operation pressure and control philosophy.

(3)

Control, sensors, and communication commands (PLCs) wiring with proper mechanical support.

(4)

The cooling system that is typically part of the holistic H2PS cooling system but can be part of one or both supplied packages.

(5)

A heat recovery system for CHP can be used in thermal management when material-based or cryogenic storage technology is used.

(6)

A battery bank or buffer unit can be included in the electrolyser package or the holistic H2PS.

6.2.2. Storage-Regenerative (FC) Packages’ Interconnection

The integration and interconnection of the storage and the regenerative (FC) packages depend on the storage outlet pressure and the FC package-specific inlet operation pressure and operations philosophy. The required interconnections and interoperability equipment can be summarised (but are not limited to) as follows:

(1)

Hydrogen transfer pipework with proper mechanical support and safety valves.

(2)

Hydrogen pressure reduction skid when high-pressure storage technology is in use or a low-pressure compressor when using low-pressure material-based or LH2 storage technology. The pressure regulation depends on the FC package inlet operating pressure and the control philosophy.

(3)

Control, sensors, and communication commands (PLCs) wiring with proper mechanical support.

(4)

The cooling system that is typically part of the holistic H2PS cooling system but can be part of one or both supplied packages.

(5)

Heat recovery system for CHP or to control the hydrogen release flow rate when using material-based or LH2 storage technology.

(6)

Reclaimed water and transferred from the FC to the water supply system, which depends on the FC technology, operating temperature, and water scarcity.

(7)

The DC/DC converter unit can be included in the FC package or associated with the external DC/AC inverter, which is the gateway to the point of connection.

(8)

The auxiliary power supply, typically a battery bank, can be included in the FC package or the holistic H2PS. Also, it could be an external power supply.

6.3. Off-the-Shelf Engineered-Packaged Systems

The literature reveals that many companies are offering pre-engineered assembly from different OEM companies and packaged as off-the-shelf products (H2PS), which are ready to be installed with fewer engineering design requirements. These systems may be provided in one or more enclosures with all the necessary operations and safety measures [128,129]. However, these systems may require external sub-systems and/or power regulation or inversion to match the site point of connection requirements. Typically, these systems comprise the same three main packages and the sub-systems of the case-specific-designed H2SP [128]. In addition, some OEM companies offer case-specific custom-built systems that may require the system engineers or the owner’s engineer contractors’ involvement [129]. Figure 15 shows two selected examples of engineered H2PS.

6.4. Technologies’ Selection: Cause and Effect

The hydrogen equipment technologies at a high level are typically decided during the preliminary stages, i.e., in the feasibility study stage. This requires informed decisions on the hydrogen equipment specifications, interconnection, and integration requirements. This study presents engineering design guidelines for the technologies’ selection and the mutual effect of the technologies’ options on holistic H2PS BoP integration and functionality, i.e., at level 3. The mutual impact is presented as a cause-and-effect matrix, as shown in

Table 3. Technology selection and specifications’ parameters cause and interdependent effect matrix.

Technology Selection Interdependent Cause & Effect Matrix Legend “√” = Can Have an Effect		Effect	Effect on the H2PS Packages
			Grid-Connected or Stand-Alone	Site Location/Constraints	New or Replacing Existing PS	Stand-Alone or Part of a System	RESS and/or Grid Stabilising	Electrolyser Technology	Storage Technology	FC Technology	Type of RE Generation	H2PS Supporting BESS	Operating Pressure	Operating Temperature	Cooling System	Water Treatment System	Hydrogen Purity	Hydrogen Compression	Hydrogen Pressure Reduction	Thermal Management System	Buffer Unit	Holistic System Response Time	Cryogenic System	Point of Connection	Holistic H2PS Safe Operations	Hazardous and Exclusion Zones	Venting and Ventilation	Holistic H2PS Footprint	Environmental Controls
Selected Technology (Cause)
Electrolyser(s)	Technology Option			√			√		√		√	√	√	√	√		√	√		√	√	√			√	√	√		√
	Production Capacity/Size in kW		√	√		√	√	√	√		√	√	√	√	√		√	√		√	√	√	√	√	√	√	√		√
	Response Time (Ramp Up/Down)		√			√	√	√			√	√
	Control/Operation Philosophy		√		√	√	√	√	√		√	√			√			√		√	√			√	√	√	√
	Housing (Outdoor/Containerised)			√				√							√					√				√	√	√	√	√
Storage	Technology Option			√			√	√	√	√			√	√	√		√	√	√	√	√	√	√		√	√	√	√	√
	Storage (H2 Mass) Capacity		√			√	√		√		√	√	√		√			√		√			√		√	√	√	√	√
	Control/Operation Philosophy		√			√	√	√	√	√			√	√	√			√	√	√		√	√		√		√
	Housing (Outdoor/Containerised)			√					√						√					√			√		√	√	√	√	√
Fuel Cell (FC)	Technology Option		√				√			√		√	√		√	√	√	√	√	√	√	√		√	√	√	√	√	√
	Regenerating Capacity/Size in kW			√		√	√		√	√		√	√	√	√	√		√	√	√	√	√		√	√	√	√	√	√
	Load Following/Steady Generating		√		√	√	√		√	√		√			√	√		√	√	√	√	√			√		√
	Response Time (Ramp Up/Down)		√		√	√	√		√	√		√						√	√	√	√	√
	Control/Operation Philosophy		√	√	√	√	√	√	√	√	√	√			√	√		√	√	√	√						√
	Housing (Outdoor/Containerised)			√						√															√	√	√	√
	Water Reclaiming			√											√	√				√									√

.

7. Conclusions and Way Forward

This study aimed to fill the literature gap concerning the Hydrogen-enabled Power Systems (H2PS) equipment, devices, auxiliary, and sub-systems Balance of Plant (BoP). The H2PS-BoP was researched regarding publicly available information such as manufacturers’ specifications and datasheets, the literature, authors’ experience, and personal communications across three critical levels. The first (level 1) encompasses the manufacturing (construction) BoP of hydrogen cells and stacks with their related devices regarding different technologies. This level was discussed briefly as it is not directly related to the engineering design of the H2PS projects. The packages (level 2) comprising the hydrogen stacks and devices, along with their auxiliary and sub-systems delivered as ready-to-use packages, were thoroughly discussed. The holistic H2PS (level 3) addressing the interconnection and interoperability BoP requirements were discussed from an engineering design perspective. This comprehensive study revealed that many common auxiliary and sub-systems are typically included in the OEM-delivered packages. However, many others vary depending on the choice of technology, which significantly affects other packages and the holistic H2PS BoP. The variation in technology choices’ effect on other packages was presented as a cause-and-effect matrix. This matrix demonstrated the expected variations in the BoP at the packages’ level 2 and the holistic H2PS level 3.

The findings of this study establish engineering design and operations guidance and accelerate the feasibility studies and implementations of H2PS worldwide. The list of equipment, devices, and sub-systems required to operate H2PS in this study introduced a database of existing and developing technologies, OEM packages, and system integration requirements. Nevertheless, expanding this study to other hydrogen systems like fuelling stations, mobility, transport, and export, and hydrogen for green industries, is vital to advance the Hydrogen Economy.