Meiling Yue
email: meiling.yue@femto-st.fr

Hugo Lambert

Elodie Pahon

Robin Roche

Samir Jemei

Daniel Hissel

Hydrogen Energy Systems: A Critical Review of Technologies, Applications, Trends and Challenges

Keywords:

The global energy transition toward a carbon neutral society requires a profound transformation of electricity generation and consumption, as well as of electric power systems. Hydrogen has an important potential to accelerate the process of scaling up clean and renewable energy, however its integration in power systems remains little studied. This paper reviews the current progress and outlook of hydrogen technologies and their application in power systems for hydrogen production, re-electrification and storage. The characteristics of electrolysers and fuel cells are demonstrated with experimental data and the deployments of hydrogen for energy storage, power-to-gas, co-and tri-generation and transportation are investigated using examples from worldwide projects. The current techno-economic status of these technologies and applications is presented, in which cost, efficiency and durability are identified as the main critical aspects. This is also confirmed by the results of a statistical analysis of the literature. Finally, conclusions show that continuous efforts on performance improvements, scale ramp-up, technical prospects and political support are required to enable a cost-competitive hydrogen economy.

Introduction

Although a considerable part of the global energy demand is currently served by fossil fuels, the harmful impacts of the combustion of fossil fuels are unignorable: greenhouse gas, acid rain, etc., which are devastating to the environment and human beings. To this end, global energy transformation is gaining momentum, which is accelerated by the rapid development of using renewable energy. To enhance this momentum and to mitigate emissions, hydrogen has been explored as a substitute energy carrier, while generating electricity from hydrogen using a fuel cell causes no local pollution because the only byproduct is pure water. Another advantage of hydrogen lies in its high specific energy density. It can provide three times more energy than gasoline combustion per unit mass [START_REF] Nicoletti | A technical and environmental comparison between hydrogen and some fossil fuels[END_REF]. Also, hydrogen can be locally produced, which reduces countries' dependence on external energy suppliers. Besides, hydrogen can be extracted from an extensive range of substances, such as water, oil, gas, biofuels, sewage sludge, etc [START_REF] Revankar | Chapter four -nuclear hydrogen production[END_REF]. In particular, the abundance of water on earth assures the production of hydrogen in a rather sustainable way. Splitting water by electrolysis offers promising opportunities for synergy with the renewable energy. The hydrogen can be produced before it is used due to the intermittent nature of some renewable energy resources so that it is suitable for distributed production and centralized production connected directly to the remote renewable resources. The hydrogen produced from an electrolyser is perfect for use with fuel cells. Stationary fuel cell technologies also facilitate the development of distributed power backup, stand-alone power plants and co-generation. It provides a substituted option of the traditional power grid because combined with a fuel cell, the electricity can be produced when and where it is needed so that the hydrogen does not necessarily to be stored.

Advances of integrating hydrogen in power systems have been gradually made in recent years ranging from production and storage to re-electrification and safety issues. Extensive descriptions of the existing progress can be found elsewhere and a number of studies are seeking to characterize the current progress in hydrogen system integration by novel methods [START_REF] Dincer | Review and evaluation of hydrogen production methods for better sustainability[END_REF]. A wide consensus has been reached that producing hydrogen from renewable energy sources (solar, wind, etc.) shows great promise for the world's sustainable development [START_REF] Chaubey | A review on development of industrial processes and emerging techniques for production of hydrogen from renewable and sustainable sources[END_REF]. Chi et al. have pointed out that changing the hydrogen production by using renewable electricity can enhance the interconversion of electricity and hydrogen and expand the hydrogen application [START_REF] Chi | Water electrolysis based on renewable energy for hydrogen production[END_REF]. Numerous researches on renewable hydrogen production technologies were launched and have generated great interest [START_REF] Rashid | Hydrogen production by water electrolysis: A review of alkaline water electrolysis, pem water electrolysis and high temperature water electrolysis[END_REF]. Producing hydrogen from renewables using photocatalysis have been reviewed in [START_REF] Ahmad | Hydrogen from photo-catalytic water splitting process: A review[END_REF] and [START_REF] Connelly | Photoreaction of au/tio 2 for hydrogen production from renewables: a review on the synergistic effect between anatase and rutile phases of tio 2[END_REF], in which the solar energy is used for water-splitting. Wang et al. have focused on the intensification technologies on the component level to save the energy consumption in the hydrogen production [START_REF] Wang | The intensification technologies to water electrolysis for hydrogen production -a review[END_REF]. On the system level, several advanced cutting-edge power-to-gas projects have been reviewed and investigated in [START_REF] Gahleitner | Hydrogen from renewable electricity: An international review of power-to-gas pilot plants for stationary applications[END_REF][START_REF] Eveloy | A review of projected power-to-gas deployment scenarios[END_REF]. Electrolysis and methanation status in terms of the cost and the capacity have been reviewed in [START_REF] Thema | Power-to-gas: Electrolysis and methanation status review[END_REF] for power-to-gas applications, while the effects of hydrogen injection on the gas infrastructure and gas quality have been studied in [START_REF]Hydrogen integration in power-to-gas networks[END_REF]. As most power-to-gas plants are located next to remote renewable energy sources, it requires the produced hydrogen to be stored and then fed to the gas distribution system, therefore, researches have been launched to improve the hydrogen storage capability [START_REF] Dutta | A review on production, storage of hydrogen and its utilization as an energy resource[END_REF]. Abe et al. pointed out that the current hydrogen storage technologies had not fully satisfied the techno-economic feasibility and further investigations on solid hydrogen storage are demanding [START_REF] Abe | Hydrogen energy, economy and storage: Review and recommendation[END_REF]. The security issue of hydrogen storage and delivery were studied in [START_REF] Moradi | Hydrogen storage and delivery: Review of the state of the art technologies and risk and reliability analysis[END_REF], as well as the reliability of the presently available technologies. A detailed review of utilising hydrides for hydrogen storage in stationary and transportation applications is found in [START_REF] Von Colbe | Application of hydrides in hydrogen storage and compression: Achievements, outlook and perspectives[END_REF].

It is noticed that recent reviews have stated the importance of integrating hydrogen in power systems, however, they tend to focus on specific hydrogen technologies. Some reviews have acknowledged the undertaking of hydrogen in various power systems. For example, Mazloomi et al. [START_REF] Mazloomi | Hydrogen as an energy carrier: Prospects and challenges[END_REF] have discussed the prospects and challenges of hydrogen as an energy carrier while explaining current hydrogen production technologies and cost potentials, while discussions on the system level are lacking. Hanley et al. [START_REF] Hanley | The role of hydrogen in low carbon energy futures-a review of existing perspectives[END_REF] have analysed the role of hydrogen in the future economy based on eight existing hydrogen powered system models. The prospects of hydrogen penetration and decarbonization are stated, however, key hydrogen technologies and the current progress of developing hydrogen technologies have not been fully addressed. Parra et al. [START_REF] Parra | A review on the role, cost and value of hydrogen energy systems for deep decarbonisation[END_REF] have analysed the current progress of hydrogen energy system from the points-of-view of cost, products, applications and control strategies, but lack detailed insights into the current development status of the hydrogen technologies and their evolution. Besides, challenges in terms of technological and social aspects are not discussed, while integrating hydrogen in power systems requires significant investment and intensive coordination in both research and social aspects.

This paper is devoted to treating hydrogen powered energy systems as a whole and analysing the role of hydrogen in the energy systems. As hydrogen has become an important intermediary for the energy transition and it can be produced from renewable energy sources, re-electrified to provide electricity and heat, as well as stored for future use, key technologies including water electrolysis, fuel cells, hydrogen storage and their system structures are introduced in this paper, in which the characteristics are described by the corresponding models and experimental results. It provides general explanations for readers who are not or partly engaged in different hydrogen technology fields. Moreover, four principle hydrogen integrated applications including energy storage, power-to-gas applications, co-and tri-generation and transportation are introduced and interpreted by remarkable projects. Current status on hydrogen applications is analysed statistically in terms of cost, consumption, efficiency and durability, which justifies the need of further progress in the related technologies. The current status is also illustrated by the level of research based on the literature survey across the time. Last but not least, this paper critically discusses the perspectives of developing hydrogen integrated energy systems in terms of performance, ramping-up scales, technical perspectives and social and political implications.

Following sections of this paper are arranged as follows: Section 2 presents the dominant technologies in hydrogen production, re-electrification and storage and their principles. Section 3 introduces the four major applications of hydrogen-integrated power systems. The current status in terms of cost, consumption, efficiency and durability are analysed in Section 4. A detailed publication survey is described in Section 5 and Section 6 presents the insights and prospects of developing hydrogen power system technologies before concluding.

Hydrogen technologies

Some hydrogen technologies that are typically used in hydrogen power systems are introduced in this section. They include electrolytic hydrogen production, hydrogen re-electrification using fuel cell, hydrogen storage and converter technologies. The characteristics of these technologies are presented and demonstrated by some experimental results.

Electrolytic hydrogen production

Hydrogen production methods like steam reforming, coal gasification and electrolysis of water are majorly used today for industrial hydrogen production. Other hydrogen production methods like reforming ethanol and sugars, water biophotolysis, photochemical water splitting and high-temperature water splitting are still in the stage of development and are rarely industrially deployed. Today, with the declining cost for renewable electricity, there is a growing interest in water electrolytic hydrogen production, which consumes electricity to extract hydrogen from water while causing no carbon byproducts like CO 2 .

Water electrolysis principle

In a water electrolysis cell, two electrodes are put in the electrolyte solution and are connected to the power supply to conduct current, as shown in Figure 1. When a sufficiently high voltage is applied between the electrodes, water is decomposed to produce hydrogen on the cathode and oxygen on the anode. The addition of an electrolyte raises the conductivity of the water, which facilitates the continuous flow of electricity. Acids and solid polymer electrolytes are commonly used in water electrolysis, and use different ions as charge carriers: + , -, 2-, etc. The reactions of water electrolysis at the electrodes with different charge carriers may be different, but the overall reaction is always the same:

2 2 + electricity + heat → 2 2 + 2 (1)

Electrolyte Solution

Anode Cathode

e - e - (Water)
O 2 H 2 H 2 O 2 out H 2 out

Gas separator

Figure 1: Water electrolysis principle: Two electrodes are placed in the electrolyte solution, which are connected to the power supply to conduct current. Water is decomposed into pure hydrogen and oxygen gas, appearing at the cathode and the anode, respectively.

Water electrolysis technologies

Three principal types of electrolysers and their characteristics are summarised in Table 1. AELs have the dominant place in today's electrolyser market and have been developing for many years. SOELs can split water at very high temperatures and they do not need as much electricity as that of other types of electrolysers. The efficiency of SOELs is, therefore, high. PEMELs are quickly gaining momentum in recent years and large PEMELs are now being commercialized. They can benefit from their low gas permeability, high proton conductivity, thin proton exchange membranes, and good compactness. Moreover, PEMELs can be operated with high efficiency at high power density, fast response, relatively low operating temperature, small footprint and balance of plant (BOP) simplicity [START_REF] Kumar | Hydrogen production by pem water electrolysis -a review[END_REF].

Electrolyser system structure

The electrolyser stack consists of several individual cells, which are connected in series so that it can reach a rather high voltage even if each individual cell has a low voltage (approximately 2 V). Moreover, electrolyser systems with stacks connected in parallel are able to reach a multi-MW scale with relatively low voltage (up to a few kV) given a high current density [START_REF] Matute | Techno-economic modelling of water electrolysers in the range of several mw to provide grid services while generating hydrogen for different applications: A case study in spain applied to mobility with fcevs[END_REF]. To make the system operational, a power supply unit and the BOP are also necessary, as shown in Figure 2. The produced gases enter into gas separators where they are separated from water and then being purified and dried. A pump is used to pump the electrolyte through the electrolyser cells and the heat exchanger is used to reach the operating temperature [START_REF] Brauns | Alkaline water electrolysis powered by renewable energy: A review[END_REF]. A transformer and a rectifier in the power supply unit are used to feed direct power into the electrolyser stack.

Electrolyser electrical characteristics

Most commercial electrolysers are current-controlled so that their hydrogen production rate can be fixed at a set current value [START_REF] Abdin | Modelling and simulation of a proton exchange membrane (pem) electrolyser cell[END_REF]. The performance of an electrolyser can be illustrated by its polarization curve, as shown in Figure 3. The measurements come from a 33-cell 5 kW PEMEL with an active area of 700 cm 2 , which is operated under The electrolyser is connected to the power supply through an AC/DC power converter and are connected to the gas-water separators and purification/drying systems to separate out pure hydrogen and oxygen gas. The supplied water is stored in the gas-water separator and is pumped into the electrolyser.

constant pressure and temperature. The experiment was realised at UBFC's FCLAB and FEMTO-ST laboratories in Belfort, France. To quantify the dynamic interactions of the electrolyser, one can model it based on mole balance on the electrodes and derive the voltage equation [START_REF] Görgün | Dynamic modelling of a proton exchange membrane (pem) electrolyzer[END_REF]. Equation (2) shows a commonly used electrolyser model, which is the sum of open circuit voltage 0, , activation overpotential , , ohmic overpotential ℎ , and concentration overpotential , .

, = 0, + , + ℎ , + , (2)
The three overpotential losses caused by electrochemical reaction, ohmic loss and mass transport, respectively, can be modelled using the following model [START_REF] Lebbal | Identification and monitoring of a PEM electrolyser based on dynamical modelling[END_REF]:

, = 0, + 2 ln ⎛ ⎜ ⎜ ⎝ 2 0.5 2 2 ⎞ ⎟ ⎟ ⎠ + + + 2 ln 0 + 2 ln 1 + (3)
where index denotes anode and denotes cathode, is the gas constant, is the operating temperature, is the Faraday constant, is the current density, 0 is the exchange current density, and are the charge transfer coefficients, is the equivalent resistance of the electrodes and the interface, is the resistance of the membrane, 2 and 2 are the oxygen and hydrogen pressure and

2
is the water activity (1 if water is liquid). The determination of the parameters is discussed in [START_REF] Han | Electrochemical performance modeling of a proton exchange membrane electrolyzer cell for hydrogen energy[END_REF]. The model validation results in Figure 3 show that measurements are in good agreement with the model. Figure 4 shows the power and efficiency curves versus the applied current. The stack voltage efficiency can be calculated using (4), which is the ratio between its reversible voltage and operation voltage:

= ⋅ (4) with = -Δ ∕2 (5)
where is the number of cells, Δ is the free enthalpy during the course of the formation of a mole of water at standard pressure, which equals 285.83 kJ/mol. It does not mean that it should operate the electrolyser at low current density to increase the efficiency because the provided power is low at low current density, leading to the high specific cost. As shown in Figure 5, the higher the current, the higher the hydrogen production rate. The theoretical calculation is based on the following equation: where is the current and is the molar volume of hydrogen, which equals to 0.022414 m 3 mol -1 . Therefore, correct current density should be selected with the objective of finding the optimum between minimizing the specific electrolyser cost and ensuring the high efficiency.

Production rate 2 = ⋅ ⋅ ⋅ 3600 2 ⋅ (6)
Figure 5: Hydrogen production rate of a 33-cell 5kW PEMEL with an active area of 700 cm 2 : A higher current defines a higher hydrogen production rate according to [START_REF] Rashid | Hydrogen production by water electrolysis: A review of alkaline water electrolysis, pem water electrolysis and high temperature water electrolysis[END_REF]. The experimental result is well adapted to the theoretical calculation.

The system efficiency of hydrogen power system concerns, on one hand, the stack voltage efficiency of the hydrogen devices, as discussed above and on the other hand, the system efficiency when considering the consumption of all auxiliaries. The efficiency of a water electrolysis system can be represented by the ratio of the high heating value (HHV) of the fuel produced over the electricity used, written as:

= HHV( ℎ ) × produced hydrogen( ) ( Stack input energy( ℎ)
Power supply efficiency ) + Ancillary losses( ℎ) [START_REF] Ahmad | Hydrogen from photo-catalytic water splitting process: A review[END_REF] Figure 6 shows the system electrical efficiency of a 5kW PEMEL system described above. The efficiency of the PEMEL system decreases with increasing current density.

Hydrogen re-electrification

Hydrogen re-electrification refers to the generation of electricity from hydrogen. Hydrogen can first be re-electrified through combustion. Similar to internal combustion engines running on gasoline, some combustion engines or turbines can also run directly on hydrogen. However, hydrogen combustion engines are less efficient than gasoline combustion engines, with a thermodynamic efficiency of around 20%-25% [START_REF] Hosseini | An overview of development and challenges in hydrogen powered vehicles[END_REF]. This is due to the fact that hydrogen has relatively a low volumetric energy density. Besides, when combusting hydrogen, nitrogen oxides are emitted even though no CO 2 is released [START_REF] Yip | A review of hydrogen direct injection for internal combustion engines: Towards carbon-free combustion[END_REF]. Compared to hydrogen combustion engines, using fuel cell is a preferable way to maximize the potential benefits of hydrogen as fuel cells convert the chemical energy of hydrogen into electrical energy directly so that their efficiency can reach 60%-80%, with only water as a byproduct [START_REF] Mekhilef | Comparative study of different fuel cell technologies[END_REF]. Fuel cells are now commercially applied in a variety of stationary and transportation applications.

Fuel cell operation principle

The fuel cell is supplied with hydrogen at the anode where ionization occurs to release electrons and H + ions and it is supplied with air on the cathode where negative ions are produced, as shown in Figure 7. Similar to electrolysers, different fuel cell types differs in their charge transfer directions and charge carriers in the electrolyte so that the water may be produced on both sides. The process is facilitated by using catalyst, which is usually carbon materials coated with platinum. Although different charge carries lead to different reactions on the electrodes, the overall reaction is always the same: 7: Fuel cell operation principle: Hydrogen and oxygen are passed through the anode and the cathode, respectively, and water molecules are produced by combining protons, electrons and oxygen at the cathode.

2 2 + 2 → 2 2 +

Fuel cell technologies

Different types of fuel cell work with different types of electrolytes. Their characteristics are summarized in Table 2. The following focuses on the PEMFC as it can be operated with a rather low temperature and it has gained a lot of interests in both transport and stationary applications.

Table 2

Comparison of different types of fuel cells in their operation temperature, stack voltage efficiency and pros and cons [START_REF] Mekhilef | Comparative study of different fuel cell technologies[END_REF]

Types

Operating temperature

Stack voltage efficiency

Pros and cons

Proton exchange membrane fuel cell (PEMFC)

Fuel cell system structure

A fuel cell system consists of a stack and its auxiliaries including a hydrogen tank, pumps, an air compressor, power electronics, a thermal management system, etc., as shown in Figure 8. A fuel cell can generate 0.6 V to 0.8 V nominal voltage at nominal load [START_REF] Barbir | Fuel cells -exploratory fuel cells | regenerative fuel cells[END_REF], while the stack voltage can be upgraded by increasing the number of cells. Similar to electrolysers, fuel cell stacks can be connected in parallel to increase the output current so as to reach multi-MW scales. Besides, the current can also be raised by increasing the active area of cells. The fuel cell system also has some auxiliaries, for example, the air is supplied by a compressor powered by a motor and the hydrogen is supplied using a pressurized tank where the flow and the pressure of the hydrogen are controlled. In addition, a cooler is used to cool off the compressed air and a humidifier is used to prevent the membrane dehydration [START_REF] Rotondo | Robust fault diagnosis of proton exchange membrane fuel cells using a takagi-sugeno interval observer approach[END_REF]. The generated DC power can be then inverted to AC power and distributed to the grid.

Fuel cell electrical characteristics

Fuel cells can be characterised by the polarization curve, which shows the voltage-current characteristics. The polarization curve of a 5-cell 300W PEMFC stack with an active area of 100 cm 2 is shown in Figure 9, measured at UBFC's FCLAB and FEMTO-ST laboratories in Belfort, France. The polarization curve is modelled as [START_REF] Wang | The intensification technologies to water electrolysis for hydrogen production -a review[END_REF]. Contrary to the model of electrolyser stack, fuel cell stack voltage is modelled as the reversible voltage 0, subtracting activation losses , , ohmic losses ℎ , , concentration losses and crossover losses

+ , . , = 0, - , -ℎ , - + , (9)
A more detailed model may also be used [START_REF] Jouin | Degradations analysis and aging modeling for health assessment and prognostics of pemfc[END_REF]:

, ( ) = 0 - 2 ln + 0, - 4 ln + 0, - -ln 1 - , ( 10
)
where is the hydrogen crossover current, 0, and 0, are the exchange current at the electrodes, is the equivalent ohmic resistance, is an empirical parameter considering the water and gas accumulation effects and , is the limiting current at the cathode [START_REF] Jouin | Degradations analysis and aging modeling for health assessment and prognostics of pemfc[END_REF]. The model validation is fitted with the measurements, as shown in Figure 9.

The stack voltage efficiency of the fuel cell is presented by the ratio of its output power over the energy flux contained in the reactants, calculated by [START_REF] Pera | Electrochemical components[END_REF]:

= ⋅ ̇ 2 ⋅ Δℎ (11)
with

̇ 2 = ⋅ 2 ⋅ ( 12
)
Figure 9: Polarization curve of a 5-cell 300W PEMFC stack with an active area of 100 cm 2 , comparing with the model identification result of ( 9) and [START_REF] Gahleitner | Hydrogen from renewable electricity: An international review of power-to-gas pilot plants for stationary applications[END_REF] where Δℎ is the free enthalpy during the complete combustion of a mole of hydrogen, which equals to 285.83 kJ/mol. Figure 10 shows the power and efficiency curves versus the applied current of the above described PEMFC stack. In fact, the maximum efficiency of the fuel cell is found to be achieved at partial load. Decreasing the current density below its maximum power density value helps to decrease the cell voltage loss and therefore, to increase its efficiency [START_REF] Marnellos | Integration of Hydrogen Energy Technologies in Autonomous Power Systems[END_REF]. Efforts have been made on the system level to make the fuel cell operated in its maximum efficiency region through system control and strategy design [START_REF] Wang | An optimized energy management strategy for fuel cell hybrid power system based on maximum efficiency range identification[END_REF][START_REF] Pirasaci | Non-uniform, multi-stack solid oxide fuel cell (sofc) system design for small system size and high efficiency[END_REF].

Besides, the net efficiency of the fuel cell system is calculated based on the following equation, which is the ratio of produced electricity over the HHV of the consumed hydrogen:

= ( Stack output energy( ℎ)
Power output efficiency ) -Ancillary consumption( ℎ)

HHV( ℎ ) × consumed hydrogen( ) (13)

Hydrogen storage

The high mass-based energy density of hydrogen makes it one of the most promising future fuels. Hydrogen contains 33.33 kWh energy per kilo, compared to 12 kWh of petrol and diesel [START_REF] Handwerker | Comparison of hydrogen powertrains with the battery powered electric vehicle and investigation of small-scale local hydrogen production using renewable energy[END_REF]. However, storing the same amount of hydrogen requires a larger volume. The development of hydrogen storage technologies is, therefore, a fundamental premise for hydrogen powered energy systems. Conventional technologies store the hydrogen as compressed gas and cryogenic liquid, while for large-scale applications, underground storage turns out to be a preferable method. In recent years, solid-state hydrogen storage has seen rapid development and is believed to be the safest hydrogen storage mode. Different technologies of hydrogen storage have been summarized in Figure 11.

Compressed gas

To store more hydrogen a smaller volume, being compressed to high pressure is one of the options. The most common way of storing hydrogen is to compress it into steel gas cylinders under a pressure of up to 700 bar [START_REF] Riis | Hydrogen storage-gaps and priorities[END_REF]. Figure 12 shows the evolution of hydrogen density as a function of pressure. By compressing the hydrogen gas under 700 bar, it reaches a volumetric density of 36 kg/m 3 [START_REF]Hydrogen compressibility at different temperatures and pressures[END_REF]. It can be realised in the state-of-the-art lightweight composite steel high-pressure gas cylinders [START_REF] Makridis | Hydrogen storage and compression[END_REF][START_REF] Dagdougui | Chapter 4 -hydrogen storage and distribution: Implementation scenarios[END_REF]. Compressed hydrogen storage is widely used when transporting hydrogen through hydrogen pipeline and hydrogen tube trailer, however, the capability of transport is largely limited by the weight of the gas cylinder. Lighter materials that can be used to compress hydrogen at high pressures are under development [START_REF] Rödl | Chapter 3 -assessment of selected hydrogen supply chains-factors determining the overall ghg emissions[END_REF]. Another technological issue to be solved may be the heat transfer process during compressing. As the temperature rises inside the tank, composite degradation may take place and cause a severe consequence. Researches on high thermal conductivity materials and structural design have been deployed to improve the heat transfer behaviour [START_REF] Zhang | A review of heat transfer issues in hydrogen storage technologies[END_REF][START_REF] Simonovski | Thermal simulations of a hydrogen storage tank during fast filling[END_REF].

Underground hydrogen storage

Various solutions have been proposed for large-scale hydrogen storage. Except for the buried tanks compressing hydrogen in gas and liquid, hydrogen underground storage solutions, such as aquifers, depleted deposits of natural gas and oil and salt caverns are the principal choices for large-scale hydrogen storage in medium and long term. The first two types are of porous structure and their capacity may be influenced by the geological conditions. Most underground hydrogen storage of the world is in depleted deposits, approximately 75% [START_REF] Tarkowski | Underground hydrogen storage: Characteristics and prospects[END_REF]. In recent years, salt caverns have seen great interests in storing hydrogen gas owing to their stability and imperviousness of their walls of salt caverns. The volume of a salt cavern can range from 100,000 to 1000,000 m 3 working at a maximum pressure of 200 bar [START_REF] Beutel | Salt deposits and gas cavern storage in the uk with a case study of salt exploration from cheshire[END_REF]. However, the development of salt cavern hydrogen storage is limited by some technical aspects where the tightness of , data from [START_REF]Hydrogen compressibility at different temperatures and pressures[END_REF] the boreholes and the transfer capacity of the surface installation are of significant importance. Besides, environmental limitations and sustainable development should also be taken into consideration when making location plans [START_REF] Tarkowski | Underground hydrogen storage: Characteristics and prospects[END_REF].

Liquid hydrogen

As hydrogen can be converted into its liquid form at a low temperature (20-21K) and ambient pressure, liquid hydrogen is another way to store hydrogen in a small volume and the realised volumetric density can reach 70.8 kg/m 3 , which is even a little bit higher than that of solid hydrogen, i.e. 70.6 kg/m 3 [START_REF] Dagdougui | Chapter 4 -hydrogen storage and distribution: Implementation scenarios[END_REF]. However, it is time and energy-consuming to liquefy the hydrogen while about 40% energy is lost during the liquefaction process. At the moment, liquid hydrogen is reserved for special high-tech applications, e.g. space travel, and has not yet been largely commercialized [START_REF] Colozza | Hydrogen Storage for Aircraft Applications Overview[END_REF].

Solid storage

Despite the above-mentioned physical-based hydrogen storage technologies, solid hydrogen is another option that is realised by combining hydrogen with solid materials through absorption and adsorption. Absorption stores the hydrogen directly into the bulk of the material to formulate chemical compounds. Among them, metal hydrides have aroused more and more interest owing to their high hydrogen storage capacity. Palladium, for example, can absorb 900 times its own volume of hydrogen at room temperature and atmospheric pressure. A detailed review of using various metal hydrides materials can be found in [START_REF] Rusman | A review on the current progress of metal hydrides material for solid-state hydrogen storage applications[END_REF] and mathematical studies of sorption/desorption process modelling in metal-hydride systems are detailed in [START_REF] Mohammadshahi | A review of mathematical modelling of metal-hydride systems for hydrogen storage applications[END_REF]. The dynamic system simulation models have been investigated in [START_REF] Raju | System simulation model for high-pressure metal hydride hydrogen storage systems[END_REF] and [START_REF] Zhu | New dynamic modeling of a real embedded metal hydride hydrogen storage system[END_REF], which simulate the high-pressure metal hydride bed and the heat exchange in vehicles. For large-scale development of metal hydrides, efforts have been made towards lowering the cost, optimizing the operation temperature and enhancing the thermal management of the system [START_REF] Moradi | Hydrogen storage and delivery: Review of the state of the art technologies and risk and reliability analysis[END_REF].

Besides, there are also complex hydrides (Mg 2 NiH 4 , LiAlH 4 , NaBH 4 , etc) and chemical hydrides (LiH, NaH, CaH 2 , etc) that store the hydrogen by absorption, however, the lack of reversibility and the complex reactions to extract hydrogen are the main challenges of these methods. Another choice of storing hydrogen is by adsorption, which physically adsorbs hydrogen using porous materials, such as metal-organic frameworks and carbon materials. The advantage of this option is that they can avoid thermal management in the charging and discharging process [START_REF] Moradi | Hydrogen storage and delivery: Review of the state of the art technologies and risk and reliability analysis[END_REF]. However, the physical adsorption hydrogen storage is still far away from large commercialization as the filling time is still under satisfaction when considering the storage capacity [START_REF] Marnellos | Integration of Hydrogen Energy Technologies in Autonomous Power Systems[END_REF].

Converters

In hydrogen powered energy systems, it is the power electronic converters that link the different parts in terms of hydrogen production and utilisation. For example, DC/DC converters are applied to step down the external delivered voltage to the level of the supply voltage of the electrolyser, and to level up the DC voltage of the fuel cell with high voltage gain. In addition, DC/AC rectifiers and inverters are applied when the electrolyser and the fuel cell are connected to the grid [START_REF] Kovacevic | Advanced dc-dc converter for power conditioning in hydrogen fuel cell systems[END_REF][START_REF] Garrigós | 5 kw dc/dc converter for hydrogen generation from photovoltaic sources[END_REF]. A typical structure of a distributed hydrogen powered energy system with power conversion is shown in Figure 13.

DC bus

Figure 13: Hydrogen powered energy system connected by power electronic converters, in which the DC/DC converters are used to lower down the high DC voltage output to meet the requirement of low DC voltage input of the electrolyser and to boost the low variable voltage from the fuel cells to regulate the voltage for grid-connection proposes. The DC-AC converters are used for grid integration.

To ensure the proper operation of hydrogen power systems, the converters must be featured with flexible voltage ratio, high conversion efficiency and low current ripples [START_REF] Guilbert | A stacked interleaved dc-dc buck converter for proton exchange membrane electrolyzer applications: Design and experimental validation[END_REF]. Different topologies of converters have been investigated in stationary power applications [START_REF] Tawfiq | Wind energy conversion system topologies and converters: Comparative review[END_REF][START_REF] Kolli | A review on dc/dc converter architectures for power fuel cell applications[END_REF] and vehicle applications [START_REF] Reddy | Energy sources and multi-input dc-dc converters used in hybrid electric vehicle applications -a review[END_REF]. For example, soft switching techniques for fuel cell applications have been proposed in [START_REF] Saha | Efficient soft-switched boost converter for fuel cell applications[END_REF]. It has achieved 95% boost converter efficiency at full fuel cell load. A comparative study in [START_REF] Guilbert | Dc/dc converter topologies for electrolyzers: State-of-the-art and remaining key issues[END_REF] has found out that other converter topology, such as interleaved half-bridge DC/DC buck converters, can effectively improve the converter performance in electrolyser applications in terms of cost, efficiency and reliability. Studies also found that compared with conventional single-phase converters, multi-phase interleaved converters can reduce the switching losses and mitigate the current ripples as they can share the current between different legs of the converter [START_REF] Naik | Analysis of ripple current, power losses and high efficiency of dc-dc converters for fuel cell power generating systems[END_REF]. Besides, isolated topology is believed to be a favourable way to maximize efficiency and reduce the cost, whereas the input of the converter is electrically insulated to the output. The voltage can be easily stepped up and down through an optimally sized transformer without losing energy efficiency [START_REF] Guilbert | Dc/dc converter topologies for electrolyzers: State-of-the-art and remaining key issues[END_REF]. However, further efforts in fault-tolerant conversion should be made to eliminate the power switch failures, i.e. open-circuit failures, short-circuit failures, etc. while at the same time, enhance the voltage ratio and ensure the reliability of the system.

Applications in power systems

As hydrogen plays an important role in various applications to store and transfer energy, in this section, four typical applications of integrating hydrogen into power systems are introduced and demonstrated with example projects: energy storage, power-to-gas system, fuel cell co-and tri-generation and vehicular applications.

Energy storage

Considering the high storage capacity of hydrogen, hydrogen-based energy storage has been gaining momentum in recent years. It can satisfy energy storage needs in a large time-scale range varying from short-term system frequency control to medium and long-term (seasonal) energy supply and demand balance [START_REF] Parra | A review on the role, cost and value of hydrogen energy systems for deep decarbonisation[END_REF].

Medium to long-term energy storage

The recent years have seen rapid growth in renewable energy generation. However, the intermittent nature of some renewable energy resources makes them time and season-dependent. Therefore, the generated renewable energy needs to be stored in a reliable form, which should be tolerant to the fluctuation and randomness of those renewable energy sources. There are several existing energy storage options, e.g., pumped hydro energy storage, compressed air energy storage, batteries, etc [START_REF] Al Shaqsi | Review of energy storage services, applications, limitations, and benefits[END_REF]. Compared with them, hydrogen has its advantages of high energy storage capacity, long storing period and flexibility. It can smooth out the energy volatility and uncertainty and absorb, especially, the excess renewable energy generation [START_REF] Mehrjerdi | Daily-seasonal operation in net-zero energy building powered by hybrid renewable energies and hydrogen storage systems[END_REF]. It can be applied to deal with:

• Energy time shift: Hydrogen is used to equilibrate the demand and supply by storing the excess of the energy generated by renewables when the supply is larger than demand and when it is needed, the hydrogen can be used for power generation or grid injection through, for example, stationary fuel cells. In particular, the energy generated during low demand and low electricity price period tends to be stored in hydrogen to lower the energy cost and in contrary, the hydrogen is used to produce electricity during high demand and high electricity price period, gaining the most benefit. Besides, the storage duration of hydrogen is much longer than batteries, up to weeks or months, compared to hourly or weekly storage of batteries [START_REF] Bocklisch | Hybrid energy storage approach for renewable energy applications[END_REF].

• Seasonal variation: Hydrogen can also be used to shift the renewable resources across the seasons due to the seasonal difference in energy production. Moreover, hydrogen storage capacity can reach up to MWh, even TWh, owing to its high energy density, while batteries tend to be used in kWh to MWh applications, i.e. one needs to expand the size of the instrument to reach a greater storage capacity [START_REF] Al Shaqsi | Review of energy storage services, applications, limitations, and benefits[END_REF][START_REF]Energy Information Administration, Battery storage in the united states: An update on market trends[END_REF].

Numerous hydrogen energy storage projects have been launched all around the world demonstrating the potential of its large industrial use. For example, DATAZERO (https://www.irit.fr/datazero/index.php/en/) is a project aiming at integrating renewable energy in data centers, solving the sizing, optimization and control problems on both software and hardware levels [START_REF] Pierson | Datazero: Datacenter with zero emission and robust management using renewable energy[END_REF]. It has proposed an all green solution to supply the electrical loads by photovoltaics (PVs) and wind turbines and using hydrogen storage, batteries and supercapacitors to handle the limited flexibility and controllability of the IT infrastructure. As shown in Figure 14, batteries and supercapacitors are used to meet the short-term and fast response requirements, while the hydrogen storage system is responsible for long-term energy storage and takes the seasonal variations into consideration. Underground Sun Storage (https://www.underground-sun-storage.at/) is an Austrian project that stores the energy from wind and solar power below ground. As the energy from renewable sources lacks flexibility and cannot be controlled to meet the demand, excess energy generated from renewables is reproduced into hydrogen through water electrolysis and stored for further use. The outcome of the project has found out that the underground gas storage tanks can tolerate hydrogen (up to 10% in gas), which has the capacity of balancing out the seasonal supplies of renewable energy [START_REF]underground sun storage" final report[END_REF]. There are also other projects that use hydrogen as the energy carrier to solve the mismatching problem between system demand and load. For example, Orsted (https://us.orsted.com/Wind-projects) in Denmark has planned to use the excess wind energy of wind farms to power the electrolysers and produce hydrogen during the wind farms' oversupplying period. SoCalGas (https://www.socalgas.com/) in the United States has launched a project to produce hydrogen by an on-campus solar electric system and the produced hydrogen is converted into methane directly through a bioreactor.

Ancillary services

Fuel cells and electrolysers can also play a role in providing ancillary services to the grid. These services mainly come in the form of flexibility, which is the main requirement to integrate renewable energy sources. Examples of ancillary services include congestion mitigation, reducing negative price occurrences, frequency regulation, voltage support and black start.

Like other types of energy storage, hydrogen can first be used to mitigate transmission and distribution line congestion which can result from an insufficient line capacity [START_REF] Bird | Wind and solar energy curtailment: A review of international experience[END_REF]. This may, for example, arise due to renewable generation exceeding export line capacity, as in the HAEOLUS project (http://www.haeolus.eu) where excess wind generation feeds an electrolyser to produce hydrogen. Congestion tends to increase market prices, as well as renewables curtailment [START_REF] Larscheid | Potential of new business models for grid integrated water electrolysis[END_REF][START_REF] Ban | Integration of power-to-hydrogen in day-ahead security-constrained unit commitment with high wind penetration[END_REF]. Redispatching costs are also incurred when such congestion occurs. Similarly, such installations can help delay investments in new and expensive transmission and distribution capacity.

The second type of service is to help reduce the number of occurrences of negative prices on markets. Such negative prices mostly occur due to inflexibility in the generation, either renewable (e.g., none or little dispatchable) or conventional (e.g., due to minimum power output or uptime). Increasing electrolysers consumption can, for example, provide a fast and efficient solution to increase demand, i.e., by providing a form of negative generation [START_REF] Gunawan | At what cost can renewable hydrogen offset fossil fuel use in ireland's gas network?[END_REF]. A similar result can be achieved by decreasing fuel cell power output or shutting them down.

Frequency regulation aims to maintain the grid frequency close to its reference value (50 or 60 Hz) by injecting or absorbing power coordinately to ensure the balance between supply and demand. Several types of such regulation reserves are distinguished in European networks. Frequency containment reserve (FCR), or primary reserve, provides constant containment of frequency when it deviates from the nominal value in a very short term. For frequency deviations longer than 30 seconds, the restoration of frequency is handled by frequency restoration reserve (FRR), with a much larger capacity. Both FCR and FRR services can be achieved by fuel cells and electrolysers, by increasing or decreasing their power setpoint following a frequency signal [START_REF] Alshehri | Modelling and evaluation of PEM hydrogen technologies for frequency ancillary services in future multi-energy sustainable power systems[END_REF]. In case of a frequency drop, e.g., due to a generator failure, a fuel cell can increase its power output and an electrolyser decrease its consumption, which makes it a form of demand response asset. Compared to conventional turbine-based generators, fuel cells and electrolysers are much more flexible with very high ramp-up and ramp-down rates, short response times (with setpoint changes in less than a second) as well as low minimum up times [START_REF] Tuinema | Modelling of large-sized electrolysers for real-time simulation and study of the possibility of frequency support by electrolysers[END_REF]. On the other hand, operating equipment in such a way for a long duration may significantly reduce their lifetime.

Another service which can be achieved by hydrogen equipment is voltage support [START_REF] Alshehri | Modelling and evaluation of PEM hydrogen technologies for frequency ancillary services in future multi-energy sustainable power systems[END_REF]. As other devices connected to the grid through power electronics-based converters, the power factor of fuel cells and electrolysers can be adapted based on the local needs for voltage support. This is in turn achieved by supplying or absorbing reactive power through inverter or rectifier control. Black start is another possibility, in the case of a blackout [START_REF] Yoldas | Blackout and blackstart on power systems[END_REF]. A conventional generator, such as a diesel genset, is typically used to bring a power station back into operation. A fuel cell can also achieve this, without any emission or noise.

While most of these services may also be provided by other types of energy storage, hydrogen benefits from its high energy storage capacity while also being able to respond to setpoint changes within a second. This high flexibility level enables hydrogen technologies to potentially benefit from increased revenue from providing such services [START_REF] You | Value assessment of hydrogen-based electrical energy storage in view of electricity spot market[END_REF], for example on the reserves or capacity markets, while reducing the payback time of the installations. In most studies, providing a series of different services and selling hydrogen for other applications does indeed seem required to enable the profitability of hydrogen installations [START_REF] Larscheid | Potential of new business models for grid integrated water electrolysis[END_REF][START_REF] You | Value assessment of hydrogen-based electrical energy storage in view of electricity spot market[END_REF].

Power-to-gas

Power-to-gas is an application which usually uses electric power to produce a combustible gas. As hydrogen is believed to be a combustible gas with rather high energy density, power-to-hydrogen applications are gaining momentum. The hydrogen produced by an electrolyser can then be methanated into methane and injected to the natural gas grid, or stored, providing a balancing service to the energy market.

Various pilot and demonstration projects have been launched or are about to be launched around the world [START_REF] Gahleitner | Hydrogen from renewable electricity: An international review of power-to-gas pilot plants for stationary applications[END_REF][START_REF] Eveloy | A review of projected power-to-gas deployment scenarios[END_REF][START_REF]Hydrogen integration in power-to-gas networks[END_REF]. According to the statistical research in [START_REF] Thema | Power-to-gas: Electrolysis and methanation status review[END_REF], around 85% of the state-of-the-art power-to-gas projects are located in Europe, while a few are located in the USA and Japan. Germany holds the highest installed power share with around 40 MW and a 100 MW power-to-gas pilot plant is being built for industrial use, which will be connected to the grid from 2022 [START_REF] Wulf | Review of power-to-x demonstration projects in europe[END_REF]. Besides, numerous power-to-gas infrastructures are being constructed in areas with abundant wind and solar resources. Due to the relatively low demand and a constrained transmission network connecting them to other areas, hydrogen finds its place in both securing stable power supply and increasing value-creation of these areas. Benefiting from storage facilities, it is possible to smooth out the variations on wind power production to secure that the loads in these areas are satisfied at all times.

Project HAEOLUS (http://www.haeolus.eu/) proposes the integration of an MW-scale electrolyser in a remote wind farm in the north of Norway. It aims to demonstrate that the combined operation of both systems can enhance the flexibility and grid integration of the remote wind farm and increase its economical profitability. A 2.5 MW PEMEL is used for converting collected wind power into hydrogen and a 100 kW PEMFC is used to distribute the hydrogen for multiple uses. As shown in Figure 15, a monitoring and control system is developed to remotely control the combined system. Project HyCAUNAIS (https://www.europe-bfc.eu/beneficiaire/hycaunais-storengy/) intends to demonstrate also the feasibility of operating a flexible power-to-gas system with the methanation process. It is equipped with a nominal 1 MW electrolysis unit, which is optimized for very flexible operation (up to + / -1 MW up / down) to meet the needs of the network and also to be controllable according to wind production. The produced hydrogen is methanated and injected to the natural gas grid. Furthermore, it is coupled with a bio-methane production unit from landfill biogas. Besides, power-to-gas produced hydrogen can be injected into the existing gas grid. It offers an efficient storage solution using existing infrastructure and saving construction cost. For example, Jupiter1000 (https://www.jupiter1000.eu/) is a power-to-gas industrial demonstrator project, which aims to transform renewable electricity into gas in order to store it. The produced hydrogen and methane are being injected into the gas grid around France. Based on studies, a volumetric level of around 15%-20% blended hydrogen should be the allowable proportion when being injected into the gas grid for eliminating hydrogen embrittlement problem. This value should be further lowered in high-pressure gas transmission grids as high pressure can add to the effects of hydrogen embrittlement [START_REF] Quarton | Power-to-gas for injection into the gas grid: What can we learn from real-life projects, economic assessments and systems modelling?[END_REF]. The additional cost of hydrogen injection is calculated as 0.39 EUR/kg assuming that the cost of hydrogen production is 5.21 EUR/kg using a 5 MW PEMEL with an average electricity cost of 30 EUR/MWh [START_REF]Development of business cases for fuel cells and hydrogen applications for european regions and cities[END_REF]. The injected hydrogen can provide the subsequent use in a range of different applications including power generation, heat provision, transport applications such as gas-fuelled urban buses or passenger cars.

Most power-to-gas projects today, with or without methanation process, tend to be pilot projects that last for 1 to 3 years, while large industrial plants are planned around the world and need more social and political supports [START_REF] Wulf | Review of power-to-x demonstration projects in europe[END_REF]. The potential improvements lie in the aspects of hydrogen and methanation producing efficiency, as well as the utilisation of the by-products, like oxygen and heat.

Co-and tri-generation

To improve the efficiency and to reduce the cost, fuel cells can be used as prime movers for combined heat and power (CHP) generation or combined cold and power (CCP) generation, known as co-generation, or to be used for combined cold heat and power (CCHP) generation, known as tri-generation. The mechanism of running a tri-generation system to produce electricity and heat from renewable energy sources through electrolysers and PEMFCs is shown in Figure 16.

Co-generation

The process of using fuel cells as prime movers to produce both electricity and heat concurrently is called cogeneration, in which the electricity is used to provide the electrical needs, while the released heat is used for heating applications so that the total efficiency can reach up to 95%. A typical fuel cell co-generation system is made up of a stack, a fuel processor (a reformer or an electrolyser), power electronics, heat recovery systems, thermal energy storage systems (typically a hot water storage system), electrochemical energy storage systems (accumulators or supercapacitors), control equipment and additional equipment (fans, pumps, communication devices, etc.).

Nowadays, a great number of commercial projects are launched to develop fuel cell co-generation applications. Japan is a leader country on small-scale co-generation installations driven by the ENE-FARM project (about 300,000 units in 2018), which provides electricity and heat for home use by deploying PEMFCs from 0.3 to 1 kW. As homes are supplied with liquefied petroleum gas (LPG), a reformer is used to convert the LPG into hydrogen and the residual heat can be used to heat up water. Then the PEMFC stack combine hydrogen with ambient oxygen into water and at the same time, produce electricity and heat to meet the electrical needs and to heat water for kitchen, bathroom, room heating, etc.

In Europe, micro-co-generation for residential applications is currently in commercial development. The first European project for micro-co-generation using fuel cells is the ene.field project (http://enefield.eu/). From 2012 to 2017, over 1,000 residential micro-CHP fuel cells were installed across 10 European countries. Project PACE (http://www.pace-energy.eu/) follows up the work of ene.field project. This project started in 2016 and ends in 2021, in which 2,800 micro-CHP fuel cells over 10 European countries are being installed. In the ene.field project, an environmental life cycle assessment of micro-CHP fuel cell has been carried out. It concludes that in all the scenarios investigated, fuel cell co-generation produced less greenhouse gas compared to gas boilers and heat pumps. For these two projects, they have reached an electrical efficiency and overall efficiency of 60% and 95%, respectively.

Tri-generation

Tri-generation is an extended application of co-generation, which couples a prime mover to thermally driven equipment to produce cooling. Typically, a heat pump is used to produce cold from a thermal sink, which contains two reactors, a condenser and an evaporator. The two reactors consist of an absorption/adsorption reactor and a desorption reactor. The vapour or gas extracted from the absorbent passes through the condenser where it transforms into a liquid by rejecting heat, then the refrigerant liquid passes through the evaporator at low pressure, where it absorbs heat to evaporate.

Compared to the traditional distributed cold, heat and electricity, fuel cell tri-generation can lower the carbon emissions and increase energy efficiency. In [START_REF] Fong | Investigation on zero grid-electricity design strategies of solid oxide fuel cell trigeneration system for high-rise building in hot and humid climate[END_REF], using a 593 kW SOFC and absorption chillers, total carbon emissions were divided by two and the overall efficiency of the system reached 75%. For large scale applications, [START_REF] Yu | Investigation on performance of an integrated solid oxide fuel cell and absorption chiller tri-generation system[END_REF] has run a simulation for a 339 kW SOFC, coupled with a combustor and a heat steam recovery, which could recover 267 kW of heat for an overall efficiency of 84%. They also studied a system of 339 kW with an absorption chiller for cold production, which could generate 303.6 kW of cold and increase overall efficiency to 89%.

In isolated applications, making full use of the fuel cell rejected heat can reduce electrical power consumed by the compressor and allows to store cold when there is no cold demand [START_REF] Ferrucci | Mechanical compressor-driven thermochemical storage for cooling applications in tropical insular regions. Concept and efficiency analysis[END_REF]. This improves the autonomy and the efficiency of the system. A combined organic rankine cycle (ORC) and a vapour compression cooling (VCC) have been used in [START_REF] Chang | Energy analysis of a hybrid PEMFC-solar energy residential micro-CCHP system combined with an organic Rankine cycle and vapor compression cycle[END_REF] to produce hot water and cold effect, where the fuel cell provides 8 kW electrical power. Meanwhile, the released heat can be used to run the ORC and/or to be stored for domestic hot water supply. Then, the ORC produces mechanical power for the compressor of the VCC. Thermal solar panels are used in addition to power the ORC. They are able to recover 70 kW of heat and 16 kW of cold, while the overall efficiency can reach 85%.

Application of hydrogen in transportation

Hydrogen-fuelled electric powertrains provide a solution for long-distance driving with clean energy, while batterypowered vehicles suffer from range limitations. 3% of global vehicle sales in 2030 are expected to be hydrogenfuelled, and this percentage could reach 36% in 2050 [START_REF]Path to hydrogen competitiveness: A cost perspective[END_REF]. Several companies are developing fuel cell powertrains in terms of their quality, reliability and dependableness to accelerate their commercialization in the vehicle market. For example, Mirai fuel cell vehicles developed by Toyota have used mass-production PEMFCs with a 3.1 km/L volume power density and a 144 kW (155 DIN hp) maximum power output, where a 1.6 kWh nickel-metal hydride battery is connected in parallel to deal with the regenerative braking and also assist during high-power demands when accelerating. The current hydrogen storage systems in most commercial hydrogen fuel cell vehicles are high-pressure compressed hydrogen fuel tanks. For example, Honda's Clarity fuel cell vehicle, Hyundai's NEXO fuel cell vehicle use such tanks, while BMW's Hydrogen 7 has used a liquid hydrogen fuel tank.

Other than fuel cell vehicles, fuel cell ships have been in development in recent years. The high pollution caused by ships, counting for around 2.5% of total global greenhouse gas emissions makes the shipping sector to shift to more sustainable sources of energy, i.e. hydrogen. Fuel cells are capable of powering ships sailing relatively long distances compared with those powered by batteries and meeting the auxiliary energy needs of larger ships. The same is true for fuel cell trains. Hydrogen-fuelled regional multi-unit trains have been put into operation in Europe and are expected to have even higher market share in the future, which may take place of 30% of the currently used diesel fleets [START_REF] Garrison | Study on the use of fuel cells and hydrogen in the railway environment[END_REF].

Moreover, instead of injecting the hydrogen into the grid, hydrogen integrated with on-board systems gives a chance for the hydrogen producer to resell the hydrogen at a higher price. NREL has revealed that the potential price of hydrogen is about 3 to 10 USD/kg, while the most common price of hydrogen fuel is 13.99 USD/kg [START_REF] Eichman | Economic Assessment of Hydrogen Technologies Participating in California Electricity Markets[END_REF]. Besides, developing on-board hydrogen applications can also help to reduce the need to increase the capacity of grid infrastructure for vehicle charging. As the hydrogen refuelling infrastructure can be considered in the light of the existing gasoline and diesel stations, the recharging stations for alternative electric vehicles equipped with batteries can be less demanded.

Current status

This section summarises the current status of hydrogen powered energy systems, in which current progress is considered in the points-of-view of capital costs, hydrogen production cost, water and rare material consumption, system efficiency and durability.

Capital cost

Capital cost is about capital investments at the beginning of the system installation. Current capital costs of electrolysers, compressed tanks and fuel cells have been summarised in [START_REF] Parra | A review on the role, cost and value of hydrogen energy systems for deep decarbonisation[END_REF] and several cost-effectiveness analysis has been conducted in recent projects and researches [START_REF] Yilmaz | Life cycle cost assessment of a geothermal power assisted hydrogen energy system[END_REF][START_REF] Zhao | Life cycle cost analysis: A case study of hydrogen energy application on the orkney islands[END_REF][START_REF] Dispenza | Evaluation of hydrogen production cost in different real case studies[END_REF][START_REF] Fang | Life cycle cost assessment of wind power-hydrogen coupled integrated energy system[END_REF][START_REF] Li | Life cycle cost and sensitivity analysis of a hydrogen system using low-price electricity in china[END_REF].

The capital cost of the current hydrogen production system using AELs ranges from 1,000 to 1,500 EUR/kW including installation and that of PEMELs is twice these numbers, i.e., 2,000 to 3,000 EUR/kW [START_REF] Bertuccioli | Development of water electrolysis in the european union[END_REF]. Although alkaline water electrolysis has been well developed, the production volume is rather low. This is because the electrolyser providers tend to fabricate small-volume electrolysers for niche market, which increases at the same time the cost of BoPs. Therefore, potential cost reduction for AELs depends on more cost-efficient production, while for further reduction in the cost of PEMEL, breakthroughs in technological developments are required.

Different dimensions of stationary fuel cell systems are deployed to meet various demands from serving residential buildings to industrial applications. Fuel cell micro-CHPs for family homes and small buildings with an installed capacity of 0.3-5 kW now cost around 10,000 EUR/kW. Some mid-sized installation for larger buildings of 5-400 kW now cost 4,500 EUR/kW to 7,500 EUR/kW and large scale installations of 0.4-30 MW costs 2,000 to 3,000 EUR/kW for specific industrial applications [START_REF]Fuel cells and hydrogen joint undertaking, State-of-the-art and future targets (kpis)[END_REF]. The capital costs have the potential to be largely reduced in the coming future owing to more mature installation technologies and economies of scale. Target capital expense (CAPEX) values for 2030 are expected to reach 3,500 EUR/kW, 1,500-4,000 EUR/kW and 1,200-1,750 EUR/kW for micro-CHP, mid-size and large scale applications, respectively [START_REF]Fuel cells and hydrogen joint undertaking, State-of-the-art and future targets (kpis)[END_REF][START_REF] Ammermann | Advancing europe's energy systems: Stationary fuel cells in distributed generation[END_REF].

With the gradual maturity of technologies, the capital costs of both electrolyser systems and fuel cell systems are expected to decrease significantly by 2030, especially the stack cost. Technological advances in increasing the active area of the stack are required, which can reduce the number of cells for producing a certain amount of hydrogen, and therefore, decrease the cost.

Hydrogen production cost

The cost of steam reforming hydrogen is mainly shaped by the gas prices, which currently ranges from 1.4 to 1.8 EUR/kg with CO 2 capture [START_REF] Van Cappellen | Oplossingen voor milieu, Feasibility Study Into Blue Hydrogen: Technical, Economic & Sustainability Analysis[END_REF]. However, driven by the exhaustion of fossil fuels and the decreasing cost of renewable electricity, electrolytic hydrogen becomes competitive and is about to see continuous increasing deployments shortly. The current hydrogen production cost of AEL is 3.2-5.2 EUR/kg, while using PEMEL, the cost is 4.1-6 EUR/kg [START_REF] Bertuccioli | Development of water electrolysis in the european union[END_REF]. Regarding hydrogen's compressing, storing and dispensing, the pipeline scenario cost is 1.8-2.6 EUR/kg and the distributed scenario cost is 2.1-3 EUR/kg. This cost is expected to be reduced to 1.6 EUR/kg as the ultimate goal [START_REF] Parks | Hydrogen station compression, storage, and dispensing technical status and costs[END_REF].

Although producing hydrogen through water electrolysis is a promising solution, the consumption of electricity should be considered. If the hydrogen is produced through water electrolysis with an assumed efficiency of 60%, all today's dedicated hydrogen demand requires 3,600 TWh of electricity consumption, which exceeds the total annual electricity generation in Europe [START_REF]The future of hydrogen[END_REF]. One promising solution for lowering down the electricity price is to generate electricity from renewables or nuclear power. As renewable energy sources, e.g. solar and wind, have been explored with declining costs, renewable electricity becomes less expensive. Although the hydrogen produced using renewable energy may suffer from high transmission and distribution cost as the locations could be remote, the final profit is considerable. A cost-benefit analysis of an integrated wind-hydrogen system in Corvo island has been conducted in, which found that the local renewable energy can cover 80% of the electricity demand of the island [START_REF] Parissis | Integration of wind and hydrogen technologies in the power system of corvo island, azores: A cost-benefit analysis[END_REF]. Projects for installing electrolysers at the locations with excellent solar and wind resources have been launched all over the world and future large-scale industrial deployments are about to be undertaken.

Consumption of water

Except for electricity, water is another necessary element for hydrogen production via electrolysis. Theoretically speaking, 0.81 litres of water is consumed to produce 1 Nm 3 of hydrogen but at least 25% more water is consumed in a practical manner, i.e. 1 litre of water is required in reality [START_REF] Shi | Quantification of fresh water consumption and scarcity footprints of hydrogen from water electrolysis: A methodology framework[END_REF]. According to the literature, to produce 1 kg of hydrogen using a PEMEL needs 18 litres of water and 54 kWh of electricity [START_REF] Mehmeti | Life cycle assessment and water footprint of hydrogen production methods: From conventional to emerging technologies[END_REF]. Supposing all of today's dedicated hydrogen production, i.e. 70 Mt was produced by water electrolysis, water consumption would represent 1.3% of the water consumption of the global energy sector [102]. One alternative solution to is to use reverse osmosis for seawater desalination. The electricity cost for desalinating 1 m 3 of water is 0.7-2.5 USD, which is believed to have little influence on the total hydrogen production cost [START_REF] Caldera | Role of seawater desalination in the management of an integrated water and 100% renewable energy based power sector in saudi arabia[END_REF]. Efforts on how to easily integrating seawater into water electrolysis process are required for the moment.

Consumption of rare materials

Rare materials can play different roles in fuel cell and electrolyser systems. For example, they could be used as catalysts and co-catalysts of electrodes, electrolytes additives, etc [START_REF] Antolini | The use of rare earth-based materials in low-temperature fuel cells[END_REF]. The consumption of rare materials has generated a wide concern on their high cost, concentrated supply and resource shortage [START_REF] Leader | The effect of critical material prices on the competitiveness of clean energy technologies[END_REF].

Rather than using Nickel-based electrocatalysts for the alkaline hydrogen oxidation reaction in alkaline technologies, PEM fuel cells and electrolysers need more expensive rare materials to formulate their bipolar plates and the catalysts of anode and cathode to achieve competitive efficiency. The commonly used bipolar plates are of titanium, which makes up 50% of the cost of the PEMEL stack [START_REF] Borgardt | Mechanical characterization and durability of sintered porous transport layers for polymer electrolyte membrane electrolysis[END_REF]. Catalysts for the anode and the cathodes make up 10% or less the total stack cost [START_REF] Bertuccioli | Development of water electrolysis in the european union[END_REF]. The electrodes of PEMFCs and PEMELs also need expensive metal elements to provide high corrosion resistance and catalytic activity. The anode normally uses iridium and ruthenium-based catalysts and the cathode uses palladium and platinum-based catalysts [START_REF] Bertuccioli | Development of water electrolysis in the european union[END_REF]. A study has shown that 7% of the world's platinum supply will be required for the fuel cell use in the Europe in 2030 [START_REF] Moss | Critical metals in the path towards the decarbonisation of the eu energy sector: Assessing rare metals as supply-chain bottlenecks in low-carbon energy technologies[END_REF].

To reduce the stack cost, catalyst-related cost reduction is regarded as a priority. Solutions exist in developing advanced support structures, mixed metal oxides and nano-scale catalysts. For example, carbon nanotube supported platinum catalyst has been developed to reduce the use of platinum [START_REF] Gupta | Development of multiwalled carbon nanotubes platinum nanocomposite as efficient pem fuel cell catalyst[END_REF]. It is expected to reduce the iridium and ruthenium used on the anode and the platinum used on the cathode to 0.4 mg/W and 0.1 mg/W, respectively, in 2030 [START_REF]Fuel cells and hydrogen joint undertaking, State-of-the-art and future targets (kpis)[END_REF].

Environmental impact

The environmental impact of hydrogen depends, most of all, on how it is produced. Current hydrogen supply relies on coal gasification and steam reforming of natural gas, rather than being generated through renewable energy because the costs of steam reforming are relatively low [START_REF] Nowotny | Impact of hydrogen on the environment[END_REF]. This kind of hydrogen is called "grey hydrogen", which is massively used in the industry nowadays. However, the process generates hydrogen, as well as CO and CO 2 gases. The produced CO are burned to be turned into CO 2 , which is the major contributor to the greenhouse gas. "Grey hydrogen" production emits at least 10 kg CO 2 per kilo hydrogen production [START_REF]Hydrogen decarbonization pathways[END_REF]. Due to the increasing carbon tax, the price of the "grey hydrogen" has become no longer attracted. Besides, the climate change due to emissions causes problem in human health and may also generate costs on the population displacement, which may be hardly quantified but add to the hydrogen cost. As the hydrogen is playing an important role in the energy transition, it is not only necessary to make the hydrogen economy economically feasible but also needed to maximize its decarbonization potential. Alternative pathways to reduce emissions in the medium term is to produce "blue hydrogen" and "yellow hydrogen". "Blue hydrogen" is reformed from the natural gas or coal-derived gas with carbon capture and sequestration (CCS) and a 90% carbon capture rate is possible with less than 1.5 kg CO 2 emitted per kilo hydrogen production [START_REF]Hydrogen decarbonization pathways[END_REF]. However, "blue hydrogen" production depends on the fossil fuel supply chain and CCS storage facilities. It reduces emissions and saves costs in the short to medium term, but it will be more expensive in the long term. The hydrogen produced electrolytically by nuclear energy is called "yellow hydrogen", which is zero-carbon. LucidCatalyst has reported in 2020 that the cost of hydrogen from nuclear power is 2 USD/kg, which is competitive to "grey hydrogen", 0.7-1.6 USD/kg without costing CO 2 emissions [START_REF] Ingersoll | Missing Link to a Livable Climate: How Hydrogen-Enabled Synthetic Fuels Can Help Deliver the Paris Goals[END_REF]. In the long run, producing "green hydrogen" using renewable electricity (e.g., solar, wind) should be promising owing to the cost reductions for electrolyser CAPEX and the increasing capacity of the renewable energy.

On the other hand, hydrogen itself can be regarded as an indirect greenhouse gas [START_REF] Derwent | Global environmental impacts of the hydrogen economy[END_REF]. The proportion of the hydrogen emitted from a hydrogen energy system during production, transport or at the point of use may range from 0.2 up to 10% [START_REF] Van Ruijven | Emission scenarios for a global hydrogen economy and the consequences for global air pollution[END_REF]. Although hydrogen technologies have the potential to replace fossil fuels that generate directly man-made greenhouse gas, the inevitable emissions through the hydrogen production, compression, storage and transportation process can lead to the indirect concentration of the greenhouse gas [START_REF] O'hayre | Chapter 14: Environmental Impact of Fuel Cells[END_REF]. This is because the hydrogen can react with hydroxyl radicals and reduce their concentration, which perturbs the oxidation reactions of hydroxyl radicals and other greenhouse gases, e.g. CH 4 and CO, and increases the greenhouse effects [START_REF] Van Ruijven | Emission scenarios for a global hydrogen economy and the consequences for global air pollution[END_REF]. The oxidation of hydrogen also increases the water content in the stratosphere and cools down the lower stratosphere. The low temperature may create more polar stratospheric clouds and impede the breaking-up of the polar vortex, causing larger and deeper ozone hole [START_REF] Tromp | Potential environmental impact of a hydrogen economy on the stratosphere[END_REF]. However, as few studies have been conducted to consider the impacts of hydrogen as a greenhouse gas in energy systems, the uncertainties should be investigated before the large-scale hydrogen deployment.

Life cycle assessment (LCA) is an efficient tool to evaluate the potential environmental impacts of hydrogen energy systems. The EU has published the International Reference Life Cycle Data System (ILCD) Handbook, in which several impacts are considered: climate change, ozone layer depletion, photochemical oxidation, acidification potential, eutrophication potential and resource and fossil fuels depletion, etc. These environmental impacts can then be translated into damage impacts on the human health, ecosystem quality and resource scarcity. For example, Mehmeti et al. have conducted midpoint and endpoint LCA, corresponding to 17 problems on the ecological level and 3 problems oriented on the damage, to evaluate different hydrogen production methods and pointed out that the electrolysis using renewable energy showed the most benefits [START_REF] Mehmeti | Life cycle assessment and water footprint of hydrogen production methods: From conventional to emerging technologies[END_REF]. If hydrogen is going to play an important role in future energy systems, life cycle criteria must be assessed to know how it might perform better and also to convince the stakeholders. As the methodological choices of LCA studies vary in goal and scope definition, life cycle invention and assessment, the results and their interpretation can be affected, therefore, a methodological framework that can facilitate the decisionmaking in the hydrogen sector should be defined from a generalized perspective [START_REF] Valente | Life cycle assessment of hydrogen energy systems: a review of methodological choices[END_REF].

Efficiency and durability

The current achievements on efficiency and durability of fuel cell and electrolyser systems are still not satisfactory and remain as challenges that hold behind the successful market introduction of many hydrogen energy systems. In fact, the efficiency of water electrolysis system is very close to the ultimate target today. The electrical efficiency of AEL systems is around 63-73%, while the ultimate goal is 70-80% [START_REF]Fuel cells and hydrogen joint undertaking, State-of-the-art and future targets (kpis)[END_REF]. A PEMEL system has a lower efficiency of around 60%, which is expected to be improved to 67%-74% in the future [START_REF] Bertuccioli | Development of water electrolysis in the european union[END_REF]. However, in real systems, the operating efficiency can be even higher as the electrolyser can work at higher efficiency with partial loads rather than full loads. Developing optimization and control strategies to aid electrolysers working at their optimal load may help to improve the operating efficiency. For fuel cell stationary applications, the thermal efficiency should be considered as well. The current achieved electrical efficiency and thermal efficiency of micro-CHP applications is around 36% and 52%, respectively [START_REF]Fuel cells and hydrogen joint undertaking, State-of-the-art and future targets (kpis)[END_REF]. For mid-size applications, the electrical efficiency and thermal efficiency are 50% and 37%, respectively and for large-scale applications, they are 55% and 32%, respectively [START_REF]Fuel cells and hydrogen joint undertaking, State-of-the-art and future targets (kpis)[END_REF].

Another challenge is due to the fact that the components of fuel cells and electrolysers may suffer from various degrees of degradation. The degradation may come from electrical, mechanical and thermal deformation, which lead to performance loss. In practice, the operating voltage of an electrolyser stack in a hydrogen production system decrease by about 0.4-5 V per operation hour and the current electrolyser system durability is around 40,000 hours, which implies un efficiency up to 10% lower compared to the initial state [START_REF] Schmidt | Future cost and performance of water electrolysis: An expert elicitation study[END_REF]. The same is applied to stationary fuel cell systems. The system life is designed to be around ten years for micro-CHP, however, the replacement of the stack must take place twice due to the stack voltage degradation in limited operation time of 40,000 hours [START_REF]Fuel cells and hydrogen joint undertaking, State-of-the-art and future targets (kpis)[END_REF]. A system lifetime of 15 years without stack replacement is expected in the industry. For mid-size and large-scale applications, the system lifetime is around 10 years with one stack replacement and is expected to reach 20 years in the near future. Besides, unsatisfied durability and reliability of the current hydrogen integration systems are usually associated with a high maintenance cost, while non-optimized operation conditions could be a critical reason leading to the unexpected shutdowns and degradation of the components [START_REF] Abdin | Solar hydrogen hybrid energy systems for off-grid electricity supply: A critical review[END_REF][START_REF] Reddy | Low-maintenance solar-hydrogen generator using alkaline water electrolysis[END_REF]. Efforts on developing system health control and management may be one solution to reduce this part of the cost.

Publication statistics

The statistical status of research publications regarding hydrogen powered energy systems is investigated in this section, which has illustrated the increasing importance of hydrogen in today's power systems. The review procedure and the criteria for screening are shown in Figure 17. Based on the above review procedure, the yearly evolution of publications investigating different hydrogen power system applications from 2000 to date and the percentage distribution is shown in Figure 18. It is noted that the researches on hydrogen-based energy storage consist of researches on storage materials and tanks, as well as researches on the system level. The two aspects are considered separately. It is found that all these applications have seen an increasing trend in research efforts. The number of researches on hydrogen-based energy storage systems has taken first place, followed by that of transportation, which has seen a rapid increase. Research on hydrogen storage materials has also aroused great interest owing to the rapid development of material engineering. Publications on the applications of power-to-gas and co-and tri-generation have shared a relatively small part, however, they are getting a rapid increase since 2010 and may continue as the research hot-spots in the coming future.

Figure 18: Yearly evolution and percentage distribution of publications from 2000 to date containing terms "hydrogen" AND "storage" AND "material", "hydrogen" AND "energy storage system" NOT "tank", "hydrogen" AND "power-togas", "hydrogen" AND "transportation", "hydrogen" AND ("co-generation" OR "tri-generation"). The search was made on the 2th June 2020.

Figure 19 shows the yearly evolution of research works regarding the different performance of hydrogen powered systems and the percentage distribution. The searched terms include cost, efficiency and durability, which concern mostly the feasibility and effectiveness of developing hydrogen powered energy systems. Researches regarding hydrogen system efficiency have attracted the most interest, followed by cost and durability, while each of them has seen a yearly increase. Researches on system durability and lifetime share the smallest part, 14% compared to 30% of cost and 56% of efficiency, and have got more emphases only after 2010. As the current durability of hydrogen powered systems and components is still on the way to reach the satisfied level, researches on diagnostic, prognostics and fault tolerant control should be further encouraged.

Discussion

This section summarises the projected performance evolution of different aspects of hydrogen power systems on the basis of current status and a trend of increasing system scale is disclosed by reviewing current power system projects. Prospects on technical perspectives and political implications are discussed based on previous analysis.

Projected performance evolution

Recent years projected cost and performance evolution of fuel cell stationary applications are shown in Figure 20. It could be seen that continuous efforts should be made to lower down the capital costs of systems in different dimensions, while the maintenance cost should be largely reduced and tends to touch the goal of 2030 target soon. The lifetime and the electrical efficiency of micro-CHP are still on the way of improvement, while the thermal efficiency goal has already been achieved. For mid-size applications, thermal efficiency is under development and has hardly been improved during years. The lifetime of mid-size applications is expected to be doubled for 2030. Efforts on improving the electrical efficiency of both mid-size and large-scale applications should be made in the coming years, whereas the lifetime and thermal efficiency of large-scale applications are about to reach a satisfying level. 19: Yearly evolution and percentage distribution of publications from 2000 to date containing terms "hydrogen" AND ("cost" OR "economy"), "hydrogen" AND "efficiency", "hydrogen" AND ("durability" OR "lifetime" OR "degradation"). The search was made on the 2th June 2020. Figure 21 shows the performance evolution of producing hydrogen from renewable electricity using AEL and PEMEL. The technologies of deploying AEL are more mature than deploying PEMEL systems. Efforts on reducing costs and enhancing the system durability and reliability for PEMEL systems are demanding in future work. Besides, for the two electrolyser systems, the use of rare materials on the electrode catalysts is expected to be further reduced, possibly through nano-structure development.

Scale ramp-up

The collected data from past decades shows that AEL systems tend to reach MW system size rather than PEMEL systems owing to the rather mature technologies [START_REF] Bertuccioli | Development of water electrolysis in the european union[END_REF]. From now on, PEMEL systems are likely to catch up as most manufacturers start to use multi-stack PEM systems rather than single-size ones. By 2030, 2-4 MWe electrolyser stacks and 7 MWe systems are expected to be the common hydrogen production scale over the world [START_REF]Fuel cells and hydrogen joint undertaking, State-of-the-art and future targets (kpis)[END_REF]. With the rapid development of renewable resources, large-scale systems can easily reach system response requirements. Figure 22 shows the trend of the increasing scale of the installed power-to-gas plants from 2000 to date around the world.

By 2030, the European Clean Hydrogen Alliance aims to install at least 40 GWe renewable hydrogen electrolysers in Europe, which is a huge leap over the current 250 MWe electrolysers installed in place [START_REF]A hydrogen strategy for a climate-neutral europe[END_REF]. Some countries have also started to install large-scale hydrogen projects, for example, the Fukushima project in Japan with an installed

Technical perspective

The development of hydrogen powered energy systems has been industrially mature to some extent, however, room for its technology improvement is still significant. The summarised performance evolution in Section 6.1 shows that enhancements on the cost, efficiency and durability are demanding to different degrees and achieving them concurrently is also important.

Research and development of approaches to reduce cost while improving system efficiency and durability should be undertaken. Studies have been carried out for developing models and designing control and optimization strategies ranging from supply chain to system structure, e.g., cost minimization [START_REF] Won | Design and operation of renewable energy sources based hydrogen supply system: Technology integration and optimization[END_REF][START_REF] Tebibel | Off grid pv system for hydrogen production using pem methanol electrolysis and an optimal management strategy[END_REF], scheduling design [START_REF] Zhang | Comparative study of hydrogen storage and battery storage in grid connected photovoltaic system: Storage sizing and rule-based operation[END_REF][START_REF] El-Taweel | Hydrogen storage optimal scheduling for fuel supply and capacity-based demand response program under dynamic hydrogen pricing[END_REF], system sizing [START_REF] Burhan | Development of performance model and optimization strategy for standalone operation of cpvhydrogen system utilizing multi-junction solar cell[END_REF][START_REF] Han | Development of a high-energy-density portable/mobile hydrogen energy storage system incorporating an electrolyzer, a metal hydride and a fuel cell[END_REF] and response optimization [START_REF] Fang | Control strategy of electrolyzer in a wind-hydrogen system considering the constraints of switching times[END_REF][START_REF] Chen | Hydrogen value chain and fuel cells within hybrid renewable energy systems: Advanced operation and control strategies[END_REF]. The topology of hybrid hydrogen power systems and technoeconomic solutions have been reviewed in [START_REF] Vivas | A review of energy management strategies for renewable hybrid energy systems with hydrogen backup[END_REF]. The analysis has pointed out that current strategies for operating a hybrid hydrogen power system are mainly designed to satisfy the power supply, while the strategies devoted to solving the techno-economic optimization criteria are demanding and should be further developed, especially for the increasing widespread distributed systems. A model predictive control method has been proposed in [START_REF] Castilla | Event-based state-space model predictive control of a renewable hydrogen-based microgrid for office power demand profiles[END_REF] for a hydrogen-based microgrid in order to meet the energy demand of an office. The proposed event-based control method can successfully track the power load using the renewable energy and the stored energy and at the same time, minimizing the power purchase from the local grid. An optimized stochastic design has been proposed in [START_REF] Coppitters | Robust design optimization and stochastic performance analysis of a grid-connected photovoltaic system with battery storage and hydrogen storage[END_REF] for a PV-battery-hydrogen system, which has proved that the integrated system can sustain an affordable electricity cost over the system's lifetime owing to the energy storage components. Similarly, a multi-optimization problem has been solved in [START_REF] Siddiqui | Optimization of a new renewable energy system for producing electricity, hydrogen and ammonia[END_REF] for an integrated solar and wind energy system, which assessed the exergoeconomic performance and derived the optimal operation conditions of the system. A modelling process has been introduced in [START_REF] Rispoli | Constrained optimal design of a reversible solid oxide cell-based multiple load renewable microgrid[END_REF] for a self-sufficient renewablebased microgrid, in which the system is used to supply electricity and hydrogen to a resident and a car fleet. The fuel cell and electrolyser modes have been realised through an reversible solid oxide cell system and a constrained optimization strategy has been developed for sizing of the plant components. A two-level multi-objective strategy consisting of the long-term planning of the system capacities and the short-term operation optimization for an integrated renewable system in microgrid has been developed in [START_REF] Ding | Multi-objective optimization for an integrated renewable, power-to-gas and solid oxide fuel cell/gas turbine hybrid system in microgrid[END_REF].

Besides, adequate system and component modelling should be another prerequisite for successful system control and optimization and ultimately, for hydrogen cost reduction. Li et al. have proposed a 2D model for the hydrogenbased microgrid system and to cooperate between different storage systems, a two-layer structure has been built to solve the allocating-and-dispatching problem, while minimizing the operation cost [START_REF] Li | Multiple hydrogen-based hybrid storage systems operation for microgrids: A combined topsis and model predictive control methodology[END_REF]. A mathematical model has been proposed in [START_REF] De | Modeling and analysis of a solar thermal-photovoltaic-hydrogen-based hybrid power system for running a standalone cold storage[END_REF] for a PV-hydrogen-based power system, which facilitate the study on the energy, economy and environment. Direct techno-economic models are also favourable for evaluating the hydrogen production cost, as proposed in [START_REF] Matute | Multi-state techno-economic model for optimal dispatch of grid connected hydrogen electrolysis systems operating under dynamic conditions[END_REF]. Moreover, when the hydrogen system is integrated with renewable energy resources, the complexity of the system augments and the uncertainties in the power load and supplies increases. Therefore, the configuration of this kind of system should be delicately designed to deal with the nonlinear characteristics and additional variables [START_REF] Abdin | Solar hydrogen hybrid energy systems for off-grid electricity supply: A critical review[END_REF]. Ren et al. have modelled the PV reactor using a 3D multiphysical coupling mathematical modelling method and further used it for studying the dynamic behaviour of an integrated system and determining the decision variables [START_REF] Ren | Machine learning-assisted multiphysics coupling performance optimization in a photocatalytic hydrogen production system[END_REF]. Planning models that integrate renewable energy in the system have been reviewed in [START_REF] Deng | Power system planning with increasing variable renewable energy: A review of optimization models[END_REF], which has justified the uncertainty analysis is necessary, especially for those systems that with different penetration degrees of renewable resources. An optimization strategy for a renewable energy microgrid with a stochastic module and Monte Carlo simulation has been proposed in [START_REF] Zheng | Optimization under uncertainty of a biomass-integrated renewable energy microgrid with energy storage[END_REF], in which the uncertainties of renewable sources are taken into consideration. Yang et al. have proposed a robust optimization method for a microgrid, which have considered both uncertainties in renewables and loads [START_REF] Yang | Robust optimization of microgrid based on renewable distributed power generation and load demand uncertainty[END_REF]. Other investigations on integrating the renewable energy into microgrids have been reviewed in [START_REF] Dawoud | Hybrid renewable microgrid optimization techniques: A review[END_REF][START_REF] Hannan | Optimized controller for renewable energy sources integration into microgrid: Functions, constraints and suggestions[END_REF].

The publication statistics in Section 5 have also indicated that there is a short of hydrogen system durability improvement in the current research interest and the current achievements on durability issues are still far away from the 2030 target (see Figure 20 and Figure 21). To meet the industrial needs and to improve hydrogen system durability and reliability, more studies on monitoring the operation and the ageing process of the system are necessary. Diagnostics and prognostics are the methods for identifying the state-of-health, locating the faulty components and predicting the residual lifetime of the system and its components under actual operating conditions. Works on fuel cell diagnostics and prognostics have gained in popularity in the past decade [START_REF] Pahon | A non-intrusive signal-based method for a proton exchangemembrane fuel cell fault diagnosis[END_REF][START_REF] Morando | Fuel cells prognostics using echo state network[END_REF], while those of electrolysers have remained as a nascent field [START_REF] Olivier | Modelling, simulation and analysis of a pem electrolysis system[END_REF]. For example, Jouin et al. have reviewed the prognostics and health management activities in the PEMFC applications [START_REF] Jouin | Prognostics and health management of pemfc -state of the art and remaining challenges[END_REF], while Lin et al. have analysed the prognostics methods used for different scenarios, i.e. health monitoring, fault diagnosis, prolonging life span, etc [START_REF] Lin | Review on hydrogen fuel cell condition monitoring and prediction methods[END_REF]. A review of electrochemical diagnostic methods of PEMFCs have been conducted in [START_REF] Zhang | A review of online electrochemical diagnostic methods of on-board proton exchange membrane fuel cells[END_REF], which has pointed out that the diagnostic methods existing in the literature cannot be simply applied to the online applications, while an optimized EIS multi-frequency analysis could be a possible solution on-board. As the operation principle of fuel cells and electrolysers are rather similar, the same techniques could be developed for both of them. The key point here is to integrate the diagnostics and prognostics to the control module and to deploy sequence corrective actions or predictive maintenance schedules according to the monitoring results. In this way, the risk of faulty operations could be reduced by detecting and mitigating degradation phenomena, and therefore, it can improve the overall durability of the system. Besides, adaptable maintenance schedule, i.e. predictive maintenance, could be conducted based on predicted system performance so that to save operational and maintenance cost, especially in the remote area.

Social and political implications 6.4.1. Over Europe

Europe has been in the leading place of developing hydrogen powered energy systems. It has also a large market potential for further profitable business. More than 300 companies in the EU have been engaged in fuel cell and hydrogen sectors, while more in related supply chains [150]. A great deal of techno-economic analysis on operating hydrogen-fuelled facilities have certified the economic feasibility of hydrogen power systems by taking into consideration all direct and indirect costs. Chiefly, marketizing hydrogen-based fuel has a two-fold impact, i.e. integrating renewable energy in current fuel infrastructure and eliminating greenhouse gas emissions. To enable a more favourable environment, some measures could be undertaken.

• A first concern is the electricity price. Taxes take a major share of the electric bill in Europe's power-to-hydrogen applications, leading to difficulties for shrinking down the cost. Exemptions on taxes and levies must be considered, at least with some regulations, for example, avoiding electricity consumption during peak period.

• Gas pipeline systems should be put into practice for hydrogen transport in the near future. Studies have shown the viability of transmitting and distributing hydrogen with the existing natural gas grid, however, the injection of hydrogen is, for the moment, largely limited by national permitting rules. Legal pathways should be considered by the authorities to support gas pipeline development, therefore, give more opportunities to the development of power-to-gas applications.

• Curtailed renewable energy source electricity production has been observed in European countries. As the development of renewable energy source is even faster than the ability to inject them into the local grid, the congestion in the grid leads to the curtailment of the renewable energy sources. A preferable solution could be undertaking regulations for putting in place the priority of connection and dispatch of the renewables and in case of curtailment, compensations should be considered by the policymakers adequately.

Around the world

Hydrogen, as a pollution-free energy carrier, tends to be an essential choice for potential power system developments in all countries around the world. For the moment, reducing its cost and pursuing wide applications are the two main priorities. Following recommendations are suggested in both social and political sectors:

• Large-scale applications should be encouraged. As the technologies are ready to a major extent, large-scale applications are believed to be an economized way providing the minimal infrastructure investments compared to the high operation capacity, especially for areas with abundant renewable energy. Hundred megawatts to gigawatts applications are expected with proper policy supports.

• New hydrogen markets should be developed in the coming decades. Hydrogen as a promising energy transition solution must be considered in all aspects that have suffered from resource exhaustion and pollution: industry, trucking, aviation, shipping, chemical sectors, etc.

• Governments hold the leadership in accelerating the hydrogen economy. Absence of specific regulations will hinder the industrial operations in various ways (impacts on time, cost, etc.). Therefore, coordination, regulation and standardisation should be supported by the governments to help allow the market entry of hydrogen powered energy system.

Conclusion

Hydrogen is playing an important role in supporting the decarbonisation of various sectors, e.g. industry, transport, power generation, etc. Efforts have been made to accelerate the process of transforming this potential into reality. This paper has reviewed the key technologies that facilitate the hydrogen integration into energy sectors in terms of production, re-electrification and storage. The applications on the system level for the stationary background are highlighted and the potential of hydrogen to store and transfer energy is recognised. The improvement of the technology readiness level makes it possible to achieve major installations of the renewable hydrogen electrolysers in the coming years.

This paper has also pointed out that the current status on the system capital cost and hydrogen production cost are still not competitive for the hydrogen's wide introduction to the industrial deployments and the consumption of water and rare materials have limited the development from the aspect of sustainability. Moreover, the efficiency and durability of electrolyser systems and fuel cell systems are not satisfied, which lead to the high operation and maintenance cost. Based on the literature survey across the time, a variety of progress is demanded in the near future.

Figure 3 :

 3 Figure 3: Polarization curve of a 33-cell 5kW PEMEL with an active area of 700 cm 2 , comparing with the model identification result of (2) and (3)

Figure 4 :

 4 Figure 4: Power and efficiency curves of a 33-cell 5kW PEMEL with an active area of 700 cm 2 : Increasing the current will raise the supplied power, however, it will sacrifice the efficiency.

Figure 6 :

 6 Figure 6: System efficiency of a 5kW PEMEL system considering ancillary losses

Figure 8 :

 8 Figure 8: Fuel cell system structure: Hydrogen and air are compressed and humidified into the fuel cell stack and the produced power is supplied to the load through an AC/DC power converter.

Figure 10 :

 10 Figure 10: Power and efficiency curves of of a 5-cell 300W PEMFC stack with an active area of 100 cm 2 : Increasing the current will raise the delivered power by the fuel cell, however, it will sacrifice the efficiency.

Figure 11 :

 11 Figure 11: Different technologies of hydrogen storage

Figure 12 :

 12 Figure 12: Hydrogen density evolution at different temperatures and pressures

Figure 14 :

 14 Figure14: Power supply infrastructure of data centers (project DATAZERO: https://www.irit.fr/datazero/index.php/en/)[START_REF] Pierson | Datazero: Datacenter with zero emission and robust management using renewable energy[END_REF]

Figure 15 :

 15 Figure 15: Integration of a 2.5 MW electrolyser in a state-of-the-art wind farm (project HAEOLUS: http://www.haeolus.eu/)

Figure 16 :

 16 Figure 16: Demonstration of the working principle of tri-generation

Figure 17 :

 17 Figure 17: Review procedure and criteria for screening

Figure

  Figure19: Yearly evolution and percentage distribution of publications from 2000 to date containing terms "hydrogen" AND ("cost" OR "economy"), "hydrogen" AND "efficiency", "hydrogen" AND ("durability" OR "lifetime" OR "degradation"). The search was made on the 2th June 2020.

Figure 20 :

 20 Figure 20: Performance evolution of different scales of fuel cell applications (indicated by normalized medium values), data from[START_REF]Fuel cells and hydrogen joint undertaking, State-of-the-art and future targets (kpis)[END_REF][START_REF] Ammermann | Advancing europe's energy systems: Stationary fuel cells in distributed generation[END_REF]

Figure 21 :

 21 Figure21: Performance evolution of different hydrogen production methods using renewable electricity (indicated by normalized medium values), data from[START_REF]Fuel cells and hydrogen joint undertaking, State-of-the-art and future targets (kpis)[END_REF]

Figure 22 :

 22 Figure22: Increasing installed scale of the worldwide power-to-gas projects (from 2000 to date), data from[START_REF] Patel | A review of global power-to-gas projects to date[END_REF]

Table 1

 1 Comparison of different types of electrolysers in their operation temperature, stack voltage efficiency and pros and cons[START_REF] Schmidt | Future cost and performance of water electrolysis: An expert elicitation study[END_REF]

	Type	Operating	Stack voltage	Pros and cons
		temperature	efficiency
	Alkaline electrolyser (AEL)	< 80 • C	62% -82%	Pros: good durability and maturity. Cons:
				low partial load range, low current density.
	Proton exchange membrane	< 80 • C	67% -82%	Pros: good compactness and efficiency, fast
	electrolyser (PEMEL)			response. Cons: more expensive and lower
				durability.
	Solid oxide electrolyser (SOEL)	> 700 • C	Around 100%	Pros: high efficiency and operation pres-
				sure, reusable heat. Cons: low maturity, not
				widely commercialized.
		O 2	Power supply		H 2
	Purification/ drying	Power converter	Purification/ drying
	system			system
	Gas-water			Gas-water
	separator			separator
	Water		Electrolyser stack Heat exchanger
			Pump

AC DC

Figure 2: Electrolyser system structure:

Unused air and/or water out O 2 H 2 O Excess gas and/or water out Air in H 2 in H 2 O Figure

	H 2
	H 2	Anode e -	Electrolyte	Cathode e -
	electricity + heat			(8)

Acknowledgment

This project has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking under the European Union's Horizon 2020 research and innovation program [grant number 779469]. Any contents herein reflect solely the authors' view. The FCH 2 JU and the European Commission are not responsible for any use that may be made of the information herein contained. This work was also supported by French research agency ANR under the RECIF project [grant number ANR-18-CE05-0043], the EIPHI Graduate School [contract ANR-17-EURE-0002] and the Region Bourgogne Franche-Comté, France.

Research and development of approaches to reduce cost while improving the system efficiency and durability should be undertaken. Furthermore, policy-makers should enhance the measures that can bring hydrogen to today's markets and promote the development of hydrogen integrated energy systems.

Highlights

Hydrogen Energy Systems: A Critical Review of Technologies, Applications, Trends and Challenges

Meiling Yue, Hugo Lambert, Elodie Pahon, Robin Roche, Samir Jemei, Daniel Hissel • Characteristics of hydrogen technologies are presented with experimental results.

• Hydrogen powered energy systems are exemplified by worldwide projects.

• Current status on cost and performance of hydrogen powered systems is investigated.

• A detailed statistical analysis of publications is presented.

• Future trends and challenges on technological and social aspects are discussed.