Hydrogen Production Methods Based on Solar and Wind Energy: A Review

Abstract

Several research works have investigated the direct supply of renewable electricity to electrolysis, particularly from photovoltaic (PV) and wind generator (WG) systems. Hydrogen (H₂) production based on solar energy is considered to be the newest solution for sustainable energy. Different technologies based on solar energy which allow hydrogen production are presented to study their benefits and inconveniences. The technology of water decomposition based on renewable energy sources, to produce hydrogen, can be achieved by different processes (photochemical systems; photocatalysis systems, photo-electrolysis systems, bio-photolysis systems, thermolysis systems, thermochemical cycles, steam electrolysis, hybrid processes, and concentrated solar energy systems). A comparison of the different methods for hydrogen production based on PV and WG systems was given in this study. A comparative study of different types of electrolyzers was also presented and discussed. Finally, an economic assessment of green hydrogen production is given. The hydrogen production cost depends on several factors, such as renewable energy sources, electrolysis type, weather conditions, installation cost, and the productivity of hydrogen per day. PV/H₂ and wind/H₂ systems are both suitable in remote and arid areas. Minimum maintenance is required, and a power cycle is not needed to produce electricity. The concentrated CSP/H₂ system needs a power cycle. The hydrogen production cost is higher if using wind/H₂ rather than PV/H₂. The green energy sources are useful for multiple applications, such as hydrogen production, cooling systems, heating, and water desalination.

1. Introduction

Many investigations have been conducted to enhance the hydrogen production and efficiency of the green energy source system. The photovoltaic (PV) system is considered to be the most appropriate technology for solar-based hydrogen production combined with water electrolysis. PV panels produce electricity to power the electrolysis system, which allows the extraction of oxygen (O₂) and hydrogen (H₂) gases from water.

Many research works have elaborated on the performance and cost of hydrogen production using green energy sources such as solar and wind energy. The studies have investigated the supplying of electricity to electrolysis from PV systems and wind turbines. In this study, we focus on the different production methods based on renewable energy by giving a comparison of the methods and the benefit of each method.

An optimization has been investigated to give maximum hydrogen production by matching the characteristics of photovoltaic panels with those of electrolysis [1]. The PV generator combined with a light concentrator has been studied and proposed and includes a water electrolysis unit. The hydrogen production cost was given after a detailed analysis of the results.

The sensitivity of the PV hydrogen production costs has been investigated [2] by modifying some of the factors influencing the cost of the PV hydrogen production system (HPS). For the best use of energy sources, four key measures have been tested, including the hydrogen production cost, the availability of the energy source, the land cost, and the reduction in carbon dioxide (CO₂) emissions [3]. The results show the different aspects of the HPS using renewable energy sources and traditional sources. Moreover, an analytic hierarchy process (AHP) has been applied for different hydrogen production processes. The results show that the processes based on renewable energy sources such as the PV system, wind, and hydropower give a better performance than the conventional processes [4].

An investigation into the use of solar thermal hydrogen production has been presented in [5]. A description has been given for powering steam methane reforming, thermochemical cycles, and high temperature water electrolysis by solar energy. A review of hydrogen production costs based on traditional systems, nuclear power, and renewable sources was presented [6]. A prediction of the production costs was made for the different technologies.

A new thermochemical method for solar hydrogen production was elaborated by giving a techno-economic report [7]. The method of the global yearly revenue requirement was used to analyze the hydrogen production cost. A hybrid process based on renewable electricity combined with electrolysis and a battery to produce hydrogen was proposed and analyzed [8]. The combined system allows the improvement of the performance of the hydrogen production system. Moreover, a model was developed to analyze the quality of a combined electrolysis/battery unit which was directly coupled using a large off-grid PV power plant.

A hydrogen production system based on a hybrid solar parabolic combined with an electrolysis system has been examined to study its techno-economic performance [9]. The impact of climate conditions such as direct solar irradiation and solar fraction has been studied to evaluate their effects on hydrogen outcomes by using two different geographical sites in Algeria. The results show that there is a direct relationship between the hydrogen cost and the cost of the energy source used.

Another study has explored hydrogen production using an electrolysis system based on a proton exchange membrane (PEM) powered by solar power [10]. Some parameters were evaluated such as solar hydrogen and electrolysis efficiencies. The results showed that the use of methanol solutions instead of water minimize the cost of hydrogen at a small size.

A technical economic study has investigated a hydrogen production system based on a PV system associated with a battery and electrolysis unit [11]. The results show the low cost of hydrogen by using a combination of technologies. The cost depends principally on the costs of the PV system and the electrolysis.

A few research works are based on the simulation and modeling of electrolysis such as alkaline electrolysis (AEL) and polymer electrolyte membrane electrolysis (PEMEL). A model has been proposed for both electrolysis using solar irradiation and wind energy data as input [12]. The results indicate that hydrogen production output, its efficiency, and its costs are related to the power source of the electrolysis. The AEL technology needs less electricity than PEMEL, and AEL produces more hydrogen than PEMEL because of its higher efficiency.

An evaluation of electrolysis powered by solar energy based on a mathematical programming framework to optimize the system cost with specific weather conditions has been conducted [13]. In fact, the results showed that on-grid PV systems affect the cost of the solar-electrolysis system. A cost analysis has been conducted on hydrogen production using wind energy [14]. This study shows that the productivity cost of hydrogen is reliant on the considered demand request. A wind/electrolysis system has been investigated to evaluate the cost of hydrogen production [15]. Different cases have been analyzed such as different hub heights and different power levels, such as 120 kW and 40 kW. It was confirmed that the total cost is low when the turbine hub is higher.

Another source of renewable energy is also used to produce hydrogen using a large-scale wind plant to drive electrolytic hydrogen [16]. The wind/H₂ system proposed has a capacity of 563 MW for electricity production destined to power electrolysis units. The result allowed a modeling and a simulation of the wind/H₂ system. An investigation has been conducted concerning an economic assessment of wind/H₂ production [17]. The feasibility analysis is based on cost indicators resulting from the comparison of renewable and non-renewable sources. A modeling of wind/H₂ with energy storage has been suggested for the development of 563 MW [18].

The wind energy data have been used for the optimal sizing of the electrolysis unit and energy storage. The best configuration used contains 81 units of 3495 kW (760 Nm³/h) of electrolysis and 60 units of 360 MWh of battery capacity. The lower production cost of hydrogen was estimated at 9.00 USD/kg, and 63% of this price was destined for the wind farm. A newly designed energy system has been elaborated for hydrogen production using electrolysis [19]. In fact, a mixed system combined solar and wind energy to generate the large quantity of energy needed for the hydrogen production unit. The price found for hydrogen productivity was around USD 4.10 per kg, which corresponds to a supplement cost while minimizing CO₂ emissions by 2%.

Recently, a combination using both wind turbines and a fuel cell has been elaborated for a drive train in Germany [20]. The water/electrolysis is powered by wind energy, which has great potential and can be used in rail transportation

Another work on the potential of wind energy development for the purpose of hydrogen production has been investigated [21]. Wind energy characteristics have been statistically analyzed to determine the potential of wind power to generate hydrogen in the studied cities. The performances of four different large-scale wind turbines for producing wind power in Abadeh city were evaluated. It was found that hydrogen from wind energy using a small hydrogen-producing unit would fuel approximately 22 cars per week if an EWT Direct wind 52/900 model wind turbine was used.

Another investigation was conducted on the CIGS absorber film, which is deposited in one step to optimize the deposition parameters, such as deposition potential, deposition time, and overvoltage of hydrogen on the surface of the deposited cathode [22].

Other technologies, such as biological or photochemical technologies, are used for hydrogen production in low-scale units for educational and research studies. These units give a low production of H₂, which is used for fuel cells with low power.

Solar biochemical energy conversion is one of the optional routes to convert solar energy into a beneficial source of energy. In fact, hydrogen can be generated from a variety of biochemical substrates; however, the efficiency of any biochemical hydrogen generation process is generally low. A hydrogen-producing enzyme is strictly necessary to generate hydrogen gas from a substrate. The most important enzymes for biochemical hydrogen are nitrogenase and hydrogenase, which are the most effective for hydrogen generation [23]. An indirect pathway to generate biochemical hydrogen is through biogas and biofuel reforming, such as methanol, ethanol, and biodiesel which can be used to generate green hydrogen [23].

Biochemical energy allows the generation of hydrogen in either the absence or presence of photonic energy, which can come from the sun or a light source. If this biochemical to hydrogen energy conversion occurs in the absence of any light, it is called dark fermentation. Dark fermentation can also take place when the light supply is reduced below a certain level [24].

Dark fermentation reactors present many advantages when compared to the photo-fermentation reactors since they do not need any photon energy-processing components. The hydrogen production ability from organic waste allows the regulation, stabilization, and elimination of waste, which can potentially eliminate any risk of biological waste pollution. Moreover, dark fermentation can be combined with wastewater management facilities to generate H₂ from wastewater. Hydrogen production from organic and biological waste and wastewater can potentially lower the hydrogen production costs as the process input is the waste of other processes; so, it is affordable, abundant, and easily available.

In modern thermochemical routes, hydrogen can be produced from biomass. It is a technology developed using the same biofuel technologies, such as that of bio-methane, adopted from steam methane reforming. Basically, gasification, pyrolysis, and aqueous phase reforming are the three simple thermochemical methods for producing hydrogen [25]. The thermochemical method is in active research, development, and commercialized system. However, it needs electricity, water, chemicals, and thermochemical methods for the hydrogen production; moreover, it can use nuclear energy as an energy source supply. Biomass gasification is a typical thermochemical route; it represents an economical method of hydrogen production. Currently, its efficiency is around 50%. Steam, oxygen, and air are the main gasification agents that can be used for this method. This pathway to produce hydrogen has fewer adverse environmental impacts than those based on fossil fuels because the CO₂ released comes from CO₂ absorbed while the organisms are still alive, and no excess carbon is entered into the atmosphere [26]. In addition, the carbon emissions of hydrogen production by fossil fuel thermochemical technology are not satisfactory, and the economic cost of carbon reduction is greater than that of other clean technologies.

Furthermore, the copper-chlorine cycle is one of the thermochemical processes to dissociate water into hydrogen and oxygen. It consists of reaction cycles such as hydrolysis, thermolysis, water separation, and hydrogen production [27].

In this work, we focus on a green hydrogen system which has two important components: a renewable energy source and water electrolysis units. The electrolyzer allows the extraction of hydrogen and oxygen from water. The electrolyzer powered with green energy sources has become the most appropriate commercial instrument for hydrogen productivity and storage. The most used electrolyzers are the high-temperature steam electrolyzer (HTSE), the proton exchange membrane electrolyzer (PEM), the alkaline anion exchange membrane (AEM), the alkaline water electrolyzer (AWE), and the solid oxide electrolyzer (SOE) [28,29].

The principal advantages of electrolyzer-based renewable energy sources are as follows:

• Minimization of the cost of storage and transportation when using stand-alone systems.
• High hydrogen production compared to photoelectrochemical system.
• No need for grid installation in remote areas.
• Availability of renewable sources [30].

In this work, we present the different methods of the hydrogen generation systems that use electrolysis supplied by the renewable energy sources which have been introduced since 2000 [31]. There are more applications for hydrogen production using solar energy than wind energy.

The main objectives of this work are summarized as follows:

• To present the different types of electrolysis system using renewable energy as input green sources.
• To describe the different systems for green electricity, such as solar and wind energy to power electrolysis for H₂ production.
• Typical examples of hydrogen production are given in this work: solar energy represented by the PV system and the concentrated solar power (CSP) system and the wind turbine).
• A comparative study of the various methods for H₂ production based on solar energy and wind energy is given.
• An economic assessment of green hydrogen production is also presented. The electricity cost (USD/kWh) and the hydrogen production cost (USD/kg) are given for different energy sources.

Section 2 describes the different types of water electrolyzers by giving their advantages and their disadvantages. In Section 3, the different methods of H₂ production based on solar energy and wind energy are described. Section 4 provides the green electricity production systems. Finally, an economic study of green hydrogen production is given in Section 5. The conclusion and the perspective are reported in the last section.

2. Different Types of Water Electrolyzers

The green hydrogen system is constituted by two important components:

• The renewable energy source.
• The water electrolyzer.

The latter allows the extraction of H₂ and O₂ from water. The best electrolyzers available are:

• The proton exchange membrane electrolyzer unit.
• The alkaline water electrolyzer.
• The alkaline anion exchange membrane.
• The high-temperature steam electrolyzer.
• The solid oxide electrolyzer.

The basic equation of water decomposition is given:

H₂ O (Liquid) + Energy = H₂ (g) + ½ O₂ (g)

(1)

The electrical energy is used from a DC power source. The water electrolysis technology can be classified into three categories based on the nature of the electrolyte used in the cells:

Use of liquid electrolyte: alkaline water electrolyzer and alkaline anion exchange membrane.

Electrolysis in acid ionomer environment: proton exchange membrane and solid polymer electrolyzer.

Steam electrolysis: high temperature steam electrolyzer and solid oxide electrolyzer (HTSE and SOE).

The fundamental principle for the electrolysis cell is shown in Figure 1. All the cited electrolyzers have the same principle. If a voltage is applied to the electrochemical cell, the water decomposes into H₂ and O₂ gas bubbles which emerge towards the cathode and anode, respectively.

The different reactions, the operating temperature intervals, and the charge carrier for each type of electrolysis technology are presented in

Table 1. Different operating temperatures and reactions of each type of electrolysis technology.

Electrolysis Technology	High-Temperature Electrolysis	Alkaline Electrolysis (AWE and AEM)	Membrane Electrolysis
Anode Reaction Oxygen Evolution Reaction (OER)	${\mathrm{O}}^{2-}\to \frac{1}{2}{\mathrm{O}}_2+2{\mathrm{e}}^{-}$	$2{\mathrm{OH}}^{-}\to \frac{1}{2}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-}$	${\mathrm{H}}_2\mathrm{O}\to \frac{1}{2}{\mathrm{O}}_2+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}$
Cathode Reaction Hydrogen Evolution Reaction (HER)	${\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2+{\mathrm{O}}^{2-}$	${\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2+2{\mathrm{OH}}^{-}$	$2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2$
Charge Carrier	${\mathrm{O}}^{2-}$	${\mathrm{OH}}^{-}$	${\mathrm{H}}^{+}$
Operating Temperature Range	700–1000 °C	40–90 °C	20–100 °C

[32].

2.1. Proton Exchange Membrane Electrolyzer (PEM)

The PEM is constituted by electrodes and the electrolyte, and the used materials for the electrodes are platinum, iridium, and ruthenium. The PEM is characterized by a compact design with a high conductivity of protons and a high pressure (15 to 30 bar at 50–90 °C) [33]. The advantages of the PEM consist in its fast response and the high purity (99.999%) of the hydrogen produced [34]. However, its disadvantage is its high price because of the noble material used inside the electrolysis [35].

2.2. Alkaline Water Electrolyzer (AWE)

The AWE is an electrolyzer constituted by a diaphragm and two electrodes inside a liquid electrolyte water solution, which is in general almost 40% sodium hydroxide (NaOH) or potassium hydroxide (KOH) [36]. The diaphragm which separates the two electrodes in the solution allows the circulation of water molecules and hydroxide ions and separates the H₂ and O₂ for safety and purity aspects [33,37]. The purity of the hydrogen produced is 99.5 to 99.9%, and it can reach 99.99% by using catalytic gas purification processes [34]. The performance of this kind of electrolysis is affected by the diaphragm, anode, and cathode material type and thickness. The AWE and the PEM use two different types of electrolytes: the PEM uses a solid polymer membrane electrolyte, but the AWE uses a corrosive liquid electrolyte.

2.3. Alkaline Anion Exchange Membrane (AEM)

The AEM is considered to be a combination of the PEM and the AWE. The advantages of the PEM and the AWE are merged in the AEM. So, the AEM uses a low-concentration alkaline solution as opposed to a 20–40% KOH or NaOH aqueous solution with a solid electrolyte (polymeric) membrane [38]. The anode in an AEM is composed of Ni-based (e.g., Ni foams) or titanium materials, and the cathode includes Ni, Ni-Fe, and NiFe2O₄ [39].

2.4. High-Temperature Steam Electrolyzer (HTSE)

The HTSE is an electrolyzer which operates at a high temperature using steam. The principal benefit of this technology is that the dissociation of steam needs less energy compared with liquid water [40]. In order to decompose the water molecule, the HTSE uses heat instead of electrical energy. This process allows the improvement of the efficiency and the decreasing of the hydrogen production cost.

2.5. Solid Oxide Electrolyzer (SOE)

The SOE works at a high temperature but due to the low temperature of electrolysis, the needed electricity to power its electrolysis is low. Therefore, the system’s efficiency is improved because it uses a low-cost thermal energy. The hydrogen production unit of the SOE is defined by the steam at the cathode side, which is reduced to hydrogen according to the cathode reaction, and then, the oxide anions generated are the path through which the solid electrolytes form oxygen on the anode side [37].

2.6. Advantages and Disadvantages

There are very few applications for H₂ production using HSTE technology.

Table 2. Advantages and disadvantages of water electrolyzers [28,41].

Electrolyzers	HTSE	PEM	AWE	AEM	SOE
Advantages	- Lower energy needed - Lower voltage - High efficiency - Provides heat required to split the water - Suitable for load fluctuation - High-pressure operation	- High hydrogen purity - Compact and simple design - Fast response - High current - High dynamic operation	- Stable and well-established - Noble materials not needed - Low capital Cost - Efficiency 70%	- High hydrogen Purity - Simple design - Low cost	- High efficiency - Used as fuel Cell - Low capital cost
Disadvantages	- Degradation phenomena - Low lifetime due to high heat - Bulk system design	- Expensive - High membrane cost - Low durability - Acidic environment	- Corrosive electrolyte - Low H2 purity. - Slow startup - Low current density - Low operational pressure	- Low ionic conductivity - Low membrane stability - Low lifetime	- Unstable electrodes - Safety and sealing problems - Bulky design - Uses brittle material

presents the advantages and limits of the different types of water electrolyzers.

2.6.1. Advantages

A comparison between the low-temperature electrolysis technologies and the high-temperature steam technology is given and discussed as follows, with both the technical and the economic considerations:

• Low energy needed for the HTSE compared with low-temperature electrolysis (PEM, AWE, AEM, and SOE).
• Low voltage is needed for the HTSE compared with the other electrolysis technologies (PEM, AWE, AEM, and SOE).
• Lower values of voltage are applied for the HTSE than for the other electrolysis technologies. This is due to the low energy required to split the steam molecule compared to the energy required to split a water molecule. The typical range of the I-V curve for different cells is presented in Figure 2.
• The HTSE has better efficiency due to the low power consumption.

Figure 3 presents the voltage (V) versus current density, which is related to the hydrogen production based on the Faraday law:

Q = I/2 F

(2)

Q represents the hydrogen flow rate produced, I is the current, and F is the Faraday constant, which is equal to 96,485 Coulombs/mol.

Thus, the higher current density corresponds to the higher hydrogen production. The HTSE is a good process which allows the improvement of the hydrogen production rate. For the same output amount of hydrogen production, the needed voltage is lower for the HTSE than the PEM and the alkaline, as shown in Figure 2.

An experimental example is given in

Table 3. Experimental comparison between HTSE, PEM, and alkaline.

Current Density (1 A/cm2)	HTSE	PEM	Alkaline
Efficiencies	96%	68%	62%
Hydrogen production of 0.5 (Nm3/h/cm2)	1 V	1.5 V	2 V
Power consumption (W)	0.5 W	0.75 W	1 W

. With a current density around 1 A/cm², less voltage is needed for the HTSE with an efficiency of 96%. For comparison, the efficiencies for the PEM and the alkaline water electrolyzer are 68% and 62%, respectively [42]. As we can see, in Table 3, to produce 0.5 Nm³/h/cm² of hydrogen, we apply 1V if using the HTSE and 1.5 V if using the PEM and 2V in the case of the alkaline.

The I-V curve of an electrolysis unit represents its performance, as indicated for the HTSE cell in Figure 3. This figure indicates the I-V curve for the HTSE with the corresponding hydrogen production. The conditions taken [32] for this characteristic correspond to the air as an anodic gas and a mixture of 90% H₂O and 10% H₂ as cathodic gas. The steam conversion is 43% at 2 A/cm².

2.6.2. Disadvantages

The last section showed that the high-temperature steam electrolyzer is better than the low-temperature electrolyzer from the point of view of efficiency and performance. However, there is degradation which appears during the HTSE operation due to the phenomena happening during steady-state operation and cycling [43]. Degradation phenomena can be found at the cell and stack component levels and are due to the physico-chemical, electrochemical, chemical, or mechanical phenomena. Moreover, the electrolyte instability and the formation of oxygen in the electrode can be the causes of some phenomena [44].

3. Hydrogen Production Methods

There are some methods that allow the production of hydrogen using the thermal decomposition of water based on clean energy sources, such as solar energy, wind energy, geothermal energy, biomass energy, hydro energy, ocean thermal energy, tidal and wave energy, and nuclear radiation. In this present work, we focus on the hydrogen production process based on solar energy and wind energy sources [37]. Figure 4 summarizes the above methods for hydrogen production.

Figure 5 presents the processes for clean hydrogen production via clean sources.

3.1. PV-Electrolysis System

This system is composed of photovoltaic cells which produce the electricity to power the electrolysis unit, as shown in Figure 6. The electrolysis of water is an electrochemical water-splitting process which allows the decomposition of water (H₂O) to H₂ and O₂ gas [45], as shown in Figure 7. The ions of H₂ and O₂ pass through the water to the cathode and anode, respectively. The produced H₂ has many benefits, such as fuel cell applications and welding applications when mixed with O₂ to obtain oxyhydrogen gas. This method produces a large amount of high-purity hydrogen without environmental impacts. This hydrogen production process is powered by the electricity from solar energy.

The objective is to improve the efficiency of the electrolyzers to more than 90%, knowing that the efficiency is currently around 75% [45]. The electrolyzers produce hydrogen with no gas emissions when using renewable energy sources as electrical energy.

The PV-electrolysis device given in Figure 6 is composed of a three-junction solar cell and two electrolysis (PEM) units in series [46]. The cell was cooled to reach the temperature of 25 °C using a water cooling system and was illuminated under a sun simulator based on white light, using a xenon arc lamp to obtain an illumination corresponding to AM 1.5D solar irradiation.

The two electrolysis units were branched in series with the PV cell. The water was pumped into the anode compartment of the first electrolysis unit, and there was no input flow for the cathode of the first electrolysis unit. The water and O₂ from the first electrolysis unit’s anode compartment flowed to the anode compartment of the second electrolysis unit. The H₂ from the cathode side of the first electrolysis unit flowed to the cathode side of the second electrolysis unit. The H₂ and O₂ produced were collected from the second electrolysis unit and the remaining water was collected in a tank and then recycled into the system.

The temperature of the electrolysis unit was fixed at almost 80 °C, which corresponded to the standard conditions for industrial water electrolysis [33].

3.2. Hybrid Photovoltaic/Thermal (PV/T)-Electrolysis System

The solar photovoltaic thermal (PV/T)-electrolysis system is constituted by PV panels and PEM electrolysis. The PV/T electrolysis system is constituted by the following parts: the PV-thermal array, the converter DC/DC, and an electrolysis unit. Figure 8 shows the hybrid PV/T-electrolysis system [47].

The performance of the (PV/T) system has also been tested using the PEM electrolysis cell (PEMEC) for hydrogen production. The PV/T gives the necessary current for the PEMEC and preheats the feed water [48]. The annual experimental data of the hydrogen production using PEMEC are given in Figure 9.

A model has been developed for the PVT-PEMEC system. This model contributed to the study of the effect of different factors, such as solar irradiation, water temperature, and the water mass flow rate, on the hydrogen production.

An experimental study has investigated hydrogen production using solar energy via hybrid PV/T, which allows the obtaining of electrical and thermal energy. The electrical energy is produced for powering the alkaline water electrolysis, and the thermal energy is necessary for heating the circulating water fixed on the back surface of the PV panel, as shown in Figure 10 [49]. The H₂ production unit was experimented on at different temperatures of electrolysis water. For the best configuration, the results gave the maximum hydrogen production rate of almost 154 mL/min with the efficiency of the system around 21%, and the daily H₂ produced was almost 221 L/day.

3.3. Wind-Electrolysis System

A wind-electrolysis device comprises a wind turbine generator, a converter (AC/DC), and a water electrolyzer [50]. Many applications can be performed using the wind-electrolysis system for the following configurations:

• Direct configuration: wind-electrolysis. This application is suitable in remote areas with wind farms [51].
• Production of hydrogen using this configuration: hybrid wind/grid-electrolysis. This application allows the contribution of a grid as the auxiliary energy to the wind turbine when there is no wind.
• The third configuration consists in the production H₂ using wind energy, and the excess of energy from the wind is provided to the grid.
• The fourth case corresponds to the third configuration with a storage system of hydrogen, which allows the production of electricity by a fuel cell [52].

Figure 11 shows the different components of the wind-electrolysis system.

A study elaborated on the use of the horizontal axis wind turbine (HAWT) to power the electrolysis of a type of AWE, and the excess of hydrogen was used to generate electricity by a fuel cell [53]. It was found to have a total efficiency of 60%. An experimental study on wind/H₂ systems contributed to the provision of electricity for 3 days for ten households [54].

As reported later in this manuscript (Section 5), the electricity cost from wind energy is higher compared to the other sources (see Figure 12). So, many actions will be performed to reduce the cost of the wind system. In fact, due to the wind speed fluctuation, a chopper circuit has been introduced to regulate the input electricity for each electrolysis system [55]. This allows the improvement of the life and efficiency of such a system. Moreover, an experimental work using the vertical axis wind turbine (VAWT) to power the electrolysis system has given acceptable results [56].

3.4. PEM Electrolysis/Photocatalysis

Hydrogen production has also been produced using a heterogenous photocatalyst. This process is based on photo-electrocatalysis or direct photocatalysis [57]. The principle of this process is the generation and transfer of electrons between electrodes under the effect of incidental light. This can be performed when the electrodes are fabricated by a photoactive material such as a semiconductor (SC). The photoanode is made of an n-type SC, while the photocathode is made of a p-type SC. The principle of the photoelectrochemical cell (PEC) is presented in Figure 12.

The principle of the photocatalysis system allows water decomposition based on solar energy, which drives the photo-material and allows the generation of photoexcited charge carriers to produce hydrogen in easy steps, as follows [57,58]:

• Solar light is absorbed by the photoanode and then generates electrons and holes.
• Electrons and holes transfer between electrodes.
• Chemical reactions allow the extraction of H₂ and O₂ from the water molecule.

Hydrogen production based on the water-splitting system can be elaborated as follow [59,60]:

• Photocatalyst (PC) system.
• Photoelectrochemical (PEC) system.
• Photovoltaic-photoelectrochemical (PV-PEC) system.

The PC system is a simple way of water splitting based on solar irradiation (see Figure 13). In the photocatalyst process, the water decomposition appears in the homogeneous phase, and there is no necessity for transparent electrodes [60]. The PC system has a limitation in the water decomposition process:

• Low efficiency due to the extra energy required in the water-splitting process for the immediate separation of H₂ and O₂.
• A photo-stationary state can be caused due to the same rate of the reactions without the splitting of water under the illumination of a PC system.
• For a large scale, the installation of PC systems is complicated.

In a PEC system, the photoanode and photocathode are made from a photoactive semiconductor material. A low voltage is applied for water splitting. The incident solar irradiation allows generations of charge carriers and the holes (n-type photoanode) and electrons (p-type photocathode) to travel to the semiconductor electrode–liquid interface for the reaction [61], as shown in Figure 14b,c. The PEC system has the benefit that no gas is needed for the separation because the generation of H₂ and O₂ is separated at different electrode sides [61,62].

The PV-PEC consists of a single photoelectrode PEC configuration, powered by a PV cell, as shown in Figure 14a.

3.5. Bio-Photolysis/Photochemical

The photo processes are used to separate hydrogen from water using sunlight.

The photo processes can be classified as follows:

• Photoelectrochemical;
• Photochemical;
• Photobiological.

The photobiological process is applied for small quantities of hydrogen production [63] due to its low efficiency. In fact, the hydrogen production systems, based on the photochemical or biological units, are small-scale units which are destined for educational and research purposes only. The small amounts of hydrogen produced by such a system are in general used to power small fuel cells around a few watts.

In the bio-photolysis system, the extraction of H₂ and O₂ from water is due to the solar energy. This process can be one of forward or reverse bio-photolysis:

• In forward bio-photolysis, the sunlight allows water splitting, and then, the electrons produced from the splitting of water are used.
• In indirect bio-photolysis, the endogenous substrates catabolize and produce electrons which are used in the indirect process.

After transferring the residual electrons to the photosystem, the hydrogenase allows the production of hydrogen with almost zero emissions of CO₂. The bio-photolysis process releases oxygen into the atmosphere based on this reaction:

2H₂O + light energy → 2H₂ + O₂

(3)

3.6. Thermolysis System

The thermolysis system or the thermal decomposition of water based on solar energy has been applied to improve the efficiency and to minimize the cost of hydrogen production. The H₂ production cost, using the solar thermal dissociation of water and gas at high-temperature electrolysis, is lower compared to the cost of hydrogen produced by PV-electrolysis [64]. The thermal energy generated by concentrators allows the heating of water or fossil fuels in thermolysis system. In fact, the thermal decomposition of natural gas at high temperature is the more appropriate procedure for hydrogen production.

3.7. Thermochemical System

The thermochemical process consists in the decomposition of water into hydrogen and oxygen by combining heat sources (thermos) with chemical reactions. The chemicals used are recycled in process called thermochemical cycles.

In this method, water is heated to very high temperature, which is around 2500 °K, until it decomposes into hydrogen and oxygen. So, three conditions are needed for this process:

• The necessity of a high-temperature source.
• The materials used in the reactions should withstand the high temperatures.
• Complex chemical methods are used to separate the hydrogen and oxygen [65].

3.8. Steam Electrolysis

Electrolysis can produce hydrogen and oxygen at a high temperature using steam electrolysis (HTSE) and at a low temperature using water [66]. The reaction of the decomposition is the same relation as that given in (1).

In the steam electrolysis process, smaller amounts of energy are required for the dissociation of steam (H₂O (g)) than liquid water (H₂O (L)). Moreover, for the high temperature, the heating could replace a part of the electrical energy necessary for water decomposition. This contribution of heat allows the reduction in the hydrogen production cost with high efficiency.

The steam electrolysis process comprises several single-repeat units, as indicated in Figure 13. The principal part in the HTSE is the electrochemical cell which is fabricated in ceramics due to the high operating temperature [60]. This electrochemical cell is named the solid oxide electrolysis cell (SOEC) and is formed by three layers in ceramic: a dense electrolyte and two porous electrodes (cathode for H₂ and anode for O₂), as shown in Figure 15.

Table 4. Comparison between different methods for hydrogen production based on solar and wind energy.

H2 Production Methods	Advantages	Disadvantages	Ref.
PV-Electrolysis system	Simple design and compact. Fast response and startup. High hydrogen purity Low Electricity production cost	Low current density Low operating pressure Slow loading response Fluctuating power which depends on weather conditions	[4,33,45,46]
Hybrid PV/T-Electrolysis system	Produces very pure hydrogen and at the same time requires much less maintenance Easily adapted to the desired rate of H2 production or to the output PV energy The feedwater is preheated	Must use distilled or deionized water instead of tap water	[5,29,30,31,32,33,47,48,49]
Wind-Electrolysis system	Suitable in remote areas Wind intermittency Low electricity production cost Technically mature and has already been commercialized	Power fluctuation changes based on wind speed	[4,50,51,52,53,54,55,56]
PEM Electrolysis/Photocatalysis (PC)	PC is the simplest, cheapest, method for water splitting Homogeneous phase for water splitting	Low efficiency of water-splitting process Needs additional energy to separate H2 and O2 A photo-stationary state Implementation still challenging	[57,58,59,60,61,62,67]
Bio-Photolysis/Photochemical	Use of solar water splitting Water resources available Low potential required (1.23 eV) No CO2 emission	Low efficiency Small amounts of hydrogen produced	[1,63,68]
Thermolysis system	High efficiency Low cost	Requires high temperatures Unsuitable for real-life applications due to the high temperature Difficult to separate hydrogen and oxygen in time Material-related limitations (caused by high temperature requirement)	[64,69,70]
Thermochemical system	High theoretical efficiency potential Low or no greenhouse gas emissions Long-term technology pathway	A high-temperature source is required Complex chemical methods are used to separate H2 and O2 Generates a lot of waste	[65,71,72]
Steam electrolysis	Lower energy required for dissociation of steam than for liquid water	Lifetime of the hydrogen electrode, which is limited by degradation	[66,72]

shows a comparison of the different methods; the advantages, the disadvantages, and the corresponding references are given.

4. Green Sources for Electricity Production

Solar energy and wind energy are used as green sources to produce electricity for powering electrolysis for hydrogen production [73]. Some applications have been conducted using solar energy as photovoltaic systems (PV), concentrated solar power (CSP) systems, and wind energy to drive electrolysis systems [74]. For matching the power of all these green energy sources to the input of electrolysis, a converter (AC/DC or DC/DC) is necessary. Renewable energy sources are the solutions to make electricity available in remote areas instead of the high cost of power transmission [41]. The excess of energy provided by green energy sources has been used to drive electrolysis for H₂ production. Figure 16 illustrates the principle of the PV system for hydrogen production (PV/H₂). PV panels drive the electrolysis via the solar charger and DC/DC converter through a maximum power point tracker (MPPT) electronic circuit [75]. The battery is considered to be the energy storage in the case of low solar radiation.

The advantage of the PV/H₂ system compared to other renewable energy sources is the use of DC voltage, and all the parts are guaranteed and do not need a lot of maintenance. The CSP/H₂ system provides heat rather than electricity to drive the electrolysis system and to convert water to steam using the SOE [31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75], as shown in Figure 17. Thermal storage in the CSP/H₂ system allows continuous hydrogen production. An investigation has been conducted [76] to study the performance of each system under the same operating conditions. The results showed that the CSP/H2 system gives better performances than the PV/H₂.

Moreover, wind energy has been used to power the electrolysis (wind/H₂) unit by providing electricity using an AC/DC converter. Wind energy can be available 24 h and not only during daylight as with solar energy, but wind is an unstable energy source due to its nature.

Figure 17 shows a combination of renewable energy sources able to produce hydrogen by driving an electrolysis unit from green sources (PV/H₂, CSP/H₂, and wind/H₂). This configuration allows the improvement of the efficiency of the whole system.

4.1. PV System for Hydrogen Production (PV/H₂)

The PV/H₂ system is a green source for hydrogen production based on the photovoltaic system which generates electricity for powering the electrolysis unit. The PV/H₂ system is the most used method for green hydrogen production due to its cost, performance, and easy feasibility [77].

This system has been tested experimentally in different locations and under various weather conditions [38]. Another study showed that using the PV tracking system gave the best performance but with a greater cost than the traditional PV system and using the concentrated PV system enhanced the efficiency compared with the classical PV system [78]. Another research work based on modeling showed the same results with an experimental study [79]. An investigation of a PV system-based MPPT indicated that the efficiencies were almost close to those of the PV system with or without MPPT [80], but the cost was a little bit higher when using MPPT. Moreover, it was demonstrated that the hydrogen production rate depended on the MPPT efficiency [81].

As reported by many research works, using the PV/H₂ system with PEM electrolysis gave interesting results, and the performances have been increasing from the first experiments until now. In fact, in 2008 only 6% efficiency was found using the PV/H₂ system with a high production cost of around 40 USD/kg [82]. Many research works focused on enhancing the efficiency of the PV/H₂ system and increasing the productivity and then reducing the cost. In fact, in 2010 the efficiency was improved and reached the value of 12.4% by using direct coupling between the PV system and the electrolysis unit [83]. The productivity cost of hydrogen decreased from 40 USD/kg in 2008 to 3.4 USD/kg in 2022 [84]. This result is due to the operating voltage system which is continuously available from the PV system to drive the electrolysis. Many other studies investigated the suitability between PV systems and electrolysis, and the results show that the PV/H₂ system is more adequate in remote areas [85]. Moreover, the so-called PV panels affect the hydrogen production due to their having higher efficiency than horizontal panels [86]. The PV/H₂ system not only allows the production of hydrogen but also can be used to provide electricity by a fuel cell, which is necessary for a nocturnal profile or the during winter season [87].

The electricity provided by a fuel cell can be purchased to power an electrolysis unit when the solar irradiation is low and insufficient to provide the necessary electricity for electrolysis [88]. A comparison between the costs of different green sources for electricity production revealed that using grid/H₂ for hydrogen production is cheaper than using grid/PV/H₂ or than using PV/H₂. In fact, the values of costs are 5.5, 6.1 and 12.5 USD/kg for grid/H₂, grid/PV/H₂, and PV/H₂, respectively [89,90]. The applications of PV systems and wind energy are more useful in remote areas where the conventional grid is not installed due to the very high price of electricity transportation. It has been found that the initial cost depends on the land and the PV system installation costs.

Due to the large potential of solar energy in remote areas, many applications of PV systems are suitable in such locations, especially in the provision of a high production of hydrogen by enhancing the efficiency of the PV/H₂ system. For this, bifacial solar panels have been used to improve efficiency and then to increase hydrogen production [91]. The results showed that the efficiency reached 13.5% for bifacial solar panels instead of 11.55% for monofacial solar panels, corresponding to the increase in hydrogen production to 4.2 g/h/m² instead of 3.7 g/h/m² in the case of monofacial panels.

Another application for the PV/H₂ system revealed that efficiency of a high concentration of PV/H₂ reached 21%, while the efficiency of a traditional PV/H₂ is around 9.4% [92].

Using a PV/H₂ system with batteries allows hydrogen production during the night, and then, it enhances the productivity of hydrogen, and the cost is minimized to around 7.47 USD/kg [93,94]. An investigation of a high-efficiency DC/DC converter allowed an increase in the global efficiency [95].

A photovoltaic thermal (PVT) system provides enough electricity and heat energy. The electricity enhances the productivity of hydrogen using electrolysis, and minimizing the cost and the heat energy allows continuous hydrogen production [96,97].

Many combined systems have been investigated for the provision of electricity (PV) or the generation of a heat energy (PVT) or cooling system (PVT/water) [85,98]. These systems present higher performances than the traditional PV/H₂ system.

4.2. Concentrated Solar Panel System for Hydrogen Production (CSP/H₂)

The concentrated solar power system provides electricity to the electrolysis unit and also heat energy to produce the steam to power the absorption cooling cycles, as shown in Figure 18. This is called the CSP/H₂ system which consists of a solar collector, a parabolic dish collector, and an electrolysis unit. It is used not only for hydrogen production but also to generate electricity, cooling, heating, and a distilled water supply. This multi-generation system improves the global efficiency when the solar radiation increases, and then, the operating temperature of the electrolysis decreases, which allows the increase in its current density [98]. In fact, an investigation on the CSP/H₂ system revealed that its exergy and energy efficiency vary from almost 21% to 36% and from 34% to 72%, respectively [13]. The CSP/H₂ using multi-generation systems uses other available energy sources such as geothermal energy to improve the efficiency [99]. In fact, the hydrogen production cost of such systems is around 2.84 USD/kg, and the generated electricity cost is about 0.03 USD/kwh. In order to produce electricity directly from a solar concentrator, a Stirling engine was installed at its focus point to drive the electrolysis unit [100]. A comparison, from the point of view of the productivity of hydrogen, has been conducted between PV/H₂ and CSP/Stirling/H₂ [101]. It was found that the PV/H₂ system produced almost 268 kg and that the CSP/Sterling/H₂ system produced almost 302 kg.

4.3. Wind Energy for Hydrogen Production (Wind/H₂) System

Wind energy does not perform as well as solar energy due to the random appearance of the wind. Therefore, the produced electricity from wind turbines remains variable during its operating period. So, in the case of excess electricity, it has been proposed to store it as hydrogen gas using wind energy for a hydrogen production (W/H₂) system, as shown in Figure 19. The produced hydrogen can be converted to electricity using a fuel cell during the period of low wind speed and during the period of high wind speed; a part of the hydrogen can be stored and sold while the other part is converted to electricity [102]. So, combining a wind turbine with an electrolysis unit and a fuel cell forms an adequate green energy source and allows the improvement of the performance of wind turbines [103].

In conjunction with the experimental works, there are theoretical and simulation experiments on the wind/H₂ system. In fact, a model has been developed for electrolysis using four numerical models [104]. A good result has been found for the proposed models under the outcome conditions of wind speed. Others research work have investigated the improvement of the performance of the wind/H₂ system using a control method and a new strategy [105,106]. Different techniques and methodologies have been elaborated to minimize the production cost of hydrogen under the wind/H₂ system. It has been observed that the prices depend not only on the initial costs but also on the geographical locations where the wind/H₂ systems are installed.

Table 5. Hydrogen production cost and electricity cost-based wind energy system.

Hydrogen Production Cost (USD/Kg)	Electricity Cost (USD/kwh)	City/Country Names	Ref.
13.28–11.52	NA	Kuwait	[110]
2.118–2.261	0.063–0.079	Afghanistan	[111]
1.375–1.59	0.0325–0.0755	Lutak City, Iran	[111]
2.1008–3.5602	0.068–0.115	Yazd City, Iran,	[112]
6.34–8.97	NA	South Africa	[113]
3.1	NA	Turkey	[114]
4.02	NA	Pakistan	[115]
1.214	NA	Adrar city, Algeria	[116]
2.36 to 2.66	NA	Morocco	[117]

NA: Not available for the studied cities.

gives an example of some locations in the world with the corresponding price of the hydrogen production and electricity cost-based wind energy system [107,108,109].

Once the hydrogen is produced by the green energy sources, it is stored and transported. The storage of H₂ is conducted safely using tanks under high pressure. The big problem that can occur is the leakage of compressed gas under high pressure, risking an explosion [118]. On the other hand, the storage system is one of the main factors affecting the production cost of hydrogen.

So, different techniques have been investigated during the installation of wind/H₂ systems in order to reduce the production cost [114]. The hydrogen production cost with its transportation by pipeline is around 5.71 USD/kg. An investigation has been conducted on the combination of hydrogen production and methane production, which allows the increasing of the total efficiency of the system [115]. A low cost of hydrogen production has been found using a wind turbine in the region characterized by the highest potential wind power in the south of Algeria [116]. The minimum cost of around 1.214 USD/kg was obtained in Adrar city.

4.4. Solar/Wind Energy for Hydrogen Production (PV–Wind/H₂) System

Applications of PV systems are more useful in remote areas to produce electricity where no grid is available. Moreover, producing hydrogen-based solar and wind energy to provide electricity for electrolysis units is very interesting, particularly in the case of excess energy when the hydrogen can be sold or stored or converted to electricity via fuel cells [76].

To improve the efficiency of the hydrogen production system, it is essential to combine solar and wind energy to obtain an optimal hybrid hydrogen production system, which allows the reduction in hydrogen cost and continuous production because two green energy sources are applied [119].

The principle of the PV–Wind/H₂ system is presented in Figure 17. The best performance is obtained via the PV–Wind/H₂ system compared with the PV/H₂ and Wind/H₂ systems since a great potential for electricity is provided by this hybrid green energy sources.

The hybrid system is formed by the PV/H₂ and wind/H₂ systems [30,120]. The hybrid system is more productive than the single systems such as PV/H₂ or Wind/H₂ [117,121]. The efficiency of the PV–Wind/H₂ system is increasing due to the increase in the input electricity to the electrolysis unit and also due to the increase in the water temperature inside the electrolysis.

Many other experimental applications have been conducted using different techniques to enhance the performance of the solar–wind/H₂ system. As examples, the following results are presented [122,123,124,125]:

• Refueling vehicles in hydrogen stations where the production of hydrogen was around 0.51 Kg/h for a cost of 13.12 USD/kg.
• Including batteries in the solar–wind/H₂ system allows continuous working without interruption and enhances system efficiency.
• The utilization factor of the hybrid system is greater than that of the single system.
• Providing electricity via fuel cells from hybrid systems of hydrogen production to supply houses.
• Providing hydrogen for vehicles with the low cost of 9.28 USD/kg.
• The production of 239 Kg/h has been reached, with an efficiency of around 61%.

So, the solar/wind energy for the hydrogen production system is useful to produce H₂ and also for electrification, cooling, heating, and desalination.

Table 6. Specifications of Solar/Wind/H₂ production systems.

Solar/Wind Hydrogen Production Systems	Performances and Specifications	H2 Production Cost (USD/kg) and Generated Electricity Cost (USD/kwh)	Ref.
PV/H2	- The most used method for green hydrogen production - Minor maintenance - Increases the hydrogen production - Reduces the production cost - Efficiency reached 12.4% in 2010 - Efficiency reached 21% in 2020	- Hydrogen production cost varies from: [3.41–16.01] USD/kg - Generated electricity cost varies from: [0.06–0.38] USD/kwh	[77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98]
CSP/H2	- Presents better performance than PV/H2 system - The system efficiency varies from 12 to 16% - Energy efficiency varied from 35.52 to 71.6% in 2021 - Exergy efficiency varied from 20.7 to 36% in 2021	- Hydrogen production cost is around 2.84 USD/kg - Generated electricity cost varies from: [0.145–0.46] USD/kwh.	[98,99,100,101]
Wind/H2	- AC/DC converter is needed to power the electrolysis - Efficiency around 5–14% - High wind-speed conditions	- Hydrogen production cost varies from: [5.27–8.01] USD/kg - Generated electricity cost varies from: [0.08–0.55] USD/kwh.	[102,103,104,105,106,107,108,109,110,111,112,113,114,115,116]
Hybrid solar–wind/H2	- Used for electrification - Used as cooling system - Used as heating system - Used as desalination system - High efficiency - H2 production is greater than wind/H2 and PV/H2 productions	- Hydrogen production cost varies from: [3.73–4.65] USD/kg - Generated electricity cost varies from: [0.06–0.55] USD/kwh.	[112,113,114,115,116,117,118,119,120]

presents a resume of the specifications of some hybrid solar–wind/H₂ systems.

5. Economic Assessment of Green Hydrogen Production

The cost of the hydrogen produced depends on the input electricity cost, which is applied to the electrolysis unit. Other parameters affect the H₂ cost, such as the installation of different equipment to build a green energy source, the land cost, and the lifetime.

The principal produced parameter from any green energy sources is the electricity which is necessary to power an electrolysis unit for generating hydrogen. So, the price of the electricity produced affects the hydrogen production cost. The electricity cost depends on many factors, such as the installation of the renewable energy source, the location and land cost, the design, and the sizing of green energy source. Figure 20 shows the cost ranges of hydrogen using different green energy sources [37,126]. It is shown that classical energy sources such as nuclear and coal present a lower cost than the renewable cost, but they pollute the environment by their gas emissions. On the other hand, green energy sources present higher hydrogen production costs than other sources, but the trend in the world is the preference for the green energy sources due to zero emissions, and all the focus is on reducing their cost. As shown in Figure 20, the renewable energy/H₂ systems have the higher hydrogen production cost compared to the traditional energy sources.

Figure 21 shows the electricity cost (USD/kWh) for different energy sources. The calculation of the electricity cost directly affects the hydrogen production cost. In fact, the conventional energy sources have a lower electricity cost, as indicated in Figure 21, and they also have a lower hydrogen production cost, as shown in Figure 20.

Hydrogen production from solar and wind energy depends on the weather conditions, which affect the produced electricity due to the variation of solar irradiation and to the instability of wind speed. So, all the results mentioned in this work are related to a specific site and to a given green energy source used. The price of the produced hydrogen depends on the location in each country in the world, even for similar designs.

Many research works have investigated the relationship between the hydrogen production cost and the type of the electrolysis unit used for the same green energy sources. It has been found in [37,127] that:

• The hydrogen cost produced by the wind/H₂ system coupled with the PEM varies from 5 to 9.37 USD/kg.
• For the AWE coupled with the wind/H₂ system, the hydrogen production cost is around 7.47 to 7.6 USD/kg.
• Using the SOE connected with the wind/H₂ system, the hydrogen production cost varies from 6 to 9.2 USD/kg.
• When using the HTSE technology, the hydrogen production cost is around 3.23 USD/kg and around 2.50 USD/kg if using steam methane-reforming technology.

So, we can state that two principal factors influence the hydrogen production cost, which are: the renewable energy sources and the electrolysis type. Moreover, the hydrogen cost produced by high-temperature electrolysis is lower than that obtained with low-temperature electrolysis. However, the HTSE electrolysis is not yet commercialized and the cost of the laboratory can be found to increase when it is sold on the market. Figure 22 shows the hydrogen cost as a function of electricity cost for the different types of electrolysis used in renewable energy source/H₂ systems. It is observed that the HTSE technology produces hydrogen with a low cost compared with the PEM and the alkaline for the same electricity price.

Finally, the green hydrogen production is a good alternative for producing not only hydrogen but also the electricity for different applications. The cost of hydrogen production should be further reduced by increasing production and developing new techniques and infrastructure to enhance the performance of the green hydrogen production systems

Due to the requirements of international climate agreements and due to the growing energy demand in emerging economies, many countries have recently considered a transition towards a clean energy system in combination with a hydrogen economy [128]. In the current Five-Year Plan (2021–2025), hydrogen is one of China’s six industries of the future. It is the leading user and producer of hydrogen; it has had more than 30 green hydrogen projects in the works. After the Republic of Korea and United States, China has the third-largest FCEV fleet in the world with around 8 400 fuel cell electric vehicles. Moreover, the European Union (EU) has identified hydrogen as a key priority for achieving the European Green Deal. Its strategy focuses on renewable hydrogen. It includes the installation of 40 GW of renewable hydrogen electrolyzers in the European Union by 2030. In August 2021, India launched its National Hydrogen Mission. It considers green hydrogen vital to making a “quantum leap” towards achieving energy independence by 2047. On the other hand, Japan adopted a national hydrogen strategy in 2017. It aims to use hydrogen across all sectors of the economy. In 2020, around USD 670 million was invested in the hydrogen and fuel cell business, and mobility targets were set for 800 000 FCEV units and 900 hydrogen fueling stations by 2030. The Republic of Korea’s 2019 hydrogen roadmap identified hydrogen as an engine of economic growth and job creation. It plans to use hydrogen to power 10% of the country’s cities, counties, and towns by 2030 and 30% by 2040.

The government expects hydrogen to become the country’s largest single energy carrier in 2050, accounting for a third of the total energy consumption, and is exploring hydrogen imports with various supplier countries, including Australia and Saudi Arabia. The United States of America is the second-largest consumer and producer of hydrogen in the world, accounting for 13% of global demand. The USA dedicates USD 9.5 billion to accelerating the development of clean hydrogen technology. The United States also launched the Hydrogen Earth Shot to bolster the development of clean hydrogen projects. It has an ambitious program to reduce the cost of clean hydrogen to 1 USD/kg by 2030. Furthermore, Chile launched a green hydrogen strategy in 2020. It aims to reach 5 GW of electrolyzer capacity by 2025 and 25 GW by 2030, to produce the world’s cheapest hydrogen by 2030, and to become one of the world’s top three hydrogen fuel exporters by 2040. Morocco created a National Hydrogen Commission in 2019 and published a green hydrogen roadmap in January 2021.

In Saudi Arabia, the Helios Green Fuel Project, in which there was a green hydrogen and green ammonia plant powered entirely by solar and wind, was announced in July 2020. The plant is expected to start operation in 2025 in the planned megacity of Neom, on the shores of the Red Sea. Figure 23 shows the hydrogen demand by country in 2050 [129].

6. Conclusions and Perspectives

Hydrogen from renewable energy can be produced everywhere, while the cost-effectiveness varies by location. Hydrogen enables renewable energy to be traded across borders in the form of molecules or commodities. The state of the art from the literature on hydrogen production under different technology routes is an important concern for researchers around the world looking at the cost and technical production potential of green hydrogen in different scenarios and assumptions. This research field provides a perspective on the role of hydrogen in climate change since H₂ will be needed to attain full decarbonization. In this sense, green hydrogen, produced from renewables, is expected to represent the main part of the production. Moreover, hydrogen allows the storage of renewable energy, which can be used to fuel transportation, generate heat for industrial processes, and send electricity to the grid.

In this study, we focused on solar energy and wind energy, which are used as green sources to produce the electricity for powering electrolysis for hydrogen production. The applications of PV systems are more useful in remote areas to produce electricity where no grid is available. Moreover, producing hydrogen-based solar and wind energy to provide electricity for electrolysis units is very interesting, particularly in case of excess energy when the hydrogen can be sold or store or converted to electricity via fuel cell. The renewable hydrogen which has been 100% converted into electricity can be returned to the grid, effectively supplying solar or wind power when the sun is not shining and the wind is not blowing.

It has been found that wind energy does not perform as well as solar energy due to the random appearance of the wind. Therefore, the produced electricity from wind turbines remains variable during its operating period. The hydrogen production cost depends on two important factors: the renewable energy sources and the electrolysis type. Electrolyzers consume a significant amount of electricity as an input, which, in addition to the cost of the equipment itself, makes them generally expensive. The electrolyzer units can be arranged into two categories:

• The low-temperature electrolyzers (PEM, AWE, and AEM). These need only an electric DC source for water decomposition.
• The high-temperature electrolyzer (SOE). This needs a heat and power source for water decomposition.

Additionally, it is worth noticing that the need to reduce global emissions to net zero and the opportunity offered by the decreasing cost of electricity from renewable sources are, nowadays, encouraging the renaissance of the electrolyzer industry.

The following recommendations have been deduced through this study:

• The most used and adequate electrolysis is the PEM unit.
• The SOE electrolysis is adequate with the CSP/H₂ system, which produces the necessary high temperature.
• The PV/H₂ and wind/H₂ systems are both suitable in remote and arid areas. Minimum maintenance is required and a power cycle is not needed to produce electricity. The concentrated CSP/H₂ system needs a power cycle.
• The hydrogen production cost is higher if using wind/H₂ rather than PV/H₂.

We conclude that green energy sources are useful for multiple applications, such as hydrogen production, cooling systems, heating, and water desalination. The cost and efficiency of hydrogen production systems depend on weather conditions, installation cost, productivity of hydrogen per day, the energy source used, and the electrolysis type. Correspondingly, understanding the role of hydrogen in the clean energy transition, such as that of solar and wind energy, is very helpful in supporting the continued cost reductions and improved performance in commercially available technologies, as well as in ensuring that the next generation of hydrogen technologies reach suitable commercialization.