Renewable Energy 216 (2023) 119041

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Renewable Energy
journal homepage: www.elsevier.com/locate/renene

The green hydrogen revolution

Gaetano Squadrito *, Gaetano Maggio , Agatino Nicita
Consiglio Nazinale delle Ricerche, Istituto di Tecnologie Avanzate per l’Energia “Nicola Giordano” (CNR-ITAE), via Salita S, Lucia sopra Contesse 5, 98126, Messina,
Italy

A R T I C L E I N F O A B S T R A C T

Keywords: Green hydrogen is considered the most suitable choice for the future energy market, both as energy storage
Green hydrogen media, energy vector and fuel for transportation, industry and other applications.
Hydrogen policies In the last twenty years, increasing efforts have been dedicated to green hydrogen technologies development,
Renewable energy
but still today a number of issues are claimed in justifying the delay in its large scale application and the star­
Water
Distributed generation
vation of its market. Moreover, some new questions seem ready to be put on the table for justifying the delay in
green hydrogen technologies applications.
In this paper, a critical analysis of recent literature and institutional reports is carried out with the aim of
understanding what is the real state of the play. In particular, peculiar advantages and shortcomings of different
green hydrogen technologies (biomass pyrolysis and gasification, water electrolysis, etc.) have been analysed and
compared, with a focus on the electrolysis process as the most promising method for large scale and distributed
generation of hydrogen.
Some geopolitical and economic aspects associated with the transition to a green hydrogen economy -
including the feared exacerbation of the water crisis - have been widely examined and discussed, with the
purpose of identifying approaches and solutions to accelerate the mentioned transition.

1. Introduction role, so that the energy generated mainly from these renewable sources
can be fully harnessed [12].
To reduce the greenhouse gases (GHGs) emissions and the depen­ Among the most widespread and common storage systems, batteries
dence of the energy market on fossil fuels, most countries in the world are the most common storage facility, characterised by high round-trip
are focusing on the development of renewable energy sources (RESs) to electrical efficiency (>90%) [8]. In a battery, the electric energy is
drive the energy transition and to reduce their dependence on external converted by an electrochemical reversible process into chemicals
supplies [1–3]. These efforts will result in an increasing share of green (charge phase), these last when necessary can be converted back to
electricity and gradual introduction of green hydrogen [2]. electric energy (discharge phase). Batteries are competitive with other
Although it is possible to produce biogas and hydrogen from residual energy storage systems also because they allow decentralised energy
biomasses [4,5], other forms of RES (like hydraulic, solar, wind, storage [13,14]. But even these devices currently have a number of
geothermal, sea waves and tidal streams) can be easily converted into technical shortcomings, such as: limited number of useful cycles (char­
electricity or heat. ge-discharge), use of critical raw materials and/or polluting compounds,
In particular, photovoltaic (PV) and wind are considered as the most difficulty of recycling (costly and/or lacking technologies) [15].
suitable and cost-effective technologies for large scale application and For this reason, interest in hydrogen as an environmentally friendly
power generation potential, and this has led to their continuous and and economically competitive solution for energy storage has been
considerable growth within the energy mix [6–8]. growing dramatically [16–18]. Hydrogen can be produced by different
Unfortunately, both photovoltaic and wind power generation are paths, each having a different environmental impact, that is usually
highly dependent on location and weather conditions. These constraints associated to a “colour” attribute. Today, a number of different colours
pose a problem since very often the timing of energy supply and demand are used to classify the hydrogen according to the CO2 emissions related
do not coincide [9–11]. For this reason, storage technologies play a key to the production path [19].

* Corresponding author.
E-mail address: gaetano.squadrito@itae.cnr.it (G. Squadrito).

https://doi.org/10.1016/j.renene.2023.119041
Received 6 March 2023; Received in revised form 6 July 2023; Accepted 14 July 2023
Available online 14 July 2023
0960-1481/© 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-
nc-nd/4.0/).
G. Squadrito et al. Renewable Energy 216 (2023) 119041

Although hydrogen is defined green when there are no CO2 emis­ analysis are summarised in Section 6.
sions, the most suitable and well developed technology is based on
electrochemical water splitting (electrolysis), then common meaning of 2. Green hydrogen production technologies
“Green Hydrogen” is as “Hydrogen made via electrolysis using renew­
able electricity”. This last definition of green hydrogen is present in Hydrogen can be produced with different methods and technologies
documents and articles which consider electrolysis to be the preferred using both fossil fuels and renewable sources. As already mentioned,
and main route for future hydrogen production. “green hydrogen” is usually conceived as hydrogen produced by RES-
As in batteries, in water electrolysis electricity is converted into based electrolysis. While the general definition of green hydrogen is to
chemicals (hydrogen and oxygen). Apart from the fact that electrolysis be related to the absence of CO2 emissions, i.e. produced through
relies on a non-spontaneous reaction driven by an external power technologies using renewable sources, or carbon neutral technologies
source, the main difference with batteries is that hydrogen has several [26–28]. Then, hydrogen made from biomass is CO2 neutral and can be
possible applications [20–24]. In fact, hydrogen can be converted back defined as “green”.
to electric energy by electrochemical process in fuel cells; it can be used Converting waste into something value makes a lot of sense and is
as a fuel in internal combustion engines and turbines for mechanical part of the circular economy concept. However, the use of agricultural
energy; it can be burned in ovens to generate heat; it can be used as a products that could be used for food production is not considered,
feedstock in many industrial applications that use hydrogen obtained by because it is not logic and can generate social conflicts. But it has to be
processing fossil fuels (e.g., methane, oil and its derivatives, carbon); noticed that there are a number of wastes from food industries and
and it could be applied in other industrial and civil applications to transformation industries that today are not valorised. Expanding the
replace natural gas. green hydrogen concept also broadens the available hydrogen produc­
For these reasons, many countries look at hydrogen as the solution tion technologies to be considered for large-scale hydrogen applications
for the future energy management and are increasingly supporting the and industrial decarbonisation. Probably, it is possible to add two new
introduction of hydrogen technologies aimed at a “decarbonised” colours, “dark green” hydrogen for the hydrogen produced by electrol­
economy. For this purpose, several “plans and strategies for the devel­ ysis, and “light green” hydrogen for the other approaches, but thereafter
opment and deployment of hydrogen” have recently been drawn up this is not considered and the terminology “green hydrogen” will be used
[25]. To support these plans, governments are launching support pol­ for all cases.
icies through the provision of incentives for both the construction of new The main environmental characteristics of the principal technologies
delivery infrastructures and the production of green hydrogen. The that could be applied to yield green hydrogen are listed in Table 1. With
production costs of green hydrogen are considered the main hurdle to the exception of electrolysis, which does not use biomass, several types
the introduction and large-scale application of green hydrogen. of feedstock are valuable: waste biomass, e.g. waste from agricultural
Furthermore, in recent years, there has been discussion of the implica­ and food production, waste wood, residual agricultural products,
tions of hydrogen production by water electrolysis in relation to water sewage sludge, bio-methane and other bio-fuels.
availability and reserves (see Section 4). This because, as a consequence
of conceiving green hydrogen just as that produced by electrolysis 2.1. Steam reforming of bio-feedstocks
powered by renewable sources, a large quantity of water is requested.
According to the authors of this article, the definition of green Steam reforming is a well-known process that today supplies large
hydrogen in relation to electrolysis technology needs to be broadened to parts of the hydrogen for industrial applications by using methane as a
include the CO2 cycle and therefore all technologies that are carbon feedstock. It is a catalytic reaction of a hydrocarbon with water vapour.
neutral. For example, hydrogen from biomass is CO2 neutral and can be In the case of methane, the reaction allows the production of a gas
called ‘green’, also without carbon capture, because the CO2 output in mixture of carbon monoxide, hydrogen and carbon dioxide usually
the final hydrogen production step is coming from a previous capture named reforming gas. Industrial systems are usually based on tubular
operated by the plants. reactors containing catalyst particles, like nickel particles supported on
From this perspective, the authors carried out an overview of the
state of play of hydrogen production technologies, and explored some Table 1
considerations related to major geopolitical and economic aspects and Green hydrogen technologies, main environmental characteristics.
implications of the hydrogen economy development, including the
Production Advantages Drawbacks
water aspects. With this research, the authors like to contribute to the technology
analysis of both technical and socio-economic factors that may hinder or
Steam Reforming No oxygen needed; high Although carbon neutral, there
instead favour the uptake and use of hydrogen. For this reason, we have
of bio- conversion efficiency; well are CO2 and CO emissions.
adopted a holistic approach in which, in addition to the technical issues, feedstocks known technology already
a number of problems have been identified and studied, such as the issue used for natural gas.
of water, which could have an impact on national and local economies, Biomass Residual and waste biomass Although carbon neutral, there
especially in some countries of the Global South. We hope that this study Gasification as feedstock. are CO2 emissions; a post
treatment of exit gases and
will contribute to the development of future research in this area, while residuals is requested.
providing policy makers with a broad vision that includes the various Biomass Pyrolysis Low water content residues Although carbon neutral, there
socio-technical impacts associated with the deployment of hydrogen and waste biomass as are CO2 and CO emissions;
technologies. feedstock; no oxygen is formation of TAR that must be
needed. treated and it is not easy to
The paper is organised as follows. After this introduction, Section 2
manage.
provides an overview of the green hydrogen production technologies. Water Electrolysis No direct emissions; oxygen Low cost green electricity is
Section 3 highlights the close correlation between green hydrogen and is a by-product. needed; low conversion
RESs, with an emphasis on the electrolysis technology. Some geopolit­ efficiency (about 60%); pure
ical implications of large-scale adoption of green hydrogen are evi­ water is requested.
Direct production Large variety of wastes Technology at low level of
denced in Section 4, with discussions of possible impacts of hydrogen- by biological could be used as a development; low production
based scenarios on global energy and fuel markets and on water re­ processes feedstock; possibility to use density; pre-treatment of the
sources. In Section 5, we focused our attention on the distributed CO2 as co-feedstock; feedstock required to fit the
hydrogen generation and the valorisation of electrolysis by-products possibility to produce applied microorganisms
useful by-products. necessities.
(oxygen, heat, etc.). Finally, the main findings and conclusions of our

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alumina silica, with K2O as activator. The catalytic particles are lapped chlorinated and sulfurised compounds in relation to the starting mate­
by the stream of reactants at an average temperature of around rial (usually known as TAR), and the management of this residuals
850–870 ◦ C. The process can be applied to a number of hydrocarbons originates socio-environmental issues [40,41].
like methane, methanol, ethanol, glycerol. Methane is the largest used
feed and steam methane reforming (SMR) is today a cheap technology 2.3. Biomass pyrolysis
for large industrial applications. In many cases, steam reforming is
coupled with the water gas shift reaction to increase the hydrogen yield Pyrolysis is a heat treatment process consisting of non-oxidative
according to the process to be fed. For example, reforming gas is largely thermal decomposition, i.e. without additional oxygen. It can be used
used for methanol and synfuel production. But, using methane (natural for the energy conversion of various organic materials as long as they
gas) and other fossil fuels there is a net CO2 emission and the produced have a low water content (<15%). The pyrolysis reaction is charac­
hydrogen is called “grey hydrogen”. To produce pure hydrogen with this terised by very complex chemical reactions occurring in the temperature
process a separation from the other components of reforming gas is range of 400–800 ◦ C. The products of the decomposition reaction consist
necessary. By this separation process it is possible also to capture the in:
produced CO2 and to dispose it in some way (like in exhaust oil field), a
process named CO2 sequestration. ● solid fraction: 20–30% by weight of the initial material, carbona­
By CO2 sequestration, it is possible to obtain a clean form of ceous based. It has a good calorific value (8000 kcal/kg) and,
hydrogen named “blue hydrogen”, but this means anyway having CO2 to therefore, it can be used as a fuel;
manage and additional production costs related to this management. ● liquid fraction (TAR): 50–60% by weight. The energy content is
While using bio-gas, bio-alcohols or bio-oils it will be carbon neutral and about 22–23 MJ/kg on a dry basis and 16–18 MJ/kg with 20% water;
the resulting hydrogen can be classified as “green hydrogen” [29–31]. ● gaseous fraction (pyrolysis gas): it constitutes 15–30% by weight,
The steam reforming technology is well assessed and largely used, and is mainly composed of hydrogen, carbon monoxide, carbon di­
although further advancements are possible. Nowadays, methane steam oxide, light hydrocarbons. It has a medium-high calorific value
reforming is the cheapest approach for hydrogen production, but the (15–22 MJ/kg) and can be used for various purposes.
result is grey hydrogen. By capturing CO2 the emissions can be strongly
reduced but not totally avoided. Moreover this kind of hydrogen is not The way of carrying out the pyrolytic process determines the balance
suitable for application in fuel cell and in other applications that require in production of bio-oil, syngas and carbonaceous residue.
very pure hydrogen, requiring further separation processes.
Using bio-hydrocarbons as feed, steam reforming can be considered ● Conventional pyrolysis takes place at moderate temperatures
green, but anyway a number of purification steps and separation of (around 500 ◦ C), with long reaction times and it results in a balance
hydrogen will be requested to obtain pure hydrogen. of the three fractions;
A solution to this aspect are the metal membrane reactors. When the ● Carbonisation, the oldest and best known pyrolysis process, has been
process is conducted by metal membrane reactors the separation of used for the production of vegetable charcoal since about 2500 years
hydrogen from other reforming gases is realised directly in the reactor ago. It occurs at temperatures between 300 and 500 ◦ C and only the
[32]. solid fraction is recovered, the other fractions are minimised;
● Fast pyrolysis, at temperatures from 500 to 650 ◦ C, is addressed to
2.2. Biomass gasification reduce the reforming of intermediate compounds, favouring the
production of the liquid fraction up to 70–80% by weight of the
Gasification is a chemical process that converts carbon-rich material incoming biomass;
(such as coal, oil or biomass) into a gas mixture usually called “syngas”, ● Flash pyrolysis is carried out at above 650 ◦ C and with contact times
containing carbon monoxide, hydrogen, carbon dioxide and other of less than 1 s, in order to favour the production of the gaseous
gaseous substances. The process is known and applied since the XIX fraction.
century, when it was largely used for supplying the city gas, a mixture of
hydrogen (about 50%), 3-6% of carbon monoxide and the remaining Although this technology is well known and largely studied, a
part as methane and carbon dioxide. City gas was produced from coal in number of issues limit its diffusion and application. First of all, the ne­
extreme conditions: 30 bar or more, and temperatures up to 1200 ◦ C cessity to have well characterised biomass feedstock to have a well
[33]. The process consists of the combustion of the feedstock in poor controlled system and products quality. The system efficiency depends
oxygen conditions (from this the name of “partial oxidation”) and in also on some feed characteristics like dimension of particles, density and
presence of steam; it is usually carried out at temperatures between 700 humidity, just to mention the simplest ones. This introduces the need of
and 800 ◦ C [33,34]. appropriate physical, chemical or biologic pre-treatments that have to
Also in this case, when fossil fuels are used by appropriate cleaning, be tailored to the used biomass. Moreover, the products of pyrolysis are
blue hydrogen can be produced. heat, bio-oil, bio-char and gases. To obtain hydrogen it is necessary a
If biomass is used as feedstock, the process is carbon neutral and the post-treatment of the products in gas and liquid phase by a reforming
hydrogen coming from effluent gases purification is green. Combustion process. Not the least, for plants of significant size, biomass storing in­
is exothermic, so that a large amount of energy is released as heat, and troduces significant fire issues and the biomass has to be collected over a
this technology becomes really interesting in cogeneration processes large area. This last introduces additional costs and issues related to
[35,36]. As high temperature heat is released during the process, it is of biomass transportation, storage and pre-treatment [42,43].
great interest when cogeneration of heat and power is considered [37,
38]. Moreover, as the presence of steam is needed, the biomass feedstock 2.4. Electrolysis
could be not dehydrated. A schematic and detailed analysis of the
different gasification reactor technologies can be found in Ref. [39]. Electrolysis is an electrochemical technology that uses direct electric
Within the issues that limit the diffusion of the technology for the current (DC) to drive a chemical reaction along a non-spontaneous re­
hydrogen production by biomass gasification there are the control of the action path. In particular, water electrolysis allows the split of water
biomass quality and water content, the ash management, syngas quality (H2O) into hydrogen and oxygen. If the water electrolysis is done only by
control and syngas purification to hydrogen, hot gas management and RES power, no CO2 emission will be present. Also this technology has
reuse. Besides, reforming of liquid residual contains paraffins, iso­ been known for a long time, and is applied on the industrial scale
paraffins, olefins, aromatic hydrocarbons and a number of oxygenated, [44–46], usually for production processes requiring high purity

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hydrogen. applications [22,58].

For every kg of hydrogen produced by electrolysis, approximately 8
kg of oxygen is produced using about 9 kg of pure water and consuming 3.1. The current state of green hydrogen in the energy transition
about 50–55 kWh of electricity (depending on the electrolysis technol­
ogy). But, usually, only hydrogen is cost-assessed without considering Nowadays, almost all hydrogen is produced by Steam Methane
oxygen; only in recent few years the valorisation of oxygen was Reforming (SMR). According to the IEA Global Hydrogen Review 2022
considered [22,23,47,48]. [59] about 82% of the hydrogen produced (94 Mt, in 2021) is directly
Due to the increasing large diffusion of PV and wind technologies derived from methane, oil and carbon. About 18% is a by-product
converting sun and wind power into electric power, and a vision for the coming from different production technologies (e.g., naphtha reform­
transition to 100% electric energy from RES, the water electrolysis is ing). Therefore, hydrogen produced by low-emission technologies is less
considered the most suitable way for hydrogen production. than 1 Mt (0.7%), with the majority of this coming from fossil fuels with
More details about water electrolysis will be discussed in the next CCUS (about 0.7%) and only 0.04% (35 kt H2) coming from renewable
sections. electricity via water electrolysis. In 2021, the emissions associated with
hydrogen production were more than 900 Mt CO2 [59]. Hydrogen
2.5. Direct production by biological processes production costs by these processes are low, $1-2/kg H2 for SMR and
$1–1.5/kg H2 for gasification, respectively.
Microorganisms were applied for biomass transformation and waste The main challenge is how to produce hydrogen for today’s and
treatment at the dawn of time, but only in the last few centuries this future uses at costs that are close to current ones, but without emitting
practice evolved and was accurately studied. Nowadays, looking at en­ CO2 into the atmosphere. Within the sector today in the hotspot for
ergy and fuels, the most important products are biogas, alcohol and bio- green hydrogen application there are: heavy transport, district heating
fuels. or the decarbonisation of certain industrial processes such as
Green hydrogen can be obtained both by reforming these products, steelmaking.
as reported before, and by direct production from organic feedstocks The two approaches currently able to meet this challenge are “blue
applying selected microorganisms [49–51]. Applicable technologies, hydrogen” and “green hydrogen”.
according to the specific microorganism, are dark-fermentation, pho­ Blue hydrogen comes mainly from methane steam reforming and, in
to-fermentation, photolysis, CO2 gas-fermentation. In fermentation, small part, from gasification. There are a number of pilot plants at the
hydrogen evolution is the result of microorganism metabolism in industrial level for the production of blue hydrogen which, with
anaerobic conditions. Moreover, in some cases CO2 is a feed for the different approaches, have demonstrated the possibility of achieving a
culture and this approach can be used for carbon dioxide capture and total production cost of blue hydrogen at around $2–2.5/kg H2 by 2050,
conversion. In photolysis, both bacteria and algae are applied, also in with around 90% as the maximum level of carbon dioxide capture
this case in anaerobic conditions, for a direct photolysis of water or for generated in the process - corresponding to an emission threshold of
CO2 fixation process with hydrogen release. around 5 t CO2/t H2 [60].
These technologies are very interesting because they offer the op­ According to the current emission thresholds set by RED II
portunity to produce hydrogen from wastewater and other wastes con­ (Renewable Energy Directive II - DIRECTIVE (EU) 2018/2001 [1]),
taining organic compounds like sugars, starches, cellulose, acetate, clean hydrogen production must keep its emissions below 3 t CO2/t H2.
butyrate, lactase. But they suffer from some issues for large-scale Under current conditions, blue hydrogen would not be able to meet
application like large volumes of the bio-reactors, the rate of hydrogen the limits of RED II if it were produced by an energy mix with a high
production, the control of the microorganism population against mu­ proportion of fossil fuels. The same is true for hydrogen produced by
tation and competitor microorganism infection, the pre-treatment of the electrolysis of water not using just RES power, which would be classified
wastewater to be used [49–51]. as yellow instead of green.
In addition to the previous processes for green hydrogen production, The compliance to RED II is granted only by green hydrogen,
other possibilities are under study, like direct solar water splitting [52, including all carbon neutral cycles, processes like pyrolysis and gasifi­
53], and electrochemical reforming of bio-compounds [54,55]. These cation of biomass considered for the recovery of hydrogen from residual
processes, although promising, are today at low development level and biomass and really interesting also as near zero pollution heat and power
require more research to reach the readiness level for commercialisa­ cogeneration systems with high efficiency. But the plants are complex,
tion; then they are not treated here. small plants are very expensive, the collection of suitable quantities of
What is interesting to note is that in talking about green hydrogen biomass could be a critical point, and the management of residual wastes
usually only water electrolysis is considered, this is due to the depen­ is not simple [35–38]. Also steam reforming of biogas, bio-oils, bio-fuels
dence of electrolysis by electric power availability, a condition that and bio-alcohols is carbon neutral if we consider that the carbon content
creates a strong link between RES and electrolytic hydrogen. is coming from carbon bio-fixed through photosynthesis, i.e. if the whole
carbon cycle is considered [55]. Moreover, if carbon capture and storage
3. Green hydrogen and RES is applied after steam reforming also negative emissions could be ob­
tained [61]. In this case the bio-feedstock comes from a previous
To face the pollution and global warming issues a shift towards treatment of biomass that introduces an additional step, then reducing
renewable energy technologies is currently running in the world energy efficiency and increasing costs. The approach could be interesting for
mix. Renewable energy in 2016 accounted for less than 15% of elec­ industries where methane steam reforming is already applied for
tricity, nowadays it represents 28% of the world’s electricity production replacing it having a carbon footprint reduction, in this case one issue
[56] and 13.8% of the global energy consumption [57]. The progressive could be to grant a continuous bio-feedstock supply.
increase of the RES power share in the electric grid introduces issues Regarding biological processes, today the mature technologies re­
related to the natural discontinuity and fluctuations of RES, that are the gard biogas, bio-alcohol production, and bio-fuels. While direct
object of an increasing number of investigations, and require energy hydrogen production is under development and requires significant
storage systems to mitigate them [9,11,58]. research efforts to become competitive.
Although there are different possible technologies for energy storage, The production of hydrogen by electrolysis of water using electricity
hydrogen is considered the most suitable choice for massive and long produced from renewable sources, is to date the only technology on the
term storage of RES power. This is because hydrogen is an energy vector market that can compete for maturity with the steam methane reform­
(fuel) and also a commodity gas and a feedstock for many industrial ing, moreover it fully complies with the emission limits imposed by RED

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II: green hydrogen is, therefore, the cornerstone on which the entire - Solid oxide electrolyser cell (SOEC)
European Hydrogen Strategy is focused. SOECs, unlike the other electrolysers described above, operate at a
Maybe, in the long term other countries, including the natural gas high temperature, over 700 ◦ C, with lower energy consumption. The
and oil exporters, will converge on this approach. technology is based on a solid oxide ceramic electrolyte, conducting
the O2− ions, and uses high-temperature steam to replace water. It is
3.2. The essential technology for green hydrogen: the electrolyser an interesting technology due to its low energy consumption,
particularly for those user sectors that have high-temperature steam
There are different types of electrolysers, some of them are already within their processes, but still suffers from a relatively low tech­
on the market and others in the research and development phase. The nology readiness level, particularly for the lifetime of ceramic oxides
technologies that are best known to date are listed in Table 2 on the basis [66,67]. Although very promising, solid oxide electrolysis is the most
of the Technology Readiness Level (TRL) and their current market problematic of the technologies related to fuel cells used in reverse
penetration. mode. The issues are related to materials, lifetime, resilience to
temperature fluctuations, number of on-off cycles. The CAPEX
- Alkaline electrolyser (AEL) requirement for this technology ranges across $2,800-5,600/kW
Alkaline electrolysis has already been employed for many years in [63].
some industries, such as chlor-alkali production, and have proven to
be very reliable. Based on the above considerations, the efforts for green hydrogen to
The advantages of AEL are their substantial reliability, their high become widespread and competitive are focusing on the AEL and PEMEL
service life (around 60,000 to 100,000 operating hours), and the use electrolysis technologies. The short- and medium-term goal is to lower
of inexpensive raw materials for their manufacture. The disadvan­ the CAPEX – by using less expensive materials and components – and the
tages are the inability to operate at low loads (<20%) - due to the OPEX – by mainly reducing consumption and electricity costs [59,63,
problem of H2/O2 mixing, that could generate explosions -, the high 68–70].
footprint, and the high resistive losses in the electrolyte that limit the In particular, attention is paid to critical raw materials that will play
efficiency to 50-70%, i.e. in energy terms requiring around 50-78 a decisive role in the growth of production costs, especially for those
kWh/kgH2 [62]. technologies that depend on scarce materials or which are produced in
As for other types of electrolysers, the current value of CAPEX has limited geographical areas [17,70–72].
a very wide range, given the low degree of dissemination and But putting electrolysis in the spotlight of current energy policies at
industrialisation, which is around $500-1,400 per kW of installed global level brings out new queries related to the global impacts of large
power [62,63]. scale application of this technology and on the new equilibrium that will
- Polymer electrolyte membrane electrolyser (PEMEL) be created between countries [73].
PEM electrolysers have an electrolyte consisting of a thin polymer
membrane that allows H+ ions to pass through it, while the elec­ 4. Some geo-political considerations on the role of hydrogen in
trodes consist of titanium coated with specific catalysts. PEM elec­ the energy transition
trolysers have a much more compact design, can be operated at low
and high loads (>100%), and have a sufficiently high service life 4.1. The potential reshaping of global energy markets by hydrogen
(around 50,000 to 80,000 operating hours). However, they require
very expensive materials (such as platinum or gold) as coatings to In recent years, there has been considerable discussion about the
protect the materials from the strongly acidic environment in the cell geopolitical implications of the energy transition. Researchers have
and, above all, materials such as platinum and iridium for the cata­ studied how renewable energies and related technologies impact inter­
lysts. Iridium in particular, one of the rarest chemical elements in the national relations and the economies of individual countries [74–77].
Earth’s crust, is feared to become a bottleneck for the entire tech­ Much of the analysis focused on wind and solar energy, but there is
nology. Today, its cost has risen by 400% compared to the 2015- growing interest in the geopolitical implications of large-scale adoption
2020 average, due to its importance in hydrogen production. The of green hydrogen. Some researchers argue that the growth of green
CAPEX value for a PEM electrolyser, also with a wide range, is hydrogen within the global economy could lead to such geo-economic
around $1,100-1,800/kW, and is currently higher than AEL [62,63]. and geopolitical changes, in which new scenarios and in­
- Anionic exchange membrane electrolyser (AEMEL) terdependencies will be shaped [19,78,79].
AEM electrolyser brings together the advantages of AEL and The consequences will be a different geography of energy trade with
PEMEL technologies. The TRL level is still low, due, in particular, to the emergence of new centres of geopolitical influence, based on the
the resistance and lifetime of the membrane. An AEM electrolyser is production and use of hydrogen. In this scenario, traditional oil and gas
similar in concept to a PEMEL, but the polymer membrane allows the trade is expected to shrink.
passage of OH- ions instead of H+. The environment in the cell is According to the outlook drawn up by IRENA, green hydrogen will
alkaline, greatly simplifying the material requirements for catalysts cover 12% of global energy consumption by 2050. This will be due to
and for chemical corrosion resistance with respect to PEM [64,65]. targeted investments in the sector that will increase economic compet­
AEM electrolysers are less evolved in respect to the PEMEL and a itiveness and change the current hydrocarbon-based relationships [17].
number of issues related to the polymeric membrane life and me­ With regard to future value chains for the production of green
chanical properties are remaining. hydrogen-based ammonia, methanol and green steel, according to Eicke
& De Blasio [80], changes can be expected within the global market that

Table 2
Comparison of water electrolysis technologies.
Alkaline electrolyser Polymer electrolyte membrane electrolyser Solid oxide electrolyser Anion exchange membrane electrolyser
(AEL) (PEMEL) (SOEC) (AEMEL)

TRL 8–9 8 5–6 3–4

Market Large scale Rapid expansion Limited development Lab scale
penetration

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G. Squadrito et al. Renewable Energy 216 (2023) 119041

will lead some countries to take other positions than they currently have. corrosion and the formation of precipitates on the electrode surface. Gao
With regard to green hydrogen ammonia, countries such as Russia et al. [84] emphasise the importance of developing highly active and
and Egypt, among the world’s leading producers, are limited in their selective catalysts for the electrolysis of seawater in the presence of
ability to produce or distribute green hydrogen at scale; Russia because contaminants such as metal ions, chloride and bio-organisms.
of infrastructure constraints, and Egypt due to limited freshwater While, regarding seawater reverse osmosis (SWRO) coupled with
availability. Whereas Mexico, Spain or Thailand, which have a good proton exchange membrane (PEM) electrolysis, in addition to the
resource endowment and high economic potential, have a high capacity presence of technical issues, Khan et al. [85] noted that there are limited
for growth. economic and environmental incentives in pursuing R&D on the
Changes are also expected with respect to the production of green up-coming technology of direct seawater electrolysis.
hydrogen-based methanol. Four countries - Saudi Arabia, Trinidad and The issues and development of these specific technologies will be
Tobago, Oman and the United Arab Emirates - with a total world market addressed in more technical detail in the following paragraphs.
share of 39%, are limited in their potential for green methanol pro­ For Pflugmann & De Blasio [78] the issue of water resources is
duction. The consequence of this will be that these countries will have to particularly important for countries where fresh water is scarce. The
rely on imports to maintain their position in the future green methanol authors focus on the case of Saudi Arabia, which can rely on an abun­
market. On the contrary, countries such as New Zealand, Norway or dance of renewable energy but limited water resources. It would be
Chile, which currently do not have significant market shares in this possible to address this shortcoming by desalinating sea water. To pro­
sector, could, given their resources and economic conditions, sharpen duce an amount of hydrogen equivalent to about 15% of Saudi Arabia’s
their positions. annual oil production, 26 million tonnes of renewable hydrogen would
The production of green steel will also bring about changes in the be required per year. This amount of hydrogen would require 230
world market for this product. China, the current largest producer million m3 of fresh water. In order to obtain the freshwater Saudi Ara­
globally, will continue to maintain its leadership. The other major steel bia’s needs, at least five desalination plants would need to be added to its
producing countries, such as India, Japan and Russia, will face a existing 31 large desalination plants.
considerable contraction of resources, increasing the import of green Referring to Africa, the World Energy Council [86] also points out
hydrogen. Furthermore, other countries such as the Baltic States, that, in the short term, access to water suitable for electrolysers might
Morocco, Turkey and Thailand, which have good resource endowments require upstream investments to desalinate water in some parts of the
and favourable economic conditions, could try to attract green steel continent. This would entail further investments, particularly in
production. water-stressed areas, and the improvement of suitable technologies.
Terlouw et al. [87] argue that the large-scale spread of hydrogen
4.2. The issue of renewable fresh water production in combination with other factors - such as additional water
demand due to climate change, population growth, economic develop­
One issue that is at the centre of the debate on what the impacts of a ment and agricultural intensification - could lead to water scarcity. They
widespread deployment of green hydrogen might be concerns water refer to what is already happening on the islands of Crete and Tenerife.
resources. Researchers from various scientific fields are comparing and Lebrouhi et al. [88] also take the issue of water scarcity into account in
assessing the effect of green hydrogen on the global water resource. The their analysis, as it is one of the biggest problems in the world today. In
key question is: will there be enough water to meet our future demand their view, it is crucial that policies for the development of green
for green hydrogen? hydrogen in some countries include an increase in the storage capacity
The views and scenarios that can be drawn from the literature and of water flows (dams and local rainwater storage systems). At the same
reports by various international bodies make different predictions. A time, it is necessary to develop strategies for the optimal management of
research conducted by Newborough & Cooley [81] states that if all available stocks, create tools for recycling the resource to optimise its
current fossil fuels used were converted to green hydrogen, the need for use and avoid waste, control water pollution and develop seawater
water for electrolysis would amount to 1.8% of the current global water desalination plants.
consumption. Furthermore, they point out two aspects that need to be Woods and al. [89] suggest using wastewater effluents for water
considered: i) the increase in demand would be outweighed by the water electrolysis, referring to the case of Australia. According to these au­
savings achieved by not having to produce fuels from oil or biomass and thors, unused tertiary effluents have significant potential to lead to
by reducing the use of conventional thermal power plants; ii) when green hydrogen production at scale. Lower investments are required
green hydrogen is oxidised by combustion plants and fuel cells, the same compared to seawater desalination systems. In addition, they constitute
amount of water as originally consumed by electrolysis is released into a security in water supply compared to stormwater. In this way, water
the environment. for hydrogen is not in direct competition with existing needs and does
A consensus on the low impact of green hydrogen production on not entail an additional water stress.
water resources can be found in the work of Beswick et al. [82], as all The World Economic Forum [90] carried out an analysis estimating
future hydrogen will be produced using renewable energy sources such what the impacts on water resources could be from the transition to a
as wind and solar, which have little or no water consumption. However, hydrogen economy. The research was carried out by analysing data,
even if the consumption of water to produce hydrogen is less than that concerning energy demand and water withdrawal, from 135 countries.
required to produce energy from fossil fuels, concerns over the scarcity According to the estimates derived from the analysis, only nine of the
of fresh water call for a reduction in the use of water sources. They see a 135 countries studied would need to increase their current freshwater
feasible and concrete solution in utilising the Earth’s vast salt water withdrawal by more than 10% to fully switch to hydrogen-based energy,
resources, which can further reduce the water footprint of hydrogen. while 62 countries would need to increase their freshwater withdrawal
Many researchers in the field believe that salt water is a potential by less than 1%. The average value for all 135 countries is 3.3%. The
optimal solution because it would prevent hydrogen production from increased demand for water resources would affect desert countries with
contributing to the increase in demand for fresh water leading to the low annual rainfall, such as Qatar, Israel, Kuwait or Bahrain. Or, small
depletion of sources, along with other factors such as population growth island states, such as Singapore, Trinidad and Tobago or Malta, which
and climate change. would also experience difficulties due to limited freshwater reserves. For
Some of them, however, highlight the technical challenges that still instance, Singapore, which is heavily dependent on neighbouring
need to be addressed in order for this technology to be fully deployed. Malaysia for freshwater resources, is expected to increase the water it
From this point of view, Mohammed-Ibrahim & Moussab [83] focus on uses to convert energy into hydrogen by about 46.4%.
the need for robust and efficient electro-catalysts to prevent chloride According to analysts at the World Economic Forum, the hydrogen

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economy can open up interesting prospects not only for the energy
system, but also for addressing the issue of water scarcity. Countries
with water shortages, such as Singapore and Qatar, are unlikely to be
able to produce their own hydrogen and will therefore have to rely on
imported hydrogen. This, which can certainly be seen as a disadvantage,
will however allow these countries to use the water produced by the
conversion of hydrogen back into energy, either through combustion or
fuel cell technology, and to reuse this high-purity water locally.

4.3. The water-hydrogen-water cycle

As reported in the previous section, although nowadays hydrogen is

largely used in a number of industrial processes only a small percentage
of it is today produced by electrolysis. The transition to green hydrogen,
obtained by RES electricity-based electrolysis or other green routes, will Fig. 1. The water-green hydrogen-water cycle. Only water (blue arrows),
introduce a competitor to fossil sources of hydrogen used as feedstock, i. hydrogen (red arrows) and oxygen paths (light blue arrows) are reported.
e. it will impact on the 82% of today’s hydrogen production and,
consequently, on the prices of fossil raw materials (oil, natural gas, nowadays, the possible approaches discussed are two. The first one is
carbon). In the same way, hydrogen is an energy vector, and it can strictly related to the efficiency of the wastewater treatment plant, the
partially or totally replace the conventional fossil fuels in transportation second one is the approach to bio-hydrogen production. In particular:
and home heating and cooking. In this last case, green hydrogen will be a
totally new actor in a market today dominated by fossil fuels and under a) the treatment of wastewater uses air as an oxidant agent; this because
transition to electric mobility [91]. anaerobic treatment is a slow process, produces bad smells, and it
Hydrogen will therefore be both a market competitor and a market needs post treatments of the exit water. Aerobic treatment can be
coupling actor, introducing a novel paradigm in market management done in open pools, it is faster, although it requires more energy, and
and pricing. Based on this, a coupling mechanism between the green the addition of oxygen to this air flow increases the treatment effi­
hydrogen market, carbon trading market, and electricity market has ciency, reduces the treatment time and, consequently, the costs. So
been formulated [92]. that it is preferred although in this case there is emission of CO2,
Green hydrogen production and diffusion are inevitable processes while in anaerobic digestion biogas can be produced. City water
that will have both positive and negative impacts, as described in the treatment plants are soil and energy consuming. By coupling an
literature we have cited. According to some researchers, the impact on electrolyser for green hydrogen production, it is possible to use the
water sources is one of these factors. This concern stems from the by-product oxygen to speed up the water treatment, and treated
assumption that fresh water used to produce hydrogen will be diverted water coming from the plant can be easily filtered, deionised and
from other essential applications or sectors for social and economic well- used as a feedstock for the electrolysers [81,93,94]. By this approach,
being. In this situation, water sources could be managed unfairly, trig­ green hydrogen production can be linked to the existing city
gering speculation. wastewater treatment plants.
Based on this assumption, water is only and exclusively consumed. It b) the bio-hydrogen approach is based on dark and photo-fermentation
does not take into account the by-product water resulting when processes of the wastewater, or bio-supported electrolysis [95,96].
hydrogen is used to generate electricity, as the World Economic Forum This approach is in the early stage of development and aims at
[90], for example, pointed out. reducing the energy cost of water treatment by valorising the
By burning hydrogen for energy generation pure water is obtained, by-products, in this case hydrogen. The main issue related to this
both using fuel cells or direct combustion (internal combustion engines, approach is the control of the wastewater composition. This is
turbines, heaters). As well as the fresh water used for electrolysis comes because the microorganism needs substrate having compositions
back as pure water (water-hydrogen-water cycle), more in general - well inside defined ranges. So that this approach is more suitable for
whatever is the green hydrogen source (sea water, waste water, industry applications, especially the food industry, where the pro­
biomass) - the hydrogen applied as a fuel will return pure water. This duced wastewater has sufficiently stable characteristics [97,98].
water can be recovered and reused, although a part of it will be in the
form of vapour and then it will be dispersed in the atmosphere. As already mentioned, another opportunity is the sea water. In
In this way, by using green hydrogen it is possible to recover a part of conventional approach, the desalinated sea water can be used as a
the fresh water consumed for human activities, such as the water used feedstock for electrolysis, but this creates a conflict with desalination
for agriculture (irrigation) or urban purposes (Fig. 1). addressed to other purposes [78,86,87]. Direct seawater electrolysis is
Water can be recovered passing through hydrogen by pyrolysis and under development, and a number of research papers proposed the
gasification of biomasses that are of great interest for valorising residual possibility of applying it for producing hydrogen/oxygen [83–85,99],
and waste biomass, and contributing also to waste management in a but this entails facing some technological challenges. Electrolysis
circular economy vision. applied to solutions of sodium chloride is a well-known industrial pro­
By producing bio-hydrogen with microorganisms, it is possible to cess for production of chlorine, sodium hydroxide and sodium hypo­
treat waste water and create hydrogen at the same time. In addition, chlorite, where hydrogen is a by-product. Industrial processes use high
hydrogen combustion produces pure water. salt concentrations and different electrolyte solutions at anode and
Hydrogen by-products from some industrial production processes, cathode to maximise the production of chlorine, sodium hydroxide and
such as chlor-alkali, or the steam reforming of biogas and bio-alcohol sodium hypochlorite. In direct seawater electrolysis to hydro­
can also be a source of pure water. gen/oxygen, oxygen evolution is in competition with chlorine evolution.
In summary, not only does green hydrogen has no impact on the Consequently, the challenge is to develop new catalysts for avoiding the
freshwater global availability, but it allows recovery of part of the fresh evolution of chlorine, which is toxic, and obtaining exclusively oxygen
water coming from human activities in the form of wastewater or evolution [83–85,99].
biomass. But there is also the possibility to apply electrolysis for producing
Wastewater is a great opportunity for hydrogen production;

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G. Squadrito et al. Renewable Energy 216 (2023) 119041

hydrogen and sodium hypochlorite at low concentrations, without the construction of new “hydrogen pipelines”. For instance, in Europe a
necessity of producing oxygen [100]. Not the least, another alternative large research project for this purpose was launched by IPCEI (Important
is to use the by-product of desalination. For example, about 70% of Project of Common European Interest) funds. Looking at centralised
desalinated water is extracted from sea water by reverse osmosis [85, hydrogen production (see Fig. 2), it also incurs energy and opportunity
101], a concentrated solution of salt is the waste of this process and, losses. In fact, long range transportation requires infrastructures and
normally, it is not used and released in the environment. In principle, energy. The energy used for transportation reduces the system efficiency
this brine can be used as chlor-alkali process feedstock, producing and increases the hydrogen costs. Moreover, centralised production
hydrogen and added-value chemicals (soda, chlorine, sodium hydrox­ could not be really useful for by-product valorisation, with a loss of
ide). Using this system, desalination for urban purposes and electrolytic market opportunities and, again, an increase in hydrogen cost. Not least,
hydrogen production do not “conflict”. Coming back to previous centralised green hydrogen production by electrolysis, as usually
geopolitical considerations, using Saudi Arabia as example, before considered, requires large-scale clean water availability close to the
planning the installation of new desalination plants for hydrogen pro­ production site.
duction, it could be possible to apply the brine exit from the existing Renewable electricity technologies, like PV and wind generators,
desalination utilities by an appropriate adaptation of the existing allow distributed electricity generation. They are simple, can be applied
chlor-alkali technology. widely and nowadays the produced energy has reached costs that are
Based on these considerations and looking at the future world trade competitive with centralised production by fossil fuels also in small-
market of hydrogen, it can be claimed that if the water-hydrogen-water scale applications. In the same way, distributed hydrogen production
cycle is implemented in the same location (local environment), no water by electrolysis is an additional opportunity for hydrogen’s wide
consumption will rise up. On the contrary, the result will be different if applications.
hydrogen is moved from the production site to another side, especially Compared to centralized production, distributed hydrogen produc­
for countries having a water stress issue. In this case, we have to rethink tion offers a number of opportunities related to both the direct appli­
the “hydrogen transport” as “water transport” and, consequently, “world cation of oxygen and/or heat by-products, increased efficiency through
hydrogen trade” as “world water trade”. As a consequence, for countries the absence (or limitation) of transportation requirements, and
with existing (or planned) “water stress”, importing hydrogen might be improved energy utilization through by-product valorisation.
more attractive than exporting hydrogen. Indeed, by importing
hydrogen, these countries will also acquire pure water, which has dual 5.1. Oxygen valorisation
utility: energy and water.
This does not mean that “a hydrogen water issue” does not exist, but As already highlighted, by water electrolysis for each kg of hydrogen
that a cunning management of the question will minimise the environ­ we also produce 8 kg of high purity oxygen without any additional de­
mental impacts. Moreover, we need to be aware that moving billions of vice and energy cost. In addition, there is no variation of the plant’s
tons of water/hydrogen from one continent to another could have an CAPEX and OPEX. Obviously, if oxygen is not used in real time and its
effect on the environment. For example, coming back to the Arabic Gulf storage is requested, an additional expenditure - with respect to the case
case, the realisation of many desalination plants dedicated to hydrogen of the system with oxygen released in the atmosphere - is expected for
production is requested, and these desalination plants will release waste the compression, or liquefaction, and storage of oxygen. As a result, at a
concentrated brine in the local environment [102]. As evidenced in very low oxygen price, the valorisation of this by-product will reduce
literature [102–104], this release is potentially harmful for the envi­ hydrogen production costs.
ronment because it can has a negative impact on the aquatic ecosystem This aspect was already considered by some authors in the financial
and related human activities. analysis of electrolysis plants using photovoltaic roof installations as
In summary, if well managed, green hydrogen will not reduce the renewable electric power supply [23,48], and by Kato and al. [47] for
availability of the fresh water for urban uses. On the contrary, if pure reducing PEM electrolysis hydrogen production costs. This configura­
water from green hydrogen oxidation is collected, freshwater avail­ tion is very interesting for distributed hydrogen production when the
ability will increase. This feature opens up new opportunities in local plant is dedicated to an oxygen user. This is because the user will become
water scarcity management. an “oxygen prosumer” (i.e. producer and user at the same time [112])
avoiding partially (or totally) buying oxygen and related transportation
5. Distributed electrolysis and valorisation of by products costs. This also reduces the oxygen storage costs because the user can
store oxygen at a lower pressure and for a limited time. As expected, the
What we would like to emphasise is that much of the scientific higher the oxygen market price, the larger the advantage. So, for com­
literature and the institutional reports we have referred to - both in panies using low quality oxygen the advantage will be limited, while for
technical, economic and geopolitical terms - concern large-scale, cen­
tralised hydrogen production. The main R&D and financial efforts are
focused on this category of systems and technologies.
These researches usually look at grid stabilisation, wind energy
farms, electric market management and large scale hydrogen production
for industry, transportation and fuel networks [105–107].
This centralised production approach creates an additional issue:
hydrogen transportation and distribution, with related safety issues and
cost increases. Moreover, as highlighted in the previous section, trans-
continental hydrogen trade will move large quantities of water with
possible unforeseen effects.
The cost increases due to transportation was already considered for
car refuelling stations, and solutions like local generation, on Mega­
watts’ scale, or the localisation of the refuelling station near wind farms
or industries that have hydrogen as a by-product have been investigated
[108–111].
More recently, several projects have been undertaken to transport Fig. 2. Scheme for centralised hydrogen production and distribution. Picture
hydrogen from production sites via existing natural gas pipelines or the realised by free images from VectorSquid.com and Vecteezy.com.

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G. Squadrito et al. Renewable Energy 216 (2023) 119041

hospitals using high-quality oxygen the advantage will be very inter­

esting. This is without quantifying the increased resilience against sup­
ply interruptions and sharp price variations due to self production, as
well as the reduced footprint associated with oxygen production by
conventional centralised methods and oxygen delivery to the user.

5.2. Heat valorisation

Electrolysers have limited energy conversion efficiency: 55-80%

depending on technology and balance of plant optimisation. This means
that there is waste heat that could be valorised. Unfortunately, excluding
the solid oxide technology (700-800 ◦ C), this heat is released at low
temperatures (50-80 ◦ C) and is not of real interest for industries. Again,
in distributed hydrogen production, this low value heat can be used for
home heating and district services, especially if this heat is coupled with
the heat output of a combined heat and power (CHP) generation unit
using hydrogen as a fuel (fuel cells or internal combustion engine based)
[20,24,113,114]. Fig. 3. Scheme of proposed polygeneration system with related energy and
Basically, we can envisage a homeowner producing electricity, heat matter flows.
and hydrogen. Thus, extending the current prosumer concept, often
associated with electricity. In the same way, it is possible to foresee RES/ desired energy balance. It should be noted that hydrogen could also be
hydrogen-based CHP services for renewable energy communities, sup­ sold as fuel or feedstock for a local enterprise, although this path is not
plemented by hydrogen refuelling infrastructures for the community’s reported in the figure. If the oxygen is not used, it could be sold or reused
own cars and small service vehicles. in the CHP unit to increase its efficiency, but also released in the
atmosphere.
5.3. Water recycling In summary, RES-based distributed generation of hydrogen is able
not only to reduce the necessity of infrastructure for hydrogen trans­
Water is not a by-product of hydrogen production, but a by-product portation and distribution, but also to supply additional services/feed­
of hydrogen applications for energy generation. Consequently, in stocks. This approach allows to minimise the footprint and can be used
distributed hydrogen production and application, water used for elec­ by single industries, industrial districts, cities and small communities
trolysis can be recovered from fuel cells, internal combustion engines, down to single homeowners, because it allows the local application of
CHP systems and hydrogen-fueled condensing boilers, according to the by-products and minimises emissions.
hydrogen application. This water can be reused in electrolysis after Technologies for distributed green hydrogen generation are ready.
deionisation, thus no water is consumed in the cycle. The remaining issues are mainly related to regulatory and economic
At city level, we can consider the utilization of a futuristic hydrogen aspects. In particular:
city gas distribution network replacing today’s natural gas distribution
network. In this future hydrogen city we can foresee a distribution of ● High CAPEX - Like for large scale centralised production, a cost
hydrogen by pipelines, like for methane today. In this case, by reduction for electrolysers is requested, this is more significant for
condensing the water produced by hydrogen-fueled devices, it will be small-scale electrolysis plants.
possible to cut the consumption of drinkable water applied for other ● Regulations about flammable gas storage in urban environments - In
purposes like home cleaning, cloth washing and watering houseplants. many countries, like Italy, there are strong limitations on the volume
This reproduces in small scale the concept previously exposed for the and pressure of flammable gases that could be stored (in liquid or
world trade, moving hydrogen means moving water. compressed form) in, or near, a building. This is a strong limitation
Not the least, it is possible to produce green hydrogen using city for energy communities, single homeowners and small craft
wastewater as previously discussed. Production of domestic (on-site) enterprises.
hydrogen from wastewater will also be possible in the near future. ● Regulation about refuelling points - Based on the information gath­
ered, no country allows/foresees the installation of hydrogen/
5.4. Polygeneration methane refuelling points close to residential buildings.
● Adaptation to hydrogen of gas-based home appliances - Natural gas
By unifying the previous points, it is possible to conceive a RES-based cookers, boilers and other utilities admit just a few percentages of
polygeneration concept involving power, heat, hydrogen and oxygen for hydrogen blended with natural gas. New burners must be produced
distributed generation of energy and fuel, with oxygen valorisation and for hydrogen rich blends or pure hydrogen.
water recycling. A simple scheme is shown in Fig. 3. This distributed ● City gas pipeline networks and related regulation - Regulation is
generation approach could be applied to industries, cities, neighbour­ needed both for methane/hydrogen blend distribution and for
hoods and energy communities for reducing, as close as possible to zero, allowing prosumers to inject the produced hydrogen into the city gas
the environmental impact of human activities. network. Many pipelines are not suitable for hydrogen blends.
As seen in Fig. 3, the proposed system arrangement considers RES
power generation, a hybrid configuration of batteries/hydrogen for the 6. Conclusions
energy storage system, and also the possibility to exchange both the
electricity and hydrogen with the local grids. It can be envisaged that the The aim of this research was to identify possible approaches and
system can work also in an island configuration. Water is recirculated solutions for a broad deployment and applicability of green hydrogen,
between electrolysis and CHP units to minimise the freshwater use and, by looking at all the technologies today available and ready for large-
if of interest, also water condensed from heat/cool services can be scale applications.
reused in the electrolyser. The CHP unit is not necessarily based on fuel Unlike other analyses conducted in the literature, the focus was not
cells, it could also be based on a hydrogen-fueled internal combustion on the cost of hydrogen or the analysis of the necessary infrastructures.
engine or turbine, depending on the user convenience in regards to

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G. Squadrito et al. Renewable Energy 216 (2023) 119041

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Declaration of competing interest [23] A. Nicita, G. Maggio, A.P.F. Andaloro, G. Squadrito, Green hydrogen as feedstock:
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