Discussion
The concept of GH
2 appears to be a relatively fresh concept, as evidenced by its first appearance 25 years ago (US Department of Energy,
1995). The US Department of Energy report (
1995) claims that H
2 produced by RES or nuclear energy would contribute to eliminating atmospheric pollution by carbon monoxide and ozone, and thus reduce global warming (US Department of Energy
1995). However, there is evidence from even earlier studies that address H
2 production from RES. For example, the use of wind in 1978 (Bilgen
1978) and solar in 1989 (Knoch
1989). In other words, it emerged much earlier as a method but without being identified with the term GH
2.
The current relevance of this energy vector responds to five aspects: Firstly, improvements in terms of efficiency of RES production processes (including production costs); secondly, improvement in the efficiency and cost of electrolyzers (Laguna-Bercero
2012); thirdly, the need to capitalize on the surplus of RES production due to intermittencies (Clark and Rifkin
2006; Jensen et al.
2007; Hall et al.
2014; Brey
2021); fourthly, the great growth possibilities of these energies (ESMAP
2020); and fifthly, the strong impact on reducing CO
2 emissions that it may provide in the future (Yu et al.
2021).
As regards GHG, it should be noted that CO
2 emissions are not the only cause of concern from an environmental point of view. There are other polluting gases that directly affect the population and cause environmental emergencies in many cities, e.g. particulate matter (PM
10) and polluting gases such as ozone (O
3), carbon monoxide (CO), nitrogen dioxide (NO
2), and sulfur dioxide (SO
2) (World Health Organization
2005), being of special concern for certain types of RES, and especially in the case of some types of biomass.
According to Hosseini and Wahid (
2016), environmentally friendly biomass is considered the best alternative fuel, potentially the major and most sustainable RES on the planet, with approximately 4,500 EJ of annual primary production (Hosseini and Wahid
2016). Nonetheless, biomass exploitation processes have significant shortcomings in terms of efficiency and emissions. For example, biomass has the lowest efficiency among RES for electricity production (30–35%), compared to solar and wind energy (achieving 40–60% efficiency) (Mohideen et al.
2021). It also presents other problems, such as ash, including silicate melt-induced slagging, alkali-induced slagging, corrosion and agglomeration (Niu et al.
2016). The adverse effects of biomass exploitation on ecosystem components rings alarm bells (Mai-Moulin et al.
2021). Another form of biomass exploitation is bio-hydrogen or H
2 produced biologically from biological waste, wastewater, forestry and agricultural residues, among others (Osman et al.
2020a). This method does not have the shortcomings of biomass burning in terms of emissions. However, if we consider the low EROI values for biofuels, it can be established that they have less potential to produce GH
2 than other RES, even compared to other RES of the same biomass family (see Table
1), in this case biomass (Physical process) being the one with the highest potential.
In general, the aim of promoting GH
2 production is to achieve a RES based energy carrier at a competitive cost with respect to traditional fuels, which contributes to solving GHG emissions. In this sense, biomasses also present fewer benefits compared to the rest of RES. According to results of a recently published study by Gemechu & Kumar, they show that the CO
2 values of wind based water electrolysis (0.69 ± 0, 04 kg CO
2 eq / kg H
2) on average has values three times lower than the best result for H
2 produced from a biomass (the case of bio-oil reforming varying between 1.57 and 3.46 kg CO
2 eq / kg H
2), and in the case of supercritical water gasification (SCWG) of algal biomass, this could increase between 10.14 to 12.72 kg CO
2 eq / kg H
2 (Gemechu and Kumar
2021). Although the above elements lead us to question whether this RES can be classified as green, the keyword map indicates that biomasses and biogas have been classified as GH
2, with several recent examples (Di Marcoberardino et al.
2018; Preuster and Albert
2018; Cholewa et al.
2018; Akroum-Amrouche et al.
2019; Minutillo et al.
2020; Gonzalez Diaz et al.
2021; Zhao et al.
2021), and especially bio-hydrogen, a trend which has been gaining ground (Abuşoğlu et al.
2017).
As regards to nuclear energy, although the US department of energy (US Department of Energy,
1995) raised this energy source in the conceptualization of GH
2, in our results it only appears in the keyword maps for SH
2 (Fig.
6) and RH
2 (Fig.
4). In any case, the appearances on the RH
2 map refer to the context where RES and nuclear energy are integrated to produce H
2. (Orhan et al.
2012; Orhan and Babu
2015; Agyekum et al.
2021; Temiz and Dincer
2021). Although nuclear energy cannot be included in the RES and green energy framework, it is called "clean energy
5" because no GHGs are emitted during the process of generating electricity from this source (Velasquez et al.
2021; Elshenawy et al.
2021; Brown
2022; Hassan et al.
2022). This, together with its efficiency levels, has led several authors to consider it as a sustainable option for the production of H
2 (Dincer and Balta
2011; Dincer and Zamfirescu
2012; Zhiznin et al.
2020; Velasquez et al.
2021). There are a wide range of scientific papers on H
2 production using nuclear energy, and it seems that plans for H
2 production stimulate the development of the symbiosis of nuclear energy and RES (Zhiznin et al.
2020). In any case it should not be forgotten that this energy source is enormously controversial due to its drawbacks in terms of perceived safety (Prati and Zani
2012; Perko et al.
2018; Deng et al.
2018), waste treatment (Ewing et al.
2016; Yano et al.
2018), and geopolitics (international political conflicts and lack of massification towards underdeveloped countries
6) (International Energy Agency (
2021); Hickey et al.
2021). These elements may somewhat contradict certain sustainability criteria under the Sustainable Development Goals (SDGs) and the Paris Agreement(UNFCCC
2015; United Nations
2015). It is important to note that among the strategies reviewed, no nation was identified as considering nuclear energy for GH
2 production.
According to the cited SDGs, sustainable development can be summarized as development that meets current needs without compromising future capabilities (United Nations
2015). The SDG 7 goal include access to energy, the incorporation of RES and improvement of energy efficiency, among others. This has given rise to three narrative imperatives that sustainable development must meet: satisfying human needs, guaranteeing social justice, and adhering to permissible environmental values (Holden et al.
2021). Among the indicators for defining sustainability, despite no single acceptance of such, in general they point to economic, technical, environmental, social and political factors (Mai-Moulin et al.
2021; Gunnarsdottir et al.
2021; Saraswat and Digalwar
2021). Therefore, we believe that the use of the term "SH
2" is correct when referring to improvements in economic, technical, environmental, social, and political factors that occur in H
2 related processes, as long as the improvement in one factor is not to the detriment of other factors.
National global strategies show clear signals on the use of energy sources and the technology to be employed in global GH
2 strategies (see Table
2). Electrolysis is the main conversion technology, as evidenced by the fact that future capacities are projected in electrolyzer capacity (in GW). On the RES side, countries with a clear focus on GH
2 production have clear targets for the use of solar and wind energy. Accordingly, the GH
2 concept could be defined as H
2 obtained from RES, using electrolysis, free of polluting emissions that guarantee an energy return that does not jeopardize its sustainability. In some cases (e.g. in Portugal and Spain), it is stated that H
2 could also be produced from biomass (i.e., through gasification processes, biochemical conversion, or biogas reforming), as long as sustainability requirements are met. Both countries also express their H
2 production targets in electrolyzer capacity (in GW) (Spain
2020; Presidência Do Conselho De Ministros
2020).
There are also other factors in the H
2 production chain that may jeopardize its sustainability. On one hand, there are the losses in the electrolyzers. In this sense, review investigations that consider the performance of electrolyzers and the specific phenomena that occur in their components are very useful for present and future research (Falcão and Pinto
2020). On the other hand, it is essential to consider the scale of the envisaged projects, which directly conditions aspects such as H
2 storage and transport, also including local community acceptance. The following projects, which are reproducing traditional centralized energy models (i.e., large-scale projects), could be used as example: i) The “green crane” project, under public–private partnership. This project, headed by the Spanish Enagas and the Italian ESNAM gas transmission companies, under which umbrella the “green spider” project seeks to launch a €2,250 million investment plan to turn Spain into an H
2 exporting country to north Europe (Enagás
2022); or ii) The case of Chile, for example, where the areas of greatest wind and solar energy potential are located in the south (Magallanes Region) and north (Antofagasta Region) respectively (Chile
2020), while the largest population and business activity are located in the center of the country (Santiago de Chile), i.e., at a distance of more than 500 km.
In general, we consider the timely monitoring of research indexed under the terms electrolyzers, H
2 storage, H
2 distribution and transport, and especially the EROIpou for H
2, to be essential. As stated by Zamani et al.
2022, surveillance of emerging issues is fundamental to the work of researchers, practitioners, and policy makers. At the same time, the need to provide more organized information in order to facilitate the transfer of knowledge to decision-makers in technological fields is indispensible (Garechana et al.
2022). It is also important to bear in mind that this knowledge is a key input for tracing technological trajectories, determining when a technology has matured, identifying existing knowledge gaps in these areas, and learning about new emerging knowledge (Alvarez-Meaza et al.
2020; Zamani et al.
2022).
From a technical–economic, environmental, and social perspective, we believe it is important to make a proposal that facilitates the appropriate use of the terms GH
2, SH
2 and RH
2. To this end, we propose five criteria: the type of energy, estimated EROI values, CO
2 emissions, other types of emissions, and the community's perception of the type of project. These were categorized in correspondence with the insights gained during this research (see Table
3). Failure to meet at least one of the criteria is sufficient not to reach the green category, therefore, it would fall between the sustainable or renewable category. Note that the renewable category is the least restrictive; its only condition is to use RES. Sustainable, on the other hand, necessarily implies that improvements will occur, at least in relative terms.
Table 3
Proposed criteria for the use of the terms GH2, SH2 and RH2.
Source: Own elaboration, 2022
GH2 | RES | e.g. EROI pou: no risk; < 10–15: 1 or low risk < 5–10: 1 | Low emissions | Emissions-free | Well valued by the community |
SH2 | RES alone or combined with other fuels | e.g. EROI pou: no risk; < 10–15: 1 or low risk < 5–10: 1 | Reduction of emissions compared to state of the art | Reduction of emissions compared to other sources | Improved community acceptance |
RH2 | RES | Any value | Any value | May emit, e.g. particles, ashes, odors, etc | Sometimes they can generate conflicts within the community (e.g. big scale projects; biomass from wast…) |
As the ultimate expression of technological progress, GH
2 should represent the cleanest form in terms of emissions. Therefore, it is not enough to use RES to classify a production method as green; it is also essential to evaluate the type of energy, the GHG emissions of the production process, the production technologies used (Dawood et al.
2020; Velazquez and Dodds
2020) and the carbon footprint of the supply chain, as well as including a NEA based on indicators such as EROI or EPBT.
Conclusions
The great expectations created around H2 use as an energy vector focus the attention of the scientific community. This is demonstrated by the exponential growth in the number of publications on the subject. Researchers play an essential role in providing the community with knowledge of high scientific value, becoming fundamental referents in the most complex challenges facing humanity. The literature produced becomes the basis for projects of the greatest technological complexity, as well as for the formulation of new policies. For this reason, it is of utmost importance to be careful about the indexing and use of terms that can be transferred to society. In the case of the use of the indexing terms "GH2, RH2 and SH2" as synonyms, this could lead to the acceptance of technologies that do not meet the standards to be classified within each term.
The most relevant authors as regards the H
2-related production, distribution and technologies belong to developed countries, which are also the ones that provide the majority of funding for this type of research. Connecting with these researchers may help foster innovation in solutions that address local priority challenges and accelerate the implementation and transfer of these technologies, responding to the commitment made by Paris Agreement signatory countries and also in line with SDG 7 (United Nations
2015), which propose increasing international collaboration to enable access to clean energy research and technology. In this sense, more developed countries should favor the financing of those research projects on GH
2 production that include the participation of organizations and researchers based in these less developed countries that have a high potential for RES generation due to their natural conditions.
The term "ecological H2" has been used very rarely and, according to its trend, it is not expected to gain relevance in the coming years. The term clean H2 shows a discrete record, however its relevance may be favored by the development of blue hydrogen and H2 produced from nuclear energy. As regards the term RH2, it has a broader scope than GH2 and may include studies on biomass or other RES for H2 production methods that are not totally clean or efficient, with low EROI values. Unlike in the RH2 map, in the GH2 map there is little use of the terms linked to certain RES such as solar and wind, while the terms “biomass” and “biofuels” are much more prominent. Research on GH2 is growing exponentially, however, its growth rate is lower than for SH2 and RH2. This is partly because there has been no standardization in the use of these terms. Since there is no delimitation or understanding of the terms sustainable, renewable and green, articles can be framed in any of them, under the assumption that they are synonyms.
Production, distribution, and consumption of GH2 is a highly complex, diverse subject given the great variety of technologies and the specific characteristics of each RES according to the conditions of each country. In this sense, it is important to promote studies that analyze H2 efficiency levels, as well as to disseminate studies of implemented cases associated with different international experiences that help to generate maturity and investor confidence in order to accelerate massification in the production of this energy carrier, also achieving the incorporation of medium and small actors in the implementation of national policies. Therefore, it is fundamental to promote the correct identification of keyword indexing terms among scientific editors and authors.
As the ultimate expression of technological progress, GH2 should be a reference for technologies that meet the acceptance criteria by the scientific community, the highest standards in terms of emissions and efficiency. In addition to the use of RES to classify it as GH2, it is essential to evaluate the type of RES, the production technologies as well as the carbon footprint of the supply chain, also including a NEA based on EROI, EBPT, or other related indicators. In turn, it is important to influence the vision of policy makers to confer a special status to GH2. Therefore, the new policies formulated would aim for H2 production models to apply for the "green" distinction, to be favored by the incorporation of incentives, possibly fiscal or commercial (tariff reduction), among others. Nonetheless, one should not be absolute when defining the type of technologies used due to the rapid development and production of new techniques. At present electrolysis and/or photoelectrolysis are the ones being used in national H2 strategies, to the point that they project their objectives referring to the installed capacity of electrolyzers (in GW). Although several authors describe certain types of biomass as very promising, we consider that any strategy for GH2 production based on these RES should pay special attention to the efficiency levels achieved in the different processes and how to control or mitigate the resulting waste and pollutant emissions.
Main national policy strategies have included the H
2 as the most promising energy carrier towards the energy transition path. In any case, the relevance of big scale projects should also be underlined. In this regard, indicators such as EROIpou may become fundamental to classify projects as RH
2 or GH
2, as well as to guide different national strategies towards a low carbon transition. Despite the fundamental progress made in estimating the EROI of different technologies (e.g., as regards the use of Integrated Assessment Models (IAMs) that allow the EROI to be estimated endogenously and dynamically), the fact that the different meta-analyses do not differentiate between different EROI metrics (e.g. EROIpou; EROIext…) is a clear sign of the necessity to establish clear boundaries and improve the methodological aspects as regards this indicator. Moreover, the present study also reveals that despite EROI being now well-established within the scientific community for evaluating different energy projects, it is at very incipient stage when it comes to evaluate projects related to H
2, especially GH
2. In any case, as Carbajales-Dale states, we consider that the moment has come for policy makers to make greater use of this fundamental tool in determining their overall long-term energy strategies towards energy transition (Carbajales-Dale et al.
2014). In this sense, EROI is a complex metric that, together with other indicators (e.g., LCA), may be fundamental in order to distinguish boundaries between GH
2, RH
2 and SH
2.
This work shows the main challenges from the point of view of indexation in relation to the types of energy used for the production of GH2. However, other aspects such as the energy loss in electrolyzers, storage, scale of the projects, transport, different technologies and RES used for its production, also including different forms of H2 consumption and the aspects related to community acceptance, should be studied further. Furthermore, future research should focus on the analysis of EROI boundaries and its different modalities (i.e., EROIpou, EROIext…), which are useful tools to evaluate the efficiency of each of the stages of the H2 chain. We should also address the use of biomasses as an energy source for the production of RH2, remarkable for the wide variety of existing materials and technologies for its exploitation. Accordingly, the application of bibliometric tools combined with sustainability indicators can be very useful for providing synthesized information to environmental policy makers and stakeholders in general.