Public Acceptance of the Underground Storage of Hydrogen: Lessons Learned from the Geological Storage of CO₂

Abstract

The successful commercialisation of underground hydrogen storage (UHS) is contingent upon technological readiness and social acceptance. A lack of social acceptance, inadequate policies/regulations, an unreliable business case, and environmental uncertainty have the potential to delay or prevent UHS commercialisation, even in cases where it is ready. The technologies utilised for underground hydrogen and carbon dioxide storage are analogous. The differences lie in the types of gases stored and the purpose of their storage. It is anticipated that the challenges related to public acceptance will be analogous in both cases. An assessment was made of the possibility of transferring experiences related to the social acceptance of CO₂ sequestration to UHS based on an analysis of relevant articles from indexed journals. The analysis enabled the identification of elements that can be used and incorporated into the social acceptance of UHS. A framework was identified that supports the assessment and implementation of factors determining social acceptance, ranging from conception to demonstration to implementation. These factors include education, communication, stakeholder involvement, risk assessment, policy and regulation, public trust, benefits, research and demonstration programmes, and social embedding. Implementing these measures has the potential to increase acceptance and facilitate faster implementation of this technology.

1. Introduction

Hydrogen is regarded as one of the most significant fuels of the future, with the potential to substantially contribute to the reduction in greenhouse gas emissions and the enhancement of energy security [1,2,3]. This gas is considered a fundamental element in the implementation of the Paris Agreement targets to reduce CO₂ emissions. Moreover, the energy policy of the European Union anticipates the utilisation of hydrogen in larger quantities within the future economy [4,5,6,7].

In order to reduce CO₂ emissions, it is essential to replace fossil fuels with clean (renewable) energy sources. Hydrogen has the potential to fulfil this function in various sectors, including energy, transportation, and heavy industry, with particular applications in metallurgy [8,9]. It is capable of not only meeting current energy demands, but also enabling long-term energy storage. The industrial use of hydrogen is currently dominated by oil refining and chemical production, and almost all gas is produced from fossil fuels (grey hydrogen) [10]. By contrast, low- or zero-carbon hydrogen (also known as blue, green, and other hydrogen types) can be used for a variety of purposes, combining the role of energy carrier and energy storage capability [11]. It is becoming a key player in the transition to renewable energy sources [12,13,14].

The demand for hydrogen storage is driven by several important considerations. First, the development of renewable energy sources is a key factor, as is the decarbonisation of the energy sector and industry. In addition, the reduction in CO₂ emissions, the diversification of energy sources, the increased demand for hydrogen in the economy, energy security, and stabilisation of the energy system are also significant drivers [5,7,15,16,17,18].

The development of hydrogen technologies, particularly within the domains of transport, energy, and industry, which are characterised by high energy demands, is contingent upon effective hydrogen storage methodologies. There are several methods of hydrogen storage, including physical (compressed, liquid, and underground storage) and chemical (metal and complex hybrids as well as liquid organic hydrogen carriers) [19,20]. The selection of the most suitable method is contingent upon the particulars of the application, the requisite gas demand, and the storage costs. One method of the large-scale and long-term storage of hydrogen is in geological structures such as salt caverns, deep aquifers, or depleted natural gas fields [7,21,22,23,24,25]. The prospect of underground hydrogen storage (UHS) is promising, representing the possibility of storing large quantities of hydrogen in a safe and secure manner at relatively low costs [26]. UHS is a technology that can be used to compensate for seasonal fluctuations in the production of energy from renewable sources in the range of GWhH₂ and TWhH₂ per year [27,28,29]. The transition from general hydrogen storage technologies, such as compressed gas and liquid hydrogen storage, to underground storage represents a significant advancement in the hydrogen economy. Underground hydrogen storage utilises geological formations like salt caverns and depleted oil and gas reservoirs to store large quantities of hydrogen at high pressures. This method leverages existing natural gas storage infrastructure, making it a cost-effective and scalable option. UHS supports the integration of renewable energy sources and the transition to a low-carbon energy system by providing a robust and resilient storage solution.

The feasibility of storing hydrogen in underground caverns leached in salt deposits has been demonstrated [30]. Examples of this type of infrastructure include salt caverns in Texas, serving refineries in the Gulf of Mexico, and the Advanced Clean Energy Storage project in Utah, where the world’s largest UHS project is located [31]. The EUH2STARS project is being carried out in Europe with the aim of demonstrating the storage of hydrogen in depleted natural gas fields in Europe. This project is supported by the Clean Hydrogen partnership and aims to increase energy security and support the energy transition in Europe [32]. Germany is implementing the H2CAST Etzel project and Hydrogen Pilot Cavern Krummhörn [33], while the Netherlands is managing a commercial storage facility developed as part of the HyStock project [34]. The French HyPSTER project involves the storage of green hydrogen in salt caverns in Alsace [35], while the UK HySecure project is developing storage systems in northern England [36]. These projects emphasise that social acceptance is crucial for the success of UHS and requires transparent communication and the involvement of local communities. In some projects, social acceptance is included in various work packages, including the assessment of hydrogen storage needs and the involvement of local communities (HyPSTER), work focusing on social involvement and social assessment (H2SECURE), and the legal feasibility and risks and benefits (EUH2STARS) of UHS. The HyStock project also takes into account public consultation and transparency in planning and implementation. In Canada, the government is currently engaged in a pilot programme that involves the storage of natural gas in geological formations located in the western region of the country. Meanwhile, Australia is implementing a strategy that integrates the storage of natural gas in depleted fields with renewable energy sources. This initiative aims to facilitate the export of energy to Asian markets [37]. In 2024, the International Energy Agency listed 59 underground hydrogen storage projects in various stages of development in its database. Most of these projects are in the concept phase, and 23 are in the front-end engineering design phase. Only five projects have the status of being operational. In two projects, storage is carried out in depleted natural gas fields, and the storage facilities are located in salt caverns in two others. One project involves storage in rock caverns [33]. The operational underground hydrogen storage facilities that have been constructed provide the foundation for the growth of a hydrogen economy. Their operation provides valuable experience and guidance on safety, technology, and the economics of UHS.

In order to realise the full potential of hydrogen, it is essential to establish effective, secure, and socially acceptable methods for its storage [38]. The introduction and utilisation of underground hydrogen storage must, therefore, be in accordance with societal expectations, and its implementation necessitates an understanding of both the technical and social aspects involved [39]. As emphasised by Mendrinos et al. (2022) and van Gesse and Hajibeygi (2023), the success of the implementation of a new technology, which UHS undoubtedly is, depends not only on its technological readiness level (TRL) but also to a large extent on its social readiness level (SRL) [40,41]. A lack of, or incomplete, social entrenchment resulting from public resistance, an absence of suitable policies and regulations, an unreliable business case, and uncertainty regarding environmental impacts has the potential to result in a slowdown or failure of UHS commercialisation, even when the technology is technically ready for implementation. A case in point is provided by a study of public perceptions of underground coal gasification in the UK, which is cited to explain the lack of deployment of this technology [42].

Purpose and Scope of This Study

Despite the elevated technological readiness level (TRL) of technologies for underground carbon dioxide storage (CCS) and underground hydrogen storage (UHS), their implementation on an industrial scale is impeded by a paucity of social readiness (SRL) for their acceptance, particularly in the case of UHS. In practice, this signifies that even with documented effectiveness and technical safety, the absence of full social acceptance can substantially delay or even forestall the implementation of the project. This discrepancy between technological and social readiness is identified as a significant impediment to the implementation of UHS technology. Additional factors contributing to the social acceptance challenges of UHS include concerns regarding safety and environmental impact, inadequate regulatory and policy frameworks, the limited utilisation of experience from analogous technologies, and a paucity of research on the social acceptance of UHS. The experience with underground CO₂ storage demonstrates that even at a high level of technological readiness, a lack of social acceptance can result in delays or even the complete failure of implementation.

A review of the literature on the social acceptance of UHS reveals a paucity of publications in this field, although there has been increased interest in the issue over the past decade, as evidenced by the growing number of scientific articles and reports devoted to the subject. The authors have experience, backed up by publications, in the area of social acceptance of the underground storage of other gases, particularly carbon dioxide disposed of using CCS technology. It is important to note that underground gas storage technologies, such as H₂ and CO₂, despite differences in the type of stored gases, demonstrate significant similarities in terms of how they are injected into the same geological structures and the resulting effects on the storage formations. Therefore, it can be hypothesised that the problems with public acceptance for this type of activity will be similar. In light of this, the authors deemed it prudent to leverage the accumulated knowledge and case studies pertaining to the public acceptance of geological CO₂ storage (CCS), a technology with a more extensive operational history, to identify the barriers and challenges hindering the implementation of UHS in the present context. This analysis was conducted by examining pertinent articles from indexed scientific journals. The primary objective of this research endeavour was to discern the elements and experiences associated with the social acceptance of CCS that can be applicable to the social acceptance of UHS. This was complemented by an analysis of the literature on UHS, which revealed a paucity of publications on the subject. The main objective of the literature analysis was to present a framework for activities influencing the social acceptance of UHS, the introduction of which will accelerate the implementation of this technology. A systematic framework for the social acceptance of CCS and UHS, detailed recommendations for various stakeholder groups, and directions for future research were additional objectives of this study.

The bibliographic database Scopus was selected for the literature search, with the search terms ‘underground hydrogen storage and social acceptance and public acceptance’ and ‘carbon dioxide storage and CCS and social acceptance and public acceptance’ employed. The search was conducted for the time period from 1970 to 2025. The search algorithm was executed on 10 January 2025. It was not constrained by any particular field or scientific discipline; rather, it was open.

2. Analysis of Publications on Public Acceptance of Underground Storage of CO₂ and H₂

The social acceptance of underground gas storage, mainly of CO₂, is discussed in reviews and research papers. The bibliometric analysis of these articles provides insight into research trends and also shows the factors influencing the implementation of geological storage and/or utilisation technologies for CO₂ (CCS and CCUS) [43]. A bibliometric analysis of the publications revealed their geographical distribution and institutional affiliation, as well as the major journals in which the articles were published. A marked increase in the number of publications was observed after 2009, with significant contributions to CCS research from countries such as the US, China, and the UK. However, it is noteworthy that key societal aspects of CCS are underrepresented in the literature [44]. A review of the peer-reviewed literature examining the public acceptance and social impacts of CCUS projects identified three areas of potential controversy. These are specifically identified as the contentious matters of the extent of CCUS impacts, and the factors that shape individuals’ perceptions of these impacts. Additionally, the importance of how these impacts are contested by the involved communities, politicians, and researchers is emphasised [45]. Conversely, a meta-analytic review of the literature indexed in the Scopus database over the last 40 years regarding the public acceptance of renewable energy identified that the public acceptance of RES is crucial for the success of the energy transition and the fight against climate change [46].

2.1. Experiences with CO₂ Storage

In the context of underground hydrogen storage, it is imperative to utilise the insights and experiences derived from social acceptance studies in related fields, including gas storage, oil and gas production [47,48], and CCS/CCUS technology [49,50,51,52], while considering the specific properties of gaseous hydrogen [21,41,53,54]. These studies can form the basis for the development of a public communication approach to enable the success of UHS technology. As the majority of publications concern the social acceptance of CCS, the focus of this article is on the experience with this technology.

The fundamental distinction between UHS and CCS lies in the necessity for hydrogen storage, as well as retrieval, in contrast to the storage of CO₂, which is inherently irreversible. A significant challenge is the presence of various processes (chemical, physical, and microbiological) that occur during UHS, which can lead to contamination, hydrogen loss, or reduced storage capacity [43,55].

A review of the literature on the subject reveals a number of key barriers to the implementation of CCS: there is a lack of public knowledge about CCS, which is compounded by a number of misconceptions; there is a paucity of communication strategies; there is competition between alternative technologies; there is a lack of long-term policy on the implementation of CCS; there is controversy surrounding the economic efficiency of the technology; there is a weak market-based mechanism; there has not been enough study of the long-term effects of the technology; there is a lack of trust in some stakeholders; there is a NIMBY reaction (“not in my backyard”); there is a failure to take into account the specifics of the local population in site selection and project design; and finally, there is the appearance of protest potential due to negative public perception [56].

Tcvetkov et al. (2019) emphasise that the public awareness of CCS technology remains very low, with lay opinion surveys having only begun in the initial decade of the 21st century. The results of the study suggest that public opinion will be an important element in the implementation of CCS projects; therefore, its needs must be given due consideration. This necessitates stakeholder engagement to establish trade-offs in all stages of project implementation [56]. This study identified nine key aspects that influence the public perception of CCS, including awareness, knowledge, the perception of benefits and risks, social and demographic factors, willingness to pay, trust, the acceptance of CCS, preference between technologies, governmental policy, and interaction between stakeholders (Figure 1) [56].

From a socio-technical perspective, McLaughlin et al. (2023) identified enabling factors and barriers to CCUS implementation, policy mechanisms, and international frameworks, as well as research gaps [57].

The implementation of CCS technology has, thus far, encountered significant public resistance in both Europe and the USA. A review of risk perceptions associated with the different stages of the CCU/CCS process chain has shown significant differences in the acceptance of the two technologies, with a greater recognition of CCU. It is highlighted that the acceptance of CCS is negatively influenced by the risks associated with carbon storage and transport [49].

There are examples of local community surveys that aim to ascertain levels of the knowledge and perception of climate change and CCS. The results that are presented here may serve as a basis for further work directed at gaining acceptance of local communities in work relating to the implementation of CCS technology [52]. The research findings on opportunities and challenges for local communities related to the location of combined CO₂ and natural gas storage (CCUS and UGS) in north-western Poland highlight four significant mechanisms for fostering trust in CCUS and UGS technologies: communicating safety, communicating lack of nuisance, communicating benefits, and transparent and competent knowledge transfer [58].

One of the reports presenting case study descriptions in the field of research into aspects of social acceptance of CCS is the WiseEuropa report ‘Social acceptance of CCUS technology in Poland’ [59]. The report contains the results of a survey that analyses the public’s awareness of and attitude towards CCUS technology. The survey revealed that 27% of respondents had prior knowledge of CCS. Moreover, the provision of additional information led to a negative shift in opinion for 34% of respondents, while 13% expressed a positive shift. Notably, more than half (53%) remained unaltered in their views following the receipt of further information.

A survey of the local (rural) community [52] was conducted to ascertain the level of knowledge regarding the perception of climate change and CCS technologies. The results indicated that the majority of respondents were not acquainted with clean coal technologies, particularly CCS. It was observed that as the level of education increased, so did the respondents’ knowledge about clean coal technologies. The survey revealed a predominant belief among respondents that CCS can contribute to the prevention of climate change.

As has been previously observed, the scientific discourse pertaining to CCS is characterised by a prevailing imaginarium (notion) that anticipates the implementation of CCS as a means to safeguard the region’s industrial foundation, while concurrently pursuing objectives related to climate change mitigation. This conceptual framework is founded on a scientific authority that justifies the necessity for carbon capture and storage (CCS) by associating it with environmental sustainability. The credibility and popularity of this framework are reinforced by its alignment with established scientific principles and the concept of ‘greening’ the industry, which refers to the integration of environmental considerations into industrial practices [60].

However, the majority of studies suggest that the provision of risk information, when integrated with other CCS-related information, has a favourable impact on acceptance. For instance, Oltra et al. (2012) observed that the provision of additional educational materials led to an enhancement in acceptance of CCS [61]. Similarly, Tokushige et al. (2007) noted that the dissemination of information regarding risks and benefits served to mitigate concerns without adversely impacting the perception of benefits [62].

The findings of research on CCS communication conducted since 2002 have led to recommendations for industry and policymakers, alongside the identification of stakeholder groups for communication activities. These groups have been categorised into the following categories: influential others, community, education, and project-specific activities [63].

The results of the studies point to the need to consider public acceptance of CCS in the research, development, demonstration, and deployment of this technology. The findings highlight the key role of stakeholders in influencing innovation, investment decisions, and the location of CO₂ and CO₂ disposal facilities, as well as derivative products [64].

It has been highlighted that narratives concerning CCUS technology can vary significantly between stakeholders and regions [65]. A study of the factors influencing the acceptance of CCS in industry (iCCS) and how this knowledge can be used in policy and industry decision-making processes showed some important differences between the public acceptance of iCCS and CCS [66]. Mendrinos et al. (2022) [40] demonstrated the application of the societal embeddedness level (SEL) methodology to CO₂ sequestration in the Netherlands, Norway, and Germany.

The risk of hydrogen leakage is likely to be a significant factor influencing the public acceptance of UHS as it affects perceptions of the overall safety of the process. It is posited that the lessons learned from CO₂ sequestration may offer valuable insights that can be utilised in the domain of hydrogen storage in salt caverns and porous rocks [39]. Risk concerns play a dual role in the decision-making process of a plant location, both formally and informally. Informally, that is to say outside the permitting and approval process, risk constitutes a significant issue for the public, influencing public opinion and general acceptance. This in turn has a major impact on whether a project will ever be successfully completed. Even if public consent is not formally considered in regulatory decisions, high-risk projects can mobilise public resistance, thereby reducing the likelihood of project success. Research indicates that informing people about the risks associated with CCS can negatively affect the acceptance of these technologies [67]. For instance, Ha-Duong et al. (2009) observed a 20-percentage point decrease in acceptance following the dissemination of risk information [68]. Similarly, Dowd et al. (2014) noted that survey participants who received risk information exhibited reduced support for CO₂ storage [69].

As Krevor et al. suggest, the public acceptance of CO₂ storage could be enhanced by financial incentives [70].

A comprehensive review of public acceptance studies on direct air carbon capture technology, coupled with storage, reveals two distinct perceptions of the technology. Opponents of the technology emphasise concerns about storage, citing the risk of leakage as a primary concern. Conversely, proponents of the technology perceive it as economically, climatically, and ethically beneficial, both in the present and for future generations [71].

Public trust, based on both integrity and competence, is a key predictor of CCS acceptance. Detailed information positively influences respondents’ initial reactions. Environmental NGOs are perceived as more credible than industry stakeholders. To build and maintain trust, it is important to strengthen communication channels and education about CCS. People often rely on trust in supporters or opponents of the technology, which shapes their attitudes towards CCS [72]. Trust is a key element of the positive perception of CCS technology. Public trust in stakeholders determines the willingness to consider implementing a project. In Germany, CCS ideas provoke negative reactions, and gaining public trust is a long and labour-intensive process. Negative experiences, as in some regions of Russia, make it difficult to change public opinion for the better [56].

The level of public acceptance of the technology is influenced by the level of trust in the actors running/organising CCS projects. Research in this area has demonstrated that public trust in relevant stakeholders influences the acceptance of CCS implementation, with individuals tending to place greater trust in environmental NGOs than in corporate entities. The public is more likely to accept CCS policy decisions when they are convinced that they have the opportunity to express their opinions in the decision-making process [73]. The necessity of disseminating information to the public regarding CCS technology gives rise to the following question: how should such information be communicated in order to enable the public to form credible opinions [74]? A study on the management of public perception regarding the CO₂ storage pilot project in Australia (Otway Basin) shows that public acceptance should be considered during planning. This is a crucial aspect to consider in order to build industry and community confidence in the utilisation of CCS technology [28].

Despite the extensive research conducted on the public awareness of CCS, no recommendations have been proposed thus far concerning the dissemination of information on CCS. Numerous assessments of the general public perception of CCS technology have been undertaken, yet there has been limited focus on regions with CO₂ storage projects. The factors deemed most significant by local communities and the alignment of project outcomes with their expectations have not been subject to analysis. The following key barriers to CCS implementation were identified: lack of public knowledge about CCS, misconceptions, lack of or poor communication strategies, competition between alternative technologies, lack of long-term policy of CCS implementation, controversial economic efficiency, capital intensity, and a weak market [56].

The results of deliberative group interviews with representatives of local communities in Poland on the opportunities and challenges associated with the theoretical location of combined CO₂ and natural gas storage facilities (CC(U)S and PMG) in their immediate vicinity [75] demonstrated four significant mechanisms for fostering trust in CC(U)S and UGS in the local communities analysed. These mechanisms pertained to the communication of safety, the communication of the absence of nuisance, the communication of benefits, and the transparent and competent dissemination of knowledge.

The importance of including non-technical components in public acceptance has been demonstrated by numerous CO₂ storage projects that have encountered opposition from regional and local communities, ultimately leading to the failure of the operation [66,76]. A review of public attitudes towards the underground storage of CO₂ reveals minimal support for the development of CCS in Canada, with a perception that CCS is less risky than normal operations in the oil and gas industry, nuclear power, or coal-fired power plants [77].

2.2. Public Acceptance of the UHS in the Literature

The results of UHS public acceptance surveys are less numerous than for CCS and cover the last ten years, with clearly more interest in the last three years [27].

The document entitled ‘Hydrogen TCP-Task 42. Underground Hydrogen Storage. Technology Monitor Report’ provides an overview of the public’s acceptance of UHS 2023 [41]. As outlined in the report, the following activities are of particular significance with regard to the public’s acceptance of the technology in question: the soliciting of feedback from relevant stakeholders and local communities; the organisation of open and frequent meetings; and the delivery of information events and visits with stakeholders, including members of the public. The authors emphasise that these activities facilitate effective channels of communication and address the concerns of the public and other stakeholders in all stages of the project lifecycle, from conception to abandonment. The public’s confidence in the UHS project is contingent upon the implementation of risk mitigation measures and solutions. Public consent is also influenced by changes in public support and regulations, as well as the media’s presentation of UHS projects. Clear communication and procedures between projects and stakeholders are important. Such measures are instrumental in fostering comprehension and enhancing safety awareness among local communities, thereby fostering greater confidence in the UHS. The social aspects of UHS projects are grouped by the report into four areas: the environmental impact, stakeholder involvement, policy, and financial resources. These factors are then linked to the four stages of a UHS project (exploration, development, demonstration, and deployment).

The imperative to incorporate social components into UHS projects from their inception is underscored [78].

Compared to the United States, Europe and Asia are leading in exploring the social and regulatory dimensions of storing green hydrogen. Nevertheless, it should be noted that a considerable number of questions with regard to the public acceptance of UHS remain unanswered and that the success of green hydrogen use will require, among other things, collaboration between the public and private sectors and research in the field of social sciences [79].

As part of the Hystories project, public perception surveys were carried out to ascertain the degree of acceptance of UHS technology and the factors determining this acceptance. The surveys have demonstrated views and opinions, as well as the social impacts associated with the deployment of large-scale underground hydrogen storage facilities and their reception by various stakeholder groups (including workers, the public, and local communities) [80].

Research from two UK projects has highlighted key gaps in the public acceptance of hydrogen. These include the unknown, current perceptions, ‘public acceptance’, willingness to pay more for public goods, and issues around information and trust [81].

A survey of French citizens has revealed their readiness to accept the underground storage of hydrogen in salt caverns. The survey findings demonstrate the influence of knowledge and experience on perceptions of hydrogen, highlighting the importance of these factors in shaping public acceptance [82]. It is important to note that the implementation of the UHS is contingent upon the presence of social, political, economic, and regulatory frameworks. These frameworks can vary considerably between countries and regions [83].

Transparent communication and clear procedures between the project and its stakeholders are key to understanding local community safety and identifying ways to protect the environment and infrastructure. Constructive dialogue with regulators is important in identifying areas of concern and highlighting gaps that require clarification at the appropriate legislative level (see e.g., [84]).

The development of communication and public engagement strategies that span the entire duration of the project, as well as promoting a favourable image of hydrogen’s role in achieving climate objectives, is identified as being of paramount importance to foster public support for the UHS [39].

As Zaunbrecher et al. (2016) have demonstrated, mental models, knowledge, and the social acceptance of hydrogen storage indicate attitudes that support trust in hydrogen storage. They also reveal misunderstandings, such as a lack of information, and concerns that will need to be addressed in future approaches to social communication [85].

‘Green’ hydrogen production technology is still in the early stages of large-scale deployment, with government initiatives playing a key role in its development [86].

The identification and active engagement of external stakeholders by the UHS project developer is to be emphasised from the project’s earliest stages [40].

It is essential to emphasise the critical importance of a comprehensive assessment of the health, safety, and environmental risks of UHS [87].

The issue of hydrogen storage in geological structures is of paramount importance in determining its social acceptance. The risk of hydrogen leakage from underground storage is related to its physical and chemical properties, with the very small molecule size and low diffusion coefficients of hydrogen potentially resulting in leakage outside the storage site [23,88]. The low viscosity of hydrogen can lead to the formation of viscosity tongues during the injection process [89]. Drilling, overburden, fault zones, and fractures are indicated as potential main leakage pathways from underground storage sites [90]. The main challenges to the integrity of a hydrogen storage facility include geochemical reactions, microbial activity, the reactivation and propagation of fractures and faults, and borehole leakage. Interactions between geochemical and microbiological processes involving hydrogen, cushion gas, reservoir fluids, and minerals can occur during underground hydrogen storage [91,92]. These reactions have the potential to result in a loss of stored hydrogen, a reduction in hydrogen purity [93,94], and alterations in the petrophysical parameters of reservoir rocks and overburden. Geochemical reactions and cyclic hydrogen injection/withdrawal can lead to a reduction in the integrity of the caprock [23,95]. Hydrogen can also have a detrimental effect on the mechanical properties of steel (hydrogen embrittlement, hydrogen-induced cracking, and blistering) [96]. Furthermore, corrosion of the injection system can be caused by the interaction of hydrogen, H₂S, or CO₂ with microorganisms [97,98,99,100,101,102]. Furthermore, elastomers utilised in boreholes have been observed to undergo degradation when exposed to the environment to H₂S [41]. Furthermore, the process of hydrogen storage has been demonstrated to compromise the impermeability of the cement in boreholes [96].

Whilst the objectives related to the development of hydrogen technologies are similar across different countries, regulatory approaches vary depending on local conditions. The USA, for example, focuses on supporting innovation and reducing investment barriers, and regulations on hydrogen storage are currently being developed at both the federal and state levels. The EU, meanwhile, is pursuing a policy of renewable hydrogen with an emphasis on harmonising standards and international cooperation. The EU has introduced a directive (Directive (EU) 2024/1788) on renewable gas, natural gas, and hydrogen. By contrast, Asian countries such as Japan are focusing on increasing energy self-sufficiency through the development of hydrogen technologies, with regulations on hydrogen storage centred on promoting the research and development of hydrogen technologies and creating safety standards [103].

The potential conflicts of interest in the simultaneous use of the rock mass for the underground storage of CO₂ and H₂ are indicated [53].

Analysis shows that broad public acceptance in Germany for hydrogen use requires the specific promotion of its everyday benefits [104].

Research and demonstration programmes should be initiated to enhance understanding of storage processes, effective gas injection and production strategies, and the evaluation of potential risks and their mitigation [41,105]. The 45+ pilot projects currently underway should provide information to reduce the technical and regulatory risks associated with UHS and positively influence perceptions of it [106].

The Hydrogen Incident and Accident Database (HIAD) is a research tool that is currently being developed as a repository of data describing in detail adverse events (incidents or accidents) related to hydrogen. The HIAD is an invaluable resource in the ongoing discourse on the public acceptance of hydrogen. It can be used for a variety of purposes, such as serving as a data source for the identification of lessons learned, the communication of risk, and, to a limited extent, the assessment of risks [107].

The significance of the societal embeddedness level in the context of the social acceptance of new technologies, including UHS, has been emphasised. The methodology focuses on the social obstacles which could delay or hinder implementation [108]. The results obtained from the SEL assessment have the capacity to demonstrate which social requirements have been met in the present stage of the project’s development and which requirements are yet to be fulfilled. The latter requirements may be considered as those that must be improved in order to facilitate the implementation of the technology under consideration. It is emphasised that social aspects related to a technology that have been correctly defined and implemented will condition the successful implementation of a project [40,109]. The structured SEL format provides operators, regulators, the public, and relevant parties with a set of guidelines to achieve satisfactory results when organising and implementing public engagement. It is emphasised that the SEL methodology should be applied to every UHS project [41]. To date, the number of examples of SEL implementation that have been published remains relatively small. However, the study by Mendrinos et al. (2022) demonstrates the methodology’s application to CCS. SEL was evaluated for CO₂ sequestration installations using national databases from the Netherlands, Norway, and Germany. The results of the study indicated that SEL could be improved through its application at the scale of a local project. The highest SEL levels were recorded in Norway (SEL = 3), the Netherlands (SEL = 2), Germany (SEL = 1), and Greece (SEL = 1). The SEL also varied in each country across four dimensions: physical and social environment, stakeholder engagement, policy and regulations, and market and financial resources [40].

3. Discussion

Drawing upon the experience of CCS, coupled with a comprehensive review of the extant literature on UHS, this paper presents a series of proposals designed to foster public acceptance of UHS technology. These include public education and awareness, transparent communication, stakeholder engagement, risk assessment, policies and regulation, public benefits, research and demonstration programmes, public trust, and societal embeddedness levels. The implementation of these actions has the potential to substantially augment the public acceptance of underground hydrogen storage, thereby facilitating the expeditious deployment of the technology.

The developing technology of underground hydrogen storage (UHS), regardless of its technological readiness level (TRL), faces a barrier due to the low level of social readiness (SRL) for its acceptance. In practice, this means that even with documented effectiveness and technical safety, the lack of full social acceptance can significantly delay or even prevent the implementation of the UHS project. Therefore, research into the social acceptance of this technology is important [26,41].

A literature review on the social acceptance of UHS shows a small number of publications in this area. There is much more experience, supported by scientific articles, in the case of the social acceptance of CCS [43,56,110], which, according to the authors, can be used in the social acceptance of UHS. This is due to the fact that the technologies of the underground storage of CO₂ and H₂ are similar. Despite the differences in the type of gases stored, they show numerous similarities related to the injection of CO₂ or H₂ into the same geological structures, as well as the resulting effects on underground storage formations [21,22,23,25]. It is therefore advisable, as the authors have recognised, to use the experience gained from the social acceptance of geological CO₂ storage (CCS), a technology with a longer operational history, in the context of underground hydrogen storage (UHS). This was achieved by analysing relevant articles from indexed journals on the subject of the social acceptance of CCS and UHS. The conducted literature analysis allowed for the identification of CCS technology elements that can be used and incorporated into the social acceptance of UHS. This type of analysis should be considered an innovative approach to the social acceptance of UHS.

The comparison of the social acceptance of CCS and UHS with regard to selected technological, social, political, regulatory, environmental, and economic factors shows that CCS and UHS differ in terms of technological advancement and social perception. Technologically, CCS is a more developed technology, but it raises concerns about the stability of storing CO₂ underground. Industrial-scale UHS, on the other hand, is in the testing phase and requires further research on safety and infrastructure. Due to social factors, CCS has more negative social reactions, mainly due to concerns about CO₂ leaks. In contrast, UHS has the potential for better acceptance, but requires building social trust and transparent communication. Both technologies require improvements in terms of social education and transparent communication. CCS has more developed legal regulations, while UHS requires the harmonisation of regulations to increase public trust. Further pilot and demonstration projects and regulatory standards for UHS are needed to increase the level of public acceptance. CCS is a more developed technology in environmental terms, but its impact on ecosystems is not fully understood. UHS can support the use of renewable energy sources, but it also requires further research. Due to economic factors, both technologies are costly and require public support. CCS is more technologically ready, but UHS may become more cost-effective in the long term [24,26,39,53,103,111].

Underground hydrogen storage is a technology of strategic importance for the energy transition, which is why stakeholder engagement is crucial to its success. UHS stakeholders are diverse groups that influence the development, regulation, and social acceptance of this technology, and include the following: society, for which education, transparent communication, benefits for society, and public trust are of key importance; scientists for whom research programs, risk assessment, and stakeholder engagement are a priority; industry for which stakeholder engagement, risk assessment, and transparent communication are important; decision makers (government, EU), whose main role is to create policies and regulations and support education and research programs; and NGOs (non-governmental organisations) and social organisations focusing on education, transparent communication, and building public trust. Each of these groups has different goals and interests, which is why cooperation, legal regulations, transparency, effective communication, and stakeholder engagement in early stages of projects will be crucial [73,104,112].

Based on the analysis of the literature on the social acceptance of CCS, in combination with the review of the literature on the social acceptance of UHS, a framework for measures to influence the social acceptance of UHS is presented (Figure 2). They include the following elements: education and public awareness, transparent communication, stakeholder involvement, risk assessment, policy and regulation, public benefits, research and demonstration programs, public trust, and the level of social embeddedness. In terms of education and public awareness, research shows that educational programs and information campaigns increase acceptance of new technologies [104,113], and the number of educational campaigns is an indicator of success. Effective, regular, comprehensible, and open communication throughout the project duration, measured using surveys and responses to the information provided, minimise misinformation and increase public trust [114,115]. Identifying and actively involving external stakeholders in all stages of the project by organising meetings and consultations with local communities to take their concerns and suggestions into account reduce their resistance [112,116]. Conducting a comprehensive and systematic assessment of health, safety, and environmental risks, and informing the public about the measures taken to reduce these risks and prevent accidents are crucial for safety and social acceptance [117,118]. Another important element is cooperation with local and national authorities to create an appropriate regulatory framework and update regulations and policies to support the development of UHS [118]. Understanding the social benefits accelerates the acceptance of the technology, which is influenced by the number of publications presenting the benefits or the number of new jobs [45,119]. Subsequent research and demonstration programs to better understand underground storage processes and pilot studies reduce uncertainty and improve technology design. Building trust in UHS projects through transparency, openness, and taking into account the opinions of local communities makes them transparent and builds long-term trust, which is measured by public opinion polls [109,120,121]. The use of the SEL methodology to systematically assess and develop social attitudes towards UHS projects enables the evaluation of social acceptance and the identification and elimination of social barriers [41,109].

A comparison of the social acceptance of CCS and UHS with regard to selected factors—technological, social, political, regulatory, environmental, and economic—shows that CCS and UHS differ in terms of technological advancement and social perception. A key element for the success of both technologies is transparency and stakeholder involvement in an early stage of the projects. Technologically, CCS is a more developed technology, but it raises concerns about the stability of CO₂ storage. UHS, on the other hand, is in the testing phase and requires further research on safety and infrastructure. Due to social factors, CCS raises more negative social reactions, mainly due to concerns about CO₂ leaks. UHS has the potential for better acceptance but requires building social trust and transparent communication. Both technologies require improvement in terms of public education and transparent communication. CCS has more developed regulations, but its social acceptance is hampered by negative connotations. UHS requires a harmonisation of regulations to increase public trust. Demonstration tests and regulatory standards for UHS are needed to increase the level of public acceptance. CCS is a more environmentally developed technology, but its impact on ecosystems is not fully understood. UHS can support renewable energy sources, but it also requires further research. Due to economic factors, both technologies are expensive and require public support. CCS is more technologically advanced, but UHS could become more cost-effective in the long term.

4. Conclusions

The ongoing research and projects on the geological storage of CO₂, which have been in progress for several decades, have provided experience with the technological and social aspects of implementing CCS technology. However, the findings of these studies indicate a general lack of knowledge regarding this technology among the public, which may be in part attributed to a paucity of effective communication strategies. As a consequence, other alternative CO₂ disposal technologies may be perceived as more attractive. Another factor hindering acceptance is the lack of a long-term policy for CCS deployment, while the lack of adequate market mechanisms to support CCS is also problematic. A lack of research into the long-term effects of the technology is also causing problems and affecting its perception. Additionally, the absence of trust among certain stakeholders, particularly industrial entities, is a salient factor. The so-called ‘not in my backyard’ (NIMBY) effect, whereby local communities tend to oppose such projects on their doorsteps, is a common occurrence. The possibility of protests due to negative perceptions of CCS technology, which has already occurred, is also a significant factor and should be taken into account in the implementation of CCS projects from the very beginning. The selection of a site and the design of a CO₂ storage site without taking into account the specific characteristics of the site and the local communities can lead to problems with the commissioning of an underground CO₂ storage site.

An analysis of publications on UHS has identified further factors related to public acceptance, which will influence the implementation of this technology. A comprehensive evaluation of health, safety, and environmental risks is deemed imperative for the public acceptance of UHS. The involvement of external stakeholders from the project’s inception is deemed essential. Effective communication and public engagement strategies throughout the life of the project are considered crucial. Transparent communication and clear procedures between the project and its stakeholders are often highlighted. Public confidence in the UHS project is contingent upon the implementation of adequate measures to mitigate risks and prevent accidents. Demonstrating the benefits of hydrogen is likely to be a key factor in the recognition of UHS technology. It is also acknowledged that changes in public support, updated regulations, and the manner in which projects are presented by the media will play a significant role. The utilisation of an SEL methodology, with its emphasis on societal barriers that may impede the implementation of UHS technology, is advocated for each project. The present day is of particular importance in this regard, as it is crucial to initiate further research and demonstration programmes in the field of UHS.

In terms of social acceptance of UHS, the following research directions are considered priorities for the coming years: the perception of risk and safety of UHS in order to understand how society perceives the risks associated with UHS, transparency and social communication to determine the best strategies for informing the public about UHS technology, and active stakeholder engagement to develop methods for involving local communities and other stakeholder groups in UHS projects. SEL research will also be of critical importance, as it allows for the implementation of UHS projects to be adapted to local socio-cultural conditions in various stages of the project.