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Accessibility, affordability, and efficiency of clean energy: a review and research agenda

• Review Article
• Published: 11 January 2022
• Volume 29 , pages 18333–18347, ( 2022 )

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• Sanjeet Singh   ORCID: orcid.org/0000-0001-6103-2346 1 , 2 &
• Jayaram Ru 1  

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Clean, affordable, and efficient energy sources are inevitable for a sustainable world. Energy crisis, especially the poor access and affordability, demand–supply mismatches, energy inequality, and high dependence on non-renewable energy sources, are the challenges before the attainment of clean energy goals for sustainable development. The 5-year review from the adoption of sustainable development goals (SDGs) by using bibliometric and thematic analysis was conducted in this review. This review is a synthesis of 175 scientific papers on SDG 7. Policy reforms and better funding; technology innovation and inclusion; and economic growth, rapid promotion of renewable, and alternative fuels are the recommendations for the achievement of the energy goals. Future research on energy-related goals should focus on energy justice, policy reforms, energy poverty, poor affordability, off-grid transmissions, renewable energy sources, alternative fuels, reforms in the energy supply chain, and international cooperation for better implementation of projects and for attracting foreign capital and private funds. This paper invites the attention of practitioners, academicians, funding agencies, and international agencies to collaborate in the initiatives for a clean, green, and energized world.

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Singh, S., Ru, J. Accessibility, affordability, and efficiency of clean energy: a review and research agenda. Environ Sci Pollut Res 29 , 18333–18347 (2022). https://doi.org/10.1007/s11356-022-18565-9

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Received : 11 October 2021

Accepted : 04 January 2022

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DOI : https://doi.org/10.1007/s11356-022-18565-9

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Mapping the Worldwide Trends on Energy Poverty Research: A Bibliometric Analysis (1999–2019)

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Energy poverty is one of the main challenges facing humanity in the 21st century. Research on energy poverty is becoming a common focus of scholars in many areas. Bibliometrics can help researchers dig deep into the information of specific research fields from a quantitative perspective. In this study, we collected 1018 research papers in the field of energy poverty published in the period 1999–2019 from the Web of Science databases and conducted a bibliometric analysis on them. Cleaning and screening of sample papers, matrix construction, and visualization were performed using Bibliometrix, VOSviewer, and HistCite, summarizing the internal and external characteristics of the papers. With regard to external characteristics, a total of 982 research institutions in 80 regions conducted research in this field. There is extensive cooperation between the countries, and the UK, the USA, Australia, and Italy play the most active role in the cooperation network. With regard to internal characteristics, we found the two most representative citation paths: one path starts from the concerns of energy-poor groups and stops at an ethical discussion on energy poverty; the second path is based on the existing technological path, continuously developing coping policies, evaluation methods, and a conceptual framework for dealing with energy poverty. Furthermore, through coupling analysis, we discovered four focuses of energy poverty research: improvement of definition, improvement of evaluation methods, effects of coping policy, and energy justice. Through a comprehensive analysis of existing papers, this paper reveals some limitations of previous studies and recommends some promising directions for future research on energy poverty.

1. Introduction

The history of human society is closely linked to the evolution of energy utilization [ 1 ]. Energy is one of the most basic conditions needed to sustain people’s daily lives [ 2 ]. “Energy poverty” refers to a situation in which it is difficult for people to obtain adequate, affordable, high-quality, safe, and environmentally friendly energy services to live a decent life [ 3 ]. Energy poverty, which is widespread in all regions of the world, is becoming a great challenge for humanity, which wants to achieve the New Millennium Development Goals [ 4 ]. In Europe, there are about 50 million to 125 million people living in energy poverty, and the size of this group has been increasing in recent years [ 5 ]. In the underdeveloped regions of Asia and Africa, more than half of the total population has to live in energy poverty [ 6 , 7 ]. According to the prediction of the International Energy Agency, if effective measures are not taken to mitigate energy poverty, there will still be about 2.5 billion people in the world who will not have access to clean and reliable energy by 2030 and can only rely on traditional biomass energy as their daily energy source [ 8 ]. Energy poverty not only affects health and diminishes happiness of residents [ 9 , 10 ] but also hinders the sustainable development of economy and environment [ 11 ]. On 9 November 2011, the United Nations designated 2012 as the International Year of Sustainable Energy for All and set the popularization of modern energy services as one of the important energy goals for 2030 [ 12 ].

Boardman [ 13 ] first gave a quantitative definition of energy poverty from an energy consumption perspective: families that cannot get enough energy services at a cost of less than 10% of their income are in a state of energy poverty. Some scholars have tried to incorporate energy efficiency and environmental impact in energy poverty research and have constructed a more complete conceptual framework for energy poverty [ 14 , 15 , 16 ]. In response to appeals from scholars and the public, European countries, especially the United Kingdom (UK), France, and Cyprus, began to address the issue of energy poverty within their borders many years ago and tried to formulate appropriate policies to alleviate energy poverty [ 17 ]. India, China, and other developing countries have gradually begun to include energy poverty in their policy agendas, but have achieved only limited effects of policy works [ 18 , 19 ]. Following the outbreak of the COVID-19 pandemic, lockdown measures taken by almost all countries and declining income of residents further exacerbated the already severe situation of energy poverty [ 20 , 21 ]. Currently, the grim reality of energy poverty requires a more in-depth research of its causes, consequences, and solutions.

In the last 20 years, a lot of research results have been accumulated in the field of energy poverty. Especially after 2015, the number of related research papers increased rapidly at a rate of more than 100 papers per year. To better understand the current situation in this field, some scholars have reviewed existing papers from different perspectives, focusing on the following four areas: (1) Definition: Castaño [ 17 ] reviewed related definitions of energy poverty, while Day [ 1 ] presented a more comprehensive and coherent conceptual framework based on previous studies; (2) Evaluation method: Nussbaumer [ 22 ] and Herrero [ 23 ] reviewed existing energy poverty evaluation indicators and summarized the advantages and disadvantages of these indicators; (3) Consequences: Sovacool [ 4 ] and Jessel [ 24 ] integrated existing research data and discussed health hazards, environmental pollution and gender discrimination caused by energy poverty; and (4) Coping policies: Bouzarovski [ 25 ] systematically collected and evaluated policies being implemented to address energy poverty in various countries. These literature reviews provide an overview of past research results, allowing researchers and government to better understand the practical condition so that they can better plan their research projects and formulate relevant policies [ 26 ]. However, the authors of these reviews generally chose a limited number of papers from renowned journals for review based on their experience, so they would inevitably miss some important papers when doing paper collection. Therefore they lack a real international perspective. For this reason, existing reviews barely reflect the overall research on energy poverty [ 27 ].

Bibliometrics provide quantitative methods for analyzing academic literature in specific research fields. Using bibliometrics methodology, researchers can conduct a quantitative analysis of the distribution structure and evolution of disciplines in papers, thus minimizing the impact of subjectivity on the quality of review [ 28 , 29 ]. In the field of energy research, some scholars have applied the bibliometric method to energy security, energy efficiency, the renewable energy supply chain, and other fields [ 30 , 31 , 32 ]. According to data from the Web of Science (WoS), more than 500 papers published from 2015 to 2019 directly relate to energy poverty, but there were no bibliometric studies of energy poverty covering the entire period. To get a clearer picture of the research on energy poverty and to explore the directions for future development, it is necessary to perform a comprehensive bibliometric study.

The characteristics of papers can generally be divided into external and internal. The external characteristics include publication time, country, institution, author, journal, etc., while the internal characteristics include keywords, research focus, and references. To ensure the comprehensiveness of the research, bibliometrics research should start from the internal and external characteristics of papers. In this study, we performed a quantitative analysis of the internal and external characteristics of papers focused on energy poverty (published from 1999 to 2019) using bibliometrics and its supporting visualization methods, to understand the commonalities and differences of previous studies in the field of energy poverty.

To comprehensively summarize the existing energy poverty studies and provide valuable insights into the practice of government policies, we try to explore the answers to the following two sets of questions:

With regard to external characteristics, which countries, institutions, journals, and authors are most influential in energy poverty research? What is the cooperative relationship between different entities?

With regard to internal characteristics, what are the focuses of energy poverty research? What are the development paths that energy poverty research has traversed? Are there any unexplored areas in this field?

The rest of this paper is organized as follows: The second part describes the data sources and research methods used in this study. The third part presents a visualization of the results of bibliometric analysis and discusses the results of the analysis. The fourth part provides the conclusions of this study and presents some suggestions for future research on energy poverty.

2. Methodology

Bibliometrics is a technology used to investigate the development process and knowledge structure of a particular field of research [ 33 ]. Bibliometrics can be used to depict the overall picture of a particular research field at the macro level, and it can also be used to analyze hot topics at the micro level, so more and more scholars have applied it in their research [ 34 , 35 ]. With the support of mathematics and statistics, bibliometrics can be applied to the analysis of various literature samples such as books, periodicals, and policy texts [ 36 , 37 ]. The standard bibliometrics procedure consists of the steps of document collection, data processing, visualization, and analysis. Relying on the research of Li [ 27 ], Zupic [ 38 ], Aria [ 39 ], and other scholars in bibliometrics, we formulated our own research framework shown in Figure 1 .

Research framework of bibliometric analysis.

2.1. Data Sources

Selection of databases: Compared to Google Scholar, PubMed, Springer, Wiley, and other databases, the Web of Science Core Collection includes more reputable journals and related papers. This database has a strict literature screening mechanism, which ensures a high academic level of collected papers [ 40 ]. Standardized literature entries exported from the Web of Science Core Collection can be directly applied in various bibliometric analysis tools. Considering the comprehensiveness and quality of the collected papers, most scholars rely on the Web of Science Core Collection to conduct a bibliometric study [ 41 , 42 ]. In this study, we collected relevant research papers with a complete research structure from four databases in the Web of Science Core Collection, namely Science Citation Index Expanded (SCI-Expanded), Social Sciences Citation Index (SSCI), Arts & Humanities Citation Index (A&HCI), and Emerging Sources Citation Index (ESCI).

Selection of keywords: There are multiple expressions of energy poverty, among which “energy poverty,” “energy poor,” and “fuel poor” are often used interchangeably [ 16 , 43 ]. There are no obvious differences between these concepts; only the referred regions or climatic conditions differ [ 44 ]. Therefore, this study follows the practice of previous studies, i.e., we used the keywords “energy poverty,” “energy poor,” “fuel poor,” and “fuel poverty” to search papers [ 45 ].

Defining the retrieval period: In the selected databases, the first paper related to energy poverty appeared in 1999, and the number of related studies has been increasing since then. In this study, the retrieval period is set to 1999–2019.

2.1.1. Methods of Retrieval

We constructed the following search strings:

#1: TS = “energy poverty” OR TS = “energy poor” OR TS = “fuel poverty” OR TS = “fuel poor”

#2: TI = (“energy poverty” OR “energy poor” OR “fuel poverty” OR “fuel poor”) OR AK = (“energy poverty” OR “energy poor” OR “fuel poverty” OR “fuel poor”) OR KP = (“energy poverty” OR “energy poor” OR “fuel poverty” OR “fuel poor”)

#3: #1 NOT #2

The rules for retrieval in Web of Science are as follows: TI means “title”; AK means “author keywords”; KP means “KeyWords Plus”; TS means “topic,” covering the following four fields: “title,” “author keywords,” “KeyWords Plus,” and “abstract”; “A AND B” means “Find all records that appear in both set A and set B,” while “A NOT B” means “Find all records in set A that are not in set B.”

2.1.2. Methods of Screening

The search string “#1” returns those studies that match the search terms in titles, author keywords, KeyWords Plus, or abstracts, totaling 1112. The search string “#2” returns those studies that match the search terms in titles, author keywords, or KeyWords Plus, totaling 730. The search string “#3” returns those studies that match the search terms in abstracts but not in titles, author keywords, or KeyWords Plus, totaling 382. As shown in Figure 2 , “#2” and “#3” constitute the entirety of “#1.”

The inclusion relations between #1, #2, and #3.

The 730 documents (returned by “#2”) are highly relevant to energy poverty, because energy poverty occurs in the titles, author keywords, or KeyWords Plus. These documents can be directly included in the study without screening.

On the contrary, we need to screen the 382 documents (returned by “#3”) since energy poverty only occurs in the abstract but not in titles or keywords. After excluding irrelevant documents, 296 documents relevant to energy poverty are obtained.

In total, 1026 documents (730 + 296) are imported into Bibliometrix for processing, and 8 documents are removed because of lack of information. Finally, we obtain 1018 documents for analysis. The retrieval and screening was conducted on 27 October 2020.

2.2. Analysis Methods

In this study, the Bibliometrix3.0.3 Package (Download from: bibliometrix.org) based on the R language was used to process the collected papers. Bibliometrix is an open source tool developed by Massimo Aria et al., which is used for data processing as well as for scientific knowledge mapping and analysis. It can be used to easily convert data exported from WoS databases into the R data frame, on the basis of which data can be cleaned, filtered, and analyzed, and the matrix construction can be performed. Currently, this tool is used in bibliometric studies by scholars in many fields [ 39 ]. VOSviewer (Download from: vosviewer.com), developed by Nees Jan van Eck and Ludo Waltman, is a bibliometric tool mostly used to construct and visualize co-occurrence relationships in the literature [ 46 ]. HistCite (Download from: clarivate.com), developed by Eugene Garfield, is a bibliometric tool mostly used to translate citation relationships into citation paths. VOSviewer1.6.15 and HistCite12.03.17 were also used to conduct more explanatory visual analysis of relevant data.

3. Results and Discussion

3.1. external characteristics of the publications, 3.1.1. overall status of the publications.

Figure 3 shows the overall trend of publications in energy poverty in the period 1999–2019. Blue and gray bars represent the annual number of total publications (TP) and total citations (TC), respectively. The red dots represent the mean total citation per year (MTCY).

Annual distribution of total publications (TP), total citations (TC), and the mean total citation per year (MTCY).

The annual number of TPs exhibited an overall increasing trend in the period 1999–2019. Following the practice of Xie [ 47 ], we segmented the entire period into three stages according to the annual publication amount, i.e., 1999–2010 (annual publications were below or around 20), 2011–2015 (annual publications were between 20 and 100), and 2016–2019 (annual publications were more than 100). Prior to 2010, the annual number of TPs on energy poverty was less than 20 per year, and sometimes it was zero. This shows that energy poverty, a completely new concept, has not attracted much attention from scholars. Research on energy poverty was in its initial stage in this period. During 2011–2015, the annual number of TP on energy poverty increased at a rate of 57 papers per year, indicating that energy poverty was gradually becoming one of the important topics in the field of energy research, and research on energy poverty entered a growing stage. After 2016, energy poverty research entered an active period with high yield. The annual number of TP on energy poverty was increasing at a rate of 100 papers per year and reached the maximum number of 239 in 2019.

Both TC and MTCY show yearly fluctuations. In the early research stage, the concept of energy poverty and the research methodology were not mature. So, there were no high-quality or high-influence papers in some years. This explains the low TC and the drastic fluctuation of MTCY in the early papers. In 2003, MTCY reached the highest value of seven, which means that the papers published in that year have been cited by seven papers each year since then. This shows that the papers published in 2003 have a high influence, laying the foundations for the field of energy poverty. After 20 years of development, MTCY exhibited a steady development trend in 2018 and 2019, indicating that research on energy poverty has entered the maturity stage. Supported by a complete set of concepts and methodologies, scholars carried out research in a more or less predictable manner, which also had a limiting effect on the groundbreaking research.

3.1.2. Contributions by Countries

The output of energy poverty research is counted from a national perspective. Papers authored by scientists from multiple countries are counted in the output of all relevant countries. Following this approach, we can figure out the full output of one country and the cooperation among countries.

From 1999 to 2019, scholars from 80 countries or regions published papers related to energy poverty. Table 1 shows the number of published papers, the proportion of published papers, the number of citations, and the h-index of the top 20 most productive countries or regions. The data map in Figure 4 shows the numbers of published papers contributed by all countries or regions in the world. With 335 published papers, the UK is the country with the highest number of published papers. The UK and the USA account for almost half of the total number of publications, far exceeding other countries or regions combined. Governments in developed countries can provide more resources and data support to scholars to conduct research on energy poverty. That is why all five top countries are developed countries. With the development of the economy and the strengthening of civic awareness, scholars from developing countries, including China, South Africa, India, and Nigeria, have begun to analyze the phenomenon of energy poverty in their countries and they have been the most active in the field of energy poverty in recent years.

Production distribution in the field of energy poverty.

The top 20 most productive countries in the field of energy poverty.

The h-index is usually defined as the following expression: A scientist has an index h if the h of his/her papers has at least h citations each, and other papers are cited no more than h times each [ 48 ]. At present, the h-index is widely used to assess the influence of publications produced by countries, institutions, scholars, and even the influence of papers published in certain years [ 31 ]. The total citations and h-index of the UK and the USA are far higher than those of other countries. The h-index of most countries is basically between 10 and 20, except the UK and the USA. China, which ranks 6th in the number of published papers, has an h-index of 12, while Denmark, which ranks 20th in the number of published papers, has an h-index of 10. This indicates that there is no positive correlation between the quantity and quality of publications. Countries with a small share of literature contribution in the total number of publications can also produce research results that are generally recognized in academic circles.

Figure 5 shows the cooperation relationship between countries or regions. The size of the node indicates the importance of a country in the whole network. The thickness of the line indicates the intensity of cooperation between two countries. Figure 5 shows that the cooperation network of countries can be divided into four clusters, and the UK, the USA, Australia, and Italy are at the core of each country cluster. The UK has established cooperative relations with almost all important regions and is the leader of the whole cooperative network. Most countries tend to cooperate with other countries in the same region. Due to similarities in geographical conditions, economy, and even culture, countries in the same region can use similar research paradigms, making it easier for them to establish cooperative relations.

Cooperation network between countries in the field of energy poverty.

3.1.3. Contributions by Institutes

A total of 982 research institutions have published papers related to energy poverty. As shown in Table 2 , The University of Manchester in the UK and Columbia University in the USA occupy the first two places in the ranking, with 28 and 22 papers, respectively. Among institutions, 11% of them published two papers, and 65% of them published only one paper. This indicates that energy poverty research is scattered across institutions. At the institutional level, there is no agglomeration trend similar to Bradford’s law or Lotka’s law.

Top 20 productive research institutions in the field of energy poverty.

Compared to the international cooperation network, the density of the institutional cooperation network is lower, which is characterized by more clusters. Figure 6 shows that among the four institutional clusters there is only bilateral cooperation, and there has been no cooperation spanning across three clusters. The University of Sussex is most active in collaborating with other institutions, but most of its partners are institutions in the same country. Similar to the University of Sussex, most institutions also tend to establish cooperative relations with other domestic research institutions. The most representative are the cooperation groups led by the University of New South Wales, the University of Michigan, and the University of Birmingham. In the institutional cooperation network, a few nodes, such as the University of Nottingham, the University of Cape Town, the University of Toronto, and so on, play the role of “bridge,” connecting cooperation groups in different regions.

Cooperation network between institutions in the field of energy poverty.

3.1.4. Distribution of Journals

A total of 283 journals published papers related to energy poverty in the study period. Among the 20 most productive journals, Renewable and Sustainable Energy Reviews is the one with the highest impact factor, representing the highest level of energy poverty research to some extent. Energy Policy , Energy Research and Social Science , Energy Building , and Renewable and Sustainable Energy Reviews are the top four most productive journals by the number of published papers. As shown in Figure 7 , the above four journals account for 37% of all published papers related to energy poverty, forming an essential part of energy poverty research, confirming Bradford’s law. Simply put, Bradford’s law is the embodiment of the Pareto effect with the following quantitative characterization: if journals in one field are divided into three parts by the number of papers, each part accounting for about one-third of all papers, then the journals in each part will be proportional to 1:n:n 2 , while the first part is usually defined as the core part.

Publication amount of journals in the field of energy poverty.

Figure 8 shows the annual variations in the number of papers published in the top 20 journals. Applied Energy , which has been publishing papers on energy poverty since 2002, is one of the reputable journals that initially paid attention to this field. Since 2011, more journals have begun to pay attention to energy poverty, and the number of publications has been steadily increasing. In the last 10 years, Energy Policy has paid attention to energy poverty. With 182 published papers, it became the main contributor to the increase of literature in the field, boasting the largest number of citations. It is worth noting that journals that publish energy poverty studies cover a wide range of disciplines, including environmental science, management, economics, architecture, etc., reflecting the interdisciplinary nature of energy poverty research.

The top 20 most productive journals in the field of energy poverty.

3.1.5. Contributions by Authors

A total of 2253 authors have published papers in the field of energy poverty. Among published papers, 210 were written by single authors, while others were co-written by several authors. In general, each paper was completed by an average of three authors. Compared to international cooperation networks and institutional cooperation networks, the co-authorship networks shown in Figure 9 lack global connections. Apart from authors in the large-scale co-authorship networks represented by Sovacool and Bouzarovski, most authors exist in networks in the form of small co-authorship networks or isolated nodes. This shows that current academic cooperation between scholars dealing with energy poverty is inadequate and that the scholars do not fully share relevant knowledge and opinions.

Cooperation network between authors in the field of energy poverty.

Figure 10 shows that among the top 20 most productive scholars, Pachauri published his first paper on energy poverty in 2004 and has maintained a stable output ever since. Sovacool started publishing relevant papers relatively late in 2010, but he published nine papers in the next year and gained a large number of citations. For most scholars, once they started the research on energy poverty, they would continuously publish papers, which indicates the high research significance of energy poverty. The papers published by Sovacool in 2011, McCauley in 2016, and Bouzarovski in 2017 have the most numerous citations, which indicates that these publications have played an important role in the history of energy poverty research. In the latter part of the analysis, we will conduct a more in-depth discussion of these important papers.

The top 20 productive authors in the field of energy poverty.

3.2. Internal Characteristics of Publications

This section discusses in detail the internal characteristics of papers related to energy poverty to shed light on development paths and research focus in this field. Keywords can effectively reflect the focuses of the study [ 47 ]. Thus, the primary task is to analyze the frequencies and evolution of keywords. Firstly, keywords with similar meanings such as “housing” and “household,” “poverty” and “deprivation” are merged. Then, the words that are used in paper retrieval or have a broad meaning, such as “energy poverty,” “fuel poverty,” “energy,” “policy,” and so on, are deleted. Finally, visualization tools are used to draw a tree map that reflects the frequencies of keywords, and a Sankey map that reflects the evolution of keywords.

The tree map shown in Figure 11 contains words in the fields of energy, economy, health, construction, and even policy, indicating that energy poverty is an interdisciplinary topic. The word “housing” appears in nearly 10% of papers, which indicates that the main research object of energy poverty is residential houses. Furthermore, “energy efficiency” and “renewable energy” appear in 8% and 5% of the papers, respectively, which shows that scholars consider the improvement of energy efficiency and the use of renewable energy as two important solutions to energy poverty. The appearance of “public health” and “thermal comfort” indicates that the ultimate goal of energy poverty research is to improve the health and ensure well-being of the population. In addition, the fact that the word “energy justice” appears in 5% of papers indicates that some researchers in the field of energy poverty have drawn attention to ethical aspect of the problem.

Tree map of high-frequency keywords in the field of energy poverty.

The Sankey map in Figure 12 shows the evolution pattern of keywords. In the history of energy poverty research, the evolution of keywords has been quite volatile. Prior to 2010, keywords in the literature were relatively concentrated, and scholars discussed the basic problems of energy poverty from the perspective of energy consumption, energy policy, housing and thermal comfort. After 2011, these keywords underwent an obvious fusion and regeneration: “energy acquisition,” “energy consumption,” and “energy policy” first evolved to “rural electrification,” and then new keywords such as “energy justice” appeared. After 2011, “biomass energy” was replaced by “climate change” and “sustainable development” and converted to “renewable energy”; the keywords “household” and “thermal comfort” evolved into energy efficiency research and then lost their dominant position; the keywords “acceptability,” “social housing,” and “green trading” first appeared after 2011 and changed after 2016. In general, the number of keywords in the field of energy poverty is constantly increasing, and keywords cover quite a few research fields, indicating that the interdisciplinary nature of energy poverty research is becoming more and more notable. In the following citation path analysis and coupling structure analysis, we explore the general law of literature evolution from a microscopic perspective.

Keywords’ evolution in the energy poverty research.

3.2.1. Analysis of Citation Path Evolution

The citation relationship between papers reveals the paths of knowledge dissemination. In the initial stage of research in a specific field, the citation relationship is relatively simple. As the research breadth expands, the citation relationship between papers will gradually evolve into a vast citation network [ 49 ]. The citation analysis was first proposed by Hummon [ 50 ]. At present, the analysis of citation path evolution has been adopted for bibliometrics research in various fields [ 51 ]. By analyzing citation relationships and extracting key citation paths, researchers can discover the development process of important papers from a microscopic perspective [ 36 ].

In this study, paper samples were sorted according to the number of citations, and citation paths of the 25 most cited papers were visualized using HistCite software. Figure 13 shows that the network begins with papers #12 [ 52 ] and #18 [ 53 ] and ends with papers #408 [ 54 ] and #440 [ 1 ]. Except for paper #197 [ 55 ] which is an isolated point, other papers entered the citation network from 2003 to 2016. The number of papers published in 2012 is higher than in any other year, which indicates that the research in that year has an important impact on the diffusion of knowledge throughout the field. Based on the network density and the importance of citations, the whole network can be divided into left and right parts. Therefore, the two most representative citation paths were identified as shown in Figure 14 .

Overall citation paths of energy poverty research from 1999 to 2019.

Key citation paths of energy poverty research.

The first path originates from the discussion of the living conditions of people suffering from energy poverty and settles on the ethical concept of energy justice. Paper #12 [ 52 ] analyzes the causes of an abnormally high number of deaths in 14 European Union countries in winter and points out that inadequate heating of residential houses can significantly increase the mortality rate. This paper, for the first time, systematically discusses the negative effects of energy poverty on residents and calls on the government to pay more attention to energy-poor groups. Paper #21 [ 56 ] and #36 [ 57 ] analyze the impact of energy poverty on quality of life, physical health, and even mental health, based on data obtained from in-depth interviews with families in various communities. Going further in the direction of the above studies, the authors of paper #96 [ 58 ] conducted a more comprehensive analysis of the potential impact of energy poverty on human health based on a large-scale survey of different groups (adults, teenagers, and children) lasting 10 years, emphasizing that children are one of the main victims of energy poverty. The authors of paper #188 [ 59 ] noticed the inequality suffered by several underprivileged groups as a result of energy poverty and suggested for the first time that energy poverty is linked to different concepts of social and environmental justice. Based on previous studies, papers #281 [ 60 ] and #323 [ 61 ] reflect the causes of energy poverty in different regions at the microscopic level. Finally, paper #408 [ 54 ] applies the principle of justice to the energy system and conducts a detailed discussion of the emerging ethical concept of energy justice.

The second path concerns the development the coping policies, evaluation methods, and a conceptual framework related to energy poverty, with the aim of effectively measuring and solving the problem of energy poverty. Paper #22 [ 62 ] analyzes the contradiction between energy poverty and the goal of the United Nations Framework Convention on Climate Change and proposes the elimination of energy poverty by levying incremental tax on oil and establishing an “energy poverty alleviation fund.” Paper #173 [ 4 ] explains the correlation between energy availability and the Millennium Development Goals, pointing out that energy poverty has a major negative impact on society, environment, and even global issues such as climate warming. Papers #18 [ 53 ], #155 [ 22 ], and #193 [ 63 ] examine existing methods for assessing energy poverty and strategies for formulating poverty reduction policies and set out reasonable procedures for assessing energy poverty and formulating countermeasures. Based on the above studies, paper #370 [ 64 ] provides a comprehensive conceptual framework for studying and mitigating energy poverty in developed and developing countries. Paper #440 [ 1 ] further expands the original conceptual framework of energy poverty from a capability perspective, so that the framework can be used to comprehensively measure energy poverty in different regions of the world.

3.2.2. Analysis of Bibliographic Coupling

Bibliographic coupling analysis is a common method for establishing network and clusters of literatures [ 65 , 66 ]. It can integrate papers throughout the whole period and explain the focus of research in a given field [ 67 , 68 ]. According to bibliographic coupling analysis, we established the coordinate diagram, as shown in Figure 15 [ 39 ]. After reviewing representative papers, we grouped them into the following six clusters.

Clustering of bibliographic coupling in energy poverty research.

Cluster 1: Improvement of the Definition of Energy Poverty

Cluster 1, represented by Thomson [ 69 ], revised and expanded the original definition of energy poverty. Moore [ 70 ], Middlemiss [ 71 ], and Robinson [ 72 ] criticized several definitions adopted by the British government. Robinson analyzed the UK’s energy poverty situations based on “10% definition” and “LIHC definition,” pointing out that neither of them can accurately reflect the complex spatial distribution of energy poverty. Moore analyzed the deviation of previous definitions and pointed out that the future definitions should take into account practical factors such as family size and energy efficiency. Finally, to make the definition operational for different environments, Day [ 1 ] proposed a broader definition based on “capacity theory.” Improving the definition not only promotes the development of energy poverty research but also helps the government to identify energy-poor groups and formulate coping policies.

Cluster 2: Improvement of Energy Poverty Evaluation Methods

Cluster 2, represented by Bouzarovski [ 64 ], pays more attention to the improvement of energy poverty evaluation methods. Pachauri [ 73 ] suggested formulating different evaluation methods at the macro, community, and household levels. Bouzarovski [ 64 ] pointed out that energy poverty is essentially the ineffective operation of socio-technical paths. Therefore, when measuring energy poverty, researchers should consider the composition of energy services provided to households. Based on previous studies, Nussbaumer [ 22 ] proposed a new index named the multidimensional energy poverty index (MEPI) and conducted a comprehensive analysis of energy poverty in African countries. Kaygusuz [ 74 ], Barnes [ 75 ], and Sadath [ 76 ] studied energy poverty in rural areas of developing countries. Barnes found that more than half of rural households in Bangladesh had some difficulty in obtaining energy. Sadath’s research in India confirmed that the existence of energy poverty often coincides with other forms of deprivations such as income poverty. Given that women play an important role in home cooking and heating, both Kaygusuz and Sadath believed that women face gender inequality in energy poverty situations. In summary, through the improvement and application of energy poverty evaluation, relevant studies have enriched the methodology for measuring energy poverty, which allows the government to better understand the state of energy poverty and thus pay more attention to vulnerable groups in energy use.

Cluster 3: Effects of Coping Policies

Cluster 3, represented by Liddell [ 58 ], seeks to analyze the effects of different policies that address energy poverty. The Warm Front Scheme implemented by the UK government has interested many scholars. Gilbertson [ 77 ] conducted semi-structured interviews with some families benefiting from the project and found that the project not only improved the physical health but also improved family relations and enhanced the emotional security of family members. A large-scale field survey conducted by Hong [ 78 ] also confirmed Gilbertson’s research results. After investigating the costs and benefits, Sovacool [ 79 ] found that the project brought energy consumption savings and annual income growth to residents. However, according to survey conducted by Critchley [ 57 ], the Warm Front Scheme failed to achieve the expected results for old houses, and families living in those houses may face a higher risk of anxiety or depression due to inadequate heating. Roberts [ 80 ], Santamouris [ 81 ], and Thomson [ 9 ] also discussed the adverse consequences caused by the lack of relevant policies. Overall, European countries are the first regions to implement policies to address energy poverty and the effects of their policies have been fully studied. By contrast, few researchers have paid attention to relevant policies in developing countries, although there may be more room for improvement in their policy practices.

Cluster 4: Ethical Discussion on Energy Poverty: Energy Justice

Cluster 4, represented by Jenkins [ 54 ], reflects the recent ethical discussion on energy poverty. Scholars pointed out that access to energy services should be a basic right of citizens in modern society, and research in the field of energy should not be conducted in a “moral vacuum” [ 82 ]. Based on this understanding, Walker [ 59 ] defined energy poverty as a type of injustice and decomposed the injustice into three dimensions: procedure, distribution, and cognition. Sovacool [ 82 ] built an energy justice framework focused on availability, affordability, due process, transparency and accountability, sustainability, equity, and responsibility. Walker [ 83 ] emphasized the dynamic characteristic of energy justice and proposed to include the public deliberations about necessities in the scope of justice. Based on previous studies, Jenkins [ 54 ] outlined the evaluative and normative reach of energy justice to help future scholars identify the regions where energy injustice occurs. Furthermore, Gillard [ 84 ] and Bouzarovski [ 85 ] emphasized the difference between region-centric energy justice and people-centric energy justice.

Cluster 5 and Cluster 6: Technology Strategy and Carbon Emission Effect

There are two more clusters with only a small number of papers. Cluster 5 discusses the possibility of using biomass resources for fuel cell production in energy-deficient areas. Cluster 6 discusses the effects of carbon emissions from urbanization and globalization in the Sub-Saharan region of Africa. As these studies are outside the mainstream of energy poverty research, they have not attracted much attention from scholars.

4. Conclusions and Future Research Directions

4.1. conclusions.

We collected papers on energy poverty published in the last 20 years (1999–2019) from the WoS databases and performed a visual analysis of external and internal characteristics. The study yielded the following findings:

With regard to external characteristics, energy poverty research went through the stages of beginning, growing, and maturity from 1999 to 2019. The year 2003 is the foundation year of energy poverty research. A total of 982 research institutions in 80 regions conducted research in this field. There is extensive cooperation between the countries, and the UK, the USA, Australia, and Italy play the most active role in the cooperation network. Compared to cooperation between countries, the intensity of inter-institutional cooperation is much lower. There is no core that connects the entire institutional network, except that the University of Nottingham, the University of Cape Town, and the University of Toronto play the role of a bridge between different clusters. Among journals that have published papers on energy poverty, Energy Policy pays attention to energy poverty longer than any other journal, and Renewable and Sustainable Energy Reviews represents the highest level of energy poverty research. Researchers in the field of energy poverty have formed small-scale co-authoring networks. Sovacool ranks first among all authors in terms of influence and research output. Of course, more countries, institutions, journals, and authors have joined the research on energy poverty in recent years, which has brought more insights and knowledge into this field.

With regard to internal characteristics, the focus of research in the field of energy poverty has been constantly changing in the last 20 years, accompanied by a constant increase in keywords and the expansion of related disciplines. The processes of integration, regeneration, expansion, and extinction are constantly occurring among the keywords. By analyzing the citation path evolution, we found the two most representative citation paths: one path starts from the concerns of energy-poor groups and stops at an ethical discussion on energy poverty; the second path is based on the existing technological path, continuously developing coping policies, evaluation methods, and a conceptual framework for dealing with energy poverty. Furthermore, through coupling analysis, we discovered four focuses of energy poverty research: improvement of definition, improvement of evaluation methods, effects of coping policy, and energy justice.

4.2. Future Research Directions

Poverty is a challenge for a human society that strives to achieve sustainable development in the 21st century. Energy poverty is an important component of poverty in modern society. At present, energy production and consumption are highly globalized, and energy poverty has become a common problem faced by developed and developing countries [ 86 ]. From the beginning, academic research on energy poverty has always progressed ahead of poverty reduction policy. Reliable research results provide the necessary support for policy making. We invite researchers around the world to carry out more extensive cross-regional cooperation. At the end of this paper, we recommend the following promising directions for future energy poverty research.

4.2.1. Energy Poverty in Developing Countries

At present, developed countries have formed a unified policy framework for energy poverty at the national and even regional level and have achieved good policy results. Most energy poverty research is related to developed countries or economies. However, in developing countries, people face more serious difficulties related to energy. Due to relevant resource constraints, it is difficult for developing countries to pay much attention to energy poverty. Meanwhile, due to the lack of a unified policy framework, coping policies in developing countries usually take the form of specific projects such as the Brightness Project and Off-grid Electrification [ 19 , 87 ]. Among existing studies, very few focus on energy poverty in developing countries, because the relevant data is difficult to obtain and the governments of developing countries do not take the problem seriously. What is the situation with energy poverty in developing countries? What are the characteristics of coping policies in developing countries? Are these policies effective? Scholars need to answer these practical questions in future research.

4.2.2. Impacts of Energy Poverty on Vulnerable Groups

Energy poverty has different negative effects on different groups [ 58 ]. The vast majority of energy poverty studies actually ignore the living conditions and real needs of vulnerable groups. Although some studies have discussed the impact of energy poverty on the health of children, the elderly and women, the scope of these studies is often limited to specific countries, and their conclusions are not universally relevant. Therefore, it is necessary to conduct studies covering wider regions and multiple vulnerable groups.

4.2.3. Root Causes of Energy Poverty

Energy poverty is caused by a multitude of factors. So far, most researchers have investigated micro factors such as low income, old houses, and energy demand. Few scholars have tracked the root causes of energy poverty at the macro level. Taking China as an example, after the founding of the People’s Republic of China, the government planned central heating areas according to the standard of the Soviet Union, drawing a boundary line along the Qinling Mountains and the Huaihe River. As a result, residents in southern China have very limited means of heating in the cold season, thus facing higher health and fire risks [ 88 ]. Exploring and reflecting on the root causes of large-scale energy poverty can help the government to correct the flaws in existing policies and more comprehensively consider the well-being of all citizens in future macro-planning.

4.2.4. Consequences of Emission Reduction Policies

Currently, when the necessity of tackling climate change and environmental pollution has become a consensus of human society, all countries must seriously fulfill their obligations to reduce emissions. However, it should be noted that it is extremely difficult for a country to achieve a balance between energy security, energy welfare, and environmental protection. The “energy trilemma” presented by the World Energy Council is a concise summary of this situation. From a policy practice perspective, some emission reduction policies not only serve the purpose of controlling carbon emissions but also help to ensure that people can enjoy reliable and cheap energy services. However, many emission reduction policies impose high heating costs on residents, leading them into a state of energy poverty [ 89 ]. It is therefore necessary to discuss the loss of energy welfare caused by emission reduction policies in order to help the government assess the rationality of current emission reduction policies and avoid the undesirable policy consequences.

Author Contributions

Y.X.: research design, data collection, results analysis, and writing; H.W.: result visualization and writing—original draft; G.W.: review; H.M.: data cleaning and screening. All authors have read and agreed to the published version of the manuscript.

This research was funded by the NSFC project of China: “Risk Identification and Prevention Mechanisms of Non-traditional Security Issues—A Case Study of Information Sharing and Use in Smart City Governance” (Project No. 71734002). It was also funded by the Fundamental Research Funds for the Central Universities, HUST: 2020JYCXJJ033.

Institutional Review Board Statement

Informed consent statement, data availability statement, conflicts of interest.

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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What the data says about Americans’ views of climate change

A recent report from the United Nations’ Intergovernmental Panel on Climate Change has underscored the need for international action to avoid increasingly severe climate impacts in the years to come. Steps outlined in the report, and by climate experts, include major reductions in greenhouse gas emissions from sectors such as energy production and transportation.

But how do Americans feel about climate change, and what steps do they think the United States should take to address it? Here are eight charts that illustrate Americans’ views on the issue, based on recent Pew Research Center surveys.

Pew Research Center published this collection of survey findings as part of its ongoing work to understand attitudes about climate change and energy issues. The most recent survey was conducted May 30-June 4, 2023, among 10,329 U.S. adults. Earlier findings have been previously published, and methodological information, including the sample sizes and field dates, can be found by following the links in the text.

Everyone who took part in the June 2023 survey is a member of the Center’s American Trends Panel (ATP), an online survey panel that is recruited through national, random sampling of residential addresses. This way, nearly all U.S. adults have a chance of selection. The survey is weighted to be representative of the U.S. adult population by gender, race, ethnicity, partisan affiliation, education and other categories. Read more about the ATP’s methodology .

Here are the questions used for this analysis , along with responses, and its methodology .

A majority of Americans support prioritizing the development of renewable energy sources. Two-thirds of U.S. adults say the country should prioritize developing renewable energy sources, such as wind and solar, over expanding the production of oil, coal and natural gas, according to a survey conducted in June 2023.

In a previous Center survey conducted in 2022, nearly the same share of Americans (69%) favored the U.S. taking steps to become carbon neutral by 2050 , a goal outlined by President Joe Biden at the outset of his administration. Carbon neutrality means releasing no more carbon dioxide into the atmosphere than is removed.

Nine-in-ten Democrats and Democratic-leaning independents say the U.S. should prioritize developing alternative energy sources to address America’s energy supply. Among Republicans and Republican leaners, 42% support developing alternative energy sources, while 58% say the country should prioritize expanding exploration and production of oil, coal and natural gas.

There are important differences by age within the GOP. Two-thirds of Republicans under age 30 (67%) prioritize the development of alternative energy sources. By contrast, 75% of Republicans ages 65 and older prioritize expanding the production of oil, coal and natural gas.

Americans are reluctant to phase out fossil fuels altogether, but younger adults are more open to it. Overall, about three-in-ten adults (31%) say the U.S. should completely phase out oil, coal and natural gas. More than twice as many (68%) say the country should use a mix of energy sources, including fossil fuels and renewables.

While the public is generally reluctant to phase out fossil fuels altogether, younger adults are more supportive of this idea. Among Americans ages 18 to 29, 48% say the U.S. should exclusively use renewables, compared with 52% who say the U.S. should use a mix of energy sources, including fossil fuels.

There are age differences within both political parties on this question. Among Democrats and Democratic leaners, 58% of those ages 18 to 29 favor phasing out fossil fuels entirely, compared with 42% of Democrats 65 and older. Republicans of all age groups back continuing to use a mix of energy sources, including oil, coal and natural gas. However, about three-in-ten (29%) Republicans ages 18 to 29 say the U.S. should phase out fossil fuels altogether, compared with fewer than one-in-ten Republicans 50 and older.

There are multiple potential routes to carbon neutrality in the U.S. All involve major reductions to carbon emissions in sectors such as energy and transportation by increasing the use of things like wind and solar power and electric vehicles. There are also ways to potentially remove carbon from the atmosphere and store it, such as capturing it directly from the air or using trees and algae to facilitate carbon sequestration.

The public supports the federal government incentivizing wind and solar energy production. In many sectors, including energy and transportation, federal incentives and regulations significantly influence investment and development.

Two-thirds of Americans think the federal government should encourage domestic production of wind and solar power. Just 7% say the government should discourage this, while 26% think it should neither encourage nor discourage it.

Views are more mixed on how the federal government should approach other activities that would reduce carbon emissions. On balance, more Americans think the government should encourage than discourage the use of electric vehicles and nuclear power production, though sizable shares say it should not exert an influence either way.

When it comes to oil and gas drilling, Americans’ views are also closely divided: 34% think the government should encourage drilling, while 30% say it should discourage this and 35% say it should do neither. Coal mining is the one activity included in the survey where public sentiment is negative on balance: More say the federal government should discourage than encourage coal mining (39% vs. 21%), while 39% say it should do neither.

Americans see room for multiple actors – including corporations and the federal government – to do more to address the impacts of climate change. Two-thirds of adults say large businesses and corporations are doing too little to reduce the effects of climate change. Far fewer say they are doing about the right amount (21%) or too much (10%).

Majorities also say their state elected officials (58%) and the energy industry (55%) are doing too little to address climate change, according to a March 2023 survey.

In a separate Center survey conducted in June 2023, a similar share of Americans (56%) said the federal government should do more to reduce the effects of global climate change.

When it comes to their own efforts, about half of Americans (51%) think they are doing about the right amount as an individual to help reduce the effects of climate change, according to the March 2023 survey. However, about four-in-ten (43%) say they are doing too little.

Democrats and Republicans have grown further apart over the last decade in their assessments of the threat posed by climate change. Overall, a majority of U.S. adults (54%) describe climate change as a major threat to the country’s well-being. This share is down slightly from 2020 but remains higher than in the early 2010s.

Nearly eight-in-ten Democrats (78%) describe climate change as a major threat to the country’s well-being, up from about six-in-ten (58%) a decade ago. By contrast, about one-in-four Republicans (23%) consider climate change a major threat, a share that’s almost identical to 10 years ago.

Concern over climate change has also risen internationally, as shown by separate Pew Research Center polling across 19 countries in 2022. People in many advanced economies express higher levels of concern than Americans . For instance, 81% of French adults and 73% of Germans describe climate change as a major threat.

Climate change is a lower priority for Americans than other national issues. While a majority of adults view climate change as a major threat, it is a lower priority than issues such as strengthening the economy and reducing health care costs.

Overall, 37% of Americans say addressing climate change should be a top priority for the president and Congress in 2023, and another 34% say it’s an important but lower priority. This ranks climate change 17th out of 21 national issues included in a Center survey from January.

As with views of the threat that climate change poses, there’s a striking contrast between how Republicans and Democrats prioritize the issue. For Democrats, it falls in the top half of priority issues, and 59% call it a top priority. By comparison, among Republicans, it ranks second to last, and just 13% describe it as a top priority.

Our analyses have found that partisan gaps on climate change are often widest on questions – such as this one – that measure the salience or importance of the issue. The gaps are more modest when it comes to some specific climate policies. For example, majorities of Republicans and Democrats alike say they would favor a proposal to provide a tax credit to businesses for developing technologies for carbon capture and storage.

Perceptions of local climate impacts vary by Americans’ political affiliation and whether they believe that climate change is a serious problem. A majority of Americans (61%) say that global climate change is affecting their local community either a great deal or some. About four-in-ten (39%) see little or no impact in their own community.

The perception that the effects of climate change are happening close to home is one factor that could drive public concern and calls for action on the issue. But perceptions are tied more strongly to people’s beliefs about climate change – and their partisan affiliation – than to local conditions.

For example, Americans living in the Pacific region – California, Washington, Oregon, Hawaii and Alaska – are more likely than those in other areas of the country to say that climate change is having a great deal of impact locally. But only Democrats in the Pacific region are more likely to say they are seeing effects of climate change where they live. Republicans in this region are no more likely than Republicans in other areas to say that climate change is affecting their local community.

Our previous surveys show that nearly all Democrats believe climate change is at least a somewhat serious problem, and a large majority believe that humans play a role in it. Republicans are much less likely to hold these beliefs, but views within the GOP do vary significantly by age and ideology. Younger Republicans and those who describe their views as moderate or liberal are much more likely than older and more conservative Republicans to describe climate change as at least a somewhat serious problem and to say human activity plays a role.

Democrats are also more likely than Republicans to report experiencing extreme weather events in their area over the past year – such as intense storms and floods, long periods of hot weather or droughts – and to see these events as connected with climate change.

About three-quarters of Americans support U.S. participation in international efforts to reduce the effects of climate change. Americans offer broad support for international engagement on climate change: 74% say they support U.S. participation in international efforts to reduce the effects of climate change.

Still, there’s little consensus on how current U.S. efforts stack up against those of other large economies. About one-in-three Americans (36%) think the U.S. is doing more than other large economies to reduce the effects of global climate change, while 30% say the U.S. is doing less than other large economies and 32% think it is doing about as much as others. The U.S. is the second-largest carbon dioxide emitter , contributing about 13.5% of the global total.

When asked what they think the right balance of responsibility is, a majority of Americans (56%) say the U.S. should do about as much as other large economies to reduce the effects of climate change, while 27% think it should do more than others.

A previous Center survey found that while Americans favor international cooperation on climate change in general terms, their support has its limits. In January 2022 , 59% of Americans said that the U.S. does not have a responsibility to provide financial assistance to developing countries to help them build renewable energy sources.

In recent years, the UN conference on climate change has grappled with how wealthier nations should assist developing countries in dealing with climate change. The most recent convening in fall 2022, known as COP27, established a “loss and damage” fund for vulnerable countries impacted by climate change.

Note: This is an update of a post originally published April 22, 2022. Here are the questions used for this analysis , along with responses, and its methodology .

• Climate, Energy & Environment
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How Republicans view climate change and energy issues

How americans view future harms from climate change in their community and around the u.s., americans continue to have doubts about climate scientists’ understanding of climate change, growing share of americans favor more nuclear power, why some americans do not see urgency on climate change, most popular.

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Zap Energy Achieves Significant Fusion Energy Milestone: Electron Temperature Exceeding 10 Million Degrees

Zap Energy and their collaborators at Lawrence Livermore National Laboratory, Los Alamos National Laboratory, University of California, San Diego, and University of Washington published a peer-reviewed article in Physical Review Letters   this month demonstrating electron temperature greater than 10 million degrees (approximately 1 keV, the unit of temperature favored by fusion scientists) in a sheared-flow-stabilized Z pinch. Electron temperatures up to 3 keV were reported. This result is a major physics milestone for Zap Energy and for the broader fusion energy research community. Although achieving roughly 10-keV ion temperature is what matters directly for enabling appreciable amounts of deuterium-tritium fusion, the 1-keV threshold in electron temperature is a significant milestone in the development of any fusion concept because heating electrons to such a high temperature requires that major instabilities and cooling mechanisms have been controlled. These characteristics are necessary for any fusion energy concept, and therefore the 1-keV electron-temperature threshold can be regarded as a dividing line between “concept exploration” and readiness for “concept development” as measured by the Lawson criterion .

Due to the inherent physics and engineering challenges, it took over 15 years after initiation in the early 1950s of major controlled-fusion research efforts around the world before 1-keV electron temperature was first achieved. In 1968, Russian physicists announced 1-keV electron temperature on the T3 tokamak , a result initially met with skepticism around the world but confirmed in early 1969 by a travelling team of physicists from the United Kingdom who used a then new technique called Thomson scattering (a technique also used by Zap, more below). This early work on the tokamak concept and the confirmatory measurement of electron temperature set in motion the tremendous worldwide interest in the tokamak concept, which continues to this day.

We note that only six fusion concepts have peer-reviewed published data showing electron temperature exceeding 1 keV before this achievement: the tokamak (including spherical tokamak), laser inertial confinement fusion (laser ICF), mirror, reversed field pinch, magnetized liner inertial fusion (MagLIF), and stellarator. Published FRC data is close, so we’ll call it seven. This publication brings this number to eight, and importantly, in a Z-pinch concept that may offer economic benefits due to its relative simplicity owing to its linear geometry. We also emphasize that while 1-keV electron temperature is an important milestone, there is still significant work remaining in the sheared-flow-stabilized Z-pinch “concept development” phase to achieve further milestones (e.g., scientific breakeven) on their path to commercial fusion energy.

This result is also a major success story for ARPA-E. Building on earlier scientific studies of sheared-flow stabilization of a Z pinch supported by DOE Fusion Energy Sciences, the University of Washington pursued initial performance scale-up of the concept in the ARPA-E ALPHA program beginning in 2015. From this seed, Zap Energy was spun off, and further performance improvements were funded in the ARPA-E OPEN 2018 program and the BETHE program starting in 2020 , before Zap raised over $200M of private capital. Additionally, ARPA-E funded a Thomson-scattering diagnostic capability team from Lawrence Livermore National Laboratory and a neutron diagnostic capability team from Los Alamos National Laboratory . These two teams’ travelling diagnostic capabilities provided independent and consistent measurements, which confirmed the achievement of 1-keV electron temperature.

The sheared-flow-stabilized Z pinch joins the handful of fusion concepts that have exceeded 1-keV electron temperature. This is a tremendous return on the $14M over eight years invested by ARPA-E in this concept and even the entire investment of $133M that ARPA-E has made into fusion energy since 2015 when placed in the context of the other fusion concepts that have reached this milestone. We look forward to further successes both from Zap Energy and from the many other promising ARPA-E funded teams pursuing fusion energy.

AI Index Report

Welcome to the seventh edition of the AI Index report. The 2024 Index is our most comprehensive to date and arrives at an important moment when AI’s influence on society has never been more pronounced. This year, we have broadened our scope to more extensively cover essential trends such as technical advancements in AI, public perceptions of the technology, and the geopolitical dynamics surrounding its development. Featuring more original data than ever before, this edition introduces new estimates on AI training costs, detailed analyses of the responsible AI landscape, and an entirely new chapter dedicated to AI’s impact on science and medicine.

Read the 2024 AI Index Report

The AI Index report tracks, collates, distills, and visualizes data related to artificial intelligence (AI). Our mission is to provide unbiased, rigorously vetted, broadly sourced data in order for policymakers, researchers, executives, journalists, and the general public to develop a more thorough and nuanced understanding of the complex field of AI.

The AI Index is recognized globally as one of the most credible and authoritative sources for data and insights on artificial intelligence. Previous editions have been cited in major newspapers, including the The New York Times, Bloomberg, and The Guardian, have amassed hundreds of academic citations, and been referenced by high-level policymakers in the United States, the United Kingdom, and the European Union, among other places. This year’s edition surpasses all previous ones in size, scale, and scope, reflecting the growing significance that AI is coming to hold in all of our lives.

Steering Committee Co-Directors

Ray Perrault

Steering committee members.

Erik Brynjolfsson

John Etchemendy

Katrina Ligett

Terah Lyons

James Manyika

Juan Carlos Niebles

Vanessa Parli

Yoav Shoham

Russell Wald

Staff members.

Loredana Fattorini

Nestor Maslej

Letter from the co-directors.

A decade ago, the best AI systems in the world were unable to classify objects in images at a human level. AI struggled with language comprehension and could not solve math problems. Today, AI systems routinely exceed human performance on standard benchmarks.

Progress accelerated in 2023. New state-of-the-art systems like GPT-4, Gemini, and Claude 3 are impressively multimodal: They can generate fluent text in dozens of languages, process audio, and even explain memes. As AI has improved, it has increasingly forced its way into our lives. Companies are racing to build AI-based products, and AI is increasingly being used by the general public. But current AI technology still has significant problems. It cannot reliably deal with facts, perform complex reasoning, or explain its conclusions.

AI faces two interrelated futures. First, technology continues to improve and is increasingly used, having major consequences for productivity and employment. It can be put to both good and bad uses. In the second future, the adoption of AI is constrained by the limitations of the technology. Regardless of which future unfolds, governments are increasingly concerned. They are stepping in to encourage the upside, such as funding university R&D and incentivizing private investment. Governments are also aiming to manage the potential downsides, such as impacts on employment, privacy concerns, misinformation, and intellectual property rights.

As AI rapidly evolves, the AI Index aims to help the AI community, policymakers, business leaders, journalists, and the general public navigate this complex landscape. It provides ongoing, objective snapshots tracking several key areas: technical progress in AI capabilities, the community and investments driving AI development and deployment, public opinion on current and potential future impacts, and policy measures taken to stimulate AI innovation while managing its risks and challenges. By comprehensively monitoring the AI ecosystem, the Index serves as an important resource for understanding this transformative technological force.

On the technical front, this year’s AI Index reports that the number of new large language models released worldwide in 2023 doubled over the previous year. Two-thirds were open-source, but the highest-performing models came from industry players with closed systems. Gemini Ultra became the first LLM to reach human-level performance on the Massive Multitask Language Understanding (MMLU) benchmark; performance on the benchmark has improved by 15 percentage points since last year. Additionally, GPT-4 achieved an impressive 0.97 mean win rate score on the comprehensive Holistic Evaluation of Language Models (HELM) benchmark, which includes MMLU among other evaluations.

Although global private investment in AI decreased for the second consecutive year, investment in generative AI skyrocketed. More Fortune 500 earnings calls mentioned AI than ever before, and new studies show that AI tangibly boosts worker productivity. On the policymaking front, global mentions of AI in legislative proceedings have never been higher. U.S. regulators passed more AI-related regulations in 2023 than ever before. Still, many expressed concerns about AI’s ability to generate deepfakes and impact elections. The public became more aware of AI, and studies suggest that they responded with nervousness.

Ray Perrault Co-director, AI Index

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Facility for Rare Isotope Beams

At michigan state university, frib researchers lead team to merge nuclear physics experiments and astronomical observations to advance equation-of-state research, world-class particle-accelerator facilities and recent advances in neutron-star observation give physicists a new toolkit for describing nuclear interactions at a wide range of densities..

For most stars, neutron stars and black holes are their final resting places. When a supergiant star runs out of fuel, it expands and then rapidly collapses on itself. This act creates a neutron star—an object denser than our sun crammed into a space 13 to  18 miles wide. In such a heavily condensed stellar environment, most electrons combine with protons to make neutrons, resulting in a dense ball of matter consisting mainly of neutrons. Researchers try to understand the forces that control this process by creating dense matter in the laboratory through colliding neutron-rich nuclei and taking detailed measurements.

A research team—led by William Lynch and Betty Tsang at FRIB—is focused on learning about neutrons in dense environments. Lynch, Tsang, and their collaborators used 20 years of experimental data from accelerator facilities and neutron-star observations to understand how particles interact in nuclear matter under a wide range of densities and pressures. The team wanted to determine how the ratio of neutrons to protons influences nuclear forces in a system. The team recently published its findings in Nature Astronomy .

“In nuclear physics, we are often confined to studying small systems, but we know exactly what particles are in our nuclear systems. Stars provide us an unbelievable opportunity, because they are large systems where nuclear physics plays a vital role, but we do not know for sure what particles are in their interiors,” said Lynch, professor of nuclear physics at FRIB and in the Michigan State University (MSU) Department of Physics and Astronomy. “They are interesting because the density varies greatly within such large systems.  Nuclear forces play a dominant role within them, yet we know comparatively little about that role.” 

When a star with a mass that is 20-30 times that of the sun exhausts its fuel, it cools, collapses, and explodes in a supernova. After this explosion, only the matter in the deepest part of the star’s interior coalesces to form a neutron star. This neutron star has no fuel to burn and over time, it radiates its remaining heat into the surrounding space. Scientists expect that matter in the outer core of a cold neutron star is roughly similar to the matter in atomic nuclei but with three differences: neutron stars are much larger, they are denser in their interiors, and a larger fraction of their nucleons are neutrons. Deep within the inner core of a neutron star, the composition of neutron star matter remains a mystery. 

  “If experiments could provide more guidance about the forces that act in their interiors, we could make better predictions of their interior composition and of phase transitions within them. Neutron stars present a great research opportunity to combine these disciplines,” said Lynch.

Accelerator facilities like FRIB help physicists study how subatomic particles interact under exotic conditions that are more common in neutron stars. When researchers compare these experiments to neutron-star observations, they can calculate the equation of state (EOS) of particles interacting in low-temperature, dense environments. The EOS describes matter in specific conditions, and how its properties change with density. Solving EOS for a wide range of settings helps researchers understand the strong nuclear force’s effects within dense objects, like neutron stars, in the cosmos. It also helps us learn more about neutron stars as they cool.

“This is the first time that we pulled together such a wealth of experimental data to explain the equation of state under these conditions, and this is important,” said Tsang, professor of nuclear science at FRIB. “Previous efforts have used theory to explain the low-density and low-energy end of nuclear matter. We wanted to use all the data we had available to us from our previous experiences with accelerators to obtain a comprehensive equation of state.”   

Researchers seeking the EOS often calculate it at higher temperatures or lower densities. They then draw conclusions for the system across a wider range of conditions. However, physicists have come to understand in recent years that an EOS obtained from an experiment is only relevant for a specific range of densities. As a result, the team needed to pull together data from a variety of accelerator experiments that used different measurements of colliding nuclei to replace those assumptions with data. “In this work, we asked two questions,” said Lynch. “For a given measurement, what density does that measurement probe? After that, we asked what that measurement tells us about the equation of state at that density.”   

In its recent paper, the team combined its own experiments from accelerator facilities in the United States and Japan. It pulled together data from 12 different experimental constraints and three neutron-star observations. The researchers focused on determining the EOS for nuclear matter ranging from half to three times a nuclei’s saturation density—the density found at the core of all stable nuclei. By producing this comprehensive EOS, the team provided new benchmarks for the larger nuclear physics and astrophysics communities to more accurately model interactions of nuclear matter.

The team improved its measurements at intermediate densities that neutron star observations do not provide through experiments at the GSI Helmholtz Centre for Heavy Ion Research in Germany, the RIKEN Nishina Center for Accelerator-Based Science in Japan, and the National Superconducting Cyclotron Laboratory (FRIB’s predecessor). To enable key measurements discussed in this article, their experiments helped fund technical advances in data acquisition for active targets and time projection chambers that are being employed in many other experiments world-wide.   

In running these experiments at FRIB, Tsang and Lynch can continue to interact with MSU students who help advance the research with their own input and innovation. MSU operates FRIB as a scientific user facility for the U.S. Department of Energy Office of Science (DOE-SC), supporting the mission of the DOE-SC Office of Nuclear Physics. FRIB is the only accelerator-based user facility on a university campus as one of 28 DOE-SC user facilities .  Chun Yen Tsang, the first author on the Nature Astronomy  paper, was a graduate student under Betty Tsang during this research and is now a researcher working jointly at Brookhaven National Laboratory and Kent State University. 

“Projects like this one are essential for attracting the brightest students, which ultimately makes these discoveries possible, and provides a steady pipeline to the U.S. workforce in nuclear science,” Tsang said.

The proposed FRIB energy upgrade ( FRIB400 ), supported by the scientific user community in the 2023 Nuclear Science Advisory Committee Long Range Plan , will allow the team to probe at even higher densities in the years to come. FRIB400 will double the reach of FRIB along the neutron dripline into a region relevant for neutron-star crusts and to allow study of extreme, neutron-rich nuclei such as calcium-68. 

Eric Gedenk is a freelance science writer.

Michigan State University operates the Facility for Rare Isotope Beams (FRIB) as a user facility for the U.S. Department of Energy Office of Science (DOE-SC), supporting the mission of the DOE-SC Office of Nuclear Physics. Hosting what is designed to be the most powerful heavy-ion accelerator, FRIB enables scientists to make discoveries about the properties of rare isotopes in order to better understand the physics of nuclei, nuclear astrophysics, fundamental interactions, and applications for society, including in medicine, homeland security, and industry.

The U.S. Department of Energy Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of today’s most pressing challenges. For more information, visit energy.gov/science.