1. Introduction
Wireless charging modules for electric vehicles (EVs) are being increasingly studied. Two techniques of transferring power to EVs via charging systems can be used: conductive charging and wireless charging. Some notable studies, as presented in this work, focused on developing more effective wireless-charging modules for electric vehicles in order to pave the way for the creation of more sustainable urban transportation [
1,
2,
3].
In 1891, Nikola Tesla developed the wireless power transfer (WPT) concept based on capacitive coupling [
4]. There is no physical connection between the vehicle and the charger in wireless charging [
5,
6]. Coupling plates known as a transmitting end (at the roadside) and a receiving end (at the vehicle side) are used in most EV wireless chargers [
7]. EV charging via an inductive power transfer (IPT) system is gaining a lot of attention [
8,
9] in recent times, though it can also be done via capacitive power transfer (CPT), laser, microwave, and other methods.
In 1977, the Lawrence Berkeley National Laboratory demonstrated the very first IPT-based EV dynamic wireless charger [
10]. Wireless charging systems are available in both mobile and fixed configurations. When using the dynamic (mobile) wireless charging technology, a vehicle can be charged while driving; when using the stationary (fixed) wireless charging technology, a vehicle must be parked. This was a revolutionary technology which begged for more research at the time. This was evenly met with numerous studies around the subject [
11,
12,
13,
14].
There are some benefits of wireless charging systems. First, it allows for convenient, adaptable, and safe EV charging without requiring direct human contact [
15]. Second, the total cost of charging is considerably lower than the traditional conductive charging [
15]. Third, the charging structure is practical, making it very suitable for residential use with minimal maintenance [
16]. Finally, dynamic WPT technology allows for vehicle charging while moving [
17,
18]. These merits make the WPT desirable technology for automotive applications towards the achievement of a more sustainable mobility in cities.
Table A1 summarize some WPT technologies, along with their operations, benefits, drawbacks, and components. In
Figure 1, Liang and Chowdhury [
7] proposed an alternative classification based on the size of the air gap between the transmitter and reception units. The length of the air gap between the transmitting and receiving ends was used to classify EV wireless charging systems in the work of Liang and Chowdhury [
7]. Also, various EV wireless charging strategies were reviewed. Each method was described, and the numerous topologies related to each method were reviewed and contrasted, with a special emphasis on power transfer efficiency. Liang and Chowdhury [
7] proposed that a dynamic wireless charging system is created for improved output power efficiency during misalignments and lower installation costs for a sustainable electric transportation system. The paper further predicted that allowing vehicle-to-grid (V2G) technology and the development of dynamic wireless charging would usher in a new era of electric-vehicle based transportation with lower battery capacity and greater vehicle range.
2. Literature Review
In a review conducted by Li and Mi [
18] on wireless charging for electric vehicles, it was evident that vehicle electrification was unavoidable due to environmental and sustainable energy supply concerns. When contrasted with plug-in (cabled) charging, wireless charging offers numerous advantages. With highways electrified to provide wireless charging, it will lay the groundwork for mass-market adoption of electric vehicles, regardless of battery technology [
19,
20]. Researchers have conducted more research on topology [
21], control [
22,
23,
24], inverter design [
25,
26,
27], and human safety [
28,
29,
30,
31].
Mohammed and Jung [
32] carried out a comparable investigation. The report provided a good overview of contemporary EV wireless power charging research in a concise manner. It also included a small-scale experimental model for deeper comprehension of the wireless proposition. The inductive power transmission efficiency was significantly higher than that of the round coil, resulting in a more efficient total wireless power transfer system. Despite the fact that square coils were far more efficient than round coils, their efficiency remained low. To counter this, it was suggested by a study [
33] that a larger diameter wire be used, as this would result in a longer coil. A longer coil length could significantly enhance inductance and magnetic field, resulting in a higher transfer efficiency.
The electromagnetic field (EMF) emission limitations mentioned in international guidelines and standards such as IEEE, ICNIRP, ACGIH, and SAE were examined and collated in a study by Asa et al. [
34]. The authors asserted that EMF emissions can be significant, especially at high-power transfer levels and misalignment circumstances, and should be kept within the ICNIRP 2010 recommendations, which are more conservative and considered safer.
In the context of EV wireless charging applications, various WPT technologies were introduced and compared in the paper of Qiu et al. [
35]. The fundamentals of inductive power transfer and strongly coupled magnetic resonance were discussed, with a focus on maximal power transmission and efficiency. A synopsis of current achievements in wireless EV charging was presented. The most up-to-date solutions for each issue were explored.
To eliminate mutual inductance fluctuation and provide output stability, including output power and efficiency, a crossed DD coil shape with double-coil excitation method was developed by Xiang et al. [
36]. The system efficiency with the crossed DD geometry and double-coil excitation was nearly unchanged (from 89.2% to 88.7%) irrespective of the EV position, whereas the system efficiency with traditional DD coils was massively diminished (from 88.5% to 81.2%) when the pick-up coil moved from the non-switching area to the switching area, according to simulations and experimental results. In comparison with the conventional DD geometry, the proposed crossed DD geometry with double-coil excitation was found to considerably improve system output stability.
In a revised version, a DD geometry called DDQ pad was proposed, which contained a third D coil in quadrature with the other two to mitigate the null point problem [
37]. The DDQ pad has boosted efficiency, but because of its high copper requirement in design, researchers have turned their attention to other pad structures. University of Auckland posited a version of the classic DD pad with some overlaps between the two coils that offers enhanced performance close to the DDQ pad with 25% reduced copper use [
38].
Jiang et al. [
39] examined safety implications for WPT applications on EVs in a study. Because of the broad region of electromagnetic field exposure between the vehicle and the primary coil, as well as the high electrical power involved in this operation, the system must be developed to fulfil the safety standard. In order to satisfy the customer’s expectations, safety must be improved, as well as the efficiency and charging cycle. Electrical shock, electromagnetic field exposure level, and fire danger should all be included in the standard test. Both computational and experimental tools were used to evaluate the near magnetic fields for the EV’s wireless charging system.
A quick review of the number of documents on WPT in the SCOPUS database showed a decline after a peak in 2020 (
Figure 2). This is one of the motives that necessitated this investigation on the status of studies in the field. Furthermore, no study has been undertaken to demonstrate the topology of the research field, as evidenced in the literature reviews to the best of our knowledge. The current study, therefore, attempts to answer the following research questions:
- (1)
What is the social structure of the field among authors, countries and organizations?
- (2)
What is the influence of authors and sources on the research domain?
- (3)
Who are the most relevant authors, sources, documents and institutions on the topic?
Consequently, in answering the aforementioned questions, co-authorship, co-citation and citation analyses were employed. The evidence of collaboration and social structure among researchers were revealed with co-authorship analysis. The influence of sources and authors on works published was depicted in the co-citation analysis. It is worth noting that since new documents take time to receive citations, it was impossible to map documents that receive little or no citations in the current study [
40]. However, this did not have any significant effect on the outcome of the analysis. A bibliometric coupling would have been suitable for acknowledging newly published documents but this technique has limited time interval for the data collected and does not necessarily show important works by citation counts. Hence making the co-citation analysis relevant. The citation analysis estimated the relevance of documents, sources, and authors in the research domain [
40,
41]. It must be pointed out that citation analysis is biased towards older published works but can quickly find the relevance of works in the research field.
Setting a threshold value for all studies involved a compromise between two competing goals: offering a broad picture of the visualization versus providing a more focused and clear depiction. If the threshold value is too high, there is a risk of missing certain smaller groups (clusters) of items that are below the threshold value but are nonetheless essential. We have a different set of issues if the threshold is set too low. It is more difficult to visualize larger sets of items. For citation analysis, less cited items have less information, which raises the likelihood of spurious links.
This study provides a bibliometric analysis of works published on WPT between 2011 and 2022. Specifically, this study conducted a bibliometric analysis on literature extracted from the SCOPUS database from 2011 to 2022.
Figure 3 outlines graphically the various stages of the bibliometric study. In addition, some emerging trends in WPT electric vehicle applications were presented.
3. Methods
3.1. Bibliometric Data Extraction
Data collection from published studies was critical for achieving the objectives of this study since this would help in defining which scholarly publications would be used to draw any conclusions based on findings. SCOPUS was chosen as the database for this study because it has a wider variety of coverage of research than other databases including Web of Science (WOS), Google Scholar, and PubMed, among others [
42]. SCOPUS is also a better alternative for inter-disciplinary areas of research, such as the one discussed in this [
43]. However, it must be noted that, the WOS and SCOPUS have similar coverage when it comes to documents published in the engineering discipline [
42].
In November 2021, a bibliometric search of WPT- and EV-based keywords through the SCOPUS database was completed to analyze published research in English Language. Using keywords like ‘Inductive Power Transmission’, ‘Energy Transfer’, ‘Electric Vehicles’, ‘Wireless Charging’, ‘Inductive Power Transfer’, ‘Magnetic Couplings’, and ‘Inductive Couplings’, the available literature relating to WPT application to electric vehicles in the SCOPUS database was retrieved. Two thousand, one hundred and sixty-three (2163) records were returned using Boolean logic operators (AND, OR) and an integrated list of targeted keywords. In order to retrieve all records matching the selected keywords in the title, abstract or selected keywords section, the keyword search in the SCOPUS database was configured to title/abstract/keywords. The search was limited to records from the past decade (2011 to 2022) on the subject under study.
3.2. Inclusion and Exclusion Criteria
The results of the preliminary keyword search were manually filtered to ensure that only relevant articles were used for the study. This was done by excluding documents without specific relevance to wireless power transfer application to electric vehicles. Other criteria for inclusion were the following:
- (1)
Document type (articles, books, lecture notes etc.) was left to include every available documents on the topic.
- (2)
Years under review was 2011–2022. Any document within the period was included. Any document published outside of this range was excluded.
- (3)
Relevance of document title and content as outlined in the abstracts. This was necessary to eliminate any document with no relevance to the topic for the study.
After screening 2163 documents returned after the initial search, 1367 records from the SCOPUS database remained for evaluation.
Table 1 depicts the distribution of document types of the data subjected to the bibliometric analysis.
3.3. Scientometric Analysis
A scientometric analysis, including co-occurrence, co-authorship, co-citation, and citation analyses using several metrics (unit of measure), was conducted. Keywords, countries, and co-authors were used as units of measurement in the co-occurrence study. Co-authorship utilized authors, countries, and organization as units of measurement. Authors and sources were used as units of measurement for co-citation. Citation analysis included the usage of authors, documents, organization and sources. The scholarly topography was mapped and visualised using density maps and network visualizations.
6. Conclusions
A bibliometric analysis of documents in the SCOPUS database has been presented. There has been a decrease in overall contribution on wireless charging for electric vehicles. This can be seen in the decrease in studies in 2019/2020 (
Figure 2). A scientometric study has been conducted to investigate the current state and global trends in wireless charging studies for electric vehicles. Despite the fact that some literature reviews have already been published, this is the first bibliometric analysis of the subject as a whole, with 1367 papers examined using a ‘science mapping’ technique.
Recent research has focused on ‘object detection’ and ‘shielding effectiveness,’ according to the findings. Early contributions looked at all aspects of wireless charging, but later research focused on specific aspects of inductive power transfer and electromagnetic shielding effects. Novel strategies including ‘compensation topology’ were investigated as well.
For the author co-authorship analysis, C. C. Mi had the highest citation counts, but one of the lowest link strength scores. V. P. Galigekere had the strongest links but had the fewest citations. More documents had been produced by O.C. Onar. This indicates that authors who are often cited do not generally publish a large number of documents or collaborate with other authors. China, the United States, the United Kingdom, Italy, and Australia have all made important contributions to the field’s publications. It was inferred that the more publications a country has, the more advanced it is, because more money may be spent for advanced research. It can also be determined that research organizations/institutions rarely collaborate on this study domain’s topics. Only three research institutes collaborated: Beijing Jiaotong University’s School of Electrical Engineering, San Diego State University’s Department of Electrical and Computer Engineering, and Northwestern Polytechnical University’s School of Automation. Only three of the 2292 research organizations had collaborations in this area. This means that fewer international co-authored research papers have been published. It should be noted, however, that some of these international co-authored works have garnered widespread recognition, such as the 632 citation counts on publications created by Northwestern Polytechnical University’s School of Automation. As a result, more collaboration between research institutes is encouraged in order to develop more valuable works.
The most often mentioned scholars were G. A. Covic, J. T. Boys, C. C. Mi, S. Li, and J. Kim. As seen in the total link strengths, there were a lot of collaborations between G. A. Covic, J. T. Boys, C. C. Mi, and S. Li. G. A. Covic is the field’s leading researcher. Furthermore, G. A. Covic appears to be the source of thought spread through literature. In terms of sources, it should be noted that IEEE Trans. on Power Electronics and IEEE Trans. on Induction Electronics are preferred by the scientific community.
C. C. Mi, J. T. Boys, and G. A. Covic were the most influential authors according to citation analysis. C. C. Mi, O. C. Onar, and S. Li had the highest citation strengths. This is an indicator of the researcher’s importance. The connections in the maps show that authors who referenced documents from Energies also cited materials from other sources. As indicated, Energies and IEEE Access appear to have published the majority of documents, however IEEE Transactions on Power Electronics and IEEE Transactions on Induction Electronics outperformed Energies and IEEE Access in terms of impact. Covic G. A. (2013a), Covic G. A. (2013b), and Forouzesh M. (2017) were the three most significant documents. Although San Diego State University’s Department of Electrical and Computer Engineering produced the most research in this discipline, it fell short of becoming the most relevant institution with the most collaborations. Only one document was generated by the Department of Electrical and Computer Engineering at the University of Auckland in New Zealand, but that document made the department immensely relevant.
Certainly, a better understanding of wireless charging methods, particularly the use of renewable energy technologies, may cultivate industry support for more in-depth and narrowly focused research in the field, which in turn may aid policy-makers’ and practitioners’ research planning and funding efforts. Furthermore, this research presents experts with crucial insights into the current lack of drive in the sector when it comes to wireless charging and renewable energy research. Besides, the trend toward wireless charging in electric vehicles, with an emphasis on reducing battery size by employing renewable energy sources, appears promising. The required battery size and charging infrastructure will be minimized when WPT is combined with these technologies. As a result, the environmental footprint associated with the transportation industry will be reduced.
Despite the study’s significance, the findings must be seen in the context of certain constraints. The findings are constrained by the initial keyword selection, which limits the scope of the published studies. Furthermore, given the study’s aims, delving into the “why” and “how” research has been undertaken thus far is outside the scope of this work. As a result, while various difficulties within the research domain have been dis-covered, tracing these problems to their source and proposing solutions are research areas that could be tackled in the future. Furthermore, performing such studies at future critical times will help to address the shifting complexity of the examined topic and track its progress.