Global Perspectives on and Research Challenges for Electric Vehicles

Abstract

This paper describes the characteristics of worldwide scientific contributions to the field of electric vehicles (EVs) from 1955 to 2021. For this purpose, a search within the Scopus database was conducted using “Electric Vehicle” as the keyword. As a result, 50,195 documents were obtained through analytical and bibliometric techniques and classified into six communities according to the subject studied and the collaborative relationships between the authors. The most relevant publications within each group, i.e., those related to the most publications, were analyzed. The result shows 104,344 authors researching on EVs in 149 different countries with 225,445 relations among them. Furthermore, the most frequent language in which these publications were written as well as the h-index values of their authors were analyzed. This paper also highlights the wide variety of areas involved in EV development. Finally, the paper raises numerous issues to consider in order to broaden knowledge about EVs, their efficiency, and their applications in the near future for the development of sustainable cities and societies.

Full text

Citation: Novas, N.; Garcia Salvador,
R.M.; Portillo, F.; Robalo, I.; Alcayde,
A.; Fernández-Ros, M.; Gázquez, J.A.
Global Perspectives on and Research
Challenges for Electric Vehicles.
Vehicles 2022,4, 1246–1276. https://
doi.org/10.3390/vehicles4040066
Academic Editors: Weixiang Shen
and Chen Lv
Received: 31 July 2022
Accepted: 28 October 2022
Published: 3 November 2022
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4.0/).
Review
Global Perspectives on and Research Challenges for
Electric Vehicles
Nuria Novas , Rosa M. Garcia Salvador *, Francisco Portillo , Isabel Robalo , Alfredo Alcayde ,
Manuel Fernández-Ros and Jose A. Gázquez
Department of Engineering, University of Almeria, ceiA3, 04120 Almeria, Spain
*Correspondence: rgarciasalvadors@ual.es; Tel.: +34-950214764
Abstract:
This paper describes the characteristics of worldwide scientific contributions to the field
of electric vehicles (EVs) from 1955 to 2021. For this purpose, a search within the Scopus database
was conducted using “Electric Vehicle” as the keyword. As a result, 50,195 documents were obtained
through analytical and bibliometric techniques and classified into six communities according to
the subject studied and the collaborative relationships between the authors. The most relevant
publications within each group, i.e., those related to the most publications, were analyzed. The result
shows 104,344 authors researching on EVs in 149 different countries with 225,445 relations among
them. Furthermore, the most frequent language in which these publications were written as well
as the h-index values of their authors were analyzed. This paper also highlights the wide variety of
areas involved in EV development. Finally, the paper raises numerous issues to consider in order
to broaden knowledge about EVs, their efficiency, and their applications in the near future for the
development of sustainable cities and societies.
Keywords: Electric Vehicles; Hybrid Electric Vehicle; Battery Electric Vehicle; Scopus
1. Introduction
Since the Scottish businessman Robert Anderson invented the first electric vehicle (EV),
it has been modified and adapted to the continuing needs of society. Originally, vehicles
were used only by the wealthy classes but gradually evolved into a basic and necessary
asset in all citizens’ personal and professional development. The first contributions to the
EV involved rechargeable energy accumulators from the French plant of Gaston in 1865
and Camille Faure in 1881 [
1
]. At that time, these vehicles were competing with fossil
fuel and steam vehicles. The first EVs with commercial functions were cabs in New York
City in the late 19th century [
2
]. Early users of electric cars outnumbered users of internal
combustion engines [
3
]. These vehicles evolved into hybrids to adapt to the locomotion
of large heavy vehicles for both passenger transport (buses) and goods transport (trucks)
where electric vehicles had insufficient locomotive power. In 1899, Ferdinand Porsche
designed the first hybrid electric vehicle (HEV) with an electric and gasoline engine [
4
] and
a range of
64 km
. Its design consisted of a gasoline engine that worked at a constant speed
feeding a dynamo that charged the electric batteries [
5
]. Internal combustion engines began
to be dynamo-started, and the excess energy was used for different purposes [6].
At the beginning of the 20th century, EV production powered by rechargeable batteries
was a reality, and different companies competed on the basis of the type of batteries [
7
].
This resulted in a change in the locomotion trend with the Ford Model T and cheap oil
leaving electric locomotion in oblivion and giving way to fossil fuel combustion [
8
]. During
the 1960s, some models of electric vehicles appeared again in the industrial sector to
promote research into electric mobility due to the oil crisis [
9
]. In 1990, General Motors
raised awareness on the environmental impact of vehicles at the Los Angeles Auto Show,
and some legislative policies appeared in order to lower emissions [
10
,
11
]. This situation,
Vehicles 2022,4, 1246–1276. https://doi.org/10.3390/vehicles4040066 https://www.mdpi.com/journal/vehicles

Vehicles 2022,41247
together with lithium battery technology and the rising cost of fossil fuels, encouraged
large automotive companies to research electric vehicles [
12
]. In the 21st century, citizens
show their concern for nature by using ecological and sustainable systems. This favors an
adequate environment for the commercialization of HEVs and EVs [13,14].
The automotive industry has been immersed in society’s demands for adapting their
vehicles to modern times, incorporating new technologies and new advances in electronics
which increase effectiveness in the transfer of power to the electric traction system and
improve the efficiency of battery charging, etc. [
15
,
16
]. Citizens are increasingly aware of
the problems of environmental pollution and the need to save energy. The transportation
industry contributes about a quarter of all greenhouse gas emissions in the USA and
globally [
17
]. This information can lead to the replacement of your combustion vehicle
with a more environmentally friendly one [18].
Within the category of EVs, we must differentiate between HEVs and fully electric
ones [
19
]. The most important brands in the automotive sector are making a significant
effort to develop efficient and environmentally friendly vehicles, but most of these vehicles
are hybrids, that is, they use electric automotive systems for low speeds (30–60 km/h)
and fossil fuel systems for high speeds in order to reduce carbon emissions into the
environment [
20
]. Plug-in HEV systems are an alternative to HEVs [
4
]. These systems
incorporate a charger next to the battery as well as powertrain management strategies to
further improve vehicle performance. It allows the driver to configure the battery power
consumption and replace fossil fuel consumption [21].
The development of an EV has several difficulties, the main one being the autonomy.
Such autonomy depends on the batteries; they contribute a great part of the vehicle’s cost
and limit the maximum distance that can be covered without recharging [
22
]. In Spain, a
vehicle travels an average of 80 km per trip [
23
], but this is not a limitation since the average
autonomy of these vehicles is around 250 km. New policies are required for charging
in public spaces and within areas of collective use such as parking areas and residential
areas [24].
Among these EVs are those that run on batteries and those that run on fuel cells.
Fuel cell EVs are based on hydrogen to produce electricity, with a grid connection for
recharging and as an emergency power backup. These vehicles have the unique feature of
being almost emission-free. There are a variety of fuel cells, such as direct methanol fuel
cells (DMFC) [
25
], proton exchange membrane fuel cells (PEMFC) [
26
], alkaline electrolyte
fuel cells (AFC) [
27
], phosphoric acid fuel cells (PAFC) [
28
], molten carbonate fuel cells
(MCFC), and solid oxide fuel cells (SOFC) [
29
]. They are based on hydrogen production
and storage, and the technical limitations of fuel cells include safety due to high voltage and
electromagnetic interference caused by high-frequency and high-current switching in the
electric propulsion system. For these reasons, these systems are not available to the public,
although they seem to be an interesting option for minimum CO2 emissions [
30
]. Although
hydrogen storage technology no longer presents safety problems, efficient production is
still pending [31].
Researchers are studying different options for storage and green generation suitable
for EVs. Research on battery-based EVs advances the use of ecological batteries [
32
],
promoting the idea of green energy [
33
], a pillar of this type of vehicle. There is a wide
variety of batteries on the market which aim to replace the non-ecological lead-acid batteries
used by conventional internal combustion vehicles, which are the cheapest and most
widespread [
34
]. The battery for the EV must be emission-free (zero emissions) and not
use polluting elements, and it must have high capacity (Ampere-hours, Ah), high stored
energy (Watt-hours, Wh), a usable state of charge (SoC), high life cycle, and maximum
discharge current capacity, etc. The most common batteries are those based on nickel
as nickel–metal hydride (Ni-MH), which are environmentally friendly [
35
], but their life
cycles are short and they have the problem of a high rate of self-discharge [
36
]. The lithium
battery has great advantages for this use such as a light weight, high energy, and specific
power or high energy density [
37
]. Their main disadvantage is their high production

Vehicles 2022,41248
cost compared to Ni-MH. At this time, work is ongoing on other lithium combinations
such as lithium–sulfur, which has a higher energy capacity at low weight but a low life
cycle [
38
]. Another combination is lithium iron phosphate, which has superior thermal
and chemical stability as well as better safety characteristics than lithium-ion batteries [
39
].
Another energy storage option is the ultracapacitor (UC): double-layer electric capacitors,
pseudocapacitors, and hybrid capacitors [
40
]. The structure of these devices is like that of a
capacitor but with high capacitance [
41
]. They do not require maintenance, have a long
service life, and are not affected by room temperature. Flywheel energy storage (FES) is
also being explored. It is a device that stores/maintains kinetic energy through the rotation
of the rotor/flywheel [
42
]. This mechanism has disadvantages such as safety issues and
gyroscopic force. Another alternative for the capture of energy is the use of photovoltaic
panels on the roofs of vehicles. Despite their low generation, they can reduce the generation
of electricity by other techniques. The use of an automotive thermoelectric generator
(ATEG), which converts thermal energy into electricity, must also be considered [
43
]. It has
a lifetime of 10 to 20 years and requires no maintenance. Regenerative braking has been
incorporated into the EVs [
44
] which makes it possible to generate electricity through its
kinetic energy.
The grid connection infrastructure for the loading/unloading of EVs is an unresolved
issue [
45
]. The scientific community introduced the vehicle-to-grid system (V2G) which is
the inverse activity of charging the vehicle, exporting energy back to the grid during the
peak energy demand or saving it as backup energy [
46
]. There are studies that suggest the
use of renewable energy generation systems as an alternative to a grid connection and as a
way of avoiding the production of non-ecological energy.
Research on EVs is ongoing. New storage systems that are more ecological and have
a longer life are incorporated considering the equilibrium of the power market exposed
to marginal prices [
47
]. Improved battery management and monitoring algorithms and
new strategies for powertrain management have been considered [
48
], as well as new grid
connection infrastructures for recharging/discharging, etc. [49].
All the advances are focused on improving the efficiency of EVs in order to make
them competitive in the automotive market, with several scientific communities making
continuous contributions in different areas. These contributions are doubled per decade
which involves the processing of an enormous number of documents. These documents
are distributed in communities according to the subject matter. Collaborations between the
different communities and authors make the innovation of the locomotion systems more
efficient as there are also relationships between institutions that support their laboratories
to conduct the necessary tests. In this paper, the different communities that have been
consolidated in this subject and their relationships over time were studied.
The following sections are structured as follows: Section 2explains the methodology
followed and the extraction of the document analyses using bibliometric techniques. These
techniques were used to group the documents according to both their subthemes and
relationships. Section 3is divided into three subsections. Section 3.1. considers the different
themes and independently analyses key concepts of each theme. Section 3.2. analyses
the global growth of the subject matter according to its geographical distribution, the
authors with the highest h-index in the subject matter, and the language of the documents.
Section 3.3 sets out future challenges for each of the communities, which emerge from
their analysis. The last section provides the overall conclusions and identifies the general
challenges for the growth of the EV in its different varieties.
2. Materials and Methods
A bibliometric method was used in this review. It was a rapid method to identify
scientific collaborations worldwide using the Scopus database. This research studied global
scientific publications indexed in the Scopus database whose subject matter was related to
EVs. There are several search engines based on scientometric indicators on the web, but the
results considered neither collaborations between authors nor the possibility of establishing

Vehicles 2022,41249
different research communities on the same subject [
50
]. Elsevier’s Scopus was selected
among the existing databases [
51
]. An API rasNetBot interface developed by the research
group of the Department of Electrical Engineering of the University of Almeria (Spain) [
52
]
was used.
Figure 1shows a flowchart of the automatic information extraction script of the Scopus
database, named Research Network Bot (ResNetBOT). The method used rasNetBot, with
the search criterion, “TITLE-ABS-KEY”; in its download phase, it downloads all the publi-
cations that meet this criterion, obtaining as a result the metadata of the publications. Then,
in phase 2, the system downloads the extended information for each of the publications,
obtaining as a result a file for each publication. Once this download is finished the system
launches the search algorithms. First, it obtains the data of each of the works: for example,
keys, authors, year of publication, etc. Further, it obtains the references among the papers
that meet the search criteria, for example, if a paper is cited by many others. However,
the papers that cite it were not included in the search; these references were discarded
because they were not important for the studied set since the community does not consider
them adequate.
Figure 1. Flowchart for ResNetBOT automated script.
The keyword used in the search was “Electric Vehicle” in the TITLE field and in the
OR AUTH KEY field. Documents and relationships between them were analyzed by means
of community detection algorithms and plotted graphically using the open source program
“Gephi” [
53
]. The size of the nodes related to each publication is proportional to the h-index.
An author may have a highly referenced document, but if working alone that publication
will have a smaller node than other papers less referenced but with more collaborations
with other documents. In the graph, lines between two nodes indicate that they are cited,
and its length shows the importance of their collaboration.
Figure 2shows the result of the search: 50,195 documents were obtained with
225,445 relations developed between 1955 and the beginning of 2021. This information
went through a process of data cleansing which allowed the disposal of documents that
were not related to other communities. After this debugging process, 48,536 publications
that did not show connections with the main collaboration nuclei were eliminated and the
96.69% of the total relations remained. Figure 2shows 6 communities, but it was difficult
to mark out their area because of their low density and small size. The “Battery Charges”
community is mixed with the “Vehicle to Grid Power” community as well as with the “HEV
Control Strategies” community. On the other hand, the “EV Market Study” community is
widely related to the “EV Battery Management” and “Vehicle to Grid Power” communities.
The “HEV Control Strategies” community is the biggest one and presents relations with the
rest of the communities. Its relations with the “EV Battery Management” and “EV Market

Vehicles 2022,41250
Study” communities create regions in which the nodes of both communities cannot be
easily distinguished.
Figure 2. Representation of the communities that investigate EVs after debugging.
Figure 3shows the number of documents that make up each community to establish
their size. There are two communities that stand out for their size. The “HEV Control
Strategies” community is the largest one with 29.25% of the total work, followed by the
“Vehicle to Grid Power” community with 23.50%. The “EV efficiency community” (15.61%)
is the next by size and is made up of half of the documents that form the largest community.
It should be noted that the “Battery chargers” community is very small compared to the
rest. It is made up of 5.94% of the documents and can be considered negligible compared
to the rest.
Figure 3. Representation of work distribution in each of the communities.
Figure 4shows the keywords used in the documents studied, which reveals the great
diversity of communities that study EV-related topics. In this figure, the acronyms have

Vehicles 2022,41251
been eliminated and replaced by their equivalent to really see the number of repetitions
and their relevance. This substitution was necessary because there are many concepts
with the same acronyms and different meanings depending on the subject matter. For this
reason, it is becoming less and less frequent to use acronyms in the keywords of an article.
After the substitution of these acronyms, 30.36% of the total number of keywords were
eliminated. The size of these words depends on its number of repetitions. The keyword
“Electric Vehicle” has the largest size because it is the one that appears most frequently, in
almost all communities. Other words considered to be of a general nature are “Hybrid
Electric Vehicle” and “Battery”, which appear in most of the communities studied.
Figure 4. Representation of the global keywords of all communities.
Figure 5shows the number of repetitions of the 20 most common keywords. “Electric
vehicle” is the most repeated, with a value which is five times higher than “Hybrid electric
vehicle”. This keyword is among the top 10 most repeated words in all communities and is
the most repeated in 4 of them. Other words such as “Lithium-Ion Battery”, “Vehicle to
Grid (V2G)”, and “Battery” have more than 300 repetitions and tend to be in almost all the
communities, but in a different order according to their subject.
Figure 5. Representation of the 20 most repeated keywords.

Vehicles 2022,41252
3. Results and Discussion
3.1. Analysis of the Communities
This section is divided by subheadings. It provides a concise and precise description
of the experimental results, their interpretation, and the experimental conclusions that can
be drawn. Each community has a common theme. The main nodes of each community
and their most significant contributions have been analyzed. The number of references of
the three most important papers of each community have been determined according to
their size.
The Battery Chargers community
focuses on the various advances in technologies
that improve energy efficiency in the transfer of energy in the charging of the batteries of
electric vehicles. The distribution of this community is shown in Figure 6a. It is a relatively
small community (5.94%), with a small central nucleus where most of the studies are
concentrated. A high number of documents are located outside the community. Despite the
small concentration of this community compared to others, its distribution is large because
the articles are strongly dispersed, and they relate to many other communities. The three
main nodes are [
54
] with 1236 citations, [
55
] with 1259 citations, and [
56
] with 863 citations.
Figure 6.
Representation of the Battery Chargers community: (
a
) Isolated distribution of publications;
(b) Keywords.
Figure 6b shows the 10 most repeated keywords in the community. “Electric Vehicle
(EV)” is the most repeated keyword with almost twice as many repetitions as the next
most repeated keyword, “Wireless Power Transfer (WPT)”. Keywords such as “Inductive
Power Transfer (IPT)” and “Battery Charger” are among the top 20 most repeated keywords
globally. The remaining keywords are related to the EV efficiency process such as “Wireless
charging”, with a similar number of repetitions.
Plug-in electric vehicles (PEVs) differ from HEVs because they incorporate a higher
capacity battery and power converters. Batteries are charged during low-power periods
and discharged during high power demand, providing an energy boost.
Advances in power electronics are present in EVs, both in the power transfer and
battery charging system [
57
]. Charging systems use two implementation-based approaches,
capacitor-based and contactless coil-based systems, which are called conductive and induc-
tive, respectively. Charging batteries via a grid connection has disadvantages such as the
gauge of the connection cable or the necessary infrastructure, among others. To overcome
this, research has been carried out on wireless power transfer (WPT) or inductive power
transfer (IPT), the main advantages of which are safety, convenience, and a fully automated
charging process [
58
,
59
]. Wireless power transfer systems (WPTS) can be classified as
inductive power transfer systems (IPTS), coupled magnetic resonance systems (CMRS),
like IPTS with an extremely high-quality factor (Q), and capacitive power transfer systems
(CPTS). CMRS present difficulties in maintaining resonance conditions due to their Q and

Vehicles 2022,41253
are bulky, making them unlikely to be a reliable candidate for future EVs. The most likely
implementation for EVs is IPTS and CPTS.
Battery chargers can also be classified according to their location as external chargers
(the charger is mounted on the charging station and is independent) and internal or
integrated unidirectional and bidirectional chargers (the charger is mounted on the EV) [
58
].
EV on-board chargers have been designed with light weight, small size, high performance,
and simplicity of control as desirable features [
60
]. Battery charging can be performed in
different modes of operation and sometimes allows interaction between them [61]:
•Grid-to-vehicle (G2V) is used in internal chargers.
•
Vehicle-to-everything (V2X) uses bidirectional integrated chargers and allows dis-
tributed energy control to share stored energy. However, V2X is vulnerable to cyber–
physical attacks and instability caused by time delay. There are proposals to solve
this by using cyber resilience techniques, authentication protocols, and delay-tolerant
techniques, through which the resilience of the V2X system to cyber–physical attacks
and time delays can be increased.
•
Vehicle-to-grid (V2G) uses the energy stored in the battery for the grid connection
to provide services to the grid (active power demand regulation, reactive power
compensation, peak shaving and valley filling of load demand, frequency and voltage
regulation, harmonic compensation of grid current, improved reliability, and stability
and efficiency of the system, among others).
•
Vehicle-for-grid (V4G) is a special case of the V2G mode of operation to compensate
harmonics in the line current and inject reactive power to improve the voltage profile
of the system; it allows the G2V/V2G mode, and the remaining energy not used in
this mode can only be used for reactive and harmonic power compensation during
the V4G mode.
•
Vehicle-to-vehicle (V2V) is used to exchange charging energy between EVs, where EV
owners can sell their surplus energy to other EV owners. This functionality can also
be realized by V2V for EVs connected to smart homes and car parks.
•
Vehicle-to-home (V2H) implements the V2G modes to provide a backup supply for
connected loads in the home (connected appliances in a smart home) and V2V.
•
Vehicle-to-load (V2L) is used to ensure a continuous supply to critical loads that cannot
be left without power in case of main grid failure such as military sites, hospitals,
data centers, etc. It is implemented as a special case of the V2H and V2V modes of
operation for electric vehicle chargers.
The battery charger is one of the main elements that define EV technology and is
implemented in the charge control algorithm and charge converter topologies. Among its
characteristics, it must optimize efficiency (energy density, cost, and the size and health
of the battery) and be reliable and affordable. Different charging techniques based on
topologies and their electrical models are presented in [58].
There are different proposals to modify the basic scheme, such as using a bidirectional
DC/DC converter for regenerative loads in case of braking, etc. [
61
]. Inductive charging
does not use a cable connection between the supply and the power system for loading.
These systems are a safe and robust bet for high-power applications (>50 kW). Inductive
charging is based on the AC/DC/AC conversion from the supply network and an AC/DC
rectifier for the battery connection; a resonant circuit (L and C) is usually used to adapt
the energy to the conditions of maximum energy efficiency. For the magnetic coupling,
different forms of cores are used (U, E, I, or W), and for the resonant circuits, depending on
the arrangement of the passive elements, different resonant topologies are obtained. These
systems allow energy supply without contact, using galvanic isolation between the primary
source and the load, with efficiencies of around 90%. The batteries used in these vehicles
have a high storage capacity. The performance of the magnetic coupler limits the energy
transfer and the viability of the system. Inductively coupled power transfer (ICPT) systems
have been extensively studied since their invention in 1995 [
62
]. The literature includes

Vehicles 2022,41254
many studies on the behavior of circular power pads [
63
], but there are other proposals
with the aim of minimizing losses in EV charging [64].
Other authors propose a theoretical inductively coupled ICPT using a variable fre-
quency controller for EV battery charging to deliver 30 kW through a 45 mm air gap at a
nominal frequency of 20 kHz and a primary current of 150 A [54]. Researchers developed
a design factor called Kre for the selection of the optimal number of coils, appropriate
section, compensation capacitors, and resonance frequency of an ICPT system for four basic
topologies [
65
]. Choosing the operating frequency is essential for maximum power transfer
to the load. If the frequency is not appropriate, it can cause stability and control deficiency.
The results show the dependence of the quality factor of the secondary resonant circuit on
the topology of the primary and secondary circuits, with parallel compensation less critical
in series connection.
Researchers have proposed the design of an integrated bidirectional AC/DC charger
and DC/DC converter for PHEV and hybrid/plug-in-hybrid conversions. The system al-
lows adapting an HEV to a PHEV and uses the AC/DC converter to charge the battery with
power factor correction and a DC/DC converter to transfer the energy of the battery [
66
].
This system adds an additional high-energy battery to the HEV system that receives or
supplies power to the bidirectional DC/DC converter, which is connected to the traction
and regenerative load recovery in the braking. Achieving a low-cost, highly efficient, and
flexible EV charging and discharging system is an ongoing development, involving both the
industrial and academic communities to make it viable and environmentally friendly. Other
proposals analyze a mathematical model based on a phase and/or voltage magnitude mod-
ulator for a 1.5 kW inductive power transfer under various operating conditions [
67
]. Other
research proposes an online electric vehicle (OLEV) center, and it has been commercialized
in the Seoul Grand Park [
61
]. This proposal includes a wireless 100 kW power transfer
system for the OLEV system obtaining a power transfer efficiency of 80% for an air space
of 26 cm, and there are currently upgraded versions such as OLEV 6G [
68
]. Reviews on the
wireless charging of electric vehicles have also been carried out, in which the benefits of the
introduction of electric roads with wireless charging capacity are discussed [55].
The V2V mode of operation requires communication models to make use of applica-
tions based on the internet of things (IoT) and intelligent transport systems (ITS). Major
electric vehicle companies such as Audi (Germany), General Motors (USA), BMW (Ger-
many), Volvo Cars (Sweden), Daimler AG (Germany), Toyota Motor Corporation (Japan),
Qualcomm Technologies, Inc. (USA), Volkswagen (Germany), and AutoTalks Ltd. (Israel)
are developing applications that support V2V communication [60].
This kind of system will encourage using these vehicles since the charge will not
depend on the type of battery, although wireless charging needs further research to make it
economically viable and safe for citizens.
The EV Market Study community
is formed by research about EV market trends. The
characteristics of this community are shown in Figure 7a, where there are higher density
areas of nodes and an external halo that relates publications within this community to
others from neighboring communities. The three main nodes are [
69
] with 880 cites, [
70
]
with 536 cites, and [
71
] with 119 cites. Figure 7b shows the 10 most repeated keywords,
among which “Electric Vehicle” is the most repeated keyword, with almost five times more
repetitions than the next keywords: “Battery electric vehicle”, with a similar number of
repetitions as the following keywords, “Life Cycle Assessment” and “Lithium-ion Battery”.
This indicates that the trend in this community is towards the study and improvement
of electric vehicle batteries. The following words are repeated a similar number of times.
Other general words such as “Plug-in hybrid electric vehicle” or “Hybrid electric vehicle”
also appear. Most studies with these words focus on battery-related issues or on China, the
country where the market is growing the most.

Vehicles 2022,41255
Figure 7.
Representation of the EV Market Study community: (
a
) isolated distribution of publications;
(b) keywords.
Issues related to EV manufacturing pollution and green recharging are discussed
in this community. In [
71
], a survey of 3029 people interested in purchasing a vehicle is
conducted to analyze whether they prefer an internal combustion vehicle (gasoline) or
the EV option. The results show that the number of potential EV consumers is higher
among younger and more educated people with greener lifestyles. They also reveal that
the incomes and the ownership of several cars are not relevant to drivers’ decisions. Saving
fuel consumption is the main motivation. Although many drivers are willing to buy an EV
at a higher cost than the internal combustion ones, a price drop is required to make these
vehicles competitive, mainly on the batteries [20].
Plug-in hybrid electric vehicles (PHEVs), which use electricity from the grid to power
part of their drives, could contribute to the reduction in greenhouse gas (GEI) emissions
from the transport sector. It would represent a 32% reduction compared to conventional
vehicles and is conditional on low-carbon electricity sources. One of the conclusions
established in [
70
] is that the electricity generation infrastructure should be long-lasting,
and the next decades’ technology decisions on electricity supply in the power sector would
affect GEI emission reductions if PHEVs were taken into account over several decades.
Potential socio-technical barriers to EV acquisition are identified in [
69
]. Other research
shows that the understanding of EVs differs by gender, age, and educational background,
although the sample (481) may not be representative [
71
]. The initiative to boost the
replacement of conventional vehicles is based on the potential for environmental value.
There is a group of potential buyers who are very familiar with the technology, and therefore
they consider these vehicles as a feasible alternative. However, others do not engage with
EVs due to uncertainty about batteries and the sustainability of the power sources.
Other researchers study the relationship between environmental impacts and EVs/HEVs
compared to conventional vehicles through 51 surveys. The results are not definitive and
require more data. Most studies analyze fuel and electricity consumption, but there is little
documentation about batteries [
72
]. Another study compares global warming in relation
to conventional diesel and gasoline vehicles with electric ones [
70
]. The EV potentially
produces lower emissions, and its environmental impact depends on producing electricity
from renewable sources or natural gas. The production of EVs can cause higher human
toxicity potential, freshwater ecotoxicity, freshwater eutrophication, and metal depletion
impacts. These problems can be counteracted by making use of effective recycling programs
and improving the lifetime of EVs, which decreases the negative long-term impact. This
community also contemplates the advances in power electronics that are present in EVs,
both in the power transfer and battery charging system [
73
]. Si-based devices are cheaper
but less efficient compared to the generation of SiC or GaN devices (broadband switching
devices), although they increase the cost of the converter.
A review of the penetration rate of HEVs and electrical PHEVs in the USA is carried
out in [
74
]. It concludes with six recommendations related to improved surveys, inclusion

Vehicles 2022,41256
of vehicle supply models and actions by automakers and state policies, the effect on
automotive markets and technology competition, an improved model of market volume
and vehicle ratings, and an improved sensitivity analysis that can support and verify model
results and provide guidance for future model improvements. EV penetration rate was also
analyzed in 30 countries, as well as its relationship to economic incentives in the policies of
different countries [
75
]. The results consider the importance of the charging infrastructure,
the economic incentives, and the presence of local production facilities with the market
share of each country, although they do not guarantee a high rate of adoption of electric
vehicles [75].
Other authors discuss the decrease in Li-ion battery packs’ costs between 2007 and
2014 for EV manufacturers [
76
]. Another paper reviews the drivers and barriers to the
deployment of PHEVs [
73
]. The study examines consumer preferences for the charging
infrastructure as well as how consumers interact with and use this infrastructure. It
establishes that the most important place for electric vehicle charging is at home, followed
by the workplace and then public places. Studies have revealed that more effort is needed
to ensure that consumers have easy access to electric vehicle charging and that charging at
home, at work, or in public places should not be free of charge. Existing research on this
topic is still insufficient to determine the amount of infrastructure needed to support the
deployment of EVs. The relationship between policy and environmental education must
be analyzed to assess the effectiveness of these policies in the adoption of EVs. Marketing
plays an important role in how manufacturers want to present EVs. Although many of them
focus on environmental claims, others are more oriented towards the superior performance
of these vehicles [70].
The EV Battery Management community
studies the autonomy of EVs since it de-
pends largely on the battery and, therefore, the battery’s SoC along with the expected
battery life.
The isolated distribution of this community is shown in Figure 8a. This community
consists of 14.58% of the documents and is made up of a central area as well as highly
concentrated areas of nodes and papers in the outer region. The three main nodes are [
77
]
with 3083 citations, [78] with 1128 citations, and [79] with 1349 citations.
Figure 8.
Representation of the EV Battery Management community: (
a
) isolated distribution of
publications; (b) keywords.
In this community, unlike the rest, the most repeated keyword is “Lithium-Ion”, with
a number of repetitions similar to the most repeated keyword globally, “Electric Vehicle
(EV)” (Figure 8b). The other keywords with the highest number of repetitions are “State of
Charge (SOC)”, “Battery Management System”, and “Battery”, all of which are specific to
the study of batteries in EVs.
The battery in EVs must provide energy to supply enough power for a long and stable
drive, as well as adequate transient power for acceleration and downhill. In addition,
long cycle life, stable voltage, high energy and power density, fast response, and short
recharge time are other specifications for the high-power energy storage system of an EV.
EV batteries require a monitoring system (battery SoC, power fade, capacity fade, etc.) and

Vehicles 2022,41257
instantaneous management of the available SoC. Good monitoring and load management
can improve performance and increase battery life if charged with a proper charger and
infrastructure. Conventional methods for detecting the state of the battery are easy to
implement, but they do not consider aging, temperature, or external disturbances [
79
]. One
review [
80
] focused on the status of lithium battery technology as the preferred energy
source for the consumer electronics market. Then, a study of their characteristics was
carried out, and the new existing challenges, which are aimed at achieving quantum leaps
in energy and power content, were raised.
Comparing Li-ion batteries with other commonly used batteries for EVs, these batteries
have high energy and power density, a long lifespan, and they are environmentally friendly,
so they are suitable for EVs [
77
]. However, they have safety problems, and their durability,
uniformity, and cost could be improved. Furthermore, there are some issues concerning
battery monitoring and battery management systems (BMS), such as battery performance,
the better use of battery models, the adaptive control techniques, and the expert battery
management system theories.
Gregory L. Plett published a series of three papers [
78
] in which he proposes a method
based on extended Kalman filtering (EKF) that can achieve the dynamic and instantaneous
management of the SoC available in Lithium-ion polymer (LiPB) batteries in PHEVs. The
Kalman filter includes a set of recurring equations that are repeatedly evaluated as a lin-
ear system or as the extended Kalman filter in the case of a non-linear system. The first
paper shows the basics of BMS and the second develops the final model with dynamic
requirements (open circuit voltage, ohmic loss, polarization time constants, electrochem-
ical hysteresis, and temperature effects). The third one discusses the use of EKT for the
battery management algorithm. In addition, EKF allows for dynamic power evaluations
that automatically compensate for recent discharge events and provide a more accurate
estimation of how much power can be consumed without exceeding voltage limits. Other
researchers study battery SoC as well as HEV and EV battery state of health estimation
strategies [
81
]. They also provide a roadmap for battery EV researchers and manufacturers.
The research community is striving to develop an advanced method of SoC estimation and
a Li-ion battery energy management system for future high-tech EV applications. There are
frequent articles where reviews based on lithium-ion batteries are presented as a rapidly
advancing field, it attracts a larger number of researchers. In [
82
], the authors make a
battery check for EV vehicles and consider the SoC as a crucial parameter that can indicate
the instantaneous energy available in a battery and inform about the charging/discharging
strategies to be followed, as well as protect the battery from overcharging/over-discharging.
Lithium-ion batteries are considered suitable for electric vehicles, although they present
some problems due to their complex electrochemical reactions, performance degradation,
and lack of accuracy in improving battery performance and lifespan. In [
83
], the application
of artificial intelligence for battery status estimation is reviewed.
Other studies address the implementation of a dedicated battery management system
which also includes Li-ion battery condition monitoring for a long-life, high-performance
electric scooter prototype [
84
]. The results show that this system increases the reliability
and autonomy. A battery’s thermal energy management is an issue of concern, since good
thermal management improves battery performance. Models shown in [
85
] respond to the
thermal behavior of high-energy batteries such as Ni-MH, Li-ion, and proton exchange
membrane fuel cells. Air and liquid cooling systems do not seem to be suitable for high-
energy batteries, as they are greater in size and cost. Therefore, a new method which uses a
pulsed heat pipe is proposed. When discharge rates and operating or ambient temperatures
are high, phase-change materials are the best choice for thermal management, although this
requires a study of thermomechanical behavior. They also present the possibility of heat
recovering to increase energy efficiency. In [
85
], the effects of low temperature on batteries
are explained, as well as possible different improvement strategies.

Vehicles 2022,41258
Other researchers provide a double polarization model of an equivalent circuit of a Li-
ion battery for use in EVs [
86
]. Voltage imbalances between individual batteries are a major
problem for premature cell degradation and safety risks, which leads to
capacity reduction
.
A summary comparison and evaluation of the different methods of active battery
equalization are presented according to their application, as well as the state-of-the-art
energy management strategies of connected HEVs/PHEVs [
84
]. An overview of new
strategies to address today’s challenges for automotive battery systems is provided in [
87
].
Smart battery systems have the potential to make battery systems more efficient and future-
proof for the next generations of electric vehicles. Currently, research is ongoing on the
construction of high-voltage batteries as well as high-capacity batteries. Zinc batteries
are seen as a promising technology for the next generation, once it overcomes insufficient
energy density [
88
,
89
]. EVs with high-capacity batteries are already available. In 2022, the
Tesla Roaster is the car with the highest battery capacity of 200 kWh; its predecessors such
as the Tesla Model X or Tesla Model S had a capacity of up to 100 kWh. There are other
brands that have models with 100 kWh capacities such as Renault (ZOE 2rs model) and
Volvo (40 series model) [90].
Battery recycling presents a drawback for EV expansion. EV manufacturing uses
valuable metallic materials that can be conveniently recycled. Using circular economy
ideology, Roy et al. (2022) review lithium-ion battery recycling procedures and the chal-
lenges remaining to make them common practice. Most material extraction has a lower
cost than the extraction of metals in nature, and some are scarce and can be used to make
new batteries from the recycled materials [91].
The Energy-Efficient Transmission in EVs community
focuses on the various ad-
vances in technologies that improve energy efficiency in EVs (Figure 9a). With energy
prices constantly on the rise, there is a need for researchers to develop energy-efficient
devices. The electric drive is the core of EVs, so its selection is a very important step
that requires special attention. Torque distribution and traction control are also important
parameters to consider. Other researchers are also investigating emerging technologies that
improve energy consumption, such as thermoelectric heat recovery, temperature control,
and regenerative braking systems. The three main nodes are [
92
] with 1571 citations, [
93
]
with 1454 citations, and [
94
] with 871 citations. Figure 9b presents the 10 most repeated
keywords in this community, with concepts such as traction control and induction motors
being parameters to be considered and of special interest. The search keyword is shown as
the most frequent, repeated almost three times more than the following word. Both words
are the most repeated words globally, as they are the ones used in the search methodology.
The following most repeated words are specifically related to the topic of this community,
such as “In Wheel Motor”, “Permanent Magnetic Machine”, and “Induction Motor”.
Figure 9.
Representation of the energy-efficient transmission in EVs community: (
a
) isolated distribu-
tion of publications; (b) keywords.

Vehicles 2022,41259
Chau and Chan (2007) identified and discussed three emerging energy-efficient tech-
nologies: the thermoelectric waste heat recovery and temperature control systems for HEVs,
the integrated starter generator for mild hybrids, and the electronic continuously variable
transmission propulsion for HEVs. They pointed out that research in energy efficiency
technologies will boost the HEV [12].
Chau et al. (2008) provided an overview of permanent-magnet brushless drives
for EVs and HEVs, with emphasis on machine topologies, drive operations, and control
strategies [
93
]. The magnetic-geared outer-rotor drive system for EVs, the integrated
starter generator system for micro and mild HEVs, and the electric variable transmission
system for full HEVs were discussed in the article. In the analysis of rule-based strategies,
Chan et al. (2010)
suggests that fuzzy rules as well as the neural network are better than
other methods based on deterministic rules due to their robustness and capacity for effective
adaptation in real time [
95
]. Wu and Zheng (2017) also used several control strategies, such
as a dynamic programming global optimization algorithm, fuzzy control, and torque equal
distribution, optimizing the motor’s working point, reducing the energy consumption,
and increasing efficiency by 4.7% [
92
]. In the same direction, various induction, switched
reluctance, and permanent-magnet brushless machines have been described in Zhu and
Howe (2007), highlighting the different topologies and merits [
94
]. They concluded that
the three different technologies can meet the performance requirements of traction drives.
Zeraoulia et al., (2006) reviewed the state of the art of four electric propulsion systems:
DC motors, induction motors, permanent-magnet synchronous motors, and switched
reluctance [
96
]. They carried out a comparative study on electric motor drive selection
problems for HEV propulsion systems, concluding that induction motors seem to be the
most efficient candidate for the electric propulsion of urban HEVs. De Santiago et al. (2012)
presented a review of drivelines in EVs, discussing the advantages and disadvantages of
each electric motor type. The authors proposed the adaption of a standardized drive cycle
or other standardized methods of efficiency measurement in order to make possible the
comparison between EVs [97].
Ivanov et al. (2015) presented an overview of the most original and cited variants
of traction control and anti-lock braking systems for full EVs [
98
]. They concluded that
there are multiples approaches to improve the traction dynamics of vehicles, but only a few
techniques had been validated and verified in conventional vehicles. The authors also en-
couraged the creation of objective procedures for benchmarking and
comparative analysis
.
Some articles analyzed the effectiveness of different technologies in prototypes. Hori
(2006) analyzed the experimental prototype “UOT MARCH II” regarding the effectiveness
of various control studies such as the model-following control and slip ratio control [
99
].
The benefits of the studies were proven to take advantage of quick, accurate, and distributed
torque generation. Chen et al. (2011) analyzed the technical requirements for the design of
a permanent-magnet electric variable transmission based on an analysis of the operation of
the Toyota Prius II [
100
]. The authors chose this HEV as the reference vehicle because it is a
well-known and efficient system, but it could be considered for the design and analysis of
other hybrid powertrains.
The Vehicle-to-Grid Power community
is formed by researchers who intend to de-
velop systems to commercialize vehicle-to-grid power. They also investigate the impact on
the connection to the electrical grid, recharge/discharge optimization strategies to reduce
its impact, and how to maximize the economic performance of the system. To further
minimize impacts in terms of climate change, the use of renewable energy for electric
vehicles could be improved by integrating energy demand interaction models.
Figure 10a shows the distribution of this community. It is the second community
in size (32.6%) and features a central zone with the greatest concentration of nodes. It
is related to other communities by the outer zone. The three main nodes are [
101
] with
2190 citations, [102] with 1697 citations, and [103] with 1581 citations.

Vehicles 2022,41260
Figure 10.
Representation of the vehicle-to-grid power community: (
a
) isolated distribution of
publications; (b) keywords.
Figure 10b shows the 10 most repeated keywords in the community. The next most
repeated keyword is “Vehicle to Grid (V2G)” with half as many repetitions as the previous
one, marking it the main theme of this community along with specific words such as “Smart
Grid”, “Microgrid”, and “Optimization”, among others. The following words are among
the top 20 most repeated words globally. Some of these words are “Plug in Hybrid Electric
Vehicle” and “Plug in Electric Vehicle”, which are present in almost all communities.
The distribution grid is being affected as electric vehicles expand. Among the consider-
ations for the distribution grid are the adequacy of electricity generation (use of renewables)
and energy efficiency. The greatest impact occurs at peak EV load demand hours, which
requires the expansion of generation capacity. This also involves overloading the substation
and service transformers, shortening their lifetime. It can cause power quality problems
such as voltage sags, power imbalances, and voltage/current harmonics, among others.
This community investigates alternatives to provide solutions to these considerations and
their implications for the distribution network [104].
The use of EVs increases when the possibility of the commercialization of the V2G
and V4G systems increases. This is the reverse activity of charging the vehicle and an
alternative to export power back to the grid during peak power demand or to use it as
backup power. V2G is not suitable for a base-load power distribution network (constant
electricity supply throughout the day), where large generators produce higher power at
lower cost. However, it does seem suitable for a fast-response and high-power services to
balance constant load fluctuations and to adapt to unexpected equipment failures. In [
102
],
it is quantitatively expressed how much EVs can become part of the electrical network, and
methods are discussed to estimate expected revenues and costs. They propose using V2G
when there is a capacity payment for being online and available along with an additional
energy payment when the energy is sent. This results in improved reliability and reduced
costs of the electrical system. Once the electric network and the EV fleet are analyzed, it is
observed that they respond inversely since the electric network has high capital costs and
low production costs, and for the EV fleet it is the other way around. The same happens
when using electric generators for 57% compared to 4% of the vehicles. The electric grid
has no storage capacity, while the car fleet must have storage to fulfil its function. In [
103
],
strategies and business models for V2G are proposed, as well as the necessary steps for
its implementation considering the comparison of the electric grid and the EV fleet. This
proposal is in line with the use during peak energy demand hours or as backup energy in
the short term. However, in the long term it proposes backup generation and storage for
renewable energy.
Another study proposes to add an aggregator as an intermediate system to manage a
vehicle’s energy while regulating the energy of the distribution network [
105
]. The proce-
dure for the alternative charging or discharging of the batteries of the vehicles belonging

Vehicles 2022,41261
to the aggregator is established to meet the power requested by the grid operator, but it is
difficult to develop an algorithm that efficiently serves vehicles with arbitrary loads. The
aggregator must control the sequence, duration, and charging rate of each vehicle based on
the price of electricity, which maximizes the aggregator’s revenue.
PHEV batteries can be recharged either in a parking lot or at home. Recharging con-
sumes a large amount of electrical energy, and this can lead to large and undesirable peaks
in electrical consumption that can cause an impact on the distribution network. In [
101
],
the impact on the distribution network of household recharging is studied. Intelligent
recharging can be planned, where the charging is coordinated remotely to shift the demand
to periods of lower PHEV recharge consumption and thus avoid higher peaks in electricity
consumption. The results of this paper show that coordinated charging, using quadratic
programming techniques, reduces power losses since dynamics do not improve results.
In [
106
], the relationship between feeder losses, load factor, and load variation in coordi-
nated PHEV loading is studied. Three optimal load algorithms are simulated to reduce
the impact on the distribution network at the load connection. The results confirm the
effectiveness of the study in maximizing the load factor and minimizing the load variation.
Moreover, [
107
] addresses advanced strategies for centralized EV load control allowing the
integration of a larger fleet into the system without network reinforcements. This allows
the network to be operated in less extreme conditions by adopting a level of local control
to operate in isolation, as the EV batteries can provide rapid compensation to the system.
In [
108
], an analytical solution is developed to predict the charge of an EV considering
the stochastic nature of the individual battery charging start time as well as the initial
SoC of the battery. This method is applied to four EV charging scenarios: uncontrolled
domestic charging, uncontrolled off-peak domestic charging, “smart” domestic charging,
and uncontrolled public charge travelers capable of recharging at the workplace. A mar-
ket EV penetration between 10% to 20% would result in a maximum daily increase in
energy demand from 17.9% to 35.8% for the uncontrolled domestic charging scenario. How-
ever, off-peak domestic charging increases electricity consumption at night. The “smart”
charging method is the most beneficial for both the distribution network operator and EV
customers. However, if they start to charge simultaneously it will impose a new peak in
the off-peak period on the distribution network. The above studies model the distribution
network on a small scale, but, for a real analysis, a large-scale distribution network must be
considered. In [
109
], two real distribution areas are studied to obtain a model for large-scale
distribution planning. The results show that with smart charging strategies it is possible
to decrease incremental investment by applying a decrease in simultaneous charging and
off-peak scheduling of the distribution network.
Other research proposes coordinating PHEV charging to minimize total generation
costs in real time [
110
]. Other papers incorporate market energy prices that vary over
time [
103
,
108
] and add load time zones based on priority selection (owner’s choice). This
allows PHEVs to start charging as soon as possible while meeting network operation
criteria. The proposed algorithm reduces system overloads and global power surges
without the need to over-size the power generation infrastructure to service the PHEV
fleet. In addition, it relates the charging infrastructure with the PHEV charger as well as
the type of battery [
110
]. Battery performance depends not only on type and design but
also the characteristics of the charger and the charging infrastructure. Common onboard
chargers restrict power to meet weight, space, and cost restrictions, except the onboard
charger which has no such restrictions and supports low-cost, high-power, and rapid
bidirectional charging with a unit power factor. The availability of a charging infrastructure
reduces battery energy storage requirements and costs. Inductive battery charging supports
wireless charging systems and were considered in the “Chargers Batteries” community,
being the foundation of V2G wireless.
The HEV Control Strategies community
. HEVs have two sources of energy which
require optimal energy management strategies that consider fuel consumption savings and
CO2emissions reduction.

Vehicles 2022,41262
Figure 11a shows the isolated distribution of this community. It is the largest one and
it has areas of high density of nodes where it is difficult to distinguish isolated nodes. It
also has a wide halo formed by documents related to other communities. The three main
nodes are [111] with 1458 cites, [112] with 1162 cites, and [113] with 1098 cite.
Figure 11.
Representation of the HEV control strategies community: (
a
) isolated distribution of
publications; (b) keywords.
Figure 11b shows the 10 most repeated keywords in this community. The search key-
word “Electric Vehicle (EV)” has a lower number of repetitions than the keyword “Hybrid
electric vehicle”, a general word frequent in almost all communities and which is specific
to this community. Other words specific to this community are “Energy Management”,
“Energy Management Strategy”, and “Dynamic Programming”. Among the top 10 most
repeated keywords are other general keywords such as “Plug in Hybrid Electric Vehicle
(PHEV)” and “Ultracapacitor”.
EVs have significant advantages over internal combustion engines as they are qui-
eter and more efficient, reliable, and durable. The EV motor controller is smaller and
lighter than the current internal combustion engine as well as cheaper to maintain and
manufacture [
112
]. EVs in general (EV, HEV, PHEV, and hydrogen cell vehicles) are more
energy efficient as they are hybrids with optimal fuel consumption strategies [
111
,
114
]. For
HEVs and PHEVs to be competitive with conventional vehicles, costs must be reduced,
efficiency improved, and the range of electric driving increased [
115
]. Power conversion
and rotating machines are similar in HEVs and PHEVs, as well as the problems associated
with them [
114
]. These vehicles require an efficient energy management system to divide
the energy demand between the powertrain components [
116
]. There are two different
methods of energy management: control strategies based on a physical model of the system
and optimization strategies that are usually based on simulations of the system under
study [
95
]. The hybrid transmission train is a discrete dynamic system, which has a time-
varying, multi-domain, and non-linear variable plant. In these vehicles, an intelligent
control algorithm is used to improve the control of low-level components [31]. The imple-
mentation of the control strategy is carried out within the vehicle’s central controller, and it
is responsible for making decisions on when to activate or deactivate certain local parts
of the transmission system. These strategies are based on different parameters to satisfy
the driver’s demand, maintain the battery charge preventing them from overloading and
overdischarging, optimize the efficiency of the transmission train, minimize engine stops
and restarts and idle time to avoid unnecessary fuel consumption, and to perform in its
optimal operating region most of the time and reduce fuel consumption and emissions [
101
].
However, emission reduction and efficiency optimization are parameters in opposition,
although a balance is sought between both. Global optimization techniques are not valid
for real-time development using heuristic control techniques, where the static ones use
the energy consumption to calculate the fuel cost and the dynamic ones optimize as a

Vehicles 2022,41263
function of a time horizon rather than for an instant of time. It is required to use control
strategies with a non-linear, variable-time, multi-domain system [
117
]. The methods are
classified into two general trends: those based on rules (deterministic and fuzzy) and
those based on optimization (global and real time). In [
111
], possible improvements on the
control strategies developed up to 2007 are analyzed. An updated review of metaheuristic
algorithms used for EV optimization is presented in [118].
In [
113
], a proposal implemented in an HEV with a dynamic strategy is presented.
The rules-based proposal considers cost, minimization of fuel consumption, and emissions.
The study shows that low fuel consumption could significantly reduce emissions. In [
30
],
the authors propose a control system that looks for the instantaneous cost considering the
two energy sources and the restrictions imposed by the state of the battery and the amount
of fuel.
3.2. Analysis of Authors and Documents on the Topic of EVs
Research based on EVs has been widely developed in 149 countries, with
104,344 authors
researching in different areas. Figure 12 shows the distribution of the researchers according
to their country of origin. The darkest country is the one with the greatest number of
researchers in the subject. There are scientific contributions from all the continents, high-
lighting European (28.6%) and Asian (48.3%) countries, the United States (14.4%), and
Canada (3.3%).
Figure 12. Distribution of authors by country.
The representation of the 20 countries with the highest number of researchers working
on EVs is shown in Figure 13. China is the country with the highest contribution, with
25,059 researchers (24.02%), almost twice as high as the contribution of the following
country, the USA, with 15,048 researchers (14.42%). It should be noted that the coun-
tries that have invested the most in technology are the countries of origin of the greatest
number of researchers, such as China or the USA, as well as world powers such as In-
dia with
6280 researchers
(6.02%), Germany with 5881 researchers (5.64%), the United
Kingdom with 4577 researchers (4.39%), and Japan with 4349 authors (4.17%). It also
shows the growing development in technology in other countries such as South Korea with
3758 authors (3.60%), or Canada with 3406 researchers (3,26%). Most of the countries
in the European Union are among the 20 countries with the greatest contributions, such
as Italy (3.12%), France (2.73%), Spain (1.58%), and Iran (1.57%). The rest have percent-
ages close to 1%, as is the case for Taiwan (1.11%), the Netherlands (1.09%), and Sweden
(1.09%), followed by Portugal with 1.05% of the total number of researchers. The sum of
the contributions by authors from the rest of the countries is 19.30%.

Vehicles 2022,41264
Figure 13. Representation of the 20 countries with the greatest number of authors’ contributions.
Of the authors who research on subjects related to EVs, 7.6% have an h-index within a
range of 0 to 132. Figure 13 shows that 83.2% of these papers have an h-index between 0
and 10. The number of publications with an h-index of 1 (12,703 papers) represent 24.3%.
Figure 14 shows the number of publications in different languages. The most used
language is English (93.80%). Although the country with the greatest number of researchers
in this area is China, the international scientific journals prefer to use English. China is
one of the few countries that publishes in its own language, at 4.74%, far more than the
papers in German (0.37%) or Japanese (0.36%). The remaining languages make up 1.46%
(see detail in Figure 14).
Figure 14. Representation of the languages of EV publications.
Table 1shows a list of the 20 authors with the highest h-index. The first two authors are
from the USA. The country with the largest number of researchers in this subject is China,
although the first Chinese author is in the fifth position. There are no German or Japanese
authors among the first 20 authors, despite these countries’ positions in Figure 12. The top
German author is in the 25th position, and the top Japanese author is in the 51st position.
The first European authors are Blaabjerg, F., Mannucci, P., and Kuss, M. Italy’s contribution
is 3.1% with 3251 documents, surpassed in Europe by Germany with 5881 documents (5.6%)
and the United Kingdom with 4577 (4.4%). Most of the researchers started publishing in
this field in 2005. The year of highest production of documents with the keyword “electric
vehicle” in Scopus is 2021 with 13,800 documents, and by September 2022 there were
9739 published
documents. There has been an incremental evolution from 1969, which
exceeded 100 documents, to the present day. An important milestone occurred in 2018,

Vehicles 2022,41265
when more than 10,000 documents were published, which is expected to be maintained
until many of the pending challenges in this subject are resolved. Until this happens, the
number of publications will not decrease.
Table 1. Authors with h-index over 100 in EV research.
Indexed
Name
H-
Index
Citation
Count
Document
Count Country University
First
Publication
(Year)
Gogotsi, Y. 180 160,412 936 United States Drexel University 2005
Dai, L. 148 86,083 662 United States Case Western Reserve University 2006
Blaabjerg, F. 148 113,037 2912 Denmark Aalborg Universitet 2005
Beck, H. 139 100,327 1460 Switzerland University of Bern 2007
Liu, J. 138 80,785 496 China Beijing Forestry University 2014
Amine, K. 136 62,151 680 United States Stanford University 2016
Chapín, F. 135 100,129 432 United States University of Alaska Fairbanks 2005
Chen, J. 134 63,266 583 China Nankai University 2005
Aurbach, D. 131 71,805 738 Israel Bar-Ilan University 2010
Poor, H. 130 78,631 2150 United States Princeton University 2013
Liu, H. 129 64,488 1206 Australia University of Wollongong 2005
Dou S. 128 75,307 1875 Australia University of Wollongong 2014
Sun, Y. 127 64,493 702 South Korea Hanyang University 2013
Liu, M. 126 54,155 732 United States Georgia Institute of Technology 2005
Gao, H. 123 46,696 719 China Harbin Institute of Technology 2005
Gao, F. 123 46,696 719 China Nanjing Agricultural University 2009
Kuss, M. 116 46,395 337 Italy Istituto Nazionale di Fisica Nucleare, Sezione di Pisa 2007
Giannakis, G. 114 52,728 1153 United States University of Minnesota Twin Cities 2010
Cho, J. 114 48,489 389 South Korea Ulsan National Institute of Science and Technology 2005
Wong, C. 114 52,241 1570 United States Georgia Institute of Technology 2005
3.3. Future Perspectives and Challenges
In this paper, the key publications that have led to advances in electric vehicles have
been analyzed. The bibliometric analysis of the EV has made it possible to visualize the
main papers and authors that have marked the breakthrough in the papers published in the
Scopus bibliographic database. Using this analysis, the research gaps and challenges can be
predicted. Furthermore, the prospects of EVs can be foreseen. In the last decade, political
and social awareness have been the main drivers for electric transport. Some issues, mainly
related to efficiency, have been solved, and sustainability, which was initially the main
driver, has been put into context. However, the market deployment of the electric vehicle
as the main mode of transport requires some crucial technical issues to be resolved. The
following are the main challenges extracted from the study communities.
The Battery Chargers community addresses challenges related to charging techniques
and charging modes of operation, which are also addressed by the vehicle-to-grid com-
munity from the perspective of their influence on the grid. These challenges include the
following:
•
Optimized charging techniques are required to balance charging time and battery life
and also to incorporate additional protection to balance battery temperature during
the charging process in order to avoid battery degradation [
58
]. Battery heating is a
serious problem in the case of external charging, as external charging to increase the
efficiency of charging stations mainly depends on the selection of power converter
topologies [119].
•
The latest generation of EVs have the vehicle-to-everything (V2X) mode of operation.
Extensive research in the domain of power density, power level, converter topologies,
and control techniques related to the V2X system is required to expand its commercial-
ization. The implementation of the V2X system has an important role to play in future
EVs [60].
•
Among the technical challenges of future EVs is the coordination between different
emerging charging technologies such as V2X, V2G, and VG4 [60].

Vehicles 2022,41266
•
The modes of operation between G2V and V2G must solve the following challenges:
transformer ageing, battery degradation and energy loss, harmonic distortion, voltage
profile deterioration, and charging curve variation [119].
•
Successful communication techniques are required, in which a communication link
is created between charging and EV systems. Communication vulnerability (cyber-
attack) and communication delay are among their challenges. In addition, it is recom-
mended to integrate various vehicular communication technologies such as wireless
access to meet the communication needs of various use cases [120].
•
The challenges facing the fast charging station are to achieve good overall efficiency,
reduced harmonics, low capital operating cost, and an efficient control algorithm to
control the charging current [58].
•
The challenges of the wireless charging station to be solved optimally are the design of
the coils, the selection of a suitable compensation network, and the ability to transfer
high power over a long distance [58]. Standardized wireless charging systems across
different types of charging infrastructure and different classes of electric vehicles also
require technological improvements [59].
•
A global standard for chargers and connectors is required to make energy transfer
more efficient and to standardize the associated systems. Currently there are standards
depending on the country and vehicle model; if we want to make progress with EVs
we must try to homogenize the criteria for selecting associated standards. Vehicle
manufacturers must also agree to use a charging connector standard, although new
EVs usually come with dual-connector models depending on the charging mode of
operation. The standardization of charging systems and their connectors is a gap that
remains to be solved [58].
•
Charging times are long, from 3 to 12 h, although 80% can be charged in 30 min when
using a fast charger. Public fast chargers are still rare in many cities due to their high
investment cost. By having fast charging stations along the roadside, fast charging
could play an important role in expanding the range of electric vehicles [121].
The EV Market Study community studies market opportunities and consumer opin-
ions to find thriving EV-related niche markets. EV-associated markets and their challenges
include the following:
•
The incorporation of autonomous driving technologies (ADT) in EVs is stimulating
for the vehicle sharing industry and EV car sharing. Remaining challenges include
planning the size of a fleet, vehicle relocation strategies such as mixed relocation
strategies based on operators and users, vehicle route optimization, and government
management policies to increase user demand such as parking fees and subsidy
strategies.
•
Research should be done to consider the spatial and temporal distribution of demand
and the influence of dynamic demand-responsive pricing schemes for car sharing
including EVs. In addition, subsidies may be the key to EV utilization for passengers
with a car sharing platform, such as Uber. How to design subsidy mechanisms to
promote EV sharing in a competitive environment, incorporating uncertainties in
last-minute bookings, charging levels, driver choice behaviors, and energy prices in
the models, are issues that need to be resolved. This topic raises many issues for future
research [122].
•
Regarding batteries and new charging technology, a battery exchange or leasing market
has emerged. The battery leasing model may be more successful than the battery swap
model during the early stages of EV adoption because the initial capital costs (land,
building a facility, and maintaining a battery inventory) are much higher than the cost
of installing a charging station [
122
]. The study of productive leasing models is based
on a standardization of batteries that would limit battery stocking.
•
Charging infrastructure can be a productive market, but there are investment and
planning issues for charging infrastructure that need to be addressed in the face of
the growing number of electric vehicles on the market [
123
], mainly due to the lack of

Vehicles 2022,41267
government regulations and subsidies to support these infrastructures. In addition,
this business requires standardization of the infrastructures and optimal planning of
their location.
•Many of the potential markets still require profitable short-term business models.
•
The social and market acceptability of a different technology than the conventional
one is an issue that needs to be addressed. Increased acceptance of EV technology
would enable mass production and could make the technology more economically
viable for the consumer [58].
The EV Battery Management community shows that most batteries used in com-
mercial EVs are lithium-ion, although low charging temperatures must be considered, as
they significantly influence the ability of Li-ion batteries to self-discharge. Research has
evaluated other alternatives such as sodium-based (Na-NiCl and Na-S), lead–zinc, and
Ni-MH batteries, but Li-ion has the highest capacity. Lithium-ion batteries are considered
suitable for electric vehicles, although they present some problems due to their complex
electrochemical reactions, performance degradation, and lack of accuracy in improving
battery performance and lifespan [
82
], as well as the drawback of using toxic and expensive
materials such as cobalt or nickel and rare earths, which makes them environmentally
unfriendly. Rare earths are mainly concentrated in China, which can act geopolitically on
their evolution and control the manufacture of batteries. In addition, their extraction poses
risks to the ecosystem, human health, and the destruction of wildlife [
124
]. Rechargeable
zinc batteries are seen as a promising technology for the next generation, once it over-
comes insufficient energy density [
59
,
60
]. Another alternative to avoid premature battery
ageing—since high-variation EV current intensifies battery ageing—is the use of a hybrid
high-energy-density storage system such as batteries with ultracapacitors (UC), super-
conducting magnetic energy storage (SMES), and flywheels. This requires optimization
strategies to manage EV energy [
7
]. While each of the hybrid systems theoretically have
utility, none have been brought to commercialization because large-scale EV deployment
is complex and time-consuming and the additional cost and complexity imposed on the
vehicle is relatively greater than the economic savings due to improved battery cycle life
and reduced energy losses. Mass production of batteries will have to take into consideration
the following challenges:
•
Research on new batteries that have higher capacity, higher energy density, better
safety, more efficient battery management, longer life cycles, and that are environmen-
tally friendly [60].
•
Higher capacity batteries will encourage the adoption of faster and more powerful
charging methods, as well as improved wireless charging technology.
•
The energy management system needs improvements to decrease costs and increase
the life cycle of batteries; the trend in recent research is hybrid energy systems, but
their commercialization requires robustness, low computational complexity, real-time
control, accuracy, and overall optimization of the energy management system.
•
Studies initially used life cycle assessment (LCA) as a method of assessing the environ-
mental impacts of emerging technologies such as EVs, but it is insufficient to consider
the economic and social impacts. Few studies assess socio-economic indicators at
the macro level, except for life cycle cost analysis. Many studies link CO2 emission
reduction as a precursor to driving EV expansion, but secondary effects, macroeco-
nomic impacts, and impacts related to the global supply chain need to be considered
as a comprehensive approach to help decision making in the event of conflicts in
technology deployment [124].
•
Another remaining challenge is the recycling of batteries, which, as noted, have toxic
materials. If batteries are not carefully designed with end-of-life management in mind,
dependence will simply shift from one non-renewable source (oil) to others (rare
earth metals), which is an important issue for further study for the world’s green
revolution [124].

Vehicles 2022,41268
The Energy-Efficient Transmission in EVs community studies the electric motor, to-
gether with power electronics and batteries, which are essential parts of the powertrain. The
automotive industry has developed different alternatives such as induction, permanent-
magnet, and wound-rotor motors, according to diverse concerns in motor design. Each
alternative has advantages and disadvantages. For example, permanent-magnet motors
are based on rare earths, which are difficult to obtain and geographically limited, mainly to
China, making them limited and dependent on availability. The efficiency of the DC–DC
converter will have a direct impact on the efficiency balance. Depending on whether
the power flow is bidirectional or unidirectional, converters vary [
121
]. The remaining
challenges that can be found include the following:
•
One remaining challenge is the coupling of the motor and battery for driving conditions
and performance requirements (cost, efficiency, driving dynamics, and
driving comfort
).
•
The selection of a power coupling architecture, together with the optimization of both
the appropriate component size according to the architecture employed and the control
strategy, will be the subject of future research. Although there are many examples
of energy-efficient control strategies in the literature, they should be investigated
to achieve dynamic coordinated control of the mode switching process, as it has a
significant impact on vehicle handling and ride comfort [125].
•
Efficiency improvement of the permanent-magnet synchronous motors (PMSM).
Among the losses in this class of motors are copper losses, iron losses, friction losses,
and dispersion losses. Iron losses have not been considered in previous works; how-
ever, several studies have found iron loss to be an important component of the total
losses [
126
]. Therefore, ignoring iron losses will overestimate motor efficiency. Pei
et al. (2022) point that copper losses and iron losses are greatly dependent on control
strategies [
127
], and in the near future the PMSM efficiency optimization strategy with
time-varying parameters should be studied.
•
Increase the power density of the motor. This can be achieved through three ap-
proaches: increasing the speed of the motor; the use of new materials in the magnetic
circuit, winding insulation, etc.; or the application of new technologies to the motor
production [128].
•
Direct torque control (DTC) has been used traditionally, but it results in large torque
fluctuation. To solve the torque ripple problem, efforts are dedicated in the literature
to overcome these issues and various improved methods are being proposed. One of
them is to calculate the effective voltage vector action time in real time to guarantee
the minimum torque ripple for current torque error [
129
]. Nasr et al. (2022) proposed
a DTC strategy based on an effective duty ratio regulation to improve the torque
performance in terms of the steady-state error and the ripple [130].
•
In general, manufacturers are further converging on permanent-magnet motor designs
for their superior efficiency and power density, but the sustainability of the permanent
magnets depends on the recovery and recycling methods for these magnets in the
automotive error [
131
]. Nasr et al. (2022) proposed a DTC strategy based on an
effective duty-ratio regulation to improve the torque performance in terms of the
steady-state error and the ripple [132].
The Vehicle-to-Grid Power community raises outstanding challenges of implementing
a standardized model that solves the interactions of charging operations with the grid to
improve grid stability. Electric vehicles not only act as transport for people and goods,
but they also have communication to interact with other electric vehicles and all smart
devices through IoT applications, which makes them very valuable for other applications
such as auxiliary backup power to the grid. There are interesting proposals addressing this
issue with different methodologies such as the EV charging scheduling and the charging
infrastructure planning for car sharing systems [
122
]. However, there is still no state-of-
the-art methodology covering different levels of decision making for EV systems. More
research is needed on smart dynamic fleet charging/discharging strategies, including
V2G and V4G technology, to improve grid stability, as well as the implementation of V2X

Vehicles 2022,41269
technologies in future EVs. These EV operating modes can earn more revenue by applying
smart charging/discharging strategies during peak and off-peak hours while serving as
backup storage power for grid fluctuations, especially for sensitive supplies such as the
military, health, and databases, etc. In addition to these outstanding issues, this community
needs to solve the following challenges:
•
Interactions of EV charging operations with the grid must be considered to improve
grid stability. In addition, a rigorous assessment of the environmental and economic
impacts of large-scale charging infrastructure could help the development of the
dynamic wireless power transfer (DWPT) [60].
•
Charging infrastructure optimized according to an assumable forecast of the EV fleet
and the distribution grid. Different studies have been conducted using AI-based
algorithms, but decisions still need to be made not only on EV charging needs and the
grid, but also considering the habits of EV users.
•
The EV Market Study Community has identified several niche markets among which
the optimal distribution of battery swapping stations (BSS), as well as the charging
infrastructure, must consider the habits of EV users. Battery swapping is an efficient
charging alternative and BSS can serve not only for battery swapping but also as an
auxiliary backup supply for the distribution network.
•
V2G technology has an outstanding challenge such as cyber security for smooth
operation and to ensure network security. Network security and integrity for secure
and seamless data transfer from electric vehicles to the grid. Another drawback is
battery degradation. Although research is being done on methods to solve this such
as battery swapping, which requires standardization of batteries and infrastructure
for swap management [133].
•
Regulatory policies on energy market prices, so that owners can consider the EV invest-
ment and its profitability by using the sale of their energy surplus to the distribution
grid or planning loads in off-peak hours of the distribution grid.
•
Research focused on the integration of electric vehicles (EVs) powered by renewable
energy sources is currently a viable option to combat climate change and advance the
energy transition [104,121].
The HEV Control Strategies community argues that for HEVs and PHEVs to be
competitive with conventional vehicles, costs must be reduced, efficiency must be improved,
and electric driving range must be increased [85].
Improving the control strategies that HEVs and PHEVs, currently different strategies
have been exposed in this community, but there is still basis for improvement where they
consider real time conditions in the efficient driving of the vehicle by operating in the
optimal region of operation most of the time and reduce fuel consumption and emissions.
Although reducing emissions and optimizing efficiency are opposing parameters, a balance
between the two should be pursued in future HEVs and PHEVs [118].
4. Conclusions
A search has been carried out on EV research from 1955 to 2021. We worked with
50,195 documents; 104,344 authors in 149 countries are researching on EVs and English is
the preferred language for scientific publication. They were classified into six communities
according to the relationships between the authors. The most representative papers of
these communities were analyzed according to their influence within and outside the
community. The studied publications fall into the following categories: “HEV Control
Strategies”, “Vehicle to Grid Power”, “Energy Efficient Transmission in EV”, “EV Battery
Management”, “EV Market study”, and “EV Battery Chargers”.
The analysis of the h-index of the authors shows that they do not fully correspond to
the distribution of authors by country. China is the country with the highest number of
authors, but the first Chinese researcher on the list is in the fifth position. China is the world
leader in rare earths, materials that are essential for the manufacture of electric motors and
batteries needed for EV development. This implies the need to search for other materials

Vehicles 2022,41270
to meet this need and avoid China’s geopolitical control over EVs. There are more than
20 researchers
with an h-index over 100, which indicates that it is a very productive topic
with great advances.
Competitions have proven to be a good incentive for future vehicle designers, intro-
ducing equipment improvements in EVs’ technological innovation. The major innovation
should focus on reducing costs and improving efficiency. Some studies from the “EV
Market Study” community highlight an understanding of potential consumers of EVs
in general. They highlight that the difficulty in switching from conventional vehicles to
EVs lies in the fact that the EV improvement is still in progress. This perception is an
issue to be addressed by manufacturers and politicians if they want to change consumers’
mindset towards zero-emission products. Some manufacturers already introduce their EVs
as more efficient vehicles than conventional ones and not as a greener alternative. They
pay attention to market studies which show that new generations see technology and fuel
savings as more attractive than ecological improvements, even assuming higher costs. In
addition to innovative technology, EV manufacturers must consider market demand and
the infrastructure and services required. Consumers should be informed that the cost of an
EV is high compared to a combustion engine vehicle, but EVs consume less fuel and have
low maintenance costs compared to combustion vehicles. In addition, electric motors have
higher efficiency. The average efficiency of combustion vehicles is between 15% and 18%,
while the efficiency of electric vehicles is in the range of 60% to 70%.
EV manufacturing should also be studied. The main driver for EVs is the decarboniza-
tion of the transport industry, but all the factors involved, from manufacturing to powering
the batteries, must be taken into consideration. Different types of energy used to generate
electricity to power electric vehicles will cause different emissions. Any study assessing
vehicle emissions must therefore consider the simultaneous impact of these variables to
arrive at a realistic estimate of vehicle emissions. Within these factors, an outstanding
challenge is the recycling of the materials involved in EVs, mainly batteries. Considering
the recent crises in the automotive industry related to the shortage of semiconductor chips,
automotive spare parts due to rising raw material prices and the shortage of lithium used
for EV batteries, it will be crucial to consider reuse, recycling, and remanufacturing in
end-of-life management. However, research into other, more environmentally friendly
materials for EV battery manufacturing is still ongoing. There is a need to analyze the
entire life cycle of EVs or the global warming beyond their emissions. There is also a need
to provide incentives to manufacturers and governments for effective recycling policies.
Another issue to consider is EVs’ charges and their relationship with renewable energy
systems. The emissions must be reduced, both in transport and in the charging process
of EVs.
Implementing a robust system for monitoring and managing the state of the available
charge is a necessary step in research on the efficient use of high-capacity batteries. There is
still much research to be done to improve battery safety using materials that allow high
energy storage capacity and longer lifespan to reduce pollution and battery replacement
(the toxic components of the battery) and lower EVs’ costs, which is one of the future goals
to make them commercially competitive.
Among the challenges for their full implementation in the automotive sector is the
management of the charging infrastructure and its effects on the distribution network; there
are already systems that allow fast charges of 20–30 min, although this has detrimental
effects on the distribution system that would be solved with a planning of the charges.
There are many proposals that implement optimization algorithms to locate charging
stations from the perspective of the distribution network or the charging station owner.
Each algorithm selects different parameters, the most common being energy loss and the
cost of energy from the network, associated with other parameters such as the use of
renewable energy or network efficiency parameters such as maximum voltage deviation,
etc., in the case seen from its optimization in the distribution network. Other authors
consider the perspective of the EV user, considering the cost of access, the cost of travel to

Vehicles 2022,41271
charge from the point of demand to the charging station, the cost of the waiting time, and
the cost of charging times, among other parameters. Other research focuses on the reliability
of the distribution network. The commercialization of effective V2G, V4G, and V2X services
is an interesting proposition. EVs can become part of the electric grid, although accurate
studies are still needed to find an effective method for estimating expected revenues and
costs with arbitrary charges and within a specific time frame. In addition to overcoming
economic challenges, it must overcome the social barrier, grid security, and battery life
degradation. One possible solution is battery swapping, although it requires pay-as-you-go
and a third party who owns the batteries while managing their charging conditions. This
can be a social barrier, where EV users do not have a guarantee on the health status of
the battery being swapped, plus a battery swap management infrastructure and battery
standardization are required.
Having a low-cost, highly efficient, and flexible EV charging and discharging system
is an ongoing research topic. For HEVs and PHEVs to be competitive with conventional ve-
hicles, costs must be reduced, efficiency improved, and the electric driving range increased.
The global fight for sustainability in the automotive sector requires developing regions
to get involved.
The conclusion of this work shows that research on this topic is still ongoing, and
there are still many issues to be solved, although the regular use of EVs would lead to the
establishment of sustainable cities and societies. Promoting energy policies is only possible
if countries are engaged through real environmental policies.
Author Contributions:
N.N. and R.M.G.S.: writing—original draft and conceptualization; F.P.
and I.R.: writing—editing; A.A.: software, validation, and conceptualization; M.F.-R. and J.A.G.:
preparation, editing and revision. All authors have read and agreed to the published version of
the manuscript.
Funding:
The Telemedicine TIC019 Research Group of the University of Almeria, Spain and in part by
the European Union FEDER Program and CIAMBITAL Group by I + D + I Project UAL18-TIC-A025-A,
the University of Almeria, and the European Regional Development Fund (FEDER).
Data Availability Statement:
The data have been obtained by members of the Telemedicine Research
Group TIC019 of the University of Almeria, Spain.
Conflicts of Interest: The authors declare no conflict of interest.
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