Wireless Power Transfer with Data Transfer Capability for Electric
and Hybrid Vehicles: State of the Art and Future Trends
Sami Barmada, Nunzia Fontana and Mauro Tucci
Department of Energy and System Engineering (DESTEC), University of Pisa, Largo Lucio Lazzarino 2, Pisa, Italy
Keywords: Powerline Communications, Wireless Power Transfer, Electric and Hybrid Vehicles, Vehicle to Grid Data
Exchange.
Abstract: This papers shows how Powerline Communication and Wireless Power Transfer technologies can be used
together to allow both power and data transfer when hybrid and electric vehicles are connected to the grid.
These two technologies have lately become popular when dealing with the Smart Grid environment (the
former) and charging of electric powered devices (the latter). The authors have dedicated their research on
the integration between them, keeping in mind their use in the automotive environment; this papers serves as
a review and a starting point for future work in the area, offering a synthetic description of the operating
principles and some results. In addition, shielding techniques for Wireless Power Transfer systems are shown
and compared with each other, in order to show different aspects of this fundamental topic.
1 INTRODUCTION
Hybrid vehicles are nowadays being considered as a
real alternative to internal combustion powered
vehicles by a great percentage of people who are in
the process of evaluating a new car. This big change
in the last few year has been driven by ideological and
political issues; in addition, most of the car makers
have now hybrid vehicles in their production. Only
considering economy of operation from the user point
of view, the main turning point is the use of full
electric vehicles or plug in vehicles, that obviously
significantly reduces the fuel consumption.
Envisioning a constant increase of such vehicles,
technologies that will allow easy data transfer from
the vehicle to the grid (and vice versa) will become of
common use; for instance, such data transmission
could be in the area of infotainment, navigation
information and statistics, diagnostics etc. Among the
different technologies widely diffused allowing data
transfer, Powerline Communication (PLC) has gained
new life together with the birth of the so-called smart
grid environment.
As for the battery charging, so far, to the authors’
knowledge, all the commercially available electric or
plug-in vehicles are equipped with a conductive
charging equipment (either AC or DC), but a great
quantity of studies and investments are directed
towards the use of Wireless Power Transfer (WPT)
technology, that will change the way we charge
vehicles in the near future. This change is already
ongoing as for consumer electronics products, since
commercial products are already available
(toothbrushes and mobile phone wireless chargers are
of common use).
The present contribution works as a review paper,
in which the basic concepts, applicatons and future
trends are shown both for PLC and WPT, keeping in
mind their use in the automotive environment. With
this goal in mind, the authors have developed a hybrid
PLC-WPT system, plus a new shielding technique
suitable to reduce exposure to the electric field and
the reuslts relative to these activities are shown here.
Section 2 is dedicated to the review of the
Powerline communication technology and show how
an implementation onboard an electric vehicle has
been tested. Section 3 deals about the Wireless Power
Transfer technology, and, in particular, describes a
system that enables both power transfer and data
transfer (with the PLC technology in mind). Section
4 shows the importance of shielding techniques, and
also describes a new concept relative to the use of
metamaterials in order to reduce exposure to the
electric field.
662
Barmada, S., Fontana, N. and Tucci, M.
Wireless Power Transfer with Data Transfer Capability for Electric and Hybrid Vehicles: State of the Art and Future Trends.
DOI: 10.5220/0010484706620669
In Proceedings of the 7th International Conference on Vehicle Technology and Intelligent Transport Systems (VEHITS 2021), pages 662-669
ISBN: 978-989-758-513-5
Copyright
c
2021 by SCITEPRESS – Science and Technology Publications, Lda. All rights reserved
2 PLC FOR VEHICLES AND
SMART GRID APPLICATIONS
2.1 Technology Background
In recent years the use of Powerline Communication
technology has gained renewed interest as a low cost
way to allow integration of devices into a smart grid
environment. With nowadays commercially available
modems, data rates above 1Mb/s can be reached,
making PLC an attractive technology that can
compete with other communication technologies.
The recent development of the Home Plug Green
PHY protocol (specifically dedicated to EVs) shows
the future trend, and communication speeds of
hundreds of Mbits per seconds can be reached (Kim
et al., 2011; Son et al., 2010 and Joo et al., 2017).
In addition, when space and weight are a
constraint, PLC technology can be fundamental to
reduce cable harness onboard vehicles. Many studies
are available showing how integration of PLC
onboard different vehicles is a viable option and the
signals that can be transmitted vary from usual
control signals to Battery Management Systems
signals. See for instance (Barmada et al., 2008,
Lallbeeharry et al., 2018, Vincent et al., 2020, Sohn
et al., 2019, Landinger et al., 2020, Krasovsky et al.,
2020, IEEE Approved Draft Standard, 2018).
2.2 Measurements on a Specific Vehicle
In a previous work the authors (Amrani et al., 2013)
have measured the performances of Vehicle to Grid
(V2G) communication system between an electric
vehicle and the power grid. In particular, the vehicle
was plugged to the power grid during battery
charging, and the communication channel was
established between the cigarette lighter inside the
vehicle and the same socked used for charging. These
two access point were clearly not optimized, in order
to be conservative in the measurements results. The
system was also equipped with a low pass filter in
order to filter-out the mains. For this specific vehicle,
the ignition key can be in the “off” position; in
“position I” (battery is connected to a limited set of
auxiliary devices); in “position II” (all electrical
devices energized and the inverter is turned on).
Figure 1 shows the transfer function of the
measured channel with the ignition key in the
positions mentioned before while Figure 2 shows the
same measurement in the CENELEC narrowband. As
known, considering usual noise scenarios,
attenuations below 40db do not allow a reliable
communication, however the figures show that above
300 kHz communication is possible, also considering
that when V2G and G2V are considered, the vehicle
can reasonably be turned off. The measurements for
other in-vehicle channel can be seen in (Amrani et al.,
2013) and are not reported here for the sake of
conciseness.
Figure 1: Transfer function of G2V channel.
Figure 2: Transfer function relative to the CENELEC band
limits.
2.3 Future Trends
As an overall comment, in most cases the
communication signal has to “pass through” at least
one converter (the AC/DC dedicated to battery
charging) but often there is also the presence of a
second one dedicated to the voltage reduction from
the battery to the loads. From the communication
point of view these are a bottleneck and in the future,
in case V2G communication (not only for BMS) will
become a standard requirement, specific circuitry
could be designed with the aim of bypassing the
inverters in specific frequency bands.
Wireless Power Transfer with Data Transfer Capability for Electric and Hybrid Vehicles: State of the Art and Future Trends
663
3 WPT WITH DATA TRANSFER
CAPABILITY
3.1 Technology Background
Wireless Power Transfer (WPT) technology is
nowadays a “hot” research topic that is witnessing
investments both from public research centres and
industry. Applications in the area of portable
consumer electronic devices are already
commercially available, while high power
applications are in the course of prototyping.
Charging of electric or hybrid vehicles is one of the
most promising application of WPT technology,
since the car manufacturers are strongly investing in
the electrification of their production (Kurs et al.,
2007, Lee et al., 2011, Madawala et al., 2011, Ahn et
al., 2011, Hee Lee et al., 2020).
WPT for automotive applications can be
implemented in different formats, from the most
common two coils systems, to the multiple coil to
achieve longer transmission distance; arrays of coils
can be used when the position of the vehicle is
affected by uncertainty, or when the vehicle charging
should be considered as “dynamic”, i.e. when the
coils are places along a specific path the vehicle
should follow.
In all these cases the trade-off between a cabled
connection allowing a V2G communication (wireless
communication is not always available, and needs
additional equipment) and a wireless power charging
is evident. This reason has led the authors in the
recent past to propose a hybrid WPT – PLC system,
that allows power and data transfer, having in mind
the interaction with a power grid in which PLC
technology (low cost) is used to implement
communication in a smart grid environment.
3.2 Two Coils System
The two coils system is described in (Barmada et al.,
2015) and it can easily be represented by Figure 3.
The two coils (that can be realized using different
technologies) are the core of the system, and both
power and data are transmitted through the inductive
coupling. The bottom part of Figure 3 shows the usual
power link (between ports 1 and 2), composed by the
high frequency amplifier (on the Tx side) and the
rectifier (on the Rx side), assuming that the system is
used for battery charging. The capacitors C
a
are
responsible for the system’s tuning at the desired
frequency. The top part is responsible for the data
transfer (between ports 3 and 4), with particular
attention dedicated to a PLC link (working in
frequency bands having tens of MHz in the upper
limit).
The role of the high-pass filter and the capacitors
C
b
, C
c
is the one usually performed by the capacitive
coupler always present in PLC modems, that is to
inject the high frequency signal into a low frequency
power grid. In this particular system, these passive
components also play the role as frequency
decouplers between data and power, allowing the
desired signal only to flow through the proper branch;
in this way, correct and secure operations are
guaranteed. The system could be of course designed
to achieve bi-directional power and/or data transfer.
Figure 4 shows the channel capacity, evaluated
with the Shannon Hartley equation, for a system
prototype, described in (Barmada et al., 2017),
characterized by two spiral coils (of square shape)
whose longer dimension was 20cm. In particular, the
channel capacity at different distances has been
calculated, showing that (as it should be), shorter
distances lead to better performances.
Figure 3: WPT – PLC system outline.
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664
Figure 4: Channel capacity versus distance.
The Shannon Hartley equation allows the
calculation of the ideal channel capacity as follows,
(1)
where
is the power spectral density of the
received communication signal,
is the random
noise spectral density at the receiver, and
is the
narrowband interference power produced by the WPT
circuit. is the channel bandwidth,
is the
transmitted signal’s spectral density, and is
the communication channel’s transfer function. In
this case additive white Gaussian noise (AWGN) has
been considered for the sake of simplicity. The use of
eq. (1) allows a simple calculation of the theoretical
channel capacity, and, despite its simplicity, is often
use to calculate reasonable figures.
The graph shows that, at distances that are of the
same order of magnitude than the coils, reasonable
bandwidth for data communication can be achieved.
3.3 Multiple Coils Systems
The scheme shown in Figure 3 does not necessarily
refer to the simple two coils systems, and the same
concept can be applied to multiple coils systems
(typically three or four) in which the additional loops
(short circuited and physically placed between the
transmitter and the receiver) play the role of repeaters.
In these cases optimal performances can be achieved
at the resonant frequency, at the cost having a
bandwidth reduction; in addition, the additional coils
contribute to the increase of joule losses, an issue that
has to be taken into account as a trade-off between
efficiency and power transmission distance.
Figure 5 shows the equivalent circuit of a four
coils resonant system (valid for non radiative WPT),
in which the coils connected to the source (
) and to
the load (represented here by the resistance
) are
(usually) called drive and load loops, while the short
circuit repeaters are called Tx and Rx coils. Usual
modelling of such system consists in a reasonable
simplification, that is to consider only the coupling
with the nearest neighbour (coupling coefficients
,
, and
).
Figure 5: Lumped equivalent circuit of a four coils system.
Figure 6 shows a realization of the scheme shown in
Figure 3, i.e. the data transmitter and receiver devices
(i.e.
and
) are connected to the same ports with
respect to the two coils through generic filters
(whose role has been explained before).
Figure 6: WPT – PLC system characterized by power and
data having the same access point.
The possibility offered by having additional coils, is
summarized in Figure 7 and proposed in (Barmada et
al., 2019), in which an access point for data transfer
has been created on the Tx and Rx coils, originally
short circuited and simply used as repeaters. Here, the
data source and data receiver (i.e.
and
) are
coupled to the resonators by using parallel filters
that have the role of short circuiting the power signal
(that should not go through the transmission and
receiver devices).
Figure 7: WPT – PLC system system characterized by
power and data having different access points.
Wireless Power Transfer with Data Transfer Capability for Electric and Hybrid Vehicles: State of the Art and Future Trends
665
Figure 8: Prototype realization.
Figure 8 shows a prototype used to evaluate the
convenience of creating the double access point as
previously described. The details of the prototype
(and the design criteria) are explained in in (Barmada
et al., 2019); a set of measurements have been
performed, at a distance and
between the coils (case 1 and case 2), with different
filter realizations (named “a” and “b”). The results
shown here are relative to the longer distance and to
a resonant frequency for the power transfer set to
, and are summarized in Figures 9, 10 and
11, in which “outer coils” refer to the system reported
in Figure 6, while “inner coils” refer to the system
shown in Figure 7.
Figure 9 shows the transfer function of the power
channel, in which it is shown that, with a proper
design of the filters, the presence of the data channel
(and the relative equipment) does not alter the power
efficiency.
Figure 9: Power channel.
Figure 10 shows the transfer function of the data
channel: in this case (data transfer) the curve’s most
important characteristics to be taken into account is
the bandwidth, and for the designed system, it can be
understood from the figure that the data channel
implemented with access on the repeaters (inner
coils) has a better behavior than the data channel
created on the power coils (outer coils).
Figure 10: Data channel.
The qualitative analysis of Figure 10, is confirmed
by the channel capacities (evaluated through the
Shannon-Hartley equation) reported in figure 11.
The results shown here are relative to a specific
system, with its own resonant system and its own set
of filters. However, when a multiple coil system is
adopted in order to achieve longer transmission
distance, the theoretical and experimental results
show that an additional data channel with better
performances could be obtained.
Figure 11: Data channel capacity.
3.4 Future Trends
As a final comment, the system proposed by the
author shows that when using WPT systems to charge
the battery of an electric or hybrid vehicle, it is also
possible to create a data link without the addition
extra elements. Such data link can be easily coupled
with a pre-existing PLC system on a power grid, in
order to achieve both power and data transfer.
The possibility of using additional ports for
information exchange, made available by the
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666
presence of extra loops, can increase the
communication performances between the vehicle
and the grid.
The use of the data link should drive the design of
the system’s parameters; i.e. the transmission of
infotainment data might require more bandwidth than
diagnostics data etc.
4 SHIELDING ISSUES
When WPT systems are designed for high power
uses, and when their use can take place close to
human beings, shielding becomes an important issue.
Several papers have been published on the topic, and
some of them are specifically dedicated to vehicles
(Cruciani et al., 2019 and Campi et al., 2020).
With the main goal to reduce cable harness, some
authors are proposing WPT systems also to be used
inside vehicles, in order to energize additional
appliances that can be installed on the car (power seat
or seat heater, for instance) or belonging to the
passenger (Abul Masrur et al., 2019 and Seong-Ming
et al., 2019).
These possible applications are of course prone to
additional investigation related to shielding, since
they “take place” inside the vehicle, where the
driver/passenger and the WPT device are in close
proximity.
In general, a lot of attention has been lately
dedicated to the reduction of the magnetic field
outside the coils; the most common approach is to use
ferromagnetic slabs (properly designed in thickness
and shape) that also contribute to the concentration of
the magnetic field in between the coils.
Also active shields, implemented as coils driven
by controlled sources, have been proposed in order to
reduce exposure to WPT generated fields
The authors in (Brizi et al., 2020) focus their
attention on the exposure to electric field (in a
standard two coils system), that cannot be always
neglected in close proximity to the coils. In particular,
the authors propose a slab of metamaterial, that, based
on experimental results and a physical interpretation,
has the ability of reducing the electric field between
the coils, retaining (actually, increasing), the power
transfer efficiency).
The heuristic explanation behind effectiveness of
the metamaterial slab in reducing the electric field, is
the following: the electromagnetic field produced by
a resonator is basically influenced by its dimensions
(smaller coils are subject to smaller emf, hence
smaller currents and consequent fields). Thus, with
respect to the more common multiple coil systems, an
array of smaller resonators (to a larger extent, a
metamaterial) would lower the electric field
measured at the surface of the transmitting side.
As a rule of thumb, there is a trade-off between
the number of elements composing the array and the
system performances. In particular, increasing the
number of spirals decreases the electric field, but
increases ohmic losses on the resonators themselves
and lowers the resulting coupling between input and
output (leading to an energy efficiency reduction).
The last two trends suggest that by employing a
configuration with a high number of elements a
further reduction of the electric field strength can be
reached; however, the resulting mutual coupling and
the efficiency of the system drops significantly.
Simulation on the system implemented by (Brizi
et al., 2020), show that at a distance of 30cm (coil
diameter is 18cm), the energy efficiency of a simple
two coils system is lower than 10%. The inclusion of
the matrix of resonators helps in increasing this value
an reducing the electric field. In particular, with a 5x5
array, efficiency reaches the level of about 35% while
the electric field is significantly reduced.
Metamaterials slabs with higher number of
resonators lead to a further reduction of the electric
field, but to a lower energy efficiency. On the
contrary, lower number of resonators are
characterized by higher efficiency but also higher
electric field.
The selected metamaterial is an array of spiral
resonators, realized with Litz wires wounded on
plastic holders, and made resonant at the same
resonant frequency as the WPT system. The
experimental setup in (Brizi et al., 2020) showing a
two coils WPT system with the 5x5 spiral arrays
matrix is shown in Figure 12.
Figure 12: Wireless Power Transfer system with
metamaterial slab.
The experimental results showed a relevant
efficiency enhancement with the slab insertion and a
Wireless Power Transfer with Data Transfer Capability for Electric and Hybrid Vehicles: State of the Art and Future Trends
667
reduction of the electric field in between the coils of
more than 60% if compared to the regular system
without slab.
As a final comment to this section, it is important
to underline that shielding, both for increasing the
power transmission and to reduce exposure to
electromagnetic field is a fundamental issue and will
be one of the most important design topic in the near
future, when WPT systems will become commonly
available.
Different frequencies are subject to different
limits (see for instance the ICNIRP guidelines) and
require different approaches and different shielding
philosophies, ranging from ferromagnetic and
conductive shields to active coils and metamaterials.
5 CONCLUSIONS
The present contribution shows how the powerline
communication and wireless power transfer
technology can work together to achieve both data
and power transfer, with little modification to be done
on either systems (if considered as stand-alone).
Application to electric and hybrid vehicles is
straightforward, and will see in the near future strong
investments and research activity.
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