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Empirical Evaluation of V2G Round-trip Efficiency

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The business case of vehicle-to-grid (V2G) technology and its potential to provide grid services is heavily dependent on the round-trip efficiency of this technology. Surprisingly, very little empirical research is conducted to determine the V2G round-trip efficiency of electric vehicles currently available in the market, resulting in a wide range of efficiency values used in V2G modelling studies. This study aims to create more insight in the current V2G round-trip efficiency to stimulate that more uniform and realistic efficiency values are used in other studies. A field experiment is executed to measure the round-trip energy efficiency of V2G for different dates, current rates and average states of charge. It was found that the average round-trip efficiency (i.e., combined inverter and battery efficiency) when charging between a state of charge 25% and 75% with 3x16 Ampere was 87.0%. However, various external factors could influence the measured efficiencies, which had a total range from 79.1% to 87.8%. Charging at lower ambient temperatures and lower current rates had a statistically significant adverse effect on the round-trip efficiency. Efficiency at high and low state of charge was found to be marginally lower than around medium state of charges. Two different electric vehicle + charging station models were tested, one with on-board AC/DC converter, which is a novel V2G setup, and one with external AC/DC converter, rendering no statistically significant different efficiency values.
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Empirical Evaluation of V2G Round-trip Efficiency
Wouter Schram*1
Utrecht University
w.l.schram@uu.nl
Nico Brinkel1
Utrecht University
n.b.g.brinkel@uu.nl
Gilbert Smink
ElaadNL
gilbertsmink@gmail.com
Thijs van Wijk
ElaadNL
thijs.van.wijk@elaad.nl
Wilfried van Sark
Utrecht University
w.g.j.h.m.vansark@uu.nl
Abstract—The business case of vehicle-to-grid (V2G) technol-
ogy and its potential to provide grid services is heavily dependent
on the round-trip efficiency of this technology. Surprisingly,
very little empirical research is conducted to determine the
V2G round-trip efficiency of electric vehicles currently available
in the market, resulting in a wide range of efficiency values
used in V2G modelling studies. This study aims to create more
insight in the current V2G round-trip efficiency to stimulate that
more uniform and realistic efficiency values are used in other
studies. A field experiment is executed to measure the round-trip
energy efficiency of V2G for different dates, current rates and
average states of charge. It was found that the average round-
trip efficiency (i.e., combined inverter and battery efficiency)
when charging between a state of charge 25% and 75% with
3x16 Ampere was 87.0%. However, various external factors could
influence the measured efficiencies, which had a total range from
79.1% to 87.8%. Charging at lower ambient temperatures and
lower current rates had a statistically significant adverse effect
on the round-trip efficiency. Efficiency at high and low state of
charge was found to be marginally lower than around medium
state of charges. Two different electric vehicle + charging station
models were tested, one with on-board AC/DC converter, which
is a novel V2G setup, and one with external AC/DC converter,
rendering no statistically significant different efficiency values.
Index Terms—V2G, V2G efficiency, EV charging efficiency,
electric vehicles, field experiment
I. INTRODUCTION
Vehicle-to-grid (V2G) technology is gaining prominence
with increasing adoption of electric vehicles (EVs). The infeed
of electricity from EV battery systems to the grid through V2G
expands the opportunities of grid operators to stabilize the
grid using EVs. It has been demonstrated that power quality
and congestion problems in low-voltage grids can be mitigated
using V2G [1], while V2G also can provide financial benefits
as well as environmental benefits [2].
In recent years, multiple car models suitable for V2G have
been introduced to the market (e.g., Nissan LEAF, Mitsubishi
Outlander), while charging stations compatible for bidirec-
tional charging are now also introduced to the streets [3]. The
round-trip efficiency of V2G charging cycles is crucial for
future adoption of V2G as it directly affects the business case
and environmental impact of batteries in general, and of V2G
specifically [4], [5].
This study was supported by the EU’s ERDF in the project ‘Smart Solar
Charging regio Utrecht’ and by the Dutch RVO in the project ‘Slim laden
met flexibele nettarieven (FLEET)’. The authors want to thank ElaadNL for
allowing their EV testing lab to be used for this study and want to thank
Bram van Eijsden for his contributions throughout the research process.
1Both authors contributed equally to this work.
*Corresponding author
An overview of the used V2G round-trip efficiencies in an
non-exhaustive list of model studies in Table I indicates that
the ambiguity about the efficiency is high, as V2G round-trip
efficiencies range from 55% to 100%. Surprisingly, the number
of empirical evaluations of the V2G round-trip efficiency is
low. Refs. [4], [6]–[8] arrived at round-trip efficiency values
of between 53-70%, which is very low compared to the
efficiency of stationary lithium-ion based battery systems.
A single measurement was carried out in [9], arriving at a
round-trip efficiency of 87%. However, this study did not
consider the effect of e.g. State-of-Charge (SoC), current
and temperature. A laboratory experiment in [10] arrived at
a round-trip efficiency of 77%, but did not use an actual
EV in determining this value. Refs. [11] and [12] arrived at
efficiencies of around 80%, but only considered the efficiency
of the charging station. The inconclusiveness on the value of
the V2G round-trip efficiency among researchers is highlighted
by the discussions in the following comment papers [4], [7].
Given the high importance of the V2G round-trip efficiency for
future research, the current research presents a re-evaluation
of the V2G round-trip efficiency in a field experiment setting,
which resembles the natural environment of V2G. The V2G
round-trip efficiency is evaluated for different dates, SoCs,
charging currents and EV charging systems. Thereby this study
includes a proof-of-concept of performing V2G using an on-
board AC/DC converter.
The paper is outlined as follows: Section II presents the
setup of the field experiment and describes the methods used
to determine the round-trip efficiency. The results of the field
experiments are presented in Section III. This is placed in a
wider context in the Discussion in Section IV. Concluding
remarks are presented in Section V.
TABLE I
NON -EXH AUS TIV E OVE RVIE W OF US ED V2G RO UN D-
TR IP EFFI CIE NC IES I N LI TER ATUR E.
Efficiency EV charging models
55% [13]
72% [14]
73% [15], [16]
77% [17]
81% [1], [18]–[21]
84% [22]
86% [23]
87% [24]
94% [25]
100% [26], [27]
978-1-7281-4701-7/20/$31.00 ©2020 IEEE
II. ME TH OD S
This study used two experimental setups considering two
types of charging systems, as depicted in Fig. 1a and Fig.
1b. The first experimental setup considered a charging system
with the AC/DC inverter inside the charging station. A Nis-
san LEAF (MY2018) was charged and discharged using the
eNovates DC V2G 10 kW charging station. The second ex-
perimental setup considered a charging system with a AC/DC
inverter on board of the EV, using a V2G prototype of the
Renault ZOE and a WeDriveSolar v1.1. charging station with
a Last Miles Solutions controller. This second setup is a novel
approach, which means that this research also serves as a
proof-of-concept of V2G technology using the AC/DC inverter
on board of the EV.
The EV batteries in both systems reported the SoC of the EV
on a two second basis. Multiple charging/discharging cycles
were performed, which were based on the communicated SoC
of the EV battery. In a charging/discharging cycle, an EV
starts charging from the predetermined starting SoC until the
predetermined final SoC is reached, after which it discharges
until the starting SoC is reached again. Discharging is enforced
by sending a computer signal to the Open Charge Point
Interface Protocol (OCPI) protocol of the charging station
to change the direction of the current. Fig. 2 illustrates the
charging power and the SoC during one charging/discharging
cycle. In both charging systems, power flows between the
charging station and the AC grid were measured on a two
second basis, at the measuring point depicted in Fig. 1a and
Fig. 1b. The efficiency of one charging/discharging cycle was
determined by taking the ratio between the energy exported
from the charging station Eout and the energy fed into the
charging station Ein in one charging/discharging cycle, as
outlined in eq. (1). Hence, the losses consist of all conversion
losses in the charging station and in the EV battery in a
full charging/discharging cycle. Ein and Eout are determined
considering the charging power over time (Pt), the duration
of one timestep (t), the starting moment of charging at the
starting SoC (tSoCmin,start), the moment the final SoC is reached
(tSoCmax) and the moment the starting SoC is reached again
(tSoCmin,end).
η=Eout
Ein
=
PtSoCmin,end
tSoCmax Ptt
PtSoCmax
tSoCmin,start Ptt(1)
Note that the discharging power, in the numerator of (1), is
negative by convention - as also visible in Fig. 2. Therefore,
the minus sign is added to obtain a positive value for the
efficiency.
III. RES ULTS
Table II provides an overview of all performed tests and
the average efficiency per experimental set-up. The measured
efficiencies ranged from 79.1% to 87.8%. Highest efficiencies
were found for the Nissan LEAF when charging and discharg-
ing at maximum current, namely 87.0%. However, various
external factors were found to have an impact on the efficiency,
which will be discussed in the next sections.
A. Impact of Date on Efficiency
Fig. 3 illustrates a comparison between the test results of
two tests: one performed on 23 April 2019 and one performed
on 28 November 2019. Average efficiency of the former was
87.0%, whereas efficiency of the latter was 85.6%. Despite
the small sample size (three and four cycles, respectively) this
difference was statistically significant (two-sample t-test; p <
0.05).
Results could be explained by the difference in temperature
between these two days; the average ambient temperature
of the test performed on 23 April was 15.3°C, while the
average of the tests performed on 28 November was 5.5°C.
The decreased performance of EVs on cold days is a well-
known factor in EV user experiences and EV modelling [28].
It is also in line with laboratory research performed on lithium-
ion battery charging and discharging, which found higher heat
generation (which indicates conversion losses) in the battery at
low ambient temperatures than at high ambient temperatures
[29].
B. Impact State of Charge on Efficiency
Fig. 4 illustrates the efficiencies of tests performed around
an SoC of 15%, 50% and 85%. Results indicate that charging
efficiency is higher for medium SoCs than for SoCs either on
the low or high extremes (84.6% versus 83.7% and 83.0%,
respectively). For low SoCs, this is in line with previous
research on lithium-ion batteries; it was found that batteries
exhibit higher internal resistance and heat generation at low
SoCs for both charging and discharging [29].
However, differences are relatively small and statistically
insignificant. This indicates that V2G can also be performed
for low and high SoCs of the EV without considerably
compromising the efficiency.
C. Impact Current on Efficiency
Fig. 5 illustrates the various efficiencies on full load and
partial load. Partial load (3x8A) significantly reduces the
round-trip efficiency of V2G (two-sample t-test, p <0.01). De-
creasing the current to 3x4A further decreases the efficiency.
These results are not in line with [29], who found that heat
generation in batteries (which indicate losses) increase with
increasing current. However, the C-rates (i.e. power-to-energy
ratio) in that study were between 1 and 4, whereas C-rates
were below 1 in our study, which makes results incomparable.
Our results are in line with [6], who explain this finding by
noting that EV chargers must be designed for a large range
of charging conditions, but cannot be optimized for all these
conditions.
D. Impact EV and Charging Station Type on Efficiency
Fig. 6 compares the efficiencies of the two tested charg-
ing systems. Results indicate that similar efficiencies can be
obtained with different V2G configurations. As the AC/DC
Fig. 1. a) Experimental setup of V2G measuring system with AC/DC inverter in the charging station, using a Nissan LEAF. Image of Nissan LEAF is taken
at testing lab of ElaadNL. b) Experimental setup of V2G measuring system with AC/DC inverter onboard of the EV, using a Renault ZOE prototype. Image
of Renault ZOE is a stock photo from Renault.
Fig. 2. One charging / discharging cycle, in this case between a SoC of 45%
and 55%. The start and end times of a half cycle are indicated by tSoCmin,start,
tSoCmax and tSoCmin,start. Note that the EV only communicates integers, which
explains the step-wise increase in SoC.
converter of the Renault ZOE is still a prototype, better effi-
ciencies can potentially be obtained with further development
of the technology.
Fig. 3. Efficiencies of tests performed in spring and in late autumn, with SoC
range 25% to 75%. Height of bars indicate the average of the tests; error bars
indicate the minimum and maximum found efficiency within the tests.
E. Charging and Discharging Power at Different SoCs
Fig. 7 illustrates the relationship between SoC and power
for charging and discharging (V2G) of the Nissan LEAF. In
general, power rates of charging are somewhat higher. This
is because of the location of the measuring point, which is
TABLE II
OVE RVIE W TES T RE SULT S.
Start Time End Time Current SoC limits EV + EVSE Number
of cycles
Average
Efficiency
(AC-to-AC)
23/04/2019,
18:17
24/04/2019,
07:33
3x16A 25%-75%Nissan LEAF + eNovates 4 87.0%
28/11/2019,
19:22
29/11/2019,
04:53
3x16A 25%-75%Nissan LEAF + eNovates 3 85.6%
1/05/2019, 15:30 2/5/2019, 06:31 3x8A 30%-70%Nissan LEAF + eNovates 3 84.6%
24/04/2019,
16:02
25/4/2019, 05:02 3x4A 25%-75%Nissan LEAF + eNovates 1 79.2%
28/11/2019,
10:06
28/11/2019,
12:04
3x16A 80%-90%Nissan LEAF + eNovates 3 83.0%
28/11/2019,
13:21
29/11/2019,
10:31
3x16A 45%-55%Nissan LEAF + eNovates 3 84.6%
28/11/2019,
15:29
28/11/2019,
16:50
3x16A 11%-19%Nissan LEAF + eNovates 3 83.7%
9/10/2019, 11:31 9/10/2019, 14:49 3x16A 25%-35%Renault ZOE (prototype) 3 85.1%
Fig. 4. Efficiency at different average SoCs, with SoC range of 11% to 19%,
45% to 55% and 80% to 90%. Height of bars indicate the average of the
tests; error bars indicate the minimum and maximum found efficiency within
the tests.
between the charging station and the AC grid. Hence, charging
power rates are before conversion losses and discharging
rates after conversion losses. What further stands out, is the
jagged shape in the charging curve - despite the fact that
the current signal that is sent to the charging station is
constant. Apparently, either the charging station or the EV
readjusts the voltage at specific SoCs. A possible explanation
is that the EV some modules of the battery pack are charged
consecutively instead of in parallel, however, the underlying
reason is difficult to verify. The discharging curve follows
the more well-known power curve of a battery, with higher
voltages at higher SoCs [30].
IV. DISCUSSION
In total, the round-trip efficiency of 23 full V2G charging
+ discharging cycles was determined. An efficiency of 87%
was found for charging the Nissan Leaf at 3x16A. This value
Fig. 5. Efficiencies of tests for different values of current, with SoC range
of 25% to 75% for 3x16A and 3x4A and 30% to 70% for 3x8A.. Height of
bars indicate the average of the tests; error bars indicate the minimum and
maximum found efficiency within the tests. The test of 3x4A was performed
only once, hence the non-existent error bar.
is in line with the values reported in [9], but substantially
higher than the highest reported V2G round-trip efficiency
in multiple other studies [4], [6], [10], [11]. The higher
efficiency values measured in this study indicate that other
studies assuming a considerably lower efficiency values might
have underestimated the business case and potential to provide
grid services of V2G.
Lower temperatures and partial load seem to have a negative
impact on V2G round-trip efficiency. The considerably lower
efficiency values with partial load has considerable impli-
cations for EV charging models. Where most EV charging
models consider that the charging and discharging efficiency is
independent of the charging load, this study indicated that this
assumption is invalid. It is recommended that the dependency
between charging current and charging efficiency is considered
in future charging models.
Fig. 6. Efficiencies of the Nissan LEAF (MY2018) combined with the
eNovates DC V2G 10kW charger, with SoC ranges between 25% and 75%
and the Renault ZOE with on-board AC/DC converter (prototype), with SoC
range of 25% to 35%. Height of bars indicate the average of the tests; error
bars indicate the minimum and maximum found efficiency within the tests.
Fig. 7. State of Charge versus a) charging power and b) discharging (V2G)
power of EV of three charging cycles at 3x16 Ampere of the Nissan LEAF
between SoCs of 25% and 75%.
It should be noted that the experimental setup was a field
experiment, which is both a strength and a weakness of
the present study. The disadvantage of this setting is that it
is impossible to make conclusive statements on the relation
between independent and dependent variables. The advantage
is that this setting resembles the natural environment of V2G,
increasing the external validity of the tests.
Inaccuracies in measurements could occur from the com-
municated SoC by the EV battery. An important factor for
the EV to determine its SoC is the voltage level in the battery
system. However, the voltage in the battery can also be affected
by different parameters, including the battery temperature and
previous operation history [30]. Therefore, the same reported
SoC does not necessarily represent the same energy level in the
battery. By performing multiple charge and discharge cycles
of a large SoC-range, the effect of this potential inaccuracy is
minimized.
Our results have important practical implications. Since
the efficiencies reported in this study are determined in a
field experiment, results give a realistic estimation of V2G
efficiencies in real-world applications. As mentioned before,
efficiency is of large importance for the financial and envi-
ronmental impact of battery operation in general, and V2G
specifically [4], [5]. Aggregators could use the efficiency
values obtained in this research to explore V2G business cases
for EV users. Furthermore, the environmental impact of V2G
can be determined more accurately.
V. CONCLUDING REMARKS
This study is the first to perform a broad field experiment
on the round-trip efficiency of V2G. Furthermore, it provides
a proof-of-concept of V2G using the AC/DC inverter on board
of the EV. The results give guidance to EV modellers on
appropriate efficiencies to assume in EV charging models and
provide insight to e.g. charging fleet operators on the factors
impacting the EV charging efficiency.
Future research could perform more sophisticated analyses
on the V2G round-trip efficiency by performing more charge
and discharge cycles and by considering different types of
charging stations and EVs. In addition, future research should
implement power flow meters behind the inverter to be able to
separate V2G efficiency losses into battery losses and inverter
losses. Also the range of experiments can be extended by
testing other potential impacting factors, including different
weather conditions and battery age.
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... According to [46], efficiencies for V2G chargers found in literature vary between 55% and 100%. The tests executed in [46] show an average V2G efficiency between 79% and 87%. ...
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... These values are very similar to the measured efficiencies in this study. Compared with the measurements in [46] in this study the V2G roundtrip efficiency also the DC to AC and AC to DC efficiencies are determined. The determined V2G efficiencies are similar to the results in [46]. ...
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... • Battery: η ch = η dis = √ η round trip [75], where η round trip = 0.87 [76], capacity E = 75 kWh, max. charging rate b in = 22 kW, depreciation C s = 20 USD/MWh-throughput [53], initial and min. ...
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... • Battery: η ch = η dis = √ η round trip [73], where η round trip = 0.87 [74], capacity E = 75 kWh, max. charging rate b in = 22 kW, depreciation C s = 20 USD/MWh-throughput [53], initial and min. ...
Preprint
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This paper proposes a novel scalable type of multi-agent reinforcement learning-based coordination for distributed residential energy. Cooperating agents learn to control the flexibility offered by electric vehicles, space heating and flexible loads in a partially observable stochastic environment. In the standard independent Q-learning approach, the coordination performance of agents under partial observability drops at scale in stochastic environments. Here, the novel combination of learning from off-line convex optimisations on historical data and isolating marginal contributions to total rewards in reward signals increases stability and performance at scale. Using fixed-size Q-tables, prosumers are able to assess their marginal impact on total system objectives without sharing personal data either with each other or with a central coordinator. Case studies are used to assess the fitness of different combinations of exploration sources, reward definitions, and multi-agent learning frameworks. It is demonstrated that the proposed strategies create value at individual and system levels thanks to reductions in the costs of energy imports, losses, distribution network congestion, battery depreciation and greenhouse gas emissions.
... This study used battery degradation parameters from [6] to model battery degradation, assuming a battery capacity of 50 kWh in case the battery capacity of the EV was unknown. A charging and discharging efficiency of √ 0.87 is used in this analysis [35]. Every scenario is run ten times using a newly-simulated set of EV charging transactions to obtain insight in the variability in results. ...
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... The HVAC and EV are controllable appliances, whereas other electrical appliances (dishwasher, water heater, etc.) are noncontrollable in the simulation. As shown in Table 2, the HVAC parameters in Equation (18) were obtained from data of a residential house located in Hillsboro, Oregon [40,41], and the EV parameters can be found in [42,43]. Although simulations are performed only in one location in this paper, the proposed HEMS model can be effectively applied to any other locations with different climate conditions and electricity tariff structures. ...
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... The −10 kW has been chosen based on an existing V2G charger on the market (Schram et al., 2020). ...
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... In [40], the authors developed an on-board charger prototype that achieves a peak efficiency value of 97.3 % in boost operation mode and 97 % in buck operation mode. The on-board charger developed by Radimov et al. is a bi-directional, three-stage, on-board charger with a peak efficiency of 96.65 %. [41] Schram et al. determined the V2G round-trip efficiency of a Renault Zoe with a bi-directional on-board charger to be 85.1 % and of a Nissan Leaf connected to an external charging station to be 87.0 %. [42] In the Parker project, grid services were offered with a V2G setup using commercial DC-chargers and commercial EVs using CHAdeMO DC-charging. The researchers set power set-points and evaluated that the provided power by the charger lagged 7 s behind the requested power and the set-point error was 8.7 %. ...
Preprint
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An electric vehicle model is developed to characterize the behavior of the Smart e.d. (2013) while driving, charging and providing vehicle-to-grid services. The battery model is an electro-thermal model with a dual polarization equivalent circuit electrical model coupled with a lumped thermal model with active liquid cooling. The aging trend of the EV's 50 Ah large format pouch cell with NMC chemistry is evaluated via accelerated aging tests in the laboratory. The EV model is completed with the measurement of the on-board charger efficiency and the charging control behavior via IEC 61851-1. Performance of the model is validated using laboratory pack tests, charging and driving field data. The RMSE of the cell voltage was between 18.49 mV and 67.17 mV per cell for the validation profiles. Cells stored at 100 % SOC and 40 $^{\circ}C$ reached end-of-life (80 % of initial capacity) after 431 days to 589 days. The end-of-life for a cell cycled with 80 % DOD around an SOC of 50 % is reached after 3634 equivalent full cycles which equates to a driving distance of over 420000 km. The full parameter set of the model is provided to serve as a resource for vehicle-to-grid strategy development.
... All simulations of charging transactions and charging behavior were repeated five times, in order to obtain insight in the variability of results. This study used an charging efficiency (η ch ) of √ 0.87 [21]. ...
... Thus, the V2G storage efficiency is 84.9%. Although [47,48] suggest that V2G round-trip efficiency may only be 50 -70%, experimental work published more recently by Schram et al [49] suggests a range of 79.2 to 87% is realistic. Schram et al also found that the effects of SOC or temperature on charging efficiency are relatively small, so these are neglected here. ...
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Shirazi and Sachs comment on our publication in Energy "Measurement of Power Loss During Electric Vehicle Charging and Discharging" [1]. Their comment discusses aspects of our methodology and findings on round-trip efficiency during charging and discharging of electric vehicles (EVs) This reply discuss the transformer used in our experimental process, resulting in a round-trip efficiency of 70% and equally inflated commercial assumptions. Also, we suggest that the transformer losses should not be included in the V2G economics.