Academic Journal of Science and Technology 
ISSN: 2771-3032 | Vol. 13, No. 3, 2024 

 

152 

Off‐grid	Automobile	Wireless	Charging	System	Based	
on	Photovoltaic	Energy	Storage	
Yifan Zi1, a 

1Department of power engineering, North China Electric Power University, Baoding, Hebei (Province), 071066, China 
a220211020930@ncepu.edu.cn 

 

Abstract: With the development and popularization of new energy vehicles, more and more electric vehicles have entered the 
public's vision, and gradually replaced the traditional fuel vehicles to become a good partner for people to travel. However, 
electric vehicles have problems such as slow charging process, short driving range, insufficient number of charging piles, and 
the energy structure of municipal power dominated by thermal power also restricts the emission reduction ability of electric 
vehicles. The establishment of off-grid optical storage wireless charging system will greatly alleviate this situation. The system 
uses photovoltaic as a source of all energy, which can be distributed in the open space on both sides of the highway and municipal 
roads, and the charging base is arranged in the safety zone on both sides of the road or at the traffic light intersection. Its energy 
storage facilities can ensure the stability of the photovoltaic system power supply and meet the needs of car charging. By 
optimizing the energy storage battery capacity, photovoltaic panel layout and number, inverter and wireless charging system 
circuit structure will greatly improve the efficiency and reliability of the charging system, making it possible to use on a large 
scale. 

Keywords: Photovoltaic, Energy storage, Wireless car charging. 
 

1. Introduction 
In recent years, with the proposal of China's "carbon peak, 

carbon neutrality" goal, and increasingly severe 
environmental problems, the electric vehicle industry has 
been rapid development, more and more vehicles have been 
put into large-scale use in all walks of life. For example, ride-
hailing, law enforcement vehicles and even special vehicles 
such as mining trucks have achieved large-scale 
electrification changes. But today, the charging problem of 
vehicles is still troubling the majority of users. The charging 
efficiency of electric vehicles has been effectively improved 
in the process of increasing the power of charging piles, but 
the shortage of charging piles has not been solved. Moreover, 
it is worth noting that due to the instability of new energy, 
thermal power is still the main source of electricity in China's 
power grid, accounting for about 66% of the total electricity 
[1]. Therefore, the energy consumption of electric vehicles is 
not zero carbon, and this problem needs to be solved. A set of 
efficient and entirely renewable energy charging technology 
will solve this series of problems: photovoltaic and energy 
storage based electric vehicle wireless charging system. First 
of all, this system introduces photovoltaic as its total energy 
source completely carbon-free, second, the energy storage 
facility will ensure the stability of the EV charging process. 
Wireless charging facilities can be arranged in temporary 
parking areas on both sides of the road, which will greatly 
reduce the problem of finding land during the charging 
process of the vehicle. In addition, the temporary roadside 
parking area that is often vacant will also ensure that the 
vehicle can have enough electricity to charge at the fast 
charging point in case of emergency, alleviating the range 
anxiety of electric vehicles over long distances. The charging 
method without charging port can also effectively avoid the 
corrosion of the charging interface such as dust and rain. 
Charging without the driver having to get out of the car can 
also keep the owner safe on the highway. 

2. Photovoltaic Part Design Principle 
Photovoltaic energy storage system mainly includes 

photovoltaic power generation array, energy storage battery 
array and charging control system. Taking highways as an 
example, photovoltaic arrays can be placed on both sides of 
the highway or in the middle of the fence, and connected to 
the charging base on both sides of the road through electrical 
wiring and control systems. The conversion process of 
photovoltaic power generation per unit area is shown 
below[2]: 

 

𝐸 𝐸 𝑅 𝐸 𝐸 𝜌      (1) 

 

Eb refers to direct solar radiation on a horizontal plane; Rb 
refers to the ratio of direct radiation flux on the inclined plane 
to the horizontal plane, refers to the tilt Angle of the 
photovoltaic array, Ed refers to the solar scattered radiation on 
the horizontal plane, refers to the reflectivity of the ground, E 
refers to the total solar radiation on the horizontal plane. 

According to the relevant literature test data, the energy 
conversion efficiency of monocrystalline silicon photovoltaic 
panel is about 20%, the energy conversion efficiency of 
inverter is 98.4%, and the energy efficiency of photovoltaic 
power generation system is about 81.14-87.87% (Win/Wout) 
[3]. 

The photovoltaic power generation per unit area per unit 
time can be obtained by calculating the following formula. 

 
𝑊 𝐸 𝑄 𝑄                  (2) 

 

Where ET is the direct radiation power, Qt is the 
photoelectric conversion efficiency of monocrystalline 
silicon photovoltaic panel, and Qn is the efficiency of 
photovoltaic power generation system. 



 

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According to the solar irradiance data of the deployment 
area, MATLAB is used to calculate the hourly irradiance and 
calculate the full-day power generation of the photovoltaic 
system. It can provide relevant data for the subsequent energy 
storage configuration. 

3. Energy Storage Part Design 
Principle 

In the photovoltaic wireless charging model, the energy 
storage system is responsible for the stable output of electric 
energy and the continuous supply of electric energy at night. 
Therefore, its main design includes battery series parallel 
design and battery capacity quota. In order to fit today's 

mainstream electrochemical energy storage methods, the 
following will take lithium iron phosphate batteries as an 
example. 

The selected battery is a commercial square lithium iron 
phosphate battery composed of positive collector fluid 
(aluminum foil), negative collector fluid (copper foil), 
positive material (nickel cobalt aluminum), negative material 
(graphite), diaphragm, electrolyte, safety valve, positive cap, 
washer, shell, etc. The battery capacity is 280 Ah. When the 
battery is working, it is charged to 3.65V, and then discharged 
to 2.50V at constant power [4]. 

Taking electric vehicle charging voltage 800V[5] and 
charging power 250kW as an example, the inverter system is 
simulated by MATLAB as shown in the following figure. 

 

 
Figure 1. Inverter model 

 

The output voltage of the battery is 800V, the charging 
power is 250kW, and the minimum output voltage of a single 
battery is 2.5V and the minimum output current is constant 
1A. The calculation results show that a single container 
cabinet should be 320S312P. In the specific implementation 
process, a battery container can be divided into several 
arrangements according to the battery types provided by the 
battery cabinet manufacturers. This will facilitate battery heat 
dissipation and fire safety design. 

The determination of the capacity of energy storage 
facilities should consider the capacity of photovoltaic power 
generation and the demand for highway vehicle charging. In 
large power stations, the quota ratio of energy storage and 
new energy is about 1/10, but the energy storage quota can be 
appropriately increased under the tram charging condition. 
This is mainly because of the inevitable loss of power 
generated by the coil during the wireless charging process. 

 

 
Figure 2. Photovoltaic wireless charging system diagram 

 

4. Wireless Charging Part Design 
Principle 

The wireless charging system selects the wireless charging 
mode of coil mutual inductance. The wireless charging 
system includes three parts: inverter, coil and compensation 

network. Its basic network is shown in the figure below. It 
includes a primary DC power supply and a secondary battery 
charging system. To simplify this part of the model, the cell 
on the secondary side is simplified into a resistance element 
and its input current and power are measured. Among them, 
the mutual inductance coil used is a generalized mutual 
inductance model. 

 



 

154 

 
Figure 3. Wireless charging system diagram 

 
The sub-side output current, voltage, power, and energy transmission efficiency are shown in the figure below. 

 

 
Figure 4. Battery charging current simulation diagram Unit 10A 

 

 
Figure 5. Battery charging voltage simulation diagram Unit 100V 



 

155 

 
Figure 6. Battery charging power simulation diagram Unit 1000W 

 

According to the final receiving resistance charging power, 
the charging power of the wireless charging system to the 
tram is about 370W when the output energy storage battery is 
800V. In order to improve the charging power, we can reduce 
the iron loss and copper loss in the coil by using reactive 
power compensation, optimizing the coil parameters, 
changing the capacitance and inductance parameters in the 
circuit. 

5. Wireless Charging System 
Optimization Process 

5.1. Photovoltaic optimization 
Photovoltaic optimization process includes photovoltaic 

panel area optimization and inverter performance 
optimization. The PV panel area optimization process should 
first investigate the charging needs of the vehicles in the road 
section and predict the daily electricity consumption. The 
process can be optimized using genetic algorithms in 
MATLAB to find the minimum PV panel area and minimum 
cost. Specific operations are as follows: 

Firstly, the daily number of charging vehicles and power 
consumption of a charging station in the road section in a year 
are investigated, and the daily average value is calculated. 
According to the energy conversion efficiency of the charging 
system, the daily required photovoltaic power generation is 
calculated, and the number of photovoltaic panels required is 
calculated based on this. 

For the selection of photovoltaic panel installation points, 
regional discretization analysis can be used [6]. Because there 
may be trees or houses blocking the sun along the highway or 
highway, the area does not have the conditions for the 
construction of photovoltaic modules. In order to facilitate the 
analysis, this paper uses a grid of specific size to discretize 
the road area. The roadside area is divided by a specific small 
square, and the circle is marked as having sufficient light area. 

 

 
Figure 7. Results of highway side discretization 

 

Taking the maximum installation area of PV modules as the 
target to calculate the PV capacity that can be installed beside 
the highway, the optimal layout model of PV modules is a 
maximum coverage model, that is, in the limited areas on both 
sides of the road, the PV modules cover as much as possible 
the areas that can receive solar radiation. The constraints of 
the objective function include boundary constraints on both 
sides of the road, foreign body constraints on the side of the 
road, photovoltaic panel coverage constraints in the radiation 
area, and photovoltaic panel orientation constraints. 

In addition, the energy conversion efficiency of 
photovoltaic panels can also be optimized, such as selecting a 
monocrystal silicon photovoltaic panel with high energy 
conversion efficiency, installing a single axis rotation device 
for the photovoltaic system and equipped with a ray tracing 
instrument. 

Inverter optimization includes the following processes.The 
inverter can assume the function of reactive power regulation. 
While supplying power, distributed PV has the ability to 
flexibly adjust reactive power, and the voltage can be adjusted 
to the photovoltaic junction point by controlling the emission 
or absorption of reactive power [7]. 

The relationship between the adjustable reactive capacity 
of the inverter and its rated capacity and active output power 
is as follows: 

 

𝑄𝒎𝒂𝒙, 𝑆 𝑃             (3) 

 

Qmax,PV is the reactive power adjustable capacity of 
photovoltaic inverter; SINV is the rated capacity of 



 

156 

photovoltaic grid-connected inverters, generally 1.0 to 1.1 
times the rated power of photovoltaic panels; PPV is the active 
power of photovoltaic connected to the grid. 

5.2. Wireless charging is optimized 
The optimization process of wireless charging technology 

can not only modify the original parameters such as 
capacitance and inductance in the circuit, but also carry out 
static anti-drift design. Sonapreetha et al. [8] studied the 
setting of non-overlapping coils as a wireless fixed electric 
vehicle charger detection for foreign bodies and vehicle 
positions. Deng et al. [9-10] detected the edge and position of 
electric vehicles by capturing the phase Angle difference of 
induced voltage and current. Hasan et al. [11] analytically 
described the time-varying power transmission and efficiency 
distribution, and proposed a power transmission control 
algorithm independent of coil position detection. Lu et al. [12] 
studied a hybrid radio energy transmission system based on 
capacitance-inductance (CL) compensation. Feng et al. [13] 
proposed a method for road traffic monitoring through 
magnetic sensors, using multiple magnetic dipoles to simulate 
vehicles in motion, and modeling local magnetic field 
disturbance caused by moving vehicles for vehicle 
identification and speed estimation. Xu Shihui et al. [14] 
studied a new type of square coil and proposed an optimized 
close-range accurate positioning method, which solved the 
problem of large error in traditional positioning methods of 
magnetic dipole model. Chen et al. [15] conducted a review 
study on the development of dynamic radio energy 
transmission technology. Baiye Hong et al. [16-17] used 
traffic simulation and nonlinear optimization methods to 
study the length of charging lanes to minimize system costs. 

5.3. Energy storage partial optimization 
In conventional new energy power plants, the capacity of 

energy storage equipment generally needs to reach 10%-20% 
of the total installed capacity of the power plant to ensure the 
safe and stable operation of the power plant, and play the role 
of "peak cutting and valley filling". 

In the field of automobile wireless charging, energy storage 
must first consider the load characteristics of the charging 
system, determine the threshold of the charging and 
discharging state of the energy storage, and build a better 
operating condition and operating conditions of the road 
vehicle charging system with optical storage. Secondly, with 
the goal of maximizing photovoltaic utilization and 
minimizing system investment and operating costs, particle 
swarm optimization algorithm is used to optimize the solution, 
and the optimal photovoltaic and energy storage capacity of 
the optical storage bus charging station is determined. He Xin 
et al. [16] concluded that a more reasonable PV and energy 
storage capacity could be obtained by considering load 
characteristics and operating conditions under the same 
configuration. In their research, the moving boundary method 
is mainly used to determine the charging and discharging 
threshold of energy storage, which can meet the requirements 
of accuracy and stability. 

The main design in this paper is an off-grid wireless 
charging system, so all energy sources and consumption are 
completed in a small range [17-19]. The specific working 
conditions are divided into the following two parts: (1) 
photovoltaic as the energy source of the charging system, 
electric vehicles and energy storage as the photovoltaic 
consumption object. (2) photovoltaic and energy storage as 

the energy source of the charging system, and electric 
vehicles as the consumption object of photovoltaic and energy 
storage. The following objective functions and constraints are 
used in the optimization process. 

The first objective function is the lowest comprehensive 
daily operating cost of the optical storage and charging system. 

 

𝐶    (4) 

 

minF 𝐶 𝐶                  (5) 

 

C1 is the equivalent daily cost of the initial construction of 
the charging system; C2 is the daily maintenance cost of the 
charging system. The following are the remaining objective 
functions. 

 
𝐶 𝐶 𝑃                      (6) 

 

𝐶 𝐶 𝐸 𝐶 𝑃 .            (7) 

 

𝐶                 (8) 

 

Cpv is the photovoltaic cost, Cbattery is the cost of the energy 
storage system, Cconversion is the cost of the wireless charging 
system, r is the discount rate, m1 is the service life of the 
charging station, m2 is the life of the energy storage battery. 
Cv, Ce and Cp are respectively the price of 182 unit capacity 
photovoltaic components (yuan /kW), the price of unit 
capacity energy storage battery (yuan /kWh) and the unit price 
of power conversion device (yuan /kW),Ppvmax, E and 
Pb.max are respectively the maximum output of photovoltaic 
system (kW), the capacity of energy storage system (kWh) 
and the maximum charge and discharge work of energy 
storage system Rate; kbat is the annual operation and 
maintenance coefficient of the energy storage system. 

The constraints of the off-grid system under two working 
conditions are as follows: 

working condition (1) 
 

𝑃 𝑃 𝑃                  (9) 

 

working condition (2) 
 

𝑃 𝑃 𝑃               (10) 

6. Conclusion 
The off-grid optical storage electric vehicle wireless 

charging system is still in the development stage, and the 
problems facing it are mainly insufficient output power, more 
energy dissipation during the charging process, insufficient 
stability of the photovoltaic system, and insufficient life of the 
photovoltaic panel and battery. However, with the progress of 
technology and the realization of related optimization, the 
advantages of wireless charging systems that can be charged 
anywhere and the charging concept of off-grid photovoltaic 
energy storage will bring people a green, safe and comfortable 
travel experience, and will also help the development of new 
energy technology in the field of transportation. 



 

157 

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