Design scheme for fast charging station for electric vehicles with distributed photovoltaic power generation

1 Introduction

In recent years,electric vehicles are increasingly being used in many countries worldwide owing to their advantages such as energy conservation,environmental protection,and high efficiency.The Chinese government continues to vigorously promote the development of the electric vehicle industry and actively construct the basic infrastructure network for charging and replacing electric vehicle batteries.It is planned that by 2020,there will be up to 12,100 newly centralized charging and replacing stations and 4.8 million dispersed charging piles to meet the changing demands of 5 million electric vehicles in China.The charging infrastructure in China is rapidly increasing.As mentioned in Reference [1],in Dec.2018 the number of charging piles exceeded 600,000.There is a large deficit in the number of charging piles as they cannot meet service demand due to rapid growth of electric vehicles.

Fast charging stations can significantly shorten the charging period,improve the utilization rate of charging equipment,and improve the driving experience of users of electric vehicles.Therefore,in the next few years,rapid construction of urban fast charging network is planned and the design of charging stations will be optimized to further improve the comprehensive charging service capability of fast charging network and fully meet the charging demand of the vehicle owner.

Many scholars have investigated relevant issues in fast charging stations for electric vehicles.In reference [2],a multi objective optimal allocation mathematical model was proposed based on the actual demand of constructing the stations taking into consideration the utilization rate of renewable energy and operating costs; constraint conditions containing the decision variable scope,power balance requirements,and upper and lower limits of power change were formulated.In reference [3],a coordinated planning model of the distribution network was established,which consists of distributed power and charging station,and an improved genetic algorithm was adopted taking the stochastic volatility of wind and light resources as well as loads into consideration in order to minimize the random expectation value of the distribution system investment,operation and maintenance costs,and environmental costs.In reference [4],a path selection model and a traffic satisfaction evaluation model were formulated,and the timing sequence characteristics and complementarity of distributed power were investigated.In reference [5],a new system of charging station service that promotes ordered development of electric vehicles in Beijing was investigated through comprehensive and systematic research and analysis of the current situation of charging stations in Beijing.Reference [6] proposed a planning model for charging stations in urban area considering the road network structure,traffic flow information,distribution network structure,capacity constraint,and other factors.In reference[7],an optimal model for charging station planning and layout was proposed based on minimization of all social costs and considering the interests of both the operators of charging stations and the users of electric vehicles as prerequisites.Reference [8] discussed the adverse impact of connecting electric vehicles to the grid and evaluation method of its potential benefits.Reference [9] analyzes the environmental benefits,economic benefits,social benefits,policies and practice conditions for the promotion of pure electric buses in cities.In reference [10-11],the application of photovoltaic power generation in charging stations was analyzed.In reference [12],benefits of low carbon reduction and planning investment were comprehensively investigated.Reference [13-15] analyzed the design of distributed photovoltaic charging piles and charging stations.In reference [16-17],the authors developed an optimal dispatch for the operation of photovoltaic charging station.In reference [18],the feasibility of integration of distributed energy and charging stations was investigated.In reference[19],distributed charging stations and the corresponding management system were investigated.Reference [20]investigated distributed photovoltaic power generation system and operating parameters of charging stations in Xi'an Expo Park.In reference [21],photovoltaic cell external characteristics,control algorithm of photovoltaic inverter,reliability of the charging station,and other issues were investigated using photovoltaic cells and photovoltaic inverters in a photovoltaic power generation system and charging stations.

The above studies focused on integration of distributed energy and charging stations based on comprehensive benefits of the renewable energy utilization rate,traffic flow information,and investment planning.However,these studies did not focus on the design of fast charging stations in a specific area taking into consideration the charging demand of electric vehicles,distributed renewable energy power generation,charging system configuration,power supply system configuration,and other factors.With the increase in the number of electric vehicles,the integration design of photovoltaic power and charging station can be considered for a fast charging station in terms of the overall energy utilization without high buildings nearby to block the sunlight.Rooftop distributed photovoltaic power generation system can provide partial power for a fast charging station.A fast charging station with photovoltaic power generation can promote local consumption of renewable energy,which is a typical application scenario of energy Internet,realize“green power”for electric vehicles,and further harness the environmental protection advantages of electric vehicles.

In this study,a zone in Beijing was set as the area for constructing the fast charging station.First,the charging service demand of fast charging stations was analyzed based on estimation of traffic flow near the fast charging station,then the layout and function structure of the charging stations were designed,and the station configuration was thoroughly analyzed,including the charging system,power supply and distribution system,and operation monitoring system.A photovoltaic power generation system was installed at the rooftop as the power source for the charging station.In this study,the functional demand and technical indexes of a fast charging station were investigated.The proposed design scheme can be used a reference for planning and construction of a fast charging network in an urban area,optimization of operating mode,and improvement of economic benefits of a fast charging station.

2 Analysis of charging demand

To date the number of licensed electric vehicles in Beijing is 63,269; furthermore,542 charging and replacing stations and 7,568 charging piles have been built,whereas only 442 charging and replacing stations and 6,484 charging piles are operational.According to Special Planning of Charging Infrastructures for Electric Vehicles in Beijing(2016-2020) released in 2016,it is predicted that by 2020,the demand for electric vehicles in Beijing will be 600,000,including 450,000 private electric vehicles and 150,000 public service electric vehicles.According to Construction Planning for Traffic Development of the 13th Five-year Period in Beijing released in 2016,by 2020,the number of motor vehicles in Beijing will be less than 6.3 million and electric vehicles will account for up to 10% of the total number of motor vehicles.

Traffic flow surrounding a fast charging station is estimated through comprehensive analysis of the service radius and traffic density of electric vehicles as well as the proportion of electric vehicles in the future.The main factor that influences the service radius of a charging station is the endurance mileage of electric vehicles.The theoretical endurance mileage of power battery of electric vehicles is presently in the range of 150-300 km.In the future,the actual service radius of charging stations will be shorter considering the construction scale of charging piles,traffic jam,aging of batteries,and other practical situations.The maximum service radius of the station was set as 40 km based on a comprehensive consideration of the practical situation of the station.

The formula to calculate the number of vehicles passing through one road section in one day is given below based on traffic and road conditions in Beijing as well as the driving speed of electric vehicles,morning and evening peak,and other factors.

where N is the number of vehicles passing through one road section in one day,SMean is the average distance of driving on the road per minute,SDistance is the distance between two vehicles (depending on change in traffic flow density),L is the number of lanes of the road,and TTotal is determined according to the operating time of the charging station.

According to the traffic condition in Beijing,the average speed of vehicles is 50 km/h and the average distance of driving on the road per minute is 830 m.The distance between two vehicles is approximately 50 m based on a combination of morning and evening peak.It was assumed that the main road nearby the fast charging station is a twoway and four-lane road,such that L= 4.With combination of many factors,such as ownership of electric vehicles in Beijing in the future,the operation time of fast charging station is temporarily determined as 17 h.TTotal= 17×60= 1020(minutes).Put all parameters in the formula (1) to obtain number of vehicles which pass through the fast charging station in one day,N= 830÷50 × 4 ×1020= 67,728.

Considering the rate of electric vehicles in central city as a studied example site,the proportion of electric vehicles that can have access to the fast charging station in Beijing was set as 50%.Most electric vehicles are scooters,and travel short distances.Most owners opt to charge at home due to high charging fees of fast charging stations.Therefore,the proportion of electric vehicles that have access to the fast charging station was set to 30%.On the basis of the above analysis,it is estimated that by 2020,the number of electric vehicles passing through the fast charging station per day requiring charging is approximately 7,171.

Traffic density refers to the number of vehicles existing at a certain moment in a lane with unit length.The charging demand is closely related to the traffic density and is restricted by the operating mode of electric vehicles.The number of electric vehicles of any shape that enter and exit any area in a certain period is the same,that is,traffic flow is conserved.Therefore,it can be concluded that the number of electric vehicles in the planned area in a certain period does not change.The planned area is divided into smaller planned areas.The total charging demand of electric vehicles in these small areas is regarded as the load points of the charging station.The traffic density within the service radius of 40 km can be categorized into (40 km/5 km)2= 64 blocks.The average charging service demand of one fast charging station is approximately 7,171/64= 112 vehicles.

3 Design scheme

3.1 General design

The station was designed to cover an area of 3,200 m2.The fast charging station is located in the middle part of the outdoor place and is above or underground in any given position.The hall of the charging station can be divided into charging area,operation area,equipment area,and distribution area.The solar photovoltaic power generation system was combined with an energy storage unit.The roof area was approximately 1,680 m2 (35 m×48 m),and the roof with photovoltaic power generation equipment covers an area of 1,500 m2,which meets the illumination and emergency power consumption demand.The layout plan of the upper and lower floors of the fast charging station is shown in Figs.1 and 2,respectively.

Fig.1 Layout plan for 1F of fast charging station

Fig.2 Layout plan for B1 of fast charging station

The vehicle entrance is in the west side of the station and vehicles can enter the charging zone through a ramp.The pedestrian entrance is in the south side of the station.The total number of charging positions in 1F and B1 is 55.The south area of 1F is designated as the operation zone to provide many value-added services and temporary rest place.The west area of the ground is designated as a temporary parking area,which is equipped with intelligent charging equipment and covers an area of approximately 60 m2.The equipment rooms of the two floors are in the north area,whereas the distribution equipment is located in B1.

3.2 Charging demand judgment

The charging station in this study refers to fast charging station.Therefore,the charging zone is only equipped with a DC charger and an intelligent AC integral charger.

3.2.1 DC charger

The DC system of the DC charger includes an AC distribution unit,a lightning protection unit,a charging module,a monitoring module,a cooling unit,a dehumidification unit,and a metering unit.The DC charger has the following technical characteristics:

(1) The main parts are high frequency switching power supply charging modules and monitoring modules;

(2) Intelligent operation management and multiple protection are adopted to significantly improve the stability and reliability of the system;

(3) Modular design,system configuration RS485,CAN,and Ethernet interface are adopted to communicate with monitoring background and realize the function of “remote control,remote detection,remote communication,and remote adjustment”.

3.2.2 Intelligent AC integral charger

The intelligent AC integral charger is mainly used by small passenger vehicles and has AC and DC charging interfaces to ensure simultaneous charging.It can be employed in charging stations with different voltage grades.The working flow chart of the intelligent AC integral charger is shown in Fig.3.The intelligent AC integral charger mainly consists of power supply input,main control unit,and AC (DC) output.The main functions parts of the system are shown in Fig.4.

The intelligent AC integral charger has the following technical characteristics:

(1) It has double functions capability (AC charging and DC charging),can provide many power supply modes for users,and has strong adaptability and interoperability;

(2) Custom service and friendly interface;

(3) Reasonable and accurate billing;

Fig.3 Schematic diagram of working flow of intelligent AC/DC integral machine

Fig.4 Schematic diagram of main functions of parts of intelligent AC/DC integral charger

(4) Reliable safety protection:the charging interface has charging control and guidance function according to international standards and can effectively prevent unreliable contact to the charging interface;

(5) The AC/DC integral charger has many types of communication interfaces,which can carry out data interaction with different equipment and upper level system and perform remote data uploading,remote dispatching,and other functions.

A comparison of the technical and economic performance of the DC charger and intelligent AC/DC integral charger is presented in Table1.BYD E6 (battery capacity of 63.4 kWh and endurance mileage of 300 km) was used as an example to compare the service capacity of the charger.If the DC charger (60 kW) is adopted,the full charging time is 70 min,whereas if the intelligent AC/DC integral charger (50 kW) is adopted,the full charging time is 80 minutes.Assuming a parking time of 10 min,and taking the operation time and other time intervals into consideration,the daily service capability of a fast charging station for a full charging time of T is given as

According to formula (2),if DC charger (60 kW) is adopted to only provide service for BYD E6,the maximum service capability is 701 vehicles; if intelligent AC/DC integral charger (50 kW) is adopted to only provide service for BYD E6,the maximum service capability is 623 vehicles.

The use of 45 DC chargers and 10 intelligent AC/DC integral chargers (arranged in the temporary parking area in 1F) was adopted in the design considering the formation cycle of the electric vehicle market.When the vehicle scale is small,the resource will be fully utilized and the AC slow charging and AC fast charging modes of the intelligent AC/DC integral chargers can be adopted.When the vehicle scale becomes large,DC chargers and the DC fast charging mode of the intelligent AC/DC integral chargers can be adopted to ensure optimization of resource configuration of the entire station.

3.3 Power supply and distribution system

According to the overall design layout of the substation,the distribution room and the separation room are arranged in B1 of the charging station.There is a two-way power supply from the substation.The distribution room is equipped with two 2,500 kVA dry transformers to provide power for the charging station.

3.3.1 10 kV part and 0.4 kV part

The 10 kV two-way supply was adopted for the incoming power of the power supply system of the fast charging station based on the requirements of Guidance Opinions of State Grid on Construction of Charging Facilities for Electric Vehicles.A single bus sectioned wiring mode was adopted for the 10 kV side,including a substation system,power distribution system,secondary side protection and monitoring,AC/DC system,safety protection,harmonic control,cable facilities,and lightning protection and grounding.The 110 kV side is the substation equipment accessed according to the actual situation of the substation.The 110 kV cannot be directly stepped down to 0.4 kV using existing technology.Therefore,the primary system of the substation is between the 110 kV side and the 10 kV side.

The single bus sectioned wiring mode was adopted for the 0.4 kV side and it was designed according to the single bus sectioned wiring mode standard.It has two low voltage incoming cabinets,two low voltage incoming metering cabinets,one low voltage sectioned cabinet,four low voltage outgoing cabinets,two 375 kVar reactive power compensation cabinets,and four 300 A active filter device.3.3.2 Main equipment lectotype scheme

Ring net cabinet was adopted for the 10 kV distribution device,which is equipped with a joint venture vacuum circuit breaker.The charging station is equipped with DC chargers and intelligent AC/DC integral chargers.The power of the DC charger is 60 kW.The rated power of the intelligent AC/DC integral charger is 50 kW; however,its charging power can be adjusted up to 60 kW by varying the voltage and current.Therefore,the limit of the output power of these two chargers is 60 kW.

The total capacity of the charger is

The total capacity of the transformer is given as

where βm is the optimal load rate of the transformer,which is 0.81.The simultaneity factor of the charger Kx is determined by the number of chargers.The typical value of the simultaneity factor is 0.5-0.8.A value of 1 is adopted considering the limit situation.Se is the total load capacity of other charging facilities except for the chargers,including 10 kW of the monitoring system at the station level,7 kW of electric vehicle intelligent charging,replacing service network,and operation monitoring system,12 kW of electrical illumination,20 kW of air conditioner,3 kW of office load,and 60 kW of battery detection and maintenance and a certain margin (180 kW).According to above data,thetotal capacity of the distribution transformer SN is 4,987 kVA.Two 2,500 kVA distribution transformers were considered to meet the above load demand.The actual load rate of the transformer is 81%.

Table1 MComparison of DC charger and intelligent AC/DC integral charger

Charging equipment Charging mode Layout mode Reference unit price(including installation and debugging costs)Intelligence ce degree DC charger DC fast charging One machine and one pile,two sets of equipment for support 80,000 yuan Higher Intelligent AC/DC integral charger Select DC fast charging mode,AC fast charging mode,and AC slow charging mode according to demand Single machine layout,convenient layout 420,000 yuan Higher

The low voltage drawer type switch cabinet was adopted for the 0.4 kV equipment,intelligent framework circuit breaker was employed for the incoming line,molded case circuit breaker of electronic release was used for the outgoing line,and the circuit breakers of the incoming line and the outgoing line have communication functions.In terms of the secondary configuration of the power supply system,a microcomputer integrated protection device was installed in the 10 kV incoming line switch for protection,detection,and control.A microcomputer protection,detection and control device was adopted and installed in the distribution transformer 10 kV switch cabinet; the fast charging station has a set of 220 V DC system to provide power for control,protection and signal,accident illumination,and power for energy storage motor.A popular brand of maintenance-free battery with a capacity of 100 Ah was used.The charging station was powered from a 0.4 kV bus and equipped with double-loop supply from different buses.An AC distribution box was used to supply power for illumination,air conditioners,air fans,and sockets.Flame retardant three-core metal copper core cables were used as power cables for the 10 kV power owing to high heat generation in the line during fast charging.

3.4 Photovoltaic power generation system

In this study,it is assumed that the lighting condition near the fast charging station is good and there is no high building.To fully utilize the land resources,the roof can be fully utilized for distributed photovoltaic power generation equipment to realize energy conservation and environmental protection and to meet the demand of emergency power consumption and daily illumination.

According to GB50797-2012 Design Code for Photovoltaic Power Station,Beijing is located at latitude 39.8°,the daily mean radiation is 18,035 kJ/m2,the daily radiation is 15,261 kJ/m2,the recommended obliquity of independent system is Φ + 4°,the recommended obliquity of grid connected system is Φ - 7°,and the obliquity location is 30°.The roof area is 1,680 m2 (35 m×48 m),and the area for the photovoltaic equipment is 1,500 m2.The peak power and the peak voltage of common solar panels are 240 W and 30.6 V,respectively.The station has 500 solar panels and each array is composed of 100 panels.Every five solar panels were connected in parallel to form a small photovoltaic power generation unit with peak voltage of 30.6 V and peak current of 39.2 A.Every 10 small photovoltaic units were connected in series mode to form a compound photovoltaic power generation unit with peak voltage of 30.6 V and peak current of 39.2 A at the end.There are 10 arrays in total.It is estimated that the predicted annual average power generating capacity is 262,800 kWh based on effective illumination period of 6 h as the sunshine angle and intensity vary at different times in a day.

The photovoltaic power generation system mainly includes photovoltaic cell assembly and its fixing device,DC lightning protection confluence distribution box,photovoltaic grid connection inverter,system communication monitoring device,environment parameter detection device,standard battery power generation monitoring device,lightning protection and grounding device,and pipe connection cable and protection system.The specific configuration of the photovoltaic system is shown in Fig.5.

The DC confluence box receives and transmits DC power sent by the solar photovoltaic panels.Its serial and parallel scheme ensures that every two small compound photovoltaic power generation units are connected in serial mode and feeds the DC confluence box.A 10-way DC input and 1-way output were adopted for the confluence box.The maximum input voltage was 1,000 V; the AC/DC distribution cabinet of the solar photovoltaic power generation system includes DC distribution unit and AC distribution unit.The DC distribution unit provides DC input/output interfaces,which are mainly used to merge DC power from the photovoltaic modules and connection to inverter or direct supply to other DC loads (such as battery and charging power).The AC distribution unit mainly provides grid connection interfaces for inverter through the cabinet and is equipped with AC circuit breaker for AC loads.Besides,a high performance PC was used as the main monitoring machine of the photovoltaic power generation system for detection of operation data of all inverters connected to grid 24 h of the day and for continuous recording of operation data and fault data.Multi machine communication software was used.RS485 or Ethernet remote communication mode was adopted to acquire the operation parameters of equipment operation state machine of the power station and upload to the main monitoring machine in real time.

3.5 Operation monitoring system

Fig.5 Photovoltaic system architecture

Fig.6 Schematic diagram of composition and configuration of station level monitoring system

The station level monitoring system is the core of the automatic system of the charging station and is a comprehensive monitoring management system based on a unified monitoring platform,with layered structure and integrated charging monitoring,distribution monitoring,and other application subsystems.Layered structure was adopted for the fast charging station monitoring system in this study.Its system structure can be categorized into station level control layer,interval control layer,and network equipment.All layers communicate using industrial highspeed Ethernet and field bus with international standard and adopt the communication protocol that complies with international standard.The station level monitoring system provides redundancy configuration and is convenient for software and hardware expansion.Besides,the operation monitoring system has many functions,such as manmachine interface monitoring,acquisition and storage of real-time data,operation monitoring and alarm,operation record,operation control,screen display and tabulation printing,communication,self-diagnosis,and self-recovery.It is characterized by a friendly interface,complete function,convenient expansion,high intelligence,and standardization.It can effectively monitor the operation situation in the station and in real time.The composition and configuration of the system is shown in Fig.6.

4 Benefit analysis

The charging station is mainly used for fast charging.Therefore,its daily vehicle flow is far more than that in an AC charging station.It is assumed that the average charging time is 30 minutes (20 kWh electricity for 100 km).As mentioned above,if the working time of the fast charging station is 17 h and with adoption of the DC charger (60 kW),the maximum service capability is 701 vehicles.According to the standard for charging electricity price of Beijing Electric Power Company,the charging fees of public charging facilities since June 15,2016 is the sum of industrial Time of Use (TOU) power price and 0.8 yuan/kWh,as presented in Tab.2.

Working time of fast charging station in Beijing is 06:00-23:00.According to the reference table of TOU power price,the average power price for charging= [1.194 6×(1)+1.495×(3+3+2)+1.8044×(5+3)] /17= 1.6229 (yuan/kWh).The annual revenue of the fast charging station (Y)can be calculated by formula (4):

Where,nEV is number of charged vehicles,S is charged power quantity,P is the charging price.

It can be calculated that the annual revenue of the fast charging station in the example site under the condition of full load (55 charging piles/chargers) operation can be up to 8.3 million yuan.Even if based on the estimation of average charging demand of 112 vehicles analyzed above,the annual revenue is more than 1.32 million yuan.With consideration of revenue of other value added services,benefits of the fast charging station will be very considerable.

Table2 Electricity price of public charging facilities in peak and valley time of Beijing Electric Power Company

Charging power price(yuan/kWh)Service price(yuan/kWh)Final charging price (yuan/kWh)Execution time Peak power price 1.0044 0.80 1.8044 10:00-15:00 18:00-21:00 7:00-10:00 15:00-18:00 21:00-23:00 Valley power price 0.3946 0.80 1.1946 23:00-7:00 Flat power price 0.6950 0.80 1.495

5 Conclusion

Fast charging station for electric vehicles with convenient charging service experience will become the mainstream of construction of charging stations in the future.This paper presented analysis of the charging demand of electric vehicles near a fast charging station in studied example site with combination of policies and plans,traffic density,and service radius of electric vehicles in Beijing.

Then,the station layout and function framework were presented for a fast charging station equipped with distributed photovoltaic power generation system.The functional framework and key technical indexes of the charging system,power supply and distribution system,photovoltaic power generation system,and operation monitoring system of the fast charging station were determined.Finally,a detailed and rational design scheme for the charging station was developed.This scheme can be used as reference for future planning and construction as well as improvement of operation benefits of fast charging stations in an urban area.