Introduction to electric vehicle energy storage and charging and discharging related technologies

The distribution of my country's energy resources is characterized by more coal, less gas and less oil. China's oil resources are heavily dependent on imports. If it is not controlled, China's oil imports may reach 850 million tons by 2030, equivalent to 50% of the current global export volume, which will bury serious hidden dangers for my country's energy security. Transportation is a major oil consumer. According to forecasts, the oil consumption of vehicles will account for more than 55% of the total oil consumption in 2030, so the transformation of vehicle energy consumption structure is imperative. With the increasing depletion of fossil energy and the intensification of climate warming caused by greenhouse gas emissions such as carbon dioxide, energy conservation and emission reduction have become a global consensus. my country's "12th Five-Year Plan" requires that energy consumption per unit of GDP should be reduced by 20% and pollution emissions should be reduced by 10%. With the development of my country's society and economy, household cars will continue to become popular, and the energy consumption and emissions of cars will continue to increase. Therefore, it is also imperative to develop and promote clean and efficient automotive power. Electric vehicles do not rely on oil resources, and their energy efficiency is 1.5 to 2.0 times that of traditional fuel vehicles. Compared with fuel vehicles, they can reduce carbon dioxide emissions by 20% over their entire life cycle. Therefore, they are considered to be the most promising means of transportation. China has included the development of electric vehicles in the "12th Five-Year Plan" and is preparing to develop electric vehicles on a large scale in the next few years.

1 Types of electric vehicles

Generally, there are three types of electric vehicles: pure electric vehicles (PEV), hybrid electric
vehicles (HEV), and fuel cell electric vehicles (FCEV). In recent years, plug-in hybrid electric vehicles (PHEV) have attracted particular attention. Domestic and foreign experts believe that PHEV is expected to be widely promoted and used in a few years.

1.1 Pure electric vehicles

A pure electric vehicle is an electric vehicle that is completely powered by power batteries, as shown in Figure 1. Currently, lead-acid batteries, nickel-metal hydride batteries and lithium-ion batteries are mainly used as driving force.

Lead-acid batteries are very mature storage batteries with relatively low prices. Although the manufacture and disposal of lead-acid batteries have the serious disadvantage of heavy metal pollution, they are still an important driving force for electric vehicles in the near future.

Nickel-hydrogen batteries have high specific energy and long service life, but they require the use of expensive nickel metal (accounting for 60% of the cost), so the manufacturing cost is high and large-scale promotion encounters great difficulties.

Lithium-ion battery technology has developed rapidly. In the past 10 years, the specific energy has increased from 100 Wh/kg to 180 Wh/kg, the specific power can reach 2 kW/kg, the cycle life can reach more than 1,000 times, and the operating temperature range can reach -40~55℃. In recent years, due to major breakthroughs in the research and development of lithium iron phosphate batteries, the safety of batteries has been greatly improved. Therefore, many developed countries have taken lithium-ion batteries as the main direction of power batteries for electric vehicles. my country has the advantage of lithium resources. In 2004, the output of lithium batteries accounted for 37.1% of the global market. It is expected that after 2015, the cost performance of lithium-ion batteries is expected to reach a level that can compete with lead-acid batteries and become the main power battery for electric vehicles in the future.

Based on the energy conversion efficiency, we can roughly compare the economic efficiency of pure electric vehicles and fuel vehicles. Assume that the maximum charge state of charge (SOC) of the battery is 0.9 and the discharge SOC is 0.2, that is, the actual available battery capacity is only 70% of the total capacity; the power supply price from the grid is 0.5 yuan/kWh, and the average charge and discharge efficiency of the battery is 0.75. Roughly calculated, the price of lead-acid batteries for each kWh of electricity provided is about 3.05 yuan (of which 2.38 yuan is battery depreciation and 0.67 yuan is grid power supply fee), the cost of nickel-hydrogen batteries for each kWh of electricity provided is 9.6 yuan, and the cost of lithium-ion batteries is 10.2 yuan.

1.2 Hybrid electric vehicles

Hybrid electric vehicles have two or more power sources, one of which can release electrical energy, as shown in Figure 2.

According to the different hybrid modes, hybrid vehicles can be divided into three types: series, parallel and series-connected; according to the different hybrid degree (ratio of motor power to internal combustion engine power), they can be divided into three types: micro-hybrid, mild hybrid and full hybrid. Among them, the external belt-driven starter/generator (BSG) type is the typical structure of micro-hybrid vehicles. The motor power used is generally only 2~3 kW. It has the function of stopping and cutting off the fuel of the engine, which can save 5%~7% of fuel. The typical structure of a mild hybrid vehicle is to install an electric/generator disc motor (ISG) at the rear end of the crankshaft of the automobile engine. Full hybrid or series-connected hybrid vehicles use pure electric drive function. Toyota's Prius sedan belongs to this type of full hybrid vehicle. At present, most of the hybrid vehicles developed in my country use ISG mild hybrid or BSG micro-hybrid solutions, mainly considering that the technical difficulty of these two solutions is relatively low and the production cost is also relatively low. However, according to research, the fuel saving rate of hybrid vehicles is almost proportional to the hybrid degree of the vehicle power. Therefore, in the long run, it is an inevitable trend to promote the development of full hybrid electric vehicles. In 1997, Toyota Motor Corporation of Japan took the lead in launching the Prius hybrid car to the market, which has achieved great success in the markets of Japan, the United States and European countries, with cumulative production and sales exceeding 600,000 units. Subsequently, Honda of Japan, Ford of the United States, General Motors and some major European companies have also launched various types of hybrid cars to the market.

As the latest generation of hybrid electric vehicles, plug-in hybrid electric vehicles (PHEV) have received widespread attention from governments, automobile companies and research institutions in recent years. PHEV uses large-capacity power batteries in hybrid electric vehicles. The battery capacity is generally 5~10 kWh, which is about 30%~50% of the battery capacity of pure electric vehicles and 3~5 times the battery capacity of general hybrid electric vehicles. Therefore, it can be said to be a transitional product between hybrid vehicles and pure electric vehicles. Compared with traditional internal combustion engine vehicles and general hybrid electric vehicles (HEV), PHEV relies more on power batteries to drive the vehicle, which can usually ensure that the vehicle can travel 50~90km by pure electric drive. The internal combustion engine needs to be started only when the vehicle exceeds this mileage. Therefore, its fuel economy is further improved, and carbon dioxide and nitrogen oxide emissions are less.

According to statistics: 80% of French urban residents drive less than 50 km per day; in the United States, more than 60% of car drivers drive less than 50 km per day, and more than 80% drive less than 90 km per day. Therefore, PHEV has good applicability, especially for salaried workers who drive to and from work 5 days a week and drive between 50 and 90 km.

1.3 Fuel Cell Electric Vehicles

Fuel cell electric vehicles use proton exchange membrane fuel cells (PEMFC) as the engine driving power source, and the typical structure is shown in Figure 3. As a hydrogen fuel cell, PEMFC emits water and water vapor, so it can be said that it has zero pollution to the environment. PEMFC has an energy conversion efficiency of up to 60%~70%, and operates without mechanical vibration, low noise, and low heat radiation. As a hydrogen fuel cell fuel, hydrogen has a high calorific value, and the calorific value of 1 kg of hydrogen is equivalent to that of 3.8 L of gasoline. In my country, the Ministry of Science and Technology has listed the research and development of fuel cell buses and fuel cell cars as major scientific and technological projects in the "10th Five-Year Plan", "11th Five-Year Plan" and "863", and has achieved a series of major scientific and technological achievements.

However, there are still many problems to be solved in the current PEMFC.

First, the durability of fuel cells is short. The service life of PEMFC currently developed in my country is generally only 1000~1200 hours (2200 hours abroad); the driving power of fuel cell vehicles will drop by about 40% after traveling 40,000~50,000 km. In comparison, traditional internal combustion engine vehicles can generally travel 500,000 km, which is a big gap.

Secondly, the manufacturing and operating costs of fuel cell engines remain high, especially since my country's PEMFC technology is relatively backward. The key materials and key components required, such as proton exchange membranes, carbon paper, platinum metal catalysts, high-purity graphite powder, hydrogen recovery pumps, and boost air pumps, can only be imported and are very expensive. At present, the manufacturing cost of fuel cell engines in my country is about 30,000 yuan/kW (the cost abroad is 3,000 US dollars/kW), which is a huge gap compared with the manufacturing cost of traditional internal combustion engines, which is only 200~350 yuan/kW. The cost of using fuel cell vehicles is also too high. For example, the high-purity (99.999%) high-pressure (more than 20MPa) hydrogen used is currently sold at a price of about 80~100 yuan/kg. Based on the calculation that 1kg of hydrogen can generate 10kWh of electricity, the fuel cost alone is about 10 yuan/kWh. The total operating power cost of a fuel cell engine includes depreciation. The depreciation cost is 30 yuan/kWh if the working life of the fuel cell is 1000 hours. Therefore, the total power cost of a fuel cell vehicle will reach 40 yuan/kWh.

Thirdly, PEMFC has poor adaptability to the working environment. Domestic PEMFC can work at temperatures between 0 and 40°C. Below 0°C, there is a problem of icing, and above 40°C, it overheats and cannot work normally. The operating PEMFC is very sensitive to dust, carbon monoxide, sulfide, etc. in the air, and the platinum catalyst is very easy to be contaminated and poisoned and become ineffective. In addition, as a gas, hydrogen has many difficulties in its storage, transportation and distribution that need to be solved.

Despite so many problems, fuel cell vehicles are still one of the cleanest and most promising new energy vehicles. As long as there are further breakthroughs in technology and the cost is greatly reduced, fuel cell vehicles are entirely possible to promote. 2 Electric vehicle charging and discharging technology With the improvement of the intelligent level of distribution networks and the advancement of demand-side management technology, the on-board batteries of electric vehicles may be used as mobile energy storage units in smart grids in the future. Vehicle-to-grid interconnection (V2G) refers to the connection of electric vehicles to the grid as mobile energy storage units, realizing two-way interaction of information and energy with the grid under controlled conditions. The construction of electric vehicle charging and discharging stations is an important part of the power consumption of smart grids. On average, cars only travel for 1 hour a day and are parked 95% of the time; when there are enough electric vehicles connected to the grid, they can be effectively used as mobile distributed energy storage devices to reduce peak loads and balance loads. Especially in microgrid systems with a high proportion of renewable energy generation that may be formed in the future, the reasonable charging and discharging of electric vehicles can effectively balance the volatility of renewable energy and help the grid effectively accept renewable energy generation.

At present, the charging and discharging technologies of electric vehicles mainly include unidirectional disordered VOG mode, unidirectional ordered TC and V1G mode, and bidirectional ordered V2G mode.

2.1 Unidirectional disordered power supply

VOG (Vehicles Plug-in without Logic/Control) refers to a mode in which electric vehicles are treated as ordinary electrical equipment and can be connected to the power grid for immediate charging at any time using mature unidirectional conversion technology. VOG is currently the most common charging method for electric vehicles, such as golf carts, airport shuttles and other special electric vehicles, as well as some newly built public charging facilities at home and abroad, and the Beijing Olympic Games electric vehicle charging station. The biggest problem with VOG at present is that electric vehicle charging is used as a high-power, unconstrained power load, which means that the operation of VOG charging increases the difficulty of peak load regulation of the power grid.

2.2 Unidirectional orderly power supply

TC (Timed Charging) mode, i.e. time control mode, is a charging mode of single-phase orderly power supply. In this mode, electric vehicles are charged in a given period of time. By controlling the start time of charging, peak-shifting charging is achieved to avoid charging during the peak load period of the power grid. At the same time, users can also enjoy the discount of valley electricity. However, due to various reasons, the current time control mode cannot fully and flexibly control the charging process according to the peak and valley state of the power grid. This mode of charging still uses unidirectional conversion technology and does not require real-time communication with the power grid. At present, the technical equipment is mature and has entered the demonstration operation stage.

V1G (Vehicles Plug-in with Logic/Control Regulated Charge) is also a charging mode for single-phase orderly power supply. In this mode, electric vehicles communicate with the power grid in real time, and charging is controlled by the power grid. Charging can be carried out at any time allowed by the power grid, and the efficiency of the power grid is improved by optimizing the charging schedule. At present, the Pacific Northwest National Laboratory (PNNL) of the United States has released a charging control device for electric vehicles called "Smart Charger Controller", which is equipped with a short-range wireless communication module, can receive information such as electricity price settings from power companies, and automatically avoid charging during peak hours in combination with smart grid technology. The ZigBee/IEEE 802.15 standard of this device has been submitted to IEC for application as an international standard, and has now been released as the first standard of the US Smart Grid 1.0.

2.3 Conversion of bidirectional orderly power

Electric vehicles can only obtain electricity from the power grid when charging with unidirectional technology, but cannot feed back excess electricity to the grid. With the bidirectional orderly power conversion charging mode, the on-board battery of electric vehicles can be used as a mobile energy storage unit to perform bidirectional power conversion with the power grid. Household cars are stopped most of the time. If there are enough electric vehicles connected to the power grid, they can be used as mobile distributed energy storage devices for peak load shaving and valley filling, load balancing, etc., to improve the efficiency of power grid operation and bring direct economic benefits to electric vehicle users.

Using the V2G (Vehicle To Grid) mode, electric vehicles communicate with the energy management system of the power grid and are controlled by it to achieve energy conversion (charging and discharging) between electric vehicles and the power grid. Currently, V2G-related research and demonstrations are mainly carried out in the United States. In October 2007, Trinidad and Tobago University successfully connected an AC Propulsion eBox (Toyota Scion modified car) to the power grid and accepted dispatching instructions. The vehicle operated as a frequency regulation and backup power generation equipment. According to the demonstration operation, each vehicle can bring about $4,000 in benefits to power companies each year.

Shanghai Electric Power Company has built two V2G-enabled electric vehicle charging and discharging demonstration stations, namely the Caoxi Electric Vehicle Charging and Discharging Station and the Shanghai World Expo State Grid Pavilion Charging and Discharging Station. Each station has a 30 kW DC V2G charger, which can be used as a conventional charger to achieve instant charging, scheduled charging, etc. It can also receive dispatching instructions from the power grid according to the background management system, dynamically adjust the working status and power, and achieve two-way energy interaction between electric vehicles and the power grid. At present, the V2G model is still in the experimental demonstration stage and does not yet have a market environment for commercial operation. To this end, it also requires the cooperation of advanced power grid communication, dispatching, control and protection technologies, peak and valley electricity price policies, and paid service policies such as peak and frequency adjustment and demand response for electric vehicles connected to the power grid.