What is the energy storage mode and main challenge of electric vehicle?

With the continuous expansion of the number of electric vehicles, electric vehicle storage has received increasing attention from the industry. Electric vehicles can also be regarded as distributed energy storage facilities. They can be combined with distributed energy sources, renewable energy sources, etc. to form microgrid systems. They can also be applied to power demand response, and real-time charge and discharge changes can be made according to system flexibility adjustment requirements.

Concept and status of vehicle electricity storage

As the proportion of renewable energy power generation continues to rise. The development of energy storage technology will be the fundamental way to make up for the lack of flexibility of the power system. However, China's installed energy storage capacity is less than 1.5% of the national power generation capacity, plus domestic natural gas power generation and storage capacity hydropower. When traditional peaking resources are scarce, limited pumped storage resources cannot meet the huge demand for future clean energy transformation. Although electrochemical energy storage technologies (such as lithium-ion batteries , flow batteries, etc.) have been developed in recent years, their installed capacity accounts for less than 1% of the total installed capacity, which is not enough to substantially fill the energy storage supply in the short term gap.

Electric vehicles are strategic emerging industries that realize the transformation of China's transportation energy and catch up with the automobile industry. China has carried out early research and development of key technologies for electric vehicles and implemented commercialization support policies, which has effectively promoted the rapid development of China's electric vehicle industry. In 2017, the national electric vehicle sales reached 777,000 units, and the cumulative sales volume exceeded 1.8 million units, accounting for more than half of the global electric vehicle market. At present, Germany, France, Britain, Norway, Netherlands, India and other countries have raised the target of fuel vehicles. China has also set a development target of 2 million vehicles and 5 million vehicles for new energy vehicles in 2020, and 2018. The implementation of the "Parallel Management Method for the Average Fuel Consumption of Passenger Vehicle Enterprises and New Energy Vehicle Points" means that traffic electrification has become an irreversible trend.

With the continuous expansion of the number of electric vehicles, electric vehicle storage has received increasing attention from the industry. Electric vehicles can also be regarded as distributed energy storage facilities. They can be combined with distributed energy sources, renewable energy sources, etc. to form microgrid systems. They can also be applied to power demand response, and real-time charge and discharge changes can be made according to system flexibility adjustment requirements. After the popularization of electric vehicles, the scale of regulation and control is very impressive. Combined with advanced power electronic communication control technology, reasonable charging and discharging facilities layout and guiding electricity price policy, electric vehicles have great application potential in improving the reliability and flexibility of power system operation.

Foreign research on vehicle electrical energy storage began earlier. Kempton and Tomic of the University of Delaware earlier compared the development scale of the US power system and transportation sector and found that if there is a quarter of the number of vehicles in the US transportation sector. For electric vehicles, the total power of charging/discharging of electric vehicles exceeds the total installed capacity of power generation in the United States. The study further found that since the electric vehicle was designed to achieve instantaneous power adjustment capability for road conditions, the electric vehicle also fully meets the response time and climb rate requirements of the power system auxiliary service. In addition, the investment cost of electric vehicle V2G hardware is relatively limited. The most important factor limiting its promotion is the high battery cycle life loss during its operation. Kempton further analyzed the feasibility of three types of electric vehicles, namely, pure electric vehicles, plug-in hybrid vehicles and fuel cell vehicles, in baseload power generation, peak load power generation, spare capacity and system frequency modulation. The study concluded that different types of electric vehicles (pure electric vehicles, plug-in hybrid vehicles, fuel cell vehicles) adapt to different types of ancillary services, and although the economics are different, there are no insurmountable technical obstacles.

In terms of technology demonstrations, the Danish government launched the three-year Edison Electric Vehicle Smart Grid project in 2009, which aims to explore the potential of electric vehicles for power demand side response. The project was completed by 7 working groups of electric vehicle technology group, power battery simulation group, technical economic evaluation team, distributed energy grid-connected technology integration group, fast charging facility group, communication system group and system function testing group. The technical feasibility of the V2G power system consists of renewable energy power generation and electric vehicles. The project is one of the largest empirical research projects for electric vehicle energy storage in the world to date.

In terms of industrialization, China's electric vehicles and charging pile enterprises have already taken the lead. The BYD e6 and other models have achieved 3.3 kW off-board discharge function, and are the first to have distributed energy storage power stations and mobile charging vehicles. The CMS active flexible intelligent charging system developed by Special Calls integrates control, protection, display and metering into the box-type substation based on the function of retaining the charging pile foundation. At the same time, it detects the load of the grid in the area, the number of vehicles to be charged, and the battery charge. State and user charging time requirements, intelligent distribution of charging power, charging the battery with optimized flexible current output, laying a technical foundation for intelligent charging and vehicle energy storage.

Vehicle electricity storage mode and main challenges

Electric vehicles can realize vehicle electric energy storage through different modes. This paper divides vehicle electric energy storage into four types: orderly charging, V2G, battery replacement and decommissioned battery energy storage. Different models have differences in scale potential, cost and infrastructure. :

1. Orderly charging. Although the electric vehicle cannot directly discharge to the grid or load under the orderly charging, it can still participate in the power grid clipping and valley filling by changing the charging time (power demand response) to realize the “virtual energy storage” function. The energy storage power of an electric vehicle in an orderly charging depends on the charging and discharging power of the vehicle, and its energy storage capacity depends on the energy efficiency of the vehicle and the intensity of the trip.

2. Vehicle electrical interconnection (V2G). At present, the power battery capacity of electric vehicles is generally limited, and the battery life is sufficient to meet the road behavior. The participation of vehicles in V2G will accelerate battery aging and bring extremely high costs to users. However, with the increase of battery capacity and the increase of cycle life, the battery life of electric vehicles will gradually exceed the daily traffic demand, and the value of V2G will appear rapidly.

3. Battery replacement. Battery replacement offers the possibility to quickly replenish electric vehicle power. Since the vehicle and the battery are separated, the battery replacement mode releases the energy storage potential of the vehicle battery to the greatest extent. The battery unloaded from the vehicle can be charged and discharged at any time according to the peaking demand of the power system. At this time, the power storage battery is similar to the fixed battery energy storage power station. For battery replacement, a long waiting time is allowed between every two battery replacements, so that the unloading battery can meet the grid regulation requirements and battery life for charging and discharging, while maximizing the value of battery energy storage, as much as possible Extend battery life.

4. Retired battery. With the end of the life of the vehicle power battery, the decommissioned battery is expected to participate in the grid energy storage indirectly through the use of the ladder. Generally speaking, after the battery capacity is reduced to less than 80% of the original capacity, the requirements for the vehicle power battery cannot be met. As the scale of the promotion of the electric vehicle continues to expand, the energy storage potential of the decommissioned battery cannot be ignored.

Based on the above four types of vehicle energy storage methods, if 100 million electric vehicles are promoted nationwide in 2030, the theoretical energy storage capacity of electric vehicles can reach more than 5,000GWh, which is much higher than the potential of pumped storage resources in China, and is fully capable and large-scale. Renewable energy forms a synergy between supply and demand and accelerates the transformation of China's energy structure.

Although the energy storage capacity of the vehicle has a potential advantage over the traditional energy storage resources, there are still many obstacles to its commercialization. In addition to the power battery cost problem, it also has a profound impact on the operation mode of the existing power system.

Taking V2G as an example, first of all, in terms of economy, the cost of vehicle electricity storage is highly correlated with the cycle life of the power battery. At present, the battery life of the vehicle before the power battery capacity retention rate is attenuated to 80% can only meet the accumulated cruising range of 150,000 kilometers, and this can only basically meet the travel demand of the general private electric passenger car users. With the increase of power capacity and cycle life, its cumulative endurance will gradually exceed the travel needs of vehicle users, which in turn will generate the economics of V2G power system services. Therefore, V2G still has certain economic constraints in the near future, but the continuous improvement of battery technology will strongly promote its cost reduction.

Secondly, V2G will also change the existing power market structure, not only adding a new type of decentralized power supply, but also changing the relationship between power grid companies and power generation individuals, making grid companies directly and charging with randomness and complexity of interest. Electric car users have a relationship. From the perspective of market transactions, in the traditional market, electric energy is directly transmitted from the power generation side to the grid company in one direction, and then transmitted to the load side by the grid company, so the capital flow is also one-way transmission; in the market with V2G, the power and The funds flow in both directions between the electric car and the grid. Due to frequent discharges to accelerate battery aging, grid companies need to influence the charging and discharging behavior of electric vehicle users by formulating anti-purchasing electricity prices.

Furthermore, from the perspective of operational mode, V2G resources can provide similar reliability services to large-scale units only when a large number of electric vehicles are concentrated and participate in system services. Therefore, V2G relies on the popularity of electric vehicles, the establishment of smart grids, and business models are mature.

Promotion of vehicle electricity storage policy recommendations

Develop a charge and discharge price that reflects the value of the system

The market mechanism and the price of charge and discharge are the decisive factors affecting the energy storage effect of vehicles. Electric vehicles have huge potential for flexible charging and discharging, and their value needs to be reflected by reasonable market and price mechanism design, thus promoting the friendly connection between electric vehicles and power systems.

Accelerate the construction of charging and discharging operation platform and standard system

At present, the monitoring systems of built-in and under-charging facilities have different levels of support for orderly charging. Even if it supports orderly charging, the system has problems in connection with the grid operating system. With the large-scale application of electric vehicles, electric vehicles will eventually become distributed mobile energy storage facilities, which will place higher demands on ordered charging and discharging. The orderly charging and discharging of electric vehicles depends on vehicles, charging facilities and power grid enterprises. Therefore, research on orderly charging and discharging technologies should fully integrate various stakeholders. For grid companies, charging facilities should be treated as power infrastructure. The charging network should be used as an integral part of the distribution network. Comprehensive consideration should be given to the planning, design, construction and operation of charging facilities and power grids, and accelerate the unified charging service operation platform and The construction of the charging and discharging standard system promotes the coordinated development of electric vehicle charging facilities and the power grid.

Carry out demonstration of synergy project between new energy power generation and electric vehicle

On the basis of technical demonstration, in key cities and regions such as Beijing-Tianjin-Hebei region, Zhangjiakou low-carbon Olympics zone, high-ratio renewable energy demonstration city, energy Internet demonstration zone, national smart city pilot area, and electric vehicles, centrally promote and apply cities, Organize the implementation of vehicle power storage technology demonstration, and accumulate experience in policy and operation mode for large-scale promotion in the future.

Accelerate the use of decommissioned battery energy storage ladders and recycling of raw materials

Lithium, cobalt and other resources are the core materials that lithium battery technology relies on. The motorization of electric vehicles and the storage of power systems will drive the above-mentioned resource consumption to grow geometrically. The competition and division of lithium and cobalt resources in the world will become more intense in the future. Retired battery energy storage and recycling of battery raw materials can alleviate the problem of insufficient supply of upstream raw materials while increasing the life cycle value of the battery. However, current decommissioned battery storage and material recovery still face many challenges. First of all, due to the variety of power battery pack designs for different models, it is necessary to subdivide for different battery types and the process flow and safety issues are quite complicated. Secondly, there are still technical obstacles to the assessment and system integration of decommissioned battery health. How to effectively control the process and material costs of disassembly, testing, grouping, grouping, battery management, etc., improve the relevant technical level and improve cross-industry standards are prerequisites for the decommissioning of battery energy and recycling of key materials.

The page contains the contents of the machine translation.