Research on Key Factors of the Life Cycle Carbon Emission for Battery Electric Vehicles

. With the ever increasing energy crisis and environmental issues globally in recent years, China is enhancing the processes of automobile electrification. Compared with traditional fuel vehicles, Battery Electric Vehicle (BEV) has lower carbon emission level during the whole life cycle, which is one of important technical routes for the low-carbon development of the automotive industry. Based on the main influencing factors of carbon emission for BEVs, this paper focuses on the correlation between the five key factors and carbon emission, including vehicle mileage, power grid cleaning, energy consumption level, battery energy density and utilization rate of renewable materials, and proposes development suggestions for carbon reduction ways for BEVs.


Introduction
Carbon neutralization in automobile industry is a systematic project, involving multiple fields of energy, industry and transportation information, as well as multiple links of R&D, production, use and recycling. In order to achieve the goal of carbon peaking and carbon neutralization in the automobile industry, not only do we need to make technological breakthroughs, but also other relevant industries need to accelerate the research and development of low-carbon technologies, accelerate the energy conservation and emission reduction in the whole life cycle of automobiles and the whole industrial chain, and jointly promote the green development of the automobile industry.
In the past year, the dual carbon target has become an important driving force to promote the rapid development of new energy vehicles and accelerate the low-carbon transformation of the automobile industry. Thanks to the technological innovation of battery structure and battery materials, the driving range of models equipped with lithium iron phosphate battery system continues to increase, which can meet the daily travel needs of consumers. At present, the installed capacity of lithium iron phosphate battery has gradually increased.
Research results at home and abroad [1][2] show that the full life cycle perspective is an important method to study the resource consumption and environmental impact of automotive products. The overall carbon emissions of automobile product life cycle consist of fuel cycle and vehicle cycle. According to research and calculation, although the carbon emission of battery electric vehicles in the vehicle cycle is higher than that of traditional fuel vehicles, the total carbon emission in the life cycle of battery electric vehicles is lower than that of traditional vehicles because they can achieve zero emissions in the operation phase, and the future emission reduction potential is greater [3].
This paper selects the representative models of the battery electric passenger vehicle market, quantitatively analyzes the relationship between key influencing factors and carbon emissions, and puts forward development suggestions on carbon reduction approaches of battery electric passenger vehicles.

Life cycle model
Life Cycle Assessment (LCA) is an assessment method to analyze the resource consumption and environmental impact of automotive products in the stages of raw material acquisition, parts production and manufacturing, vehicle production and manufacturing, fuel production, fuel transportation, fuel use and scrap recovery from the perspective of the entire life cycle. Considering the availability of input data and its contribution to the overall carbon emissions of the vehicle model, this paper only considers the four upstream stages [4][5] of traditional component raw material acquisition, battery raw material acquisition, vehicle production and manufacturing, and vehicle driving process power for the calculation of carbon emissions of battery electric passenger vehicles, and other stages are ignored. The calculation formula is shown in Formula (1).
(1) Where, CBEV is the total carbon emission of BEV; CM is the carbon emission in the raw material acquisition stage of traditional components; CBM is the carbon emission in the battery raw material acquisition stage; CP is the carbon emission in the production and manufacturing stage of the whole vehicle; CF is the carbon emission upstream of the electric power during vehicle driving.
The carbon emission of traditional components in the raw material acquisition stage (CM) is calculated according to formula (2).
(2) Where, n is the number of material types of BEV traditional parts; Wi is the ith material quality of traditional components; Ei is the carbon emission factor of the ith material of traditional components; L is the life cycle mileage of BEV.
The carbon emission in the battery raw material acquisition stage (CBM) is calculated by formula (3).
(3) Where, n is the number of BEV battery material types; BWi is the mass of the ith material of the battery; BEi is the carbon emission factor of the ith battery material; L is the life cycle mileage of BEV.
Carbon emissions in the production and manufacturing stage of the whole vehicle (CP), the calculation formula [6] is shown in Formula (4).
(4) Where, n is the amount of fossil energy consumed in the production and manufacturing process of BEV vehicles; FCi is the consumption of the ith fossil energy; FEi is the carbon emission factor of the ith fossil energy use process; ECP is the power consumption in the whole vehicle production process; EP is the carbon emission factor in the power production process; L is the life cycle mileage of BEV.
The calculation formula of carbon emissions upstream of the electric power during vehicle driving (CF) is shown in Formula (5). × (5) Where, EC is the power consumption per unit mileage during BEV driving; EP is the carbon emission factor of power production process.

Correlation analysis method
Correlation analysis is a method used to evaluate the impact of input factors on output results in life cycle research. It can be subdivided into single factor correlation analysis and multi factor correlation analysis. Single factor correlation analysis is used to measure the impact of a single input factor on output results, and multi factor correlation analysis is used to measure the impact of two or more input factors on output results. In this paper, single factor correlation analysis is used to study the impact of different factors on carbon emissions.

Impact factors
It can be seen from the life cycle model that the carbon emissions of battery electric passenger vehicles are jointly determined by vehicle material quality, vehicle material carbon emission factor, energy consumption in the production and manufacturing process, vehicle power consumption, upstream carbon emission factor of electric power, and travel characteristics. According to the calculation, from the weight of carbon emissions in each stage in the total carbon emissions of the BEV life cycle, the upstream power of the vehicle is the largest, followed by the traditional component raw material acquisition stage, battery raw material acquisition stage, and the vehicle manufacturing stage is the smallest. In addition, the mass of a battery electric vehicle is negatively correlated with the battery energy density. The higher the battery density, the lower the vehicle mass [7]. Therefore, this paper focuses on the analysis of the correlation between the five impact factors of vehicle mileage, power grid cleaning, energy consumption level, battery energy density and renewable material utilization rate and carbon emissions.

Representative vehicle model
In order to distinguish the correlation between the carbon emissions of vehicles equipped with different types of power batteries and various influence factors, and at the same time, it is necessary to take into account the principle of comparability of vehicle parameters of different objects, this paper selects different versions of a company's A-class battery electric car as the research object, and records the driving range of 410 km, the vehicle model equipped with lithium iron phosphate battery as BEV①, the driving range of 510 km and the model equipped with ternary lithium battery as BEV②, and the driving range of 602 km and the model equipped with ternary lithium battery as BEV③. Table 1 shows some parameters of representative vehicle models, mainly referring to the Catalogue of Recommended Models for the Promotion and Application of New Energy Vehicles in the 9th batch in 2020 and the 5th batch in 2021 [8][9]. Assuming that the vehicle life cycle mileage is 150,000 km, the carbon emissions per unit mileage of BEV①, BEV② and BEV③ are 115.6g·km -1 , 127.5g·km -1 and 133.3g·km -1 .

Vehicle mileage
Vehicle mileage reflects the use intensity of a vehicle throughout its life cycle. With the increase of vehicle use intensity, the carbon emissions of battery electric vehicle cycle are greatly diluted, so the total carbon emissions will be significantly reduced. In this paper, the total carbon emissions of the three BEV models are calculated with 50,000 km as the division interval. It can be seen from Figure 1 that when the driving mileage in the life cycle increases from 50,000 km to 200,000 km, the total carbon emissions of all models show a rapid downward trend. For every additional 10,000 km, the carbon emissions per unit mileage of BEV① decrease by 5.5 g·km -1 , and that of BEV② and BEV③ decrease by 7~8 g·km -1 . It can be seen that with the increase of vehicle driving mileage, The carbon emission of BEV equipped with ternary lithium battery decreased more than that of BEV equipped with lithium iron phosphate battery; When the driving mileage in the life cycle is from 200,000 km to 500,000 km, the total carbon emissions of vehicle models still keep a downward trend, but the decline is slowing down, which also shows that for commercial vehicles, due to the long driving mileage, the total carbon emissions per mile are significantly lower than private vehicles, and the carbon reduction effect is very obvious, so the taxi field should vigorously promote electrification.

Power grid cleaning
Due to the rich coal resources, thermal power generation is the main power generation mode in China, accounting for about 70% of the total power generation in China [10]. The carbon emission factor of 0.6101 kg·(kW·h) -1 used in this paper is from the Accounting Methods and Reporting Guidelines for Greenhouse Gas Emissions of Enterprises issued by the Ministry of Ecology and Environment in March 2021 [11]. The upstream of electric power, that is, the upstream of fuel, is the main component of carbon emissions during the whole life cycle of battery electric vehicles. According to the current level of power production, the carbon emissions in the upstream of fuel account for about 60% of the total. Therefore, the degree of grid cleanliness is crucial to improve the emission reduction performance of battery electric vehicles in their life cycle. It is estimated that the carbon emission factor of electric power production will decrease by 0.1 kg·(kW·h) -1 for every 10% decrease in the proportion of thermal power generation. Figure 2 shows the relationship between grid cleaning and carbon emissions. It can be seen that there is a linear correlation between the two. For every 0.1 kg·(kW·h) -1 reduction in carbon emission factor of power production, the total carbon emissions of vehicle models will decrease by about 13 g·km -1 . By changing the power structure of China, the proportion of clean energy power generation (such as wind power, nuclear power and hydropower) will gradually increase [12], the carbon emissions in the upstream of fuel will be significantly reduced, and the proportion in the entire life cycle will also be reduced synchronously, and the carbon emissions in the full life cycle of battery electric vehicles will be gradually reduced. kW·h·100km -1 [13]. Compared with traditional vehicles, the carbon reduction effect of battery electric vehicles is better. It is estimated that a battery electric car with a power consumption of 16~18 kW·h·100km -1 has the same carbon emissions in its life cycle as a gasoline car with a fuel consumption of 4~5 L·100 km -1 . It can be seen from Figure 3 that for every 1 kW·h·100km -1 decrease in energy consumption level of BEV, carbon emissions will decrease by about 6 g·km -1 . In the future, the energy consumption level of electric vehicles will still be further reduced. It is estimated that by 2025, the power consumption of a typical class A battery electric car with advanced technology under comprehensive working conditions will be less than 11 kW·h·100km -1 [13].

Battery energy density
At present, in the battery electric passenger vehicle market, for lithium iron phosphate battery, the sales of models with energy density of 120~140 Wh·kg -1 account for the largest proportion. For ternary lithium batteries, the sales volume of models with energy density of 160~180 W·h·kg -1 accounted for the largest proportion, and some models reached 190~200 W·h·kg -1 . With the increase of battery energy density, the battery quality will decline, and the carbon emissions generated by the battery will decrease. At the same time, the decline of the vehicle's curb weight will also lead to a decrease in power consumption [14], and the carbon emissions of the entire vehicle model will gradually decrease. It can be seen from Figure 4 that when the energy density increases from 80 W·h·kg -1 to 120 W·h·kg -1 , the carbon emissions of BEV① decrease by 10%, and those of BEV ② and BEV③ decrease by about 15%; When the energy density increases from 120 W·h·kg -1 to 160 W·h·kg -1 , the carbon emission of BEV① decreases by 5.5%, and that of BEV② and BEV③ decreases by 8%~9%; In general, with the increase of battery energy density, the carbon reduction effect of BEV equipped with ternary lithium battery is better than that of BEV equipped with lithium iron phosphate battery. Therefore, starting from the green design of power battery, improving its energy density will play a certain role in emission reduction of battery electric vehicle life cycle [15].

Utilization rate of renewable materials
Automobile production needs to consume a large amount of steel, nonferrous metals, plastics, rubber, glass and textile resources. The reasonable reuse of scrapped automobile resources can effectively promote energy conservation and consumption reduction in the automobile industry [16]. At present, the utilization rate of renewable materials for domestic automotive products is not high, and the application scope is limited. There is no authoritative organization to calculate the specific value of this proportion. This paper mainly considers the steel, aluminum, copper and plastic traditional materials used on chassis and body, and takes the current level and assumed future proportion increase as variables to calculate the change trend of vehicle carbon emissions. It can be seen from Figure 5 that every 5% increase in the utilization rate of renewable materials will reduce the carbon emissions by about 1 g·km -1 . With the increase in the utilization rate of renewable materials, the carbon emissions of electric vehicles will decrease, but the reduction is limited. In June 2021, the four ministries and commissions jointly issued the Implementation Plan for the Pilot Program for the Extension of Producer Responsibility of Automotive Products [17], which proposed that by 2023, the green supply chain system for automobiles should be fully established, with the recyclability rate of automobiles reaching 95%, and the utilization rate of renewable raw materials for key components no less than 5%. In the future, with the progress of recycling technology, automotive renewable materials will be widely used in automobiles, thereby reducing production costs.

Summary
Based on the life cycle research theory, this paper selects different versions of typical models of battery electric passenger vehicles, and focuses on the correlation between vehicle mileage, grid cleaning, energy consumption level, battery energy density, renewable material utilization and carbon emissions. According to the quantitative research conclusion, the changes of these five influencing factors are positively or negatively correlated with the carbon emissions of vehicle models. The main conclusions are as follows.
(1) The change of vehicle mileage has the greatest impact on carbon emissions of vehicle models, and taxi electrification is of great significance to carbon emission reduction; (2) Power grid cleaning is an important way to control carbon emissions and total carbon emissions of single vehicles. In the medium and long term, there is great room for carbon emission intensity of power production to decline; (3) The energy consumption level of electric vehicles will directly affect the carbon emissions upstream of the fuel. In the next five years, there is still 10% room for energy consumption to decline, and then the energy consumption will enter a slow decline period; (4) On the premise of meeting the requirements of battery safety, power, driving range, service life and cost, the carbon reduction effect of electric vehicles equipped with higher energy density power batteries is better; (5) In the short term, the improvement of the utilization rate of renewable materials has a weak effect on reducing carbon emissions in the life cycle of electric vehicles.
In a word, compared with traditional fuel vehicles, battery electric vehicles have obvious carbon reduction advantages. Under the dual carbon goal, the development of battery electric vehicles will become a favourable measure for energy conservation, emission reduction and carbon reduction in the field of transportation.