1. Introduction

In the context of global efforts to combat climate change and achieve carbon neutrality goals, the widespread adoption of renewable energy sources and energy storage technologies has become crucial for the transition to a low carbon energy system. Residential battery energy storage systems (RBESS) play an increasingly important role in this transition. They can store excess electricity generated from renewable sources such as solar panels during the day and supply it to households during peak demand periods or when renewable energy generation is low. This not only enhances the self consumption of renewable energy but also reduces the reliance on the grid, especially from fossil fuel based power generation.

However, while RBESS offer significant benefits in terms of energy efficiency and renewable energy utilization, it is essential to understand their environmental impact, specifically their carbon footprint. The carbon footprint represents the total amount of greenhouse gas emissions (expressed as carbon dioxide equivalent, CO₂e) associated with a product, service, or activity throughout its life cycle. Calculating the carbon footprint of RBESS accurately is of great significance. It helps policymakers and energy planners make informed decisions regarding the promotion and development of these systems. For example, by knowing the carbon footprint, they can assess whether the use of RBESS truly contributes to overall carbon reduction targets. It also provides guidance for manufacturers to optimize the design and production processes of batteries to minimize their environmental impact.

Moreover, with the growing market for residential energy storage, consumers are becoming more environmentally conscious. Understanding the carbon footprint of RBESS allows them to make more sustainable choices when selecting energy storage products. This research on the carbon footprint calculation method for RBESS aims to fill the existing knowledge gaps in this area, providing a comprehensive and accurate approach to measure the environmental impact of these systems.

2. Life Cycle of Residential Battery Energy Storage Systems

The life cycle of an RBESS encompasses several distinct stages, each of which contributes to its overall carbon footprint. These stages include raw material extraction, battery manufacturing, transportation, installation, operation, and end of life treatment.

2.1 Raw Material Extraction

The extraction of raw materials for battery production is a critical and often energy intensive stage. For lithium ion batteries, which are commonly used in RBESS, key raw materials include lithium, cobalt, nickel, and graphite. Lithium extraction, for example, can involve various methods such as brine extraction and hard rock mining. Brine extraction typically requires large amounts of water and energy for evaporation and purification processes, while hard rock mining involves excavation, crushing, and concentration operations, all of which consume significant energy and can lead to environmental degradation.

Cobalt mining, especially in some regions with less regulated mining practices, has raised concerns not only about environmental pollution but also about human rights issues. The extraction of cobalt often involves the use of heavy machinery and chemical reagents, which can result in soil and water contamination. Nickel and graphite extraction also have their own environmental impacts, including deforestation, soil erosion, and greenhouse gas emissions from the energy used in the mining operations. The carbon emissions associated with raw material extraction are mainly from the energy consumption of mining equipment, which is often powered by fossil fuels, and the chemical processes involved in purifying the raw materials.

2.2 Battery Manufacturing

The battery manufacturing stage is another major contributor to the carbon footprint of RBESS. This stage includes multiple processes such as electrode production, cell assembly, and battery pack integration. In electrode production, the synthesis of active materials, coating of electrodes, and drying processes consume a large amount of energy. For instance, the synthesis of lithium based cathode materials often requires high temperature sintering, which is typically carried out in furnaces powered by electricity or fossil fuels.

Cell assembly involves the precise handling and combination of electrodes, separators, and electrolytes. This process requires cleanroom environments and specialized equipment, all of which contribute to energy consumption. Battery pack integration, where individual cells are connected to form a functional battery system, also requires energy intensive processes such as welding and encapsulation. Additionally, the manufacturing of battery components often involves the use of solvents and other chemicals, the production and disposal of which can also have environmental implications.

2.3 Transportation

Transportation of raw materials from the extraction sites to the manufacturing facilities, as well as the transportation of finished batteries from the factories to the installation sites, contributes to the carbon footprint. The carbon emissions from transportation depend on various factors such as the mode of transportation (e.g., trucks, ships, airplanes), the distance traveled, and the fuel efficiency of the transportation vehicles. Long distance transportation of heavy battery materials, especially when using fossil fuel powered ships or trucks, can result in significant greenhouse gas emissions.

2.4 Installation

The installation of RBESS at residential sites also has an associated carbon footprint. This includes the transportation of installation equipment, the energy used by installation tools, and the potential emissions from any additional construction or modification work required at the installation site. Although the carbon emissions during the installation stage are generally relatively smaller compared to other stages, they still contribute to the overall environmental impact of the system.

2.5 Operation

During the operation stage of RBESS, the carbon footprint is mainly related to the energy consumption of the battery management system, cooling systems (if any), and the charging and discharging efficiency of the battery. If the electricity used to charge the RBESS comes from fossil fuel based power plants, a significant portion of the carbon emissions during operation will be associated with the grid electricity. However, if the charging is primarily from renewable energy sources, the carbon emissions during operation can be greatly reduced. Additionally, the degradation of the battery over time can also affect its performance and energy consumption, which in turn impacts the carbon footprint during operation.

2.6 End of Life Treatment

When an RBESS reaches the end of its useful life, proper treatment is essential to minimize environmental impact. End of life treatment options include recycling, reuse, and disposal. Recycling batteries can recover valuable raw materials, reducing the need for virgin material extraction and associated carbon emissions. However, the recycling process itself requires energy, and if not properly managed, can lead to environmental pollution. Reuse of batteries in less demanding applications can extend their life cycle and reduce the overall carbon footprint. Disposal, especially in landfills, can pose significant environmental risks due to the potential leakage of toxic chemicals from the batteries.

3. Carbon Footprint Calculation Models

There are several established carbon footprint calculation models that can be adapted or applied to RBESS. These models provide a structured approach to quantify the greenhouse gas emissions associated with different stages of the RBESS life cycle.

3.1 Life Cycle Assessment (LCA) Model

The Life Cycle Assessment (LCA) model is one of the most comprehensive and widely used methods for calculating the carbon footprint of products and systems. It takes into account all the inputs and outputs throughout the entire life cycle of the RBESS, from raw material extraction to end of life treatment. The LCA model is based on the principle of mass and energy balance, where all the materials and energy flows are traced and their associated greenhouse gas emissions are calculated.

In the context of RBESS, the LCA model first defines the system boundaries clearly. This includes determining which processes and stages are included in the assessment. For example, it may choose to include the entire supply chain from raw material extraction to the installation of the RBESS at the residential site, or it may focus only on the manufacturing and operation stages. Once the system boundaries are defined, data on material inputs, energy consumption, and emissions from each stage are collected. This data can be obtained from various sources such as industry reports, manufacturer data, and academic research. Then, through a series of calculations based on emission factors (which represent the amount of greenhouse gas emissions per unit of activity), the total carbon footprint of the RBESS can be determined.

The LCA model has the advantage of providing a holistic view of the environmental impact of RBESS. It allows for comparisons between different types of batteries, manufacturing processes, and end of life treatment options. However, it also has some limitations. The data collection process can be time consuming and costly, and the accuracy of the results depends heavily on the quality of the data. Additionally, the LCA model may not fully capture some dynamic factors, such as changes in energy mix over time during the operation stage.

3.2 Input Output Model

The Input Output (IO) model is another approach that can be used to calculate the carbon footprint of RBESS. This model is based on economic input output tables, which show the inter relationships between different industries in an economy. In the case of RBESS, the IO model can be used to trace the flow of goods and services related to the production, use, and disposal of the system through the economy and estimate the associated carbon emissions.

The IO model has the advantage of being able to capture the indirect emissions associated with the RBESS. For example, it can account for the emissions from the production of the machinery used in battery manufacturing, which may not be directly included in a traditional LCA. However, the IO model also has limitations. It provides a more aggregated view of the economy and may not be as detailed as the LCA model in terms of specific processes and stages within the RBESS life cycle. Additionally, the accuracy of the IO model depends on the quality and timeliness of the input output tables, which may not be updated frequently enough to reflect the latest technological changes in the RBESS industry.

3.3 Hybrid Model

To overcome the limitations of the LCA and IO models, a hybrid model that combines the strengths of both can be developed for calculating the carbon footprint of RBESS. The hybrid model starts with a detailed LCA based analysis of the key processes and stages within the RBESS life cycle, such as raw material extraction and battery manufacturing. Then, the IO model is used to account for the indirect emissions that are not fully captured by the LCA, such as the emissions from the broader economic activities related to the industry.

The hybrid model offers a more comprehensive and accurate approach to calculating the carbon footprint of RBESS. It can provide detailed information on the direct emissions from specific processes while also considering the indirect emissions from the overall economic system. However, developing and implementing a hybrid model requires a significant amount of data collection, integration, and computational resources.

4. Data Acquisition and Analysis

Accurate data acquisition is the foundation for reliable carbon footprint calculation of RBESS. The data required for the calculation covers various aspects of the RBESS life cycle, and different methods are used to obtain and analyze this data.

4.1 Data Sources

4.1.1 Manufacturer Data

Manufacturers of RBESS can be an important source of data. They often have detailed information about the materials used in battery production, the energy consumption during the manufacturing process, and the transportation methods employed. For example, a battery manufacturer can provide data on the exact composition of the cathode and anode materials, the amount of electricity consumed in each production step, and the type of vehicles used to transport the finished batteries. However, the data provided by manufacturers may be subject to biases, as they may be more inclined to present data that shows their products in a favorable light. Therefore, it is necessary to verify and cross check this data with other sources.

4.1.2 Industry Reports and Databases

Industry reports and databases, such as those from international organizations, research institutions, and industry associations, can offer valuable data on the RBESS industry. These sources may provide general information on average material consumption, energy efficiency, and emission factors for different types of batteries and manufacturing processes. For example, the International Energy Agency (IEA) and the European Battery Alliance often publish reports that contain data relevant to the environmental impact of battery energy storage systems. However, the data in these sources may be more aggregated and may not be specific enough for detailed carbon footprint calculations for individual RBESS products.

4.1.3 Academic Research

Academic research plays a crucial role in data acquisition. Many studies focus on specific aspects of the RBESS life cycle, such as raw material extraction technologies, battery manufacturing processes, and end of life treatment methods. These research papers can provide in depth data on the energy consumption, emissions, and environmental impacts associated with these processes. For example, academic studies may investigate the carbon emissions of different lithium extraction methods or the efficiency of battery recycling processes. However, the data from academic research may be based on laboratory scale experiments or specific case studies, and it may need to be adjusted and scaled up for practical application in carbon footprint calculations.

4.2 Data Analysis

Once the data is collected, it needs to be analyzed carefully. The first step in data analysis is data cleaning, which involves removing any incorrect, inconsistent, or missing data. For example, if there are outliers in the energy consumption data during battery manufacturing, they need to be identified and either corrected or removed.

After data cleaning, the data is processed according to the selected carbon footprint calculation model. For the LCA model, the data on material inputs and energy consumption are used to calculate the emissions from each stage of the RBESS life cycle using appropriate emission factors. These emission factors are usually obtained from established databases or literature. For the IO model, the data is integrated into the economic input output framework to estimate the indirect emissions. In the case of the hybrid model, a combination of these data processing methods is used.

Sensitivity analysis is also an important part of data analysis. It helps to understand how changes in input data, such as variations in energy prices, raw material costs, or emission factors, affect the calculated carbon footprint. By performing sensitivity analysis, researchers can identify the key factors that have the most significant impact on the carbon footprint of RBESS and focus on improving the accuracy of the data collection and calculation for these factors.

5. Case Studies

To illustrate the practical application of the carbon footprint calculation method for RBESS, several case studies can be conducted.

5.1 Case Study 1: A Lithium Ion Based RBESS in a European Country

In this case study, a typical lithium ion based RBESS installed in a residential building in a European country is selected. The system boundaries are defined to include raw material extraction, battery manufacturing in Asia, transportation to Europe, installation at the residential site, a 10 year operation period, and end of life treatment in a local recycling facility.

Data on raw material extraction is obtained from industry reports on the mining operations of lithium, cobalt, nickel, and graphite in the relevant regions. The battery manufacturing data is provided by the manufacturer, including the energy consumption in each production process and the types of materials used. Transportation data is estimated based on the average distances and transportation modes used from the manufacturing site to the installation site.

During the operation stage, the electricity consumption of the battery management system and the charging and discharging data are collected from the system's monitoring records. The electricity used for charging the RBESS comes from a mix of renewable energy (30%) and grid electricity (70%). At the end of the 10 year operation period, the battery is sent to a recycling facility, and data on the energy consumption and emissions during the recycling process are obtained from the recycling plant.

Using the LCA model, the carbon footprint of this RBESS is calculated. The results show that the raw material extraction and battery manufacturing stages contribute the largest proportion of the total carbon footprint, accounting for about 60% of the total emissions. The operation stage contributes about 30% due to the grid electricity related emissions, and the remaining 10% is from transportation, installation, and end of life treatment.

5.2 Case Study 2: A Sodium Ion Based RBESS in an Asian Country

This case study focuses on a sodium ion based RBESS installed in a residential area in an Asian country. Sodium ion batteries are considered as a potential alternative to lithium ion batteries due to their lower cost and more abundant raw materials. Similar to the first case study, the system boundaries are defined, and data is collected from various sources.

The raw material extraction data for sodium ion batteries is based on the latest research on sodium based raw material mining and processing. The manufacturing data is obtained from a local manufacturer that specializes in sodium ion battery production. During the operation stage, the RBESS is mainly charged by solar panels installed on the residential roof, with only a small portion (10%) of electricity coming from the grid.

Using the hybrid model for carbon footprint calculation, the results show that the carbon footprint of this sodium ion based RBESS is significantly lower than that of the lithium ion based RBESS in the first case study. The raw material extraction and manufacturing stages contribute about 40% of the total carbon footprint, while the operation stage contributes about 50% due to the high proportion of renewable energy charging. The remaining 10% is from other stages.

These case studies not only demonstrate the practical application of the carbon footprint calculation methods but also highlight the differences in carbon footprints between different types of RBESS and the importance of factors such as raw material selection, energy source during operation, and end of life treatment options in reducing the overall environmental impact.

6. Conclusion and Future Research Directions

In conclusion, calculating the carbon footprint of residential battery energy storage systems is essential for understanding their environmental impact and promoting sustainable development in the energy storage industry. Through the analysis of the life cycle of RBESS, the exploration of different carbon footprint calculation models, the study of data acquisition and analysis methods, and the implementation of case studies, a comprehensive approach to carbon footprint calculation has been presented in this research.

It has been found that the raw material extraction and battery manufacturing stages are the major contributors to the carbon footprint of RBESS, highlighting the need for more sustainable mining and manufacturing practices. The energy source during the operation stage also has a significant impact on the carbon footprint, emphasizing the importance of promoting the use of renewable energy for charging RBESS.

However, there are still several areas that require further research. Firstly, as the technology of RBESS continues to evolve, new types of batteries and manufacturing processes are emerging. Research is needed to update and improve the carbon footprint calculation methods to adapt to these technological changes. Secondly, more accurate and detailed data on the RBESS life cycle, especially for emerging technologies and regions with limited data availability, should be collected. This can be achieved through better cooperation between researchers, manufacturers, and industry organizations. Thirdly, the dynamic factors during the operation stage, such as changes in the energy mix of the grid over time and the degradation of battery performance, need to be further studied and incorporated more comprehensively into the carbon footprint calculation models.

In the future, with continuous research and improvement in the carbon footprint calculation method for RBESS, it will be possible to make more informed decisions in promoting the development and application of these systems, ultimately contributing to the global goal of reducing greenhouse gas emissions and achieving a low carbon energy future.