1. Introduction
The transportation industry has become the department with the fastest growth in energy consumption in China, and the development of any industry cannot be separated from the coordination of the transportation links, which makes the realization of energy conservation and emission reduction crucial in the process of logistics transportation. The transportation industry is one of the most energy-consuming and polluting sectors, typically accounting for 30–35% of the total fuel demand in most industrialized countries [
1]. Statistics from the International Energy Agency (IEA) showed that the transportation industry has the fastest growth in carbon emissions in all economic departments [
2]. Some scholars also pointed out that the transportation industry is the fastest-growing energy consumption industry in the world [
3]. The greenhouse gas emissions generated during car operation leads to global warming [
4]. According to the report of the Intergovernmental Panel on Climate Change (IPCC), the continuous emission of fossil fuels will lead to a temperature increase of 1.4 to 5.8 degrees Celsius between 1990 and 2100 [
5], among which the transportation industry has accounted for 13.5% of global warming [
6]. Therefore, implementing the strategy of sustainable development has become an inevitable trend in the progress of the international society. The United Nations Conference on Environment and Development was held in Rio de Janeiro, Brazil, reaching the “United Nations Framework Convention on Climate Change” (UNFCCC) on climate change. The agreement emphasized that action on climate change must be carried out within the framework of sustainable development and that reducing greenhouse gas emissions is a global common society responsibility [
7]. The Energy Charter Treaty had a restrictive effect on energy consumption generated by transportation activities and aimed to improve energy efficiency [
8]. Due to increasing environmental degradation and severe resource shortages in China [
9], in order to promote the healthy development of the industry, the 13th Five-Year Plan for the Development of Renewable Energy stipulated a series of principles to improve the energy structure [
10]. The Chinese government proposed the sustainable development of a circular economy strategy [
11] and became the first country in the world to promulgate the relevant laws and regulations.
2. Literature Review
The increase in energy consumption is mainly due to the fuel consumption generated in the process of transportation. Therefore, it is necessary to select an appropriate index to evaluate the impact of energy consumption on the environment of a country or region. In order to understand the credibility of the emergy analysis model, Giannetti et al. [
12] used an emergy comprehensive method to analyze the reverse logistics network of steel recovery and pointed out that when the recovery price of steel reverse logistics is the lowest, fuel consumption was more important than investment in goods and services. Combining emergy from the perspective of the transportation system, Cristiano et al. [
13] applied an emergy analysis to evaluate the convenience of civil infrastructure construction in transportation research and found that part of the transportation mode is converted to railway transportation ways that can improve the sustainability of the transportation system. Huang et al. [
14] conducted an exploratory study on land transportation modes from the perspectives of emergy and found that the public transportation mode is an inevitable choice to improve environmental efficiency. Similarly, Liu et al. [
15] used the emergy analysis method to explore the efficiency and sustainability of rapid pyrolysis and large-scale transportation fuel production and set up two programs for hydrogenation production and hydrogenation purchase. The study found that the purchase of hydrogen is the best choice for planning a biofuel production system, which provides a feasible solution for large-scale transportation fuel production. In addition, Meng et al. [
16] discussed the overall development of urban public transportation systems. The study indicated that a rapid bus transit system can show high energy and environmental performances, and higher transportation speeds and technologies can improve transportation efficiency. The above research mainly focused on the use of emergy analysis to evaluate the environmental performance of energy consumption in logistics and transportation. Based on the consideration of the sustainability dimension, we summarized the current research fields of emergy analysis and the corresponding emergy evaluation indicators in
Table 1.
However, it is difficult to study the dynamic interactions of the system. A small change in any one factor may cause a huge change in the entire system, so it is necessary to conduct a dynamic analysis on the internal structure of the system and the internal relations of different subsystems. From the perspective of policy recommendations, we divided the measures to optimize the transportation system into three categories. The first category is policy incentive measures. Liu D et al. [
26] used a system dynamics model to establish an electric vehicle development scenario under policy incentives. The results confirmed that the Chinese electric vehicle market is mainly dominated by national policies, especially financial support, followed by environmental policy and energy policy support. The second category is technological advancement measures. Gómez Vilchez et al. [
27] conducted a comprehensive analysis of the development history of system dynamics in transportation vehicles and found that technical research has been the main direction of exploration in recent years. Liu X et al. [
28] found that technological progress is the optimal choice to achieve energy conservation and emission reduction in the short term. Mallick et al. [
29] established a road system dynamics model for sustainable development based on the depletion of natural aggregate reserves. The results showed that policy makers and technological innovation are the key links to achieve sustainable development of road environments. The third category is the development of clean energy. Hao et al. [
30] used the multiplier energy consumption as an object and the results showed that changing the fuel structure and improving fuel efficiency are important ways to reduce transportation carbon emissions. Later, Kuo et al. [
31] proposed the use of biofuels to support the renewable development of fuel consumption in the transportation sector. They found that the increase in the proportion of biofuels should be achieved through technological development. Cao et al. [
32] established the situational development trends of the thermal coal supply chain. The results showed that the effective ways to reduce carbon emissions include improving transportation structure and increasing investment in biodiesel. The above research mainly adopted system dynamics to conduct dynamic analyses on the whole transportation system and showed the ways to reduce energy consumption, as shown in
Table 2. However, these studies only considered the sustainable development of the transportation industry from the perspective of the environment, and such results cannot fully reflect the situation of a country or region.
Fang et al. [
36] combined the emergy theory with the system dynamics model and used the system dynamics model to simulate the urban emergy flow. Lei et al. [
37] used the emergy flow analysis method to analyze and describe the development of Macau, and the dynamic simulation software simulated the emergy trend. Based on the above research content, it can be seen that some scholars used emergy analysis method to carry out research in logistics, and others used a system dynamics model to optimize the transportation system. However, few papers link the emergy analysis method with system dynamics when applying it to the transportation industry. Due to the lack of research in this field, we combined emergy analysis with system dynamics and selected Qinghai as the research area to collect energy consumption data, system modeling and scenario analysis in the transportation industry.
The emergy analysis method enables the unified description and quantitative measurement of ecological objectives and effectively simplifies the complex interactions between human social activities and natural systems. The emergy evaluation index can be used to analyze the sustainable development status in specific fields. Therefore, in order to reflect the ecological status of the transportation industry more objectively and truly, this study introduces the emergy analysis method for the sustainable development evaluation of the transportation industry. However, the emergy analysis method ignores the contribution of technical progress and economic development to the sustainable development of transportation industry and thus cannot describe the dynamic change trend of the complex feedback relationships and ecological conditions in the transportation system. To this end, this study introduces the system dynamics method that can simulate the structure and function of the system and build the system dynamics model of the transportation industry to realize the simulation of the development trend of the transportation ecosystem. Thus, we combined emergy theory with the system dynamics method to construct a simulation evaluation model for regional development. As such, the evolution mechanism of the industrial ecosystem can be investigated qualitatively and quantitatively, which can not only evaluate the current development situation, but also explore the future development mode. The research objective is to improve the sustainable performance of the transportation system through scenario analysis.
3. Data and Methods
3.1. Study Area and Data
Qinghai is located in the west of China (geographical position is between 89°35′ E–103°04′ E and 31°36′ N–39°19′ N), situated in the northeast of the Qinghai–Tibet Plateau, with an area of 722,300 square kilometers, ranking the fourth largest province in China in terms of land area. Qinghai is a province of natural resources, including sufficient energy such as renewable resources and non-renewable resources, which is the unique advantage of Qinghai. Although the modern service industry started late, it has developed rapidly, with investment in fixed assets and infrastructure construction showing positive growth, among which investment growth is the fastest in the new energy industry. However, to maintain the growth of the GDP, many people need to be supported. The increase in population leads to an accelerated consumption of resources, including internal and external resources. According to the Qinghai Province Railway Development Plan, the railway to Chengdu forms a convenient passage connecting with the southwest, and the railway to Dunhuang constitutes a channel to connect with the northwest. Due to the planning and construction of the Qinghai–Tibet Railway and the implementation of the western development strategy, the construction of transportation infrastructure has continued to expand within Qinghai Province. From the perspective of these characteristics, Qinghai is a representative province in terms of the abundance of natural resources and the development trend of the transportation industry in recent years. Therefore, this paper selects Qinghai as the object of case study.
In this article, most of the original data comes from the Qinghai Statistical Yearbook (2011–2018), the China Energy Statistical Yearbook (2011–2018) and the China Statistical Yearbook (2011–2018) (the National Bureau of Statistics, 2011–2018). The remaining data was collected from the Qinghai Yearbook (2011–2018) [
38,
39,
40,
41].
3.2. Emergy Analysis Method
The concept of emergy was introduced by Odum and is sometimes considered as an expansion of the embodied energy concept [
42]. It is defined as “the available energy of one kind previously used directly or indirectly to generate a service or a product” [
43]. Emergy allows for the measurement of system performances based on a common energy metrics. Emergy is the product of the original data and the emergy conversion rate. The emergy conversion rate is named UEV, which is the amount of solar energy required to produce 1J product or service. The emergy index can quantitatively analyze the degree of sustainable development of natural resources. The emergy analysis involves the flow, conversion and storage of energy. As an emergy method based on thermo dynamics, the total emergy is obtained by converting fuel, energy, electricity, human labor and services.
3.2.1. Emergy Flow
Emergy flow can be used to analyze the flow of resources and goods between various departments in the city, as well as the exchange between the socioeconomic system and the ecological environment. The emergy analysis method has been used to quantitatively assess the carrying capacity of the urban ecological environment. Generally speaking, the energy flow diagram includes the interaction between inputs, system components, symbolic components and the final product. The emergy flow is mainly concentrated on the conversion of internal and external resources [
44]. Combined with the actual situation, we have depicted the energy system diagram of the transportation industry in Qinghai Province on the Qinghai–Tibet Plateau, as shown in the figure below.
Figure 1 shows the energy source and flow path of the transportation system in Qinghai Province. On the left is the renewable resource, which is the wealth foundation of the system. It is connected with the internal natural ecosystem, providing a variety of goods and services for human beings and supporting the survival and development of mankind. The exterior of the system includes the resource consumables and labor services of the transportation industry, which are fuel, electricity, vehicles and people in a hierarchical order respectively. Inputs purchased from outside the system are introduced along with their associated services to purchase sufficient resources, energy and labor through trade. Within the system, the natural ecosystem acts as a carrier for heat loss associated with the construction and maintenance of facilities, passenger and cargo transportation and vehicles. Electricity is an excellent energy with a very wide range of sources and uses. The actual production and life cannot be separated from the support of electricity. The region converts materials and goods into infrastructure through specific resource consumption, vehicle assistance, labor support and road investment. The road infrastructure built and maintained allows passengers and goods to pass. Transportation is the core part of the system, driven by incoming vehicles, driver’s labor, fuel and electricity and renewable resources. The final system exports passenger and freight services on the right side, resulting in heat loss below the system.
3.2.2. Emergy Analysis Index
Taking into account the actual local conditions in the process of calculation, this paper calculated the emergy flow during the operating years from 2010 to 2017. The following is an explanation of the formula for calculating the emergy index, and references for emergy conversion rates are shown in
Table 3.
In order to satisfy the convenience of the emergy analysis research, we divided the data into renewable resources (R), non-renewable resources (N), economic feedback input (F) and output emergy (Y) [
51]. A summary of the emergy index and its meanings is shown in
Table 4.
Combined with the basic situation of the development of transportation industry in Qinghai Province, based on emergy theory, this paper analyzes its characteristics and operating state from a new perspective, and evaluates the development status and future development potential of transportation industry. The evaluation index system should fully consider the availability and completeness of the data, which can not only reflect its characteristics but also make full use of the relevant statistical data.
3.3. System Dynamics
System dynamics is a methodology for the systematic analysis of social and economic problems proposed by Professor Frates of the Massachusetts Institute of Technology. It uses a computer as a tool to analyze and study the structure and behavior of information feedback system. It is known that dynamics can be defined as the speed of change over time. System dynamics studies the overall behavior of the system by analyzing the feedback structure relationship between various variables in the socioeconomic system.
The variables include level variables and rate variables, as well as auxiliary variables. Level variables are generated by activities in the system, which explain the activities of the system at a certain point in time. Rate variables are variables of control flow, which indicate the variable of the system activity at a certain point. The basic idea is that feedback control can be carried out. Therefore, system dynamics can be used to deal with dynamic and complex problems [
52].
3.3.1. Causal Cycle Diagram
We constructed a causal loop diagram, as shown in
Figure 2. The causal relationship diagram shows the main variables of the model and can identify some positive feedback and negative feedback loops so that we can deeply understand the feedback and control processes of the system [
53]. In order to gain a deeper understanding of the causal cycle diagram, it is necessary to analyze the causal relationship between the variables. For example, there is a positive relationship between the emergy of non-renewable resources and the environmental load rate in transportation industry. In other words, it means that if the energy consumption of non-renewable resources increases in the transportation industry, its environmental load rate will increase. Conversely, there is a negative relationship between renewable resources and environmental load rate. Therefore, if the area has an increase in renewable resources, the environmental load rate will decrease. The closed loop of causality forms the feedback loop, which defines the dynamics of the model [
54].
3.3.2. System Flow Diagram of the Transportation Industry
Based on the causal cycle diagram, we built a dynamic model of urban emergy flow system based on the energy flow diagram and emergy evaluation index system, as shown in
Figure 3. This model is useful for combining causality, feedback, and delay between variables [
55]. In the system flow diagram of the transportation industry, we set horizontal variables such as regional GDP, output value of the tertiary industry, population, industrial output value of the logistics industry, and volume of goods turnover. Due to the rapid economic development in recent years, the development of the transportation industry has been promoted. The investment in logistics infrastructure and the enhancement of logistics supply capabilities have increased the output value of the logistics industry. The rapid economic development, especially the rapid growth of GDP per capita, will have a great impact on the transportation system. Population is one of the level variables which will have a great impact on the system if it changes slightly. The increase in population will lead to the increase in logistics transaction volume, the increase in population welfare will lead to an increase in the new born population, and the improvement in the level of medical resources will reduce the number of deaths in the population. At the same time, the consumption of various resources by urban residents drives the flow of resources, which will have a series of impacts on the environment. On the left side of the system, there are mainly activities related to the economy and logistics industry. Taking road, railway, air and other major modes of transportation as an opportunity, the choice of different modes of transportation and the corresponding energy consumption are displayed at the bottom right of the system. At the upper right of the system flow diagram, renewable resources and non-renewable resources are added to the emergy index system and the concept of emergy is introduced. The emergy yield rate, environmental load rate, emergy sustainability indices and other economic and environmental perspectives can measure the sustainable development of the transportation industry.
We mentioned the emergy analysis method and integrated the emergy evaluation index into the system dynamics model as its variable to evaluate the sustainability of the transportation system. The modeling objectives are described below. To make the approach clearer, the methodological framework of the SD model is shown in
Figure 4.
Through the construction of the causality diagram, the regional transportation system is deeply analyzed to clarify its internal influencing factors and the causality between each influencing factor and further realize the development trend of the regional transportation system.
We carry out a simulation prediction analysis and comparison on the output results of variables in the transportation system dynamics model. A better simulation effect can be achieved by using the dynamic simulation analysis method to describe the flow process of variables in the transportation system. On this basis, we can explore the way to achieve the sustainable development of regional transportation.
3.3.3. Model Test
We calculated the data of energy storage and the conversion of transportation industry in Qinghai Province. In order to verify the validity of the model, the historical data of the output emergy of the transportation industry in Qinghai was input into the model from 2010 to 2017 and the data from 2018 to 2050 was predicted. The error between the simulated value and the actual value is less than 5%, indicating the validity of the flow standard deviation. The verification results are shown in
Table 5.