A positive effect of urbanization was the building of which public transportation systems?

2.4.2.5 Subway (metro) systems

The subway (metro) system represents the backbone of urban transportation system in many large urban areas around the world. The layout and length of its (mostly underground) infrastructure network can be different, mainly depending on the size of a given urban area, its population and intensity and type of particular activities, including the presence and scale of operations of other urban mass transport systems. In many cases, these other systems, as mentioned earlier, have complemented to the subway (metro) system. The spatial layout of the system's infrastructure networks can be one of those shown in Fig. 2.7B. The lines constituting the networks are the exclusive right-of-way enabling frequent, punctual, reliable, and fast transport services compared to other urban mass transport systems.

Some of the characteristics of these infrastructure networks are shown in Fig. 2.17. This is an example of the relationship between the length of network and the number of stations of 29 subway (metro) systems operating worldwide (UITP, 2014; http://www.railway-technology.com/features/featurethe-worlds-longest-metro-and-subway-systems-4144725/) (Fig. 2.15).

As can be seen, the number of stations increases with increasing of the network length at decreasing rate. One of the rather strong influencing factors is that the larger networks also cover the parts of larger urban areas with lower spatial density of population as the prospective demand, thus requiring less dense stations along the corresponding lines.

Some statistics indicate that the subway (metro) systems currently operate in 148 urban areas (cities) around the world with about 540 lines. The total daily passenger demand served by these systems has been about 150 × 106 passengers per day (UITP, 2014). Specifically in Europe, the total length of network of 2800 km in 45 cities and the fleet of 21,500 vehicles (cars) serves more than 30 million passengers per day.

TYPICAL ENERGY EXPENDITURE VALUES

The following tabulations provide “typical” unit values of the energy expended in the construction, operation, and maintenance of various urban transportation systems. These values are “typical” only in the sense that they represent a range of often widely disparate estimates published in various sources. As such, they should be viewed as no more than order-of-magnitude guides to the amounts of energy that would be expended in the creation and continuing operation of proposed urban transportation systems.

TABLE 1. Guideway Construction

Sources for Tables 1 through 4: CBO (1977), Apostolos (1978).

TABLE 2. Vehicle Construction

TABLE 3. Operating Energy, Total

Abstract

Massive transportation development in mass urbanization has accelerated fossil fuel energy consumption, and greenhouse gas emissions from vehicles account for nearly 30% of global warming. We need clean energy for urban transportation sectors to meet their energy demand and avoid global warming. In this study, a wind energy system model was developed by integrating advanced technological and mathematical aspects to obtain a potential solution for the total energy demand of all urban transportation systems. Detailed analysis of the theoretical wind energy systems was modeled by a series of mathematical equations that were then proposed for use in transportation sectors to naturally meet their energy demand. To better explain this technology and its application in transportation sectors, and taking the practical scale and components into consideration, a sample experimental model of a car was also described as a hypothetical experiment. Interestingly, both the theoretical modeling and experimental analysis of the car confirm that a turbine can be a promising tool to utilize wind energy to generate electricity from self-renewing resources to power the car; importantly, wind energy is clean and globally abundant. The proposed wind energy system could be an innovative large-scale technology in energy science that can enable vehicles that produce energy from the wind when the vehicle is in motion, thus meeting 100% of the vehicle's energy demand.

Perspectives for Sustainable Urban Transportation

Reflecting on the above perspectives, it becomes clear that sustainable development is an anthropocentric concept. The driving force of this vision is the needs of current and future generations. With this approach, sustainable development becomes an inherently open and process-oriented concept.

Human needs are complex and dynamic with various interdependencies and constantly evolving specific needs. Therefore, the development of needs-oriented urban transportation systems will never be a linear process. Mistakes are a necessary part of the learning process. Doing, being, and interacting allow us to realize and fulfill different needs in different ways (Rauschmayer et al., 2011). This autonomous and culturally embedded search for improved quality of life is an important part of human development and the sustainability concept. Different means of mobility are used in the numerous local contexts to fulfill various needs. The degree of fulfillment is unknown beforehand for current users and even more so for future generations. Designing sustainable urban transportation is an ongoing task that guides policy making, today and in the future, and that leads to a variety of transportation practices and systems.

Specific boundaries are necessary for this learning process and help to avoid the arbitrary application of the sustainability concept. In transportation, the limitations set to reduce environmental impacts are often exceeded. There is broad consensus that the current amount of natural resources used and the amount of greenhouse gases, noise, and air pollutants emitted cannot be regarded as sustainable. Limitations can also be formulated for social and economic dimensions in order to ensure basic accessibility for all transportation users and to develop transportation systems that are affordable for current and future generations. The premise for social equity from an intra- and intergenerational perspective should be that everyone has access to activities, to mobility, and to destinations in order to participate in social, economic, and cultural life.

Indicator systems are helpful for monitoring progress. Their influence in policy processes, however, currently seems to be limited. They sometimes serve “more to rationalisen than to generate decisions or even as mere symbolic gestures” (Gudmundsson et al., 2016, p. 213).

Cities are hubs for ideas, commerce, culture, science, productivity, and social development. They are important drivers for innovation in all sectors of society. Local stakeholders need support from higher levels of policy making in order to achieve the right framework conditions. Local stakeholders have, at the same time, the chance and the responsibility to exploit their position and ability to cope with the urban challenges, such as congestion and environmental problems. We hope that this article creates interest and initiative for stakeholders to convene and to continue the journey toward sustainable urban transportation.

Sustainable urban transportation is an ongoing process that relates to basic human needs in an open and process-oriented manner. The overarching goal of sustainable urban transportation is to provide everyone the opportunity to be mobile while protecting our resources for the future. This ensures the fulfillment of personal needs while also respecting the set limitations of these transportation and mobility practices.

12.5.2 Under the Covid-19 pandemic

When we look back to the year 2020, the Covid-19 pandemic has dramatically changed the way people live and also brought huge negative effects on enterprises and supply chains. Many enterprises went bankrupt because they cannot operate normally because of being affected by the Covid-19 pandemic, as well as the supply chains. It is believed that continuous awareness of rapid-changing events is absolutely significant during the Covid-19 pandemic. Social distancing and the virtualization of work have become the mainstream lifestyle of the world. For PLM, contactless logistics is becoming the new norm. Blockchain-enabled PLM also showed great potential. One representative scenario is the dispatching and management of epidemic prevention supplies.

In that outbreak, the shortcomings of the traditional supply chain, such as information asymmetry, information opacity, low traceability, and lack of effective coordination, have become increasingly prominent. At the beginning of the epidemic, the urban transportation system was restricted, logistics and supply chains were correspondingly severely constrained, and the dispatching and management of materials and supplies were in chaos. For example, due to the unclear supply and demand in the early stages, there were problems such as the detention of donated materials, the surge pressure on storage management, the misallocation of resources, and even some cases of abusing, reselling, and withholding of donated materials.

The blockchain-enabled logistics and supply chain have great prospects for solving these problems. Blockchain technology can be used to integrate the business flow, logistics, information flow, capital flow, and other data of the supply chain into the same industrial chain with a real-time record, realizing the integration of the streams of the supply chain. This will make the whole operation open and transparent, increase the reliability of data in the supply chain, and improve the dispatching and management of the epidemic relief supplies.

The Covid-19 vaccine supply chain is another good case for blockchain applications. With the advent of vaccines, how to distribute them around the world has become one of the most important questions. Proper preservation of vaccines to maintain their effectiveness can be critical or even a matter of life or death in the distribution process. Traditionally, vaccines need to be transported by trucks and planes in rigid temperature-controlled conditions throughout their full lifecycle from production to final vaccination. Besides, they also need to be stopped and stored at multiple distributors before arriving at their destinations, then refrigerated once they arrive. For the last mile, they may need to be transported by lorry to nearby towns, and in some remote areas, donkeys, bicycles, and camels may even be needed for final delivery. Across this lifecycle, all the cold storage, including refrigeration house, cold-chain vehicles, cold containers, and vaccine distributors used in this process must be qualified in accordance with corresponding specifications. However, it is really a huge challenge for transportation and storage compliance over the lifecycle. It is assumed by the World Economic Forum (WEF) that there will be a 20%–30% loss during storage and transit due to preexisting problems (Shukla et al., 2020).

Blockchain technology is believed to be a necessary and indispensable tool for the successful and equitable global distribution of Covid-19 vaccines in the future. It is emphasized by WEF that a fair and reasonable allocation mechanism needs to be established for the epidemic prevention vaccines supply chain. Besides, optimal vaccines distribution should be global rather than national or regional. Therefore, a fully trusted, publicly verifiable, consensus-driven information system is necessary to prevent the interference of vested interests, and to ensure that the data cannot be tampered with. Blockchain and distributed ledger technology will therefore be indispensable tools for the equitable distribution of vaccines globally since they can build trust and transparency, and can reduce vaccine wastage rates, avoid shortages, and equitably distribute vaccines to the global population. Moreover, it is also pointed out that it is necessary to build a global alliance organization including vaccine researchers, pharmaceutical companies, manufacturers, wholesalers, health-care workers, and governments to facilitate the response to outbreaks and corresponding measures. For example, it can be realized by using IoT technology and blockchain to track the authenticity and logistics of each dose of vaccine, as well as various relevant supporting measures to ensure an equitable distribution of vaccines. Blockchain, without doubt, is the essential technology to build trust among multiple participants and organizations.

3.2.1 Power sources for electric vehicles

With a goal of tackling the air pollution problem brought about by urban transportation systems, Electric Fuel Ltd. (EFL, part of the Arotech Corporation) developed full-sized all electric, zero-emission vehicles utilizing Zn-air fuel cell technology in 1999. This Zn-air system used “reconstructable cells”, i.e., one type of the mechanically rechargeable Zn-air fuel cells discussed in Section 3.1.1. Fig. 3-8 shows a network for operating electric buses/trucks run with EFL's Zn-air system, which requires three linked system elements (denoted as (a), (b) and (c)) for powering electric vehicles [97].

The first element (a) is a “discharge-only” Zn-air fuel cell pack with a replaceable Zn fuel cassette. The anodic cassettes are made of Zn particles in a KOH aqueous electrolyte. A Zn-air cell is constructed by inserting the anode into a separator envelope, which is sandwiched between two air reduction cathodes. The specific energy of this fuel cell is 200 Wh kg−1. EFL's transit buses were powered with three trays of 6 modules each (47 Zn-air cells/module), with a total on-board energy of 312 kWh and peak power of 99 kW. An auxiliary NiCd battery pack (22 kWh) was integrated in the buses to improve the peak power to 125 kW and allow energy recovery during braking. The on-road testing of these buses showed a driving range of 280 km at a speed of ∼50 km h−1 between Zn refueling. The maximum speed for the buses is limited to ∼80 km h−1.

The second element (b) is a refueling station for mechanical exchange of detachable cassettes, including the Zn fuel and current collector frame. One advantage of EFL's Zn-air fuel cell system is that the mechanical exchange of Zn-air modules facilitates a short refueling time (within 10 min), which is far more efficient compared with hours of plug-in recharging for vehicles powered by electrically rechargeable batteries.

The third element (c) is a central regeneration facility for recycling Zn anodes, functioning as a fuel refinery comparable to an oil refinery but with minimal adverse impact on the environment. EFL adopted an electrowinning cell to electroextract Zn (with high surface area and low corrosion rate) from the zincate solution that is produced by dissolution of the ZnO discharged product in a KOH electrolyte.

EFL successfully ran field tests of a transit bus in the United States (2000) and a postal fleet vehicle in Germany (1996–1998) using the developed Zn-air technology, and had three Zn regeneration plants established in Bet Shemesh in Israel, Trofarello in Italy and Bremen in Germany. However, the electric vehicle program applying Zn-air technology at EFL is inactive now, with a transition to developing Li batteries for hybrid EVs that started in 2012 [98]. Limited driving speed and acceleration, together with the cost of establishing central regeneration plants, are the primary challenges that EFL's Zn-air fuel cell technology faces when competing with the current automotive market, especially compared with the well-developed Li battery technology.

More energy companies have emerged in the 21st century with a focus on developing electrically rechargeable Zn-air battery technology with application to EVs. Innovations made on improving each battery component, including the Zn anode, electrolyte and air cathode, are gradually making this goal achievable. One of the pioneering companies, ZAF Energy Systems (US) founded in 2011, have claimed a Zn-air battery system using a solid polymer electrolyte, which exhibits at least two times the energy (400 Wh kg−1) and one-third the price of a Li-ion battery [5]. ZAF researchers have developed a highly efficient bi-functional air cathode (with catalytic perovskites particles [99]) and a proprietary Zn electrode that limits dendrite growth. These developments ensure that the Zn-air battery can be recharged for at least 500 cycles. ZAF's chief technical officer has predicted commercialization of this battery for EVs by 2016–2018 [100]. Similar advancements are claimed by the Israeli company Phinergy, which is devoted to developing high-energy metal-air technology, with an emphasis on Zn-air rechargeable batteries for transportation applications. They have cited an innovation breakthrough on fabricating an extremely durable air cathode, with a lifespan of thousands of hours, and a proprietary Zn anode which is immune to dendrite formation [101]. However, Phinergy has not commercialized their Zn-air batteries or applied them to EVs yet.

4.2.2 CNG/LNG transit buses

In China, many transit bus companies are state-owned or partially state-owned. The decision-makings of the transit bus companies are highly affected by local governments. Under the increasing pressure from urban air pollution, many local governments considered CNG/LNG transit buses as a part of clean urban transportation system, and announced CNG/LNG transit bus development targets. Table 6 summarizes the local development targets and financial incentives for CNG/LNG transit buses [72–79]. CNG and LNG transit buses are different in terms of range capacity, vehicle cost, fuel cost, infrastructure requirement, etc. Specifically, as the prices of CNG and LNG fuels are determined through different mechanisms, the comparison of fuel cost can change significantly over time. Generally, LNG transit buses show higher market potential in the long term. As a result of both financial and administrative factors, China׳s CNG/LNG transit buses have been growing rapidly over recent years, and are expected to maintain the current growth trend in the coming years.

Table 6. Local targets and incentives for promoting natural gas vehicles.

Before CNG/LNG transit buses, LPG transit buses have been regionally promoted as a possible alternative to diesel transit buses [80]. However, due to the limited production capacity of LPG fuel, LPG transit buses faced the challenge of high fuel cost. The tailpipe emissions from LPG buses are also very controversial. As a result, many of such deployment projects have been canceled.

Abstract

China is now in the process of rapid urbanization. City׳s operating efficiency was directly determined by the scale and efficiency of energy consumption and flow. The pattern, scale and efficiency of urban carbon flow are not only important indicators that reflect urban efficiency and sustainable development, but also important references in the formulating low-carbon and sustainable energy polices for cities. Through establishing a theoretical framework and calculation method, this paper studied the carbon flows of Nanjing urban system in three different levels. It shows that urban production and transportation system, urban living system, rural production system and rural living systems are the major part of urban system in the carbon flow. The carbon flows between Nanjing and the external system, was much higher than the carbon flows among different internal subsystems. If the embodied carbon is taken into account, carbon flow from the urban to rural system of Nanjing was clearly greater than the flow in the opposite direction. With economic development and the implement of energy-saving and emission reduction policy, the carbon productivity and carbon flow efficiency in Nanjing has improved significantly since 2000. Fossil energy consumption, urbanization, agricultural activities, rural life demands and trade are key factors with major impact on urban carbon flows in Nanjing. Therefore, adjusting industrial structure, urban expansion control, and developing renewable energy are main measures to realize sustainable development of Najing city. Furthermore, the dual urban–rural structure in Nanjing brought large exchanges of products and embodied carbon between urban and rural areas, indicates that urban carbon flow and its efficiency was highly influenced by urban–rural structure, which will further aggravate carbon flow burden of urban systems.

3.1 General information

Annual Scientific Production – The research on EB is not new. However, the scientific community engaged in more in-depth research on this topic in this century. The articles collected in this review were published in twelve years, from 2008 to 2020. There is some minimal scientific production before 2008 and the content of those articles is not very relevant in the scope of this work. Fig. 4 depicts the number of publications by year and the cumulative publication concerning the EB research.

From 2008 to 2013, little attention was paid to EB studies, mainly reflecting the early development of technologies that could bring feasibility and economic attractiveness to vehicle deployment in the market. However, since 2017 the annual article publication rates have risen sharply. In the last four years, 63.4% of the total articles (282 out of 445) were published. The average annual growth rate of the analysed timeframe is 26.2%. The analysis indicates the attention that has been paid to the field, which is likely to increase in the forthcoming years. As aforementioned, the penetration of EB in the market is increasing fast; therefore, the scientific community is developing research on a large scale to address the main challenges concerning the deployment of those vehicles in urban transportation systems.

Journal Publications – The analysis developed in this subsection aims to present representative sources in which researchers can find relevant content. Fig. 5 presents an assessment of the number of papers per year in the top-10 journals with the highest number of articles on EB research.

Scientific production and collaboration sorted by country – This analysis evaluates the regions where research is well developed. Fig. 6 displays the distribution of publications by country.

The findings highlight the countries investing more in developing EVs globally and creating policies to deploy this technology. China, the USA, and the European Union (EU) present the largest EV markets. Considering just buses, China has the biggest fleet – Chinese cities hold roughly 98% of the deployed EB in the world [49]. The pursuit of a more electrified transportation sector by those countries has also been associated with the fact that they present the highest CO2 emission levels globally, and vehicle electrification can help diminishing the transportation sector's environmental impact [50]. As a result, the USA, China, and Europe implemented several policies and legislation to promote the market and technology development. In the USA, a series of programs have been applied to stimulate and enhance EB research and production, including financial incentives in the forms of tax credits, tax exemptions, and other different forms of subsidies [51,52]. Since 2010, China is also adopting incentive policies covering favourable laws, national sales targets, municipal air-quality targets, and subsidies to intensify core technologies development (including EB) [49,53]. Consequently, the Chinese EV industry rose around 360% from 2010 to 2016 in sales and production [54,55]. The European Parliament adopted in 2019 new regulations on public procurement to encourage investment in clean buses (namely electric, gas, or hydrogen) [56]. Further, the Zero Emission Urban Bus System (ZeEUS) project financed the deployment of EB in the EU [57,58]. As a result, the EB registrations increased by 170.5% from 2018 to 2020 [59]. These examples shed light on the dominance of the research in the field by those regions.

2.4 The mechanism of urbanization/industrialization on energy consumption and emissions

Industrialization usually refers to an increase in industrial activity, and most papers assume that industrialization leads to higher energy usage because higher value added manufacturing uses more energy than does traditional agriculture or basic manufacturing [29]. Thus, the relationship between industrialization and energy consumption/emissions is mandatory—positive (Tables 1 and 2). So the literatures pay more attention to the mechanism of urbanization on energy consumption and emissions.

The process of urbanization/industrialization can increase energy consumption through three ways: (a) energy conversion from one form to another—typically, from bioenergy to fossil fuels; (b) indirect energy consumption in goods-producing and transporting activities; and (c) direct energy consumption in final uses such as household and transportation [14,17,87]. However, there are also significant mechanisms reducing energy demand in urban buildings and urban transportation systems, such as more efficient home appliances, and shortened travel distances because of compact city layouts and economies of scale [55].

Madlener and Sunak [4] found that different processes and mechanisms of urbanization substantially affect urban structures as well as human behavior, and thereby affect energy demand differently in developed and developing countries as well as within developing countries. Additionally, urbanization is closely associated with economic growth because of the industrialization process through which rural agricultural labor force transferred to the industrial and service sectors located in the urban areas [30]. Thus, urbanization usually leads to greater energy consumption since energy consumption is a normal good. Zhou et al. [6] believed that urbanization affects energy consumption patterns in four ways and they include adjustmentof the industrial structure (the process of industrialization), optimization of energy supply, technological improvements, and more efficient use of resources.

Why urbanizationincreases residential energy consumption? Holtedahl and Joutz [39] presented two arguements. First, moving to urban areas increases household accessibility to electricity, since households can be more readily connected to the grid. Second, households who already had access to electricity in rural areas are likely to increase their consumption in urban areas because of increased use of existing appliances and purchase of new ones.

Some studies investigate how urbanization affects China׳s energy-consumption patterns [6,88]. Three energy-consuming sectors intricately associated with urbanization are identified and analyzed: residential households, transportation, and the building materials industry. Urbanization has profoundly affected each of the sectors. Moreover, the latter two are high energy consumption and potentially high carbon producing. Further, an important reason for the positive relationship between urbanization/industrialization and energy consumption in China is the investment-driven economic growth model [52].

It is argued that the use of fossil fuels and cement in estimated CO2 emissions is hypothesized to be related to the heightened transportation and construction requirements of urbanization, and may be expected to exhibit a positive association with the percentage share of urban population in total population which is referred to as 'percent urban’ [15,17]. O׳Neill et al. [24]concluded that urbanization can affect emissions through several channels: the effect of urbanization on economic growth (through labor supply), the direct effect of urbanization on consumption preferences, and the indirect effect of urbanization on preferences through income growth. Poumanyvong and Kaneko [11] considered three relevant theories to explain the environmental impacts of urbanization: ecological modernization, urban environmental transition,and compact city theories. Zhu and Peng [78] found that urbanization generally affects CO2 emissions in three ways: (a) energy used for production and residential consumption increases in line with urbanization as both situations increase energy demand, resulting in increased emission; (b) the requirements levels of infrastructure and reidential dwellings grow with urbanization, increasing demand for building materials (especiallycement products), which are sizeable sources of emissions; (c) urbanization involves conversion of grasslands andwoodlands, land-use changes that increase emissions.