Energy Sources for Road Transport in the Future ince hominid started to utilize fire as a power source, the entire human society has advanced so much in civilization and economy. Until now, 4/5 of the world energy consumption relies on fossil fuel, especially for automobiles. Our interest is how to make the emerging new energy vehicles save more cost, function more efficiently, and release fewer pollutants than traditional vehicles with internal combustion engines as we enter a new stage dominated by emerging energy vehicles. Transportation is a major consumer of energy, and it is also a major source of carbon dioxide. Supply, demand, and distribution in the whole transportation system are interrelated, but each is also independent. Therefore, it is crucial to optimize petroleum extraction, electricity generation, and energy transformation efficiency of distinctive automobiles and to further improve the energy efficiency of automobiles, especially electric vehicles, which will greatly mitigate our reliance on raw energy. By evaluating tank-to-wheel (TTW) efficiency, we found that the efficiency of traditional internal combustion engines is only 12.6%. On the contrary, because electric vehicles have simple structures and use electricity to power either part or all of their wheels, they have higher efficiency. The TTW efficiency of hybrid and pure electrical vehicles can reach 26.6 and 54%, respectively.1 However, because hybrid cars also need fossil fuels, their well-to-wheel (WTW) efficiency is only 20%, still higher than the efficiency of the traditional model.2 Currently, to improve the energy efficiency of automobiles, we need to consider differences in each country’s energy structure and characteristics of different electric vehicle models. We believe that electric vehicles still have much room for improvement in efficiency with the advancement of energy technology and technology in electric vehicles. Emerging Energy Vehicles and Fuel Consumption. To better compare the energy efficiency of different vehicle models, we calculated the energy consumption of traditional, hybrid, pure electric, and fuel cell automobiles with the same wheel power output, as discussed below (see Figure 1). Traditional internal combustion engine (ICE) vehicles are the least energy efficient and have an efficiency of only 12.7%. Pure electric vehicles have an efficiency of 51.6%, which is the highest. Fuel cell vehicles (FCVs) and hydrogen ICE (HICE) vehicles have efficiencies of 28.3 and 26.6%, respectively. From TTW, the efficiencies of hybrid electrical vechicles (HEVs), battery electric vehicles (BEVs), FCVs, and ICE are 22.6, 21.8, 21.4, and 10.8%, respectively. Clearly, we can conclude that hybrid vehicles have an advantage if we only consider using petroleum to power automobiles. Figure S1 lists the electricity conversion efficiency of major energy sources (including raw oil, natural gas, coal, nuclear power, and other new energy) and sources of electricity production in several countries in Table S1.3
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© XXXX American Chemical Society
Taking the China power grid as an example (the overall efficiency is 42.3%), if the power is directly from the grid, the WTW energy efficiencies of the EV and HEV are 21.8 and 22.6%, respectively. However, if the electricity comes from a hydro or gas-steam combined cycle unit, the WTW energy efficiency of EV (46.44 or 30.96%) is much higher than that of the HEV (22. 6%). Under the same energy in wheel rim, the energy utilization of the EV is the highest, but the source of power needs to be considered under the view of TTW efficiency. Much work needs to be undertaken in renovating ICE vehicles to save energy. Currently, the WTT efficiencies of EVs, HEVs, and FEVs are 42.3, 75.5, and 84.9%, respectively. Yet, there exist sharp regional differences in the distribution of the oil and hydrogen fuel energy. Therefore, to the countries that are short on oil resources or are reliant on imports, EVs have obvious advantages in energy efficiency, environmental protection, and cost efficiency. EVs with electric energy as the energy source will be further developed along with the increasing proportion of hydropower, wind power, nuclear power, and other renewable energy. Though generating hydrogen from industrial byproducts has an efficiency of 75%, TTW efficiency of FCV models is 28.3%, and yet, the total efficiency is only 21.4%. To date, the main obstacle to further promoting the FCV models in the market is the extraction and preservation of hydrogen. Improvements in energy efficiency of vehicles can greatly reduce oil dependency. These improvements include increased use of lightweight materials, automatic energy recovery technology, and more advanced batteries. Energy Structure Disparity among Dif ferent Countries. Petroleum, coal, and natural gas are still major sources of world energy even though the amount of nuclear, water, and renewable energy is increasing. On the basis of the current situation, people need not only improve the efficiency of traditional vehicles that utilize fossil fuels but also investigate electric vehicles that can rely on new energy sources. After more discoveries of petroleum reserve and advancement in drilling and digging technology, the distribution of fossil fuel storage has changed. However, the supplier and consumer of this energy still geographically stay far apart, and such disparities even bring issues related to trade balance and national security. Further, importing energy cannot be consistent and economical, and therefore, each country consumes different kinds and amounts of energy, as shown in Table.1. On the other hand, although fossil fuels still dominate the whole energy supply, new energy sources including nuclear, water, and others will be more and more adopted. By 2050, Received: April 13, 2017 Accepted: May 5, 2017
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DOI: 10.1021/acsenergylett.7b00291 ACS Energy Lett. 2017, 2, 1334−1336
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Figure 1. Energy flow of different types of vehicles: (A) 85% efficiency of gas and diesel extracted from petroleum; (B) a hybrid vehicle that includes an electric motor and parallel drive train, which eliminates idling loss and captures some energy of braking; (C) overall efficiency of 42.5% for electric power in China is (including power from coal, water, new energy, and their combination); (D) using byproducts from manufacturing to produce hydrogen.
Table 1. Proportion of Primary Energy Consumption by Fuel in the Major Countries4,a U.S. China India Saudi Arabia Japan Germany Brazil total world
OIL (%)
natural gas (%)
coal (%)
nuclear energy (%)
hydroelectricity (%)
renewables (%)
total (MTonnes)
36.4 17.5 28.3 59.3 43.1 35.9 48.1 32.6
30.2 5.6 7.1 40.7 22.2 20.5 12.1 23.7
19.7 66 56.5
8.3 1.0 1.2
2.6 8.1 4.6
2.8 1.8 2.2
27.7 24.9 5.2 30.0
7.1 1.2 4.4
4.3 1.5 28.2 6.8
2.5 10.2 5.2 2.5
2298.7 2972.1 456.1 239.5 456.1 311.0 296.0 12928.4
a
BP Statistical Review of World Energy. Primary energy comprises commercially traded fuels, including modern renewables used to generate electricity. Note: Oil consumption is measured in million tonnes.
forth. Therefore, it demands a new type of hydrogen cell catalyst and a more efficient and economical source of hydrogen in order to lower the cost of fuel cells. Biofuels are an ideal substitution for fossil fuels. The wide use of biofuels will solve the environmental problem caused by using straw fuel in the countryside. It has been reported that second-generation biofuels can reduce 96% of carbon dioxide release. Figure S3 compares the hydrogen conversion efficiency of several major energy sources. It knows that electrolyzing water to produce hydrogen is not economic until we can use renewable nuclear or water energy to produce electricity with low costs. On the contrary, industrial and fuel extraction methods for hydrogen production are more energy efficient. Therefore, we propose that three types of countries and regions can focus on developing FCVs:
5.3% of the world energy generation will come from these new sources compared to 0.4% now. This improvement, therefore, will simulate the development of electric and hybrid vehicles that are energy cost efficient and environmentally friendly, as shown in Figure S2.5 However, generating electricity through those new energy sources mentioned above is still costly and unlikely to be widely used. According to a report by the UN Intergovernmental Panel on Climate Change, only 2.5% is being utilized on the potentials of renewable energy in the world. 6 Under appropriate public policy support, renewable energy can provide 70% of the global electricity need by 2050, which can reduce a total of 2200−5600 tons of carbon dioxide release. Thus, electricity generation technology using renewable energy still faces many economic and technical challenges.7 The FCV has advantages in driving mileage and fast charging, but some technical and social issues limit its promotion in the market, such as catalysts in hydrogen cells, proton exchange membranes, construction of hydrogen charging stations, and so
(1) Countries with abundant hydrogen resources and regions that can produce hydrogen at lower cost (e.g., Brazil) (2) Regions that enjoy low-cost electricity or large storage of electricity. 1335
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(2) Righolt, C. H.; Rieck, G. F. Energy chain and efficiency in urban traffic for ICE and EV. EVS27, Barcelona, Spain, November 17−20, 2013. (3) World Development Indicators: Electricity production, sources, and access. http://wdi.worldbank.org/table/3.7 (2017). (4) BP Statistical Review of World Energy June 2015. http://www.bp. com/content/dam/bp/pdf/energy-economics/statistical-review-2015/ bp-statistical-review-of-world-energy-2015-primary-energy-section.pdf (2015). (5) More information is available at the World Energy council. World Energy Scenarios: Composing energy futures to 2050. https://www. worldenergy.org/publications/2013/world-energy-scenarioscomposing-energy-futures-to-2050/ (2013). (6) Special Report on Renewable Energy Sources and Climate Change Mitigation. https://www.ipcc.ch/pdf/special-reports/srren/SRREN_ FD_SPM_final.pdf (2012). (7) Chu, S.; Majumdar, A. Opportunities and challenges for a sustainable energy future. Nature 2012, 488, 294.
How to best adjust the energy sources for vehicles is not easy for any country. When fossil fuel energy is not enough to meet people’s demand, we have to use renewable energy to fill the gap. In this context, district energy networks of automobiles with new energy have been shaped. Such networks will greatly reduce consumption of nonrenewable energy. Each country should further encourage creation and innovation and also take the market into consideration.
Yanhui Zhang*,†,‡ Sukwon Cha† Wei Feng‡ Guoqing Xu*,‡,§
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† Department of Mechanical and Aerospace Engineering, Seoul National University, Gwanak-ro 1, Gwanak-gu, Seoul 151-744, Republic of Korea ‡ Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055, People’s Republic of China § School of Mechatronic Engineering and Automation, Shanghai University, Shanghai 200444, People’s Republic of China
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00291. Detailed description of power conversion efficiency of major energy sources and hydrogen production with various energy and details of world development indicators and world energy scenarios to various energies (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (Y.Z.). *E-mail:
[email protected] (G.X.). ORCID
Yanhui Zhang: 0000-0002-1244-5723 Notes
Views expressed in this Energy Focus are those of the authors and not necessarily the views of the ACS. The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (No. 61603377, No. 61573337). This research was also supported by the visiting fellows program of CAS PIFI (No.2017VCA0032), the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20153010031930). The Brain Korea 21 Plus program should also be acknowledged for their partial support.
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REFERENCES
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DOI: 10.1021/acsenergylett.7b00291 ACS Energy Lett. 2017, 2, 1334−1336
