Tsinghua University provincial-level lifecycle study finds fuel-cycle criteria pollutants of EVs in China could be up to 5x those of natural gas vehicles due to China’s coal-dominant power mix
12 January 2013
|Consumption-based power mixes and NG transmission distances by Chinese province in 2010. Credit: ACS, Huo et al. Click to enlarge.|
A province-by-province life cycle analysis of natural gas and electric vehicles by a team from Tsinghua University concludes that while, from the perspective of reducing greenhouse gas (GHG) and criteria pollutant emissions, natural gas vehicles (CNGVs) are “an option with no obvious merits or demerits”, electric vehicles (EVs) are “an option with significant merits and demerits in this regard” due to China’s heavily coal-based electricity generation (national average of about 77%).
In regions where the share of coal-based electricity is relatively low, EVs can achieve substantial GHG reduction, the team reports in a paper in the ACS journal Environmental Science & Technology. However, the fuel-cycle PM10, PM2.5, SO2, and NOx emissions of EVs could be up to five times higher than those of ICEVs (internal combustion engine vehicles) and CNGVs. While the increases in PM10 and PM2.5 emissions are less important because of the low contribution of light duty vehicles to national PM10 and PM2.5 emissions, the NOx and SO2 increases are significant enough to notably change total national emissions, they conclude.
...China has made a firm commitment to improve its air quality. In February 2012, the China State Council approved its first national ambient air quality standard for PM2.5, which will come into effect by the end of 2015. Since substantial coal use (equivalent to 50% of global coal consumption) is the primary reason for the poor air quality, China is considering setting a cap on its use. According to the 12th Five-Year Plan of the China Coal Industry (2011−2015) formulated by the National Development and Reform Commission of China (NDRC), coal consumption will be limited to 3900 million metric tons (MMT) by 2015, which will present a challenge given the tremendous increase in coal use within the last 5 years (2300 MMT in 2006 and 3200 MMT in 2010).
Setting a cap on coal use while maintaining economic growth at a high level means that China must resort to other energy sources, such as natural gas (NG) and renewable fuels, to fill the energy supply gap. Since coal-use control is a near-term goal, NG appears more adequate than other energies to replace coal in the short term, because of its greater availability and technological maturity. Under such circumstances, there will be a battle between coal and NG in many sectors, particularly the on-road transport sector, which is exclusively petroleum-dependent but currently facing a worldwide oil shortage.
The enormous growth in vehicle population in China (from 5 million in 1990 to 100 million in 2011) has raised serious concerns about energy supply security...Many fuel-substitute measures have been taken, both coal-based and NG-based, with electric vehicles (EVs) and compressed natural gas vehicles (CNGVs) as their representatives. China recently launched several EV demonstration programs (e.g., the Ten Cities, Thousand Vehicles Program) and issued numerous economic policies favoring the purchase of EVs. In April 2012, the State Council approved the “Development Plan of Energy-Efficient and New-Energy Vehicles (2012−2020),” which plans to achieve accumulated sales of 500,000 new-energy vehicles (including hybrids and EVs) by 2015, and 5 million by 2020. [Earlier post.] Since EVs use electricity as fuel and Chinese electricity is primarily generated from coal-fired power plants (national average about 77% in 2010), EVs can be regarded as a coal-based option.
...The tremendous vehicle growth and the oil shortage portend competition between coal and NG for vehicle fuel. The preferred fuel path(s) will depend to a great extent on their climate and environmental performance, which is of national concern. The fuel paths may perform differently by region because energy-use characteristics differ significantly by region.—Huo et al.
The Tsinghua study examines fuel-cycle (well-to-wheels, WTW) emissions of greenhouse gases (GHGs, including CO2, CH4, and N2O) and criteria pollutants (PM10, PM2.5, SO2, and NOx of conventional gasoline internal combustion engine vehicles (ICEVs), gasoline hybrid vehicles (HEVs), EVs, and CNGVs in present-day China (2010) and projected in the future (2030). It also assesses the strengths and weaknesses of EVs and CNGVs from a perspective of climate change and environmental impact.
The team chose to conduct the study at the provincial level because using national average data might severely underestimate or overestimate results for technologies that primarily operate in designated cities—i.e., EVs and CNGVs in China—because energy-use characteristics differ significantly by region.
Fuel-cycle emissions of EVs are generated mainly from power generation processes, while those of CNGVs are from NG recovery and processing, NG transportation (via pipelines), NG compression (using electricity), and vehicle operation (NG combusted in engines). Fuel-cycle emissions of both EVs and CNGVs in China are highly subject to a variety of regional factors.
For the study, they applied the GREET (Greenhouse gases, Regulated Emissions, and Energy use in Transportation) model from Argonne National Laboratory, adjusted with Chinese energy and emission data.
Among the findings were:
GHG emissions of gasoline ICEVs and HEVs are generally the same across provinces, since the emissions occur mainly during vehicle operation. EVs and CNGVs show “remarkable” variations in such emissions across provinces. Although EVs have slightly higher GHG emissions than ICEVs and CNGVs in the North and Northeast regions where the share of coal-based electricity is large, EVs can achieve a greater GHG reduction benefit than HEVs in the Central and South regions, owing to their large hydropower shares.
Compared to EVs, CNGVs have very limited benefit in the reduction of GHG emissions, no more than 10% in most provinces; CNGVs might even increase such emission in eastern provinces that obtain NG from thousands of miles away. On a national average, EVs and CNGVs can reduce fuel-cycle GHG emissions per kilometer traveled by 20% and 6%, relative to ICEVs. In the future (the year 2030), improving vehicle fuel economy will reduce GHG emissions of ICEVs, CNGVs, and EVs.
If China decreases the share of coal-based electricity in the mix from 75% to 65% and increases combustion efficiencies of coal-fired power plants from 35% to 40%, EVs are expected to have a 27% lower fuel-cycle GHG emission level than ICEVs.
Although EVs have a greater GHG reduction benefit than CNGVs, they may increase life-cycle emissions of criteria pollutants, simply because coal is heavily involved. Improving the fuel efficiency of EVs from 20 kWh/100km to 14 kWh/100km, decreasing the share of coal-based electricity from 75% to 65%, increasing combustion efficiency of coal-fired power plants from 35% to 40%, and the new emission standard for new-build power plants will help reduce the PM emission of EVs. Nonetheless, EVs still have 2−3 times higher PM10 emissions than ICEVs, and coal mining will be the dominant source, contributing more than 70% of the fuel-cycle PM10 emissions.
On a national average, CNGVs can reduce NOx and SO2 emissions by 18% and 22% compared to ICEVs, respectively, but EVs will increase NOx and SO2 emissions by 120% and 370%. EVs have lower NOx fuel-cycle emissions where hydropower accounts for a large share. Similar to the PM results, EVs in provinces importing a large percentage of coal and electricity (e.g., Beijing, Shanghai, and Guangdong) could have local NOx and SO2 emissions comparable to ICEVs.
|Fuel-cycle GHG emissions of EVs, CNGVs, gasoline ICEVs and HEVs. Credit: ACS, Huo et al. Click to enlarge.|
|Fuel-cycle NOx and SO2 emissions of EVs, CNGVs, gasoline ICEVs, and HEVs. Credit: ACS, Huo et al. Click to enlarge.|
The original reason for developing EVs and CNGVs is to substitute oil, not to reduce emissions of GHGs or any pollutant. Both EVs and CNGVs will clearly achieve this goal by saving over 98% of oil use. Any additional benefits will help these vehicles to earn more credits, but any negative environmental influences caused could make them lose in competition, especially at the present time when China is working very hard to clean its air.
...fuel-cycle NOx and SOx emissions of EVs are expected to substantially decline in the near future. Until then, however, these NOx and SO2 emissions are a disadvantage of EVs in comparison with CNGVs. One current solution is to develop EVs in accord with the cleanness levels of regional power plants, first in places where electricity is cleaner, e.g., with ample non-coal electricity or more emission control equipment installed in coal-fired power plants.
Ranking provinces by emission levels of their power plants is helpful in formulating the EV promotion plan. In this way, EVs are competitive with CNGVs. Further, using EVs in the large coal or electricity-importing provinces (e.g., Beijing, Shanghai, and Guangzhou) may not exacerbate local air quality because the majority of emissions originate in other provinces.
The competition between CNGVs and EVs will continue. In addition to their potential impact on the climate and ambient environment, it is important to compare the potential impacts of developing CNGVs and EVs on the energy supply system. Assuming LDGVs travel 18000 km per year, replacing all LDGVs with CNGVs will increase national NG demand by 70%, but EVs will only increase electricity demand by 5% and coal demand by 2%. Obviously, EVs have much smaller impacts on national energy supply. However, CNGVs may have a much lower cost than EVs, which requires further study.—Huo et al.
Hong Huo, Qiang Zhang, Fei Liu, and Kebin He (2012) Climate and Environmental Effects of Electric Vehicles versus Compressed Natural Gas Vehicles in China: A Life-Cycle Analysis at Provincial Level. Environmental Science & Technology doi: 10.1021/es303352x
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